G. Fantozzi

Crack growth resistance of alumina, zirconia and zirconia toughened alumina ceramics for joint prostheses.

Mono-phase bio-ceramics (alumina and zirconia) are widely used as femoral heads in total hip replacements (THR) as an alternative to metal devices. Unfortunately, the orthopaedic community reports significant in-vivo failures. Material scientists are already familiar with composites like alumina zirconia. Since both are biocompatible, this could prove to be a new approach to implants. This paper deals with a new generation of alumina-zirconia nano-composites having a high resistance to crack propagation, and as a consequence may offer the option to improve lifetime and reliability of ceramic joint prostheses. The reliability of the above mentioned three bio-ceramics (alumina, zirconia and zirconia toughened alumina) for THR components is analysed based on the study of their slow crack-growth behaviour. The influence of the processing conditions on the microstructure development, of the zirconia toughened alumina composites and the effect of these microstructures, on its mechanical properties, are discussed.

Determination by nanoindentation of elastic modulus and hardness of pure constituents of Portland cement clinker

The mechanical properties, particularly the elastic modulus and the hardness of the major phases of Portland cements (C3S and alite, C2S and belite, C3A, C4AF) are measured at the microscopic scale by nanoindentation and at the macroscopic scale by the resonance frequencies technique. At the microscopic scale, the elastic moduli of these phases range between 125 and 145 GPa. The hardness of the calcium aluminate (C3A) is around 10 GPa while those of the silicates are lower (8 to 9 GPa). There are no significant differences between the elastic properties of the pure phases (C3S, C2S) and those of their solid solution alite and belite found in industrial clinker. The results found at the macroscopic scale takes into account the bulk porosity of the samples. The elastic moduli of the C3S, C2S, C3A phases decrease by about 35% when the porosity decreases by 15%. The elastic moduli obtained at the macroscopic scale, by extrapolation at zero porosity, are in good agreement with the results obtained at the microscopic scale by nanoindentation.

Ceramic foams by powder processing

Ceramic foams show a significant potential of development and application, essentially due to the emergence of environmental preoccupations. A brief overview of the state of the art in cellular ceramic application, preparation and characterization is presented in order to introduce some new data concerning the elaboration of mullite and PZT foams by a replication and a bubble generation method, respectively. Some discrepancies between the theory, developed for describing the properties of open-cell foams, and the experimental mechanical behaviour of these semi-closed cell materials were also shown.

Evaluation by indentation of fracture toughness of ceramic materials

A transition fracture mode from Palmqvist to median has been observed in a number of ceramic materials. A new expression to determine the fracture toughness (KIC) by indentation is presented. The KIC values calculated by this formula are independent of the crack profile (median or Palmqvist) and of the applied load. This formula has been obtained by modifying the universal curve of Evans and Charles to incorporate Palmqvist and median cracks over a wide range of loads in the case of brittle materials with different mechanical properties (elastic properties: E, v, KIC).

Fracture toughness, strength and slow crack growth in a ceria stabilized zirconia-alumina nanocomposite for medical applications.

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-K(I) curves) are obtained under static and cyclic fatigue using the double torsion method. The static V-K(I) curve in air reveals the three stages characteristic of stress corrosion with a threshold K(I0) approximately 4.5 MPa m(1/2) and a fracture toughness of 8.8 MPa m(1/2) 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 m(1/2)) still stands above that of current biomedical grade alumina and zirconia.

A critical comparison of methods for the determination of the aging sensitivity in biomedical grade yttria-stabilized zirconia

Since the recent failure events of two particular series of zirconia femoral heads for total hip replacement prosthesis, a large decrease in the use of zirconia ceramics for orthopaedic implants has been observed. In spite of the biomedical success of this material during the last 10 years, this decrease in use was required for safety reasons, until the cause of the failures is known. It has been shown that these failures were related to the low temperature hydrothermal degradation (also known as aging). Thus, it is crucial to better understand the aging behavior, in order to be able to assess its importance and then control it if required. In this study, various techniques relevant to assess the hydrothermal degradation sensitivity of biomedical grade yttria-stabilized zirconia are discussed and compared. The expected outputs of conventional methods, that is, X-ray diffraction and scanning electron microscopy are examined. More recent methods like optical interferometry and atomic force microscopy are presented, with their respective benefits and drawbacks. An up-to-date comparison of these different techniques is provided, and their use for ensuring the long-term reliability of a particular batch of zirconia in terms of aging degradation is demonstrated.

Low-temperature ageing of zirconia-toughened alumina ceramics and its implication in biomedical implants

Changes in crystalline phases resulting from low-temperature ageing of different yttria doped and non-doped zirconia-toughened alumina composites and nanocomposites were investigated under controlled humidity and temperature conditions in autoclave. A classical powder mixing processing route and a new modified colloidal processing route were used to process the composites. Different compositions ranging from 2.5 wt.% zirconia in a matrix of alumina to pure zirconia (3Y-TZP) were studied. It was observed that Al2O3+yttria stabilised ZrO2 composites exhibited significant ageing. However, ageing was much slower than traditionally observed for Y-TZP ceramics, due to the presence of the alumina matrix. Ageing was clearly limited for zirconia content beyond 25 wt.%. On the other side of the spectrum, Al2O3+2.5 wt.% ZrO2 initially presented a monoclinic fraction but did not show any ageing degradation. These composites seem to represent the best choice between slow crack growth and ageing resistance.

Neutron powder diffraction studies of transition metal hemicarbides M2C1−x—II. In situ high temperature study on W2C1−x and Mo2C1−x

Abstract In the first part of this paper [Acta metall.36, 1891 (1988)], we have presented the experimental background for a high temperature neutron powder diffraction study of transition metal hemicarbides M2C1−x having various compositions. In this second part, we report the results: in Mo2C1−x, a first order transformation between an orthorhombic ζ-Fe2N-type superstructure and a hexagonal ϵ-Fe2N-type one has been identified in a middle temperature range (1100–1400°C); in the case of W2C1−x, the same ϵ-Fe2N-type phase appears to be the major constituent in our powders. In both carbides, the disordering of this ϵ-phase occurs through a second order transition at elevated temperatures. The ζ-Fe2N-type phase is also observed as a minor constituent of a W2C0.91 powder; a consistent discussion of these results points out a general scheme for the structural evolution of these M2C1−x compounds as a function of the temperature; this conclusion is also correlated to the refinement of the ζ and ϵ-type structures which has been performed at room temperature.

High temperature mechanical properties of reaction-sintered mullite/zirconia and mullite/alumina/zirconia composites

Strength and fracture toughness of reaction-sintered mullite/zirconia composites (RSMZ) and reaction-sintered mullite/alumina/zirconia composites have been investigated as a function of temperature. Thermal shock resistance has also been determined. It was found that dispersion of zirconia particles and the particular microstructure of mullite obtained by means of anin situ reaction process leads to improved properties, with a room temperature fracture toughness of about 5.25 MPa m1/2. Up to 1000° C fracture strength and toughness values are quite high, which make these materials potential candidates for high temperature applications.

Microstructure development in calcium hexaluminate

Abstract Calcium hexaluminate (CA 6 ) was prepared from alumina and calcium carbonate powders. The influence of processing method and firing temperature on calcium hexaluminate grain morphology was studied. A significant correlation was found between grain morphology and green density, porosity distribution and presence of agglomerates. Platelet grains were observed for low green densities and large pores, while more equiaxed grains were found when green density was increased. A model is proposed for the formation of equiaxed or platelet grains. The model is based on the number of contact areas between alumina and calcium carbonate grains in green specimens as well as on the free space available for calcium hexaluminate to grow.