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.

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.

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.