R. D. Rawlings

Bioactive glasses and glass-ceramics.

Bioactive materials are designed to induce a specific biological activity; in most cases the desired biological activity is one that will give strong bonding to bone. A range of materials has been assessed as being capable of bonding to bone, but this paper is solely concerned with bioactive glasses and glass-ceramics. Firstly, the structure and processing of glasses and glass-ceramics are described, as a basic knowledge is essential for the understanding of the development and properties of the bioactive materials. The effect of composition and structure on the bioactivity is then discussed, and it will be shown that bioactivity is associated with the formation of an apatite layer on the surface of the implant. A survey of mechanical performance demonstrates that the structure and mechanical properties of glass-ceramics depend upon whether the processing involves casting or sintering and that the strength and toughness of glass-ceramics are superior to those of glasses. Attempts to further improve the mechanical performance by the use of non-monolithic components, i.e. bioactive coatings on metal substrates and glass and glass-ceramic matrix composites, are also reviewed and are shown to have varying degrees of success. Finally, some miscellaneous applications, namely bioactive bone cements and bone fillers, are briefly covered.

Effect of chemical vapor infiltration on erosion and thermal properties of porous carbon/carbon composite thermal insulation

Abstract A highly porous carbon/carbon composite, known as carbon bonded carbon fiber (CBCF) and used as thermal insulation, was densified by chemical vapor infiltration (CVI). The erosion resistance, thermal conductivity and thermal expansion coefficient were measured with interest to utilization of the CVI densified composite as erosion protection in furnaces that employ inert gas quenching. It was found that the erosion rate decreased exponentially as bulk density increased from 0.18 to 1.00 Mg·m−3 and this was related to the Ryshkewitch and Duckworth relationship which relates mechanical properties to the porosity of ceramic materials. The thermal conductivity of the CVI densified composite both parallel and perpendicular to the fiber array was greater than that of CBCF in the measured range between 1000 and 2000°C. The measured values of thermal conductivity showed reasonable agreement with values calculated from a model structure. For CBCF and CVI densified composite the contribution of solid conduction was calculated to be dominant compared to the contribution of radiative heat transfer and heat transfer taking a path through the gas present in the pores of the composite. As a function of temperature thermal conductivity decreased for the CVI densified composite whereas an increase was found for CBCF. This opposite behavior is a result of the intrinsic properties of the dominant component in the composites: fibers in CBCF and pyrolytic carbon deposit in the case of CVI densified composite. Both materials exhibited anisotropy in the thermal conductivity; the thermal conductivity parallel to the fiber array being about 2.8 times that in the perpendicular direction in the case of CBCF. The thermal expansion coefficient in both directions increased as CBCF was densified by CVI; the increase was greater in the direction perpendicular to the fiber array which increased from 1.8×10−6 K−1 for CBCF to 5.3×10−6 K−1 for the CVI densified composite (1.10 Mg·m−3). The thermal expansion anisotropy was consistent with a c-axis radial orientation of the pyrolytic carbon around the fibers in the CVI densified composite.

Dislocation structures in deformed single-crystal Ni3(Al, Ti)

Abstract A transmission electron microscopy investigation of deformed Ni3(Al, Ti) single crystals has shown that, over the temperature range − 107°C to 917°C, there exist five distinct regions, each having a characteristic dislocation structure. No primary cube slip was observed at temperatures below approximately 450°C, the temperature of the peak proof stress. This observation makes untenable any theory which attributes the anomalous, positive dependence of the proof stress to an interaction between primary cube and octahedral slip.

Mechanical behaviour of zirconia and zirconia-toughened alumina in a simulated body environment

The mechanical properties of a zirconia-toughened alumina (ZTA) and three tetragonal zirconia polycrystal ceramics (TZPs), together with a biograde alumina, have been assessed in a simulated body solution (Ringers solution). The hardness and fracture toughness of these materials were unchanged from the values in air when the tests were carried out in Ringers solution; there was an instantaneous fall in strength in Ringers solution but this was considered acceptable. However, ageing for long periods in Ringers solution promoted a surface layer of monoclinic zirconia. This was accompanied by a strength decrement and it is concluded that these yttria-stabilized ZTA and TZP materials are unsuitable as implant materials.

