Reactions between hydrogen-containing carbon films and substrate metals at elevated temperature

Changes in physicochemical properties of hydrogen-containing carbon films deposited on beryllium, molybdenum and tungsten were studied by means of infrared, Raman, x-ray photoelectron (XPS) and thermal desorption (TDS) spectroscopy as well as x-ray diffraction (XRD). Infrared and Raman spectroscopy showed that as-prepared hydrogen-containing films were composed of carbon atoms with sp2 and sp3 hybridized orbitals, where hydrogen was bound to carbon as -CH3 and >CH2. Vacuum heating of the carbon films caused two major thermal desorption peaks. One appeared at about 715 K, irrespective of substrate metals, and the other at higher temperatures, depending on the substrates; 873, 1050 and 1173 K for Be, Mo and W, respectively. The low temperature peak was composed of H2, CO and hydrocarbons, whereas the high temperature peaks consisted essentially of H2. The former was due to the reaction of the carbon films with adsorbed water, and evaporation of volatile fragments and/or thermal decomposition of the films. On account of the XPS and XRD observations, the latter was attributed to a solid-state reaction between the films and the substrates to form M2C (M=Be,Mo,W). Good linear relations were found between the reactivity and material properties of the substrates, such as melting point, surface tension, etc. The over-all kinetics of the solid-state reaction could be explained by a nucleation-growth model. (author)