U.S. patent number 3,892,890 [Application Number 05/358,647] was granted by the patent office on 1975-07-01 for process for forming carbon coatings.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Akira Misumi, Takayoshi Onodera, Kazuo Sunahara, Kiyoshi Watanabe.
United States Patent |
3,892,890 |
Watanabe , et al. |
July 1, 1975 |
Process for forming carbon coatings
Abstract
A nickel-phosphorus alloy layer having an amount of 4-12 percent
by weight phosphorus is first allowed to adhere to the surface of a
substrate and the nickel-phosphorus alloy layer-coated substrate is
heated in an atmosphere of a noncombustible hydrocarbon-containing
gas to effect thermal decomposition of the gas, thereby depositing
carbon onto the surface of said substrate to form a dense carbon
coating thereon without causing thermal deformation of the coated
substrate or decrease in stiffness of the resulting carbon coating.
It is also possible to first subject a nickel-phosphorus alloy
layer formed on the surface of a substrate to preliminary oxidation
and then heat the substrate in the presence of a
hydrocarbon-containing gas to effect thermal decomposition of said
gas.
Inventors: |
Watanabe; Kiyoshi (Mobara,
JA), Misumi; Akira (Mobara, JA), Onodera;
Takayoshi (Mobara, JA), Sunahara; Kazuo (Mobara,
JA) |
Assignee: |
Hitachi, Ltd.
(JA)
|
Family
ID: |
27292593 |
Appl.
No.: |
05/358,647 |
Filed: |
May 9, 1973 |
Foreign Application Priority Data
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May 12, 1972 [JA] |
|
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47-46394 |
May 12, 1972 [JA] |
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47-46395 |
May 12, 1972 [JA] |
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47-46396 |
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Current U.S.
Class: |
427/249.1 |
Current CPC
Class: |
C23C
16/0281 (20130101); C23C 16/26 (20130101) |
Current International
Class: |
C23C
16/02 (20060101); C23C 16/26 (20060101); C01b
031/00 () |
Field of
Search: |
;117/46CB,46CC,13E,169R,71M,71R,49,16R,16C |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Martin; William D.
Assistant Examiner: Bell; Janyce A.
Attorney, Agent or Firm: Craig & Antonelli
Claims
What is claimed is:
1. A process for forming a carbon coating on an iron surface of an
article to be coated, which process comprises the steps of first
forming on the iron surface of said article a nickel-phosphorus
alloy layer having an amount of 4 - 12 percent by weight of
phosphorus, and of then heating the nickel-phosphorus alloy
layer-coated article in a non-combustible gas mixture containing a
thermally decomposable hydrocarbon gas, thereby effecting a thermal
decomposition reaction of said gas at a temperature below
700.degree.C to deposit carbon contained in said gas on the surface
of said nickel-phosphorus alloy layer-coated article to form the
carbon coating thereon, said nickel-phosphorus alloy layer
effecting catalytic action to deposit carbon with a
nickel-phosphorus compound of said alloy layer, so that said carbon
coating is firmly bonded to said surface at said temperature.
2. A process according to claim 1, wherein the nickel-phosphorus
alloy layer is formed by the electroless plating method.
3. A process according to claim 2, wherein an amount of phosphorus
to be contained in the nickel-phosphorus alloy layer is controlled
by adjusting a pH value of a solution of the electroless
plating.
4. A process according to claim 1, wherein the amount of phosphorus
contained in the nickel-phosphorus alloy layer is at least 5
percent by weight based on the nickel.
5. A process according to claim 1, wherein a thickness of the
nickel-phosphorus alloy layer is 0.1 to 6 microns.
6. A process according to claim 1, wherein the hydrocarbon is an
acetylene-containing gas.
7. A process according to claim 6, wherein the non-combustible gas
containing 0.1 to 1.5 percent by volume of acetylene gas.
8. A process according to claim 7, wherein the non-combustible gas
contains an inert gas selected from the group consisting of
nitrogen, helium, and argon.
9. A process according to claim 7, wherein the non-combustible gas
also contains Liquefied Petroleum Gas or methane gas.
10. A process according to claim 6, wherein the non-combustible gas
is composed mainly of an inert gas selected from the group
consisting of nitrogen, helium, and argon, said non-combustible gas
containing at least one member selected from the group consisting
of 0.1 - 1.5% by volume of acetylene gas, 0.1% or more of Liquefied
Petroleum Gas, and 0.1 percent or more of methane gas in an amount
within such a range that the resulting whole mixed gas is
non-combustible.
11. A process according to claim 10, wherein the non-combustible
gas contains further at least one member selected from the group
consisting of ethane and ethylene gases in an amount within such a
range that the resulting whole mixed gas is non-combustible.
12. A process according to claim 6, wherein the non-combustible gas
comprises acetylene gas and at least one member selected from the
group consisting of hydrogen and methane gases.
13. A process according to claim 1, wherein the non-combustible gas
constitutes a reduced pressure atmosphere surrounding the article
to be coated.
14. A process according to claim 13, wherein the reduced pressure
is 100 Torr or less.
15. A process according to claim 1, wherein the nickel-phosphorus
alloy layer coated-article is heated in the presence of the
non-combustible gas at such a rate of temperature elevation that a
catalytic ability of the nickel-phosphorus alloy layer may not
markedly be reduced, thereby effecting at a temperature of
450.degree.-700.degree.C a thermal decomposition of said
non-combustible gas to deposit carbon contained in said gas on the
surface of said nickel-phosphorus alloy layer-coated article to
form a carbon coating thereon.
16. A process according to claim 15, wherein the article to be
coated with carbon is a shadow mask for color picture tube of
television.
17. A process according to claim 15, wherein the nickel-phosphorus
alloy layer is applied to the article to be coated by the
electroless plating method.
18. A process according to claim 17, wherein an amount of
phosphorus to be contained in the nickel-phosphorus alloy layer is
controlled by adjusting a pH value of a solution of the electroless
plating.
19. A process according to claim 15, wherein the non-combustible
gas contains 0.1 to 1.5 percent by volume of acetylene gas.
20. A process according to claim 19, wherein the non-combustible
gas contains an inert gas selected from the group consisting of
nitrogen, helium, and argon.
21. A process according to claim 19, wherein the non-combustible
gas is composed mainly of an inert gas selected from the group
consisting of nitrogen, helium, and argon, said
hydrocarbon-containing gas containing at least one member selected
from the group consisting of 0.15-1.5 percent by volume of
acetylene gas, at least 0.1% by volume of Liquefied Petroleum Gas,
and at least 0.1% by volume of methane gas in an amount within such
a range that the resulting whole mixed gas is non-combustible.
22. A process according to claim 21, wherein the non-combustible
gas contains further at least one member selected from the group
consisting of ethane and ethylene gases in an amount within such a
range that the resulting whole mixed gas is non-combustible.
23. A process according to claim 21, wherein the nickel-phosphorus
alloy layer has a thickness of at least 1 micron.
24. A process according to claim 15, wherein the non-combustible
gas constitutes a reduced pressure atmosphere surrounding the
article to be coated with carbon.
25. A process according to claim 24, wherein the reduced pressure
is 100 Torr or less.
26. A process according to claim 1, wherein the nickel-phosphorus
alloy layer-coated article is heated at such a rate of temperature
elevation that a catalytic ability of said nickel-phosphorus alloy
layer may not be markedly reduced, thereby effecting at a
temperature of 500.degree.-700.degree.C a thermal decomposition of
said non-combustible gas to deposit carbon contained in said gas on
the surface of said nickel-phosphorus alloy layer-coated article to
form a carbon coating thereon.
27. A process according to claim 26, wherein the article to be
coated with carbon is a shadow mask for color picture tube of
television.
28. A process according to claim 26, wherein the nickel-phosphorus
alloy layer is applied to the article by the electroless plating
method.
29. A process according to claim 28, wherein an amount of
phosphorus to be contained in the nickel-phosphorus alloy layer is
controlled by adjusting a pH value of a solution of the electroless
plating.
30. A process according to claim 26, wherein the hydrocarbon is an
acetylene-containing gas.
31. A process according to claim 30, wherein the
acetylene-containing gas comprises acetylene gas and at least one
member selected from the group consisting of hydrogen and methane
gases.
32. A process according to claim 26, wherein the non-combustible
gas constitute a reduced pressure atmosphere surrounding the
artilce to be coated with carbon.
33. A process according to claim 32, wherein the reduced pressure
is a pressure of 100 Torr or less.
34. A process according to claim 26, wherein an atmosphere of the
non-combustible gas is an acetylene-containing gas having a
pressure of 100 Torr or less.
35. A process according to claim 34, wherein the
acetylene-containing gas comprises acetylene gas and at least one
member selected from the group consisting of hydrogen and methane
gases.
36. A process according to claim 34, wherein the nickel-phosphorus
alloy layer has a thickness of at least 1 micron and the
acetylene-containing gas comprises acetylene gas and at least one
member selected from the group consisting of hydrogen and methane
gases.
37. A process according to claim 1, wherein the nickel-phosphorus
alloy layer coated on the surface of the article is oxidized and
then heated in the presence of said non-combustible gas, thereby
thermally decomposing said gas to deposit carbon contained in said
gas on the surface of the nickel-phosphorus alloy layer-coated
article to form a carbon coating thereon.
