In-situ TiC-TiB2 coating on Ti-6Al-4V alloy by powder suspension electro-discharge coating (EDC) and tungsten inert gas (TIG) cladding processes

Abstract

Ti-6Al-4V is one of the most attractive and frequently used Ti alloy in various industrial applications due to its excellent properties including high specific strength, corrosion resistance, and high melting temperature. However, the application of Ti-6Al-4V alloy is restricted under severe wear and frictional conditions owing to its low hardness and poor wear resistance. Hence, surface modification of Ti-6Al-4V alloy becomes important to extend its possibility of application.
In the present research work, TiC-TiB2 composite coating has been fabricated on Ti-6Al-4V alloy to enhance its tribological properties by powder suspension electro-discharge coating (EDC) method and TIG cladding process through in-situ reactions between Ti and B4C powder mixture. The microstructure analysis of the coating has been executed through scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM) images, electron probe micro-analyzer (EPMA) and electron dispersive spectroscopy (EDS). The compound phases formed via in-situ reaction of Ti and B4C have been identified by X-ray diffraction (XRD) technique. The micro-hardness values of the coating have been measured using Vickers micro-indentation hardness tester and wear behaviour of the coating has been analyzed by pin-on-disc and ball-on-disc sliding wear test. With reference to the available literature, SEM images of the blended precursor powder mixture and coating layer, EPMA images and XRD analysis of the produced coating, the formation mechanism of the present TiC-TiB2 composite coating during TIG cladding process has been elucidated.
The coating produced by EDC process shows upto three times increase in the hardness value and seven times higher wear resistance against WC-Co ball than the uncoated Ti-6Al-4V substrate. Further, the coating produced by TIG cladding method under different processing condition exhibited up to four times increased hardness and three times higher sliding abrasive wear resistance (against alumina abrasive disc) than the as-received Ti-6Al-4V alloy substrate. The fracture toughness of the coatings produced by TIG cladding
method as measured by indentation method was found (5.65–6.97 MPa.m1/2), much higher than that of individual TiB2 or TiC phases. The effect of processing current employed during the TIG cladding and the effect of Ti and B4C ratio on the morphology and mechanical behavior of the produced TiC-TiB2 coatings have also been investigated. The microstructural analysis of the produced composite coating revealed that at relatively low current, the coating layers are not appropriately bonded with the substrate; whereas, for employing higher current, owing to high heat input, mixing of the coating material with the substrate surface becomes appreciable. Significant variation in the hardness and wear value was recorded for the alteration of the processing current, and Ti and B4C powder ratio, which was mainly attributed to the variation in the proportion of hard TiC and TiB2 phases and dilution of the substrate material with the clad layer.