Effects of annealing temperatures on the morphological, mechanical, surface chemical bonding

37 Pages·2016·4.11 MB·English

by  M. Mahbubur Rahman

University of Birmingham

Effects of annealing temperatures on the
morphological, mechanical, surface chemical
bonding, and solar selectivity properties of
sputtered TiAlSiN thin films
Rahman, M. Mahbubur; Jiang, Zhong-tao; Zhou, Zhi-feng; Xie, Zonghan; Yin, Chun Yang;
Kabir, Humayun; Haque, Md. Mahbubul; Amri, Amun; Mondinos, Nicholas; Altarawneh,
Mohammednoor
DOI:
10.1016/j.jallcom.2016.02.077
License:
Creative Commons: Attribution-NonCommercial-NoDerivs (CC BY-NC-ND)
Document Version
Peer reviewed version
Citation for published version (Harvard):
Rahman, MM, Jiang, Z, Zhou, Z, Xie, Z, Yin, CY, Kabir, H, Haque, MM, Amri, A, Mondinos, N & Altarawneh, M
2016, 'Effects of annealing temperatures on the morphological, mechanical, surface chemical bonding, and solar
selectivity properties of sputtered TiAlSiN thin films', Journal of Alloys and Compounds, vol. 671, pp. 254-266.
https://doi.org/10.1016/j.jallcom.2016.02.077
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Accepted Manuscript
Effects of annealing temperatures on the morphological, mechanical, surface
chemical bonding, and solar selectivity properties of sputtered TiAlSiN thin films
M. Mahbubur Rahman, Zhong-Tao Jiang, Zhi-feng Zhou, Zonghan Xie, Chun
Yang Yin, Humayun Kabir, Md. Mahbubul Haque, Amun Amri, Nicholas Mondinos,
Mohammednoor Altarawneh
PII: S0925-8388(16)30346-2
DOI: 10.1016/j.jallcom.2016.02.077
Reference: JALCOM 36688
To appear in: Journal of Alloys and Compounds
Received Date: 21 December 2015
Revised Date: 5 February 2016
Accepted Date: 8 February 2016
Please cite this article as: M.M. Rahman, Z.-T. Jiang, Z.-f. Zhou, Z. Xie, C.Y. Yin, H. Kabir, M.M.
Haque, A. Amri, N. Mondinos, M. Altarawneh, Effects of annealing temperatures on the morphological,
mechanical, surface chemical bonding, and solar selectivity properties of sputtered TiAlSiN thin films,
Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.02.077.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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ACCEPTED MANUSCRIPT
1 Effects of annealing temperatures on the morphological, mechanical,
2 surface chemical bonding, and solar selectivity properties of sputtered
3 TiAlSiN thin films
4
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5 M. Mahbubur Rahman1*, Zhong-Tao Jiang1*, Zhi-feng Zhou2, Zonghan Xie3, Chun Yang
6 Yin4, Humayun Kabir5, Md. Mahbubul Haque6, Amun Amri7, Nicholas MondPinos1,
7 Mohammednoor Altarawneh8 I
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8 1 Surface Analysis and Materials Engineering Research Group
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9 School of Engineering & Information Technology, Murdoch University, Murdoch, Western
10 Australia 6150, Australia
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11 2Department of Mechanical and Biomedical Engineering, City University of Hong Kong,
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12 Kowloon, Hong Kong, China
13 3School of Mechanical Engineering, University of ANdelaide, SA 5005, Australia
14 4School of Science and Engineering, Teesside UniveArsity, Borough Road, Middlesbrough,
15 TS1 3BA, United Kingdom
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16 5School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham

17 B15 2TT,D United Kingdom
18 6Materials Science Division, AtomEic Energy Centre Dhaka, Dhaka 1000, Bangladesh
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19 7Department of Chemical Engineering, Riau University, Pekanbaru, Indonesia
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20 8School of Engineering & Information Technology, Murdoch University, Murdoch, Western
21 Australia 6150, Australia
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22 *Corresponding Authors: [email protected] (M.M. Rahman)
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23 [email protected] (Z.-T. Jiang)
24
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25
26 ABSTRACT
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27 Quaternary sputtered TiAlSiN coatings were investigated for their high temperature structural
28 stability, surface morphology, mechanical behaviors, surface chemical bonding states, solar
29 absorptance and thermal emittance for possible solar selective surface applications. The
30 TiAlSiN films were synthesized, via unbalanced magnetron sputtered technology, on AISI M2
31 steel substrate and annealed at 500 °C - 800 °C temperature range. SEM micrographs show
32 nanocomposite-like structure with amorphous grain boundaries. Nanoindentation analyses
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33 indicate a decrease of hardness, plastic deformation and constant yield strength for the
34 coatings. XPS analysis show mixed Ti, Al and Si nitride and oxide as main coating
35 components but at 800 °C the top layer of the coatings is clearly composed of only Ti and Al
36 oxides. Synchrotron radiation XRD (SR-XRD) results indicate various Ti, Al and Si nitride
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37 and oxide phases, for the above annealing temperature range with a phase change occurring
38 with the Fe component of the substrate. UV-Vis spectroscopy, FTIR spectroscPopy studies
39 determined a high solar selectivity, s of 24.6 for the sample annealed at 600 °C.I Overall results
40 show good structural and morphological stability of these coatings at tempeRratures up to 800
41 °C with a very good solar selectivity for real world applications.
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42 Keywords: magnetron sputtering; thin film coatings; optical properties; solar absorptance;
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43 solar emittance; selective solar surface.
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44 1. Introduction N
45 Transition metal nitride based quaternary TiAlSiN coatings are attractive candidates
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46 as cutting tools, protective and decorative coatings due to their many outstanding properties
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47 [1]. In recent years they have seen significant interest as solar selective absorbers for
48 harvesting solar energy in various applications such as thermal solar collectors, solar steam

49 generators and steam turbines for prodDucing the electricity at mid and mid-to high
50 temperatures [2-13]. A good selective surface must have high absorptance (α) in the visible
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51 spectrum up to 2.5 µm and low emittance (ε) in the infra-red (IR) region ≥ 2.5 µm at the
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52 operating temperatures. In recent years, transition metal nitride based tandem coatings (e.g.,
53 TiAlN/AlON, and TiAl/TiAlPN/TiAlON/TiAlO) have been suggested for use in solar selective
54 surfaces to be used in photothermal applications [4, 14]. Barshilia et al. [15] developed a high
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55 thermal stable TiAlN/TiAlON/Si N tandem absorber on a copper substrate for high
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56 temperature solar selective applications that exhibited an absorptance of 0.958 and an
57 emittance of 0.0C7. Until now these materials have not been commercially produced [4, 11,
58 16]. TransitAion metal oxides based thin film coatings with good optical properties have been
59 also developed for solar selective surface applications [17-22].
60 Most of the solar selective coatings exhibit good stability in a vacuum but in air they
61 have very limited thermal stability. However, in a high temperature or for long period
62 application purposes, these solar selective absorbers should have stable structural
63 configuration minimal degradations. The oxidation resistance behaviour of these coatings is
64 also very important as they are frequently exposed at high temperature atmospheres in air.
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65 The addition of Al and Si to the TiN coatings increases their oxidation resistance by forming
66 oxide layers around the surface which eventually work as a barrier for further oxygen
67 penetrations at high temperatures. The formation of amorphous phases also provides better
68 stability against degradation, corrosion and oxidation than that of crystalline metallic nitride
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69 phases. TiAlSiN coatings provide good thermal stability at temperatures above 800 °C [26,
70 27]. These coatings have been explored mostly for their extraordinary mechanicalP properties,
71 but as applications in solar selective surfaces in thermal collector devices Iare relatively
72 unexplored [28-31]. R
73 The synchrotron radiation X-ray diffraction (SR-XRD techniqueC is successfully used
74 to probe the crystalline and electronic structure of various systems in a wide range of fields
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75 such as physics, chemistry, environmental sciences, materials sciences, biology, medicine,
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76 and geophysics. The SR-XRD offers many advantages over the conventional laboratory
77 based XRD techniques such as: highly collimated and inteNnse photon beams, photon-energy
78 tune ability, exceptional photon wavelength resolutionA (∆λ/λ ≈ 2 × 10-4), polarization control,
79 coherence, very high signal-to-noise and signal-to-background ratio. The synchrotron
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80 techniques are extensively used in the identification of phases in compounds and unknown
81 structural forms developed during the synth esis processes of the films. In recent years these
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82 techniques successfully investigated the local electronic structure of metal nitride thin films
83 in pure and doped states [23-25]. E
84 To the best of our knowTledge, investigations on structural thermal stability and
85 oxidation resistance behaviours of these coatings via SR-XRD technique are yet to be
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86 established. This study addresses the temperature dependent surface morphology, mechanical
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87 properties, surface chemical bonding states, high temperature solar selective behaviours, and
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88 high temperature structural stability of magnetron sputtered TiAlSiN coatings via
89 mechanical, SEMC, XPS, UV-Vis and FTIR, and SR-XRD techniques.
90 A
91 2. Experimental techniques
92 2.1 Film deposition process
93 TiAlSiN films were deposited onto AISI M2 tool steel substrates via a closed field
94 unbalanced magnetron sputtering system (UDP650, Teer Coating Limited, Droitwich,
95 Worcestershire, UK). The magnetron sputtered system is equipped with a four-target
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96 configuration. Before coating, the substrates were ultrasonically cleaned in an acetone and
97 methanol solution and then dried using high purity nitrogen gas. A pressure of 0.24 Pa was
98 maintained in the chamber throughout the deposition process. Prior to synthesizing the
99 samples, all of the targets were cleaned properly and the steel substrates were safeguarded by
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100 shutters through the magnetrons. In order to achieve a homogeneous film thickness, the
101 substrate was rotated at a speed of 10 rpm throughout the synthesis process. TheP substrates
102 were sputter cleaned with Ar plasma at a bias of -450 V for 30 minutes. The Icoatings were
103 deposited in an Ar+N mixed gas atmosphere. The partial pressure of Ar anRd N were 0.133
2 2
104 Pa and 0.106 Pa throughout the deposition processes. The bias currents for Ti, Al and Si
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105 target were 5 A, 7 A, and 5 A, respectively. The final products were then annealed at 500,
106 600, 700 and 800 °C. Details of the deposition conditions of TiAlSSiN coatings are displayed
107 in Table 1. U
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108 2.1 Characterisations
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109 Scanning electron microscopy (SEM) was conducted to determine the surface
110 morphology of the thin film coatings with a high Mresolution microscope (SEM, PHILIPS XL
111 20, Eindhoven, The Netherlands). A secondary electron (SEI) detector was used to

