Stresses, strains and cracks in a helium-implanted SiC/C composite

Helium was implanted at room temperature and at 1000 °C into 3 × 3 mm2 bars of a SiC/C particulate composite, uniformly to depths of 117 and 254 μm, respectively. Profilometry showed strong bending of the bars due to volume expansion in the implanted layer, which is ascribed to concurrently produced displacement defects. For uniform helium concentrations above ≈350 appm in layers of 254 μm thickness, scanning electron microscopy revealed spontaneous cracking just below the implanted region where tensile stress was maximum, while this critical concentration was above ≈700 appm for the 117 μm implanted layer. Below these critical concentrations the influence of helium on strength of the material was studied by 3-point bending tests. In a detailed analysis of the dependence of cracking on specimen geometry and applied stress, internal stresses from volume expansion were included. This analysis indicated hardening by implantation, in contrast to the apparent reduction of strength. The dose and temperature dependence of volume expansion was fitted by a defect recovery model.