- Email: [email protected]
SIMS investigation of nitride coatings
ARTICLE IN PRESS
Vacuum 78 (2005) 545–550 www.elsevier.com/locate/vacuum
SIMS investigation of nitride coatings V.M. Anischika, V.V. Uglova,, S.V. Zlotskia, P. Konarskib, M. Cwilb, V.A. Ukhovc a
Department of Physics, Belarusian State University, F.Skariny Ave. 4, 220080 Minsk, Belarus b Industrial Institute of Electronics, ul. Dluga 44/50, 00-241 Warszawa, Poland c Belmicrosystems, Korzhenevsky 12, 220064 Minsk, Belarus
Abstract The Ti–Cr–N coatings were formed by condensation from a plasma phase in a vacuum with ion bombardment of sample surfaces while combining Ti and Cr plasma flows of variable density in a residual nitrogen atmosphere. The elemental and phase composition and also microstructure were studied by secondary-ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SIMS and AES investigations show that the system of gradient by composition was formed. It is found that the gradient coating consists of basic layer of variable composition, in which concentration of Ti and Cr elements increase or decrease with the coating thickness and Cr-rich interlayer. XRD and TEM measurements show that a continuous series of solid solution TixCr1xN (0.60oxo0.84 and 0.25oxo0.67) of variable composition with preferred orientation (2 0 0) was formed. Cross-section SEM shows that the dense finegrained structures with the average grain size of 10–50 nm were formed. r 2005 Elsevier Ltd. All rights reserved. Keywords: CAVD gradient Ti–Cr–N coatings; SIMS investigations; Microstructure
1. Introduction Thin hard coatings of nitrides of transitional metals fabricated by the cathodic arc vapour deposition (CAVD) are now widely used as corrosion and wear resistance protective coatings. This is due, in the first place, to their improved Corresponding author. +375 17 2095512;
fax: +375 17 2265552. E-mail address: [email protected] (V.V. Uglov).
mechanical properties, such as high oxidation and corrosion resistance, wear resistance and enhanced hardness. Ternary transition metal nitride coatings provide a wide range of structures that enable control of mechanical and electronic properties. Moreover, ternary compound coatings often exhibit fine-grained and distorted structures resulting in high hardness [1]. But, the deposition of coatings directly onto substrate surfaces usually reduces the adhesion strength due to higher brittleness and residual
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.083
ARTICLE IN PRESS V.M. Anischik et al. / Vacuum 78 (2005) 545–550
546
stress [2]. The formation of gradient ternary systems with a variable concentration of elements, both on interphase boundary and inside the coating allows to increase adhesion and to improve mechanical properties [3]. But, gradient systems are complex enough for investigation of coatings with non-homogeneous changing of elemental and phase composition. Thus, the investigation of elemental composition of such films requires the application of several methods like secondary-ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), Rutherford backscattering, etc. in combination with X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thus, this work is devoted to studying of the element, phase composition and also microstructure of Ti–Cr–N gradient coatings, formed by vacuum-arc deposition with the use of combining Ti and Cr plasma flows.
Cr cathode (ICr) 100 A, the vacuum in the chamber was P ¼ 103 Pa: Cr was chosen for ion clearing because of good adhesion on steel substrates [5]. This stage provided the heating of the substrate to 450 1C. Film deposition was initiated by injecting N2 gas at a pressure PN ¼ 101 Pa into the vacuum chamber with the substrate bias equal to 120 V. The gradient Ti–Cr–N system was deposited by varying the arc currents of Ti and Cr cathodes (Table 1). The total thickness of the formed coatings is about 0.8–1.2 mm depending on condition of deposition. Combination of plasma flows of material evaporated from cathodes at the surface of substrate was provided with two cathodes situated at an angle to each other [4]. Relation of concentrations of component of coatings is linearly dependent on the ratio of plasma flow density in these conditions. SIMS analyses were performed on SALW-05 apparatus equipped with 06-350E Physical Electronics ion gun and QMA-410 Balzers 16 mm quadrupole analyser. Argon ion beam of 5 keV energy and 100 mm full-width at the halfmaximum (FWHM) beam diameter, was digitally rastered in a spiral mode over 1 mm 1.65 mm area using the 128 128 pixel frame. In order to calibrate sputtering rate of the coating layers, the eroded craters were analysed ‘‘ex situ’’ using the Tencor alpha-sep 100 profilometer. The elemental composition of Ti–Cr–N coatings was determined by AES with the step-by-step sputtering of the samples surface layer by argon ions with the energy of 3 keV. The scanning microprobe PHi-660 was used for measurements.
