Wear behavior and self tribofilm formation of hot-pressed Al2O3/TiC/CaF2 ceramic composites sliding against cemented carbide

Al2O3/TiC ceramic composites with the additions of CaF2 solid lubricants were produced by hot pressing. Sliding wear tests against cemented carbide were performed on these ceramic composites using the ring-block method. The friction coefficient and wear rates were measured. Results showed that the friction coefficient and the specific wear rate decreased with the increase of CaF2 content up to 10 vol.%, with further increases in CaF2 content, both the friction coefficient and the wear rate increased rapidly. SEM observations showed that self tribofilms were consistently formed on the wear surfaces. Two types of tribofilms are present on the worn surface depending on the CaF2 content. A dense self tribofilm with a smooth surface associated with small friction coefficient and low wear rate was formed by the releasing and smearing of CaF2 solid lubricants on the wear surface when with low CaF2 content (less than 10 vol.%). This dense self tribofilm acted as solid lubricant film between the sliding couple, protected the ceramics from severe wear by brittle microfracture, and thus significantly reduced the friction coefficient and the wear rate. A broken tribofilm associated with high friction coefficient and large wear rate was formed when with high CaF2 content (more than 10 vol.%). The reason is that large degradation of mechanical properties occurred in samples with high CaF2 contents.