Juanli Deng

REACTION THERMODYNAMICS IN CHEMICAL VAPOR DEPOSITION OF BORON CARBIDES WITH BCl3–C3H6 (PROPENE)-H2 PRECURSORS

The gas phase reaction thermodynamics in the chemical vapor deposition (CVD) process of preparing boron carbides via the precursors of BCl3–C3H6(propene)–H2 is investigated with a set of 325 gaseous species, in which the data for 135 species are evaluated in this work. The thermochemistry data are calculated with accurate model chemistry at G3(MP2) and G3//B3LYP levels. The concentration distribution of all of the 325 species is obtained with the principle of chemical equilibrium. The thermochemistry data include the heat capacities, entropies, enthalpies of formation, and Gibbs free energies of formation. The heat capacities and entropies at temperatures in 298.15–2000 K are evaluated with the standard statistical thermodynamics. The Gibbs free energies of formation in 298.15–2000 K are calculated with the classical thermodynamics based on the developed heat capacities and entropies. By including the crystal B4C, C(graphite), and B, the results for an example of the 3:1:2 precursors of BCl3:C3H6:H2 show that the crystal B4C can be produced at temperatures higher than 700 K while the graphite has a higher molar value and can be produced at lower temperatures. It is also examined that the production of graphite can be controlled by changing the ratio of the injected reactants or pressure. It is interesting that BHCl2, BCH3Cl2, BH2Cl, and B2Cl4 are found to be the most effective species in the CVD process, which is similar to those in the BCl3–CH4–H2 system. The results predicted in this work are consistent with the experiments.

Effect of Braking Speed on Frictional Properties of Short Fiber C/C-SiC Brake Materials and Grey Cast Iron

Short fiber C/C-SiC brake materials were prepared by dipping the fibers in resin, chopping, warm pressing, and pyrolyzing, followed by a liquid silicon infiltration process. The effect of braking speed (initial braking speed) on the frictional properties of a pair of short fiber C/C-SiC brake materials and grey cast iron (HT250) (C/C-SiC-HT250) was investigated. The results indicated that the average friction coefficient of the C/C-SiC-HT250 increased to the maximum value at 10 m/s and then decreased with an increase in braking speed. The wear rate of the C/C-SiC increased with increasing braking speed when it was lower than 10 m/s and decreased when it was higher than 10 m/s. The wear was the result of the brittle fracture of asperities when the braking speed was lower than 10 m/s and induced granular debris. The wear for the tough cutting by C/C-SiC asperities or hard phase when the braking speed was higher than 10 m/s and resulted in ribbon debris. When the braking speed was higher than 20 m/s, the texture of the friction surface of the HT250 changed, which would affect the tribological properties.