SEVERE EVALUATION OF MECHANICAL PROPERTIES AND PHASE STABILITY IN ADVANCED CERAMIC COMPOSITE FOR ARTIFICIAL JOINTS

Abstract

Combined techniques of fracture mechanics and confocal Raman microprobe spectroscopy were applied to characterize, after increasing periods of environmental exposure, bulk and surface toughness values in an advanced alumina/zirconia composite. This material is used in joint prostheses (BIOLOX^® delta femoral heads, manufactured by CeramTec AG). Besides conventional fracture mechanics characterizations, including different types of fracture toughness test, Raman and fluorescence microprobe spectroscopy provided a microscopic insight into the effect of environmentally assisted processes of zirconia phase transformation at the surface on the fracture toughness of the material. We have found that the tetragonal-to-monoclinic polymorphic transformation occurs in the studied composite material as a consequence of an environmentally assisted process, although severe exposures are needed for to obtain a substantial increase of the monoclinic content. Such severe exposures in vitro correspond to exposures in human body of several lifetimes. The effect of an exposure of 10 h in autoclave (in vitro accelerated test) was carefully examined, because this span of time corresponds:

1. to the period of time recommended for testing in vitro by ISO standard; and,

2. to approximately the lifetime expected for a prosthesis in vivo.

The main experimental outcomes of confocal Raman spectroscopy and fracture mechanics assessments can be summarized as follows:

1. the crack-tip toughness level measured in the as-received material was comprehensive of a tangible contribution by transformation toughening, thus showing that phase transformation in the zirconia dispersoids plays a positive role in the toughening behavior of the material;

2. after the material was environmentally aged in vitro for periods of the order of hundreds of hours, its surface toughness was reduced by about one-third; but, even in the case of such a severe exposure, the surface toughness of the composite was at least the same as that of monolithic alumina;

3. the observed decrease of fracture toughness by about one-third was limited to the very surface of the material (i.e., to a layer of the order of the tens of microns) and did not affect the bulk fracture behavior of the composite.

It appears that concerns arising from the brittleness of alumina-based materials and, thus, from their vulnerability to fracture due to unexpected load situation, can be successfully counteracted by properly adding a dispersion of zirconia particles to the alumina matrix. Such an addition enables the obtainment of a composite material, whose fracture resistance is greatly enhanced by a crack-shielding effect due to phase-transformation processes occurring in the zirconia dispersoids.