in Materials for High Performance Applications. The University of Birmingham, England, U.K., B15 2TT.

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1 Wear behaviour of plasma sprayed TiC-15%Ni coatings R. J. C. Cardoso*, M. A. Ashworth\ M. H. Jacobs* ^Department of Science and Technology ofmaterials, The Federal University of Bahia, RuaAristides Novis, 2, Federagao, Salvador, in Materials for High Performance Applications. The University of Birmingham, England, U.K., B15 2TT. Abstract Wear resistance properties of TiC reinforced Ni thermally sprayed coatings have been investigated. The powder used was nickel coated titanium carbide particles (TiC-15%Ni), size: Jim. The starting powder was characterized via metallography, SEM and X-ray diffraction analysis for identification of its morphology and the phases present. The TiC-15%Ni powder was then deposited on to AISI 1019 carbon steel substrates (60 mm dia. and 10 mm thick wear discs) using an APS (Air-Plasma-Spray) Plasma Technik gun. The plasma gun produced a plasma jet as a result of ionization of Ar/H gas mixture. The structure of the coatings were studied by optical microscopy and the phases were determined with X-ray diffraction. Also, the microhardnesses of the coatings were measured. The results for TiC-15%Ni coatings were compared to those obtained for a standard WC-12%Co (ANDRY 301) coating as deposited by APS. A ball-on-disc wear testing method was employed to evaluate all material coatings under unlubricated and room temperature sliding wear conditions. The results include wear volumes and coefficients of friction. TiC- 15% Ni coatings had a structure of high porosity, lower values of hardness and coefficient of friction when compared to the corresponding values of WC-12% Co. The X-ray diffraction (XRD) patterns of the starting powder and plasma sprayed TiC- 15% Ni coatings consisted only of TiC and Ni phases. The results of volume loss for TiC- 15% Ni coatings deposited by atmospheric plasma spraying were higher than the WC-12% Co sprayed coatings. TiC reinforced metal powder coatings studied here promise to provide a lower coefficient of friction and better wear performance in coatings of lower porosity. Certain applications may benefit from these attributes. 1 Introduction Wear is recognised as a serious problem in many areas of industry/ '. Coatings technologies have steadily gained in importance in response to the need to conserve expensive materials by replacement with cheaper substrates whose surfaces are enhanced to withstand the required conditions of service. Coating technology is an important enabling manufacturing process that could permit many industrial companies to enhance their competitiveness and, hence profitability significantly. Cobalt-based thermal spray deposits with additions of

