Laser processing of ceramics of the SiO2-Al2O3 system

Abstract

An investigation has been made of the constitution of laser-processed ceramics from the SiO2-Al2O3 system. Samples were produced in the form of pellets a few millimetres in diameter by pulsed laser melting of mixtures of silica and alumina powders containing 40, 60 and 70 mol % Al2O3. X-ray diffraction identified the main crystalline phases as Al2O3 in the pellets produced from the 70 mol % Al2O3 mixture and mullite from the 60 and 40 mol % Al2O3 mixtures. The proportion of glassy phase present increased with increasing SiO2 content. Microstructural observations on the 60 mol % Al2O3 pellet showed primary mullite crystals and a lamellar structure interpreted as a eutectic of Al2O3 and mullite. Pellets prepared by melting kaolin powder consisted essentially of a glassy phase and much porosity. Cladding of an alumina substrate, carried out using a continuous powder feed into a laser-generated melt pool, was carried out using the same silica-alumina mixtures as those employed for pellet production. A clad layer was also produced by preplacing a kaolin coat on the alumina substrate prior to laser processing. The effects of traverse speed over the range 3.7 to 7.4 mm s−1 inclusive, power density (44.4 and 111 W mm−2) and powder flow rate (0.13 to 0.47 g s−1 inclusive) were investigated. It was found that the phases present in the clad layer depended on the composition of the precursor powder and the processing conditions. Microstructural examination of the clad layers produced from SiO2-60 mol % Al2O3 and kaolin that had completely melted during processing exhibited various growth morphologies.