Influence of silicon concentration on the nanoindentation and nanoscratch behavior of pyrolytic carbon

The influences of silicon on the nanoindentation and nanoscratch behavior of pyrolytic carbon were studied using a Nanoindenter XP. The elastic behavior was compared to polycrystalline POCO graphite, single crystalline Ticonderoga graphite, and four actual heart valve components, two of which had high porosity and two which had low porosity. The elastic modulus and hardness were relatively constant below a critical silicon concentration of approximately 8 wt%. Above this critical concentration, the modulus and hardness increase with increasing silicon concentration. This behavior appears to be related to the formation of silicon carbide particles that are embedded in and surround the carbon growth features in the microstructure of an 8 wt% silicon sample. The scratch behavior in the “snow plow” mode also changes abruptly at about 8 wt% silicon. It was observed that pyrolytic carbon is scratch resistant at silicon concentrations less than about 8 wt%, while deep troughs are produced above about 8 wt%. The abrupt change in scratch behavior may be due to the lower fracture toughness of silicon carbide and the distribution of silicon carbide in the growth features. Indentation deformation with the Berkovich indenter in pyrolytic carbon is almost fully elastic and recoverable, due to the sliding of weakly bonded hexagonal planes of carbon atoms that are constrained by neighboring crystallites. Indentation of single crystals of Ticonderoga graphite, which have no constraint, resulted in only a small amount of elastic recovery during unloading. The mechanical behavior of the actual heart valve components can be explained based on their silicon content, with the exception of the hardness of the high porosity samples was about 15 % lower than expected.