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carbon composites
Surface & Coatings Technology 190 (2005) 287 – 292 www.elsevier.com/locate/surfcoat
The effect of plasma spraying power on the structure and mechanical properties of hydroxyapatite deposited onto carbon/carbon composites Jin-Ling Sui a,b, Mu-Sen Li a,*, Yu-Peng Lu a, Yun-Qiang Bai a a
School of Materials Science and Engineering, Shandong University, South Campus, 73 Jing Shi Road, Jinan 250061, PR China b School of Power and Control Engineering, Shandong University of Science and Technology, Jinan 250031, PR China Received 15 August 2003; accepted 26 February 2004 Available online 10 May 2004
Abstract This paper deals with the effect of plasma spraying power on hydroxyapatite (HA) coatings on carbon/carbon composites (C/C composites). The microstructure and phase composition of the as-sprayed coatings have been examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The shear strength of the HA coatings – C/C substrates was detected on a RGD-5 tensile testing machine. Results indicate that the melting extent and the shear strength of the coatings were evidently improved with the increasing of spraying power. Moreover, the amount of decomposed phases is increased and the content of crystalline HA of coatings was slightly changed. Observation of fracture surfaces shows that carbon fiber bundles can bond well with HA coatings using 40 kW spraying power. D 2004 Elsevier B.V. All rights reserved. PACS: 61.10 Nz; 68.37.Hk; 81.15.Rs; 81.05.Uw Keywords: Plasma spraying; Carbon fiber reinforced carbon composites; Hydroxyapatite coating; Spraying power
1. Introduction Carbon fiber reinforced carbon composites (C/C composites) possess good mechanical properties and stability, excellent biocompatibility, as well as appropriate elastic modulus similar to that of human cortical bones. Therefore, they are also promising materials for loaded artificial bones. However, they cannot form chemical bond with human bones due to their bioinert property. In order to overcome this drawback, many studies have been focused on depositing hydroxyapatite (HA) onto it. Some authors reported that the surface bioactivity of C/C composites was evidently improved after grafting with NH2 –PEG – NH2 or phosphorylation, and a collagen/apatite coating can be formed on C/C substrates under ambient environmental conditions with a bioactivating medium by a biomimetic process, but the adhesion strength between the coatings and substrates is very weak [1]. Lucie et al. [2] coated C/C composites with a carbon/titanium layer by using a plasma-enhanced physical vapor deposition method. In the * Corresponding author. Tel.: +86-531-8395693; fax: +86-5312955999. E-mail address: [email protected] (M.-S. Li). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.02.042
present work, hydroxyapatite was coated onto C/C substrates by a plasma spraying method. The aim is to investigate the effect of spraying power on the microstructure, phase composition and bond strength of HA coatings – C/C substrates.
2. Experimental materials and methods C/C composites obtained from the National Science Center, Kharkov Institute of Physics and Technology, Ukraine, were sliced to 51010 mm blocks with a diamond saw, and then the blocks were rubdown with abrasive papers, sandblasted by SiO2 to form a rough surface. Sulzer Metco 9M plasma spraying equipment was employed to prepare the HA coatings in air. Table 1 lists the plasma spraying parameters. The starting HA powder with a mean size of 38 –75 Am used for coatings was prepared by a wet chemical precipitation method. Fig. 1 gives the morphology of the powder, which has an essentially irregular shape. The X-ray diffraction pattern of it is shown in Fig. 2, suggesting that it composes of highly crystalline HA and a little htricalcium phosphate (h-TCP) that is present probably
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Table 1 Plasma spraying parameters Plasma gas N2 Plasma gas H2 Spray distance
40 l/min 10 l/min 110 mm
Powder carrier gas N2 Powder feed rate Spray power
Coating thickness
100 Am
Powder transport mode
2.0 l/min 20 g/min 25, 30, 35, 40, 45 kW External
because the pH value in aqueous solutions is less lower than the standard value. A JXA-840 scanning electron microscope (SEM) with an acceleration voltage of 20 kV was used to study the morphology of coatings. The phase compositions of coatings were determined by a D/max-gB X-ray diffractometer (XRD) with a scan speed of 4j/min from 10j to 60j of 2h angles. A Cu target was used as X-ray source using Cu Ka radiation at 40 kV and 100 mA. The shear strength of the HA coatings-C/C substrates was measured on a RGD-5 tensile testing machine with a special device. The experimental details for measuring shear strength were described in the previous work [3].
