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Influence of residual stress on bonding strength of the plasma-sprayed hydroxyapatite coating after the vacuum heat treatment
Surface & Coatings Technology 201 (2007) 7187 – 7193 www.elsevier.com/locate/surfcoat
Influence of residual stress on bonding strength of the plasma-sprayed hydroxyapatite coating after the vacuum heat treatment Yung-Chin Yang Institute of Materials Science and Engineering, Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 106, Taiwan, ROC. Received 30 November 2006; accepted in revised form 15 January 2007 Available online 23 January 2007
Abstract Plasma-sprayed HACs with a fair amount of impurity phases and amorphous calcium phosphate will induce the dissolution and the dissociation for a period of implantation. Besides, the compressive residual stress in the coating would promote the tendency of plasma-sprayed coating to de-bond. For the biological stability, a significant crystallization and stress release of plasma-sprayed HACs with an appropriate post vacuum heating treatment is an effective method. According to the experimental results, post vacuum heating treatment could improve the cohesive strength of HACs by sintering and re-crystallizing. Meanwhile, the heat treatment also releases the compressive residual stress and replaced by the tensile stress. Hence, the HAC shows the highest bonding strength value when heating up to 600 °C. Then, the heating temperature higher than 600 °C led to the deterioration of the bonding strength of HA/Ti-substrate. This could be contributed to the reverse compressive residual stress in HAC. With regard to the variation of stress state in HACs might due to the various thermal expansion coefficients, which as a result of the crystallization during vacuum heating and the volume contraction of HACs. These will be discussed in detail. © 2007 Published by Elsevier B.V. Keywords: Residual stress; Plasma spray; Hydroxyapatite; Coating; Vacuum heat treatment; Coefficient of thermal expansion
1. Introduction Residual stresses occurred near the interface of the plasmasprayed ceramic coating and the metal substrate [1–3] when the coating rapidly solidified after plasma spraying due to the mismatch of thermal expansion coefficients of coating and substrate [4,5]. The thermal expansion coefficient of the metal substrate is generally higher than that of the ceramic coating; hence a compressive residual stress would be developed after cooling down. Compressive stress promotes the tendency of coatings to de-bond [6,7]. Residual stress in the coating might vary with coating thickness [6,8], spraying parameter and substrate cooling effect (i.e. temperature of substrate) [4,5]. In general, residual stresses increase with the thickness of coating and the temperature of the specimen during plasma spraying. In addition to the variable such as the structure of the coating and the roughness of the substrate surface, whether residual stress
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might be an important factor influencing the bonding strength between a ceramic coating and the metal substrate is not clear. More relevantly, the relation ship between residual stress and the bonding strength of the hydroxyapatite coating, crucial to the clinical success of the implants, has not been addressed in the literature. The higher crystallinity of HA coating (HAC) enables the higher biocompatibility to induce the bone tissue growth in the literatures [9,10]. Nonetheless, HAC prepared through plasma spray process from high temperature to rapid cooling contains plenty of amorphous phases and displayed the low crystallinity. Therefore, in order to improve the mechanical properties of plasma-sprayed HA coating, this paper employed the post-vacuum heating treatment to advance the strength and the biocompatibility of HA coating. This study addressed the properties of plasma-sprayed HACs after the post-vacuum heating (400–900 °C), including the crystallinity, Young's modulus, the residual strain and the bonding strength between coating and substrate. The plasmasprayed HAC is equipped with different material properties from HA bulk due to the coating formed from the solidification
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of rapid cooling from high temperature. We focused on the precipitation of impurity phases, the change of the crystallinity, Young's modulus, the coefficient of thermal expansion (CTE) and the microstructure of the post-vacuum heating HAC. This study aimed to measure the bonding strength between the plasma-sprayed HAC and substrate after the vacuum heating treatment with the various temperatures, besides, the stress relaxation method (mechanical destructive method) [11,12] was employed to measure the residual strains of HACs in order to understand the change of both the residual strains existed in HACs and the bonding strengths between the HAC and Ti-alloy substrate under various heating temperatures and to clarify the mechanism of the influence factor in bonding strength between the HAC and substrate after post-vacuum heating. 2. Materials and experimental 2.1. Atmospheric plasma spray and vacuum heating treatment Commercial high purity HA powder (Amdry 6021, Sulzer Metco Inc.), with particle sizes ranging from 45 to 125 μm, was used in the coating process. Plates of Ti-6Al-4Valloy (ASTM F136), measuring 50 mm (l) × 15 mm (w) × 3.3 mm (t) were selected as substrates. Prior to spraying, their surfaces were degreased with dilute acid (HCl, 1 mM) to remove organic contaminants and grit blasted with Al2O3 (A030#, 710–600 μm) to roughen the surface. The roughness of Ti-6Al-4V substrate (Ra = 3.5 μm) was an average with ten tests, which determined by the roughness-meter (Surfcorder SE-30H, Kosaka Laboratory Ltd.). The HA powder was carried by high purity argon gas to the plasma torch of a plasma spraying system (PT M-1100 C). HACs with an average thickness of 180 ± 20 μm were prepared using the spraying parameters shown as the Table 1. For postvacuum heating treatment, the samples namely HAC400, HAC500, HAC600, HAC700, HAC800 and HAC900 were heated at temperatures of 400–900 °C respectively with a heating rate of 10 °C/min and held for 3 h. The pressure in the vacuum chamber was lower than 1 × 10− 5 torr. 2.2. Characteristics of HACs The surface morphologies of the as-sprayed and vacuum heated HACs were examined with a secondary electron image (SEI) of a scanning electron microscope (FE-SEM, Philips XL-
