U.S. patent number 3,881,910 [Application Number 05/388,434] was granted by the patent office on 1975-05-06 for chromium-chromium carbide powder.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to John Franklin Pelton.
United States Patent |
3,881,910 |
Pelton |
May 6, 1975 |
Chromium-chromium carbide powder
Abstract
A reinforced chromium coating produced by plasma spraying or
detonation-gun deposition of powder consisting essentially of a
mixture of chromium and at least one chromium carbide taken from
the class consisting of Cr.sub.23 C.sub.6 ; Cr.sub.7 C.sub.3 and
Cr.sub.3 C.sub.2 wherein said chromium carbides are present in the
range of from 3 to 30 volume percent of said mixture.
Inventors: |
Pelton; John Franklin (Yorktown
Heights, NY) |
Assignee: |
Union Carbide Corporation (New
York, NY)
|
Family
ID: |
23534105 |
Appl.
No.: |
05/388,434 |
Filed: |
August 15, 1973 |
Current U.S.
Class: |
75/252;
75/255 |
Current CPC
Class: |
C23C
4/06 (20130101) |
Current International
Class: |
C23C
4/06 (20060101); B22f 001/00 () |
Field of
Search: |
;75/.5AC,.5BC,.5R,.5B
;117/105,105.2,105.1,93.1PF:1M ;29/192CD,191.2 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3150938 |
September 1964 |
Pelton et al. |
|
Primary Examiner: Rutledge; L. Dewayne
Assistant Examiner: Crutchfield; O. F.
Attorney, Agent or Firm: Terminello; Dominic J.
Claims
What is claimed is:
1. A powder for deposition by a process taken from class consisting
of plasma and detonation-gun type processes consisting essentially
of a mixture of chromium and at least one chromium carbide taken
from the class consisting of Cr.sub.23 C.sub.6 ; Cr.sub.7 C.sub.3
and Cr.sub.3 C.sub.2 wherein said chromium carbides are present in
the range of from 3 to 30 volume percent of said mixture.
2. A powder according to claim 1 wherein said chromium carbides are
present in the range of about 15 volume percent to 30 volume
percent of said mixture.
Description
This invention relates to a novel coating and article produced by
plasma spraying or detonation gun deposition of a chromium and
chromium carbide powder. More particularly this invention relates
to a coating or article having unique wear and frictional
characteristics produced from a mechanical mixture of chromium and
chromium carbide powders.
Chromium metal has been used as an electroplated coating (i.e.,
"hard chromium plating") for many years to restore worn or damaged
parts to their original dimensions, to increase wear resistance,
reduce friction, and provide corrosion resistance. Chromium's
excellent wear and frictional characteristics have been attributed
to its low ratio of energy of adhesion to hardness when mated
against a number of materials that are commonly used in engineering
applications. Hard chromium electroplate, however, has a number of
limitations. The electroplating of chromium is economically
feasible when the configuration of the part is relatively simple
and the number of parts and/or their size is relatively small. When
the configuration of the part becomes complex, obtaining a uniform
coating thickness by electrodeposition is difficult and requires
precise placement of electrodes and thieves. Without a uniform
coating thickness, grinding to a finished surface configuration
becomes necessary, and it is both difficult and expensive with
electroplated chromium because of its inherent brittleness and
hardness. The rate of deposition by electroplating is relatively
low, and thus for a large number of parts and/or large areas and/or
thick coatings a very substantial capital investment in plating
tanks and power supplies is required. In chromium electroplating it
is often necessary to use expensive surface cleaning and etching
procedures to prepare substrates. Further, with many substrate
materials it is not possible to directly apply chromium
electroplating and one or more undercoats of other metals must be
used. Spent plating baths present a disposal problem because they
are a serious pollution source, and hence handling them adds
significantly to the cost of the process.
An alternative method of depositing chromium metal is by metal
spraying, such as plasma or detonation gun coating processes. These
methods offer a number of processing advantages. Surface
preparation is relatively simple and inexpensive. The coatings can
be applied to almost any metallic substrate without using
undercoats. The rate of deposition is very high so that a large
volume of parts can be coated with a minimal capital investment.
The coating thickness can be controlled very closely so that any
subsequent finishing can be kept to a minimum. The overspray can be
easily contained and recovered making pollution control a simple
matter.
