Soldering of diamond to molybdenum using a Zn–15Al alloy with semisolid phase assisted ultrasonic vibration

Abstract

Diamond was soldered to a molybdenum plate using a semisolid solder of Zn–15Al and ultrasonic vibration at brazing temperatures not exceeding 900 K. The joint interface was formed by a 10% aluminum solid phase in a semisolid solder when an ultrasonic vibration was applied to the interface. Several joints reached a shear strength of 40 MPa, although the shear strength results for the joints had a wide distribution. The Weibull plot of the strength test results revealed two categories of fracture interfaces. One category contained a flat diamond surface and a flat solder surface. The other category contained remaining solder on the diamond-joined surface. The first category had a shape that contained regions that were not joined. The second category reached a high joint strength over 40 MPa and had remaining solder on the fractured surface, a Weibull modulus m = 1.5 and a scale parameter η = 52 MPa. Adjacent to the joint interface, spherical alpha aluminum grains could be observed in the semisolid solder via applied ultrasonic vibration. Transmission electron microscopy was performed to observe the bonded interface of the diamond using the solder. The electron scattering images of the diamond that was cross sectioned with a focused ion beam revealed the formation of a few layers of crystallized alumina on the diamond interface. The alumina structure was spinel due to its formation at the low temperature of 900 K because zinc atoms were included in the layer. The reaction layer included carbon atoms that grew epitaxially from the diamond. The composition was considered to be (Zn, Al)2(O=C)3. It is hypothesized that ultrasonic vibration energy generates a reaction between the oxide on the diamond with the aluminum.