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INSIGHT For example, brush-plated Ni-W has a hardness of 495 VHN, which compares with 819 VHN for tank-plated hexavalent Cr. [4] Conversely, it has been suggested that a coating based on a metal-matrix composite with ceramic particles as reinforcement phases should exhibit much better wear resistance.[1,5] The wear resistance of a Ni-Co-ZrO^sub 2^ composite coating was shown to be 1.81 times greater than that of an unreinforced Ni-Co coating because the ZrO^sub 2^ particles readily bear the operating load and the Ni-Co matrix acts to hold the particles in place.[6] Unfortunately, only a few composite materials are known that can be formed by codepositing alloys and ceramic particles.[6] In high- temperature coating processes, the ceramic particles may be partially melted during the coating processes and may not be uniformly distributed in the final deposits.[7] To overcome these two problems (the low hardness/wear resistance and nonuniformly distributed ceramic particles), a new hybrid coating process, in which an alloy is first coated on substrates and ceramic particles are formed in situ in the deposit during a subsequent heat treatment, is proposed. A hybrid process may be defined as the application of two individual surface modification processes to engineer the surfaces of components in order to produce combined mechanical, metallurgical, and chemical properties that cannot be obtained by any individual process.[8] One of the examples is to use nitrided substrates for hard coating to achieve higher wear resistance and better adherence of the coating.[9] To prove the concept of the new hybrid process, a relatively inexpensive coating process, brush plating, and a relatively simple alloy system, Ni-W, were used. Brush plating, which allows metal deposition onto selected areas, is a variation of electroplating.[10] Its process characteristics make it suitable for many applications where normal electroplating cannot supply the desired result or is too costly.[11] Brushplated Ni-W alloys have excellent soundness and corrosion resistance but much lower hardness compared with Cr alloys.[1] Nitrocarburizing has been selected as the second part of the hybrid process, causing nitride and carbide particles to form in the brush-plated coating. Therefore, the objective of this work was to develop a combined brush plating and nitrocarburizing process to produce composite coatings with improved wear resistance compared to simple Ni-W alloy coatings. The chemical composition, microstructure, and wear resistance of the composite coatings produced by this hybrid process were studied. The nominal compositions of the substrate material to be coated (a cold work die steel) and the counterpart steel for wear testing (a 12CrMoV cold work tool steel) are listed in Table I. The composition of the Ni-W brush plating solution is listed in Table II. The composition (in weight percent) of the brush-plated deposits was 98.5 to 99.2 Ni, 0.6 to 1.2 W, and 0.05 to 0.3 Co. The specimens used for microhardness testing, X-ray diffraction (XRD) and scanning electron microscopy (SEM)/energy dispersive x-ray spectroscopy (EDS) analysis, were of rectangular shape (25 X 50 mm and 10-mm thick). The wear specimen used for wear testing was cylindrical, of 45-mm outside diameter, 16 mm inside diameter and 10 mm thickness, with the coating on the outer rim. The substrate steel for the wear specimen was heat treated by quenching from 1120 deg C into oil and tempering twice at 540 deg C for 60 min to produce the desired core mechanical properties. The counterpart specimen for use in the wear test, which was identical in geometrical design as the wear specimen, was heat-treated by quenching from 840 C into oil and tempering twice at 240 deg C for 60 min. In this work, the sequence of surface processing involved surface machining, electro-cleaning, cold water flushing, electro- activating with a strong activating solution, cold water flushing, electro-activating with a weak electro-activating solution, cold water flushing, brush plating, cold water flushing and nitrocarburizing. The applied voltage during electro-cleaning, electro-activating, and brush plating was 14 to 16, 12 to 14 and 10 to 12 V respectively. In electrocleaning and brush plating, the specimen was the cathode; the reverse polarity was used in electro- activating. The moving velocity of the brush electrode was 6 to 8 m/ min. The temperature of nitrocarburizing was 540 deg C, which is the same as the tempering temperature of the substrate steel. The nominal composition (in volume percent) of the nitrocarburizing atmosphere was 52 NH^sub 3^, 38 N^sub 2^, and 10 CO. The XRD analysis using a Dmax/Rc type diffractometer with Cu K^sub a^ radiation was performed on the coating surface to identify the phases present. Surface morphology of the wear-tested coatings was observed using SEM. The EDS analysis was used to investigate the element distribution across the coating/substrates interface. Vickers hardness of the coating surface was determined with a load of 150 gf and a loading time of 20 seconds; each hardness value represented an average of ten tests. Wear tests were carried out using an M-200 type wear test machine, with a wheel on wheel configuration, a load of 2000 N, and an oil lubricant at a drip rate of 10 mL/min. The rotation speeds of the upper specimen wheel and lower counterpart wheel were 400 and 360 rpm, respectively, so that there was a 10 pct sliding occurring between the two contact surfaces. The XRD spectra of coated surfaces clearly indicate that new phases have formed in the Ni matrix during nitrocarburizing and they have been identified as WN and WC. Three of the most intense peaks of WC and WN are identified, i.e., the peaks at 2theta = 31.4, 35.6, and 48.2 resulting from the (001), (100), and (101) planes of WC, and the peaks at 2theta = 36.9, 41.8, and 57.0 resulting from the (001), (100), and (101) planes of WN.