CA1036390A

CA1036390A - Chromium-chromium carbide powder method for producing same and articles made therefrom

Info

Publication number: CA1036390A
Application number: CA206,050A
Authority: CA
Inventors: John F. Pelton
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1973-08-15
Filing date: 1974-07-31
Publication date: 1978-08-15
Also published as: DE2438998B2; IT1018969B; SE7410371L; JPS5050415A; GB1484583A; IL45473A0; IL45473A; JPS548361B2; FR2245774B1; FR2245774A1; US3846084A; AU7130674A; ES429279A1; DE2438998A1; CH594740A5

Abstract

ABSTRACT OF THE INVENTION
A composite powder and process for making same for use in producing articles or coatings having unique wear and frictional characteristics consisting essentially of a chromium matrix with at least one chromium carbide taken from the class of carbides consisting of Cr23C6; Cr7C3; and Cr3C2 and each particle containing from about 0.2 wt % to about 5.4 wt %
carbon.

Description

_ D-9435 ~036390 This invention relates to a novel powder for use in producing articles and coatings having unique wear and frictional characteristics. More particularly this invention relates to powders which are to be applied as a coating on a substrate using metal spraying techniques and to the articles and coatings made thereby.
Chromium metal has been used as an electro-plated 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 the parts and/or their size is relatively small. When the configuration of the part becomes complex, obtaining a uniform coating thickness by electro-deposition is dif-ficult 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 - - .

- - , -_ D-9435 1 ~ 39~D

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 andtor large areas and/or thick coatings a very substantial capital investment in plating tanks and power supplies i8 required. In chro-mium electroplating it is often necessary to use expensive surface cleaning and etching procedures to prepare sub-strates. 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 with 8 pla~ms or detonation gun. These methods offer a number of process-ing advantages. Surface prepartion 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.

. _ , .

- : D-9435 1 ~ 3 ~

Unfortunately, plasma-deposited chromium is not as wear-resistant at ambient temperature as hard electro-plated chromium. This is because the wear-resistance of chromium plate is not an inherent property of elemental chromium but is believed to arise largely from impurities and ~tresqes incorporated in the coating turing plating.
Plasma deposited chromium being a purer form of chromium thus lacks the wear resistances of hard chromium plate while retaining the corrosion-resistance characteristics ?
of chromium.
It has now been discovered that coatings made by the plasma or detonation-gun process can be made that are remarkably superior to hard chromium electroplate in compatibility, frictional characteristics and wear resist-ance by incorporating 9 dispersion of chromium carbide particles in a chromium matrix.
Coatings of this type have been made from mechan-ical mixtures of powders as described in my co-pending Canadian application Serial No. 206,051, filed July 31, 1974.
While such mechanical mixtures are advantageous, there are certain limitations to the quality of coatings made from them. ;-~
Both plasma and detonation-gun deposition result in a coating --with a multilayer structure of overlapping, thin, lenticular -particles or "splats." Each coating particle or splat is derived from a single particle of the powder used to ~-. ' , . . :

~ 036390 produce the coating. There is little, if any, combining or alloying of two or more powder particles during the coating deposition process. This results in some of the splats being completely chromium and some being completely chromium carbide, with the "fineness" or interparticle spacing being controlled by the ~izes of the initial chromium and chromium carbide powder particles, Thus, the "fineness" of the chromium csrbide dispersi~ in the coating is limited by the fineness of the powder that can be handled by the coat-ing process. Since many desirable properties of the coatingare improved by reducing the interparticle spacing or increasing the "fineness" of the dispersion and since it is desirable from a coating application standpoint to use powders with particles much larger than desired from the coating "fineness" standpoint, it woult be ~d~an~ageous to produce a coating in which each splat is a mixture of chromium and chromium carbide. This in turn requires that each powder particle contain a mixture of chromium metal and chromium carbide.
Accordingly, it is an object of this invention to provide a powder which, when sprayed by a plasma or detonatlon-gun, will produce an article or coating wherein each "splat" is a mixture of chromium metal and chromium carbides.

