Stress relaxation through thermal crack formation in CVD TiCN coatings grown on WC-Co with different Co contents. (January 2020)

• Record Type: Journal Article
• Authors: Stylianou, Rafael; Velic, Dino; Daves, Werner; Ecker, Werner; Stark, Andreas; Schell, Norbert; Tkadletz, Michael; Schalk, Nina; Czettl, Christoph; Mitterer, Christian
• Is Part Of: International journal of refractory metals & hard materials. Volume 86(2020)
• Publication Date: 2020-01
• Subjects: Cemented carbide -- Chemical vapor deposition -- Hard coatings -- Titanium carbonitride -- Thermal cracks -- Stress relaxation; Heat resistant alloys -- Periodicals; Refractory materials -- Periodicals; Metallography -- Periodicals; Alliages réfractaires -- Périodiques; Matériaux réfractaires -- Périodiques; Métallographie -- Périodiques; Heat resistant alloys; Metallography; Refractory materials; Periodicals; Electronic journals; 669.73
• DOI: 10.1016/j.ijrmhm.2019.105102
• Languages: English
• ISSNs: 0263-4368

Abstract

Abstract: 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 the difference between room and deposition temperature (ΔT ≈ −780 °C), and the mismatch of the coefficient of thermal expansion (CTE) between substrate and coating. Co contents of 6, 7.5, 10, 12.5, and 15 wt% were utilized to adjust the CTE of the substrate, and therefore tune the stress in TiCN coatings. Dilatometry of the substrates and high temperature X-ray diffraction of a powdered TiCN coating indicate a decreasing CTE-mismatch for increasing substrate Co contents. In consequence, residual stress in TiCN determined by X-ray diffraction increases 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 electron microscopy, coincides with 10 wt% Co. Stress relaxation in TiCN coatings through the formation of thermal cracks becomes evident. A finite element simulation utilized for the calculation of residual stress distributions reveals shielding effects, which occur with the introduction of thermal cracks. Discrepancies between experimental and simulated thermo-elastic stresses imply the presence of secondary relaxation sources. High temperature residual stresses in TiCN, determined up to 1000 °C (i.e. above deposition temperature), suggest additional thermal crack formation for substrate Co contents of 6 wt%. Highlights: Coefficients of thermal expansion (CTEs) of CVD TiCN and WC-Co substrates CTE mismatch tuning of TiCN and WC-Co through adjustment of Co content Elimination of thermal crack networks in TiCN for 12.5 and 15 wt% Co Illustration of crack shielding effects by finite element simulations High temperature residual stress of TiCN coatings grown on WC-Co substrates