- Email: [email protected]
Electrodeposition of [001] oriented TiB2 coatings
Materials Letters 59 (2005) 3234 – 3237 www.elsevier.com/locate/matlet
Electrodeposition of [001] oriented TiB2 coatings Jun Li a,*, Bing Li a, Zheng’e Dong b a
School of Resource and Environment Engineering, East China University of Science and Technology, Shanghai 200237, China b School of Textile and Material, Xi_an Institute of Science and Technology, Xi_an 710048, China Received 21 January 2005; accepted 23 May 2005 Available online 16 June 2005
Abstract This paper focused on the preparation of titanium diboride (TiB2) coatings by electroplating in fluoride – chloride electrolytes (KF – KCl) containing K2TiF6 and KBF4 as the electrochemically-active components. X-ray diffraction (XRD) analyses indicated that the preferred orientation of TiB2 coatings is [001], which is in accordance with the prediction of the two-dimensional crystal nuclei theory. The effect of the current density on average crystal size and texture coefficient was also studied. Thermodynamic predictions and experimental results showed the substrate and TiB2 coatings bind physically. D 2005 Elsevier B.V. All rights reserved. Keywords: Electrodeposition; Titanium diboride; Coatings
1. Introduction The shortcomings of carbonaceous materials in smelting cells have stimulated an extensive investigation into the possibility of replacing both the carbon cathode and the cell lining by other materials. Some research results have shown TiB2 comes closest to meet the strict requirements to cathode material and ideal cathodes lining [1,2]. As compared with other possible techniques of production of refractory borides, molten salts generally can be considered as a very promising medium for chemical and electrochemical synthesis of different refractory borides [3]. The electrochemical synthesis techniques consist of the continuous current plating (CCP) and the pulse current plating (PCP) electrochemical techniques. The two most frequent current profiles at PCP are the periodically interrupted current (PIC), in which the current is a unidirectional square-wave, and the periodic reverse current (PRC), where a bidirectional square waves is used. Compared with the CCP technique, the PCP technique has more advantages [4]. * Corresponding author. Tel.: +86 21 64252601; fax: +86 21 64252601. E-mail address: [email protected] (J. Li). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.050
Grounded on above analyses and considerations, the paper adopted the electrodeposition methods (CCP and PIC) to synthesize the TiB2 coating on the graphite substrate, for the thermal expansion coefficient (4.3 Am/m/-C)of graphite is close to that (4.6 Am/m/-C) of titanium diboride [5]. It is an initial attempt to explore the preferred orientation, average crystal size, texture coefficient and adherence property of coatings, previous studies focus mainly on the topography of coatings, and have little detailed discussion about these.
2. Experimental 2.1. Coated process The supporting electrolytes used in the electrodeposition were the mixture of fluorides and chlorides (KF – KCl, 94– 6 mol%) and electroactive constituents employed were a mixture of K2TiF6 and KBF4 in the molar ratio of 1 : 5. The experiment was performed in an electrolytic container at 800 -C under high-purity dry argon. The deposition was carried out for 60 min at CCP with current density varying from 0.6 to 0.8 A/cm2, and for 30 min at PIC with the same
J. Li et al. / Materials Letters 59 (2005) 3234 – 3237
-50
-150
∆Go (kJ/mol)
3. Results and discussion
B4C
-100
-200
TiC TiB2
3.1. Thermodynamic predictions
TiB
The chemical reactions occurring in the coatings are very essential because they are helpful for predicting the nature and properties of the coatings. The application of thermodynamics requires the consideration of all possible reactions occurring in the coatings. For the system of Ti – B – C, the reactions between Ti – B, Ti – C and B – C are considered. Ti – B system:
-250 -300 -350 -400 -450
Ti þ B ¼ TiB;
-500 200 400 600 800 1000 1200 1400 1600 1800 2000
Temperature (K) Fig. 1. Gibbs free energy of formation of borides of Ti, boron carbide and titanium carbide.