.' :

~ D-9435 ~Q36390 ;
Another object is to provide such a powder which contains chromium and chromium carbide in each particle.
A further object is to provide a method for making such powder.
Yet another object is to provide a chromium/
chromium carbide coating having superior property to hard chromium electroplate.
Still another ob~ect is to provide a coated tro-choid surface for a rotary combustion engine.
These and other objects will either be pointed out or become apparent from the following description and drawings wherein:
Figure 1 i9 a pictorial representation of the structure obtained by depositing mechanical mixture of chromium and chromium carbides;
Figure 2 is a pictorial representation o~ the type structure obtained by depositing the powder of this invention;
Figures 3, 4 and 5 show possible distribution of the carbide phases in the powder particles;
Figure 6 shows the variation of wear scar volumes with carbon content of the powder used to produce the coat- -ing tested, compared to coatings of hard chrome plate; and, Figure 7 shows the hardness of coatings obtained with powders of various carbon content compared to hardness of hard chrome plate. 6 .. . .~ . .
. ~ - - , .

lV36390 The methods of this invention, which will be described shortly, produce a compositepowder containing the desired amount of chromium carbide and chromium in which substantially each particle contains at least some chromium and chromium carbide. Examples of the possible distribution~ of the carbide phases in the powder particles are shown in Figures 3, 4 and 5. For use in producing plasma or detonation-gun coatings, the exact composition of the carbide phases in the powder or the distribution of the carbide phases as shown in Figures 3, 4 and 5 are not important, only the total carbon content, since during deposition the particles become essentially completely molten. As the individual splats Rolidify during deposi-tion, the carbides reprecipitate from the melt forming Cr23C6, Cr7C3, or Cr3C2, or a combination of these, depend-ing on the totsl 8mount of C present and the rate of solidi-fication. The preferred composition results in a predomin-antly Cr23C6 dispersion.
Basically, the material is prepared by chemical reaction of an intimate mixture of a source of Cr and a source of C; temperatures of 1000-1400C are suitable for hov~s ~ ~p solid state reactions. Times of from about 1-50~are suit- a~ /73 able. Temperatures in excess of 1500C are required for production of the powder by melting referred to hereinafter.

- . .~ . .

The principal reaction involved is xCr + yC ~ CrxCy (1) The principal product is Cr23C6, with minor amount of Cr7C3 and Cr3C2 When oxygen is present in the Cr (as Cr203) or Cr203is used as the Cr source, reaction (1) is preceded or accompanied by Cr23 + 3C ~ 2Cr ~ 3C0 (2) The Cr formed in reaction (2) may react with C present in excess of the amount required to bring reaction (2) to completion to form Cr carbide by reaction (1).
The source of Cr may be commercial Cr powder (e.g., Union Càrbide Mining and Metals Division electro-lytic chromium powder), Cr203 as in reaction (2), or any compound that decomposes on hesting or by reaction with C
or H2 on heating to form essentially Cr and volatile products.
The source of carbon may be any commercial carbon consisting of essentially elemental C and volatile impuri- -ties. Decolorizing carbon, lampblack, and powdered graphite .
have been used with equal success. In addition, a higher carbide of Cr may be used as the C source, since it may react with Cr to form another carbide, the resulting prod-uct having the characteristic intimacy of the invention.
As an example, ... .. ... . . . . .. .. ..
: .~ , - , .
~- ~
' ' ~ ~" ' ~36390 14Cr ~ 3Cr3C2 > Cr23C6 ( ) would produce a carbide on the surface of the Cr particles (present in excess of the amount consumed in reaction (3)).
A gaseous hydrocarbon or hydrocarbon/hydrogen gas mixture is also a suitable carbon source, provided its composition is such that the carbon activity is high enough to permit carbide formation. This reaction has not been u8ed directly, but powdered mixtures of Cr and C heated in a H2 atmosphere are found to consist, after reaction, of two-phase particles in which the c~rbide phase essentially encapsulates the original Cr particles as shown in Figure 3. This structure differs from that found in similar mixtures heated in the absence of H2, which show mainly isolated areas of carbide formation on the Cr particles, a8 shown in Figure 4, corresponding to points o 801id-801id contact of the original Cr and C partlcle8. The difference instructure is clear evidence that carbon has been transported through the vapor phase in the H2 atmos-phere, by the reaction xC + ~ H2 > CXHy (4) occurring at the carbon particles and the reaction CXHy+ zCr ~ CrzCx+ 2Y H2 (5) occurring at the Cr particles. This vapor transport re- -action may be the principal source of Cr carbide formation or it may supplement reaction (1). Some oxygen removal ', - ;.

