Polycrystalline c-BN

A new approach in this area is the synthesis of high purity polycrystalline c-BN sintered bodies [155]. To get a full conversion of h-BN into pure c-BN, pressure and temperature must be relatively high and pure h-BN (<0.03 wt% B203) is necessary. Typical conversion parameters are 7.7 GPa, 1900-2700 °C, and 15 min reaction time. Above 2400 °C no h-BN was observed by X-ray diffraction in the sintered bodies. The c-BN grain size increases from 0.5 pm at 2300 °C to 5 pm at 2700 °C [155],... 

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

A polycrystalline compact - containing c-BN powder and a second phase - can be sintered at parameters where c-BN is the stable phase (to prevent h-BN formation). Therefore, high-pressure high-temperature sintering is necessary, and the maximal diameter for the produced parts is limited by the dimension of the high pressure apparatus. 

With a high-pressure hot-pressing method c-BN-TiC/TiN composites are prepared by sintering and subsequently heat treatment between 1000 and 1400 °C. The samples exhibit a dense polycrystalline structure of c-BN-TiN/ TiC, and a thin layer of fine TiB2 is visible at the c-BN-binder interface. Hardness decreases significantly after heat treatment [256]. 

Fig. 4.1-58 c-BN (BNcub). Electrical conductivity vs. reciprocal temperature for several different polycrystalline samples above RT [1.52,53]... 

P-BN tools work satisfactorily in hardened steel up to contact temperatures of 1000°C, since there is no chemical reaction between boron nitride and iron. This, however, also depends on the binding phase of the polycrystalline materials and can lead to adhesive wear [24, 25]. In hard steel, the main wear mechanism on the tool is abrasion by hard alloy carbide particles [26]. In the case of Co-based super alloy (Vitallium), the results on hard-BN tool wear are somewhat incongruous [27, 28], while Inconel 718 can be machined under proper selection of the cutting conditions [29]. Apparently, austenitic steels containing a high percentage of Co are difficult to cut by hard-BN tools, due to the formation of cobalt nitrides which leads to high tool wear [8]. 

Boron Nitride (BN) parallel to c axis Thorium Dioxide (Th02) Beryllium Oxide (BeO) — polycrystalline Thorium Dioxide (Th02) 10.15xl0 for25to 350 C 10.17x10 for 25 to 1500 C 10.3x10 for 25-1500 C 10.43x10 for 25 to 1700 C... 

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