Date of Award
Master of Science
Materials Science and Engineering
Raymond A. Buchanan
Peter Liaw, Roberto Benson
In the present research, the wear and corrosion properties of a heat treated Ti-8Mo alloy are compared to that of a Ti-6Al-4V, an alloy commonly used as the material for the stem component of hip prosthesis. Ti-6Al-4V has the attractive properties of high strength to weight ratio and excellent corrosion resistance. However, the alloy has poor wear resistance and there have been biocompatibility concerns with Al and V. The heat treated Ti-8Mo is an alloy that has not yet been examined for biomaterial applications. However, the low toxicity of Mo and the possibility of improved wear resistance due to heat treatment warrant the consideration of Ti-8Mo as an alternative to Ti-6Al-4V in hip replacement implants.
Ti-6Al-4V ELI plates were donated by Titanium Industries Inc. The Ti-6Al-4V plates were machined at The University of Tennessee into flat disks. Ti-8Mo was fabricated by arc melting in an inert argon atmosphere. The material was held at 925˚C for 16 hours, immediately transferred to a furnace at 776˚C and isothermally aged for 2 hours in the α + β region, then water quenched to room temperature. This heat transfer was performed as described by James and Moon  to obtain a microstructure of a precipitated ω phase in a α + β matrix. James and Moon demonstrated that the ω phase was associated with increases in hardness and strength, which inferred that the ω phase could lead to improved wear resistance. Ti-8Mo was also machined into flat disks at The University of Tennessee.
Pin-on-disks wear tests with PMMS pins were performed on Ti-6Al-4V and Ti-8Mo in a bovine serum (simulation of synovial fluid) at room temperature. The contact stress of the pin was 3.4 MPa with an interfacial velocity of 150 mm/s. During wear tests, a hold potential of 100 mV above the open-circuit-potential was applied, and the current was measured. Profilometry was used to quantify the total material loss for each alloy. Cyclic-anodic-polarization experiments were conducted in a phosphate buffered saline (PBS) solution at 37˚C. The oxygen concentration of the solution was controlled by aerating the solution with a N2 gas mixture containing 0.4 % O2. The flow rate of the gas mixture was approximately 50 mL/s. Corrosion rate calculations were performed for each alloy to predict the amount of material lost in one year.
The total mass loss of Ti-6Al-4V was more than 10 times that of Ti-8Mo. The average mass loss value due to wear for Ti-6Al-4V was greater than 10 times that of Ti-8Mo. For both alloys, the amount of material loss due to corrosion was significantly lower than that lost due to wear. Cyclic-anodic-polarization results supported that both Ti-6Al-4V and Ti-8Mo were extremely resistant to localized corrosion in the simulated biological environment. And, both alloys exhibited low corrosion current densities and low corrosion penetration rates. Overall, the pin-on-desk wear behavior of Ti-8Mo was superior to Ti-6Al-4V, and Ti-8Mo exhibited excellent corrosion resistance that was comparable to that of Ti-6Al-4V.
Green, Brandice Annette, "Ti-8Mo as an Alternative to Ti-6Al-4V in Hip Replacement Implants. " Master's Thesis, University of Tennessee, 2004.