Hardness and friction behavior of bulk CoAl2O4 and Co–Al2O3 composite layers formed during Spark Plasma Sintering of CoAl2O4 powders

Materials made up of a Co–Al2O3 composite coating over a CoAl2O4 core are prepared during Spark Plasma Sintering of CoAl2O4 powders. The Co particles are precipitated because of a combination of high temperature and low O2 partial pressure. The precipitation and densification processes hamper each other and thus the way the uniaxial pressure is applied during the sintering cycle is an important parameter to control the microstructure of composite layer and its thickness (about 100 μm) and obtain a dense sample (about 4 g/cm3). The friction coefficient of the Co-Al2O3 composites against an Al2O3 ball is lower than that found for an Al2O3 specimen, which could reveal the lubricating role of submicrometer Co particles. However, increasing the load from 5 to 10 N load causes major changes in the friction contact, which are detrimental. Bulk CoAl2O4 was found to have a Vickers microhardness about 15.5 GPa and an average friction coefficient lower than that of an Al2O3 sample.