3. Results and discussion 3.1. Morphology and mechanical properties of C/C composites The properties of C/C composites are given in Table 2. The SEM image of the cross-section of C/C composites is shown in Fig. 3a, in which there are two kinds of crosssection of carbon fiber bundles: longitudinal and transversal. Fig. 3b gives the rough surface of C/C composites sandblasted by SiO2, in which there are many cracked carbon fiber bundles. 3.2. Surface morphology of HA coatings In order to optimize the spraying power, five powers were selected to prepare the coatings at a spraying distance of 110 mm. Fig. 4a – e are the SEM micrographs of as-
Fig. 2. X-ray diffraction spectra of starting HA powder (n: h-TCP; unmarked peaks: HA).
sprayed coatings corresponding to spraying power of 25, 30, 35, 40, 45 kW, respectively. It was observed that the coatings were composed of tightly adhered irregular splats with pores and microcracks. Some particles obtained at 25 kW remained unmelted or partially melted as seen in Fig. 4a, indicating that the power of 25 kW could not provide sufficient energy to melt all particles, which results in a non-uniform microstructure in the coatings. When sprayed at 30 kW (Fig. 4b), more particles were completely melted, and well-flattened splats were produced. Almost all of the HA powders were totally melted or super-melted at 35– 45 kW (Fig. 4c – e), and the flattened splats and non-uniform particles (which suggests that some melted particles could have sparked after they reached substrates due to the excessive plasma energy input) can be clearly seen. There are more pores in the coatings of 40 and 45 kW caused by the overlap of sparked particles. These microscopic irregularities are found to improve the fixation of HA coating layer [4]. Cracks in the coatings often appear as white lines because they are in the closed state. Under a high magnification, the cracks seemed to open up and spread along or across several particles as indicated in Fig. 5. Fig. 5a demonstrates that these particles have melted into a whole locally. Then the stress may cause the separation of particles with weak bond rather than crack spreading across particles. This is caused not only by the thermal expansion coefficient mismatch between the HA coatings and C/C substrates, but also by the residual stress induced during cooling. Moreover, these cracks could lead to local stress concentration and give rise to further mechanical and physical – chemical instability of the coatings [5]. 3.3. Phase composition of HA coatings Hydroxyapatite coated by plasma spraying has been the subject of many studies due to its biocompatibility, direct bone bonding capability and ability to enhance bone formaTable 2 The properties of C/C composites
Fig. 1. SEM micrograph of starting HA powder.
Density Tensile strength Compressive strength Elastic modulus
1.67 g/cm3 225.7 MPa 186.3 MPa 56.8 GPa
J.-L. Sui et al. / Surface & Coatings Technology 190 (2005) 287–292
289
Fig. 3. SEM morphologies of C/C composites: (a) starting material and (b) sandblasted.
tion [6]. Since the plasma spraying process involves a high temperature and a fast cooling rate, it usually produces amorphous calcium phosphate, which was found to dissolve faster than crystalline HA phase, even if the starting powder is 100% crystalline HA [7]. XRD patterns for as-sprayed coatings corresponding to 25, 30, 35, 40, 45 kW are given in Fig. 6. It can be seen that coatings contain some amorphous phase
and crystalline phases. Amorphous phase resulted from the quenching of molted particles on C/C substrates during spraying. The crystalline phases are primarily a mixture of HA, a-tricalcium phosphate (a-TCP), h-tricalcium phosphate (h-TCP) and a little calcium oxide (CaO). The intensity of the HA peak corresponding to 25 kW was lower than that to 30– 45 kW within which the content of crystalline HA is
Fig. 4. SEM micrographs of as-sprayed coatings under different spraying powers: (a) 25 kW, (b) 30 kW, (c) 35 kW, (d) 40 kW, and (e) 45 kW.
