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Microstructure and mechanical properties of ceramic coatings on Ti and Ti-based alloy
Applied Surface Science 238 (2004) 288–294
Microstructure and mechanical properties of ceramic coatings on Ti and Ti-based alloy B. Surowskaa,*, J. Bienias´a, M. Walczaka, K. Sangwalb, A. Stochc a
Department of Materials Engineering, Lublin University of Technology, ul. Nadbystrzycka 36, 20-618 Lublin, Poland b Department of Applied Physics, Lublin University of Technology, ul. Nadbystrzycka 38, 20-618 Lublin, Poland c Faculty of Materials Science and Ceramics, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Krako´w, Poland Available online 20 July 2004
Abstract Results of a study of silica and silica–titania sol–gel coatings for the creation of intermediate interfaces between commercially pure Ti or titanium alloy Ti6Al4VELI and dental porcelain are presented. Coatings of SiO2 on Ti6Al4V alloy and SiO2–TiO2 on Ti were deposited using sol–gel method. Surface microstructures and wear behaviour of the coatings were studied by using scanning electron microscopy with electron diffraction spectroscopy and pin-on-disc method. It is found that (1) Ti6Al4V/SiO2 and Ti/SiO2–TiO2 coatings obtained by the sol–gel method are compact, chemically homogeneous and relatively rough, and (2) the smaller wear of SiO2 coatings than that of SiO2–TiO2 coatings is associated with differences in their microstructure and roughness. # 2004 Elsevier B.V. All rights reserved. Keywords: Titanium; Ceramic coatings; Sol–gel process; Pin-on-disc; Surface morphology
1. Introduction Ceramic coatings on metal-based alloys are a modern trend in biomaterials. Preparation of intermediate layers between a metal and porcelain, characterised by a very good adhesion, is an area of modern investigations in dental implantation [1]. Therefore, production of durable joints composed of metal–porcelain systems via intermediate layers is a result of application of advanced technologies in materials engineering to stomatology [2]. Due to their attractive properties, such as biocompatibility, corrosion resistance, low density and high mechanical properties, titanium
* Corresponding author. E-mail address: [email protected] (B. Surowska).
and its alloys are basic materials for applications in metal–ceramic coatings [3,4]. In recent years many studies on the methods of obtaining intermediate layers on titanium have been carried out [4,5]. However, the main problems are: insufficient bonding between titanium and porcelain coatings, higher thickness, and influence of high temperature during manufacturing process [6]. One of the new techniques of producing intermediate layers used in biomaterials is sol–gel method [7–9]. These intermediate layers are characterised by low thickness, high homogeneity, and satisfactory mechanical and chemical stability. The aim of this paper is to investigate silica or silica– titania sol–gel coatings for the creation of intermediate interfaces between commercially pure Ti or titanium alloy Ti6Al4VELI and low-melting dental porcelain.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.219
B. Surowska et al. / Applied Surface Science 238 (2004) 288–294
Fig. 1. SEM microphotographs of surface of different coatings: (a) Ti6Al4V/SiO2 and (b) Ti/SiO2-TiO2.
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The intermediate layers were characterised from the standpoint of their microstructure and wear resistance.
2. Experimental Commercially pure titanium (ASTM-grade 2) and Ti6Al4VELI alloy (ASTM-grade 5) were used. Ti was
forged and annealed. Titanium alloy was hot rolled and solution treated. Coatings of SiO2 on Ti6Al4V alloy and SiO2–TiO2 on Ti were deposited using sol– gel method. Silica sol was prepared by the hydrolysis of tetraethoxysilane (Si(OC2H5)4; hereafter abbreviated as TEOS) diluted in ethanol with the addition of HCl as a catalyst. The molar composition of H2O:
Fig. 2. SEM-EDX spectra of different coatings: (a) Ti6Al4V/SiO2 and (b) Ti/SiO2-TiO2.
