Development of dynamic friction polishing technique for CVD diamond

Abstract

Diamond films produced by the chemical vaporised deposition (CVD) technique, often called CVD diamond films, have been widely used in industry because of their extraordinary properties. However, the film surfaces are usually very rough and cannot be used directly in most cases. Many methods have been developed to polish such diamond films, among which the dynamic friction polishing (DFP) method has been found to be efficient. Generally speaking, the DFP combines the mechanisms of mechanical rubbing and the thermo-chemical reaction created by frictional heating for highly efficient polishing. This thesis investigates the feasibility of using the DFP technique to polish CVD diamond surfaces, both experimentally and numerically. Two types of specimens were studied; the research focus was on the finished surface quality and the material removal rates in relation to different polishing conditions. Both experimental and theoretical methods were used to explore the polishing mechanism. Experimental results proved that the DFP technique can be successfully used to polish both CVD diamond wafers and CVD diamond thin films. It was found that to minimise the cracking of specimens, a stepwise polishing process should be introduced. It was also found that specimen cracking is sensitive to the polishing pressure applied, and that the polishing window for CVD thin films is smaller. Polishing time is a critical factor to avoid cracking, because a longer polishing time means a higher thermal stress. With the proper parameters obtained in this study, very smooth, high quality surfaces of CVD wafers and thin films could be obtained in minutes. On the other hand, simulation results proved that both the CVD diamond wafer and thin film experience similar temperature distributions, and that thermal expansion has an obvious effect on stress distribution. For the CVD diamond thin film, stress discontinuity is obvious across the interface of thin film, which could result in interface debonding. The CVD diamond wafer experiences a clear bulk bending and stressing. The maximum tensile stress can destroy a specimen easily if it is high enough. It was found that controlling the heat generation in polishing is instructionally useful for reducing stress.