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Fig. 5. Microcracks around (a) and across (b) particles in HA coatings.
about the same. However, the amount of decomposed phases using 40 and 45 kW is more than that using 30 and 35 kW. This result does not accord with popular viewpoints [8,9]. It can be explained as follows: the crystalline HA of coatings is arisen from partially melted starting particles determined by the plasma energy input and recrystallized phase determined by the temperature of coatings and substrates during spraying. More particles are partially melted at 25 kW, and the lower temperature of plasma flame caused the cooler coatings and substrates, which reduces the content of the recrystallized HA. On the contrary, with the increasing of spraying power from 30 to 45 kW, the higher temperature increased the recrystallizing of HA and the decomposing of the melted particles simultaneously. In fact, since a-TCP, h-TCP and CaO are both biocompatible materials in physiological environment, so the existence of small amounts of these phases is not assumed to affect significantly biocompatibility of the coatings [10,11]. In addition, the diffraction peak of h-TCP was found to strengthen slightly with the increasing of the spraying power, owing to the enhancing of the decomposed extent of melted HA particles.
3.4. Shear strength of HA coatings – C/C substrates As the property of the coating is directly related to the combinative state between the coatings and substrates, many studies have been conducted on the structural characterization of the coatings [12,13]. It is also well known that the bond strength of the coatings and substrates depends on the interfacial state and residual stress. The shear strength between C/C composites and bone is 2.44 MPa, after implantation in mouse for 20 weeks [14]. We tested the shear strength of HA coatings – C/C composites after plasma spraying, which is described in Fig. 7. Each point of the curve is the average of six reliable results. It is found that the mean value increases from 2.1 to 7.4 MPa. On the other hand, with the increasing of spraying power from 20 to 40 kW, the relative bond strength is significantly enhanced. The images of fracture surfaces in the direction parallel to the interface after shear test are given in Fig. 8, corresponding to the power of 25, 35 and 40 kW, respectively. Fig. 8a is the fracture morphology of
Fig. 6. X-ray diffraction spectra of HA coatings at different spraying powers: (a) 25 kW, (b) 30 kW, (c) 35 kW, (d) 40 kW, and (e) 45 kW (z: a-TCP ;n: hTCP;.: CaO; unmarked peaks: HA).
J.-L. Sui et al. / Surface & Coatings Technology 190 (2005) 287–292
Fig. 7. Shear strength of HA coatings – C/C substrates under different spraying powers.
the substrates at 25 kW; it can be seen that some coating particles are remained on the surface and attached on the substrates, showing that interfacial failure exists during the bond strength evaluation. Under a high magnification given in Fig. 8b, it is noted that the fracture of HA is
291
along the splat boundaries and the carbon fibers have no influence on the shear strength, thus, the mean value of strength is relatively lower. The typical fracture morphologies of coatings at 35 and 40 kW are shown in Fig. 8c and e. Fig. 8d and f is that of the substrates, respectively. As indicated in Fig. 8c and d, carbon fibers are not bonded well with HA coating during plasma spraying, resulting in the pulling off of some fibers from the substrate and the porous coating material can be clearly seen. On the contrary, it can be observed from Fig. 8e and f that the coating is tightly adhered to the substrates, and coating particles impinged into carbon fiber bundles. The coating and the carbon fibers are simultaneously fractured under external load. Therefore, carbon fibers have improved the shear strength between the HA coatings and C/C substrates. In addition, observation of fracture surfaces as indicated in Fig. 8d and f revealed that the interface is clear and there are no new phases produced, suggesting that the chemical bonding is hardly to occur. When HA powders is sprayed
Fig. 8. Morphologies of shear fracture surfaces of HA coatings – C/C substrates. (a) C/C substrate at 25 kW, (b) high magnification of (a). (c) HA coating at 35 kW, (d) C/C substrate at 35 kW. (e) HA coating at 40 kW, (f ) C/C substrate at 40 kW.
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J.-L. Sui et al. / Surface & Coatings Technology 190 (2005) 287–292
onto a smooth substrates surface, sandblast that is a general pre-treating method for metal substrates. It is usually performed to give microscopically rough surface in advance, although it is probably not the optimal option for C/C composites. This means that the coated layer is sustained by physical bonding to the substrates. At last, it can be concluded from the above discussions that the optimal spraying power in this work is 40 kW.
4. Conclusions Hydroxyapatite can successfully be coated onto carbon/ carbon composites by a plasma spraying method. XRD patterns of coatings indicate that the coatings contain some amorphous apatite, crystalline HA , a-TCP, h-TCP and a little CaO. The spraying power has little effect on the content of crystalline HA phase, whereas the melting extent of coating particles and the amount of decomposed phases are enhanced with increasing spraying power. Moreover, the mean shear strength of the HA coatings– C/C substrates increases from 2.1 to 7.4 MPa. It was found physical bonding plays an important role in the interfacial bond as indicated from the SEM images of the fracture surfaces. Results indicate that the best spraying power is 40 kW under the tested conditions, in which carbon fiber bundles bond well with HA coatings and can accordingly enhance the adhesive strength.
