Microstructure and mechanical properties of plasma sprayed HA/YSZ/Ti-6Al-4V composite coatings

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

a</sup> School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Ave, Singapore 639798, Singapore
^b Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore
^c School of Materials Engineering, Nanyang Technological University, Nanyang Ave, Singapore 639798, Singapore

Received 29 January 2003;  accepted 10 October 2003.  Available online 10 January 2004.
Biomaterials
Volume 25, Issue 18 , August 2004, Pages 4009-4017

1. Abstract

Plasma sprayed hydroxyapatite (HA) coatings on titanium alloy substrate have been used extensively due to their excellent biocompatibility and osteoconductivity. However, the erratic bond strength between HA and Ti alloy has raised concern over the long-term reliability of the implant. In this paper, HA/yttria stabilized zirconia (YSZ)/Ti-6Al-4V composite coatings that possess superior mechanical properties to conventional plasma sprayed HA coatings were developed. Ti-6Al-4V powders coated with fine YSZ and HA particles were prepared through a unique ceramic slurry mixing method. The so-formed composite powder was employed as feedstock for plasma spraying of the HA/YSZ/Ti-6Al-4V coatings. The influence of net plasma energy, plasma spray standoff distance, and post-spray heat treatment on microstructure, phase composition and mechanical properties were investigated. Results showed that coatings prepared with the optimum plasma sprayed condition showed a well-defined splat structure. HA/YSZ/Ti-6Al-4V solid solution was formed during plasma spraying which was beneficial for the improvement of mechanical properties. There was no evidence of Ti oxidation from the successful processing of YSZ and HA coated Ti-6Al-4V composite powders. Small amount of CaO apart from HA, ZrO₂ and Ti was present in the composite coatings. The microhardness, Young's modulus, fracture toughness, and bond strength increased significantly with the addition of YSZ. Post-spray heat treatment at 600°C and 700°C for up to 12 h was found to further improve the mechanical properties of coatings. After the post-spray heat treatment, 17.6% increment in Young's modulus (E) and 16.3% increment in Vicker's hardness were achieved. The strengthening mechanisms of HA/YSZ/Ti-6Al-4V composite coatings were related to the dispersion strengthening by homogeneous distribution of YSZ particles in the matrix, the good mechanical properties of Ti-6Al-4V and the formation of solid solution among HA, Ti alloy and YSZ components.

Author Keywords: Author Keywords: Plasma spraying; Hydroxyapatite; Yttria stabilized zirconia; Ti alloy; Mechanical property; Composite coating

2. Article Outline

1. Introduction

2. Experimental procedures

2.1. Powder preparation

2.2. Composite coating production

2.3. Coating characterization

3. Results and discussion

3.1. Characteristics of composite powders for plasma spraying

3.2. Phase composition and crystallinity of composite coatings

3.3. Microstructure of composite coatings

3.4. Mechanical properties of composite coatings

4. Conclusions

References

3. 1. Introduction

Bioactive calcium phosphate ceramics as coatings on bioinert metallic substrate have received worldwide attention in both orthopaedic and dental applications due to their biocompatibility and their ability to bond directly to bone [1 and 2]. Laboratory animal studies and experiences with human implants suggested that plasma sprayed hydroxyapatite (HA) coated titanium implants could induce a chemical bond with bone, and achieve biological fixation to bone [3 and 4]. The direct bone bonding capacity of HA is known as osteoconductivity, which means that HA allows direct formation of bone on its surface by acting as a template. Other advantages of HA coated Ti-6Al-4V system include prevention of metallic ion release in the physiological environment, and its good mechanical stability due to the tougher nature of metals [5]. Despite these advantages, the bond strength of the HA/metal interface could be the potential weakness in prosthesis. Previous studies on bonding between HA coating and bone showed the importance of the strength of HA in the coating [6].