current density range. For the PIC deposition, the research results indicated 100 Hz was the most suitable at the range from 10 to 1000 Hz [4], so the 100 Hz for a ratio of t c / t off = 9 / 1 was selected in this paper. 2.2. Characterization 2.2.1. XRD analyses A Rigaku D/mas 2550 X-ray diffractometer with CuKa1 (k = 0.15406 nm) radiation was used for phase identification. The tube voltage was 40 kV, the current was 100 mA, and scanning speed was 2.5 deg/min. Some parameters can be calculated by following formulas: Average crystal size: Dhkl
Kk ðnmÞ ¼ bcosh
3235
Ti þ 2B ¼ TiB2 ;
3Ti þ 4B ¼ Ti3 B4 :
The thermodynamic data for Ti3B4 are unknown, but according to the Ti – B phase diagram [6], there is no possibility of formation of Ti3B4 at the experiment conditions, henceforth only TiB and TiB2 will be considered for further analyses and discussion. Ti C system :
Ti þ C ¼ TiC;
B C system :
4B þ C ¼ B4 C:
Fig. 1 shows the Gibbs free energy of formation of borides and carbides as a function of temperature. These data are based on the Ref. [7]. It is evident that the standard Gibbs free energies of formation are negative up to 2000 K, which indicates the formation of these compounds is spontaneous, and with the increase of the temperature, the standard Gibbs free energies of formation gets more negative, which shows the spontaneous trend of these compounds gets more obvious. During the same temperature, the spontaneous trend of these compounds is followed: TiB2 > TiC > TiB > B4C, that is to say, the TiB2 is the most stable boride in the Ti – B – C system. 3.2. XRD analyses
ð1Þ
where K is the Scherrer constant, the calculation was performed with a value K = 0.89, k is the X-ray wavelength, b is the width of the X-ray peak when the height of the Xray peak is half, h is the Bragg angle. Texture coefficient:
The X – ray diffraction spectrums reveal the coatings obtained by CCP and PIC are composed of the relatively pure TiB2 at above experimental conditions. Fig. 2 illustrates the X – ray pattern of the coating prepared by PIC. Clear peaks can be detected by glancing 30000
TiB2[001]
ð2Þ
i¼1
where I [hkl] and I 0[hkl] are the integrated intensities from an [hkl] peak, obtained for the TiB2 deposits and for TiB2 with random texture, respectively. 2.2.2. Interface and element analyses Interface and element analyses were carried out on a JSM-6360LV Scanning Electron Microscope (SEM) coupled with a FALCON Energy Dispersive X-ray Spectrum (EDS).
Intensity (a.u.)
25000
I½hkl =I0½hkl T:C: ¼ X 100% n I½hkl =I0½hkl
20000 15000 TiB2[002]
10000 5000 0 10
20
30
40
50
60
70
80
2Theta (deg.) Fig. 2. X-ray diffraction of the coating for PIC, frequency = 100 Hz; i c = 0.8 A/cm2; t c / t off = 9 / 1.
3236
J. Li et al. / Materials Letters 59 (2005) 3234 – 3237
3.0 [001] [101] [101] [001] [110] [100]
40
CCP PIC
Generation energy W[hkl]
Average crystal size D001 (nm)
42
38 36 34 32 30 28
2.5 2.0 1.5 1.0 0.5 0.0
0.60
0.65
0.70
0.75
[112]
0.80
3
5
4
Fig. 3. Average crystal size D 001 variation with the current density.
angle. The diffraction pattern corresponds well to the JCPDS standard (PDF# 35-741) for TiB2. Some low-intensity peaks can_t be identified, which may be attributed to adhesion of a trace of electrolytes such as KF, KCl, K2TiF6 and KBF4. According to the Eq. (1), the average crystal size can be evaluated, and the result is showed in Fig. 3. At the same current density, the coatings deposited by PIC possess finer grains than that deposited by CCP, and for PIC or CCP, with the increase of the current density, the average crystal size gets smaller. It is well known that the crystal size depends on two factors: nucleation speed and grain growth speed. If nucleation speed is faster than grain growth speed, the grain size will get small. The increase of over potential is propitious to nucleation not grain growth. Increase of the current density means increase of the over potential, so the crystal size gets smaller with increase of the current density. The over potential consists of two parts: concentration polarization and electrochemical polarization, the former makes no contribution for grain refining, and grain refining should be attributed to the latter. Compared with CCP, PIC can remarkably reduce the effective thickness of the diffusion layer, decrease concentration polarization and enhance the concentration of metal ions in the diffusion layer. Therefore, PIC leads to smaller grains. The preferred orientations of the TiB2 coatings prepared by CCP and PIC are [001] and [002] at above experimental conditions, the texture coefficient of the other peaks is less than
70 65 60 55 50 45 40 35
0.60
0.65
0.70
0.75
0.80
Current density (A/cm2) Fig. 4. Texture coefficient variation with the current density.