2. Experimental Gradient Ti–Cr–N coatings were formed by condensation from a plasma phase in a vacuum with the ion bombardment of the samples surface combining the plasma flows of titanium and chromium of variable density in a residual nitrogen atmosphere [4]. Carbon steel St3 ðo0:18 wtCÞ and silicon (1 0 0) orientation were used as substrate material. Prior to the deposition, the surface of substrates was cleaned and heated by chromium ion bombardment for 1 min, with the substrate bias (Ub) being 1 kV, the arc current of Table 1 Deposition conditions of the Ti–Cr–N coatings Sample
Pretreatment by chromium ions
Deposition of coating Ub [V]
G1 G2 G3
U b ¼ 1 kV; P ¼ 103 Pa
120 120 120
PN, 101 [Pa]
1 1 1
Variation of arc current of cathodes ITi [A]
ICr [A]
50–100 100–50 100
100–50 50–100 100
ARTICLE IN PRESS V.M. Anischik et al. / Vacuum 78 (2005) 545–550
The phase composition and preferred orientation in Ti–Cr–N films were investigated by X-ray diffraction using CuKa characteristic X-ray radiation. A TEM Hitachi H800 was used to obtain micrographs, and a 200-kV instrument was used for nanoprobe diffraction. The microstructure of the coatings was studied by SEM Hitachi S806.
3. Results and discussion 3.1. Chemical composition Fig. 1 shows SIMS concentration profiles of elements in the Ti–Cr–N coating deposited on steel substrate (Fig. 1a, c and e), obtained under different conditions of deposition. N+, Cr+, Ti+ secondary ion of the main components of the layer are normalised with respect to their values measured prior to reaching the interface. Fe+ secondary ion current is normalised to the sable value measured during sputtering the St3 steel substrate. Assuming the direct relation of ion current and concentration, we can conclude that in coatings (G1, G2) the Cr and Ti concentration increases (decreases) with the thickness of the deposited films. In all conditions the nitrogen concentration remains constant (Fig. 1a and c). But, for the condition G3 the contents of Ti, Cr, N elements do not change with the depth of coating (Fig. 1e). These data indicate that the gradient by composition Ti–Cr–N coatings (Fig. 1a and c) and the film with the constant composition (Fig. 1e) are formed. At the Ti–Cr–N/steel interface we can see higher signals both of Cr+ and Ti+ currents (Fig. 1a, c and e). It is due to the presence of oxide to oxide layer on the surface of substrate, which enhances the emission of both positive metallic ions. This oxide layer appears by absorption of residual gases in the initial stage of film formation [6]. Higher signal of Cr+ currents is connected also with formation of transition Cr layer, which is created by pretreatment of substrate with chromium ion. The analysis of SIMS concentration profiles of Ti–Cr–N coatings indicates that the correlation
547
between the conditions of deposition (variation of arc currents ITi and ICr) and the concentration of titanium and chromium is observed. Application of SIMS method allows to obtain concentration profiles with high precision. But, the main problem consists in quantitative assessment of layer composition. Though this method provides high sensitivity in analysis of coatings, we can determine the absolute concentration only by comparing it with the standard sample. As the investigations show, the formed gradient coatings are complicated and non-homogeneous systems, and standard samples are difficult to be made for these films. Thus, the AES method was used for elemental analysis of Ti–Cr–N coatings. This method has also deficiency in approach to the system Ti–Cr–N (KVV peak of nitrogen overlaps LMM peak of titanium). Thus, the data of SIMS investigations of concentration profile (distribution of nitrogen in coating) were used for segregating of these peaks. Fig. 1 shows that concentration profiles, obtained by SIMS method, are correlated with the results of AES investigations