Mechanical behaviour of Ti-12Mo-6Zr-2Fe (TMZF) β-titanium alloy in physiological environments

2017-02-26T23:58:03Z (GMT) by Yang, Xueyuan
Titanium and its alloys are popular metallic implant biomaterials because of their excellent corrosion resistance and satisfactory biocompatibility. Among them, the β titanium alloys have the advantage of low Young’s moduli (compared with α and α+β titanium alloys), which can help stress transfer to the bone and reduce stress shielding. Ti-12Mo-6Zr-2Fe (TMZF) is one of the β titanium alloys which was developed for orthopaedics in the 1990s. Based on the mechanical properties of TMZF in dry conditions, this alloy compares very favourably with the best of the available metallic implant materials and has been applied in total hip replacements (THR) since the 2000s. However, after having been applied for nearly a decade, the US Food and Drug Administration (FDA) recalled TMZF implants in 2011 [1]. The reason for recall is an unacceptable levels of ‘debris’ from the implant. This project is focussed on understanding the reasons and the mechanisms leading to such unacceptable levels of debris from TMZF implants in the body. THR’s operate in an aggressive saline environment, are subject to cyclic stresses for cycles of the order of 1×107 and the surfaces of the implant are abraded by minerals in bone, or by other artificial surfaces for multi-component implants. Any of the processes of corrosion, wear or fatigue, or synergistic combinations of them, may be responsible for the debris that led to the recall of these implants (and not of similar modular implants constructed of α+β titanium alloys). In this project, an experimental program was designed to deconstruct the complicated phenomena of corrosion-wear-fatigue into individual experiments on corrosion, wear and fatigue, followed by a systematic examination of the couplings of corrosion-wear and corrosion-fatigue. The corrosion process leads to the spontaneous formation of hydroxyapatite on the TMZF in SBF with nearly neutral pH, just as it does on Ti-6Al-4V (Ti64) and the formation kinetics of hydroxyapatite are similar on both materials. This suggests TMZF does not have an intrinsically poor corrosion response in SBF and therefore this is unlikely to be the sole cause of the unacceptable levels of debris that led to the recall. The wear behaviour of TMZF and Ti64 was compared in both dry conditions and in SBF (i.e. corrosion-wear). Since corrosion is an inherently time-dependent process, the coupling of corrosion with wear introduces a characteristic time scale into the wear problem. A type of ‘time dependent wear test’ was developed and performed in SBF and it is shown that unlike Ti64 which tends to reduce its wear rate in SBF (because of lubricating properties of the fluid), the wear rate of TMZF increases dramatically in SBF, compared to dry conditions. The corrosion process in SBF leads to the deposition of particles of hydroxyapatite on the surface of the alloys which act as abrasive elements (like sand between two articulating surfaces) and greatly accelerates the wear rate of the TMZF alloy in SBF. Similar particles are formed on Ti64 but in this case the alloy exhibits sufficient strain hardening to further strengthen the alloy and resist wear. TMZF is a β-Ti alloy and exhibits no strain hardening-any abrasive elements accelerate the wear process significantly and this is what is observed with the TMZF alloy. It is likely this is the origin of the debris that have led to the recall of this implant. The fatigue performance of TMZF was also measured in dry conditions and in SBF. Fatigue loading leads to the roughening of the component surfaces in SBF and accelerates fatigue failure. More importantly, the cyclic loading causes the cracks of the hydroxyapatite layer formed on TMZF surface leading to the release of hydroxyapatite particles from the surface layer into the surrounding environments or between the articulating surfaces. These particles act as third-body abrasives (just like the hydroxyapatite particles formed between the two sliding surfaces during corrosion-wear) to further accelerate the wear rate of TMZF alloy since it exhibit an inability to resist the abrasive wear due to the lack of strain hardening capacity. This lack of strain hardening is a general feature of β-Ti alloys and similar corrosion-wear problems should be expected for all β-Ti alloys being considered for implant applications.