Low friction and galling resistant coatings and processes for coating
GOVERNMENT CONTRACT 
     The invention described herein was made during the course of, or in the performance of work, under U.S. Government Contract No. DE-AC06-76FF02170 under the auspices of the Dept. of Energy. 
    
    
     The present invention pertains to the fields of coating processes and resultant coatings on structural alloys which provide low friction and galling resistant surfaces. It especially pertains to ESD (electro-spark deposited) coatings for use in rubbing contact with similarly coated structural alloys or uncoated structural alloys, especially in liquid metal environments. 
     Over the years a number of commercial as well as developmental alloys have been evaluated for use in the elevated temperature liquid metal environments encountered in a liquid metal fast breeder reactor (LMFBR) and its associated systems which also handle liquid metal (e.g. pump components and steam generator components). Within the LMFBR itself, a high energy neutron (0.1 MeV to 1.0 MeV) environment exists in which fuel rod cladding and duct components see service in contact with flowing, low oxygen (&lt;1 ppm) sodium. The fuel cladding has surface temperatures of about 400° C. (˜750° F.) to about 650° C. (˜1200° F.). Ducts, surrounding each bundle of fuel pins, see service at about 380° C. (˜715° F.) to about 550° C. (˜1020° F.). These components may be exposed at the aforementioned elevated temperatures to neutron fluxes on the order of 10 15  n/cm 2 .s (E&gt;0.1 MeV), and should be capable of performing adequately to fluences on the order of 2 to 3×10 23  n/cm 2  (E&gt;0.1 MeV). Additional information about LMFBR materials, components, conditions, and concerns are outlined in Bennett and Horton, &#34;Materials Requirements for Liquid Metal Fast Breeder Reactors,&#34; Metallurgical Transactions A, Vol. 9A, February 1978, pp. 143-149. 
     Included among the commercial austenitic stainless steels evaluated is AISI 316 whose chemistry is described in U.S. Pat. No. 4,421,572. The developmental austenitic stainless steels evaluated include the D9 type alloys and are exemplified by the alloys described in U.S. Pat. No. 4,407,673 (D9--B1) and U.S. patent application Ser. Nos. 414,167 and 484,322, filed on Sept. 2, 1982 and Apr. 12, 1983, respectively. One feature that is common to all the foregoing austenitic stainless steel alloys is that for use as LMFBR fuel cladding or ducts it is desired that they be in a cold worked microstructural condition in order to delay the onset of neutron irradiation induced swelling. 
     Among the commercial ferritic stainless steels considered have been HT-9 (a trademark of Sandvik Corporation) for its martensitic alloy which nominally contains 11.5 wt.% Cr--1 wt.% Mo--0.2 wt.% C--0.4 wt.% Si--0.5 wt.% Ni--0.5 wt.% Mn--0.3 wt.% V--0.5 wt.% W--balance essentially iron). 
     Other ferritic stainless steels considered have been 9 Cr--1 Mo type alloys (ASME-SA812, Grade F9, UNS. No. K90941, 2.25 Cr--1Mo type alloys (ASME-SA182, Grade F22, UNS. No. K21590) and those alloys described in U.S. Pat. Nos. 4,049,431; 4,049,432; 4,075,010; 4,435,231; and U.S. patent application Ser. No. 370,439, filed on Apr. 21, 1982. These alloys have been considered for use as ducts, fuel cladding, and steam generator tubing and tubing support structures. 
     Other alloys considered include austenitic precipitation hardening nickel base and iron base alloys described in U.S. Pat. Nos. 4,129,462; 4,231,795; 4,377,555; 4,359,349; 4,359,350; 4,236,943; 4,362,696; and U.S. patent application Ser. Nos. 370,438, and 180,770, filed on Apr. 21, 1982 and Aug. 22, 1980, respectively. Similar commercial alloys that have been considered for LMFBR related applications include Niomonic PE16 and Inconel 706 and 718. (These are nickel base alloys marketed by the International Nickel Co.) 
     One concern that has been common to the usage of all of the above structural materials in the liquid sodium environments described, has been their ability to resist damage caused by rubbing contact with other components. 
