HPHT sintering of binderless Si3N4: structure, microstructure, mechanical properties and machining behavior

Abstract

Silicon nitride (Si3N4) is widely used in the manufacture of cutting tools due to the combination of properties such as high hardness, fracture toughness and wear resistance. This ceramic is usually sintered by liquid phase, causing the reduction of thermo-mechanical properties. This work investigated the sintering of pure (binderless) Si3N4 tools by a high pressure and high-temperature (HPHT) technology. Sub-micrometric α-Si3N4 powder was hot-pressed at 1700 °C for 3 min, using extreme pressures of 5, 6 or 7 GPa. The sintered samples were characterized by XRD and atomic force microscopy (AFM); hardness and fracture toughness were measured by the indentation fracture method (IF). Preliminary machining tests with binderless-Si3N4 tools were performed using AISI 4140 hardened steel. Commercial TiN-coated hard metal insert was used as a comparative tool material. Turning tests were carried out using coolant/lubricant, cutting time of 12 min, cutting speed of 150 m/min, cutting depth of 0.3 mm and feed of 0.11 mm/rev. The results showed the presence of α-Si3N4 and β-Si3N4 crystalline phases after sintering, indicating partial α → β phase transformation, with elongated β-Si3N4 grain. The relative density after sintering was near 90–97%. The best results for the mechanical properties were hardness of 21 GPa and fracture toughness of 8.9 MPa m1/2. The machining results indicated an improvement on surface quality when using Si3N4 ceramic tool. The flank wear of the hard metal insert was 0.7 mm, while the wear of binderless Si3N4 insert was substantially lower, 0.1 mm. The roughness (Ra) measured in AISI 4140 steel was 17.6 μm (HPHT Si3N4 insert) and 19.7 μm (TiN-coated hard metal insert), after machining tests.