U.S. patent application number 13/125622 was filed with the patent office on 2011-08-18 for method for deposition of hard chrome layers.
This patent application is currently assigned to ENTHONE INC.. Invention is credited to Helmut Horsthemke.
Application Number | 20110198226 13/125622 |
Document ID | / |
Family ID | 40427109 |
Filed Date | 2011-08-18 |
United States Patent
Application |
20110198226 |
Kind Code |
A1 |
Horsthemke; Helmut |
August 18, 2011 |
METHOD FOR DEPOSITION OF HARD CHROME LAYERS
Abstract
The present invention concerns a method for galvanic deposition
of a hard chrome layer on a substrate surface at high rates of
deposition. According to the invention, the substrate surface being
coated makes contact at reduced pressure relative to the ambient
pressure with a chromium-containing electrolyte suitable for
galvanic deposition and a relative motion between substrate surface
and electrolyte is produced during the depositing of the chrome
layer on the substrate surface.
Inventors: |
Horsthemke; Helmut;
(Solingen, DE) |
Assignee: |
ENTHONE INC.
West Haven
CT
|
Family ID: |
40427109 |
Appl. No.: |
13/125622 |
Filed: |
October 22, 2009 |
PCT Filed: |
October 22, 2009 |
PCT NO: |
PCT/US09/61683 |
371 Date: |
April 22, 2011 |
Current U.S.
Class: |
205/50 ;
205/88 |
Current CPC
Class: |
C25D 21/00 20130101;
C25D 5/12 20130101; C25D 21/04 20130101; C25D 5/04 20130101; C25D
5/18 20130101; C25D 7/10 20130101; C25D 5/003 20130101; C25D 5/08
20130101 |
Class at
Publication: |
205/50 ;
205/88 |
International
Class: |
C25D 5/14 20060101
C25D005/14; C25D 5/18 20060101 C25D005/18 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 2008 |
EP |
08018462.5 |
Claims
1-9. (canceled)
10. A method for depositing of a hard chrome layer on a substrate
surface comprising: making contact between the substrate surface
and a chromium-containing electrolyte; applying a pulsed current
between the substrate surface and a counterelectrode for the
deposition of a first hard chrome layer on the substrate surface,
wherein the deposition of the first hard chrome layer occurs in a
container essentially gas-tight to the surroundings and at a
reduced pressure relative to ambient pressure, the substrate
surface and chrome-containing electrolyte are moved with a velocity
of between 0.1 m/s to 5 m/s relative to each other; and applying a
second hard chrome layer over the first hard chrome layer.
11. The method of claim 10 wherein the second hard chrome layer is
deposited on the first deposited hard chrome layer using a direct
current.
12. The method of claim 10 wherein the reduced pressure has a
pressure difference of 10 mbar to 800 mbar relative to ambient
pressure.
13. The method of claim 10 wherein the reduced pressure has a
pressure difference of 20 mbar to 200 mbar relative to ambient
pressure.
14. The method of claim 10 wherein the pulsed current has a
frequency of 5 Hz to 5000 Hz.
15. The method of claim 10 wherein the pulsed current has a
frequency of between 50 Hz and 1000 Hz.
16. The method of claim 10 wherein the first hard chrome layer is
deposited at a current density between 25 A/dm.sup.2 and 1000
A/dm.sup.2.
17. The method of claim 10 wherein the first hard chrome layer is
deposited at a current density between 50 A/dm.sup.2 and 500
A/dm.sup.2.
18. The method of claim 10 wherein the second hard chrome layer is
deposited by direct current at a current density in the range
between 25 A/dm.sup.2 and 1000 A/dm.sup.2.
19. The method of claim 10 wherein the layers are deposited at an
electrolyte temperature between 30.degree. C. and 85.degree. C.
20. The method of claim 10 wherein the electrolyte has a pH of
.ltoreq.pH 3.
21. A method for depositing of a hard chrome layer on a substrate
surface comprising: making contact between the substrate surface
and a chromium-containing electrolyte; applying a pulsed current
between the substrate surface and a counterelectrode for the
deposition of a first hard chrome layer on the substrate surface,
and applying a direct current between the first hard chrome layer
and the counterelectrode for deposition of a second hard chrome
layer; wherein the deposition of the first hard chrome layer and
second hard chrome layer occurs with a single electrolyte in a
container essentially gas-tight to the surroundings and at a
pressure reduced by between 10 mbar and 800 mbar relative to
ambient pressure, the substrate surface and chrome-containing
electrolyte are moved with a velocity of between 0.1 m/s to 5 m/s
relative to each other, the pH is <3, the pulsed current
frequency is from 5 to 5000 Hz, the current density is between 25
and 1000 A/dm.sup.2, and the electrolyte temperature between
30.degree. C. and 85.degree. C.
