Fabrication And Properties Of Laminated ZrB₂-Mo₅SiB₂ Ultra-High Temperature Ceramics

ZrB₂ ceramics become one of the top candidates for non-ablative ultra high temperature thermal protection material due to outstanding properties,such as high melting point,high strength,high hardness,good thermal conductivity and so on.However,their application and development are limited by the intrinsic brittleness and low oxidation resistance.In this paper,in order to improve the poor toughness and oxidation resistance of ZrB₂ ceramics,laminated ZrB₂–Mo₅SiB₂ ultra-high temperature ceramics were designed and prepared.The effects of the processing parameters,microstructure and layer thickness ratio on the strength,toughness and oxidation resistance have been studied.The toughening mechanism of the laminated material has been also discussed.The main results were as follows:（1）The pure Mo₅SiB₂ powder was firstly synthesized.In the tape casing,anhydrous ethanol,polyethylene glycol（PEG）,polyvinyl butyral（PVB）and tricresyl phosphate（TCP）were selected as the solvent,the dispersant,the binder and the plasticiser,repectively.In order to obtain a good-dispersed,low-viscosity,well-moulded slurry,the optimum content of PEG,PVB and TCP were 0.2 wt.%,4 wt.%and 4 wt.%of the solid phase,respectively.（2）The ZrB₂–Mo₅SiB₂ matrix sheet was prepared by tape casting,then Mo–Mo₅SiB₂ was coated on the matrix sheet.The layer thickness ratio between the matrix layer and the interlayer could be adjusted via controlling the coating amount.The laminated ceramics were finally obtained by hot pressing sintered at 1900°C for 2 h.The sample with layer thickness ratio of 13 had the best flexural strength（431.6±15.1MPa）and fracture toughness(9.89±0.26 MPa·m^1/2).The toughening mechanism of the laminated ceramics may come from crack deflection and bifurcation caused by residual stresses between the matrix layer and the interlayer.（3）ZrB₂–SiC–Mo₅SiB₂ composites were prepared by spark plasma sintering at1900°C for 10 min.The mechanical and oxidation resistance properties of the ZrB₂–Mo₅SiB₂ layer was obviously improved by the addition of SiC particle.The density,hardness,flexural strength,fracture toughness of the composites was up to 99.48%,19.87±0.93 GPa,755.1±62.8 MPa and 8.01±0.48 MPa·m^1/2,respectively.This phenomenon may have a relationship with the high relative density of the composites and the uniform distribution of fine SiC particles.The oxidation of the composites was also improved.In the antioxidation experiment,when the testing temperature was below 1200 ^oC,a liquid layer could be formed by Si O₂ and B₂O₃,which were the oxidation products of Mo₅SiB₂.When the testing temperature was above 1200 ^oC,dense Si O₂ layer was formed due to the oxidation of SiC.（4）The ZrB₂–SiC–Mo₅SiB₂ matrix sheet was prepared by tape casting,then Mo–Mo₅SiB₂ was coated on the matrix sheet.The laminated ceramics were sucessfully fabricated by SPS at 1900 ^oC for 10 min.The specimen with layer thickness ratio of 9exhibited the highest flexural strength（494.2±13.2 MPa）and fracture toughness(11.92±0.29 MPa·m^1/2).The improved toughness may have a relationship with crack deflection,bifurcation and particle pull-out induced by residual stress,microcracking and SiC.（5）Rod–shaped ZrB₂ powders were synthesised by molten salt assisted carbon thermal reduction.The molar ratio of Zr OCl₂–8H₂O,B₄C and C was 2:1.2:3.Independently dispersed rod–shaped ZrB₂ with 0.2～1μm in diameter,10～40μm in length and up to 200 in aspect ratio were sucessfully prepared by heat treatment to 1500^oC for 60 min.Laminated ZrB₂–SiC–Mo₅SiB₂/Mo–Mo₅SiB₂ ceramics with layer thickness ratio of 9 were fabricated by hot pressing sintering at 1900°C.The best performance was obtained for the specimen held for 120 min,with a flexural strength of 545.67±10.4 MPa and fracture toughness of 12.03±0.31 MPa·m^1/2.Besides crack deflection and bifurcation caused by interlayer residual stresses,the toughening mechanisms of the composites may be explained by reinforcement of crack deflection,bridging and pull-out by large aspect ratio ZrB₂ particles.