Acknowledgements This work was supported by the Key Project of Science and Technology Development Plan of Shandong Province (011120106).
References [1] S.H. Li, Z.G. Zheng, Q. Liu, J.R. de Wijn, K. de Groot, J. Biomed. Mater. Res. 40 (1998) 520. [2] B.K. Lucie, S. Vladimı´r, K. Olga, L. Ve˘ra, J. Biomed. Mater. Res. 54 (2001) 567. [3] J.L. Sui, M.S. Li, Y.P. Lu, L.W. Yin, Y.J. Song, Surf. Coat. Technol. 176 (2004) 188. [4] R.G.T. Geesink, K. De Groot, C.P.A.T. Klein, J. Bone Jt. Surg. 70-B (1988) 17. [5] Y.C. Yang, E. Chang, Biomaterials 22 (2001) 1827. [6] I.H. Arita, V.M. Castano, D.S. Wilkinson, J. Mater. Sci., Mater. Med. 6 (1995) 19. [7] E. Park, A. Robert, S. Condrate, D. Lee, Mater. Lett. 36 (1998) 38. [8] Y.C. Tsui, C. Doyle, T.W. Clyne, Biomaterials 19 (1998) 2015. [9] S. Radin, P. Ducheyne, J. Mater. Sci. 3 (1992) 33. [10] G. Lewis, Biomed. Mater. Eng. 10 (2000) 157. [11] Y. Cao, J. Weng, J. Chen, J. Feng, Z. Yang, X. Zhang, Biomaterials 17 (1996) 419. [12] S.S. Tzeng, W.C. Lin, Carbon 37 (1999) 2011. [13] I.S. Leea, H.E. Kima, S.Y. Kim, Surf. Coat. Technol. 131 (2000) 181. [14] D.F. Williams, J. Biomed. Eng. 11 (1989) 185.
The effect of plasma spraying power on the structure and mechanical properties of hydroxyapatite deposited onto carbon/carbon composites Jin-Ling Sui a,b, Mu-Sen Li a,*, Yu-Peng Lu a, Yun-Qiang Bai a a
School of Materials Science and Engineering, Shandong University, South Campus, 73 Jing Shi Road, Jinan 250061, PR China b School of Power and Control Engineering, Shandong University of Science and Technology, Jinan 250031, PR China Received 15 August 2003; accepted 26 February 2004 Available online 10 May 2004
Abstract This paper deals with the effect of plasma spraying power on hydroxyapatite (HA) coatings on carbon/carbon composites (C/C composites). The microstructure and phase composition of the as-sprayed coatings have been examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The shear strength of the HA coatings – C/C substrates was detected on a RGD-5 tensile testing machine. Results indicate that the melting extent and the shear strength of the coatings were evidently improved with the increasing of spraying power. Moreover, the amount of decomposed phases is increased and the content of crystalline HA of coatings was slightly changed. Observation of fracture surfaces shows that carbon fiber bundles can bond well with HA coatings using 40 kW spraying power. D 2004 Elsevier B.V. All rights reserved. PACS: 61.10 Nz; 68.37.Hk; 81.15.Rs; 81.05.Uw Keywords: Plasma spraying; Carbon fiber reinforced carbon composites; Hydroxyapatite coating; Spraying power
1. Introduction Carbon fiber reinforced carbon composites (C/C composites) possess good mechanical properties and stability, excellent biocompatibility, as well as appropriate elastic modulus similar to that of human cortical bones. Therefore, they are also promising materials for loaded artificial bones. However, they cannot form chemical bond with human bones due to their bioinert property. In order to overcome this drawback, many studies have been focused on depositing hydroxyapatite (HA) onto it. Some authors reported that the surface bioactivity of C/C composites was evidently improved after grafting with NH2 –PEG – NH2 or phosphorylation, and a collagen/apatite coating can be formed on C/C substrates under ambient environmental conditions with a bioactivating medium by a biomimetic process, but the adhesion strength between the coatings and substrates is very weak [1]. Lucie et al. [2] coated C/C composites with a carbon/titanium layer by using a plasma-enhanced physical vapor deposition method. In the * Corresponding author. Tel.: +86-531-8395693; fax: +86-5312955999. E-mail address: [email protected] (M.-S. Li). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.02.042
present work, hydroxyapatite was coated onto C/C substrates by a plasma spraying method. The aim is to investigate the effect of spraying power on the microstructure, phase composition and bond strength of HA coatings – C/C substrates.
