Preliminary results on a novel fabrication route for α-Al 2O 3 single crystal monofilament-reinforced reaction-bonded mullite (RBM)

Abstract

Owing to their excellent properties, continuous-fibre reinforced mullite-matrix composites are good candidates for applications in which oxidation resistance and damage tolerance at high temperatures (> 1000 °C) are required. To avoid fibre damage, near net-shape fabrication techniques of the composite are required. This has been achieved by using the reaction-bonding process which benefits the oxidation of metal powders producing volume expansion, and hence fully or partially compensating for the sintering-induced shrinkage. Starting materials include Al-Si alloy (80:20), Si metal, α-Al 2O 3 and mullite precursor powders. Due to the variety of starting compounds with different reaction and sintering kinetics, composite fabrication becomes a complex process. Differential scanning calorimetry (DSC) measurements, scanning electron microscopy (SEM) observations, and X-ray diffractometry (XRD) data show that effective milling of metal powder leads to a high degree of mullite formation (≈ 84%) at temperatures as low as 1500 °C, although densification of the ceramic compacts remains rather low (≈ 45% of theoretical density). Single crystal α-Al 2O 3 monofilaments were used to reinforce the reaction-bonded mullite (RBM) matrix. Although no intense reaction between the matrix and the fibres was observed at process temperature, strong bonding develops between uncoated fibres and the matrix. In order to produce a weaker fibre-matrix interface, which is necessary for improvement of the damage tolerance, the fibres were coated with ZrO 2 by means of high frequency sputtering. Microstructural observations of the fibre surfaces before and after the reaction-bonding process indicate that thick coatings (> 10 μm) produce very weak bonding, insufficient for matrix-fibre load transfer due to shrinkage of the low density ZrO 2 layers. Thinner layers (1 μm) produce a better interfacial relation with suitable pull-out of fibres.