Design And Development Of Self-lubricating Ceramic Cutting Tool And Study On Its Self-lubricating Mechanism

The concept of self-lubricating tool was proposed firstly, namely that solid lubricants were added into ceramic matrix to improve its tribological behaviors. A self-lubricating Al₂O₃/CaF₂ (ATF) ceramic tool material has been developed successfully with adding solid lubricant CaF₂ to Al₂O₃/TiC matrix. Tribological behaviors and self-lubricating mechanisms were detailed studied as well as its design theory, hot pressing parameters, mechanical properties and microstructure.On the base of analyzing frictional characteristics of cutting process, a model of decreasing friction was established for self-lubricating ceramic cutting tool. If there was a solid lubricating film on the tool surface, the friction will be decreased, even there was no a full film on tool surface, the friction still was decreased to a certain extent. The chemical compatibility of the self-lubricating ceramic cutting tool material was analyzed and the critical volume fraction of solid lubricants was determined. The optimum content of solid lubricant for self-lubricating ceramic material was determined theoretically. The microstructure of self-lubricating cutting tool material was designed. It shows that if the self-lubricating cutting tool material had good properties, the base must formed net frame. So the ratio of the radius of solid lubricant grains to the radius of the ceramic base grains should be from 0.225 to 0.414.Al₂O₃/TiC/MoS₂, Al₂O₃/TiC/BN and Al₂O₃/TiC/Caf₂ self-lubricating ceramic materials were produced by hot pressing with Al₂O₃/TiC as matrix and CaF₂, BN and MoS₂ as additives. No trace of MoS₂ was found in the sintered Al₂O₃/TiC/MoS₂ composite owing to the low melting point of MoS₂. There are a lot of obvious pores or cavities located on the polished surface resulted from the melting and escaping of MoS₂ during the hot-pressing process. The flexural strength, fracture toughness and hardness of Al₂O₃/TiC/MoS₂ composite continuously decreased with increasing of MoS2 content. AlN phase resulted from the reaction of Al₂O₃ with BN was formed in Al₂O₃/TiC/BN composite after sintering. Significant micro-cracks resulted from the residual stressowing to the difference in the thermal expansion coefficient were found on the polished surface, and caused large mechanical properties degradation for AI2O/I1C/BN composite. A^O/HC/C^ ceramic composite showed higher flexural strength, toughness, and hardness compared with that of AI2O3/T1C/BN and Al2O3/TiC/MoS2 composites owing its porosity absent and finer microstructure. Under the optimum conditions, a flexure strength of 673MPa, a fracture toughness of 3.6MPaml/2 and a hardness of 16.1GPa were obtained for ATF self-lubricating ceramic cutting tool material.The effects of CaF2 content and test conditions on the tribological behaviors of ATF self-lubricating ceramic cutting tool material were studied in detail. Results indicated that the friction coefficient of ATF composites continuously decreased with the increasing of CaF2 content, while, it had the least wear rate when CaF2 content is 10%. The friction coefficient of ATF composite decreased with the increasing of load and frictional speed. While wear rate of ATF composites decreased with the increasing of frictional speed and increased with increasing of load. The difference of friction coefficients was little when ATF composites coupled with 45# steel and cemented carbide respectively. When coupled with 45# steel, the wear rate of ATF composites was larger than that when coupled with cemented carbide at low speed. While at high speed, the difference of wear rate of ATF composites was little when coupled with 45# steel and cemented carbide respectively.A model of forming solid lubricating film was established by analyzing the forming process of self-lubricating film. Elements of solid lubricating film were analyzed by EDS and the results indicated that there were many CaF2 in the solid lubricating film. Several transferring ways of solid lubricating film were analyzed and lubricating mechanisms of the ways were studied. The stress in solid lubricating film was analyzed in theory and calculated with FEM. The results indicated that the stresses in ATF self-lubricating ceramic materials were lower than those in other A^O/TiC-based ceramic materials without solid lubricants. One important reason was that the solid lubricating film formed on the wear surface of ATF ceramic material decreased the friction coefficient and then reduced the stresses in the ATF. The maximum value of the main stress in the film was on the center of the contact zone of the sliding couple and the maximum value of the shearing stress was at the edge. The stress was high in the film when the temperature of the film altered much and difference in temperature between the film and base was high. Thus the film should be destroyed. The failure mechanism of film was studied as well as failure ways. The action of abrasive grains to the film's destruction was analyzed.A model for decreasing friction of solid lubricating film on the rake face during cutting process was established. SEM photos of solid lubricating film during different forming phases were used to validate the model. The evolvement law of self-lubricating film on rake face in cutting process was studied. Self-lubricating film entered into a forming, failure and re-forming cycle, thus the ATF cutting tool has self-lubricating function in its lifetime. Machining tests were carried out. By analyzing SEM photos of rake face, it indicates that a solid lubricating film was formed on the ATF rake face and the friction coefficient of ATF rake face was lower than that of LT55. There was no solid lubricating film on the flank face of ATF cutting tool and has typical characteristic of abrasive wear.