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We demonstrate in this paper the influence of surface finish on the ageing kinetics of biomedical grade zirconia. The critical influence of polishing has been systematically investigated by optical microscopy, atomic force microscopy and X-ray diffraction. The stress state around polishing scratches gives rise to preferential transformation in the zone of the induced scratches and consequently to accelerated ageing. The influence of residual stresses is analyzed semi-quantitatively by preparing samples with various surface finish, thus with various stress states. Rough polishing produces a compressive surface stress layer beneficial for the ageing resistance, while smooth polishing produces preferential transformation nucleation around scratches. When a thermal treatment of 2 h at 1200 {\deg}C is applied to relax the residual stresses, all the surfaces states exhibit the same sensitivity to ageing. These results demonstrate that roughness alone cannot be used for ensuring a long-term stability. The variation of ageing sensitivity is indeed related indirectly to the surface roughness via the induced surface stress state. The current ISO standards are not able to take these effects into account. Indeed, great variations in ageing kinetics were observed for samples with different surface states, although all well below the ISO requirements., Comment: 13 pages, 7 figures

Among materials for medical applications, hydroxyapatite is one of the best candidates in orthopedics, since it exhibits a composition similar to the mineral part of bone. Double torsion technique was here performed to investigate slow crack growth behaviour of dense hydroxyapatite materials. Crack rate, V , versus stress intensity factor, K I , laws were obtained for different environments and processing conditions. Stress assisted corrosion by water molecules in oxide ceramics is generally responsible for slow crack growth. The different propagation stages obtained here could be analyzed in relation to this process. The presence of a threshold defining a safety range of use was also observed. Hydroxyapatite ceramics appear to be very sensitive to slow crack growth, crack propagation occurring even at very low K I . This can be explained by the fact that they contain hydroxyl groups (HAP: Ca 10 (PO 4 ) 6 (OH) 2 ), favouring water adsorption on the crack surface and thus a strong decrease of surface energy in the presence of water. This study demonstrates that processing conditions must be carefully controlled, specially sintering temperature, which plays a key role on V–K I laws. Sintering at 50 °C above or below the optimal temperature, for example, may shift the V–K I law towards very low stress intensity factors. The influence of ageing is finally discussed.

cited By 32; International audience; In order to fulfill the clinical requirements for strong, tough and stable ceramics used in dental applications, we designed and developed innovative zirconia-based composites, in which equiaxial α-Al2O3 and elongated SrAl12O19 phases are dispersed in a ceria-stabilized zirconia matrix. The composite powders were prepared by an innovative surface coating route, in which commercial zirconia powders were coated by inorganic precursors of the second phases, which crystallize on the zirconia particles surface under proper thermal treatment. Samples containing four different ceria contents (in the range 10.0-11.5mol%) were prepared by carefully tailoring the amount of the cerium precursor during the elaboration process. Slip cast green bodies were sintered at 1450°C for 1h, leading to fully dense materials. Characterization of composites by SEM and TEM analyses showed highly homogeneous microstructures with an even distribution of both equiaxial and elongated-shape grains inside a very fine zirconia matrix.Ce content plays a major role on aging kinetics, and should be carefully controlled: sample with 10mol% of ceria were transformable, whereas above 10.5mol% there is negligible or no transformation during autoclave treatment.Thus, in this paper we show the potential of the innovative surface coating route, which allows a perfect tailoring of the microstructural, morphological and compositional features of the composites; moreover, its processing costs and environmental impacts are limited, which is beneficial for further scale-up and real use in the biomedical field. © 2015 Elsevier Ltd.

Mechanical properties and slow crack growth (SCG) behavior of a 10Ce-TZP/Al2O3 nanocomposite currently developed as a biomaterial are considered. Fracture toughness is determined for sharp, long (double torsion) and short (indentation) cracks and a good agreement is found between the two types of cracks. The main toughening mechanism in the nanocomposite is the tetragonal to monoclinic phase transformation of the ceria-stabilized zirconia (Ce-TZP) phase. Transformation at the surface of ground specimens leads to surface compressive induced stresses and an increase in strength. Crack velocity curves (V–KI curves) are obtained under static and cyclic fatigue using the double torsion method. The static V–KI curve in air reveals the three stages characteristic of stress corrosion with a threshold KI0 - 4.5 MPa m1/2 and a fracture toughness of 8.8 MPa m1/2 significantly higher than those of currently used inert bioceramics (i.e. alumina and Y-TZP). A crack growth accelerating effect is shown under cyclic loading, correlated with a decrease in the threshold. However, the cyclic fatigue threshold (4 MPa m1/2) still stands above that of current biomedical grade alumina and zirconia.