Study On In Situ Growth Whisker Toughening Alumina Matrix Ceramic Tool And Cutting Performance

SiC whiskers are generally used in whisker toughening Al₂O₃ ceramic tool materials. However, the compound SiC can react with the element Fe when machining hardened steels in high cutting speed and its thermal expansion coefficient is too low to be an ideal toughening additive to the Al₂O₃. Moreover, the application of whisker in ceramic tool materials is still limited by such disadvantages as difficult dispersion, a high cost, healthy hazard, etc. Therefore, a TiC whisker toughening Al₂O₃ matrix ceramic tool material is successfully fabricated by an in situ growth technology. The growth mechanisms of TiC whisker, the fabrication for mixture powder of in situ growth whisker toughening Al₂O₃ matrix ceramic tool material, as well as the fabrication, mechanical properties, microstructure, toughening mechanisms, cutting performance and tool wear mechanisms of the in situ growth whisker toughening Al₂O₃ matrix ceramic tool are investigated.The growth mechanisms of TiC whisker synthesized by a carbothermal reduction technology are investigated. A flux globe surface transport mechanism and a SLLS (solid-liquid-liquid-solid) whisker growth mechanism are proposed. The growth patterns and the factors affecting the yield and aspect ratio of the whisker in the SLLS mechanism are investigated. It is shown that the Ni is the accelerant phase to induce the growth of TiC whiskers and the NaCl is the transport phase in the carbothermal reduction technology. Based on the surface wettability and evaporation of NaCl flux globes, the raw materials can be transported as solid particles directly to the Ni droplet to sustain the growth of TiC whiskers. There are three whisker growth patterns in the SLLS mechanism such as reaction, Ostwald ripening and coalescence growth. The yield of whiskers in the SLLS mechanism can increase by the means of mixing the raw materials in a blender, increasing the transport phase content to adhere the majority of raw material particles, increasing the accelerant phase content, elevating the heating rate and adjusting the exchanging rate of the protective atmosphere between inside and outside of the reaction container according to the evaporating rate of the transport phase and the dissolving rate of the accelerant phase to raw material particles. The aspect ratio of whiskers can be controlled by such a method as to adjust the content ratio of the accelerant phase to the transport phase in the precursor. Because that the growth of whiskers is reduced by accelerant droplets from solid raw materials, the SLLS mechanism has the advantages of both the VLS mechanism and the SLS mechanism such as wide application in growth of varied whiskers, controllable growth state and commercial foreground, will be an important whisker growth mechanism just as the VLS mechanism.The fabrication for mixture powder of in situ growth TiC whisker toughening Al₂O₃ matrix ceramic tool material is investigated. The results show that the carbothermal reduction technology can be used to prepare the mixture powder, the chemical compatibility between the Al₂O₃ matrix and the precursor for TiC whiskers is ideal. Ball milling in ethonal medium can be used to prepare the precursor to synthesize homogeneou mixture powder in which the TiC whiskers is well dispersed. The ideal growth temperature for TiC whiskers in Al₂O₃ matrix powder is 1480℃. The mechanical properties of the Al₂O₃ ceramic tool material toughened by in situ growth of TiC whiskers can be improved when the carbon content in the precursor decreases to be 97% of the stoichiometric content. Synthesizing at a higher temperature can decrease the element carbon content in the mixture powder by increasing the compound carbon content in the TiC whiskers, which can also improve the mechanical properties of the tool material, therefore the ideal fabrication technology of the mixture powder is to be firstly heated to 1480℃and held for 60min duration, subsequently heated to 1700℃and held for 30min duration. The ideal content (molar ratio) of Ni and NaCl in the precursor is that TiO₂:Ni:NaCl=1:0.05:0.5. A vibration operation to full the precursor powder into react container and the phase transformation fromγ-Al₂O₃ toα-Al₂O₃ can both be used to control the growth and aggregation of the Al₂O₃ matrix grains, improve the mechanical properties of the ceramic tool material.An in situ growth TiC whisker toughening Al₂O₃ ceramic tool material ATW30 is successfully fabricated. The flexure strength, fracture toughness and Vickers hardness of ATW30 are 800MPa, 7.63MPa·m^1/2 and 19.5GPa, respectively. A new concept is suggested that the interface bonding strength of whisker-matrix can be divided into two kinds such as the thermal bonding strength and the mechanical bonding strength. The fracture toughness of ATW30 can be improved by fabrication at a higher temperature because that the whisker-matrix interface bonding strength in ATW30 is mainly formed by the thermal bonding strength that can be strengthened at a higher temperature. Large amount of intragranular TiC whiskers and dislocation configuration are observed in ATW30, which can improve the density and mechanical properties of the material.The model for the ratio of practical area to effective area of crack propagation is established in which the relationship among the ratio of practical area to effective area of crack propagation, elastic modulus, fracture surface energy and fracture toughness is investigated. The results show that the main factor affecting the elastic modulus and fracture surface energy of ATW30 is the composition. The ratio of practical area to effective area of crack propagation of ATW30 is increased by the intergranular fracture and the debond on the whisker-matrix interface. The verification of the model on the ceramic tool material ATW30 shows that the calculated fracture toughness value (8.85MPa·m^1/2) is in consistent with the measured value (7.63MPa·m^1/2). Because that the concrete toughening mechanism need not to be related, the model has more universal applicability and important theoretical guidance significance to the study on advanced ceramic tool materials.The cutting performance and tool wear mechanisms of the ceramic tool material ATW30 when machining hardened steels such as 40Cr, 45^# and T10A are investigated and compared to the ceramic tool material SG4. The results show that the wear resistance ability of ATW30 is superior to SG4. The wear resistance ability of ATW30 when machining hardened 40Cr in low cutting speed is superior to that in high cutting speed. The main wear mechanism is abrasive wear in low cutting speed whereas abrasive wear and adhesive wear in high cutting speed. The wear resistance ability of ATW30 when machining hardened 45^# and T10A in high cutting speed is superior to that in low cutting speed. The wear mechanism is abrasive wear, indicating that the ceramic tool material ATW30 has excellent chemical stability and very low affinity when machining such hardened steels as 45^# and T10A.