B. Surowska et al. / Applied Surface Science 238 (2004) 288–294
TEOS:HCl in the silica sol was 4:1:0.01, while the final concentration of SiO2 in the silica sol was 3– 5 wt.%. Titania–silica sol was prepared by the hydrolysis of titanium propoxide (Ti(OC3H7)4) and TEOS with the addition of HCl as a catalyst. The final concentration of TiO2 þ SiO2 was 7.63 wt.%. Prior to obtaining the sol–gel coatings, Ti or titanium alloy samples in the form of discs of 25 mm diameter were mechanically polished, degreased and etched. Deposition of the layers consisted of withdrawing the metal discs from sol solution with a constant rate of 20 cm/ min. Thickness of the deposit was controlled by multiple dipping. After the completion of deposition, the as-deposited coatings were carefully dried and annealed in an argon atmosphere. The heating treatment removed water and residual organic substances from the coatings. The wear resistance of the coatings was determined using the pin-on-disc method. The test apparatus consisted of a 0.5 mm diameter WC–6% Co ball. A load (normal force) of 0.29 N was applied to the ball. The ball was rubbed against the sample fixed to a rotating disc. The samples used for this purpose were in the form of discs of about 25 mm diameter and 0.5 mm thickness. The sliding speeds between the rubbing surface and the ball were between 37 and
291
53 mm/s. The tests were performed at 150, 300 and 600 cycles. All wear experiments were conducted under atmospheric humidity conditions ranging between 29 and 34% at the room temperature (about 25 8C). The wear test were evaluated from the crosssectional profiles of the wear tracks measured by means of a Taylor Hobson Profilometer. After wear tests the microstructure of the layers was examined using the scanning electron microscope LEO 1430VP with an EDX-Roentec attachment.
3. Results and discussion The microstructures of the layers of SiO2 on Ti6Al4V alloy and SiO2–TiO2 on Ti are presented in Fig. 1. From this figure it may be observed that Ti6Al4V/SiO2 and Ti/SiO2–TiO2 layers are compact and chemically homogeneous [9,10], but Ti6Al4V/ SiO2 layer contains visible microcracks on their surfaces (see Fig. 1a). The microstructure of Ti/SiO2– TiO2 layers, as seen from Fig. 1b, consists of a mixture of SiO2 and TiO2 formed as a result of their deposition and heat treatment. The thickness of the intermediate layers is about 3 mm. The coatings have the value of the roughness factor Ra equal to 0.63 mm for Ti6Al4V/
Fig. 3. Wear measurements of the coatings with numbers of cycles.
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Fig. 4. Worn surfaces of: (a) SiO2 coatings after 300 cycles the test against WC-6% Co pin and (b) SiO2-TiO2 coatings.
B. Surowska et al. / Applied Surface Science 238 (2004) 288–294
SiO2 and 0.82 mm for Ti/SiO2–TiO2. However, the nature of the bond existing in the intermediate layer was not investigated in detail. Numerous microcracks observed in the microstructure of Ti6Al4V/SiO2 coating may be attributed to the effect of high soaking temperature during the sol–gel process. However, we found that these microcracks do not go deep into the SiO2 coatings and do not influence their mechanical and physical properties. According to Milella et al. [6], microcracks formed due to shrinkage occurring during thermal treatment can develop points of ‘‘mechanical interlocking’’ to promote osteintegration. Guille´ n et al. [11] proposed that the formation of microcracks is affected by the number of layers deposited one on another and the thickness of each deposited layer. According to them, mechanical stresses are accumulated during the production of each layer followed by heat treatment. Fig. 2 illustrates the results of microanalysis of the chemical constitution of the surfaces of Ti6Al4V/SiO2 and Ti/SiO2–TiO2 coatings. The EDX analysis revealed the presence of Si and/or Ti in the coatings. As seen from Fig. 2, in the case of Ti6Al4V/SiO2 coating an increased amount of Si and O is present, whereas in the Ti/SiO2–TiO2 coating one observes an increase in Si, O and Ti. It has been reported [6] that Xray diffraction study of TiO2/hydroxyapatite