Ageing effects on the nanoindentation response of sub-micrometric 3Y-TZP ceramics

Yttria-stabilized tetragonal zirconia (Y-TZP) ceramics are used as prosthesis material in hip replacement in virtue of their strength, fracture toughness and biological compatibility [1–3]. A critical issue of the zirconia ceramics is the martensitic transformation from tetragonal to monoclinic (t fi m) phase, which can be triggered by mechanical stresses or humid environments [4]. This transformation leads to surface roughening [5], microcraking and grain pullout [6, 7]. Moreover, the presence of monoclinic zirconia changes the hardness of the prosthesis surface. In retrieved zirconia femoral heads, it was reported that the nanoindentation hardness decreased with the increase of the monoclinic phase content [8]. The t fi m transformation is not an issue limited to bulk zirconia ceramics as it was also observed in alumina–zirconia composites [9]. Since one of the most important factors affecting the nucleation and growth of the t fi m transformation is the grain size [4], in this work the surface mechanical response, specifically hardness and Young’s modulus, of two thermally aged 3Y-TZP ceramics with different grain size is evaluated by depth-sensing indentation tests. Ageing treatments have been shown to replicate the same t fi m transformation effects observed in retrieved prostheses [10] and the assessment of the surface mechanical behaviour can give useful information about the wear and contact properties of these aged ceramics. Two 3 mol% yttria-stabilized zirconia ceramics (3Y-TZP) were obtained starting from two different commercial powders (Kyocera, Japan). They were both mixed with 0.3 mol% of Al2O3, CIPed and HIPed. The final densities were almost 100% of the theoretical one and the mean grain sizes were 0.7 lm and 0.2 lm, respectively. In the following, these ceramics will be labelled as ZR07 and ZR02. Samples were cut, machined and polished. The ageing treatments were carried out in autoclave at 121 C with a water vapour pressure of 0.235 MPa for different times: 12, 24, 36, 48 and 60 h. For each ageing time, one sample for each material was used. The depth-sensing indentation tests were carried out using a Berkovich diamond tip on a commercial nanoindenter (Nano Indenter XP, MTS Systems Corporation, Oak Ridge, TN, USA). Four peak loads were used to investigate the indentation hardness and Young’s modulus as a function of penetration depth: 10, 100, 200 and 400 mN. In each sample, at least 10 indentations were made at each peak load. Indentation hardness (H) and Young’s modulus (E) were calculated by the data acquisition software of the nanoindenter (TestWorksTM ver. 4.06A) based on the model of Oliver and Pharr [11]. For the Young’s modulus calculation, a Poisson ratio of 0.32 [12] was considered. The experimental data were statistically scrutinized by ANOVA [13]. As a deeper characterization of the nanoindentation response of these ceramics in un-aged state was S. Guicciardi (&) CNR-ISTEC, Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018 Faenza, Italy e-mail: stefano@istec.cnr.it