Study On The Preparation And Properties Of Glass Matrix Composites With Low-expansion And High Mechanical Strength

With the development of science and technology,especially the rapid development of national defense,aerospace and automobile industry,the demand for comprehensive performance of materials is also increasing.It is not only required that the materials have higher mechanical strength,but also relatively small density,lower coefficient of thermal expansion?CTE?,and better thermal shock resistance in order to use these materials in the environment with a large temperature difference.At present,the preparation of these materials which applied in these fields is still dominated by metal matrix composites and ceramic matrix composites.However,because the thermal expansion coefficient of metal are high,and its synthesis temperature is high,when the material with negative thermal expansion coefficient is introduced to adjust metal's thermal expansion coefficient,it will cause damage to these material.The preparation temperature requirement of the ceramic matrix composites is also high,and it is generally necessary to bond different compounds together at a temperature above 1000?to produce a composite having a higher density.Glass has small density and good viscous flow characteristics at high temperature,and the temperature of the prepared composite is low.However,since the mechanical strength of the glass is worse than metal and ceramic,it is necessary to introduce a fiber or powder which having larger mechanical strength to enhance its mechanical strength,while the coefficient of thermal expansion of the glass matrix composites can be adjusted by adjustment of the glass component or introduction of a substance having negative thermal coefficient expansion.In nature,there are fewer substances with lower coefficients of thermal expansion,and most materials with low expansion coefficients have poor mechanical properties.At present,the most widely used substances with negative thermal expansion coefficients are ZrV_2-xP_xO₇,ZrW₂O₈and?-eucryptite.When these compounds are combined with other materials,the mechanical strength of the composites will usually be reduced.Therefore,in order to improve the mechanical strength of the composites,it is usually necessary to introduce the third phase with higher mechanical strength.In this paper,glass matrix composites with lower density,lower coefficient of thermal expansion and higher mechanical strength can be prepared at a lower sintering temperature of 800?by using glass as matrix,?-eucryptite and polycrystalline mullite fiber as reinforcing phase.And the difficult problem of the preparation condition of the ceramic-based low-expansion composite material is solved.At present,the glass powder is obtained mainly through ball-milling the glass which prepared by melting-quenching method.The preparation process is tedious,long period and the particle size is uneven.Therefore,the preparation method of glass powder with uniform particle size,adjustable composition and simple steps is particularly important.In this paper,glass powder was prepared by sol-gel method at a lower temperature,and the glass powder own higher purities and more homogeneous distribution of chemical composition.The glass powder prepared by sol-gel method and is easy to adjust the glass composition,and it is easier to crush the powder into smaller size samples.Using this glass powder as the matrix of the composites,the composites with more uniform composition and lower density can be obtained.The main research contents and results are summarized as follows:1)The uniform SiO₂-B₂O₃-Na₂O-ZnO-Al₂O₃?SBNZA?glass powder was prepared by sol-gel method.According to the results of X-ray diffractometer?XRD?,the optimum conditions for the preparation of amorphous SBNZA glass powder were determined,and the glass transition temperature?Tg?of SBNZA gel was determined to be 552?by thermo-gravitometry and different scanning calorimetry analysis?TG-DSC?.The network structure of SBNZA gel and glass were analyzed by Fourier Transform Infrared Spectroscopy?FT-IR?.The phase composition of gel and glass prepared at different sintering temperatures were analyzed by X-ray diffractometer?XRD?.The surface morphology and element content of SBNZA gel powder and glass powder were analyzed by Field Emission Scanning Electron Microscope?FE-SEM?and Energy Dispersive Spectrometer?EDS?.?1?Preparation of amorphous gel powder.The precursors and solvents for the preparation of SBNZA gel were selected through literature and experiments.The precursor of Si sol is tetraethyl orthosilicate,and the precursor for preparing component B sol is boric acid,the solvents are ethyl alcohol.The precursor for preparing Na sol was sodium methoxide,the solvent is methyl alcohol.The precursor for the preparation of Zn sol is zinc acetate dehydrate,using the distilled water as solvent,and the precursor for the preparation of Al sol is aluminum isopropoxide,the solvent is isopropanol.Amorphous SBNZA gel powder was successfully prepared under the follow conditions:the reaction temperature is 80?,the catalyst is HNO₃,hydrolysis-polycondensation and aging time is 13 days,and the final pH value of the reaction system is 3.?2?Characterization of SBNZA powder.According to the TG-DSC analyses results,the glass transition temperature?Tg?of SBNZA powder is 552?,but no obvious crystallization temperature?Tc?was detected.It can be seen from the FT-IR results that the glass network structure is mainly formed by the interaction between[SiO₄],[BO₄]and[AlO₄]groups in SBNZA gel glass,Na⁺and Zn²⁺ions are filled into the network as modification ions.The amorphous gel and sol-gel glass can be obtained by heat-treated below 700?,and beyond this temperature,the glass will be converted into ceramics.When the heat treatment temperature is as high as 900?,only SiO₂and Zn₂SiO₄ phases was existed in the ceramics.The glass powder prepared by sol-gel method is loose and porous,which can effectively reduce the sintering temperature and is beneficial to obtain finer size glass powder than ball-milling fused glass.2)The composites with low density,high mechanical strength and low coefficient of thermal expansion was prepared by the follow steps:mixing the SBNZA glass powder,polycrystalline mullite fiber and?-eucryptite powder through ball-milling,pressing the mixture into a green body,and then sintered the green body at 800?.?1?Effect of sintering process on density and mechanical strength of composites.The influence of vacuum,sintering temperature,sintering time and annealing on the properties of composites were investigated.The results showed that when the vacuum is 30 KPa,the sintering temperature is 800?,the holding time is 8 h and after annealing,the composite with complete shape and great shrinkage can be obtained.Low sintering temperature is not beneficial to the three-phase bonding of glass powder,?-eucryptite and polycrystalline mullite fiber,which leads to the poor fracture toughness and flexural strength of the composites.While too high sintering temperature will result in composites expand and exhibit a porous structure.It has been demonstrated that at sintering temperature of 800?,the as-prepared composite containing 10 wt.%fiber has the largest density of 2.387 g/cm³ and high mechanical strength,the fracture toughness and flexural strength are 1.26 MPa·m^1/2 and 78.5 MPa,respectively.?2?The effect of composition on the properties of the composites.The effects of fiber aspect ratio and fiber content on the density and mechanical strength of the composites were investigated.The larger aspect ratio of the fiber is and the more fiber content is,the easier to form a three-dimensional rigid network skeleton between the fibers,which can increase the porosity and decrease the density of the composites.The results showed that when the fiber aspect ratio is 100³33,the density of the composite with 10 wt.%fiber content gives a highest value of 2.240 g/cm³.In addition,when the fiber content of all the composites are 20 wt.%,the composite whose fiber aspect ratio is 13³3 exhibits the highest density of 2.301 g/cm³.In this paper,the mechanical properties of the composites with fiber aspect ratio of 13³3were investigated.With the increase of fiber content,the flexural strength and fracture toughness of the composites increased at first and then decreased.When the fiber content is 15 wt.%,the fracture toughness and flexural strength of the composite are the highest,which is 1.30 MPa·m^1/2 and 90.2 MPa,respectively.The coefficient of thermal expansion of the composite can be effectively adjusted by introducing?-eucryptite.With the increasing of?-eucryptite content,the coefficient of thermal expansion of the composite decreases gradually.When the content of?-eucryptite is31 wt.%,the mean thermal expansion coefficient of the composite is as low as1.59×10^-6/K in the temperature range of-100²00?.