D~9435 ~ ~ 6 39HD
reaction, either reaction (2) or reaction (6) y CrxCy ~ Cr23 > (2 ~ 3yx) Cr + 3C0 (6) also occurs.
The intimately mixed Cr/Cr carbide structure may also be prepared by melting Cr and C (present either as the element or as a Cr carbide) mixtures of appropriate totfll analy8is, allowing thé homogeneous liquid to freeze and the Cr carbide to precipitate out, and then crushing the solidified melt to powder. Temperatures greater than 1500C are required for this method. Limitations of higher melting temperatures and difficulty in crushing the ~olidi-fied melt practically limit this method of prepar8tion to carbon content of 3% by weight or more The reaction of Cr and C is preferably carried out in vacuum because this promotes the removal of the gaseous C0 formed in reaction ~2) or (6). The vacuum does not h8ve to be extraordinarily good, ultimate system pressures be-tween 0.01 and 100 microns having been found to yield products of essentially the same oxygen content. The re-action can also be carried out in any atmosphere with oxygen potential sufficiently low to prevent oxidation of Cr. A hydrogen atmosphere i8 quite suitable and is par-ticularly useful for the preparation of a composite of low C content with a uniform carbide distribution, since the H2 takes part in the reaction and promotes uniform distribu-tion. - 10 -, '~ -: ' ;

1C~36 39~D

The product of the Cr ~ C or Cr203 + C reaction is a sintered cake, however the reaction i8 carried out.
Sintering is least, and reduction to powder by ball-milling, hammer-milling, and other conventional techniques is easier, when the Cr203 ~ C reaction i9 used or when the Cr ~ C
reaction is carried out in H~. Lower reaction temperatures favor ease of reduction when the Cr + C reaction is carried out in vacw m.
The carbide distribution within the powder par-ticle is a function of the method of production. When a mixture of solid carbon and chromium is heated in vacuum, the predominant form is that shown in Figure 4 because the carbon tends to react with the chromium surface closest to it. The finer and more uniform the distribution of carbon in the starting mixture, the more uniform the distribution of carbides around the surface of the chromium will be.
The ultimate extension of this trend is achieved when a gaseous source of carbon is used either by directly supply- p ing a hydrocarbon gas or (-Vh~} by heating the solid car- ~/ l/73 bon plus chromium in a hydrogen atmosphere (which results in a hydrocarbon gas). The carbide distribution which results is like that in Figure 3. A distribution of carbon particles throughout the powder particle, Figure 5, ~sy result when a solid ingot of the proper total composition is reduced to powder.

~Q36390 ~ o3~
Oxygen content (in the range ~ rb) does not ~ 3 affect the wear properties of coatings made from powders of this invention. The carbon content of the powder of this invention ma~ be between 0.2% and 5.4% by weight. At thelower limit, plasma deposits made from the powder are superior in tests to similar deposits made from commercial electrolytic chromium powder. The high end of the range is defined by the complete conversion to the compound Cr23C6, which contains 5.6% by weight; at this point, the material no longer contains free Cr. The wear resistance of coatings made from the powder varies with carbon content as shown in the band curve on Figure 6. The range of values observed for commercial hard chrome plate i8 also shown in the Figure 6 by the cross-hatched area ad~acent to the vertical ~Xi8.
The optimum composition is believed to lie in the range 0.8-1.7% C by weight, and may vary somewhat with the method of preparation. Coatings, made from powders in this composition range,are equivalent to or superior to com-mercial electrolytic Cr plate in laboratory lubricatedrubbing wear tests at high load (see Figure 6). Further-more, the hardness, see Figure 7, is at a minimum, making it possible to readily fini~h the coating with conventional grinding or honing tools. Low-surface-speed, high-deposition-rate plasma plating produces well-bonded, uncraeked coatings.

. . .

~ .~

~a36390 Specifically, it has been found that powders containing about 1 wt % carbon produce plasma deposited coatings on interior trochoid surfaces of rotary com-bustion engines which have remarkedly and unexpectedly superior propertles, as shown hereinafter in Example 9 The coating of this invention i8 characterized by the presence in substantially every splat of both Cr and Cr carbide. As pictorially illustrated in Figure 2, the relative amounts of Cr and Cr Carbides will vary between splats as a necessary result of the use of powder with a range of partial sizes and adventitious difference in the degree to which esch Cr particle is carburized and in the conditions to which the various particles are sub-~ected in passing through the coating device Neverthe-less, the coating of this invention is distinguished from tha~ produced from a powder which is a simple mixture of ~hlcl. ~ f~
Cr and Cr carbide,~E~ is pictorially represented in U
Figure 1, in that the splats in the latter type of coating are each individually either all Cr or all Cr carbide.
Figure 2 i8 to be understood as being merely illustrative of one feature of the distribution of the carbides in the coating. Upon extraction by chemical methods of carbides from the invention and examination of these carbides by optical and electron micro~copy, it has been found that at least some, and probably most, of the , - . , .

