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Comparison of TiN and Ti1−xAlxN coatings deposited on Al using plasma immersion ion implantation assisted deposition
Surface & Coatings Technology 200 (2005) 2459 – 2464 www.elsevier.com/locate/surfcoat
Comparison of TiN and Ti1x Alx N coatings deposited on Al using plasma immersion ion implantation assisted deposition S. Mukherjee*, F. Prokert, E. Richter, W. Mfller Institute for Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, PF 510119, 01314 Dresden, Germany Received 20 May 2004; accepted in revised form 19 November 2004 Available online 12 January 2005
Abstract TiN and Ti1x Alx N coatings were deposited on Al substrates using the plasma immersion ion implantation and deposition technique, employing a filtered Ti and Ti0.5Al0.5 cathodic arc in a nitrogen atmosphere. Negative pulsed bias voltages between 0 to 4.0 kV were applied with varying duty cycles, at a constant time-averaged bias. Stress measurements using X-ray diffraction reveal an increase and then a decrease in the intrinsic compressive stress at increasing on-time bias, more pronounced for Ti1x Alx N coatings. A bias dependent preferred orientation is observed for both the coatings, with [200] being the preferred orientation at higher bias. The hardness always reduces for TiN coatings with increase in bias, whereas for Ti1x Alx N it shows a reverse trend. The results are qualitatively explained by the role played by Al in Ti1x Alx N. The results indicate that the peak bias plays a more dominant role than time averaged bias. D 2004 Elsevier B.V. All rights reserved. Keywords: Plasma immersion ion implantation assisted deposition; Titanium nitride; Preferred orientation; Stress
1. Introduction Plasma immersion ion implantation and deposition (PIIIAD) is a widely used technique to deposit and simultaneously implant a wide variety of metallic and ceramic coatings [1–7]. In most cases a cathodic arc is used as a source of metallic ions [1–5], which can be filtered to remove macroparticles using curved magnetic fields. Most of the work in PIIIAD has been focussed on developing Ti based coatings, of which TiN is one of the coatings having been one of the most extensively investigated [8]. In PIIIAD the substrates are biased to negative voltages varying from few hundred to few thousand volts with various repetition rates, so as to enable deposition during the off-time of the pulse and implantation and deposition during the on-time of the pulse. In addition, PIIIAD experiments employing a cathodic arc plasma guided by a magnetic field * Corresponding author. FCIPT, Institute for Plasma Research, B15-17/P GIDC, Gandhinagar 382044, Gujarat, India. Tel.: +91 79 3230908; fax: +91 79 3235024. E-mail address: dr_ s _ [email protected] (S. Mukherjee). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.11.028
exhibit an inherent directed ion energy of the order of 10– 100 eV, with which the arc plasma strikes the substrate even during the off-time phase of the substrate bias [1,2,8]. The growth of TiN coatings by the cathodic arc has been studied very recently in context of controlling the intrinsic stress of the coating [4,8]. It has been shown that by applying a pulsed bias on the substrates during deposition, the intrinsic stress can be controlled and also reduced. However, not much work is done in this direction for Ti1x Alx N coatings. Recently, the authors have shown that in spite of time averaged energy remaining identical, the TiN coatings have various stress levels, indicating that the peak energy plays a more dominant role than time averaged energy [8]. It has also been reported that modelling using time-averaged bias underestimates the coating thickness and predicts a composition that is different from what is actually found in the coating [1]. Ti1x Alx N coatings are superior to TiN with respect to hardness and oxidation resistance. Thus, they find wide applications and represent the state-of-the-art used on cutting tools. In the present investigation, the PIIIAD technique has been applied to develop Ti1x Alx N coatings
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on Al, using different peak amplitudes of pulsed substrate bias at constant time averaged bias. The intrinsic compressive stress in the coatings, their hardness and their coating thickness were measured in dependence on various experimental parameters. The results are compared with those obtained for TiN coatings deposited under identical parameters. The choice of Al substrate served dual purpose. Firstly, because of good thermal conductivity, the temperature difference between the front (the deposited surface) and the back (where the thermocouple is connected) of the substrate was minimised. Secondly, the feasibility of depositing Ti1x Alx N and TiN coatings by a physical vapour deposition technique was investigated, which can be an alternate technique to harden Al surfaces, which is conventionally performed using chemical vapour deposition techniques.
2. Experimental Disc shaped Al substrates of 2 mm thickness and 25 mm diameter were polished to mirror finish. The substrates were positioned with their polished surface normal to the cathodic arc plasma emanating from the end of a 908 magnetic filter. After attaining a base pressure of about 1.0103 Pa with the help of a turbomolecular pump, the pressure was increased to 1.1 Pa by filling with Ar. The substrates were biased to 1.5 kV for 10 min for sputter cleaning of the surface. The DC cathodic arc source, positioned in the front of the magnetic filter [1,8–10], was equipped with a Ti watercooled cathode for deposition of TiN coatings. For deposition of Ti1x Alx N coatings, a Ti0.5Al0.5 water-cooled cathode was used. The arc current was kept constant at 120 A at an operating pressure fixed at 0.1 Pa of nitrogen. The substrate was biased by negative pulses between 0.5 and 4.0 kV. The repetition rate was kept fixed at 10 kHz, with
the pulse on time varied from 40 to 5 As, so that the time averaged bias remained constant. To keep the pulse rise time less than 400 ns, a solid state switching technique was used. Thus, the time averaged bias remained identical within less than 10%. The reported flat-top bias voltages have been measured at the substrate holder. The Al substrate was firmly fixed on the metallic substrate holder using screws. Due to the resulting contact resistance of b1 V and the high conductivity of the substrate, the potential drop between the substrate holder and the substrate surface was negligible. The deposition time was 4 min. The temperature was measured with an electrically isolated thermocouple connected to the rear side of the substrates (Table 1). The temperature is considered to be uniform throughout the substrate because of high thermal conductivity of Al. After implantation the substrates were characterised by Bragg–Brentano X-ray diffraction (XRD) for phase identification. On the measurable X-ray reflections in Bragg– Brentano geometry, stress was evaluated using the d hkl sin 2 W technique. The X-ray elastic constants for TiN, needed for stress evaluation, were taken from Ref. [11]. Lacking better knowledge, the same X-ray elastic constants were also used for Ti1x Alx N coatings. This might introduce a systematic error. Profilometry was used to measure the coating thickness. Hardness was measured using a load of 3 mN, which gave an indenter penetration depth less that one tenth of the coating thickness, so that an influence of the soft Al substrate can be excluded.
