Advanced ceramic coatings on aluminum by laser treatment of filled organosilazane-based composites

Abstract

Aluminum and their alloys are particularly suitable for structural components due to their high specific strength and stiffness. However, the low surface hardness and wear resistance limits the lifetime and the use of aluminum. To increase the surface hardness, ceramic-based coatings are frequently the most suitable solutions. A suitable method for the preparation of ceramic coating is the precursor technology. Nevertheless, the required high temperature for the ceramization of the precursor-based coatings in a furnace limits this approach to high temperature-resistant substrates. Laser radiation as a heat source for the pyrolysis of the preceramic polymer is an approach to overcome this restriction. In this paper, we report on a coating system for an aluminum substrate, consisting of a polysilazane (Durazane 2250) bond-coat and a hard top-coat composed of an organosilazane (Durazane 1800) with tetragonal ZrO2 and aluminum fillers pyrolyzed using Nd:YVO4 laser. The bond-coat was applied by dip-coating and thermally cross-linked at 150 °C. Afterwards, the top-coat was applied by spraying. The coatings were subsequently ceramized by using laser radiation. Therefore, the laser parameters were adjusted with regard to the low-melting aluminum substrate. The laser irradiation led to pyrolysis of the silazane and to a predominantly dendritic solidification structure, indicating complete melting of the used fillers. Despite the high temperatures within the coating, thermally-activated processes in the aluminum substrate as a result of laser irradiation could be excluded by determining the tensile strength and Young's modulus both before and after laser irradiation. The laser-treated coatings exhibited good adhesion to the substrate. Even under bending load, the coating showed good adhesion and no spalling occurred. Finally, hardness was investigated and compared with coatings of conventional manufacturing processes. These investigations showed that the coatings exhibited very high hardness and are therefore suitable for wear protection.