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Novel chemical metathesis route to prepare TiCN nanocrystallites at low temperature
Materials Chemistry and Physics 94 (2005) 58–61
Novel chemical metathesis route to prepare TiCN nanocrystallites at low temperature Xin Feng a,b,∗ , Li-Yi Shi a,c b c
a College of Sciences, Shanghai University, Shanghai 200444, China School of Material Science and Engineering, Shanghai University, Shanghai 200072, China Nano Science & Technology Research Center, Shanghai University, Shanghai 200444, China
Received 9 October 2004; received in revised form 28 March 2005; accepted 7 April 2005
Abstract Nanocrystalline titanium carbonitride (TiCN) was successfully synthesized at low temperature (420 ◦ C) via a chemical metathesis route using the source materials of TiCl4 , CCl4 and NaN3 , which are readily attainable. X-ray powder diffraction (XRD) indicated that the product ˚ Transmission electron microscopy revealed that the crystals were composed of spherical was cubic TiCN with a lattice constant a = 4.244 A. particles with an average diameter of 13 nm. X-ray photoemission spectra analysed that the atomic ratio was in good agreement with the TiC0.2 N0.8 stoichiometry. © 2005 Published by Elsevier B.V. Keywords: Chemical synthesis; Nanostructures; Ceramic; X-ray photoemission spectroscopy
1. Introduction Nowadays, titanium nitride (TiN) and titanium carbide (TiC) are becoming increasingly important ceramic hard materials, but the properties of the two hard materials are inferior in many ways to the properties of TiCN [1–4]. Ternary TiCN has some more excellent properties, such as high melting point and thermal conductivity, significant hardness–toughness compromise, good oxidation and wear resistance and low electrical resistivity, which make it wide applications in strengthening ferrous matrices for wear-resistant surfaces, in fabricating advanced engineering ceramic-based composites, scratch-resistant protective coating on watches and jewellery and electrical or automatic refractory devices [5–10]. TiCN was traditionally synthesized by high-temperature solid-state diffusion from TiN–TiC and TiN–C powder blends [11], or by a self-propagating reaction between Ti and C in a N2 atmosphere [12]. Jha and Yoon prepared TiCN via the reduction of TiO2 with C in the presence of N2 over a ∗
Corresponding author. Tel.: +86 21 66133800; fax: +86 21 66135066. E-mail address: [email protected] (X. Feng).
0254-0584/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2005.04.018
range of temperatures between 900 and 1500 ◦ C [13]. Kerr et al. synthesized TiCN using high-energy ball milling of TiO2 and graphite for 100 h under a N2 atmosphere and subsequent isothermal annealing at up to 1200 ◦ C [14]. Xiang et al. prepared TiCN at 1550 ◦ C by carbonthermal reduction of TiO2 derived from sol–gel process [15]. Lichtenberger et al. reported the synthesis of nanocrystalline TiCN by pyrolysis of poly(titanylcarbodiimide) above 1100 ◦ C [16]. Other physical and chemical methods, such as ion implantation process [17] and chemical vapor deposition [18–20], were also adopted to prepare TiCN. As a consequence, all these methods available for the synthesis of TiCN should carried out at high temperature or high pressure with complicated manipulation. Thus, it is necessary to search facile, economical synthesis routes to achieve this important material. In this work, we describe a novel chemical metathesis reaction to prepare ternary TiCN nanocrystallites at 420 ◦ C using TiCl4 , CCl4 and NaN3 as source materials. To the best of our knowledge, this is the first time to prepare nanocrystalline TiCN at such low temperature (420 ◦ C) under conveniently controllable conditions. The crystals were characterized by X-ray powder diffraction, X-
X. Feng, L.-Y. Shi / Materials Chemistry and Physics 94 (2005) 58–61
59
ray photoemission spectra and transmission electron microscopy.
