Sliding wear behavior of Co-Cr-Mo alloys with C and B additions for wear applications

The present study analyzes the effect of boron and carbon additions on the microstructure and wear performance of a Co-30Cr-5Mo alloy. This alloy was based on the chemical composition of the ASTM F75 alloy which is frequently used in gas turbines and high temperature applications subject to wear. For this study, a 0.5 wt% B and C additions were used to modify the microstructure and obtain a 37 %vol of reinforcing phases composed by M23C6 carbides and M3B2 borides. The microstructure of this alloy showed a network formed by the reinforcing phases, similar to the microstructure observed in the Co-30Cr-8.5W alloys, exhibiting hardness values between 43 and 48 HRC and excellent wear properties. Characterization was made through optical and electronic microscopy and X ray diffraction for phase identification in the as-cast and after heat treatment at 1200 °C for 3 h. Hardness was measured in both conditions in HRC scale. Sliding wear tests based on G77 standard were undertaken with 50 and 100 N load during 5 km sliding distance. The friction coefficient was monitored during the tests by measuring the normal and tangential forces. Wear depth, surface roughness, and volume wear losses were measured by using a non-contact 3D profiler. Results show an increase on the hardness and the reduction of the wear losses, in the alloy with the simultaneous addition of 0.5% wt of B and 0.5% wt of C. A decrease in the average friction coefficient with the increase in the volume of reinforcing phases was also observed. The base alloy showed a high degree of adhesion and higher roughness values. On the other hand, the predominant wear mechanism in the reinforced alloys was oxidative wear followed by a small degree of adhesion and micro-ploughing caused by the wear products between sliding surfaces. Cross-sectional analysis showed a lower degree of deformation under the surface in the B and C added alloys, attributed to the presence of the reinforcing phases and the strengthening of the matrix promoted the transition from adhesive wear to oxidative wear. •C and B addition form a network of hard phases surrounded by Co-ε in a Co-γ matrix.•Microstructure and hardness are comparable with high C and W stellite alloys.•Additions B and C considerably increase the hardness and reduce the wear losses.•Microstructural refinement increased the stability of the protective oxide layer.•The increase in hardness favored a transition from adhesive to oxidative wear.