Elsevier

Journal of Alloys and Compounds

Volume 671, 25 June 2016, Pages 254-266
Journal of Alloys and Compounds

Effects of annealing temperatures on the morphological, mechanical, surface chemical bonding, and solar selectivity properties of sputtered TiAlSiN thin films

https://doi.org/10.1016/j.jallcom.2016.02.077Get rights and content

Highlights

  • TiAlSiN sputtered coatings were characterized for solar selective applications.

  • In situ synchrotron radiation XRD were studies show the occurrence of multiple stable phases.

  • A high selectivity of 24.63 has been achieved for the coatings annealed at 700 °C.

  • Existence of XRD phases were also confirmed by XPS measurements.

  • At high temperature annealing the mechanical properties of films were governed by the utmost surfaces of the films.

Abstract

Quaternary sputtered TiAlSiN coatings were investigated for their high temperature structural stability, surface morphology, mechanical behaviors, surface chemical bonding states, solar absorptance and thermal emittance for possible solar selective surface applications. The TiAlSiN films were synthesized, via unbalanced magnetron sputtered technology, on AISI M2 steel substrate and annealed at 500 °C - 800 °C temperature range. SEM micrographs show nanocomposite-like structure with amorphous grain boundaries. Nanoindentation analyses indicate a decrease of hardness, plastic deformation and constant yield strength for the coatings. XPS analysis show mixed Ti, Al and Si nitride and oxide as main coating components but at 800 °C the top layer of the coatings is clearly composed of only Ti and Al oxides. Synchrotron radiation XRD (SR-XRD) results indicate various Ti, Al and Si nitride and oxide phases, for the above annealing temperature range with a phase change occurring with the Fe component of the substrate. UV–Vis spectroscopy, FTIR spectroscopy studies determined a high solar selectivity, s of 24.6 for the sample annealed at 600 °C. Overall results show good structural and morphological stability of these coatings at temperatures up to 800 °C with a very good solar selectivity for real world applications.

Introduction

Transition metal nitride based quaternary TiAlSiN coatings are attractive candidates as cutting tools, protective and decorative coatings due to their many outstanding properties [1]. In recent years they have seen significant interest as solar selective absorbers for harvesting solar energy in various applications such as thermal solar collectors, solar steam generators and steam turbines for producing the electricity at mid and mid-to high temperatures [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. A good selective surface must have high absorptance (α) in the visible spectrum up to 2.5 μm and low emittance (ε) in the infra-red (IR) region ≥ 2.5 μm at the operating temperatures. In recent years, transition metal nitride based tandem coatings (e.g., TiAlN/AlON, and TiAl/TiAlN/TiAlON/TiAlO) have been suggested for use in solar selective surfaces to be used in photothermal applications [4], [14]. Barshilia et al. [15] developed a high thermal stable TiAlN/TiAlON/Si3N4 tandem absorber on a copper substrate for high temperature solar selective applications that exhibited an absorptance of 0.958 and an emittance of 0.07. Until now these materials have not been commercially produced [4], [11], [16]. Transition metal oxides based thin film coatings with good optical properties have been also developed for solar selective surface applications [17], [18], [19], [20], [21], [22].

Most of the solar selective coatings exhibit good stability in a vacuum but in air they have very limited thermal stability. However, in a high temperature or for long period application purposes, these solar selective absorbers should have stable structural configuration minimal degradations. The oxidation resistance behavior of these coatings is also very important as they are frequently exposed at high temperature atmospheres in air. The addition of Al and Si to the TiN coatings increases their oxidation resistance by forming oxide layers around the surface which eventually work as a barrier for further oxygen penetrations at high temperatures. The formation of amorphous phases also provides better stability against degradation, corrosion and oxidation than that of crystalline metallic nitride phases. TiAlSiN coatings provide good thermal stability at temperatures above 800 °C [26], [27]. These coatings have been explored mostly for their extraordinary mechanical properties, but as applications in solar selective surfaces in thermal collector devices are relatively unexplored [28], [29], [30], [31].

The synchrotron radiation X-ray diffraction SR-XRD technique is successfully used to probe the crystalline and electronic structure of various systems in a wide range of fields such as physics, chemistry, environmental sciences, materials sciences, biology, medicine, and geophysics. The SR-XRD offers many advantages over the conventional laboratory based XRD techniques such as: highly collimated and intense photon beams, photon-energy tune ability, exceptional photon wavelength resolution (Δλ/λ2×104), polarization control, coherence, very high signal-to-noise and signal-to-background ratio. The synchrotron techniques are extensively used in the identification of phases in compounds and unknown structural forms developed during the synthesis processes of the films. In recent years these techniques successfully investigated the local electronic structure of metal nitride thin films in pure and doped states [23], [24], [25].

To the best of our knowledge, investigations on structural thermal stability and oxidation resistance behaviors of these coatings via SR-XRD technique are yet to be established. This study addresses the temperature dependent surface morphology, mechanical properties, surface chemical bonding states, high temperature solar selective behaviors, and high temperature structural stability of magnetron sputtered TiAlSiN coatings via mechanical, SEM, XPS, UV–Vis and FTIR, and SR-XRD techniques.

Section snippets

Film deposition process

TiAlSiN films were deposited onto AISI M2 tool steel substrates via a closed field unbalanced magnetron sputtering system (UDP650, Teer Coating Limited, Droitwich, Worcestershire, UK). The magnetron sputtered system is equipped with a four-target configuration. Before coating, the substrates were ultrasonically cleaned in an acetone and methanol solution and then dried using high purity nitrogen gas. A pressure of 0.24 Pa was maintained in the chamber throughout the deposition process. Prior to

Surface morphology of TiAlSiN films

SEM images of sputtered TiAlSiN coatings before annealing and after being annealed at 500–800 °C in steps of 100 °C are presented in Fig. 1. SEM examination of the microstructure showed nanocomposite-like structure deformed by formation of an amorphous phase and facilitated by fine grains. As the annealing temperature progresses from 500 to 600 °C, there originated some random pores around the surface of the coatings together with non-uniform grains. At the same time, the surface morphology and

Conclusions

State-of-the-art magnetron sputtered TiAlSiN thin films, on a M2 steel substrate, were investigated, before and after annealing over temperature range of 500 °C–800 °C, for their structural, microstructural morphology, mechanical properties, surface chemical bonding states and solar selective behaviors. Images from SEM analysis show nanolayers forming dense and closely packed structures with amorphous grain boundaries. Increasing annealing temperature results in higher surface roughness of the

Acknowledgments

This research was supported by School of Engineering & Information Technology at Murdoch University. The authors gratefully acknowledge funding by the Australian Synchrotron beamtime award AS141/PD/7582. M. Mahbubur Rahman gratefully acknowledges Murdoch University for providing with the financial support under the Murdoch International Postgraduate Research Scholarship (MIPRS) program to carry out his PhD studies.

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