High temperature new hard nanocomposite coatings: study of tribofilm formation and effect of added elements on spinodal mechanism


CALL: 2009

DOMAIN: MS - New Functional and Intelligent Materials and Surfaces

FIRST NAME: Nathalie





KEYWORDS: Tribochemistry, nanocomposite hard coating, spinodal decompo

START: 2010-05-01

END: 2013-04-30

WEBSITE: https://www.list.lu/

Submitted Abstract

Although Ti-Al-N coatings are widely used in industry, their good wear resistance at high temperature is not sufficient for specific applications like dry high speed machining of composite materials. Different alloying elements were then added to improve wear and oxidation resistance at high temperature of such coatings. Among the alloying elements, Si and Ta are particularly interesting for high temperature applications. They lead to nanocristalline structure which consists of 3–10 nm nanocrystals of a hard transition metal nitride such as TiAlN surrounded by an amorphous monolayer Si3N4 phase which gives the cohesion strength to the system. Even if significant progress on the development of such quaternary coatings was achieved in the past five years at laboratory scale, the correlation between the kinetics of the growth mechanisms, the microstructure and properties/performances of these coatings, especially at high temperature, is not yet fully understood.In order to get better wear and oxidation properties at high temperature (>850°C), we propose an original approach consisting in the addition of one Reactive Element (Y, La..) or Cr in Al-Ti-Ta-N-based nanocomposite coatings. The idea is to use doping to improve the chemical de-mixing energy of the nanocristalline (e.g. (Ti,Al)N) phase and the amorphous phase (e.g. TaN), to increase the coherence of the interface between amorphous and crystalline phase and thus to improve the chemical coating stability and tribological properties at high temperature. The main effects of RE known for alloys are to decrease the outward diffusion of metals in the grains boundaries, limiting the oxidation kinetics, and to favour the formation of finer oxide grains leading to better mechanical properties of the oxide scales. An optimized addition of a few atomic percent is expected to prevent the formation of rutile and favour the formation of Al2O3, which can act as barrier layer. Although the positive effects of RE or Cr, have already been demonstrated on oxidation resistance of alloys, they have never been verified on nanocomposite hard coatings such as Al-Ti-Ta-N. This project aims at verifying if the results obtained on alloys are applicable to Al-Ti-Ta-N coatings. In particular, the influence of the nature and the quantity of the added element on the coating growth mechanism and the formation of the nanocomposite structure will be studied.This study will focus on the elaboration of such a new category of Al-Ti-Ta-N-based nanocomposite coatings by magnetron sputtering from elemental high-purity targets in the reactive mode (Ar/N2) and their characterization in their initial, oxidized and worn states using conventional analytical techniques (TEM, XPS, XRD, Auger, GDOES,..). Moreover, the potentialities of Atom Probe and new generations of SIMS instruments (optimized lateral or depth resolution) will be used to investigate the distribution of RE and Cr, the tribofilms and wear tracks. By studying microstructure, composition, high temperature oxidation and wear resistance of the deposited coatings, the nanostructure, which generated the best-performing coatings will be identified. Moreover, this study will give us a deeper knowledge for the growth of nanocomposite coatings, the mechanisms of oxidation and the tribofilm formation.

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