38. A process according to claim 37, wherein the article to be
coated with carbon is a shadow mask for color picture tube of
television.
39. A process according to claim 37, wherein the nickel-phosphorus
alloy layer is applied to the article to be coated with carbon by
the electroless plating method.
40. A process according to claim 37, wherein an amount of the
nickel-phosphorus alloy layer is controlled by adjusting a pH value
of a solution of the electroless plating.
41. A process according to claim 37, wherein the nickel-phosphorus
alloy layer contains at least 5 percent by weight of phosphorus
based on the nickel.
42. A process according to claim 37, wherein the non-combustible
gas contains 0.01-5 percent by volume of acetylene gas.
43. A process according to claim 37, wherein the non-combustible
gas contains an inert gas selected from the group consisting of
nitrogen, helium, and argon.
44. A process according to claim 37, wherein the non-combustible
gas is composed mainly of an inert gas selected from the group
consisting of nitrogen, helium, and argon, said non-combustible gas
containing at least one member selected from the group consisting
of 0.01-5 percent by volume of acetylene gas, at least 0.1 percent
by volume of Liquefied Petroleum Gas, and at least 0.1 percent by
volume of methane gas in an amount within such a range that the
resulting whole mixed gas is non-combustible.
45. A process according to claim 37, wherein the nickel-phosphorus
alloy layer contains at least 5 percent by weight of phosphorus
based on the nickel, and the non-combustible gas contains 0.01-5
percent by volume of acetylene gas.
46. A process according to claim 45, wherein the non-combustible
gas contains an inert gas selected from the groups consisting of
nitrogen, helium, and argon.
47. A process according to claim 45, wherein the non-combustible
gas is composed mainly of an inert gas selected from the group
consisting of nitrogen, helium, and argon, and containing at least
one member selected from the group consisting of Liquefied
Petroleum Gas and methane gas in an amount within such a range that
the resulting whole mixed gas is non-combustible.
48. A process according to claim 45, wherein the nickel-phosphorus
alloy layer has a thickness at least 1 micron.
49. A process according to claim 37, wherein the oxidation is
carried out in an oxygen-containing atmosphere while increasing
temperature.
50. A process according to claim 1, wherein said temperature is
650.degree.C or lower, such that thermal deterioration of said
article to be coated is prevented.
51. A process according to claim 1, wherein said temperature is in
the range of about 450.degree.C to near 700.degree.C.
52. A process according to claim 1, wherein said carbon coating has
a carbon density of at least 1.9 g/cm.sup.3.
53. A process according to claim 1, wherein said carbon coating is
a stable crystalline film.
54. A process according to claim 1, wherein said nickel-phosphorus
compound is Ni.sub.x P wherein x is about 2.
55. A process according to claim 1, wherein said nickel-phosphorus
alloy acts to deposit an initial small amount of carbon particles
thereon such that said carbon particles become nuclei for carbon to
be deposited.
56. A process according to claim 1, wherein said temperature is
achieved within one minute or less.
57. A process according to claim 1, wherein said thermal
decomposition reaction is maintained for at least about 40
minutes.
58. A process according to claim 57, wherein said thermal
decomposition reaction is maintained for at least about 20
minutes.
59. A process for forming a carbon coating a surface comprising
coating a carbon layer on a surface comprising a substrate of iron
coated with a nickel-phosphorus alloy layer containing 4 - 12
percent by weight of phosphorus by heating a non-combustible
hydrocarbon containing gas at a temperature below 700.degree.C to
thermally decompose said gas, thereby depositing carbon on said
surface, said nickel-phosphorus alloy layer effecting catalytic
action to deposit carbon with a nickel-phosphorus compound of said
alloy layer, so that carbon is firmly bonded to said surface at
said temperature.
60. A process according to claim 59, wherein the hydrocarbon is an
acetylene-containing gas.
61. A process according to claim 60, wherein the non-combustible
gas contains 0.1-1.5 percent by volume of acetylene gas.
62. A process according to claim 61, wherein the non-combustible
gas contains an inert gas selected from the group consisting of
nitrogen, helium, and argon.
63. A process according to claim 59, wherein the non-combustible
gas is composed mainly of an inert gas being at least one of
nitrogen, helium, and argon, said non-combustible gas containing at
least one of 0.1 -1.5 percent by volume of acetylene gas, 0.1% or
more of Liquefied Petroleum Gas, and 0.1percent or more of methane
gas in an amount within such a range that the resulting whole mixed
gas is non-combustible.
64. A process according to claim 63, wherein the non-combustible
gas contains further at least one of ethane and ethylene gases in
an amount within such a range that the resulting whole mixed gas is
non-combustible.
65. A process according to claim 59, wherein the non-combustible
gas comprises acetylene gas and at least one of hydrogen and
methane gases.
66. A process according to claim 59, further comprising the step of
oxidizing said nickel-phosphorus alloy layer prior to said
deposition of carbon on said surface.
67. A process according to claim 59, wherein said surface is a
shadow mask surface for a color picture tube.
68. A process according to claim 59, wherein said temperature is
650.degree.C or lower, such that thermal deterioration of said
article to be coated is prevented.
69. A process according to claim 59, wherein said temperature is in
the range of about 450.degree.C to near 700.degree.C.
70. A process according to claim 59, wherein said nickel-phosphorus
compound is Ni.sub.x P wherein x is about 2.
Description
This invention relates to a process for forming a graphitic carbon
coating at relatively low temperatures on the surface of an object
composed of a metal or other material. More particularly, the
invention pertains to a novel and efficient process for forming a
carbon coating at a relatively low temperature below 700.degree. C
on the surface of an object (hereinafter referred to as
"substrate") composed of such metal, for example, as an iron sheet
or other substance in such a state that the carbon coating formed
has a predetermined thickness, is high in density, and is firmly
stuck to the surface of the substrate.
Carbon has not only exellent corrosion resistance to chemical
agents but also good physical properties such as heat radiation
ability. Carbon is also low in secondary-electron emission, and
hence in the field of electronic appliances and parts thereof,
those appliances or parts are frequently used after being coated on
the surface thereof with carbon with the view of preventing said
surface from secondary-electron emission. A process for coating the
surface of substrates with carbon is roughly divided into three
methods, i.e. spreading, evaporating or spattering, and
decomposing. The spreading method, i.e. the first one of the
above-mentioned three methods, is to spread or spray a
carbon-containing paint on a substrate. This method is very simple,
but involves such problem as uniformity in thickness of the spread
coatings or reproducibility. The said method has also such drawback
that because of weak adhesion between the substrate and the spread
carbon layer, in the spread substrate obtained by this method the
carbon layer easily peels off from the substrate when said
substrate is heat-treated. This method is not a practical means to
obtain a uniform and dense carbon layer on the surface of
substrate. As the second method, there is a vacuum evaporation
coating. According to this method, a dense and strongly adhesive
carbon coating can be obtained. In order to evaporate and deposit
carbon in vacuo, it is necessary to maintain the temperature of
carbon necessary therefor at a high temperature of at least
2,500.degree.C. In the case where treatment of a large number of
substrates relatively large in size is intended to be carried out
economically, application of this method thereto is extremely
difficult. Moreover, it is almost impossible to deposit carbon to
form a carbon coating having a uniform thickness on a substrate of
complicated shape, because carbon shows its directional qualities
when it evaporates. In either method, there are problems as to
uniformity and adhesivity of the carbon coating to be formed on the
surface of substrate, and also there is such drawback that the
carbon coating formed on the surface of substrate is apt to peel
off therefrom. Such condition as to adhesivity is required also
even under such severe conditions where organic compound type
binders cannot be used, when a substrate, for example, is a part of
electron tube, because the part is used in vacuo. For example, in
case an anode of receiving tube is coated with carbon with the view
of maximizing heat radiation, the anode is heated in vacuo at any
temperatures between about 600.degree. and about 900.degree.C, and
the heating is repeated. For instance, when a shadow mask for color
picture tube of television is coated with carbon with the view of
preventing secondary-electron emission, the condition under which
the shadow mask is used is such that the use of inorganic type
binders having characteristics to promote secondary-electron
emission is absolutely not permissible, and also there is required
such adhesive strength of the carbon coating that said coating is
not substantially worn by combustion even when heated in air at a
temperature of about 400.degree. to 500.degree.C for 1 to 2 hours,
and even when the coating has a thickness of about 1 micron or
thicker, said coating will not peel off from the said mask due to
slight contact or compact during handling thereof in the assembling
operation. Further, when a carbon-coated substrate is used
particularly in an electron tube within which is applied a high
voltage, it is an extremely important requirement for the carbon
coating formed on the substrate that not only the carbon coating is
firmly stuck to the surface of said substrate but also carbon
particles themselves in said carbon coating adhere to one another,
because if a carbon coating forms carbon dusts that float within
the tube, the dusts eventually cause spark to give serious damages
or serious troubles in the circuit of the electron tube. In order
to cope with such requirement, graphitic carbon (hereinafter
referred to as "graphite") is desirably used in forming a carbon
coating for use in such application. Furthermore, in the case of a
corrosion resisting carbon coating formed on a boat for use in the
liquid place method of GaAs production, for example, a graphite
coating is preferably used so that the coating may not peel off
from the boat due to mechanical rubbing during operation at an
elevated temperature.