2. Experimental The starting materials used in this work were analytical pure grade anhydrous TiCl4 , CCl4 and NaN3 . In a typical procedure, 3.5 ml TiCl4 , 2.3 ml CCl4 and 14 g NaN3 were put into a stainless steel autoclave of 50 ml capacity. All the manipulations were carried out in a protecting N2 gas flow. After the autoclave was sealed, it was heated to 420 ◦ C and held 8 h at the temperature and then allowed to cool to ambient temperature in furnace. The product was collected, filtered and washed with absolute ethanol, hot hydrochloric acid and distilled water successively to remove unreacted reagent, byproduct of NaCl and other impurities. After being dried fully in vacuum at 60 ◦ C, brown powders were ultimately obtained, the yield relative to the theoretical value was calculated as about 60%. X-ray powder diffraction (XRD) was performed on a Rigaku D max-␥A X-ray diffractometer with Ni filtered Cu K␣ radiation (V = 50 kV and I = 100 mA) at a scanning rate of 4◦ min−1 . The composition of TiCN was analysed by Xray photoelectron spectra (XPS), which was recorded on a KARTOS XSAM800 X-ray photoelectron spectrometer, Al K␣ (hν = 1486.6 eV) radiation was employed as the excitation source with an anode votage of 12 kV and an emission current of 10 mA. The morphology of TiCN nanocrystals was examined using a Hitachi H-800 transmission electron microscope (TEM) at an accelerating voltage of 150 kV.
Fig. 1. XRD pattern of the as-prepared TiCN crystals from the reaction of TiCl4 , CCl4 and NaN3 at 420 ◦ C for 8 h.
3. Results and discussion Fig. 1 shows the X-ray powder diffraction pattern of the sample, the peaks can be indexed to cubic phase with a lattice ˚ which is similar to TiC and TiN belongconstant a = 4.244 A, ing to Fm3m space group. The average grain size calculated by the Scherrer formula is near 13 nm. The X-ray photoelectron spectra of the product are shown in Fig. 2. The three strong peaks at 282.4, 396.5 and 456.2 eV correspond to C 1s, N 1s and Ti 2p binding energies, respectively. It is obvious that the C 1s and N 1s binding energies in TiCN are slightly higher than those in TiC and TiN, which is
Fig. 2. XPS spectra of the as-prepared TiCN crystals from the reaction of TiCl4 , CCl4 and NaN3 at 420 ◦ C for 8 h: (a) survey spectrum, (b) C 1s, (c) N 1s and (d) Ti 2p. The three strong peaks at 282.4, 396.5 and 456.2 eV correspond to C 1s, N 1s and Ti 2p binding energies, respectively.
60
X. Feng, L.-Y. Shi / Materials Chemistry and Physics 94 (2005) 58–61
Fig. 3. Typical TEM morphology and the corresponding EDP of TiCN nanocrystallites from the reaction of TiCl4 , CCl4 and NaN3 at 420 ◦ C for 8 h.
consistent with the result drawn from the literature [21–25]. The change of binding energy compared with TiN and TiC verifies the formation of ternary TiCN compound, rather than the composite of TiN and TiC. Calculation of the peak areas gives an atomic ratio of Ti:C:N = 1:0.21:0.79, which is very close to the stoichiometry of TiC0.2 N0.8 . From the C regions, the other peaks can be attributed to free carbon (284.3 eV) and CO2 adsorbed at the sample surface. Fig. 3 is the typical TEM morphology of the TiCN nanocrystals and the corresponding electron diffraction pattern (EDP). It can be seen that the crystals consist mainly of uniform spherical particles of 13 nm or so in size, which is consistent with the result from the XRD pattern. From the EDP, the d-values corresponding to the diffraction rings (from ˚ the inner to the outer) are 2.448, 2.122, 1.500 and 1.279 A, which can be assigned to the planes of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of the cubic TiCN, respectively. It is essential in this method that the mechanical and functional properties of the ternary TiCN strongly depend on the structure and stoichiometry. The density of as-prepared TiC0.2 N0.8 crystals was measured by the Archimedes method, the Vickers indentation method was used with an applied load of 98 N to measure hardness and fracture toughness at room temperature. The electrical resistivity was measured by a dc four-probe method passing currents up to 10 mA with accuracy better than 1 × 10−8 m. The results are shown in Table 1. The possible formation mechanism of TiCN was also investigated as a function of the heating temperature. With raising the temperature, the CCl4 vaporized at the boiling point Table 1 Experimental properties of the TiC0.2 N0.8 after sintering Sample Density (g cm−3 ) Hardness Hv (GPa) Fracture toughness (MPa m1/2 ) Electrical resistivity ( cm)
TiC0.2 N0.8 5.23 18.4 4.3 25
of 76.8 ◦ C, subsequently, TiCl4 vaporized at 136.4 ◦ C, so the gas–solid interface was begun to form between the gaseous CCl4 , TiCl4 and solid NaN3 . When the temperature elevated to 420 ◦ C, the activity of gaseous CCl4 and TiCl4 increased constantly and reacted with NaN3 on the interface simultaneously, resulting in the formation of TiCN. The process can be actually regarded as a chemical metathesis reaction in the following: TiCl4 + CCl4 + 8NaN3 = TiCN + 8NaCl +
23 2 N2
Here the by-product of NaCl played a significant role in the experiment, which not only decreased the maximum reaction temperature but also avoided the agglomeration of the TiCN nanocrystals. Just as a role of the by-product CaCl2 as reported by Gillan and Kaner [26], in this literature once the reaction initiates, it gives out heat energy rapidly, resulting in the molten CaCl2 (melting point, 775 ◦ C), which is favorable to the formation and crystallization of the resultants.
