High-entropy ceramics such as oxides, borides and carbides having a single-phase microstructure have been subject of recent research. There are several reports on transition metal high entropy carbides (HECs) having 5 or more metals in equiatomic ratio and carbon resulting in formation of a single-phase Rock Salt structure. HECs have been shown to have superior mechanical properties and good resistance to oxidation, corrosion and wear, making them promising candidate for aerospace applications as machining tools. Preparation of HEC using metal carbide precursors by conventional powder metallurgy routes requires very high temperatures more than 2000 °C. The objective of the work is to study the synthesis of single phase (TiWTaMoV)Cx high entropy carbide by powder metallurgy route using reactive spark plasma sintering of ball milled metal and carbon mixtures. The effect of milling time, SPS parameters and C/M ratio on the HEC phase formation, microstructure, mechanical and wear properties were evaluated.
Elemental powders of Ti, W, Ta, Mo, V (purity 99.5%, size 40–200 µm) were taken in equiatomic ratio and mixed with Carbon black in different C/M ratio of 0.45, 0.6, and 0.75 and subjected to mechanical alloying. Mechanical alloying was carried out using a Fritsch Pulverisette 5 planetary mill in toluene medium for 2, 6, and 10 h using WC balls (10 mm dia.) and a WC lined vial, with a ball-to-powder ratio of 10:1 by weight. Reactive sintering was carried out using a Spark plasma sintering equipment at 1400 °C, 1600 °C, and 1800 °C with a heating rate of 50 °C/min and an applied uniaxial pressure of 50 MPa and holding time of 5 min at peak temperature. Carbon measurements were performed on milled powder as well as SPS compacts for determining the final C/M ratio. Out of the total 27 conditions, 9 resulted in the formation of dense HEC while others had secondary phases in them. For the C/M ratio of 0.6, single phase solid solution was achieved for samples prepared from 6h and 10h milled powders at 1600 °C and 1800 °C. 0.75 C/M ratio composition formed single phase solid solution for all milling durations except 10h when sintered at the lower 1600 °C temperature, while single phase was achieved across all milling times when sintered at 1800 °C. Atom Probe Tomography studies showed homogeneous distribution of elements throughout the matrix with segregation of Fe and Co at grain boundaries which could be impurities in the metal powder and helped in densification. Also, presence of TiO2 was observed in the microstructure. The grain size of the spark plasma sintered compacts exhibited a decreasing trend with increasing ball milling time of the starting powders, while the Vickers microhardness showed an increasing trend with both extended milling time and higher sintering temperature. The compact sintered at 1800 °C with C/M ratio of 0.6 and milling time of 10h exhibited a microhardness of 26.6 GPa. The specific wear rate of HEC with C/M=0.6 was found in the range of 2-6x10-7 mm3/N·m which is comparable to the high entropy carbides synthesized using refractory metals such as Hf and Zr. The results show promise of RSPS as a low temperature sintering method for the synthesis of HECs.