Fabrication And Properties Of Zirconium-based Ultra High Temperature Ceramic Composites By Reactive Melt Infiltration At Relative Low Temperature

The extreme operational conditions encountered for hypersonic and space vehiclesas well as rocket propulsion systems present a great challenge to the development ofultra high temperature materials. New and innovative structural materials capable ofprolonged operation at temperatures above2000℃are required for future spacesystems. In this paper, zirconium based ultra high temperature ceramics compositeswere proposed to be prepared by reactive melt infiltration at relative low temperature,furthermore, the properties of the composites were also investigated.Firstly, the W-ZrC, ZrB₂-ZrC, and ZrB₂composites were chosen as the resultingmaterials standing for metal-ceramic, ceramic-ceramic, and single-phase ceramicmaterials, respectively. Zr₂Cu was chosen as the infiltrant, because its melting point isonly1000℃, the zirconium content is74.2wt%, copper can be extruded out of theresulting composites easily, and the residual copper can be used as transpiration cooling.WC, B4C, and boron were chosen as the raw materials for the three preforms, theycould react with Zr₂Cu at1200℃from thermodynamics calculations and experiments.After reactions, solid volumes increased, to fill all pores and keep the shapes of thepreforms, the theoretical porosities were50.1%、58.3%and49.3%, respectively.Preparation of W-ZrC composites by the low temperature reactive melt infiltrationmethod was investigated. The best preform processing was that the raw WC particlesize was about4μm, the preform pressure was20MPa, the content of adhesive PCSwas3wt%. After the preforms were fabricated, they had better to infiltrate with Zr₂Cuat1400℃for5h in vacuum. The resulting W-ZrC composites had a content of5.4vol%W,57.7vol%ZrC, and6.9vol%residual alloy.The low temperature reactive melt infiltrated W-ZrC composites had excellentmechanical properties, ablation resistance, and thermal shock resistance. The fracturetoughness was7.2MPa·m1/2, the1800℃flexural strength was1.2times of the roomtemperature value. The thermal shock resistance was better than the reported Mo-ZrC,ZrC-SiC composites, the thermal shock critical temperature was450℃. When thecomposites were ablated by oxyacetylene flame for30s, the linear ablation rate was0.0033mm·s-1, the mass ablation rate was0.0012g·s-1.The reaction mechanism between WC and Zr₂Cu was investigated. It was foundthat when the Zr₂Cu melt came in to contact with WC, carbon atoms diffused out to form W2C and then W phase. The diffused carbon atoms in melt reacted with zirconiumatoms to form ZrC, dispersed in the melt. Because of the effect of pore structure in theprefoms, zirconium might be excessive or insufficient in some local place. When themelt was excessive, zirconium atoms diffused into the W grains to form W2Zr phase andW-Zr solid solution phase. While the melt was insufficient, tungsten atoms diffused intothe ZrC grains to form （W,Zr）C solid solution.Preparation of ZrB₂-ZrC based composites by reactive melt infiltration at relativelow temperature was firstly investigated. The composites were prepared with150μmB4C particle and15wt%PCS at1200oC had the best properties. They had a content of65.0vol%ZrB₂,28.1vol%ZrC, and6.9vol%residual alloy.The ZrB₂-ZrC based composites had outstanding high temperature resistance andablation resistance. The composites had a flexural strength of360.3MPa, a flexuralmodulus of200.8GPa, and a fracture toughness of12.1MPa·m1/2, after the compositeswere treated at1600oC in argon, the flexural strength, modulus, and toughnessincreased to30.4%,26.2%, and12.4%, respectively. After the composites were ablatedby oxyacetylene flame for300s, the mass ablation rate was-0.00063g·s-1, the linearablation rate was0.0033mm·s-1, and the main ablation mechanism wasthermal-chemical process.The reaction mechanism between Zr₂Cu and B4C involved surface reaction andsolution-precipitation was proposed. When the infltrating Zr₂Cu metal came intocontact with B4C particles in the preform, a surface reaction happened. Boron atom hada better affinity to react firstly to form ZrB₂, the residual carbon then reacted to formZrC layer. As the melt infiltrated though the product layer to the unreacted B4C surface,the activity of the infiltrant decreased with zirconium atoms consumed continually.Copper rich melt was left in the interstice of products and provided an easier route formass transfer, the melt also attacked B4C substrate and made boron dissolve in the melt,zirconium atoms might react with the dissolved boron in the melt to precipitate ZrB₂grains.ZrB₂based composites were also firstly prepared by reactive melt infiltration atrelative low temperature. The amorphous boron powder together with15wt%PCSwere pressured at100MPa and then were sintered at1600℃in argon to produceporous preforms. ZrB₂based composites could be prepared with the preforms andZr₂Cu at1200℃for3h. The composites were mainly composed of200nm ZrB₂grains. The mechanical properties and ablation resistance of ZrB₂based composites werealso investigated. The composites had a flexural strength of414.3MPa, a flexuralmodulus of183.6GPa, a fracture toughness of5.5MPa·m1/2. When the compositesprepared with SiC filler, the ablation resistance increased, they had a linear ablation rateof0.0013mm·s-1and a mass ablation rate of0.0028g·s-1. The oxidation coating on thesample was useful to resistance ablation more.The processing and the three composites were compared with each other at last. Itwas found that B4C was the most active, the cost of frabrication of ZrB₂-ZrC basedcomposites was the lowest, and the composites had a low density, a good fracturetoughness and ablation resistance.