Chemical vapor deposition of amorphous boron carbide coatings from mixtures of trimethylboron and triethylboron

Amorphous B_xC films were deposited from the coreaction of triethylboron (TEB) and trimethylboron (TMB) at 700 °C in H₂. We observed that combining both precursors allows us to balance their deposition kinetics and yields higher growth rates. Quantitative analysis by x-ray photoelectron spectroscopy shows that a wide range of B/C ratios between 0.7 and 4.1 could be obtained by varying the TEB:TMB ratio. Raman spectroscopy was used to assess the bonding in the films that gradually evolved from a structure similar to that of a-B, to a mixture of half-icosahedra embedded in a carbon matrix to a graphitic structure, as the carbon content increased. The addition of TMB in the gas phase was found to result in a decrease in elasticity and hardness but an improved adhesion, resulting in complex crack patterns upon cleaving, such as sinusoidal cracks and loops. On the one hand, the incorporation of carbon from TMB leads to an increasing contribution of the softer carbon matrix, to the detriment of polyhedral B–C structures, which in turn decreases Young’s modulus and hardness. On the other hand, it suggests that near the film-substrate interface, the presence of the carbon matrix affords a high density of strong carbon-based bonds, resulting in improved adhesion and preventing delamination of the coatings.