8
0.3% and lower than JCPDS standard other than [001] and [002]. [001] is parallel with [002], so [001] is used to represent the preferred orientation. Fig. 4 indicates the relationship of the texture coefficient with the current density, compared with CCP, the [001] texture coefficient of the TiB2 coatings prepared by PIC is greater, and the [002] texture coefficient gets smaller, the reason for that change is not very clear and needs to be studied further. According to the two-dimensional crystal nuclei theory, it is well known that preferred orientation is determined by the type of two-dimensional crystal nuclei, that is to say that growing style of crystal nuclei depends on the formation process of two-dimensional crystal nuclei determined by the electrodeposition conditions. W [hkl] is the growing energy of two-dimensional crystal nuclei with [hkl], it is clear that growing speed of two-dimensional crystal nuclei with the lowest W [hkl] value is fastest. W [hkl] satisfies the following equation: W½hkl ¼
zF N
B ½hkl g A½hkl
ð3Þ
in which z is the electric charge of electrodeposition ions, F is the Faraday constant, N is the Avogadro constant, g is the over
Concentration (wt%)
Texture coefficient (%)
75
7
Fig. 5. Generation energy W [hkl] as a function of over potential g for the hexagonal crystal system.
80 CCP [001] PIC [001] CCP [002] PIC [002]
6
Over potential η
Current density (A/cm2)
110 100 90 80 70 60 50 40 30 20 10 0 -10
B content Ti content C content
Interface
Substrate
-40
-20
Coating
0
20
40
Distance from the interface (µm) Fig. 6. Elemental depth profile of Ti, B and C in the coating and substrate for CCP, i c = 0.8 A/cm2.
J. Li et al. / Materials Letters 59 (2005) 3234 – 3237
potential, A [hkl] is the dimension of energy, B [hkl] is the dimension of square energy. For the different types of crystals nuclei, the value of A [hkl] and B [hkl] is different. TiB2 belongs to the hexagonal crystal system, the theoretical curve is offered in Fig. 5. According to the curve, it is easy to predict the preferred orientation under the given electrodeposition conditions. In this paper, the over potential is comparatively low, the preferred orientation of the TiB2 coatings should be [001], which accords well with the test results.
3237
TiB2
Graphite
3.3. Adherence property Referring to adherence property, it is necessary to consider the nature of possible reactions occurring within both the coating and the interface. This would reveal the nature of the interaction whether it is chemical or physical. All the possible chemical reactions for Ti – B – C system have been considered earlier, and it is found that the formation of TiB2 is the most thermodynamically favorable reaction. However, there is some possibility of formation of some phases such as Tix Cy and Ba Cb . XRD spectrums reveal the present phases in the coating and interface. Only TiB2 is showed, and TiB, TiC and B4C are absent, which indicates the substrate doesn_t adhere to the coatings by the transition layer adherence. That is to say the binding between the coatings and the substrate is possibly the physical binding. Concentration profiles for Ti, B and C further substantiate above analyses from the other hand, the experimental result is shown in Fig. 6. It is clear that the element Ti and B distribute uniformly throughout the whole coating, and the concentration of the element Ti and B and the ratio of Ti / B are close to the theoretical value. At the side of substrate, a trace of Ti and B is detected, which should be attributed to the penetration of K2TiF6 and KBF4 into the cathode, at the side of the coating, there is almost no element C detected, which indicates Ti and B deposited by electrochemical reaction and C don_t diffuse toward the cathode and the coating. It is well known that particles with enough kinetic energy can diffuse into the substrate, evidently, Ti and B deposited by electrochemical reaction lack enough kinetic energy. As shown in Fig. 7, the interface of the coating and the substrate is very clear and continuous, and is not gradually alterative, there is also no the transition layer, which accords with the above analyses.
