Stress relaxation through thermal crack formation in CVD TiCN coatings grown on WC-Co with different Co contents

TiCN coatings were grown by chemical vapor deposition (CVD) on WC-Co substrates with different Co contents,in order to control thermal stress. The driving force for the development of thermal stress is attributed to thedifference between room and deposition temperature (ΔT≈−780 °C), and the mismatch of the coefficient ofthermal expansion (CTE) between substrate and coating. Co contents of 6, 7.5, 10, 12.5, and 15 wt% wereutilized to adjust the CTE of the substrate, and therefore tune the stress in TiCN coatings. Dilatometry of thesubstrates and high temperature X-ray diffraction of a powdered TiCN coating indicate a decreasing CTE-mismatchfor increasing substrate Co contents. In consequence, residual stress in TiCN determined by X-ray diffractionincreases up to 662 ± 8 MPa with decreasing Co contents down to 10 wt%. For Co contents below 10 wt%, the residual stress decreases. The formation of thermal crack networks in TiCN, analyzed by scanning electronmicroscopy, coincides with 10 wt% Co. Stress relaxation in TiCN coatings through the formation of thermalcracks becomes evident. A finite element simulation utilized for the calculation of residual stress distributionsreveals shielding effects, which occur with the introduction of thermal cracks. Discrepancies between experimentaland simulated thermo-elastic stresses imply the presence of secondary relaxation sources. High temperatureresidual stresses in TiCN, determined up to 1000 °C (i.e. above deposition temperature), suggest additionalthermal crack formation for substrate Co contents of 6 wt%