In clinical applications, the brittle nature of HA coating often results in rapid wear, and premature fracture of the coated layer. To overcome this problem, several attempts have been made [7, 8 and 9]. One of these approaches is the application of composite coatings, and thermal sprayed bioceramic composite coatings were successfully prepared by composite powders formed by a mechanically strong biocompatible Ti-6Al-4V metal alloy and a bioactive but mechanically fragile HA [10 and 11]. Previous results on HA/Ti-6Al-4V composite coatings have shown that the bond strength was improved significantly with the addition of Ti-6Al-4V. Plasma sprayed partially stabilized ZrO₂ containing 6-8 wt% Y₂O₃ (yttria stabilized zirconia—YSZ) has superior performance due to the formation of non-transformable tetragonal (t′) phase during plasma spraying. It has been used to strengthen the brittle bulk HA and bioglass and the results showed promise for composite materials [12 and 13].

In this study, HA/YSZ/Ti-6Al-4V composite coatings are produced by plasma spraying technique with blended HA-YSZ-Ti-6Al-4V powder feedstock. The influences of net plasma energy and plasma spray standoff distance on the coating microstructure, phase composition and mechanical properties are investigated.

4. 2. Experimental procedures

2.1. Powder preparation

HA powders were made through reacting orthophosphoric acid with calcium hydroxide, followed by spray drying. The spray dried hydroxyapatite (SDHA) powders collected were calcined at 900°C for 2 h to increase the powder's crystallinity. The heat-treated SDHA powders were sieved and, the powders with the particle size smaller than 20 
m were used. ZrO₂−8 wt% Y₂O₃ powders (YSZ, AI-1075, Praxair Thermal Alloys International Division, Baytown, TX, USA) with a powder size range of 10-60 
m were ball milled using a P-5 (Fritsch, GmbH, Germany) planetary ball mill in a zirconia bowl with zirconia balls at a rotating speed of 100 rpm for 3 h. The average powder size after ball milling was found to be in the size range 1-5 
m. Commercial Ti-6Al-4V powders with particle size of 45-110 
m (B&S Aircraft Alloys Inc. USA) were used in this study.

Ceramic slurry mixing method was used to prepare 35 wt% HA/15 wt% YSZ/50 wt% Ti-6Al-4V composite powders. The rationale behind this approach is to use Ti-6Al-4V slurry added with a certain amount of polyvinyl alcohol (PVA) to cause mixed HA/YSZ particles that are finer than 20 
m to be coated onto the Ti-6Al-4V powders. SDHA and ball-milled YSZ powders were premixed before being poured into the slurry. The binder was prepared by mixing 20 wt% PVA with 150 ml of distilled water. Ti-6Al-4V powders were stirred into the binder for 2 h to form the slurry by means of a mechanical stirrer. HA/YSZ mixed powders were then added into the Ti-6Al-4V slurry, stirred for 5 h. The HA/YSZ coated Ti-6Al-4V composite powders were debound at 450°C for 1 h to volatilize the PVA binder and sintered in vacuum at 600°C for 6 h to consolidate the particles.

2.2. Composite coating production

The composite coatings were deposited using a robot controlled, computerized 100 kW direct current plasma torch equipped with an advanced computerized closed loop powder feed system for accurate powder feed rate. Argon was used as the main plasma forming gas while helium forms the auxiliary gas. The selection of plasma spraying parameters was based on the optimum parameters of 50% HA/50% Ti-6Al-4V [14] and 50% HA/50% YSZ [15] composite coatings from the previous study. Table 1 lists the parameters for plasma spraying of composite powders. Coatings were post-spray heat-treated at 600°C and 700°C for 3, 6, 9 and 12 h, respectively, to study the influence of heat treatment on the performance of coatings.

Table 1. Plasma spraying parameters of HA/YSZ/Ti-6Al-4V

2.3. Coating characterization

A scanning electron microscope (JEOL JSM-5600LV) was used to study the morphology of the composite powders and coatings. Energy dispersive X-ray analysis (EDX) was performed to identify the elements within the powders and coatings. Phase analysis was performed on the Philips MPD 1880 X-ray diffractometer system (XRD) using CuK
radiation at 40 kV and 30 mA. The relative crystallinity of HA was determined by comparing the integrated area intensity of the [2 1 1] peak of the coating with the integrated area intensity of [2 1 1] peak of the composite powders.

