TMS QUICK LINKS:
  • TMS ONLINE
  • MEMBERS ONLY
  • MEMBERSHIP INFO
  • MEETINGS CALENDAR
  • PUBLICATIONS

JOM QUICK LINKS:
  • JOM HOME PAGE
  • MORE HTML ARTICLES
  • COMPLETE ISSUES
  • BOOKS REVIEWS
  • SUBSCRIBE

Metal matrix syntactic foams are composites that incorporate hollow particles in a matrix, where enclosing porosity inside the thin shell of the particle leads to low density without large decreases in mechanical properties. Studies on Al, Mg, Pb, and Zn alloy matrix syntactic foams are available in the published literature. A large stress plateau region appears in the compressive stress-strain graphs of metal matrix syntactic foams. The height and length of stress plateau can be tailored by means of particle wall thickness, volume fraction, and size, and the total compressive energy absorption can be controlled. Metal matrix syntactic foams seem promising in various energy absorbing applications including automobile parts since their energy absorption capability per unit weight is better than other foams and lightweight materials.

Metal foams are used in lightweight structures, especially as core materials in sandwich constructions.^1,2 Mechanical properties of metal foams are much lower compared to the base metal, restricting their applications to where tensile or compressive strength is not the primary design criterion.³ An innovative method of incorporating porosity in materials is the use of hollow particles as fillers. Enclosing porosity inside stiff and strong shells and incorporating those shells in matrix metals leads to porous materials that have significantly higher modulus and strength than foams containing gas porosity. These hollow particle filled materials are called syntactic foams and are a class of particulate composites.^3,4 Syntactic foams with over 50 vol.% porosity have been synthesized, providing substantial weight saving compared to the matrix material. Apart from weight reduction, the presence of controlled size porosity of a spherical shape with uniform distribution helps in providing high energy absorption under compression in syntactic foams. Therefore, available studies are mainly related to either the processing aspects or compressive property characterization. This review focuses on the compressive properties of metal matrix syntactic foams. Numerous studies are available on other aspects such as corrosion and electrical and high strain rate properties, which are not covered in this review. Due to the interest in lightweight materials, most of the published literature is focused on aluminum alloy matrix syntactic foams. In addition, developing lightweight composites of high density metals such as lead⁵ and zinc⁶ is also of interest. Compressive properties of all these syntactic foams are compiled and analyzed to find structure– composition–property relationships.

Thin-walled ceramic particles are beneficial in synthesizing low-density syntactic foams. Numerous types of ceramic hollow particles are now available that can be used for this purpose. Apart from mechanical property modification, the use of ceramic hollow particles provides higher dimensional stability to the composite by reducing the thermal expansion coefficient.⁷ In general, two types of hollow particles are widely used in synthesizing syntactic foams.

The first type of particles, microballoons, are high-quality hollow ceramic microspheres that are commercially produced and have controlled properties such as diameter, size distribution, and wall thickness. Ceramic microballoons have been used in several studies.^8–11 An example of microballoons is shown in Figure 1a. These particles go through several quality control steps such as pressurization at given pressure levels to fracture and eliminate weaker and defective particles, flotation or air classification to select only the intact low density particles, and sieving to obtain given size ranges. These processing steps ensure that high-quality particles, with a narrow distribution of properties, are obtained. However, these particles are still not completely free from defects. Figure 1b shows an example where porosity and small size solid particles are embedded inside the wall of a large microballoon. In addition, some variation in the wall thickness is also observed in this broken microballoon. Such irregularities are not widespread in glass microballoons. The commercial microballoons are available in the density range of about 100–1,000 kg/m³. In addition to glass, microballoons of several ceramics such as silica, alumina, zirconia, and carbon are also available now.

