J. Martinez-Fernandez

A new generation of bio-derived ceramic materials for medical applications

A new generation of bio-derived ceramics can be developed as a base material for medical implants. Specific plant species are used as templates on which innovative transformation processes can modify the chemical composition maintaining the original biostructure. Building on the outstanding mechanical properties of the starting lignocellulosic templates, it is possible to develop lightweight and high-strength scaffolds for bone substitution. In vitro and in vivo experiments demonstrate the excellent biocompatibility of this new silicon carbide material (bioSiC) and how it gets colonized by the hosting bone tissue because of its unique interconnected hierarchic porosity, which opens the door to new biomedical applications.

High-temperature compressive creep of liquid phase sintered silicon carbide

Creep of liquid phase sintered SiC has been studied at temperatures between 1,575 and 1,700 C in argon under nominal stresses from 90 to 500 MPa. Creep rates ranged from 3 {times} 10{sup {minus}8} to 10{sup {minus}6}/s, with an activation energy of 840 {+-} 100 kJ/mol (corresponding to carbon and silicon self-diffusion), and a stress exponent of 1.6 {+-} 0.2. The crept samples showed the presence of dislocation activity, generally forming glide bands and tangles. Degradation of the mechanical properties due to cavitation or reaction of the additives was not detected. SEM and TEM microstructural characterization and analysis of the creep parameters leads to the conclusion that the creep mechanisms operating are grain boundary sliding accommodated by lattice diffusion and climb-controlled dislocation glide operating in parallel. Other possible operating mechanisms are discussed and the data are compared with published data.

Deformation mechanisms for high-temperature creep of high yttria content stabilized zirconia single crystals

Abstract Creep of 21 mol.% yttria-stabilized zirconia single crystals has been studied between 1400 and 1800°C. The creep parameters have been determined indicating a change of the controlling mechanism around 1500°C. At higher temperatures recovery creep is found to be the rate controlling mechanism, with a stress exponent ≌ 3 and an activation energy ≌ 6 eV. Transition to glide controlled creep occurs below 1500°C, associated with larger stress exponents (≌ 5) and activation energies (≌ 8.5 eV). TEM observations of the dislocation microstructure confirm this transition. The influence of the high yttria content, which is at the origin of the high creep resistance of these crystals, is discussed for each range of temperatures.

Fabrication, chemical etching, and compressive strength of porous biomimetic SiC for medical implants

BioSiC is a biomimetic SiC-based ceramic material fabricated by Si melt infiltration of carbon preforms obtained from wood. The microstructure of bioSiC mimics that of the wood precursor, which can be chosen for tailored properties. When the remaining, unreacted Si is removed, a SiC material with interconnected porosity is obtained. This porous bioSiC is under study for its use as a medical implant material. We have successfully fabricated bioSiC from Sipo wood and studied the kinetics of Si removal by wet etching. The results suggest that the reaction is diffusion-limited, and the etch rate follows a t −0.5 law. The etching rate is found to be anisotropic, which can be explained attending to the anisotropy of the pore distribution. The compressive strength was studied as a function of etching time, and the results show a quadratic dependence with density. In the attainable range of densities, the strength is similar or better than that of human bone.

Erosion and strength degradation of biomorphic SiC

Abstract Solid-particle-erosion studies were conducted on biomorphic SiC based on eucalyptus and pine, reaction-bonded (RB) SiC, and hot-pressed (HP) SiC. The erodents were angular SiC abrasives of average diameter 63, 143, or 390 μm and the impact velocity was 100 m s −1 . Impact occurred at normal incidence. Material loss in all targets occurred by brittle fracture. The biomorphic specimens eroded by formation of both lateral and radial cracks and their erosion rates were higher than both conventional SiCs. The RB SiC eroded as a classic brittle material, by formation and propagation of lateral cracks. The HP SiC, the hardest target, was the most erosion resistant. In erosion of the HP SiC, the abrasive particles, especially the largest ones, fragmented upon impact. The resulting dissipation of energy led to relatively low erosion rates. Flexural strength before and after erosion was measured for the biomorphic eucalyptus, RB SiC, and HP SiC. Erosion damage reduced the flexural strengths of all of the specimens. The relative strength reductions were lowest for the biomorphic eucalyptus and highest for the HP SiC. The hot-pressed SiC responded as predicted by accepted models of impact damage in brittle solids. The responses of the biomorphic and reaction-bonded SiC specimens were modeled as if they consisted of only SiC and porosity. This approximation agreed reasonably well with observed degradations of strength.