coatings does not reveal the presence of amorphous phases. Therefore, it may be argued that initial amorphous titania gel crystallises to anatase during heating. However, SiO2 coating is in the amorphous glassy phase [12]. The amorphous or very low crystalline character of silica deposits favours their good chemical reactivity with the porcelain enamel, ensuring bonds of high mechanical strength [13]. The pin-on-disc is one of the methods to study wear tests of coatings [14–20]. A measure of wear of coatings is the transverse cross-section of the trace wiped out by the pin on the sample [19,20]. Results of the pin-on-disc testing are summarized in Fig. 3. This figure shows that the wear of SiO2 coatings is smaller than that of SiO2–TiO2 coatings. Such a dependence was observed in all cycles. In the case of SiO2 coatings the test shows that the total layer ruptures near 600 cycles. During this trial the ball begins to wear off Ti6Al4V material. However, in the case of SiO2–TiO2 coating a complete rupture of the layer occurs above 300 cycles. The hardness of the coatings is similar
293
(about 460 HV 0.1). The difference in the wear behaviour of SiO2 and SiO2–TiO2 coatings is may be attributed to differences in their microstructure and roughness. SEM analysis of wear tracks showed that the rupture of layers occurs by the detachment of coating fragments from the sites of higher microcrack concentration. This feature is clearly seen in Fig. 4. In the initial stage of the wear, the material of summits of the roughness of the coatings is torn away. The behaviour of this wear is similar in both types of coatings. The visible sites of torn out fragments are in the vicinity of the traces of the wear. This type of behaviour is typical for bioceramics [21]. However, until now there is no published work on the nature of wear of SiO2 and SiO2–TiO2 coatings deposited by using sol–gel method on titanium and titanium alloys. From this study it may be concluded that Ti6Al4V/ SiO2 and Ti/SiO2–TiO2 sol–gel coatings are compact and chemically homogeneous. However, Ti6Al4V/ SiO2 coatings contain microcracks and are smoother than Ti/SiO2–TiO2 coatings. The smaller wear of SiO2 coatings than that of SiO2–TiO2 coatings suggests that the former sol–gel layers have better adhesive properties than the latter.
Acknowledgements The work was financed by State Committee for Scientific Research (grant no. 4T08A04523). References [1] M. Yoda, T. Konno, Y. Takada, K. Iijima, J. Griggs, O. Okuno, K. Kimura, T. Okabe, Biomaterials 22 (2001) 1675. [2] C.M. Kreulen, H. Moscovich, K.A. Dansen, N.H.J. Creugers, J. Dent. 28 (2000) 429. [3] M. Niinomi, Mater. Sci. Eng. A 243 (1998) 231. [4] J. Breme, Y. Zhou, L. Groh, Biomaterials 16 (1995) 239. [5] M. Metikos-Hukovic, E. Tkalcec, A. Kwokal, J. Piljac, Surf. Coat. Technol. 165 (2003) 40. [6] E. Milella, F. Cosentino, A. Licciulli, C. Massaro, Biomaterials 22 (2001) 1425. [7] J. Livage, Chem. Scripta 28 (1988) 9. [8] A. Stoch, W. Lejda, A. Rakowska, Metall. Foundry Eng. (Poland) 18 (1992) 233. [9] A. Stoch, C. Paluszkiewicz, T. Gibala, A. Bolek, J. Mol. Struct. 293 (1993) 287.
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[10] C. Paluszkiewicz, A. Stoch, Vibrat. Spectrosc. 35 (2004) 183. [11] C. Guille´ n, M.A. Martı´nez, G. San Vicente, A. Morales, J. Herrero, Surf. Coat. Technol. 138 (2001) 205. [12] R.N. Viswanath, S. Ramasamy, Colloids Surf. 133 (1998) 49. [13] H. Matraszek, A. Stoch, Cz. Paluszkiewicz, A. Broz˙ ek, E. Dlugon˜ , Eng. Biomater. (Poland) 23–25 (2002) 72. [14] S. Wilson, H.M. Hawthorne, Q. Yang, T. Troczyn˜ ski, Surf. Coat. Technol. 133/134 (2000) 389. [15] X. Zhao, J. Li, B. Zhu, H. Miao, Z. Luo, Ceram. Int. 23 (1997) 483.
[16] N.M. Renevier, V.C. Fox, D.G. Teer, J. Hampshire, Surf. Coat. Technol. 127 (2000) 24. [17] B.V. Cockeram, W.L. Wilson, Surf. Coat. Technol. 139 (2001) 161. [18] A. Borruto, G. Crivellone, F. Marani, Wear 222 (1998) 57. [19] A.A. Youssef, P. Budzyn˜ ski, J. Filiks, B. Kamien˜ ska, D. Maczka, Vacuum 68 (2003) 131. [20] P. Budzyn˜ ski, P. Tarkowski, E. Jartych, A.P. Kobzev, Vacuum 63 (2001) 737. [21] S.H. Teoh, Int. J. Fat. 22 (2000) 825.
