Microstructure and crack in color Al2O3 samples by laser engineered net shaping

High-purity Al2O3 ceramic was formed on a Ti-6Al-4V substrate via laser engineered net shaping (LENS). The black and white thin-wall samples were produced, respectively. The microstructure and chemical elements of the samples' cross section were analyzed by scanning electron microscope. The phase compositions of the original Al2O3 powder, the black sample and the white sample, the color difference of the samples and the relationship between the amount of surface cracks and the color of the samples were investigated. The results show that the black thin-wall sample has many evenly-distributed surface cracks, and the white thin-wall sample contains a few surface cracks only on the upper part. The formation process affects the volatility degree of oxide impurities in the particles, and the chemical elements and phase compositions in the samples. The emergence of the second phase is a major cause to obtain the black sample and promote the formation and propagation of the surface cracks in the black sample.