Synthesis of free standing, one dimensional, AlSiC based functionally gradient materials using gradient slurry disintegration and deposition

Materials Science and Engineering A276 (2000) 210–217

Synthesis of free standing, one dimensional, Al�SiC basedfunctionally gradient materials using gradient slurry disintegration

and deposition

M. Gupta *, C.Y. LokeDepartment of Mechanical and Production Engineering, National Uni6ersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

Received 24 March 1999; received in revised form 21 April 1999

Abstract

In the present study, aluminum–silicon carbide based functionally gradient materials were successfully synthesized using a newtechnique termed here as gradient slurry disintegration and deposition process. Gradients of SiC were successfully establishedusing this technique for the starting silicon carbide weight percentages ranging between 15–20%. The results were confirmed usingmicrostructural characterization techniques and microhardness measurements. The results further revealed that an increase in theweight percentage of silicon carbide particulates along the deposition direction lead to a concurrent increase in porosity, degreeof clustering and microhardness while the silicon carbide–aluminum interfacial integrity remained the same. An attempt is madeto interrelate the processing methodology, microstructure and microhardness results obtained in the present study. © 2000Elsevier Science S.A. All rights reserved.

Keywords: Functionally gradient materials; Processing; Microstructure; Microhardness; Gradient slurry disintegration and deposition

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1. Introduction

The vision of rapid advancement in technology trans-lates into the superior performance of the devices thatcan principally be met by the use of new and innovativematerials. Functionally gradient materials (FGMs) orgradient materials are one such class of materials.Functionally gradient materials can technically bedefined as the class of engineering materials that exhibita variation in chemical composition and/or microstruc-tural parameters over definable geometrical distances[1–5]. The variation in composition/microstructuralparameters lead to the variation in the property/proper-ties of interest allowing the material to withstand twodistinct service conditions, with ease, at the same timeacross the cross section.

Functionally gradient materials are presently beingsynthesized using a number of techniques that can

broadly be classified as [2–15]: (a) vapor phase tech-niques; (b) liquid phase techniques and (c) solid phasetechniques. These techniques besides having their suit-ability for certain specific applications suffer from oneor more of the following limitations [2–15]: (i) lowthickness of the functionally gradient bulk material orcoating; (ii) low deposition rate; (iii) complex processrequirement and (iv) high cost of the process. Theresults of existing literature search revealed that noattempt is made to synthesize FGMs using a moltenmetal based technique that can collectively circumventthese limitations to synthesize free standing, one dimen-sional, aluminum (Al)/silicon carbide (SiC) based func-tionally gradient materials.

Accordingly, in the present study an attempt is madeto synthesize Al/SiC based FGMs using a new processtermed as gradient slurry disintegration and deposition(GSDD) process. The microstructure of the FGMsalong the deposition direction was characterized usingacid dissolution test, density measurement and scanningelectron microscopy while the mechanical response wasassessed in terms of the matrix hardness using micro-

* Corresponding author. Tel.: +91-65-7726358; fax: +91-65-7791459.

E-mail address: [email protected] (M. Gupta)

0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.

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M. Gupta, C.Y. Loke / Materials Science and Engineering A276 (2000) 210–217 211

hardness measurements. Particular emphasis was placedto interrelate the increasing presence of SiC particulateson the porosity and microhardness values along thedeposition direction.

2. Experimental

2.1. Materials

In this study, an aluminum alloy AA1050 (]99.5wt.% Al) was used as the matrix material and siliconcarbide (a-SiC) particulates, with an average size of 34.4mm, were used as the reinforcement phase.

