José F. Bartolomé

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.

Nanostructured ceramic oxides with a slow crack growth resistance close to covalent materials.

Oxide ceramics are sensitive to slow crack growth because adsorption of water can take place at the crack tip, leading to a strong decrease of the surface energy in humid (or air) conditions. This is a major drawback concerning demanding, long-term applications such as orthopaedic implants. Here we show that a specific nanostructuration of ceramic oxides can lead to a crack resistance never reached before, similar to that of covalent ceramics.

Percolative mechanism of aging in zirconia-containing ceramics for medical applications

Recently, several episodes of fracture of zirconia ceramic femoral heads of total hip prostheses have alarmed the medical and scientific community regarding aging problems in zirconia prostheses. Such fractures cause immediate local tissue reactions, which require urgent medical intervention to prevent further complications. As a result, it has been promoted that yttria-stabilized zirconia (Y-TZP) hip prostheses be substituted by alumina and alumina/Y-TZP ceramics. In the present investigation, we have found an upper limit of Y-TZP concentration in alumina/Y-TZP composites (16 vol.%) to avoid future aging problems. This limit coincides with the percolation threshold measured by infrared (IR) reflectance in a series of alumina/Y-TZP composites.

Reliability assessment in advanced nanocomposite materials for orthopaedic applications.

Alumina-zirconia nano-composites were recently developed as alternative bearing materials for orthopedics. Previous, preliminary reports show that such alumina-zirconia nanocomposites exhibit high crack resistance and low wear rate. In this paper, additional information is given in terms of wear, crack resistance and ageing behaviour: femoral heads are inspected after 7 million cycles of wear testing on a hip simulator, crack resistance is measured and compared to other ceramics used today in orthopedics, slow crack growth is reported under static and cyclic fatigue, and aging resistance is assessed. We also report on the load to failure of femoral heads prototypes during compression tests. This overall reliability assessment ensures a potential future development for these kinds of new nanocomposites in the orthopedic field.

Microstructural Investigation of the Aging Behavior of (3Y‐TZP)–Al2O3 Composites

The low-temperature autoclave aging behavior of zirconia-toughened alumina composites processed by a classical powder mixing processing route was analyzed using atomic force microscopy (AFM), scanning electron microscopy, and X-ray diffraction (XRD). The transformation was evaluated in terms of nucleation and growth, assessed by XRD. The time–temperature equivalency of the transformation was used to measure an apparent activation energy of the nucleation stage of the transformation of 78 kJ/mol. The microstructural features influencing the transformation were identified, and the influence of the alumina matrix on the transformation was investigated. Transformation progression grain by grain was observed by AFM. Transformation does not only occur in zirconia agglomerates but also in isolated zirconia grains. The matrix could partially inhibit the transformation. This behavior could be rationalized considering the constraining effect of the alumina matrix, shape strain accommodation arguments, and microstructural homogeneity effects.

Zirconia/stainless-steel continuous functionally graded material

Abstract A close to theoretical density zirconia/stainless steel continuous functionally graded material has been fabricated starting from coarse (≈20 μm) commercial metal powder by a wet processing method (pressure slip casting). The shape of the metal concentration profile has been characterized by image treatment. The dependence of the electrical properties of the material with the metal concentration presents a percolative behaviour with a metal-insulator transition, in addition to an increment of the capacity in the neighbourhood of a critical volume concentration, f c ≈0.285, approximately. Finally, the Vickers hardness vs metal concentration has been determined. Likewise, the results were compared with those obtained from composites with uniform metal concentration.

Cyclic fatigue crack growth resistance of Al2O3Al2TiO5 composites

Abstract The role of the aluminium titanate and the grain size of alumina in the flaw tolerance of Al 2 O 3 Al 2 TiO 5 composites has been studied. The fabrication of these in situ formed composites with different grain sizes of alumina with homogeneous and heterogeneous microstructure, is described. An investigation was carried out regarding whether a correlation exists between the cyclic fatigue crack growth behaviour in composites with different distribution and quantity of aluminium titanate and different average grain size of the alumina matrix, and the magnitude of the bridging forces developed when the cracks propagate at similar rates.

Sliding wear of ceramics and cermets against steel

Abstract The wear resistance of ceramics and ceramic/metal hybrid composites against steel was studied under dry sliding condition by the use of a pin-on-disc type wear test. The results were compared not only on the basis of the specific wear rates of the various ceramic based materials, but also on the basis of the total cumulative wear rates, which show accumulated wear losses of both sample material as a sliding body and its steel counterpart. From this point of view, it can be considered whether the tribo-materials are optimised with regard to the whole tribo-system or not. The specific wear rates of metal and ceramic/metal composites showed roughly 1.7–16 times higher values than the monolithic ceramics. But the total cumulative wear rate of the ceramic/metal composite, which contained larger sized metal particles, exhibited more than twice better total wear performance than the other systems. The mechanisms responsible for these behaviours were discussed by means of microscopical observations on the worn surfaces and the microstructures of the samples.

Wet processing of mullite/molybdenum composites

The main objective of this work is to show that the difficulties involving substantial differences in particle size and density of commercial molybdenum and mullite powders, when attempts are made to produce homogeneous and/or functionally graded materials, can be overcome by intelligent choice of processing parameters. The flocculant effect of an anionic polyelectrolyte on Mo particles in water and in alcohol media has been shown. On this basis, an appropriate selection of solids loading and surfactant addition allows us to achieve the processing of mullite/molybdenum composites with a controlled metal distribution and microstructure.

Nanotechnology in joint replacement

This paper reviews the most relevant achievements and new developments in the field of nanomaterials and their possible impact on the fabrication of a new generation of reliable and longer lasting implants for joint replacement. Special emphasis is given to the role of nanocomposites with different microstructural designs: micro-nano composites, nano-nano composites, macro-micro-nano composites as well as bioinspired hierarchical composite materials. These nanostructured materials have opened up an exciting avenue in the design of non-metallic biocompatible, crack growth resistant, tough, and mechanically resistant implants with a lifespan close to the life expectancy of the patients.