| MgAlON composites sintered by SPS with fine particles distributed uniformly havecompact structure，but the harsh conditions restricte the development and adaptation of that. Sofar, generally through setting up experiments to study on the synthetic mechanism, structure andproperties of MgAlON composites prepared by SPS. It has not been reported how to analyze thegrain refinement mechanism of MgAlON composites sintered by SPS. This paper studied thegrain refinement mechanism under the direct pulse current in the sintering process by setting upmathematic model. Hoping to optimize the technological parameters based on the simulationanalysis results before.Firstly, according to the ratio of raw materials Al2O3: MgO: Al=70:15:15(mt%) and thetechnological parameters of MgAlON composites perpared by SPS, used Comsol Multiphysicssoftware to simulate the discharge plasma reactors in the sintering process of MgAlONcomposites preparation. And study the electron density distribution and electric energy densitydistribution between grains when the distances between electric grains different. Thenspeculated how the heating patterns change in the sintering process. Results indicated that directpulse current conveyance by samples lead to the plasma discharge between Al grains. Andelectric breakdown occured easily along the line connecting grains center. The shorter thedistance between grains was, the easier the electric breakdown occured. Then the necks couldformed between electric particles with particles surface fusing, plenty of Al grains connectedcould be a continuous conductor, and improved the proportion of the joule heat generated bycurrent in the sintering process.The joule heat generated by pulse current would become the main heat in the sinteringprocess of SPS. According to the raw material properties parameters, equipment conditionparameters and current distribution method, the current conveyance by sample or single Al graincould be computed at different ratioes of raw materials. And the relation between the rawmaterial configuration and the current conveyance by sample or single Al grain could be workedout. Then analyzed the current control principle, speculated how the current distributioninfluence the microstructure and properties of the sintered samples. It turned out that the more the Al volume fraction was, the larger the electric current conveyance by samples, the smallerthe electric current conveyance by single Al grain. The temperature between particles would riserapidly and the necks between particles could formed dynamically in the sintering process.In the process of SPS sintering, the necks could formed between MgAlON particles withelectric current heating rapidly by discharge plasma and pulse current, the particle radius woulddecreased with the necks forming. According to the current self-regulation mechanism betweenparticles in SPS sintering process, the larger the neck size was, the less the joule heat generatedby current was, and the slower the necks growed. The grain refinement effects of MgAlONcomposites could be strengthened under high pulse electric current, and a plenty of microparticles would uniformly distributed in the sintered material.The raw materials for verification experiment was Al2O3, MgO and metallic Al, and theburden ratio of raw materials was Al2O3: MgO: Al=70:15:15(wt%). Set the sintering pressureat38Mpa, the holding time at5min, the sintering temperature at800℃,950℃,1000℃,1100℃,1200℃and1300℃, then analyzed the phase composition and microstructure of samples.Results shown that: When the sintering temperature was950℃and above, MgAlON couldformed and increased with sintering temperature increasing. MgAlON become the vast majorityphase in samples at1300℃. The SEM patterns of the sintered samples at different sinteringtemperature shown that: Some little particles could agglomerated and adhered to large particlesat950℃. Little particles agglomerated also existed at1000℃and decresed at1100℃, andsome fibrous crystallization formed between particles. So, the speculation could be comfirmedthat plasma discharge could generated between powder sample particles under the DC pulsecurrent, and the necks could formed between electric particles and become a continuousconductor in SPS sintering process. Particles distributed more uniformly and the structure wasmore compact with the sintering temperature increasing, a large number of MgAlON tinyparticles distributed uniformly in samples and the structure was quite compact at1300℃, thatechoed the grain refinement mechanism of MgAlON composites sintered by SPS based on themathematical model analysis results before. Then the simulation analysis results have a certaingaiding role for MgAlON composites sintered by SPS. |
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