The thermal stability of the composite sheet can be said to be poor. This is due to the use of the underlying alloy (WC) powder, polycrystalline single-crystal diamond micro-nano powder between the diamond and tungsten carbide powder and served with an adhesive within the diamond, cobalt or cobalt compounds generally, sometimes sometimes need to use graphite powder with the right amount of catalytic agent, sintering at a high temperature of about 1500 ℃ and 60,000 atmospheres. Because of this bond and cobalt from a catalytic role will not only result in polycrystalline diamond graphitization, but also due to the thermal expansion coefficient of cobalt or cobalt compounds are very different, so that the polycrystalline layer easily damaged by heat. In addition to the influence of cobalt, polycrystalline layer and the different thermal expansion coefficients of tungsten carbide layer is also liable to cause serious defects in the polycrystalline layer from the bottom (or substrate) off. Thus, the composite sheet of such a specific cutting block inserts welded on the drill bit body has become a key technology. Because the polycrystalline diamond composite sheet at high temperature above 700 ℃ will cause the decline of, so the melting point of the brazing metal should be less than 700 ℃; if the brazing metal melting point is too low, but also affect the bonding strength after soldering, resulting in the drill bit the composite sheet easy to fall off in the course of their work, which is the main problem PDC drill.