The second commonly used particles are fly ash cenospheres. Fly ash is produced during coal combustion and is an industrial waste by-product. Due to the pozzolanic properties, fly ash is added to cement and construction material as its most significant application.¹² However, nearly half of the fly ash produced in the United States is dumped in landfills. A review article is available on fly ash, which provides details on its classification, composition, and applications.¹³ One of the challenges in developing applications of this waste product is to separate the useful hollow particles, called cenospheres, from the coal combustion by-products, which include a wide variety of impurities. These processing methods add cost to this otherwise freely available material; however, the cost of fly ash cenospheres is still much lower than synthetically made microballoons. Incorporating cenospheres in metals can lead to substantial savings on the cost of raw materials and reduce the pollution that is generated in the production of metals like aluminum, which consume a lot of energy in their production.¹⁴ Figure 2a shows a sample of cenospheres obtained from Trelleborg Offshore Boston. These processed cenospheres show a uniform size distribution. Some of the particles can be defective as shown in Figure 2b, where porosity, non-uniform size and shape, and poor surface finish are among the defects. Fly ash particles have predominantly SiO₂, Al₂O₃, and Fe₂O₃ in their structure.¹⁵ An example of a composition of fly ash obtained from Wisconsin Electric Power Company in Milwaukee, in wt.%, is SiO₂ – 61.0%, Al₂O₃ – 25.80%, Fe₂O₃ – 4.99%, K2O – 3.59%, MgO – 1.58%, TiO₂ – 1%, Na₂O – 0.74% CaO – 0.82%, and SO3 – 0.31%. Trace amounts of several toxins may be present in fly ash particles, including As, Cd, Pb, and Zn, depending upon the origin of coal and the combustion reactions.¹³ Leaching of these toxins from the particles is a significant concern, especially in landfills. Such possibilities should be considered while developing processing techniques and applications for cenosphere filled syntactic foams.

Three methods of syntactic foam synthesis are widely used: pressure infiltration, stir casting, and powder metallurgy. These methods have their advantages and limitations and are selected based on the material system and composition.

In pressure infiltration, a preform or a bed of loosely packed particles is prepared and placed in a mold.^9,16 Molten metal is infiltrated in the mold by applying either high pressure or vacuum or a combination of both to fill the interparticle spaces and form a near-net-shaped syntactic foam component.^17,18 The advantages of this method include synthesis of foams containing high volume fraction of particles (up to 70 vol.%), net-shaped or near-net-shaped component fabrication, and low porosity in the composite. The limitations of this process include use of high pressure for melt infiltration leading to fracture of particles, difficulty in synthesizing syntactic foams with low particle volume fraction, and additional cost associated with preparing a preform. There has been some work on infiltrating loose beds of cenospheres with molten alloys to form syntactic foams to eliminate the need of preparing preforms. This process requires a very close control over melt superheat temperature and particle preheat temperature to avoid freeze choking of the melt and incomplete infiltration. Low infiltration pressure can lead to incomplete filling of pores and high residual porosity, whereas high infiltration pressure can cause particles to fracture or liquid metal to infiltrate inside the hollow spaces within the cenospheres due to defects. Studies have shown effects of all these processing parameters on material quality.

In stir casting, the molten melt is stirred using a high shear impeller and particles are slowly added in the vortex formed in the melt.^19,20 This process can be conducted in a conventional foundry and requires very little new infrastructure. Low cost and easy implementation have resulted in widespread use of stir casting for synthesizing syntactic foams. Flotation of low density particles is a concern in this method, especially when the particle volume fraction is low. On the contrary, at high particle volume fractions high shear processing can lead to substantial particle fracture. Wetting of particles with liquid melts is also a concern. In several studies particles are coated with suitable materials, including metals like nickel, to increase their wettability with the molten melt. The coating also has the advantage of sealing the porosity in the hollow microballoons and cenospheres. For example, nickel-coated fly ash particles have been incorporated in aluminum alloy melts. This method has been used for fabrication of aluminum alloy syntactic foams.^19,21,22 Use of this method for lead and zinc matrix syntactic foams is especially difficult because of the large density difference between the particles and the matrix.⁵ However, the stir casting method can be followed by slow solidification or centrifugal casting processes, which lead to a high concentration of particles in the top part of the casting. The top part can be used as the highly filled syntactic foam, while the bottom part can be recycled in the subsequent heats.

