Characterization and adhesion strength of diamond films deposited on silicon nitride inserts by d.c. plasma jet chemical vapour deposition

Abstract

Abstract Diamond films have been deposited on silicon nitride and cemented carbide inserts using the d.c. plasma jet chemical vapour deposition (CVD) technique. The morphologies and structures of the diamond films were characterized by scanning electron microscopy (SEM) and Raman spectroscopy. It was found that some of the diamond films deposited on WC-8%Co cemented carbide inserts peeled off immediately after shutting off the plasma torch owing to the large thermal stress accumulated inside the diamond films; this was caused by the different thermal expansion coefficients of diamond and cemented carbide inserts as well as the rapid heating and cooling rates in the d.c. plasma jet CVD process. In contrast, diamond films deposited on silicon nitride inserts, at deposition times ranging from 10 min to 3 h, showed no evidence of flaking even under severe thermal shock, since the similar thermal expansion coefficients of diamond and silicon nitride greatly reduce the thermal stress inside the diamond films. However, further indentation test results under loads of 31.25, 60 and 100 kgf revealed that almost all of the diamond films deposited on silicon nitride inserts spalled around the indentation centres even under a 31.25 kgf load (the lowest limit of the apparatus). This may be caused by the low toughness of the silicon nitride inserts. As a result, cracks initiated by the indentation load in the substrate propagate rapidly and cause chipping and loss of silicon nitride as well as the diamond film.