J. Requena

Control of the Photonic Crystal Properties of fcc-Packed Submicrometer SiO2 Spheres by Sintering

We acknowledge M. Planes for his help during SEM characterization. This work was partially financed by the Spanish CICyT project No. MAT97-0698-C04 and the Fundacion Ramon Areces

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.

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.

Powder processing of mullite/Mo functionally graded materials

Graded or layered materials have been used for many years in specific applications to improve reliability for bonds between dissimilar materials, for improved coating adherence and to obtain better compatibility between fibers and matrices in composites. Recently, the concept has been formalized, and such materials entitled FGMs. Molybdenum and mullite are two ideal candidates to fabricate ceramic/metal composites and FGMs because of their similarity in thermal expansion, lack of reactivity at high temperature and high refractoriness. In the present work a systematic investigation of powder processing for the fabrication of mullite/Mo FGMs is described. It was found that the processing of mullite/Mo in water resulted in a formation of a homogeneous composite, whereas mixtures processed in ethyl alcohol produced FGMs with continuous gradient. Compositional gradients, fracture toughness, hardness and electrical conductivity of the FGMs were also evaluated.

Functionally gradient ceramics by sequential slip casting

Abstract Sequential slip casting is proposed as a route for future developing of a new family of dense functionally gradient ceramics (FGCs) with complex shapes and tailored microarchitectures. Following this route an alumina/yttria tetragonal zircona polycrystal (Y-TZP) FGC with close to theoretical density, homogeneous layers and sharp layer interfaces has been obtained.

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.

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.

Slip casting of zircon by using an organic surfactant

Abstract The effect of an organic surfactant on slip casting of a zircon powder has been studied. Zeta potential measurements have been made for zircon suspensions with different surfactant concentrations and pH values. Rheological properties have been determined in order to achieve the best casting conditions. Drying behaviour, densification evolution, and microstructural observations of the sintered compact are also reported.

Acid-basic stability of Y-TZP ceramics☆

Abstract The chemical stability of different Y-TZP (3 mol. % Y 2 O 3 ) powders has been studied in both acid and basic media by using HCl and NH 4 OH respectively. The determination of solubilized yttrium has been made by the inductively coupled plasma-atomic emission spectrometry technique. The results indicate that a partial dissolution of Y 2 O 3 takes place in the acid region. The amount of solubilized yttrium can be correlated with the specific surface area and heating temperature of Y-TZP powders.

Development of Advanced Zirconia-Toughened Alumina Nanocomposites for Orthopaedic Applications

Current ceramic solutions for orthopaedic applications are nowadays questioned. Great concern is rising for thermal stability problems of zirconia containing ceramics. Thus new zirconia containing composites are developed today. Properties of advanced zirconia-alumina nanocomposites is presented here. The use of a colloidal processing route allows to get a very fine distribution of zirconia grains within the matrix. By tailoring the microstructure, it is possible to get better mechanical properties without any thermal degradation sensitivity.