40 FEG). For the observation of cross-sectional coatings, the specimens were cross-sectioned and mounted in epoxy. The mounted specimens were polished carefully to avoid inducing extra pores and cracks. The polished specimens were coated with carbon, and then examined by a backscattering electron image (BEI) of a SEM. The phases of the as-received HA powder, the as-sprayed and vacuum heated HACs were identified by X-ray diffractometry (Rigaku D/MAX III. V), using CuKα radiation, operated at 30 kV and 20 mA. Moreover, the index of crystallinity (IOC %) of the HACs was evaluated from the ratio of the main peak intensities of the HAC (Ic) and the HA powder (Ip) by the relation of IOC = (Ic/Ip) × 100% [9]. For the thermal dilatometry, the free sprayed-HA coatings measuring 15 (l) × 3 (w) × 3 (t) mm3, which were separated from the Ti−6Al−4V substrates, were prepared and then examined with a dilatometer (Netzsch DIL 402 C). They were heated at a heating rate of 10 °C/min to a maximum temperature of 900 °C and then furnace cooled. 2.3. Measurement procedures of the residual strain The residual strain and stress formed in the HACs were analyzed by a technique according to our previous work [12]. A 180 ± 20 μm thick coating was deposited on a Ti substrate with dimensions of 50 mm (l) × 15 mm (w) × 3.3 mm (t). The gauge lengths (l) of the HA coatings separated from the substrates according to the procedure described in [12] were measured, and the curvatures of the free HAC strips determined from sideview photographs. From the mechanics of a flexural beam, the residual strain at the top surface and the HAC/Ti-substrate interface can be calculated for a concave coating using the equation [12]: etop ; einter ¼ −½ðl−l0 Þ=l0 Fðt=2rÞ
ð1Þ
where t is the coating thickness, and r is the radius of curvature measured at the neutral surface of the HA coating. The first term − (l − l0) / l0 in Eq. (1) is defined as the linear strain and the second term (t / 2r) is the curvature strain. For convex coatings the calculated symbol “±” before the second term (t / 2r) in the Eq. (1) should be reversed, i.e. the Eq. (1) will change to: etop ; einter ¼ −½ðl−l0 Þ=l0 bðt=2rÞ
ð2Þ
2.4. Young's modulus measurement of HACs Table 1 Plasma spraying parameters employed for preparing HACs Parameter
HAC
Flow rate of primary gas (Ar) (1 min− 1) Flow rate of secondary gas (H2) (1 min− 1) Flow rate of powder carrier gas (Ar) (1 min− 1) Powder feed rate (g min− 1) Power (kW) Stand-off distance (cm) Surface speed (cm min− 1) Transverse speed (cm min− 1)
41 8 3 20 40.2 7.5 1200 60
Plasma spraying was performed with Plasma-Technik system.
The Young's modulus of the thermally-sprayed materials, due to their microstructure and inhomogeneity, might be different from the comparable bulk materials [13–15], and the HA coating would be sintered partially during the post-vacuum heating. Therefore, the real Young's moduli are needed for evaluating the mechanical properties of post-vacuum heating HA coatings. For measurement of Young's modulus in HACs, the 1.0 mm thick coating was coated on Ti-substrate of dimension 50 mm (l) × 10 mm (w) × 0.7 mm (t). Then, the width of the specimen with coatings and substrates was cut by a low
Y.-C. Yang / Surface & Coatings Technology 201 (2007) 7187–7193 Table 2 The roughness, index of crystallinity, residual strain and bonding strength of various HACs after vacuum heat treatment
HAC25 HAC400 HAC500 HAC600 HAC700 HAC800 HAC900 a b
Roughness IOC (Ra, μm) a (%)
Residual strain (%) b
Bonding strength (MPa) b
10.0 ± 1.4 10.2 ± 0.8 9.8 ± 0.8 9.6 ± 0.9 9.8 ± 1.8 10.4 ± 1.1 10.0 ± 1.2
− 0.25 ± 0.09 0.21 ± 0.04 0.17 ± 0.12 0.13 ± 0.07 − 0.14 ± 0.07 − 0.16 ± 0.06 − 0.17 ± 0.09
28.7 ± 1.7 38.7 ± 4.0 42.3 ± 3.4 41.1 ± 3.6 34.8 ± 1.7 27.9 ± 2.9 20.2 ± 4.6
44 53 57 68 71 73 70
Each value was the average of ten tests. Values are given as mean ± S.D. Each value was the average of five tests. Values are given as mean ± S.D.
speed diamond saw (Isomet) into 7–8 mm to trim the thinner portion of HA coatings near the edge. Finally, the coating was carefully separated from the Ti-substrate by grinding the latter from the back of specimen [13], leaving the free HA coating. Measurement of Young's modulus of HACs was carried out using a standard three-point bending test (ASTM E-855) [16]
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and each measured value of Young's modulus represented an average of three to five tests. After test, the value of Young's modulus was calculated from the relationship [16]: E¼
P L3 4 b h3 d
ð3Þ
where E is Young's modulus, P is load, L is span length between supports, b is specimen width, h is specimen thickness, and δ is deflection at midspan. 2.5. Bonding strength measurement Besides, the bonding strengths of the HACs were tested using a standard adhesion test (ASTM C-633) that was especially designed for plasma-sprayed coatings. Each test specimen was an assembly composed of a substrate fixture, to which the HACs of 180 ± 20 μm were applied, and a loading fixture. The facings of the loading fixtures were grit-blasted and attached to the surface of the HACs using special adhesive glue
Fig. 1. SEM surface morphologies of (a) the as-sprayed HAC, (c) HAC700; and the cross-sectional feature of (b) the as-sprayed HAC, (d) HAC700, (e) HAC900.