4. Conclusions (1) High-quality TiB2 coatings can be successfully prepared by CCP and PIC in fluoride – chloride electrolytes (KF – KCl). Compared with CCP, the grain size of the TiB2 coatings prepared by the PIC is smaller.
25µm Fig. 7. Cross-section of the TiB2 coating for CCP, i c = 0.8 A/cm2.
(2) The preferred orientation of TiB2 coatings prepared by CCP and PIC is [001] at the experimental conditions, which can be well explained by two-dimension crystal nuclei theory. (3) Thermodynamic predictions, XRD and EDS analyses show the substrate and TiB2 coatings bind physically.
Acknowledgement This research was supported by the National Natural Science Foundation of China (No. 50204006).
References [1] C. Pfohl, A. Bulak, K.-T. Rie, Develop of titanium diboride coatings deposited by PACVD, Surface and Coatings Technology [J] (2000) 131 – 141. [2] Bing Li, Zhuxian Qiu, Jun Li, Yifu Ye, Zuxin Zhao, Study on electrodeposition of TiB2 coating on graphite substrate, Rare Metal Materials and Engineering [J] 33 (7) (2004) 764 – 767. [3] G. Kaptay, S.A. Kuznetsov, Electrochemical synthesis of refractory borides from molten salts, Plasmas & Ions [J] (1999) 2 – 45. [4] Gerhard Ett, Elisabete J. Pessine, Pulse current plating of TiB2 in molten fluoride, Electrochimica Acta [J] 44 (1999) 2859 – 2870. [5] G. Ett, E.J. Pessine, Titanium diboride (TiB2) formation by electroplating in molten salt with continuous & pulsed current, Plating and Surface Finishing [J] (2000 June) 118. [6] Jueqi Yu, Wenzhi Yi, Bangdi Chen, Hongjian Chen, Phase Diagrams for Binary Alloys [M], Shanghai Science and Technology Press, 1987, pp. 206 – 207. [7] Dalun Ye, Practical Inorganic Thermodynamics Manual [M], Metallurgy Industry Press, 1981, pp. 113 – 983.
Electrodeposition of [001] oriented TiB2 coatings Jun Li a,*, Bing Li a, Zheng’e Dong b a
School of Resource and Environment Engineering, East China University of Science and Technology, Shanghai 200237, China b School of Textile and Material, Xi_an Institute of Science and Technology, Xi_an 710048, China Received 21 January 2005; accepted 23 May 2005 Available online 16 June 2005
Abstract This paper focused on the preparation of titanium diboride (TiB2) coatings by electroplating in fluoride – chloride electrolytes (KF – KCl) containing K2TiF6 and KBF4 as the electrochemically-active components. X-ray diffraction (XRD) analyses indicated that the preferred orientation of TiB2 coatings is [001], which is in accordance with the prediction of the two-dimensional crystal nuclei theory. The effect of the current density on average crystal size and texture coefficient was also studied. Thermodynamic predictions and experimental results showed the substrate and TiB2 coatings bind physically. D 2005 Elsevier B.V. All rights reserved. Keywords: Electrodeposition; Titanium diboride; Coatings
1. Introduction The shortcomings of carbonaceous materials in smelting cells have stimulated an extensive investigation into the possibility of replacing both the carbon cathode and the cell lining by other materials. Some research results have shown TiB2 comes closest to meet the strict requirements to cathode material and ideal cathodes lining [1,2]. As compared with other possible techniques of production of refractory borides, molten salts generally can be considered as a very promising medium for chemical and electrochemical synthesis of different refractory borides [3]. The electrochemical synthesis techniques consist of the continuous current plating (CCP) and the pulse current plating (PCP) electrochemical techniques. The two most frequent current profiles at PCP are the periodically interrupted current (PIC), in which the current is a unidirectional square-wave, and the periodic reverse current (PRC), where a bidirectional square waves is used. Compared with the CCP technique, the PCP technique has more advantages [4]. * Corresponding author. Tel.: +86 21 64252601; fax: +86 21 64252601. E-mail address: [email protected] (J. Li). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.050
Grounded on above analyses and considerations, the paper adopted the electrodeposition methods (CCP and PIC) to synthesize the TiB2 coating on the graphite substrate, for the thermal expansion coefficient (4.3 Am/m/-C)of graphite is close to that (4.6 Am/m/-C) of titanium diboride [5]. It is an initial attempt to explore the preferred orientation, average crystal size, texture coefficient and adherence property of coatings, previous studies focus mainly on the topography of coatings, and have little detailed discussion about these.