2 382 Transactions Surface on Treatment, Engineering Sciences Computer vol 17, Methods 1997 WIT Press, and Experimental ISSN Measurements tungsten carbide, from Co-WC powders, are used widely in Europe and America as wear resistant coatings. Alternative deposits with similar or better technical performance, under specific conditions, coupled with either processing advantages and/or cost benefits, would be of appreciable interest. Carbide reinforcement of metals has been used as a successful technique for improving the wear resistance of surfaces/"*. These materials are known as "Ceremets" and the idea is combine ceramic phases with metallic matrices in order to produce materials with superior properties to those attained with ceramics or metals independently/. To a first approximation, the hard carbide particles provide resistance to abrasion, whereas the matrix confers cohesion and toughness. The first generation of ceremets appeared after publication of the Schroter patent covering WC-CO hard metals A Since the late 1950s, thermal spray techniques have been shown to be a successful method for depositing coating surfaces,". Carbide particles such as WC, or Cr^Cy and TiC have proved to be effective when added to tougher, more ductile binder matrices/. WC or Cr^C reinforced materials, are used in many applications to decrease sliding or erosive wear. TiC reinforced materials, however, produce a lower weight and a superior temperature wear resistant coating,*. If TiC phase is stable and well bonded and dispersed into the matrix, a hard, low coefficient of friction surface is produced. Recently, there have been many studies on TiC reinforced metal/" ^. These works show that the coatings made from TiC reinforced powders, are more resistant to sliding wear than either WC/Co or Stellite,'-. This type of carbide has been selected due to its lower weight, higher compatibility, and its lower friction coefficients. Compared to tungsten carbide and chromium carbide based coatings, titanium carbide particle based coatings have a lower weight and superior high temperature behaviour. Cr^Cy or WC may decompose at higher operating temperatures and would not provide the equivalent low friction coefficient. WC or W%C tend do decompose partially at temperatures higher than 540 C over long term. The present study was conducted to evaluate the wear behaviour of thermal sprayed TiC-15%Ni, which is a cobalt-free system and which was compared with a thermal sprayed tungsten carbide cobalt, WC-Co coating. The starting powders and spray coatings were tested and characterized via metallography, SEM and X-ray diffraction analysis for identification of their morphologies and the phases present. Microhardness testing (Vickers, 300 g load) and ball-on-disc wear testing were carried out on all materials, which yielded results that included wear volumes and coefficients of friction. 2 Experimental Procedures Powder Manufacturing: The spray powder used in this study was TiC-15%Ni powder in which TiC-particles was coated with nickel. The particles size distribution of the powder is im. This powder was supplied by London & Scandinavian Metallurgical Co. Limited and the other two powders were supplied from ANDRY, as given in Table 1. Deposition Processes: Plasma gun spraying was performed at the IRC in Materials for High Performance Applications at The University of Birmingham, UK. The powders shown in Table 1 were deposited using an Air Plasma Spray - APS process. Plasma spraying was performed with the Plasma-Technik-AG gun, using argon and hydrogen as plasma gases for all coatings. The main spray parameters used are presented in Table 2. Two layers, a bond coat followed by a trial coating, were deposited on a rolled mild steel (AISI 1019) 50 mm dia. X 10 mm thick wear discs. All the discs were grit blasted with 500 microns alumina

3 Surface Treatment, Computer Methods and Experimental Measurements 383 (AUOO particles and ultrasonic ally cleaned in methanol prior to spraying. A bond coat of Al-5%Ni powder was the first layer deposited on to all specimens. The powders particles were injected internally into the plasma jet environment, where they melted and accelerated towards a moving substrate. The powders were consolidated onto discs. Nominal coatings thickness on wear discs were 0.25 mm. Powder Reference Composition Ni-85%v.TiC Ni-5%Al WC-12%Co Manufacture Method Agglomerated Spheroidal/Granular Ni clad with Al Fused and crushed Comments jam ANDRY im ANDRY 301 Table 1. Powder Compositions. Powder Arc gas 1 (1/min) Ar/65 Ar/45 Ar/65 Arc gas 2 (1/min) H/9.5 H/ll H/3 Carrier gas (1/min) Ar/3.5 Ar/3.3 Ar/2 Powder feed rate (g/min) Current (A) Spray distance (mm) Table 2. Process Parameters and Spray Conditions. Characterization and Testing: Characterization and coating property tests were conducted on the TiC-15%Ni powders and all coated specimens. The TiC-15%Ni starting powder was characterized via SEM analysis to identify its morphology and X-ray diffraction (XRD) was performed for identification of the phases present. Microhardness tests, metallographic evaluation, wear properties and X- ray diffraction were conducted on the plasma coated test discs. The sprayed coatings were grounded up to 120 mesh SiC paper. Microhardness measurements (Vickers diamond indentor at 300 g load) were performed on the coated surfaces; twelve (12) values were recorded and an average was taken. X-ray diffraction patterns were obtained from the coated surfaces with a Dyane diffractometer, in order to identify the phases present and then relate them to the features observed by optical and scanning electron microscopy Friction and wear tests on the coatings were performed at room temperature with a pin-on-disc tribometer. Sliding ball-on-disc tests were conducted on each coated disc under unlubricated (dry) conditions. The test pair used was Co-WC balls, 6 mm in diameter as counter body. The coated discs, 50 mm in diameter, 10 mm thick, ground to an average surface roughness of 5 micron. The tests were conducted at 23 mm track diameter and 125 rpm, giving a surface speed of 0,30 m/sec. A 23 N load was used in all tests. Each test was carried out for a total of 5,000 cycles. The frictional forces and coefficient of friction values were continuously recorded on a computer and sampled at a frequency of 1 Hz. The ball -on-disc wear test set-up is shown schematically in Figure 1. Wear-volume measurements on the coatings, at the end of each test, were made by calculating the area of the wear track from a profile trace measured by a surface profilometer.

