Microstructure And Properties Of3Y-ZrO₂Fibers Reinforced ZrB₂Ultra High Temperature Ceramics

Ultrahigh temperature ceramics （UHTCs） possess physical-chemical stability inultra-high temperature environment （>2000℃） and reaction atmosphere （atomic oxygenand plasma）. As a new potential candidate for high temperature structural applications,ZrB₂-based ultra-high temperature ceramics （UHTCs） exhibit unique properties, such ashigh melting point, high strength, as well as great thermal shock resistance. In this study,ZrO_2fiber was used as toughening phase in ZrB₂-based ceramics in order to improvethe low toughness and unsatisfactory thermal shock resistance, since ZrO_2fiber hadboth the virtue of fiber-toughening and phase transformation toughening. Highly denseand good-properties ZrB₂-based ceramics toughened by ZrO_2fibers were prepared byhot-pressing. The effect of each factor on the microstructure and mechanical propertiesof the final bulk was thoroughly investigated to find the most suitable preparationprocess, including milling time, sintering rate, sintering temperatures, dwelling time,morphology of raw powders and constituents. By means of X-ray diffraction （XRD）,scanning electron microscopy （SEM） transmission electron microscopy （TEM）, andelectron backscatter diffraction （EBSD） with simultaneous chemical analysis by energydispersive spectroscopy （EDS）, the microstructure were investigated. Based on fracturemechanics theory, the damage resistance and R-curves behavior were evaluated by usingthe indentation-strength in bending （ISB） technique, and compared with the envelopemethod, the difference between the two being analyzed. With the combination ofanalysis of microstructure, the strengthening and toughening mechanism were alsodiscussed. In addition, self-crack healing and pre-oxidation were further presented bymeans of muffle furnace.Introduction of ZrO₂fibers was significantly beneficial for the restriction of graingrowth of ZrB₂during densification. However, in order to obtain nearly full densematerial, higher sintering temperature and longer dwelling time were needed whichwould cause degradation of ZrO₂fibers and was also harmful to the mechanicalproperties. The optimum proportion of ceramic materials was studied, both SiC particleor SiC whisker and ZrO₂fiber were added into ZrB₂matrix ceramics. The preparationprocess was studied, the production of a homogeneous ceramic could be successfullyachieved by using a combination of20h milling time and hot-pressing at1850℃for1hunder a uniaxial load of30MPa with heating velocity of3.75℃·min^-1and coolingvelocity of1.67°C·min^-1. The flexural strength and fracture toughness of thehot-pressed ZrB₂-20vol.%SiC_p-15vol.%ZrO_2f（Z20S_p15Z_f） and ZrB₂-20vol.%SiC_w- -15vol.%ZrO_2f（Z20S_w15Z_f） ceramics reached1084MPa,680MP and6.8MPa·m1/2,8.0MPa·m1/2, respectively. And the flexural strengths after testing at900℃were604MPa and724MPa, respectively.A transmission electron microscopy study on the characteristics of grain boundaryof ZrB₂-SiC-ZrO_2fceramics was presented. The contour of grain boundary ofZ20S_p15Z_fceramic was illegible, which was attributed to the weak reaction of SiCparticle and ZrO_2fiber. On the contrary, no reaction between the SiC whisker and ZrO₂fiber was existed in Z20S_w15Z_fceramic. The analysis of single crystal electrondiffraction pattern was presented, showing that the ZrO₂in fibers was of the tetragonalcrystalline structure, and the phase relationship between tetragonal and monoclinicphase was consistent to the crystal orientation relationship of100m//100t，001m//001t. The existence of tetragonal phase was the necessary condition fortransformation toughening. In addition, the orientation distribution of ZrO_2fibers inhot-pressed ZrB₂-SiC-ZrO_2fexhibited the same preferred orientation obtained from themeasured data of electron backscatter diffraction. The distribution of ZrB₂and SiCparticles in Z20S_p15Z_fceramic was random, which could reach the macro isotropy. Butthe appearance of preferred orientation was found in Z20S_w15Z_fceramic, which wouldshow the macro anisotropy. The low energy boundaries related to mechanical propertiesof materials, such as low angle and low ΣCSL grain boundaries, were found in bothmaterials.In order to predict the instability of ceramics, characterizing and understandingcrack resistance curve （R-curve） behavior are very important. The traditional ceramicswithout R-curve behavior exhibit a large strength scatter. In this study, R-curves forZrB₂-SiC-ZrO_2fceramics were evaluated by using the indentation-strength in bending（ISB） technique and the envelope method. The analytical results revealed thatZrB₂-based ceramics toughened by ZrO_2fibers provided obvious R-curve. Based on thetheoretic models for toughening mechanisms, quantification of every tougheningmechanism was analyzed, and the total fracture toughness （Ktot） was calculated by usingthe linear superposition method. Compared to Z20S_p15Z_fceramics, Z20S_w15Z_fceramics had higher fracture toughness, which attributed to improvements oftoughening effects of crack deflection （97%）, crack bridging （5%）, fiber pull-out （26%）and transformation toughening （15%）. The introduction of SiC whisker was benefit tothe crack deflection and also had promotion impact to other toughening mechanisms.Self-crack healing and preoxidation treatment of ZrB₂-SiC-ZrO_2fceramics weresystematically studied. The oxidized sensitivity of Z20S_p15Z_fand Z20S_w15Z_fwere different. The initial oxidation temperature of Z20S_p15Z_fceramic was800℃, while thatof Z20S_w15Z_fwas1000℃, and the oxidation rate of Z20S_w15Z_fwas lower than that ofZ20S_w15Z_fceramic. Overquick oxidation rate would produce too much surface oxide,which was harmful to the improvement of mechanical properties. Flexural strength ofZ20S_w15Z_fwas greatly improved to757±23MPa after preoxidation at800℃for30min in air, which was improved by11%compared to the original strength. The effect ofpreoxidation on the mechanical properties was investigated. Microstructure analysissuggested that less oxide was produced because of low oxidation rate, and the formedoxide could heal and blunt the surface defects i.e. microcraks, porosities, which wasalso benefit for self-crack healing. The optimum healing condition for both Z20S_p15Z_fand Z20S_w15Z_fwas treated at1200℃for30min, and the flexural strength wasimproved to630±51MPa and632±29MPa, respectively.