Thus, it is apparent that a graphite coating deposited on a
substrate by thermal decomposition of a graphitic carbon-containing
reaction gas is most preferable as the graphite coating as
mentioned above, which does not substantially subject to loss in
weight even when heated in vacuo and/or scrubbed mechanically or
heated in air at a temperature of about 400.degree. to about
500.degree.C, and which does not also peel off from a substrate
even placed in such state that neither organic nor inorganic
binders can be used, and which does not produce carbon dusts.
As the third method, there is a method in which a carbon-containing
gas is allowed to react under heating to deposit carbon, thereby
coating a substrate with the depositing carbon. According to this
method, the carbon coating formed on the substrate strongly sticks
to the substrate and is high in density. This method is widely used
in general. The greatest drawback of this method, however, is that
the carbon coating is formed by thermal reaction wherein a
substrate is heated in the presence of the reactive gas stream
comprising methane, propane, etc. at such elevated temperature as
high as 1,200.degree. to 2000.degree. C. Because of such high
heating temperatures employed in this method, when a substrate is a
thin metallic sheet or a material low in heat resisting property,
the carbon-coated substrate suffers from its deformation or change
in quality of the substrate itself, with the results that the
coated substrate obtained thereby cannot be used. Even a substrate
is a material which is sufficiently resistant to the aforesaid
temperature, the substrate is liable to damages such as thermal
deformation because it is heated at high temperature. Particularly,
in the case of a shadow mask for color picture tube of television,
which is very thin (usually about 0.2 mm or thinner) and in which
even a slight deformation of a body of the mask is not permissible,
the heating temperature employed therefor should be suppressed at
least to 700.degree.C or below.
There have heretofore been proposed various methods as processes to
improve a high temperature decomposition method, wherein
hydrocarbon-containing compounds are subjected to thermal
decomposition at a relatively low temperature such as 850.degree.C
or thereabout to deposit carbon onto a substrate to form a carbon
coating which is high in density and has an excellent adhesion.
Some of the proposed method have been put into practice. However,
in case a substrate to be coated with carbon is, for example, such
a thin iron sheet as a shadow mask for color picture tube of
television, the substrate which is to be treated under such
temperature as 850.degree.C or higher brings about such various
disadvantages as carburizing, formation of carbide of iron, thermal
deformation, decrease in stiffness or spring-back. In order to
avoid such disadvantages, one must think of the possibility of
effecting carbon-coating at lower temperatures than 850.degree.C,
for example, about 650.degree.C or lower. There are few known
techniques as to carbon-coating effected at low temperatures, but
they are all found unsatisfactory. To cite one example of the said
known techniques, for example, there is a method wherein a nickel
disc of 30 mm in diameter, and 2 mm in thickness and of a porosity
of about 51 percent, formed from nickel powder, is heated at about
650.degree.C for 20 minutes in a gas stream prepared by mixing
propane gas with nitrogen in a volume proportion of 1:1, thereby
depositing carbon on the surface of disc to form a carbon coating
(refer to U.S. Pat. No. 3,311,505). That is, this method is to coat
the very porous nickel disc formed from nickel powder on its
surface with carbon, and hence this method cannot be applied at
all, for example, to the case where such a sheet as a thin iron
sheet having a thickness of 0.5 mm or less is intended to be coated
on its surface with carbon high in accuracy.
To mention another example of the aforesaid known techniques, there
is a method in which an iron substrate is heated at a temperature
of below about 650.degree.C for 0.5-2 minutes in the presence of a
mixed gas comprising 10 percent by volume of hydrocarbon and
0.75-2.0 percent by volume of carbon dioxide gas, thereby
depositing carbon on the surface of the substrate to form a carbon
coating thereon (refer to U.S. Pat. No. 2,344,908). In this method,
however, when the concentration of hydrocarbon in the mixed gas is
increased in order to form a carbon coating having a sufficient
thickness and, on the contrary, if the hydrocarbon concentration is
decreased in order to form a dense carbon coating, no carbon
coating having a sufficient thickness is obtained. According to
this method, therefore, it is not possible to coat a substrate with
carbon to form a dense carbon coating having a density of 1.9
g/cm.sup.3 and a coating amount of 1 mg/cm.sup.2.
The present invention has been made in the light of various
problems mentioned above which are associated with the prior art
techniques. A primary object of the present invention is to provide
a novel and effective process in which the surface of substrate
such as an iron sheet may be coated with a dense carbon to form
thereon without forming any carbide on the surface of the substrate
and without bringing about such disadvantages as thermal
deformation of the coated substrate or decrease in stiffness of the
resulting carbon coating.
Another object of the present invention is to provide a process in
which a carbon-containing gas is allowed to react at a relatively
low temperature of about 700.degree.C or below, thereby forming on
the surface of a substrate a dense carbon coating having a firm
adhesion between said carbon coating and the surface of the
substrate.
Other object of the present invention is to provide a process,
according to which even the surface of a thin iron sheet substrate
having a thickness of about 0.5 mm or less with carbon to form
thereon a carbon coating having a thickness of 1 mg/cm.sup.2 with
uniformity and with accuracy, said carbon coating being extremely
high in density, having a carbon density of about 1.9 g/cm.sup.3,
and having a high adhesive strength so that said carbon coating
does not peel off from the surface of the substrate even when
subjected to quick heating and cooling tests at a temperature of
600.degree.C or below.
The present inventors have found that in a process for depositing
carbon on the surface of a substrate by the reaction of a
hydrocarbon-containing gas the above-mentioned objects can be
accomplished by previously forming a suitable catalyst material on
the surface of the substrate, thereby depositing carbon utilizing a
strong catalytic action of said catalyst material to form a dense
carbon coating which adheres strongly to the surface of the
substrate.
The present invention is illustrated in more detail below with
reference to the accompanying drawings.
In the drawings, FIG. 1 is a graph showing the relationship between
a pH value of a nickel-phosphorus alloy plating solution and an
amount of phosphorus contained in the nickel-phosphorus alloy layer
obtained from said plating solution.
FIG. 2 is a sketch showing an example of the manufacturing
apparatus illustrating a process for preparing a carbon
coating.
FIG. 3 is a graph showing the relationship between an amount of
phosphorus contained in the nickel-phosphorus alloy layer, an
amount of carbon deposited, and a porosity of carbon.
FIG. 4 is a graph showing the relationship between an amount of
carbon deposited and a treatment time.
FIG. 5 is a graph showing the relationship between a Liquefied
Petroleum Gas (hereinafter referred to as "L.P.G.") concentration
in the reactive gas and an amount of carbon deposited.
FIG. 6 is a graph showing the relationship between an acetylene
concentration in the reactive gas and an amount of carbon
deposited.
FIG. 7 is a graph showing the relationship between a preliminary
heating time at 600.degree.C of a nickel-phosphorus alloy layer and
an amount of deposited carbon resulting from a thermal
decomposition reaction of acetylene gas.
FIG. 8 is a graph showing the relationship between partial
pressures of the resulting hydrogen associated with thermal
decomposition of a sample having a catalytic action and a sample
which lost its catalytic action in an atmosphere of acetylene gas,
and a heat reaction time.
FIG. 9 is a graph showing the relationship between amounts of
carbon deposited when an iron sheet substrate plated on its surface
with a nickel-phosphorus alloy as a catalyst is heated in an
atmosphere of each of the three reaction gases having different
compositions, and heating temperature.
FIG. 10 is a graph showing an amount of carbon deposited and an
amount of phosphorus contained in a nickel-phosphorus alloy layer
having a thickness of about one micron formed on the surface of an
iron sheet substrate.
FIG. 11 is a graph showing the relationship between a concentration
of oxygen in a nitrogen stream and an amount of carbon deposited in
the case where an iron sheet substrate is plated on the surface
with a nickel-phosphorus alloy to form a layer thereof having a
thickness of one micron and then this nickel-phosphorus alloy layer
is subjected to a preliminary oxidation treatment in a nitrogen
oxygen atmosphere at about 600.degree.C for 10 minutes.
As stated hereinabove, the present invention intends to coat the
surface of a substrate such as a thin iron sheet or a heat
resistant material with a dense carbon film having a firm adhesion
force, to the substrate by effecting reaction of a
hydrocarbon-containing gas at a low heat treatment temperature such
as 700.degree.C or below, which temperature was not conceivable at
all in the prior art processes. It is impossible to deposit on the
surface of a substrate by merely effecting a reaction of a
carbon-containing gas at such low temperature as intended in the
present invention. As explained previously, in order to accomplish
this object, it is necessary to utilize a strong catalytic action
of a material formed on the surface of a substrate intended to be
coated with carbon. The present inventors conducted experiments,
wherein the surface of a substrate on which a carbon coating layer
is intended to be formed was subjected to surface treatment of
every sort, and also carbon coatings were formed on the surface of
the substrates thus heated to a relatively low temperature of about
450.degree. to about 700.degree.C while bringing various types of
carbon-containing gases into contact with said substrates. As the
result, they have found that when a nickel-phosphorus alloy layer
is provided on the surface of the substrate, a reaction of a
carbon-containing gas can be effectively brought about even at a
low temperature of about 450.degree. to 700.degree.C owing to a
catalytic action of said alloy and a dense carbon coating layer
having a firm adhesion force can be obtained by deposition of
carbon resulted from the reaction. That is, process of the present
invention is based on the above finding, wherein the surface of a
substrate is plated with a nickel-phosphorus alloy having an
appropriate composition and the thus plated substrate is heated in
a gas atmosphere comprising a predetermined amount of a hydrocarbon
gas up to a predetermined reaction temperature within the range
from 450.degree. to 700.degree.C preferably within a fixed time to
carry out the reaction under heating, thereby forming a dense
carbon coating on the surface of said substrate. Moreover,
according to the process of the present invention, it is possible
to obtain dense carbon coatings formed on substrates on a large
commercial scale and moreover at low treatment temperatures even by
use of such flammable and highly explosive gas such as acetylene
gas is used as a starting hydrocarbon gas for forming a carbon
coating in combination with an appropriate diluent gas or by mixing
it with other gases in suitable proportions, thereby rendering the
reactive mixed gas inflammable as well as non-explosive.