4. Conclusion TiCN nanocrystals was successfully prepared by the direct reaction of TiCl4 , CCl4 and NaN3 at low temperature (420 ◦ C) in autoclave. XRD, TEM and XPS results verified that the product was pure and crystallized very well. The advantage of this process was low reaction temperature and easy to control, so the similar route may be extended to prepare other carbonitrides, which is underway.
Acknowledgement This work was financially supported by Shanghai Nano Science and Technology Special Project (0359nm001).
X. Feng, L.-Y. Shi / Materials Chemistry and Physics 94 (2005) 58–61
References [1] Y. Maatsumura, T. Chujo, H. Uchida, H.H. Uchida, Surf. Coat. Technol. 60 (1993) 489. [2] K.J.A. Brookes, World Directory and Handbook of Hard Metals and Hard Materials, fifth ed., International Carbide Data, East Barnet, Hertfordshire, UK, 1992. [3] C.K. Narula, Ceramic Precursor Technology and Its Application, Mareel Dekker, New York, 1995, p. 235. [4] S.Y. Zhang, Mater. Sci. Eng. A 163 (1993) 141. [5] L.M. Berger, W. Gruner, E. Langholf, S. Stolle, Int. J. Refract. Met. Hard Mater. 17 (1999) 235. [6] R. Shaviv, Mater. Sci. Eng. A 209 (1996) 345. [7] P. Ettmayer, H. Kolaska, W. Lengauer, K. Dreyer, Int. J. Refract. Met. Hard Mater. 13 (1995) 343. [8] R. Koc, J.S. Folmer, J. Mater. Sci. 32 (1997) 3101. [9] M. Yoshimura, M. Nishioka, S. Somiya, J. Mater. Sci. Lett. 6 (1987) 1463. [10] A.W. Weimer, Carbide, Nitride and Boride Materials Synthesis and Processing, Chapman and Hall, 1997. [11] H. Pastor, Mater. Sci. Eng. A 401 (1988) 105. [12] M. Eslamloogrami, Z.A. Munir, J. Mater. Res. 9 (1994) 431.
61
[13] A. Jha, S.J. Yoon, J. Mater. Sci. 34 (1999) 307. [14] A. Kerr, N.J. Welham, P.E. Willis, Nanostruct. Mater. 11 (1999) 233. [15] J.H. Xiang, Z.P. Xie, Y. Huang, H.N. Xiao, J. Eur. Ceram. Soc. 20 (2000) 933. [16] O. Lichtenberger, E. Pippel, J. Woltersdorf, R. Riedel, Mater. Chem. Phys. 81 (2003) 195. [17] M. Guemmaz, A. Mosser, J.J. Grob, Appl. Phys. A 64 (1997) 407. [18] H. Holzschuh, Int. J. Refract. Met. Hard Mater. 20 (2002) 143. [19] D.H. Kuo, W.C. Liao, Appl. Surf. Sci. 199 (2002) 278. [20] K.T. Rie, J. W¨ohle, Surf. Coat. Technol. 112 (1999) 226. [21] H. Ihara, Y. Kumashiro, A. Itoh, K. Maeda, Jpn. J. Appl. Phys. 12 (1973) 1462. [22] L. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman, C. Nordling, J. Phys. Chem. Solids 30 (1969) 1835. [23] A.A. Galuska, J.C. Uht, N. Marquez, J. Vac. Sci. Technol. A 6 (1988) 110. [24] H. Goretzki, P.V. Rosenstiel, S. Mandziej, Z. Fres, Anal. Chem. 333 (1989) 451. [25] B.J. Burrow, A.E. Morgan, R.C. Ellwanger, J. Vac. Sci. Technol. A 4 (1986) 2463. [26] E.G. Gillan, R.B. Kaner, J. Mater. Chem. 11 (2001) 1951.
