Biomaterials
Volume 24, Issue 9 , April 2003, Pages 1603-1611

In vitro studies of plasma-sprayed hydroxyapatite/Ti-6Al-4V composite coatings in simulated body fluid (SBF)

Y. W. Gu^a, K. A. Khor^{, , a} and P. Cheang<sup>b

a</sup> School of Mechanical & Production Engineering, Nanyang Technological University, North Spine Block N3, Level 250, Nanyang Avenue, Singapore 639798, Singapore
^b School of Materials Engineering, Nanyang Technological University, North Spine Block N3, Level 250, Nanyang Avenue, Singapore 639798, Singapore

Received 12 August 2002;  accepted 11 November 2002. ; Available online 23 January 2003.

1. Abstract

The bioactivity of plasma-sprayed hydroxyapatite (HA)/Ti-6Al-4V composite coatings was studied by soaking the coatings in simulated body fluid (SBF) for up to 8 weeks. This investigation was aimed at elucidating the biological behaviour of plasma-sprayed HA/Ti-6Al-4V composite coatings by analyzing the changes in chemistry, and crystallinity of the composite coating in a body-analogous solution. Phase composition, microstructure and calcium ion concentration were analyzed before, and after immersion. The mechanical properties, such as tensile bond strength, microhardness and Young's modulus were appropriately measured. Results demonstrated that the tensile bond strength of the composite coating was significantly higher than that of pure HA coatings even after soaking in the SBF solution over an 8-weeks period. Dissolution of Ca-P phases in SBF was evident after 24 h of soaking, and, a layer of carbonate-apatite covered the coating surface after 2 weeks of immersion. The mechanical properties were found to diminish with soaking duration. However, slight variation in mechanical properties was found after supersaturation of the calcium ions was attained with the precipitation of the calcium phosphate layers.

Author Keywords: Biomaterials; Simulated body fluid; Hydroxyapatite; Composite coating; Plasma spraying

2. Article Outline

1. Introduction

2. Materials and methods

2.1. Preparation of HA/Ti-6Al-4V coating

2.2. Characterization of HA/Ti-6Al-4V composite coatings

2.3. Soaking of HA/Ti-6Al-4V composite coatings in SBF

3. Results and discussion

3.1. Characterization of HA/Ti-6Al-4V composite powders and coatings

3.2. Soaking of HA/Ti-6Al-4V composite coatings in SBF

3.2.1. Morphological and chemical analysis

3.2.2. Phase analysis

3.2.3. ICP analysis

3.2.4. Bond strength

3.2.5. Microhardness and Young's modulus

4. Conclusions

References

3. 1. Introduction

Hydroxyapatite (HA) can bond to living bony tissues and, it is being widely used in clinical applications [1 and 2]. One major application of HA is to serve as a cover material for titanium or other metals used in implants [3]. In this case, the biocompatibility of implants is assured by HA, while the mechanical properties are provided by the metal substrate. Various deposition techniques, such as sol-gel [4 and 5], sputtering [6 and 7], pulsed laser deposition [8], and thermal spraying [9, 10 and 11], are available for the deposition of HA coatings. Among them, plasma spraying is the most popular method due to its feasibility and good coating mechanical properties [12].

Bioactivity is widely accepted as the essential requirement for an artificial biomaterial to exhibit chemical bonding to living tissues upon the formation of a bone-like apatite layer on its surface in any simulated body environment [13 and 14]. Previous studies have shown that the simulated body fluid (SBF) has almost the same ion concentrations as those of the human blood plasma and can well reproduce the in vivo surface changes [15]. The in vitro tests of plasma-sprayed HA-coated Ti-6Al-4V coating have found new bone-like apatite layer formation on the surface of HA coating [16, 17 and 18]. Clinical studies of HA-coated total hip prosthesis and dental implants have shown satisfactory results [19 and 20].