2.2. Processing

In the present study, the synthesis of functionallygradient/gradient materials was successfully accom-plished using gradient slurry disintegration and deposi-tion technique. Three Al/SiC based functionallygradient/gradient materials were successfully synthesizedand they are designated as Al/15SiC, Al/18SiC andAl/20SiC in the forthcoming sections. These designationsindicate that they were synthesized with the starting SiCweight percentages of 15, 18 and 20, respectively. Thesynthesis was carried out according to the followingprocedure. The aluminum ingots were properly cleanedusing water and acetone, prior to melting, to eliminatesurface impurities. After weighing, the cleaned pieceswere placed in graphite crucible and superheated to atemperature of 950°C. SiC particulates, preheated to950°C for one hour were added into the molten alu-minum melt stirred using zirconia coated stirrer at 294rpm. The selection of stirring speed was made so as toallow controlled sedimentation of SiC particulates. Thetotal addition time of SiC particulates was limited to amaximum of 20 min. The gradient slurry thus obtainedin the crucible was allowed to flow into a 11.2 mmdiameter stream through a centrally drilled hole in thecrucible and was disintegrated using argon gas jets at adistance of �0.18 m from the pouring point using a gasflow rate of 25 l min−1 and subsequently deposited ontoa metallic substrate located at a distance of 0.25 m fromthe disintegration point. The solidified FGMs wereobtained in the form of an ingot with a diameter of 55mm and a thickness of about 40 mm. The deposition timefor all the three FGMs synthesized in the present studydid not exceed beyond 5 s. Fig. 1 shows a conceptualschematic diagram of the gradient slurry disintegrationand deposition technique used in the present study.

2.3. Specimen preparation

FGMs ingots obtained following processing weredefaced to remove the shrinkage cavity located on the top

of the ingot and sliced in 0.005 m thick discs along thethickness (deposition direction) at 0.005 m distanceinterval. The sectioning was carried out using FORM2-LC Charmilles Technologies electric discharge wire cut-ting machine to minimize the gap of cut. All thespecimens required for quantitative assessment of SiCparticulates, density measurement, microscopic analysisand microhardness measurements were obtained fromthese slices.

2.4. Quantitati6e Assessment of SiC Particulates

Quantitative assessment of SiC particulates was car-ried out using the chemical dissolution method [16,17].This method involved: (i) measuring the mass of thecomposite samples; (ii) dissolving the samples in hy-drochloric acid, followed by (iii) filtering to isolate theceramic particulates. The particulates were then driedand the weight fraction determined.

2.5. Density measurements

The densities of the extruded composite samples weremeasured using Archimedes principle to quantify the

Fig. 1. Schematic diagram showing the gradient slurry disintegrationand deposition technique.

M. Gupta, C.Y. Loke / Materials Science and Engineering A276 (2000) 210–217212

volume fraction of porosity [16,17]. The densitymea-surements involved weighing polished cubes in airand when immersed in distilled water. The densities,derived from the recorded weights, were then com-pared to the theoretical Rule of Mixtures densitiesfrom which the volume fractions of porosity werecalculated. The samples were weighed using anA&D ER-182A electronic balance to an accuracy of90.0001 g.

2.6. Microstructural characterization

Microstructural characterization was carried out us-ing a JEOL JSM-T330A scanning electron microscope(SEM) on the metallographically polished samples toinvestigate the: (i) distribution of SiC particulates; (ii)interfacial bonding, and (iii) presence of porosity. Mi-crostructural characterization was conducted in boththe etched and unetched condition. Keller’s reagent(0.5 HF+1.5 HCl+2.5 HNO3+95.5 H2O) was usedas the etchant.

2.7. Microhardness measurements

Microhardness of samples was determined using theMatsuzawa MXT50 automatic digital microhardnesstester. Microhardness measurements were made using apyramidal diamond indenter with a facing angle of136°, 200 g indenting load, load applying speed of 50mm s−1 and a load holding time of 15 s.

3. Results

3.1. Quantitati6e assessment of SiC particulates

The results of quantitative assessment of SiC partic-ulates are graphically plotted as a function of distancefrom the base for all the three FGMs synthesized inthe present study (see Figs. 2–4). The results reveal anincreasing trend of amount of SiC particulates with anincrease in the distance from the base for all the threeFGMs.

