| Polycrystalline yttrium aluminum garnet?Y3Al5O12,YAG?ceramic has been studied for many years and is widely known as an important solid-state laser host material.In addition to this,due to its high melting point,high hardness,low creep rate,low oxygen diffusivity as well as excellent chemical and phase stability at high temperature,YAG is also a desirable candidate for high temperature structural applications.However,the low fracture toughness of monolithic YAG ceramic is the vital factor that hinders its practical application as structural material.To improve the fracture toughness of monolithic YAG ceramic,three-dimensional carbon fiber preform was employed as reinforcement,and the C/YAG composites were fabricated through“vacuum impregnation-drying-heat treatment”?SIDH?route,using the Y2O3-Al2O3 sol with a high solid content as starting material.The properties of sol were studied firstly.In order to vertify the feasibility of SIDH route,the laminated and stitched carbon fiber cloth reinforced YAG composite was prepared by the SIDH route.From the application point of view,low cost 3D carbon fiber needled felt reinforced YAG?3DN C/YAG?composite by SIDH route was employed as main subject for research,and its properties and failure behaviors in simulative environment were investigated.In addition,the effects of interfacial coatings on the properties of 3DN C/YAG composite were studied.The synthesized Y2O3-Al2O3 sol is a satisfying starting material for SIDH route because it has high stability,low viscosity,nano-sized colloid particle and a high ceramic yield of 23.1%.When heat treatment was conducted at 1000?C,the amorphous Y2O3 and Al2O3 were entirly transformed into YAG.Moreover,the YAG phase was well developed and showed high sintering activity with increasing temperature.The linear shrinkage of pressed wafer was exhibited as a ladder-type growth from 800?C to1800?C.An linear shrinkage of 23.5%was obtained by increasing temperature to1600?C.The laminated and stitched carbon fiber cloth reinforced YAG composites showed a flexural strength of 294.6 MPa and a fracture toughness of 9.3 MPa?m1/2.The flexural strength was comparable to the full dense monolithic YAG ceramics.The fracture toughness was increased by 4?5 times.The 3DN C/YAG composite showed a total porosity of 10.8%.In addition,the flexural strength,the fracture toughness and the fracture work were 113.5MPa,4.5MPa·m1/2/2 and 2280J/m2,respectively.The microstructure and mechanical properties of as-fabricated 3DN C/YAG composite were mainly decided by the structural characteristics of 3D carbon fiber needled felt as well as the thermal mismatch between carbon fiber and YAG.Thanks to the addition of 3D carbon fiber needled felt,the as-fabricated 3DN C/YAG composite showed non-catastrophic fracture behavior with a fracture toughness which was twice more than that of monolithic YAG ceramic.The thermal-physical properties and thermal conductivity mechanism of 3DN C/YAG composite were studied from RT to 1300?C.The results showed that the composite had a medium thermal expansion coefficient,which was 2?8×10-6K-1.Thermal diffusivity decreased exponentially with increasing temperature.The thermal conductivity of 3DN C/YAG composite was 5.4-3.14W/?m·K?,which was very similar to the value of YAG ceramic.The thermal stability,oxidation resistance and thermal stock resistance of 3DN C/YAG composite were investigated.The 3DN C/YAG composite could be chemically stable up to 1800?C under inert atmosphere.However,depending on the evolution of sintering state of matrix and physical bonding of interface,original flexural strength and fracture work were decreased by 14.2%and 14.3%at 1700?C and 26.4%and 90.6%at1800?C,respectively.After oxidation at 1200?C,1400?C and 1600?C for 30 min,the3DN C/YAG composite retained 83.3%,77.1%and 67.8%of original flexural strength,respectively,which was decided by the oxidation behavior of carbon fibers.The composite had an undesirable thermal stock resistance.After 10 thermal cycles,the strength retention rates of 3DN C/YAG composites at 1200?C and 1400?C were only34.7%and 30.8%,respectively.The poor thermal shock resistance of uncoated 3DN C/YAG composites is mainly due to the coupling of oxidation and thermal stress caused by thermal alternation.A layer of coating with a thickness of about 10?m was prepared on the surface of 3DN C/YAG composites by introducing silica sol in the infiltration process.The introduction of the coating effectively reduced the formation of cracks on the surface,inhibited oxygen diffusion and thus improved the thermal shock resistance of 3DN C/YAG composite.After thermal shock under the same conditions,the strength retention rates were improved to 66.7%and 60.2%,respectively.The interfacial coatings including PyC,SiC and PyC-SiC were introduced and their effects on the properties of 3DN C/YAG composite were studied.The mechanical properties of 3DN C/YAG composite at room temperature could be enhanced by the interfacial coatings.Thereinto,the PyC coating showed the best effects.The flexural strength,fracture toughness and fracture work were increased by 23%,33%and 65%,respectively,by the addition of PyC interfacial coating.At the same time,interfacial coatings were beneficial to the thermal stability of 3DN C/YAG composite at 1700?C.In the same way,the PyC coating showed the best effect.However,due to their deficient thermal stability at 1800?C,the interfacial coatings had negative influence on the thermal stability of 3DN C/YAG composite at 1800?C.The interfacial coatings played a key role in alleviating the thermal stress derived from the thermal mismatch between carbon fiber and YAG matrix.At the same time,the interfacial coatings can protect carbon fibers from oxidation to a certain degree.Accordingly,the 3DN C/YAG composites with interfacial coatings exhibited superior oxidation resistance and thermal shock resistance.Among the four interfacial coatings,PyC-SiC coating showed the best effects.No strength loss was observed for C/PyC-Si C/YAG composite after oxidation at 1200?C for 30min.After oxidation at1400?C and 1600?C for 30min,the strength losses were only?10%.The C/PyC-Si C/YAG composite retained 50.6%of original strength after 10 cycles of thermal shock from 1200?C to room temperature,which was higher than that?43.0%?of the C/YAG composite without interfacial coating. |
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