Study On Molybdenum Carbides Synthesized At High Temperature And High Pressure

Molybdenum carbides were reported as refractory hard materialswith the similar structure of tungsten carbide which is widely used as ahard material. However, there are few reports on the physical propertiesof molybdenum carbides. We use high pressure and high temperature（HPHT） to synthesize molybdenum carbide. We estimated the pressure bya calibration curve which was fixed by the phase transformation pressuresof bismuth, thallium and barium, and we estimated the temperature by therelationship between input heater power and temperature, which wasmeasured by the platinum-rhodium thermal couple. In this work, weintroduce our work such as:（1） The raw materials for preparation of Mo-C compounds wereas follows: molybdenum powders （purity99.9%） and amorphouscarbide powders （purity99.5%）. We mixed the molybdenum powderand amorphous carbide powder in a stoichiometric ratio carefully inan agate mortar with some alcohol to help intimate mixing. Then, wecarried out high pressure and high temperature （HPHT） synthesis on aSPD6×600T cubic anvil apparatus. Different Molybdenum carbidesincluding α-Mo₂C and γ-MoC have been successfully synthesized at2200K,5.2GPa,15min and σ-MoC synthesized at1750K,5.2GPa,1h.（2） The samples were encased in a slow-curing epoxy resin and preliminary polishing was carried out using silicon carbide abrasivepaper. Diamond films （3,1and0.5μm particle sizes） were used forfine polishing in order to obtain a mirror finish. A minimum of10indentations were made on each sample at different loads between0.25and0.98N. The residual indentation diagonal lengths weremeasured using an Axiotech100reflected-light microscope （CarlZeiss, Inc.） under400×magnification. Vickers micro-hardness （Hv）was calculated with HV=1854.4P d2（1） where d is the arithmeticmean of the two diagonals of the indentation and P is the applied load.and we know the hardness of the samples （σ-MoC:7.8GPa，γ-MoC:10.5GPa，α-Mo₂C:9.0GPa）. At last, we get the hardness of γ-MoC，σ-MoC and WC theoretically by VASP, and know their electrondensity. We want to get clear the hardness different of WC andσ-MoC that have the same structure by analyzed the distribution ofbonding electron.（3） We use Van Der Pauw method to characterized all synthesizedcompounds to know their electrical conductivity. Firstly, we polishedthe samples by abrasive paper less than0.5mm, then we fixed fourthin copper wire around the samples edge, and get the DC powerbetween50mA and120mA （plus10mA each time）, At last we can getthe current from the multi-meter, and we get the resistivity of thesamples all about10-8which is much better than molybdenum.（4） We crushed the samples into power, and measured theiroxidation resistance with DTA-TG. The samples are not changed untilabout400oC, which could be satisfied nearly all the normal use.