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

Diamond coatings were deposited on commercial silicon nitride (Si3N4) cutti ng tools by low pressure microwave plasma assisted chemical vapor depositio n (MPCVD) at various processing conditions to assess the quality and adhesi on of diamond coatings. A four-factor, three-level fractional factorial exp erimental design methodology was used to determine the significance. of the process variables, such as the microwave chamber pressure, substrate tempe rature, microwave power and % CH4 concentration in the hydrocarbon-hydrogen mixture. The reactor pressure was found to have the strongest influence on nucleation followed by the temperature and the % CH4 concentration. Microw ave 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 % CH4 concentration. Substrate temperature and % CH4 concentration were found to be the most crucial parameters dictat ing the overall growth rate as well as the quality of the diamond films. Di amond-coated Si3N4 tools were characterized by mu -Raman spectroscopy and s canning electron microscopy. The former was used to assess the quality of t he 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 t he tools. The relative intensity of the non-diamond to the crystalline diam ond was found to decrease with increase in the microwave chamber pressure. Higher microwave power was found to increase the intensity of the crystalli ne diamond and decrease the non-diamond peak intensity. The morphology of d iamond coatings was assessed using an SEM. The diamond film deposited were qualitatively assessed for adhesion using the Rockwell hardness tester with a Brale indenter and an abrasion wear tester. A chamber pressure of simila r to 20 torr (2.65 kPa); a microwave power of similar to 1000 W; a CH4 conc entration of 0.5-1 %; and a substrate temperature in the range of 850-900 d egreesC, were identified as the optimum deposition conditions for producing good quality diamond coatings on Si3N4 substrate. (C) 2001 Elsevier Scienc e B.V. All rights reserved.