Self-lubricating mechanisms via the in situ formed tribofilm of sintered ceramics with CaF2 additions when sliding against hardened steel

Sliding wear tests against 45# hardened steel were performed on Al2O3/TiC ceramic composites with the additions of CaF2 solid lubricants. The friction coefficient and wear rates were measured. The wear surface features were examined by scanning electron microscopy. Results showed that the friction coefficient and the wear rate decreased with the increase of CaF2 content up to 10 vol%, with further increases in CaF2 content, the wear rate increased rapidly. The mechanisms responsible were determined to be an in-situ formed self tribofilm between the ring-block sliding couple. SEM observations showed that two types of tribofilms are formed on the wear 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 10 vol% CaF2 content. This dense self tribofilm acted as solid lubricant film between the sliding couple, and thus significantly reduced the friction coefficient and the wear rate. Breakdown of the tribofilm on the surface associated with a large wear rate was observed on samples with 15 vol% CaF2 content. This is due to the large degradation of mechanical properties of the composite with higher CaF2 contents.