of Ti–Cr–N coatings on Si substrate (Fig. 1b, d and f). The formation of the main layer Ti–Cr–N with variable concentration of Ti and Cr elements in depth and Cr-rich transition layer is observed. So, in Fig. 1b, d the relative concentration of titanium rises (decreases) and the concentration of chromium decreases (increases) with the increasing sputtering time. Fig. 1d shows the different dependence: the concentration of Ti and Cr does not change with the thickness of coating and is equal to 28 and 22 at%, respectively. In all cases, nitrogen concentration remains constant and is equal to 50 at%. The obtained AES data of concentration of elements in the main layer allow to present the composition of gradient coatings in the form of (TixCr1x)N, where x is changing in the range 0.60–0.84 (G1, Table 1), 0.25–0.67 (G2, Table 1) and also coating of the constant composition Ti0.56Cr0.44N (G3, Table 1). Thus, Ti–Cr–N coatings formed with combining the plasma flows of titanium and chromium of variable density present the gradient in its
ARTICLE IN PRESS V.M. Anischik et al. / Vacuum 78 (2005) 545–550 2.0
100
1.8 Ti
1.4
Fe
1.2 N
1.0 0.8
Cr
0.6
70 Si
60 N
50 40
Ti
30 20
0.2
10
0.0
0 0.7 0.8 0.9 1.0 1.1
Cr
0
120 240 360 480 600 720 840 960 1080 1200 1320
(b)
Depth [µm]
Sputter time [s] 100
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
90
(TiXCr1-X)N (0.25
Normalized secondary ion current [arb.]
(TiXCr1-X)N (0.60
80
0.4
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Cr
N
Fe
Ti
(TiXCr1-X)N (0.25
80 70
Si
60 N
50 40
Ti
30 20
Cr
10 0 0.0
0.2
0.4
(c) Normalized secondary ion current [arb.]
Concentration [at.%]
1.6
(a)
0.6
0.8
1.0
1.2
0
120 240 360 480 600 720 840 960 1080 1200
(d)
Depth [µm]
Sputter time [s] 100
1.4
90 Ti
1.2
Cr
Fe
1.0 N
0.8 0.6
(Ti0.56Cr0.44)N 0.4
(Ti0.56Cr0.44)N
80 70
Si
60 N
50 40
Ti
30 20
0.2
Cr
10 0
0.0 0.0
(e)
90
(TiXCr1-X)N (0.60
Concentration [at.%]
Normalized secondary ion current [arb.]
548
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Depth [µm]
0
(f)
120
240
360
480
600
720
840
960
Sputter time [s]
Fig. 1. Concentration profiles of Ti–Cr–N coatings, obtained by SIMS (a, c, e) and AES methods (b, d, f).
composition system TixCr1xN (0:60oxo0:84 and 0:25oxo0:67) with a variable concentration of elements Ti and Cr and the system Ti0.56Cr0.44N with a constant composition.
The research results of phase composition and microstructure of the obtained gradient systems for investigations of main and transition layers were obtained.
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3.2. XRD analysis and microstructure (TixCr1– x)N (222)
Fig. 2 presents the fragment of the XRD pattern of Ti–Cr–N coatings (G1 and G2) and silicon substrate, after being treated by chromium ions. The basic diffraction peak of coating (Fig. 2a) is located between the peaks of nitrides of titanium TiN and chromium CrN, which indicates the formation of the solid solution with the B1 NaCl crystal structure and preferred orientation (2 0 0) whose composition is Ti0.56Cr0.44N (Fig. 1f). The wide diffraction peak is observed between the peaks of nitride of titanium and chromium (Fig. 2b), which is formed evidently with the superposition of the peaks of the solid solutions of different concentration TixCr1xN ð0:60oxo0:84Þ (Fig. 1b) with the B1 NaCl crystal structure and preferred orientation (2 0 0). The appearance of a wide diffraction peak of TixCr1xN gradient coating is connected with the formation of fine-grained structure. Grain size from the peak broadening FWHM of gradient coating is estimated to be 10 nm. The TEM analysis of gradient (G1) coatings shows that actual solid solution with the B1 NaCl crystal structure and preferred orientation (2 0 0) is formed (Fig. 3a). Fig. 3b presents the TEM micrograph of gradient coating TixCr1xN (0.60oxo0.84). One can see that the grain size of
38
36
2θ
40
44
Ti2N (111)
Intensity [a.u.]
