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Journal of Materials Processing Technology 155156 (2004) 18391846Lightweight, hollow-sphere-composite (HSC) materials formechanical engineering applicationsE. Baumeistera, S. Klaegera, A. KaldosbaInstitute for Manufacturing Technology and Quality Management, Otto-von-Guericke-University, Magdeburg, GermanybSchool of Engineering, Liverpool John Moores University, Liverpool, UKAbstractLightweight structure is a new trend in machine tool design to ensure higher speed and higher acceleration of elements. The drive andcontrol systems in mechanical engineering requires lightweight design provided by the recently developed light materials thus resultingin economical advantages. The hollow-sphere-composites (HSCs) consist of hollow spheres up to 80 of the volume and a reactive resinsystem as binder. The recently developed HSC materials, the hollow sphere bodies, are made from ceramics, silicates, plastics or metals andprovide a range of structural materials of different chemical composition, grain size distribution, density, bulk density, softening temperatureand compression. Therefore, a vast palette of HSC-variants can be obtained with different properties for a variety of applications. Themechanical properties of HSC materials depend on the properties of the spherical hollow bodies. The mechanical and thermal behaviourof HSC materials can be characterised by using dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) andthermomechanical analysis (TMA). The thermal and mechanical properties of selected HSC structures, e.g. machine tool components,robot arms, demonstrate the flexibility and application feasibility of this new material. 2004 Elsevier B.V. All rights reserved.Keywords: Lightweight materials; Hollow-sphere-composites (HSCs); Mechanical properties; Mechanical engineering1. IntroductionIn mechanical engineering, including automotive and air-craft manufacture, the same lightweight building principlesare used to meet various and often complex demands inshape-, structure-, material coupled with the need for opti-mised production process selection for technology needs andfinancial considerations. The optimised design of machinetools using finite element methods may lead to substantialimprovements in the acceleration or damping behaviours.The application of new, alternative materials in machinetool design provides dramatic improvements in mass reduc-tion through the full utilisation of material, high strengthand stiffness as well as maximum functional integrity andeconomy 1. The requirements for the lightweight machinestructures are characterised by the optimal use of materialquantity. These demands can rarely be satisfied with mono-lithic structures. As a result, the application of cellular ma-terials, e.g. honeycomb, metal foams or syntactic foams willsoon gain significance. A combination of metals and fibrousCorresponding author.E-mail addresses: erika.baumeistermasch-bau.uni-magdeburg.de(E. Baumeister), a.kaldoslivjm.ac.uk (A. Kaldos).materials can be used adaptively to different conditions, sim-ilar to natural structures, like the hand bones as shown inFig. 1. This is a foam structure connected with the sup-porting system, where muscles and sinews are utilised formovements.2. Hollow-sphere-compositesAn alternative method in reducing the mass of materialsis to use a mixture of high percentage volume of hollowspheres containing air or gas, and a reactive resin system2. In this research hollow-sphere-composites consistingof corundum based (0.51 mm) macro-hollow-spheres andaluminium-silicate Fillite (5300H9262m) micro-hollow-spheresare used as shown in Fig. 2 3.In the recent research programme 12 different types ofhollow spheres were used in combination with cold andwarm hardener epoxy resin (EP) and with and without fibrereinforcement, resulting in excess of 20 HSC-variants withdifferent properties. The hollow spheres vary in diameterbetween 10 and 2000H9262m and the wall thickness is only 10%of the diameter size. The round shape of the spheres providesa high package density and a minimal viscous drag.0924-0136/$ see front matter 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2004.04.3851840 E. Baumeister et al. / Journal of Materials Processing Technology 155156 (2004) 18391846Fig. 1. Cellular structure of human hand.3. Properties of hollow-sphere-compositesIn order to establish the application areas of HSC in me-chanical engineering, it is extremely important to charac-terise the thermal and mechanical behaviour of the materialand to determine the characteristic values, which are neces-sary for the FE-calculations of the machine elements. Due tolack of standards on HSC materials, the thermal and mechan-ical tests must be governed by the appropriate standards forpolymer concrete and plastic materials. The German Stan-dards DIN 51290 prescribe that the minimum dimensions ofFig. 