| Ceramic-metal composites have received increasing attention because of the combination of elevated strength and toughness that can't be obtained in single-phase materials.The most important factor in the manufacture of ceramic-metal composites is the compatibility of ceramic and metal.Mullite?3Al2O3·2SiO2?and molybdenum both have high melting point and excellent high-temperature properties,so they have been widely used in high-temperature fields.Especially,they show outstanding physical and chemical compatibilities:?i?they have similar thermal expansion coefficients(?Mull=5.13×10-6°C-1,?Mo=5.75×10-6°C-1,at 1000°C),and thus the residual stresses due to thermal expansion mismatch are expected to be small;?ii?mullite and Mo are compatible up to 1650°C without generating new phases at a range of p?O2?.As a result,mullite-Mo composites constitute a highly preferred structural material and have been studied extensively,especially,with regard to the toughening mechanism.However,the resaerch on low-temperature sintering and microstructure control of mullite-Mo composites have not been reported widely.This work synthesized mullite gels by a sol-gel process,and studied the influence of mullite precursors with different phases and morphologies on the sintering ability and microstructure of mullite-Mo composites by using PAS.The result shows that homogeneous precursor with a granular morphology can be synthesized when gel is heated by SPS at 1000°C first,then ball mills.This precursor contains Si-Al spinel but no mullite phase and shows an excellent sintering activity.Mullite-Mo composites obtained from this precursor have a more uniform microstructure.By analysing sintering shrinkage curves of 1500°C,a two-stage sintering process was designed to enhance viscous flow sintering of amorphous SiO2 which happens at relative low temperature.When the holding temperature at the first stage is 1060°C,the relative density of composite sintered at 1350°C reaches 99.15%.Mullite-Mo composites produced by this process obtain a homogeneous microstructure,and the average size of mullite grain is about 300 nm.By heating composites with different densities in air,the result shows Mo containing in the surface of composites begins to oxidize at about 400°C.At 800°C,mass of composites decreases because of the sublimation of MoO3,and relative change of mass is related to the density of composite.Composites with high relative density show a smaller mass change.At 1200°C,nearly all Mo that contains in composites disappears because of the fast oxidation and sublimation.The hardness of mullite-Mo composites is deeply dependent on relative density.The diffusion bonding between mullite and Mo was also conductd by PAS in this study.The inflence of such factors whether interlayers are used,the composition of interlayers and bonding temperature on the joint strength and thermal shock resistance has been studied.Result shows the good joint of mullite and Mo cannot obtain without interlayer even at 1600°C.Flaws appear at the interface.The introduction of interlayers and two-stage sintering process achieves good joint at1350°C.Shear strength of joint is control by temperature and interlayer.The highest shear strength reaches 61.3MPa when a linear interlayer is used at 1350°C.The mode of crack propagation is different by using different interlayers.When the interlayer is molybdenum-rich cracks are more inclined to diffuse in mullite matrix,and it's disadvantageous to increase shear strength of joint.Compared with 1450°C,thermal shock resistance of the joint bonded at 1350°C increase to some extent. |
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