Although the plasma-sprayed HA coatings on titanium alloy implants show strong bonding between the HA coating and the bone structure, it has been recognized that the mechanical stability of the interface between the HA coating and titanium alloy substrate could be a problem either during surgical operation or after implantation [21, 22 and 23]. It cannot be used at highly loaded places such as femoral and tibial cortical bones due to the brittleness of HA. One way to circumvent this inherent deficiency is to form HA-based composites by reinforcing with a mechanically superior secondary phase. Various HA-based composites prepared by plasma spraying have been produced to solve this problem [24, 25 and 26].

In the present work, novel HA/Ti alloy composite coatings prepared by the composite powders were developed to improve the mechanical stability of HA coating. The composites prepared using HA and Ti alloy represent a good combination to yield concomitant mechanical strength and biocompatibility. The HA/Ti-6Al-4V composite powders were prepared using a ceramic slurry mixing method. The proposed approach of the composite powder, with an inner core of Ti-6Al-4V wrapped by an outer layer of HA, offers many advantages. It improves the fracture toughness of HA alone, improves the adhesion strength between HA and the underlying Ti-6Al-4V substrate, and prevents the oxidation and corrosion of Ti-6Al-4V. It also improves the mechanical stability of the coating after exposing the coating to the simulated body fluid.

In this paper, plasma-sprayed HA/Ti-6Al-4V composite coatings were produced using HA-coated Ti-6Al-4V composite powders as feedstock. An in vitro dissolution study was conducted to study the bioactivity and mechanical stability of the composite coatings when subjected to a physiological medium. Microstructure and phase composition of HA/Ti-6Al-4V composite coatings were examined by scanning electron microscopy (SEM) and X-ray diffractometer (XRD), respectively. The microhardness, Young's modulus and tensile bond strength of the composite coatings before and after soaking in SBF were also characterized. The chemical composition of the apatite layer formed on the surface of the composite coating was determined by X-ray photoelectron spectroscopy (XPS) analysis. The use of XPS allows the precise determination of the chemical composition of the surface apatite layer.

4. 2. Materials and methods

2.1. Preparation of HA/Ti-6Al-4V coating

Composite powders (50 wt% HA/50 wt%) were prepared by a novel ceramic slurry mixing method [27]. A robot-controlled 100 kW direct current plasma torch (SG-100 Praxair Thermal Inc., USA) equipped with an advanced computerized closed-loop powder feeder system was used for the deposition of composite powders onto the Ti-6Al-4V substrates. The plasma spraying parameters were listed in Table 1. Two shapes of bioinert Ti-6Al-4V were used as substrates: cylindrical stubs, 25.4 mm in diameter and 25.4 mm in length were used for bond strength measurement. Plate specimens (10×10×2.5 mm) were employed for coating characterization.

Table 1. Plasma spraying parameters

2.2. Characterization of HA/Ti-6Al-4V composite coatings

The morphology and elemental analysis of the composite coatings were carried by JEOL JSM-5600LV (Japan) SEM equipped with energy dispersive X-ray spectrometry (EDX). The bond strength of HA/Ti-6Al-4V coating was evaluated using standard tensile adhesion test (ASTM-C633), which was especially designed for thermal-sprayed coatings. Two identical cylindrical Ti-6Al-4V stubs were used as a set, one with the coating on the surface and the other without. A high performance DP-460 Epoxy Adhesive (3M, USA) with a maximum bond strength of 40 MPa was used to join the two stubs. The surface of the uncoated stub was sand blasted to enhance the adhesion strength. The two stubs were aligned and a weight of about 420 g was applied to ensure an intimate contact between the two surfaces. After 12 h of curing at room temperature, the bond strength was measured using an Instron 4302 tester at a cross-head speed of 1 mmmin⁻¹. Five specimens were tested and an average was obtained.

