Development Of Modified Self-lubricating Ceramic Tool By Coated Nano CaF₂ Powders And ZrO₂ Whiskers

The self-lubricating ceramic cutting tool has the ability to reduce the friction in the process of dry cutting,but the self-lubricating ceramic cutting tool are not available to possess rational combination of anti-wear and anti-friction properties.This paper aims to obtain high-performance self-lubricating ceramic tools,proposing to change the microstructure and mechanical properties of the ceramic tool by surface coverage with modified nano-solid lubricant and the synergistic effect of ZrO2 whisker.The main research contents include the preparation of calcium fluoride nano-particles,the surface modification of nano-calcium fluoride,the preparation of ceramic tools with CaF2@Al(OH)3,the preparation of ceramic tools with ZrO2 whiskers,and the preparation and cutting experiments of the synergistically modified self-lubricating ceramic tools.In this paper,the process of preparing nano-CaF2 was studied.Through chemical reaction,CaF2 particles with a size of about 10 nm were obtained.The dispersant was used to disperse the CaF2 nano-particles,and the particle dispersibility was the best when the concentration of PVP in the mixed solvent was 5 g/L.Nano-coated powder CaF2@Al(OH)3 was prepared by heterogeneous nucleation method.Selecting different process parameters has a great influence on the morphology,and when the process parameters were determined to be pH=7 and C(Al3+)=0.03mol/L respectively,a good coating effect was achieved on the nano-particles,with the average particle size of the coated powder below 30nm.According to the HRTEM test,the particle size of CaF2 as the core was below 10 nm,and the thickness of the amorphous Al(OH)3 cladding layer was about 15 nm.The tests of ceramic specimens with volume ratios of 5%,10%,and 15%with coated CaF2@Al(OH)3 powders showed that when the volume ratio of CaF2@Al(OH)3 was 10%,the bending strength of the material reached the maximum value of 471 MPa,and hardness and fracture toughness were 18.58 GPa and 6.50 MPa · m1/2,respectively.As compared with ceramic materials to which nano-CaF2 was directly added,the hardness increased by 3.2%,the fracture toughness by 12%,and the bending strength by 9%.Through the SEM and HRTEM tests of the bulk material,the presence of intragranular nano-particles could be observed.The intragranular nano-particles were uniformly distributed with a particle size of less than 10 nm.The Al2O3 shell was fused with the matrix material,the nano-particles were dispersed to the inside of matrix grains during the sintering process,and the surface coating design prevented the nano-particles from agglomerating and growing.Al2O3/Ti(C,N)ceramic tools with ZrO2 with volume ratios of 3%,6%and 9%of were prepared,and the density of the samples decreased with the increase of ZrO2 whiskers.When the addition amount of whiskers was 6%,the comprehensive mechanical properties of the samples obtained by hot-pressed sintering were optimal,and the values of hardness,fracture toughness,and bending strength were:19.1 GPa,584 MPa,and 6.61 MPa.m1/2,respectively.Relative to the ceramic tool without ZrO2 whiskers,the hardness slightly decreased,but the fracture toughness increased by 14%,and the bending strength increased by about 5%.The SEM measurement of the powder material before sintering showed that the ball-milling time was 6 h,the whisker dispersibility was good,and the particle size was uniform.The fracture mode of the material was a mixed mode of intergranular fracture and transgranular fracture.Toughening mechanisms included phase transformation toughening,whisker extraction and whisker bridging caused by the addition of whisker materials,and crack deflection.The preparation of the modified tools showed that when the content of ZrO2 whisker and nano-coated powder was 10%and 6%respectively,the comprehensive mechanical properties were optimal,with the bending strength of 520 MPa,the fracture toughness of 7.16 MPa m1/2,and the hardness of 16.72 GPa.As compared to ATCN tools,ATCN-Z-C achieved lower surface roughness in cutting operations.At cutting speed 200m/min,the back cutting depth ap=0.2mm,and the feed rate f = 0.102 mm/r,after comparison of ATCN-Z-C with a traditional tool ATCN:the cutting temperature was reduced by 31%,the main cutting force was reduced by 21%,the friction coefficient of the rake face of the ATCN tool was 0.485,while that of ATCN-Z-C was 0.39.Wear patterns of the rake face of ATCN-Z-C tool included adhesive wear and micro tipping.The rake face of ATCN-Z-C had a smaller wear area than that of ATCN tool.Moreover,the formation of the lubricating film could be observed,the wear area of the flank was flat,and there was a stripe sticky area,with boundary wear.