Development Of High-Performance Si₃N₄/TiC Nanocomposite Ceramic Tool Materials And Investigation Of Their Properties

Compared to Al₂O₃-based ceramic materials, Si₃N₄-based ceramic materials have more excellent mechanical properties, such as low coefficient of thermal expansion, high chemical reliability, creep resistance, flexural strength and thermal-shock resistance. Due to its covalent nature of bonding, it's difficult to get densified. Simultaneity, pure Si₃N₄ ceramics have low hardness, bad wear resistance. So it's imperative to improve their mechanical properties with different additives.In order to prepare good-quality, dense, TiC-reinforced Si₃N₄ nanocomposites by hot-pressing, many techniques were developed to prevent agglomeration of the nanopowders during powder processing. For this purpose, the surfaces of the nanoparticles were selectively covered with long organic molecular chains by soaking in a solution of polyethylene glycol(PEG) suspensions, assisted with ultrasonic dispersion and mechanical agitation. The result powder not only resisted re-agglomerateion, but was also readily dispersible within the Si₃N₄ matrix. Mechanical properties of materials with different combination of compositions were compared, such as Si₃N₄/Si₃N_4(n)/TiC_n,Si₃N₄/TiC_nå’Œ Si₃N₄/Si₃N_4(n)/TiC_sub-μ. The microstructure and composition of Si₃N₄/TiC nanocomposites have been investigated by means of H-800 transmission electron microscope (SEM), S-750 scanning electron microscope (SEM) and RAX-10A X-ray diffraction (XRD). The brittle fracture mechanism of nanocomposite is discussed. Due to TiC nanoparticles and β-Si₃N₄ quasiwhiskers with a distinct bidomal grain diameter distribution, the synergetic toughening mechanism was also studied. The thermal shock resistance and R curve of nanocomposites were investigated by means of indentation-quenching and indentation quenching-retained strength testing methods. The high temperature oxidation and mechanisms ofnanocomposites has been investigated. Finally, machining tests were also carried out.The dispersion experiments indicated that a good-quality, re-agglomerated composite powder may be prepared by the optimization of the molecular weight and concentration of dispersant, pH value and concentration of dispersion solution. By TEM, it is found that good dispersion of single and composite powders has been achieved by using PEG as a surfactant.The fabrication technique principles and sintering mechanism of nanocomposites were discussed. By staged heat rate and sintering pressure, densified nanocomposites with high density over 98% were fabricated.Phase composition and microstructure of nanocomposites indicated that Si3N4/TiC nanocomposite is combined with P-SiaN4, TiC and glass phase. TiC nanoparticles distribute in the matrix and at the grain boundaries. The nano-sized particles refined the matrix grains and promoted the forming of quasiwhiskers duplex distribution with internesting microstructure.With 10% weight fraction of nano-SisN4 and 15% of TiC powder, the Si3N4/TiC nanocomposite tool materials show the best mechanical properties. The flexural strength, fracture toughness and Vicker's hardness were 1025MPa, 8.5MPam1/2 and 16.95GPa respectively.In this paper, microcrack propagation Involves crack deflection, crack bridging and quasiwhiskers pullout, bridging, resulting in small amount of fracture within P-SisN4 matrix and TiC hardening phase.The thermal shock resistance of the Si3N4/TiC nanocomposites was measured by the indentation-quench testing, and the crack grow resistance behavior(R-curve) of nanocomposites were determined by the indentation-strength method. The results of the test were compared with the results of quenching test, showing that the two methods are consistent. The thermal shock temperature of composites with TiC and Si3N4 nanoparticles is over 700 °C with rising R curve. The distinct interrelation between R curve, the thermal shock resistance and strengthening as well as toughening mechanisms was qualitative analyzed also.The oxidation behavior at high temperature of nanocomposites has been investigated. The results indicate the oxidation weight gains of nanocomposites follows parabolic law as the time increases. With the increase of TiC content, the oxidation resistance decreased. Measurements of flexural strengths show that the residual strengths of the nanocomposites after oxidation for 100 hours at the temperature of 1000°C decrease slightly.Compared with SNM88, machining tests indicate that SiaN^TiC nanocomposite ceramic tools are suitable for continuous cutting iron and hardened steel. The performance of wear and impact resistance of SiaN^TiC is higher than that of SNM88.