2. Experimental 2.1. Coated process The supporting electrolytes used in the electrodeposition were the mixture of fluorides and chlorides (KF – KCl, 94– 6 mol%) and electroactive constituents employed were a mixture of K2TiF6 and KBF4 in the molar ratio of 1 : 5. The experiment was performed in an electrolytic container at 800 -C under high-purity dry argon. The deposition was carried out for 60 min at CCP with current density varying from 0.6 to 0.8 A/cm2, and for 30 min at PIC with the same
J. Li et al. / Materials Letters 59 (2005) 3234 – 3237
-50
-150
∆Go (kJ/mol)
3. Results and discussion
B4C
-100
-200
TiC TiB2
3.1. Thermodynamic predictions
TiB
The chemical reactions occurring in the coatings are very essential because they are helpful for predicting the nature and properties of the coatings. The application of thermodynamics requires the consideration of all possible reactions occurring in the coatings. For the system of Ti – B – C, the reactions between Ti – B, Ti – C and B – C are considered. Ti – B system:
-250 -300 -350 -400 -450
Ti þ B ¼ TiB;
-500 200 400 600 800 1000 1200 1400 1600 1800 2000
Temperature (K) Fig. 1. Gibbs free energy of formation of borides of Ti, boron carbide and titanium carbide.
current density range. For the PIC deposition, the research results indicated 100 Hz was the most suitable at the range from 10 to 1000 Hz [4], so the 100 Hz for a ratio of t c / t off = 9 / 1 was selected in this paper. 2.2. Characterization 2.2.1. XRD analyses A Rigaku D/mas 2550 X-ray diffractometer with CuKa1 (k = 0.15406 nm) radiation was used for phase identification. The tube voltage was 40 kV, the current was 100 mA, and scanning speed was 2.5 deg/min. Some parameters can be calculated by following formulas: Average crystal size: Dhkl
Kk ðnmÞ ¼ bcosh
3235
Ti þ 2B ¼ TiB2 ;
3Ti þ 4B ¼ Ti3 B4 :
The thermodynamic data for Ti3B4 are unknown, but according to the Ti – B phase diagram [6], there is no possibility of formation of Ti3B4 at the experiment conditions, henceforth only TiB and TiB2 will be considered for further analyses and discussion. Ti C system :
Ti þ C ¼ TiC;
B C system :
4B þ C ¼ B4 C:
Fig. 1 shows the Gibbs free energy of formation of borides and carbides as a function of temperature. These data are based on the Ref. [7]. It is evident that the standard Gibbs free energies of formation are negative up to 2000 K, which indicates the formation of these compounds is spontaneous, and with the increase of the temperature, the standard Gibbs free energies of formation gets more negative, which shows the spontaneous trend of these compounds gets more obvious. During the same temperature, the spontaneous trend of these compounds is followed: TiB2 > TiC > TiB > B4C, that is to say, the TiB2 is the most stable boride in the Ti – B – C system. 3.2. XRD analyses
ð1Þ
where K is the Scherrer constant, the calculation was performed with a value K = 0.89, k is the X-ray wavelength, b is the width of the X-ray peak when the height of the Xray peak is half, h is the Bragg angle. Texture coefficient:
The X – ray diffraction spectrums reveal the coatings obtained by CCP and PIC are composed of the relatively pure TiB2 at above experimental conditions. Fig. 2 illustrates the X – ray pattern of the coating prepared by PIC. Clear peaks can be detected by glancing 30000
TiB2[001]
ð2Þ
i¼1
where I [hkl] and I 0[hkl] are the integrated intensities from an [hkl] peak, obtained for the TiB2 deposits and for TiB2 with random texture, respectively. 2.2.2. Interface and element analyses Interface and element analyses were carried out on a JSM-6360LV Scanning Electron Microscope (SEM) coupled with a FALCON Energy Dispersive X-ray Spectrum (EDS).