3. Results and discussion Though the average deposited energy was kept identical by keeping the product of the applied bias and the duty cycle identical, there was a rise in the substrate temperature with increasing magnitude of the applied bias for TiN and
Table 1 Operating parameters of the PIIIAD of TiN and Ti1x Alx N on Al Coating
Bias (V)
On-time (As)
Max. temp (C)
r of (111) (GPa)
r of (200) (GPa)
r of (220) (GPa)
r mean (GPa)
r th (GPa)
r intrinsic (GPa)
TiN TiN TiN TiN TiN TiN TiN Ti1x Alx N Ti1x Alx N Ti1x Alx N Ti1x Alx N Ti1x Alx N Ti1x Alx N
No bias 500 1000 1500 2000 2500 4000 No bias 500 1000 1500 2000 4000
– 40 20 13.3 10 8 5 – 40 20 13.3 10 5
50 104 109 128 132 135 150 60 115 130 146 211 290
6.34 11.1 – – – – – – – – – – –
6.46 – – 8.5 3.9 9.8 9.4 5.8 – – 7.2 6.0 6.7
– 11.6 12.1 10.9 4.56 4.4 9.2 – 12.0 8.7 7.5 – –
6.4 11.35 12.1 9.7 4.23 7.6 9.3 5.8 12.0 8.7 7.35 6.0 6.7
0.22 0.62 0.65 0.79 0.82 0.84 0.96 0.29 0.70 0.81 0.92 1.4 1.98
6.18 10.73 11.45 8.91 3.41 6.76 8.34 5.51 11.3 7.89 6.38 4.6 4.72
The pulse repetition rate is fixed at 10 kHz with total deposition time of 240 seconds. The compressive stress, has been measured by the d hkl -sin 2 W technique using the following parameters; Y (111)=418 GPa, Y (200)=445 GPa, Y (220)=424 GPa and m (111)=0.21, m (200)=0.19, m (220)=0.2, where Y (hkl) corresponds to the Young’s modulus and m (hkl) corresponds to the Poisson ratio of an hkl reflection [11] for both the coatings. r th is calculated using Eq. (2), with T r=293 K.
S. Mukherjee et al. / Surface & Coatings Technology 200 (2005) 2459–2464
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Ti1x Alx N coatings (Table 1). Bilek [12,13] have shown that temperatures do not rise appreciably with increase in bias in pulsed conditions. However, it needs to be noted that the repetition rates in their experiments were much lower than that used here, and hence there was enough time between the pulses for cooling down. In the present experiment the duty cycle used was always higher than 5%. In addition, in the case of Ti1x Alx N the temperature rise was more than that of TiN. Though the arc discharge current was kept constant at 120 A, the arc erosion rates may be higher for the Ti0.5Al0.5 cathode, thus having more ionic flux bombarding the substrate. However, the temperature was still sufficiently low to avoid any mechanical distortion of the Al substrates. An increase in temperature with bias might also indicate that there is an enhanced ionisation with bias, that of the background gas and/or the metallic species. A temperature rise with bias indicates that peak bias plays a more crucial role than time averaged bias. XRD measurements for both TiN and Ti1x Alx N are shown in Fig. 1. For TiN, prominent (200) reflections are observed at high substrate bias. For lower substrate bias (111) and (220) reflections are also observed. For Ti1x Alx N, prominent (200) reflections are observed at high substrate bias. For lower substrate bias (111) and (220) reflections are also observed. (220) reflections are very strong for 1 kV bias. Compared to TiN, the (111) peak is weak for Ti1x Alx N. The reflections are also very broad indicating the nanocrystalline nature of the coating. They are also asymmetric indicating the presence of strain and/or compositional differences with depth. The texture coefficient (Tc) is defined as Tc ¼ ½nIm ðhklÞ=I0 ðhklÞ=½RIm ðhklÞ=I0 ðhklÞ
ð1Þ
where I m(hkl) is the measured intensity of reflection from a given (hkl) plane, I 0(hkl) is the relative intensity of the reflection from the same plane as indicated in a standard sample (PDF No. 38-1420) and n is the total number of reflections observed which is 3 in the present investigation [1,14]. The highest value of Tc is 3 for a perfectly oriented coating and its value is 1 for a randomly oriented one. Tc is plotted against the on-time bias in Fig. 2. An additional 40 V is added to all the bias assuming that the ions from the plasma strike the substrate surface even in unbiased condition with an energy corresponding to 40 V [1,4,5,9,10]. For TiN coatings, for the unbiased case, the texture coefficient is close to unity for all three orientations, whereas there is a pronounced deviation for non-zero substrate bias. For low (0.5 kV) substrate bias, (111) is dominant. At intermediate bias (1 kV and 1.5 kV), (220) dominates, whereas (200) texture prevails at further increasing bias. For Ti1x Alx N coatings, for the unbiased case, the texture coefficient is close to unity only for (111), and close to 3 for (200). For low (0.5 kV) substrate bias, (111) and (220) are dominant. At 1 kV, (220) dominates, and at
Fig. 1. XRD reflections for (top) TiN and (bottom) Ti1x Ax N coatings deposited by PIIIAD on Al. The intensity is plotted on a logarithmic scale. The unlabelled peaks are from the Al substrate. (111), (200) and (220) correspond to the various TiN peak positions as given in the powder diffraction data base (PDF No. 38-1420). The curves are shifted vertically for clarity.
