Abstract
INTRODUCTION
Ceramics are excellently suited for applications in arthroplasty, mainly total hip, knee and shoulder replacement. As the most prominent representative of this demanding type of material, BIOLOX®delta is widely used and very successful in the market for more than 10 years. The ability of zirconia phase transformation (t-ZrO2→m-ZrO2) in zirconia-platelet toughened alumina (ZPTA) ceramics is an indispensable prerequisite for their excellent mechanical properties. The degree of stabilization of the zirconia tetragonal phase at body temperature is essential for the desired toughening mechanism. Y2O3 is the most widely used t-ZrO2 chemical stabilizer; also microstructure and grain size contribute to t-ZrO2 phase stabilization. Stabilization must be achieved such that no material degradation will occur in body environment, i.e. in aqueous liquid (synovia), which is known to potentially trigger phase transformation at the surface of ceramic components. In this study, it is shown how phase stabilization in BIOLOX®delta as a reference material is excellently balanced by means of optimal mechanical performance and environmental stability.
OBJECTIVES
To assess the influence of t-ZrO2 chemical stabilization on ZPTA properties, in terms of fracture toughness (i.e. the ability to resist crack extension), wear resistance and environmental stability.
METHODS
Three ZPTA compositions with increasing yttria content (Y2O3/ZrO2 2–4mol%) were produced and compared to the reference.
Hardness and fracture toughness were assessed by Vickers indentation method.
A micro scratch tester (CSM Instruments, Peseux Switzerland) loaded with a Rockwell C diamond tip with 50µm radius was used to assess the scratch resistance of the ceramic compositions. The scratch load was linearly increased from 0 to 30N, which simulates extremely heavy local wear conditions.
The morphology and depth of the scratches as well as local damage has been analysed with a laser microscope (Olympus LEXT-OLS4000) and scanning electron microscope (Hitachi S4700).
The hydrothermal aging resistance was measured by autoclaving the compositions up to 150 hours at 134°C and 2.2bar and the monoclinic zirconia volume fraction measured by XRD using the Garvie formula at each time interval (i.e. 0, 10, 50, 100 and 150h). Surface roughness after hydrothermal ageing was also evaluated by atomic force microscopy.
RESULTS
As expected, t-ZrO2 stabilization improved with increasing yttria content. Consequently, hydrothermal aging resistance increased and fracture toughness decreased strongly to almost monolithic alumina values. The scratch resistance performances also decreased showing gradually lower critical load Lc1, where grain pull-out phenomenon appeared earlier with the higher zirconia stabilization composition. The materials kept almost the same hardness (about 18GPa). Surface roughness also remained unchanged for all compostions, even in extreme hydrothermal conditions.
CONCLUSIONS
Higher tetragonal zirconia stabilization leads to the suppression of zirconia phase transformation toughening and consequently of low temperature degradation; annihilating almost all the improvements of ZPTA over the monolithic alumina material. It was demonstrated that the best performance is achieved by properly triggering the tetragonal zirconia transformation. This result is explaining the successful performance of BIOLOX®delta bearing couples already observed in the clinical setting.