Microstructure and mechanical properties of ceramic coatings on Ti and Ti-based alloy B. Surowskaa,*, J. Bienias´a, M. Walczaka, K. Sangwalb, A. Stochc a
Department of Materials Engineering, Lublin University of Technology, ul. Nadbystrzycka 36, 20-618 Lublin, Poland b Department of Applied Physics, Lublin University of Technology, ul. Nadbystrzycka 38, 20-618 Lublin, Poland c Faculty of Materials Science and Ceramics, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Krako´w, Poland Available online 20 July 2004
Abstract Results of a study of silica and silica–titania sol–gel coatings for the creation of intermediate interfaces between commercially pure Ti or titanium alloy Ti6Al4VELI and dental porcelain are presented. Coatings of SiO2 on Ti6Al4V alloy and SiO2–TiO2 on Ti were deposited using sol–gel method. Surface microstructures and wear behaviour of the coatings were studied by using scanning electron microscopy with electron diffraction spectroscopy and pin-on-disc method. It is found that (1) Ti6Al4V/SiO2 and Ti/SiO2–TiO2 coatings obtained by the sol–gel method are compact, chemically homogeneous and relatively rough, and (2) the smaller wear of SiO2 coatings than that of SiO2–TiO2 coatings is associated with differences in their microstructure and roughness. # 2004 Elsevier B.V. All rights reserved. Keywords: Titanium; Ceramic coatings; Sol–gel process; Pin-on-disc; Surface morphology
1. Introduction Ceramic coatings on metal-based alloys are a modern trend in biomaterials. Preparation of intermediate layers between a metal and porcelain, characterised by a very good adhesion, is an area of modern investigations in dental implantation [1]. Therefore, production of durable joints composed of metal–porcelain systems via intermediate layers is a result of application of advanced technologies in materials engineering to stomatology [2]. Due to their attractive properties, such as biocompatibility, corrosion resistance, low density and high mechanical properties, titanium
* Corresponding author. E-mail address: [email protected] (B. Surowska).
and its alloys are basic materials for applications in metal–ceramic coatings [3,4]. In recent years many studies on the methods of obtaining intermediate layers on titanium have been carried out [4,5]. However, the main problems are: insufficient bonding between titanium and porcelain coatings, higher thickness, and influence of high temperature during manufacturing process [6]. One of the new techniques of producing intermediate layers used in biomaterials is sol–gel method [7–9]. These intermediate layers are characterised by low thickness, high homogeneity, and satisfactory mechanical and chemical stability. The aim of this paper is to investigate silica or silica– titania sol–gel coatings for the creation of intermediate interfaces between commercially pure Ti or titanium alloy Ti6Al4VELI and low-melting dental porcelain.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.219
B. Surowska et al. / Applied Surface Science 238 (2004) 288–294
Fig. 1. SEM microphotographs of surface of different coatings: (a) Ti6Al4V/SiO2 and (b) Ti/SiO2-TiO2.
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B. Surowska et al. / Applied Surface Science 238 (2004) 288–294
The intermediate layers were characterised from the standpoint of their microstructure and wear resistance.
2. Experimental Commercially pure titanium (ASTM-grade 2) and Ti6Al4VELI alloy (ASTM-grade 5) were used. Ti was
forged and annealed. Titanium alloy was hot rolled and solution treated. Coatings of SiO2 on Ti6Al4V alloy and SiO2–TiO2 on Ti were deposited using sol– gel method. Silica sol was prepared by the hydrolysis of tetraethoxysilane (Si(OC2H5)4; hereafter abbreviated as TEOS) diluted in ethanol with the addition of HCl as a catalyst. The molar composition of H2O:
Fig. 2. SEM-EDX spectra of different coatings: (a) Ti6Al4V/SiO2 and (b) Ti/SiO2-TiO2.
