Development of duplex nitrided and closed field unbalanced magnetron sputter ion plated CrTiAlN-based coatings for H13 aluminium extrusion dies
Introduction
There is increasing demand for extruded aluminium-based metal matrix composites (MMCs) with improved stiffness and wear resistance to meet a range of industrial applications. Wear of extrusion dies by the hot extrudate is technologically and economically significant, affecting both the dimensions and surface finish of the product [1]. Extrusion dies are conventionally fabricated from hot work tool steels, such as H13, and surface treated to reduce wear. MMCs containing significant quantities of hard, second-phase particulates place particular demands on the extrusion die. Critical mechanical and tribological characteristics must be maintained, in an environment of chemical and abrasive wear in the flowing aluminium MMC under conditions of high temperature and pressure. Many advanced applications demand the fine control of both the dimensional tolerance and surface finish of the extrudate, while maximising the productive lifetime of the associated die, and minimising scrap, setup costs, etc. The development of an advanced duplex treatment for H13 steel extrusion dies is described below. The treatment combines gas nitriding and closed field unbalanced magnetron sputter ion plating (CFUBMSIP) of high-performance CrTiAlN-based coatings. The intended application and the detailed laboratory testing of coated substrates, using the pin-on-disc and abrasive wear testing are also reported below.
Gaseous nitriding [2] is a well-established technique to increase the surface hardness, chemical and abrasive wear resistance of die steels. Nascent nitrogen is liberated by the thermal decomposition of gaseous ammonia, catalysed on the iron surface, and diffuses into the steel surface forming an insertion solid solution. The nitrided surface provides an effective foundation for a subsequent coating, producing a gradation in hardness and elasticity from the bulk alloy to the hard coating. The exceptional versatility of CFUBMSIP in the deposition of high-quality, well-adhered films has been recognised [3]. Losses of electrons from the discharge to the chamber walls are considerably reduced by the closure of the magnetic fields of two or more magnetrons, resulting in enhanced ionisation in the discharge and greater ion current density (ICD) at a biased substrate [4], [5]. The technique is readily compatible with the production of alloys or multilayers from a number of single element sources.
Ternary TiCrAl coatings have been recommended for improving the high temperature oxidation resistance of TiAl alloys [6] due to the so-called “Cr effect” which promotes the growth of a thin, stable passivating oxide scale. It is assumed that this beneficial effect may also accrue to similar nitride coatings. An increased CrN content (up to a maximum of 30 at.% Cr [8]) in a TiAlN/CrN arc-sputtered coating reduced the already very low oxidation rate (up to ∼900 °C) [7]. Huang et al. [9] found that nitrogen content in reactively deposited TiAlCr(N) films, sputtered from a Ti–50Al–12Cr alloy target, strongly influenced their hardness and crystal structure. Yang et al. [10] demonstrated that nano-structured CrTiAlN coatings, reactively deposited by CFUBMSIP, achieved high hardness and wear resistance, and that these properties were preserved following heat treatment in air at up to 900 °C for 3 h. The deliberate addition of an oxynitride layer can further reduce the friction of the resulting hard coating, and such cermets are readily produced by reactive magnetron sputtering [11].
The pin-on-disc testing geometry has been used to study the behaviour of a hard tool surface against a deformable workpiece [12]. Micro-abrasion testing [13], which allows the abrasion resistance of small volumes of material and thin coatings to be determined, is of increasing interest in characterising hard coatings. The application of both of these techniques, at room temperature, to investigate the potential of the CrTiAlN-based coating is described below. The results justify the future practical evaluation of these coatings in a demanding aluminium extrusion process.
Section snippets
Experimental
After machining to size, the H13 hot work die steel discs were heat treated to achieve a hardness of 4.8–4.9 GPa (i.e., 48–49 HRc). All the discs were then heated in a turbulent flow of nitrogen 540 °C, followed by a soak in an ammonia atmosphere (with flow to waste) for 4 h at 540 °C. The samples were cooled to 150 °C while still under ammonia, and finally cooled to room temperature in air before removal from the nitriding furnace. Previous detailed metallurgical examination of EN41 steel
Results
To determine the wear produced by the 6-mm-diameter Al–TiC MMC, flat-faced pins, profilometry traces of the worn 100-mm-diameter, nitrided, CrTiAlN- and CrTiAlOxNy-coated H13 discs were compared with those from the unworn specimens, and the results are shown in Fig. 2. The wear rate of the nitrided surface was estimated as 1.4×10−13 m3 m−1, i.e., a specific wear rate (SWR) of 4.7×10−15 m3 N−1 m−1 for the 30-N load. The wear of the coated discs was so small that it could not be measured by this
Discussion
The selection of a hard coating on a substrate is fundamental to obtaining the maximum performance for the coating/substrate/workpiece material system. The combination of the hard coating and a nitrided layer on extrusion dies is designed to resist abrasive and chemical wear [1] and the coating must retain high hardness, high toughness and good adhesion simultaneously, and at high temperatures, in the extrusion process.
To address the demanding set of coating requirements (high hardness, high
Conclusions
CrTiAlN-based coatings with a nano-multilayer structure have been successfully deposited by reactive CFUBMSIP on H13 hot work die steel in a duplex process, following gaseous thermal nitriding. The duplex approach produces a gradual transition in mechanical properties from the substrate to the hard coating surface, the benefits of which are further amplified by the judicious design of the hard coating itself, as exemplified, for example, by the coating's high adhesion.
Both the CrTiAlN and
Acknowledgements
The support of the EU under the following collaborative projects for several aspects of this study is gratefully acknowledged: AMMCEP (MMC extrusion, Contract No. G5ST-CT2002-50329), HIPERCUT (development of the CrTiAlN-based coating, Contract No. G5RD-CT-2000-00333) and CRATER (determination of abrasive wear, Contract No. G6RD-CT2000-00415). The PoD2 and AT-1 wear and abrasion test were kindly performed at Teer Coatings by Joanne Stallard and Stéphane Poulat, respectively.
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2019, Tribology International