Novel chemical metathesis route to prepare TiCN nanocrystallites at low temperature Xin Feng a,b,∗ , Li-Yi Shi a,c b c
a College of Sciences, Shanghai University, Shanghai 200444, China School of Material Science and Engineering, Shanghai University, Shanghai 200072, China Nano Science & Technology Research Center, Shanghai University, Shanghai 200444, China
Received 9 October 2004; received in revised form 28 March 2005; accepted 7 April 2005
Abstract Nanocrystalline titanium carbonitride (TiCN) was successfully synthesized at low temperature (420 ◦ C) via a chemical metathesis route using the source materials of TiCl4 , CCl4 and NaN3 , which are readily attainable. X-ray powder diffraction (XRD) indicated that the product ˚ Transmission electron microscopy revealed that the crystals were composed of spherical was cubic TiCN with a lattice constant a = 4.244 A. particles with an average diameter of 13 nm. X-ray photoemission spectra analysed that the atomic ratio was in good agreement with the TiC0.2 N0.8 stoichiometry. © 2005 Published by Elsevier B.V. Keywords: Chemical synthesis; Nanostructures; Ceramic; X-ray photoemission spectroscopy
1. Introduction Nowadays, titanium nitride (TiN) and titanium carbide (TiC) are becoming increasingly important ceramic hard materials, but the properties of the two hard materials are inferior in many ways to the properties of TiCN [1–4]. Ternary TiCN has some more excellent properties, such as high melting point and thermal conductivity, significant hardness–toughness compromise, good oxidation and wear resistance and low electrical resistivity, which make it wide applications in strengthening ferrous matrices for wear-resistant surfaces, in fabricating advanced engineering ceramic-based composites, scratch-resistant protective coating on watches and jewellery and electrical or automatic refractory devices [5–10]. TiCN was traditionally synthesized by high-temperature solid-state diffusion from TiN–TiC and TiN–C powder blends [11], or by a self-propagating reaction between Ti and C in a N2 atmosphere [12]. Jha and Yoon prepared TiCN via the reduction of TiO2 with C in the presence of N2 over a ∗
Corresponding author. Tel.: +86 21 66133800; fax: +86 21 66135066. E-mail address: [email protected] (X. Feng).
0254-0584/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2005.04.018
range of temperatures between 900 and 1500 ◦ C [13]. Kerr et al. synthesized TiCN using high-energy ball milling of TiO2 and graphite for 100 h under a N2 atmosphere and subsequent isothermal annealing at up to 1200 ◦ C [14]. Xiang et al. prepared TiCN at 1550 ◦ C by carbonthermal reduction of TiO2 derived from sol–gel process [15]. Lichtenberger et al. reported the synthesis of nanocrystalline TiCN by pyrolysis of poly(titanylcarbodiimide) above 1100 ◦ C [16]. Other physical and chemical methods, such as ion implantation process [17] and chemical vapor deposition [18–20], were also adopted to prepare TiCN. As a consequence, all these methods available for the synthesis of TiCN should carried out at high temperature or high pressure with complicated manipulation. Thus, it is necessary to search facile, economical synthesis routes to achieve this important material. In this work, we describe a novel chemical metathesis reaction to prepare ternary TiCN nanocrystallites at 420 ◦ C using TiCl4 , CCl4 and NaN3 as source materials. To the best of our knowledge, this is the first time to prepare nanocrystalline TiCN at such low temperature (420 ◦ C) under conveniently controllable conditions. The crystals were characterized by X-ray powder diffraction, X-
X. Feng, L.-Y. Shi / Materials Chemistry and Physics 94 (2005) 58–61
59
ray photoemission spectra and transmission electron microscopy.
