Morphology of laser treated polycrystalline α-Al2O3

Applying metallic coating to ceramics via laser surface processing is an attractive process, due to the ability to reach relatively thick surface coatings, which include reaction products formed between the metal and ceramic. Two main effects must be characterized to implement such processes: damage to the ceramic due to thermal shock, and changes in the ceramic microstructure due to the process. This study focuses on the influence of laser surface processing on the microstructure of α-Al2O3. A CO2 laser with a maximum power of 3 kW was used for the study. The laser beam was scanned across the polycrystalline alumina substrate under argon at a rate of 0.66–2.0 cm/s. Specimens were prepared at laser powers ranging from 200 to 800 W, with a constant beam diameter of 0.2 cm. The substrates were preheated to 400 °C before the laser treatments to minimize thermal shock. The alumina substrates were 96% pure alumina plates, with a thickness of 2 mm. The microstructure within the melt pool varied strongly as a function of distance from the free surface. Different regions were identified by electron microscopy, with grain sizes varying from 2–3 μm up to 60 μm long needle-shaped grains having a {112̄6} preferred orientation with respect to the substrate surface. Due to the presence of Si, Ca, and Mg in the alumina, an amorphous phase was found at the grain boundaries throughout the melt-pool region. The amorphous phase primarily appeared as a relatively thin film at the grain boundaries. However, large spheres (≅ 50 μm) of amorphous regions also appeared in the melt pool, which contained small grains of alumina. The reasons for the strongly varying microstructure will be discussed.