Acetylene gas is, as is well known, one of the most unstable gases
among hydrocarbon gases, and is large in change of free energy
associated with the decomposition thereof within a relatively low
temperature range such as about 1,000.degree.C or below. That is,
generally at a low temperature of about 1000.degree.C or below
(hereinafter referred to as "low temperature"), acetylene gas
belongs to those which are most liable to thermal decomposition
among hydrocarbon gases. The reaction of acetylene is an exothermic
reaction and, it is represented, for example, at room temperature
by the following equations.
C.sub.2 h.sub.2 .fwdarw. 2c + h.sub.2
-.DELTA.h.sub.298.sub..degree.k = 54.19 kcal/mol
The generation of heat associated with such reaction will further
accelerate the thermal decomposition reaction of acetylene gas, and
hence said thermal decomposition reaction proceeds explosively. The
carbon results from such abrupt reaction, however, will not form
crystal because of its excessively fast rate of deposition and
tends to become amorphous. In case such reaction is intended to be
carried out on an industrial scale, it is a crux to control
skillfully such exothermic reaction to avert a danger of thermal
decomposition explosion. That is, an important point for making
possible a process in which acetylene gas is used on an industrial
scale and thermal decomposition of said gas is carried out is to
decrease the amount of acetylene gas used, or to slow down the
reaction by increasing partial pressure of hydrogen, or to prevent
a chain-like thermal decomposition associated with the reaction of
acetylene gas by use of methane gas together with acetylene gas
with an eye to the fact that the decomposition reaction of methane
gas is an endothermic reaction. As explained above, in order to
make it possible to effect selectively a thermal decomposition of
acetylene gas in a controlled atmosphere comprising acetylene gas
at a relatively low temperature in the presence of an optional
substrate, thereby depositing carbon on the surface of the
substrate to form thereon a stable crystalline carbon film, it is
most effective to coat the surface of said substrate under the
conditions as explained previously with a catalyst material which
will act in response to the progress of the selective thermal
decomposition reaction of acetylene gas. Such catalyst material is
preferably a material which can be coated on the surface of an
optional substrate in a simple as well as an economical manner.
The present invention has been made with an eye to various
problems: such technical difficulties associated with prior art
processes as illustrated in detail previously, characteristics of
the thermal decomposition reaction of acetylene gas, utilization of
the catalyst material as a countermeasure relative to the acetylene
gas reaction and so on. Accordingly, the present invention, as
explained previously, is to provide a novel process for coating the
surface of a substrate with a crystalline carbon layer which is
dense and having a high adhesion strength in a safe and stabilized
operational condition by utilizing a catalytic action of the
nickel-phosphorus alloy layer formed previously on the surface of
the substrate in response to the thermal decomposition reaction of
acetylene gas.
The nickel-phosphorus alloy layer formed on the surface of a
substrate in accordance with the present invention can be
chemically plated in a simple manner on an optional substrate,
irrespective of conductive and non-conductive materials, according
to plating technique known as the electroless plating method
without necessitating a direct current power source as may be seen
in the case of common plating procedure. Thus, the
nickel-phosphorus alloy layer of the present invention satisfies
the requirements as catalyst and is sufficiently of commercial
value. In the case of electroless nickel, nickel in which a solid
solution of phosphorus is supersaturated coexists with a compound
of nickel with phosphorus and, they have such properties that they
eventually form, when heated, crystalline nickel and a
nickel-phosphorus compound of Ni.sub.3 P. As to catalytic action of
the nickel-phosphorous alloy, amorphous nickel and a
nickel-phosphorus compound of Ni.sub.x P in which x is about 2,
play individually the most effective role as catalyst. From a
practical point of view, any nickel-phosphorous alloys can display
their efficiency as catalyst in practicing the process of the
present invention so long as the amount of the total phosphorus
component is 4% by weight or more. However, it has been found that
this alloy is converted on heating into crystalline nickel and the
compound of Ni.sub.3 P, whereby catalytic action of said alloy for
formation of graphite due to thermal decomposition of acetylene gas
is markedly reduced and finally its catalytic action completely
disappears. Therefore, in case a hydrocarbon gas is subjected to
thermal decomposition, it is necessary to employ such heating
temperature or rate of temperature elevation as may not lose a
catalytic action of the nickel-phosphorus alloy layer.
According to an embodiment of the present invention, there is
provided a process for forming a carbon coating on the surface of a
substrate, characterized by comprising the steps of plating the
surface of said substrate with a nickel-phosphorus alloy containing
4-12 percent by weight of phosphorus to form thereon a
nickel-phosphorous alloy layer, and of heating the
nickel-phosphorus alloy layer-coated substrate in a non-combustible
reaction gas composed mainly of at least one of such inert gases as
nitrogen, helium and argon and admixed with 0.1-1.5 percent by
volume of acetylene gas at a temperature of
450.degree.-700.degree.C and at a rate of temperature elevation so
that a catalyst ability of said nickel-phosphorus alloy layer in
formation of carbon from said acetylene gas.
According to another embodiment of the present invention, there is
provided a process according to the embodiment mentioned above,
which process is characterized by using the reaction gas composed
mainly of at least one of such inert gases as nitrogen, helium, and
argon and containing 0.1-1.5 percent by volume of acetylene gas
and, in addition thereto, at least one member selected from 0.1
percent by volume or more of L.P.G. and 0.1 percent by volume or
more of methane gas in an amount within such a range that the
resulting whole mixed gas may become non-combustible.
By virtue of adopting the above-mentioned processes, it is possible
to coat with a dense carbon having a high adhesion force the
surface of a substrate which is thin and should indispensably be
free from post-deformation, particularly the surface of a thin
metal sheet, and more particularly, for example, in the case of a
shadow mask for color picture tube of television, which mask is
thin and, in which no post-deformation is permissible and no
heating at a temperature above about 700.degree.C is
permissible.
The following procedure may be applicable to the preparation of a
plating solution from which is provided a nickel-phosphorus alloy
plated layer to be utilized as a catalyst material in the reaction
of a carbon-containing gas in accordance with the process of the
present invention. As a principal material, "SUMER" (a trade name
of a product of Nihon Kanizen K.K.) is used, to which a suitable
amount of hydrochloric acid or aqueous ammonia is added so that a
pH value of the mixture may be maintained within the range from
4-10. Subsequently, a substrate (having a thickness of 0.1 mm) to
be subjected to carbon coating treatment, for example, an iron
sheet, is subjected to degreasing and rinsing according to usual
procedure and the substrate is then immersed on the said plating
solution to effect plating while maintaining the temperature of
said plating solution at 80.degree.C according to the electroless
plating method, whereby a nickel-phosphorus alloy plated layer is
obtained, the phosphorus content in said layer being different
according to the pH value of said plating solution.
FIG. 1 is a graph showing the relationship between a pH value of a
plating solution and an amount of phosphorus contained in the
resulting alloy plated layer.
FIG. 2 is a sketch showing an example of the manufacturing
apparatus illustrating a process for preparing a carbon coating. In
this figure, substrates 1 plated with a nickel-phosphorus alloy in
the manner mentioned above are inserted into reaction tube 3 into
which is introduced reaction gas 2 comprising a mixture containing,
for example, 1.0 percent by volume of acetylene gas, 10% by volume
of methane gas, 0.5 percent by volume of a gas obtained by
vaporizing L.P.G. and nitrogen gas as the remainder (88.5 percent
by volume). In this case, the temperature inside reaction furnace 4
is previously fixed at an optional temperature within the range
from 450.degree. to 700.degree.C, and the amount of said reaction
gas 2 is previously and sufficiently controlled so that a
predetermined amount thereof may accurately flow therethrough. The
nickel-phosphorus alloy plated substrate is heated in a very short
time such as 0.5 to 1 minute up to a suitable heating temperature
within the said range from 450.degree. to 700.degree.C, and the
heating of said substrate is effected in an atmosphere of said
reaction gas 2 for a predetermined time. After completion of the
heating, the said substrates 1 are withdrawin from the said
reaction tube 3 through one end 5 of the tube. By virtue of
carrying out a series of treatments as illustrated above, it
becomes possible to deposit varying amount of carbon on the surface
of the nickel-phosphorus layer according to the composition of the
nickel-phosphorus alloy plated layer on the substrate, the reaction
time and the reaction temperature.