2. Experimental The starting materials used in this work were analytical pure grade anhydrous TiCl4 , CCl4 and NaN3 . In a typical procedure, 3.5 ml TiCl4 , 2.3 ml CCl4 and 14 g NaN3 were put into a stainless steel autoclave of 50 ml capacity. All the manipulations were carried out in a protecting N2 gas flow. After the autoclave was sealed, it was heated to 420 ◦ C and held 8 h at the temperature and then allowed to cool to ambient temperature in furnace. The product was collected, filtered and washed with absolute ethanol, hot hydrochloric acid and distilled water successively to remove unreacted reagent, byproduct of NaCl and other impurities. After being dried fully in vacuum at 60 ◦ C, brown powders were ultimately obtained, the yield relative to the theoretical value was calculated as about 60%. X-ray powder diffraction (XRD) was performed on a Rigaku D max-␥A X-ray diffractometer with Ni filtered Cu K␣ radiation (V = 50 kV and I = 100 mA) at a scanning rate of 4◦ min−1 . The composition of TiCN was analysed by Xray photoelectron spectra (XPS), which was recorded on a KARTOS XSAM800 X-ray photoelectron spectrometer, Al K␣ (hν = 1486.6 eV) radiation was employed as the excitation source with an anode votage of 12 kV and an emission current of 10 mA. The morphology of TiCN nanocrystals was examined using a Hitachi H-800 transmission electron microscope (TEM) at an accelerating voltage of 150 kV.
Fig. 1. XRD pattern of the as-prepared TiCN crystals from the reaction of TiCl4 , CCl4 and NaN3 at 420 ◦ C for 8 h.
3. Results and discussion Fig. 1 shows the X-ray powder diffraction pattern of the sample, the peaks can be indexed to cubic phase with a lattice ˚ which is similar to TiC and TiN belongconstant a = 4.244 A, ing to Fm3m space group. The average grain size calculated by the Scherrer formula is near 13 nm. The X-ray photoelectron spectra of the product are shown in Fig. 2. The three strong peaks at 282.4, 396.5 and 456.2 eV correspond to C 1s, N 1s and Ti 2p binding energies, respectively. It is obvious that the C 1s and N 1s binding energies in TiCN are slightly higher than those in TiC and TiN, which is
Fig. 2. XPS spectra of the as-prepared TiCN crystals from the reaction of TiCl4 , CCl4 and NaN3 at 420 ◦ C for 8 h: (a) survey spectrum, (b) C 1s, (c) N 1s and (d) Ti 2p. The three strong peaks at 282.4, 396.5 and 456.2 eV correspond to C 1s, N 1s and Ti 2p binding energies, respectively.
60
X. Feng, L.-Y. Shi / Materials Chemistry and Physics 94 (2005) 58–61
Fig. 3. Typical TEM morphology and the corresponding EDP of TiCN nanocrystallites from the reaction of TiCl4 , CCl4 and NaN3 at 420 ◦ C for 8 h.
consistent with the result drawn from the literature [21–25]. The change of binding energy compared with TiN and TiC verifies the formation of ternary TiCN compound, rather than the composite of TiN and TiC. Calculation of the peak areas gives an atomic ratio of Ti:C:N = 1:0.21:0.79, which is very close to the stoichiometry of TiC0.2 N0.8 . From the C regions, the other peaks can be attributed to free carbon (284.3 eV) and CO2 adsorbed at the sample surface. Fig. 3 is the typical TEM morphology of the TiCN nanocrystals and the corresponding electron diffraction pattern (EDP). It can be seen that the crystals consist mainly of uniform spherical particles of 13 nm or so in size, which is consistent with the result from the XRD pattern. From the EDP, the d-values corresponding to the diffraction rings (from ˚ the inner to the outer) are 2.448, 2.122, 1.500 and 1.279 A, which can be assigned to the planes of (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of the cubic TiCN, respectively. It is essential in this method that the mechanical and functional properties of the ternary TiCN strongly depend on the structure and stoichiometry. The density of as-prepared TiC0.2 N0.8 crystals was measured by the Archimedes method, the Vickers indentation method was used with an applied load of 98 N to measure hardness and fracture toughness at room temperature. The electrical resistivity was measured by a dc four-probe method passing currents up to 10 mA with accuracy better than 1 × 10−8 m. The results are shown in Table 1. The possible formation mechanism of TiCN was also investigated as a function of the heating temperature. With raising the temperature, the CCl4 vaporized at the boiling point Table 1 Experimental properties of the TiC0.2 N0.8 after sintering Sample Density (g cm−3 ) Hardness Hv (GPa) Fracture toughness (MPa m1/2 ) Electrical resistivity ( cm)
TiC0.2 N0.8 5.23 18.4 4.3 25
of 76.8 ◦ C, subsequently, TiCl4 vaporized at 136.4 ◦ C, so the gas–solid interface was begun to form between the gaseous CCl4 , TiCl4 and solid NaN3 . When the temperature elevated to 420 ◦ C, the activity of gaseous CCl4 and TiCl4 increased constantly and reacted with NaN3 on the interface simultaneously, resulting in the formation of TiCN. The process can be actually regarded as a chemical metathesis reaction in the following: TiCl4 + CCl4 + 8NaN3 = TiCN + 8NaCl +
23 2 N2
Here the by-product of NaCl played a significant role in the experiment, which not only decreased the maximum reaction temperature but also avoided the agglomeration of the TiCN nanocrystals. Just as a role of the by-product CaCl2 as reported by Gillan and Kaner [26], in this literature once the reaction initiates, it gives out heat energy rapidly, resulting in the molten CaCl2 (melting point, 775 ◦ C), which is favorable to the formation and crystallization of the resultants.
