MICROSTRUCTURAL AND TRIBOLOGICAL CHARACTERIZATION OF TIN COATED ALUMINIUM ALLOY (AL6061)

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1 MICROSTRUCTURAL AND TRIBOLOGICAL CHARACTERIZATION OF TIN COATED ALUMINIUM ALLOY (AL6061) Anil Kumar H C 1, N.K. Udayashankar 2, Sudheendra P 3, H.S. Hebbar 4 1 Dept. of Mechanical Engineering, Anjuman Engg. College Bhatkal, 2 Dept. of Physics, 3 Dept. of Materials & Metallurgical Engineering, 4 Dept. of Mechanical Engineering, National Institute of Technology, Karnataka- Surathkal, Corresponding e - mail address: satwale@yahoo.com, anilkumarhc@yahoo.co.in. Abstract: Titanium Nitride (TiN) was deposited on aluminium alloy Al6061 using reactive DC magnetron sputtering technique. X-Ray diffraction and EDAX confirmed the presence of TiN phase in the coating. Optical microscopy showed the golden bronze coloured TiN coating at a N 2 /Ar ratio of Maximum composite Microhardness hardness of 2210 (HK) was obtained at 5g load for the coating deposited at a ratio (N 2 /Ar) of The dry sliding wear behaviour was studied by pin-on-disc machine. Oxidation wear prevailed during the sliding test. Coatings deposited at a ratio (N 2 /Ar) of 0.47 and 0.60 showed better wear resistance as compared to uncoated specimens. Keywords: Titanium Nitride; Aluminium (Al6061); DC Magnetron sputtering INTRODUCTION Aluminum alloys are used in many applications due to their excellent properties such as high strength-to-weight ratio, good ductility, easy availability and low cost [1]. However surface properties of aluminium alloy such as the hardness and wear resistance are poor and do not stisfy the tribological requirements in weight critical applications like aircraft and automobiles. Surface coating with ceramics can be a promising approach to this problem. The nitride compounds of Titanium (TiN x ) are the unique materials exhibiting both metallic (Ti-Ti) and covalent (Ti-N) bond characteristics. The covalent properties like high melting point, extreme hardness, brittleness, excellent thermal and chemical stability [2] are observed in the TiN coatings. Progress in the development and utilization of thin wear resistant coatings has been particularly marked over the past ten years, lead mainly by the rapid technical advances in Physical Vapour Deposition (PVD) processes [3]. The most widely used process for the PVD coating of thin films is Sputtering. In this present work, TiN coatings were deposited with different N 2 /Ar ratios on aluminium alloy Al6061 by reactive dc magnetron sputtering. The deposited film was characterized using Optical microscope, X- Ray Diffraction, SEM and EDAX. The aim of this work is to study the effect of TiN coatings on microhardness and wear behaviour of the aluminium alloy Al6061. EXPERIMENTAL DETAILS Deposition, Composition and Morphology TiN coatings were deposited on aluminium Al6061 and glass substrate by reactive dc magnetron sputtering technique using VRTSU04D model. Samples of Al6061 were machined to 8 mm 8 mm 8 mm size and mirror polished. Along with these samples glass substrate (25 mm 75 mm 1 mm) were used for the study of the optical properties of the coated films. The prepared samples and glass substrate were precleaned and kept in coating chamber. Titanium (99.7% 2 dia.) was used as the sputtering target material. Argon ( %) and Nitrogen (99.99%) were used for sputtering at different input ratios. Initially a vacuum of 10-5 mbar was created in the chamber. N 2 and Ar gas mixture was allowed in the chamber with working pressure of mbar. Flow rate of the Ar was kept constant at 14ml/min, while that of N 2 was varied to obtain ratios of 0.3, 0.47 and 0.60 keeping other parameters constant. Substrate to target distance was maintained at 40 mm. and the substrates were preheated to 230º C. Target voltage was maintained between -375 V to -400 V. Then the deposition was carried out for 60 minutes in all the trials. The composition of the coated film was analyzed using X-ray diffractometer (Bruker D8) with Cu Kα radiation (λ= ǻ) at an advancing angle of 2º/min and a voltage 30 kv and current 20 ma was used. 73

