Surface Engineering of Tungsten Carbide Tool Material by Nano and Microcrystalline Diamond Coatings

Abstract

Diamond is generally accepted as a material with individual properties such as superlative hardness, low coefficient of friction and very high thermal conductivity. Synthetic diamond can be achieved in the form of thin and thick films using experimental chemical vapor deposition (CVD) methods. Cemented tungsten carbide (WC–Co) is the commonly used tool material with high hardness (~18 GPa) and high elastic modulus (~550 GPa) and, also compatible to the growth of synthetic diamond films. Presently, synthetic diamond coatings have been widely used on carbide cutting tools from long time to prevent the abrasive wear occurred on conventional tools. Based upon the size of the grains, synthetic diamond coatings are basically classified into nanocrystalline diamond (NCD) and microcrystalline diamond (MCD). In the work reported in the present chapter, smooth and adhesive thin NCD and MCD coatings were deposited on chemically treated tungsten carbide substrates using pre-determined process parameters in the hot filament chemical vapor deposition (HFCVD) method. Tungsten carbide with 6% Co is the mostly accepted grade of base material used for the successful growth of synthetic diamond films on its surface and, also to minimize the thermal residual stresses existing during the deposition and cooling down process between the interfaces of coating and substrate. Mostly, these thermal residual stresses are produced due to difference in thermal expansion coefficients between the coating and substrate. During deposition process, the process parameters such as methane concentration (%CH₄/H₂) and chamber pressure were controlled automatically using pre-programmed recipe for the growth of NCD and MCD films. The structural characteristics and quality of the synthetic diamond films were confirmed using X-ray diffraction and Raman spectroscopy techniques, respectively. The surface morphology was studied using a high resolution scanning electron microscope (HRSEM) and atomic force microscope (AFM). Moreover, the hardness measurement of coatings were done using a Berkovich nanoindenter. After that, a comparative evaluation between these two types of coatings was done.