Plasma Spray Deposition

PLASMA SPRAY DEPOSITION OF BIOACTIVE COMPOSITE COATINGS

A. Gnedovets, V.I. Kalita, D.I. Komlev

A.A. Baikov’s Institute of Metallurgy and Materials Science of the Russian Academy of Sciences, Moscow, Russian Federation

A. Yerokhin and A. Matthews

Department of Engineering Materials, University of Sheffield, Sheffield, U.K.

Abstract. A new technology was developed for deposition of composite coatings, in which a hydroxyapatite (HA) layer is arc-plasma sprayed on the top of thick 3D capillary porous (3DCP) Ti underlayer in the atmosphere of air. The parameters of plasma spray process were optimised using Monte-Carlo simulation of the coating formation on each stage. The composite coatings with total thickness of 0.5-0.8 mm were deposited on testpieces and real titanium implants.

INTRODUCTION

Plasma assisted methods of surface engineering are widely used for coating of intrabone prosthetic implants. It is important that these coatings possess high strength, convoluted surface profile and proper chemical composition to ensure bioactive properties. These considerations were taken into account when developing a method of plasma spray deposition of thick porous functionally graded Ti coatings. The coating structure is featured by alternate hills and valleys forming 3-dimensional capillary porous (3DCP) network. To enhance bioactivity of 3DCP Ti coatings a thin film of bioactive ceramic material should be formed on the coating surface. In earlier work, a method of plasma electrolytic oxidation was utilised for this purpose [1]. Despite demonstrating a good coating performance, this approach has some technological drawbacks associated with application of hybrid coating methods and dissimilar equipment that makes the coating production process complicated. Therefore it is worthwhile considering alternative approaches based on application of duplex coatings deposited using similar equipment and tooling. In this work, research and development of new technology for plasma spray deposition of a bioactive ceramic layer on the top of 3DCP Ti coating is discussed.

EXPERIMENTAL

The coatings were deposited on the following two types of testpieces: wires with initial diameter of 2 mm (Fig. 1) and barrels with external and internal diameters of 8 mm and 5 mm respectively (Fig. 2). Prior to coating, the substrates were cleaned and grip-blasted. Additionally the coatings were deposited on appropriate surfaces of real femoral prosthetic implants, e.g. sides of stems and outer surfaces of acetabular cups.

1 Fig. 1. Typical appearence of composite 3D-CP Ti + HA coating on titanium wire testpieces.

Fig. 2. Typical examples of coated barrel testpieces: bottom – 3D-CP Ti coating with average thickness of 500 m; middle and top – composite 3D-CP Ti + HA coatings with thickness of HA layer 80 m and 550 m respectively.

Commercially pure titanium and hydroxyapatite (HA) powders with particle dimensions ranging from 25 to 63 m supplied by Sulzer-Metco were used as precursor materials for coating deposition. The coatings were deposited in air using a UPU-3D universal arc-plasma spray installation. Parameters for the coating deposition on the testpieces were similar to those on real implants. To adjust temperature conditions a mass of tooling was varied. The temperature increases during deposition of HA layer could cause its dehydration and decomposition. To restore HA content on the surface, the as deposited coatings were subjected to a hydrothermal treatment in an autoclave at 150-190 oC.

2 A modelling of deposition of the composite coatings was performed to reduce amount of experimentation and identify optimal ranges of process parameters for deposition of each layer. The modelling was performed using a 3D discrete ballistic model [2] which was modified to allow simulation of deposition multilayer 3DCP coatings. For each layer, a deposition of 104-105 particles was considered. The incidence angle  of the plasma flux was varied within 15o to 90o range. Initial velocity and temperature of sprayed particles were accepted to be 200 m s-1 and 2000 oC respectively. Deposited coatings were characterised in terms of surface morphology and phase composition using conventional methods of optical microscopy and X-ray diffraction analysis.

RESULTS AND DISCUSSION

Results of simulation of the coating formation during different stages of deposition of the composite coatings are shown in Fig. 3. Investigations of the effects of incidence angle on the deposition of Ti coatings indicated that appropriate 3DCP structures can be formed at the angles <30o. Owing to the shadowing effect, formation of the coating ridges occurs right at the beginning of the coating deposition process (Fig. 3a). 3DCP coatings deposited within the 15o to 25o range of incidence angles possess the highest porosity and the largest specific surface area (Fig. 3b); this provides reliable implant anchoring in the bone tissue.

Fig. 3. Macrostructure formation stages during plasma spray deposition of composite 3DCP Ti + HA coating under different incidence angles  of plasma flux at deposition of Ti layer: (a) and (b) – at the beginning and at the end of deposition of Ti layer; (c) and (d) – at the beginning and at the end of deposition of HA layer; (e) – cross- sectional morphology of the as deposited composite coating.

3 Evolution of the coating macrostructure during deposition HA layer is shown in Fig. 3c and d. It can be seen that the layer formation begins at the tops of the hills of 3DCP Ti layer. The optimum conditions correspond to the range of  = 75o to 90o, although it is not possible to achieve 100% surface coverage in the valleys of 3DCP coating. At the thicknesses of HA layer above 50…70 m, the islands of forming coating begin merging to form an independent skeleton (Fig 3d). This thickness range appears to represent the lower limit for the thickness of the bioactive layer. Phase composition was evaluated for the top layer of composite coating deposited by plasma spraying under optimum conditions. Main results are collated in the Table 1 for two different coating thicknesses in as deposited conditions and after hydrothermal treatment. The analysis of phase composition has showed that, depending on thickness, as deposited coatings contain 65% to 78% HA. In addition to this main constituent, the coatings comprised some tri- and tetra-calcium phosphates (typically < 10%). After the hydrothermal treatment, the concentration of hydroxyapatite increased up to 90- 96% and the secondary phase developed was calcium oxide (CaO). The final HA content depended upon its initial concentration determined mainly by the thickness of HA layer.

Table 1. Coating Phase Composition in Various Stages of Deposition (wt%).

Stage Ca10(PO4)6(OH)2 Ca3(PO4)2 Ca4O(PO4) CaO

Precursor Powder 100 - - - As Deposited (80 m) 66.7 9.1 11.8 12.4 Post-treated (80 m) 90.9 - - 9.1 As deposited (550 m) 79.6 11.2 0.1 9.1 Post-treated (550 m) 95.2 - - 4.8

Fig. 4. Femoral stem and acetabular cup with composite 3D-CP + HA coatings deposited by plasma spray technique.

In terms of hydroxyapatite content, these results are similar to those claimed by the world leading coating providers, e.g. Sulzer-Metco. At the same time, the hydrothermal treatment is carried out in a simpler and a cheaper way. It can be seen

4 that as a result of the hydrothermal treatment, the coating transfers from amorphous into crystalline state. This is accompanied with attachment to the coating material of hydroxyl groups from water vapour. This leads to the transformation of anhydrous calcium phosphates into hydroxyapatite phase. Such hydrothermal treatment results in optimum composition and crystalline size of HA phase [3,4]. In vivo studies of HA behaviour in the animal body show that amorphous hydroxyapatite phase is easily dissolved whilst forming less amount of new bone tissue. In contrast, desorption of nanocrystalline HA is hindered and nucleation of the new bone is facilitated.

CONCLUDING REMARKS

Based on the above studies, original technology was developed for deposition of composite 3DCP + HA coatings. The technology was implemented for coating femoral implants (Fig. 4.).

ACKNOWLEDGEMENTS

The work was supported by the Russian State contract #02.523.11.3007. International collaboration was supported by the Royal Society and is acknowledged with thanks.

REFERENCES

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