Thermal dissolution mechanisms of AlN/CrN hard coating superlattices studied by atom probe tomography and transmission electron microscopy Darius Tytko, ⇑ Pyuck-Pa Choi ⇑ and Dierk Raabe Max-Planck-Institut fu ¨ r Eisenforschung GmbH, Max-Planck Str. 1, 40237 Du ¨ sseldorf, Germany Received 28 July 2014; revised 15 October 2014; accepted 3 November 2014 Abstract—AlN/CrN superlattices with a B1 cubic crystal structure and a bilayer period of 4 nm were deposited by reactive radiofrequency magnetron sputtering. The coatings were investigated with respect to their thermal stability and changes in microstructure and chemical composition at 900 °C. The AlN layers show high chemical stability but undergo dissolution by pinching off at grain boundaries. A transformation from cubic to hexagonal AlN with subsequent coarsening at grain boundary triple junctions is observed. In contrast to AlN, the CrN layers show poor chemical stability and their compositions are shifted towards Cr 2 N upon annealing in a protective argon atmosphere due to nitrogen loss. However, even after establishing Cr 2 N stoichiometry the crystal structure of the layers remains cubic. Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: AlN/CrN; Hard coatings; Thermal stability; Layer pinch-off; Atom probe tomography 1. Introduction Thin films based on metal nitrides show superior hardness and wear resistance and are therefore of great interest as protective coating layers [1–5]. Such coatings are important for cutting tool applications, which require high performance as well as extended lifetimes. In order to meet the increasing demands for material properties (e.g. wear, corrosion and oxidation resistance at elevated temperatures) novel material systems need to be explored. It has been found that nanoscale multilayer hard coat- ings result in further improvement of hardness as well as corrosion and oxidation resistance as compared to their single-layered counterparts [3,4]. However, in some dry- cutting applications temperatures up to 1000 °C may arise and lead to deterioration of the cutting tool performance. This is because multilayered coatings are susceptible to microstructural changes and layer dissolution at elevated temperatures. AlN/CrN multilayers are promising materials for cut- ting tool applications due to their outstanding hardness of 40 GPa [6,7] and remarkable oxidation resistance [6,8–12]. Although the oxidation and tribological [13,14] resistance of these multilayers have been intensively stud- ied, only a few reports exist on their thermal stability [9,14]. Duh and Tien [9] investigated the thermal stability of AlN/CrN superlattices with a bilayer period of 4 nm by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Their XRD results indicated a pre- served multilayer structure after annealing at 850 °C for 1 h in vacuum as well as in air. No structural information could be gathered after annealing at 900 °C for 1 h due to delamination of the coating from the substrate. However, XRD results obtained by Mayrhofer et al. [14] on similar AlN/CrN multilayers showed minor changes of the coat- ing microstructure after exposure to 900 °C for 20 min. Since typical application times of such hard coatings may exceed this limited time frame, we have used atom probe tomography (APT) in conjunction with TEM to study the microstructural changes of AlN/CrN multilay- ers after isothermal exposure to 900 °C also at longer times. In general, multilayered materials are susceptible to layer dissolution due to their high density of internal interfaces. At grain boundaries (GBs) at elevated temper- atures the layer dissolution mechanism is controlled by interfacial (c i ) and GB (c GB ) energies, where the layer with the higher c GB /c i ratio is prone to become interrupted at the GB. This effect is often referred as layer “pinch-off” [15–17]. In this work we will reveal that dissolution of the AlN/ CrN structure occurs by AlN layer pinch-off at GBs. The resulting nanostructure after exposure to 900 °C consists of nanocrystalline hexagonal AlN and cubic Cr 2 N/AlN layers. http://dx.doi.org/10.1016/j.actamat.2014.11.004 1359-6462/Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. Tel.: +49 211 6792 180 (D. Tytko). Tel.: +49 211 6792 167 (P.-P. Choi); e-mail addresses: d.tytko@mpie. de; p.choi@mpie.de Available online at www.sciencedirect.com ScienceDirect Acta Materialia 85 (2015) 32–41 www.elsevier.com/locate/actamat