Compressive Characterization of Single Porous SiC HollowParticles

VASANTH CHAKRAVARTHY SHUNMUGASAMY,1,3

STEVEN E. ZELTMANN,1 NIKHIL GUPTA,1 and OLIVER M. STRBIK III2

1.—Composite Materials and Mechanics Laboratory, Mechanical and Aerospace EngineeringDepartment, New York University Polytechnic School of Engineering, Brooklyn, NY 11201, USA.2.—Deep Springs Technology, LLC, Toledo, OH 43615, USA. 3.—e-mail: vs860@nyu.edu

Silicon carbide hollow spheres are compression tested to understand theirenergy absorption characteristics. Two types of particles having tap densitiesof 440 kg/m3 and 790 kg/m3 (referred to as S1 and S2, respectively) weretested in the present study. The process used to fabricate the hollow spheresleads to porosity in the walls, which affects the mechanical properties of thehollow spheres. The porosity in the walls helps in obtaining mechanicalbonding between the matrix material and the particle when such particles areused as fillers in composites. The single-particle compression test results showthat the S1 and S2 particles had fracture energies of 0.38 9 10�3 J and3.18 9 10�3 J, respectively. The modulus and fracture energy of the particleswere found to increase with increasing diameter. However, the increasingtrend shows variations because the wall thickness can vary as an independentparameter. Hollow particle fillers are used in polymer and metal matrices todevelop porous composites called syntactic foams. The experimentally mea-sured properties of these particles can be used in theoretical models to designsyntactic foams with the desired set of properties for a given application.

INTRODUCTION

Hollow particles are widely used for a variety ofapplications, including in composite materials,pharmaceutical products, and sensing.1–4 Therehave been considerable advancements in recentyears in the synthesis of hollow particles from nanoto macro-scale diameters and wall thicknesses.Characterization of synthesized particles is criticalin understanding the relation between their com-position, structure, and properties. Final applica-tions guide the testing that is conducted on thehollow particles. For example, uniaxial compressionis conducted for composite materials used in groundtransportation vehicles and hydrostatic compres-sion is conducted on spheres used in deepwaterthermal insulation and vehicle structures.

Characterization of single hollow particles andinterpretation of results are challenging because oftheir small size and wall thickness, porosityembedded in their wall, and the presence of defects.Direct measurements of particle diameter can beconducted, but measurement of internal diameter of

hollow particles is a challenge. The true particledensity measurements can be used for estimatingthe wall thickness of the particle only if the wallsare fully dense. However, the presence of porosity inthe shell makes the true particle density measure-ments unreliable. Although surface roughness andsurface porosity can help in promoting themechanical bonding between the particle and thematrix at the interface in a composite material,these features makes it difficult to determine theparticle characteristics. Given the diverse set ofapplications of hollow and porous particles, it isdesirable to develop methods that can help in esti-mating the properties of individual hollow particles.Understanding the mechanical properties and fail-ure characteristics of hollow particles has been ofgreat interest in several existing studies.5–9

Silicon carbide (SiC) is an important material thatis used in composite materials,10 electronic pack-aging,11 and medical implants.12 There is a stronginterest in developing lightweight hollow particlesof SiC for these applications. Hollow particles canprovide the advantages of SiC material but keep the

JOM, Vol. 66, No. 6, 2014

DOI: 10.1007/s11837-014-0954-7� 2014 The Minerals, Metals & Materials Society

892 (Published online April 24, 2014)

weight low in composite materials. Porous hollowparticles can also be useful in biomedical implantsfor improving osseointegration and reducing stressshielding effects. The present work is focused oncharacterizing SiC hollow particles (SiCHP) that aresynthesized by powder sintering. During process-ing, the hollow sphere assembly is heated in an in-ert atmosphere to sinter the SiC particles andobtain a hollow shell. The types of SiCHP charac-terized in the present work have been used recentlyas fillers in composite materials.13–15 Improvedunderstanding of the structure and properties ofthese particles will help in developing compositematerials with the desired set of properties.

The sintering temperature is one of the parame-ters that can be used as a means of controlling theporosity in the particle shell. The process of manu-facturing these particles is very versatile in terms ofobtaining a wide range of shell thickness andkeeping a close control over the shell thickness in agiven batch. Two types of SiCHP are tested in thepresent study under compression using a micro-electromechanical system designed to test singleparticles. The experimental results of the particlecompression are used to estimate the modulus of theporous SiC particle wall.

