Synthesis and coating of aluminium nitride on mild steel by combined laser sol-gel technique

In this work a novel method for synthesising Aluminium Nitride (AlN) coatings is reported. A high-power diode laser was applied at different powers and traverse speeds to a mild steel substrate, coated with slurry of aluminium hydroxide sol-gel, urea and graphite. The reaction chemical thermodynamics were investigated to estimate the compositions, temperature range, and the required reaction enthalpy for producing AlN. A three-dimensional heat transfer model by CFD technique was used to predict the processing parameters (speed and power) required to obtain specific temperature ranges during experimentation. Surface morphology and microstructure of the deposited coatings and substrate surface layers were examined using an optical microscopy and scanning electron microscopy, which revealed the presence of a very thin layer of AlN coating. In addition, sub-micron crystalline structures of AlN form a metal matrix composite interlayer with the substrate below the film, which suggests good metallurgical bonding with the substrate. The thin AlN layer phases were identified by X-ray diffraction. Results of micro-hardness measurements revealed a hardness value of the order of 876 HV. The surface roughness of the best coated samples was found to be in the order of 1.8-2.1 μm Ra. The coating is found to exhibit a very low coefficient of friction (<0.1). This makes it an ideal film for applications requiring low friction contact surfaces.