Powder metallurgy methods are used in several studies. These methods are versatile because a wide variety of particle volume fractions can be incorporated in composites. Even reactive metals, which are not amenable to liquid state processing, can be used as the matrix material in the powder metallurgy method. Hollow particles and powder of matrix metal are mixed together in required volume fractions, followed by compaction and sintering to obtain syntactic foams.²³ However the powder metallurgy methods have the disadvantage that fracture of weak hollow particles can be significant in the compaction stage at high volume fractions. This method is especially suitable in synthesizing syntactic foams containing low microballoon volume fraction.

Syntactic foams have a two-component microstructure, including matrix material and hollow particles. However, the microstructure can have several phases. As the third phase, porosity entrapped in the matrix alloy can be significant in some composites and affect the mechanical properties. Figure 3a and b shows microstructures of two aluminum matrix syntactic foams, where entrapped air porosity in the matrix alloy can be observed in the regions between hollow microspheres. Additional phases may be present in the matrix alloy. For example, in magnesium- aluminum alloys intermetallic precipitates of Mg₁₇Al₁₂ are present in the matrix.

Figure 3c shows that the grain size of the matrix alloy is refined in the vicinity of the fly ash cenospheres. A similar effect is observed in magnesium alloy matrix syntactic foams. In AZ91 alloy intermetallic precipitates are present in the matrix along grain boundaries. The size of these precipitates is refined by an order of magnitude in the AZ91/fly ash syntactic foams as shown in Figure 3d. In ZC63/fly ash composites, the dendrite arm spacing was reduced compared to the matrix alloy cast under the same conditions. Numerous elements present in fly ash can diffuse out in the matrix and lead to grain refinement. Diffusion of elements from the particle matrix interface to the interparticle region depends on the type of element, concentration, melt temperature, and processing time.

Apart from the grain size effects, reactions at the particle-matrix interface are also observed in several syntactic foams.²⁴ Interfacial reactions in aluminum matrix composites can result in brittle phases that can be detrimental to the syntactic foam properties. In an aluminum/fly ash system, transmission electron microscopic analysis showed the presence of two interpenetrating crystalline networks in the matrix comprising a-Al₂O₃ particles surrounded by a continuous metallic network.²⁵ The silicon level was increased in the matrix in the A356/ fly ash composites as a result of reaction between the silica of fly ash and the matrix alloy.²² In magnesium alloy ZC63/fly ash syntactic foams the main interfacial reaction product phase was detected as MgO. These reactions can be controlled by coating particles with appropriate metals.

Unlike fly ash cenospheres, commercially produced glass and ceramic microballoons have closely controlled composition and undesired interfacial reactions are not a major concern with them. Microballoons can also be coated with appropriate metals to increase their wetting characteristics. Nickelcoated particles have been used for aluminum and lead alloys.⁵ In addition, nickel-coated fly ash cenospheres have been incorporated in aluminum alloys.26 It was observed that wetting characteristics and dispersion of hollow particles improved as a result of the coating that was compatible with the matrix alloy.

Stress-strain Graphs
Compressive properties have been widely studied for a variety of syntactic foams.²⁷ Representative compressive stress-strain curves for A356 alloy matrix syntactic foams are presented in Figure 4.¹⁷ In Figure 4a the compressive response of the matrix alloy is compared with syntactic foams containing different volume fractions of cenospheres, whereas Figure 4b presents the effect of cenosphere particle size on the properties of the composite.

The compressive stress-strain curves are characterized by a linear region, followed by a long stress plateau. At the end of the plateau region stress starts increasing again. The initial linear region is normally considered linear elastic behavior, where modulus is calculated. However, in syntactic foams where particles of a wide variety of wall thicknesses and size are present, it is not necessary that this region is truly elastic. Some of the particles can fail at low stress levels and cause some variation within this region. Usually a small stress drop is observed at the end of the linear region before the stress plateau appears. Initiation of cracks in the specimens at the end of the linear region is usually responsible for the stress drop. The energy absorption capabilities of syntactic foams are mainly related to the height and length of the stress plateau. Detailed studies are available in polymer matrix syntactic foams where the effect of particle wall thickness and volume fraction on the strength and energy absorption in syntactic foams has been studied.²⁸ Although such systematic studies are not yet available in metal matrix syntactic foams, trends similar to those observed in polymer matrix syntactic foams can be expected.4 Experimental results have shown that using thick-walled particles can increase the strength and modulus of syntactic foams without causing significant increase in the density.^29,30 The plateau region is where sequential crushing of microballoons takes place and the material absorbs energy without any significant change in the strength. The particle crushing and compaction result in the densification of the composite material. When the densification is complete, then the stress starts rising again as visible in Figure 4a.

