Arc sprayed coatings are attractive means to protect components from abrasion wear provided they contain enough hard phases. Because of their hardness and toughness 316LTiB 2 cermets were selected as the basis for developing wear resistant coatings. Core wires composed of 304 stainless steel sheaths filled with 10 to 65 wt % TiB 2 , 1 to 15 wt % additives and remaining 316L stainless steel were fabricated and arc sprayed with air. The arc sprayed stainless steel-TiB 2 coatings were submitted to the ASTM G65-B abrasion test and the volume loss was measured with an optical profilometer. As expected, the volume loss decreases and the proportion of TiB 2 increases. However, large differences in volume loss between coatings that contain about the same volumetric proportion of hard phases cannot be explained by a linear reationship. The inverse rule of mixing was proposed. This inverse rule of mixing was found particularly useful for determining the influence of additives. Tin, added in the core as a fugitive liquid transfer agent, was found to improve the wear resistance of coatings. These advanced arc sprayed stainless steel-TiB 2 coatings can be favorably compared with coatings obtained by arc spraying commercially available solid and core wires.

The surfaces of machine components can be effectively protected against wear by highly resistant hardmetal-like coatings, such as WC-Co and Cr 3 C 2 -NiCr, deposited by different thermal spray processes. These composite materials are characterized by the presence of hard carbide particles embedded in a ductile metal binder matrix which have also found many applications as sintered parts (cutting tools, wear resistant parts, mining drills and others) obtained by a powder metallurgy route. Conclusions on the potentials of the different systems for coating applications can be made on the base of experiences and fundamental research from sintered hardmetals. In this paper a comparison of the properties of sintered parts and thermally sprayed coatings of the WC-Co, Cr 3 C 2 -NiCr and (Ti,Mo)C-NiCo systems is given. The structure and properties of the coatings depend strongly on the technology of spray powder preparation, the combination of spray process temperature and particle velocity, and other spray process parameters. It is shown that the TiC-Ni based system can be significantly improved by alloying. This makes the system suitable for coating applications where simultaneous high wear and corrosion resistance in combination with high temperature stability are required. This system can partially substitute the commercially introduced systems but has also the potential to explore new applications.

WC-Co-Cr represents an important composition for hardmetal-like coatings which is appHed when simuhaneous wear and corrosion resistance is required. In this paper five commercially available spray powders obtained by various production techniques (sintered and crushed as well as agglomerated and plasma-densified) of the composition WC-10%Co- 4%Cr have been thoroughly characterized and were sprayed by DCS, HVOF (CDS process) and APS. The microstructures of the coatings were characterized and their wear behaviour was investigated by means of an abrasion wear test. For the best of these powders the wear resistance was nearly equal for the DGS and HVOF coatings. Other powders show significant differences with respect to their processabilities in these spray processes. APS coatings from all powders, obtained with an Ar/H2 plasma showed inferior microstructures and significant lower wear resistance. The spray powder compositions, grain sizes and structures were found to determine the processability of the powders and the microstructure and properties of the coatings. COMPOSITE MATERIALS of the type hard phase - metallic binder with WC and CoCr as constituents are widely used for the preparation of hardmetal-like coatings. The chromium addition to the metallic binder is thought to improve its corrosion resistance in comparison with pure WC-Co. This has led to many applications of WC-CoCr coatings where simultaneous wear and corrosion resistance is required. Despite of its significant practical importance only a limited number of publications is devoted to detailed questions of structure and properties of WC-CoCr coatings (1-3). In some comparative studies such coatings have been investigated together with WC-Co and Cr3C2-NiCr coatings (4-8). However, systematic investigations of spray powder compositions and morphologies as well as investigations of the influence of different thermal spray processes on coating structures and properties which have repeatedly been provided for WC-Co (for example (9, 10)) are missing for WC-CoCr. In this paper a short survey of literature on the phase relationships in the WC-CoCr system and the effect of chromium additions on the properties of sintered parts and thermally sprayed coatings compared to WC-Co is given. In the experimental part a systematic study of the influence of the preparation process on composition and morphology of commercially available WC-10%Co-4%Cr spray powders was provided. These powders have been sprayed by DGS, HVOF and APS and the microstructure and basic properties of the coatings have been studied.

To maintain surface roughness of process rolls in cold rolling steel plants, WC-Co coatings have been known to be effective ones. In this study, a high pressure/high velocity oxygen fuel (HP/HVOF) process was used to obtain WC-Co coatings. To get the best quality of coatings, WC-Co coatings are sprayed with numerous powders made by various processes. These powders include agglomerated sintered powders, fused-crushed powders, extra high carbon WC-Co powders and (W 2 C, WC)-Co powders. After spraying, properties of coatings such as hardness, wear resistance. X-ray diffraction, and microstructures were analyzed. For coatings produced by agglomerated-sintered powders, hardness of the coating increased as power levels and the number of passes were increased. In case of the coatings produced by fused-crushed powders, a very low deposition rate was obtained due to a low flowablity of the powders. In addition, the WC-Co coatings sprayed with extra carbon content of WC-Co did not show improved hardness and wear resistance. Also, some decomposition of WC was observed in the coating. Finally, the coatings produced by (W 2 C, WC)-Co powders produced higher hardness and lower wear resistance coating.

