Preparation of α-alumina coated carbide tools by the sol-gel process

Ceramic-coated cemented carbide cutting tools were prepared from aluminum alkoxide solution and carbide substrates by the sol-gel route. The carbide substrates was coated with boehmite sol by a dip-coating process and dried to get gel film. Multiple coating was necessary for high quality and sufficient thickness of the coatings. After heat treatment at 1200°C, the coating with a thickness up to more than 6 μm could be obtained without any macroscopic defect and showed good performance in turning medium carbon steel, which presented a novel and promising coating method for cutting tools. To determine the heat treatment schedule, boehmite gel was characterized by differential thermal analysis (DTA). The coating morphology was examined with the aid of a scanning electron microscope (SEM). The composition of the coating and interface layer was characterized by X-ray diffraction (XRD). The results suggested that the coating convert to α-alumina at about 1140°C and sintered to a relatively high density at 1200°C. The coating was polycrystalline and isotropic and there was no weak phase in the interface layer. It was found in electron probe microanalysis (EPMA) that cobalt diffused into the coating, which would be conducive to enhancing the bonding strength between the substrate and the coating.