Weight saving in the transportation, automotive and aerospace sectors is an effective way to reduce greenhouse gas emissions. 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.
Protective ceramic-based coatings are frequently the most suitable solutions to increase the surface hardness. However, the traditional methods for the preparation of ceramic coatings are relatively expensive and have some limitations. As an alternative, the polymer-derived ceramics (PDC) technology has been successfully applied for the processing of various ceramic coatings. In this method, preceramic polymers are converted into amorphous ceramics by a pyrolysis between 500 and 1000 °C in a furnace. Therefore, the furnace based PDC-technology can only applied in combination with temperature-resistant, high melting substrate materials like steels, super-alloys and ceramics. A very innovative approach to overcome this restriction is the use of laser radiation as a heat source for the pyrolysis of the preceramic polymer on the low-melting aluminum substrate.
For this reason, a composite coating system composed of an organosilazane (Durazane 1800) with ZrO₂ particles and aluminum flakes as fillers was developed suitable for pyrolysis with a Nd:YAG laser. Firstly, the aluminum substrates were dip-coated with perhydropolysilazane (PHPS) as a bond coat, onto which the composite coating slurry was applied by spraying. After drying, laser pyrolysis led to the formation of a dense ceramic-based coating system with a thickness up to 20 µm and a mainly cellular/columnar-dendritic microstructure. Thereby the aluminum fillers led to a significant increase in absorption of the laser energy and prevent the interaction of the laser radiation with the substrate. The applied laser energy led to the melting of the fillers and the ceramization of the organosilazane. Despite the high temperatures reached within the coating, thermal damage to the substrates could be excluded by determining the mechanical properties of the substrate before and after laser treatment of the coating. The coating itself showed a high adhesion strength of 22.5 ± 2.3 MPa and a hardness of 19.18 ± 4.82 GPa (HV 0.01). Due to the excellent adhesion to the substrate and high hardness, the coating exhibited high wear resistance, which was investigated using a test procedure based on the pin-on-disc method.