The effect of small additions of Nb and Ti on the sliding wear behavior of a Co–30Cr–5Mo alloy

Abstract

This work studies the addition of 0.5%Nb and 0.5%Ti in a Co–30Cr–5Mo alloy as well as the increase of carbon from 0.1 to 0.3%. The main goal is to increase the wear resistance of the alloy with such additions due to the formation of primary carbides (TiC and NbC). The additions of carbon, niobium and titanium were as ferroalloys, which implies the addition of iron into the chemistry of the alloy. Microstructural prediction studies by JMatPro indicate that the addition of all of these elements promotes an increase in the amount of the sigma phase, an undesirable phase because it is a hard brittle intermetallic compound. However, this undesirable phase could be eliminated from the structure by means of a solubilization heat treatment undertaken at 1200 °C for 2 h; this heat treatment lowered the values of hardness of the alloys. An additional aging heat treatment promoted the precipitation of chromium carbides of the type M23C6 as dispersed elongated bars within the cobalt matrix, which notably increased the hardness of the alloys. Each alloy was wear tested under two loads 52 and 78 N and under dry sliding and also under lubricated sliding using Hank solution as lubricant; the tests were undertaken at a sliding speed of 0.5 m/s. Results indicate that the addition of Ti and Nb produced small amounts of TiC and NbC in addition to a light increase in the amount of the sigma phase which increased hardness in the as-cast conditions. These alloys, with Ti and Nb additions, shown the higher hardness values in the as-aged conditions. The wear resistance of the alloys are in agreement with the hardness results particularly for the dry sliding tests. Wear resistance was higher for the harder alloys (those with Nb and Ti additions in the as-aged conditions) and it is lower for the higher applied load. For the case of the lubricated wear tests, such a trend is also observed but not so clearly as for the unlubricated tests. However, friction and wear for the lubricated experiments shown values extremely low compared to these observed for the unlubricated sliding wear tests.