Morphology of laser treated polycrystalline alpha-Al2O3

Applying metallic coating to ceramics via laser surface processing is an at tractive process, due to the ability to reach relatively thick surface coat ings, 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 microstruc ture due to the process. This study focuses on the influence of laser surfa ce processing on the microstructure of alpha-Al2O3. A CO2 laser with a maxi mum 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 d egrees C before the laser treatments to minimize thermal shock. The alumina substrates were 96% pure alumina plates, with a thickness of 2 mm. The mic rostructure within the melt pool varied strongly as a function of distance from the free surface. Different regions were identified by electron micros copy, with grain sizes varying from 2-3 mu m up to 60 mu m long needle-shap ed grains having a {11 (2) over bar 6} preferred orientation with respect t o the substrate surface. Due to the presence of Si, Ca, and Mg in the alumi na, an amorphous phase was found at the grain boundaries throughout the mel t-pool region. The amorphous phase primarily appeared as a relatively thin film at the grain boundaries. However, large spheres (congruent to 50 mu m) of amorphous regions also appeared in the melt pool, which contained small grains of alumina. The reasons for the strongly varying microstructure wil l be discussed. (C) 1999 Laser Institute of America.