2 The surface morphology of the coated films were studied by using optical microscope and Scanning Electron Microscope (SEM) (JEOL JSM6380LA). Optical microscope with colour imaging camera was used to investigate the colour variation of the coated film at different N 2 /Ar ratios. The elemental analysis was carried out by using EDAX feature of the SEM. Microhardness tests The composite microhardness of the uncoated and TiN coated Al6061 specimens, at different N 2 /Ar ratio were measured using CLEMEX microhardness tester with Knoop indentation. The load was varied as 5 g, 10 g, 25 g, 50 g, 100 g with a dwell time of 13 seconds. Sliding tests Dry sliding wear test was performed on uncoated and TiN coated samples of Al6061 using Pin-On-Disc (TR-201CL) machine with a variable load of 200 g to 1000 g and sliding velocity of 0.4 m/sec. All the experiments were carried out at room temperature. Hardened steel was used as the counterface material. Test load and sliding velocity was varied for each type of coating and the results were compared with uncoated samples of Al6061. The morphology of wear scars and debris were observed by optical microscope and scanning electron microscope. RESULTS AND DISCUSSION Deposition, composition and morphology TiN coatings were deposited on aluminium Al6061 with a gas (N 2 /Ar) ratio of 0.30, 0.47 and The colour shift of titanium nitride film depends mainly on the ratio of N 2 /Ar composition with Titanium sputter target [4]. Optical microscope showed the colour change in the coating from yellowish grey to golden bronze to violet grey with increase in N 2 content as shown in figure1. A golden bronze colour was observed for TiN deposited at a gas ratio of 0.47 (N 2 /Ar) which is a typical colour for TiN [4]. Fig. 1. Optical images at N 2 /Ar a)0.3 b)0.47 c)0.6 X-ray diffraction pattern for coating deposited at N 2 /Ar =0.3 shows a number of peaks from 2θ = 36.97º to 37.35º corresponding to (111) plane which indicates presence of Ti, TiN, TiAlN phases. The lattice parameter varies from to

3 nm. Figure 2b shows a sharp peak along (111) direction and a low intensity peak along (200). TiN formed shows preferred orientation along (111) plane. The calculated lattice parameter of the coated film was nm which is almost incongruence with the lattice parameter (0.424 nm) of the stoichiometric TiN [5]. Figure 2c shows complete absence of (111) plane and presence of number of peaks along (200) plane. With increasing N 2 flow the orientation of the film was changed from (111) to (200) which is consistant with the similar reports [6, 7]. a b c Fig.2. XRD pattern of coatings at N 2 /Ar a) 0.30, b) 0.47, c) The microstructural characterization of the coated film was carried out using SEM. Figure 3a reveals a number of oval shaped bubble like structures of about 2 to 5µm size on the coated film which contains composite phases of Titanium and Nitrogen along with Aluminium. This was confirmed by EDAX (figure 3b) carried out on the coated films. The compositions of these films were Titanium (61.22At%) and Nitrogen (36.23At%) along with the traces of Al (2.55At%). It was observed that as the amount of N 2 increased these bubbles or ovalities are reduced as shown in figure 3c. The same figures also shows ridges formed over the surfaces. These ridges seen on the coated surface might be formed due to substrate ridges or irregularities present on the surface of the substrate. Figure 3d shows the EDAX of coating with a gas ratio of 0.47 (N 2 /Ar) with higher percentage of N (48.64At%). Also in the same film it was observed that Al content (0.69At%) is very less and the coating consisted mainly of TiN. Figure 3e shows the structure of the film coated at a N 2 /Ar ratio of At this ratio less number of sputtered particles were observed. EDAX (figure3f) taken over the particle area shows an Al (3.41At%). Aluminium percentage is reduced over the matrix portion and oxygen being nil in the sample. The coating mainly consist of TiN with TiAlN particles with higher percentage of nitrogen. 75

4 a CPS OKa TiLa NKa TiLl AlKa TiKa TiKb µm kev b TiKa c CPS TiLl TiLa NKa OKa AlKa TiKb d kev TiKa e CPS TiLa NKa TiLl AlKa TiKb f µm kev 76

