Study On The Fast Sintering And Densification Mechanisms Of High-melting-point Ceramics

Controlling grain growth and keeping densification during sintering is a key factor to obtain a highly dense and fine-structured ceramic with better performance.However,densification and grain growth share the same thermodynamical driving force and they compete with each other during sintering.The only way to separate the two process needs to seek dynamic differences by regulating the sintering parameters.Field-assisted sintering techniques have attracted much attention due to their superiority in fast heating,particle surface activation,flexible adjustment of processing parameters.In addition,large amounts of previous researches show the field-assisted sintering technique is favor of promoting densification and retarding grain growth,which makes field-assisted sintering suitable for the preparation of structural ceramics.Ceramic with high melting point is one of the most-hard-to-densify cermics due to its strong covalent bond,low self-diffusion coefficient and oxygen contamination in starting powders.The competition between densification and grain growth in high-melting-point ceramics is hard to be completely avoided since the grain growth is more prone to dominate the kinetics at final stage.Besides,rapid coarsening can easily produce trapped porosity and limit the final density.Therefore,it is hard to achieve dense and fine-grained high-melting-point ceramics.The present work focuses on the fast preparation of two typical high-melting-point ceramics TaC and ZrB₂ through field-assisted sintering technique.The evolution of microstructural features,densification and grain growth,as well as their mechanisms were studied during fast sintering.The sintering process was also optimized in order to provide some guidance for application.The main contribution of this work is shown below:(1)The sintering behavior of commercial submicron-scale TaC powder during spark plasma sintering(SPS)was studied.By properly choosing a specific temperature range and coupled with a relatively high pressure,a highly dense TaC ceramic(98.6%)with the average grain size of 1.48?m was prepared under 250 MPa via high pressure spark plasma sintering at 1850°C.It was suggested that to achieve full densification,one the one hand,grain boundaries(GBs)need to remain stable or migrate at a low speed,so as to avoid trapped pores produced by pore-boundary separation.One the other hand,the pressure needs to be high enough to keep the plastic deformation involved mechanisms(including GB sliding and diffusion creep)active to ensure the proceeding of densification even at final stage.(2)The densification mechanisms were further studied by comparing the microstructural features for TaC ceramics sintered at different temperatures and pressures.It was suggested that low temperature and high pressure favored the acquisition of fined-grained structure.The grain orientation observations showed no texture.The distributions of small-angle GBs and large-ange GBs for TaC ceramics prepared at different conditions were also similar.The TEM observations showd that there exists a certain amount of dislocations in all specimens.However,the TaC ceramics sintered at low temperature and low pressure seems to have lower density of dislocations.Several phenomenological analytical models were also used and compared to determine the densification and grain growth mechanisms.With the increase of pressure,the densification mechanisms at initial sintering stage changes from lattice diffusion/GB sliding to GB diffusion/GB sliding/plastic flow.The controlling densification mechanisms at final sintering stage changes from lattice diffusion to GB diffusion.At low pressure,grain growth prefers to be controlled by GB migration,while at high pressure,grain growth seems to relate to stress-enhanced dynamic grain growth by GB sliding and grain rotation.(3)The densification mechanisms of ZrB₂ ceramics were also investigated by comparing the sintering behavior of commercial staring powder,self-prepared powder by borothermal reduction,the heat-treated commercial powder.The commercial ZrB₂powder shows the best sinterability,while the grain growth at final-stage is also prominent.The rapid GB migration cannot be avoided regardless of the mechanical pressure.Nevertheless,the higher pressure still favors densification.The ZrB₂ ceramics sintered at 1900°C/200 MPa reaches the relative density of 98.0%.The self-prepared powder by borothermal reduction shows the second sinterability next to the commercial one.However,the rapid grain growth still cannot be avoided at final stage,although not so preminent compared to the aforementioned one.the heat-treated commercial powder shows the poorest sinterability,as well as the grain growth.The reason for the good sinterability of commercial ZrB₂ powder should be attributed to the ultra-fine particles within it.Compared to TaC ceramics,the mechanical pressure contributes much less to the promotion of density for ZrB₂.This could be explained by the higher GB strength of ZrB₂,which provides more drag force for grain rearrangement and GB sliding.(4)The densification behavior of high-melting-point using ultra-fast heating rate was studied by designing a novel flash spark plasma sintering(FSPS)apparatus.The apparatus is available for applying pressure during ultra-fast sintering,which allows for sintering from powder compacts without preheating,as well as promoting the densification.Taken TaC ceramics as an example,densification and grain growth behavior with different currents,pressures and loading strategies were firstly investigated.Then,by applying a pressure of 80 MPa at the intermediate sintering stage,dense TaC ceramics(95.18%)with fine-grained microstructure(4.09?m)were prepared using a current of 1250A for 90s.Our work provides an efficient method to quickly prepare refractory ceramics.The total time consumption of FSPS technique in this work only takes 5%of common SPS and 6.1%of traditional FSPS.The total energy consumption only takes 9.8%of common SPS and 18.5%of traditional FSPS.