The Research Of Brazing Processing Of CVD Diamond Thick Film

In recent years, as one kind of new function material, CVD diamond thick film has been used in many field such as cutting tool, cooling device of super-high speed computer CMOS chip, cooling device of semiconductor and microwave parts, and shown unique superiorities. However, because of the special crystal structure and physical, chemical, mechanical characteristic of diamond, some obstacles still exist in the field of its process. Moreover, the bond between diamond and substrate is the key in application and the theoretic research, the theories of diamond bonding need more research by absorbing opinions of many scholars.In this paper, the object of the research is to bond between CVD diamond thick film and cemented carbide substrate with active metal brazing in vacuum furnace. Regarding the problem of the non- wetting between diamond and filler metal and residual stress in the joint, the brazing theory and the processes have been studied by the means of combining theoretical studies with experiments .As a result, the shear strength of the joint has reached about 267 MPa which is higher than that of diamond.The ideal composition of brazing filler metal has been obtained by investigating the wetting between diamond and filler metal, the bond strength of joint and observing the microstructure of interface. The wetting angle has been about 20?.The connection between the thickness of filler metal, the ingredients and the distribution of the internal stress has been studied by the method of finite element, ANSYS non-linear software. The result is that the thickness of filler metal has one optimal value and it is the effective method adding Cu into filler metal .It has been pointed out that the characteristic of diamond, filler metals and substrate metal influence the distribution and magnitude of internal stress in the brazed joint between diamond and filler metal. Among the characteristic parameters, the heat expand coefficient ( a ) is the most important factor.When the filler layer is thin , the maximal pressural stress is in the central region of diamond layer and when the filler layer is thick, the maximal pressural stress is in the verge region. The maximal pressural stress in the diamond is near the interface along the direction vertical to interface. The more the thickness of filler metals layer is, the less the pressural stress is. The most suitable size of filler layer exists.It is found that the composition of filler metal and microstructure of interlayer affect the thermal stress of brazing joint specially and for the first time it is confirmed that adding Cu interlayer to the Ag-Cu-Ti filler metal is a good method to reduce thermal stress and avoid fracture of diamond. The optimal thickness of Cu interlayer exists. The more thin the thickness of Ag-Cu-Ti filler metal layer is, the less the thermal stress is, so the layer should be thin under the condition to wet the surface of diamond.For the analysis of brazed interface of diamond and filler metal, the fracture surface and across surface of brazed joint were examined by SEM, EPMA, EDX the morphologies of TiC was obtained.It is indicated that the main factor to result in the joint strength reduce is not only the TiC, but also the increase of thickness of brittle compound TiCu. So the quantity of Ti in the filler metal must be controlled strictly.