     Development of low-friction and wear-resistant materials for liquid sodium environments has long been recognized as one key to the successful design and operation of LMFBR components. Low friction surfaces provide reliable operation of mechanical components, ease of assembly of complex and close tolerance parts, reduced operating forces and power requirements, lower stresses in components, and reduced surface damage and wear. Some nuclear applications impose a maximum allowable friction coefficient that makes a low-friction material or coating virtually mandatory. A variety of industrial bearings, lubricants, and materials are available to provide low friction or low wear, but when the special environments and temperatures of LMFBR&#39;s are considered, the list of candidate materials for low-friction applications becomes severely restricted. Besides being a poor lubricant, sodium is a highly reactive coolant, which precludes the use of nearly all known liquid or solid lubricants. High-temperature sodium is also exceptionally effective in stripping oxide layers from most material surfaces. The resulting highly-cleaned surfaces can exhibit much higher friction and self-welding tendencies than the same surfaces would show in most other environments. The U.S. Dept. of Energy National Tribology Program has been developing, evaluating, and qualifying low friction and wear resistant materials for advanced energy systems, with most efforts so far directed toward liquid metal environments. Materials include structural materials and alloys, weld-deposited hardfacings, coatings and platings, cast bearing materials, and lubricants. LMFBR applications for such wear-resistant materials include fuel assembly interfaces, control rod drive mechanisms, fuel handling machines, heat exchangers, valve seats and bearings, coolant pump bearings, test loop components, and many others. 
     Through the foregoing program a number of coatings have emerged. For example it has been found that nickel--aluminide coated (i.e. aluminized) Inconel 718 has superior wear, friction, and corrosion properties compared to uncoated Inconel 718 in liquid sodium. This coating is formed by diffusion of aluminum into the nickel alloy surface forming a surface layer of NiAl. The coating process used requires that the part be held at about 1000° C. or higher, for a significant period of time. These heat treatment requirements limit the uses to which this coating can be applied. For example, this high temperature coating process can anneal out the desired cold worked microstructure of a 316 or D9 type alloy component. 
     Another coating developed is chromium carbide and 15 volume percent nichrome (80 wt.% Ni--20 wt.% Cr) applied by the detonation gun process, directly to 20% cold worked 316 duct wear pads. The detonation gun process is a method of producing high mechanical bond strength coatings on most metallic substrates. In the detonation gun process, a mixture of oxygen and acetylene, along with powdered coating material, is introduced into the chamber of the gun. The gas is detonated by a spark plug and the resulting explosion heats the powder to near melting and propels the hot particles out the barrel of the gun at supersonic velocities. The impingement of these high kinetic energy particles at high temperatures on the substrate produces a dense, well-bonded coating. The process is repeated at about 250-500 shots/minute to build up the coating to the desired thickness level. 
     The limitations of this coating are those common to other mechanically bonded coatings, i.e., proper substrate preparation is critical to achieving adequate bonding, rigid process and quality controls are exceptionally important, and high contact stresses (above the yield strength of the substrate) must be avoided. 
     Additional information with respect to the application of the aforementioned and other coatings are provided in R. N. Johnson et al., &#34;Development of Low Friction Materials for LMFBR Components,&#34; Proceedings of the International Conference on Liquid Metal Technology in Energy Production, USERDA Conf. Publ. 760503-Pl, U.S. Energy Research and Development Administration, Washington, DC (1976) and R. N. Johnson et al., &#34;Wear Resistant Coatings for Reactor Components in Liquid Sodium Environments&#34;, Journal of Vacuum Science &amp; Technology, Vol. 11, No. 4, July/August 1974 pp. 759-764. 
     I have found that one of the most promising advances in coating technology for reactor component applications has been ESD (Electro-Spark Deposited) coatings. The ESD coatings are produced by discharging stored energy from high voltage capacitors through an electrode of the material to be deposited. In the resulting spark, a small amount of material is melted and removed from the electrode and welded to the substrate material. An ESD coating apparatus and its operation are described in Sheldon U.S. Pat. No. 4,405,851. 