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900 °C the bonding force between the coating and the substrate substantially decreased as evidenced by the destruction of the interface between coating and substrate (Fig. 1e). The X-ray diffraction pattern of HA coatings after postvacuum heating are displayed in Fig. 2 showing that the peak intensities of impurity phases (TetrCP, TCP, CaO) increase with increasing temperature. The impurity phases after heating at 900 °C reveal the highest content. Table 2 shows the index of crystallinity (IOC) of HA and Fig. 3 presents the relation between IOC and temperature, indicating that the IOC advanced as the temperature went up to attain a maximum at 500–600 °C owing to a maximum nucleation rate. However, Fig. 3 also shows that the IOC decreased at temperatures above 800 °C suggesting an acceleration of the phase transformation of crystalline HA into impurity phases as also confirmed by Fig. 2. Fig. 2. The diffractograms of the various vacuum heating HACs.
(METCO EP-15) with an adhesive strength of about 60 MPa. The assembly was held perpendicularly and placed in an oven at 180 °C for 2 h. After curing and hardening of the bonding glue, the assembly was loaded in an Instron machine for measurement of the tensile bonding strength. 3. Results and discussions 3.1. Characteristics of HA coating
3.2. Residual strain of HA coating To measure the residual stress we employed the specimen curvature method. The Ti alloy substrate was removed [12] to obtain the free HA coating (Fig. 4). By measuring the variation of length and curvature of the free coatings we obtain the residual strain (ε) of the coating as shown in Table 2 and Fig. 5. We learn from Fig. 5 that the as-sprayed HA coating represents a compressive residual strain but the HAC400 to 600 tensile ones. However, higher heating temperature (HAC700 to 900) again produced compressive residual strains. In our previous study [13], plasma-spayed HA coating (HAC25) showed the
The values of the roughness (Ra, μm) of HACs through six kinds of vacuum heating temperatures and the as-sprayed HAC are listed in Table 2; there is no apparent difference of roughness of HACs both before and after the heat treatment. The typical surface and cross-sectional morphologies of as-sprayed HACs are shown respectively, in Figs. 1a and c, the latter with pronounced lamellar structure and microlaminar cracks within the coating. Since coatings vacuum-heat treated at various temperatures were very similar in appearance only one typical example of a HAC annealed at 700 °C is being displayed (Figs. 1b and d, respectively). However, in HAC heated to
Fig. 3. The index of crystallinity (IOC) of the as-sprayed and vacuum heating HACs.
Fig. 4. (a) Illustration of the effect of removing the substrate on the shape of HAC. (b) Profiles of the separated HA coatings.
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Fig. 5. The residual strains of the as-sprayed and vacuum heating HACs.
compressive residual strain due to the complex coating deposition and cooling process during the plasma spraying. However, considering the residual stresses in post-vacuum heated coatings is much simpler because coating and substrate were at the same temperature. Indeed, after cooling down from a high temperature to room temperature formation of residual stresses could simply be explained by the mismatch in the coefficients of thermal expansion (CTE). Theoretically, the CTE of bulk HA is larger than that of Ti-alloy substrate (αHA = 11.5 × 10− 6 K− 1, αTi-6Al-4V = 8.9 × 10− 6 K− 1 [17,18]). Hence according to the difference of CTE a tensile residual stress ought to be induced after post vacuum heating. Cofino et al. [19] invoked the concept of the “real” coating morphology, i.e. coating roughness to explain why a thermally sprayed HA coating exhibits predominately compressive stresses rather than tensile ones. Also, the sinusoidal interface between substrate and coating may lead to a reversal of the sign of stress [20,21]. But the porosity, crack and low crystallinity of plasma sprayed coating result in the difference between the value of CTE of HAC and the theoretical value of CTE of HA bulk. So the practical CTE of plasma-sprayed HAC must be realized first if CTE were utilized to explain the generation and status of the residual stress. The dilatation curve of HACs and Ti-substrate during heating is shown in Fig. 6, the average CTE of each temperature interval were shown in Table 3. Fig. 6a and Table 3 indicated that the coefficient of thermal expansion (CTE) of Ti6A14V substrate was between 9.9–10.5 × 10− 6 K− 1 , higher than the value reported in the literature. Fig. 6b showed the dilatation of HAC volume during heated from room temperature to 900 °C which was separated into three intervals of temperatures. In the interval 100–400 °C (I) showed that HAC expanded with increasing temperature and according to the definition of the coefficient of thermal expansion (CTE) α: (δL/Lo) = α × ΔT, the value of the slope in Fig. 6 which could seen as CTE was 14.24 × 10− 6 K− 1 . In the interval 400–700 °C (II) showed that non-linear relation between the dilatation and the temperature; as the temperature was higher than 550 °C, the volumes of HACs were contracted with the increasing temperature. So we inferred that the interval II might be the major range of crystallization
Fig. 6. The dilatation curve of (a) Ti-6Al-4V substrate and (b) sprayed-HA coating for a single heating process.