4 384 Surface Treatment, Computer Methods and Experimental Measurements r applied 1 Environment Coaling composition structure Test Measurtmtnts Wear Track - rpm - time / cydea - diameter Fig. 1 Schematic presentation of sliding wear test: pin-on-disc (POD) apparatus. a) 150X b) 350X Fig. 2 SEM micrographs of the TiC-15% Ni agglomerated powder.

5 Surface Treatment, Computer Methods and Experimental Measurements 385 Powder section 400X; Powder section 850X. Fig. 3 SEM micrographs of the TiC-15% Ni agglomerated powder. The light phase is nickel and the grey phase is TiC. *# ' ^ a) TiC-15%Ni coating b) WC-12%Co coating Fig. 4 Optical microstructures of APS sprayed coatings, as polished (x200).

6 386 Surface Treatment, Computer Methods and Experimental Measurements The average wear cross sectional area were then multiplied by the circumference of the wear track to obtain the wear volume. 3 Results and Discussion Powder characterization: Fig. 2 shows SEM micrographs of the TiC-15% Ni agglomerated powder as supplied by London & Scandinavian Metallurgical Co. Limited. The Fig. 3 shows a cross section of a particle of powder. The light phase is nickel and grey phase is TiC. The X-ray diffraction patterns of the starting powder are shown in Fig. 5 (a), which revealed the TiC particles and Ni phase. Hardness: the microhardness results of the TiC-15% Ni and WC-12% Co plasma sprayed coatings are shown in Table 3. The microhardness value of the sprayed TiC-15% Ni coatings was 603 HV/0.3 Kg. It is remarkably lower than the corresponding value of 824 HV/0.3.Kg obtained for WC-12% Co coatings. Nominal thickness ( Jim ) Microhardness Vickers ( 300 g ) Wear resistance/loss of volume (mnf ) Coefficient of friction ( COF ) Average roughness Ra ( im ) TiC-15% Ni WC-12% (*) Co ( * ) No volume loss was detected on the coatings Table3. As-sprayed Coating Properties Microstructure: The optical micrographs of the TiC-15% Ni and WC-12% Co plasma sprayed coatings are presented in Fig. 4. The micrograph (a) shows a high level of porosity and unmelted particles of powder for TiC-15% Ni coatings. Deposit evaluations have not shown acceptable densities of all TiC reinforced materials with plasma conditions given in Table 2. Possibly coarse powder of TiC-15% Ni materials were not effectively melted by the Ar/H jet conditions used and lower deposit densities resulted. The lower results of hardness for TiC 15% Ni coatings can be correlated to the high porosity revealed in micrograph Fig. 4 (a). The WC-12% Co coatings display a structure much more dense as seen in Fig. 4(b). Wear result: Wear volumes and coefficients of friction for all tested coatings are displayed in Table 3. The volume loss values show that the coatings made from TiC-15% Ni alloys ( 2,7 mnf ) were higher as those of WC-12% Co coatings, which were negligible. No wear track was detected by profilometer on WC-12% Co coatings. After each wear test on WC-12% Co coatings, the Co-WC balls were visibly worn. The lower wear performance for TiC reinforced Ni coatings may be related to the high porosity of the TiC-15% Ni coatings. The Table 3 shows that the coefficient of friction - COF values for TiC-15% Ni coatings are slightly lower than the WC-12% Co coatings. However, it is very hard to correlate coefficient friction (COF) with any other properties of the coating. X-Ray Diffraction: the results of the X-ray diffraction for the plasma sprayed coatings are shown in Fig. 5 (b). It shows that the main phases present are TiC

