Solid state diffusion in the lubricious film formation on iron by mixed tertiary butyl phenyl phosphates under static conditions

The reaction between vaporous aryl phosphate esters and iron is known to yield a lubricious product film at high temperatures. The rate law exhibited in the growth of this film is consistent with a diffusion controlled process. In this work the actual movement of elements during this growth process was characterized using laminated metal foil samples consisting of a 1,000 A iron film deposited on a platinum substrate. The interface between the platinum and iron served as a reference datum against which the movement of the elements was observed. Iron was found to be the lone mobile element that diffuses through the product layer where it reacts with the vapor at the exposed surface generating more product. The change in the concentration profiles of the iron within the product served as a means to evaluate the diffusivity of the iron during the product growth. In this case the diffusivity of iron was found to be greater than its self diffusivity and within the range of values typical of iron oxide. This data in combination with the estimated activation energy for the process suggested that the diffusion was of a nature similar to the vacancy diffusion displayed in the oxidation of iron. The data suggests that the diffusion of iron through the product is the rate controlling step in the product growth process.