TY - BOOK

T1 - Novel Multifunctional Coatings for Advanced Tribological Applications

AU - Jäger, Nikolaus

N1 - embargoed until null

PY - 2021

Y1 - 2021

N2 - Hard, wear-resistant coatings for the protection of surfaces exposed to wear have been in the focus of research for several decades. However, ever-increasing demands in modern machining applications like high-speed cutting or dry-cutting require advanced coating materials with extraordinary properties. Besides the mechanical properties, thermal stability and resistance against corrosion and oxidation are of utmost importance, as the coatings regularly face very high temperatures above 1000°C during operation. Within the scope of this thesis, several novel multifunctional coatings with specific compositional, microstructural and residual stress designs were developed and investigated by modern, advanced characterization methods. In a first approach, alternating layers of hard Al70Cr30N with cubic and soft Al90Cr10N with hexagonal crystal structure were combined in a multi-layered coating with specific architecture. The coating was studied in the as-deposited state and after vacuum annealing at 1000°C by cross-sectional X-ray nanodiffraction in order to access the microstructure, texture, phase composition and residual stress state and their changes upon annealing. The results reveal the influence of the complex coating architecture as the columnar grain growth is interrupted at the interfaces of layers exhibiting different phases, the decomposition of the metastable cubic phase is governed and a residual stress depth distribution with pronounced stress gradients is formed. The second approach is based on nanocomposite coatings consisting of immiscible nitrides. Three AlCr(Si)N coatings with Si-contents of 0at.%, 2.5at.% and 5at.% Si were synthesized and investigated regarding their thermal stability by in-situ high-temperature high-energy grazing incidence transmission X-ray diffraction. The results reveal a change from a columnar to a nanocomposite microstructure, a diminished decomposition of CrN into Cr2N and an increase of lattice defects accompanied by higher compressive residual stress with increasing Si-content. Furthermore, the addition of Si to AlCrN resulted in enhanced mechanical properties at elevated temperatures, demonstrated by an increase in hardness of the Si-containing coatings, while the hardness for AlCrN decreased after annealing. Finally, the oxidation behavior of the nanocomposite AlCr(Si)N coatings was investigated by differential scanning calorimetry, thermogravimetric analysis and ex-situ X-ray diffraction. The results reveal an improved oxidation behavior with increasing Si-content by a retarded onset of oxidation to higher temperatures from 1100°C to 1260°C and a simultaneously decelerated oxidation process. Additionally, a sample of the AlCrSiN coating with 5at.% Si was partially oxidized by annealing it at 1400°C in ambient air to study the elemental and structural changes along the coating thickness by scanning transmission electron microscopy and cross-sectional X-ray nanodiffraction. Outwards diffusion of Cr, Al and Si to the surface and inward diffusion of oxygen resulted in the development of three zones throughout the coating. An oxide layer consisting of an Al-rich and a Cr-rich part formed at the very top, followed by a transition zone with fine grains and incomplete oxidation and a non-oxidized zone depleted of Cr underneath. Overall, this thesis presents several successful attempts to improve the mechanical properties, thermal stability and oxidation behavior of wear-protective coatings by sophisticated microstructural and compositional designs. The results establish a deeper understanding of the process–structure–property correlations of coatings with a complex, multi-layered architecture or a nanocomposite structure. Simultaneously, the study points out the necessity of advanced, position resolved and in-situ characterization techniques for the analysis of coatings with structural and compositional heterogeneities.

AB - Hard, wear-resistant coatings for the protection of surfaces exposed to wear have been in the focus of research for several decades. However, ever-increasing demands in modern machining applications like high-speed cutting or dry-cutting require advanced coating materials with extraordinary properties. Besides the mechanical properties, thermal stability and resistance against corrosion and oxidation are of utmost importance, as the coatings regularly face very high temperatures above 1000°C during operation. Within the scope of this thesis, several novel multifunctional coatings with specific compositional, microstructural and residual stress designs were developed and investigated by modern, advanced characterization methods. In a first approach, alternating layers of hard Al70Cr30N with cubic and soft Al90Cr10N with hexagonal crystal structure were combined in a multi-layered coating with specific architecture. The coating was studied in the as-deposited state and after vacuum annealing at 1000°C by cross-sectional X-ray nanodiffraction in order to access the microstructure, texture, phase composition and residual stress state and their changes upon annealing. The results reveal the influence of the complex coating architecture as the columnar grain growth is interrupted at the interfaces of layers exhibiting different phases, the decomposition of the metastable cubic phase is governed and a residual stress depth distribution with pronounced stress gradients is formed. The second approach is based on nanocomposite coatings consisting of immiscible nitrides. Three AlCr(Si)N coatings with Si-contents of 0at.%, 2.5at.% and 5at.% Si were synthesized and investigated regarding their thermal stability by in-situ high-temperature high-energy grazing incidence transmission X-ray diffraction. The results reveal a change from a columnar to a nanocomposite microstructure, a diminished decomposition of CrN into Cr2N and an increase of lattice defects accompanied by higher compressive residual stress with increasing Si-content. Furthermore, the addition of Si to AlCrN resulted in enhanced mechanical properties at elevated temperatures, demonstrated by an increase in hardness of the Si-containing coatings, while the hardness for AlCrN decreased after annealing. Finally, the oxidation behavior of the nanocomposite AlCr(Si)N coatings was investigated by differential scanning calorimetry, thermogravimetric analysis and ex-situ X-ray diffraction. The results reveal an improved oxidation behavior with increasing Si-content by a retarded onset of oxidation to higher temperatures from 1100°C to 1260°C and a simultaneously decelerated oxidation process. Additionally, a sample of the AlCrSiN coating with 5at.% Si was partially oxidized by annealing it at 1400°C in ambient air to study the elemental and structural changes along the coating thickness by scanning transmission electron microscopy and cross-sectional X-ray nanodiffraction. Outwards diffusion of Cr, Al and Si to the surface and inward diffusion of oxygen resulted in the development of three zones throughout the coating. An oxide layer consisting of an Al-rich and a Cr-rich part formed at the very top, followed by a transition zone with fine grains and incomplete oxidation and a non-oxidized zone depleted of Cr underneath. Overall, this thesis presents several successful attempts to improve the mechanical properties, thermal stability and oxidation behavior of wear-protective coatings by sophisticated microstructural and compositional designs. The results establish a deeper understanding of the process–structure–property correlations of coatings with a complex, multi-layered architecture or a nanocomposite structure. Simultaneously, the study points out the necessity of advanced, position resolved and in-situ characterization techniques for the analysis of coatings with structural and compositional heterogeneities.

KW - Röntgen-Nanobeugung am Querschnitt

KW - Dünnschichten

KW - Oxidationsverhalten

KW - Hochtemperaturröntgenbeugung

KW - Lichtbogenverdampfung

KW - Thermische Stabilität

KW - Cross-sectional X-ray nanodiffraction

KW - Thin films

KW - Oxidation behaviour

KW - High-temperature X-ray diffraction

KW - Cathodic arc evaporation

KW - Thermal stability

M3 - Doctoral Thesis

ER -