Preparation And Mechanical Properties Of B₄C Ceramics By Hot Oscillating Pressing

As a member of special ceramics,B₄C has been widely used in various industrial fields due to its various excellent characteristics,and has a very wide application prospect.However,due to the low fracture toughness of B₄C,its further application is limited.The reason for this phenomenon is that it is very difficult for the material to be sintered to compact.Therefore,how to densify B₄C is a current research focus.In order to solve this problem,non-pressure sintering,hot pressing,hot isostatic pressing and discharge plasma sintering are adopted.Non-pressing sintering and hot pressing sintering often fail to densitize B₄C at very high temperatures.Hot isostatic pressing sintering cannot be widely used due to its complex process and expensive equipment.The structure of B₄C prepared by spark plasma sintering is not uniform and large-size samples cannot be prepared.In recent years,it has been proposed that applying dynamic alternating load in sintering process can effectively promote material densification and inhibit grain growth so as to improve material properties,but there is still a lack of relevant research on sintering B₄C with alternating load.Therefore,this paper adopts the method of hot oscillating pressing to prepare pure B₄C ceramics,introduces alternating load in its sintering process,and studies its sintering behavior and the mechanical properties of the prepared sample,to explore how the alternating load affects the microstructure and mechanical properties of B₄C.The main research work is as follows:（1）Hot oscillating pressing of boron carbide was studied and its sintering behavior was analyzed.The results show that under the same sintering conditions,the porosity and grain size of B₄C powder under alternating load are smaller than that under static load.Compared with the static load,the alternating load promotes the faster elimination of pores between powders and inhibits the growth of grains in the sintering process of B₄C,and the application of alternating load has greater advantages in the sintering process of B₄C.（2）The densification mechanism and grain growth mechanism of hot oscillating pressing B₄C were analyzed.It was found that the densification mechanism of B₄C was diffusion and grain boundary slip in the stage of rapid pore removal,and the densification mechanism of B₄C was grain boundary slip in the stage of pore removal deceleration,and the grain growth mechanism was evaporation/condensation mechanism.The activation energy of grain growth of B₄C under alternating load and static load was calculated to be 443KJ mol^-1and 395 KJ mol^-1respectively,indicating that alternating load can increase the threshold of grain growth of B₄C and inhibit the grain growth of B₄C.（3）The influence of the median pressure and frequency of alternating load on B₄C during sintering process was studied.The results show that the porosity between B₄C powders decreases with the increase of the median pressure of alternating load.The grain size increases firstly and then decreases with the increase of the median pressure.At 1800℃,the porosity of B₄C powder increases first and then decreases with the increase of alternating load frequency.At 1900℃,the porosity of B₄C powder does not change with the change of alternating load frequency.（4）Through the test of mechanical properties,microstructure analysis,the results show that the same situation in other sintering conditions,the effect of alternating load the sample on the hardness and fracture toughness is better than the static load of the specimen,the porosity of the samples and under the action of alternating load is lower and the grain size is smaller;The hardness of B₄C increases gradually with the increase of the median pressure,which is mainly caused by the decrease of porosity when the pressure is lower than 50 MPa,and the decrease of grain size when the pressure is higher than 50 MPa.The fracture toughness increases first and then decreases with the increase of the median pressure,because the porosity decreases and the grain size increases first and then decreases with the increase of the pressure.At 1800℃,the hardness and fracture toughness of B₄C first decreased and then increased with the increase of the frequency of alternating load.At1900℃,the hardness of B₄C gradually increased with the increase of the frequency of alternating load,which may be related to the smaller grain size caused by high frequency.