1.5 kV, both (200) and (220) are present. With further increase in bias only (200) texture prevails. The average compressive stress was obtained from the d hkl -sin 2 W technique using appropriate values of Young’s modulus and Poisson’s ratio for each reflection [11] (Table 1), for both TiN and Ti1x Alx N coatings. The measured stress consists of the intrinsic stress and contribution from the thermal stress. The thermal stress (r th) is estimated from the following relation [15,16]; ð2Þ rth ¼ Yavg = 1 mavg ðaTiN aAl ÞðTs Tr Þ where Yavg is the average Young’s modulus of the various reflections under consideration (=429 GPa in the present case), m avg is the average Poisson ratio of the reflections under consideration (=0.2 in the present case), a TiN and a Al are the thermal expansion coefficients of TiN and Al,
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Fig. 2. Texture coefficients for the (111), (200) and (220) reflections of (top) TiN and (bottom) Ti1x Alx N, as dependent on the peak bias plus 40 V, which is assumed to be equivalent to the ion energy delivered by the arc plasma, as evaluated by Eq. (1). (200) is the dominant texture for higher substrate bias for both TiN and Ti1x Alx N.
respectively, and Ts and Tr are the deposition and room temperatures, respectively. As a TiN=9.4106 K1 and a Al=23.1106 K1, a TiNa Al=13.7106 K1. As TsNTr, in all the cases, the thermal stress is negative and hence compressive in nature. The same values of various constants are also used to estimate r th for Ti1x Alx N also. The thermal stress, which is small compared to the measured stress, is subtracted from the latter, to calculate the intrinsic stress. The intrinsic stress is plotted against the peak bias (on-time bias plus 40 V) in Fig. 3 for both TiN and Ti1x Alx N. Both the coatings show a compressive stress around 6 GPa for unbiased case, which increases for 0.5 kV bias. With further increase in bias, for TiN, the stress marginally increases, and then reduces at a bias of 2 kV. The stress increases further with increase in bias. For Ti1x Alx N, the stress continuously decreases with increase in bias. In an earlier paper [8], we proposed to describe the behaviour of stress with bias by the so-called subplantation
model, [17], for the case of energetic deposition during pulsed substrate bias. The model indicates that for identical time averaged bias applied on a substrate during deposition, the stress depends more on the peak bias rather than on the time-averaged bias. It is low for the unbiased case and then peaks at some intermediate value and then reduces. The Ti1x Alx N coatings show a better qualitative agreement to the subplantation model than TiN. Fig. 4 shows a comparison of the coating thickness and the composite hardness measured with an indenter load of 3 mN. For both TiN and Ti1x Alx N, the coating thickness is maximum for the unbiased substrate. Due to sputtering, it decreases with applied pulsed bias and exhibits an average of ~630 nm for TiN and an average of ~1.2 Am for Ti1x Alx N. As the deposition is performed at identical operating conditions, the larger thickness of Ti1x Alx N indicates larger erosion rates of the Ti0.5Al0.5 arc cathode. The composite hardness has a very different behaviour for TiN and Ti1x Alx N. For TiN, the hardness reduces with increase in peak bias, whereas it increases for Ti1x Alx N. It has been shown that valence electron concentration, plays a major role in determining the hardness of ternary coatings, where 2 elements balance each other’s atomic fraction, i.e., the composition can be expressed as Ax B1x C [18]. It has also been shown that when the valence electron concentrations of the binary coatings, i.e., AC and BC are 8 and 9, the hardness of the ternary coating, Ax B1x C, maximises in between. This is proven to be valid for coatings with a cubic crystal structure. The results have been evaluated for TiCx N1x , Zrx Nb1x C, HfCx N1x , etc., and the hardness in these cases maximises for valence electron concentration between 8.3 and 8.5. In each of the cases it has been observed that the hardness is more than what is the hardness for x=0 or 1, i.e., when the coatings are binary.
Fig. 3. Absolute value of the in-plane intrinsic stress, compressive in nature, plotted against the peak on time bias plus 40 V, assumed to be equivalent of energy coming from the arc plasma. For Ti1x Alx N, the stress level is lower with increase in bias, whereas for TiN it again increases, though both of them show a high degree of [200] preferred orientation at higher bias.
S. Mukherjee et al. / Surface & Coatings Technology 200 (2005) 2459–2464
Fig. 4. (top) Dependence of the coating composite hardness for a load of 3 mN and (bottom) the coating thickness, on the peak on-time bias plus 40 V. The coating thickness is the highest for the unbiased case, and is lower when there is substrate bias. The composite hardness is highest for an unbiased case for TiN, whereas for Ti1x Alx N, it increases with bias.
Ti1x Alx N satisfies all the criteria needed for extension of the above argument. AlN has a hardness of ~15 GPa whereas TiN has a hardness ~20 GPa, whereas Ti1x Alx N has a hardness much higher than both of them. Ti1x Alx N maintains the overall cubic crystal structure of TiN. The valence electron concentration for AlN is 8 and for TiN is 9. Thus, the hardness should also maximise between 8.3 and 8.5 corresponding to x=0.7 and 0.5. In the present experiments, the value of x=0.5 as evident from the composition of the arc cathode. Glow discharge optical emission spectroscopy measurements of the coating, for composition analysis, reveal an average value of x=0.5 for the unbiased case. With increase in bias there is a marginal increase in the average value of x to 0.55 [19]. Going by the above arguments it is evident that for stress relieved condition, Ti1x Alx N should have a higher hardness than TiN and AlN, and more precisely Ti0.45Al0.55N (formed at higher bias) to be harder than Ti0.5Al0.5N. The above arguments are to be utilised, as indicated in Ref. [18], as long as the overall crystal structure is cubic.
