Preparation And Characterization Of Rare Earth Silicate Bolts Used On Thermal Protection System

With the increase of the aircraft working temperature,metal thermal protection system cannot meet the high requirements of temperature resistance and it's necessary to develop a new ceramic matrix thermal protection system,which can tolerate higher temperatures.The traditional metal bolts used on thermal protection system have low temperature resistance and high thermal expansion coefficient,which is prone to heat short;and the thermal expansion coefficient of the metal bolts mismatches that of the ceramic matrix composites,which would damage the ceramic matrix thermal protection system.So it's also necessary to develop new ceramic matrix material bolts to fit the new ceramic matrix thermal protection system.In this paper,Y₂SiO₅ is selected as the matrix material with low thermal conductivity,low thermal expansion coefficient and stable high temperature performance.By adding reinforcement phase and designing the structure of the materials,the new ceramic matrix composites with high strength and low thermal conductivity are successfully prepared.Firstly,the high purity nanoscale Y₂SiO₅ powder was prepared by sol-gel method,and then ZrO₂ fiber was selected as the enhancement phase,ZrO_2?f?/Y₂SiO₅ composites were fabricated through cold isostatic pressing and pressureless sintering at last.Results show that the composites with 20vt%ZrO₂ fibers exhibit the optimum strengthening effects,including a high bending strength?65.7 MPa?and a low thermal conductivity?0.557W/?m-K??,which are higher than those of the specimens without added fibers.The sol-gel method isn't suitable to apply on the engineering to prepare Y₂SiO₅ powder because of the low efficiency and the damage to environment.So the micron Y₂SiO₅ powder is used in the next research,which is fabricated by the powder solid state reaction method,ZrO_2?f?/Y₂SiO₅ and Ni/Y₂SiO₅ composites are successfully prepared by micron Y₂SiO₅ powder and enhanced phases?ZrO₂ fiber or Ni?,the properties of these composites are also characterized.Results show that 20vt%ZrO₂?t?/Y₂SiO₅ composites?sintering at 1450??and 20vt%Ni/Y₂SiO₅ composites?sintering at 1400??have the best mechanical properties,the flexural strength of these two types is 124.71 MPa and 78.00 MPa,respectively,which is 29.41%and 47.09%higher than that of Y₂SiO₅ ceramics.The shearing strength of them is 86.10 MPa and 76.51 MPa,respectively,which is 65.20%and 69.65%higher than that of Y₂SiO₅ ceramics.Tensile strengths of them is 53.52 MPa and 50.67 MPa,respectively,which is 57.27%and 74.72%higher than that of Y₂SiO₅ ceramics.And these two types of composites have low thermal conductivity,which are 0.542 W/?m-K?and 0.456 W/?m·K?,respectively.In the gradient ZrO_2?f?/Y₂SiO₅ and Ni/Y₂SiO₅ composites,porous Y₂SiO₅ ceramics are used as interlayers for insulation,whereas outer fiber-ceramic or metal-ceramic layers are used as outer layers to bear loads.The shearing strength at the weak interface and the transition layer is characterized to evaluate the shearing resistance of the gradient composites.The results show that the shearing strength at weak interface of two gradient composites is 45.79 MPa and 32.34 MPa,which is lower than that of Y₂SiO₅ ceramics prepared by the same process.When the shearing planes are adjusted from the weak interface to the transition layers,the shearing strength of these two series of the gradient composites?78.55 MPa and 61.93 MPa?is both greater than that of Y₂SiO₅ ceramics prepared by the same process.The bending and tensile strength of the gradient composites is also characterized.The maximum bending strength of them is 67.55 MPa and 77.81 MPa,respectively;the maximum tensile strength of them is 26.62 MPa and 23.07 MPa,respectively.We successfully prepared the gradient composite bolts by diamond tools in our research.According to the macroscopic and microscopic appearances of the bolts,adding ZrO₂ fiber will damage the machinability of Y₂SiO₅ ceramics;adding Ni will elevate the machinability of Y₂SiO₅ ceramics.Results of the tensile test for bolts show that 20vt%Ni/Y₂SiO₅ composites bolts have better tensile strength?12.46 MPa?compared with Y₂SiO₅ ceramic bolts?9.46 MPa?.During the shearing test for bolts,these two types of the gradient composites bolts have better shear-bearing capacity than Y₂SiO₅ ceramic bolts.The maximal shearing strength of the gradient composites bolts are 36.53 MPa and 16.35 MPa,respectively.