Room and high temperature failure mechanisms in solid oxide fuel cell electrolytes

The microstructural stability at elevated temperatures and the mechanical properties of 8 mol% yttria stabilised zirconia (YSZ) has been investigated. The YSZ was supplied by two manufacturers as 150 μm thick sheet suitable for the electrolyte in a planar solid oxide fuel cell (SOFC). The two materials had the cubic structure and this crystal structure was maintained up to 1100°C, which was the highest temperature investigated. However, there were differences, albeit small in most cases, in composition, density, grain size and surface finish between the products from the two manufacturers. Although the compositional and structural variations resulted in some differences in mechanical performance, the general trends shown by both materials were similar. The strength, as determined by biaxial flexure, fell by 23–30% on increasing the test temperature from room temperature (RT) to the SOFC operating temperature of 950°C and there was evidence for changes in crack initiation and propagation mechanisms at the higher temperature. The loading rate dependence of the RT strength yielded a low value for the stress exponent n in the standard lifetime equation, which indicated that sub-critical crack growth (sccg) was sensitive to applied stress. Constant load tests at 950–1000°C illustrated the sensitivity of sccg to stress and revealed that creep was also operative in this temperature range. It was concluded that the creep probably occurred by a grain boundary diffusion mechanism.

Metal-ceramic functionally gradient material produced by laser processing

A technique of injection with a powder feed of a mixture of metal + ceramic which combines the processes of laser alloying, cladding and injection, has been applied to study the feasibility of using a continuous wave CO2 laser to produce a functionally gradient material. A 2 kW CO2 laser has been used to produce, on a nickel alloy substrate, single alloy/clad tracks and three totally overlapping clad tracks using powder mixtures of Al-10 wt% SiC, Al-30 wt% SiC and Al-50 wt% SiC, respectively. The variation of composition and structure with position in the processed material has been investigated with reference to the effect of processing traverse speed and the powder feed rate.

Steady-state creep of an alloy based on the intermetallic compound Ni3Al(γ′)

An investigation of the steady-state creep of a Ni3Al.10 at% Fe alloy (γ′) has shown that two creep mechanisms were operative over the temperature range 530 to 930° C. The experimental data at low temperatures (below 680° C) were not consistent with any of the established creep theories. However, the experimental data were in good agreement with a proposed model for cross-slip from octahedral {111} planes on to cube {100} planes in Li2 crystals. Above 680° C, the rate-controlling mechanism, which had an activation energy of 3.27eV atom−1, is considered to be the removal/production of APBs during climb.

Glass-ceramic matrix composites

Glass-ceramics are produced by crystallizing a glass to produce a polycrystalline material. They have considerable potential as the matrices of composites due to their relatively low processing temperatures compared with those required for engineering ceramic matrices. Results of investigations on both fibre- and particulate-reinforced glass-ceramics are presented with particular emphasis on mechanical performance. As well as discussing strength and toughness, more complex properties such as ballistic resistance and thermal shock are covered where appropriate.

Microstructural investigation of low-density carbon-carbon composites

The microstructure of low-density (0.13–0.64 Mg m−3) carbon-carbon composites was investigated using optical microscopy, scanning electron microscopy and image analysis. All samples initially contained varying proportions of rayon precursor carbon fibres, recycled fibrous material and phenolic resin precursor matrix, and were manufactured utilizing a vacuum moulding technique. Some of the composites were densified using the chemical vapour deposition (CVD) of pyrolytic carbon. All the composites were shown to have a two-dimensional planar random microstructure, with a distinct layering effect being seen on the microscopic (and sometimes macroscopic) level. The degree of layering in the composites was quantified utilizing image analysis and was found to be most pronounced in samples containing no recycled material, and least pronounced in samples containing all of its fibrous constituent as recycled material. The composites were found to be very porous, the pores consisting of mainly interconnecting open pores (typically 65–85% of the sample volume). In non-CVD samples the fibrous material was held together by thin (<5 μm) discrete “matrix bonds”, with a few large (typically 100 μm×200 μm×800μm) fibre bundles also existing within the structure. In the CVD-processed material the deposit coat on the fibres was of even thickness throughout the composite and joined together fibrous material that was not previously in contact.

A Mössbauer effect study of Ni3Al with iron additions

Abstract A Mossbauer effect study has been carried out on two Ni3Al (Fe) alloys containing 2.5 and 9.3 at.% Fe. The magnetic parameters associated with the iron atoms have been measured and in particular, it was found that the local magnetic moment reaches a limiting value of 12.0 ± 2 μB in these high iron content alloys. The substitutional behaviour of iron in Ni3Al was determined from an analysis of the low temperature ferromagnetic sextets. The analysis showed that the proportions of iron on the aluminium and nickel sub-lattices varied with iron content.