Sintering Process And Densification Mechanism Of The Novel SiC/Ti Gradient Material

SiC/Ti lamellar composite,exhibiting the high ductility and the favorable thermal conductivity of Ti,as well as the high hardness,the excellent wear resistance and the thermal insulation performance of SiC,shows a bright prospect in its application.To reduce the stress concentration on the interface of the SiC/Ti lamellar composite,the compositions of SiC and Ti were designed through the gradient transition.In the present study,SiC/Ti gradient materials were fabricated by the hot press sintering process to address the problems,e.g.,the difference of sintering temperature between SiC and Ti and the stress concentration on the interface.First,the sintering temperature of SiC was reduced to comply with that of Ti.Subsequently,the proper component and gradient were clarified to fabricate SiC/Ti gradient material.Moreover,an investigation was conducted on the sintering mechanism and the sintering kinetics.To down-regulate the high sintering temperature of SiC,Mg alloy additive was added to conduct the liquid sintering for SiC.In addition,this study clarified the effect of Mg element on the reduction of the sintering temperature of SiC,and the SiC ceramic was sintered based on the densification at 1300?.An investigation was conducted on the effect of the initial particle size,the sintering additive and the holding time on the densification of SiC.Moreover,the SiC ceramic exhibiting the relative density over 90%and the prominent mechanical properties was fabricated.Furthermore,this study clarified the dissolution-precipitation and plastic deformation processes of Mg during the sintering process.The pores located on the grain boundaries were suggested to be capable of facilitating the motion of grain boundary and the mass transfer.To conduct the solid sintering of SiC at lower temperatures,Mg2Si was added to introduce the Mg element into SiC.The SiC ceramic was sintered based on the densification at 1300?,which exhibited a high relative density and the prominent mechanical properties.In addition,the microstructure evolutions of SiC and Mg2Si were investigated to reveal the effect of Mg2Si on the low-temperature sintering of SiC.Under the sintering time of 2 h?4 h,the SiC ceramic could exhibit the relative density over 96.8%,as well as the excellent mechanical properties.During the sintering process,the Mg2Si kept a random orientation,while rare misorientation was identified between(0-111)sic and(1-11)Mg2Si,(10-10)sic and(1-1-1)Mg2Si,(10-10)sic and(400)Mg2Si,(10-12)sic and(222)Mg2Si,(10-11)sic and(311)Mg2Si formed on the interface of SiC and Mg2Si.The densification rate decreased with the prolongation of sintering time,and the grain growth was identified during the final sintering between 2 and 4 h.The SiC/Ti gradient material without any transitional composition was fabricated,and the microstructure and densification mechanism were elucidated.The thickness and relative density of the respective layer were not found to be uniform.The layers containing only SiC or Ti were dense,and more pores were formed on the layers containing 20%and 40%Ti.With the increase in Ti,the quantity and area of pores decreased.At the mass ratio of Ti reaching 80%,the pores were formed on the boundaries of SiC.The densification process primarily took place during the heating and holding processes.Moreover,the reaction between SiC and Ti was largely based on the lamellar diffusion.Furthermore,the metallurgical combined interface between SiC and Ti was depended on the diffusion without any intermetallic layer.The novel Ni3Al additive was added to SiC to improve the relative density of low-temperature sintered SiC ceramics,and the reaction mechanism was clarified.The A1 element exhibited the uniform diffusion into the SiC matrix,and the Ni-rich regions were located on the SiC matrix.At the mass ratio of Ni3Al reaching 10%,the SiC ceramic could exhibit the optimal comprehensive properties.The microhardness was improved by 174.55%,and the fracture toughness was improved by 27.29%.The SiC particles were bonded closely,which could prevent the cracks during the bending process.The prolongation of the cracks introduced by the microhardness indentation could be prevented by the Ni-rich regions.The reaction products,the mechanisms of pores formation and reaction were clarified.The Ni3Al reacted to the Ti component.As a result,the fine concentrated pores,the fine distributed pores and the larger pores were formed.The fine concentrated pores were located on the regions with high Ni3Al,which was attributed to the uncompleted sintering.The fine distributed pores and larger pores were voids during the sintering,and some Ti-rich particles with tens of microns precipitated from the large pores.Through the reaction of Ni3Al and Ti,Ti-Ni-Al,Ti-Al and a small number of Ni-Ti phases were formed.The Ti-Ni-Al was formed initially with the decrease in the volume ratio,while the volume ratio of Ti-Al phase was elevated with the rise of the temperature.The SiC/Ti gradient material with Ni3Al transitional composition was fabricated,and the microstructure and the densification mechanism were clarified.The Ti and Si elements were distributed in a gradient manner,while the Ni and Al elements prevented the direct compact between Si and Ti elements.The four interfaces in the SiC/Ti gradient material were formed by the solid-liquid interface between Ti and Ni-Al,the difference of Ni and Al,the solid-liquid interface between Ni-Al and SiC,as well as the difference of Ni,Al and Si,respectively.The densification process,with the activation energy of 184.6 kJ/mol,was controlled by the lamellar diffusion.To separate the SiC and Ti compositions,TiAl layer was added to the SiC/Ti gradient material with Ni3Al transitional composition.The intermetallic layer was formed on the interface between Ni3Al and TiAl,while the elements were diffused uniformly on the interface between TiAl and Ti.The densification rate was kept in a low level initially and increased significantly with the prolonging of the sintering time.The densification process,with the activation energy of 338.28 kJ/mol,was largely controlled by diffusion,which was combined with interface reaction.