22. The method of claim 21 wherein the pressure reduced by between
20 mbar and 200 mbar relative to ambient pressure, the substrate
surface and chrome-containing electrolyte are moved with a velocity
of between 1 m/s to 5 m/s relative to each other, the pH is <1,
the pulsed current frequency is from 50 to 1000 Hz, and the current
density is between 50 and 500 A/dm.sup.2.
23. A substrate comprising a hard chrome layer deposited by the
method of claim 10.
Description
FIELD OF THE INVENTION
[0001] The present invention concerns a method for deposition of a
hard chrome layer on a substrate surface. In particular, the
present invention concerns a method for deposition of hard chrome
layers at high rates of deposition.
BACKGROUND OF THE INVENTION
[0002] Hard chrome layers are widespread as coatings on technical
parts. Thus, for example, it is known how to provide valve bodies,
bushings, brake pistons or axle hubs with hard chrome layers. The
deposited chrome layer serves, on the one hand, as corrosion
protection for the substrate surface located underneath, and on the
other hand also as a protective layer against friction and wear,
since the deposited hard chrome layers have great hardness.
[0003] For galvanic deposition of chrome layers, the substrate
surfaces being coated after a suitable pretreatment to prepare the
surface are brought into contact with an electrolyte having at
least the metal (chromium) being deposited, while a deposition
voltage is applied between the cathodically contacted substrate
surface and an anode. As a result, the chromium dissolved in the
electrolyte is deposited as a layer on the substrate surface.
[0004] The layers so deposited can have tensile or compressive
internal stresses. Compressive internal stresses can lead to
microcracks in the deposited layers, which means that the layers
are not continuously closed, but rather possess a network of
microcracks.
[0005] Tensile internal stresses, on the other hand, can lead to
deep cracks in the deposited layers, into which moisture or
corrosive substances can migrate and thus lead to corrosion effects
in the substrate surface underneath the chrome layer, ultimately
resulting in damaging of the chrome layer, even a flaking off.
[0006] Furthermore, the tensile internal stress found in such
layers is detrimental to many applications, such as the chrome
plating of axle hubs, since this has negative impact on the fatigue
strength under reversed bending stresses of the substrate or the
structural part. Furthermore, it is presumed that the unavoidable
occurrence of gaseous H2 during the deposition of chrome layers
leads to an incorporation of hydrogen in the layer and the
substrate, which in turn can lead to formation of cracks in the
layer and a damaging of the substrate.
[0007] In order to relieve the deposited chrome layers of the
tensile internal stresses occurring in them, the coated substrate
surfaces undergo subsequent machining according to the prior art,
e.g., by grinding or honing, in order to do away with the internal
tensile stresses occurring in the layers. Besides the fabricating
expense which this entails, the machining can also lead to a
damaging of the deposited chrome layers, which ultimately reduces
their property as a corrosion protection layer drastically.
[0008] Although chromium in itself is a relatively non-noble metal
in chemical respect, thanks to the formation of a thin oxide layer
on the surface and the concomitant very positive potential chromium
layers act to protect against corrosion and exhibit corrosion
protection properties comparable to noble metals such as gold,
silver or platinum in regard to their corrosion and tarnish
protection.
[0009] In the industrial fabrication of galvanically coated
mass-produced articles such as valves for four-stroke internal
combustion engines, shock absorbers, axle hubs or similar
mechanical parts it is necessary to deposit chrome layers with a
sufficiently high rate of deposition on substrate surfaces in order
to ensure an economically reasonable production process. Higher
rates of deposition are generally achieved by setting higher
current densities in the galvanic deposition process. However, the
formation of hydrogen at the cathode occurs as one side reaction in
the galvanic deposition of chrome layers. Since the substrate
surfaces being coated serve as the cathode in the galvanic coating
processes, the resulting hydrogen can lead to the formation of
bubbles on the substrate surfaces, which strongly affects the
outcome of the galvanic chrome deposition. Thus, pores or flaws can
form due to the resulting hydrogen bubbles, which quite adversely
affect the corrosion protection properties of the deposited chrome
layers.