112 characterize the overall surface morphologDy of thin film structures. The SEM machine was
113 operated at 25 kV. The samples were glued with double sided carbon tape affixed to the
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114 sample holders.
115 T

116 A depth-sensing indentation system (Ultra-Micro Indentation System, UMIS-2000,
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117 CSIRO, Australia) equipped with a Berkovich indenter was used to measure the Young’s
118 modulus and hardness ofE the coatings. The indenter tip was calibrated by conducting single-
119 cycling indentation tests on fused silica (a standard material having a known modulus of 72
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120 GPa) at different loads. Load–unload tests were run in a closed-loop under load control to
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121 determine the mechanical properties of the samples. A maximum load of 20 mN was applied
122 in 10 incremAents. The loading rate set to 2.5 mN/s represents the static response of the
123 materials. Following each increment there were 10 decrements, from which the Young’s
124 modulus and hardness were calculated. Five indentation tests were performed on each
125 specimen. The load vs displacement response obtained by nanoindentation is a very powerful
126 technique for analysing mechanical properties. The elastic behaviour of, and deformation
127 mechanisms within, the coating systems can be realized by analysing the nanoindentation
128 data. In order to investigate the thermal stability of the sputtered films, mechanical hardness
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129 of the coatings was tested with the samples before annealing and after being annealed
130 between 500 °C and 800 °C in steps of 100 °C in air atmosphere.
131

