Spark Plasma Sintering And Strengthening And Toughening Of Silicon Carbide Based Ceramics

The SiC ceramic has excellent properties such as high hardness,high strength,and low density.It has been extensively studied in for the past few decades.It is a promising engineering material for high-temperature applications and is widely used in the fields of anti-wear parts,precision seals,and light ceramic armor.Although SiC has superior performance,the fracture toughness of SiC is low and needs to be further improved.In this research,Al₂O₃and ZrO₂ are used as composite sintering aids,SPS sintering is employed to prepare dense SiC ceramics,and toughening agents are added to improve the fracture toughness of SiC ceramics.Also,the microstructure and phase composition of SiC ceramics are analyzed with SEM and XRD technique,and the mechanical properties of ceramic samples are measured.The contents of this research are mainly divided into the following sections:（1）Explore the influence of different sintering temperatures on SiC-Al₂O₃-ZrO₂ ceramics.The XRD results of the sample show that the main phase of the sample isβ-SiC.When the sintering temperature drops to 950℃,t-ZrO₂ begins to transform into m-ZrO₂.However,when compressive stress is applied to the sample during SPS sintering,all t-ZrO₂ cannot be converted into m-ZrO₂,and part of t-ZrO₂ remains in the SiC matrix.Therefore,both of t-ZrO₂and m-ZrO₂ phases are detected in the sample.The fracture morphology of the sample shows that as the temperature increases,the content of pores in the sample gradually decreases,and the pull-out of a small amount of ZrO₂crystal grains becomes apparent.The SiC-Al₂O₃-ZrO₂ceramics prepared at 1800℃exhibit the fine and uniform crystal grains with an average grain size of 0.5μm.Comparing the overall performance of all of the samples,the SiC sample with a sintering temperature of 1800℃showed a better overall mechanical performance(H_v=20.2GPa,K_IC=4.12 MPa·m^1/2).（2）Explore the influence of ZrO₂ content on SiC-Al₂O₃-ZrO₂ ceramics.With the increase in content of zirconia,the residual stress of SiC-Al₂O₃-ZrO₂ ceramics increases,and the characteristic XRD peak intensities of t-ZrO₂ and m-ZrO₂ also increase.All samples have reached a relative density of over 94%.The relative density of the samples is the highest（the relative density is 99.5%）when the content of ZrO₂ is 5 wt.%.The fracture toughness value also reaches the highest.There is a compressive stress field around ZrO₂.When the crack propagates,it encounters ZrO₂ particles,the crack deflects,and the fracture mode then changes from transgranular fracture to intergranular fracture.The effective fracture energy of the material increases,thereby improving the brittle fracture resistance of the material.In summary,when the ZrO₂ content is 5 wt.%,SiC-Al₂O₃-ZrO₂ ceramics can obtain better mechanical properties.（3）Carbon nanotubes（CNTs）are introduced as the toughening agent of SiC-Al₂O₃-ZrO₂ceramics to improve the fracture toughness of the samples in this study.Results of XRD show All samples containβ-SiC,m-ZrO₂ and t-ZrO₂ three phases.There is no additional carbide phase,except SiC in the sample,indicating that the CNTs do not undergo a chemical reaction with matrix,which ensures the chemical stability of SPS sintered CNTs.Due to the lubricating effect of CNTs,the reduction of grain boundary diffusion activation energy and the phenomenon of grain boundary pinning caused by CNTs,the grain size of SiC matrix,in general,is reduced.When the content of CNTs is relatively small,there is little agglomeration phenomenon observed,and the CNTs can be uniformly dispersed in the matrix.Due to the existence of toughening mechanisms such as CNTs pullout phenomenon,crack deflection,and crack bridging caused by the synergistic effect of ZrO₂ and CNTs,the toughness of the sample is improved.When the content of CNTs is 0.5 wt.%,the fracture toughness of the sample is the highest,with a toughness value of 4.98 MPa·m^1/2.When the content of CNTs is less than1 wt.%,agglomeration is easy to occur,which could result in internal defects of the sample and,thus,reduce the mechanical properties of the sample.When the content of CNTs is less than 1 wt.%,the density and hardness of the sample show a downward trend.Fracture toughness and flexural strength increase first and then decrease.（4）Study the effect of different content of SiC whisker（SiC_w）on SiC-Al₂O₃-ZrO₂ceramics.The SiC_w has a hindering effect on the grain growth of SiC.With the addition of SiC_w,the whisker was pulled out of the sample along the crack path.Since the thermal expansion coefficient of ZrO₂ is greater than that of SiC_w,part of the zirconia is subjected to the compressive stress of the sample matrix,and the t-ZrO₂ cannot be completely transformed into m-ZrO₂,and part of the t-ZrO₂ remains in the SiC matrix.With the increase of SiC_wcontent,The diffraction peaks with a diffraction angle of 49°-51°tend to move to the right.The increase of SiC_w content will cause more compressive stress and inhibit the phase transition of t-ZrO₂,so that the content of t-ZrO₂ will increase with the increase in SiC_w content.The SiC_w hinders the ability of SiC atoms to diffuse.In the later stage of sintering,the rearrangement ability of SiC grains is reduced,resulting in a decrease in the density of the sample.With the increase of SiC_w,the density of the sample shows a downward trend.In terms of fracture toughness and flexural strength,the samples showed a tendency to increase first and then decrease.When the SiC_wcontent is 25wt.%,fracture toughness and flexural strength is 5.35 MPa·m^1/2 and 650 MPa.When the crack encounters SiC_w,significant deflection occurs,accompanied by the phenomenon of ZrO₂ surrounding bridging.Therefore,the addition of SiC_w plays a synergistic effect of whisker toughening and ZrO₂phase transformation to improve fracture toughness.