Development of new ceramic nano-composites for the next generation prosthetic devices

The international market for hip and knee joint arthroplasty is expected to continue to grow into the foreseeable future as people live longer and expect a higher quality of life. Therefore, designing and production of a material with higher mechanical reliability is targeted by the international market. Ceramics are the latest materials in this regard, which offer the potential for the lowest wear rate and therefore the longest lifespan. The strength and toughness of ceramic prostheses have been greatly enhanced through the emergence of a zirconia toughened alumina (ZTA) composite. However, there remains a clear drive to reduce fracture rates through yet more reliable mechanical properties. This research focuses on processing of a new ceramic composite with fine, well dispersed and high-density microstructure and also evaluating the effect of grain refinement on wear resistance. Furthermore, it seeks to further investigate the contribution of ternary and quaternary metal oxides, namely SrO and Cr2O3, to wear behaviour of this nano-composite. Therefore, nano-ceramic composites of zirconia toughened alumina (ZTA) and ZTA containing SrO and Cr2O3 additives (ZTA-additives) were formed using Spark Plasma Sintering (SPS) and were optimised, in terms of mechanical properties and grain size. The prepared specimens were almost the same, in terms of grain size, but differed in constituent components. In addition to these specimens, a commercial sample called BIOLOX® delta (with the same composition, but coarser grain size) was also investigated. All materials were subjected to lubricated reciprocating sliding wear testing. To define the operative lubrication regime for each load, the Stribeck curve was plotted for each individual specimen. The results obtained from this curve, along with AFM and SEM images, revealed the operation of full fluid film and mixed lubrication in the ZTA composite and the operation of mixed lubrication in the ZTA-additives and commercial specimens. Upon applying a load of 32 N, the specific wear rate of ZTA presented an abrupt increase of two orders of magnitude, but for the other two specimens, only a small change was observed. This highlighted the effect of the ternary oxide, Cr2O3, on changing the chemistry of the material and therefore the behaviour of the surface. The addition clearly had a beneficial effect on the transition point from mild to severe wear, which was shifted to higher loads. The specific wear rates, SEM and AFM images of the worn surfaces revealed that the commercial specimen was mid transition to severe wear at 32 N load, while the ZTA-additives composite did not show any sign of transition. This was probably due to the smaller grain size of the ZTA-additives, compared to the commercial specimen. The grain size refinement would decrease the thermal mismatch stresses and slip length and therefore dislocation accumulation. In all three materials tested at 8N, TEM results exhibited limited dislocation activity in the surface alumina grains. This was in line with the specific wear rate and the features of the worn surfaces. The polymorphic phase transformation in zirconia grains was observed only in the commercial specimen, which was consistent with the results obtained from Raman spectroscopy. This was probably due to the smaller grain size of zirconia in the ZTA and the ZTA-additives compared to the commercial specimen. The SEM and AFM images presented contradictory results about the effect of platelet grains on the wear resistance of the composite. However, the constructive effect of these grains on the fracture toughness was observed in this study, as it was demonstrated by other researchers. The AFM, SEM and TEM results suggested that the dominant wear mechanism pre-transition was tribochemical wear for all materials. This led to the presence of a thin (nm) tribo-layer which may have affected the coefficient of friction (COF) and the specific wear rate.