Cross-section microhardness of the composite coatings was determined with the Shimadzu HMV-2000 Vicker's hardness tester. The Knoop indenter was used to determine the Young's modulus (E) as shown in the following equation [16]:

(1)

where b′/a′ denotes the indent diagonal after elastic recovery during indentation; b/a is the ratio of the Knoop indenter dimension (1/7.11),
is a constant having a value of 0.45. H_k is the Knoop hardness value and E is the Young's modulus. The test load of 300 gF was used and a sample size of 10 was adopted to evaluate E and H_k values. With a multiphase structure within which residual stresses exist, the materials are prone to cracking. Precaution must be taken to ensure the accurate measurement of the dimension of the indentation.

The bond strength of coatings was evaluated with conventional tensile bond test (ASTM C-633) 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 the maximum bond strength of 40 MPa was used to join the two stubs. 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 mm min⁻¹. Five specimens were tested and the average was taken as the bond strength of the composite coating.

5. 3. Results and discussion

3.1. Characteristics of composite powders for plasma spraying

Fig. 1 shows the morphology of HA/YSZ/Ti-6Al-4V composite powders prepared by slurry mixing method. EDX analysis of the composite powders shows that the small white particles are YSZ and the gray spherical particles are HA (Ca and P elements). Ti-6Al-4V powders are fully coated by HA powders and YSZ particles. The advantages of using HA and YSZ coated Ti-6Al-4V composite powders are to ensure uniform phase composition within the plasma sprayed coating, and prevent the Ti alloy from oxidation during plasma spraying. The composite powders have a particle size of 45-106 
m and this particle size range is favorable to plasma spraying, as there would be uniform melting of the powders in the plasma flame. Fine powders <30 
m may have difficulty penetrating the plasma column to get into the hot zone, while large particles (>120 
m) are unlikely to be melted. The XRD pattern of the composite powders is shown in Fig. 2. It is clear that well-crystallized HA and YSZ coated Ti-6Al-4V powders are obtained after the sintering and consolidating processes, indicating that the composite powders consist of tetragonal/cubic zirconia (t-ZrO₂+c-ZrO₂), highly crystalline HA and oxide-free Ti peaks. Moreover, there was no sign of decomposition of HA, and the reaction of phases is not observed after debinding and consolidation.

(20K)

Fig. 1. Morphology of HA/YSZ/Ti-6Al-4V composite powder prepared by slurry mixing method.

(7K)

Fig. 2. XRD pattern of HA/YSZ/Ti-6Al-4V composite powder.

3.2. Phase composition and crystallinity of composite coatings

The results of XRD analysis of plasma sprayed HA/YSZ/Ti-6Al-4V coatings are shown in Fig. 3. The XRD pattern of as-sprayed composite coatings retains the characteristics of powders in Fig. 2 except a small amount of CaO and
-TCP. For plasma sprayed pure HA coatings, the formation of additional calcium phosphate phases such as
-TCP,
-TCP, TTCP and CaO are likely to be induced by high-temperature plasma flame. However, in the composite coatings produced using the composite powders, there are no indications of
-TCP and TTCP formation except a small amount of CaO and
-TCP, as shown in Fig. 3. The absence of TTCP and
-TCP in the composite coatings is probably due to different degrees of decomposition of HA and the interdiffusion of HA and YSZ [17]. The thermal conductivity of YSZ (0.007 J s⁻¹ cm K) is lower than that of HA (0.013 J s⁻¹ cm K), which can affect the thermal diffusivity and molten state of the materials during plasma spraying. Moreover, there is no oxidation of Ti compound in the as-sprayed composite coating. This proves the effectiveness of the design of the composite powder in preventing of the oxidation of Ti alloy, in which Ti-6Al-4V powders are fully covered by HA and YSZ.

(14K)

Fig. 3. XRD patterns of as-sprayed and heat treated coatings (spraying parameters: 8.5 cm, 12 kW).

In comparison between Fig. 2 and Fig. 3, the as-sprayed coating exhibits much lower apatite crystallinity than the initial powders. YSZ and Ti alloys remain stable during plasma spraying. The plasma spraying process results in the formation of amorphous coating that is indicated by the broad peaks with low intensities.