MATERIALS AND METHODS

In the present work, SiCHP single particles aretested under uniaxial compression using a custom-built micromechanical test setup, which is sche-matically illustrated in Fig. 1. The tests are con-ducted at a constant compression rate of 5 lm/suntil particles fracture. A 10-N load cell is used fortesting, and data on load and displacement are ac-quired using an in-house developed LabView pro-gram (National Instruments, Austin, TX). Twotypes of SiCHP having tap densities of 440 kg/m3

and 790 kg/m3 are selected for testing. The 440- and790-type particles are referred to as S1 and S2,respectively. The diameter of these particles ismeasured through optical image analysis methodsusing a Nikon D7000 camera fitted with an AF-SMicro Nikkor 105-mm lens and NIS-Elements D3.0imaging software (Nikon Instruments, Melville,NY). The wall thickness measurements are taken

using image analysis on micrographs of brokenSiCHP obtained through Hitachi S-3400N scanningelectron microscope (Hitachi USA, Schaumburg,IL). The typical properties of SiC material taken incalculation in this work are density of 3200 kg/m3,elastic modulus of 410 GPa, and compressivestrength of 3900 MPa.

RESULTS AND DISCUSSION

The structures of the shells of the two types of par-ticles tested in the study are shown in Figs. 2 and 3.The figures show that both types of particles have alarge void at the center, which is surrounded by arelatively thin shell. The close observations showporosity that exists in the shell wall. Visual observa-tions of these figures provide an indication that the S1particles have a relatively smaller amount of porositycompared with the S2-type particles. It is anticipatedthat the modulus of the particle material would belower than the modulus of SiC as a result of theporosity.

The ratio of the internal (Ri) to the external (Ro)radii of the hollow sphere is denoted as the radiusratio (g = Ri/Ro). The radius ratio g is calculatedusing the measured values of the outer radius andthe wall thickness and is shown in Table I.

The average radii of the two particles, S1 and S2, aremeasured as 0.4 mm and 0.51 mm, respectively(Table I). The particle wall thickness of S1 and S2 ismeasured as 36.1 lm and 81.6 lm, respectively. Theideal true particle density (qHS), considering themeasured diameter and wall thickness are of particlescontaining fully dense walls, can be calculated using16

qHS ¼ qSiC 1� g3� �

(1)

where qSiC is the density of the SiC material. Theideal true particle density evaluated using Eq. 1 isfound to be 787 kg/m3 and 1293 kg/m3 for the S1and S2 particles, respectively, as shown in Table I.It is expected that the actual true particle densitywould be lower than these values because of poros-ity embedded in the particle shell.

A set of typical force–displacement graphs obtainedin the uniaxial compression testing of particles isshown in Fig. 4a for one representative particle ofeach type. The graphs show a linear variation in load

Load cellDisplacement stage

Hollow particle

Fig. 1. Schematic illustration of the compression test setup used for characterizing SiC hollow particles.

Compressive Characterization of Single Porous SiC Hollow Particles 893

with respect to deflection and a brittle failure. Thearea under the load deflection plot is calculated as thefracture energy. Table II provides an overview of themeasured properties of hollow particles.

The standard deviations in the experimentalvalues are rather large for S1 particles. The fractureenergy of the particles is a function of both mea-sured load and displacement, and it shows a largevariation. The diameter of each particle is measuredbefore compression testing. As a result of the largevariation in the compressive properties of the par-ticles, the correlation between the properties andthe diameter of the particle is analyzed.

The fracture energy for individual particles is plot-ted as a function of the particle diameter in Fig. 4b. Asa general trend, the larger diameter particles arefound to have higher fracture energy as observed inFig. 4b. The variation in this trend is likely becausethe internal diameter can be different for the particlesof the same outer diameter. Similar observation weremade in a previous study on the compression testing ofsingle hollow glass microballoons (GMBs),6 where thefracture energy increased by 200 nJ as the particlediameter increased from 5 lm to 90 lm. The GMBsused in that study were of considerably smallerdiameter and had fully dense walls. Study on thecompression testing of carbon microballoons (CMB)

observed a fracture energy of 42.41 nJ for uncoatedCMBs.5 The GMBs are widely used as filler materialin polymers to fabricate porous composites calledsyntactic foams.17–19 Syntactic foams find applica-tions as buoyancy aids,20,21 weight-saving structuralmembers in automotive and aerospace22 fields, and inthe electronics field owing to tunable coefficient ofthermal expansion (CTE) and dielectric properties.23

The CTE and dielectric constant of syntactic foamscontaining GMBs can be tailored based on the volumefraction and the radius ratio (g) of GMBs.24,25 Severalof these applications can benefit from the availabilityof high-performance SiCHP.

The experimental measurements of the geometricand mechanical properties of the particle can be usedto obtain the modulus of the hollow particle and toobtain an estimate of the porosity present in its walls.The effective modulus E* of the hollow spheres, ob-tained using the shell theory, is given by26,27

E� ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3ð1� m2Þ

p

4

PRo

dt2(2)

where P is the load (N), Ro is the radius of the hollowSiC sphere (mm), m is the Poisson’s ratio of SiC mate-rial (m = 0.14), d is the displacement (mm), and t is thewall thickness of the hollow SiC particle (lm). The

Fig. 2. Scanning electron micrographs showing the structure of an S1-type SiC hollow particle at successively increasing four differentmagnifications.