of the amorphous phase in HAC. As for the interval III higher than 700 °C, the coating was subjected by the thermal expansion rather than the crystallo-contraction and the curve of CTE represented the positive linear relation with the increasing temperature, the slope of which was 8.5 × 10− 6 K− 1 . Comparing the slopes of the interval I and II, we confirmed the diversity of CTEs in accordance with the crystallinity of HA coatings. Table 3 indicated clearly the steady value of CTEs of Ti-alloy substrate in diverse thermal intervals, but CTEs of HACs differed much in diverse thermal intervals. Through the Table 3 The average coefficient of thermal expansion (CTE) of HAC in various temperature range Temperature
25–400 °C 25–500 °C 25–600 °C 25–700 °C 25–800 °C 25–900 °C
CTE (×10− 6 K− 1) HA coating
Ti6Al4V substrate
14.24 12.77 10.64 8.75 8.41 8.24
9.89 10.03 10.15 10.34 10.45 10.50
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comparison of CTEs of both substrate and coating, we found that in three intervals of 25–400 °C, 25–500 °C, 25–600 °C, the average CTE of HAC was larger than that of Ti-alloy substrate, so the volume contraction of HAC was larger than that of Tialloy substrate as the specimens returned to the room temperature from high temperature, leading to the tensile residual strain in HAC. On the contrary, in three intervals of 25– 700 °C, 25–800 °C, 25–900 °C, the average CTEs of HACs were smaller than that of Ti-alloy substrate, leading to the compressive residual strain. This explained clearly why the tensile residual strain appeared on HAC through vacuum heating treatment at 400 °C, 500 °C and 600 °C and the compressive residual strains appeared on HACs through vacuum heating treatment at 700 °C, 800 °C and 900 °C. Fig. 8. The Young's moduli of the as-sprayed and vacuum heating HACs.
3.3. Bonding strength between HAC and Ti-substrate The bonding strengths of various vacuum heating HACs on Ti-substrate were listed in Table 2. The bonding strength between the coating and substrate pertaining to the as-sprayed HAC25 was 28.7 ± 1.7 MPa. Fig. 7 showed the variation of the bonding strengths between the post-vacuum heating HACs and substrate according to the increasing temperature. Up to 600 °C, the bonding strengths increased with increasing temperature to 42.3 ± 3.4 MPa. Above 600 °C, the bonding strengths declined to 20.2 ± 4.6 MPa, even lower than that of the as-sprayed HAC (HAC25). Details will be discussed below. The bonding strength of coating on substrate includes two factors: (1) the adhesive strength of coating and substrate interface and (2) the cohesive strength of coating itself [22]. The affecting factors of the adhesive strength of coating and substrate interface include the roughness of substrate surface and the interfacial residual stress. As for the cohesive strength of coating, the factors include the crystallinity and densification of coating, appearing on Young's modulus of coating. In this paper, we discussed the impact of the cohesive strength on the bonding strength of HAC/Ti. Fig. 8 showed the Young's moduli of HACs after post-vacuum heating. The value of Young's modulus (E) of as-sprayed HAC was 22.2 ± 1.3 GPa; the values of Young's moduli of post-vacuum heating HAC were advanced with the increasing heating temperature. This was contributed to the re-crystallization of amorphous HAC (Fig. 3)
Fig. 7. The bonding strength of the as-sprayed and vacuum heating HACs.
and the obvious effects of sintering densification of coating at higher temperature. Therefore, the cohesive strengths of HACs were enhanced with the increasing temperatures of post-vacuum heating. However, the comparison of Figs. 7 and 8 showed the bonding strengths between HAC and Ti-alloy substrate did not increase in accordance with the increasing temperature; they lowered along with the increasing temperatures after attaining the maximum value at 500–600 °C. This was resulted from another critical factor, the impact of the interfacial adhesive force in coating and substrate besides the cohesive strength of HAC. In the past studies [23], we found the in-plane compressive residual stress would induce through-thickness tensile stress acting in the direction normal to the interface of the HA coating and the Ti-substrate, and this stress would neutralize the bonding force of coating on substrate and reduce the interfacial adhesive force as well, that declined the bonding strength between coating and substrate. Therefore, from the demonstration in Fig. 7, the bonding strengths of HAC/Ti-substrate increased with the increasing temperature when the postvacuum heating treatment under 600 °C, the reasons are not only the sintering densification of HAC, but also the advanced interfacial adhesive force resulting from the release of compressive residual strain of HAC25 through vacuum heating treatment. When the post-vacuum heating over than 600 °C, the CTEs of HACs were smaller than that of Ti-alloy substrate and the larger compressive residual strain were occurred and increased with the increasing temperatures. The bonding strengths of HAC/Ti-substrates were weakened with the increasing temperatures higher than 600 °C since the declined interfacial adhesive force between coating and substrate was caused by the compressive residual strain. To sum up, the discussion concerning the bonding strengths of HAC/Tisubstrate in accordance with heating treatment temperatures in Fig. 7 can be separated into two parts: (1) When the postheating temperatures remaining under the 600 °C, the compressive residual strains of HACs were released. Therefore, the bonding strengths of HAC/Ti-alloy were advanced as the effects of the crystallization and the sintering densification of HAC because the cohesive strength impacted dominantly the
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bonding strength of HAC/Ti-substrate. (2) While heating temperature attaining higher than 600 °C, even though the advanced Young's moduli (cohesive strength) of HACs existed with increasing heating temperatures, the bonding strengths of HAC/Ti-substrate declined because the compressive residual stresses of HACs reduced the partial interfacial adhesive force of coating on substrate through increasing heating, making the total bonding strengths of HAC/Ti-substrate declined. 4. Conclusion Based on the results of the Young's modulus evaluation, residual strain measurement and the bonding strength test between the coating and substrate, the factor affecting the bonding strengths of HAC/Ti-substrate were influenced by the densification effect of coating during the heating process and residual strain of inner coating. The initial vacuum heating treatment (b 600 °C) can release the compressive residual strain of HAC after plasma spray, so the advance of Young's modulus enabled HAC to obtain the optimal bonding strength at the heating temperature of 500–600 °C. Nonetheless, the postvacuum heating temperature higher than 600 °C led to the deterioration of the bonding strength of HA/Ti-substrate. With the increasing temperature, the increased compressive residual strain will weaken the interfacial adhesive force of coating to substrate, resulting in the lower bonding strength with higher temperature. The results in this paper demonstrated that the proper temperature of post-vacuum heating treatment advanced effectively the bonding strength between plasma sprayed HAC and Ti-alloy substrate, this would enhance the durability and quality of artificial implant materials.