FIG. 3 shows an amount of carbon deposited (curve A), according to
the treatment methods explained above, on the surface of a
substrate and a porosity (curve B) of said carbon deposited. It is
shown from this figure that both the amount of carbon deposited and
the porosity thereof change according to the amount of phosphorus
contained in the nickel-phosphorus alloy layer formed on the
substrate. Generally, it is seen that the more is increased the
amount of phosphorus contained in the alloy layer, the more are
increased both the amount of carbon deposited and the porosity
thereof. The relationship between the rate of deposition of carbon
and the composition of the nickel-phosphorus alloy plated layer
does not change so much correlatively even when the absolute value
of the rate of reaction changes according to change in composition
or reaction temperature of the reaction gas. Thus, it is apparent
that the catalytic action of the nickel-phosphorus alloy layer
exerts a great influence on the carbon-depositing reaction at a low
temperature of 450.degree.-700.degree.C. The catalytic action of
the nickel-phosphorus alloy layer is closely related to the
existence of a nickel-phosphorus compound, Ni.sub.2 P, which is
separated at the very early stage of reaction from the
nickel-phosphorus alloy plated layer. The higher is the content of
phosphorus, the more is the amount of Ni.sub.2 P separated and the
larger is the catalytic ability of the alloy layer. From this, it
follows that the amount of carbon deposited is increased. However,
if the content of phosphorus exceeds 11 percent by weight, the
porosity of the deposited carbon coating tends to excessively
increase, as is clear from the porosity of carbon deposited (curve
B shown in FIG. 3). When the phosphorus content exceeds 12 percent,
the porosity of the carbon deposited becomes 35 percent or more,
with the result that the strength of the carbon film obtained
thereby is markedly reduced. Accordingly, in order to obtain a
dense carbon coating, the upper limit of the phosphorus content in
the nickel-phosphorus alloy plated layer is preferably 12 percent
by weight. On the other hand, if the phosphorus content becomes
less than 4 percent, the amount of carbon deposited thereby is
extremely small and, moreover, the thickness of the carbon coating
obtained is uneven. Thus, this carbon coating is considered
unsuitable for practical application. As will be appreciated from
the foregoing, the phosphorus content in the nickel-phosphorus
alloy plated layer is preferably within the range from 4 to 12
percent by weight for the purpose of obtaining a dense and stiff
carbon film.
In accordance with the process of the present invention, the
thickness of nickel-phosphorus alloy plated layer provided on the
surface of a substrate does not affect so much the amount of carbon
deposited and others. This is because the reaction is carried out
at a relatively low temperature such as 450.degree.-700.degree.C,
and hence the alloy layer is heated from room temperature up to the
reaction temperature in a relatively short time, whereby the alloy
reacts with the carbon-containing gas prior to the alloy layer
diffuses into the substrate, and then acts to deposit a very slight
amount of carbon particles which become henceforth nuclei for
carbon to be deposited. That is, the nickel-phosphorus alloy plated
layer acts as catalyst for the deposition of carbon even when the
thickness of the layer is extremely thin. In practice, however, the
thickness of the nickel-phosphorus alloy plated layer on the
substrate is preferably about 0.1 micron or thicker, whereby the
presence of the alloy layer on the substrate can be confirmed at
least by the visual inspection and the subsequent operational steps
can be stably assured. Even if the plated layer is thick, no
substantial adverse effect on the carbon deposition operation is
seen. Since the nickel-phosphorus alloy plating solution is
relatively expensive, the use of an unnecessarily thick layer
increases the cost of production and is not economically
advantageous. Taking all these considerations into account together
with other factors, the thickness of the nickel-phosphorus alloy
plated layer is advantageously about 6 microns or thinner. The
alloy layer having an unnecessarily large thickness is undesirable,
because such thick layer sometimes peels off from the substrate
during the carbon-deposition step.
As fully stated above, the nickel-phosphorus alloy plated layer
plays a very important role in the deposition of carbon at a low
temperature and it is also a fact that the properties of said layer
exert great influence upon the deposition of carbon. In order to
check the above fact, the following experiment was conducted.
Specimens were obtained by plating iron sheet substrates with
nickel-phosphorus containing 8 percent by weight of phosphorus to
form thereon a nickel-phosphorus alloy layer having a thickness of
1 micron. The specimens were subjected to heat treatment in pure
nitrogen gas stream for a predetermined time at the heating
temperatures of 400.degree.C, 500.degree.C and 600.degree.C,
respectively. Using each of the thus treated specimens, an
experiment of carbon coating was effected under heating in a mixed
reaction gas comprising 1.5 percent by volume of acetylene, 14
percent by volume of methane gas, 1 percent by volume of L.P.G. and
nitrogen gas as the remainder (83.5 percent by volume) at a
temperature of 560.degree.C for 20 minutes. In each case, the
temperature elevation from room temperature up to 560.degree.C was
effected in a very short time such as within 30 seconds. The
experimental results, i.e. relationship between the heat treatment
time (minute) and the amount of carbon deposited (mg/cm.sup.2),
were as shown in FIG. 4. In this Figure, curves C, D and E show the
cases where the heating temperatures of the nickel-phosphorus alloy
plated substrates in the nitrogen gas stream were 400.degree.C,
500.degree.C and 600.degree.C, respectively. As is clear from FIG.
4, the longer is the heat treatment time in the nitrogen gas
stream, the smaller is the amount of carbon deposited.
Particularly, this figure shows that when the heat treatment
temperature of the alloy layer is higher, the amount of carbon
deposited by decomposition is sharply decreased by heating for an
extremely short time. It has been understood that when heat is
applied in the manner as described above to the nickel-phosphorus
alloy plated layer prior to the deposition of carbon, the catalytic
ability of said layer is reduced. Such phenomenon is considered
ascribable to the fact that the catalytic ability of the said layer
is reduced due to conversion attended by heating of the compound
Ni.sub.2 P in the said layer into Ni.sub.3 P and other compounds,
and to extinction attended by heating of various defects in the
lattice which have been introduced into said layer (the
nickel-phosphorus alloy plated layer obtained by the method as
explained previously is well known to be in a non-crystalline
state). Accordingly, in the case of the present invention where a
nickel-phosphorus alloy plated layer is utilized as a catalyst,
heat treatment of said layer prior to deposition of carbon to form
a carbon coating should be avoided as much as possible. From this,
it follows that the rate of temperature elevation up to the
reaction temperature, i.e. 450.degree.-700.degree.C, be effected as
quickly as possible.
Subsequently, on the basis of the abovementioned results an
experiment was conducted to confirm the rate of temperature
elevation. In this experiment, specimens obtained by plating an
iron sheet substrates with a nickel-phosphorus alloy containing 8
percent by weight of phosphorus to form thereon the alloy layer
having a thickness of 1 micron were individually heated in a mixed
reactive gas comprising 1.5 percent by volume of acetylene, 14
percent by volume of methane gas, 1 percent by volume of L.P.G. and
nitrogen gas as the remainder (83.5 percent by volume) from room
temperature up to 560.degree.C at varying rates of temperature
elevation and the temperature of 560.degree.C was maintained for
about 20 minutes, thereby carrying out coating said specimens with
carbon. The results obtained showed that when the temperature
elevation time was within one minute the best performance of
coating was achieved, when the time exceeds 1 minute, the
performance became poor very sharply, when the time is more than 30
minutes, formation of the carbon coating became very small, and if
the time exceeds 120 minutes the effect of the alloy plated layer
was scarcely observed. That is, it has been understood that the
rates of temperature elevation confirmed in this experiments tend
to give the results substantially identical with those shown in
FIG. 4.
From the results of the experiment mentioned above, it is certainly
necessary that when a nickel-phosphorus alloy-coated iron substrate
is heated in a carbon-containing reaction gas, the temperature
elevation in a heating furnace up to the reaction temperature
should be effected as quickly as possible so that the catalytic
ability for carbon deposition of said alloy layer may not be lost.
Particularly, at a temperature below about 450.degree.C, no
deposition of carbon on the nickel-phosphorus alloy layer is
observed and the layer manifest itself, on the contrary, to act
only to lose its catalytic ability. It is an effective measure as
well as an important requirement that the temperature within a
heating furnace be elevated in a short time as shortly as possible
up to the reaction temperature necessary for the deposition of
carbon.
The results of studies on the composition of reaction gas used in
the process of the present invention are illustrated below.
Generally speaking, carbon-containing gases are combustible and apt
to explode when mixed with air or oxygen. Accordingly, in order to
put the present invention into practice on an industrial scale, the
practice of the process thereof should be sufficiently safe from
all possible dangers. From this viewpoint, in the present
invention, the carbon-containing gas, such as acetylene gas and
carbon monoxide gas as starting materials for deposition of carbon
which tend to decompose at relatively low temperature is mixed into
such inert and incombustible gases as helium, argon and nitrogen,
and the mixing ratios of these gases are limited to such a range
that the resulting mixed reaction gas may be rendered non-explosive
even when a large amount of air flows into the apparatus due to
destruction of the heating reaction tube. Using such
carbon-containing gases having varying compositions as mentioned
above, which are completely free from the possibility of danger of
explosion, experiments were conducted to carry out the
decomposition reaction of said gases under various conditions. As
the result, it has been confirmed that in each case of said gases,
carbon can be deposited by the reaction on a nickel-phosphorus
alloy plated layer so long as said layer has a high catalytic
ability and only when the suitable reaction conditions are
selected. For example, it is possible to obtain a dense and stiff
carbon coating in a stabilized state without any danger of
explosion of the reaction gas by use of a gas prepared by mixing at
least one of such inert gases as helium, argon and nitrogen.
