Tribo-oxide Competition and Oxide Layer Formation of Ti3SiC2/CaF2 Self-Lubricating Composites during the Friction Process in a Wide Temperature Range

Ti3SiC2/CaF2 composites were prepared by the spark plasma sintering (SPS) process. Both the microstructure of Ti3SiC2/CaF2 and the influence of test temperature on the tribological behavior of the Ti3SiC2/CaF2composites were investigated. The synergistic effect of friction and oxidation was evaluated by analyzing the worn surface morphology. The results showed that Ti3SiC2/CaF2 were still brittle materials after adding CaF2, which was in agreement with Ti3SiC2. The hardness, relative density, flexural strength and compressive strength of the Ti3SiC2/CaF2 composites were slightly lower than those of Ti3SiC2, and the addition of CaF2 decreased the decomposition temperature of Ti3SiC2 from 1350 to 1300 °C. Simultaneously, as the temperature of the test increased, the friction coefficient of Ti3SiC2/CaF2 showed a downward trend (from 0.81 to 0.34), and its the wear rate was insensitive.

Experimental Analysis of Friction and Wear of Self-Lubricating Composites Used for Dry Lubrication of Ball Bearing for Space Applications

Lubricating space mechanisms are a challenge. Lubrication must be sustained in different environments, for a very long period of time, and without any maintenance possible. This study focuses on the self-lubricating composite used in the double transfer lubrication of ball bearing. Ball/races contacts are lubricated via the transfer of materials from the cage that is made of the composite. A dedicated tribometer has been designed for the study. A specificity of the tribometer is to not fully constrain the composite sample but to let it move, as the cage would do in the bearing. Four composites (PTFE, MoS2, glass or mineral fibers) where tested in ultrahigh vacuum and humid air environments. Transfer was achieved with morphologies and composition similar to what is observed on real bearings. Adhesion measurements performed on composite materials before and after friction allowed one to explain the differences in tribological behaviors observed (transfer quality and contact instabilities). Beyond strengthening the composites, fibers are shown to be critical in trapping mechanically and chemically the transferred material to lubricate and prevent instabilities. Equilibrium between internal cohesion of transferred material, and adhesion to counterparts must be satisfied. Mass spectrometry showed that water appears also critical in the establishment of stable transfer film, even in vacuum.