B. Surowska et al. / Applied Surface Science 238 (2004) 288–294
TEOS:HCl in the silica sol was 4:1:0.01, while the final concentration of SiO2 in the silica sol was 3– 5 wt.%. Titania–silica sol was prepared by the hydrolysis of titanium propoxide (Ti(OC3H7)4) and TEOS with the addition of HCl as a catalyst. The final concentration of TiO2 þ SiO2 was 7.63 wt.%. Prior to obtaining the sol–gel coatings, Ti or titanium alloy samples in the form of discs of 25 mm diameter were mechanically polished, degreased and etched. Deposition of the layers consisted of withdrawing the metal discs from sol solution with a constant rate of 20 cm/ min. Thickness of the deposit was controlled by multiple dipping. After the completion of deposition, the as-deposited coatings were carefully dried and annealed in an argon atmosphere. The heating treatment removed water and residual organic substances from the coatings. The wear resistance of the coatings was determined using the pin-on-disc method. The test apparatus consisted of a 0.5 mm diameter WC–6% Co ball. A load (normal force) of 0.29 N was applied to the ball. The ball was rubbed against the sample fixed to a rotating disc. The samples used for this purpose were in the form of discs of about 25 mm diameter and 0.5 mm thickness. The sliding speeds between the rubbing surface and the ball were between 37 and
291
53 mm/s. The tests were performed at 150, 300 and 600 cycles. All wear experiments were conducted under atmospheric humidity conditions ranging between 29 and 34% at the room temperature (about 25 8C). The wear test were evaluated from the crosssectional profiles of the wear tracks measured by means of a Taylor Hobson Profilometer. After wear tests the microstructure of the layers was examined using the scanning electron microscope LEO 1430VP with an EDX-Roentec attachment.
3. Results and discussion The microstructures of the layers of SiO2 on Ti6Al4V alloy and SiO2–TiO2 on Ti are presented in Fig. 1. From this figure it may be observed that Ti6Al4V/SiO2 and Ti/SiO2–TiO2 layers are compact and chemically homogeneous [9,10], but Ti6Al4V/ SiO2 layer contains visible microcracks on their surfaces (see Fig. 1a). The microstructure of Ti/SiO2– TiO2 layers, as seen from Fig. 1b, consists of a mixture of SiO2 and TiO2 formed as a result of their deposition and heat treatment. The thickness of the intermediate layers is about 3 mm. The coatings have the value of the roughness factor Ra equal to 0.63 mm for Ti6Al4V/
Fig. 3. Wear measurements of the coatings with numbers of cycles.
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Fig. 4. Worn surfaces of: (a) SiO2 coatings after 300 cycles the test against WC-6% Co pin and (b) SiO2-TiO2 coatings.
B. Surowska et al. / Applied Surface Science 238 (2004) 288–294
SiO2 and 0.82 mm for Ti/SiO2–TiO2. However, the nature of the bond existing in the intermediate layer was not investigated in detail. Numerous microcracks observed in the microstructure of Ti6Al4V/SiO2 coating may be attributed to the effect of high soaking temperature during the sol–gel process. However, we found that these microcracks do not go deep into the SiO2 coatings and do not influence their mechanical and physical properties. According to Milella et al. [6], microcracks formed due to shrinkage occurring during thermal treatment can develop points of ‘‘mechanical interlocking’’ to promote osteintegration. Guille´ n et al. [11] proposed that the formation of microcracks is affected by the number of layers deposited one on another and the thickness of each deposited layer. According to them, mechanical stresses are accumulated during the production of each layer followed by heat treatment. Fig. 2 illustrates the results of microanalysis of the chemical constitution of the surfaces of Ti6Al4V/SiO2 and Ti/SiO2–TiO2 coatings. The EDX analysis revealed the presence of Si and/or Ti in the coatings. As seen from Fig. 2, in the case of Ti6Al4V/SiO2 coating an increased amount of Si and O is present, whereas in the Ti/SiO2–TiO2 coating one observes an increase in Si, O and Ti. It has been reported [6] that Xray diffraction study of TiO2/hydroxyapatite