132 The XPS data will reveal information on chemical structure, elemental compositions
133 and bonding states in the outermost 5 nm surface of the films. The XPS measurements of the
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134 TiAlSiN coatings were taken using an XPS (Kratos Axis Ultra XPS spectrometer,
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135 Manchester, UK) machine operating with an Al-Kα monochromatic radiation (hν = 1486.6
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136 eV) source with a power of ~10 mA and ~15 kV. The XPS machine was also equipped with a
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137 cold stage, and an Ar ion gun for etching the coatings. The samples were mounted on a steel
138 sample holder and put in the analysis chamber where the pressure was Creduced to 2.9 × 10-9
139 Torr. The XPS survey spectra were collected before etching, and after 6 minutes etching. Ar
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140 ion sputtering was used for etching the films. Etching was done to remove any surface oxide
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141 layers and to reduce the Ti4+ ions in order to lower oxidation states. The high resolution XPS
142 data was recorded after a 6 minute Ar+ sputtering. The typNical high resolution XPS core level
143 spectra of TiAlSiN samples before and after being annAealed show Ti2p, Al2p, Si2p, N1s and
144 O1s energy regions. Information on the existence of various chemical bonding states was
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145 acquired by subtracting the background with the Shirley’s method and deconvoluting the
146 spectra with curve-fitting methods using CA SA XPS software (version 2.3.1.5) [32].
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147 The optical properties of the thin film coatings were analysed by measuring the
148 optical reflectance as a function of waEvelength from ultraviolet through the visible to infrared
149 range of the solar spectrum via UV-Vis and FTIR spectrometers. A double-beam UV–Vis
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150 spectrophotometer (Model: UV-670 UV-Vis spectrophotometer, JASCO, USA) equipped
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151 with a unique, single monochromator design covering a wavelength range from 250 to 2500
152 nm was used to measure Ethe solar absorptance of the coatings. The monochromator itself was
153 designed with 1200 grooves/mm grating and a photo multiplier tube (PMT) detector. A
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154 PERKIN Elmer Spectrum 100 FTIR Spectrometer (USA) was used for measuring the
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155 reflectance, R(λ), of the coatings in the wavelength range of 2.5 to 15 µm. Using the R(λ) data
156 the solar abAsorptance (a ) and the solar emittance (e) of a material can be conveniently
157 estimated [20]. For a range of solar wavelengths, the total solar absorptance, α is defined as a
158 weighted fraction between absorbed radiation and incoming solar radiation (I ), while
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159 thermal emittance, ε is defined as a weighted fraction between emitted radiation and the
160 Planck black-body distribution (I ). The figure of merit of a selective surface is expressed in
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161 term of a parameter known as the selectivity, s. The solar selectivity of a material is defined
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162 by the ratio of the solar absorptance, α to the thermal emittance, ɛ, with expression (s) given
163 in Ref. [20].
164 The SR-XRD measurements of the TiAlSiN coatings were carried out with the
165 powder diffraction (PD) beamline 10-BM-1 at the Australian Synchrotron, in Melbourne. The
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166 PD beamline operated at 2.5 GeV and used a bending magnet source. A continuous spectrum
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167 of photons were generated in the range 5–30 keV with a maximum flux of about 1013
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168 photons/sec and a beam size of 1 mm (W) × 8 mm (L) x 1 mm (H). The general beam size of
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169 the sample for the focused and unfocussed beam arrangements was 0.5 mm × 0.5 mm and 5
170 mm × 2 mm respectively. An X-ray wavelength of 0.8265 Å, estimated Cwith a LaB6 standard
171 NIST 660a, was used for the measurements. The high temperature system was an Anton Parr
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172 HTK20 furnace. The furnace was calibrated and monitored by a thermocouple and the
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173 heating and cooling cycles were remotely controlled via a computer interface. The high
174 resolution XRD data was acquired at room temperature, 50N0 °C, 600 °C, 700 °C, and 800 °C.
175 A MYTHEN II microstrip detector system was used tAo record the high resolution XRD data
176 in the range of 2θ = 15° to 35° in steps of 0.02°. At each detector setting SR-XRD data was
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177 acquired for a period of 5 minutes. Data collection was started after a 3 min thermal
178 equilibration period at which the control sen sor had reached the set temperature. The acquired
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179 data was processed with PDViPeR software for further analysis.
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180 3. Results and discussion
181 3.1 Surface morphology of TTiAlSiN films
182 SEM images of sputtered TiAlSiN coatings before annealing and after being annealed
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183 at 500-800 °C in steps of 100 °C are presented in Fig. 1. SEM examination of the
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184 microstructure showed nanocomposite-like structure deformed by formation of an amorphous
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185 phase and facilitated by fine grains. As the annealing temperature progresses from 500 to 600
186 °C, there originaCted some random pores around the surface of the coatings together with non-
187 uniform grains. At the same time, the surface morphology and shape of the grains of the films
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188 were altered together with the rise in irregular mound like structures around the film’s
189 surfaces. Further inspection confirms that the higher surface roughness is attained by the
190 coatings as the annealing temperature is increased. This surface roughness hinders the
191 diffusion of titanium and nitrogen atoms and slows down their diffusions, helps to grain
192 growth and develop amorphous like structures around the grain boundaries. However, the
193 new phase is extremely small in size and cannot be detected by either XRD or SEM. Because
194 of this lower mobility, the crystallization process is disrupted and disordering is enhanced for
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195 further amorphization by Si N phases around the boundaries. This amorphous grain
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196 boundaries influence the optical properties of these coatings [33]. The annealed films
197 exhibited nanolayers which are observed as darker and lighter contrasts in the SEM
198 micrographs together with the fine-grained structure of the coatings. This is due to the
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199 systematic increase on the oxide thickness growing on top of the coating with the rise in
200 annealing temperatures. The oxides layers which are observed are composed ofP Al2O3 and
201 SiO2 at the outermost layer of the coating followed by TiO2 layers (as seen inI XPS studies).
202 These nano-sized protective layer formed on the nitride film hinders the diffRusion of oxygen
203 into the coating due to their small size and polycrystalline nature. These nanolayers did not
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204 vanish or alter their structure as a consequence of the heat treatment; rather, they seem to be
205 very stable [34]. The dense and closely packed structures show Smore stability as selective
206 surfaces at high temperatures. These kind of sputtered coatingsU are more reliable than similar
207 coatings with large columns and open pores [35]. These microstructural features have
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208 substantial impacts on the residual stress and deformation mechanisms in the coatings and
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209 could assist with the interpretation of mechanical hardness of these coatings.
210 M
211 3.2 Mechanical properties of TiAlSiN coatings before annealing and after being

