Micromechanical Characterization of Self-Organized Ti1-xAlxN Nanolamellae: The Influence of Interface Coherency and Phase Alteration on Fracture Behaviour

Research output: ThesisMaster's Thesis

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Abstract

Mechanical properties of nanostructured hard coatings depend primarily on coating phase, microstructure and residual stress state. Besides hardness, there is a strong effort to enhance fracture properties of the coatings, in particular fracture toughness and fracture stress. In the case of monolithic ceramic coatings, the usually observed intergranular fracture originates from columnar grain morphology and results in brittle coating behaviour. Apart from nanostructured and multi-layered coatings, periodic phase alteration provides an opportunity to enhance fracture properties of brittle materials. Thereby the nacre of molluscs is a leading model of fracture toughness enhancement, originating from alternating hard mineral and soft protein sublayers. In this work, mechanical properties of three different, self-organized nanostructured Ti1-xAlxN coatings are investigated. The coatings, which were synthetized using chemical vapour deposition, possess a very unique microstructure which was formed as a result of self-organized growth. The microstructure was characterized using transmission electron microscopy and the phase composition is evaluated using X-ray diffraction. The microstructure of the first coating shows a cubic-wurtzite alteration of the Ti-rich and Al-rich nitride phases, whereas the second coating is composed completely of cubic phases. In the third sample the phases were altered leading to a multi-layered coating with both cubic-cubic and cubic-wurtzite phases. Furthermore hardness and indentation modulus were characterized using nanoindentation. For all samples, Young’s modulus, fracture stress and fracture toughness were determined by micro-cantilever bending experiments in a scanning electron microscope. These experiments were carried out for the in-plane as well as the out-of-plane bending force orientation. The results reveal outstanding mechanical properties for all three coatings investigated. Hardness is in the range of 26–37 GPa. Fracture stress depends on the phase of the nanostructured coatings and reaches a maximum of 7.9 GPa for the multi-layered sample. The micromechanical bending experiments show nearly isotropic elastic response for the monolithic coatings whereas only a slight anisotropy was found in the case of the multi-layered sample. In the case of fracture toughness a complex dependence on the coating morphology is observed. Moreover, both the in-situ analysis of the crack propagation in the multi-layered structures during the cantilever bending experiments as well as the ex-situ analysis of the fracture surfaces helps to improve our understanding of fracture behaviour of nanostructured coatings.
Translated title of the contributionMikromechanische Charakterisierung von selbstorganisierten Ti1-xAlxN Nanolamellen: Der Einfluss von Grenzflächenkoherenz und Phasenalterierung auf die Brucheigenschaften
Original languageEnglish
QualificationDipl.-Ing.
Supervisors/Advisors
  • Keckes, Jozef, Supervisor (internal)
Award date1 Jul 2016
Publication statusPublished - 2016

Bibliographical note

embargoed until 03-05-2017

Keywords

  • CVD
  • micromechanics
  • fracture properties
  • bioinspired self-organized multi-layered coating
  • Ti1-xAlxN

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