Mechanical properties of polymer-derived ceramics constituted from silicon-carbon-oxygen-nitrogen and their tribological behavior in dry and humid environments.

摘要

Polymer-derived ceramics (PDC's) are a new class of ceramics that are obtained by controlled pyrolysis of highly crosslinked, silicon based polymers. The PDC's are predominantly amorphous structures that apparently contain graphene networks and mixed bonds of silicon, oxygen, nitrogen and carbon. These structures are chemically inert and remain stable up to ultrahigh temperatures. The presence of large amounts of carbon in the PDC's, up to one third molar, motivated the study of the tribological properties of the PDC's. This work represents the first such investigation of PDC's in dry and humid environments. Experiments on both bulk and thin film forms are reported.; The tribological properties of bulk PDC's varied significantly in dry and humid environments. In dry environments, the behavior is divided into two regimes, a low friction regime (mu ∼ 0.2), with negligible wear, and a high friction regime (mu ∼ 0.7) that is also accompanied by a high rate of wear. The transition occurs at a critical value of the applied load. The magnitude of the contact stress at this transition is apparently related to the onset of fracture, and moves to a higher value with the increase in the modulus and the hardness of the ceramic. (The density, the elastic modulus and the hardness increase with the molar ratio of nitrogen to oxygen). In humid environment, the transition stress moves beyond the experimentally accessible regime, so that m remains unchanged (mu ∼ 0.2), and wear rate remains undetectable, throughout the experimental range. The higher resistance to fracture in the humid environment is ascribed to a change in the work of fracture arising from the influence of humidity on the surface energy of these polymer-derived ceramic materials.; Preliminary results on the tribological behavior of thin films of PDC's deposited on various substrates are reported. These results, while showing promise in the use of PDC's as tribological coatings, also included a high degree of variability. A careful study of the influence of processing conditions, especially during pyrolysis, is needed to produce films with reliable tribological properties. The low coefficient of friction regardless of the environment can be an attractive feature of these coatings for long-term applications in harsh conditions.