Mechanical and Tribological Behavior of Polymer‐Derived Ceramics Constituted from SiCxOyNz

Abstract

Polymer‐derived ceramics (PDCs) are a new class of ceramics that are obtained from direct pyrolysis of densely crosslinked polymers. In this article, we report the mechanical and tribological properties of silicon‐based PDCs. The density, the elastic modulus, the hardness, and the fracture toughness of silicon oxycarbonitride ceramics are related to their tribological (friction and wear) behavior. The mechanical properties show a strong relationship with the oxygen/nitrogen ratio in the ceramic, which was varied by annealing the specimens in nitrogen at high temperatures and pressure. The properties are enhanced by a higher nitrogen‐to‐oxygen ratio. In dry environments, the tribological behavior is divided into two regimes: a low‐friction regime with a coefficient of friction, μ, of about 0.2, and a high‐friction regime with μ∼0.7. The transition occurs at a critical value of contact stress. This transition stress appears to be related to the onset of fracture of the ceramic, and moves to a higher value with higher modulus and hardness of the ceramic. The transition stress is successfully analyzed in terms of the influence of the elastic modulus on the fracture stress. The analysis leads to the suggestion of a residual tensile stress in the surface of the specimens of approximately 1 GPa equivalent of the contact stress. In a humid environment, the transition stress apparently moves beyond the experimentally accessible regime. In this environment, the coefficient of friction remains unchanged at μ∼0.2. Two hypotheses, one related to the effect of humidity on the work of fracture, and the other to the formation of a hydrated film on the surface of silicon nitride, the counterface in the tribological experiments, are proposed for this behavior.