Unfortunately, plasma-deposited chromium is not as wear-resistant
at ambient temperature as hard electroplated chromium. This is
because the wear resistance of chromium electroplate is not an
inherent property of elemental chromium but is believed to arise
largely from impurities and stresses incorporated in the coating
during plating. Plasma deposited chromium, being a purer form of
chromium, thus lacks the wear resistance of hard chromium
electroplate while retaining the corrosion resistance
characteristics of chromium.
In seeking an alternative method of depositing a coating equivalent
to that of hard electroplated chromium, and recognizing that plasma
deposition of chromium metal by itself is not successful, it would
be normal engineering practice to evaluate materials such as the
well-known plasma-deposited cermets; e.g., tungsten carbide-cobalt.
Cermet-type materials consist of a relatively high-volume fraction
of hard phase particles, such as chromium carbide or tungsten
carbide, bonded together by a relatively low-volume fraction of a
ductile, soft matrix such as cobalt, nickel or nichrome. While
these cermets exhibit outstanding wear characteristics, they lack
the usual frictional and compatibility characteristics that
electrolytic chromium plate exhibits and hence, although a
relatively hard coating may be obtained, many of the other
advantages exhibited by electroplated chromium metal are lost. An
inherently brittle material, such as chromium, would not normally
be chosen as the matrix however, because it would not provide
sufficient toughness or impact resistance. Chromium has been used
as a binder in some cermets but these are intended for
high-temperature use where chromium is a ductile metal.
Attempts to improve the plasma-deposited chromium coating by
intentionally spraying with oxidizing or nitriding conditions to
introduce a dispersion of hard precipitates that might strengthen
the deposits and improve their wear resistance was unsuccessful.
Both plasma and detonation gun deposition result in a multilayer
structure of thin, overlapping, lenticular particles or "splat."
Oxidation or nitridation of the chromium particles during spraying
results in weakened splat-to-splat bonding and a friable coating
with low strength in which particles tend to "pull-out" during
wear. Nitriding the chromium powder prior to deposition also
created a very friable coating and increased the tendency of
particles to pull out during grinding.
It has not been unexpectedly discovered that the compatibility,
frictional characteristics, and corrosion-resistance normally
associated with hard chrome electroplate can be obtained in plasma
or detonation-gun coatings and superior wear resistance and
finishability obtained as well by incorporating a low-volume
dispersion of chromium carbide particles, which are brittle in a
chromium matrix, which is brittle. One method of dispersing the
carbide phase in the chromium matrix is by mechanically mixing
particles of chromium carbide and chromium and co-depositing the
mixture.
Accordingly, it is the main object of this invention to provide a
coating which is predominantly chromium reinforced with chromium
carbides.
Another object is to provide a coating which is equivalent to
electrolytic chrome plate in resistance to wear and which can be
finished with the same type of tooling used for finishing chrome
plate to final dimensions.
Yet another object is to provide a powder for making such coatings
or articles.
A further object is to provide a mechanical mixture of chromium
powder and chromium carbide powder to provide the coating of this
invention.
These and other objects will either be pointed out or become
apparent from the following description and drawings where:
FIG. 1 is a pictorial representation of the structure obtained by
depositing the powder of this invention; and
FIG. 2 shows the variation of wear scar volumes with carbide
content of the powder used to produce the coating tested and
compared to coatings of hard chrome electroplate.
FIG. 1 illustrates the structure of a coating or article of this
invention achieved by depositing the powder of the invention by the
plasma or detonation-gun process. The structure is characterized by
inhomogeneity, that is, regardless of how fine the powder particles
are, some of the lenticular particles or splats will be completely
chromium metal and some completely chromium carbide. It has been
found that volume fractions of 3 to 30 percent chromium carbide are
the most desirable. Above about 30 volume percent the coating
becomes harder, more brittle and more difficult to grind. In
addition, the powder becomes more difficult to plate.
The preferred volume fraction is about 15 percent to 30 percent. In
this range the wear behavior of the material is equivalent to or
superior to that of commercial electrolytic chromium plate, the
hardness is at a minimum making the coating readily finishable with
conventional grinding or honing tools, and low-surface-speed,
high-deposition-rate plasma plating produces well-bonded, uncracked
coatings. In this composition range the coating contains far less
hard phase material than conventional cermets.
