Design of reinforced interfacial structure in brazed joints of C/C composites and Nb by pre-oxidation surface treatment combined with in situ growth of CNTs

A surface modification method of C/C composites involving a combination of pre-oxidation treatment and in situ growth of CNTs was developed to form a reinforced interfacial structure in Nb-C/C brazed joints. The interfacial structure under different surface conditions of the C/C composites and its effects on the mechanical properties of brazed joints at both room temperature (RT) and elevated temperatures were studied. Through pre-oxidation, annular gaps with an adjustable size were uniformly formed around the carbon fibers, and these gaps acted as artificial channels for the liquid brazing alloy to infiltrate into. The RT shear strength of the Nb-C/C joints increased from 29 MPa to 57 MPa after oxidation of the C/C composites at 800 °C for 5min. The growth of CNTs was able to improve the wettability, since it was beneficial for liquid brazing alloy filling in the annular gaps. Further, a reinforced joint with fine TiC particles was formed by the reaction between Ti and the CNTs. The shear strength of the brazed joints reached 62 MPa after pre-oxidation and CNT growth and remained at 48  MPa at 500 °C. The improvement in the joint shear strength after surface modification of the C/C composites was attributed to the reduced residual stress, increased bonding area, and enhanced pinning effect.