Available at: http://digitalcommons.calpoly.edu/theses/42
Date of Award
MS in Engineering - Materials Engineering
Ti-6Al-4V anterior cervical plates (ACP) are used in spinal fusion surgeries to fixate cervical vertebrae during graft adhesion. However, documented cases of implant failure and the potential for ACP corrosion raise concerns regarding any degradation of material resulting from extended implantation. In addition, abrasion during implantation may damage a section of the protective oxide layer, potentially exposing surrounding tissues to the harmful effects of bare titanium, aluminum, and vanadium. Thermal oxidation has been shown to improve corrosion-resistance and wear-resistance, depending on temperature and time. To quantify the attributes of the thermally grown oxide layer, Ti-6Al-4V coupons underwent thermal oxidation treatments in an atmosphere environment at 600 and 675 ˚C for 1, 4, 8, and 16 hours. Two sample types were produced: non-abraded and abraded.
Non-abraded samples underwent potentiodynamic polarization according to ASTM F2129, which included open circuit potential tests. Open circuit potentials (EOC) increased with increasing treatment time, indicating that longer treatment time resulted in thicker oxides. All samples treated at 675˚C displayed higher EOC than samples treated at 600˚C, indicating an increase in oxide thickness with higher temperature. During the first hour of treatment at 675˚C, the rate of oxide growth was greater than the rate of oxide growth of all samples treated at 600˚C. Samples treated at 600˚C for 4 and 8 hours displayed pitting during potentiodynamic polarization, but all other samples withstood the applied potentials and surfaces were further passivated.
To simulate damage during surgery, a single abrasion was made across samples in the abraded group with a diamond-tip indenter under a load of 471g at 4.4 mm/s. Abraded samples were subjected to potential-step tests to assess repassivation ability after abrasion. All samples displayed repassivation ability, except for the sample treated at 600˚C for 4 hours.
Surface roughness was measured with atomic force microscopy before and after thermal oxidation treatments. Lower surface roughness was desired to discourage osseointegration, or the growth of bone cells. No isothermal surface roughness trends were observed, as high surface roughness outliers were seen in samples treated at 675˚C for 8 hours and 600˚C for 4 hours. Rockwell hardness and Vickers microhardness were also measured to assess bulk changes in mechanical properties and hardness of the oxidized surfaces. No statistical change was seen in Rockwell hardness. Vickers hardness increased with increasing temperature and time, with the exception of the sample treated at 600˚C for 4 hours. Metallography of the thermally oxidized samples was analyzed to determine if a change in microstructure had occurred due to thermal processing. No major change in grain size or the amount of alpha and beta grains was seen in samples treated at 600˚C, but samples treated for extended times at 675˚C showed equiaxed enlarged alpha grains and a reduction in beta grains.
The breakdown of samples treated at 600˚C exemplified possible differences in the alpha-beta oxide behavior during thermal oxidation and corrosion. Outlying surface roughness and microhardness values related to the thermal oxidation treatments and resulting oxide structure. Due to delamination of oxides grown at 675˚C for 4, 8, and 16 hours, the treatment parameters would not be effective in the ACP application. Therefore, through corrosion resistance, repassivation ability, low surface roughness, increased microhardness, and no microstructural change, thermal oxidation treatments at 600˚C for more than 16 hours, and 675˚C for 1 hour or less would be suitable treatments for anterior cervical plates.