coatings does not reveal the presence of amorphous phases. Therefore, it may be argued that initial amorphous titania gel crystallises to anatase during heating. However, SiO2 coating is in the amorphous glassy phase [12]. The amorphous or very low crystalline character of silica deposits favours their good chemical reactivity with the porcelain enamel, ensuring bonds of high mechanical strength [13]. The pin-on-disc is one of the methods to study wear tests of coatings [14–20]. A measure of wear of coatings is the transverse cross-section of the trace wiped out by the pin on the sample [19,20]. Results of the pin-on-disc testing are summarized in Fig. 3. This figure shows that the wear of SiO2 coatings is smaller than that of SiO2–TiO2 coatings. Such a dependence was observed in all cycles. In the case of SiO2 coatings the test shows that the total layer ruptures near 600 cycles. During this trial the ball begins to wear off Ti6Al4V material. However, in the case of SiO2–TiO2 coating a complete rupture of the layer occurs above 300 cycles. The hardness of the coatings is similar
293
(about 460 HV 0.1). The difference in the wear behaviour of SiO2 and SiO2–TiO2 coatings is may be attributed to differences in their microstructure and roughness. SEM analysis of wear tracks showed that the rupture of layers occurs by the detachment of coating fragments from the sites of higher microcrack concentration. This feature is clearly seen in Fig. 4. In the initial stage of the wear, the material of summits of the roughness of the coatings is torn away. The behaviour of this wear is similar in both types of coatings. The visible sites of torn out fragments are in the vicinity of the traces of the wear. This type of behaviour is typical for bioceramics [21]. However, until now there is no published work on the nature of wear of SiO2 and SiO2–TiO2 coatings deposited by using sol–gel method on titanium and titanium alloys. From this study it may be concluded that Ti6Al4V/ SiO2 and Ti/SiO2–TiO2 sol–gel coatings are compact and chemically homogeneous. However, Ti6Al4V/ SiO2 coatings contain microcracks and are smoother than Ti/SiO2–TiO2 coatings. The smaller wear of SiO2 coatings than that of SiO2–TiO2 coatings suggests that the former sol–gel layers have better adhesive properties than the latter.
Acknowledgements The work was financed by State Committee for Scientific Research (grant no. 4T08A04523). References [1] M. Yoda, T. Konno, Y. Takada, K. Iijima, J. Griggs, O. Okuno, K. Kimura, T. Okabe, Biomaterials 22 (2001) 1675. [2] C.M. Kreulen, H. Moscovich, K.A. Dansen, N.H.J. Creugers, J. Dent. 28 (2000) 429. [3] M. Niinomi, Mater. Sci. Eng. A 243 (1998) 231. [4] J. Breme, Y. Zhou, L. Groh, Biomaterials 16 (1995) 239. [5] M. Metikos-Hukovic, E. Tkalcec, A. Kwokal, J. Piljac, Surf. Coat. Technol. 165 (2003) 40. [6] E. Milella, F. Cosentino, A. Licciulli, C. Massaro, Biomaterials 22 (2001) 1425. [7] J. Livage, Chem. Scripta 28 (1988) 9. [8] A. Stoch, W. Lejda, A. Rakowska, Metall. Foundry Eng. (Poland) 18 (1992) 233. [9] A. Stoch, C. Paluszkiewicz, T. Gibala, A. Bolek, J. Mol. Struct. 293 (1993) 287.
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[10] C. Paluszkiewicz, A. Stoch, Vibrat. Spectrosc. 35 (2004) 183. [11] C. Guille´ n, M.A. Martı´nez, G. San Vicente, A. Morales, J. Herrero, Surf. Coat. Technol. 138 (2001) 205. [12] R.N. Viswanath, S. Ramasamy, Colloids Surf. 133 (1998) 49. [13] H. Matraszek, A. Stoch, Cz. Paluszkiewicz, A. Broz˙ ek, E. Dlugon˜ , Eng. Biomater. (Poland) 23–25 (2002) 72. [14] S. Wilson, H.M. Hawthorne, Q. Yang, T. Troczyn˜ ski, Surf. Coat. Technol. 133/134 (2000) 389. [15] X. Zhao, J. Li, B. Zhu, H. Miao, Z. Luo, Ceram. Int. 23 (1997) 483.
[16] N.M. Renevier, V.C. Fox, D.G. Teer, J. Hampshire, Surf. Coat. Technol. 127 (2000) 24. [17] B.V. Cockeram, W.L. Wilson, Surf. Coat. Technol. 139 (2001) 161. [18] A. Borruto, G. Crivellone, F. Marani, Wear 222 (1998) 57. [19] A.A. Youssef, P. Budzyn˜ ski, J. Filiks, B. Kamien˜ ska, D. Maczka, Vacuum 68 (2003) 131. [20] P. Budzyn˜ ski, P. Tarkowski, E. Jartych, A.P. Kobzev, Vacuum 63 (2001) 737. [21] S.H. Teoh, Int. J. Fat. 22 (2000) 825.