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

• Published in: Journal of the Brazilian Society of Mechanical Sciences and Engineering 2018-03, Vol.40 (3), p.1-9, Article 118
• Main Authors: Filgueira, Marcello, Nascimento, Águida Luzia Nery, Oliveira, Michel Picanço, Souza, Djalma, Guimarães, Zulmira Alice Soares, dos Santos, Claudinei
• Format: Article
• Language: English
• Subjects: Atomic force microscopy; Atomic structure; Beta phase; Ceramic tools; Cutting parameters; Cutting speed; Elongation; Engineering; Fracture toughness; Hardness; High strength steels; Indentation; Mechanical Engineering; Mechanical properties; Phase transitions; Silicon nitride; Sintering; Sintering (powder metallurgy); Surface properties; Technical Paper; Test procedures; Thermomechanical properties; Tool wear; Turning (machining)
• ISSN: 1678-5878; 1806-3691
• DOI: 10.1007/s40430-018-1035-6

Silicon nitride (Si 3 N 4 ) 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) Si 3 N 4 tools by a high pressure and high-temperature (HPHT) technology. Sub-micrometric α-Si 3 N 4 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-Si 3 N 4 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-Si 3 N 4 and β-Si 3 N 4 crystalline phases after sintering, indicating partial α → β phase transformation, with elongated β-Si 3 N 4 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 m 1/2 . The machining results indicated an improvement on surface quality when using Si 3 N 4 ceramic tool. The flank wear of the hard metal insert was 0.7 mm, while the wear of binderless Si 3 N 4 insert was substantially lower, 0.1 mm. The roughness ( R a ) measured in AISI 4140 steel was 17.6 μm (HPHT Si 3 N 4 insert) and 19.7 μm (TiN-coated hard metal insert), after machining tests.