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CorrosionManagement | July/August 2011 TECHNICALARTICLE Hardness – The presence of the cobalt chromium carbide coating on the titanium alloy increased the surface microhardness from 330 HV to over 400 HV as deposited. Pure chromium carbide, known to be very hard with 1200 VHN (Vickers hardness number) contributed to the increase of the hardness. The carbide particles in the metal matrix serve as the aggregates contributing to the higher hardness whereas the metal matrix functions like a carrier for the hard aggregates. The higher concentration of the chromium carbide content in the deposit the harder the deposit should be. However, this trend did not always appear in the brush-plated deposits. The size of the carbide particle may have had a role in contributing to the hardness. The thermal stability of the Co-Cr 3 C 2 composite was tested by heat treatments at a series of temperatures up to 815° C, Table 2. Hardness measurements indicated the Co-Cr 3 C 2 composite was thermally stable. Typically after heating a pure metal to an elevated temperature the hardness of the metal decreases. The hardness of the composite did not decrease after heating to elevated temperatures, in fact a somewhat higher hardness was measured after heat treatment. Taber wear index data of MMC Co-Cr 3 C 2 coated and uncoated Ti-6Al-4V panels is presented in Figure 7. The MMC Co-Cr 3 C 2 coated material has a significantly lower wear rate than uncoated Ti-6Al-4V. The decrease in wear index as the number of cycles increases is believed to be due to a decreasing in roughness of the coating. The roughness of the as-plated deposit is decreased by the abrasive action of the CS-17 wheels. A smoother surface has a lower wear rate. The Taber wear index for a cobalt metal coated panel is compared to a MMC Co-Cr 3 C 2 coating in Figure 8. The wear index of the composite Table 2. Hardness (VHN) of Co-Cr 3 C 2 deposit after 1 hour heat treatment. Cr 3 C 2 wt% As deposited Heat Treatment Temperature 400° C 750° C 815° C 12 - 15 442 505 498 485 25 - 30 418 485 476 482 Figure 7. Taber wear index measured for MMC Co-Cr 3 C 2 and Ti-6Al-4V. Figure 9. Taber wear index for MMC Co-Cr 3 C 2 and Ti-6Al-4V after exposure to 400°C. composite coating. The Ti-6Al-4V wear index is not changed by exposure to this elevated temperature. Residual Stress – Residual stress is an important factor involved in reliability of high temperature coatings. It may influence some key performances of coatings such as the resistance to spalling, delamination, and surface cracking. The residual stress of MMC Co-Cr 3 C 2 on titanium 6Al-4V alloy measured by x-ray diffraction is presented in Table 3. The residual stresses were tensile. The residual stress of Co-Cr 3 C 2 coating was less than that of plated Co metal. The high residual stress appears to be due to the cobalt metal and not the chromium carbide or its combination with the cobalt metal matrix. Coating Stress, kpsi Co–Cr 3 C 2 31 ± 3 Normally a metal coating is oxidized upon exposure to high temperatures. The MMC Co- Cr 3 C 2 coating exhibited a strong oxidation resistance at high temperatures. The MMC Co-Cr 3 C 2 plated parts after exposure to 800°C for 30 hours did not show any sign of being oxidized. A Ti-6Al-4V panel showed yellow dust indicating some extent of oxidation occurred after 1 hour at 800°C. Interestingly, a steel panel was severely oxidized after exposure to 800°C for one hour. Taber Wear Index – The Taber wear tests were performed following ASTM Test Method D 4060- 95. The wear index (μg/cycle) is calculated as follows: (A – B) 1000 Wear index = C where A is the weight of test specimen before abrasion, mg, B is the weight of the test specimen after abrasion, mg, and C is the number of cycles. A 1000 g load was applied to the CS-17 abrasive wheel. Figure 8. Taber wear index measured for MMC Co-Cr 3 C 2 coating and metal Co coating. coating was significantly lower than that of plated cobalt metal. Exposure to an elevated temperature of 400°C leads to a decrease in wear index of the composite coating, Figure 9. This data further confirms the good high temperature performance of the brush plated Co–Cr 3 C 2 Co 33 ± 3 Table 3. Residual stress of Co-Cr 3 C 2 and Co coatings on Ti-6Al-4V alloy determined by XRD. Key Characteristics – Table 4 summarizes the surface characteristics of the cobaltchromium carbide brush plated composite. The key characteristics of Ti-6Al-4V alloy were improved by a brush plated MMC coating. The MMC improvements are more significant than those of a Co metal plated coating especially after exposure to high temperatures. The improved surface characteristics should lead to increased resistance to fretting, adhesive wear, and abrasive wear. Future Work Future work should investigate the cause of the high residual stress. The chemistry of the cobalt plating solution and the deposition parameters should be examined for their contribution to internal stress. 10
TECHNICALARTICLE Table 4. Key characteristics of the brush plated metal-matrix composite Co-Cr 3 C 2 coating on titanium alloy compared to brush plated cobalt. Property Brush Plated Co-Cr 3 C 2 Brush Plated Co Ti-6Al-4V Cr 3 C 2 (wt. %) 10 – 50 0 0 Uniformity of Cr 3 C 2 (wt.%) ± 3% - - Hardness (VHN) 360 – 500 360 - 390 330 - 340 Hardness change after > + 10 % > - 10% > -5% 400°C exposure Taber wear index(μg/cycle) 8.0 17 21.1 Taber wear index after 4.0 --- 21.5 400°C exposure ( μg/cycle) Surface finish as deposited, 0.5 - 1.5 0.5 - 1.5 --- Ra (μm) Residual stress (kpsi) 31 ± 3 33 ± 3 --- First presented as Surfair Biarritz, France 10-11 June 2010. References 1. D. Radatz, A. Zhecheva, and S. Clouser, Proceedings of the National Association For Surface Finishing, Annual Technical Conference, (SUR/FIN 2009) Louisville, Kentucky. 2. E. C. Kedward, and K. W. Wright, 14th Annual Airlines Plating Forum, (1978), American Electroplaters’ Society, Winter Park, Florida. 3. ASM Ready Reference: Thermal Properties of Metals, Fran Cverna, ed. ASM International, Materials Park, Ohio, USA (2002). 4. J. E. Masse and J. F. Mathieu, Materials and Manufacturing, Vol. 11(2), 207 (1996). visit the Icorr website www. icorr.org 11
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