Mechanical Properties
Some of the data presented here have been extracted from the published graphs. Although precautions have been taken in image processing, there may be small variations in the absolute values. Properties of matrix alloy are not reported in all studies. In the absence of matrix properties from the same study, literature values of the same composition are taken.

Results obtained from the published studies on the compressive properties of syntactic foams are summarized in Figures 5–7. The data included in the graphs correspond to aluminum,^10,17,31,32 magnesium,^33,34 titanium,^35,36 and zinc^6,37 matrix syntactic foams. Not every study reports all compressive properties, so the reported properties are extracted and included in appropriate graphs. There are numerous factors that can be controlled in designing a syntactic foam microstructure, which include matrix and particle material, particle wall thickness, diameter and volume fraction, and heat treatment of the composite. These parameters can be used to minimize the porosity entrapped in the matrix during the synthesis of composites and fraction of microballoons in which liquid metal has entered and solidified. All these parameters affect the compressive properties as well as density of the composite. Therefore, the comparison presented in Figures 5–7 is illustrative of overall properties of syntactic foams and also the weight saving potential in selected applications.

Figure 5a compares the compressive modulus of syntactic foams over a large range of density values. Usually aluminum and magnesium matrix syntactic foams have the lowest densities, while titanium and zinc alloy matrix syntactic foams have higher density values. The general trend of the data shows that the higher density syntactic foams have a higher modulus. The vertical spread of data for the same density value shows the possibility of selecting higher modulus foam having the same density. In general, the data are confined within a narrow band and the choice of different compositions of the same density is small. Figure 5b includes the modulus of syntactic foam divided by the modulus of its matrix metal or alloy. This comparison is more illustrative of the weight saving potential in syntactic foams comprising of various matrix materials. It is observed that the titanium matrix syntactic foams have higher modulus for the same density values. However, the titanium and zinc matrix syntactic foams have densities over 3 g/cc and for lower density materials aluminum and magnesium matrix syntactic foams appear to be the only options. The syntactic foam modulus is much lower than that of the matrix material due to the porosity present in the foams.

Compressive yield strength of syntactic foams and the yield strength normalized with that of the matrix material are shown in Figure 6. It is possible to tailor the properties of syntactic foams over a wide range of strength values as evident from this figure. It can be noted in Figure 6b that the strength of several lightweight syntactic foams is equal to or close to the strength of the matrix material. This figure shows that several low density syntactic foams can replace their matrix alloys in loadbearing applications, which can result in weight saving. In general, higher density foams show higher strength. Unlike modulus, strength values are not confined to a narrow band and scatter over a wide range for a given density. Therefore, several compositions of syntactic foams are available having the same density but a variety of strength levels. Data available for titanium matrix syntactic foams produced by powder metallurgy method at different compaction pressures and microballoon volume fractions is very illustrative of the change in syntactic foam properties with density.

Figure 7 summarizes plastic stress and plateau stress values of syntactic foams. In the plastic stress, two different regions are observed. Plastic stress of aluminum follows one trend line, whereas zinc and titanium follow a different trend line. A wide distribution of strengths is observed for aluminum alloy syntactic foams within a narrow density range. Plateau stress is an important property of syntactic foams because it determines the energy absorption capability of the material. In general, plateau stress up to 250 MPa is observed in various types of syntactic foams.

The compressive properties of metal matrix syntactic foams are strain rate sensitive.^38,39 Evaluation of composite properties at strain rates relevant to a given application is required for correct materials selection. In addition, in polymer matrix syntactic foams the failure mode is found to be strain rate sensitive,⁴⁰ which should be evaluated for metal matrix syntactic foams also.

