The quality of PCBN cutting tools is affected by many factors, among which changes in various process parameters during the sintering process have the greatest impact on the quality of PCBN cutting tools. Binder/catalyst materials, matrix materials and other constituent materials, as well as particle surface cleanliness and granularity, all determine the performance of PCBN cutting tools.
(1) Selection of substrate
Tungsten carbide cemented carbide which contains Co is often used as the matrix of PCBN cutting tools because it has higher hardness, better thermal conductivity and toughness. Co, as a solvent and binder, can significantly increase the sintering degree of CBN and enhance the strength of PCBN, but the content of Co in tungsten carbide cemented carbide should not be too high, otherwise it will affect the wear resistance of PCBN cutting tools and shorten its service life.
(2) Cleanliness and granularity of CBN crystal grains
The surface cleanliness of CBN grains will directly affect the sintering quality of PCBN. Therefore, the CBN grains must be strictly treated before sintering to remove impurities such as moisture and oxides on the surface of the grains. The main method used is heating under vacuum or reducing gas such as hydrogen and ammonia for 1 to 2 hours. Otherwise, excessive impurities will affect the bonding between the CBN-CBN particles and between the CBN and the binder, which will reduce the strength of the PCBN cutting tools and the wear resistance.
Since the sintering of PCBN cutting tools is achieved through the "capillary phenomenon", that is, various binder elements penetrate between the CBN particles. If the CBN particles are too small, the gap between the CBN particles will be reduced, thereby making the the amount of penetration of binder element is reduced. Therefore, the CBN particles should be selected larger during sintering. Considering the above two factors comprehensively, the mixed particle size should be selected more when CBN sintering, and different particle size ranges should be determined according to the different bonding capabilities of the selected binder.
(3) CBN grain content
CBN content has a great influence on the hardness and thermal conductivity of PCBN tools. The higher the CBN content, the higher the hardness of the tool and the better the thermal conductivity. High-content PCBN cutting tools (generally CBN content is 80% to 90%) are mainly direct bonding between CBN, with high hardness and high thermal conductivity. This kind of PCBN cutting tools is suitable for processing high-hardness alloys and materials with high hard points in the structure, such as chilled cast iron, heat-resistant alloys.
Binder required for CBN sintering:
①The closer the physical and chemical properties to CBN, the better, so that the cutting performance of PCBN cutting tools after sintering will not be weakened too much;
②It is easy to reach the melting point temperature or has good plasticity at this temperature;
③Compared with CBN, it has sufficient chemical activity and has the catalytic ability to convert hexagonal boron nitride (HBN) to CBN.
Due to different binders, PCBN cutting tools tend to have two characteristics: one is high wear resistance (high content of CBN, catalytic binder), and the other is good impact resistance (low content of CBN, ceramic binder).