Is it possible to sinter binderless Si3N4 bodies for metalworking purpose?

Reference Presenter Authors
14-026 Marcello Filgueira Filgueira, M.(UENF); Nascimento, A.L.(uenf); Souza, D.(Universidade Estadual do Norte Fluminense Darcy Ribeiro); Guimarães, R.d.(Universidade Estadual do Norte Fluminense); SANTOS, C.(UERJ); 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 High Pressure and High-Temperature (HPHT) technology. Sub-micrometric alpha-Si3N4 powder was hot-pressed at 1700ºC for 3 minutes, using extremme 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 indentation fracture method (IF). Preliminary machining tests with binderless-Si3N4 tools were performed using AISI 4140 hardened steel. Commercial TiN-coated hardmetal insert was used as a comparative tool material. Turning tests were carried out using coolant/lubricant, cutting time of 12 minutes, cutting speed of 150 m/min, cutting depth of 0.3mm and feed of 0.11 mm/rev. The results showed presence of alpha-Si3N4 and beta-Si3N4 crystalline phases after sintering, indicating partial alpha-beta phase transformation, with elongated beta-Si3N4 grains. The relative density after sintering was near to 90-97%. The best results for the mechanical properties were hardness of 21GPa, 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 hardmetal insert was 0.7mm while the wear of binderless Si3N4 insert was substantially lower, 0.1mm. The roughness (Ra) measured in AISI 4140 steel was 17.6 microns (HPHT Si3N4 insert) and 19.7 microns (TiN-coated hardmetal insert), after machining tests.
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