~ 9-9435 1(~36390 carbides are much finer than suggested by Figure 2. The majority of the carbide particles were found to be of sub-micronsize and most were predominantly in the shape of a lace-like network, suggesting that the coatings contained ~ine-grained interlocking, continuous networks of both carbide and Cr, the separation between the interstices of these networks being so small that they are not resolv-able in optical microscopy.
The coatings produced with the powder of this invention have a number of advantages in addition to the general processing advantages previously described as being associated with metal spray deposition.
1) Coatings are superior to those formed by the plflsma deposition of commercial electrolytic chromium powder in that increased wear resistance and resistance to spalling are found, though there is minimal increase in hardness as measured by diamond pyramid indentations.

2) Coatings are superior to coatings in which nitrogen rather than carbon is the strengthening additive, in that carbide-strengthened material is much less brittle and much less prone to spalling.

3) In the laboratory lubricated rubbing wear test described in Example I, the coatings of this invention with a carbon content in the preferred range of 0.8 - 1.7 wt % C,performed as well as or better than commeri~al electrolytic chrome plate. - 14 -. .
~, ' ' 1C~39N~

4~ Coatings of this invention performed far superior to electrolytic chrome plate coating on internal trochoid surfaces in rotary combustion engines as described in detail in Example 9.
The following examples illustrate the invention but are not intended to limit the variations in processing that would be apparent to those skilled in the art. More-over, the use of the powder of this invention is not in-tended to be limited to plasma or detonation-gun deposition.
EXAMPLE 1.
8879 grams of Union Carbide Mining and Metals Division electrolytic chromium, screened through a 230-mesh sieve, was mixed with 200 grams of Fisher Scientific Com-;pany Norit A~decolorizing carbon, similarly screened, and blended for two hour8 in a cone blender. A portion o~ this mixture wa8 used to fill eight pans, each about 0.6 cm deep, so that each pan contained between 210 and 230 grams of the mixture. The pans were vertically stacked in a vacuum furnace so that there was about 0.4 clearance between pans.
The furnace was evacuated slowly to about 500-micron pressure and then more rapidly to about 0.5 micron, using an oil-diffusion pump. Power was then applied to tantalum strip heaters s~rrounding the stack of pans and the pans heated over a period of about 80 minutes to a temperature of 1080C as indicated by a thermocouple in contact with ~tra~e~a~

1~6390 the powder in the uppermost pan; system pressure was main-tained below 50 microns during this period by adjusting the rate of heating. The powder was maintained at 1080C
for four hours, during which time the pressure gradually dropped to about 0.3 micron. The furnace was then allowed to cool to room temperature with pumping continued. When the pan8 were re~oved from the ~urnace, the material was in the form of sintered cakes of a significantly more metallic appearance than the original powder mix. These cakes were crushed in a mechanical pulverizer until about 95% of the material was reduced to powder that would pass a 325-mesh screen. The balance of the original mixture of chromium and carbon powders was processed identi¢ally in four additional furnacings.
The -325 mesh powders from the five furnace run8 were individually analyzed for combined carbon, ~ree carbon, and oxygen. All showed less than 0.1% free carbon, between 300 and 420 ppm oxygen, and 1.05-1. 08% combined carbon. The distribution of carbides on the chromium was similar to that in Figure 4.
The products of the five runs were blended to-gether and used to produce coatings by deposition through a plasma torch. Coatings so produced, when separated from the substrates on which they were plated, analyzed 1.03-1.06% C. The wear resistance of these plasma-deposited ... . ~ ...... . ...... .. ~ ........... .

