Study Of Core-Shell Coated Self-Lubrication Ceramic Cutting Tool And Its Cutting Performance

This PhD dissertation focuses on the area of dry cutting tools. The material design concept of particles surface coating technology was introduced to the research and development of self-lubrication ceramic cutting tools. The core-shell coated ceramic self-lubrication cutting tools with self-lubrication properties and improved mechanical properties were fabricated. The combination of lubrication properties and mechanical properties was realized.Fabrication mechanism of core-shell CaF₂@Al（OH）3 coated powders were studied based on the heterogeneous nucleation theory. The process and mechanism of the stable nucleation and growth of Al（OH）3 crystals on the surface of CaF₂ particles was analysed. The supersaturation degree △C was the driving force for Al（OH）3 crystallization. The larger the AC was, the more easier the Al（OH）3 nucleation, and hence more beneficial for the generation and growth of CaF₂@Al（OH）3 coated particles. The generation process of CaF₂@Al（OH）3 coated particles was simulated using Masterial Studio.The CaF₂@Al₂O₃ core-shell coated particles were prepared using heterogeneous nucleation theory. The core-shell coated particles prepared under optimum conditions were characterized and analysed, using XRD, IR spectrum, SEM and TEM, to identify its structure and morphologies. Influences of process parameters on morphologies and coated rate of CaF₂@Al（OH）3 coated particles were analysed. The highest coat rate and the best coat result were obtained at the optimal parameters:pH was 7.5, C（Al3+） was 0.15mol/L, T was 75℃, titration speed of ammonium hydroxide was 3 mL/min.Self-lubrication cutting tool materials with different CaF₂@Al₂O₃ contents were designed and fabricated by using Al₂O₃/TiC and Al₂O₃/Ti（C,N） as matrix, respectively. The best mechanical properties of Al₂O₃/TiC/CaF₂@Al₂O₃ were obtained when the content of CaF₂@Al₂O₃ was 10 vol.%. The flexure strength, fracture toughness and hardness were measured to be 662 MPa,6.34 MPa·m1/2 and 16.86 GPa, which were 19.33%,36.93% and 12.25% higher than the material with the addition of CaF₂, respectively. The best mechanical properties of Al₂O₃/Ti（C,N）/CaF₂@Al₂O₃ were obtained when the content of CaF₂@Al₂O₃ was 10 vol.%. The flexure strength, fracture toughness and hardness were measured to be 680 MPa,6.50 MPa·m1/2 and 17.29 GPa, which were 12.21%,29.48% and 14.50% higher than the material with the addition of CaF₂, respectively. The microstructure analysis of surface and fracture face of the material indicates that the fracture mode of the material was inter/transgranular fracture. The strengthening mechanism of the cutting tool materials was the intragranular structure induced by the firm constraint of ceramic grain Al₂O₃ for solid-lubrication CaF₂ grain during the sintering process. This was the main reason for the improvement in mechanical properties.The influences of CaF₂@Al₂O₃ content on the friction and wear performance of cutting tool materials were studied. Tests indicates that the friction coefficient of AT-C@X and ATCN-C@X cutting tool materials decrease with the increase of CaF₂@Al₂O₃ content. The wear rate decrease first and then increase with the increase of CaF₂@Al₂O₃ conten. The smallest wear rate was observed when the content of CaF₂@Al₂O₃ was 10 vol.%, which coincides with the mechanical properties of the cutting tool materials. With the increase of load, the friction coefficient decreases and the wear rate increases. The friction coefficient and the wear rate decrease with the addition of speed.Cutting performance of AT-C@X and ATCN-C@X cutting tool materials with the addition of core-shell coated solid-lubricants CaF₂@Al₂O₃ when cutting 40Cr quenched and tempered steel was studied. The results showed that it could be improved the surface roughness, decrease cutting force and cutting temperature wtih the addition of core-shell coated solid-lubricants CaF₂@Al₂O₃. But the flank wear is a little bigger because of the decrease of hardness. The main cutting force of ATCN-C@X and AT-C@X tools increased with the increasing of cutting speed and back cutting depth. The cutting temperature increased first and then decreased with the increasing of cutting speed and increased with the increment of the back cutting depth. The flank wear increased with the addition of cutting speed, as well as the back cutting depth. The surface roughness of the workpiece increased with the increasing of the back cutting depth and cutting length, but decreased with the increasing of cutting speed.The rake face wear of AT-C@X cutting tools consisted of micro chipping and crater wear and the slight crater wear were also observed on rake face of ATCN-C@X cutting tools. The flank wear of AT-C@X cutting tools consists of micro chipping and adhesion wear and the flank wear of ATCN-C@X cutting tools consisted of abrasive wear and adhesion wear. The chip of AT-C@X and ATCN-C@X cutting tools was continuous chip. The chip oxidization of ATCN-C@X was severe than that of AT-C@X cutting tools.