Study On Precision Preparation Of Alumina Ceramic Products

Advanced ceramics have excellent properties such as high strength,high hardness,high temperature resistance,wear resistance,acid and alkali corrosion,and have a wide application prospect.in petrochemical,aerospace,national defense and military,ocean and other fields.However,advanced ceramics are polycrystalline and multiphase materials,which are hard and brittle,and have poor performance in response to mechanical shock and rapid temperature changes,so higher requirements for processing methods are put forward.At present,the precision preparation of advanced ceramic products is still dominated by mechanical（abrasive）processing,which is characterized by large processing volume,high cost,and long cycle time.Microcracks are inevitably generated on the surface after processing,which affects the performance and reliability of advanced ceramic products.These problems limit the application and promotion of advanced ceramic products.The main crystal phase of alumina ceramics is corundum,which has excellent characteristics of high hardness,high mechanical strength,high-temperature resistance,and wears resistance,and the price of raw materials is lower.It is currently the main engineering ceramics in the industry.The gel casting technology combines organic chemistry and traditional ceramic molding technology to prepare a near-net size ceramic body with high strength,uniform density,low organic content,easy degumming,and certain plasticity,which can meet the requirements of mechanical precision machining.In this study,alumina ceramics were used as the research object,and high-strength plastic bodies were prepared by gel casting technology.The precision machining process of advanced ceramic products was advanced from post-firing cold working to pre-firing precision machining.After the low deformation sintering process,advanced ceramic precision parts are obtained.In this study,using N-methylol acrylamide as the monomer and imported industrial pureα-Al₂O₃ as the raw material,hydrogel and alumina ceramic bodies were prepared by gel casting technology,respectively.Various factors that affect the safe drying of hydrogel and alumina ceramic green bodies,such as the amount of monomer added,the ratio of monomer to crosslinking agent,drying rate,drying shrinkage,firing shrinkage,had been studied in detail.The sintering shrinkage rate of alumina ceramics was reduced by adding the coated aluminum powder to the billet,and the influence of the amount of the coated aluminum powder added on the green body morphology,green strength,and sintering shrinkage was explored.Precision ceramic parts was prepared by digital machining.The conclusions of the study are as follows:（1）Starting from the hydrogel,the fast and safe drying of alumina green body was studied by changing the amount of monomer,the ratio of monomer and crosslinking agent,and the drying method,the effects of various factors on the drying process of hydrogel and alumina green body were studied in detail.Results show that the strength of hydrogel increased with the increase of monomer and cross-linking agent,and the hydrogel is not easy to deform during drying,but the risk of hydrogel cracking is also increased,earlier drying speed is slower,but the drying safety is good.when the monomer content is 3～10 wt%,and the monomer and the crosslinking agent is 75:1,the dry gel with intact structure without cracking risk is obtained by hot-wet drying.The alumina green body follows the rule of hydrogel safe drying.When the monomer content is 5 wt%,the ratio of monomer and cross-linking agent is 75:1,the temperature gradient drying method is adopted,the first drying is at 30°C for 24 h,the second drying is at50°C for 48 h,the green body with complete structure and high performance is obtained.The green body strength is 54.74 MPa,the bulk density is 1.67 g/cm~3,and the drying shrinkage is 4.35%.（2）Ceramic shrinkage inevitably occurs during high-temperature densification,which adversely affects the accuracy of ceramic products.In this experiment,to achieve the purpose of reducing the firing shrinkage,the coated aluminum powder with Si O₂coated on the surface of aluminum powder was prepared by hydrolysis of ethyl orthosilicate,and was added to the billet.The volume expansion caused by the high-temperature oxidation of the coated aluminum powder to form Al₂O₃ can offset part of the volume shrinkage during the high-temperature sintering process of the body.Results show that the firing shrinkage decreases with the increase of the addition amount of the coated aluminum powder.When the coated aluminum powder is added with 10 wt%,the firing shrinkage is 14.54%,and the reduction range is 25%.However,the addition of coated aluminum powder will reduce the uniformity and bending strength of the green body,which is not conducive to the machining of the green body.（3）When gel casting technology is used to prepare large-sized and complex-shaped ceramic parts,the effects of product weight and safe drying must be comprehensively considered.A lower ratio of monomer to cross-linking agent will easily lead to deformation and fracture of the green body during the drying process.The more complex the shape and weight of the green body,the more organic monomer content should be.In this study,the monomer addition amount was 3 wt%,and the ratio of monomer to crosslinking agent was 10:1.The green ceramic bolts and nuts were prepared and successfully processed by a digital machine tool.After firing,the shrinkage rate of the outer diameter of the ceramic nut is 20.73±0.02%,the shrinkage rate of the inner diameter is 20.32±0.08%,and the firing shrinkage of the bolt thread part is 19.86±0.38%,which meets the precision requirements of similar metal products.（4）The green body is dried by using polyethylene glycol with a molecular weight of 800.During the drying process,PEG molecules enter the green body,resulting in a significant reduction in the strength of the green body,which is easily damaged during processing.The surface of the green body dried in liquid is rough after processing.The firing shrinkage is unstable and fluctuates between 19%and 24%,which is not conducive to the precision preparation of ceramic parts.