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Influence of residual stress on bonding strength of the plasma-sprayed hydroxyapatite coating after the vacuum heat treatment Yung-Chin Yang Institute of Materials Science and Engineering, Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 106, Taiwan, ROC. Received 30 November 2006; accepted in revised form 15 January 2007 Available online 23 January 2007
Abstract Plasma-sprayed HACs with a fair amount of impurity phases and amorphous calcium phosphate will induce the dissolution and the dissociation for a period of implantation. Besides, the compressive residual stress in the coating would promote the tendency of plasma-sprayed coating to de-bond. For the biological stability, a significant crystallization and stress release of plasma-sprayed HACs with an appropriate post vacuum heating treatment is an effective method. According to the experimental results, post vacuum heating treatment could improve the cohesive strength of HACs by sintering and re-crystallizing. Meanwhile, the heat treatment also releases the compressive residual stress and replaced by the tensile stress. Hence, the HAC shows the highest bonding strength value when heating up to 600 °C. Then, the heating temperature higher than 600 °C led to the deterioration of the bonding strength of HA/Ti-substrate. This could be contributed to the reverse compressive residual stress in HAC. With regard to the variation of stress state in HACs might due to the various thermal expansion coefficients, which as a result of the crystallization during vacuum heating and the volume contraction of HACs. These will be discussed in detail. © 2007 Published by Elsevier B.V. Keywords: Residual stress; Plasma spray; Hydroxyapatite; Coating; Vacuum heat treatment; Coefficient of thermal expansion
1. Introduction Residual stresses occurred near the interface of the plasmasprayed ceramic coating and the metal substrate [1–3] when the coating rapidly solidified after plasma spraying due to the mismatch of thermal expansion coefficients of coating and substrate [4,5]. The thermal expansion coefficient of the metal substrate is generally higher than that of the ceramic coating; hence a compressive residual stress would be developed after cooling down. Compressive stress promotes the tendency of coatings to de-bond [6,7]. Residual stress in the coating might vary with coating thickness [6,8], spraying parameter and substrate cooling effect (i.e. temperature of substrate) [4,5]. In general, residual stresses increase with the thickness of coating and the temperature of the specimen during plasma spraying. In addition to the variable such as the structure of the coating and the roughness of the substrate surface, whether residual stress
E-mail address: [email protected]. 0257-8972/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2007.01.027
might be an important factor influencing the bonding strength between a ceramic coating and the metal substrate is not clear. More relevantly, the relation ship between residual stress and the bonding strength of the hydroxyapatite coating, crucial to the clinical success of the implants, has not been addressed in the literature. The higher crystallinity of HA coating (HAC) enables the higher biocompatibility to induce the bone tissue growth in the literatures [9,10]. Nonetheless, HAC prepared through plasma spray process from high temperature to rapid cooling contains plenty of amorphous phases and displayed the low crystallinity. Therefore, in order to improve the mechanical properties of plasma-sprayed HA coating, this paper employed the post-vacuum heating treatment to advance the strength and the biocompatibility of HA coating. This study addressed the properties of plasma-sprayed HACs after the post-vacuum heating (400–900 °C), including the crystallinity, Young's modulus, the residual strain and the bonding strength between coating and substrate. The plasmasprayed HAC is equipped with different material properties from HA bulk due to the coating formed from the solidification
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of rapid cooling from high temperature. We focused on the precipitation of impurity phases, the change of the crystallinity, Young's modulus, the coefficient of thermal expansion (CTE) and the microstructure of the post-vacuum heating HAC. This study aimed to measure the bonding strength between the plasma-sprayed HAC and substrate after the vacuum heating treatment with the various temperatures, besides, the stress relaxation method (mechanical destructive method) [11,12] was employed to measure the residual strains of HACs in order to understand the change of both the residual strains existed in HACs and the bonding strengths between the HAC and Ti-alloy substrate under various heating temperatures and to clarify the mechanism of the influence factor in bonding strength between the HAC and substrate after post-vacuum heating. 2. Materials and experimental 2.1. Atmospheric plasma spray and vacuum heating treatment Commercial high purity HA powder (Amdry 6021, Sulzer Metco Inc.), with particle sizes ranging from 45 to 125 μm, was used in the coating process. Plates of Ti-6Al-4Valloy (ASTM F136), measuring 50 mm (l) × 15 mm (w) × 3.3 mm (t) were selected as substrates. Prior to spraying, their surfaces were degreased with dilute acid (HCl, 1 mM) to remove organic contaminants and grit blasted with Al2O3 (A030#, 710–600 μm) to roughen the surface. The roughness of Ti-6Al-4V substrate (Ra = 3.5 μm) was an average with ten tests, which determined by the roughness-meter (Surfcorder SE-30H, Kosaka Laboratory Ltd.). The HA powder was carried by high purity argon gas to the plasma torch of a plasma spraying system (PT M-1100 C). HACs with an average thickness of 180 ± 20 μm were prepared using the spraying parameters shown as the Table 1. For postvacuum heating treatment, the samples namely HAC400, HAC500, HAC600, HAC700, HAC800 and HAC900 were heated at temperatures of 400–900 °C respectively with a heating rate of 10 °C/min and held for 3 h. The pressure in the vacuum chamber was lower than 1 × 10− 5 torr. 2.2. Characteristics of HACs The surface morphologies of the as-sprayed and vacuum heated HACs were examined with a secondary electron image (SEI) of a scanning electron microscope (FE-SEM, Philips XL-
