Study On The In-situ IntegrativeFabrication Of Ceramic Tools Toughened By The Elongated Grains And Tool Wear Reliability

Aimed at the limitations of fabrication process of the ceramic tools prepared by the hot-pressing sintering technology, the design scheme for the in-situ integrative fabrication of ceramic tools toughened by the elongated grains was proposed. Moreover, the TiB2-TiCx ceramic tools toughened by the elongated TiB2grains and the ZrB2-ZrCx ceramic tools toughened by the elongated ZrB2grains were fabricated successfully. The mechanical properties, phase composition, microstructure, growth mechanism of the elongated grains, densification and toughening mechanism, cutting performance and cutting reliability were studied systematically. The research results enhanced the application domain of the preparation technology of the ceramic tools and laid the theoretical and application foundation for the future development of ceramic tools with higher mechanical properties.The material components and the reaction system were designed based on the transient plastic phase process and the Ti-B-C/Zr-B-C ternary phase diagram. The Ti-B4C was chosen as the reaction system to synthesis TiB2-TiCx ceramic tools toughened by the elongated TiB2grains and the Zr-B4C was chosen as the reaction system to synthesis ZrB2-ZrCx ceramic tools toughened by the elongated ZrB2grains. The thermodynamic calculations on Ti-B4C and Zr-B4C systems indicated that the reactions were spontaneous, self-propagating and exothermic. The fabrication process was proposed based on the SHS(Self-propagating High-temperature Synthesis, SHS) method, which was ignited by the thermal explosion method. According to the theory of thermal explosion and crystallography, the experiment optimization focused on the ignition method, ignition temperature, delay time for the pressure, the pressure after the delay time and the sintering temperature, etc. The fabrication process and the mechanical properties of the ceramic tools toughened by the elongated grains were studied. The TBw(the elongated TiB2grains/TiB2/TiCx) and ZBw(the elongated ZrB2grains/ZrB2/ZrCx) ceramic tools were developed successfully. When the molar ratio of Ti/B4C is4/1, the sintering temperature is1750℃, the duration time is60min, the delay time is5s, the pressure after delay time is16MPa and the final pressure is35MPa, TBw had the highest comprehensive mechanical properties. The flexural strength, fracture toughness, Vickers hardness and relative density are890.3MPa,7.5MPa·m1/2,20.6GPa and99.3%, respectively. When the molar ratio of Zr/B4C is4/1, the sintering temperature is1750℃, the duration time is60min, the delay time is5s, the pressure after delay time is16MPa and the final pressure is35MPa, ZBw had the highest comprehensive mechanical properties. The flexural strength, fracture toughness, Vickers hardness and relative density are898.1MPa,9.1MPa·m1/2,17.6GPa and98.2%, respectively. The relationship among the microstructure, phase composition and mechanical properties was investigated.The anisotropic grain growth mechanisms during the in-situ fabrication process, densification and toughening mechanism of ceramic tools were studied. There were two aspects for the growth mechanism of the elongated grains. The first one is involved in the inherent crystallographic orientation of TiB2and ZrB2.The other one is that the nonequilibrium ripening nucleation process controlled the nucleation and initial growth of the elongated grains. With the decrease in the degree of supercooling, the growth of elongated grains was dominated by the equilibrium ripening growth process and developed completely during the duration time. The densification was attributed to the volume changes during the reaction, the content of carbon vacancies in the non-stoichiometric phases (TiCx and ZrCx), the wetting of the liquid to the matrix and the high temperature yield behavior of TiCx and ZrCx. The toughening mechanisms with the discontinuous elongated grains were crack deflection, bridging and grain pullout. The interlocking structures, platelets, whiskers and the nanoparticles array on the surface of the grains improved the toughness accordingly. The cutting performance and tool wear reliability of TBw ceramic tools in dry machining stainless steel1Cr18Ni9Ti was studied. The highest tool wear life of TBw was obtained when the cutting speed is60m/min, the feed rate is O.lmm/r and the depth of cut is0.1mm. The main tool wear mechanisms of TBw were abrasive wear, diffusion wear and oxidation wear at a low cutting speed. The main tool wear mechanisms of TBw were abrasive wear, diffusion wear, oxidation wear and adhesion wear at a high cutting speed. The Gamma distribution was appropriate for modeling the tool wear life of TBw. The reliability of the tool wear life (11min) is46.54%. The reliable tool wear life of TBw is8.71min when the wear reliability was90%. The variation coefficient of wear life is0.155, thus the tool wear reliability of TBw is good.The cutting performance and tool wear reliability of ZBw ceramic tools in dry machining stainless steel1Cr18Ni9Ti was studied. The highest tool wear life of ZBw was obtained when the cutting speed is40m/min, the feed rate is0.1mm/r and the depth of cut is0.1mm. The main tool wear mechanisms of ZBw were abrasive wear and oxidation wear at a low cutting speed. The main tool wear mechanisms of ZBw were abrasive wear, diffusion wear and oxidation wear at a high cutting speed. The Lognormal distribution was appropriate for modeling the tool wear life of ZBw. The reliability of the tool wear life (15min) is36.51%. The tool reliable wear life of ZBw is12.53min when the wear reliability was90%. The variation coefficient of wear life is0.1099, thus the tool wear reliability of ZBw is good.