1. The Effect of Pressure on the Microstructure and Mechanical Properties of Spark Plasma Sintered Silicon Nitride Anne Ellis, Leah Herlihy, William Pinc, and Erica Corral Materials Science and Engineering Department, The University of Arizona, Tucson, AZ Properties of Silicon NitrideSpark Plasma SinteringSPS ConditionsGrain Size of Si3N4 PartsAbstractDensification Behavior Alpha to Beta Phase Transformation Mechanical PropertiesDiscussion and Conclusion Silicon nitride (Si 3 N 4 ) is a useful ceramic in research and industry due its high thermal conductivity, high strength within a wide range of temperatures, and high fracture toughness. Si 3 N 4 has two different phases that exhibit different mechanical properties. α-Phase Crystal Structure Trigonal β-Phase Crystal Structure Hexagonal Pressure-less sintering, hot pressing, and spark plasma sintering of Si 3 N 4 can result in fully dense microstructures with room temperature strengths > 1 GPa. Liquid forming sintering aids are required to achieve full density and beta growth. The alpha phase has high hardness and the beta phase forms long rods which increases the fracture toughness in the ceramic. PowderManufacturer% α-Si 3 N 4 %-Additives Average Particle Size (nm) Theoretical Density (g/cc) RTP Grade P H.C. Starck91% 1% MgO 5% Al 2 O 3 5% Y 2 O 3 580 ± 2203.22 Silicon Nitride Powder Information Spark plasma sintering is an ideal method for the rapid densification of high temperature ceramics due to the high heating rates and short sintering times required. Sample Size20 mm diameter Temperature1800°C Sintering Pressures5 MPa, 20 MPa, 30MPa Preload5 MPa Loading ScheduleLoad applied at start of heating. Load removed at the end of the hold. Heating / Cooling Rate Used 100°C/min Chamber Environment N 2 Gas (~ 1 atm) SPS uses a pulsing direct current to rapidly heat powders contained in graphite dies while simultaneously applying load. The die is heated through joule heating and the powder is heated through heat transfer. - 10-3 Thermal Technology - 3,000 Amps at up to 10V - Max load of 10 tons - Heating rate 600°C/min+ Spark plasma sintering (SPS) is used to densify powder blends, which allows us to examine a large number of sintering parameters and control the microstructure of the material. Displacement of Si 3 N 4 Powders During SPS Densification H.C. Starck Si 3 N 4 powder densifies by liquid phase sintering with oxide sintering aids and a higher sintering pressure results in a faster initial rate of densification. The onset of densification begins at the same temperature (1200°C) for all three pressures. Higher pressures result in higher initial rates of densification; as the glass is softening at these temperatures, higher pressures are able to better flow the glass. Samples sintered at higher pressures completely densify sooner. All samples reach full density. 5 MPa 20 MPa 30 MPa Early stages of densification Liquid phase formation -Onset of densification Completion of densification XRD Plot for SPS Si 3 N 4 at various pressures SPS Pressure Condition (MPa) % α-Si 3 N 4 % β- Si 3 N 4 52179 203070 304258 XRD patterns show that a lower sintering pressure results in a higher β-Si 3 N 4 concentration in the sintered part. Alpha and Beta Si 3 N 4 Content After Sintering Higher sintering pressures are inhibiting the α to β-Si 3 N 4 phase transformation. Grain Size (nm) The Effect of Pressure on Grain Size of SPS HC-Starck Si 3 N 4 SPS Pressure Condition (MPa) Average Grain Size (nm)% β- Si 3 N 4 535879 2028670 3027058 Increases in grain size with lower sintering pressures correlate with increasing β-Si 3 N 4 content. Increase in grain size is likely driven by the formation of elongated, rod-like β-Si 3 N 4 grains. Grain size is found to increase with lower sintering pressures. Average Grain Size of Si 3 N 4 After Sintering 1800°C – 2 minute 5 MPa sample 358 nm SEM images of SPS samples 1800°C - 2 minute 20 MPa sample 286 nm 1800°C - 2 minute 30 MPa sample 270 nm Pressure Increases Grain Size Increases SPS Pressure Condition (MPa) % β-Si 3 N 4 Average Grain Size (nm) Vickers Hardness (GPa) Average Flexural Strength (MPa) Average Toughness (MPa m 1/2 ) 57935813.79 ± 0.25720.4 ± 99.8_ 207028615.02 ± 0.34542.2 ± 119.2_ 305827014.31 ± 0.51869 ± 87.610.57 ± 0.74 The hardness and room temperature flexural strength of sintered Si 3 N 4 is dependent on both β-Si 3 N 4 content and average grain size. Literature shows Si 3 N 4 strength increases with increasing β-Si 3 N 4 content and decreasing grain size. Highest strength obtained with 30 MPa which gave the lowest β-Si 3 N 4 content and lowest grain size. The highest strength sample that was sintered at 1800°C, 2 minute hold, and 30 MPa had a toughness of 10.57 ± 0.74 MPa m 1/2. Our maximum strength and toughness of the H.C. Starck powder are comparable to values in literature. Using spark plasma sintering and altering the sintering pressure we were able to control the microstructure of sintered Si 3 N 4. By using SPS we are able to manipulate the grain size and beta concentration of the silicon nitride. The rapid heating and cooling of the SPS allows us to create specific microstructures. Using this sintering method allows us to investigate the effect of different microstructures of Si 3 N 4 on the flexural strength. Increasing the sintering pressure resulted in higher initial rates of densification, lower β- Si 3 N 4 concentration of the sintered part and smaller grain size while increasing the flexural strength. A ready to press blend of silicon nitride from H.C. Starck is spark plasma sintered (SPS) at varying pressures to determine the effect of pressure on grain size, alpha to beta phase transformation, flexural strength, and toughness. With SPS we are able to tailor the microstructure of the silicon nitride and examine how different microstructures affect mechanical properties. We spark plasma sintered the silicon nitride at 1800°C, with a 2 minute hold, and varying pressures of 5 MPa, 20 MPa, and 30 MPa. XRD was used to determine the phase composition of the sintered parts and the grain size was determined using SEM micrographs. The flexural strength and toughness of the sintered parts were measured. 3.00μm Fracture surface of SPS Si 3 N 4 Beta phase Long rods Increases toughness Undergraduate student support is provided by the National Science Foundation-Southwest Materials Research Training Program in High Temperature Materials under an NSF Early Faculty CAREER Award number NSF-DMR 0954110. Acknowledgement also goes to Luke S. Walker and Kimberlin Schnittker for help with sample preparation and calculations. Intensity Heating Rate 20°C/min <50°C/min <500°C/min Radiant Joule Hold Time & Temp for Si 3 N 4 Densification 6-12 Hours; 1900 °C+ < 1 Hour; 1750 °C+ 0-20 Minutes; 1550 - 1800°C Sample Shape Near Net Shape Simple Geometries Heating Method Pressure-less Sintering Hot pressingSPS Si 3 N 4 Pressure-less Sintering Si 3 N 4 Punch Sample i Punch V Control Pyrometer Sample Graphite Die Schematic of SPS 2θ2θ 5 MPa 20 MPa 30 MPa