Residual stresses in chemically vapor deposited coatings in the Ti-C-N system

Abstract
Residual stresses in chemically vapor deposited monolithic and graded coatings in the Ti-C-N system were investigated as a function of substrate material and coating composition by X-ray diffraction Sin² ψ method. The thermal expansion coefficients (CTEs) of the graphite substrates ranged from 2.5×10^-6 K^-1 to 8.6×10^-6 K^-1. Titanium nitride (CTE 9.35×10^-6 K^-1) and titanium carbide (CTE 7.5×10^-6 K^-1) coasings deposited on the low-expansion substrates (CTEs 2.5-3.5×10^-6 K^-1) exhibited crack networks which accounted for low stress levels measured in the coatings. A phenomenalogical explanation of the crack patterns was given. The coatings grown on the substrates with high coefficients of thermal expansion (CTEs 7.8-8.6×10^-6 K^-1) had no cracks. Residual stresses in the TiN coatings on these substrates were measured to be tensile. Whereas TiC coatings always exhibited compressive stresses ranging from -54±10 MPa to -288±18 MPa. The TiC_xN_y coatings deposited on the substrate with a thermal expansion coefficient of 8.6×10^-6 K^-1 also had compressive stresses increased with increasing TiC mole fraction in the TiC_xN_y up to about 0.9 above which stresses decreased. The residual stresses in the top TiC layers of the graded TiN/TiC coatings with linear, parabolic and exponentional composition profiles grown on the the same substrate (CTE 8.6×10^-6 K^-1) were measured to be compressive and about 475 MPa. Stresses in the coatings were calculated and attributed to the thermal expansion mismatch between the coating and the substrate. It was shown that the measured stresses were, in general, found to be in good agreement with the calculated ones.