Stress-Dependent Elasticity of TiAlN Coatings

Marcus Hans,Damian M Holzapfel,Daniel Primetzhofer,Bastian Stelzer,Simon Evertz,Jochen M Schneider,Denis Music,Beno Widrig,Volker Schnabel,Bernhard Völker,Lena Patterer,Anders O Eriksson,Helmut Rudigier,Mirjam Arndt,Jürgen Ramm

doi:10.3390/coatings9010024

Abstract

We investigate the effect of continuous vs. periodically interrupted plasma exposure during cathodic arc evaporation on the elastic modulus as well as the residual stress state of metastable cubic TiAlN coatings. Nanoindentation reveals that the elastic modulus of TiAlN grown at floating potential with continuous plasma exposure is 7%–11% larger than for coatings grown with periodically interrupted plasma exposure due to substrate rotation. In combination with X-ray stress analysis, it is evident that the elastic modulus is governed by the residual stress state. The experimental dependence of the elastic modulus on the stress state is in excellent agreement with ab initio predictions. The macroparticle surface coverage exhibits a strong angular dependence as both density and size of incorporated macroparticles are significantly lower during continuous plasma exposure. Scanning transmission electron microscopy in combination with energy dispersive X-ray spectroscopy reveals the formation of underdense boundary regions between the matrix and TiN-rich macroparticles. The estimated porosity is on the order of 1% and a porosity-induced elastic modulus reduction of 5%–9% may be expected based on effective medium theory. It appears reasonable to assume that these underdense boundary regions enable stress relaxation causing the experimentally determined reduction in elastic modulus as the population of macroparticles is increased.

Highlights

Metastable cubic transition metal aluminum nitrides such as TiAlN have been well known as hard protective coatings for more than 30 years [1], and are still used nowadays as state-of-the-art materials for cutting and forming applications
The chemical composition of TiAlN coatings with 5 μm thickness, synthesized on sapphire substrates with periodically interrupted and continuous plasma exposure, was identified as 27 ± 2 at.% Ti, 24 ± 2 at.% Al, 48 ± 2 at.% N and
We have investigated the effect of continuous vs. periodically interrupted plasma exposure during cathodic arc evaporation on the elastic modulus as well as the residual stress state of metastable cubic

Summary

Introduction

Metastable cubic transition metal aluminum nitrides such as TiAlN (space group Fm3m, NaCl prototype) have been well known as hard protective coatings for more than 30 years [1], and are still used nowadays as state-of-the-art materials for cutting and forming applications. Coatings 2019, 9, 24 design parameter density functional theory-based predictions at the atomic scale with mechanical testing techniques such as nanoindentation It has been demonstrated for Cr0.8 Al0.2 N that a compressive stress state of −4 GPa results in an elastic modulus increase of 150 GPa, when compared to the stress-free material [4]. Very often, rotating substrates are assembled in industrial batch production plants in order to coat as many substrates as possible in a single deposition process In such a deposition geometry, the growing coating surface is periodically moved in and out of regions of high plasma density characterized by large fluxes of film-forming species. In the present work we establish the relationship between deposition geometry and mechanical coating properties by means of elasticity, as well as the residual stress state of TiAlN

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Results

Conclusion