TY - BOOK

T1 - CVD TiCN/α-Al2O3 coated cemented carbide cutting tools

AU - Stylianou, Rafael Panayiotis

N1 - no embargo

PY - 2020

Y1 - 2020

N2 - TiCN/α-Al2O3 bilayer coatings grown by chemical vapor deposition (CVD) are widely employed on WC-Co-based cutting tools, due to their excellent wear resistance, hot-hardness, thermal stability, and chemical inertness. Especially, technological advancements concerning crystallographic texture in α-Al2O3 layers have increased wear resistance significantly. Within the first part of this thesis, the effect of reference materials on the determination of texture coefficients (TCs) was demonstrated for a CVD α-Al2O3 coating layer. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) pole figures were used to establish the (001) fiber texture of the α-Al2O3 coating. TCs were then calculated, utilizing XRD intensities from three reference materials for the same as-deposited coating. Based on the results obtained, useful guidelines were provided regarding texture coefficients, that can be accurately determined using pure α-Al2O3 powders as a reference. Within the second thesis part, TiCN monolayer coatings were grown by CVD on WC-Co substrates with different Co contents, in order to investigate thermal crack network formation through the thermal stress build-up. The driving force for the development of thermal stress was attributed to the difference between room and deposition temperature, 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 the coatings. Dilatometry of the substrates and high temperature XRD of a powdered TiCN coating indicated a decreasing CTE-mismatch for increasing substrate Co contents. In consequence, residual stress in TiCN determined by XRD increased up to 662 ± 8 MPa with decreasing Co contents down to 10 wt.%. For Co contents below 10 wt.%, the residual stress decreases, that marked the formation of thermal crack networks. Similarly, the third topic of the thesis concerned thermal crack formation in CVD TiCN/α-Al2O3 bilayer coatings. 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 of α-Al2O3. This consideration indicated a lower CTE mismatch of α-Al2O3 with WC-Co, compared to TiCN with WC-Co. XRD was further utilized for the determination of residual stress in TiCN and α-Al2O3, that demonstrated a decrease in both layers for Co contents below 12.5 wt.%. Decreasing stress signaled the formation of thermal crack networks. In addition, 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.

AB - TiCN/α-Al2O3 bilayer coatings grown by chemical vapor deposition (CVD) are widely employed on WC-Co-based cutting tools, due to their excellent wear resistance, hot-hardness, thermal stability, and chemical inertness. Especially, technological advancements concerning crystallographic texture in α-Al2O3 layers have increased wear resistance significantly. Within the first part of this thesis, the effect of reference materials on the determination of texture coefficients (TCs) was demonstrated for a CVD α-Al2O3 coating layer. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) pole figures were used to establish the (001) fiber texture of the α-Al2O3 coating. TCs were then calculated, utilizing XRD intensities from three reference materials for the same as-deposited coating. Based on the results obtained, useful guidelines were provided regarding texture coefficients, that can be accurately determined using pure α-Al2O3 powders as a reference. Within the second thesis part, TiCN monolayer coatings were grown by CVD on WC-Co substrates with different Co contents, in order to investigate thermal crack network formation through the thermal stress build-up. The driving force for the development of thermal stress was attributed to the difference between room and deposition temperature, 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 the coatings. Dilatometry of the substrates and high temperature XRD of a powdered TiCN coating indicated a decreasing CTE-mismatch for increasing substrate Co contents. In consequence, residual stress in TiCN determined by XRD increased up to 662 ± 8 MPa with decreasing Co contents down to 10 wt.%. For Co contents below 10 wt.%, the residual stress decreases, that marked the formation of thermal crack networks. Similarly, the third topic of the thesis concerned thermal crack formation in CVD TiCN/α-Al2O3 bilayer coatings. 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 of α-Al2O3. This consideration indicated a lower CTE mismatch of α-Al2O3 with WC-Co, compared to TiCN with WC-Co. XRD was further utilized for the determination of residual stress in TiCN and α-Al2O3, that demonstrated a decrease in both layers for Co contents below 12.5 wt.%. Decreasing stress signaled the formation of thermal crack networks. In addition, 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.

KW - Hartmetall

KW - Chemische Gasphasenabscheidung

KW - Hartstoffschichten

KW - Kristallographische Textur

KW - Thermische Ausdehnung

KW - Eigenspannungen

KW - Thermische Rissnetzwerke

KW - Cemented carbide

KW - Chemical vapor deposition

KW - Hard coatings

KW - Crystallographic texture

KW - Thermal expansion

KW - Residual stress

KW - Thermal crack networks

M3 - Doctoral Thesis

ER -