Influence of Silicon Carbide Layers On the Mechanical Behavior of Silicon-Alloyed Isotropic Pyrolytic Carbon

Objective: Silicon-alloyed isotropic pyrolytic carbon is a coating used extensively for prosthetic heart valves. Ideally, the coating consists of silicon carbide (SiC) particles homogeneously distributed in the carbon matrix to enhance hardness. However, particle segregation often occurs, creating SiC layers considered detrimental to mechanical performance. The influence of layers on mechanical performance was evaluated.

Methods: Non-conforming production components were evaluated using several material analysis techniques to characterize the morphology and physical properties of typical layers. Subsequently, a coating process was developed to fabricate layers in test slabs for mechanical evaluation via four point bending (FPB) and indentation fracture toughness (IFT).

Results: Typical layers were characterized as diffuse (particle clusters), discrete (solid layers), or mixed mode, with varying physical properties. FPB showed reduction in flexural strength and strain to failure for discrete layers exposed at the surface (p < 0.05). IFT showed increased fracture toughness for multiple subsurface discrete layers (p < 0.05).

Conclusions: Surface exposure of a discrete layer introduced surface finish anomalies, reducing flexural properties. Multiple subsurface discrete layers increased fracture toughness via extrinsic toughening, involving crack deflection at layer interfaces. Layer type and position were identified as primary factors controlling mechanical behavior.