Nitrogenation of hafnium carbide powders in AC and DC plasma by Electrical Discharge Assisted Mechanical Milling
Abstract Metal carbonitride powders and coatings are well known as superhard materials which are widely applied in many engineering products. Of this group of materials hafnium carbonitride properties are one of the least known due to difficulties with preparation. In this study, the electric discharge assisted mechanical milling (EDAMM) method involving a nitrogen plasma together with mechanical milling was used to synthetise hafnium carbonitride powders within minutes. It was found that difficulties in conventional manufacturing methods, resulting in the formation of only thin layers instead of bulk materials can be overcome by applying the new technique, and HfC x N y powder could be successfully engineered with controlled amount of nitrogen uptake. In this investigation we studied the effects of AC and DC discharges on nitrogen solubility in HfC powder. Microstructure and phase evolution were determined using XRD and SEM, while the nitrogen content in the grains was evaluated by EDS and bulk nitrogen content by CHN analysis. Maximum solubilities of 3.42 wt% and 2.95 wt%N under DC an AC discharge currents, respectively, were obtained. Reaction paths varied depending on processing conditions with HfC x N y formed under DC discharge processing and mixed products containing HfC, several HfC x N y phases and HfN like phase formed after AC processing. After nitrogenation the most highly nitrided powders were subjected to 1 min additional EDAMM processing using high power discharges to allow the formation of large, sintered particles of sufficient size to perform hardness measurements on. Vickers harnesses of fully sintered large particles after 5 and 10 min of EDAMM were 1808 and 2169HV, respectively.