, '' '- ~

_ D-9435 1C~36 3 ~

coatings were measured using a Dow-Corning LFW-l Friction and Wear Test Machine according to ASTM Standard Method D2714-68. Coatings deposited 12 mils thick on the wear surface of mild steel wear blocks were ground to a final thickne~s of 5 mils and tested against carburized AISI 4620 steel rings (surface hardness 58-63 Rockwell "C") at 450 lb specific load for 5400 ring revolutions at about 180 rpm;
MIL-5606A hydraullc fluid was used as lubricant. Wear scar volumes, calculated from the projected scar aress and the known ring diameter, ranged between 24 and 49 x 10 6cm3.
These test results are included in Figure 6.
EXAMPLE 2.
Numerous mixtures differing only ln the amounts of electrolytic chromium and decolorizing carbon used were processed a8 described in Example 1. The re8ulting powders, which r~nged in c~rbon content from 0.6 to 5.4%, were used to form plasma-deposited coatings and tested for wear re-sistance using the techniques and procedures described in Example 1. Results of these tests are included in Figure 6.
EXAMPLE 3.
5400 grams of the same electrolytic chromium powder used in preceding examples was mixed with 87 grams of lampblack for one hour in a ceramic ball mill and then further mixed for 30 minutes in a cone blender. The mixed powders were loaded into pans and heated in the vacuum .
furnace exactly as described in Example 1. The product, after reduction to -325 me~h powder, analyzed 0.81/~ carbon and 335 ppm oxygen. Plasma-deposited coatings were made and tested as described in Example 1. Scar volumes of 21 to 34 x 10 6cm3 were observed; these results are included in Figure 6.
EXAMPLE 4.
1476 grams of the same electrolytic chromium powder used in p~evious examples and 24 grams of the same screened decolorizing carbon used in previous examples were blended for two hours in a cone blender. Two boats, each 0.6 cm deep and about 25 cm long, were filled with this powder and placed in a 10 cm diameter ceramic tube furnace which was then ~ealed and evacuated with a mechan-ical pump for several hours. The furnace was then filled with hydrogen, heated to 1150C, and maintained 8t thi8 temperature for 22 hours, a flow of 15 scfh of hydrogen being maintained during the entire cycle. The product was a sintered cake much more readily reduced to -325 mesh powder than the products of the vacuum processing previously described. This powder analyzed 1.06% carbon, 630 ppm ;~
oxygen. Plasma-deposited coatings made and tested as de-scribed earlier had scar volumes of 21 to 42 x 10 6cm3. A
portion of the powder was mounted and polished to metal-lographic examination; at 500X magnification, it appeared .
- ~ .

that most, and possibly all~ of the powder particles con-sisted of a shell of chromium carbide surrounding a core of chromium metal similar to that in Figure 3. In this respect, the structure differed from that of powders pre-pared by vacuumprocessing; in the latter carbide and metal were observed in the same particles, but complete encap-sulation was not observed.
EXAMPLE 5.
1773 grams of the same electrolytic chromdum powder used in previous examples and 27 grams of the same screened decolorizing carbon used in earlier examples were blended by shaking and rolling in a 32-oz ~lass jar. Using this powder, eight separate heats, each with between 80 and 105 grams of mix, were made in a 4 cm diameter tube furnace. Each heat was for five hours at 1140C in a flow of about 110 9cfh hydrogen without preliminary evacuation.
The eight cakes were easily powdered by light hammering and when blended together and screened yielded a -325 mesh powder containing 1.13% C and 1730 ppm oxygen. The micro-structure of this powder was very similar to that of the powder described in Example 4, consisting of chromium carbide surrounding chromium; in addition, a small amount of very fine precipitates was noted decorating the!carbide-chromium interface.

- . -~036390 EXAMPLE 6.
.
A powder analyzing 1.13% C prepared by the method described in Example 1 was plated onto test blocks using a detonation gun. Microstructural differences between these coatings and those formed by plasma deposition were observed consistent wlth the difference in method of coating forma-tion, For wear-test conditions identical with those employed for the plasma-deposited materials, scar volumes of 15-19 x 10 6cm3 were measured on the detonation-gun coatings.
EXANPLE 7.
Four hundred lb of Cr203 was blended with 94.8 lb of lampblack in a twin-shell blended and then more thor-oughly blended in a vibratory ball mill. This product was then mixed with 9.5 lb cornstarch binder and enough water to mske a mix ~uitable for forming briquettes in a standard briquetting pres8. It w~ then pres~ed into briquette8 of about 2-inchmaximum dimension and dried to remove excess water. The briquetted mix, charged to a large vacuum fur-nace in an l9-inch-deep bed covered with graphite plates, was heated to 1000C without letting the pressure exceed 5000 microns, held one hour at 1000C after the pressure had dropped below 2000 microns, then heated to 1400C and held at that temperature for 50 hours, at the end of which time the pressure had dropped to less than 150 microns.

