Abstract
The behaviour of fibre-reinforced ceramic matrix composites (CMCs) under conditions of thermal shock was investigated in this study. A series of water-quench tests were carried out on samples of cross-ply and plain- weave (PW) woven Nicalon/CAS (calcium aluminosilicate) composites. Damage modes due to thermal shock were characterised by examining polished surfaces of the quenched materials using optical and scanning electron microscopy. In addition, the effect of thermal shock on the mechanical properties of the woven composite was assessed under tensile and flexural loading. Predictions of the critical temperature change (DeltaTc) for the onset of thermal shock fracture in both unidirectional (UD) and 2-D CMCs (cross-ply and woven) were made. In terms of the critical temperature change, the thermal shock resistance of surfaces with (0°/90°)3s, (90°/0°)3s and woven configurations was found to be comparable (DeltaTc>400+/-50°C) with that of UD Nicalon/CAS of similar thickness - the effect of ply architecture was minor. By contrast, surfaces with configurations of much smaller thickness (i.e. (0°/90°)s, (90°/0°)s) exhibited significantly higher thermal shock resistance (by >100°C) and much smaller crack densities with increasing severity of shock. Damage, unless temperature-induced microstructural changes occurred, was in the form of matrix cracking that left the fibres unaffected, and originated in the central plies of each CMC. Although on application of more severe shocks cracking extended to the outer plies, crack density always exhibited a gradient across the material thickness and was higher towards the central region. All phenomena could be understood in terms of the interaction of temperature gradients of adjacent material surfaces. The orientation and the extent of matrix cracking depended on the type of ply (i.e. longitudinal, transverse or, in the woven CMC, matrix-rich layer-'ply'). Longitudinal and matrix plies contained matrix cracks perpendicular to the horizontal (length) direction and exhibited 'composite' behaviour by virtue of a surface stress transfer mechanism - on application of more severe shocks, cracks increased in number but remained surface features. Matrix cracks in transverse plies ran parallel to the horizontal, and, although always small in number, increased significantly in length and depth at higher temperature differentials. Thus, transverse plies exhibited behaviour similar to monolithic ceramics and particulate CMCs. Thermal shock caused a small and gradual reduction in the mechanical properties of PW woven Nicalon/CAS. the onset of which occurred at higher temperature differentials than the onset of thermal shock damage. It was shown that the reduction in properties was associated with the propagation under load of the shock- induced matrix cracks in transverse plies. Thus, transverse plies were determined to be the weaker elements of the 2-D materials as they contained cracks that extended significantly in length and depth and affected mechanical properties. The extent of such cracks, as well as of other types of thermal shock damage, was found to be smaller for the woven CMC, perhaps due to the undulating nature of its microstructure. The onset of thermal shock fracture in UD and 2-D Nicalon/CAS CMCs was analysed by considering the anisotropic nature of the applied stress field as well as the presence of residual thermal stresses. A strength- based criterion, combined with a model for the effect of the biaxial nature of shock-induced stresses on the effective value of the interfacial shear stress, enabled satisfactory predictions to be made of the thermal shock resistance of the surface of UD Nicalon/CAS that contained longitudinal fibres, as well as of the central longitudinal plies of faces with (90°/0°)3s,(90°/0°)s and woven configurations. The heat transfer conditions during fracture were also determined. A fracture mechanics-based criterion combined with a modified analytical result from the literature was used to treat the situation of the transverse (end) face of UD Nicalon/CAS, as well as for the central transverse plies of faces with(0°/90°)3s,(0°/90°)sand woven configurations. The success of this approach depended on accurate knowledge of two material parameters, i.e. the relevant fracture toughness and the critical dimension beyond which thermal shock resistance becomes independent of material dimensions. Based on sufficient knowledge of the critical dimension, a method was also devised that allowed the effect of material dimensions to be incorporated into predictions of the thermal shock resistance.