2. (a) Bulk material of corundum 0.51 mm; (b) interior of Fillite (SEM); (c) hollow-sphere-composite (corundum and Fillite); (d) interior ofhollow-sphere-composite (SEM).the sample shouldnt be smaller than three times the maxi-mum grain size of the used filler 4. The result is that thepreferred sample geometry based on plastic standards mustbe modified to apply to HSC.3.1. Thermal propertiesInvestigations were carried out to obtain the thermal be-haviour and hardening of epoxy resins 5 and HSC usingdifferential scanning calorimeters (DSC 200). The obtainedtypical temperatures are: glass transition temperature (Tg),cured temperature (Tcure), temperature at the beginning ofthermal degradation (Tox). The obtained temperatures andtheir effects on residual reaction heat of the remaining re-actants (Delta1Hr) are shown in Fig. 3. It can be stated that thethermal behaviour of HSC is mainly governed by the epoxyresin used.The linear thermal expansion coefficient for Tg(1) andlinear thermal expansion coefficient over Tg(2) can bemeasured using thermomechanical analysis (TMA). Fig. 4demonstrates, that with increased percentage volume offillers from 65% (Sample 2) to 78% (Sample 3) the 1-and 2-values will be smaller, which is attributable to thesmaller thermal expansion values of the 78% HSC materialused in the research.In order to minimise the thermal distortion of machinetool elements it is important to know the 1-values. Table 1includes the 1-, Tgand 2-values for some HSC variants.E. Baumeister et al. / Journal of Materials Processing Technology 155156 (2004) 18391846 1841Fig. 3. DSC-scan of 11.1 mg epoxy resin Ebalta (1) and 12.3 mg HSC consist of corundum and Fillite (2) with heating rate of 20 K/min in air.Fig. 4. TMA-curves of epoxy resin and HSC-variants with different resin volume fractions.These values depend on the base materials used and can bedetermined from the following equations 6: =summationdisplayvii(1)where viis the volumetric percentage, ithe thermal expan-sion coefficient.The viand ivalues of the components are normallyavailable, but in this case the thermal expansion coefficientof the fillers and the influences of the encapsulated gasin HSC on 1are unknown. However, the -value of thecorundum (H9251-Al2O3)is9.5 106K1according to 7.Table 11-Values of epoxy resin and HSCSample Composition 1(106K1) Tg(C) 2(106K1) Curing time (days)1 Only epoxy resin Ebalta 70.9 60 105.3 302 65 vol.% Fillite + corundum 33.1 51.5 64.1 283 78 vol.% Fillite + corundum 22.3 52.4 51.9 304 78 vol.% Fillite 34.5 62.6 49.1 215 78 vol.% corundum 02 mm 23.4 51.3 30.8 19The calculated -value of epoxy resin is 70 106K1.The calculated 1-value of Sample 5 is 22.8 106K1,which agrees well with the experimentally obtained value of23.4106K1. The dynamic mechanical analysis (DMA)investigations of three-point-bending-samples of epoxy resin(a) and of HSC-Sample 3 (b) are shown in Fig. 5. At higherfrequencies the Tgmoves to higher temperature values 8and due to the sensitivity of the DMA-methods two Tgpointsare found for the semi-cured samples. At the start of the Tgarea the microbrown movements takes place followed by anentropy elastic state, where the dependence of the elastic1842 E. Baumeister et al. / Journal of Materials Processing Technology 155156 (2004) 18391846Fig. 5. Elastic bending modulus (Eprime), loss modulus (Eprimeprime) and log decrement (D) of epoxy resin (Sample 1) (a) and HSC (Sample 3) of (b).modulus on the temperature is less significant. It is notablethat the fillers improve the stiffness (Eprime) of Sample 3 (HSC)in comparison to Sample 1 (epoxy resin).3.2. Mechanical propertiesThe elasticity modulus (E) of epoxy resin Ebalta 120/TL(EP) and HSC were obtained from mechanical tests and areshown in Table 2. The mechanical properties of epoxy resinand HSC-samples are shown in Table 2, along with steel (St),glass fibre (GF) and carbon fibre (CF) materials for purposeof comparison. The density () of materials indicates thatHSC are lightweight materials. The ratio of stiffness (E)to density is an important parameter for material selection.To compare the compression strength of two bars of equaldimension but different materials the equation is simplifiedto3E/g 9. It is clear from the table that HSC-Samples24 have higher compression modulus than either steel orglass fibre 10. If GF or CF is manufactured as laminate,then its mechanical properties becomes much smaller. Aclear disadvantage of CF is its anisotropy, whereas HSC isisotropic in all directions.E. Baumeister et al. / Journal of Materials Processing Technology 155156 (2004) 18391846 1843Table 2Density and Youngs modulus (E) of epoxy resin and HSC in comparison to steel (St), glass fibre (GF) or carbon fibre (CF)Value EP, Sample 1 Hollow-sphere-composites St GF CFSample 2 Sample 3 Sample 4 Sample 5 (g/cm3) 1.15 0.95 0.9 0.65 1.16 7.8 2.6 1.78E (GPa) 3.5 7.8 6.8 4.1 8.7 210 73 2353E/g(3MPa cm3/g) 13.2 21.4 21 24.6 18.7 7.6 16 34.5Fig. 6 shows the tensile strength (t) and specific strengthof epoxy resin and HSC-Samples 25. The tensile test speci-men was 250 mm in length, 10 mm in thickness and 25 mm inwidth. The tensile strength tests were carried out with a speedof 5 mm/min according to DIN EN ISO 527-3. The specificstrength of Sample 3 (Fillite and corundum 0.51 mm) andSample 4 (Fillite) are higher than that of epoxy resin. Theresult is, than using the same mass of material, a higher vol-ume of component can be made when using Samples 24,and it withstands the same tensile strength as a componentmade from Sample 1.Compression tests were conducted with test pieces hav-ing a length of 100 mm, a thickness of 30 mm and a width of30 mm. The speed of compression tests was 1 mm/min. Thecompressive stressstrain curves of selected HSC-variantsare presented in Fig. 7. The symbols of circle, square, etc.mark the mean values of the compressive strength (c) andthe corresponding mean values of compression-strain ofSamples 510 of each variant. The c-values in Fig. 7 aregreater than that of tin Fig. 6 because in compression teststhe pores will be closed and they stop the propagation of thecracks. Samples 4 and 5 in Fig. 7 show that two typical stagesoccur during deformation in the course of compression testof cellular solids such as polymer foams or metal foams11,12. Following an almost linear-elastic behaviour at lowstrains the curve shows a long plateau with almost constantload, but in comparison to the another cellular solids the HSCmaterial is superior in withstanding compression. Sample 4filled with the smaller filler type Fillite behaves better underFig. 6. Tensile strength and specific strength of EP (Sample 1) and HSC (Samples 25).compression than the filled with corundum, because Sample5 has a higher porosity. Samples 2 and 3 have high packingdensity thus providing higher compressive strength values.The increase in the volumetric percentage of resin in Sam-ple 2 improves the c-values 13. The smaller the size ofthe spheres the more marked the plateau areas are, as in thiscase the crack propagation can be rapidly stopped by imped-iments (spheres or pores). This explains why the samplesfilled with smaller particles cracks appear to be diagonal,while samples filled with greater fillers develop transversalcracking develops. It has to be noted that adhesion bondsbetween fillers and binders are of paramount importance. Ifthe stiffness of the spheres is higher than the stiffness of theresin then cracking starts in the resin and vice-versa.The damage propagation can be explained using the scan-ning electron micrograph (SEM) images of the fracture sur-faces of Samples 3, 4 and 5 in Fig. 8. The Fillite spheresof Sample 4 in Fig. 8a are broken. Due to the differentwall thickness of the ceramic hollow spheres of Sample 5in Fig. 8b, the spheres are broken at different levels. Thespace between the greater corundum spheres of Sample 5are greater than the space between the smaller Fillite