The Preparation And Study Of ZrO₂ Grinding Media Ball

ZrO₂ ceramic ball is a kind of outstanding grinding media,which is used extensively and has a promising feature.Although the existing forming techniques have low efficiency, high cost and low product quality,which has become the bottleneck that limits its commercial scale production.Recently,much attention has been focused on the investigation of high efficient forming and production techniques all over the world. Rolling ball method,which has been recognized for its low cost,high efficiency and automatable fabrication,has become a kind of excellent technique for producing small size ceramic balls.But ZrO₂ is a kind of lean stuff and the particles bond imfirmly,that make it difficult to form balls,also it is the mainly reason why rolling method has not been used extensively to make ZrO₂ ceramic balls up to now.In this paper,ZrO₂ ceramic grinding media balls were made from ZrO₂ powder,which contains 3 mol%Y₂O₃ by rolling ball method and pressureless sintering technique.High solid loading and low viscosity slurry which is suitable to spray Granulating was made by the investigation on the effects of the factors such as pH value of the slurry,dispersant and solid loading to the flow properties of the slurry and the influences of milling time to the particles size;relative mathematical model was built through the analysis of the growth mechanism and stress of the zirconia green ball during the rolling process.Also the fabrication of ball core and the effects of various parameters such as the content of binder, the diameter,obliquity and roll velocity of ball-forming machine,the green balls' diameter, rolling time and the polishing time to the behaviors of zirconia green ball were studied; The effects of sintering system on the wearable modulus of zirconia ceramic media balls were analyzed by mechanical test and SEM analytic methods.The results of the study showed that zirconia slurry possesses different properties with different pH value and have different viscosities.Slightly alkaline system is in favor of the dispersion of zirconia particles.The viscosity increases with the solid loading of the slurry. Therefore appropriate amount of dispersant was added to make slurry with good flow properties and the excess of dispersant gets the opposite effect.The size of ball core affects the manufacture of green ball,in this study,optimum size of ball cores were made by isostatic compaction under 150MPa pressure and thereafter crushing and grading the particles.The particles with the size between 30 mesh to 60 mesh can be used as ball core.The analysis on the link of powder,stress and growth mechanism of balls and the relative mathematical model show that the diameter,obliquity and rolling velocity of ball-forming machine and the size of green balls interact and support each other.The rolling velocity and obliquity of ball-forming machine should be enhanced as the green ball grows gradually.Different sizes of ball-forming machine need different rolling velocity and obliquity.The addition of polyvinylalcohol(PVA) in aqueous solution can improve the interfacial bonding force and contribute to the forming performance of zirconia powder obviously.The highest density and strength of the green ball can be obtained when the PVA content is 1wt‰and the addition of powder and aqueous solution should have a set ratio.The experiment results show that the optimal ratio of the aqueous solution which contains 1wt‰PVA to the weight of powder is 1:9 and the amount of addition is in direct ratio to the quadratic of the size of green ball.The green balls become compacter as they grow gradually and have more homogeneous size and circularity.The green balls should be polished after stop charging for get the compactest and lubricious surface.The optimal polishing time is 2 hours.The surface of the green balls have high temperature and saturated humidity during rolling,if the green balls were taken out immediately after finishing rolling,the water on the surface would volatilize quickly which would lead to surface shrinking and cracking. Hence it is necessary to leave the green balls in the ball-forming machine after finishing rolling for 20 to 40 minutes for keeping humidity and letting the temperature decrease gradually.The laying aside time is relative to the temperature and relative humidity of the atmosphere and it is shorter if there is high temperature and humidity.During the dry process,the temperature should keep from low to high at the beginning(the relative humidity is more than 60%,RT) and then keep from high to low(The relative humidity is less than 10%and temperature is 110℃),the dry cycle is 40 hours and the content of water in green balls is less than 0.5%.Sintering temperature and tempering time effect the properties of zirconia ceramic media balls distinctly,the ceramic media balls can't be sintered absolutely with low sintering temperature and short tempering time,which turn to possess low crushing strength and volume density but high wearable modulus;also overly high sintering temperature and long tempering time would make zirconia grains grow tremendously and have inhomogeneous size and the cubic zirconia which contribute to toughening effect decrease too,and that balls have low crushing strength and high wearable modulus, although the volume density is high.Zirconia ceramic media balls has the optimal wear performance when they were sintered in 1550℃for 2h and the volume density is 5.91g/cm³,crushing strength is 2416N,wearable modulus is 2.6ppm/h.