Spontaneous formation of multilayer refractory carbide coatings in a molten salt media.

Refractory materials hold great promise to develop functional multilayer coating for extreme environments and temperature applications but require high temperature and complex synthesis to overcome their strong atomic bonding and form a multilayer structure. Here, a spontaneous reaction producing sophisticated multilayer refractory carbide coatings on carbon fiber (CF) is reported. This approach utilizes a relatively low-temperature (950 °C) molten-salt process for forming refractory carbides. The reaction of titanium (Ti), chromium (Cr), and CF yields a complex, high-quality multilayer carbide coating composed of 1) Cr carbide (Cr3C2), 2) Ti carbide, and 3) Cr3C2 layers. The layered sequence arises from a difference in metal dissolutions, reactions, and diffusion rates in the salt media. The multilayer-coated CFs act as a permeable oxidation barrier with no crystalline degradation of the CFs after extreme temperature (1,200 °C) and environment (oxyacetylene flame) exposure. The synthesis of high-quality multilayer refractory coating in a fast, efficient, easy, and clean manner may answer the need for industrial applications that develop cheap and reliable extreme environment protection barriers.

Ultraviolet laser photolysis of hydrocarbons for nondiamond carbon suppression in chemical vapor deposition of diamond films.

In this work, we demonstrate that ultraviolet (UV) laser photolysis of hydrocarbon species alters the flame chemistry such that it promotes the diamond growth rate and film quality. Optical emission spectroscopy and laser-induced fluorescence demonstrate that direct UV laser irradiation of a diamond-forming combustion flame produces a large amount of reactive species that play critical roles in diamond growth, thereby leading to enhanced diamond growth. The diamond growth rate is more than doubled, and diamond quality is improved by 4.2%. Investigation of the diamond nucleation process suggests that the diamond nucleation time is significantly shortened and nondiamond carbon accumulation is greatly suppressed with UV laser irradiation of the combustion flame in a laser-parallel-to-substrate geometry. A narrow amorphous carbon transition zone, averaging 4 nm in thickness, is identified at the film-substrate interface area using transmission electron microscopy, confirming the suppression effect of UV laser irradiation on nondiamond carbon formation. The discovery of the advantages of UV photochemistry in diamond growth is of great significance for vastly improving the synthesis of a broad range of technically important materials.