Several present and potential applications of syntactic foams are discussed in the available literature. The damping capacity of 6061Al/fly ash syntactic foams is found to be much higher than that of the matrix alloy, which is beneficial in automotive applications.⁴¹ Al-Si alloy/fly ash composites also showed higher damping than the matrix alloy.⁴² Aluminum matrix syntactic foams have been explored for making automotive brake rotors and differential covers.¹⁴ Studies also suggest their use in crash energy absorption zones.⁴³ Phenolic resins filled with fly ash have been tested for automotive break lining applications.⁴⁴ Metal matrix syntactic foams can also find similar applications. Superior wear resistance of A356/fly ash45 and AA6061/fly ash46 composites compared to the matrix alloy can be helpful in such applications. Ni-P/ fly ash,47 Ni-Co/fly ash,47 and Al/fly ash⁴⁸ composite coatings have been used for their wear-resistance properties. These coatings reduced the wear of 5083 wrought aluminum alloy. Electromagnetic (EM) shielding effects of 2024Al/fly ash composites were found to be better than the matrix alloy.⁴⁹ In the frequency range of 1–600 MHz the EM shielding property of 2024Al alloy was in the range -36 to -46 dB while that of the composites was in the range of -40 to -102 dB. These results show suitability of such composites for lightweight electronic packaging applications. Nickel coated fly ash cenospheres have been studied separately for EM shielding and microwave absorption applications.⁵⁰ Syntactic foams of these coated cenospheres can be effective in electronic packaging applications.

Most studies show substantially low modulus of syntactic foams compared to the matrix alloy. The strength of syntactic foams can be nearly equal to that of the matrix. The plateau and yield stress can be tailored over a wide range by selecting appropriate volume fraction and particle type. Metal matrix syntactic foams are expected to have better properties compared to open or closed-cell metallic foams since the former have controlled size and geometry of porosity and the ceramic shells contribute to stiffness and strength. Some of the barriers to widespread use of metal matrix syntactic foams can be overcome by the availability of:

1. Low-cost defect-free hollow microspheres in narrow size ranges, including nanosize ranges
2. Mechanical and physical property data for hollow microspheres
3. Mechanical and physical property data for metal matrix syntactic foams, especially under high strain rates
4. Processing methods enabling manufacture of large near-netshaped parts of syntactic foams
5. Understanding of mechanisms of deformation and fracture, and energy absorption to develop quantitative relationships between structure-processing-property and predictive capability to design microstructures
6. Understanding of solidification structure formation in the matrix in the presence of microballoons, and the influence of microballoons on grain size, dendrite size, micro-, and macrosegregation, and porosity in the matrix
7. Understanding the reactions between the surfaces of microballoons and molten alloys and developing strategies of preventing undesirable reactions

Aluminum/fly ash cenosphere composites have been cast into selected shapes and their superior properties have been demonstrated. In certain cases metal matrix syntactic foams have been encapsulated in hollow steel frames for lightweight structures. However, more research is needed before metal matrix syntactic foams will be widely used. Potential of weight saving in structural applications by using syntactic foams is evident from the published data through the graphs plotted in this review. It is recognized that these composites may not be suitable where high modulus is required. However, in applications where mechanical properties of these materials are suitable enough, ability to tailor their properties is a signifi cant advantage.

This work is supported by the National Science Foundation grant CMMI–0726723 and Office of Naval Research grant N00014-10-1-0988. The views expressed in the article are those of the authors, not the funding agencies. Authors thank William Ricci at TOB for providing fly ash sample.