Ti0.54Cr0.46N (111)
46
48
Ti0.56Cr0.44N (200) Ti2N (200)
TixCr1-xN (200)
a
(0.60
b TiN
CrN
Cr2N (200) c
Cr2N (110) Ti2N (200)
TiSi2 (150)
CrSi2(110)
CrSi2(003)
CrSi2(111)
Cr3Si
Fig. 2. X-ray diffraction patterns of coatings.
(TixCr1– x)N (311) (TixCr1– x)N (111)
(TixCr1– x)N (200) (TixCr1– x)N (220) (a)
60 nm (b) Fig. 3. TEM investigations of gradient coating TixCr1xN ð0:60oxo0:84Þ:
the coating is about 10 nm. This result is in good agreement with the data obtained from the XRD measurements. The fine-grained structure of gradient coatings is also confirmed by the results of SEM investigations (Fig. 4). The columnar structure could be clearly seen in this coating. However, the gradient coating was composed of columnar grains with the intermittent structure and the size of grains about 30–50 nm. The high contrast variations are consistent with a high lattice defect density, which is typical of the coatings obtained by the CAVD method with high-energy bombardment [7]. Besides the solid solution, the phases of titanium and chromium silicide and nitrides (CrSi2, Cr3Si, TiSi2 and Ti2N and Cr2N) are formed in the Ti–Cr–N coatings. The formation of silicides occurs in the stage of pretreatment of substrate
ARTICLE IN PRESS 550
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4. Conclusions
600 nm Fig. 4. SEM image of the cross-section of the gradient coating TixCr1xN ð0:60oxo0:84Þ:
by chromium ions and in the initial stage of the coating deposition. The thermal (temperature of substrate is about 450 1C) and radiation-enhanced diffusion of film and substrate components occurs in the stage of pretreatment [8]. In addition, relatively high energy of ions converts to heat, which increases mobility and stimulates surface diffusion of adsorbed atoms. This is confirmed by concentration profiles of AES investigations. One can see the interlayer on the substrate boundary, which contained Ti, Cr, N and Si elements (Fig. 1b, d and f). Moreover, the high content of Cr is shown by large amount of chromium silicide. The investigations of gradient coatings structure show that the main layer represents the continuous series of the solid solutions TiN–CrN of different concentration with the B1 NaCl crystal structure and preferred orientation (2 0 0). This layer has a columnar structure with the grain size about 10–50 nm. The interlayer represents the polyphase system: Ti2N, Cr2N and silicide of titanium and chromium of different stoichiometry.
The gradient in its composition systems is formed by vacuum-arc deposition combining Ti and Cr plasma flows of variable density. The formed coatings are continuous series of solid solutions TixCr1xN (0:60oxo0:84 and 0:25oxo0:67) of the B1 NaCl crystal structure and with the preferred orientation of grains (2 0 0). The polyphase system (nitrides and silicides of titanium and chromium (Ti2N, Cr2N and TiSi2, CrSi2, Cr3Si)) is formed on the interface of coating-substrate (Si). It is found, that the correlation between the conditions of deposition (variation of arc currents of Ti and Cr) and the behaviour of concentration of titanium and chromium with the coating thickness is observed. It is shown that the formed coatings possess the dense, fine-grained structure. Microstructure of the coatings has some features of a columnar structure. At the same time, the broken structure of column with the grain size 10–50 nm is observed for the gradient coating TixCr1xN ð0:60oxo0:84Þ: References [1] Knotek O, Loffler F, Kramer G. Surf Coat Technol 1993;59:14–20. [2] Wen F, Dai H, Huang N, Sun H, Leng YX, Chu PK. Surf Coat Technol 2002;156:208–13. [3] Jehn HA. Coat Technol 2000;131:433–40. [4] Anishchik VM, Uglov VV, Zlotski SV, Khodasevich VV, Ukhov VA. New electrical and electronic technologies and their industrial implementation: proceedings III international symposium 2003:13–6. [5] Xianting Zeng, Sam Zhang, Joe Hsieh. Surf Coat Technol 1998;102:108–12. [6] Cherepin VT. Ionni zond A. Kiev: Navuk dumk; 1981. p. 328. [7] Ljungcrantz H, Hultman L, Sundgren J-E. J Appl Phys 1995;78:832–8. [8] Marin PJ, Macleod HA, Neterfild RP. Appl Opt 1983;22:178–81.
