Processing and characterization of laser clad NiCrBSi/WC composite coatings — Influence of microstructure on hardness and wear

Abstract

Abstract NiCrBSi/WC composite coatings containing various amounts of WC/W2C particles were laser cladded on low carbon steel substrate S235JR. Coatings were processed using two different laser systems, a 1 kW Nd:YAG and a 3.8 kW high power diode laser (HPDL). Coatings obtained with the Nd:YAG source demonstrate significant changes in the matrix microstructure with WC/W 2 C particle addition. Specific analysis shows the formation of new carbides (W,Cr) x C y and boride phases (W,Cr) x B y resulting from a partial dissolution of the WC/W 2 C particles within the metal matrix. The Brinell macrohardness of the coatings reveals surprisingly low values for coatings containing 10 vol.% and 20 vol.% WC/W 2 C. Through nanoindentation measurements, it is suggested that the low hardness of these new carbide and boride phases most likely counteracts the WC/W 2 C addition and may explain this unexpected behavior. On the contrary, the same coatings deposited using the HPDL source exhibits no change in the microstructure of the NiCrBSi matrix and display an expected monotonic increase of composite hardness with WC/W 2 C amount. It is suggested that the microstructural appearance of new carbide and boride phases may not be related to the type of laser used but to the specific laser energy during the coating process. Contrarily to hardness, measurements show that the erosive wear is marginally affected by the microstructural differences of the coatings. These results demonstrate that evaluating the quality of laser cladded coatings by simply assessing their density and the absence of a crack (as usually done) is insufficient as it does not automatically guarantee reaching optimal mechanical performance.