Multilayer Design for Increased Toughness of CrN-based Coatings

Manfred Schlögl

Research output: ThesisDoctoral Thesis

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Abstract

Ceramic-like coatings are widely used for various industrial applications because of their outstanding properties like high thermal stability, oxidation resistance and abrasion resistance. Particularly, transition metal nitrides, such as CrN are well known and investigated with respect to their microstructure, morphology, thermal and mechanical properties. Due to the demand of versatile requirements smart architectural designs such as nanocomposite and multilayers become more important during the last decades. This study focuses on the structural, mechanical and thermal properties of CrN/AlN multilayer coatings. The influence of the individual layer thickness is investigated by preparing multilayer coatings composed of 1, 2 and 3 nm thin AlN layers and CrN layers with thicknesses ranging from 1 to 10 nm. Based on X-ray diffraction (XRD), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) it can be concluded that a fully stabilization of the AlN layers in their metastable cubic structure can be achieved up to layer thicknesses of 3 nm with the condition that the CrN layers need to be at least as thick as the AlN layers. Otherwise the AlN layers crystallize also in their stable ZnS wurtzite type structure. The superlattice coatings exhibit a characteristic hardness profile as a function of the bilayer period Λ with a pronounced hardness maximum of 31 and 28 GPa. Highest thermal stability for these superlattice coatings is obtained when the columnar growth is inhibited. Further investigations concentrate on the influence of AlN layers in their metastable cubic structure and stable wurtzite structure on the columnar growth of CrN-based multilayers and the resulting mechanical and thermal properties. Therefore, multilayer coatings were developed composed of 3 or 10 nm thin AlN layers and combined with ~100 nm thin CrN, CrAlN, or CrAlYN layers. The study clearly demonstrates improved mechanical and thermal properties for coatings composed of 10 nm thin AlN layers. If the AlN layers are thicker than 3 nm, coherency strains to the CrN, CrAlN, or CrAlYN layers are too weak to allow for a fully coherent cubic stabilization. Hence, they tend to crystallize in their stable wurtzite structure and thereby inhibit a pronounced columnar growth structure. The brittleness of ceramic-like coatings often negatively influences their performance especially when used in conditions with an increased need for crack resistance. Therefore, the influence of AlN layers (in their metastable cubic or hexagonal structure) on the fracture behavior of ceramic-like CrN/AlN multilayers is investigated. In-situ micro-compression, bending and tensile tests were conducted for such multilayers and compared to monolithic CrN. The study clearly demonstrates crack deflection, crack arrest and crack stop mechanisms for the fully cubic stabilized CrN/AlN multilayer coating and highest fracture stress which can be attributed to a strain induced phase transformation of the metastable cubic to the stable wurtzite AlN phase. In contrast CrN/AlN multilayers with mixed cubic/wurtzite structured AlN layers showed no crack deflection and lowest fracture stresses. The study shows extensive in-situ fracture tests in a micro-scaled range providing necessary information on the fracture behavior of hard coatings.
Translated title of the contributionMultilagen Design zur Verbesserung der Zähigkeitseigenschaften von auf CrN basierenden Schichten
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Keckes, Jozef, Assessor A (internal)
  • Mayrhofer, Paul Heinz, Supervisor (internal)
Publication statusPublished - 2012

Bibliographical note

embargoed until null

Keywords

  • multilayer
  • crack
  • toughness
  • CrN
  • AlN
  • compression test
  • tensile test
  • bending test
  • in-situ fracture test

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