Vacuum 78 (2005) 545–550 www.elsevier.com/locate/vacuum
SIMS investigation of nitride coatings V.M. Anischika, V.V. Uglova,, S.V. Zlotskia, P. Konarskib, M. Cwilb, V.A. Ukhovc a
Department of Physics, Belarusian State University, F.Skariny Ave. 4, 220080 Minsk, Belarus b Industrial Institute of Electronics, ul. Dluga 44/50, 00-241 Warszawa, Poland c Belmicrosystems, Korzhenevsky 12, 220064 Minsk, Belarus
Abstract The Ti–Cr–N coatings were formed by condensation from a plasma phase in a vacuum with ion bombardment of sample surfaces while combining Ti and Cr plasma flows of variable density in a residual nitrogen atmosphere. The elemental and phase composition and also microstructure were studied by secondary-ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SIMS and AES investigations show that the system of gradient by composition was formed. It is found that the gradient coating consists of basic layer of variable composition, in which concentration of Ti and Cr elements increase or decrease with the coating thickness and Cr-rich interlayer. XRD and TEM measurements show that a continuous series of solid solution TixCr1xN (0.60oxo0.84 and 0.25oxo0.67) of variable composition with preferred orientation (2 0 0) was formed. Cross-section SEM shows that the dense finegrained structures with the average grain size of 10–50 nm were formed. r 2005 Elsevier Ltd. All rights reserved. Keywords: CAVD gradient Ti–Cr–N coatings; SIMS investigations; Microstructure
1. Introduction Thin hard coatings of nitrides of transitional metals fabricated by the cathodic arc vapour deposition (CAVD) are now widely used as corrosion and wear resistance protective coatings. This is due, in the first place, to their improved Corresponding author. +375 17 2095512;
fax: +375 17 2265552. E-mail address: [email protected] (V.V. Uglov).
mechanical properties, such as high oxidation and corrosion resistance, wear resistance and enhanced hardness. Ternary transition metal nitride coatings provide a wide range of structures that enable control of mechanical and electronic properties. Moreover, ternary compound coatings often exhibit fine-grained and distorted structures resulting in high hardness [1]. But, the deposition of coatings directly onto substrate surfaces usually reduces the adhesion strength due to higher brittleness and residual
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.083
ARTICLE IN PRESS V.M. Anischik et al. / Vacuum 78 (2005) 545–550
546
stress [2]. The formation of gradient ternary systems with a variable concentration of elements, both on interphase boundary and inside the coating allows to increase adhesion and to improve mechanical properties [3]. But, gradient systems are complex enough for investigation of coatings with non-homogeneous changing of elemental and phase composition. Thus, the investigation of elemental composition of such films requires the application of several methods like secondary-ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), Rutherford backscattering, etc. in combination with X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thus, this work is devoted to studying of the element, phase composition and also microstructure of Ti–Cr–N gradient coatings, formed by vacuum-arc deposition with the use of combining Ti and Cr plasma flows.
Cr cathode (ICr) 100 A, the vacuum in the chamber was P ¼ 103 Pa: Cr was chosen for ion clearing because of good adhesion on steel substrates [5]. This stage provided the heating of the substrate to 450 1C. Film deposition was initiated by injecting N2 gas at a pressure PN ¼ 101 Pa into the vacuum chamber with the substrate bias equal to 120 V. The gradient Ti–Cr–N system was deposited by varying the arc currents of Ti and Cr cathodes (Table 1). The total thickness of the formed coatings is about 0.8–1.2 mm depending on condition of deposition. Combination of plasma flows of material evaporated from cathodes at the surface of substrate was provided with two cathodes situated at an angle to each other [4]. Relation of concentrations of component of coatings is linearly dependent on the ratio of plasma flow density in these conditions. SIMS analyses were performed on SALW-05 apparatus equipped with 06-350E Physical Electronics ion gun and QMA-410 Balzers 16 mm quadrupole analyser. Argon ion beam of 5 keV energy and 100 mm full-width at the halfmaximum (FWHM) beam diameter, was digitally rastered in a spiral mode over 1 mm 1.65 mm area using the 128 128 pixel frame. In order to calibrate sputtering rate of the coating layers, the eroded craters were analysed ‘‘ex situ’’ using the Tencor alpha-sep 100 profilometer. The elemental composition of Ti–Cr–N coatings was determined by AES with the step-by-step sputtering of the samples surface layer by argon ions with the energy of 3 keV. The scanning microprobe PHi-660 was used for measurements.