3.2. Porosity measurements

The results of porosity computation derived fromthe experimental results of density measurements andquantitative assessment of SiC particulates are showngraphically in Figs. 2–4. The results reveal an increasein the volume percent of porosity with an increase inthe distance from the base for all the three FGMs. Itmay however be noted that the variation in the averageamount of porosity particularly in the initially de-posited material (up to 10 mm thickness) of Al/15SiCand Al/18SiC FGM ingots was minimal and within the

Fig. 2. Graphical representation of: a) gradient of SiC particulatesand volume percent porosity and b) variation in microhardness as aresult of presence of gradient of SiC particulates in the case ofAl/15SiC functionally gradient material.

standard deviation of the values obtained from theprevious sections (see Figs. 2 and 3).

3.3. Microstructural characterization

The results of microstructural characterizationconducted on all the three FGMs investigated in thepresent study revealed columnar-equiaxed grainstructure and a fairly uniform distribution of SiCparticulates along the deposition direction. Clusters wereobserved and the clustering tendency (number ofparticulates per cluster) was found to be minimum at thebase level and maximum at the top section of the ingots.Fig. 5 shows the distribution of SiC particulates observedat different sections of Al/20SiC ingot. Microstructuralcharacterization results also revealed the presence ofinterdendritically located and clusters-associatedporosity. The clusters-associated porosity was found toincrease from the base to the top of the ingot in the caseof all the three FGMs investigated in this study (see Fig.5 for Al/20SiC FGM). The SiC � Al interfacial integritywas found to be good in all the locations in all thesamples investigated in the present study. Fig. 6 showsthe representative SEM micrographs taken fromAl/20SiC FGM ingot illustrating the nature of the


SiC � Al interface at two locations containing relativelylower and higher weight percentages of SiCparticulates.

3.4. Microhardness measurements

The results of microhardness measurements areshown in Figs. 2–4. The results revealed, in common, asignificantly different microhardness values of the basewhen compared to the top of the ingot. The microhard-ness of the metallic matrix was found to increase as afunction of distance from the base of the ingot in thecase of all the three FGMs investigated in this study.The only exception being the microhardness values inthe deposition distance ranging between 20–30 mmfrom the base of the ingot in the case of Al/15SiC FGMingot. These values, however, were still in each othersstandard deviation (see Fig. 2).

4. Discussion

4.1. Processing

The results of microstructural characterizations andmicrohardness measurements clearly revealed that



GSDD process which is a variation of disintegratedmelt deposition technique can be successfully utilized tosynthesize Al/SiC based functionally gradient materialswith the starting weight percentages of SiC particulatesranging between 15 and 20%. The results further indi-cate the capability of GSDD processing technique tobring together the simplicity (no complex process re-quirement) and cost effectiveness (no overspray pow-ders and high yield of starting materials) associatedwith conventional casting methods [18] and the scien-tific innovativeness (disintegration-associated mi-crostructural refinement) and technological potentialassociated with the spray processes [19–23]. It mayfurther be noted that the GSDD process resembles thespray forming processes in a way that the molten metalslurry is gas-disintegrated and subsequently depositedon a metallic substrate [16,17,19–23], however, differ-ences include use of high melt superheat temperature,disintegration mode, and complete absence of over-spray powders. Moreover, unlike disintegrated meltdeposition process [24], GSDD process involves ex-tremely controlled stirring so as to permit the formationof the SiC particulates’ gradient in the molten metal in


the crucible prior to pouring and disintegration. Theformation of gradient during the GSDD process can beattributed to the coupled effects of stirring conditions

used in the present study and comparatively higherdensity of SiC particulates (3200 kg m−3) when com-pared to that of aluminum (2700 kg m−3). The stirring

Fig. 5. Representative SEM micrographs showing the microstructural features of the samples taken from: a) 5 mm, b) 10 mm, c) 15 mm, d) 20mm, e) 25 mm, f) 30 mm, and g) 35 mm from the base of the ingot.