As a result of a further detailed experimental studies on the above
subject, it has been found that when a small amount of L.P.G. is
added to the aforesaid three-component gas, the rate of deposition
of carbon is more rapidly accelerated and thereby a dense carbon
layer can be obtained in a very short time and hence the addition
of L.P.G. is extremely advantageous from an economical point of
view.
It has also been found that when a part of such inert gases as
argon, helium and nitrogen is replaced with methane gas in an
amount within such a range that the resulting mixed reaction gas
may remain incombustible, the influence of flow of the reaction gas
on the deposition of carbon by heat reaction becomes negligibly
small, and hence the addition of methane gas is extremely
preferrable, in particular, in obtaining a uniform carbon coating
on the surface of parts having complicated shapes.
The upper limits of L.P.G. and methane gas illustrated above have
been established with consideration for such safety in the
operation that the reaction mixed gas containing L.P.G and/or
methane gas will never explode even when mixed with air in any
proportions. If the object is only to obtain a dense and uniform
carbon coating, the whole portion of argon, helium or nitrogen in
the reaction mixed gas may be replaced with methane gas, L.P.G., or
a mixture thereof. The effect of addition of methane gas and L.P.G.
does not change in a wide range of their mixing proportion from
several percent to several tens of percent. Further, it should give
proper consideration to the problem of toxicity of the starting
gas, and from this viewpoint, acetylene gas is safer than carbon
monoxide gas and is advantageous when used in the operation carried
out on an industrial scale.
The effect on the amount of carbon deposited of the mixed gases of
varying compositions explained above is illustrated below. From an
economical reason, nitrogen gas was used as an inert gas, i.e. a
carrier gas. Even in case of various hydrocarbon gases, for
example, L.P.G., methane gas, acetylene gas, etc., actually only
acetylene gas decomposes to deposit carbon, and the other gases
added act to promote the decomposition of acetylene gas. The above
fact has been confirmed by experiment. First of all, the results
shown in FIG. 5 are to indicate the effect of the concentration
(percent by volume) of L.P.G. on the amount of carbon deposited
(mg/cm.sup.2) in the case where a mixed gas containing 0.15 percent
by volume of acetylene gas is introduced at a rate of 1 l per
minute into a reaction tube having an inside diameter of 35 mm to
deposit carbon by reaction of acetylene gas under heating to coat
an iron sheet substrate of 23 mm in length, 23 mm in width and 0.1
mm in thickness plated with a nickel-phosphorus alloy containing 8
percent by weight of phosphorus. In this case, the reaction
temperature, temperature elevation time, and reaction time are
540.degree.C, less than 30 seconds, and 20 minutes, respectively.
As is clear from the said figure, the addition of a small amount of
L.P.G. has a great effect on the amount of carbon deposited, and it
is understood that a sufficiently satisfactory carbon coating can
be obtained by the addition of L.P.G. in an amount of at least 0.1
percent by volume. Furthermore, FIG. 5 shows the results of an
experiment where commercially available L.P.G. is used. This L.P.G.
contains about 60 to about 80 percent by volume of propane gas, and
the remainder thereof contains butane gases and impure gases such
as propylene, butadiene, butene and others. These contaminant
gases, however, do not act disfavorably on the deposition of carbon
by the reaction of the reaction gas of nitrogen + acetylene +
L.P.G. system, but serve to bring about good results as a whole.
The upper limit of the amount of L.P.G. to be added should be
limited to such a range that the starting mixed gas may remain
incombustible even when brought into contact with any amount of
air. Such range is 7-15 percent by volume, though it varies
depending on the concentration of acetylene in the starting mixed
gas.
On the other hand, when a part (about 10 percent by volume or less)
of nitrogen gas in the mixed gas of nitrogen + acetylene + L.P.G.
is replaced with methane gas, the resulting mixed gas of nitrogen +
acetylene + L.P.G. + methane system brings no unevenness of coating
due to flow of the gas without substantially changing of the rate
of deposition of carbon, and hence the use of the mixed gas of this
system is extremely effective in the case of the treatment of parts
having very complicated shapes, where the gas is difficult to
uniformly pass through every portion of said parts. If an
inexpensive natural gas is used as a supply source of methane gas,
the cost of the starting mixed gas can further be reduced, and
hence a marked economical effect can be expected.
The amount of acetylene gas used as a starting gas for deposition
of carbon is explained below. FIG. 6 shows change in the amount of
carbon deposited when the proportion of acetylene gas in the
nitrogen + L.P.G. (0.5 percent by volume) mixed gas. In this case,
the reaction (heating) temperature is 560.degree.C, the temperature
elevation time and the reaction (heating) time are less than 30
seconds and 20 minutes, respectively, the flow rate of the gas is 6
liters per minute, and other conditions are the same as shown in
the embodiment illustrated by FIG. 5. As is clear from the results
shown in FIG. 6, the larger is the amount of acetylene, the higher
is the rate of deposition of carbon deposited. However, if the
amount of acetylene is more than 1.5 percent by volume, there is
observed a tendency that the resulting carbon coating becomes
somewhat porous. When the amount of acetylene gas is more than 2
percent by volume, dust-like carbon particles come to attack to the
surface of the carbon film deposited. Accordingly, in order to
obtain a dense and stiff carbon film, the upper limit of the amount
of acetylene gas to be mixed with a carrier gas is preferably 1.5
percent by volume. On the other hand, in order to obtain a
practically useful rate of reaction, at least 0.1 percent by volume
of acetylene gas should be mixed with the carrier gas.
When such hydrocarbon gases as ethylene and methane are added to a
mixed gas of nitrogen-acetylene-L.P.G. or
nitrogen-acetylene-L.P.G.-ethane in an amount within such a range
that the resulting mixed gas remains incombustible even when mixed
with air, the addition will have no particular adverse effect on
the deposition of carbon.
The embodiments of the present invention illustrated hereinbefore
refer to the cases where the plating of the nickel-phosphorus alloy
onto the surface of substrates is effected according to the
electroless plating method using SUMER solution, but it goes
without saying that the invention is not limited thereto.
As is apparent from the foregoing, the present invention has such
advantages, as compared with the conventional reduced pressure
method in which the deposition of carbon is effected in an
atmosphere under reduced pressure, that because of the practice of
the deposition of carbon under atmospheric pressure the apparatus
used therefor is very simple, the size of the object to be treated
is not restricted, and the desired carbon coating can be obtained
on a large scale with low cost. While the present invention has
been described with reference to the cases where the objects
(substrates) to be coated with carbon are limited to iron sheet,
any materials can be used so long as the materials as substrates
can be plated with the present nickel-phosphorus alloy because the
deposition of carbon can be achieved due mainly to the properties
of said alloy used, and as long as said materials as substrates can
resist to heating at a temperature of about 450.degree. to about
700.degree.C. This is also one of the characteristic features of
the present invention.
According to another embodiment of the present invention, there is
provided a process for forming carbon coatings, which process is
characterized by comprising a step of heating a nickel-phosphorus
alloy-plated substrate in an atmosphere of a reaction gas
comprising acetylene gas, said atmosphere being reduced in pressure
to 100 Torr or less, at a temperature of about 500.degree. to about
700.degree.C and at such a rate of the temperature elevation that
the nickel-phosphorus plated layer may not lose its catalytic
ability for deposition of carbon.
According to a still further embodiment of the present invention,
there is provided a process for forming carbon coatings, which
process is characterized by using in the above-mentioned process a
reaction gas containing acetylene gas and at least one of hydrogen
and methane gas.
FIG. 7 shows the results of the formation of carbon coatings on
substrates, wherein specimens of iron sheets of 0.18 mm in
thickness were coated with a nickel-phosphorus alloy containing 8
percent by weight of phosphorus by use of SUMER plating solution
adjusted pH to 6.0 to form thereon an alloy layer of 1 micron in
thickness, the specimens were then individually heated at a
temperature of about 600.degree.C in vacuo of 10.sup..sup.-6 Torr
for 5 minutes, 10 minutes, and 20 minutes, respectively, and
thereafter acetylene gas of a pressure of about 10 Torr was
introduced to bring about thermal decomposition reaction. As is
clear from FIG. 7, the longer is the preliminary heating time prior
to the practice of the thermal decomposition reaction of acetylene
gas, the smaller is the amount of carbon deposited, and when the
preliminary heating time is prolonged up to 20 minutes, no
substantial deposition of carbon is brought about. That is, this
means that in such a state as mentioned above, the effect of
utilization of a catalytic material, the utilization of which is an
original object of the present invention, is totally lost.
