Hafnium carbide as a barrier in multilayer coatings by chemical vapor deposition (CVD)

Abstract
Hafnium Carbide (HfC) is used as a wear resistant coating for hard metal cutting tools as well as a barrier for oxygen in multilayer coatings to protect Carbon Fibre reinforced Carbon composite materials (CFC) at high temperatures. Chemical Vapor Deposition (CVD) allows the production of this refractory carbide above temperatures of 850°C, far away from its melting point of 3930°C. Hafnium (IV) chloride (HfCl₄) and methane (CH₄) in an excess of hydrogen (H₂) are used as precursors. The reproducibility and the standard deviation of the process was tested at constant process conditions. The effect of the deposition conditions such as total pressure, molar fraction of hydrogen and the influence of additional hydrogen chloride in the reaction gas on the HfC - deposition rate (dpr) is investigated. The composition of the HfC layers was determined using Electron Probe Micro Analysis (EPMA). The oxidation stability of multilayer coated CFC composites is tested in a thermobalance, in a temperature range between 380° and 868°C.