Effects of sigma phase and carbide on the wear behavior of CoCrMo alloys in Hanks' solution
Introduction
Cobalt–chromium–molybdenum (CoCrMo) alloys have been widely used in gas turbines, dental and orthopedic implants [1], [2], [3], [4], [5], as the group of cobalt-base alloys can be commonly described as wear, corrosion and heat resistant [6]. Up to now, the largest use of wear resistant components and/or their applications appeal for persistent and further studies of CoCrMo alloy.
Properties of CoCrMo alloys can be improved by change of cobalt crystallographic nature, solid-solution-strengthening effect of alloying elements (Cr, Mo, W et al.), and formation of second phase such as carbides [7]. However, the alloys' compositions of today are little changed from the early ones of Elwood Haynes but for tremendous amount and type of carbide formation in the microstructure [6]. Therefore, plenty of researches focusing on how carbides affect the wear behavior of the CoCrMo alloy have been conducted. Fulcher et al. [8] and Desai et al. [9] found that the abrasive wear resistance increased monotonically with increasing volume fraction and size of the carbide phase, respectively, for a series of CoCr alloys. Cawley et al. [10] also reported that the as-cast CoCrMo alloy containing the highest volume fraction of blocky carbide showed the lowest wear rate. Similar conclusions were drawn by Scholes and Unsworth [11] and Tipper et al. [12], who suggested that the improved hardness and wear resistance could be attributed to the presence of carbides in wrought CoCrMo alloys. However, the opposite viewpoint that the hard precipitate phases (e.g., M23C6-type carbides) are detrimental to the wear behavior was also espoused by Wimmer et al. [13] and Chiba et al. [14], because hard carbide is prone to being torn off from the rubbing surfaces, resulting in three-body abrasive wear. It could be concluded that the type, distribution, morphology and volume fraction of carbide have significant influence to wear resistance for various CoCrMo alloys.
In the typical carbon- and nitrogen-containing CoCrMo alloy system, the precipitation of carbide and σ phases is unavoidable since most hot working and heat treatments are carried out below ~1373 K [15], [16]. However, no detailed observation on interaction between the carbide and σ phases on the wear behaviors of these alloys has been identified so far. Therefore, the aim of the present study is to elucidate the synergy effects of the σ phase and carbide on the wear behavior of CoCrMo alloys by performing pin-on-disc wear tests in Hanks' lubricated environment.
Section snippets
Materials
The materials used in this study are low-carbon (LC) and high-carbon (HC) Co–28Cr–6Mo alloys, with the compositions listed in Table 1. Hot forging process for the ingots were: (1) vacuum induction melting process; (2) heating to 1523 K at a heating rate of 1 K/s with a homogenization holding for 18 h, followed by air cooling; (3) hot forging at 1273 K with a total reduction of ~45%; and (4) water quenching. The mechanical properties were evaluated in hardness tests (Shimadzu, HMV) and Young's
Material characterization
The properties of the alloys are tabulated in Table 3. Both LC and HC alloys have densities of ~8.3 g/cm3 and Young's moduli of ~220 GPa. The initial Vickers hardness, HV1, of the HC alloy is a little higher that of LC alloy as the strength hardening effect of carbide.
Fig. 2 shows the microstructure with equiaxed grains of ave. ~20 μm for (a) LC and (b) HC alloys, in which globular precipitates were observed predominantly precipitating along the grain boundaries, or some of them inside the grains.
Discussion
In this study, microstructure is proved to affect the CoCrMo alloys wear behavior significantly. Different volume loss amounts, surface topographies and wear mechanisms were obtained for LC and HC alloys, because that different carbon contents lead to different properties of, and precipitates in the matrix.
The CoFs were comparable to those observed by other workers under similar saline lubrication [7], [14]. For all the samples the dynamic CoFs were increased gradually with prolonged sliding
Conclusions
In the present study, the synergy effects of the σ phase and carbide on the wear behavior of CoCrMo alloys under Hanks' lubricated condition has been evaluated. The following conclusions can be drawn from this study:
- (1)
The HC bearings, owing to harder precipitates and rougher surfaces, exhibited higher CoF than the LC ones.
- (2)
The LC bearings resisted better than the HC ones with less wear factor. Worn surface topographies were characterized by scratched grain basins with raised σ-phase precipitates
Acknowledgements
The authors gratefully acknowledge the financial support of the Ministry of Education, Culture, Sports, Science and Technology of Japan, as part of the Regional Innovation Cluster Program 2010. The authors also acknowledge I. Narida, Y. Kodama and S. Ito for assistance with the EPMA, FIB and TEM assistance, respectively.
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2022, Additive ManufacturingCitation Excerpt :Co–Cr–Mo alloys have been widely used in biomedical applications such as artificial hip/knee joints, spinal instrumentation devices, and dental restorations owing to their excellent combination of wear resistance, corrosion resistance, mechanical properties, and biocompatibility [1–5].