212 annealed D
213 The hardness, H and Young’s modulus, E values of the sputtered TiAlSiN coatings
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214 before and after annealing are presented in in Fig. 2. Table 2 represents the hardness, elastic
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215 modulus, Yield strength and corresponding plastic deformation resistance of the TiAlSiN
216 coatings as acquired from theP nanoindentation measurements. Before annealing, the TiAlSiN
217 coating showed a hardneEss of 28.1 ± 0.6 GPa and an elastic modulus of 369 ± 9 GPa. Similar
218 values for hardness and modulus have been reported in other studies [36-38].
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219

220 The high hardness and high elastic modulus of TiAlSiN coatings before annealing
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221 arises due to the biaxial compressive stress developed by high energetic ions bombardment
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222 during the coating deposition. Effects of biaxial compressive stress on the hardness of
223 superhard multilayer coatings were examined by Veprek et al [39-41]. An increase in the
224 development of the intergranular amorphous phase in the nanocomposite matrix leads to a
225 definite enhancement of the hardness of the coatings. This is also true of the elastic modulus
226 values. At nano-scale level, the obstruction of dislocation movements and the nonappearance
227 of dislocations also contribute to enhance the hardness of the coatings [42]. The dissolution
228 of Al and Si atoms in the TiN crystallites produce solid solutions that are also responsible for
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229 the greater hardness of these coatings [43, 44]. In addition, the grain boundary strengthening
230 at interphase boundaries also predominate to a higher hardness in the films [37, 45].
231 According to Veprek [39-41], a superhard microstructure with high elasticity and thermal
232 stability can be formed by fine nanocrystallites (<10 nm) enclosed by a thin amorphous phase
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233 (<1 nm) along with a strong boundary between the amorphous and crystalline phases. This
234 interface successfully hinders the grain boundary sliding’s, the formation of dislocPations and
235 their movements, and the spread of cracks. Generally a hard material hIas high wear
236 resistance. The material’s hardness and elastic modulus are also related to thRe wear behavior
237 [46]. It is known that the wear of a material is coupled with the elastic limit of materials that
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238 define the ability of a material to deform under an applied stress and regain its initial state
239 without being deformed permanently [46]. Experimental valuSe of the elastic limit is
240 expressed by the H/E ratio. The H3/E2 ratio, which is known as Uthe plastic deformation factor,
241 indicates the coating’s resistance to plastic flow [47]. Each of the parameters play an essential
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242 role in the coating wear behaviors. The linear decrease of H, H/E and H3/E2 are well
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243 correlated with the reduction of wear resistance of TiAlSiN coatings during high temperature
244 annealing. A high value of the H3/E2 ratio of a coMating means that it offers high resistance to
245 plastic deformation. This high value of the H3/E2 ratio is also associated with hindering

246 dislocation formation or movement due toD nano-grains and the solid impeding of the crack
247 dissemination in the amorphous Si N phase [48]. From an engineering application
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248 viewpoint, coating hardness must be accompanied by appropriate toughness and the ability of
249 a material to absorb energy underT an applied stress until fracture [49]. Musil and Jirout [50]
250 reported their findings that resistance against crack formation of thin film coatings increases
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251 with increasing H3/E2 ratio. From Table 2, it can be seen that both the hardness and elastic
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252 modulus strongly depend on the annealing temperatures. The hardness decrement of the
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253 TiAlSiN coatings at elevated annealing temperatures may result from diverse mechanisms.
254 Since the coatinCgs were heated in air, the composition at the surface is expected to change
255 due to oxidation, and therefore, it is reasonable to have lower hardness as the annealing
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256 temperature is enhanced. At high temperature annealing, the reduction in hardness and elastic
257 modulus values of TiAlSiN coatings may be also ascribed to the softening of steel substrates,
258 changes in composition of the films due to the diffusion, formation of new phases and stress
259 relaxations. High temperature annealing causes grain refinement, and when combined with
260 the formation of point defects, interstitials, biaxial compressive stress, and other effects of the
261 ion bombardment may result in a reduction of hardness and elastic modulus as the
262 temperature is increased. According to Petrov et al. and Barna et al., oxygen and other