The Role of Hydrothermal Degradation in the Wear Behaviour of Zirconia

Yttria stabilised tetragonal zirconia polycrystal (Y-TZP) ceramics have attracted particular attention in medical applications due to their outstanding mechanical properties. In particular they have increased toughness compared to other ceramics such as alumina. Transformation toughened zirconia has the highest fracture toughness of the oxide ceramics, but its engineering applications are currently limited because hydrothermal degradation can catastrophically degrade its strength. Hydrothermal degradation is the autocatalytic transformation of the metastable tetragonal (t-ZrO2) to the equilibrium monoclinic (m-ZrO2) which occurs at a free surface when held in a moist atmosphere at elevated temperature (typically 100-200oC). The underlying cause of this degradation process is still a matter of considerable debate and attempts to improve the resistance of Y-TZPs to hydrothermal degradation have generally resulted in unacceptable loss of toughness. The most prominent example of zirconia hydrothermal degradation is the failure of femoral heads in hip prosthetics. Hydrothermal degradation is not the only cause of hip joint failure, but tribological behaviour is also important, and the interaction between wear and ageing is crucial to developing zirconia applications. The origin of the detrimental effects of ageing on the wear of zirconia remains controversial. In the current work the wear behaviour of zirconia against alumina was investigated. In particular, a series of Y-TZPs have been developed with ternary and quaternary dopant additions designed specifically to reduce the rate of hydrothermal degradation to see whether these additions affected the wear behaviour. Ceramics with 0.1%wt Al2O3 and 0.1%(La2O3+Al2O3) doped 3Y-TZP ceramics were manufactured under conditions that yielded materials with the same grain size and fracture toughness. TEM of sintered materials demonstrated that the ternary oxide
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additions segregated to grain boundaries. Prior to degradation the surfaces were prepared by careful polishing to produce an extremely flat surface, followed by thermal etching to minimise residual surface strains and to ensure the starting surface was fully tetragonal zirconia. After degradation surfaces were examined using atomic force microscopy and X-ray diffraction and measurements were taken of the transformation depth on cross-sectional samples. Materials with ternary oxide additions exhibited superior degradation resistance to the base zirconia material. To study the ageing behaviour on wear behaviour in zirconia the specimens were aged at 134 ᵒC in superheated steam (one hour in these conditions is believed to equate to approximately one year in vivo). A UMT tribometer in pin–on–disk configuration was used for the tests. The effect of two different lubricants (water and bovine serum) was also investigated. The specific wear rates were measured to be at or below 10-6 mm3/Nm for all samples, indicating that mild wear predominated throughout. Surprisingly, the wear rate of some of the aged specimens was lower than that for the non-aged samples. The worn surfaces exhibited evidence of surface fracture and grain pull-out. Cross-sectioning the surface by FIB and examination in the TEM showed that transformation from tetragonal to monoclinic zirconia occurred, with grain pull out resulting from loss of material from within the monoclinic zirconia layer. TEM also revealed an amorphous tribolayer formed on top of the worn surface, which appeared to be a combination of the substrate material and the lubricant.