     One of the principal advantages of the ESD process is that thin (usually 25 to 50 μm) layers of nearly any electrically conductive material can be fused to a metal surface with such a low heat input that the bulk substrate material remains at or near ambient temperature. This essentially eliminates thermal distortions or changes in metallurgical structure (except for a small diffusion zone) of the substrate. It also produces a coating that, because of its metallurgical bonding, is significantly more resistant to damage and wear than coatings applied by most other low-heat-input processes, such as detonation-gun, plasma spray, electro-chemical deposition, etc. (see my paper, &#34;Coatings for Fast Breeder Reactor Components&#34; published in Thin Solid Films Vol. 118 no. 1 (August 1984) pp. 29-45). 
     I have further surprisingly found that in accordance with the present invention the following ESD coatings, having superior wear and friction properties, can be provided by the following ESD processes. 
     A substrate of an iron base alloy or a nickel base alloy is ESD coated to an average thickness between about 7.5 and 100 microns (hereinafter all coating thicknesses referred to are average values). This ESD coating, in accordance with the present invention, is formed by ESD depositing material from a cemented carbide electrode of chromium carbide in a nickel base matrix and ESD depositing material from an electrode selected from the group of aluminum and its alloys containing at least 95 wt.% aluminum, or nickel--molybdenum--chromium alloys containing greater than 20 wt.% molybdenum. 
     In the ESD coating utilizing aluminum, it is most preferred that the layer formed by the cemented carbide electrode be about 15 to 50 microns thick and that the aluminum be lightly applied onto it, thereby maintaining about the same coating thickness while forming an aluminum rich alloy zone of about 3 to 10 microns thick adjacent to the coatings surface. After the ESD coating is complete, it is also preferred that the roughness of the aluminized surface be reduced (by light buffing, for example) and the coating surface then oxidized. 
     In the ESD coating utilizing a Ni--Mo--Cr alloy, it is most preferred that the layer formed by the cemented carbide electrode be about 15 to 50 microns thick and that a Ni--Mo--Cr alloy, such as Tribaloy 700 (a trademark of the Cabot Corporation for its laves hardened hardfacing alloy containing nominally: Ni--32 wt.% Mo--15 wt.% Cr--3 wt.% Si) be applied as an overlayer of about 2 to 6 microns, and most preferably about 5 microns thick. After coating, the roughness of the surface is preferably reduced, for example, by light buffing. 
     The substrate metallurgically bonded to the foregoing ESD coatings may be any iron base or nickel base alloy. Preferably, the iron base alloys are ferritic or austenitic stainless steels or superalloys, such as those previously referred to herein. The nickel base alloys may include any of the known commercial nickel base alloys such as Inconel 600, 706, or 718, in addition to the alloys previously referred to herein. 
     These and other aspects of the present invention will become more apparent upon review of the drawings briefly described below, in conjunction with the detailed description of the invention which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the average dynamic friction coefficient of an embodiment of the present invention at various temperatures in low oxygen sodium. 
     FIG. 2 shows a metallographic cross section through one embodiment of an article coated in accordance with the present invention. 
     FIG. 3 shows a metallographic cross section through another embodiment of an article coated in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The ESD process is a micro-welding process and therefore does not simply deposit a pure coating of whatever electrode material is used. Since the surface of the substrate is also melted (at least momentarily) the deposit consists of a fused mixture of the substrate and the molten material transferred from the electrode. 
     Subsequent layers, of course, can be used to further modify the surface composition, and, if the same electrode is used, the surface composition of the deposit will eventually approach the composition of the electrode. Usually, process parameters may be adjusted to produce coatings with varying levels of substrate dilution in the coating. For example, other factors being equal, the coating composition tends to approach the electrode composition (i.e. minimal substrate dilution effect) with increasing thermal conductivity of the substrate, increasing speed of travel of the electrode over the substrate, decreasing electrode mass, increasing substrate melting point and heat capacity, decreasing electrode melting point and heat capacity, decreasing substrate temperature, and increasing number of coating layers. For example, in my work in which ESD deposited material from a C815 cemented carbide electrode (C815 is a fully dense, sintered, chromium carbide (about 85 volume percent) in a matrix of about 15 volume percent nickel, marketed by Carmet Inc. of Spartanburg, S.C.) on to either ferritic or austenitic stainless steel, that at least about a 7.5 micron, and more preferably about 12.5 microns, thick coating was required to avoid excessive dilution of the coating with material from the substrate causing a significant decrease in hardness. However, I have found that if the coating is too thick (i.e. above about 100 microns), excessive spalling and/or cracking of the coating may occur, probably caused by excessive internal stresses in the coating. I have found that my best results can be achieved when the coating thickness is between about 15 to about 50 microns, in that excellent hardness is obtained and cracking and/or spalling can be minimized. 