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It was been widely mentioned in the literature [3,4,8,15,16,19,20], for a TiN coating when the preferred orientation is [200], the hardness reduces, which matches with the observation presented here. However, the same has not been observed for Ti1x Alx N, because of the role that Al plays. In Ti1x Alx N, Al partially substitutes Ti, maintaining an overall cubic structure and because of its smaller volume, reduces stress. The reason for hardness enhancement is already described based on the valence electron concentration. Harder coatings are usually more difficult to sputter. Hence the sputtering rates during energetic deposition is lower in the case of Ti1x Alx N than that for TiN. This is evident from the fact that the thickness is more reduced due to bias sputtering more for TiN. Thus, for Ti1x Alx N, with increase in bias, stress and sputtering are reduced, and simultaneously the hardness is increased. In addition to that, there may be density compaction, phase segregation forming a nanocomposite of AlN and TiN, though not directly evidenced from the XRD measurements, which may also contribute to enhanced hardness [21–23]. The results can be summarized as follows. Partial substitution of Ti with Al, maintaining the overall TiN like structure, is always beneficial for enhancement of hardness. Ti1x Alx N coatings deposited at identical time averaged energy, show a preferred [200] orientation with increase in bias indicating that peak bias plays a more dominant role than time-averaged bias. Even though for Ti1x Alx N, the [200] texture is dominant with peak bias, the hardness increases. The results also indicate, that Ti1x Alx N coatings can be deposited with hardness enhancement, without increasing bulk temperature as done in conventional deposition devices, by manipulating the pulse bias characteristics. The results also demonstrate the feasibility of deposition of hard coatings by PIIIAD on soft Al substrates.
4. Conclusion The pulse voltage critically determines the properties of TiN and Ti1x Alx N coatings deposited on Al by plasma immersion ion implantation assisted deposition from arc plasma. The time-averaged bias is an ill-defined parameter for the determination of coating properties. The coating properties depend more on peak bias and effects caused by it. The result also indicate, that for Ti1x Alx N coatings, it is possible to reduce stress, at a preferred [200] orientation, while still maintaining a high hardness, unlike for TiN.
Acknowledgement S. Mukherjee is grateful to Alexander von Humboldt foundation for financial support during his stay in Forschungszentrum Rossendorf. Technical assistance from Mr. A. Reichel is gratefully acknowledged.
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References [1] S. Mukherjee, F. Prokert, E. Richter, H. Reuther, W. Mfller, Surf. Coat. Technol. 160 (2002) 93. [2] A. Anders, Surf. Coat. Technol. 93 (1997) 158. [3] R.N. Tarrant, C.S. Montross, D.R. McKenzie, Surf. Coat. Technol. 136 (2001) 188. [4] M.M.M. Bilek, D.R. McKenzie, R. Tarrant, S. Lim, D.G. McCulloch, Surf. Coat. Technol. 156 (2002) 136. [5] M.M.M. Bilek, P. Evans, D.R. McKenzie, D.G. McCulloch, H. Zreiqat, C.R. Howlett, J. Appl. Phys. 87 (2000) 4198. [6] S. Schoser, J. Forget, K. Kohlhof, Surf. Coat. Technol. 93 (1997) 339. [7] C. Ruset, E. Grigore, Surf. Coat. Technol. 156 (2002) 159. [8] S. Mukherjee, F. Prokert, E. Richter, W. Mfller, Thin Solid Films 445 (2003) 48. [9] R. Gqnzel, N. Shevchenko, W. Matz, A. Mqcklich, J.P. Celis, Surf. Coat. Technol. 142 (2001) 978. [10] J. Brqckner, R. Gqnzel, E. Richter, W. Mfller, Surf. Coat. Technol. 103–104 (1998) 227. [11] B. Eigenmann, E. Macherauch, Mat-Wiss. Werkstofftech. 27 (1996) 426.
[12] M.M.M. Bilek, Personal communication. [13] S.H.N. Lim, D.G. McCulloch, M.M.M. Bilek, D.R. Mckenzie, Surf. Coat. Technol. 174–175 (2003) 76. [14] M.I. Jones, I.R. McColl, D.M. Grant, Surf. Coat. Technol. 132 (2000) 143. [15] H. Ljungcrantz, L. Hultman, J.E. Sundgren, L. Karlsson, J. Appl. Phys. 78 (1995) 322. [16] Y. Pauleau, in: H.S. Nalwa (Ed.), Handbook of Thin Film Materials, vol. 1, Academic Press, New York, 2002, p. 455. [17] C.A. Davis, Thin Solid Films 226 (1993) 30. [18] S. Hoon, J.J. Ihm, S.G. Louie, M.L. Cohen, Nature 399 (1999) 132. [19] S. Mukherjee, F. Prokert, E. Richter, W. Moeller, bMechanical properties of Ti based coatings developed by plasma immersion ion implantation assisted depositionQ, Surf. Coat. Technol, in preparation. [20] W.J. Chou, G.P. Yu, J.H. Huang, Surf. Coat. Technol. 149 (2002) 7. [21] S. Veprek, J. Vac. Sci. Technol., A Vac. Surf. Films 17 (1999) 2401. [22] P.H. Mayrhofer, A. Hoerling, L. Karlsson, J. Sjoelen, T. Larrson, C. Mitterer, L. Hultman, Appl. Phys. Lett. 83 (2003) 2049. [23] L. Hultman, Vacuum 57 (2000) 1.