Intensity (a.u.)
25000
I½hkl =I0½hkl T:C: ¼ X 100% n I½hkl =I0½hkl
20000 15000 TiB2[002]
10000 5000 0 10
20
30
40
50
60
70
80
2Theta (deg.) Fig. 2. X-ray diffraction of the coating for PIC, frequency = 100 Hz; i c = 0.8 A/cm2; t c / t off = 9 / 1.
3236
J. Li et al. / Materials Letters 59 (2005) 3234 – 3237
3.0 [001] [101] [101] [001] [110] [100]
40
CCP PIC
Generation energy W[hkl]
Average crystal size D001 (nm)
42
38 36 34 32 30 28
2.5 2.0 1.5 1.0 0.5 0.0
0.60
0.65
0.70
0.75
[112]
0.80
3
5
4
Fig. 3. Average crystal size D 001 variation with the current density.
angle. The diffraction pattern corresponds well to the JCPDS standard (PDF# 35-741) for TiB2. Some low-intensity peaks can_t be identified, which may be attributed to adhesion of a trace of electrolytes such as KF, KCl, K2TiF6 and KBF4. According to the Eq. (1), the average crystal size can be evaluated, and the result is showed in Fig. 3. At the same current density, the coatings deposited by PIC possess finer grains than that deposited by CCP, and for PIC or CCP, with the increase of the current density, the average crystal size gets smaller. It is well known that the crystal size depends on two factors: nucleation speed and grain growth speed. If nucleation speed is faster than grain growth speed, the grain size will get small. The increase of over potential is propitious to nucleation not grain growth. Increase of the current density means increase of the over potential, so the crystal size gets smaller with increase of the current density. The over potential consists of two parts: concentration polarization and electrochemical polarization, the former makes no contribution for grain refining, and grain refining should be attributed to the latter. Compared with CCP, PIC can remarkably reduce the effective thickness of the diffusion layer, decrease concentration polarization and enhance the concentration of metal ions in the diffusion layer. Therefore, PIC leads to smaller grains. The preferred orientations of the TiB2 coatings prepared by CCP and PIC are [001] and [002] at above experimental conditions, the texture coefficient of the other peaks is less than
70 65 60 55 50 45 40 35
0.60
0.65
0.70
0.75
0.80
Current density (A/cm2) Fig. 4. Texture coefficient variation with the current density.
8
0.3% and lower than JCPDS standard other than [001] and [002]. [001] is parallel with [002], so [001] is used to represent the preferred orientation. Fig. 4 indicates the relationship of the texture coefficient with the current density, compared with CCP, the [001] texture coefficient of the TiB2 coatings prepared by PIC is greater, and the [002] texture coefficient gets smaller, the reason for that change is not very clear and needs to be studied further. According to the two-dimensional crystal nuclei theory, it is well known that preferred orientation is determined by the type of two-dimensional crystal nuclei, that is to say that growing style of crystal nuclei depends on the formation process of two-dimensional crystal nuclei determined by the electrodeposition conditions. W [hkl] is the growing energy of two-dimensional crystal nuclei with [hkl], it is clear that growing speed of two-dimensional crystal nuclei with the lowest W [hkl] value is fastest. W [hkl] satisfies the following equation: W½hkl ¼
zF N
B ½hkl g A½hkl
ð3Þ
in which z is the electric charge of electrodeposition ions, F is the Faraday constant, N is the Avogadro constant, g is the over
Concentration (wt%)
Texture coefficient (%)
75
7
Fig. 5. Generation energy W [hkl] as a function of over potential g for the hexagonal crystal system.
80 CCP [001] PIC [001] CCP [002] PIC [002]
6
Over potential η
Current density (A/cm2)
110 100 90 80 70 60 50 40 30 20 10 0 -10
B content Ti content C content
Interface
Substrate
-40
-20
Coating
0
20
40
Distance from the interface (µm) Fig. 6. Elemental depth profile of Ti, B and C in the coating and substrate for CCP, i c = 0.8 A/cm2.
