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

Rafael Panayiotis Stylianou, Dino Velic, Werner Daves, Werner Ecker, Michael Tkadletz, Nina Schalk, Christoph Czettl, Christian Mitterer

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Within this work, the thermal stress build-up of chemically vapor deposited TiCN/α-Al 2O 3 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-Al 2O 3. High temperature X-ray diffraction was employed to determine thermal expansion of an α-Al 2O 3 powder. Crystallographic texture of the α-Al 2O 3 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-Al 2O 3 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-Al 2O 3, 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-Al 2O 3 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-Al 2O 3 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-Al 2O 3, while TiCN undergoes further relaxation through secondary mechanisms.

Original languageEnglish
Article number125687
Number of pages11
JournalSurface & coatings technology
Issue number25 June
Early online date21 Apr 2020
Publication statusPublished - 25 Jun 2020

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