The corrosion of alumina‐toughened zirconia (ATZ) as a consequence of hydrothermal treatment was investigated, especially the transformation kinetics from tetragonal zirconia to the monoclinic phase. For this purpose, polished ATZ specimens (Ra<5 nm) were aged in water vapor at different temperatures ranging from 70° to 134°C. The fraction of the monoclinic phase was determined using X‐ray diffraction and Rietveld refinement. The isothermal transformation curves obtained were fitted to the Mehl–Johnson–Avrami equation by least squares. An Arrhenius plot of the fitted transformation rates was used to determine the activation energy and the pre‐exponential factor. Following this procedure, the kinetic parameters of the phase transformation were extrapolated down to body temperature and the formation of the monoclinic phase was simulated. In addition, optical interferometry on well‐polished specimens (Ra<2 nm) was alternatively used to calculate the monoclinic fraction from the histogram dataset. The results agree very well with those of the X‐ray measurements. Additionally, the development of surface roughness with increasing aging time is discussed.

Further development of ceramic materials for total hip replacement aim to increase fracture toughness and further reduce the incidence of bearing fracture. Edge loading due to translational mal positioning (microseparation) has replicated stripe wear, wear rates, and bimodal wear debris observed on retrievals. This method has replicated the fracture of early zirconia ceramic-on-ceramic bearings. This has shown the necessity of introducing microseparation conditions to the gait cycle when assessing the tribological performance of new hip replacement bearings. Two novel ceramic matrix composite materials, zirconia-toughened alumina (ZTA) and alumina-toughened zirconia (ATZ), were developed by Mathys Orthop€ adie GmbH. In this study, ATZon-ATZ and ZTA-on-ZTA bearing combinations were tested and compared with alumina-on-alumina (Al 2 O 3 -on-Al 2 O 3 ) bearings under adverse microseparation and edge loading conditions using the Leeds II physiological anatomical hip joint simulator. The wear rate (695% confidence limit) of ZTA-on-ZTA was 0.14 6 0.10 mm 3 /million cycles and that of ATZ-on-ATZ was 0.06 6 0.004 mm 3 /million cycles compared with a wear rate of 0.74 6 1.73 mm 3 /million cycles for Al 2 O 3 -on-Al 2 O 3 bearings. Stripe wear was evident on all bearing combinations; however, the stripe formed on the ATZ and ZTA femoral heads was thinner and shallower that that formed on the Al 2 O 3 heads. Posttest phase composition measurements for both ATZ and ZTA materials showed no significant change in the monoclinic zirconia content. ATZon-ATZ and ZTA-on-ZTA showed superior wear resistance properties when compared with Al 2 O 3 -on-Al 2 O 3 under adverse edge loading conditions.

Zirconia (Y-TZP) is used as material for components of implants and prostheses because of its high short-term strength. The mechanical long-term reliability, however, is limited for Y-TZP because of hydrothermal aging effects and a pronounced tendency for subcritical crack growth. The hypothesis of this study was that a substantial amount of alumina in a zirconia matrix can help to significantly suppress subcritical crack growth and thereby improve the mechanical long-term reliability. The Weibull parameters as well as the parameters of the subcritical crack growth were determined for Alumina, Y-TZP, and two dispersion ceramics, that is Alumina Toughened Zirconia (ATZ, 20% alumina/80% Y-TZP), and Zirconia Toughened Alumina (ZTA, 75% alumina/25% Y-TZP). The long-term failure probability as a function of service time was predicted for the four ceramics. The parameter n of the subcritical crack growth was approx. 80% higher for ATZ compared to Y-TZP. In consequence, the estimated lifetime revealed a significant better mechanical long-term reliability for ATZ. It can be concluded that tailored dispersion oxide ceramics can address the aging problem of monolithic zirconia. This makes ATZ very interesting for components of joint replacement as well as for dental prostheses and implants.

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The present study has been aimed to combine the advantages of the two single-phase materials, Al2O3 and ZrO2 while avoiding their disadvantages, by producing a mixed ceramic. Within a study of an ATZ ceramic we tested the influence of aging by a hydrothermal treatment on the strength, phase composition and surface properties. We found that the hydrothermal treatment of the ATZ ceramic did not significantly influence these parameters, unlike those of a single-phase ZrO2 ceramic.