Microstructure and wear mechanisms of textured CVD alumina and titanium carbonitride coatings
Doctoral thesis, 2022
The aim of this thesis is to find the reasons behind how the wear performance of hard coatings produced by chemical vapor deposition (CVD) is influenced by their texture, as this is today not fully known. Therefore, differently textured coatings were synthesized, and subsequently analyzed both before and after specially designed machining tests. The main research methods used are analytical electron microscopy, transmission Kikuchi diffraction, electron backscattered electron diffraction, X-ray diffraction, atom probe tomography and Schmid factor simulations.
The microstructure, texture and facet development of the as-deposited α-Al2O3 coatings were determined, and the effect of catalyzing gas and diffusion of heavy elements from the substrate were described. After machining tests, terrace formation at the edges is attributed to crystallographic dependent etching. More deformation occurs for the textured coatings in the transition zone, with an associated sub-surface dislocation formation coupled to the number of activated slip systems. The surface morphology in the sliding zone is mainly affected by the surface developed in the previous zones. For the (0001)-textured coating, the observed low wear rate is attributed to homogeneous basal-slip dominating plastic deformation, while for the (01-12) and (11-20) textures the main deformation mechanism is heterogeneous plastic deformation, causing micro-rupture and abrasion, leading to higher wear-rates.
For titanium carbonitride coatings, the (211)-textured coating exhibits a more significant and non-uniform deformation than (110), which is related to a heterogeneous response of the relevant slip system.
In conclusion, the results presented in this thesis reveal the complex relationships between local wear mechanisms and coating texture. This fundamental
understanding can facilitate future development of texture-controlled CVD α-alumina and titanium carbonitride coatings, with the potential of further improving the performance of coated cutting tools.
The microstructure, texture and facet development of the as-deposited α-Al2O3 coatings were determined, and the effect of catalyzing gas and diffusion of heavy elements from the substrate were described. After machining tests, terrace formation at the edges is attributed to crystallographic dependent etching. More deformation occurs for the textured coatings in the transition zone, with an associated sub-surface dislocation formation coupled to the number of activated slip systems. The surface morphology in the sliding zone is mainly affected by the surface developed in the previous zones. For the (0001)-textured coating, the observed low wear rate is attributed to homogeneous basal-slip dominating plastic deformation, while for the (01-12) and (11-20) textures the main deformation mechanism is heterogeneous plastic deformation, causing micro-rupture and abrasion, leading to higher wear-rates.
For titanium carbonitride coatings, the (211)-textured coating exhibits a more significant and non-uniform deformation than (110), which is related to a heterogeneous response of the relevant slip system.
In conclusion, the results presented in this thesis reveal the complex relationships between local wear mechanisms and coating texture. This fundamental
understanding can facilitate future development of texture-controlled CVD α-alumina and titanium carbonitride coatings, with the potential of further improving the performance of coated cutting tools.
Plastic deformation
Texture
TKD
Electron microscopy
Coatings
CVD
TiCN
α-alumina
Wear mechanisms
PJ-salen, Fysik Origo byggnad, Fysikgränd 3
Opponent: Paul H. Mayrhofer, Institute of Materials Science and Technology, Technische Universität Wien, Austeria
Author
Siamak Shoja
Chalmers, Physics, Microstructure Physics
Many of the mechanical systems that we frequently use in our daily life such as automobiles, home appliances, and airplanes are made of a large number of distinct components. Many of these components are cut (machined) to the desired shape and dimension by various metal cutting processes such as turning, milling and drilling. One of the key problems when cutting materials is the excessive wear of the cutting tools. Wear of cutting tools cannot be avoided, so it is critical to create a tool that doesn't wear soon but lasts for a long period.
Coating the cutting tools with a few layers of thin, hard, and wear-resistant materials is one technique to increase the lifetime of cutting tools. These coatings are in the micrometer range, i.e. about 1/100th the width of human hair. They also have a texture meaning that the small crystals (grains) forming these coatings are aligned in a certain direction. The textured coatings have a large impact on the cutting performances, e.g. the tool life can be extended as much as 20 times by using specific textures in certain applications. However, the reason for this is not known.
In order to build cutting tools with improved performance and longer lifetime, it is very important to know how these coatings degrade when cutting metals and thus, to know the best coating microstructure to withstand the degradation. The aim of this thesis is to increase the fundamental understanding of the underlying coating degradation mechanisms. This was achieved by performing controlled cutting tests on differently textured coatings, and studying the worn coatings at the surface and in cross-section at a microscopic level, using a combination of experimental and theoretical methods. The mechanisms that cause wear and degradation were identified. The knowledge obtained can contribute to the optimization of cutting tools for improved performance and longer lifetimes. In this approach, in addition to the economic gain, the environment will benefit, resulting in more sustainable production.
Coating the cutting tools with a few layers of thin, hard, and wear-resistant materials is one technique to increase the lifetime of cutting tools. These coatings are in the micrometer range, i.e. about 1/100th the width of human hair. They also have a texture meaning that the small crystals (grains) forming these coatings are aligned in a certain direction. The textured coatings have a large impact on the cutting performances, e.g. the tool life can be extended as much as 20 times by using specific textures in certain applications. However, the reason for this is not known.
In order to build cutting tools with improved performance and longer lifetime, it is very important to know how these coatings degrade when cutting metals and thus, to know the best coating microstructure to withstand the degradation. The aim of this thesis is to increase the fundamental understanding of the underlying coating degradation mechanisms. This was achieved by performing controlled cutting tests on differently textured coatings, and studying the worn coatings at the surface and in cross-section at a microscopic level, using a combination of experimental and theoretical methods. The mechanisms that cause wear and degradation were identified. The knowledge obtained can contribute to the optimization of cutting tools for improved performance and longer lifetimes. In this approach, in addition to the economic gain, the environment will benefit, resulting in more sustainable production.
Subject Categories
Materials Engineering
ISBN
978-91-7905-653-7
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5119
Publisher
Chalmers
PJ-salen, Fysik Origo byggnad, Fysikgränd 3
Opponent: Paul H. Mayrhofer, Institute of Materials Science and Technology, Technische Universität Wien, Austeria