Comparison of TiN and Ti1x Alx N coatings deposited on Al using plasma immersion ion implantation assisted deposition S. Mukherjee*, F. Prokert, E. Richter, W. Mfller Institute for Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, PF 510119, 01314 Dresden, Germany Received 20 May 2004; accepted in revised form 19 November 2004 Available online 12 January 2005
Abstract TiN and Ti1x Alx N coatings were deposited on Al substrates using the plasma immersion ion implantation and deposition technique, employing a filtered Ti and Ti0.5Al0.5 cathodic arc in a nitrogen atmosphere. Negative pulsed bias voltages between 0 to 4.0 kV were applied with varying duty cycles, at a constant time-averaged bias. Stress measurements using X-ray diffraction reveal an increase and then a decrease in the intrinsic compressive stress at increasing on-time bias, more pronounced for Ti1x Alx N coatings. A bias dependent preferred orientation is observed for both the coatings, with [200] being the preferred orientation at higher bias. The hardness always reduces for TiN coatings with increase in bias, whereas for Ti1x Alx N it shows a reverse trend. The results are qualitatively explained by the role played by Al in Ti1x Alx N. The results indicate that the peak bias plays a more dominant role than time averaged bias. D 2004 Elsevier B.V. All rights reserved. Keywords: Plasma immersion ion implantation assisted deposition; Titanium nitride; Preferred orientation; Stress
1. Introduction Plasma immersion ion implantation and deposition (PIIIAD) is a widely used technique to deposit and simultaneously implant a wide variety of metallic and ceramic coatings [1–7]. In most cases a cathodic arc is used as a source of metallic ions [1–5], which can be filtered to remove macroparticles using curved magnetic fields. Most of the work in PIIIAD has been focussed on developing Ti based coatings, of which TiN is one of the coatings having been one of the most extensively investigated [8]. In PIIIAD the substrates are biased to negative voltages varying from few hundred to few thousand volts with various repetition rates, so as to enable deposition during the off-time of the pulse and implantation and deposition during the on-time of the pulse. In addition, PIIIAD experiments employing a cathodic arc plasma guided by a magnetic field * Corresponding author. FCIPT, Institute for Plasma Research, B15-17/P GIDC, Gandhinagar 382044, Gujarat, India. Tel.: +91 79 3230908; fax: +91 79 3235024. E-mail address: dr_ s _ [email protected] (S. Mukherjee). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.11.028
exhibit an inherent directed ion energy of the order of 10– 100 eV, with which the arc plasma strikes the substrate even during the off-time phase of the substrate bias [1,2,8]. The growth of TiN coatings by the cathodic arc has been studied very recently in context of controlling the intrinsic stress of the coating [4,8]. It has been shown that by applying a pulsed bias on the substrates during deposition, the intrinsic stress can be controlled and also reduced. However, not much work is done in this direction for Ti1x Alx N coatings. Recently, the authors have shown that in spite of time averaged energy remaining identical, the TiN coatings have various stress levels, indicating that the peak energy plays a more dominant role than time averaged energy [8]. It has also been reported that modelling using time-averaged bias underestimates the coating thickness and predicts a composition that is different from what is actually found in the coating [1]. Ti1x Alx N coatings are superior to TiN with respect to hardness and oxidation resistance. Thus, they find wide applications and represent the state-of-the-art used on cutting tools. In the present investigation, the PIIIAD technique has been applied to develop Ti1x Alx N coatings
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on Al, using different peak amplitudes of pulsed substrate bias at constant time averaged bias. The intrinsic compressive stress in the coatings, their hardness and their coating thickness were measured in dependence on various experimental parameters. The results are compared with those obtained for TiN coatings deposited under identical parameters. The choice of Al substrate served dual purpose. Firstly, because of good thermal conductivity, the temperature difference between the front (the deposited surface) and the back (where the thermocouple is connected) of the substrate was minimised. Secondly, the feasibility of depositing Ti1x Alx N and TiN coatings by a physical vapour deposition technique was investigated, which can be an alternate technique to harden Al surfaces, which is conventionally performed using chemical vapour deposition techniques.
2. Experimental Disc shaped Al substrates of 2 mm thickness and 25 mm diameter were polished to mirror finish. The substrates were positioned with their polished surface normal to the cathodic arc plasma emanating from the end of a 908 magnetic filter. After attaining a base pressure of about 1.0103 Pa with the help of a turbomolecular pump, the pressure was increased to 1.1 Pa by filling with Ar. The substrates were biased to 1.5 kV for 10 min for sputter cleaning of the surface. The DC cathodic arc source, positioned in the front of the magnetic filter [1,8–10], was equipped with a Ti watercooled cathode for deposition of TiN coatings. For deposition of Ti1x Alx N coatings, a Ti0.5Al0.5 water-cooled cathode was used. The arc current was kept constant at 120 A at an operating pressure fixed at 0.1 Pa of nitrogen. The substrate was biased by negative pulses between 0.5 and 4.0 kV. The repetition rate was kept fixed at 10 kHz, with
the pulse on time varied from 40 to 5 As, so that the time averaged bias remained constant. To keep the pulse rise time less than 400 ns, a solid state switching technique was used. Thus, the time averaged bias remained identical within less than 10%. The reported flat-top bias voltages have been measured at the substrate holder. The Al substrate was firmly fixed on the metallic substrate holder using screws. Due to the resulting contact resistance of b1 V and the high conductivity of the substrate, the potential drop between the substrate holder and the substrate surface was negligible. The deposition time was 4 min. The temperature was measured with an electrically isolated thermocouple connected to the rear side of the substrates (Table 1). The temperature is considered to be uniform throughout the substrate because of high thermal conductivity of Al. After implantation the substrates were characterised by Bragg–Brentano X-ray diffraction (XRD) for phase identification. On the measurable X-ray reflections in Bragg– Brentano geometry, stress was evaluated using the d hkl sin 2 W technique. The X-ray elastic constants for TiN, needed for stress evaluation, were taken from Ref. [11]. Lacking better knowledge, the same X-ray elastic constants were also used for Ti1x Alx N coatings. This might introduce a systematic error. Profilometry was used to measure the coating thickness. Hardness was measured using a load of 3 mN, which gave an indenter penetration depth less that one tenth of the coating thickness, so that an influence of the soft Al substrate can be excluded.
