The present invention relates to a cutting tool insert including cubic boron nitride with an excellent resistance to chipping and edge fracture when machining hard materials like hardened steel and alike materials.
Cubic boron nitride (cBN)-based ceramics sintered under high pressure and high temperature are known.
Generally, cBN-based materials for hard part machining comprise cBN as the hard dispersed phase and a ceramic binder which forms a sintered hard cutting tool insert. The ceramic binder phase of the cBN cutting tool insert with 40-80% cBN by volume comprises generally a nitride, carbonitride or carbide of titanium with smaller amounts of borides of Ti, W, Co, Al or a solid solution of these, alumina and other unavoidable reaction products. By varying the relative amounts of the ingredients, cBN tools can be designed for optimum performance in different applications, e.g. continuous or interrupted cutting. cBN tools with relatively high cBN content are recommended for interrupted to heavy interrupted cutting applications, whereas a high ceramic binder content gives high wear resistance in continuous cutting. The severe conditions in interrupted cutting usually cause edge failure and thus determine the lifetime for the tool rather than other wear modes such as notch or crater wear. Even in a continuous application, machine instability may cause an intermittent behaviour causing early edge failure. In particular, the above mentioned cBN cutting tool with a cBN content in the range of 40-80% by volume comprising a nitride, carbonitride or carbide of titanium are commonly subjected to a wide range of cutting applications ranging from continuous ones with high demands of wear resistance and interrupted ones with high demands of failure resistance. Thus it is of great interest to gain both edge failure and wear resistance in the abovementioned cBN cutting tool.
In the past, it has been proposed to use an intermediate adhesion phase between the ceramic binder and the hard dispersed phase (EP-A-1498199) in order to increase chipping resistance. A bonding phase surrounding the cBN grains has also been proposed (EP-A-974566) in order to prevent a direct cBN-cBN contact. The bonding phase is formed due to a chemical reaction between cBN or B2O3 residuals coating the cBN grains and the ceramic binder forming TiB2. Furthermore, cBN grains have been pre-coated by a nitride or boride of Ti and Al by a PVD-process in order to enhance the reinforcing rim surrounding the cBN grains (U.S. Pat. No. 6,265,337).
It has now been found that an intermediate phase between the ceramic binder phase and the hard dispersed cBN phase can actually decrease the edge toughness of the insert material because one very important mechanism for toughening, namely crack deflection, is minimized. If the bonding between the different phases in the material is too strong, a formed crack will easily propagate through the material in a very straight manner leading to a low value of fracture toughness. If the bonding is too low, it would mean a significantly reduced wear resistance. If, however, the bonding is balanced, meaning it should be lower than the intrinsic strength of the grains, the crack will preferably propagate along the grain boundaries meaning higher toughness. The desired strength of the bonding of the cBN grains and the ceramic binder can be achieved by carefully controlling the sintering temperature and the reactivity of the raw materials.
U.S. Pat. No. 4,343,651 disclose a sintered compact having a cBN concentration above 80 wt % wherein the TiB2 can be minimized by the addition of Cu and/or Fe.
There is a need of further improved cBN-based tools since a higher productivity with reduced costs is required from the industry. Generally this implies higher cutting speeds and in particular higher cutting depths and feeds often in combination with interrupted cuts. Consequently both improved wear and edge failure resistance are desired in order to meet the demands from the machining industry.