Design And Development Of In-situ Reaction Self-lubricating Ceramic Tools And Study On Their Antifriction Mechanisms

The concept of self-lubricating ceramic tool base on in-situ reaction is proposed in this paper, namely that by the friction chemical reaction under high-temperature, generate the lubricating film on the cutting tool surface to achieve the self-lubricating tools. A new self-lubricating cutting tool material has been developed successfully with adding reinforcement phase-ZrB2 and dispersed phase-ZrO2 to Al2O3 matrix, as well as its ratio design, fabrication process, properties, microstructures, tribological behaviors and cutting performance more over, the antifriction mechanism are studied in detail.On the analyzing of friction characteristics in cutting process, the design principles of self-lubricating ceramic tool material is proposed and the material system is also determined. Optimize the composition of the materials, meanwhile, the Al2O3/ZrB2/ZrO2 self-lubricating ceramic tool material is designed. Analysis of physical and chemical compatibility of self-lubricating material determine that there is no chemical reaction in the hot pressing sintering process, and make sure the maximum volume content of additions. Tribological design of self-lubricating ceramic tool material is proposed, and thermodynamic results show that in-situ reaction could occur in machining proces, meanwhile, the antifriction mechanism is analyzed.Self-lubricating tool materials of Al2O3/ZrB2/ZrO2 ceramic (AZ series) are fabricated by technology of the hot-pressing sintering. Test results of material properties give a preferred value for ratio of materials, sintering pressure and holding time. By detailed X-ray diffraction and microstructure analysis, toughening and strengthening mechanisms are derived. Based on the above conclusions, the effect of ball milling refinement on properties and microstructure of Al2O3/ZrB2/ZrO2 ceramic materials are investigated. AZ20 ceramic composite shows higher synthetic mechanical properties in AZ series self-lubricating materials with double milling refinement technology, when the sintering temperature is 1700℃, pressure is 30MPa, holding time is 20min and the volume of ZrB2/ZrO2 is 20%. The optimized properties are:Vicker's hardness 23.1GPa, bending strength 760.9MPa and fracture toughness 6.19 MPa-m1/2.The oxidation model of self-lubricating ceramic tool materials is proposed. According to the relevant physical properties of the composite, simplified oxidation surface of Al2LO3/ZrB2/ZrO2 at oxidation temperatureθ<1000℃is simulated, and the weight gains of AZ series self-lubricating ceramic materials are predicted. High temperature oxidation experiments of AZ series self-lubricating ceramic materials are conducted under different conditions, and oxidation behavior of the composite is studied, and the weight gains accord with parabolic law characteristics, and the general trend of the oxidation curves experimental weight gain is obviously greater than the forecast model, the reason is that the theoretical models is idealistic and many more factors can affect the oxidation process. Oxidation rate of the self-lubricating materials decreases with increasing the oxidation time, and the oxidation process change to slow gradually. This is because the generated ZrO2 on the surface can prevent continued oxidation of the composite materialsin a sence. The mechanical properties of AZ series ceramic materials decrease in varying degrees when oxidate 20 hours at different temperatures, and has obvious decline at 700℃. Oxidation test and XRD analysis indicated that the composites began to oxidate at 500～700℃.The tribological properties and antifriction mechanisms of in-situ reaction self-lubricating ceramic tool materials are studied. Analytic equations of the average pressure and maximum temperature in contact area are established. The effects of ZrB2/ZrO2 content, friction velocity and environmental temperature on the tribological behaviors of AZ self-lubricating ceramic tool materials are studied in detail, and the relevant researchs about the transfer and failure mechanisms of self-lubricating film are analyzed. Moreover, antifriction functions of self-lubrication in different conditions are discussed. The results show that:the friction coefficient of AZ self-lubricating ceramic tool material decrease with the increasing of ZrB2/ZrO2 content, and the wear rate can reach to the minimum data when the ZrB2/ZrO2 content is 20%. The reason is that the mechanical properties of AZ self-lubricating material is the best at this time, and the lubricating film generated on the friction surface can improve the tribological behavior and reduce the wear rate. The friction coefficient and wear rate of AZ20 composite decrease with the increasing of friction speed or environmental temperature. This is because the in-situ reaction can generate the ZrO2 and B2O3 self-lubricating film, which can improve the tribological behavior and prevent the adhesive wear at high temperature.The cutting performance and anti-friction mechanism of self-lubricating ceramic tool base on in-situ reaction are studied. Dry cutting of normalized and hardened 45# steel in air with AZ20 self-lubricating ceramic tool, when the cutting depth ap= 0.2mm, feed rate f= 0.lmm/r, cutting speed v exceeds 160m/min, a self-lubricating film of ZrO2 and B2O3 is formed on the friction surface of AZ20 in cutting process, which has an anti-friction behavior and can prevent the adhesive wear. With the self-lubricating base on in-situ reaction, it was found that the tool wear is homogeneous and no chipping situation. According to the effect of self-lubricating, the tool flank wear, cutting force, friction coefficient and surface roughness of machined workpiece when dry cutting of normalized and hardened 45# steel in air with AZ20 tool are reduced compared SG4 in the air and AZ20 in nitrogen environment. Under the same cutting conditions that the cutting depth ap= 0.2mm, feed rate f= 0.1mm/r, cutting speed v= 160m/min, the tool life of AZ20 ceramic tool is prolonged more than 17.6% compared the SG4 tool when machining normalized 45# steel, and 20% when machining hardened 45# steel. A simulation way of cutting temperature field considering property of materials and friction is is proposed, and the results of simulation agree with experiment.