A Comprehensive study on Boride-Nitride-Carbide composite coatings developed through laser processing

Abstract

Ceramic matrix composite coatings with in-situ reinforcements are developed over steel substrates with the process combination of laser triggered chemical reaction and subsequent laser treatment (laser surface alloying). A precursor powder mixture, comprising of a mixture of titanium-di-oxide, hexa-boron nitride, silicon-di-oxide and graphite powders, is pre-placed over a steel substrate. A chemical reaction in the preplaced powder mixture is initiated by the heat input from the scanning of a high power continuous wave laser and its reaction products (TiB2, TiN and SiC) subsequently form an in situ ceramic matrix composite coating (TiB2-TiN-SiC) over the steel substrate (AISI 1025). Following the same procedure, two more coatings (TiB2-TiN-SiC-hBN and TiB2-TiN-SiC (with additional SiC)) are developed with minor alterations in constituents of the preplaced precursor mixture. Minor modifications in the constituents of the preplaced precursor mixture helped in developing coatings different in terms of the amount of hBN and SiC present in it. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) are used to confirm the presence of all the reaction products in these coatings. More insight is obtained in a thorough evaluation of various microstructural, mechanical and tribological properties of the coatings. Addition of hBN and SiC in composite coatings brought significant changes in mechanical and tribological properties of the coatings. On successful incorporation of hBN in the coating matrix as a self-lubricating phase helped in imparting significant changes in coating’s frictional properties. Composite coating developed with additional SiC improved the specific wear rate. A study with larger compositional variations is also carried out to determine compositions of precursor mixtures capable of imparting maximum benefits in terms of the material properties (tribological and mechanical) of coatings. To do that, several composite coatings are developed over AISI 1025 steel substrates with precursors containing variable amounts of hBN and SiC added to precursor mixture along with the other reactants. Results of the study indicates that addition of certain weight percentage of hBN and SiC in precursor gained advantageous developments in certain properties of coatings and further addition beyond that specific amount caused declination in many of those. Composite coatings developed with separately added hBN or SiC are studied to identify compositions achieving satisfactory mechanical and tribological characteristics. On that basis, possibilities of manufacturing a new composite coating with simultaneous additions of hBN and SiC are explored. With this motivation, additional amounts of hBN and SiC are simultaneously added to the precursor along with other reactants to gain solid lubrication without compromising on different mechanical properties. This study shows effectiveness of additional amounts of hBN and SiC in resulting favourable frictional properties alongside improvement in both microhardness and wear resistance. Studies on various mechanical and tribological properties of laser pre glazed (sintered) and laser glazed metal matrix composite with reinforcements of TiB2, TiN and SiC provided the essential idea about the probable character of the coating-substrate interlayer zone.