Effect of bulk microstructure of commercially pure titanium on surface characteristics and fatigue properties after surface modification by sand blasting and acid-etching

Abstract

Surface modification techniques are widely used to enhance the biological response to the implant materials. These techniques generally create a roughened surface, effectively increasing the surface area thus promoting cell adhesion. However, a negative side effect is a higher susceptibility of a roughened surface to failure due to the presence of multiple stress concentrators. The purpose of the study reported here was to examine the effects of surface modification by sand blasting and acid-etching (SLA) on the microstructure and fatigue performance of coarse-grained and ultrafine-grained (UFG) commercially pure titanium. Finer grain sizes, produced by equal channel angular pressing, resulted in lower values of surface roughness in SLA-processed material. This effect was associated with greater resistance of the UFG structure to plastic deformation. The fatigue properties of UFG Ti were found to be superior to those of coarse-grained Ti and conventional Ti–6Al–4V, both before and after SLA-treatment.

Introduction

Titanium and its alloys are widely used to produce medical implants for replacing damaged or missing load-bearing tissues. Titanium is ideally suited for such applications due to a combination of high corrosion resistance, light weight and good mechanical properties it possesses (Rack and Qazi, 2006). The surface of an implant plays a crucial role in cell response, particularly in bone replacement. The characteristics of a titanium implant surface govern cell attachment, spreading and proliferation in the process of osseointegration. Ultimately, this determines the quality of bonding between the newly grown bone tissue and the implant (Schenk and Buser, 1998). Therefore, a significant effort of researchers in recent years has been aimed at developing effective surface modification techniques to promote osseointegration. Most of the techniques make use of one of the following processes or a combination thereof: plasma-spraying, grit-blasting, acid-etching, and anodization (Le Guéhennec et al., 2007). One particular variant of these processes, called SLA (which stands for “sand-blasting with large grit and acid-etching”), combines positive effects of grit-blasting and etching to create a macroscopically rough surface with micro-pits (Hyeongil et al., 2008). It was shown by some researchers that SLA-treatment can enhance the biological performance of the surface, giving rise to increased differentiation of stem cells (Wall et al., 2009), enhanced osteoblastic cell viability (Le Guehennec et al., 2008), and increased bone anchorage (Szmukler-Moncler et al., 2004). In addition, a very high survival rate of implants with SLA-treated surface was found in clinical tests of dental implants: 98.7% after 15.2 months (Hyeongil et al., 2008) and 95.41% after 5 years of implantation (Kim et al., 2014).

Mechanical performance of implant materials, which is of paramount importance for their application in implants, also depends on the surface quality. Predicting the effect of surface roughening by SLA treatment on mechanical properties, in particular, fatigue life, is a challenging task, though. On one hand, rough surfaces are detrimental to mechanical properties of materials. Pits and irregularities create stress concentrations and provide sites for crack nucleation (Leinenbach and Eifler, 2006). On the other hand, the impact of grit particles during blasting was shown to produce a near-surface layer with altered structure and high compressive stresses, which may retard the nucleation and propagation of fatigue cracks at the surface (Javier Gil et al., 2007, Pazos et al., 2010, Conforto et al., 2004). The microstructure of the material prior to SLA is a crucial factor, as blasting-induced surface distortion will be a function of strength of the material, which depends on the grain size and local texture. For example, it was shown (Javier Gil et al., 2007) that pure titanium samples with different initial microstructure were affected by grit-blasting in different ways. While the fatigue performance of samples with equiaxed microstructure was improved owing to compressive stress in the vicinity of the surface produced by grit-blasting, this kind of surface treatment did not have a significant effect on the structure with acicular martensite. Rather, crack propagation along martensite α′ plates was found to occur.

Altering the microstructure of dental implant materials, in particular titanium and its alloys, has been a target of numerous studies in the past years, especially with the advent of a number of techniques for refining the grain size of these materials down to a submicron/nano-range. These techniques made it possible to produce materials with exceptional mechanical properties (Estrin and Vinogradov, 2013, Valiev et al., 2012, Valiev et al., 2002) and enhanced biological performance (Bindu et al., 2009, Estrin et al., 2011, Estrin et al., 2009, Valiev et al., 2008). However, to our knowledge, no systematic experiments dedicated to examining the effect of surface modification of such ultrafine grained (UFG) materials on their mechanical performance, notably fatigue properties, were undertaken to date.

The aim of the present study was to investigate the surface characteristics of commercially pure titanium of two different grades treated by SLA and their effect on its fatigue performance. Both grades were available in two distinctly different microstructural states – coarse-grained and ultrafine-grained ones. The fine crystallinity of the UFG Ti was achieved by equal channel angular pressing (ECAP).