The effect of active elements on the microstructure and properties of graphite and copper-brazed joints

Purpose To obtain better properties of joints of graphite/Cu brazing, brazing of graphite to oxygen-free copper with different coating has been employed. Methods Graphite is brazed to copper with surface coatings containing Cr, Ni or no element at 860 degrees C for 10 min. Results The shear strength of all joints exceeds 20 MPa. In addition to the interaction between the Ti element in the filler and the C element leading to the formation of TiC, the coating materials Cr and Ni also form Cr7C3 and Ni3C, respectively. Compared to the Cr-coated and no-coated brazed samples, the filler is distributed more evenly in the Ni-plated brazed sample. TEM diffraction patterns have been utilized to calibrate the granular metal phase forming at the brazing filler metal boundary, revealing its composition Ti (Cu, Ni)(2). The thermal conductivity of the brazed samples has been tested at different temperatures; the Cr-coated samples show significantly lower thermal conductivity than the uncoated samples. Conversely, the thermal conductivity of the Ni-coated samples is significantly higher than that of the uncoated samples. Conclusion Graphite and copper with different coatings can be successfully joined by using AgCuTi at 860 degrees C for 10 min. The graphite/Cu joint with Ni coating achieves better thermal conductivity with a more even distribution of the filler.