Pushdown Method Research Articles

Stability of interfaces in calcium aluminosilicate matrix/Nicalon SiC fibre composites

The high toughness exhibited by aluminosilicatebased ceramic matrix composites (CMCs) reinforced with SiC fibres is well documented [1-4]. This high toughness has led to interest in their use for aerospace applications, where the reduced weight and high temperature stability of the matrix would improve engine efficiency. Because the present generation of glass-ceramic matrix composites utilize an in situ reacted carbon-rich layer to create a low-cohesion interface, their mechanical properties after long periods at elevated temperatures, espedally in oxidizing conditions, requires careful study. The system most extensively studied is lithium aluminosilicate (LAS)/Nicalon, which has shown susceptibility to high-temperature oxidation, particularly in a microcracked matrix [5, 6]. These studies have shown that for times as short as 16 h at 800 °C carbon-rich fibre-matrix interfaces are fully oxidized, resulting in brittle monolithic behaviour. However, the relative importance of oxygen transport via matrix or interface diffusion has not been fully identified. This letter describes the property changes in calcium aluminosilicate (CAS)/SiC (Nicalon) composites, fabricated by Corning (USA), subjected to oxidizing heat treatment with varying temperature. For heat treatment of the material, the original hot-pressed plate was cut into sections 5 0 m m x 3 0 m m x 3 m m in size. These sample plates were heat-treated in air for 100 hours at 600, 700,800, 1000 and 1200 °C. The plates were sliced to the dimensions 50mm x 3 m m x 3 mm before three-point bend testing on a span of 44 mm at a crosshead speed of 1 mm min -1 . The interfacial shear stresses were determined using the microindentation fibre push-down method first reported by Marshall [7]. The microhardness of Nicalon varied from 18.5 to 23.5 GPa, due primarily to variations in indentation load but possibly also to composite heat treatment. The fibre microhardness was measured by indenting fibres in longitudinally cut specimens with loads that produced indents of the same size as the pushed-down fibres. Fibres that were pushed down were indented with loads between 0.49 and 1.03 N. After heat treatment the plate sections were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1 shows the modulus of rupture (MOR) and microcracking stress variation for both unidirectional (UD) and 0/90 ° materials, with the different