2. Experimental Gradient Ti–Cr–N coatings were formed by condensation from a plasma phase in a vacuum with the ion bombardment of the samples surface combining the plasma flows of titanium and chromium of variable density in a residual nitrogen atmosphere [4]. Carbon steel St3 ðo0:18 wtCÞ and silicon (1 0 0) orientation were used as substrate material. Prior to the deposition, the surface of substrates was cleaned and heated by chromium ion bombardment for 1 min, with the substrate bias (Ub) being 1 kV, the arc current of Table 1 Deposition conditions of the Ti–Cr–N coatings Sample
Pretreatment by chromium ions
Deposition of coating Ub [V]
G1 G2 G3
U b ¼ 1 kV; P ¼ 103 Pa
120 120 120
PN, 101 [Pa]
1 1 1
Variation of arc current of cathodes ITi [A]
ICr [A]
50–100 100–50 100
100–50 50–100 100
ARTICLE IN PRESS V.M. Anischik et al. / Vacuum 78 (2005) 545–550
The phase composition and preferred orientation in Ti–Cr–N films were investigated by X-ray diffraction using CuKa characteristic X-ray radiation. A TEM Hitachi H800 was used to obtain micrographs, and a 200-kV instrument was used for nanoprobe diffraction. The microstructure of the coatings was studied by SEM Hitachi S806.
3. Results and discussion 3.1. Chemical composition Fig. 1 shows SIMS concentration profiles of elements in the Ti–Cr–N coating deposited on steel substrate (Fig. 1a, c and e), obtained under different conditions of deposition. N+, Cr+, Ti+ secondary ion of the main components of the layer are normalised with respect to their values measured prior to reaching the interface. Fe+ secondary ion current is normalised to the sable value measured during sputtering the St3 steel substrate. Assuming the direct relation of ion current and concentration, we can conclude that in coatings (G1, G2) the Cr and Ti concentration increases (decreases) with the thickness of the deposited films. In all conditions the nitrogen concentration remains constant (Fig. 1a and c). But, for the condition G3 the contents of Ti, Cr, N elements do not change with the depth of coating (Fig. 1e). These data indicate that the gradient by composition Ti–Cr–N coatings (Fig. 1a and c) and the film with the constant composition (Fig. 1e) are formed. At the Ti–Cr–N/steel interface we can see higher signals both of Cr+ and Ti+ currents (Fig. 1a, c and e). It is due to the presence of oxide to oxide layer on the surface of substrate, which enhances the emission of both positive metallic ions. This oxide layer appears by absorption of residual gases in the initial stage of film formation [6]. Higher signal of Cr+ currents is connected also with formation of transition Cr layer, which is created by pretreatment of substrate with chromium ion. The analysis of SIMS concentration profiles of Ti–Cr–N coatings indicates that the correlation
547
between the conditions of deposition (variation of arc currents ITi and ICr) and the concentration of titanium and chromium is observed. Application of SIMS method allows to obtain concentration profiles with high precision. But, the main problem consists in quantitative assessment of layer composition. Though this method provides high sensitivity in analysis of coatings, we can determine the absolute concentration only by comparing it with the standard sample. As the investigations show, the formed gradient coatings are complicated and non-homogeneous systems, and standard samples are difficult to be made for these films. Thus, the AES method was used for elemental analysis of Ti–Cr–N coatings. This method has also deficiency in approach to the system Ti–Cr–N (KVV peak of nitrogen overlaps LMM peak of titanium). Thus, the data of SIMS investigations of concentration profile (distribution of nitrogen in coating) were used for segregating of these peaks. Fig. 1 shows that concentration profiles, obtained by SIMS method, are correlated with the results of AES investigations