| Aimed at the low fracture toughness of TiB2-based ceramics, the lack of study on high-temperature mechanical properties and the limitation of the traditional fabrication process, the Reactive Hot Pressing (RHP) technology was proposed to fabricate TiB2-based ceramic cutting tools toughened by in-situ growth elongated TiB2 grains. Advanced ceramic cutting tools with good mechanical properties at both room and elevated temperatures as well as good cutting performance were fabricated. The phase composition, microstructure evolution, mechanical properties, high-temperature flexural strength, growth mechanisms of the elongated grains, toughening mechanisms, cutting performance and wear reliability evaluation of the ceramic cutting tools were investigated systematically. The research results enriched the fabrication technologies for ceramic cutting tools and laid the foundamental for the development and application of advanced ceramic cutting tools with higher mechanical properties.The Reactive Hot Pressing (RHP) technology was proposed to fabricate in-situ growth elongated TiB2 grains toughened ceramic cutting tools. The integrative fabrication of ceramic cutting tools reinforced by in-situ growth elongated TiB2 grains was achieved.TS series (TiB2-SiC) ceramic cutting tools were fabricated successfully. With an increase in the content of SiC, the fracture toughness increased while the flexural strength and hardness decreased. TS10 (TiB2-10wt.%SiC) had the optimum comprehensive mechanical properties at room temperature. Its flexural strength, fracture toughness, Vicker’s hardness and relative density were 705MPa,6.6MPa·m1/2,22.5GPa and 98.7% respectively. The elongated TiB2 grains (single crystals) with a diameter of 1～2μm, an aspect ratio of 3-6 and a yield of 12vol.% were in-situ synthesized. The toughening mechanisms of the elongated TiB2 grains were crack deflection, crack bridging, crack branching and grain fracture.TSN series (TiB2-SiC-Ni) ceramic cutting tools were fabricated successfully. The content of Ni had a significant effect on the mechanical properties of the ceramic cutting tools. TSN5 (TiB2-SiC-5wt.%Ni) had the optimum comprehensive mechanical properties at room temperature. Its flexural strength, fracture toughness, Vicker’s hardness and relative density were 1121MPa,7.9MPa-m1/2,21.3GPa and 98.6% respectively. The elongated TiB2 grains (single crystals) with a diameter of 0.2-1.5μm, an aspect ratio of 3～10 and a yield of 18vol.% were in-situ synthesized. The toughening mechanisms of the elongated TiB2 grains were crack deflection, crack branching, crack bridging and grain fracture. The ductile phase toughening effect of Ni improved the fracture toughness of the ceramic cutting tools.TTS series (TiB2-TiC-SiC) ceramic cutting tools were fabricated successfully. The growth of TiB2 grains was inhibited by the in-situ synthesized TiC and the ceramic composites with fine and homogenous microstructure were obtained. TTS (TiB2-TiC-SiC) had the optimum comprehensive mechanical properties at room temperature. Its flexural strength, fracture toughness, Vicker’s hardness and relative density were 768MPa,7.3MPa-m1/2,21.8GPa and 99.1% respectively. The elongated TiB2 grains (single crystals) with a diameter of 0.5-1μm, an aspect ratio of 3-8 and a yield of 20vol.% were in-situ synthesized. The toughening mechanisms of the elongated TiB2 grains were crack deflection, crack bridging, crack branching, grain fracture and grain pullout.The flexural strength of the advanced ceramic cutting tools at elevated temperatures and its increment/degradation mechanisms were investigated. The flexural strength of the TS series ceramic cutting tools decreased with an increase in the temperature. TS10 had an optimum flexural strength at both room and elevated temperatures. Dramatical strength degradation at elevated temperatures was found for TSN series ceramic cutting tools. The evidence of plastic deformation was observed for TSN at the temperatures higher than 800℃. The flexural strength of the TTS series ceramic cutting tools was increased and subsequently decreased with an increase in the temperature. The strength increment was attributed to the healing of the preexisting flaws and the relief of the residual tensile stress. The strength degradation was attributed to the softening effect of the grain boundaries, the surface oxidation and the elastic modulus degradation. The in-situ synthesized SiC improved the oxidation resistance of the TiB2 ceramics. The in-situ synthesized TiC inhibited the grain growth and improved the flexural strength at both room and elevated temperatures. Ni increased the mechanical properties at room temperature while decreased the flexural strength at elevated temperatures. The grain sliding and grain rotation were inhibited by the in-situ synthesized elongated TiB2 grains and the high-temperature flexural strength of the advanced ceramic cutting tools was improved.A geometric model of the microstructure evolution was established. A Solid-Liquid-Solid (SLS) in-situ growth model of the elongated TiB2 grains was established based on the diffusion-controlled mechanism. The strengthening and toughening model of the elongated TiB2 grain bridging and pullout was established. The sequence of the microstructure evolution included the mixed raw powders, the instantaneous liquid, the transitional plastic phase, grain rearrangement and the growth of the elongated grains. The molten Ti during the reaction provided a liquid environment for the growth of the elongated TiB2 grains. The growth kinetics of the elongated TiB2 grains was studied. The length, diameter and aspect ratio of the elongated grains increased with an increase in the growth time at a given temperature. The aspect ratio increased with an increase in the temperature at a given growth time. The contribution of the grain bridging and pullout to the fracture toughness of the ceramic cutting tools was studied. In addition, the toughening mechanisms of the intragranular grain in an elongated grain, the intra-type elongated grain, the intergranular and intragranular nano grains, dislocations, metallic binder and the interface were studied.The cutting performance of the advanced ceramic cutting tools when cutting the hard-to-cut meterials was investigated. The statistical distribution pattern of the tool life was established. The cutting performance and wear reliability evaluation of the advanced ceramic cutting tools when continuously wet-turning Invar36 alloy were studied. The results showed that the optimum cutting parameters for TS10 were v=140m/min, f=0.1mm/r and ap=0.4mm. The tool life was 26.2min and the wear mechanisms were adhesive wear, diffusion wear, oxidation wear and abrasive wear. The tool life of TS10 obeyed the Log-normal distribution and the coefficient of variation was 0.085, indicating a good reliability. The cutting performance and wear reliability evaluation of the advanced ceramic cutting tools when continuously wet-turning austenitic stainless steel (1Crl8Ni9Ti) were studied. The results showed that the optimum cutting parameters for TSN5 were v=100m/min, f=0.1mm/r and ap=0.35mm. The tool life was 24.4min and the wear mechanisms were adhesive wear, diffusion wear and slight oxidation wear. The tool life of TSN5 obeyed the Weibull distribution and the modulus was 1.633, indicating a low reliability. The cutting performance and wear reliability evaluation of the advanced ceramic cutting tools when continuously wet-turning hot die steel (H13) were studied. The results showed that the optimum cutting parameters for TTS were v=100m/min, f=0.1mm/r and ap=0.25mm. The tool life was 45.4min and the wear mechanisms were adhesive wear, oxidation wear and abrasive wear. The tool life of TTS obeyed the Log-normal distribution and the coefficient of variation was 0.069, indicating a good reliability. |
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