Advanced TiAlTaN hard coatings for cutting tools

Birgit Großmann

Research output: ThesisDoctoral Thesis

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Abstract

Although many modern alternative manufacturing processes are on the rise, cutting represents still one of the most important shaping technologies. A demand of industry for higher cutting speed and therefore higher throughput results in increased thermal loading at the cutting edge. Hard, protective coatings in the thickness range of a few micrometers applied to the cutting tool lead to pronounced higher tool performance and lifetimes; however, the operation temperature up to 1000 °C or even higher is also demanding for the coating material, leading to oxidation and material softening. Therefore, enhancement of the oxidation resistance as well as thermal stability is a major goal. Ti1-xAlxN is a benchmark material system for hard coatings on cutting tools, providing good oxidation and wear resistance. Oxidation leads to formation of a porous TiO2-rich layer and a dense Al2O3-rich toplayer, which acts on the one hand as protection layer to hinder further oxidation, and on the other hand as wear protection due to its hardness. At elevated temperatures, age hardening processes due to spinodal decomposition of the metastable Ti1-xAlxN solid solution result in a hardness increase and peak performance of the cutting tool. However, the occurring phase transformation at temperatures above ~900 °C results in a deterioration of mechanical properties, and leads to pronounced oxidation, wear and finally tool failure. The addition of Ta to Ti1-xAlxN coatings is a promising approach to enhance the coating performance at elevated temperatures. Ta is known for its high melting point, yielding high hardness and thermal stability. In this thesis it was shown that an increasing Ta content in Ti1-x-yAlxTayN coatings stabilizes the metastable Ti1-xAlxN phase, and shifts the phase transformation to higher temperatures, which leads to enhanced thermal stability and high hardness values up to ≥1000 °C. The oxidation resistance was considerably increased, since the Ta atoms densify the porous TiO2-rich layer, which hinders diffusion of the oxidative agents. Additionally, Ta promotes the formation of lubricious oxides, causing a decrease of the friction coefficient. However, when the Ta content in Ti1-x-yAlxTayN exceeds a critical level, friction increases again. In summary, the addition of Ta is beneficial up to y~0.08 in Ti1-x-yAlxTayN hard coatings; however, even very small amounts (i.e., y≥0.01) yield significant improvements of thermal stability and oxidation resistance.
Translated title of the contributionHochentwickelte TiAlTaN Hartstoffschichten für Zerspanungswerkzeuge
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Schneider, Jochen, Assessor B (external), External person
  • Mitterer, Christian, Assessor A (internal)
Publication statusPublished - 2018

Bibliographical note

embargoed until null

Keywords

  • hard coatings
  • TiAlN
  • TiAlTaN
  • oxide layer
  • tribology
  • thermal stability
  • cutting tool

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