Kenneth Chyung

Structure and chemistry of fibre-matrix interfaces in silicon carbide fibre-reinforced glass-ceramic composites: an electron microscopy study

Silicon carbide continuous fibre-reinforced glass and glass-ceramic matrix composites showing high strength and fracture toughness have been studied using thin-foil transmission electron microscopy and scanning transmission electron microscopy (AEM). The outstanding mechanical behaviour of these materials is directly correlated with the formation of a cryptocrystalline carbon (graphite) reaction-layer interface between the fibres and the matrix. A solid-state reaction involving relatively rapid diffusion of silicon and oxygen from fibre to matrix correlates well with the experimental observations. Silica activity in the glass-ceramic matrix is suggested to play a primary role in the ability to control the chemical reaction which creates the graphitic interface. AEM results are used to comment upon a possible mechanism for the high-temperature embrittlement behaviour noted for these materials when they undergo rupture in an aerobic environment.

Role of microstructure on contact damage and strength degradation of micaceous glass-ceramics

OBJECTIVES This study examines the hypothesis that microstructure plays a critical role in the accumulation of strength-degrading damage in dental ceramics. A series of micaceous glass-ceramics crystallized from a common glass composition, using heat treatments to increase the diameter and aspect ratio of mica platelets, is used as a model ceramic system. METHODS Damage modes are investigated by Hertzian contact testing. Four-point bend tests on indented specimens quantify the influence of single-cycle and multi-cycle damage on strength. RESULTS Two competing damage modes are observed: fracture, by tensile-induced cone cracking at the macroscopic level; and quasi-plastic deformation, by shear-induced yield at the microscopic level. The quasi-plastic mode becomes more dominant as the microstructures become coarser and more elongate. Bend tests show severe strength losses in the finer grain structures where cone cracking dominates, but relatively small losses in the coarser grain structures where quasi-plasticity dominates. Whereas natural strengths decline with increasing crystallization temperature, the strengths after indentation damage attain a maximum at intermediate crystallization temperatures. Multiple-cycle contact loading reduces strengths even further, and at relatively low indentation loads, indicating susceptibility to fatigue. Finite element modelling is carried out to evaluate the stress components that drive the damage modes. SIGNIFICANCE Microstructure is confirmed to be a controlling factor in determining the nature and degree of strength-impairing damage accumulation in dental ceramics. The Hertzian test provides a means of characterizing such damage in the context of clinical function.

Effect of test method and crack size on the fracture toughness of a chain-silicate glass-ceramic

The fracture toughness of a canasite glass-ceramic with a highly acicular, interlocked grain structure was measured by a number of different methods. The values at room temperature obtained by the chevron-notch, short-bar and notched-beam methods ranged from 4 to 5 M Pa m−1/2, well above literature values for other glass-ceramics. Similar values of toughness were obtained by the fracture of bars with indentation cracks introduced with loads ranging from 1.96 to 400 N, but only for crack sizes >200 μm, with lower values for cracks of smaller size. The toughness values obtained by the direct measurement of the size of the indentation cracks were appreciably lower than the values obtained by all other methods over the total range of indentation loads and corresponding crack size. SEM fractography showed that the surface within the indentation cracks was appreciably smoother than the surrounding fracture surface. The high values of fracture toughness were attributed to the combined mechanisms of crack-deflection and microcrack-toughening due to the stress-enhanced creation of microcracks caused by the residual stresses which arise from the thermal expansion anisotropy of the canasite monoclonic crystal structure. The strong negative temperature dependence of the fracture toughness suggests that at room temperature microcrack toughening represents the primary contributing mechanism to the fracture toughness. The combined effects of crack-deflection and microcrack-toughening can lead to the development of glass-ceramics with greatly improved resistance to crack propagation.

Fracture Energy and Thermal Shock Resistance of Mica Glass-Ceramics

Recent development of mica glass-ceramics provides a family of materials whose microstructures consist of highly interlocked two-dimensional mica flakes in a brittle matrix of glass.1–3 Some of the most interesting properties of mica glass-ceramics include a high degree of machinability, high values of fracture surface energy, and a wide range of strength.

Fracture of Sic Fiber/Glass-Ceramic Composites as a Function of Temperature

SiC fiber reinforced LAS and BMAS glass-ceramic matrix composites have been recently developed. They show a combination of high strength and exceptional toughness over a wide temperature range. However, they suffer from significant embrittlement over the temperature range of 800°-1100°C when tested in air. The objective of the work was to understand this embrittlement behavior.

Effect of fibre orientation on the thermal conductivity of a uniaxial carbon fibre-reinforced aluminoborosilicate glass-matrix composite for various specimen geometries

The development of fibre-reinforced brittle-matrix composites has led to major improvements in such properties as fracture toughness, brittleness and strength [1-6]. Generally, the thermal conductivity of the fibres exceeds that of the matrix, so increases in thermal diffusivity and/or conductivity of the composite are observed as well [7-11]. The thermal conductivity of fibreor whisker-reinforced composites has received a great deal of theoretical interest [11-18]. In general, the thermal conductivity of such composites has been shown to be a function of the thermal conductivity values and the volume fractions of the fibres and matrix, the fibre orientation and the degree of thermal contact at the fibre-matrix interface. The effective thermal conductivity of a uniaxial composite in any direction relative to the fibres can be calculated from the values of the thermal conductivity parallel and perpendicular to the fibre direction [18]. The above theories generally have assumed that the composite is infinite in extent and have not taken into account the possible effect of finite size of the composite specimen. As pointed out by Nye [19] and Carslaw and Jaeger [20], the net direction of heat flow within single crystals with anisotropic thermal conductivity does not necessarily coincide with the direction of the imposed temperature gradient. This should also be relevant for fibre-reinforced composites, which in general are expected to exhibit anisotropic thermal conductivity. Depending on the fibre angle, the presence of insulated boundaries parallel to the imposed temperature gradient can interfere with the net heat flow within the composite. In this respect the recent analysis of Hasselman et al. [21] showed that depending on the orientation of the fibres, the effective thermal conductivity of a uniaxially reinforced composite strip of finite width can be much less than the value for an infinitely large plate. For a composite strip with insulated sides with a temperature gradient imposed along the length of the strip, the effective thermal conductivity along the length of the strip was derived to be [21]

Hybrid ceramic matrix composites

A model hybrid glass-matrix composite has been studied. The system investigated was Corning Code 1723 glass matrix (an alkaline earth aluminosilicate glass) with silicon carbide whiskers and Nicalon® fibres. It was found that a 10 wt % whisker loading of the matrix gave optimum composite properties. The optimized hybrid composite, when compared to an optimized non-hybrid composite, showed increases in microcrack yield stress from 330 to 650 MPa, interlaminar shear strength from 47 to 130 MPa, and transverse strength from 12 to 50 MPa, while the ultimate strength decreased from 965 to 900 MPa.