Mechanical properties of cBN-Al composite materials dependence on grain size of cBN and binder content

McKie, Amanda Lynne
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PcBN materials have been widely used as cutting tools for ferrous materials for which PcD materials have limitations. Like diamond, cBN has excellent properties, a hardness in excess of 40 GPa, good thermal and chemical stability and good thermal conductivity. Several research studies have gone into the development of PcBN materials, but very little has been done on determining the mechanical properties affecting them. It is important to know the mechanical properties of polycrystalline cBN materials such as hardness, fracture toughness and strength ( f) in order fully to understand the behaviour of these materials in application. There has not yet been an extensive mechanical testing out on cBN-Al composites. The aim of this project is to investigate the relationship between the microstructure and mechanical properties for a wide range of composite materials based on polycrystalline cubic boron nitride and aluminium as a binder phase (PcBN-Al). The PcBN-Al composites were made using high-pressure high-temperature (HPHT) sintering methods, yielding materials with grain sizes of cBN of between 2 – 20 μm and an amount of Al binder of between 15 – 25 vol.%. Mechanical properties tested were hardness, fracture toughness, KIC, R-curve behaviour and transverse rupture strength (TRS). Hardness ranged between 15 – 40 Abstract - iv - GPa, while fracture toughness and strength were between 6.4 – 8.0 MPa.m1/2 and 355 – 454 MPa, respectively. Fractography was employed to rationalize the scatter in fracture strengths, to understand the nature of the flaws and correlate fracture strength with fracture toughness through the size of the fracture origins. The main findings concerning structure properties relationships of the PcBN-Al composite materials are: There is a strong dependence of hardness on grain size and binder phase, but a weaker dependence exists for fracture toughness and strength. The results confirm that the hardness increases with increasing cBN grain size and decreasing binder content. Fracture toughness generally increases with increasing cBN grain size for low binder contents and decreases with increasing binder content. R-curve behaviour was also found. It is suggested that the toughening mechanisms involved in the cBN-Al composites are due to crack and grain bridging. Strength decreases with increasing cBN grain size and there is no relationship with a change in binder content. Large strength limiting flaws were found to exist in the materials, these flaws ranged between 100 – 500 μm. The strength limiting flaws are caused mainly by large binder pools.