     The coatings in my invention are (1) a mixture of chromium, carbon, nickel, aluminum, and whatever portion of the substrate material appears as a diluent in the final coating and (2) a mixture of chromium, carbon, nickel, molybdenum, silicon and whatever portion of the substrate appears as a diluent in the final coating. I believe, however, that the substrate effects in the coatings we have tested are minimal, at least for steel substrates ranging from 21/4Cr--1Mo steel to Type 316 stainless steel, and for coating thicknesses greater than about 25 microns, since coatings on these and similar steels have yielded similar test results. It is expected, therefore, that this coating would provide similar tribological performance when applied to most other structural alloys as long as substrate dilution effects are controlled to avoid significant modification of the coating composition and structure. The other structural alloys specifically under contemplation includes the AISI TYPE 304 stainless, D9 type alloys, the HT-9 alloy, 12 Cr--1Mo alloys, as well as the other ferrous alloys previously mentioned for use in LMFBR systems and the commercial (e.g. Inconel 600, 706, and 718) as well as experimental nickel base alloys I have previously referred to. 
     While in the examples that I will later describe herein the coating was created by depositing a layer from a C815 electrode and then alloying that coating surface with aluminum or Tribaloy 700 in a second ESD deposit, it is my belief that it may also be possible that a sufficiently similar structure would be created by use of a prealloyed electrode of all the constituents (Cr, C, Ni and Al; or Ni--Mo--Cr--Si--Cr--C) or by a duplex deposit where the aluminum or Tribaloy 700 is applied first, followed by an ESD coating from a C815 electrode in such a way that the aluminum or Ni--Mo--Cr--Si alloy is effectively allowed to dilute the deposit--surface. 
     The characteristic structure of the deposit, inherent to the ESD process, is believed to be a significant contributor, if not a requisite, to the performance of the coating. The ESD process results in extremely rapid solidification of the deposit, which is characterized by an extremely fine grain structure that approaches (and with some materials actually is) an amorphous layer of metallic glass. Although a compositional gradient may exist between the surface and the substrate, typically the individual layers are very homogenous. 
     The ESD coated articles are further characterized by a substrate which is essentially unaffected by the ESD coating process which metallurgically bonds the coating to the substrate. Except for a minor zone extending inwardly up to about 25 microns from the original substrate surface in which diffusion and heat affects may be observed, the bulk of the substrate retains its original composition microstructure and other properties. For example, this allows cold worked 316 stainless or cold worked D9 type alloys to be coated without annealing out this cold worked microstructure. 
     The invention will be further clarified by considering the following examples which are intended to be purely exemplary of the present invention. 
     One embodiment of my low friction coating was developed for use in sodium by applying two hardfacing materials in such a way that the friction and wear performance of the combination was superior to either material by itself. The coating consists of a 25 to 50 μm layer of chromium carbide--15Ni cermet (C815) applied to a metal surface by ESD, with about a 5 μm overlayer of Ni--32--15Cr--3Si hardfacing alloy (Tribaloy 700) also applied by ESD. 
     Previous tests on ESD coatings made from cemented chromium carbide alone showed unacceptably high friction coefficients at 200° to 400° C. in sodium but desirably low friction at higher temperatures. That coating also showed excellent wear and galling resistance at all temperatures. ESD coatings of the Tribaloy 700 alone, conversely, showed low friction coefficients at lower sodium temperatures but increasing friction and tendencies for galling at the higher temperatures. 
