Elsevier

Materials Letters

Volume 147, 15 May 2015, Pages 75-78
Materials Letters

Effect of a blast coating process on the macro- and microstructure of Grade 5 titanium foam

https://doi.org/10.1016/j.matlet.2015.02.044Get rights and content

Highlights

  • Deposited hydroxyapatite coating onto titanium foam using a blast coating process.

  • Not adverse effect to the macrostructure of the titanium foam.

  • No obvious signs of excessive abrasion or collapsing of the structure.

  • Microstructure unaffected by the coating process.

Abstract

This work investigates the deposition of a hydroxyapatite (HA, Ca10(PO4)6(OH)2) coating onto titanium foam using a blast process and the effect of the process on the macro- and microstructure of the titanium foam. Light microscopy, scanning electron microscopy and energy-dispersive x-ray spectroscopy were used to examine the coating deposition and the macro- and microstructure of the titanium foam. After blasting the macrostructure did not exhibit signs of excessive abrasion or collapsing of the structure that may affect functionality of the foam. Similarly, microstructure was unaffected by the coating process as there is gross plastic deformation and no thermally induced change such as α-case.

Introduction

Titanium and titanium alloys are commonly used in the fabrication of artificial joints due to their light weight, high biocorrosion resistance, biocompatibility and mechanical properties [1]. However, the relatively high stiffness and bioinert nature of titanium may result in stress shielding and poor bonding between the implant and surrounding bone which in turn, may result in loosening and failure of the implant. Porous titanium foams have been developed to address the issue of high stiffness and to provide surface area for bone ingrowth to enhance fixation [2]. These foams are manufactured using various powder sintering and pressurised pore expansion methods. A comprehensive review of these productions methods is provided by Dunand [3].

Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is a biocompatible, biodegradable, osteoconductive, mineral phase and one of the primary constituents of bone. It has been used as a coating for orthopaedic implants as its presence has been shown to promote bone formation [4], [5]. Various techniques have been used to deposit HA coatings onto bulk titanium such as plasma spray [6], electrophoretic deposition [7] and sol–gel deposition [8] however these techniques exhibit relatively poor bond strengths (≈25 MPa [6], ≈10 MPa [7] and ≈25 MPa [8] respectively). In conjunction with this, the high temperature of the plasma spray technique gives rise to the formation of unwanted phases and amorphisation of the coating while the coating produced is relatively thick (≈70 µm) making it unsuitable for coating titanium foams without altering or occluding pore size [9].

The present work investigates the application of HA onto titanium foam using an ambient temperature coating technique known as CoBlastTM. CoBlast was developed to address the problems associated with high temperature coating techniques, such as the formation of unwanted phases, amorphisation of the coating and poor adherence of the coating [9], [10]. The CoBlast process utilises a co-incident stream of abrasive blast medium and a stream of coating medium particles to modify the substrate surface. The proposed mechanism for adhesion is that the abrasive roughens the surface while simultaneously disrupting the surface passivation layer of the substrate thereby exposing the reactive metal. The coating particles then react with the reactive metal to form an intimate chemical bond [9]. The adhesion of the coating to the substrate is believed to be due to a combination of tribo-chemical bond formation and mechanical interlock between the coating and the metal substrate [11]. The CoBlast process produces a thin (<10 µm), highly adherent coating (≈60 MPa) [9] making it potentially suitable for coating titanium foams.

Previous work where coating of Grade 2 titanium with HA resulted in cold working of the substrate and significant grain deformation [9], [10]. Therefore, the abrasive nature of the process could potentially alter the macrostructure of the foam by crushing pores and this in turn could inhibits the functionality of the material. Thus the key aim of this work is to examine what influence the coating process has on the macro- and microstructure of the titanium foam and to assess the reach of coating with this line of sight process.

Section snippets

Materials and methods

Titanium foam (Grade 5; Ti6Al4V) (Ceramed, Lisbon, Portugal) 20×20×20 mm3 was used as the substrate. Hydroxyapatite (HA) (SAI, France) with a particle size of 25–60 μm and a sintered apatite (sHA) (HIMED, USA) with a particle size <106 μm were used as the coating medium and blast medium respectively. The samples were prepared by spraying the coating medium and blast medium simultaneously in the same stream through a single de Laval nozzle at the titanium foam with a jet pressure of 0.55 MPa, at a

Results and discussion

Surface characterization: Fig. 1 shows SEM of the surface of the as-supplied titanium foam and titanium coated with HA. The surface appears well coated as there is little of the underlying titanium visible. Although the CoBlast process is an abrasive process, there are no obvious signs of excessive abrasion or collapsing of the structure that may affect the mechanical properties or functionality of the foam.

Fig. 2 shows a cross-section of the coated titanium foam and EDX line profiles of

Conclusions

The CoBlast process can be used to deposit HA onto titanium sponge while retaining the macrostructure of the foam. The microstructure is unaffected by the coating process as there is gross plastic deformation and no thermally induced change such as α-case. The extent of coating is limited to sites directly exposed to the blast stream.

Acknowledgements

This research is funded under the Programme for Research in Third-Level Institutions and co-funded under the European Regional Development Fund (ERDF).

References (15)

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