2.3. Soaking of HA/Ti-6Al-4V composite coatings in SBF

SBF was prepared as described in [28] by dissolving reagent grade CaCl₂, KH₂PO₄·3H₂O, NaCl, KCl, MgCl₂·6H₂O, CaHCO₃ and Na₂SO₄ in distilled water. The ion concentration in SBF closely resembles the concentration of human blood plasma, as shown in Table 2. The solution was buffered at physiological pH of 7.25 with
Tris (hydroxymethyl) aminomethane and
hydrochloric acid (HCl). The coated specimens were soaked in SBF for various periods of 1, 7, 14, 28 and 56 days before they were taken out for testing and analysis. The experiment was performed at 37°C in a water bath. The test tubes were sealed to remain sterile.

Table 2. Ion concentration of SBF in comparison with human blood plasma

After soaking for various periods, the samples were removed from the solution, gently rinsed with distilled water and dried at room temperature. The morphological and elemental analysis of the coatings was studied by SEM/EDX. The phases present in the coating were determined by Shimadzu 6000 Lab XRD system using CuK
at 40 kV and 30 mA. The X-ray data were collected in the 2
range of 20-80° in steps of 0.01°. The chemical composition of the apatite layer formed on the surface was determined by X-ray photoelectron spectroscopy (XPS).

The calcium (Ca²⁺) concentration in the solutions was measured directly after the samples were removed, using Perkin Elmer Plasma 400 inductively coupled plasma atomic emission spectrometer (ICP-AES). Three readings were taken to obtain the average value.

The mechanical properties, such as bond strength, microhardness and Young's modulus were measured after soaking in SBF. The Shimadzu HMV-2000 Vicker's hardness tester determined cross-section microhardness of the samples after soaking in SBF. An indentation load of 300 g with a loading time of 15 s was used for the indentation tests.

The Knoop indenter was used to determine the calculated Young's modulus E as shown in Eq. (1) [29 and 30].

(1)

where H_k denotes the Knoop microhardness; b′/a′ denotes the indent diagonal after elastic recovery during indentation; b/a is the ratio of the known Knoop indenter geometry (1/7.11);
is a constant, having a value of 0.45. A sample number of 10 was adopted to evaluate the hardness and modulus values, and an average was collected.

5. 3. Results and discussion

3.1. Characterization of HA/Ti-6Al-4V composite powders and coatings

The HA-coated Ti-6Al-4V composite powders prepared by ceramic slurry mixing approach is shown in Fig. 1. It can be observed that Ti-6Al-4V powders are uniformly coated by HA powders, forming overall HA-coated Ti-6Al-4V powders. These composite powders are beneficial for the plasma-spray process and the mechanical properties of the sprayed coating. It was found that the composite coatings are denser than HA coating. The composite coatings have a density of 3.89 g/cm³ and a porosity of 15.82%, while the density and porosity of HA coatings are 3.41 g/cm³ and 18.94%, respectively.

(28K)

Fig. 1. HA-coated Ti-6Al-4V composite powders.

Fig. 2a shows the SEM micrograph of untreated, as-received HA/Ti-6Al-4V coating. It shows a rough, heterogeneous and melt-like structure. There are some microcracks on the surface and these surface cracks could form due to both the thermal expansion mismatch between the coating and substrate and the release of thermal stress generated on cooling. HA/Ti-6Al-4V coating has a higher coefficient of thermal expansion (CTE) than the Ti alloy substrate; therefore, the coating is in tension and the substrate in compression. Some cracks formed due to tensile stresses build up are observed to initiate at the interface and run through the coating thickness as shown from the cross-section examination of the coating (Fig. 2b). It is, therefore, important to control carefully the spray parameters in order to reduce surface cracks. EDX analysis shows that the darkish gray area in Fig. 2b is HA and the light gray area is Ti alloy. HA/Ti-6Al-4V solid solution formed during high temperature plasma spraying can be detected as shown in Fig. 2b.

(28K)

Fig. 2. Microstructure of HA/Ti-6Al-4V composite coatings before soaking in SBF. (a) Surface morphology of composite coating. (b) Cross-section of composite coating.