spheresof Sample 5. A better packing density of the fillers is shownin Fig. 8c, where Sample 3 is filled with different grain sizeof spheres of known volumetric percentage fraction, thuscausing to improve mechanical properties of Sample 3 incomparison to Samples 4 or 5.The bending stress in Fig. 9 was determined usingthree-point-bending samples with following dimensions:1844 E. Baumeister et al. / Journal of Materials Processing Technology 155156 (2004) 18391846Fig. 7. Typical compressive stressstrain curves of HSC variants and test samples after compression test.Fig. 8. SEM images of fracture surfaces among bending HSC-samples.240 mm length, 20 mm width and 12 mm height, accordingto the DIN EN ISO 178, with a proof-speed of 4.8 mm/min.The bending strength values of HSC are smaller than thatof epoxy resin. Some HSC variants at the opposite side ofthe applied force were reinforced with carbon or glass fibreto improve tensile properties.Sample 3 is a mixture of ceramic and aluminium sili-cate hollow spheres and presents better mechanical proper-ties than Samples 4 or 5, which were filled with a singlefiller type. The thermal expansion coefficient of Sample 3 issmaller in comparison to Sample 2 or 4. Sample 3 was se-Fig. 9. Bending strength values for epoxy resin, HSC with and without carbon fibre or glass fibre.lected as construction material for machine tool componentsand other engineering parts.4. Application of HSC in mechanical engineeringOn the research programme a number of machine ele-ments, such as jigs of milling tables and robot arms forSCARA Adept robots were developed. These componentswere successfully tested and the application of HSC materi-als in mechanical engineering was demonstrated. The finiteE. Baumeister et al. / Journal of Materials Processing Technology 155156 (2004) 18391846 1845Fig. 10. Finite element models and robot arms made from aluminium alloy (a) and HSC (b).Fig. 11. Table of a milling machine made from HSC, steel plate and carbon laminates.element program COSAR provided indications for the needof design changes regarding the direction of carbon fibre re-inforcements and the aluminium connection elements. Themodels in Fig. 10 were loaded with 1000 MPa bending forceand the developed stresses remained below acceptable limit14. Based on the results obtained, two robot arms weremade from HSC reinforced with carbon fibre or aluminiumalloys bars. These robot arms were 10 and 25% lighter inweight than as the original aluminium alloy arms.A milling machine table was successfully developed fromHSC to replace a steel table. The developed HSC tablewas designed with reinforcing steel elements and carbon fi-bre laminates to withstand the typical tensile strengths. Theachieved mass reduction is between 30 and 80%, thus en-hancing dynamic characteristics. The damping properties ofthe HSC table are superior to that of cast iron table, which ispartly attributed to the ply structure as shown in Fig. 11 15.5. ConclusionIt can be stated that HSC materials combined with metal orfibre reinforcements promise a successful alternative to lightmetals or metal foams. In this research a number of machinebuilding parts with good dimensional accuracy have beenproduced and tested with good results. The spherical formof the hollow materials provided a considerably smoothersurface than that of fibrous or irregular fillers and the resinconsumption was significantly reduced. The application ofHSC materials is advantageous for the user because of thelow material and production costs. The excellent vibrationand damping properties coupled with very low heat con-ductivity and resultant heat distortion predestines the HSCmaterials to be used successfully in a variety of engineeringareas. The chemical resistance and the ease of recycling arefurther advantages of this material by changing the compo-sition of the matrix material and the volume.AcknowledgementsThe authors wish to thank the Ministry of Culture, LandSachsen-Anhalt in Germany. Many thanks go to the col-leagues in the Institute for Manufacturing and Quality Man-agement and the Institute of Material and Material testingat the Otto-von-Guericke-University, Magdeburg. The finan-1846 E. Baumeister et al. / Journal of Materials Processing Technology 155156 (2004) 1839184