Study On The Preparation Process And Performance Of Silicon Nitride Ceramics Based On Additive Manufacturing And Isostatic Pressure Composite Technology

Silicon nitride ceramics are increasingly used in aviation,aerospace,cutting tools,automotive and other fields because of their high temperature strength,high hardness and fracture toughness,as well as wear and corrosion resistance and many other excellent properties.However,with the rapid development of science and technology,the comprehensive performance and structural requirements of silicon nitride ceramics in various fields are becoming more and more demanding,and it is difficult to manufacture complex structural silicon nitride ceramic parts with integral and functionally integrated structure by traditional molding process and mechanical processing.They mainly face many problems such as complicated mold making,high cost,long cycle time,difficult mechanical processing,and poor surface quality and dimensional accuracy.To address the above background,this paper introduces the direct write forming(DIW)technology in the field of additive manufacturing,and systematically investigates the material system,preparation method and direct write forming process of high solid phase content silicon nitride ceramic slurry for DIW.The cold isostatic pressing(CIP)technology was also introduced into the DIW field,and the effects of different CIP pressures on the denseness,shrinkage and strength of the molded blanks were studied and analyzed.The effect of different HIP temperatures on the densities,hardness,fracture toughness,flexural strength,shrinkage,microscopic morphology and thermal shock resistance of silicon nitride sintered specimens was investigated and analyzed under certain conditions of holding time and holding pressure.The DIW+CIP+HIP composite molding preparation process for silicon nitride ceramic parts was developed.Finally,the performance of the prepared high-performance,highly dense silicon nitride ceramic specimens was co MPared with the currently commercially available silicon nitride ceramic tool material(RCGV-1207).The main conclusions are as follows.(1)The high-purity α-Si3N4 powder was selected as the matrix material,Al2O3-Y2O3 as the composite sintering aid,polyvinyl alcohol as the binder,polyacrylamide as the dispersant,and deionized water as the solvent,and the optimal ratios of dispersant,lubricant,and binder and the optimal p H values were determined as follows: when the content of dispersant was 0.8 wt%,the content of lubricant was 4.0 wt%,the content of binder is1.5wt% and p H=10,the ceramic slurry has the lowest zeta potential,better dispersion stability and rheology,and is suitable for true-written 3D printing.(2)When the nozzle diameter is 0.6mm,the screw speed is 50r/min,the layered thickness is 0.48 mm,and the scanning speed is 25mm/s,the quality and precision of the direct-written shaped blanks are optimal,as well as the optimal drying temperature is determined to be 50℃.(3)The best overall performance and surface quality of the molded silicon nitride blanks were achieved at a CIP pressure of 250 MPa under the condition of a certain holding time.(4)Under the conditions of holding time and pressure,the best denseness and thermal shock resistance of the silicon nitride sintered specimens were obtained when the HIP temperature was 1750°C.The density of the sintered specimens was 3.28g/cm3,hardness17.25 Gpa,fracture toughness 7.82MPa-m1/2,and flexural strength 1282.5MPa.(5)The performance co MParison shows that the bending strength and denseness of the silicon nitride ceramic parts prepared by DIW+CIP+HIP composite technology are much better than those of commercially available silicon nitride ceramic materials,except for the hardness and fracture toughness,which are slightly worse than those of commercially available silicon nitride ceramic materials.