Magnetron sputtering TiZrNbTa HEA coating on CVD single crystal diamond for improving its solderability with Cu

The connection between diamond and other materials is always a key problem for its applications in microwave energy transmission windows. In this paper, a TiZrNbTa high-entropy alloys (HEAs) coating was prepared on single-crystal diamond (SCD) sheet by magnetron sputtering and post-annealing for enhancing the connection strength between diamond and Cu. Thereafter, the TiZrNbTa modified SCD was brazed onto a Cu sheet by vacuum brazing using Cu-Sn-Ti alloy as solder. The results showed that TiZrNbTa HEA coating with nanocrystalline structure and single orientation of (111) was formed after magnetron sputtering. The annealing treatment transformed the single (111) orientation to multi-orientations including (110), (200) and (211), and significantly enhanced the coating adhesion. The shear strength of the SCD/Cu joint with annealed TiZrNbTa HEA coating is about 102.6 MPa, which is about 49 % higher than that of the joints without HEA coating. The annealed TiZrNbTa coated joints mainly fracture in the deep inner of the solder. The fracture modes include the ductile fracture of Cu solid solution and the brittle fracture of CuSn3Ti5 compounds. This is much better than the un-coated joints and the as-deposited TiZrNbTa coated joints, which mainly fracture at the SCD/solder interface (the un-coated joints), the TiZrNbTa/solder interface and in the solder close to the TiZrNbTa coating (the asdeposited TiZrNbTa coated joints). In conclusion, preparing TiZrNbTa HEA coating on the diamond surface by magnetron sputtering combined with post-annealing is very effective in improving the weldability of SCD.