3. Results and discussion Though the average deposited energy was kept identical by keeping the product of the applied bias and the duty cycle identical, there was a rise in the substrate temperature with increasing magnitude of the applied bias for TiN and
Table 1 Operating parameters of the PIIIAD of TiN and Ti1x Alx N on Al Coating
Bias (V)
On-time (As)
Max. temp (C)
r of (111) (GPa)
r of (200) (GPa)
r of (220) (GPa)
r mean (GPa)
r th (GPa)
r intrinsic (GPa)
TiN TiN TiN TiN TiN TiN TiN Ti1x Alx N Ti1x Alx N Ti1x Alx N Ti1x Alx N Ti1x Alx N Ti1x Alx N
No bias 500 1000 1500 2000 2500 4000 No bias 500 1000 1500 2000 4000
– 40 20 13.3 10 8 5 – 40 20 13.3 10 5
50 104 109 128 132 135 150 60 115 130 146 211 290
6.34 11.1 – – – – – – – – – – –
6.46 – – 8.5 3.9 9.8 9.4 5.8 – – 7.2 6.0 6.7
– 11.6 12.1 10.9 4.56 4.4 9.2 – 12.0 8.7 7.5 – –
6.4 11.35 12.1 9.7 4.23 7.6 9.3 5.8 12.0 8.7 7.35 6.0 6.7
0.22 0.62 0.65 0.79 0.82 0.84 0.96 0.29 0.70 0.81 0.92 1.4 1.98
6.18 10.73 11.45 8.91 3.41 6.76 8.34 5.51 11.3 7.89 6.38 4.6 4.72
The pulse repetition rate is fixed at 10 kHz with total deposition time of 240 seconds. The compressive stress, has been measured by the d hkl -sin 2 W technique using the following parameters; Y (111)=418 GPa, Y (200)=445 GPa, Y (220)=424 GPa and m (111)=0.21, m (200)=0.19, m (220)=0.2, where Y (hkl) corresponds to the Young’s modulus and m (hkl) corresponds to the Poisson ratio of an hkl reflection [11] for both the coatings. r th is calculated using Eq. (2), with T r=293 K.
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Ti1x Alx N coatings (Table 1). Bilek [12,13] have shown that temperatures do not rise appreciably with increase in bias in pulsed conditions. However, it needs to be noted that the repetition rates in their experiments were much lower than that used here, and hence there was enough time between the pulses for cooling down. In the present experiment the duty cycle used was always higher than 5%. In addition, in the case of Ti1x Alx N the temperature rise was more than that of TiN. Though the arc discharge current was kept constant at 120 A, the arc erosion rates may be higher for the Ti0.5Al0.5 cathode, thus having more ionic flux bombarding the substrate. However, the temperature was still sufficiently low to avoid any mechanical distortion of the Al substrates. An increase in temperature with bias might also indicate that there is an enhanced ionisation with bias, that of the background gas and/or the metallic species. A temperature rise with bias indicates that peak bias plays a more crucial role than time averaged bias. XRD measurements for both TiN and Ti1x Alx N are shown in Fig. 1. For TiN, prominent (200) reflections are observed at high substrate bias. For lower substrate bias (111) and (220) reflections are also observed. For Ti1x Alx N, prominent (200) reflections are observed at high substrate bias. For lower substrate bias (111) and (220) reflections are also observed. (220) reflections are very strong for 1 kV bias. Compared to TiN, the (111) peak is weak for Ti1x Alx N. The reflections are also very broad indicating the nanocrystalline nature of the coating. They are also asymmetric indicating the presence of strain and/or compositional differences with depth. The texture coefficient (Tc) is defined as Tc ¼ ½nIm ðhklÞ=I0 ðhklÞ=½RIm ðhklÞ=I0 ðhklÞ
ð1Þ
where I m(hkl) is the measured intensity of reflection from a given (hkl) plane, I 0(hkl) is the relative intensity of the reflection from the same plane as indicated in a standard sample (PDF No. 38-1420) and n is the total number of reflections observed which is 3 in the present investigation [1,14]. The highest value of Tc is 3 for a perfectly oriented coating and its value is 1 for a randomly oriented one. Tc is plotted against the on-time bias in Fig. 2. An additional 40 V is added to all the bias assuming that the ions from the plasma strike the substrate surface even in unbiased condition with an energy corresponding to 40 V [1,4,5,9,10]. For TiN coatings, for the unbiased case, the texture coefficient is close to unity for all three orientations, whereas there is a pronounced deviation for non-zero substrate bias. For low (0.5 kV) substrate bias, (111) is dominant. At intermediate bias (1 kV and 1.5 kV), (220) dominates, whereas (200) texture prevails at further increasing bias. For Ti1x Alx N coatings, for the unbiased case, the texture coefficient is close to unity only for (111), and close to 3 for (200). For low (0.5 kV) substrate bias, (111) and (220) are dominant. At 1 kV, (220) dominates, and at
Fig. 1. XRD reflections for (top) TiN and (bottom) Ti1x Ax N coatings deposited by PIIIAD on Al. The intensity is plotted on a logarithmic scale. The unlabelled peaks are from the Al substrate. (111), (200) and (220) correspond to the various TiN peak positions as given in the powder diffraction data base (PDF No. 38-1420). The curves are shifted vertically for clarity.
