| NiAl intermetallic is a kind of very promising thermal structure material for its high melting point, high thermal conductivity, low density, good oxidation resistance and other excellent performance. Using NiAl to modify C/C composites is expected to make up for the defects of C/C composites at high temperature such as poor anti-oxidation performace, poor matching with outer coatings and so on.This paper systematically investigated the selection and content optimization of additives of NiAl modified C/C composites. NiAl modified C/C composites were prepared by vacuum molten infiltration, and its microstructure, infiltration mechanism, mechanical properties and oxidation mechanism were investigated. The main contents and conclusions are as follows:(1) In NiAl-C system, molten NiAl and C were non-wetted; In NiAl-MC (M=Si, Zr, Ti) system, the wettability relationship between molten NiAl and carbides was TiC≥ZrC＞＞SiC; Adding Ti could improve the wettability between C/C composites and NiAl intermetallic. NiAl with about15%Ti(mass fraction) possesses the best wettability on the surface of C/C composites, while excessive Ti shows the bad effect.(2) NiAl modified C/C composites were prepared by vacuum molten infiltration while the density of C/C perfom was1.35g/cm3. The depth of NiAl permeation layer on C/C composites is about0.8-0.9mm, and the density increased from1.35g/cm3to2.47g/cm3while the porosity decreased from27%to15.1%after the infiltration of NiAl. On the other hand, the Rockwell hardness of NiAl modified C/C composites increased by30.7%in the direction parallel to fiber fabric plane compared with that of C/C composites. NiAl-infiltrated C/C composites were composed by C, TiC and NiAl while the TiC particles and NiAl melt present network distribution in C/C composites internal. Different kinds of interfaces like Cf-C, Cf-TiC, TiC-NiAl, TiC-C existed in the composites. The factors which influence NiAl infiltrating into C/C composites are as follows:It is easy to plug the capillary channel for the strong reaction between Ti and C; When TiC interface layer formed, molten Ti is hard to permeate deeper for its very low diffusion coefficient in TiC, which will affect the infiltration of molten NiAl.(3) NiAl modified C/C composites show typical pseudoplastic fracture behavior under bend load, while C/C composites show a short time of plastic fracture and then quickly convert to brittle fracture. NiAl permeation layer entraining carbon fibers from C/C matrix leads to the pseudoplastic behavior of NiAl modified C/C composites. The transverse fracture strength decreased by24.4%after NiAl infiltrated, probably due to the certain degree damage of carbon fibers by Ti infiltration. Tensile fracture show that fracture occurs near C/C matrix region. The fracture morphology of which is close to C/C matrix is similar to the fracture of C/C composites. But in the area near NiAl permeation layer, the fracture of carbon fibers occurs at the interface between permeation layer and C/C matrix. It shows good bonding strength of NiAl-TiC-Cf interface.(4) β-Al2Ti05and TiO2appeared on the surface of NiAl modified C/C composites after oxidation in ambient at1500℃. The behaviors of internal region of NiAl modified C/C composites are as follows:Carbon fibers were oxided from interface and formed a typical wedge structure. Some carbon fibers were exhausted oxided. Outer ring of NiAl-TiC layer was oxided into Al2Ti05, while the inner ring remained partial unoxidized NiAl-TiC. Oxide films show a certain degree of loss. |
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