of Ti–Cr–N coatings on Si substrate (Fig. 1b, d and f). The formation of the main layer Ti–Cr–N with variable concentration of Ti and Cr elements in depth and Cr-rich transition layer is observed. So, in Fig. 1b, d the relative concentration of titanium rises (decreases) and the concentration of chromium decreases (increases) with the increasing sputtering time. Fig. 1d shows the different dependence: the concentration of Ti and Cr does not change with the thickness of coating and is equal to 28 and 22 at%, respectively. In all cases, nitrogen concentration remains constant and is equal to 50 at%. The obtained AES data of concentration of elements in the main layer allow to present the composition of gradient coatings in the form of (TixCr1x)N, where x is changing in the range 0.60–0.84 (G1, Table 1), 0.25–0.67 (G2, Table 1) and also coating of the constant composition Ti0.56Cr0.44N (G3, Table 1). Thus, Ti–Cr–N coatings formed with combining the plasma flows of titanium and chromium of variable density present the gradient in its
ARTICLE IN PRESS V.M. Anischik et al. / Vacuum 78 (2005) 545–550 2.0
100
1.8 Ti
1.4
Fe
1.2 N
1.0 0.8
Cr
0.6
70 Si
60 N
50 40
Ti
30 20
0.2
10
0.0
0 0.7 0.8 0.9 1.0 1.1
Cr
0
120 240 360 480 600 720 840 960 1080 1200 1320
(b)
Depth [µm]
Sputter time [s] 100
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
90
(TiXCr1-X)N (0.25
Normalized secondary ion current [arb.]
(TiXCr1-X)N (0.60
80
0.4
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Cr
N
Fe
Ti
(TiXCr1-X)N (0.25
80 70
Si
60 N
50 40
Ti
30 20
Cr
10 0 0.0
0.2
0.4
(c) Normalized secondary ion current [arb.]
Concentration [at.%]
1.6
(a)
0.6
0.8
1.0
1.2
0
120 240 360 480 600 720 840 960 1080 1200
(d)
Depth [µm]
Sputter time [s] 100
1.4
90 Ti
1.2
Cr
Fe
1.0 N
0.8 0.6
(Ti0.56Cr0.44)N 0.4
(Ti0.56Cr0.44)N
80 70
Si
60 N
50 40
Ti
30 20
0.2
Cr
10 0
0.0 0.0
(e)
90
(TiXCr1-X)N (0.60
Concentration [at.%]
Normalized secondary ion current [arb.]
548
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Depth [µm]
0
(f)
120
240
360
480
600
720
840
960
Sputter time [s]
Fig. 1. Concentration profiles of Ti–Cr–N coatings, obtained by SIMS (a, c, e) and AES methods (b, d, f).
composition system TixCr1xN (0:60oxo0:84 and 0:25oxo0:67) with a variable concentration of elements Ti and Cr and the system Ti0.56Cr0.44N with a constant composition.
The research results of phase composition and microstructure of the obtained gradient systems for investigations of main and transition layers were obtained.
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3.2. XRD analysis and microstructure (TixCr1– x)N (222)
Fig. 2 presents the fragment of the XRD pattern of Ti–Cr–N coatings (G1 and G2) and silicon substrate, after being treated by chromium ions. The basic diffraction peak of coating (Fig. 2a) is located between the peaks of nitrides of titanium TiN and chromium CrN, which indicates the formation of the solid solution with the B1 NaCl crystal structure and preferred orientation (2 0 0) whose composition is Ti0.56Cr0.44N (Fig. 1f). The wide diffraction peak is observed between the peaks of nitride of titanium and chromium (Fig. 2b), which is formed evidently with the superposition of the peaks of the solid solutions of different concentration TixCr1xN ð0:60oxo0:84Þ (Fig. 1b) with the B1 NaCl crystal structure and preferred orientation (2 0 0). The appearance of a wide diffraction peak of TixCr1xN gradient coating is connected with the formation of fine-grained structure. Grain size from the peak broadening FWHM of gradient coating is estimated to be 10 nm. The TEM analysis of gradient (G1) coatings shows that actual solid solution with the B1 NaCl crystal structure and preferred orientation (2 0 0) is formed (Fig. 3a). Fig. 3b presents the TEM micrograph of gradient coating TixCr1xN (0.60oxo0.84). One can see that the grain size of
38
36
2θ
40
44
Ti2N (111)
Intensity [a.u.]