The bond strength of as-sprayed composite coatings and HA coatings is tabulated in Table 3. It can be seen that the bond strength of the Ti-6Al-4V/HA composite coatings is significantly higher than that of pure HA coatings. One plausible cause for the high bond strength of the composite coatings is metallic joints among Ti-6Al-4V lamellae within the coating as well as the strong adhesion of the Ti-6Al-4V in the incipient layer of splats onto the grit-blasted Ti-alloy surface. Besides the fact that Ti alloy has better bond strength, the relative higher bond strength of the HA/Ti-6Al-4V composite coatings also arises from the release of thermal mismatch in the CTE between HA and Ti alloy substrate. The linear CTE of HA is about 15×10⁻⁶ K⁻¹, while the CTE of Ti-6Al-4V substrate is 8.8×10⁻⁶ K⁻¹. The residual stress at the HA coating/substrate interface generated during the cooling process of plasma spraying is relatively higher in pure HA coatings due to the mismatch in CTE values between HA and Ti-6Al-4V. The CTE of the composite coatings reduces significantly due to the addition of Ti-6Al-4V. Therefore, the residual stress is reduced during plasma spraying and the bond strength of the coating is improved. Also, the formation of HA/Ti-6Al-4V solid solution during high temperature plasma spraying is considered to increase the bond strength of the composite coatings. As can be observed from Fig. 2b, the solid solution shows a denser structure, while HA is relatively porous in morphology.

Table 3. Bond strength of HA and HA/Ti-6Al-4V composite coatings

3.2. Soaking of HA/Ti-6Al-4V composite coatings in SBF

3.2.1. Morphological and chemical analysis

The surface morphologies of the HA/Ti-6Al-4V coatings after soaking in SBF for various periods are shown in Fig. 3. The dissolution observed in the coatings corresponded to the soaking time. After 1 day of soaking, the as-received HA/Ti-6Al-4V coatings show signs of surface dissolution and increased surface roughness, as can be observed from Fig. 3. Weng et al. reported the same phenomenon for the plasma-sprayed HA coatings after soaking in SBF for a few hours [31]. The increase in surface roughness of the coating provides the nucleation sites with lower interface energy for bone-like apatite to anchor [31]. Some tiny granular precipitates have grown on the surface as can be seen from Fig. 3a. In comparison with the coating surface morphology before soaking, the coating displays more microcracks on the surface ( Fig. 3a). The cracks are resulted from diffusion and reaction between the coating surface and the SBF. Some micropores are also observed, which is due to the diffusion of ions from the coating surface to the surrounding SBF. The quantity and size of the precipitates increase as the soaking duration increases. After 2-week soaking, the surface of the coating is covered by a newly formed layer consisting of small granular structures ( Fig. 3b). This dune-like layer is characterized by many large cracks of tortoise-shell character with 0.1
m in size that propagates along the whole surface of the coating. As the soaking duration increases, this layer becomes denser and the granules in the layer become larger, as shown in Figs. 3b, c. At higher magnification, it can be observed that this layer composes of many small crystallites as shown in Fig. 3d. Other authors have also observed similar findings in pure HA coatings [32]. The apatite layer formed on the surface of the coating is composed of spherulites with very fine crystallites, suggesting a high nucleation rate of calcium and phosphorus. Some spherulites are formed directly on the surface of other spherulites or at their interface. This suggests that the front of a growing layer is also a preferential nucleation site for other spherulites [33]. These features show that the HA/Ti-6Al-4V composite coatings have the ability to induce the bone-like apatite nucleation and growth on their surfaces from SBF. Bone-like apatite layer has been found to be important to establish the bone-bonding interface between bioactive materials and living tissues [34]. Therefore, it is suggested that the bone-like apatite layer formed on the surface of the composite coatings may promote the bone bonding with living tissues and increase the longevity of coatings during implanting in vivo.

(110K)

Fig. 3. Surface morphology of composite coatings after soaking in SBF. (a) 1 day. (b) 2 weeks. (c) 8 weeks. (d) higher magnification for (b).