XRD revealed t-ZrO₂/c-ZrO₂ and Ti as the major phases in the as-sprayed coating (Fig. 3). Heat treatment at 600°C and 3 h resulted in a prominent re-emergence of HA peaks in the XRD pattern followed by traces of the TiO₂ (rutile) peaks. The TiO₂ peaks became increasing prominent as the heat treatment duration increased (maximum 12 h), and as the temperature rose to 700°C. Concurrently, the intensity of the Ti peaks decreases. A distinguishing feature in the XRD pattern is the absence of supplementary calcium phosphate phases that often accompany plasma sprayed HA.

The influences of plasma net energy and standoff distance on HA relative crystallinity of heat-treated HA/YSZ/Ti-6Al-4V composite coatings are listed in Table 2. The crystallinity of HA firstly increases with an increase in power net energy, reaching a maximum value at a net energy of 12 kW, then decreases with further increase in net energy. The influences of power net energy and standoff distance on the relative crystallinity are the combined effects of amorphorization and recrystallization during plasma spraying. At higher-power net energy of 12-14 kW and a shorter standoff distance of 7-8.5 cm, the high temperature of the plasma flame and the high-temperature gradient among substrate, coating and the surrounding accelerate the transformation of crystalline HA to amorphous calcium phosphate, thus decreasing the relative crystallinity of HA. However, at lower-power net energy of 8-12 kW, recrystallization of the amorphous calcium phosphate may be more significant than the amorphorization of HA, resulting in an increase in the crystallinity of HA.

Table 2. Relative crystallinity of HA in composite coatings

3.3. Microstructure of composite coatings

Fig. 4 shows the surface morphologies of the coatings sprayed at power net energies of 8 and 12 kW with a spraying distance of 10 cm. Numerous cavities, macropores, partially melted and unmelted ZrO₂ and HA particles exist on the coating surface sprayed at 8 kW (Fig. 4a). The molten state of the particles has been improved significantly with an increase in power net energy to 12 kW. There is no apparent indication of the presence of cavities and most particles are melted by plasma flame at a power net energy of 12 kW. However, much more surface cracks can be observed at the net energy of 14 kW, probably due to the overheating of the deposited layer.

(50K)

Fig. 4. Surface morphologies of HA/YSZ/Ti-6Al-4V coatings sprayed at a standoff distance of 10 cm: (a) 8 kW; and (b) 12 kW.

The cross-sectional views of the coatings sprayed at 8 and 12 kW are shown in Fig. 5. EDX was performed to identify the elements in different layers. For the coating sprayed at 12 kW as shown in Fig. 5b, EDX analysis on region `A' shows strong evidence of HA elements (Ca and P) and region `B' shows the major element of Ti. YSZ is evident by the presence of white strips and small white specks within HA and Ti alloy as indicated by region C. Region D consists of lighter gray area of Ca, P, Zr and Ti, represented by a solid solution of HA, YSZ and Ti alloy. The formation of solid solution is considered to be beneficial for the improvement of mechanical properties [18]. Typical lamellar structure can be seen in the coatings. The composite coating sprayed at 8 kW reveals many cavities, macropores and unmelted particles. Whereas the coatings obtained under high-power net energy of 12 kW are relatively dense and quite homogeneous with evenly melted composite particles. The better interlamellar contacts, the less pores and unmelted particles compared to the coating sprayed at 8 kW could significantly improve the mechanical properties of the coatings.

(49K)

Fig. 5. Cross-section microstructure of HA/YSZ/Ti-6Al-4V coatings sprayed at a standoff distance of 10 cm: (a) 8 kW; and (b) 12 kW.

Standoff distance is one of the sensitive parameters involved in plasma spraying. Fig. 6 shows the cross-section microstructure of the composite coating sprayed at a standoff distance of 8.5 cm. The coating reveals better interlamellar contact with less amount of pores and unmelted particles compared to the coating sprayed at 10 cm as shown in Fig. 5b. At a longer standoff distance of 10 cm, the longer period of flight and lower particle velocity upon impact result in the particles resolidifying partially. Therefore, an increasing amount of porosities and unmelted particles with non-uniform deposition in the coatings are formed, leading to poor coating adhesion. Such a microstructural deformity could deteriorate the coating integrity and mechanical properties.

