Hardness and Strengthening Effect of Low-Pressure Chemical Vapor Deposition BCxNy Coatings Deposited on Ti-6Al-4V Alloy

Hardness and strengthening effect of the BC x N y coatings synthesized by low-pressure chemical vapor deposition (LPCVD) on the Ti-6Al-4V titanium alloy were studied. The BC x N y coatings were deposited from borane triethylamine vapor at the temperature range of 500-800 °C. It was found that the BC x N y coatings deposited at 600-700 °C have maximum hardness values. Strengthening effect was evaluated from the stress–strain characteristics under cyclic (periodic) loading. The coated samples showed greater resistance to fatigue failure compared to uncoated ones; the greatest strengthening effect was observed for the coating deposited at 600 °C. Mechanical stresses in the coatings were measured to clarify the nature of the strengthening effect. Most of the coatings were found to be under tensile stress; the maximum stress values of 800-1000 MPa were characteristic for the coatings deposited at 550-650 °C. The critical strains which lead to the failure of the coatings were determined. A brittle destruction of the coating occurred simultaneously with the irreversible deformation in the substrate. The interpretation of the obtained results was made using the data on the chemical and phase composition of the coatings.