The Structure Design And Thermal Shock Resistance Of Mo-SiBCN Gradient Composites

Mo-SiBCN gradient materials and monolayer Mo-SiBCN composites were prepared by hot pressing. Phases, microstructures, influences of sintering parameters on the residual stress of Mo-SiBCN materials were studied. The influences of chemical composition on the mechanical properties and thermal expansion coefficient of monolayer Mo-SiBCN were investigated. The influences of the incorporation of MAS on the phases, microstructures and mechanical properties were also examined.The results show that higher sintering temperatures lead to high density, good interface bonding and low residual stress in Mo-SiBCN gradient materials. Sintering temperatures, pressures and dwell time have little influence on the phase types. Mo-SiBCN gradient materials consist mainly of Mo4.8Si3C6, Mo C, Mo2 C, Si C, Mo Si2, Mo5Si3 and BCN. At 1800 ?C, higher sintering pressures and longer dwell time lead to more Mo4.8Si3C6. Higher sintering temperatures are beneficial to the formation of Mo C, Mo2 C and Si C. Additionally, longer dwell time cannot change significantly the relative content of Mo C, Mo2 C and Si C.With the increasing volume fraction of SiBCN, the phase composition of hot pressed monolayer Mo-SiBCN composites show regular changes. Mo can react completely with SiBCN in Mo-30%SiBCN composites. Higher content of SiBCN results in the change of phase types. Mo reacts with C generating Mo C and Mo2 C. With the content of SiBCN increasing from 40 vol% to 70 vol%, Mo reacting with Si produces mainly Mo3 Si,(Mo3Si+Mo5Si3+Mo Si2),(Mo4.8Si3C6+Mo5Si3+Mo Si2), and Mo4.8Si3C6.Increases in volume fraction of SiBCN lead to decreases in density, elastic modulus and flexural strength. The increasing content of SiBCN from 10 vol% to 30 vol% leads to increases in hardness, while higher volume fraction than 40 vol% causes decreases in hardness. The coefficient of thermal expansion(CTE) shows a change similar to the hardness change. Incorporation of MAS improves the density, flexural strength and elastic modulus of Mo-90%SiBCN and SiBCN materials, decreases CTE of Mo-90%SiBCN composites, but influences very slightly CTE of SiBCN ceramic.Thermal shocks increase the flexural strength of Mo-10%SiBCN and Mo-20%SiBCN. Higher thermal shock temperatures lead to increases in residual flexural strength. Microcracks appear often at pores, micopor es and interfaces where the residual thermal stress higher than intrinsic strength of materials induces formation of cracks.