Mechanism And Technology For Polishing CVD Diamond With Chemical And Mechanical Synergistic Effects

Diamond possesses excellent physical, thermal, optical and chemical properties, such as highest hardness, good chemical inertness, high thermal conductivity, high elasticity modulus, large electrical resistance, wide electronic gap, wide-range transparency and low friction coefficient. As a21st century engineering material, it has many promising applications and potential markets. The debut of CVD (Chemical vapor deposition) diamond will break through the limitations of rare, expensive and small size natural diamond, and greatly expands the application field of diamond from the traditional tool, mold to the fields of optical, thermal, electronic semiconductors, acoustics, and so on. However, the columnar growth of CVD diamond results in a polycrystalline nature and the grain sizes increase with film thickness. Therefore, if CVD diamonds are to be used in these fields, it is extremely important for their surfaces to be precisely machined and highly polished.The extreme hardness and good chemical inertness of a CVD diamond bring a great challenge to its polishing methods:The traditional mechanical polishing method yields extremely low polishing rate and may cause micro scratches on the polished surface. Other methods including laser polishing, electrical discharge machining and chemical etching may introduce markable damage layer to diamond surface. Nevertheless, Chemical and mechanical polishing is expected to provide new ideas in planarizing CVD diamonds with untrapresicion and damage-free surface. It is necessary to investigate the mechanical removal mechanism and develop a chemically assisted mechanical polishing and planarization technique for CVD diamonds.Base on the theory of chemical thermodynamics and kinetics, this study focues on polishing CVD diamonds with tribochemical polishing (TCP) method and chemical and mechanical polishing (CMP) method by investigating material removal mechanism involved in the two methods. Main contents and results are as follows:(1) According to the analysis of chemical thermodynamics and kinetics, diamond could convert into graphite with the catalysis of transition metals. Based on this analysis, the study investigates the catalytic mechanisms of TCP and dentifies four requirements for the polishing plate, i.e. unpaired d electrons, vertical alignment rule, high hardness and oxidation resistance at elevated temperature. Based on the thermokinetic analysis of diamond oxidation, the study proposes a chemical thermokinetic model and predicts that mechanical scratching, surface energy and strong oxidizing reaction are responsible for material removal in the CMP process of diamond polishing.(2) Based on the requirements for TCP process, an FeNiCr alloy polishing plate is designed and prepared by using the mechanical alloying and hot-press sintering techniques. The FeNiCr alloy polishing plate has higher hardness and oxidation resistance than stainless steel304and high-speed steel. It provides a material removal rate of3.7μm/min in CVD diamond polishing, which is higher than that of stainless steel304plate, high-speed steel plate and TiAl alloy plate. The study futher interprets the material removal mechanism in the TCP process as diamond first converts into graphite and other non-diamond carbons under the heating and the catalytic actions of the transition metal, then the graphite and non-diamond carbons are then removed by mechanical friction, oxidation and diffusion into the polishing plate.(3)Slurry preparation is necessary for the CMP process. According to the standard electrode potentials of common oxidants, eight oxidants including K2FeO4、KMnO4、 Na2MoO4、K2Cr2O7、CrO3、KIO4、H2O2、(NH4)2S2O8are chosen to prepare CMP slurries. Theoretical analysis and experimental results show that K2Fe04is the best oxidant, and2μm boron carbide abrasive and glass plate are the best abrasive and polishing plate, respectively. The physical and chemical stability and oxidability of the slurries are also investigated. The study identifies that K2FeO4and phosphoric acid have their ideal concentrations of15g/100ml and7.5ml/100ml, respectively. Catalyst T shows the best catalytic effect in the CMP process. X-ray diffraction spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy are used to analyze the surface chemical composition of CVD diamonds and investigate the mechanisms of material removal. The results shows that the "C-O","C-OH","C-O-C","C=O" and "O=C-OH" groups form on diamond surface. The study illustrates the step by step carbon oxidation process as:radical oxygen is formed as a result of potassium ferrate decomposition in the acid solution, and then oxidizes carbon by adsorbing onto the surface of diamond and solid catalyst. The mechanical scratching action of an abrasive produces a damage layer with a thickness of about2nm to promote the oxidation process of the diamond carbon.(4) In order to study the influences of temperature, polishing pressure, the rotational speed of polishing head and the concentration of oxidant on material removal rate and friction, a local heating device and a friction measurement device are designed and fabricated for the CMP processes. The study finds that an ideal polishing pressure of6.5MPa and rotational speeds of polishing plate of11000r/min in TCP process, and an ideal polishing pressure of266.7kPa, polishing temperature of50℃, rotational speeds of polishing plate and head of 70r/min and23r/min in CMP process, respectively, with the coefficient of friction in the range of0.060～0.065. With the utilization of the polishing process, CVD diamonds are polished with surface roughness of0.187nm with no surface scratches nor damages.