Environmental Phase Stability of Next Generation Ceramic Composite for Hip Prostheses

Abstract

A hip-joint femoral head, made of alumina matrix composite, has been evaluated with respect to its surface degradation upon increasing elongation time in water vapor environment. A microscopic evaluation of surface degradation phenomena was obtained according to laser microprobe Raman and fluorescence spectroscopies. According to a confocal configuration of the optical probe, the spectroscopic assessments were performed in volumes limited to the very neighborhood of the material surface, thus minimizing the effect of the sub-surface, which was less affected by environmental degradation. Two main features were observed: (i) significant transformation of zirconia occurred in moist environment from the tetragonal to the monoclinic polymorph; such an environmentally induced phase transformation, conspicuously increased the fraction of monoclinic polymorph (i.e., ≅ 18 vol.% in average) already present in the as-received femoral head; (ii) the equilibrium residual stress field stored at the joint surface changed from a tensile field in the as-received material to a slightly compressive stress field after several hours aging in moist atmosphere and, after exposures >50 h to an increasingly tensile stress state. A residual stress field of tensile nature in the joint surface may trigger wear degradation in the femoral head in presence of microscopic (local) weight impingements due to micro-separation and third-body wear phenomena.