Fig. 5. (Continued)

during the stirring was however strong enough so asnot to allow the complete settling of SiC particulateswithin the time of addition of SiC particulates whichwas significantly higher than the theoretically computedvalue of settling time under the assumption of nostirring. Further work is continuing to understand moreclearly the role of additional factors such as the stirrergeometry and initial weight percentage of SiC particu-lates besides the stirring speed on the formation of SiCparticulates gradient so as to exercise more control onit along the deposition direction.

4.2. Microstructure

The microstructure of the three FGMs synthesized inthe present study revealed three common salientfeatures:

Fig. 6. Representative SEM micrographs taken from Al/20SiC FGMingot illustrating the nature of SiC–Al interface at two locations: (a)5 mm and (b) 35 mm from the base of the ingot.

conditions used in the present study assisted in theincorporation of SiC particulates while simultaneouslyallowing the formation of SiC gradient due to the weakcirculating currents and higher density of SiC particu-lates. Assuming spherical shape of SiC particulates andignoring the effects of stirring, the drag on a sphere inStoke’s flow [25] can be expressed as follows:

Drag=6pmrU (1)

where m is the viscosity of molten aluminum (1×10−3

K m−1 s−1), r is the radius of sphere (17.2×10−6 m)and U is the relative terminal velocity of sphere andfluid. The drag forces acting on sphere can be equatedwith the gravitational forces as follows:

4/3pr3(rSiC−rAl)×9.81=6pmrU (2)

where rSiC and rAl represent the density of SiC particu-lates and aluminum, respectively. Using Eq. (2), a valueof 3.22×10−4 m s−1 was obtained for U. Since thelevel of the molten metal in the crucible was 0.055 m,the time for the particulates to settle down was foundto be about 2 min 50 s. Since the total addition andstirring time of SiC particulates varied between 13 and16 min, the settling of SiC particulates was expected inthe present study. This inference pertaining to the set-tling tendency of SiC particulates are also consistentwith the related studies conducted by various investiga-tors indicating the settling tendency of SiC particulatesgoverned by their density, size and shape in the alu-minum matrix [26–28]. Complete settling of SiC partic-ulates was primarily avoided by judicious selection ofstirring speed which generated insufficient circulationcurrents to realize complete homogenization of SiCparticulates in the aluminum melt contrary to themethodology practised in disintegrated melt depositionprocessing to synthesize metal matrix composites [24].The magnitude of the recirculating currents generated


Table 1Results of minimum and maximum weight percentage of SiC particulates and average gradient of SiC particulates achieved in Al/15SiC, Al/18SiC,and Al/20SiC functionally gradient materials

Material Maximum wt.% SiC (High SiC end)Minimum wt.% SiC (High Al end) Average gradient (Wt.% SiC/mm)

12.3Al/15SiC 0.311.5Al/18SiC 1.9 15.9 0.40

16.6 0.41Al/20SiC 2.4

1. columnar-equiaxed grain structure,2. interdendritic and clusters associated porosity, and3. predominant grain boundary location of SiC

particulates.The columnar-equiaxed grain structure is also com-

monly referred as ‘ingot’ type of structure and isevolved when the remaining liquid temperature afterthe onset of solidification from the mold wall remainedabove the nucleation temperature [29]. The grain struc-ture observed was expected considering that a highsuperheat temperature and a comparatively lower disin-tegration gas flow rate was used in the present study.

The formation of interdendritically located micropo-rosity in the matrix was inevitable since the experimen-tal conditions used in the present study lead tocolumnar-equiaxed grain structure. The association ofinterdendritically located microporosity with the colum-nar-equiaxed grain structure is widely established in theexisting literature [29]. The association of porosity withSiC clusters can be attributed to the inability of liquidaluminum alloy used in the present study to infiltratethe micrometer sized crevices in the inefficiently packedSiC particulates clusters formed ahead of moving so-lidification front. The results are consistent with thesimilar findings reported elsewhere [24].

