Scripta Materialia 50 (2004) 593–596 www.actamat-journals.com
Compression testing of hollow microspheres (microballoons) to obtain mechanical properties M. Koopman a
a,*
, G. Gouadec b, K. Carlisle a, K.K. Chawla a, G. Gladysz
c
Department of Materials Science and Engineering, University of Alabama at Birmingham, 1530 Third Ave. South. BEC 254, Birmingham, AL 35294-4461, USA b LADIR, UMR 7075, CNRS––Univ. P. et M. Curie, 94320, Thiais, France c Los Alamos National Laboratory, New Mexico, 87545, USA Received 26 August 2003; received in revised form 18 November 2003; accepted 19 November 2003
Abstract Glass microballoons (GMBs), which are hollow microspheres, were tested to obtain uniaxial compressive properties. Individual GMBs with diameters between approximately 5 and 90 lm were tested in compression using a nanoindentation instrument equipped with a flat ended sapphire tip. GMBs failed at loads ranging from 2.4 to 49.7 mN, and showed a direct relation between diameter and load to failure and between diameter and fracture energy. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Compression test; Nanoindentation; Microballons; Toughness; Non-metallic glasses (silicates)
1. Introduction Glass, metal, and carbon microballoons (MBs) are used in syntactic foam composites for applications ranging from aerospace to buoyancy in deep-sea submersibles [1–3]. The composites often have densities well below 1.0 g/cm3 , primarily as the result of the incorporation of MBs. Due to their generally spherical shape, MBs transmit forces through their curved surfaces and exhibit high specific strengths, (i.e. strength divided by density). Since the mechanical properties of the MBs directly affect the properties of the syntactic foams, their behavior is of interest to the materials engineering community. Data relating MB diameter to mechanical properties such as load to failure, strain to failure, and fracture energy can be used in modeling of syntactic foam microstructures and may aid in improving design of the syntactic foams, as well as designing for particular properties. Prior investigations on the strength of MBs have used techniques that test the properties of large quantities of MBs, which have been packed into a suitable fixture, *
Corresponding author. Tel.: +1-205-9341545; fax: +1-2059348485. E-mail address:
[email protected] (M. Koopman).
exposed to a particular stress, and then analyzed for wt.% of survivors [1]. GMBs are often tested in isostatic pressure vessels and then analyzed for percent survivors. These data are appropriate to the use of GMBs in viscous liquids under pressure, such as polymers during injection molding for two-phase foams. However, in three-phase foams, where there is a high volume fraction of voids between MBs, it may be useful to have data on uniaxial or multiaxial loading. The current paper describes in some detail a new technique for compression testing of individual MBs. Trends in the uniaxial compressive properties of the GMBs are presented here to demonstrate the viability of the technique and support the introduction of this experimental procedure.
2. Experimental procedure The materials tested were glass microballoons (GMBs), with a methacrylato chromic chloride surface treatment, manufactured by 3 M (St. Paul, Minnesota). All MBs tested were between 5 and 90 lm with wall thicknesses ranging from submicrometer to several micrometers. Specimens of GMBs were examined by scanning electron microscopy (SEM) to study the morphology of the MBs, as well as to examine the walls of broken MBs.
1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.11.031
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GMBs were sputter coated with Au–Pd prior to examination in SEM to provide a conductive coating. Fig. 1 shows a schematic of the experimental configuration. All tips and substrates were cleaned with acetone and/or ethanol. The MBs were sprinkled over a polished aluminum alloy substrate and inserted into the sample stage of the nanoindenter, an MTS-Nano Instruments XP-II. The calibration procedure for load was performed with a standard Berkovich tip, a threesided pyramid. The procedure uses fused silica, examining at least ten data points for hardness and modulus. In the present work, the Berkovich tip was then replaced with a cylindrical sapphire indenter of 89 lm diameter and another procedure was conducted for calibrating the position of the tip in the horizontal plane of the experiment, the plane perpendicular to the loading axis. Generally, in standard nanoindentation the procedure involves placing five indentations in a pattern, with the center indentation used for x–y position alignment. Since the flat ended tip has a much larger surface area, 6,221 lm2 , a polished sample of Sn, which is sufficiently soft to show the locations of flat-ended indentations, was used for this alignment. Fig. 2 shows the arrangement of five indentations, using the cylindrical indenter. The positioning of the stage for alignment with an
Sapphire Indenter (Diameter = 89 µm)
individual MB and the position calibration described above were both accomplished with an optical microscope, which is part of the nanoindenter. Proper alignment is critical to uniaxial testing of the individual MBs. Optical images were acquired of the individual MBs and their positions were recorded in the software for use in subsequent testing. Tests were run at a strain rate of 0.05/s.
3. Results Fig. 3 shows GMBs, which were generally spheres of various sizes. GMBs were selected for testing, which were free of observable debris and were uniform in shape. An example of a load–displacement curve for an individual MB is given in Fig. 4. The initial loading segment of the curve is linear in nature. The MB fractures when the load reaches a critical value, the ultimate compressive strength. This is followed by a horizontal portion in the load–displacement curve, until the tip impacts the aluminum substrate, which is indicated by a near vertical line on the graph.