Cermet (WC-Co) coatings have been produced on steel substrates by plasma spraying in vacuum and in air. These have been examined microstructurally and characterised in terms of porosity content, stiffness, microhardness and abrasion resistance. Particular attention has been paid to the phase constitution, as revealed by X-ray diffraction and scanning electron microscopy. High precision densitometry has been used to study porosity levels. Coatings with three different metal contents (9, 12 and 17wt.%Co) have been examined. There is a strong tendency for chemical reactions to occur within the plasma plume, particularly for spraying in air. These reactions can result in the formation of various carbides and even of metallic tungsten. Thermodynamic and kinetic aspects of the reactions involved are briefly examined. Such reactions are strongly promoted by the presence of oxygen, and are much less marked during vacuum plasma spraying. Plasma power and substrate temperature have secondary effects on the degree of reaction which occurs. A marked correlation was observed between degree of reaction and resistance to abrasive wear. This is consistent with the reaction products being brittle and causing poor interfacial cohesion. It was also found that wear resistance was greater for the coatings with lower metal contents. This behaviour can be attributed to the wear occurring predominantly by ploughing of the metallic phase and consequent release of ceramic particles. This occurred more readily when the metal content was higher. In coatings which had undergone pronounced chemical reaction, however, metal had been replaced by reaction products which conferred poor cohesive strength, leading to poor wear resistance.

Properties of an erosion resistant Cr 3 C 2 - NiCr coating have been studied as a function of both plasma spraying process variables and heat treatment. The as-sprayed Cr 3 C 2 - NiCr coating revealed low hardness value of 380-470 Hv, which provided the coating with a poor erosion resistance. This is directly attributed to the decomposition of Cr 3 C 2 constituent into Cr 7 C 3 and graphite phases during the spraying. It was not the effective way to control the process variables such as arc current and stand-off distance for preventing the decomposition of Cr 3 C 2 constituent. A proper heat treatment on the as-sprayed coating increases the hardness of the coating in a great extent up to 900Hv so that the erosion resistance of the coating is clearly improved. This was confirmed to be attributed to the recovery of Cr 3 C 2 at the expense of graphite phase and the formation of Cr 2 O 3 by the heat treatment. In addition, the formation of Cr 2 O 3 phase plays an important role of increasing the erosion resistance of the coating by healing the microcracks of the as-sprayed coating. These are the microstructural features responsible for the high erosion resistance of the coating after a proper heat treatment.

Chromium oxide coatings are currently produced predominantly by the plasma spray process utilizing the high process temperatures required to fully soften the high melting point chromium oxide powder. The development of the HVOF process, combining the relatively high flame temperatures of hydrogen, propylene or propane fuel gases with the notably high particle velocities generated by the process, is known to produces dense, low porosity coatings. By utilizing acetylene, the highest flame temperature fuel gas commercially available, and acetylene based mixtures, the HVOF process can be used to successfully spray chromium oxide powder previously impractical for HVOF systems. This paper describes the results of a programme of work carried out to study the effect of gas related parameters on the properties of Cr 2 O 3 , coatings deposited by HVOF using acetylene and acetylene based mixtures as fuel gases. It further describes the engineering of gas supply systems to overcome the working limitations of acetylene pressures and flowrates to achieve acceptable gas pressures and flow rates and subsequent particle temperature and velocity.

The experimental results of developing tribological surface coatings for high temperature application are presented. The primary focus of this work was in the area of high output advanced, low heat rejection (LHR) diesel engines, where high temperature lubrication between the piston ring and the cylinder liner wall surface is essential. The target temperature focused upon in our research is an operating top ring reversal temperature of approximately 1000° F. The technology developed typically involves treating a porous thermal spray coating with chemical binders improving coating strength and integrity and eliminating open porosity to form an almost monolithic appearing coating. The effectiveness of the densification through the coating thickness was studied. It has been shown that densification process improves mechanical properties and dramatically extends coating wear resistance. Good results were obtained using densified plasma spray Iron oxide (hematite) for a cylinder liner coating versus plasma sprayed Tribaloy T 800 for piston ring. Single Cylinder LHR engine test successfully demonstrated the feasibility of this tribological pair for the possible future applications.