EDX analysis of the dune-like layer formed on the surface of the coating shows the presence of C, O, Ca, P and Ti. The existence of carbon in the layer implies that the layer may be carbonate-containing HA phase. In order to verify the chemical composition of the dune-like layer, XPS analysis was carried out. The XPS results show that the elements present on the coating surface after 4-week soaking in SBF are comprised of C, Ca, P and O. The XPS C1s spectrum is shown in Fig. 4, which consists of three peaks. The peak at 284.5 eV and the peak at 285.5 eV corresponding to C-C and C-O peaks, respectively, which exist on the surface layer due to the carbon contamination. The C 1s peak at 287.8 eV is carbonate group CO₃²⁻. The SEM pictures shown in Fig. 3 and the XPS results reveal that the layer formed on the sample surface is carbonate-containing apatite.

(7K)

Fig. 4. XPS C 1s spectrum of HA/Ti-6Al-4V coating after immersion in SBF for 6 weeks.

3.2.2. Phase analysis

Fig. 5 shows the XRD patterns of the HA/Ti-6Al-4V coatings before and after soaking in SBF. Before soaking in SBF, Ti alloy phase and a small amount of HA phases are present in the coating. The appearance of CaO peak can be observed. CaO is formed as a result of the high temperature of plasma spray, during which a small portion of HA is decomposed [35]. The surfaces of HA splats are heated above the decomposition temperature of HA during plasma spraying and a fraction of the HA surface is transformed into other apatites. After soaking the coating in SBF for 1 day, the intensity of CaO decreases. With increasing soaking duration, the CaO peak continues to decrease. After 2-week soaking, XRD shows HA and Ti peaks only, indicating that most CaO phases have been dissolved in the solution. This is explained in terms of the reaction of CaO with water and their dissolution in water. No any additional phases are observed after soaking. The crystallinity of HA is observed to increase significantly during the first 4 weeks, and thereafter a gradual increase in the crystallinity after 4-week soaking in SBF. This finding is consistent with the research work done by de Groot, who also reported that the coating crystallinity increased after incubation in Gomori's buffer solution for 4 weeks [36].

(12K)

Fig. 5. XRD patterns of coatings after soaking in SBF.

3.2.3. ICP analysis

The calcium concentrations in the immersion solutions are shown in Fig. 6. The average calcium ion concentration in the solution increases by 54.4% after 1 week of immersion, which indicates the dissolution of calcium ions. With the further immersion, calcium ion concentration decreases exponentially, indicating a continuous precipitation. It was reported that the pure HA coatings exhibit similar trends to the composite coatings, but the rate of dissolution was more severe [37]. The initial rate of dissolution for the pure HA coating [16] is much higher than the dissolution rate of the composite coating. The dissolution of ions in the composite coatings is affected by various causes, such as phase, crystallinity, porosity and thickness of material [37]. The rate of dissolution for HA/Ti-6Al-4V composite coating decreases because Ti-6Al-4V reduces the porosity in the composite coating and the porosity in the coating will accelerate dissolution. In addition, as HA dissolves, the un-dissolved Ti alloy remaining in the coating may hinder contact between HA and SBF, which in turn decreases the dissolution rate of the composite coatings.

(5K)

Fig. 6. Calcium ion concentration as a function of soaking time.

The ICP results are consistence with the SEM and XRD results shown in Fig. 3 and Fig. 5. As observed with SEM, after 2-week immersion in SBF, the morphology of the coating changes significantly, from a heterogeneous, melt-like structure to spherulites structure on the surface. The results of ICP study indicate that the dissolution of calcium and the precipitation of calcium phosphate occur concurrently, which leads to a carbonate-containing calcium phosphate layer on the coating. In the composite coatings before soaking, CaO is observed in the XRD pattern. The intensity of CaO peak decreases significantly after immersion in SBF, and it is assumed that the CaO phase, which may be more easily soluble than HA, dissolves in SBF, which leads to the increase in the calcium ions in the SBF solution.