1.5 kV, both (200) and (220) are present. With further increase in bias only (200) texture prevails. The average compressive stress was obtained from the d hkl -sin 2 W technique using appropriate values of Young’s modulus and Poisson’s ratio for each reflection [11] (Table 1), for both TiN and Ti1x Alx N coatings. The measured stress consists of the intrinsic stress and contribution from the thermal stress. The thermal stress (r th) is estimated from the following relation [15,16]; ð2Þ rth ¼ Yavg = 1 mavg ðaTiN aAl ÞðTs Tr Þ where Yavg is the average Young’s modulus of the various reflections under consideration (=429 GPa in the present case), m avg is the average Poisson ratio of the reflections under consideration (=0.2 in the present case), a TiN and a Al are the thermal expansion coefficients of TiN and Al,
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Fig. 2. Texture coefficients for the (111), (200) and (220) reflections of (top) TiN and (bottom) Ti1x Alx N, as dependent on the peak bias plus 40 V, which is assumed to be equivalent to the ion energy delivered by the arc plasma, as evaluated by Eq. (1). (200) is the dominant texture for higher substrate bias for both TiN and Ti1x Alx N.
respectively, and Ts and Tr are the deposition and room temperatures, respectively. As a TiN=9.4106 K1 and a Al=23.1106 K1, a TiNa Al=13.7106 K1. As TsNTr, in all the cases, the thermal stress is negative and hence compressive in nature. The same values of various constants are also used to estimate r th for Ti1x Alx N also. The thermal stress, which is small compared to the measured stress, is subtracted from the latter, to calculate the intrinsic stress. The intrinsic stress is plotted against the peak bias (on-time bias plus 40 V) in Fig. 3 for both TiN and Ti1x Alx N. Both the coatings show a compressive stress around 6 GPa for unbiased case, which increases for 0.5 kV bias. With further increase in bias, for TiN, the stress marginally increases, and then reduces at a bias of 2 kV. The stress increases further with increase in bias. For Ti1x Alx N, the stress continuously decreases with increase in bias. In an earlier paper [8], we proposed to describe the behaviour of stress with bias by the so-called subplantation
model, [17], for the case of energetic deposition during pulsed substrate bias. The model indicates that for identical time averaged bias applied on a substrate during deposition, the stress depends more on the peak bias rather than on the time-averaged bias. It is low for the unbiased case and then peaks at some intermediate value and then reduces. The Ti1x Alx N coatings show a better qualitative agreement to the subplantation model than TiN. Fig. 4 shows a comparison of the coating thickness and the composite hardness measured with an indenter load of 3 mN. For both TiN and Ti1x Alx N, the coating thickness is maximum for the unbiased substrate. Due to sputtering, it decreases with applied pulsed bias and exhibits an average of ~630 nm for TiN and an average of ~1.2 Am for Ti1x Alx N. As the deposition is performed at identical operating conditions, the larger thickness of Ti1x Alx N indicates larger erosion rates of the Ti0.5Al0.5 arc cathode. The composite hardness has a very different behaviour for TiN and Ti1x Alx N. For TiN, the hardness reduces with increase in peak bias, whereas it increases for Ti1x Alx N. It has been shown that valence electron concentration, plays a major role in determining the hardness of ternary coatings, where 2 elements balance each other’s atomic fraction, i.e., the composition can be expressed as Ax B1x C [18]. It has also been shown that when the valence electron concentrations of the binary coatings, i.e., AC and BC are 8 and 9, the hardness of the ternary coating, Ax B1x C, maximises in between. This is proven to be valid for coatings with a cubic crystal structure. The results have been evaluated for TiCx N1x , Zrx Nb1x C, HfCx N1x , etc., and the hardness in these cases maximises for valence electron concentration between 8.3 and 8.5. In each of the cases it has been observed that the hardness is more than what is the hardness for x=0 or 1, i.e., when the coatings are binary.
Fig. 3. Absolute value of the in-plane intrinsic stress, compressive in nature, plotted against the peak on time bias plus 40 V, assumed to be equivalent of energy coming from the arc plasma. For Ti1x Alx N, the stress level is lower with increase in bias, whereas for TiN it again increases, though both of them show a high degree of [200] preferred orientation at higher bias.
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Fig. 4. (top) Dependence of the coating composite hardness for a load of 3 mN and (bottom) the coating thickness, on the peak on-time bias plus 40 V. The coating thickness is the highest for the unbiased case, and is lower when there is substrate bias. The composite hardness is highest for an unbiased case for TiN, whereas for Ti1x Alx N, it increases with bias.
Ti1x Alx N satisfies all the criteria needed for extension of the above argument. AlN has a hardness of ~15 GPa whereas TiN has a hardness ~20 GPa, whereas Ti1x Alx N has a hardness much higher than both of them. Ti1x Alx N maintains the overall cubic crystal structure of TiN. The valence electron concentration for AlN is 8 and for TiN is 9. Thus, the hardness should also maximise between 8.3 and 8.5 corresponding to x=0.7 and 0.5. In the present experiments, the value of x=0.5 as evident from the composition of the arc cathode. Glow discharge optical emission spectroscopy measurements of the coating, for composition analysis, reveal an average value of x=0.5 for the unbiased case. With increase in bias there is a marginal increase in the average value of x to 0.55 [19]. Going by the above arguments it is evident that for stress relieved condition, Ti1x Alx N should have a higher hardness than TiN and AlN, and more precisely Ti0.45Al0.55N (formed at higher bias) to be harder than Ti0.5Al0.5N. The above arguments are to be utilised, as indicated in Ref. [18], as long as the overall crystal structure is cubic.
