| Zirconium diborides ceramic has many significant advantages such as low relative density(6.09g/cm3), high melting temperature(3245 oC), high hardness(9-22 GPa), high modulus of elasticity(410-500 GPa), high bending strength(200-375 MPa), low resistivity(7-24 ??·cm), excellent chemical stability and so on. The ZrB2-SiC composites was regarded as a very promising ultra high temperature ceramic, and was widely used in both aerospace and military field. But this composites also has a very obvious shortcoming: low fracture toughness(4-5 MPa·m1/2), this weakness also limits the further application of ZrB2-SiC composite ceramic. That is why improving fracture toughness is the major work in the research of this composites.The add of metal with high fracture toughness was a very effective way solving this problem. But the question is that high melting temperature metal is easily get reaction with ZrB2. For this reason, the effect of toughening is not so obvious. The research use Nb foil with ZrC film as a middle layer,ZrB2-SiC layer as the matrix layer, sinter both in a layer way. The ZrC film can prevent the reaction between Nb and ZrB2-SiC.Firstly, sinter ZrC target by using pure ZrC powder as raw material through FCT. in the condition of 1900 oC sintering temperature, 30 MPa sintering pressure, 5min holding time. The relative density of the ceramic reaches 96.3%.Secondly, the film was prepared by magnetron sputtering at different temperatures and at different time, and the influence of the deposition conditions on the structure of the film was studied. The deposition rate decreases with the increase of temperature, and the crystallinity of the films increases with the increase of temperature, and the density increases with the increase of temperature. The density of the film reached 74% at 500 oC.Thirdly, consider the Nb foil with ZrC film as the toughening middle layer and ZrB2 with 25vol%SiC powder as the matrix layer. Sinter these two layers in stack way by plasma active sinter. It is clear that only a little Nb remains when the sinter temperature is under 1800 oC. but the relative density of ceramic phrase is very low, that leads to the poor mechanical properties. the fracture toughness is just 7.1 MPa.m1/2. Otherwise the sinter temperature reach 1800 oC, the relative density of the ceramic phrase is nearly full density. But metal phrase remains nothing, there is almost no Nb remains. The fracture toughness reaches 6.3MPa.m1/2, the toughening effect is very little.In order to solve this situation, ZrB2-25vol%SiC composites ceramic was sintered at 1800 oC, 30 MPa, 5 min by plasma active sinter(PAS) previously, which was prepared as the matrix ceramic layer. The relative density of ZrB2-SiC composite ceramic is 98%. The fracture toughness reaches 5.8MPa·m1/2. Then the ZrB2-SiC composite ceramic and Nb foil with ZrC film were sintered in different temperatures, different pressure and different holding time by PAS. The fracture toughness is 9.7MPa·m1/2, the counterpart of Nb foil without Zr C film is only 7.7MPa·m1/2, the differences between these two samples show ZrC film plays an important role in protection. The effect of sintering temperature, sintering pressure, holding time were investigated. The best parameter is 1500 oC, 20 MPa, 3 min. This sample get the highest fracture toughness of 11.2MPa·m1/2. |
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