| Aiming at the problem that existing self-lubricating ceramic cutting tool are not available to possess rational combination of anti-friction and anti-wear properties, nano-sized solid lubricant is proposed to replace the traditional micro-sized solid lubricant and form the intra-granular nanostructure, and the gradient design is used to make the nano-sized solid lubricant content decrease gradually from the material’s exterior into its interior and residual compressive stress exists on its exterior. Based on the synergistic modification effect of nano-sized solid lubricant and gradient design on the ceramic cutting tool, the new ceramic cutting tool was developed which have both anti-friction and anti-wear properties. Moreover, the effect of nano-sized solid lubricant and gradient design on the microstructure, mechanical property, friction and wear behavior and cutting performance was studied and analyzed.Based on the study of self-lubricating ceramic material and the toughening mechanism of nano-sized particles, the design idea of modification of nano-sized solid lubricant on the ceramic cutting tool is proposed. When the nano-sized solid lubricant enter the matrix grain and form the intra-granular nanostructure, the elastic modulus and hardness is better than that with the addition of micro-sized solid lubricant. The theoretical calculation shows that the flexural strength will not decline when the content of nano-sized solid lubricant is lower than4.33vol.%. Nano-sized solid lubricant has the toughening mechanism of the crack deflection and the microcrack toughening.Nano-sized CaF2which meets the needs of self-lubricating ceramic cutting tool is parpered, and is used in the preparation of ceramic cutting tool which has intra-granular nanostructure. The prepared nano-sized CaF2has good dispersity, high purity and uniform particle size, the average grain size is about20.4nm. The nano-sized CaF2dispersion is used in the preparation of ceramic material and form almost completely intra-granular nanostructure. The grain size of nano-sized CaF2 which located in the matrix grains is about20-50nm, and is nearly spherical. Compared with the addition of micro-sized CaF2, the flexural strength, hardness, fracture toughness and elastic modulus of the composite with the addition of nano-sized CaF2was increased by28%,24%,45%and29%, respectively. The intra-granular nanostructure is the main reason of high mechanical properties. With the nano-sized CaF2content increased, the flexural strength is first increased and then decreased, the hardness reduces, and the fracture toughness increases. When more than10vol.%content of nano-sized CaF2was used, the hardness and the flexural strength decreased rapidly.The gradient design was used to design the material content of different layers, make the content of nano-sized CaF2decreases gradually from the surface layer to interior layer and form the residual compressive stress on the surface layers. The residual thermal stress was analyzed by the finite element methods, the results show that the maxinum compressive residual stress of the surface layers of G-ATCn cutting tool and G-ATWCn cutting tool was130MPa and213MPa. The microstructure and the material content of sintered gradient ceramic cutting tools is gradually changed, the components in adjacent graded layers bonded with each other at the interfaces, and the interfaces are clear and no defect can be found at the interfaces. The results obtained from the indentation method have a similar trend with the finite element results. The mechanical properties of gradient cutting tool are higher than that of homogeneous cutting tool, and the effect of gradient design on the flexural strength is the most significant. The flexural strength, fracture toughness and hardness of G-ATCn cutting tool are745MPa,7.66MPa·m1/2and19.6GPa, respectively. Compared with the homogeneous cutting tool, that is increased by19.8%,6.7%and2.8%, respectively. The flexural strength, fracture toughness and hardness of G-ATWCn cutting tool are865MPa,6.73MPa·m1/2and17.1GPa, respectively. Compared with the homogeneous cutting tool, that is increased by74.0%,3.9%and5.0%, respectively.The friction coefficient of the ceramic cutting tool modified by nano-sized solid lubricant is mainly affected by the content of nano-sized CaF2, the wear rate is not only affected by it, but also affected by the mechanical properties which also caused by nano-sized CaF2. With the nano-sized CaF2content increased, the friction coefficient of ATCn cutting tool decreases, the wear rate firstly decreased and then increased. When the content of nano-sized CaF2is10vol.%, the wear rate reach the minimum, only5.4×10-7mm3/N·M. Gradient design has almost no effect on the friction coefficient, but the wear rate can be improved due to the improved mechanical properties and residual compressive stress exists in the surface layer, and the improvement effect of wear rate become more obvious with the increase of friction speed and load. With rising of ambient temperature, the friction coefficient and wear rate of G-ATCn cutting tool and H-ATCn cutting tool are decreased. With rising of ambient temperature, the abrasive wear of H-ATCn cutting tool decreased and the adhesive wear hard, gradient design can significantly improve the abrasive wear and adhesive wear.The addition of nano-sized CaF2can improve the machining quality and reduce the cutting force, the cutting temperature and the friction coefficient in machining. Gradient design can improve the anti-wear property, lower the surface roughness of the work-piece and reduce the cutting force and cutting temperature. The wear mechanisms of G-ATCn cutting tool rake faces are micro chipping and crater wear, and that of flank faces is micro chipping and adhesive wear. The wear mechanisms of G-ATWCn cutting tool rake faces are crater wear, and that of flank faces is abrasive wear and adhesive wear. The addition of nano-sized CaF2can relieve the adhesion wear, gradient design can improve the micro chipping and crater wear by the residual compressive stress in the surface layer. |
PDF Full Text Request