The wear resistance of plasma-deposited coatings prepared from
mixtures of chromium and chromium carbide was measured using a
Dow-Corning LEW-1 Friction and Wear Test Machine according to ASTM
Standard Method D-2714-64. Coatings deposited 12 mils thick on the
wear surface of mild steel wear blocks were ground to a final
thickness of 6 mils and tested against carburized AISI 4620 steel
rings (surface hardness 58-63 Rockwell "C") for 5400 rings
revolution at about 180rpm; MIL-5606A hydraulic fluid was used as
lubricant.
Wear tests conducted at 450 lbs. load on coatings of this
invention, which were made with mixtures of -325 mesh chromium and
chromium carbide, resulted in the data of Table I and band curves
of FIG. 2. The range of values observed for commercial hard
chromium electroplate is also shown on FIG. 2 by the cross-hatched
area adjacent to the vertical axis.
TABLE I
__________________________________________________________________________
WEAR TESTS OF COATINGS AT 450 LB. LOAD Coating Coating Wear Scar
Mating Hardness Volume % Volume Ring Weight VPN.sub.100 Coating
Type Carbide 10.sup.-.sup.6 cm.sup.3 Change, mg
__________________________________________________________________________
Mixture, Cr+Cr.sub.23 C.sub.6 24 26 to 32 -0.5 to -2.0 301 .+-. 113
100 14 to 17 -1.3 to -1.9 900 .+-. 125 Mixture, Cr+Cr.sub.3 C.sub.2
20 27 to 37 -0.9 to -1.6 374 .+-. 84 35 20 to 21 0 to -1.1 -- 50 20
to 30 -1.0 to -3.9 465 .+-. 40 Electroplated Cr -- 43 to 53 -0.6 to
+2.9 --
__________________________________________________________________________
It is evident that at 450 lb. load, there was little significant
difference in wear characteristics of the coating as a function of
volume fraction of carbide additions of 18 percent or greater, but
the wear scar volumes were significantly lower than hard
electroplated chromium. Equally good results were obtained with
Cr.sub.23 C.sub.6 additions and with Cr.sub.3 C.sub.2 additions.
Plasma-deposited chromium without chromium carbide additions tends
to fail structurally at 450 lb. loads early in the test cycle.
Differences in wear behavior for different compositions were
observed when a 600 lb. load was used. Referring to Table II below,
it is evident that as the Cr.sub.3 C.sub.2 volume fraction
increases, the wear of the mating surface (Ring Weight Change
Column) increases significantly. A similar increase occurs when a
larger Cr.sub.3 C.sub.2 particle size is used, as also shown in
Table II, by comparing the wear of -230/+325 mesh Cr.sub.3 C.sub.2
particle additions to -325 mesh additions. Even at a very
low-volume fraction, -120/+230 Cr.sub.3 C.sub.2 caused
significantly greater mating surface wear in one test, with coating
failure probably limiting the mating surface wear in another test.
These test data demonstrate that carbide particle size and volume
fraction should be kept relatively low to minimize mating surface
wear. The low-volume fraction of carbide also tends to improve
finishability since the hardness is minimized.
TABLE II
__________________________________________________________________________
WEAR TESTS OF COATINGS AT 600 LB. LOAD Volume Particle Coating
Fraction Size Wear Scar Mating Coating Coating of (Mesh) of Volume
Ring Weight Hardness* Type Carbide,% Carbide in 10.sup.-.sup.6
cm.sup.3 Change in mg VPN.sub.100
__________________________________________________________________________
Cr+Cr.sub.3 C.sub.2 Mix 21 -325 59, 75 -2.7,-3.9 374.+-. 84 50 .325
46, 59 -6.1,-7.4 465.+-. 40 80 -325 92,125 -5.3,-6.0 427.+-.101 100
-325 67, 75 -8.1,-8.6 599.+-.160 18 -230/ 82, 82 -7.3,-5.6 318.+-.
54 +325 45 -230/ 52, 75 -6.9,-5.9 407.+-. 75 +325 10 -120/ 156, C**
-4.8,-4.0 337.+-. 56 +230
__________________________________________________________________________
*Some cracking was observed around the indentations. **C implies
chipping of the coating occurred and an accurate scar volume could
not be measured.
In summary the invention provides a coating in which a low-volume
fraction of chromium carbide substantially enhances the wear and
compatibility properties of plasma deposited chromium.
Having desicrbed the invention with reference to certain preferred
embodiments, it should be understood that modification can me made
thereto without departing from the spirit and scope of the
invention.
* * * * *