2. Experimental materials and methods C/C composites obtained from the National Science Center, Kharkov Institute of Physics and Technology, Ukraine, were sliced to 51010 mm blocks with a diamond saw, and then the blocks were rubdown with abrasive papers, sandblasted by SiO2 to form a rough surface. Sulzer Metco 9M plasma spraying equipment was employed to prepare the HA coatings in air. Table 1 lists the plasma spraying parameters. The starting HA powder with a mean size of 38 –75 Am used for coatings was prepared by a wet chemical precipitation method. Fig. 1 gives the morphology of the powder, which has an essentially irregular shape. The X-ray diffraction pattern of it is shown in Fig. 2, suggesting that it composes of highly crystalline HA and a little htricalcium phosphate (h-TCP) that is present probably
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Table 1 Plasma spraying parameters Plasma gas N2 Plasma gas H2 Spray distance
40 l/min 10 l/min 110 mm
Powder carrier gas N2 Powder feed rate Spray power
Coating thickness
100 Am
Powder transport mode
2.0 l/min 20 g/min 25, 30, 35, 40, 45 kW External
because the pH value in aqueous solutions is less lower than the standard value. A JXA-840 scanning electron microscope (SEM) with an acceleration voltage of 20 kV was used to study the morphology of coatings. The phase compositions of coatings were determined by a D/max-gB X-ray diffractometer (XRD) with a scan speed of 4j/min from 10j to 60j of 2h angles. A Cu target was used as X-ray source using Cu Ka radiation at 40 kV and 100 mA. The shear strength of the HA coatings-C/C substrates was measured on a RGD-5 tensile testing machine with a special device. The experimental details for measuring shear strength were described in the previous work [3].
3. Results and discussion 3.1. Morphology and mechanical properties of C/C composites The properties of C/C composites are given in Table 2. The SEM image of the cross-section of C/C composites is shown in Fig. 3a, in which there are two kinds of crosssection of carbon fiber bundles: longitudinal and transversal. Fig. 3b gives the rough surface of C/C composites sandblasted by SiO2, in which there are many cracked carbon fiber bundles. 3.2. Surface morphology of HA coatings In order to optimize the spraying power, five powers were selected to prepare the coatings at a spraying distance of 110 mm. Fig. 4a – e are the SEM micrographs of as-
Fig. 2. X-ray diffraction spectra of starting HA powder (n: h-TCP; unmarked peaks: HA).
sprayed coatings corresponding to spraying power of 25, 30, 35, 40, 45 kW, respectively. It was observed that the coatings were composed of tightly adhered irregular splats with pores and microcracks. Some particles obtained at 25 kW remained unmelted or partially melted as seen in Fig. 4a, indicating that the power of 25 kW could not provide sufficient energy to melt all particles, which results in a non-uniform microstructure in the coatings. When sprayed at 30 kW (Fig. 4b), more particles were completely melted, and well-flattened splats were produced. Almost all of the HA powders were totally melted or super-melted at 35– 45 kW (Fig. 4c – e), and the flattened splats and non-uniform particles (which suggests that some melted particles could have sparked after they reached substrates due to the excessive plasma energy input) can be clearly seen. There are more pores in the coatings of 40 and 45 kW caused by the overlap of sparked particles. These microscopic irregularities are found to improve the fixation of HA coating layer [4]. Cracks in the coatings often appear as white lines because they are in the closed state. Under a high magnification, the cracks seemed to open up and spread along or across several particles as indicated in Fig. 5. Fig. 5a demonstrates that these particles have melted into a whole locally. Then the stress may cause the separation of particles with weak bond rather than crack spreading across particles. This is caused not only by the thermal expansion coefficient mismatch between the HA coatings and C/C substrates, but also by the residual stress induced during cooling. Moreover, these cracks could lead to local stress concentration and give rise to further mechanical and physical – chemical instability of the coatings [5]. 3.3. Phase composition of HA coatings Hydroxyapatite coated by plasma spraying has been the subject of many studies due to its biocompatibility, direct bone bonding capability and ability to enhance bone formaTable 2 The properties of C/C composites
Fig. 1. SEM micrograph of starting HA powder.
