| Recently,hypersonic aircraft has become a research hotspot due to its great military value.The hypersonic aircraft will suffer intense aerodynamic heating effect and the temperature of the surface can reach to 2000 °C when it flights through atmosphere layer.Traditional thermal protection material can hardly be used in such harsh working conditions.ZrB2 ceramic can be used in thermal protection system of hypersonic aircraft due to its combination of high melting point,low density,excellent oxidation and ablation resistance,etc.However,the use of ZrB2 ceramic is seriously limited by its poor sintering behavior and low fracture toughness.To solve the problems mentioned above,the addition of sintering aids and short fiber into the matrix of ZrB2 to fabricate ZrB2-based ceramics has become one of the most effective method in the current research state.However,there are three common problems during the hot pressing of shrot fiber-toughened ZrB2-based ceramics.Firstly,the required sintering temperature is too high even though with the presence of sintering aids.Hence,a further optimization of sintering aids is quite necessary.Secondly,the added fiber tend to react with matrix during sintering process and the reaction will become more serious with the increase of sintering temperature and hold time.In addition,the added short fibers tend to form fiber aggregations during the dispersing process.The dipersity of short fibers in matix is very important which can influence the microstructure and mechanical peoperties of hot-pressed composites.Therefore,the research of low temperature fabrication and toughening mechanism of fiber-toughened ZrB2-based ceramics is extremely important to improve the service performance.To reduce sintering temperation and realize fast densification of ZrB2,ZrSi2 was used as sintering aids in this paper.Firstly,the sintering aids and sintering temperature were optimized through the adjustment of sintering aids content and sintering temperature.Then,short carbon fibers(Csf)were used as toughening phase to prepare CSf/ZrB2-ZrSi2 ceramic matrix composites.The fracture toughness of Csf/ZrB2-ZrSi2 was improved through the optimization of fiber content,fiber surface activity and fiber/matrix interface.The main research contents and results are summarized as follows:(1)The sintering temperature and hold time were both reduced through the addition of ZrSi2 as sintering aids.The densification behavior of ZrB2-ZrSi2 was influenced by ZrSi2 content and sintering temperature.Firstly,densification speed became faster and onset temperature of densification became lower with the increase of ZrSi2 content.Secondly,densification speed became faster and densification process tend to complete with the increase of sintering temperature.ZrB2-ZrSi2 ceramics can reach to a high relative density of?99%through hot pressing at 1550 ? when 20 vol%ZrSi2 was added.(2)The fracture toughness of ZrB2-ZrSi2 was improved through the addition of Csf as toughening phase.The added fiber can be dispersed homogeneously in the matrix of ZrB2-ZrSi2 when the content of fiber was 5,10 and 20 vol%.However,serious fiber aggregations emerged as the additive amount reached 30 vol%,which had an adverse effect on the properties of the sintered samples.In general,Csf/ZrB2-ZrSi2 composites show the highest fracture toughness of 6.89±0.16 MPa·m1/2 when 20 vol%Csf was added,which increased by?55%compared with ZrB2-ZrSi2.Additionally,during the fabrication of Csf/ZrB2-ZrSi2,fiber suffered abrasion damage among ball milling procedure and underwent chemical corrosion among hot pressing procedure.It was found that the properties of fiber was degenerated due to the abrasion damage and chemical corrosion,which could limit the toughening effect of the added fibers.(3)The surface activity of fiber was reduced through high temperature treatment,and the chemical corrosion of fiber during hot pressing was obviously restrained.When heat treated under the condition of 2000 ?/N2/60 min,the surface activity of fiber was reduced due to the improved high temperature thermal stability,high graphitization degree,low oxygen-contening functional groups and high C/O.As a result,the fracture toughness of Csf/ZrB2-ZrSi2 was measured to be 7.62 ± 0.12 MPa·m1/2 which was 11%higher than the composites reinforced by as-received fibers.(4)In order to reduce the abrasion damage of fiber caused by ball milling,the dispersion of short fiber was accomplished through a novel colloidal dispersion method.Results found that the colloidal dispersion can not only disperse fiber homogeneously in the matrix of composites,but also improve the length maintenance of fiber after sintering.In addition,phenolic resin was introduced into the matrix by colloidal dispersion and phenolic resin could transform into pyrolytic carbon during sintering.As a result,the Csf/ZrB2-ZrSi2 which was processed by colloidal dispersion showed the following advantages:1)the compositions of the matrix changed from ZrB2+ZrSi2 into ZrB2+ZrC+SiC which is conducive to high temperature mechanical properties;2)the corrosion of fiber was further inhibited and fiber retained its original cross-section shape and size after sintering;3)the relative density was further improved and the grain dimension of ZrB2 was reduced from micro-size to nano-size;4)the fiber/matrix interface bonding was released and the work of fracture was measured to be 509± 18 J/m2. |
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