CoFeCrNiCu 高熵钎料钎焊ZrB₂-SiC/Nb 接头组织及性能研究

ZrB₂-SiC ceramic composites exhibit excellent properties, such as high chemical stability, high electrical and thermal con-ductivity and strong corrosion resistance, making them an ideal high temperature resistant material in supersonic flight, spacecraft and rocket propulsion. It is common to join ceramics with high melting point metals such as Nb to combine the properties of metal and ce-ramic together. Compared to diffusion welding, friction welding, fusion welding and other connection methods, brazing methods have the advantages of high reliability, stable joining performance, simple process and high processing accuracy. Traditional Ni-based hightemperature filler generates a large amount of brittle intermetallic compounds and residual stress at the interface, which significantly reduces the joint performance. The Ag-based filler becomes soft at high temperature and thus the high-temperature performance of the joint is also poor. The high temperature resistant CoFeCrNiCu high-entropy alloy, composed of solid solution structure, was used as the brazing filler in this study to braze the ZrB₂-SiC ceramics and Nb alloy together under vacuum for different holding time. Scanning electron microscope(SEM), energy dispersive spectrometer(EDS)and X-ray diffraction(XRD)were used to analyze the influence of the microstructure, product of the interface and holding time on the performance of the joint. The fracture position and method of the joint were determined. The results showed that the typical interface structure of the brazed joint was ZrB₂-SiC/Cr₂B(/Cr, Fe)₂B+fcc+ Cr₂B+Laves+Cu_{(s, s)}/Nb_{(s, s)}/Nb. There was no effect of holding time on the types of interface products of brazed joints, but the volume fraction, size and thickness of the precipitated phases changed with holding time. The amount of Nb alloy dissolved in the liquid filler and the amount of Cr enriched in the ZrB₂-SiC ceramic to determine the formation and evolution of the brazed joint structure. For short holding time, the amount of Laves+fcc eutectic structure formed was less due to lack of dissolved Nb. They were mainly distributed on the side of Nb alloy and there was only 2% in the brazing joint. Short holding time also made the interface reaction insufficient, leading to a double-layer(Cr, Fe)₂ B+Cr₂B structure and a thickness of 30 μm. As holding time increased, the thickness of Cr₂B reaction layer on the ZrB₂-SiC ceramic side increased and the amount of Laves phase in the joint increased with the increasing of Nb alloy in the liq-uid filler. As holding time increased to 60 min, more Nb alloy dissolved and the amount of Laves+fcc eutectic structure in the joint greatly increased, occupied most of the central area of the joint and the content of Nb in the brazing joint was about 16%. Long holding time allowed the interface react sufficiently and the reaction layer on the ZrB₂-SiC ceramic side was only composed of jagged Cr₂B due to the substitution effect of B atoms on the C atoms in the carbide. The thickness of reaction layer was increased to 50 μm. As holding time increased to 90 min, the solubility of Nb alloy was higher, and the Laves+fcc eutectic structure occupied most of the central area of the joint(the average content of Nb in the brazing joint was about 18% at this time). The growth of the jagged Cr₂B layer toward the center of the joint was hindered and the thickness of the Cr₂B layer was reduced to 35 μm. The shear strength of the joint was deter-mined by the thickness and shape of the reaction layer on the ceramic side interface and the shape and distribution of the Laves phase in the braze join. The highest shear strength, ₂16 MPa, could be obtained when brazed at 1160 ℃ and maintained for 60 min. The shear strength of the joint could still reach 58 MPa at a high temperature of 600 ℃. This was due to the good mechanical properties of the eutectic structure(composed of hard Laves+soft fcc phase)at room temperature and high temperature and the strong bond of the jagged Cr₂B layer and the matrix. In this case, the joint would break along the ZrB₂-SiC ceramic matrix.