5 Fig.3. SEM images and EDAX spectra respectively for TiN coated Al6061 with N 2 /Ar a),b) 0.3, c),d) 0.47,e),f) 0.6 Microhardness test Composite microhardness of the TiN coated Al6061 was found using microhardness tester for different N 2 /Ar ratios and compared with uncoated Al6061 as shown in figure 4. The column chart in figure 4 shows there is an increase in microhardness with N 2 /Ar ratio. TiN coating on Al6061 at N 2 /Ar = 0.6 showed a maximum composite microhardness of 2210 HK with 5g load, followed by 2129 HK for N 2 /Ar = The high hardness values for the above coatings are due to the presence of substantial amount of TiN phase. Fig 4. Composite micro-hardness (at 5g) of uncoated and TiN coated Al6061 at different N 2 /Ar ratios. Sliding test Dry sliding wear test showed the tribological behaviour of TiN coated Al6061 samples with gas ratios (N 2 /Ar) of 0.3, 0.47 and 0.6 and uncoated Al6061 as shown in figure 5. TiN coated Al6061 with N 2 /Ar = 0.6 shows minimum specific wear rate as compared to other samples and uncoated Al µm The high wear resistance is due to the higher micro hardness of the TiN coated sample as mentioned in microhardness test results. Fig. 5.Variation of Specific wear rate with test load for different N 2 /Ar. Wear is caused by gradual removal of the coating by microcutting and due to the formation of oxides. EDAX performed on the wear debris showed presence of oxygen and transferred carbon from the counterface. TiN oxidation leads to the formation of titanium oxide [8], since the free energy of formation for titanium oxides are much more favourable than for TiN [9,10]. The poor performance of the coating at N 2 /Ar=0.3 is due to the poor adhesion of the coating to the substrate. Visual observation showed peeled and delaminated coating which might have occurred due to compressive stresses in the coating. a) b) 77

6 30 µm c) 0.2 mm Fig6. SEM images of Wear debris produced by sliding coated Al6061 against hardened steel, N 2 /Ar a) 0.3 b) 0.47 c) 0.6 CONCLUSIONS X-ray diffraction and EDAX confirmed the presence of TiN and other phases in the coating obtained by varying the N 2 /Ar ratio. Visual and microscopic observation revealed the golden colour of TiN at N 2 /Ar=0.47. All the coated samples showed higher composite hardness as compared to uncoated ones. A max. of 2210(Hk) composite hardness at 5g load has been reported for samples coated at N 2 /Ar=0.6.Specific wear rate is mainly influenced by composite micro hardness of the coating. Al6061 coated with N 2 /Ar=0.6 outperformed the other coated samples. REFERENCES 1. Y. Fu, A.W. Batchelor, Surf. Coat. Technol. 102, 119. (1998). 2. Y.L. Jeyachandran, Sa.K.Narayandass, D. Mangalraj, Sami Areva, J.A. Milczarski,,Materials Science and Engineering A , (2007). 3. K. Holmberg, H. Ronkainen, A. Matthews, Ceram. Int. 26 (7) , 92000). 4. Huang, Y. C, Characterization of color for titanium nitride prepared by HCD process, Tokai University, Hiratsuka Kanagawa, 184, (1984). 5. P. Patsalas, C. Charitidis, S. Logothetidis,, Surface and Coatings Technology , (2000). 6. S Mahieu and D Depla, J., Phys. D: Appl. Phys (16pp), (2009). 7. Xu, XU Xuan-gian, YE Hui, ZOU Tong J Zhejiang, Univ. Science A 7(3): , (2006). 8. Sivitski, Ajaots, M.; Põdra,. April 2008, wear of pvd hard coatings in sliding contacts,6th International DAAAM Baltic Conference industrial engineering 24-26, Tallinn, Estonia. 9. J.A. Dean, Lange's Handbook of Chemistry, McGraw-Hill, New York, (1992). 10. National Institute of Standards and Technology, NIST Chemistry Webbook, No.69, (1998). 78