Density Tensile strength Compressive strength Elastic modulus
1.67 g/cm3 225.7 MPa 186.3 MPa 56.8 GPa
J.-L. Sui et al. / Surface & Coatings Technology 190 (2005) 287–292
289
Fig. 3. SEM morphologies of C/C composites: (a) starting material and (b) sandblasted.
tion [6]. Since the plasma spraying process involves a high temperature and a fast cooling rate, it usually produces amorphous calcium phosphate, which was found to dissolve faster than crystalline HA phase, even if the starting powder is 100% crystalline HA [7]. XRD patterns for as-sprayed coatings corresponding to 25, 30, 35, 40, 45 kW are given in Fig. 6. It can be seen that coatings contain some amorphous phase
and crystalline phases. Amorphous phase resulted from the quenching of molted particles on C/C substrates during spraying. The crystalline phases are primarily a mixture of HA, a-tricalcium phosphate (a-TCP), h-tricalcium phosphate (h-TCP) and a little calcium oxide (CaO). The intensity of the HA peak corresponding to 25 kW was lower than that to 30– 45 kW within which the content of crystalline HA is
Fig. 4. SEM micrographs of as-sprayed coatings under different spraying powers: (a) 25 kW, (b) 30 kW, (c) 35 kW, (d) 40 kW, and (e) 45 kW.
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J.-L. Sui et al. / Surface & Coatings Technology 190 (2005) 287–292
Fig. 5. Microcracks around (a) and across (b) particles in HA coatings.
about the same. However, the amount of decomposed phases using 40 and 45 kW is more than that using 30 and 35 kW. This result does not accord with popular viewpoints [8,9]. It can be explained as follows: the crystalline HA of coatings is arisen from partially melted starting particles determined by the plasma energy input and recrystallized phase determined by the temperature of coatings and substrates during spraying. More particles are partially melted at 25 kW, and the lower temperature of plasma flame caused the cooler coatings and substrates, which reduces the content of the recrystallized HA. On the contrary, with the increasing of spraying power from 30 to 45 kW, the higher temperature increased the recrystallizing of HA and the decomposing of the melted particles simultaneously. In fact, since a-TCP, h-TCP and CaO are both biocompatible materials in physiological environment, so the existence of small amounts of these phases is not assumed to affect significantly biocompatibility of the coatings [10,11]. In addition, the diffraction peak of h-TCP was found to strengthen slightly with the increasing of the spraying power, owing to the enhancing of the decomposed extent of melted HA particles.
3.4. Shear strength of HA coatings – C/C substrates As the property of the coating is directly related to the combinative state between the coatings and substrates, many studies have been conducted on the structural characterization of the coatings [12,13]. It is also well known that the bond strength of the coatings and substrates depends on the interfacial state and residual stress. The shear strength between C/C composites and bone is 2.44 MPa, after implantation in mouse for 20 weeks [14]. We tested the shear strength of HA coatings – C/C composites after plasma spraying, which is described in Fig. 7. Each point of the curve is the average of six reliable results. It is found that the mean value increases from 2.1 to 7.4 MPa. On the other hand, with the increasing of spraying power from 20 to 40 kW, the relative bond strength is significantly enhanced. The images of fracture surfaces in the direction parallel to the interface after shear test are given in Fig. 8, corresponding to the power of 25, 35 and 40 kW, respectively. Fig. 8a is the fracture morphology of
Fig. 6. X-ray diffraction spectra of HA coatings at different spraying powers: (a) 25 kW, (b) 30 kW, (c) 35 kW, (d) 40 kW, and (e) 45 kW (z: a-TCP ;n: hTCP;.: CaO; unmarked peaks: HA).