(22K)

Fig. 6. Cross-section microstructure of composite coating sprayed at (12 kW, 8.5 cm).

3.4. Mechanical properties of composite coatings

The effect of heat treatment on the mechanical properties of HA/YSZ/Ti-6Al-4V composite coatings is shown in Fig. 7a. Fig. 7b shows the mechanical properties of composite coatings sprayed at different net plasma energies. The effects of standoff distance and post-spray heat treatment (600°C, 6 h) on microhardness and Young's modulus are shown in Fig. 7c. The microhardness and Young's modulus of pure HA coating are plotted in Fig. 7b for comparison.

(60K)

Fig. 7. Variation of microhardness and Young's modulus of HA/YSZ/Ti-6Al-4V composite coatings as a function of: (a) heat treatment temperature and holding time; (b) power net energy; and (c) standoff distance (HZT: HA/YSZ/Ti-6Al-4V coatings).

It can be seen from Fig. 7b that the microhardness and Young's modulus values of the HA/YSZ/Ti-6Al-4V composite coatings are notably higher than those of pure HA coatings. The attractive mechanical properties exhibited by the HA/YSZ/Ti-6Al-4V coatings compared to pure HA coatings are likely due to the improved bonding among splats and lamellae, and the presence of YSZ particles as secondary phase within the coating. The strengthening mechanism of the composite coating is unlikely to be attributed to transformation toughening of YSZ particles, since the particles are effectively above the critical size for spontaneous transformation of t-ZrO₂ to monoclinic ZrO₂, and the zirconia present is a mixture of cubic and tetragonal phases. Rather, the enhanced strength of the composite coating is likely due to the dispersion strengthening by homogeneous distribution of YSZ particles in the matrix with a good particle-matrix interface [19]. In addition, plasma sprayed Ti-6Al-4V-based composite coating has a decidedly lower porosity than the pure HA coating due to better cohesion amongst metallic lamellae through a more extensive melting of the metals in the plasma flame. Furthermore, solid solution formed amongst HA, Ti alloy and YSZ components has been observed from Fig. 5. It is believed that the formation of such solid solution series gives greater strength to the composite coatings. Overall, the mechanical properties of the composite coatings are superior to HA coatings.

As can be observed from Fig. 7a, the mechanical properties of composite coatings are significantly improved after heat treatment at 600°C, compared to those of as-sprayed coatings. The mechanical properties increase with increasing holding time and reach a maximum at a holding time of 9 h. At a relatively higher temperature of 700°C, 3 h of annealing improves the mechanical properties over that of the as-sprayed coatings. A longer holding time of 6 h was found to have a detrimental effect on the mechanical properties of coatings. Post-spray heat treatment of coatings at a suitable heat treatment condition is found to be an effective method for improving the mechanical properties of coatings. Heat treatment causes the densification of microstructure through removal of inter-lamellae voids, enhanced contacts amongst lamellae, and overall reduction of the porosity level [16]. It is capable of releasing the residual stresses generated during coating deposition and causing atomic diffusion among lamellae. Furthermore, amorphous calcium phosphate phases formed in the as-sprayed coatings transforms to crystalline HA after heat treatment. The amorphous phase formed in the composite system is undesirable because amorphous phase is resorbable in body fluid and this in turn weakens the interface between coatings and implant.

The microhardness and Young's modulus values increase with increasing net plasma energy and plasma spray standoff distance. Maximum values are attained at net plasma energy of 12 kW and standoff distance of 8.5 cm, and subsequently decrease with further increase in net plasma energy and standoff distance. The mechanical properties are significantly influenced by the microstructure of coatings. At the net plasma energy of 8-12 kW, the mechanical strength increases correspondingly with increase in the net plasma energy levels. Poor splat formation and unmelted YSZ particles are the result of decreasing mechanical properties when the coatings are sprayed at low net plasma energy levels. The high mechanical strength of the coatings sprayed under a net plasma energy of 12 kW is attributed to the better mechanical interlocking of the splats formed by the fully melted particles. The mechanical properties become poor with increasing net energy due to the high temperature of the plasma flame, which induces the build up of the thermal residual stresses. An increase in the CaO phase in the coating at high net energy may also be a factor influencing the mechanical properties of coatings. On the other hand, the standoff distance also influences the microstructure of coatings. At a standoff distance of 10 cm, the melted particles impact on the substrate at a relatively lower speed. Therefore, poor splat formation and presence of interlamella pores are the result of decreasing mechanical properties. The mechanical properties become poor at a spray distance of 7 cm because of the crack formation induced by residual stresses. The plasma flame is too close to the substrate surface and the heating up of the deposited layers induces large thermal stresses.