Ti0.54Cr0.46N (111)
46
48
Ti0.56Cr0.44N (200) Ti2N (200)
TixCr1-xN (200)
a
(0.60
b TiN
CrN
Cr2N (200) c
Cr2N (110) Ti2N (200)
TiSi2 (150)
CrSi2(110)
CrSi2(003)
CrSi2(111)
Cr3Si
Fig. 2. X-ray diffraction patterns of coatings.
(TixCr1– x)N (311) (TixCr1– x)N (111)
(TixCr1– x)N (200) (TixCr1– x)N (220) (a)
60 nm (b) Fig. 3. TEM investigations of gradient coating TixCr1xN ð0:60oxo0:84Þ:
the coating is about 10 nm. This result is in good agreement with the data obtained from the XRD measurements. The fine-grained structure of gradient coatings is also confirmed by the results of SEM investigations (Fig. 4). The columnar structure could be clearly seen in this coating. However, the gradient coating was composed of columnar grains with the intermittent structure and the size of grains about 30–50 nm. The high contrast variations are consistent with a high lattice defect density, which is typical of the coatings obtained by the CAVD method with high-energy bombardment [7]. Besides the solid solution, the phases of titanium and chromium silicide and nitrides (CrSi2, Cr3Si, TiSi2 and Ti2N and Cr2N) are formed in the Ti–Cr–N coatings. The formation of silicides occurs in the stage of pretreatment of substrate
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4. Conclusions
600 nm Fig. 4. SEM image of the cross-section of the gradient coating TixCr1xN ð0:60oxo0:84Þ:
by chromium ions and in the initial stage of the coating deposition. The thermal (temperature of substrate is about 450 1C) and radiation-enhanced diffusion of film and substrate components occurs in the stage of pretreatment [8]. In addition, relatively high energy of ions converts to heat, which increases mobility and stimulates surface diffusion of adsorbed atoms. This is confirmed by concentration profiles of AES investigations. One can see the interlayer on the substrate boundary, which contained Ti, Cr, N and Si elements (Fig. 1b, d and f). Moreover, the high content of Cr is shown by large amount of chromium silicide. The investigations of gradient coatings structure show that the main layer represents the continuous series of the solid solutions TiN–CrN of different concentration with the B1 NaCl crystal structure and preferred orientation (2 0 0). This layer has a columnar structure with the grain size about 10–50 nm. The interlayer represents the polyphase system: Ti2N, Cr2N and silicide of titanium and chromium of different stoichiometry.
The gradient in its composition systems is formed by vacuum-arc deposition combining Ti and Cr plasma flows of variable density. The formed coatings are continuous series of solid solutions TixCr1xN (0:60oxo0:84 and 0:25oxo0:67) of the B1 NaCl crystal structure and with the preferred orientation of grains (2 0 0). The polyphase system (nitrides and silicides of titanium and chromium (Ti2N, Cr2N and TiSi2, CrSi2, Cr3Si)) is formed on the interface of coating-substrate (Si). It is found, that the correlation between the conditions of deposition (variation of arc currents of Ti and Cr) and the behaviour of concentration of titanium and chromium with the coating thickness is observed. It is shown that the formed coatings possess the dense, fine-grained structure. Microstructure of the coatings has some features of a columnar structure. At the same time, the broken structure of column with the grain size 10–50 nm is observed for the gradient coating TixCr1xN ð0:60oxo0:84Þ: References [1] Knotek O, Loffler F, Kramer G. Surf Coat Technol 1993;59:14–20. [2] Wen F, Dai H, Huang N, Sun H, Leng YX, Chu PK. Surf Coat Technol 2002;156:208–13. [3] Jehn HA. Coat Technol 2000;131:433–40. [4] Anishchik VM, Uglov VV, Zlotski SV, Khodasevich VV, Ukhov VA. New electrical and electronic technologies and their industrial implementation: proceedings III international symposium 2003:13–6. [5] Xianting Zeng, Sam Zhang, Joe Hsieh. Surf Coat Technol 1998;102:108–12. [6] Cherepin VT. Ionni zond A. Kiev: Navuk dumk; 1981. p. 328. [7] Ljungcrantz H, Hultman L, Sundgren J-E. J Appl Phys 1995;78:832–8. [8] Marin PJ, Macleod HA, Neterfild RP. Appl Opt 1983;22:178–81.