Design,Fabrication And Characterization Of Zirconium-based Precursors For Ultra High Temperature Ceramics

Ultra high temperature ceramic matrix composites?UHTCMCs?are kinds of promising candidate materials with excellent high temperature resistance,oxidation resistance,thermal shock resistance,favorable mechanical properties and low density,which have wide prospects in the application at hypersonic aircraft vehicles,heat protective barriers and rocket engines.Precursor infiltration pyrolysis?PIP?method is the most effective way to produce ceramic matrix composites and it is the decisive factor to obtain a class of precursors with controllable composition,superior melt and soluble propertiy and high ceramic yield.In this study the reaction path has been designed to introduce Zr to the ZrC-SiC and ZrC-ZrB₂ multiphase preceramic polymers through Zr-C and Zr-N bonds.The obtained polymers have controllable composition,advantageous melt and soluble property and high ceramic yield through crosslinking process and the precursor derived ceramics exhibit outstanding high temperature and oxidation resistance.The detailed results are as follows:The bond formation process of Zr-C and Zr-N,the structure of the reagents and the energy changes have been analyzed and simulated by the DFT method.The Grignard reaction has been chosen as the Zr-C formation process and the computational appearance of HOMO orbitals reveal that the four replacements could happen;the amine exchange reaction is confirmed as the Zr-N formation process and the simulated results of HOMO orbitals represent that the two or three replacements always occur;the C3 position in the allyl groups could be the reactive site for hydrosilation and hydroboration reactions by the simulated LUMO results in both the Grignard and amine exchange reactions.The Zr-C monomers have been synthesized by the reaction of ZrCl₄ and Grignard reagent.The effect on the Zr-C monomers of the Grignard reagent variety,the ratio of raw materials,the reaction temperature and time has been investigated and the composition and structure of the monomers have been measured.Results show that the monomers could formed with ZrCl₄ and allyl-MgCl at 75?for 3 hours.The structure could be expressed as Zr?CH₂CH=CH₂?₄.These monomers have abundant reactive groups to fabricate the Zr containing multiphase precusors.The PZCB precursors have been synthesized at 75?for 2h through the reaction of Zr-C monomers and boranes,with the yield of 85.3%and the solubility of 26.7wt%in solar solvents.The precursors are composed of Zr,C,B and H elements in the form of Zr-C and B-C framework.The main weight loss should be the break of B-C bond and the crosslinking process at 220?are used to improve the ceramic yield?58.3%?.XRD and FTIR results reveal that the ZrCB ceramics have amorphous state until ZrC and ZrB₂ crystallize at 1400?and 1600?respectively.The composition of ZrC-ZrB₂ are higher than 80wt%and the weight ratio of ZrC to ZrB₂ is between 1:1 and 1:5.The ceramics present dense structure at 1800?and it has grain size of 54.8nm at 2000?.The Zr-C monomers have been used to react with PMS to form PZCS precursors at90?for 2h,which has high solubility?37.4wt%?and moderate molten temperature?230²80??.The precursors are composed of Zr,Si,C and H in the form of Si-C and Zr-C.The ceramization process has a weight loss at 300⁵00?for the backbone rearrangement and the ceramic yield?65.4%?has been improved through crosslinking at240?.The derived ZrCSi ceramics keep amorphous until 1400?and have high ZrC-SiC composition?>80wt%?,with the weight ratio of ZrC to SiC between 1:1 to 3:1.High temperature test demonstrate that the ZrCSi ceramics have relatively good high temperature stability with the weight loss of 4.4wt%and ZrC grain size of 66.4nm at2000?.The Zr-N monomers has been synthesized through ZrCl₄ and allyl-amine at 180?and then the hydrosilation reaction is used to obtain the PZNCS precursor through the reaction of Zr-N monomers and PMS.The precursors has high yield?77.5%?and solubility?28.2wt%?.The main elements composed of the precursor are Zr,Si,N,C and H in the form of Zr-N,N-C and Si-C.The rearrangement at 200⁴50?and the escape of NH₃ at 500⁷50?lead to the most weight loss during the ceramization process.XRD results show that the ZrNCSi ceramics keep amorphous until 1400?and the Zr?C,N?-SiC composition is about 75⁸5wt%with the weight ratio of ZrC and SiC as2:1.The phase separation can be observed in the SEM image and the weight loss is4.39wt%until 2000?.The PZNCB precursors have been synthesized through the reaction of Zr-N monomers and borane at 130?,which has yield of 75.4%and solubility of 27.4wt%.The precursors are composed of Zr,N,C,B and H in the groups of Zr-N,N-C and B-C.Results show the main weight loss of the precursor is the backbone rearrangement at300⁵00?and the NH₃ evaporation at 500⁷50?.The structure and composition are discussed by XRD and SEM methods.It can be seen that the ZrNCB ceramics have porous structure and the Zr?C,N?-ZrB₂ is about 60⁷5wt%with the ratio of ZrC:ZrB₂as 2:1³:4.The weight loss of the ceramic is 3.01wt%and the ZrC grain size is about32.6nm at 2000?.In order to improve the phase separation and porous structure of the precursor derived ceramics,the PZNCS and PZNCB precursors have been modified by the Grignard reagent and then the hydrosilation and hydroboration reactions are managed in a further step.Results demonstrate that the Grignard reagent could replace the residual Zr-Cl bonds and introduce more allyl groups.The obtained m-PZNCS-2 and m-PZNCB-2 precursors have higher ceramic yield?48.9%and 68.8%respectively?and dense structures at 1800?with the UHTCs composition higher than 85wt%.The high temperature resistance at 2200?under Ar of the precursor derived ceramics has been tested as wafers.The weight loss of all ceramics is less than 2%and the volume keep stable with line shrinkage rate less than 0.6%,which means that the ceramics has good thermal resistance.The introduction of Si/B could improve the high temperature resistance,as the ZrCSi-2 and ZrCB-2 present the smallest weight loss?0.07%and 0.12%respectively?.The multiphase ceramics exhibit favorable sintering properties with the dense structure at 1800?,especially for the N containing ceramics.The multiphase ceramics have nano scale grain size at 2200?,which should be attributed to the uniform dispersion of elements and the restraint during crystalline process between different phases.The oxidation test shows that the ceramics has a stable weight and volume condition.The ceramics start to be oxidized at 600⁷00?and keep stable until 1400?.The protective layer can be formed by the oxidation of SiC and ZrB₂ when temperature increases until 1700?with the ZrCSi-2 and ZrCB-2 for the lest weight changes?8.35%and 5.02%respectively?.The introduction of N could offer a dense structure?m-ZrNCSi-2 and m-ZrNCB-2?,which exihibit favaroble oxidation resistance.