[0010] Increasing the current density to achieve sufficiently high
speeds of deposition also results in a greatly intensified
formation of hydrogen on the substrate surfaces.
[0011] However, the network of cracks occurring in galvanically
deposited chrome layers due to internal compressive stresses does
not have merely negative influence on the corrosion protection
property of the deposited layer, but instead leads positively to
improved properties of the so coated moving parts, since lubricants
for reducing the frictional resistance between moving parts can
become embedded in the microcracks, and thus they have a depot
effect for the lubricants. This capability of the layers is known
as oil carrying capacity and it is absolutely desirable for such
mechanical parts. For example, this is important in the case of
piston rings, to maintain the fire stability.
[0012] GB 1 551 340 A discloses the depositing of a hard chrome
layer on a substrate surface at a temperature of 60.degree. C. and
a current density set at 80 A/dm.sup.2 in a low-pressure chamber
with chromium deposition electrolytes flowing through it.
[0013] U.S. Pat. No. 2,706,175 A discloses a device for coating the
insides of hollow cylinders, wherein a chrome layer is deposited
under low pressure.
[0014] EP 1 191 129 A discloses a method for depositing a hard
chrome layer under low pressure, wherein electrolyte and substrate
move with a velocity of 0.4 m/sec relative to each other.
[0015] US 2001/054557 A1 discloses a method for the galvanic
deposition of hard chrome layers, in which the chrome layer is
likewise deposited under low pressure at a current density of 30 to
40 A/dm.sup.2 and a pulse frequency of 5 to 700 Hz.
[0016] EP 0 024 946 A discloses a method for depositing of hard
chrome layers at low pressure with a current density in the range
of 200 A/dm.sup.2 and the addition of a relative motion between
electrolyte and substrate being coated.
[0017] U.S. Pat. No. 5,277,785 discloses a method and a device for
depositing of hard chrome layers by means of a brush
deposition.
SUMMARY OF THE INVENTION
[0018] Taking into account the above remarks, it is therefore the
problem of the present invention to indicate a method for the
depositing of hard chrome layers by which hard chrome layers with
high corrosion resistance and good mechanical properties can be
deposited at high rate of deposition.
[0019] Briefly, therefore, in one aspect the invention is directed
to a method for galvanic depositing of a hard chrome (i.e.,
chromium-based) layer on a substrate surface, having the steps of
making contact between the substrate surface being coated and a
chromium-containing electrolyte suitable for galvanic deposition;
and applying a voltage between the substrate surface being coated
and a counterelectrode for the galvanic deposition of a hard chrome
layer on the substrate surface; wherein the deposition occurs in a
container essentially gas-tight to the surroundings, and at least
during the applying of the voltage a low pressure is established in
the container essentially gas-tight to the surroundings and wherein
substrate surface and chrome-containing electrolyte are moved with
a velocity of 0.1 m/s to 5 m/s, preferably >1 m/s to 5 m/s
relative to each other.
[0020] Other objects and features will be in part apparent and in
part pointed out hereinafter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] This application claims priority from European application
08018462.5 filed 22 Oct. 2008, the entire disclosure of which is
incorporated by reference.
[0022] This problem is solved by a method for galvanic depositing
of a hard chrome layer on a substrate surface having the steps:
[0023] making contact between the substrate surface being coated
and a chromium-containing electrolyte suitable for galvanic
deposition; [0024] applying a voltage between the substrate surface
being coated and a counterelectrode for the galvanic deposition of
a hard chrome layer on the substrate surface, [0025] wherein the
deposition occurs in a container essentially gas-tight to the
surroundings, and at least during the applying of the voltage a low
pressure is established in the container essentially gas-tight to
the surroundings and wherein substrate surface and
chrome-containing electrolyte are moved with a velocity of 0.1 m/s
to 5 m/s, preferably >1 m/s to 5 m/s relative to each other,
characterized in that a second hard chrome layer is deposited on a
first deposited hard chrome layer, and for the depositing of the
first hard chrome layer a pulsed current is applied between
substrate surface and counterelectrode, and for the depositing of
the second hard chrome layer a direct current is applied to the
first hard chrome layer.
