Finite element design and fabrication of Al2O3/TiC/CaF2 gradient self-lubricating ceramic tool material
Introduction
Ceramic tool materials are attracting significant attention as high-performance cutting tool materials. However, their inherent brittleness, low tensile strength, and poor toughness limit their further application. Significant efforts have been exerted to enhance the mechanical properties, tool life, and reliability of ceramic tool materials [1], [2], [3]. Considering the harsh friction conditions of cutting, particularly high-speed dry cutting, high cutting temperature and thermal wear of cutting tool become important problems. Lubrication and friction conditions of cutting tools should be improved while maintaining the mechanical properties of the tool materials and simultaneously reducing the friction and wear. With this method, the tool life can also be improved.
Self-lubricating cutting tool is being considered as an effective and significant lubrication technology in cutting process. The addition of solid lubricant in ceramic matrix is found to be a method to realize self-lubrication. Al2O3/TiC ceramic cutting tool with CaF2 solid lubricant, which improves the tribological properties of ceramic tool, was produced by hot pressing technique [4], [5].
The addition of solid lubricant on cutting performance has two main effects. On one hand, the solid lubricant forms a lubricating film on the friction interface, thereby improving the contact state of cutting tool and the workpiece, resulting in decreased friction coefficient and increased cutting tool life. However, on the other hand, the solid lubricant can decrease the mechanical properties of the cutting tool material, and then decrease its wear resistance and shorten the tool life. The current research recognizes that balancing both the antifriction property and wear resistance of self-lubricating ceramic tools is difficult. Some results [4] indicated that with different contents of solid lubricant, the flexural strength, hardness, and fracture toughness of Al2O3/TiC/CaF2 ceramic tool significantly decreased compared with those of common A12O3/TiC ceramic tool. When 5% to 10% CaF2 was added, the flexural strength, hardness, and fracture toughness of the developed self-lubricating ceramic cutting tool materials decreased by 24–39%, 27–37% and 31–35%, respectively.
Functionally gradient materials (FGMs) were proposed and developed by Japanese scholars in the middle of 1980s to satisfy the material requirements in aerospace technologies. FGM is a kind of new composite material with composition, structure, and properties that change gradually from one surface to the other. Increasing attention has been given to FGMs because of their potential for novel designs and outstanding properties. It becomes one of advanced subjects of material science research nowadays [6], [7], [8]. An Al2O3/TiC functionally gradient ceramic tool material was designed using finite element method (FEM) and prepared by hot pressing [1]. Cho [9] determined the optimum design of the material composition in FGM lathe bit to minimize the thermal stress level; he also simulated the distributions of thermal stress and determined the material composition distribution, which had smaller values both in global and local thermal stresses.
In the present study, the FGM concept is utilized in designing the self-lubricating ceramic tools. Models for the design of gradient self-lubricating ceramic tool materials are presented. The distribution regularities and characteristics of residual thermal stresses with different distribution exponents are analyzed and simulated with FEM. An Al2O3/TiC/CaF2 gradient self-lubricating ceramic tool material is developed based on the optimum design results.
Section snippets
Design thought
The idea of FGM is used to develop self-lubrication for ceramic tools based on the problem that the antifriction property and wear resistance of self-lubricating ceramic tools cannot be considered simultaneously. Ceramic tool material systems are tailored and designed using gradient composite technology of solid lubricant. During the design process of gradient composites, the solid lubricant content is reduced gradually from the surface to internal regions of the cutting tool materials. In such
Material fabrication
The starting powders used in fabricating the gradient self-lubricating ceramic tools are listed in Table 2 with their particle sizes, purities, and manufacturers. Four kinds of Al2O3/TiC/CaF2 (marked as ATC) composite powders at different mixture ratios (Al2O3/TiC with volume ratio 3:7 and 14 vol% CaF2, Al2O3/TiC with volume ratio 9:11 and 13 vol% CaF2, Al2O3/TiC with volume ratio 6:4 and 12 vol% CaF2, Al2O3/TiC with volume ratio 3:1 and 11 vol% CaF2) were prepared with small amounts of sintering
Radial residual stresses
Given that the FEM model is rotated along the Z axis (Fig. 1) and the radial stress is exactly the same with the circumferential stress, when one stress is analyzed, the other is known. The maximum radial tensile and compressive stresses versus the component distribution exponent n are shown in Fig. 3. When n=0.6, the maximum radial tensile and compressive stresses are 141.7 MPa and −52.4 MPa, respectively. With the increase in n, the maximum radial tensile stress decreases monotonically, whereas
Conclusions
- (1)
The physical, composition distribution, micromechanical model and FEM models of Al2O3/TiC/CaF2 gradient self-lubricating ceramic tool materials are established based on the materials design objective.
- (2)
When n=1.8, the Al2O3/TiC/CaF2 gradient self-lubricating ceramic tool materials show relatively smaller tensile stresses in middle layer, higher compressive stresses in the surfaces, and the lowest magnitude of the maximum Von Mises stress. The optimum n is finally determined to be 1.8.
- (3)
An Al2O3
Acknowledgements
The authors acknowledge the support provided by the Program for New Century Excellent Talents in University (Grant no. NCET-10-0866), the Shandong Provincial Natural Science Foundation for Distinguished Young Scholars, China (Grant no. JQ201014), the National Natural Science Foundation of China (Grant no. 51075248) and the Key Project of Chinese Ministry of Education (Grant no. 212097).
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