4. Conclusion TiCN nanocrystals was successfully prepared by the direct reaction of TiCl4 , CCl4 and NaN3 at low temperature (420 ◦ C) in autoclave. XRD, TEM and XPS results verified that the product was pure and crystallized very well. The advantage of this process was low reaction temperature and easy to control, so the similar route may be extended to prepare other carbonitrides, which is underway.
Acknowledgement This work was financially supported by Shanghai Nano Science and Technology Special Project (0359nm001).
X. Feng, L.-Y. Shi / Materials Chemistry and Physics 94 (2005) 58–61
References [1] Y. Maatsumura, T. Chujo, H. Uchida, H.H. Uchida, Surf. Coat. Technol. 60 (1993) 489. [2] K.J.A. Brookes, World Directory and Handbook of Hard Metals and Hard Materials, fifth ed., International Carbide Data, East Barnet, Hertfordshire, UK, 1992. [3] C.K. Narula, Ceramic Precursor Technology and Its Application, Mareel Dekker, New York, 1995, p. 235. [4] S.Y. Zhang, Mater. Sci. Eng. A 163 (1993) 141. [5] L.M. Berger, W. Gruner, E. Langholf, S. Stolle, Int. J. Refract. Met. Hard Mater. 17 (1999) 235. [6] R. Shaviv, Mater. Sci. Eng. A 209 (1996) 345. [7] P. Ettmayer, H. Kolaska, W. Lengauer, K. Dreyer, Int. J. Refract. Met. Hard Mater. 13 (1995) 343. [8] R. Koc, J.S. Folmer, J. Mater. Sci. 32 (1997) 3101. [9] M. Yoshimura, M. Nishioka, S. Somiya, J. Mater. Sci. Lett. 6 (1987) 1463. [10] A.W. Weimer, Carbide, Nitride and Boride Materials Synthesis and Processing, Chapman and Hall, 1997. [11] H. Pastor, Mater. Sci. Eng. A 401 (1988) 105. [12] M. Eslamloogrami, Z.A. Munir, J. Mater. Res. 9 (1994) 431.
61
[13] A. Jha, S.J. Yoon, J. Mater. Sci. 34 (1999) 307. [14] A. Kerr, N.J. Welham, P.E. Willis, Nanostruct. Mater. 11 (1999) 233. [15] J.H. Xiang, Z.P. Xie, Y. Huang, H.N. Xiao, J. Eur. Ceram. Soc. 20 (2000) 933. [16] O. Lichtenberger, E. Pippel, J. Woltersdorf, R. Riedel, Mater. Chem. Phys. 81 (2003) 195. [17] M. Guemmaz, A. Mosser, J.J. Grob, Appl. Phys. A 64 (1997) 407. [18] H. Holzschuh, Int. J. Refract. Met. Hard Mater. 20 (2002) 143. [19] D.H. Kuo, W.C. Liao, Appl. Surf. Sci. 199 (2002) 278. [20] K.T. Rie, J. W¨ohle, Surf. Coat. Technol. 112 (1999) 226. [21] H. Ihara, Y. Kumashiro, A. Itoh, K. Maeda, Jpn. J. Appl. Phys. 12 (1973) 1462. [22] L. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman, C. Nordling, J. Phys. Chem. Solids 30 (1969) 1835. [23] A.A. Galuska, J.C. Uht, N. Marquez, J. Vac. Sci. Technol. A 6 (1988) 110. [24] H. Goretzki, P.V. Rosenstiel, S. Mandziej, Z. Fres, Anal. Chem. 333 (1989) 451. [25] B.J. Burrow, A.E. Morgan, R.C. Ellwanger, J. Vac. Sci. Technol. A 4 (1986) 2463. [26] E.G. Gillan, R.B. Kaner, J. Mater. Chem. 11 (2001) 1951.