FIG. 8 shows the results of the investigation on the efficiency as
catalyst of the nickel-phosphorus alloy plated having a thickness
of about 0.6 microns layers formed on substrates, wherein each
substrate was introduced into an atmosphere of acetylene gas having
a pressure of 10 Torr heated at 600.degree.C to effect reaction and
thereby to check the relationship between the partial pressure of
hydrogen formed by thermal decomposition of acetylene gas and the
reaction time. In the said figure, curve A indicates the partial
pressure of hydrogen of one of the specimens as measured by
previously subjecting said specimen to preliminary heating in vacuo
at about 600.degree.C for 30 minutes, thereby rendering the
specimen inactive as a catalyst. While curve B shows the partial
pressure of hydrogen of the other specimen as measured in its state
as plated without subjecting to the said preliminary heating. In
this measurement, the temperature of the specimens during the
reaction time is represented by curve C. As is apparent from the
change in the partial pressure of hydrogen shown in FIG. 8, in the
case of the specimen having its catalytic action, the reaction
begins to take place when the heating temperature has reached about
500.degree.C, the thermal decomposition proceeds rapidly for the
first 10 minutes of the reaction time and, thereafter a mild
reaction continues for 30 minutes or more, i.e. even after
extinction of said catalytic action. In contrast thereto, in the
case of the specimen, from which its catalytic action has been
completely deprived of, evolution of hydrogen is not observed at
all. As a natural consequence, no deposition of carbon is observed
at all. The above facts shows not only the catalyst plays at the
initial stage of deposition of carbon a very important role but
also mean that when the deposition of carbon has once started, the
carbon successively deposits even after the catalytic action has
disappeared. In the cases illustrated above, the thickness of the
nickel-phosphorus alloy plated layer is sufficiently 0.1 micron or
thicker. In order to obtain a dense carbon coating, the initial
deposition of carbon by the catalytic action as mentioned above is
very important. If the time required to elevate the temperature of
400.degree.C up to the temperature at which the thermal
decomposition of acetylene gas begins to start excessively long, a
dense carbon coating can be obtained no longer. This critical time
is called a critical heating time. The critical heating time at
varying heating temperatures starting from 400.degree.C were
investigated to obtain the results as shown in the following table.
Heating temperature Critical heating time
______________________________________ 400.degree.C 200 minutes
500.degree.C 120 minutes 600.degree.C 20 minutes
______________________________________
Within the critical heating time as shown in the above table, a
dense carbon coating of apparent gravity of about 2.0 is formed in
each case and the carbon deposited is observed as having crystal
form according to X-ray diffraction.
FIG. 9 shows the relationship between the heating time and the
amount of carbon deposited (mg/cm.sup.2) which is obtained by
subjecting each of specimens prepared by plating according to the
electroless plating method iron sheet of 20 mm in length, 20 mm in
width and 0.2 mm in thickness with a nickel-phosphorus alloy to
reaction in a furnace heated at the temperature range from
550.degree. to 640.degree.C in three kinds of acetylene gas for
about 30 minutes. In the figure, there are shown the results
obtained in the reactions individually effected under heating in an
atmosphere of acetylene gas (C.sub.2 H.sub.2) having a pressure of
10 Torr in the case of curve D, of a mixed gas comprising acetylene
gas having a pressure of 5 Torr and methane gas having a pressure
of 5 Torr, and of a mixed gas comprising acetylene gas having a
pressure of 5 Torr and hydrogen (H.sub.2) having a pressure of 5
Torr. As is clear from the figure, in the case where acetylene gas
is used alone an effective thermal decomposition reaction due to
the action of the catalyst material takes place at the ambient
heating temperatures of about 600.degree.C, as shown in curve D,
and an increase in the amount of carbon deposited is observed. In
the case where carbon in amount of 2 mg/cm.sup.2 or more is
deposited within a very short time in this thermal decomposition
reaction, however, the carbon coated thus obtained is quite apt to
become porous and the carbon comes to take a dust-like form which
easily peels off from the substrate in many cases. In the light of
the above fact, it is also an important significance in the present
invention in effectively practicing the present process to include
a means to suppress the thermal decomposition reaction with the
view of adjusting and controlling the properties of carbon coating
formed thereby. As the means of suppressing the thermal
decomposition reaction, it has been clarified that altering the
components of the reaction gas is rather effective as the
suppressing means than control of the furnace temperature and
change of the state of activation of the catalyst, and that the
introduction of hydrogen or methane gas into acetylene gas is
effective as the concrete means for suppressing the reaction,
particularly preferred means is the introduction of H.sub.2 into
acetylene. That is, in FIG. 9, the curves E and F are to verify the
above fact. Particularly, as is apparent from the results as
indicated by the curve E, said results being obtained by thermal
decomposition of a mixed gas comprising acetylene gas and hydrogen,
it is possible to control the deposition of carbon deposited to a
given amount at each heating temperature. Furthermore, the addition
of methane gas has the effect to that of the case of hydrogen, the
decomposition reaction of methane gas is an endothermic reaction.
As shown by the curve E in FIG. 9, when the heating temperature is
below about 600.degree.C, a rapid deposition of carbon from
acetylene gas due to the action of the catalyst has been
suppressed. As the means for suppressing the rapid deposition of
carbon resulted from the thermal decomposition of acetylene gas
under the action of the catalyst, it is preferable, including a
method in which the amount of acetylene is reduced and the reaction
is effected under reduced pressure, in addition to the
above-mentioned means, that the temperature of below about
700.degree.C is selected as the heating temperature. At a
temperature above about 700.degree.C, the reaction is undesirably
effected because there are brought about not only an undesirable
effect of the heating temperature on the substrate but also
decomposition of methane gas in the case the mixed reaction
contains methane gas, whereby other factors arise, which factors
should also be taken into consideration.
According to another embodiment of the present invention, there is
provided a process for forming a carbon coating on the surface of a
substrate, characterized by comprising a step of plating the
surface of the substrate with a nickel-phosphorus alloy to form
thereon a nickel-phosphorus alloy layer containing at least 5
percent of phosphorus, a step of oxidizing said nickel-phosphorus
alloy layer, and a step of heating the substrate having thereon the
oxidized nickel-phosphorus alloy layer at a temperature of
500.degree. to 650.degree.C in a non-oxidative reaction gas
containing 0.015 to 5 percent by volume of acetylene gas.
According to this process, formation on a substrate of a desired
carbon coating can be accomplished by the following four
procedures, the first one is to limit the heating temperature of
the substrate, such as an iron sheet, employed at the time of
coating step to a relatively low temperature of below about
650.degree.C, the second one is to previously provide a
nickel-phosphorus alloy layer on the surface of the substrate, such
as an iron sheet, by means of plating, the third one is to carry
out a preliminary oxidation treatment of said nickel-phosphorus
alloy layer prior to a coating treatment with thermally decomposed
carbon, and the fourth one is to the optimum composition of a mixed
gas from which the carbon is deposited by reaction under
heating.
Referring first to FIG. 10, there is indicated an influence on the
amount of carbon deposited of the content of phosphorus in a
nickel-phosphorus plated layer having a thickness of about 1 micron
formed on the surface of an iron sheet substrate. In the embodiment
of the present invention shown in FIG. 10, the carbon coating
treatment was carried out and the conditions mentioned hereinbelow.
After plating of a nickel-phosphorus alloy, a preliminary oxidation
treatment to oxidize the nickel-phosphorus alloy plated layer was
effected by heating in a nitrogen gas containing 0.005 percent by
volume of oxygen at a temperature of about 580.degree.C for 10
minutes. Subsequently, the carbon coating treatment was carried out
in an incombustible reaction gas containing 0.05 percent by volume
of acetylene gas at about 600.degree.C for 20 minutes. It may be
said from the results of the above embodiment that at least 4
percent by weight of phosphorus should indispensably be contained
in the nickel-phosphorus alloy layer in order to deposit carbon to
be coated on the surface of the substrate.
FIG. 11 shows the influence on the amount of carbon deposited of
the concentration of oxygen in the nitrogen gas stream as an
atmosphere gas when the aforesaid nickel-phosphorus alloy layer has
been subjected to the aforesaid preliminary oxidation treatment,
provided that the content of phosphorus in the aforesaid
nickel-phosphorus alloy layer was 8 percent by weight. In this
case, the treatment subsequent to the preliminary oxidation
treatment was carried out in the manner similar to that in the case
of FIG. 10, i.e. the subsequent carbon coating treatment was
effected in a reaction gas containing about 0.05 percent by volume
of acetylene at about 600.degree.C for 20 minutes. According to the
results of this embodiment, it is indicated that when the oxygen
concentration in the nitrogen gas stream is less than 1 .times.
10.sup..sup.-4 percent by volume, the amount of carbon deposited
extremely decreases. Accordingly, it is understood that the oxygen
concentration in the nitrogen gas stream should necessarily be more
than 1 .times. 10.sup..sup.-4 percent by volume.
The above-mentioned embodiment is considered as merely
illustrative, and hence the present preliminary oxidation treatment
is not limited to such treatment conditions as illustrated above.
For example, a preliminary oxidation of the nickel-phosphorus alloy
layer in air of about 0.1 Torr at about 600.degree.C for 10 minutes
leads to formation of a favorable carbon coating. Furthermore, it
should be understood that the heating temperature in the
preliminary oxidation treatment is not limited to 600.degree.C. Any
conditions under which the oxidation of the said alloy layer is
effected to a slight extent may be preferably employed for
obtaining favorable carbon coatings in a manner as illustrated in
the aforesaid case. The effect of the preliminary oxidation
treatment of the alloy layer is that by virtue of such oxidation
treatment, the catalytic ability of said alloy layer is
revived.
The present invention is illustrated below with reference to
examples. All the nickel-phosphorus alloy layers used in these
examples were formed according to the electroless plating method
using a commercially available plating solution known as a trade
name, "SUMER." In Examples 1-4, all the alloy layers used contain 8
percent by weight of phosphorus. The preliminary oxidation
treatment was carried out in each case in a nitrogen atmosphere
comprising 0.005 percent by volume of oxygen at a temperature of
about 600.degree.C for 10 minutes.