Objective lens
CMB
Al alloy substrate
Fig. 1. Schematic showing the configuration of the nanocompression test.
Fig. 3. The GMBs have a generally spherical morphology with variation in size (SEM).
20 Load (mN)
Failure of MB 15 10 5
Loading
Indenter impinges substrate
0 0
Fig. 2. An optical image of five indentations made with a cylindrical indenter on a polished Sn sample. The middle indentation was used to align the mechanical axis of the experiment.
2
4 6 Displacement (µm)
8
10
Fig. 4. An example of a load–displacement curve for a nanocompression test on a hollow glass sphere of less than 10 lm shows the typical behavior of a GMB.
M. Koopman et al. / Scripta Materialia 50 (2004) 593–596
60
Load, mN
50 40 30 20 10 0 0
20
40 60 Vertical diameter, µm
80
100
Fig. 5. The GMBs showed a trend of increasing load to failure with diameter.
Fracture energy(nJ)
250 200 150 100 50 0 0
20
40 60 Vertical Diameter(µm)
80
100
Fig. 6. Fracture energy of GMBs in compression increased with diameter.
In addition to the ultimate compressive load of the MB, several other parameters may be obtained from the graph, which give insight into the mechanical behavior and characterization of the MB. The vertical diameter of the MB can be taken as the x-axis displacement from the position where loading begins to the position where the substrate is impacted. Compressive strain at failure is calculated by dividing the displacement at failure by the original vertical diameter of the MB. In addition, the slope of the initial linear loading segment, the pseudostiffness, k, has shown correlations to other characteristics of the MB. The value of k is dependent on the Young’s modulus of the material, the thickness of the wall, and the diameter of the MB. The fracture energy is the area under the load-displacement curve up to the fracture point, or approximately half the product of the ultimate compressive load and the displacement to failure. The GMBs showed a trend with a direct relationship between diameter and ultimate compressive load, Fig. 5. Although there is scatter in the data, a trend is readily observed. Fig. 6 shows the same experimental data set plotted as fracture energy vs. diameter, exhibiting once again a direct relationship.
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geometric factors, as well as issues related to the material and production. The GMBs selected for compression testing appeared uniform, exhibiting a spherical morphology with smooth surfaces. However, the relative uniformity of wall thickness and presence of flaws within the walls of compression tested GMBs are unknown. The GMBs fail catastrophically into a myriad of pieces, which are not readily collected or reconstructed to determine wall thickness at the point of failure. As a ceramic material, their failure is expected to follow a Weibull distribution, as well as potentially being dependent on other physical characteristics, such as variations in wall thickness. Given the dynamics of the production process, as explained below, it is reasonable to assume that the larger GMBs generally have thinner walls than the smaller GMBs. During processing, fine particles of glass are taken to a high enough temperature that the glass flows easily and surface tension causes the particles to become spherical. A latent blowing agent within the glass then evolves to the gaseous state, essentially blowing the microsphere into its hollow form. Since the process begins with a given particle size distribution, GMBs which are blown to larger diameters should have thinner walls (i.e., a given initial mass of glass is spread over a much larger surface area in a larger GMB). Direct observations by SEM have shown wall thicknesses ranging from submicrometer to several micrometers, but a detailed study relating diameter to wall thickness has yet to be performed. Similar to fibrous materials [4], it is statistically less likely for a thin walled GMB to contain a flaw of critical size to initiate a crack under a given load, compared to a thicker walled GMB. This may play a role in the higher loads to failure in larger diameter GMBs. Geometric considerations may contribute, as well, to the observed trends of GMB diameter to both load to failure and fracture energy. Since larger spheres have a smaller radius of curvature compared to smaller spheres, it is reasonable to assume that as a test proceeds there is an increase in contact area between a larger GMB and the platen. As the balloon deflects under load, the surface of the balloon near the point contact with the platen increases and becomes a circular contact area. This would lower the stress on the sample by distributing a comparable load over a larger area. Despite the scatter in the results, the trends in Figs. 5 and 6 support the viability of the experiment in determining compressive properties of MBs.
5. Conclusions 4. Discussion The observed trends between the GMBs diameter and both failure load and fracture energy may be affected by
1. A new technique has been developed to test the compressive properties of hollow spheres (microballoons).
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2. This paper demonstrates the technique on glass microballoons between 5 and 90 lm in diameter. 3. From the data collected by this new technique, the maximum load or strength of the MB, as well as their fracture energy, increased with increasing sphere diameter.
Acknowledgements The authors would like to acknowledge Los Alamos National Laboratory for funding the project under subcontract #44277-SOL––02 4X. The authors would
like to thank MTS, Nano Instruments Innovation Center of Oak Ridge, TN, and particularly Erik Herbert for his help.
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