TY - JOUR

T1 - Thermal crack formation in TiCN/α-Al2O3 bilayer coatings grown by thermal CVD on WC-Co substrates with varied Co content

AU - Stylianou, Rafael Panayiotis

AU - Velic, Dino

AU - Daves, Werner

AU - Ecker, Werner

AU - Tkadletz, Michael

AU - Schalk, Nina

AU - Czettl, Christoph

AU - Mitterer, Christian

N1 - Publisher Copyright: © 2020 Elsevier B.V.

PY - 2020/6/25

Y1 - 2020/6/25

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=85083421355&partnerID=8YFLogxK

U2 - 10.1016/j.surfcoat.2020.125687

DO - 10.1016/j.surfcoat.2020.125687

M3 - Article

VL - 392.2020

JO - Surface & coatings technology

JF - Surface & coatings technology

SN - 0257-8972

IS - 25 June

M1 - 125687

ER -