EXAMPLE 1
An iron sheet substrate of 0.15 mm in thickness plated on its one
surface with nickel-phosphorus alloy to form thereon a
nickel-phosphorus alloy layer of about 1 micron in thickness was
subjected to a preliminary oxidation. The substrate was then heated
at a temperature of 560.degree.C for 20 minutes in the presence of
a reaction gas stream comprising nitrogen gas containing 0.05
percent by volume based on the nitrogen gas of acetylene gas,
thereby allowing carbon contained in said reaction gas to deposit
on the surface of said nickel-phosphorus alloy layer coated
substrate to form a carbon coating thereon. The thus obtained
carbon coating was found to be extremely dense and have an amount
of carbon deposited of 1 mg/cm.sup.2 and a carbon density of 1.94
g/cm.sup.3.
EXAMPLE 2
An iron sheet substrate of 0.15 mm in thickness plated on its one
surface with nickel-phosphorus alloy to form thereon a
nickel-phosphorus alloy layer was subjected to a preliminary
oxidation treatment. The substrate was then heated at a temperature
of 560.degree.C for 20 minutes in the presence of a reaction gas
stream comprising methane gas containing 0.5 percent by volume
based on the methane gas of acetylene gas, thereby allowing carbon
contained in said reaction gas to deposit on the surface of the
nickel-phosphorus alloy layer-coated substrate to form a carbon
coating thereon. The carbon coating thus obtained was found to be
extremely dense and have an amount of carbon deposited of 1.2
mg/cm.sup.2 and a carbon density of 2.20 g/cm.sup.3.
In this example, the concentration of acetylene gas contained in
the methane gas may be increased upmost to 5 percent by volume. In
that case, however, a density of carbon in the resulting carbon
coating tends to decrease to a some extent.
EXAMPLE 3
An iron sheet substrate of 0.15 mm in thickness plated on its one
surface with nickel-phosphorus alloy to form thereon a
nickel-phosphorus alloy layer was subjected to a preliminary
oxidation treatment. The substrate was then heated at a temperature
of about 560.degree.C for 20 minutes in the presence of a reaction
gas stream comprising a gas obtained by vaporizing L.P.G., said gas
containing 0.7 percent by volume based on L.P.G. of acetylene gas,
thereby allowing carbon contained in said reaction gas to deposit
on the surface of the nickel-phosphorus alloy layer
coated-substrate to form a carbon coating thereon. The carbon
coating thus obtained was found to be relatively thick in its
thickness and dense, and have an amount of carbon deposited of
about 2.5 mg/cm.sup.2 and a carbon density of 2.18 g/cm.sup.2.
In this example, the concentration of acetylene contained in L.P.G.
may be increased upmost to 5 percent by volume. In that case,
however, a density of carbon in the resulting carbon coating tends
to decrease to some extent.
EXAMPLE 4
An iron sheet substrate of 0.15 mm in thickness plated on its one
surface with nickel-phosphorus alloy to form thereon a
nickel-phosphorus alloy layer of about 1 micron in thickness was
subjected to a preliminary oxidation treatment. The substrate was
then heated at a temperature of about 560.degree.C for 20 minutes
in the presence of a reaction gas stream comprising a mixture of
0.2 percent acetylene gas, 1 percent L.P.G., 4 percent methane gas
and 94.8 percent nitrogen gas in terms of percent by volume based
on the mixture, thereby allowing carbon contained in said reaction
gas to deposit on the surface of the nickel-phosphorus alloy
layer-coated substrate to form a carbon coating thereon. The carbon
coating thus obtained was found to be dense and have an amount of
carbon deposited of about 1.8 mg/cm.sup.2 and a carbon density of
1.96 g/cm.sup.2.
Since the reaction gas used in this example was non-combustible, it
has been found quite effective to use said reaction gas in
promoting a safe operation.
That is, it has been found possible to use a reaction gas having
any composition, from which the carbon contained therein is allowed
to deposit, so long as said reaction gas contains 0.01-5 percent by
volume based on the reaction gas of acetylene gas and is of a
non-oxidative composition free from such an oxidative gas as
oxygen. In that case, the reaction gas may be mixed suitably with
such combustible gas as L.P.G. or methane gas, however, it is
preferably from the standpoint of handling to use a reaction gas
comprising a mixture of such an inert gas as nitrogen or argon and
such a combustible gas as L.P.G. and/or methane gas in an amount
within such a range that the resulting whole mixed gas may become
non-combustible.
While there have been described the preferred embodiments of the
present invention with reference to the cases where the
nickel-phosphorus alloy layer was coated on the surface of the
substrate by the electroless plating using "SUMER" solution (a
trade name of a plating solution). It is needless to say, however,
that the invention is not limited thereto.
As fully explained hereinabove, according to the process of the
present invention it is possible to coat the surface of a very thin
iron sheet substrate having a thickness of 0.15 mm with dense
carbon in any given thickness with unerring precision and in a
simple and safe manner, while it was almost impossible in the prior
art processes to practice such coating as in the present invention.
The process of the present invention, moreover, has such
characteristic that since the temperature necessary for effecting a
carbon-depositing reaction in the present process is low as
compared with that used in the prior art processes, various
disadvantages which may be associated with a high temperature
heating employed in the prior art processes can be effectively
avoided.
Furthermore, the process of the present invention is extremely
effective as a process, in particular, for coating with carbon a
shadow mask for color picture tube of television and leads to
excellent results in an improvement in technique for the production
of the shadow mask.
EXAMPLE 5
A 0.15 mm thick iron sheet shadow mask for color picture tube of
television was washed with hydrochloric acid to remove completely
from the surface thereof the oxide formed thereon. The mask thus
washed as such was immersed for about 2 minutes in "SUMER"
electroless nickel plating bath (temperature of the bath had been
controlled to be maintained at 80.degree.C), a pH value of which
had been previously adjusted to 6.0. In this case, special
consideration was given so that the plating bath was thoroughly
stirred so as to extend sufficiently uniformly over the
circumferences of a large number of small pores provided on the
shadow mask, thereby uniformly plating every portion of the shadow
mask with a nickel-phosphorus alloy in thickness of about 0.3-0.5
micron. After completion of the plating treatment, the shadow mask
was pulled out from the plating bath and washed thoroughly with
pure water and thereafter dried quickly. Subsequently, the inside
of a heating furnace was once evacuated until a pressure of 1
.times. 10.sup.-.sup.6 to 1 .times. 10.sup.-.sup.5 Torr was
attained, said heating furnace being constructed by use of such
material, for example, as quartz which may not deposit carbon by
reaction with acetylene at about 600.degree.C. Therafter, acetylene
was introduced into the heating furnace so that the degree of
vacuum indicated by Pirani indicator provided in the said furnace
may reach 5 Torr. Into the furnace, the inside temperature of which
had been heated to about 600.degree.C, was then placed the shadow
mask while giving sufficient consideration not to destruct the
above-mentioned inside atmosphere of the furnace (in this case, the
shadow mask had not been preheated). The reaction was carried out
at that temperature for about 30 minutes, thereby forming on the
surface of said shadow mask a carbon coating having an amount of
carbon deposited of about 1 mg/cm.sup.2 and a thickness of about
4.5 microns. Thereafter the carbon-coated shadow mask was conveyed
to a cooling chamber and then taken out of the chamber. Microscopic
observation of the thus obtained carbon coating under magnification
of about 600 times showed in some cases that small spheres of
carbon in the form of a ball of waste thread were present on the
surface of the carbon coating. If such carbon spheres are actually
present on the surface of the carbon coating formed on a shadow
mask, they come readily to float with a color picture tube and
cause spark when a high voltage is applied thereto, and hence it is
necessary to remove completely such carbon spheres, prior to use of
the carbon-coated shadow mask, by washing with water or by rinsing
treatment with ultrasonic wave. The carbon-coated shadow mask after
being subjected to the above-mentioned treatment was actually
fitted in a color picure tube, and a test was effected. As a
result, there was obtained such excellent effects that
secondary-electron emission from the shadow mask due to heat
electron jetted under accelerated voltage of 25-30 KV could be
prevented and a good colored picture could be provided without
observing substantial halation.
While there has been described what is at present considered to be
a preferred embodiment of the present invention with reference to
the case where the shadow mask substrate was plated with
nickel-phosphorus alloy by the electroless plating method using
"SUMER" solution (a trade name of a plating solution). It is
needless to say, however, that the invention is not limited
thereto.
As explained hereinabove, in accordance with the process of the
present invention it is possible to plate a substrate, on the
surface of which a carbon coating is to be provided, with a
nickel-phosphorus alloy to form thereon a layer thereof, to deposit
from acetylene on the surface of the nickel-phosphorus alloy
layer-coated substrate by thermal decomposition reaction of the
acetylene gas at relatively low temperatures utilizing catalytic
action of the said nickel-phosphorus alloy layer, and to suppress
or control the thermal decomposition reaction of the acetylene gas
by adjusting the composition of the reaction gas containing the
acetylene gas, thereby forming a dense and uniformly thick carbon
coating on the surface of the substrate in a simple manner with
high operational stability. The present invention, therefore,
greatly contributes to advancement and improvement in the
industrial technology concerned and also brings about large
economical advantages.
* * * * *