TY - BOOK

T1 - Mechanical Properties of Al¿O¿ Coatings for Cutting Tools Deposited by Chemical Vapor Deposition

AU - Konstantiniuk, Fabian

N1 - no embargo

PY - 2023

Y1 - 2023

N2 - Due to the further development of engineering materials and rising productivity demands in the global manufacturing industry, hard protective coatings have been the subject of intensive research. To meet the challenge of higher production rates, cemented carbide substrates are frequently coated with wear-resistant coatings using chemical or physical vapor deposition (CVD, PVD). In case of turning of steel and cast iron, where temperatures close to 1000 °C emerge at the cutting edge, CVD TiN/TiCN/Al2O3 is the material system of choice, enabling the cutting tool to withstand these severe conditions. The goal of this work is to allow a further knowledge-based development of the rather well investigated TiN/TiCN/Al2O3 coating system for industrial cutting applications, focusing on the determination of the mechanical properties, including hardness, Young¿s modulus, fracture stress and fracture toughness of the Al2O3 toplayer. The Al2O3 toplayer can be realized either as ¿- or as ¿-Al2O3 phase. In a first step, single crystalline ¿-Al2O3 coatings with different orientations and polycrystalline ¿- and ¿-Al2O3 coatings were synthesized using CVD and their microstructure and mechanical properties were evaluated and compared to each other. It was found that among the ¿-Al2O3 phase, the single crystalline coating, which was aligned for fracture on the c-plane, exhibits the highest fracture stress and fracture toughness. The fracture stress and fracture toughness of the (0001) textured polycrystalline ¿-Al2O3 coating is lower compared to the (0001) oriented single crystalline ¿-Al2O3 coating, demonstrating the unfavorable influence of grain boundaries on the fracture behavior. Furthermore, it was shown that the ¿-phase has a higher fracture stress in contrast to the ¿-phase, while the opposite was observed for the fracture toughness. Having evaluated the mechanical properties of both phases, the analysis was extended to the coating-substrate system. In more detail, the influence of a Co-enriched surface zone and its thickness in cemented carbide substrates on the microstructure and the mechanical properties of a TiN/TiCN/¿-Al2O3 coating was investigated. It was found that the microstructure and the mechanical properties, including hardness and Young¿s modulus of the ¿-Al2O3 toplayer, are independent of the Co-enriched surface zone. However, with increasing thickness of the Co-enriched surface zone, the tensile residual stress in both, the ¿-Al2O3 and the TiCN layer, decreases. Furthermore, it was observed that the thermal crack network in the coatings, which arises from different thermal expansion coefficients of substrate and coating material and the high deposition temperature during the CVD process, is independent of the thickness of the Co-enriched surface zone. Thus, it was assumed that the stress relaxation is associated with the Co binder phase in the Co-enriched surface zones. This assumption was further supported by cross-sectional hardness measurements, which showed that the hardness in the Co-enriched surface zones is lower compared to the bulk material and hence, these regions also have a higher capability to deform plastically. The last part of this thesis deals with both, the evaluation of the Young¿s modulus of single crystalline (0001) oriented ¿-Al2O3 coatings and a reference material (SiO2, fused silica), using micro-cantilever experiments and the improvement of the characterization method itself. It was shown that the aspect ratio (bending length/width) of the micro-cantilever has a major influence on the obtained Young¿s modulus, which can be calculated from the slope of the load-displacement curve using the classical Euler-Bernoulli equation. For quasi-static and dynamic micro-cantilever experiments and for both materials, an aspect ratio > 6 results in Young¿s modulus values which are comparable with literature. Finite

AB - Due to the further development of engineering materials and rising productivity demands in the global manufacturing industry, hard protective coatings have been the subject of intensive research. To meet the challenge of higher production rates, cemented carbide substrates are frequently coated with wear-resistant coatings using chemical or physical vapor deposition (CVD, PVD). In case of turning of steel and cast iron, where temperatures close to 1000 °C emerge at the cutting edge, CVD TiN/TiCN/Al2O3 is the material system of choice, enabling the cutting tool to withstand these severe conditions. The goal of this work is to allow a further knowledge-based development of the rather well investigated TiN/TiCN/Al2O3 coating system for industrial cutting applications, focusing on the determination of the mechanical properties, including hardness, Young¿s modulus, fracture stress and fracture toughness of the Al2O3 toplayer. The Al2O3 toplayer can be realized either as ¿- or as ¿-Al2O3 phase. In a first step, single crystalline ¿-Al2O3 coatings with different orientations and polycrystalline ¿- and ¿-Al2O3 coatings were synthesized using CVD and their microstructure and mechanical properties were evaluated and compared to each other. It was found that among the ¿-Al2O3 phase, the single crystalline coating, which was aligned for fracture on the c-plane, exhibits the highest fracture stress and fracture toughness. The fracture stress and fracture toughness of the (0001) textured polycrystalline ¿-Al2O3 coating is lower compared to the (0001) oriented single crystalline ¿-Al2O3 coating, demonstrating the unfavorable influence of grain boundaries on the fracture behavior. Furthermore, it was shown that the ¿-phase has a higher fracture stress in contrast to the ¿-phase, while the opposite was observed for the fracture toughness. Having evaluated the mechanical properties of both phases, the analysis was extended to the coating-substrate system. In more detail, the influence of a Co-enriched surface zone and its thickness in cemented carbide substrates on the microstructure and the mechanical properties of a TiN/TiCN/¿-Al2O3 coating was investigated. It was found that the microstructure and the mechanical properties, including hardness and Young¿s modulus of the ¿-Al2O3 toplayer, are independent of the Co-enriched surface zone. However, with increasing thickness of the Co-enriched surface zone, the tensile residual stress in both, the ¿-Al2O3 and the TiCN layer, decreases. Furthermore, it was observed that the thermal crack network in the coatings, which arises from different thermal expansion coefficients of substrate and coating material and the high deposition temperature during the CVD process, is independent of the thickness of the Co-enriched surface zone. Thus, it was assumed that the stress relaxation is associated with the Co binder phase in the Co-enriched surface zones. This assumption was further supported by cross-sectional hardness measurements, which showed that the hardness in the Co-enriched surface zones is lower compared to the bulk material and hence, these regions also have a higher capability to deform plastically. The last part of this thesis deals with both, the evaluation of the Young¿s modulus of single crystalline (0001) oriented ¿-Al2O3 coatings and a reference material (SiO2, fused silica), using micro-cantilever experiments and the improvement of the characterization method itself. It was shown that the aspect ratio (bending length/width) of the micro-cantilever has a major influence on the obtained Young¿s modulus, which can be calculated from the slope of the load-displacement curve using the classical Euler-Bernoulli equation. For quasi-static and dynamic micro-cantilever experiments and for both materials, an aspect ratio > 6 results in Young¿s modulus values which are comparable with literature. Finite

KW - CVD

KW - hard coatings

KW - TiN/TiCN/Al2O3

KW - Micro-cantilever experiments

KW - CVD

KW - hard coatings

KW - TiN/TiCN/Al2O3

KW - Micro-cantilever experiments

M3 - Doctoral Thesis

ER -