[12] It follows that the hybrid two-stage process has created a composite coating (with Ni as matrix and WN and WC as reinforcement particles). Hardness testing indicated that this composite layer has a microhardness of 1229 +/-14 HV compared with that of 473 +/- 11 HV for as-plated deposit and of 826 + /-12 HV for the substrate steel. Clearly, either brush plating or nitrocarburizing can improve the wear resistance of the substrate material. However, a combined process comprising brush plating and nitrocarburizing can further improve the wear resistance compared with substrates modified by either individual surface modification technique. This can also be seen from the morphologies of the wear-tested surfaces (Figure 3), where much smaller wear marks were produced on the coated and nitrocarburized surface than on the as-plated coating surface. The improved wear resistance of the coated and nitrocarburized surface is fully consistent with the formation of a composite coating containing particles of WC and WN. Such a material can resist wear effectively because of the high hardness of the ceramic particles. The Ni matrix provides a tough and ductile base to hold these particles in place. In addition, the heat treatment during nitrocarburizing may produce metallurgical bonding at the coating/ substrate interface, which was only quasi-mechanically bonded in the brush plating process (Figure 4). This enhancement of the bond could also be inferred from the interdiffusion of Ni and Fe across the plating/substrate interface during nitrocarburizing (Figure 4). A 10- (mu)m-thick conversion layer with uniformly distributed ceramic particles at about 0.5 (mu)m in size was produced when the coating thickness was about 20 (mu)m and the holding time for nitrocarburing was 80 minutes at 540 deg C. Preliminary experiments were also performed to investigate the optimum coating thickness and nitrocarburizing time. As shown in Figure 5(a), the optimum coating thickness was found to be 20 (mu)m. Clearly, a minimum coating thickness is required to change the structure and wear properties of the substrate surface. However, if the brush-plated deposit is too thick, high stresses developed in the coating due to differential thermal expansion may cause delamination of the coating, which was observed in subsequent nitrocarburizing. Figure 5(b) shows that the optimum holding time of nitrocarburizing at 540 deg C is 80 minutes. It is believed that, if the holding time of nitrocarburizing is too short, the formation of WC and WN and the metallurgical bonding at the substrate/coating interface would not be complete because diffusion takes time. But, further work is needed to determine whatcontributed to the reduced wear resistance when the nitrocarburing time was too long. It is suggested that this may be due to the formation of brittle compounds in the coating layer; however, XRD analysis so far has been unable to confirm that. While this work has clearly shown that the composite coating formed by the new hybrid process can produce dramatic increases in wear resistance, further work needs to be carried out to optimize the process conditions. In summary, a new hybrid surface modification process has been developed combining brush plating with nitrocarburizing to create in situ a composite coating with Ni as the matrix and WC and WN as reinforcement particles. This composite coating has shown an improved wear resistance compared with that produced by either individual surface modification technique. The improved wear resistance is consistent with the improved hardness in the composite coating and the possibly improved metallurgical bonding at the coating/substrate interface. The optimum plating thickness was found to be 20 (mu)m and the optimum holding time for nitrocarburizing was 80 minutes at 540 deg C. While this work has clearly shown that the composite coating formed by the new hybrid process is responsible for the dramatic increases in wear resistance, further work needs to be carried out to understand the effects of process conditions. REFERENCES 1. J.W. Dini: Met. Finishing, 1997, June, pp. 88-93. 2. R.M. Krishnan and C.J. Kennedy: Met. Finishing, 1995, July, pp. 33-39. 3. W.H. Hui, J.J. Liu, YS. Chaug, and J.K. Dennis: Wear, 1996, vol. 192, pp. 165-69. 4. H.B. Durham and A.M. Hooper: Environ. Progr., 1995, vol. 14 (4), pp. 220-23. 5. Y. Naerheim, C. Coddet, and P. Droit: Surface Eng., 1995, vol. 11 (1), pp. 66-70. 6. W. Lin, J. Chen, J. Chen, and S. Chen: Met. Finishing, 1998, May, pp. 46-51. 7. J.M. Guilemany, J. Nutting, J.R. Miguel, and Z. Dong: Scripta Metall. Mater, 1995, vol. 33 (1), pp. 55-61. 8. J. Peng, H. Dong, T. Bell, F. Chen, Z. Mo, C. Wang, and Q. Peng: Surface Eng., 1996, vol. 12 (2), pp. 147-51. 9. H.J. Spies, B. Larisch, and K. Hoeck: Surface Eng., 1995, vol. 11 (4), pp. 319-23. 10. M. Rubinstein: Mater. Manufacturing Processes, 1989, vol. 4 (4), pp. 561-78. 11. J. Gering and J.P.G. Farr: Trans. IFM, 1996, vol. 74 (4), pp. 226-32. 12. JCPDS: "Selected Powder Diffraction Data for Metals and Alloys," Data book, JCPDS International Centre for Diffraction Data, Swarthmore, PA, 1978, vol. 11, pp. 638-59. S.J. DONG, Postdoctoral Fellow, and Y. ZHOU, Assistant Professor, are with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Contact e-mail: nzhou@uwaterloo.ca YW. SRI, Professor, is with the School of Materials Science and Engineering, Beijing Polytechnic University, Beijing, PR. China 100022. L. FAN, Engineer, is with DongFeng Automobile Corporation, Shiyan, Hubei, PR. China. 442002. Manuscript submitted December 11, 2001.
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Source: Copyright Minerals, Metals & Materials Society Jul 2002
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