Synthesis And Studys On Polycrystalline Cubic Boron Nitride (PCBN) Composite Under HPHT

The development of tools is the motive force of human civilization, and the naissance of cBN tools has brought tremendous changes for the processing of hardned iron and their alloys, has an epoch-making significance. Polycrystalline Cubic Boron Nitride(PCBN)composite is a new super-hard cutting material which is sintered composit by some cBN crystals and horniness alloy (WC-Co) in the temperature range of 1200-2000℃under high pressure range of 4.5-9.0 GPa. It has become on kind of the most popular cutting material and was widely used for cutting hardened steels especially iron group metal, because of their unique properties,such as high hardness, good thermal conductivity and chemical inertness to iron and iron alloys since it has been sintered by General Electric Co firstly in 1975. In recently year,cutting developed to the direction of the high-speed cutting,dry cutting, hard cutting,turning instead of grinding and so on,with the CNC(Computere Numerical Control)technology rapid development and extensive use of CNC,PCBN cutting toolas a major new genetation tool has been used to replace part of the common machining tools , and will get more vastly applying space.The researche and manufacture of PCBN composite material are not late in our contury,but the pace of development is not ideal,and there is a significant gap compare with foreign countries.The reason is that our concepts fall behind,besides,we also have some difficult on sintering technic.At present, there are a large number of manufacturers who can produce PCBN tool material, but most of them are not very strong,The product exists some problem such as single variety, too small diameter, lack of toughness, poor heat resistance inconsistent quality and so on.we have to import a lot of high-property PCBN tool,when we want to machining some cutting difficult materials which is used in the automotive industry and other key the user field.So it is significance to study on PCBN sintering process and sintering theory.In this paper, firstly,we emphatically researched the effect of technical parameters and raw material selection on synthesis of PCBN composite material, We referenced the common rules about synthesis PCBN composite material and finished the experiment by a China-type SPD cubic high-pressure apparatus under the conditions of 4.5～5.5GPa and 1150～1400℃. The main results were as below:(1)The density of polycrystalline Cubic Boron Nitride(400 mesh)is increases with the pressure,it reached 3.2g/cm3 at 5.2 GPa;cBN particles slip intensification of rearrangement and fragmentation is the main reason for increased density polycrystalline;the higher the pressure,the stronger wear resistance;pressure is conducive to rapid and uniform penetration of metal binder.(1) when the temperature leans low, the sintering is not fully; while the temperature leans high, cuase"over-fever",both them will reduce the wear resistance and increasing the risk of delamination of PCBN.(3)The response of Al and cBN can finished in 90s at appropriate temperature and pressure conditions;the extension will help binder phase uniformly distributed,but reduce the wear resistance of PCBN composite.(4)The apppropriate refinement cBN powder and or use mixde grain can improve the wear resistance effectively.(5)Binder content have effect on wear resistance;different size cBN powder need different binder in weight,the smaller size,the more binder; the conductivity increased with the binder content.(6)The wear resistance of PCBN is sharp decline with the increaseing of cBN layer thickness,it achieve maximun when the thickness is about 1mm.(7) In this paper, the assembly,Infiltration methods and "annealing" process can regulate pressure field, reduce the "off side" and "delamination" probability.Then,we study the sintering mechanism of PCBN composites based on the fact of substantive experiments, proposed the"infiltration"model,and analysis composite mechanism,chemical reactions and material migration in the sintering process.The main results are as follows:(1)Many factors can influence the metal infiltration,such as synthesis pressure, temperature,reaction time,power size and so on; the higher pressure and pressure difference,the faster infiltration,the higher pressure,smaller solvent,the better liquidity and faster infiltration;the smaller powder size and greater pore diameter,the faster infiltration.(2) PCBN composite are compounded hard mainly through the formation of a metal transition layer.(3)The sintering process of PCBN composite material contain two stages: solid-state sintering and liquid-state sintering; mass transfer process contain:particle rearrangement,plastic deformation,flow,infiltration,segregation and proliferation.In the end of the paper,We study on some promblem which are frequently exist in the infiltrating way and give the corresponding solution.Study concluded that: there are many factors influencing quality stability of PCBN.The unstability of wear resistance of PCBN,crushing and edge dregs,metallic line leaking out on surface of PCBN,cBN layer separating from hard alloy body and so on.we can effectively solve these problems by improveing the repeatability of experiment conditions,polishing up the uniformity of temperature field and pressure field and introduction of "annealing" process.