Shunmugasamy, Zeltmann, Gupta, and Strbik III894

estimated E* is then used to compute the modulus ofthe porous SiC material of the hollow particle as28

E ¼E� ð1� 2mÞ þ ð1þm

2 Þg3� �

ð1� 2mÞð1� g3Þ (3)

The average modulus calculated for the particlematerial is presented in Table III, and the variationof the modulus values with respect to the particlediameter is presented in Fig. 5.

It could be observed from Fig. 5 that the modulusof the S1-type particles increases as the particleradius is increased. The modulus of S2-type parti-cles shows smaller variation in the values, reflectedby the smaller standard deviation observed in

Table III. The obtained modulus of the porous SiCmaterial is low compared with the elastic modulusof 410 GPa.29 The reduction in the modulus of theSiCHP can be attributed to the porosity embedded inthe walls of the hollow particles. Comparing Figs. 2and 3, it could be observed that the S2-type particleshave higher wall thickness and possess higherporosity, in comparison with S1-type particles.These figures also show that the S2 particles seemto have some cracks in their internal surface; simi-lar cracks are not visible on the surface of the S1particle. The presence of cracks can decrease themodulus of the particle and reduce the fracturestrength and energy.

Controlled sintering of the material during theparticle manufacturing can help in modulating the

Fig. 3. Scanning electron micrographs showing the structure of an S2-type SiC hollow particle at successively increasing four differentmagnifications.

Table I. Average outer radius, particle wall thickness, and ideal true particle density of the two SiCHS

Particle type

Tap densityof particlebed (kg/m3)

Averageouter radius

Ro (mm)Particle wall

thickness (lm)Radiusratio g

Ideal trueparticle density

(kg/m3)

S1 440 0.40 ± 0.03 36.1 ± 4.4 0.91 787S2 790 0.51 ± 0.04 81.6 ± 6.8 0.84 1293

Compressive Characterization of Single Porous SiC Hollow Particles 895

density and mechanical properties of particles. Inapplications where higher strength and modulusare required, a higher level of sintering with lowerporosity in the walls can help. On the contrary,applications that use foams for energy absorptioncan benefit from having tailored porosity in thewalls to fracture at a given stress level for maxi-mum energy absorption capability. Syntactic foamscan greatly benefit from the availability of suchparticles with a predetermined set of properties.Although glass particles are widely used in polymermatrix syntactic foams, their use in metal matrix

syntactic foams is restricted by their low softeningand melting point. SiCHP can be very useful forsynthesizing tailored metal matrix syntactic foams.

CONCLUSION

Compression testing of single SiC hollow particlesis conducted using an in-house developed compres-sion test setup. Two types of particles having a dif-ferent size and density are tested. The energyabsorbed until fracture is found to be higher for theparticle with a higher density. The nano andmicroscale pores on the walls of the SiC affect themodulus of the hollow particle. The measuredmodulus of hollow particles is used to calculate themodulus of the particle material, which is porousSiC. The measured values are considerably lowercompared with that of fully dense SiC material as aresult of the porosity. The use of SiC hollow parti-cles as fillers in syntactic foams can benefit from theavailability of the properties of the particles. Suchmeasured properties are used in theoretical modelsto design syntactic foams as per the requirement ofthe applications.

ACKNOWLEDGEMENTS

The work is supported by the Office of Naval Re-search grant N00014-10-1-0988 with Dr. Yapa D.S.Rajapakse as the program manager and cooperative

(a)

(b)

Fig. 4. (a) Representative load–deflection data for the two types ofSiC hollow particles tested under compression. (b) The fracture en-ergy obtained from the compression testing of the particles plotted asa function of the particle diameter. The triangles and squares rep-resent S1 and S2 particles, respectively.

Table II. Average values of the radius, force,displacement, and the work of fracture for eachtype of SiC particles tested

Particletype

Maximumforce (N)

Maximumdeflection

(mm)

Fractureenergy

(3 1023 J)

S1 4.6 ± 1.8 0.14 ± 0.05 0.38 ± 0.17S2 16.5 ± 0.8 0.41 ± 0.04 3.18 ± 0.61

Table III. Shell thickness, particle modulus, andmodulus obtained for the wall materials (SiC) usingthe equivalent sphere approach from Refs. 24 and28

Particletype

Averageparticle

modulus (GPa)

Averageporous SiC

modulus (GPa)

S1 4.4 ± 1.1 28.5 ± 7.3S2 1.3 ± 0.1 4.9 ± 0.4

Fig. 5. Variation of the evaluated modulus for the SiC hollow parti-cles with respect to the particle diameter. The triangles and squaresrepresent S1 and S2 particles, respectively.

Shunmugasamy, Zeltmann, Gupta, and Strbik III896

agreement contract W911NF-10-2-0084 with theU.S. Army Research Laboratory, Dr. VincentHammond program manager. The authors thankthe MAE Department for providing facilities andsupport. Kevin Chen and Nahal Mustafa arethanked for help with experiment setup and imageanalysis.

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