     I anticipated that the effect of using the duplex coating of the two materials as described above was to achieve a low initial friction coefficient at low temperatures at an expected sacrifice in high temperature friction performance. The chromium carbide layer was intended to provide a backup wear protection and to limit the galling possible with the Tribaloy 700. The actual result, however, was low friction (see FIG. 1) and complete elimination of galling at all temperatures. FIG. 1 shows the average dynamic friction coefficient measured in low oxygen sodium for coated D9--B1 in rubbing contact with itself. The results shown in FIG. 1 were obtained from pin-on-plate rubbing tests using a load of 1000 psi, a stroke length of 1/4 to 1 inch, a total rubbing length of at least 100 inches at each temperature, and a total rubbing length for all temperatures of at least 5000 inches. 
     The mechanism by which this synergistic improvement occurs is not yet fully understood. While not wishing to be bound by theory, I believe that the relative thicknesses of the layers in the duplex coating are important to the performance, but further investigation is necessary to confirm that hypothesis. It also appears that the structure and composition of the surface layer may be modified by alloying effects with the chromium carbide during the coating process so that a different and possibly unique set of properties result. 
     FIG. 2 shows a cross section of an Ni--Mo--Cr modified chromium carbide coating bonded to a 316 stainless steel substrate at 400X. Occasional stress-relief cracks perpendicular to the interface occur in the coating, and have not been found to be detrimental to the tribological performance. 
     In another embodiment, an ESD coating of chromium carbide--15%Ni (C815) was treated by ESD with aluminum to achieve a significant benefit in tribological performance in sodium. The coating is created by applying 25 to 50 μm of chromium carbide--15%Ni by ESD to the metal surface, and then, alloying the coating surface with a light application of aluminum. The resulting coating remains at approximately the same thickness (25 to 50 μm) but includes an aluminum-rich alloy zone about 3 to 10 μm thick in the surface. 
     The chromium carbide coating (without the aluminum treatment) shows good wear performance and galling resistance when rubbed against itself or other hard surfaces in sodium. But when the chromium carbide is rubbed against uncoated reactor structural materials such as the austenitic or ferritic steels, severe galling of the steel surface occurs. The addition of aluminum to the chromium carbide--15%Ni by the process described provides a unique coating structure and composition that appears to be highly resistant to galling, self-welding, or adhesive wear in contact with other uncoated metallic surfaces in sodium. The only other materials known to consistently exhibit this behavior are nickel aluminide diffusion coatings that are created by high temperature (˜1000° C.) diffusion treatments of nickel-base alloys using the pack cementation process. The process described in this invention is performed at room temperature on nearly any metallic surface without the restrictions inherent to the diffusion coating process. This new ESD coating also exhibits excellent wear an galling resistance in contact with itself or other hard surfaces. 
     Since the use of this new coating provides good tribological performance in sodium without the necessity of coating both members of a rubbing pair of surfaces, the potential uses are numerous. The coating can be applied to the most convenient surface of a rubbing pair, thus avoiding the treatment of complex or difficult-to-coat components. Steam generator tubes or tube-supports can be coated without the distortion problems associated with high temperature diffusion treatments. Less expensive materials can be used, such as substituting 21/4Cr--1Mo steel for Alloy 718, since the coating can be used on nearly any metal surface, i.e., high-nickel materials are not required. This coating can also be applied to duct wear pads, and control rod and duct components that must rub against stainless steel in low oxygen sodium. 
     FIG. 3 shows a cross section of an aluminized modified chromium carbide coating on an alloy D9--B1 substrate at 400X. Occasional stress-relief cracks perpendicular to the interface occur in the coating, and have not been found to be detrimental to the tribological performance. 
     The foregoing coatings were applied using an ESD coating apparatus essentially as described in the specification of Sheldon U.S. Pat. No. 4,405,881 and as shown in FIG. 3 of that patent, with the exception that I rotated my electrodes. I ESD coated materials using the deposition parameters described in Table I as explained below. 
     
                                           TABLE 1                                 
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DEPOSITION PARAMETERS FOR APPLYING ALUMINIZED CHROMIUM                    
CARBIDE COATINGS AND Ni--Mo--Cr MODIFIED CHROMIUM CARBIDE COATINGS BY     
ESD                                                                       
Parameter        Successful Selections                                    
                                      Expected Acceptable Soultions       
                                      (Untried)                           
__________________________________________________________________________
Substrates       21/4 Cr--Mo steel, 316 SS, D9 HI-9                       
                                      Most structural alloys and steels   
Electrodes       (1) Chromium carbide - 15% Ni (C815)                     
                                      (1) Chromium carbide (or chromium   
                                      carbide with                        
                 tube, 0.25&#34; O.D. × 0.125&#34; I.D.                     
