SPS Fabrication Of ZrB₂-based Ceramics Toughened By Zr_xAl_yC_z Compounds And Evaluation Of Their Properties

Zirconium diborides (ZrB2)-based composites have significant applications in aerospace, military and other fields due to their extremely high melting point, excel-lent oxidation resistance, low theoretical density and other advantages at high tem-peratures. However, the low toughness has restrained further development and ap-plication of ZrB2-based composites in ultra high-temperature ceramics (UHTCs). How to improve the fracture toughness of ZrB2-based ceramics, which already be-came hot point for researchers of various countries. In this paper, ZrxAlyCz layered compounds were used to improve the fracture toughness of ZrB2-based UHTCs through controlling the size and distribution of ZrxAlyCz, which is resulted from the design of adding manner.Firstly, the Zr2Al4C5composite was synthesized successfully by an in situ reac-tive spark plasma sintering (SPS) process at1800℃. The Zr2Al4C5composite to be synthesized successfully by4wt0Si and/or a certain amount of SiC. The reason is that the Si solved into the Zr2Al4C5composite, which can steady crystal lattice.On the base of the above work, the ZrB2/ZrxAlyCz and ZrB2/SiC/ZrxAlyCz mul-ti-phase ceramics with different volume contents of ZrxAlyCz were synthesized by SPS at1800℃. The adding of ZrxAlyCz not only improved sintering properties but also inhibited the coarsening of grains. The fracture toughness of ZrB2/ZrxAlyCz mul-tiphase ceramics with40vol%ZrxAlyCz is3.22MPa·m1/2, which is70%higher than the monolithic ZrB2ceramic. The fracture toughness of ZrB2/SiC/ZrxAlyCz multi-phase ceramics with30vol%ZrxAlyCz is4.81MPa-m1/2, which is20%higher than the ZrB2/ZrxAlyCz multiphase ceramic. The addition of30vol%ZrxAlyCz promoted the formation of an Al-B-O glass layer on exposed surfaces of ZrB2and reduced the oxidation rate of ZrB2surface. The oxidation resistance of ZrB2/SiC/ZrxAlyCz multi-phase ceramics was improved with20vol%SiC because the denser SiO2, ZrSiO4, Al-B-O and Al-Si-O glass phase around the surface can resist oxidation.The ZrB2/ZrxAlyCz and ZrB2/SiC/ZrxAlyC2multiphase ceramics with different volume contents of ZrxAlyCz were fabricated successfully by an in situ reactive spark plasma sintering process at1800℃. The in situ ZrxAlyCz composite not only improved sintering properties but also inhibited the coarsening of grains. The high aspect ratio and uniform disperse of ZrxAlyCz play an important role in the im-provement of fracture toughness for ZrB2-based ceramics. The optimal condition is at1800℃and under20MPa for fabricating ZrB2/SiC/ZrxAlyCz multiphase ceram-ics. The fracture toughness of the multiphase ceramic with30vol%ZrxAlyCz reach-es5.26MPa-m1/2, which is30%higher than the ZrB2/SiC ceramic and10%higher than the sample with direct mixture of ZrxAlyCz.Further studies showed that control the structure of ZrxAlyCz have effect on the fracture toughness of multiphase ceramics. Firstly, the Zr/Al@ZrB2powders were prepared by high-energy ball milling with Zr、Al and ZrB2powder, and the mixture of the milled powders and C (SiC) were used to prepare ZrB2/ZrxAlyCz and ZrB2/SiC/ZrxAlyCz multiphase ceramics with coating structure by SPS at1800℃. When the content of in situ ZrxAlyCz is30vol%, the fracture toughness reaches the maximum of5.96MPa·m1/2. The fracture toughness value of multiphase ceramic with coating structure is higher than the samples with direct mixture of ZrxAlyCz and without coating structure. In addition, when the Al was added in raw materials it is easier to obtain powders with a coating structure. As the addition of Al increased, the oxidation of coated powders was reduced. As the milling time increased, the powders were refined and the oxidation was more serious. So, the optimal time for milling is4hours and the milling powders are Zr, Al and ZrB2powder.