40 FEG). For the observation of cross-sectional coatings, the specimens were cross-sectioned and mounted in epoxy. The mounted specimens were polished carefully to avoid inducing extra pores and cracks. The polished specimens were coated with carbon, and then examined by a backscattering electron image (BEI) of a SEM. The phases of the as-received HA powder, the as-sprayed and vacuum heated HACs were identified by X-ray diffractometry (Rigaku D/MAX III. V), using CuKα radiation, operated at 30 kV and 20 mA. Moreover, the index of crystallinity (IOC %) of the HACs was evaluated from the ratio of the main peak intensities of the HAC (Ic) and the HA powder (Ip) by the relation of IOC = (Ic/Ip) × 100% [9]. For the thermal dilatometry, the free sprayed-HA coatings measuring 15 (l) × 3 (w) × 3 (t) mm3, which were separated from the Ti−6Al−4V substrates, were prepared and then examined with a dilatometer (Netzsch DIL 402 C). They were heated at a heating rate of 10 °C/min to a maximum temperature of 900 °C and then furnace cooled. 2.3. Measurement procedures of the residual strain The residual strain and stress formed in the HACs were analyzed by a technique according to our previous work [12]. A 180 ± 20 μm thick coating was deposited on a Ti substrate with dimensions of 50 mm (l) × 15 mm (w) × 3.3 mm (t). The gauge lengths (l) of the HA coatings separated from the substrates according to the procedure described in [12] were measured, and the curvatures of the free HAC strips determined from sideview photographs. From the mechanics of a flexural beam, the residual strain at the top surface and the HAC/Ti-substrate interface can be calculated for a concave coating using the equation [12]: etop ; einter ¼ −½ðl−l0 Þ=l0 Fðt=2rÞ
ð1Þ
where t is the coating thickness, and r is the radius of curvature measured at the neutral surface of the HA coating. The first term − (l − l0) / l0 in Eq. (1) is defined as the linear strain and the second term (t / 2r) is the curvature strain. For convex coatings the calculated symbol “±” before the second term (t / 2r) in the Eq. (1) should be reversed, i.e. the Eq. (1) will change to: etop ; einter ¼ −½ðl−l0 Þ=l0 bðt=2rÞ
ð2Þ
2.4. Young's modulus measurement of HACs Table 1 Plasma spraying parameters employed for preparing HACs Parameter
HAC
Flow rate of primary gas (Ar) (1 min− 1) Flow rate of secondary gas (H2) (1 min− 1) Flow rate of powder carrier gas (Ar) (1 min− 1) Powder feed rate (g min− 1) Power (kW) Stand-off distance (cm) Surface speed (cm min− 1) Transverse speed (cm min− 1)
41 8 3 20 40.2 7.5 1200 60
Plasma spraying was performed with Plasma-Technik system.
The Young's modulus of the thermally-sprayed materials, due to their microstructure and inhomogeneity, might be different from the comparable bulk materials [13–15], and the HA coating would be sintered partially during the post-vacuum heating. Therefore, the real Young's moduli are needed for evaluating the mechanical properties of post-vacuum heating HA coatings. For measurement of Young's modulus in HACs, the 1.0 mm thick coating was coated on Ti-substrate of dimension 50 mm (l) × 10 mm (w) × 0.7 mm (t). Then, the width of the specimen with coatings and substrates was cut by a low
Y.-C. Yang / Surface & Coatings Technology 201 (2007) 7187–7193 Table 2 The roughness, index of crystallinity, residual strain and bonding strength of various HACs after vacuum heat treatment
HAC25 HAC400 HAC500 HAC600 HAC700 HAC800 HAC900 a b
Roughness IOC (Ra, μm) a (%)
Residual strain (%) b
Bonding strength (MPa) b
10.0 ± 1.4 10.2 ± 0.8 9.8 ± 0.8 9.6 ± 0.9 9.8 ± 1.8 10.4 ± 1.1 10.0 ± 1.2
− 0.25 ± 0.09 0.21 ± 0.04 0.17 ± 0.12 0.13 ± 0.07 − 0.14 ± 0.07 − 0.16 ± 0.06 − 0.17 ± 0.09
28.7 ± 1.7 38.7 ± 4.0 42.3 ± 3.4 41.1 ± 3.6 34.8 ± 1.7 27.9 ± 2.9 20.2 ± 4.6
44 53 57 68 71 73 70
Each value was the average of ten tests. Values are given as mean ± S.D. Each value was the average of five tests. Values are given as mean ± S.D.
speed diamond saw (Isomet) into 7–8 mm to trim the thinner portion of HA coatings near the edge. Finally, the coating was carefully separated from the Ti-substrate by grinding the latter from the back of specimen [13], leaving the free HA coating. Measurement of Young's modulus of HACs was carried out using a standard three-point bending test (ASTM E-855) [16]
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and each measured value of Young's modulus represented an average of three to five tests. After test, the value of Young's modulus was calculated from the relationship [16]: E¼
P L3 4 b h3 d
ð3Þ
where E is Young's modulus, P is load, L is span length between supports, b is specimen width, h is specimen thickness, and δ is deflection at midspan. 2.5. Bonding strength measurement Besides, the bonding strengths of the HACs were tested using a standard adhesion test (ASTM C-633) that was especially designed for plasma-sprayed coatings. Each test specimen was an assembly composed of a substrate fixture, to which the HACs of 180 ± 20 μm were applied, and a loading fixture. The facings of the loading fixtures were grit-blasted and attached to the surface of the HACs using special adhesive glue
Fig. 1. SEM surface morphologies of (a) the as-sprayed HAC, (c) HAC700; and the cross-sectional feature of (b) the as-sprayed HAC, (d) HAC700, (e) HAC900.
