Influence of surface finish and residual stresses on the ageing sensitivity of biomedical grade zirconia
Introduction
Hard on hard materials combinations are increasingly used for the head/cup couple of orthopaedic implants. The reasoning behind this choice is to reduce wear effects and particle release usually associated with polymer (usually, ultra-high molecular weight polyethylene, UHMWPE) and metallic alloy combinations. Although alumina was the first ceramic used for the cup, its development has been, at least for the first generations of alumina implanted, limited by its inherent brittleness. Yttria-stabilized zirconia (Y-TZP), with its better mechanical properties, was introduced subsequently as an alternative. Of particular interest is the fracture toughness, which is at least twice as high as that of alumina, and a higher fatigue-crack propagation threshold, potentially ensuring a longer implant lifetime. The use of zirconia has allowed new, improved implant designs not possible with the more brittle alumina. Owing to the stabilizing effect of yttria (Y2O3), Y-TZP materials can be processed in the metastable tetragonal (t) structure. The retention of the t phase at ambient temperature allows it to transform to the monoclinic (m) structure under external applied stresses. This transformation and the resulting compressive stresses generated in the vicinity of a propagating crack, combined with the volume increase associated with the transformation, slow down further propagation of the crack, resulting in the enhancement of the mechanical properties [1], [2]. Hence, this phase transformation is the key factor for obtaining materials with increased toughness and fatigue threshold.
On the other hand, the transformation can also be induced by environmental stresses, leading to the so-called ageing phenomenon [3], [4]. The degradation resulting from this phenomenon is characterized by surface roughening, microcracking at the surface and particle release in the body [5]. Ageing of zirconia is indubitably the main factor limiting further development of Y-TZP as a biomaterial. In fact, several clinical failures of zirconia-based femoral heads have recently been reported. These 343 in vivo failures [6], associated with two particular batches of Prozyr® heads, were related to an unexpected acceleration of ageing at the surface due to a modification of processing conditions. Consequently, in absence of a complete and rational description and analysis of this ageing, there is a general trend of reverting back to the older materials solutions.
Ageing in zirconia was investigated by ceramists for the last two decades, and a number of influencing factors have been identified, the most important ones being grain size [7], density [7] and phase assemblage [8]. However, the recent failure events have clearly demonstrated there is room for additional factors to be assessed, since the involved materials satisfied the ISO requirements based on the existing known factors [9]. One of these factors, the role of residual stresses has been underestimated so far. However, as pointed out recently by Basu [10], ‘It is evident that tensile residual stresses decrease the critical transformation stress and thereby increase the driving force for transformation by lowering the nucleation barrier, so contributing to enhance transformability of t-ZrO2’. Though some early studies suggested that, residual stresses were not relevant to ageing [11], other experimental reports clearly demonstrated the beneficial effects of relaxing residual stresses, for instance by the formation of a glassy phase at triple junctions resulting from oxide doping [12]. Since stresses theoretically play a key role in the transformation, residual stresses should have some effect on ageing. In fact, it was reported that the transformation always nucleates at triple junction in 3Y-TZP, a location where residual stresses are known to concentrate [13], [14]. This effect of residual stress on the transformation propagation at the surface has been recently illustrated in ceria-doped zirconia, at the scale of the first transformed grains [15]. Furthermore, since ageing starts at the surface of the components, the type and magnitude of the residual stresses at surface should be of prime importance. The surface stress state is directly related to the machining and polishing conditions [16], which vary significantly with manufacturer/technique used. The ISO requirements for this aspect are very surprisingly inadequate, as it has never been fully investigated.
Therefore, the aim of the paper is to attempt a quantitative analysis of the role played by residual stresses and surface finish on the ageing sensitivity of biomedical grade 3Y-TZP at the macroscopic scale.
Section snippets
Materials processing
Samples were processed from an atomised 3 mol% Y2O3 zirconia powder (TZ3Y, Tosoh, Tokyo, Japan), to obtain biomedical grade materials, according to ISO 13356:1997. Samples were sintered 5 h at 1450 °C, with heating and cooling rates of 300 °C/h. The microstructure of the material is illustrated in Fig. 1. The grain size (intercept segment length) and density were 0.4 μm and 6.08 g/cm3, respectively. Toughness and hardness values, measured via 10 kg indentations, were 6 MPa√m and 13 GPa, respectively.
Low-temperature autoclave ageing
The
Qualitative observations: preferential transformation around polishing scratches
It is plausible that some deep scratches introduced during machining and/or polishing operations, might influence the internal stress state even up to 20 μm below the surface [16], hence the ageing sensitivity. The influence of such scratches on the ageing degradation could be observed at the mesoscopic scale, as shown in Fig. 2. A partially transformed surface was observed by optical microscopy after ageing for 75 min (equivalent to 3.5 years in vivo). The formation of monoclinic spots at the
Discussion
From these results, it can be first concluded that the residual stresses induced by scratches, more than their topography, are responsible for the influence of polishing stages on ageing. The stress state induced by the polishing stages can be further understood by analyzing the behavior of the different samples. For the 1 and 3 μm finish samples, it is obvious that the thermal treatment is favorable to the increase of stability—the transformation rate of the relaxed samples being lower than the
Conclusions
The critical influence of polishing stages on the ageing sensitivity of 3Y-TZP has been systematically investigated by optical microscopy, atomic force microscopy and X-ray diffraction. The ageing sensitivity of biomedical grade zirconia is directly linked to the type (compressive or tensile) and amount of residual stresses. Rough polishing produces a compressive surface stress layer beneficial for the ageing resistance, while smooth polishing produces preferential transformation nucleation
Acknowledgements
Financial support of the Rhône-Alpes region and the European Union (GROWTH2000, Project BIOKER, Reference GRD2-2000-25039) are acknowledged. The authors would also like to thank the Consortium de Laboratoires pour l’Analyse par Microscopie à Sonde Locale (CLAMS) for using the nanoscope.
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