Nucleation and Growth of Environment-Induced Monoclinic Phase in Yttria–TZP Ceramics

Abstract

By careful control of grain size and by the addition of small amounts of stabilizing oxide, usually yttria, zirconia ceramics may be produced with a tetragonal structure that is metastable at both room and intermediate temperatures. These ceramics are termed tetragonal zirconia polycrystalline (TZPs) and can have excellent room-temperature mechanical properties, e.g., strength in excess of 1000 MPa and a toughness approaching 10 MPa m. These outstanding properties are the consequence of the wellestablished transformation toughening mechanism associated with a stress-induced shear transformation from the metastable tetragonal to the equilibrium monoclinic phase at the crack tip. The good mechanical properties of TZPs have led to their use in applications ranging from large cutting blades for processing equipment to femoral heads in hip prostheses. Unfortunately, the transformation from the tetragonal to the monoclinic may also be induced by the environment. This environment-induced transformation proceeds from the surface into the bulk of the material as a function of time. The monoclinic layer so produced is heavily cracked and does not exhibit any transformation toughening, and, consequently, mechanical performance is degraded. In acid solutions, the transformation in TZPs and zirconiatoughened aluminas (ZTAs) occurs rapidly at, and slightly above, room temperature [1±3], but in water, temperatures in excess of 80 8C are required for a readily measurable transformation rate [3±8]. There is much information on the environment-induced transformation at, what may be termed, the macroscopic level, e.g., data on the timeand temperaturedependence of the thickness of the monoclinic layer and the associated degradation in mechanical properties, but there is a dearth of data at the microstructural level. While early work [9] demonstrated the possibility of the transformation taking place slowly in simulated body solution at 37 8C, more recent studies on the small-grain-size, high-purity TZPs currently used for femoral heads have produced no evidence for the transformation in vivo at 37 8C or in vitro [10, 11]. Nevertheless, for the future development of TZPs for prosthetic applications, it is necessary to carry out studies to improve knowledge of the transformation at the microscopic level. This letter reports the results of such a study. Standard femoral heads that had been produced from powder containing 3 mol % yttria by cold isostatically pressing, pressureless sintering and hot-pressing to give a high density of 6.1 Mg my3 were supplied by Norton Desmarquest Fine Ceramics (France). The heads had been polished to a ®ne ®nish of Ra , 3 nm. Twenty heads were aged for up to 30 h in an autoclave operating at 134 8C and 2 bars; for future ease of reference, the polar and equatorial positions are de®ned in Fig. 1. The volume fraction of the monoclinic phase, Xm, produced by the aging treatment was determined from X-ray diffraction (XRD) data using [12]: