Hardfacing of AISI304 steel: fabrication of oxide-boride-nitride ceramic matrix composite layer by laser-assisted high temperature chemical reaction

Abstract

Composition, structure and properties of the products of self-propagating high-temperature synthesis (SHS) are characterised by some distinctive features. High heating rate, fast cooling after rapid completion of the reactions and steep temperature gradients make SHS very effective in producing in situ composites with ceramic reinforcements. In the present work, hardfacing of AISI304 substrates has been done by fabricating a hard ternary ceramic matrix composite layer of Al2O3–TiB2–TiN by laser surface treatment at different scan speeds. The formation of the surface layer is due to laser-triggered SHS followed by laser melting. A mixture of Al, TiO2 and hBN has been used as a precursor for the SHS reaction. The study of the microstructure of the as-fabricated composite layer reveals the co-existence of TiB2 and TiN phases in the nanometric size range in Al2O3 matrix. The presence of all the phases has been confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The average grain sizes were calculated for the reinforcing phases and found to be 36 and 66 nm for TiB2 and TiN, respectively, for the ceramic layer fabricated with a scan speed of 10 mm s−1, whereas 21 and 53 nm have been observed for TiB2 and TiN, respectively, for the ceramic layer fabricated with the scan speed of 5 mm s−1. The understanding of the chemical synthesis in the SHS reaction mentioned here and the process of development of the reinforced composite in the fabrication of the hardfaced layer over steel surface will be immensely helpful in the discernment of the mechanical properties and, thus, finding the target area for the usage of this product. The virtues of the process and formation of the hard composite are reflected well in the microhardness achieved in the fabricated layers, as it is significantly higher than that of the substrate (AISI304 steel). In addition, indentation with a Berkovich tip in a nano-indentation set-up helped in further evaluation of the composite’s hardness and elastic modulus. The property spectrum of the composite, as reported here, indicates its suitability in various wear-intensive applications.