Study On The Fabrication And Cutting Performance Of Ceramic Tool Materials Based On Multi-phase And Multi-scale Nanocomposites

From the requirement for ceramic tool materials applied in the high speed and dry machining, a new thought for designing the multi-phase and multi-scale nanocomposites was firstly proposed to improve the comprehensive mechanical properties. Based on this thought, four types of advanced Al₂O₃ matrix ceramic tool materials with high mechanical properties were fabricated successfully. The strengthening and toughening mechanisms of the ceramic tool materials based on multi-phase and multi-scale nanocomposites were investigated from the respects of the correlations among the hot pressing process, the microstructures and mechanical properties. It reveals that the intra/inter granular microstructures and the trans/inter granular fracture modes are the main causes for improving the flexural strength and fracture toughness. The good cutting performance of the developed ceramic cutting tools indicated that the study on the ceramic tool materials based on multi-phase and multi-scale nanocomposites has offered a new path to the further research.The new thought for designing the multi-phase and multi-scale nanocomposites was firstly proposed to improve the comprehensive mechanical properties. Multi-phase and multi-scale particles are added to the matrix, and one of the additives is nano-scale particle, thus the comprehensive mechanical properties can be improved by the synergic effects of micro-scale toughening, nano-scale strengthening and mutual benefit of multi-phases. The ideal microstructure of multi-phase and multi-scale nanocomposites was designed, which was composed of intergranular distributed different micro-scale phases and intra/inter granular nano-scale particles. With this ideal microstructure, the trans/inter granular fracture modes can be formed, which will consume more fracture energy during the crack propagation, therefore both the flexural strength and fracture toughness can be improved.Based on the stabilization mechanisms for suspensions in the colloidal chemistry, the dispersions of different nano-scale ceramic powders in liquid suspension were discussed. With the optimal parameters such as the type of dispersant, dispersingmedium, the pH value and the content of dispersant, the nano-scale powders and mixed powders were well dispersed with the combination of ultrasonic vibration and mechanical agitation.Multi-phase nanocomposites ceramic tool material of AkCtyAhCWSiCnOnarked as AAS) was firstly fabricated successfully, its flexural strength, fracture toughness and Vickers hardness are 796MPa, 5.01MPa-m1/2 and 21.32GPa respectively. The coactions of nano-scale AI2O3 and nano-scale SiC led to the reduction of the sintering temperature and the shorter preserving time, and therefore the composite with high density and fine grains was obtained. The improvement of the mechanical properties is mainly derived from the transition from intergranular to transgranular fracture mode.Multi-scale nanocomposites ceramic tool material of Al2O3/SiCM/SiCn(marked as ASs) was successfully fabricated, its flexural strength, fracture tougliness and Vickers hardness are 715MPa, 8.2MPa-m1/2 and 22.57GPa respectively. The comprehensive mechanical properties are much higher than that of any composite added with single nano-scale SiC or with single micro-scale SiC. The micro-scale SiC particles are located between AI2O3 matrix, and the nano-scale SiC particles are located on the grain boundary or within the matrix grain. Thus the typical intra/inter granular microstructure is formed in the dense compacts, which resulted in the trans/inter granular fracture modes. The zigzag crack path, which is from the grain boundary into the grain and then turning to the boundary, can result in higher consumption of fracture energy and the increase of fracture toughness.An advanced multi-phase and multi-scale nanocomposites ceramic tool material Al2O3/TiCn/TiNn(marked as LTN) was fabricated with the flexural strength, fracture toughness and Vickers hardness of 731MPa, 7.8MPam1/2 and 19.76Pa accordingly.. Because of the addition of micro-scale TiC with high elastic modulus and high harness, it is apt to form the framework structures which inlay each other with AI2O3 and improve the flexural strength of composite. Proper content of nano-scale TiN can lead to the further refinement of matrix grains, the remarkable strengthening and toughening effects are resulted from the crack pinning, deflection and branching.The strengthening and toughening mechanisms for multi-phase and multi-scale nanocomposites ceramic tool materials were thoroughly researched. The main strengthening and toughening mechanisms of the materials include the effects of grain fining and grain boundary strengthening caused by the nano-scale particles, residualstress toughening and the toughening effect caused by particular microstructure. The strengthening and toughening mechanisms for multi-phase and multi-scale nanocomposites ceramic tool materials were emphasized on the toughening effects caused by residual stress and the particular microstructures in the present study.A new mathematics model is firstly established to calculate the residual stress in the composites, the advantage of the new model over the existing simple model consist in considering different-sized particles and the irregular distribution of grains. Two finite element models for analyzing the residual stress were set up. The results analyzed with the ANSYS software clearly showed that the stress characters and values were significantly influenced by the particles' size, grain distribution and the content of the added particles. Combined with the residual stress and the corresponding crack propagation path, the toughening mechanisms of multi-scale nanocomposites ceramic tool materials were discussed, which were well consistent with the experimental results.AI2O3 nanobelts were firstly observed in the material ASs with HRTEM, the forming of ductile nanobelts is helpful for the improvement of fracture toughness. In addition, the special microstructure such as dislocations and compound twins were also observed in the nanocomposites AAS and ASs with TEM, the complicated dislocation configurations and the forming of twins in larger SiC particles also contributed to the fracture toughness.The investigation for the cutting performance showed that, the novel tool AAS is preferable to LT55 when cutting 40Cr materials. While for novel ceramic "tool ASs and LTN appeared preferable when cutting hardened T10A with middle or a little lower cutting speed, their cutting performances were higher than that of SG4 ceramic tool, which was related to the higher comprehensive mechanical properties.