Room and Elevated Temperature Tensile Properties of Single Tow Hi-Nicalon, Carbon Interphase, CVI SiC Matrix Minicomposites

Abstract Single tow Hi-Nicalon™, C interphase, CVI SiC matrix minicomposites were tested in tension at room temperature, 700, 950, and 1200°C in air. Monotonic loading with modal acoustic emission monitoring was performed at room temperature in order to determine the dependence of matrix cracking on applied load. Modal acoustic emission was shown to correlate directly with the number of matrix cracks formed. Elevated temperature constant load stress-rupture and low-cycle fatigue experiments were performed on precracked specimens. The elevated temperature rupture behavior was dependent on the precrack stress, the lower precrack stress resulting in longer rupture life for a given stress. It was found that the rupture lives of C-interphase Hi-Nicalon™ minicomposites were superior to C-interphase Ceramic Grade Nicalon™ minicomposites and inferior to those of BN-interphase Hi-Nicalon™ minicomposites.

Extensive studies on biomorphic SiC ceramics properties for medical applications

Biomorphic silicon carbide ceramics are light, tough and high-strengt h materials with interesting biomedical applications. The fabrication method of the biomor phic SiC is based in the infiltration of molten-Si in carbon preforms with open porosity. The fina l product is a biostructure formed by a tangle of SiC fibers. This innovative process allows the fabrication of complex shapes and the tailoring of SiC ceramics with optimised properties and cont rollable microstructures that will match the biomechanical requirements of the natural host tiss ue. An interdisciplinary approach of the biomorphic SiC fabricated from beech, sapelly and eucalyptus is presented. Their mechanical properties, microstructure and chemical composition were evaluated. The biocompatible behaviour of these materials has been tested in vitro .

High-temperature compressive strength of reaction-formed silicon carbide (RFSC) ceramics

Abstract High-temperature compressive strength of reaction-formed silicon carbide ceramics, fabricated by the melt infiltration of silicon, and silicon-5 at% niobium alloy into microporous carbon preforms, have been evaluated. The values of maximum strength ranged from 1000 MPa at 1250 °C to 500 at 1400 °C. These maximum strengths were reached for the ceramic fabricated by silicon infiltration into a low silicon yielding carbon preform. The ceramics fabricated by silicon infiltration into a higher volume fraction carbon preform showed a decrease on the strength. The infiltration of silicon-5 at% niobium alloy into a carbon preform also resulted in a decrease of strength compared to the ceramic fabricated by silicon infiltration into the same carbon preform. The dependence with temperature and composition of the high temperature compressive behavior is discussed.

Microstructural evolution and stability of tetragonal precipitates in Y2O3 partially-stabilized ZrO2 single crystals

Abstract The microstructure and morphology of tetragonal precipitates (t-ZrO2) in yttria partially-stabilized zirconia single crystals containing various amounts of Y2O3(3.4, 4.7 and 5.8 mol%) have been studied as a function of aging time at 1600°C in air. The precipitate size and volume fraction of t-ZrO2 phase were determined using transmission electron microscopy. The evolution of the precipitate volume fraction with aging time indicated that the precipitation reaction was completed after 24 h of annealing, in agreement with the values of the Y2O3 composition in the matrix measured by analytical electron microscopy. Further aging coarsened the precipitates which joined together forming fiber-like particles of several micrometers in length and remaining untransformed despite their large size. The stability of the t-ZrO2 precipitates against the tetragonal to monoclinic transformation in the ZrO2-Y2O3 system seems to be related to interactions between precipitates due to coherency stresses rather than with their morphological characteristics.

Thermal and electrical properties of a white-eucalyptus carbon preform for SiC/Si ecoceramics

The thermal conductivity κ and electrical resistivity ρ of a white-eucalyptus cellular carbon preform used to fabricate silicon-carbide-based (SiC/Si) biomorphic ceramics have been measured in the 5-to 300-K temperature interval. The carbon preform was obtained by pyrolysis (carbonization) of white-eucalyptus wood at 1000°C in an argon ambient. The κ(T) and ρ(T) relations were measured on samples cut along the tree growth direction. The experimental data obtained were processed.