Selection of the implant and coating materials for optimized performance by means of nanoindentation Saeed Saber-Samandari a,⇑ , Christopher C. Berndt a , Karlis A. Gross b a IRIS, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia b Riga Biomaterials, Innovation and Development Centre, Pulku 3/3, Riga LV-1007, Latvia article info Article history: Received 9 July 2010 Received in revised form 20 September 2010 Accepted 20 September 2010 Available online 29 September 2010 Keywords: Nanoindentation Substrate effects Single droplet Hydroxyapatite Thermal spray abstract Mechanical compatibility between a coating and a substrate is important for the longevity of implant mate- rials. While previous studies have utilized the entire coating for analysis of mechanical compatibility of the surface, this study focuses on the nanoindentation of a uniformly thermally sprayed splat. Hydroxyapatite was thermally sprayed to create a homogeneous deposit density, as confirmed by microRaman spectros- copy, of amorphous calcium phosphate. Substrates were commercially pure Ti, Ti–6Al–4V, Co–Cr alloy and stainless steel. Nanoindentation revealed that splats deposited on the different metals have similar hardness and elastic modulus values of 4.2 ± 0.2 GPa and 80 ± 3 GPa, respectively. The mechanical proper- ties were affected by the substrate type more than residual stresses, which were found to be low. It is rec- ommended that amorphous calcium phosphate is annealed to relieve the quenching stress or that appropriate temperature histories are chosen to relax the stress created in cooling the coating assembly. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction The load-bearing requirement of biomedical implants has led to the choice of metals that can sustain large forces and retain their shape after patient physical activity. Metal alloys used in artificial joints (i.e. hip, knee and shoulder), dental implants and orthopae- dic screws include commercially pure titanium (CP Ti) [1–3], a tita- nium-based alloy (i.e. Ti–6Al–4V) [4,5], a cobalt based alloy (i.e. Co–Cr alloy) [6,7] and stainless steel [8,9]. The selection of these metals is based on the properties, cost, manufacturing ability and ease of making surface modifications for improved bone growth and attachment. Titanium-based alloys offer a high strength to weight ratio, low modulus, high biocompatibility and resistance to corrosion [10]. Ti–6Al–4V is the most widely used titanium alloy for surgical and odontological applications due to its lightness (4.5 g cm –3 ) com- pared with stainless steel (7.9 g cm –3 ) and Co–Cr alloy (8.3 g cm –3 ). One of the main applications for Co–Cr alloys is in dental skeletal structures. These biomaterials have shown superior wear resistance over a long time in vivo, uniform surface finishing and a high resis- tance to bone debris formation [11]. Stainless steel finds wide use in orthopaedic applications because of its cost effectiveness, avail- ability and process ability. On the other hand, the release of ions (see Table 1) from metals, as well as their high elastic modulus compared with bone (see Table 2), are major concerns. Studies have shown that the release of alumin- ium and particularly vanadium ions may cause long-term health problems due to adverse tissue effects [12–15]. The introduction of foreign ions into the body arises from surface film dissolution and corrosion [16,17]. These reactions and the side effects of these products in the human body must top the list of considerations dur- ing material selection. The toxicity of metal ions, ranked in order from least to most toxic, is as follows: Cr < Mo < Al < Co < Ni < Fe < V [13]. Aluminium has been associated with dementia [18], while vanadium is considered to be an essential element in the body, becoming toxic at excessive levels [19]. Titanium forms a 1–4 nm thick oxide layer resulting in high corrosion resistance [20]. This pro- tective TiO 2 film forms particularly quickly for Ti alloys [21,22], however, it may break down on contact with physiological fluids [23]. Overall, CP Ti causes the minimum amount of side effects and has received increasing interest for the fabrication of fixed and removable restorations [24]. Alloys such as Ti–6Al–4V are the pre- ferred biomedical alloys because the second generation titanium orthopaedic alloys (i.e. Ti–12Mo–6Zr–2Fe, Ti–15Mo–5Zr–3Al, Ti– 15Mo–3Nb–3O, Ti–15Zr–4Nb–2Ta–0.2Pd, Ti–15Sn–4Nb–2Ta– 0.2Pd and Ti–35Nb–5Ta–7Zr) [25] are not yet widely available. Another key problem that has been reported is loosening of the prosthetic device, a consequence of low adhesion between metallic implants and bone and insufficient load transfer [26]. This phe- nomenon could be attributed to the mismatched elastic moduli of bone and the artificial joint replacement, which is exacerbated 1742-7061/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2010.09.023 ⇑ Corresponding author. Tel.: +61 3 9214 4345; fax: +61 3 9214 5050. E-mail address: ssabersamandari@swin.edu.au (S. Saber-Samandari). Acta Biomaterialia 7 (2011) 874–881 Contents lists available at ScienceDirect Acta Biomaterialia journal homepage: www.elsevier.com/locate/actabiomat