The results of bond strength are shown in Fig. 8. Each value in the figure represents an average of five measurements. Pure HA coating has the lowest bond strength (18.46 MPa) due to its brittleness, limited bonding to Ti, and low fracture toughness. The bond strength is improved significantly by the addition of YSZ and Ti-6Al-4V. YSZ has the attributes of high strength and enhancing the overall toughness of the coating, and Ti-6Al-4V is a metallic alloy and a ductile material. Therefore, the bond strength of HA/YSZ/Ti-6Al-4V can be expected to be significantly higher than that of the brittle HA. From Fig. 8, the variations of bond strength to net plasma energy and standoff distance are similar to those of the microhardness and Young's modulus. The fracture surface of the coatings shows a mixed mode failure of cohesive and adhesive failure occurring at the substrate-coating interface, at the coating itself and the coating-glue interface. Cohesive failure is more commonly found in the high-strength coatings. The improved bond strength values of the composite coatings suggests a much-improved interface bonding between the lamellae and the coating/substrate compared with pure HA coatings. One of the reasons for the better bonding between lamellae is the formation of metallic bonds amongst Ti-6Al-4V splats with the Ti-6Al-4V substrate, and the formation of solid solution series within the composite coatings. The release of residual thermal stresses generated by the thermal mismatch between HA and Ti-6Al-4V during the cooling process of plasma spraying is another reason for the improvement of the interfacial bonding between the composite coating/substrate interface. The coefficients of thermal expansion (CTE) of HA and Ti-6Al-4V are 16.0×10⁻⁶ and 8.1-9.8×10⁻⁶ K⁻¹, respectively [20 and 21], and the CTE of YSZ is 7.2×10⁻⁶ K⁻¹ [22]. With the application of HA/YSZ/Ti-6Al-4V composite coatings, the residual thermal stress is reduced by the addition of YSZ and Ti-6Al-4V components into the coating.

(26K)

Fig. 8. Variation of bond strength of HA/YSZ/Ti-6Al-4V composite coatings as a function of: (a) power net energy; and (b) standoff distance.

6. 4. Conclusions

YSZ/Ti-6Al-4V/HA blended powder has been plasma sprayed to produce composite coatings. The YSZ and HA coated Ti-6Al-4V powders were prepared by a ceramic slurry mixing method, and used as the feedstock for the plasma spray process. The microstructures, phase composition, microhardness, Young's modulus, and tensile adhesion strength of the composite coatings were found to respond correspondingly to variations in net plasma energy and the plasma gun-substrate standoff distance. The following findings have been made in the study:

(1) The application of HA and YSZ coated Ti-6Al-4V composite powders has been proven to ensure satisfactory chemical homogeneity and prevent Ti alloy from oxidation during plasma spraying. There was no detection of
-TCP, TTCP except a small amount of CaO and
-TCP through XRD.

(2) HA, YSZ and Ti-6Al-4V were distributed uniformly in the coatings. HA/YSZ/Ti-6Al-4V solid solution was formed during plasma spraying which could be helpful towards the improvement of the mechanical properties, as indicated by the microhardness measurements.

(3) Post-spray heat treatment at 600°C and 700°C, was found as a potential approach to further improve the mechanical properties of composite coatings. However, significant amount of TiO₂ was, however, formed when heat treatment was performed at 700°C.

(4) The microhardness, Young's modulus and bond strength were found to increase significantly with the addition of YSZ. The mechanical properties were associated with the microstructure of the coatings, such as porosity, interlamellar structure. Coatings sprayed at a net plasma energy of 12 kW and a standoff distance of 8.5 cm possessed the most favorable mechanical properties.

7. References

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