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It was been widely mentioned in the literature [3,4,8,15,16,19,20], for a TiN coating when the preferred orientation is [200], the hardness reduces, which matches with the observation presented here. However, the same has not been observed for Ti1x Alx N, because of the role that Al plays. In Ti1x Alx N, Al partially substitutes Ti, maintaining an overall cubic structure and because of its smaller volume, reduces stress. The reason for hardness enhancement is already described based on the valence electron concentration. Harder coatings are usually more difficult to sputter. Hence the sputtering rates during energetic deposition is lower in the case of Ti1x Alx N than that for TiN. This is evident from the fact that the thickness is more reduced due to bias sputtering more for TiN. Thus, for Ti1x Alx N, with increase in bias, stress and sputtering are reduced, and simultaneously the hardness is increased. In addition to that, there may be density compaction, phase segregation forming a nanocomposite of AlN and TiN, though not directly evidenced from the XRD measurements, which may also contribute to enhanced hardness [21–23]. The results can be summarized as follows. Partial substitution of Ti with Al, maintaining the overall TiN like structure, is always beneficial for enhancement of hardness. Ti1x Alx N coatings deposited at identical time averaged energy, show a preferred [200] orientation with increase in bias indicating that peak bias plays a more dominant role than time-averaged bias. Even though for Ti1x Alx N, the [200] texture is dominant with peak bias, the hardness increases. The results also indicate, that Ti1x Alx N coatings can be deposited with hardness enhancement, without increasing bulk temperature as done in conventional deposition devices, by manipulating the pulse bias characteristics. The results also demonstrate the feasibility of deposition of hard coatings by PIIIAD on soft Al substrates.
4. Conclusion The pulse voltage critically determines the properties of TiN and Ti1x Alx N coatings deposited on Al by plasma immersion ion implantation assisted deposition from arc plasma. The time-averaged bias is an ill-defined parameter for the determination of coating properties. The coating properties depend more on peak bias and effects caused by it. The result also indicate, that for Ti1x Alx N coatings, it is possible to reduce stress, at a preferred [200] orientation, while still maintaining a high hardness, unlike for TiN.
Acknowledgement S. Mukherjee is grateful to Alexander von Humboldt foundation for financial support during his stay in Forschungszentrum Rossendorf. Technical assistance from Mr. A. Reichel is gratefully acknowledged.
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References [1] S. Mukherjee, F. Prokert, E. Richter, H. Reuther, W. Mfller, Surf. Coat. Technol. 160 (2002) 93. [2] A. Anders, Surf. Coat. Technol. 93 (1997) 158. [3] R.N. Tarrant, C.S. Montross, D.R. McKenzie, Surf. Coat. Technol. 136 (2001) 188. [4] M.M.M. Bilek, D.R. McKenzie, R. Tarrant, S. Lim, D.G. McCulloch, Surf. Coat. Technol. 156 (2002) 136. [5] M.M.M. Bilek, P. Evans, D.R. McKenzie, D.G. McCulloch, H. Zreiqat, C.R. Howlett, J. Appl. Phys. 87 (2000) 4198. [6] S. Schoser, J. Forget, K. Kohlhof, Surf. Coat. Technol. 93 (1997) 339. [7] C. Ruset, E. Grigore, Surf. Coat. Technol. 156 (2002) 159. [8] S. Mukherjee, F. Prokert, E. Richter, W. Mfller, Thin Solid Films 445 (2003) 48. [9] R. Gqnzel, N. Shevchenko, W. Matz, A. Mqcklich, J.P. Celis, Surf. Coat. Technol. 142 (2001) 978. [10] J. Brqckner, R. Gqnzel, E. Richter, W. Mfller, Surf. Coat. Technol. 103–104 (1998) 227. [11] B. Eigenmann, E. Macherauch, Mat-Wiss. Werkstofftech. 27 (1996) 426.
[12] M.M.M. Bilek, Personal communication. [13] S.H.N. Lim, D.G. McCulloch, M.M.M. Bilek, D.R. Mckenzie, Surf. Coat. Technol. 174–175 (2003) 76. [14] M.I. Jones, I.R. McColl, D.M. Grant, Surf. Coat. Technol. 132 (2000) 143. [15] H. Ljungcrantz, L. Hultman, J.E. Sundgren, L. Karlsson, J. Appl. Phys. 78 (1995) 322. [16] Y. Pauleau, in: H.S. Nalwa (Ed.), Handbook of Thin Film Materials, vol. 1, Academic Press, New York, 2002, p. 455. [17] C.A. Davis, Thin Solid Films 226 (1993) 30. [18] S. Hoon, J.J. Ihm, S.G. Louie, M.L. Cohen, Nature 399 (1999) 132. [19] S. Mukherjee, F. Prokert, E. Richter, W. Moeller, bMechanical properties of Ti based coatings developed by plasma immersion ion implantation assisted depositionQ, Surf. Coat. Technol, in preparation. [20] W.J. Chou, G.P. Yu, J.H. Huang, Surf. Coat. Technol. 149 (2002) 7. [21] S. Veprek, J. Vac. Sci. Technol., A Vac. Surf. Films 17 (1999) 2401. [22] P.H. Mayrhofer, A. Hoerling, L. Karlsson, J. Sjoelen, T. Larrson, C. Mitterer, L. Hultman, Appl. Phys. Lett. 83 (2003) 2049. [23] L. Hultman, Vacuum 57 (2000) 1.