Two different W-Co-C powders were used in three deposition devices, the Super D-Gun, Jet Kote, and JP-5000 to produce coatings for laboratory immersion tests in molten zinc and %55Al-Zn. Resistance was evaluated as time to failure. Scanning electron microscopy and X-ray diffraction were used to characterize the structures ssid failure mechanism. All coatings were found to fail when the molten metal breached the coating thickness at weak spots and spread out over the underlying interface to lift the coating away from the underlying 316L substrate. These weak spots were "pits" on one Super D-Gun coating (the most resistant coating) and cracks on all the other coatings. No diffusion of zinc through the tungsten carbide coatings was observed.

High velocity oxy-fuel (HVOF) experiments were carried out with Diamond Jet (DJ) 2600 and 2700, P-5000, Jet Kote and Top Gun to investigate the influence of the spray system and of the spray parameters on microstructure and properties of Cr3C2-NiCr coatings. The results show that with all applied HVOF systems Cr3C2-NiCr coatings of high density, high bond strength and high wear resistance can be produced. However, microstructure and properties of the coatings mainly depend on the degree of oxidation and carbon loss of the material during the spray process. Due to the relatively low heating of the spray material the decarburization of CrsCa-NiCr was found to be very low using the HVOF systems DJ 2600, DJ 2700 and JP-5000. Additionally favored by the increased particle velocities coatings sprayed with the Diamond Jet systems and the JP- 5000 exceed coatings produced with the traditional HVOF systems with regard to hardness, wear resistance and bond strength.

The proposed paper reports a series of experiments to investigate the cavitation erosion mechanism of HVOF coatings. Vibratory cavitation erosion tests according to ASTM G 32 have been carried out with several HVOF coatings including cermets, oxides and metallic alloys. The steady state erosion rate for each coating was determined and the effect of coating composition and microstructure on the erosion rate was investigated. The morphology and microstructure of the various coatings before and after cavitation testing were analyzed by means of light optical and scanning electron microscopy in order to study the erosion mechanism. The results demonstrate that HVOF coatings of NiCrFeBSi, WC-17Co, Cr 3 C 2 -25NiCr and Cr 2 O 3 can exhibit a rather high resistance against cavitation erosion and should be considered for application as a protective surface layer against cavitation. Furthermore, it is shown that cavitation testing can provide a useful tool to study and characterize the bond strength between individual splats as well as the brittleness of the individual phases present in the coating.

Escalating operation and maintenance costs and increasing intervals between outages place a heavy burden upon electric power producing components. To meet this demand, component life cycles must be extended with either material upgrades or utilization of surface protection products. This paper will discuss the experiences of the Tennessee Valley Authority in the application of thermal spray coatings and try to relate some of these experiences to component performance in fossil power plants' steam turbine components. The development of high velocity thermal spray processes has given coatings an advantage over the use of high priced material upgrades. Chromium carbide coatings have proven the most economical of the surface protection products for use in high temperature applications where solid particle erosion occurs. These coatings have received extensive laboratory testing where limited field results are now just becoming available. Various thermal spray coatings will be described. The development of newer coatings and laboratory test data will be discussed. Optical microscopy and wear studies will be included in the discussion. Where appropriate and available, comparisons to standard plasma sprayed coatings and uncoated substrata are made.

The poblems of metal-titanium carbide coatings processing by air, low pressure and underwater plasma as well as high velocity oxygen fuel spraying are under consideration. Among the different methods of metal-TiC powders production, like mixing of carbides with scale structure metals, agglomeration with binders, a matter of special interest is the high temperature synthesis of TiC in presence of metallic alloy. The characteristic features of these materials include the carbide phases forming, their bonding with the alloy and reactions during spraying, grain size and their distribution, alloy behavior during synthesis and spraying. Finally, the abrasive wear and erosion resistance of Al-Si/TiC, Fe-Cr/TiC and Ni-Cr/TiC coatings is analyzed.

The high quality of the thermally sprayed tungsten carbide coatings has been attributed to high particle velocity and relatively low particle temperature. Such thermal spray conditions can be obtained with the HVOF spray process. In comparison to the plasma spray process, in the HVOF spray process the high particle velocity and optimum particle temperature have been associated with very high gas velocity (>1000 m/s) and a relatively low gas temperature (< 2700 °C). In this work tungsten carbide coatings (WC-17Co) were sprayed by the HVOF process with a low and a high gas velocity of 1050 and 1560 m/s, respectively. The spray tests were carried out also with different hydrogen/oxygen ratios. The coatings were abrasion tested in order to find out how gas velocity and the fuel/oxygen ratio affect the coating quality and wear rate. Wear rates of the HVOF sprayed coatings were found to decrease with the higher combustion gas velocity. The coating quality and wear rate became also less sensitive to gas parameters with the increasing gas velocity. The coating microhardness and wear rate were also compared to hot isostatic pressed (HIP) reference material from the same spray powder lot. The HIP sintered test piece was found to be less wear resistant than the corresponding thermally sprayed coatings.