The predominantly observed grain boundary locationof SiC particulates and the noticeable presence of clus-ters especially in the sections of FGM ingots containinghigher weight percentages of SiC particulates can beattributed to the coupled effects of less-than-ideal dis-tribution of SiC particulates in the molten aluminum asa result of slow stirring and lower-than-required so-lidification front velocity to entrap SiC particulates [26].

4.3. Gradients of SiC particulates, porosity andmicrohardness

The results of microstructural and microhardnesscharacterization revealed that the FGM synthesized inthe present study exhibited a continuously increasingbut non-steady gradient of SiC particulates (see Figs.2–4). The average gradient computed in terms ofweight percentage SiC per millimeter across the totalthickness increased with an increase in the startingweight percentages of SiC particulates (see Table 1).The results also revealed a significant difference in the

amount of SiC particulates in the opposite faces of allthe FGM ingots synthesized in the present study. Theincrease in the average gradient of SiC particulates andthe amount of SiC particulates on the opposite faces ofthe FGM ingots with an increase in the starting weightpercentage of SiC particulates can be attributed to aprogressively higher retention of SiC particulates by thealuminum melt during addition and subsequent distri-bution of SiC particulates in the form of gradient bythe stirrer’s action. The results also indicated, in com-mon, that the overall amount of SiC particulates incor-porated in the three FGMs synthesized in the presentstudy were typically less than the starting weight per-centage of SiC particulates (see Table 1). This canprimarily be attributed to the lower-than-critical veloc-ity required for maximum possible incorporation of SiCparticulates. In related studies, for example, investiga-tors have clearly revealed the existence of critical veloc-ity for maximum possible incorporation ofreinforcement in the composite formulations synthe-sized using liquid-phase routes [30,31].

The increase in porosity with an increase in theamount of SiC particulates from the base to the top ofthe FGM ingots can be attributed to the coupledinfluence of: (a) increase in the apparent viscosity of thealuminum melt limiting its ability to vent entrapped air[32], and (b) increase in the formation of SiC clustersand the difficulty of liquid aluminum to flow throughnarrower channels [24,33]. The results are also consis-tent with the similar findings of other investigatorsobtained on the homogeneous metal-ceramic formula-tions [34,35].

The results of microhardness measurements revealed,in common, an increase in the microhardness of themetallic matrix from the base to the top of the ingot inthe case of all the three FGMs synthesized in thepresent study (see Figs. 2–4). The variation of themicrohardness, however, did not follow a steady trend(see Figs. 2–4). The two opposite sides of the FGMingots synthesized in the present study exhibited signifi-cantly different microhardness values which can pri-marily be attributed to the significant variation in theweight percentages of SiC particulates (see Table 1).These results are consistent with the fundamental prin-cipals established for SiC particulates associatedstrengthening of the metallic matrices [26,36,37]. The


significantly different microhardness values obtained onthe opposite faces of all the three FGMs synthesized inthe present study indicates that significantly differenttribological and mechanical response can be expectedfrom the two ends of the FGM ingots. Further work iscontinuing in this area.

5. Conclusions

(i) Gradient slurry disintegration and deposition pro-cess can be successfully utilized to synthesize free stand-ing, one dimensional, Al/SiC based functionallygradient materials with the starting weight percentagesof SiC particulates ranging between 15–20%.

(ii) The average gradient of SiC particulates alongthe deposition direction increases with an increase inthe starting weight percentage of SiC particulates.

(iii) An increase in the weight percentage of SiCparticulates along the deposition direction leads to aprogressive increase in the porosity levels and matrixmicrohardness.

(iv) Significantly different microhardness values canbe realized at the opposite ends of the functionallygradient ingots as a result of significantly differentweight percentages of SiC particulates.

Acknowledgements

MG would like to thank Thomas Tan, Tung SiewKong and Boon Heng for their valuable assistance inexperimental work and for many useful discussionsduring the course of this investigation.

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Synthesis of free standing, one dimensional, AlSiC based functionally gradient materials using gradient slurry disintegration and deposition