Titanium diboride coatings and their interaction with the substrates

This study is the continuation of an experimental investigation of the chemical vapor deposition of titanium diboride (TiB 2) on metallic substrates using the hydrogen reduction of TiCl 4 and BCl 3 at 1 atm and at temperatures between 850 and 1050 °C. To obtain a coating we found that the substrate had to meet the following requirements: (1) it had to withstand the deposition temperature without detrimental transformation; (2) it had to be chemically inert to the byproducts of the reaction (mostly HCl); (3) its thermal expansion coefficient had to match reasonably well with that of TiB 2. Requirement (3) may be partially circumvented by using a ductile intermediate coating such as copper or nickel. Substrates meeting these requirements were tungsten, tantalum, molybdenum, WC, TiC, Kovar and some high chrome steels. Coatings on these substrates were examined by metallographic techniques, scanning electron microscopy, X-ray diffraction and electron microprobe. The structures and the degree of interdiffusion were determined. In most cases intermediate borides of the type M 3B and M 2B were formed. The hardness of the coatings was 3333 ± 310 kg mm −2 (VHN 50). Coatings of TiB 2 have already been used successfully on letdown valves in a bench-scale coal liquefaction reactor at Sandia Laboratories.