SIC-SI3N4 COMPOSITE COATINGS PRODUCED BY PLASMA-ENHANCED CHEMICAL-VAPOR-DEPOSITION

Silicon carbonitride coatings have been produced by plasma-enhanced chemical vapour deposition (CVD) on AISI 440C steel in a hot-wall reactor at 250-degrees-C from a mixture of SiH4, N2-NH3 and C2H4, and analysed by electron probe microanalysis and Rutherford backscattering spectroscopy-elastic recoil detection. Coating with different ratios of silicon carbide to silicon nitride and silicon sub-and superstoichiometries have been deposited. Stoichiometric coating show a maximum in their mechanical properties. Depending on the SiC-to-Si3N4 ratio, the Knoop hardness values vary between 1500 and 2800 HK0.025. Internal stress is low at a level of 100-300 MPa. The pinhole density is less than 2 cm-2. The fracture toughness as determined from indention tests is 4 MPa m1/2. Linear polarization testing results show excellent protection of the substrate material against chemically aggressive media as compared with conventional CVD.