Low temperature diffusion of oxygen in titanium and titanium oxide films

Abstract

Work in our laboratory and elsewhere indicates that the thermal ignition characteristics of Ti-based pyrotechnics are controlled by diffusion of oxygen from a surface coating into the bulk of the metal. Diffusion of oxygen in Ti at temperatures greater than 700 K has previously been modeled using Fick's law. No work has been reported at lower temperature, and the results from previous high temperature oxidation studies are both inconsistent and insufficient to define diffusion behavior for the temperature range 400 to 800 K. We have conducted Ti oxidation experiments in oxygen ambients of 3 to 700 Torr, at temperatures from 400 to 800 K, for periods from 1 to 100 h. Oxygen concentration profiles were determined by Auger analysis combined with sputter depth profiling. Calibration of the Auger atomic concentrations were confirmed by Rutherford backscattering spectroscopic measurements. Results show growth of four distinct oxide regions which are consistent with the titanium-oxygen phase diagram. Growth of the oxide regions is independent of the oxygen partial pressure and appears to be diffusion controlled. Fick's law with a constant diffusion coefficient was assumed, and an Arrhenius expression for the oxygen diffusion coefficient in each region was evaluated from the experimental data. Activation energies for oxygen diffusion range from about 9 to 43 kcal/mol, and indicate that the thermal ignition mechanism is controlled by growth of a TiO 2 region adjacent to the gas-solid interface.