Research on the Cutting Performance of Cubic Boron Nitride Tools

There were only two kinds of superhard tool material at the past, i.e. diamond and cubic boron nitride (CBN). Manmade diamond and CBN are manufactured by the middle of 20th century. Various manufacturing methods and manmade superhard materials were developed later. They were widely used in different industry and science areas. Recently, a new kind of superhard tool material, C 3N 4 coating film, had been developed. American physical scientists, A. M. Liu and M. L. Cohen, designed a new kind of inorganic compound C 3N 4 with the theory of molecule engineering. According to calculation, it can reach or even exceed the hardness of diamond, so material scientists and technique circles draw their attention to it. A high speed steel twist drill coated with C 3N 4 film is applied to the drilling hole process on steel workpiece in cutting tests, the tool life is increased greatly. When the C 3N 4 film is coated on the cemented carbide inserts, the cutting performance is improved, but is not good enough. The data of mechanical performance and cutting tests about this kind of new tool material is given in this paper, it shows that C 3N 4 has a promising future. The anti-wear ability of cutting tool increases sharply after C 3N 4 being coated on HSS tool. Coated HSS drill also has some benefit after being reground. The tool life prolongs after C 3N 4 being coated on cemented carbide inserts, but is not so long as that of C 3N 4 coated HSS tool. When machining PRCM with C 3N 4 thin-film coated cemented carbide tool, the cutting performance is poor and it is much better when machining PRCM with PCBN, PCD compound plates and CVD thick-film coated cutting tool. Some relative aspects need to be deeply discussed and researched, e.g. the existing coating techniques is not good enough and should be improved in the future, the film thickness should be optimized and try to find out the most effective value, the binding force and mutual effect between coated film and substrate need to be studied furtherly, etc.

Superior mechanical and thermal properties than diamond:Diamond/lonsdaleite biphasic structure

It has been found recently in experiments that diamond/lonsdaleite biphase could possess excellent thermal-mechanical properties,implying that the properties of carbon materials can be improved by reasonably designing their internal structures.The mechanism of the excellent performance arising from biphasic structure is still unknown and needs to be revealed.In this paper,we established a series of possible diamond/lonsdaleite biphasic structures and revealed the optimization mechanism of the biphasic structure using first principles calculations.It shows in our ab-initio molecular dynamics simulations that the lonsdaleite cannot exist stably at room temperature,which could explain why pure lonsdaleite can hardly be found or synthesized.Detailed analysis shows that partial slip would occur in the lonsdaleite region if the applied strain is sufficiently large,leading to the transition from biphasic phase to cubic phase.Then,further shear strain would be applied along the hard shear direction of the cubic structure,resulting in an ascent of stress.The results presented could offer an insight into the structural transformation at high temperature and large strain.