7 Surface Treatment, Computer Methods and Experimental Measurements 387 a) Powder : phases detected were TiC and Ni. _ti_ H_ ~"- - -^~ f^^^^^^^y^^^s^^^i^:^^^^^^^^ b) APS coating : phases detected were TiC and Ni Fig. 5 Principal phases in (a) TiC-15% Ni powder, and (b) in TiC - 15% Ni coating, as determined by X-ray diffraction method.

8 388 Surface Treatment, Computer Methods and Experimental Measurements and Ni phases. Coatings analysis did not show phase transformation during the thermal spray process. 4 Conclusions TiC-15% Ni and WC-12% Co coatings were deposited onto mild steel substrates by atmospheric plasma spraying - APS. TiC-15% Ni coatings had a structure of high porosity, lower values of hardness and coefficient of friction when compared to the corresponding values of WC- Co. The X-ray diffraction (XRD) patterns of the starting powder and the plasma sprayed TiC-15% Ni coating consisted only of TiC and Ni phases. The results of volume loss for TiC-15% Ni coatings deposited by atmospheric plasma spraying were higher than the WC-12% Co sprayed coatings. TiC reinforced metal powder coating studied here promise to provide lower coefficient of friction and better wear performance in coatings of lower porosity, but this must be confirmed by futher work. Certain applications may benefit from these attributes. Acknowledgments The work described in this paper was supported by Commission of the European Communities. The authors would like to thank also the London & Scandinavian Metallurgical Co Limited for supplying the powders; the Professor Daniel Biazolli from Federal University of Umberlandia for providing wearing test equipment, Mrs. Launora Franca for X-ray diffractograms, Mr. Elias Candido and Adelson Profeta for the mellographic preparation. 5 References 1. Arnell, R.D., Davies, P.B., Hailing, J., Whorney, T.L., Tribology Principles and Design Applications, McMillan, London, Lugscheider, E., in NTS/87, pp , Proceedings of the National Spray f, Orlando-FL, USA, ASM Int., Hutchings, I.M., Tribology: Friction and Wear Engineering Materials, pp , Edward Arnold, London, Ericsson, T., Acfa MgfaZ/wrg);, 1966, 14, Holleck, H., Swr/hcz EMg/Mffnfzg, pp , Vol.7, No. 2, Schroter, K., German fafenf No (1923). 7. Atamert, S., Stekly, J., Surfacing Engineering, 1993, Vol. 9, No. 3, pp Smith, R. W., Mutasim, Z., Lugscheider, E., Limbach. R., Jungklaus, H., Ross, J., Cells, J.P., de Bonte, M., Economous, S., in NTS/91, pp 59-66,

9 Surface Treatment, Computer Methods and Experimental Measurements 389 Proceedings of the International Thermal Spray Conference, Orlando-FL, USA, ASM Int., Mutasim, Z., Smith, R.W., Mohanty, M., in NTS/92, pp , Proceedings of the International Thermal Spray Conference, Orlando-FL, USA, ASM Int., Lugscheider, E., Jungklaus, H., Limbach, R., Smith, R.W., in NTS/92, pp , Proceedings of the International TJiermal Spray Conference, Orlando- FL,USA, ASM Int., Vityaz, P., Verstak, A., Sobolevsky, S., Lugscheider, E., Jokiel, P., Pursche, Chemintz, G., Yushchenko, K., pp 36-40, in Proceeding of the International DVS - Thermal Spray Conference, Aachen, Germany, ASM Int., Smith, R.W., Gentner, D., Harzenski, E., Robisch, T., pp , in NTS/88, Proceedings National Thermal Spray Conference, Cincinnati, Ohio, USA, SM Int., 1988.