Preparation And Tribological Properties Of CNTs Reinforced Al₂O₃/TiC Ceramic Conical Die

Conical die is worked in bad condition, including violent friction between the wire and conical die, unstable lubrication, and the influence of boundary lubrication, fluid lubrication and dry friction with each other, during wire-drawing process. The dimension and the quality of the surface of wires decreased when the working surface of the conical die doesn't work well caused by the wear of the working face. It is an urgent need to improve the tribological and mechanical properties of conical die. The strengthening and toughening of conical die materials, the self-lubrication properties and wearreductionandwearresistance of bore surfaceare the key to solving this problem, and are also the basic science questions during the application of conical die in practice. Because carbon nanotubes?CNTs? have both high strength and self-lubrication, carbon nanotubes were added into Al₂O₃/TiC ceramic matrix, andcarbon nanotubesreinforced Al₂O₃/TiC/CNTs ceramic composites conical die material were prepared byvacuum hot pressing sintering. The preparation, the structure of die, the tribological and mechanical properties, crack propagation,and strengthening mechanism of conical die were investigated.Selected Al₂O₃/TiC particle as the matrix, added CNTs and other trace elements to the matrix, and sintered on hot-pressuring, Al₂O₃/TiC/CNTs ceramic composites with high hardness and wear resistance was fabricated. Experiments testing on physical and mechanical properties of this compositeswere tested and analyzed. The result shows that Al₂O₃/TiC/CNTs ceramic composites have a high relative density and its bending strength and microhardness was decreased comparing with Al₂O₃/TiC ceramic composites without CNTs. With the contents of CNTs increased, the bending strength and microhardness was decreased, and the fracture toughness first was decreased, reaching to 6.04 ± 0.1MPa·m1/2, and then increased subsequently. It has an average dimension of grains and transcrystalline fracture of the material.Based on the model?and model?-?fracture propagation theory, Crack pinning and crack deflection FEM analysis model were established and the effects of CNTs on the crack tip of Al₂O₃/TiC ceramic composites were evaluated. The result shows that the stress concentration around the carbon nanotube is found when the crack is pinned on the carbon nanotube. With the prolonging of the time, the stress at the tip of this material was increased, indicating that a large number of strain energy at the crack tip of ceramic matrix was absorbed by carbon nanotube; therefore the stress of ceramic matrix reduced, the crack growth was prevented and the resistance of crack propagation increased. Due to the addition of CNTs, the stress field of crack growth path was changed when crack appear somewhere near the CNTs. The crack was preferred to propagate along the weakest region. Consequently, the crack bypassed the CNTs and went on in the previous direction. At last, the total length of crack increased. If The factors such as strain energy, potential energy and so on are not considered, the surface energy of the newly formed crack consumed a large number of fracture energy and the resistance of crack propagation increased. After finishing crack data obtained from Abaqus, the coordinate values of the point on elements that crack passing through were obtained, where the value of signed distance function is zero, the length of deflected crack was solved with an approximation solution method. It can be seen that the length of the crack increased by 25% with the addition of a single carbon nanotube.The finite element model is established for drawing process using Abaqus with Al₂O₃/TiC/CNTs ceramic material properties. The result shows that the optimum cone angle of die is 7°; the drawing force increased as angle increased when the cone angle was more than 7°; the drawing force was decreased as angle increased when the cone angle was less than 7°. When the degree was more than some value, the wire can not completely contact with die inner surface due to the elastic plastic deformation of wire. Gaps was found at the sizing area and lower friction force it was. So the drawing force showed decreases. The force at working section was bigger than fixed diameter section of the die and the maximum force position at the minimum diameter of working section.Under dry friction, friction and wear tests of Al₂O₃/TiC/CNTs ceramic material was made and its tribologicalproperties. Result shows that CNTs can reduce the friction coefficient, so that the friction coefficient of Al₂O₃/TiC/CNTs ceramic material is lower than Al₂O₃/TiC ceramic material without CNTs. The friction coefficient with CNTs contents is 0%>?1%,2%,3%,4%?>5%. With different loads and rotational speed, the friction coefficient was decreased with increased loads and rotational speed. The wear loss of Al₂O₃/TiC/CNTs ceramic material is low and the change of wear loss was not obvious with different loads and rotational speed. It can be seen that the wear loss first was decreased and then increased subsequently. When the content of CNTs is 4%, the wear loss reached the minimum. During the friction, CNTs were smeared on the worn surface, prevent the contact between the friction pairs, and show a good performance in self-lubrication, which could improve the friction-reducing and antiwear. Abrasive and fatigue wear were the major wear form of Al₂O₃/TiC/CNTs ceramic conical die composite.