Low pressure microwave plasma assisted chemical vapor deposition (MPCVD) of diamond coatings on silicon nitride cutting tools

Abstract

Diamond coatings were deposited on commercial silicon nitride (Si 3N 4) cutting tools by low pressure microwave plasma assisted chemical vapor deposition (MPCVD) at various processing conditions to assess the quality and adhesion of diamond coatings. A four-factor, three-level fractional factorial experimental design methodology was used to determine the significance of the process variables, such as the microwave chamber pressure, substrate temperature, microwave power and % CH 4 concentration in the hydrocarbon–hydrogen mixture. The reactor pressure was found to have the strongest influence on nucleation followed by the temperature and the % CH 4 concentration. Microwave power was found to have a minimum influence on nucleation. It was also found that nucleation density decreases with increase in pressure, while it increases with increase in the % CH 4 concentration. Substrate temperature and % CH 4 concentration were found to be the most crucial parameters dictating the overall growth rate as well as the quality of the diamond films. Diamond-coated Si 3N 4 tools were characterized by μ-Raman spectroscopy and scanning electron microscopy. The former was used to assess the quality of the diamond coatings (phase purity as well as the amount of amorphous carbon associated with the sp 2 structure) as well as the residual stresses on the tools. The relative intensity of the non-diamond to the crystalline diamond was found to decrease with increase in the microwave chamber pressure. Higher microwave power was found to increase the intensity of the crystalline diamond and decrease the non-diamond peak intensity. The morphology of diamond coatings was assessed using an SEM. The diamond films deposited were qualitatively assessed for adhesion using the Rockwell hardness tester with a Brale indenter and an abrasion wear tester. A chamber pressure of ∼20 torr (2.65 kPa); a microwave power of ∼1000 W; a CH 4 concentration of 0.5–1%; and a substrate temperature in the range of 850–900°C, were identified as the optimum deposition conditions for producing good quality diamond coatings on Si 3N 4 substrate.