81
a 45 MPa axial load to obtain cylindrical discs of 40 mm diameter and 3.9 mm thick. The disc was then cut into cuboidal
specimens (3.1 × 3.1 × 4.1 mm) by PremaTech Advanced Ceramics™ (Worcester, MA, USA) in preparation for mechanical
testing. A few additional specimens were successively polished down to a 0.05 μm finish using a colloidal silica solution for
microstructural examination and Vickers hardness testing.
15.2.1 Pulse-Echo Ultrasonic Measurements
The density and elastic moduli of the samples were measured using the Archimedes method and the pulse-echo ultrasonic
technique, respectively. The shear and elastic modulus of each material were calculated based on measurements of elastic
wave speed from the Olympus 5072PR Pulser/Receiver (Olympus Co., Shinjuku, Tokyo, Japan) according to ASTM E494
specifications.
15.2.2 Quasi-static Vickers Indentation Hardness
Quasi-static Vickers hardness measurements were conducted using a Wilson Instruments Tukon™ 2100B hardness tester
with a diamond indenter tip (Wilson Instruments, Norwood, MA, USA). The indentation loads ranged from 1.5 to 10 N with
15 indents at each load. Large amounts of spallation were observed at loads above 10 N. The indented regions were then
imaged using an Olympus BX51 optical microscope (Olympus, Tokyo, Japan).
15.2.3 Quasi-static and Dynamic Compression
Quasi-static uniaxial compression tests were performed on an Instron (Norwood, MA, USA) 5969 dual-column testing
machine at a strain rate of 5 × 10
−4
/s. One specimen of each material was bonded with two strain gages to measure its longitudinal and lateral strains during loading. The specimens were sandwiched between two tungsten carbide platens to avoid
stress concentration in the steel loading columns. All contact surfaces were lubricated in order to minimize friction.
Dynamic compression tests were performed using a split- Hopkinson pressure bar (SHPB) at strain rates ranging from 10
2
to 10
4
s. This test procedure employed the use of an incident bar and a transmission bar, both of length 1219 mm and diameter
19.05 mm. The SHPB uses a compressive wave transmission phenomenon to deliver high loading rates to the sample. A
pressurized gas gun launches a striker of the desired length towards the incident bar. Strain gages bonded at the mid-length
on the incident and transmission bars measure the passing waves and allow for the determination of the stress on the sample
at failure [55]. A set of two strain gages bonded directly to the sample measure the strain experienced by the sample. The
samples were sandwiched between two tungsten carbide platens, which are impedance-matched to the incident and transmission bars to ensure that the entire compressive wave is transmitted between the sample and the bars [7]. All interfaces were
lubricated and a 3D-printed polylactic acid (PLA) support is used for centering the specimen as was shown in previous studies [7, 56, 57]. Copper disks of diameter 5 mm and thickness 0.5 mm were used at the impact end of the incident bar to shape
the indent pulse to achieve ramp loading so as to provide constant strain rate deformation in the ceramic sample.
15.2.4 Scanning Electron Microscopy (SEM)
A scanning electron microscope (FEI Nova NanoSEM 430, Hillsboro, OR, USA) was used to analyze the microstructure and
extent of phase segregation in the composite sample. Energy Dispersive Spectroscopy (EDS) was used to determine the
composition of the phases observed. The fracture surfaces resulting from quasi-static and dynamic compression as well as
high-load indentation were also imaged to characterize the failure mechanisms of the two materials.
15 Static and Dynamic Mechanical Characterization of a Spark Plasma Sintered B 6 O–B 4 C Composite
a 45 MPa axial load to obtain cylindrical discs of 40 mm diameter and 3.9 mm thick. The disc was then cut into cuboidal
specimens (3.1 × 3.1 × 4.1 mm) by PremaTech Advanced Ceramics™ (Worcester, MA, USA) in preparation for mechanical
testing. A few additional specimens were successively polished down to a 0.05 μm finish using a colloidal silica solution for
microstructural examination and Vickers hardness testing.
15.2.1 Pulse-Echo Ultrasonic Measurements
The density and elastic moduli of the samples were measured using the Archimedes method and the pulse-echo ultrasonic
technique, respectively. The shear and elastic modulus of each material were calculated based on measurements of elastic
wave speed from the Olympus 5072PR Pulser/Receiver (Olympus Co., Shinjuku, Tokyo, Japan) according to ASTM E494
specifications.
15.2.2 Quasi-static Vickers Indentation Hardness
Quasi-static Vickers hardness measurements were conducted using a Wilson Instruments Tukon™ 2100B hardness tester
with a diamond indenter tip (Wilson Instruments, Norwood, MA, USA). The indentation loads ranged from 1.5 to 10 N with
15 indents at each load. Large amounts of spallation were observed at loads above 10 N. The indented regions were then
imaged using an Olympus BX51 optical microscope (Olympus, Tokyo, Japan).
15.2.3 Quasi-static and Dynamic Compression
Quasi-static uniaxial compression tests were performed on an Instron (Norwood, MA, USA) 5969 dual-column testing
machine at a strain rate of 5 × 10
−4
/s. One specimen of each material was bonded with two strain gages to measure its longitudinal and lateral strains during loading. The specimens were sandwiched between two tungsten carbide platens to avoid
stress concentration in the steel loading columns. All contact surfaces were lubricated in order to minimize friction.
Dynamic compression tests were performed using a split- Hopkinson pressure bar (SHPB) at strain rates ranging from 10
2
to 10
4
s. This test procedure employed the use of an incident bar and a transmission bar, both of length 1219 mm and diameter
19.05 mm. The SHPB uses a compressive wave transmission phenomenon to deliver high loading rates to the sample. A
pressurized gas gun launches a striker of the desired length towards the incident bar. Strain gages bonded at the mid-length
on the incident and transmission bars measure the passing waves and allow for the determination of the stress on the sample
at failure [55]. A set of two strain gages bonded directly to the sample measure the strain experienced by the sample. The
samples were sandwiched between two tungsten carbide platens, which are impedance-matched to the incident and transmission bars to ensure that the entire compressive wave is transmitted between the sample and the bars [7]. All interfaces were
lubricated and a 3D-printed polylactic acid (PLA) support is used for centering the specimen as was shown in previous studies [7, 56, 57]. Copper disks of diameter 5 mm and thickness 0.5 mm were used at the impact end of the incident bar to shape
the indent pulse to achieve ramp loading so as to provide constant strain rate deformation in the ceramic sample.
15.2.4 Scanning Electron Microscopy (SEM)
A scanning electron microscope (FEI Nova NanoSEM 430, Hillsboro, OR, USA) was used to analyze the microstructure and
extent of phase segregation in the composite sample. Energy Dispersive Spectroscopy (EDS) was used to determine the
composition of the phases observed. The fracture surfaces resulting from quasi-static and dynamic compression as well as
high-load indentation were also imaged to characterize the failure mechanisms of the two materials.
15 Static and Dynamic Mechanical Characterization of a Spark Plasma Sintered B 6 O–B 4 C Composite