J. Li et al. / Materials Letters 59 (2005) 3234 – 3237
potential, A [hkl] is the dimension of energy, B [hkl] is the dimension of square energy. For the different types of crystals nuclei, the value of A [hkl] and B [hkl] is different. TiB2 belongs to the hexagonal crystal system, the theoretical curve is offered in Fig. 5. According to the curve, it is easy to predict the preferred orientation under the given electrodeposition conditions. In this paper, the over potential is comparatively low, the preferred orientation of the TiB2 coatings should be [001], which accords well with the test results.
3237
TiB2
Graphite
3.3. Adherence property Referring to adherence property, it is necessary to consider the nature of possible reactions occurring within both the coating and the interface. This would reveal the nature of the interaction whether it is chemical or physical. All the possible chemical reactions for Ti – B – C system have been considered earlier, and it is found that the formation of TiB2 is the most thermodynamically favorable reaction. However, there is some possibility of formation of some phases such as Tix Cy and Ba Cb . XRD spectrums reveal the present phases in the coating and interface. Only TiB2 is showed, and TiB, TiC and B4C are absent, which indicates the substrate doesn_t adhere to the coatings by the transition layer adherence. That is to say the binding between the coatings and the substrate is possibly the physical binding. Concentration profiles for Ti, B and C further substantiate above analyses from the other hand, the experimental result is shown in Fig. 6. It is clear that the element Ti and B distribute uniformly throughout the whole coating, and the concentration of the element Ti and B and the ratio of Ti / B are close to the theoretical value. At the side of substrate, a trace of Ti and B is detected, which should be attributed to the penetration of K2TiF6 and KBF4 into the cathode, at the side of the coating, there is almost no element C detected, which indicates Ti and B deposited by electrochemical reaction and C don_t diffuse toward the cathode and the coating. It is well known that particles with enough kinetic energy can diffuse into the substrate, evidently, Ti and B deposited by electrochemical reaction lack enough kinetic energy. As shown in Fig. 7, the interface of the coating and the substrate is very clear and continuous, and is not gradually alterative, there is also no the transition layer, which accords with the above analyses.
4. Conclusions (1) High-quality TiB2 coatings can be successfully prepared by CCP and PIC in fluoride – chloride electrolytes (KF – KCl). Compared with CCP, the grain size of the TiB2 coatings prepared by the PIC is smaller.
25µm Fig. 7. Cross-section of the TiB2 coating for CCP, i c = 0.8 A/cm2.
(2) The preferred orientation of TiB2 coatings prepared by CCP and PIC is [001] at the experimental conditions, which can be well explained by two-dimension crystal nuclei theory. (3) Thermodynamic predictions, XRD and EDS analyses show the substrate and TiB2 coatings bind physically.
Acknowledgement This research was supported by the National Natural Science Foundation of China (No. 50204006).
References [1] C. Pfohl, A. Bulak, K.-T. Rie, Develop of titanium diboride coatings deposited by PACVD, Surface and Coatings Technology [J] (2000) 131 – 141. [2] Bing Li, Zhuxian Qiu, Jun Li, Yifu Ye, Zuxin Zhao, Study on electrodeposition of TiB2 coating on graphite substrate, Rare Metal Materials and Engineering [J] 33 (7) (2004) 764 – 767. [3] G. Kaptay, S.A. Kuznetsov, Electrochemical synthesis of refractory borides from molten salts, Plasmas & Ions [J] (1999) 2 – 45. [4] Gerhard Ett, Elisabete J. Pessine, Pulse current plating of TiB2 in molten fluoride, Electrochimica Acta [J] 44 (1999) 2859 – 2870. [5] G. Ett, E.J. Pessine, Titanium diboride (TiB2) formation by electroplating in molten salt with continuous & pulsed current, Plating and Surface Finishing [J] (2000 June) 118. [6] Jueqi Yu, Wenzhi Yi, Bangdi Chen, Hongjian Chen, Phase Diagrams for Binary Alloys [M], Shanghai Science and Technology Press, 1987, pp. 206 – 207. [7] Dalun Ye, Practical Inorganic Thermodynamics Manual [M], Metallurgy Industry Press, 1981, pp. 113 – 983.