[0026] The reduction of the pressure relative to ambient pressure
during the galvanic deposition leads to an improved detachment of
the hydrogen bubbles on the substrate surface that are formed
during the galvanic separation process. This detachment is
supported by the relative movement between substrate surface and
electrolyte. Together, this leads to the depositing of a hard
chrome layer which is also essentially free of pores or flaws, even
at high deposition current densities.
[0027] By suitable measures, such as pumps, an appropriate low
pressure can be created. Advantageously, the pressure difference to
be established lies in a range of 10 mbar to 800 mbar, preferably
20 mbar to 200 mbar.
[0028] In the method of the invention, a second hard chrome layer
is deposited on a first deposited hard chrome layer, wherein for
the depositing of the first hard chrome layer a pulsed current is
applied between substrate surface and counterelectrode, and for the
depositing of the second hard chrome layer a direct current is
applied to the first hard chrome layer.
[0029] In one embodiment of the method of the invention, a first
hard chrome layer is deposited, having no internal stresses and
being free of microcracks thanks to the pulsed current applied. By
subsequent applying of a direct current between the substrate
surface being coated and the counterelectrode, a second hard chrome
layer is deposited on the already deposited first hard chrome layer
free of cracks and internal stresses, the second layer having
internal tensile stress and the mechanically desirable
microcracks.
[0030] The resulting compound layer structure has excellent
corrosion resistance and furthermore excellent mechanical
properties as running or sliding surfaces, thanks to the
microcracks occurring in the upper chrome layer.
[0031] For the depositing of the first chrome layer, the pulsed
current can be applied with a pulse frequency of 5 Hz to 5000 Hz,
preferably 50 Hz to 1000 Hz. A current density between 25
A/dm.sup.2 and 1000 A/dm.sup.2, preferably 50 A/dm to 500
A/dm.sup.2, is adjusted for this.
[0032] For the depositing of the second chrome layer, a direct
current can be adjusted with a current density in the range between
25 A/dm.sup.2 and 1000 A/dm.sup.2, likewise with a preferred range
between 50 A/dm.sup.2 and 500 A/dm.sup.2.
[0033] According to the invention, the substrate surface being
coated makes contact with the chromium-containing electrolyte at a
temperature between 30.degree. C. and 85.degree. C., and the
electrolyte can have a pH value in the range of .ltoreq.pH 3,
preferably .ltoreq.pH 1.
[0034] According to the invention, the chromium-containing
electrolyte can have a conductivity K of 200 mS/cm to 550 mS/cm (at
20.degree. C.).
[0035] Advantageously, the method can be carried out with only one
electrolyte in a single coating cell.
[0036] According to the invention, a relative motion can be
produced at least temporarily between the electrolyte and the
substrate surface being coated. According to the invention, the
relative motion can lie in a range between 0.1 m/s and 5.0 m/s.
[0037] To produce the relative motion between electrolyte and
substrate surface, the substrate surfaces can be moved or the
electrolyte can be appropriately delivered. Stirring devices or
pumps are suitable for the delivery of the electrolyte.
[0038] Relative motion between electrolyte and substrate surface so
produced encourages a detachment of the forming hydrogen bubbles,
in addition to the low pressure applied.
[0039] In an especially advantageous embodiment of the method of
the invention, the substrate surface being coated makes contact
with the electrolyte in a cell, in which the chromium-containing
electrolyte flows in from below and can flow away across a
spillway, and a sufficient flow velocity is adjusted to sustain the
detachment of the resulting hydrogen bubbles.
[0040] To carry out the method of the invention, a coating reactor
is especially suitable, having the shape of a cylinder and being
outfitted with a cylindrical inner anode of platinum-coated metal,
such as platinum-coated titanium, niobium or tantalum. At the top
and bottom of the coating reactor, there can be supports for the
structural part being chrome plated. A coating reactor of this kind
is particularly suitable for the coating of cylindrical parts. At
least one of the two supports serves to supply current to the part
being coated and is accordingly configured as an electrical
contact.
[0041] By means of a suitable pump, an electrolyte is suctioned
from a reservoir tank through the reactor to the top part of the
reactor and from there back to the reservoir tank. In the reservoir
tank, the electrolyte can be degasified by means of suitable
devices. The gas mixture separated in this way is taken to the
outside via a drop separator. Alternatively, a separate
degasification tank can be provided.