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900 °C the bonding force between the coating and the substrate substantially decreased as evidenced by the destruction of the interface between coating and substrate (Fig. 1e). The X-ray diffraction pattern of HA coatings after postvacuum heating are displayed in Fig. 2 showing that the peak intensities of impurity phases (TetrCP, TCP, CaO) increase with increasing temperature. The impurity phases after heating at 900 °C reveal the highest content. Table 2 shows the index of crystallinity (IOC) of HA and Fig. 3 presents the relation between IOC and temperature, indicating that the IOC advanced as the temperature went up to attain a maximum at 500–600 °C owing to a maximum nucleation rate. However, Fig. 3 also shows that the IOC decreased at temperatures above 800 °C suggesting an acceleration of the phase transformation of crystalline HA into impurity phases as also confirmed by Fig. 2. Fig. 2. The diffractograms of the various vacuum heating HACs.
(METCO EP-15) with an adhesive strength of about 60 MPa. The assembly was held perpendicularly and placed in an oven at 180 °C for 2 h. After curing and hardening of the bonding glue, the assembly was loaded in an Instron machine for measurement of the tensile bonding strength. 3. Results and discussions 3.1. Characteristics of HA coating
3.2. Residual strain of HA coating To measure the residual stress we employed the specimen curvature method. The Ti alloy substrate was removed [12] to obtain the free HA coating (Fig. 4). By measuring the variation of length and curvature of the free coatings we obtain the residual strain (ε) of the coating as shown in Table 2 and Fig. 5. We learn from Fig. 5 that the as-sprayed HA coating represents a compressive residual strain but the HAC400 to 600 tensile ones. However, higher heating temperature (HAC700 to 900) again produced compressive residual strains. In our previous study [13], plasma-spayed HA coating (HAC25) showed the
The values of the roughness (Ra, μm) of HACs through six kinds of vacuum heating temperatures and the as-sprayed HAC are listed in Table 2; there is no apparent difference of roughness of HACs both before and after the heat treatment. The typical surface and cross-sectional morphologies of as-sprayed HACs are shown respectively, in Figs. 1a and c, the latter with pronounced lamellar structure and microlaminar cracks within the coating. Since coatings vacuum-heat treated at various temperatures were very similar in appearance only one typical example of a HAC annealed at 700 °C is being displayed (Figs. 1b and d, respectively). However, in HAC heated to
Fig. 3. The index of crystallinity (IOC) of the as-sprayed and vacuum heating HACs.
Fig. 4. (a) Illustration of the effect of removing the substrate on the shape of HAC. (b) Profiles of the separated HA coatings.
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Fig. 5. The residual strains of the as-sprayed and vacuum heating HACs.
compressive residual strain due to the complex coating deposition and cooling process during the plasma spraying. However, considering the residual stresses in post-vacuum heated coatings is much simpler because coating and substrate were at the same temperature. Indeed, after cooling down from a high temperature to room temperature formation of residual stresses could simply be explained by the mismatch in the coefficients of thermal expansion (CTE). Theoretically, the CTE of bulk HA is larger than that of Ti-alloy substrate (αHA = 11.5 × 10− 6 K− 1, αTi-6Al-4V = 8.9 × 10− 6 K− 1 [17,18]). Hence according to the difference of CTE a tensile residual stress ought to be induced after post vacuum heating. Cofino et al. [19] invoked the concept of the “real” coating morphology, i.e. coating roughness to explain why a thermally sprayed HA coating exhibits predominately compressive stresses rather than tensile ones. Also, the sinusoidal interface between substrate and coating may lead to a reversal of the sign of stress [20,21]. But the porosity, crack and low crystallinity of plasma sprayed coating result in the difference between the value of CTE of HAC and the theoretical value of CTE of HA bulk. So the practical CTE of plasma-sprayed HAC must be realized first if CTE were utilized to explain the generation and status of the residual stress. The dilatation curve of HACs and Ti-substrate during heating is shown in Fig. 6, the average CTE of each temperature interval were shown in Table 3. Fig. 6a and Table 3 indicated that the coefficient of thermal expansion (CTE) of Ti6A14V substrate was between 9.9–10.5 × 10− 6 K− 1 , higher than the value reported in the literature. Fig. 6b showed the dilatation of HAC volume during heated from room temperature to 900 °C which was separated into three intervals of temperatures. In the interval 100–400 °C (I) showed that HAC expanded with increasing temperature and according to the definition of the coefficient of thermal expansion (CTE) α: (δL/Lo) = α × ΔT, the value of the slope in Fig. 6 which could seen as CTE was 14.24 × 10− 6 K− 1 . In the interval 400–700 °C (II) showed that non-linear relation between the dilatation and the temperature; as the temperature was higher than 550 °C, the volumes of HACs were contracted with the increasing temperature. So we inferred that the interval II might be the major range of crystallization
Fig. 6. The dilatation curve of (a) Ti-6Al-4V substrate and (b) sprayed-HA coating for a single heating process.
of the amorphous phase in HAC. As for the interval III higher than 700 °C, the coating was subjected by the thermal expansion rather than the crystallo-contraction and the curve of CTE represented the positive linear relation with the increasing temperature, the slope of which was 8.5 × 10− 6 K− 1 . Comparing the slopes of the interval I and II, we confirmed the diversity of CTEs in accordance with the crystallinity of HA coatings. Table 3 indicated clearly the steady value of CTEs of Ti-alloy substrate in diverse thermal intervals, but CTEs of HACs differed much in diverse thermal intervals. Through the Table 3 The average coefficient of thermal expansion (CTE) of HAC in various temperature range Temperature
25–400 °C 25–500 °C 25–600 °C 25–700 °C 25–800 °C 25–900 °C
CTE (×10− 6 K− 1) HA coating
Ti6Al4V substrate
14.24 12.77 10.64 8.75 8.41 8.24
9.89 10.03 10.15 10.34 10.45 10.50
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comparison of CTEs of both substrate and coating, we found that in three intervals of 25–400 °C, 25–500 °C, 25–600 °C, the average CTE of HAC was larger than that of Ti-alloy substrate, so the volume contraction of HAC was larger than that of Tialloy substrate as the specimens returned to the room temperature from high temperature, leading to the tensile residual strain in HAC. On the contrary, in three intervals of 25– 700 °C, 25–800 °C, 25–900 °C, the average CTEs of HACs were smaller than that of Ti-alloy substrate, leading to the compressive residual strain. This explained clearly why the tensile residual strain appeared on HAC through vacuum heating treatment at 400 °C, 500 °C and 600 °C and the compressive residual strains appeared on HACs through vacuum heating treatment at 700 °C, 800 °C and 900 °C. Fig. 8. The Young's moduli of the as-sprayed and vacuum heating HACs.