J.-L. Sui et al. / Surface & Coatings Technology 190 (2005) 287–292
Fig. 7. Shear strength of HA coatings – C/C substrates under different spraying powers.
the substrates at 25 kW; it can be seen that some coating particles are remained on the surface and attached on the substrates, showing that interfacial failure exists during the bond strength evaluation. Under a high magnification given in Fig. 8b, it is noted that the fracture of HA is
291
along the splat boundaries and the carbon fibers have no influence on the shear strength, thus, the mean value of strength is relatively lower. The typical fracture morphologies of coatings at 35 and 40 kW are shown in Fig. 8c and e. Fig. 8d and f is that of the substrates, respectively. As indicated in Fig. 8c and d, carbon fibers are not bonded well with HA coating during plasma spraying, resulting in the pulling off of some fibers from the substrate and the porous coating material can be clearly seen. On the contrary, it can be observed from Fig. 8e and f that the coating is tightly adhered to the substrates, and coating particles impinged into carbon fiber bundles. The coating and the carbon fibers are simultaneously fractured under external load. Therefore, carbon fibers have improved the shear strength between the HA coatings and C/C substrates. In addition, observation of fracture surfaces as indicated in Fig. 8d and f revealed that the interface is clear and there are no new phases produced, suggesting that the chemical bonding is hardly to occur. When HA powders is sprayed
Fig. 8. Morphologies of shear fracture surfaces of HA coatings – C/C substrates. (a) C/C substrate at 25 kW, (b) high magnification of (a). (c) HA coating at 35 kW, (d) C/C substrate at 35 kW. (e) HA coating at 40 kW, (f ) C/C substrate at 40 kW.
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J.-L. Sui et al. / Surface & Coatings Technology 190 (2005) 287–292
onto a smooth substrates surface, sandblast that is a general pre-treating method for metal substrates. It is usually performed to give microscopically rough surface in advance, although it is probably not the optimal option for C/C composites. This means that the coated layer is sustained by physical bonding to the substrates. At last, it can be concluded from the above discussions that the optimal spraying power in this work is 40 kW.
4. Conclusions Hydroxyapatite can successfully be coated onto carbon/ carbon composites by a plasma spraying method. XRD patterns of coatings indicate that the coatings contain some amorphous apatite, crystalline HA , a-TCP, h-TCP and a little CaO. The spraying power has little effect on the content of crystalline HA phase, whereas the melting extent of coating particles and the amount of decomposed phases are enhanced with increasing spraying power. Moreover, the mean shear strength of the HA coatings– C/C substrates increases from 2.1 to 7.4 MPa. It was found physical bonding plays an important role in the interfacial bond as indicated from the SEM images of the fracture surfaces. Results indicate that the best spraying power is 40 kW under the tested conditions, in which carbon fiber bundles bond well with HA coatings and can accordingly enhance the adhesive strength.
Acknowledgements This work was supported by the Key Project of Science and Technology Development Plan of Shandong Province (011120106).
References [1] S.H. Li, Z.G. Zheng, Q. Liu, J.R. de Wijn, K. de Groot, J. Biomed. Mater. Res. 40 (1998) 520. [2] B.K. Lucie, S. Vladimı´r, K. Olga, L. Ve˘ra, J. Biomed. Mater. Res. 54 (2001) 567. [3] J.L. Sui, M.S. Li, Y.P. Lu, L.W. Yin, Y.J. Song, Surf. Coat. Technol. 176 (2004) 188. [4] R.G.T. Geesink, K. De Groot, C.P.A.T. Klein, J. Bone Jt. Surg. 70-B (1988) 17. [5] Y.C. Yang, E. Chang, Biomaterials 22 (2001) 1827. [6] I.H. Arita, V.M. Castano, D.S. Wilkinson, J. Mater. Sci., Mater. Med. 6 (1995) 19. [7] E. Park, A. Robert, S. Condrate, D. Lee, Mater. Lett. 36 (1998) 38. [8] Y.C. Tsui, C. Doyle, T.W. Clyne, Biomaterials 19 (1998) 2015. [9] S. Radin, P. Ducheyne, J. Mater. Sci. 3 (1992) 33. [10] G. Lewis, Biomed. Mater. Eng. 10 (2000) 157. [11] Y. Cao, J. Weng, J. Chen, J. Feng, Z. Yang, X. Zhang, Biomaterials 17 (1996) 419. [12] S.S. Tzeng, W.C. Lin, Carbon 37 (1999) 2011. [13] I.S. Leea, H.E. Kima, S.Y. Kim, Surf. Coat. Technol. 131 (2000) 181. [14] D.F. Williams, J. Biomed. Eng. 11 (1989) 185.