                                      minor additions of other carbides,  
                                      such as                             
                 (2) Pure Al, rod, 0.125&#34; diam. or 6061                   
                                      TiC) with 5-25% Ni or Nichrome;     
                 Al, tube, 0.25&#34; O.D., 0.125&#34; I.D.                        
                                      (2) Most Al alloys with at least    
                                      95% Al                              
                 (3) 50 Ni--32 Mo--15 Cr--3Si (Tribaloy-700),             
                                      (3) Various Ni--Mo--Cr alloys with  
                                      &gt;20 wt. % Mo                        
                 rod, 0.125&#34; diam.                                        
Electrode Motion Rotating, 300 rpm    Rotating, 50 to &gt; 1000 rpm, or      
                                      oscillating                         
                                      or vibrating                        
Specific Coating C815: 0.8 to 3.5/AL: 0.8 to 1.2/                         
                                      0.1 to &gt; 10 (depending on energy    
                                      input)                              
Time, min/cm.sup.2                                                        
                 Tribaloy - 700: 0.6 to 1.2                               
Environment      Air, Argon (preferred)                                   
                                      N, He, Ar + 4% H, others            
Substrate Temperature 20-40° C.                                    
                                      &lt;0° C. to 100° C. or  
                                      more                                
Energy Input     0.1 to 0.2 Joules (0.12 preferred)                       
                                      .03 to 0.3 Joules                   
Voltage          40 to 200 V (155 V preferred)                            
                                      25 to 250 V                         
Capacitance      10 to 216 mf (10 mf preferred)                           
                                      5 to 350 mf                         
Frequency        200 to 1400 Hz (850 Hz preferred)                        
                                      30 to &gt; 1400 Hz                     
Post-Coating     Light buffing with SiC/nylon wheel                       
                                      No treatment                        
Treatment        (preferably followed by heat treat-                      
                 ment in air at 700° C. for 1 hr. * for            
                 aluminized coating only)                                 
__________________________________________________________________________
 * Normallizing treatment for 21/4 Cr--1 Mo steel, also results in light  
 oxidation layer on coating believed beneficial in rapidly establishing   
 NaAlO.sub.2 film in sodium. (Same film likely to form without the        
 oxidation treatment, but would take longer sodium exposure times).       
 
    
     In applying the C815, aluminum and aluminum alloy, material, I passed the gas used down the center of the tubular electrodes. Preferably argon was used since it prevents oxidation, keeps the electrodes cool, helps control the plasma and results in a coating having a much smoother surface finish than air. Reactive gases such as air or nitrogen may be used where it is desired to produce a coating containing oxides or nitrides. 
     During coating, the electrode was held in contact with the surface being coated under an apparent pressure of about 100 grams and rastered back and forth at a linear speed of 0.7 inches/second and a stepover of between 0.002 to 0.015 inches and more typically 0.005 to 0.010 inches. During coating the electrode was held at a slight tilt (about 12°) to the substrate surface normal. At the same time, the electrode was rotated about its axis at 300 rpm. I have found that rotation of the electrode and increasing the speed of rotation increases the smoothness of the coating. In this manner, the entire surface to be coated is scanned by the electrode, normally 2 to 3 times to build up coating thickness and to fill in any holes in the coating. When applying the aluminum in accordance with the present invention only one pass was made. 
     Table 1 shows the ESD deposition parameters used to apply the low friction aluminized chromium carbide coatings and the low-friction Ni--Mo--Cr modified chromium carbide coatings. The parameters selected for use on the specimens which I tested in sodium were those that provided the smoothest surfaces, i.e., lower power levels, higher frequencies, rotating electrode, etc. 
     Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims. 
     I hereby incorporate by reference all the foregoing patents, patent applications, articles, and documents which I have previously cited herein.