3.2.4. Bond strength

Following soaking in SBF, the bond strength of HA/Ti-6Al-4V coating may degrade through chemical dissolution. The interlamellar microstructure of the coating will be weakened, and the bonding at the HA/Ti-6Al-4V coating and the substrate will decrease. The bond strength data measured from the adhesion tests are shown in Fig. 7. Average bond strength of 27.38 MPa is reached for the coating without soaking. Following soaking in SBF, the bond strength of the coatings decreases. It can be observed that after 1-week soaking, the bond strength decreases about 15.8%. With increasing soak duration, it is evident that the coating shows a continuous degradation up to the second week with a total of 31.6% reduction in original strength. There is only slight variation in bond strength after 2-week soaking. The reduction of bond strength comes mostly from the continuation of chemical dissolution of coatings, which weakens the bonding of the lamella in the coating and the bonding of the interface between coating and substrate. Fast dissolution of the coating will result in more bonding decay at the interface of coating and substrate. Previous work on in vitro study of pure HA coatings showed a 75% decrease in bond strength after soaking in SBF [38]. Compared with pure HA coating, the composite coating dissolves slowly and hence the decrease in bond strength for the composite coating is significantly lower. This indicates that the HA/Ti-6Al-4V composite coatings possess much superior physio-stability and are less prone to biological demineralization. Therefore, the reduction in bond strength for HA/Ti-6Al-4V composite coatings is significantly lower than those for pure HA coatings.

(5K)

Fig. 7. Bond strength of the HA/Ti-6Al-4V composite coatings after soaking in SBF.

After the bond test, a mixed failure mode is commonly observed, revealing cohesive failure had occurred in lamellae of the coatings and adhesive failure occurred at coating-substrate interface, as shown in Fig. 8. This finding indicates that the result of this bond test is meaningful and matches well with the criterion of the ASTM C-633.

(25K)

Fig. 8. Fracture surface of HA/Ti-6Al-4V composite coating.

3.2.5. Microhardness and Young's modulus

Fig. 9 shows the Young's modulus and microhardness of HA/Ti-6Al-4V composite coatings after soaking in SBF for different periods. Both these values decrease as the soaking duration increases, though the decrease in Young's modulus starts to diminish after 2 weeks while the Vicker's hardness is still decreasing at a steady rate even after 6 weeks. After 8-week soaking, a drop of about 21.5% in Vicker's microhardness and a corresponding drop of about 22.4% in elastic modulus values can be observed. The decrease in the microhardness and Young's modulus after soaking is due to the dissolution of the coatings in SBF. The fast dissolution at the initial 1 week causes the depletion of the calcium phosphate layer during soaking, which weakens the bonding of the lamella. The weakened structure is not able to resist the indentation force during the testing. This in turn causes the significant reduction in microhardness and Young's modulus. After 2-week soaking in SBF, only slight decreasing in microhardness and Young's modulus can be observed due to the gradual decrease in dissolution rate and the continuous precipitation.

(8K)

Fig. 9. Microhardness and Young's modulus of HA/Ti-6Al-4V composite coatings after soaking in SBF.

6. 4. Conclusions

An in-vitro study was used to investigate the biological response of HA/Ti-6Al-4V composite coatings under SBF. The coatings were found to undergo two biointegration processes, i.e., dissolution during the initial 4 weeks soaking in SBF and the subsequent bone-like apatite crystal precipitation. Complete dissolution of impurity phases such as CaO took place during the initial soaking period (up to 7 days) and as a consequence, an increase in calcium ion concentration is observed. Mechanical properties of the coatings were found to deteriorate with soaking time. However, the coatings showed superior mechanical stability than the pure HA coatings, indicating the much better long-term stability of the composite coatings in physiological environment.

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38. Sidney SWK. Thermal spraying of bioceramic (hydroxyapatite) coatings. Master of Engineering thesis, Nanyang Technological University, Singapore, 2000.

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