' _ D-9435 A portion of this product was pulverized to -325 mesh size and found to contain 1.14% C and 460 ppm oxygen. The car-bide dispersion in the powder was similar to Figure 4. Wear samples formed from this material by plasma deposition and te~ted as described previously exhibited wear scars of 21-24- x 10 6cm3 volume.
EXAMPLE 8.
A mixture of 9900 grams of commercial grade elec-trolytic chromium sized to pass through a 65-me~h screen and 100 grams of lampblack was blended dry, then mixed with water and cornstarch binder and formed into briquettes as described in Example 7. The briquetted mixture was then furnaced in vacw m under graphite covers for one hour at 1000C snd for eight hours at 1385C. The pressure in the ~urnace was maintained below 500 microns and was 50 microns at the end of the heating period. This mat~ria~ was then crushed, yielding about 30% -325 mesh material that analyzed 1.3% C and 721 ppm oxygen, with a carbide dispersion similar to Figure 4. Wear samples made from this powder by plasma deposition and tested in the standard manner exhibited wear scars of 18-23 x 10 6cm3.
EXAMPLE 9.
Plasma deposited coatings produced in a manner similar to Example 1 were applied to the interior trochoid surfaces of rotary combustion engines fitted with graphite-- : , aluminum composite rotor apex seals. The engines were run in laboratory test stands and in test vehicles. The tro-choids were made of several different types of materials and of two different sizes, examples of which are shown in Table I. Over 3113 hr of test stand operation have accumulated on the small en8ine size and 331 hr of test stand and 7000 hr of vehicle operation on the large engine size. In comparison with hard electropleted chromium the coatings of this invention showed the following advantages:
a) Essentially no wear of the coated surface has been observed and no roughening or "wash boardlng,"
as occurs with electroplated chromium, has developed.
b) The wear of the mating seal surface is approximately one-half that caused by hard electroplated chromium, which is greater than .005~' per 100 hr.
c) Performance of the coating is less sensi-tive to surface finish than hard electroplated chromium.
There was no appreciable difference in wear of either the coated surface or the seal surface between as-ground coat-ing surfaces of 16 to 32 microinches rms and honed sur-faces of approximately 6 rms. In comparison, a hardelectroplated chromium surface must be finished to better than 6 microinches rms to perform satisfactorily.
d) Finishing of the plasma-deposited ooating is far simpler and may be cheaper since it can be used : ..
.- ~ .

_ D-9435 i036390 as-ground while a hard electroplated chromium coating must be ground, then etched to enhance the micro cracked texture of the surface, and then honed to improve the ~urface fin-ish. Because thickness control is better with plasma deposition than with electroplating, the amount of material that must be removed in finishing is also less.
e) The performance of englnes with the coatings of this invention i9 far less sensitive to fluctuation ln coolant temperatures than those coated with hard electro-plated chromium.f) The performance of engines with the coatings of this invention is far less sensitive to fluctuation in oil lubrication than that of engine~ coated with hard electroplated chromium. The latter require continuou8 addition of oil to the combustion chamber, but an engine provided with the coating of this invention continued to perform satisfactorily when the oil addition was stopped.
g) The cost of engines and the vehicles can be reduced using the coatings of this invention because they can be applied directly to aluminum trochoid housings while the use of hard electroplated chromium requires a steel liner. This reduces not only the cost of the housing, but also the weight of the engine and therefore the cost of vehicle frame, suspension, etc. In addition, engine cooling is more efficient. With lower total vehicle weight, fuel - .
. ..

., ~Q3~

efficioncy is increased.
TABLE I

Coating Total Average Seal Trochoid Trochoid Thickness Time of Wear Rate T~e Size*(in.) (in.) Test(hr.) (in./100 hr,) Aluminum with 9.5 x 7.1 .015 to800 .0026 Steel Liner x 2.75 .017 Aluminum with 9.5 x 7.1 .019 to444 .0026 No Liner x 2.75 .020 Aluminum with 11.5 x 8.6 .018 to220 .0026 Steel Liner x 2.75 .015 Aluminum with 11.5 x 8.6 .016 to76 .0026 No Liner x 2.75 .018 Cast Iron 9.5 x 7.1 .010 to200 .006 x 2.75 .0115 * M~or Axis x Minor Axis x Width Having described the invention with reference to certain preferred embodiments, it shou~d be understood that minor modifications can be made thereto without departing from the spirit ard ~cope thereof.

