Thermal crack formation in TiCN/α-Al2O3 bilayer coatings grown by thermal CVD on WC-Co substrates with varied Co content

• Published in: Surface & coatings technology 2020-06, Vol.392, p.1-11, Article 125687
• Main Authors: Stylianou, Rafael, Velic, Dino, Daves, Werner, Ecker, Werner, Tkadletz, Michael, Schalk, Nina, Czettl, Christoph, Mitterer, Christian
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Bilayers; Cemented carbide; Cemented carbides; Chemical vapor deposition; Coatings; Cobalt; Cracks; Crystallography; Electron backscatter diffraction; Hard coatings; High temperature; Residual stress; Stress relaxation; Substrates; Thermal crack network; Thermal expansion; Thermal stress; TiCN; Titanium carbonitride; Tungsten carbide; Vapor deposition; X-ray diffraction; α-Al2O3
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2020.125687

Within this work, the thermal stress build-up of chemically vapor deposited TiCN/α-Al2O3 bilayer coatings was controlled by tuning the coefficient of thermal expansion (CTE) of the substrate material. This was implemented through a Co content variation from 6 to 15 wt.% in WC-Co substrates, which exhibit higher CTEs with increasing Co contents and thereby approach the CTE values of TiCN and α-Al2O3. High temperature X-ray diffraction was employed to determine thermal expansion of an α-Al2O3 powder. Crystallographic texture of the α-Al2O3 coating layer was evaluated by electron backscatter diffraction and taken into consideration in order to assign the appropriate in-plane CTE. This consideration indicated a lower CTE mismatch of α-Al2O3 with WC-Co, compared to TiCN with WC-Co. X-ray diffraction was further utilized for the determination of residual stress in TiCN and α-Al2O3, demonstrating a decrease in both layers for Co contents below 12.5 wt.%. Decreasing stress signaled the formation of thermal crack networks confirmed by scanning electron microscopy surface images. Lower residual stresses were determined in TiCN compared to α-Al2O3 layers of bilayer coatings, contradicting finite element simulations of thermo-elastic stress, that were carried out to illustrate the stress relaxation effects caused by thermal cracks. Monolayer TiCN coatings were annealed at 1000 °C, to replicate stress relaxation taking place during α-Al2O3 deposition, exhibiting a similar residual stress state to TiCN base layers of bilayer coatings. Thermal crack formation was found to be the dominating stress relaxation mechanism in α-Al2O3, while TiCN undergoes further relaxation through secondary mechanisms. •Temperature-dependent coefficients of thermal expansion (CTEs) of TiCN and α-Al2O3•Stress build-up in TiCN and α-Al2O3 layers through increasing CTE mismatch•Thermal crack formation signaled by the decrease of residual stress•Finite element simulations of thermo-elastic stress in TiCN/α-Al2O3 coatings