[0042] Devices for temperature control of the electrolyte can be
provided in the reservoir tank, such as heating and/or cooling
systems. The reservoir tank can be connected via dispensing pumps
to other reservoir tanks, which contain compositions to supplement
the electrolyte located in the reservoir tank, insofar as a further
dispensing of the electrolyte is needed. To reduce the volume, the
electrolyte heated by the applied deposition voltage can be taken
across an evaporator unit, where water is removed from the
electrolyte while cooling it at the same time.
[0043] Advantageously, such a reactor configured according to the
invention is outfitted with at least one movable end face,
facilitating the bringing up and taking away of the part being
coated. Furthermore, the usual handling systems and seals can be
provided for an automation of the process.
[0044] In one embodiment of such a coating reactor, the part being
coated in the reactor can be rinsed with rinse water or steam, or
at least prerinsed. For this, the supply of electrolyte to the
reactor can be interrupted and replaced by rinse water or steam. In
the case of a simple prerinsing of the coated part in the reactor,
the final rinsing can occur in a second reactor, which is basically
identical in design to the first reactor, but does not have any
anode or current supply.
[0045] The method of the invention shall be presented hereafter in
the context of sample embodiments, although the notion of the
invention cannot be confined to the sample embodiments.
EXAMPLES
Example 1
[0046] A workpiece being chrome plated (piston rod of steel type CK
45) was brought into contact in a reactor configured according to
the invention with an electrolyte for deposition of a hard chrome
layer, having 370 g/l of chromic acid and 5.3 g/l of sulfuric acid,
the electrolyte flowing into the respective reactor from the bottom
and it was taken away across a spillway at the top of the reactor.
The relative velocity established in this way between the substrate
surface of the workpiece being coated and the electrolyte was 4
m/s. The electrolyte had a temperature of 70.degree. C. By suitable
devices, a pressure of 50 mbar was established inside the reactor.
After an appropriate conditioning and activation of the workpiece
by applying a suitable current ramp, a hard chrome layer was then
deposited by adjusting a current density of 235 A/dm.sup.2 in the
space of 300 seconds. The substrate was then rinsed.
[0047] The obtained chrome layer had a layer thickness of 11 .mu.m,
it had around 40 cracks per cm, and it had a corrosion resistance
in the neutral salt spray test of less than 100 h.
Example 2
[0048] A workpiece being chrome plated was brought into contact
with an electrolyte in a reactor configured according to the
invention, as in example 1. The electrolyte contained 370 g/l of
chromic acid, 5.3 g/l of sulfuric acid, and 6 g/l of methane
sulfonic acid. The deposition conditions corresponded to example 1.
A shiny chrome layer with a layer thickness of 11 .mu.m was
obtained, which had around 250 cracks/cm and a corrosion resistance
in the neutral salt spray test of less than 100 h.
Example 3
[0049] A workpiece being chrome plated was brought into contact
with the electrolyte per example 2 under the conditions mentioned
in example 2, wherein a pulsed current with a current density
during the pulse of 235 A/dm.sup.2, a frequency of 1000 Hz and an
On time of 50% was applied for 400 seconds.
[0050] A shiny, crack-free chrome layer with a layer thickness of
11 .mu.m was obtained, which had 0 cracks/cm and a corrosion
resistance in the neutral salt spray test of more than 500 h.
Example 4
[0051] A workpiece being chrome plated was coated under the
deposition conditions per example 3, at first applying a pulsed
current with a current density of 235 A/dm.sup.2 during the pulse,
a frequency of 1000 Hz and an On time of 50% for 400 seconds and
then applying a direct current in the same electrolyte with a
current density of 235 A/dm.sup.2 for 100 seconds, other conditions
being equal.
[0052] The obtained shiny chrome layer had a layer thickness of 17
.mu.m and around 25 cracks/cm, with a corrosion resistance in the
neutral salt spray test of more than 500 h.
[0053] When introducing elements of the present invention or the
preferred embodiments(s) thereof, the articles "a", "an", "the" and
"said" are intended to mean that there are one or more of the
elements. The terms "comprising", "including" and "having" are
intended to be inclusive and mean that there may be additional
elements other than the listed elements.
[0054] In view of the above, it will be seen that the several
objects of the invention are achieved and other advantageous
results attained.
[0055] As various changes could be made in the above compositions
and processes without departing from the scope of the invention, it
is intended that all matter contained in the above description
shall be interpreted as illustrative and not in a limiting
sense.
* * * * *