Claims

WHAT IS CLAIMED IS:

Claim 1. A powder containing from about 0.2 wt % to about 5.4 wt % carbon and wherein substantially every particle of said powder consists essentially of chromium and at least one chromium carbide taken from the class consisting of Cr23 C6;
Cr7 C3 and Cr3 C2.
Claim 2. A powder according to Claim 1 wherein said powder contains from about 0.8 - 1.7 wt % carbon.
Claim 3. A powder according to Claim 1 wherein said powder contains about 1 wt % carbon.
Claim 4. A powder according to Claim l wherein each particle has a core of chromium substantially completely surrounded by a shell of said chromium carbides.
Claim 5. A powder according to Claim 1 wherein each particle contains chromium and said chromium carbides on the surface of said chromium.
Claim 6. A powder according to Claim 1 wherein each particle contains chromium and said chromium carbides dispersed within said chromium.
Claim 7. A method for producing a powder comprising:

A) heating a source of chromium with a source of carbon in a non-oxidizing environment until the carbon diffuses and reacts with the chromium;

B) comminuting the product formed in step A to a powder containing from about 0.2 wt % to about 5.4 wt % Carbon and wherein substan-tially every particle of said powder consists essentially of chromium and at least one chromium carbide taken from the class con-sisting of Cr23C6; Cr7C3 and Cr3C2.
Claim 8. A method for producing a powder comprising:
A) mixing a source of chromium powder with a source of carbon;
B) heating said mixture in a non-oxidizing environment until the carbon diffuses and reacts with the chromium; and C) comminuting the product formed in Step (B) to a powder containing from about 0.2 wt %
to about 5.4 wt % carbon and wherein sub-stantially every particle of said powder consists essentially of chromium and at least one chromium carbide taken from the class consisting of Cr23C6; Cr7C3 and Cr3C2.
Claim 9. Method according to Claim 8 wherein the source of chromium is Cr2O3 or chromium metal.
Claim 10. Method according to Claim 8 wherein the source of carbon is elemental carbon or a carbide of chromium contain-ing a greater percentage of carbon than Cr23C6.
Claim 11. Method according to Claim 8 wherein said mixture is heated to a temperature in the range of from 1000 to 1400°C
for a period of from 1-50 hours.
Claim 12. Method according to Claim 8 wherein said non-oxidizing environment is a vacuum.
Claim 13. Method according to Claim 8 wherein said non-oxidizing environment is hydrogen.
Claim 14. Method according to Claim 7 wherein said source of carbon is a carbon containing gas.
Claim 15. Method according to Claim 14 wherein said carbon containing gas is a hydrocarbon gas.
Claim 16. An article consisting of a metal substrate having a coating thereon consisting essentially of a composite of chromium and at least one chromium carbide taken from the class consisting of Cr23C6; Cr7C3 and Cr3C2 wherein the composite coating contains from 0.2 wt % to about 5.4 wt % Carbon and the composite is characterized by a multilayer structure of over-lapping thin, lenticular particles, each particle containing a mixture of said chromium and chromium carbides.
Claim 17. Article according to Claim 16 wherein the carbon content is in the range of 0.8 - 1.7 wt %.
Claim 18. A coated trochoid surface of a rotary combustion engine comprising a metallic surface having a coating thereon consisting essentially of a composite of chromium and at least one chromium carbide taken from the class consisting of Cr23C6;

Cr7C3 and Cr3C2 wherein the composite coating contains from 0.2 wt % to about 5.4 wt % Carbon and the composite is characterized by a multilayer structure of over-lapping thin, lenticular particles, each particle containing a mixture of said chromium and chromium carbides.
Claim 19. A coated trochoid surface of a rotary combustion engine according to Claim 18 wherein the carbon content is in the range of 0.8 - 1.7 wt %.
Claim 20. A coated trochoid surface of a rotary combustion engine according to Claim 18 wherein the carbon content is about 1 wt %.
Claim 21. An article consisting of-a composite of chromium and at least one chromium carbide taken from the class consisting of Cr23C6; Cr7C3 and Cr3C2 wherein the composite contains from 0.2 wt % to about 5.4 wt % carbon and the composite is charac-terized by a multilayer structure of over-lapping thin, lenticular particles, each particle containing a mixture of said chromium and chromium carbides.