The Effect of Pressure on the Microstructure and Mechanical Properties of Spark Plasma Sintered Silicon Nitride

Anne Ellis, Leah Herlihy, William Pinc, and Erica CorralMaterials Science and Engineering Department, The University of Arizona, Tucson, AZ

Properties of Silicon Nitride

Spark Plasma Sintering

SPS Conditions Grain Size of Si3N4 PartsAbstract

Densification Behavior

Alpha to Beta Phase Transformation

Mechanical Properties

Discussion and Conclusion

Silicon nitride (Si3N4) 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. Si3N4 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 Si3N4 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.

Powder Manufacturer % α-Si3N4 %-Additives Average Particle Size (nm)

Theoretical Density (g/cc)

RTP Grade P H.C. Starck 91%

1% MgO5% Al2O3

5% Y2O3

580 ± 220 3.22

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 Size 20 mm diameterTemperature 1800°C

Sintering Pressures 5 MPa, 20 MPa, 30MPaPreload 5 MPa

Loading Schedule Load applied at start of heating. Load removed at the end of the

hold.

Heating / Cooling Rate Used

100°C/min

Chamber Environment

N2 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 Si3N4 Powders During SPS Densification

• H.C. Starck Si3N4 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 Si3N4 at various pressures

SPS Pressure Condition

(MPa)% α-Si3N4 % β- Si3N4

5 21 79

20 30 70

30 42 58

XRD patterns show that a lower sintering pressure results in a higher β-Si3N4 concentration in the sintered part.

Alpha and Beta Si3N4 Content After Sintering

• Higher sintering pressures are inhibiting the α to β-Si3N4 phase transformation.

260

280

300

320

340

360

0 5 10 15 20 25 30 35

SPS Pressure (MPa)

Gra

in S

ize

(nm

)

The Effect of Pressure on Grain Size of SPS HC-Starck Si3N4SPS Pressure

Condition (MPa)

AverageGrain Size (nm) % β- Si3N4

5 358 7920 286 7030 270 58• Increases in grain size with lower sintering pressures

correlate with increasing β-Si3N4 content.

• Increase in grain size is likely driven by the formation of elongated, rod-like β-Si3N4 grains.

Grain size is found to increase with lower sintering pressures.

Average Grain Size of Si3N4 After Sintering

1.00 μm 1.00 μm 1.00 μm

1800°C – 2 minute 5 MPa sample

358 nm

SEM images of SPS samples1800°C - 2 minute

20 MPa sample 286 nm

1800°C - 2 minute30 MPa sample

270 nm

Pressure Increases

Grain Size Increases

SPS Pressure Condition

(MPa)% β-Si3N4

Average Grain Size

(nm)Vickers Hardness

(GPa)Average Flexural Strength (MPa)

Average Toughness (MPa

m1/2)

5 79 358 13.79 ± 0.25 720.4 ± 99.8 _

20 70 286 15.02 ± 0.34 542.2 ± 119.2 _

30 58 270 14.31 ± 0.51 869 ± 87.6 10.57 ± 0.74

The hardness and room temperature flexural strength of sintered Si3N4 is dependent on both β-Si3N4 content and average grain size.

• Literature shows Si3N4 strength increases with increasing β-Si3N4 content and decreasing grain size.

• Highest strength obtained with 30 MPa which gave the lowest β-Si3N4 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 m1/2.

• 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 Si3N4.

• 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 Si3N4 on the flexural strength.

• Increasing the sintering pressure resulted in higher initial rates of densification, lower β-Si3N4 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 Si3N4

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.

Inte

nsity

Heating Rate20°C/min <50°C/min <500°C/min

Radiant Radiant Joule

Hold Time & Temp for Si3N4 Densification6-12 Hours; 1900 °C+ < 1 Hour; 1750 °C+ 0-20 Minutes; 1550 - 1800°C

Sample ShapeNear Net Shape Simple Geometries Simple Geometries

Heating MethodPressure-less Sintering Hot pressing SPS

Si3N4 Si3N4

Pressure-less Sintering

Si3N4

Punch

Sample

i

Punch

V

Control Pyrometer

Sample

Gra

phite

Die

Schematic of SPS

2θ

5 MPa 20

MPa 30

MPa