3.3. Bonding strength between HAC and Ti-substrate The bonding strengths of various vacuum heating HACs on Ti-substrate were listed in Table 2. The bonding strength between the coating and substrate pertaining to the as-sprayed HAC25 was 28.7 ± 1.7 MPa. Fig. 7 showed the variation of the bonding strengths between the post-vacuum heating HACs and substrate according to the increasing temperature. Up to 600 °C, the bonding strengths increased with increasing temperature to 42.3 ± 3.4 MPa. Above 600 °C, the bonding strengths declined to 20.2 ± 4.6 MPa, even lower than that of the as-sprayed HAC (HAC25). Details will be discussed below. The bonding strength of coating on substrate includes two factors: (1) the adhesive strength of coating and substrate interface and (2) the cohesive strength of coating itself [22]. The affecting factors of the adhesive strength of coating and substrate interface include the roughness of substrate surface and the interfacial residual stress. As for the cohesive strength of coating, the factors include the crystallinity and densification of coating, appearing on Young's modulus of coating. In this paper, we discussed the impact of the cohesive strength on the bonding strength of HAC/Ti. Fig. 8 showed the Young's moduli of HACs after post-vacuum heating. The value of Young's modulus (E) of as-sprayed HAC was 22.2 ± 1.3 GPa; the values of Young's moduli of post-vacuum heating HAC were advanced with the increasing heating temperature. This was contributed to the re-crystallization of amorphous HAC (Fig. 3)
Fig. 7. The bonding strength of the as-sprayed and vacuum heating HACs.
and the obvious effects of sintering densification of coating at higher temperature. Therefore, the cohesive strengths of HACs were enhanced with the increasing temperatures of post-vacuum heating. However, the comparison of Figs. 7 and 8 showed the bonding strengths between HAC and Ti-alloy substrate did not increase in accordance with the increasing temperature; they lowered along with the increasing temperatures after attaining the maximum value at 500–600 °C. This was resulted from another critical factor, the impact of the interfacial adhesive force in coating and substrate besides the cohesive strength of HAC. In the past studies [23], we found the in-plane compressive residual stress would induce through-thickness tensile stress acting in the direction normal to the interface of the HA coating and the Ti-substrate, and this stress would neutralize the bonding force of coating on substrate and reduce the interfacial adhesive force as well, that declined the bonding strength between coating and substrate. Therefore, from the demonstration in Fig. 7, the bonding strengths of HAC/Ti-substrate increased with the increasing temperature when the postvacuum heating treatment under 600 °C, the reasons are not only the sintering densification of HAC, but also the advanced interfacial adhesive force resulting from the release of compressive residual strain of HAC25 through vacuum heating treatment. When the post-vacuum heating over than 600 °C, the CTEs of HACs were smaller than that of Ti-alloy substrate and the larger compressive residual strain were occurred and increased with the increasing temperatures. The bonding strengths of HAC/Ti-substrates were weakened with the increasing temperatures higher than 600 °C since the declined interfacial adhesive force between coating and substrate was caused by the compressive residual strain. To sum up, the discussion concerning the bonding strengths of HAC/Tisubstrate in accordance with heating treatment temperatures in Fig. 7 can be separated into two parts: (1) When the postheating temperatures remaining under the 600 °C, the compressive residual strains of HACs were released. Therefore, the bonding strengths of HAC/Ti-alloy were advanced as the effects of the crystallization and the sintering densification of HAC because the cohesive strength impacted dominantly the
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bonding strength of HAC/Ti-substrate. (2) While heating temperature attaining higher than 600 °C, even though the advanced Young's moduli (cohesive strength) of HACs existed with increasing heating temperatures, the bonding strengths of HAC/Ti-substrate declined because the compressive residual stresses of HACs reduced the partial interfacial adhesive force of coating on substrate through increasing heating, making the total bonding strengths of HAC/Ti-substrate declined. 4. Conclusion Based on the results of the Young's modulus evaluation, residual strain measurement and the bonding strength test between the coating and substrate, the factor affecting the bonding strengths of HAC/Ti-substrate were influenced by the densification effect of coating during the heating process and residual strain of inner coating. The initial vacuum heating treatment (b 600 °C) can release the compressive residual strain of HAC after plasma spray, so the advance of Young's modulus enabled HAC to obtain the optimal bonding strength at the heating temperature of 500–600 °C. Nonetheless, the postvacuum heating temperature higher than 600 °C led to the deterioration of the bonding strength of HA/Ti-substrate. With the increasing temperature, the increased compressive residual strain will weaken the interfacial adhesive force of coating to substrate, resulting in the lower bonding strength with higher temperature. The results in this paper demonstrated that the proper temperature of post-vacuum heating treatment advanced effectively the bonding strength between plasma sprayed HAC and Ti-alloy substrate, this would enhance the durability and quality of artificial implant materials.
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