1. P. Colombo and H.P. Degischer, Materials Science and Technology, 26 (10) (2010), pp. 1145–1158.
2. J. Banhart, Progress in Materials Science, 46 (6) (2001), pp. 559–632.
3. L.J. Gibson and M.F. Ashby, Cellular Solids: Structure and Properties (New York: Cambridge University Press, 1999).
4. M. Kiser, M.Y. He, and F.W. Zok, Acta Materialia, 47 (1999), pp. 2685–2694.
5. A. Daoud, M.T.A. El-Khair, A.Y. Shenouda, E. Mohammed, and P.K. Rohatgi, Materials Science and Engineering: A, 526 (1-2) (2009), pp. 225–234.
6. A. Daoud, Materials Science and Engineering: A, 488 (1-2) (2008), pp. 281–295.
7. P.K. Rohatgi, N. Gupta, and S. Alaraj, J. Composite Materials, 40 (13) (2006), pp. 1163–1174.
8. O. Couteau and D.C. Dunand, Materials Science and Engineering: A, 488 (1-2) (2008), pp. 573–579.
9. D.K. Balch and D.C. Dunand, Acta Materialia, 54 (6) (2006), pp. 1501–1511.
10. X.F. Tao, L.P. Zhang, and Y.Y. Zhao, Materials & Design, 30 (7) (2009), pp. 2732–2736.
11. M.Y. He, M. Kiser, B. Wu, and F.W. Zok, Mechanics of Materials, 23 (2) (1996), pp. 133–146.
12. N. Chandra, P. Sharma, G.L. Pashkov, E.N. Voskresenskaya, S.S. Amritphale, and N.S. Baghel, Waste Management, 28 (10) (2008), pp. 1993–2002.
13. M. Ahmaruzzaman, Progress in Energy and Combustion Science, 36 (3) (2010), pp. 327–363.
14. P.K. Rohatgi, D. Weiss, and N. Gupta, JOM, 58 (11) (2006), pp. 71–76.
15. B.G. Kutchko and A.G. Kim, Fuel, 85 (17-18) (2006), pp. 2537–2544.
16. D.K. Balch, J.G. O'Dwyer, G.R. Davis, C.M. Cady, G.T. Gray III, and D.C. Dunand, Materials Science and Engineering A, 391 (1-2) (2005), pp. 408–417.
17. P.K. Rohatgi, J.K. Kim, N. Gupta, S. Alaraj, and A. Daoud, Composites Part A: Applied Science and Manufacturing, 37 (3) (2006), pp. 430–437.
18. L.P. Zhang and Y.Y. Zhao, J. Composite Materials, 41 (17) (2007), pp. 2105–2117.
19. T.P.D. Rajan, R.M. Pillai, B.C. Pai, K.G. Satyanarayana, and P.K. Rohatgi, Composites Science and Technology, 67 (15-16) (2007), pp. 3369–3377.
20. A. Daoud, M.T. Abou El-khair, M. Abdel-Aziz, and P. Rohatgi, Composites Science and Technology, 67 (9) (2007), pp. 1842–1853.
21. D.P. Mondal, S. Das, N. Ramakrishnan, and K. Uday Bhasker, Composites Part A: Applied Science and Manufacturing, 40 (3) (2009), pp. 279–288.
22. Sudarshan and M.K. Surappa, Materials Science and Engineering: A, 480 (1-2) (2008), pp. 117–124.
23. M. Hrairi, M. Ahmed, and Y. Nimir, Advanced Powder Technology, 20 (6) (2009), pp. 548–553.
24. R.Q. Guo, D. Venugopalan, and P.K. Rohatgi, Materials Science and Engineering A, 241 (1-2) (1998), pp. 184–190.
25. N. Sobczak, J. Sobczak, J. Morgiel, and L. Stobierski, Materials Chemistry and Physics, 81 (2-3) (2003), pp. 296–300.
26. P.K. Rohatgi, R.Q. Guo, H. Iksan, E.J. Borchelt, and R. Asthana, Materials Science and Engineering: A, 244 (1) (1998), pp. 22–30.
27. R.A. Palmer, K. Gao, T.M. Doan, L. Green, and G. Cavallaro, Materials Science and Engineering: A, 464 (1-2) (2007), pp. 85–92.
28. N. Gupta, E. Woldesenbet, and P. Mensah, Composites Part A: Applied Science and Manufacturing, 35 (1) (2004), pp. 103–111.
29. N. Gupta, R. Ye, and M. Porfiri, Composites Part B: Engineering, 41 (3) (2010), pp. 236–245.
30. M. Porfi ri and N. Gupta, Composites Part B: Engineering, 40 (2) (2009), pp. 166–173.
31. D.P. Mondal, M.D. Goel, and S. Das, Materials Science and Engineering: A, 507 (1-2) (2009), pp. 102–109.
32. X.F. Tao and Y.Y. Zhao, Scripta Materialia, 61 (5) (2009), pp. 461–464.
33. Z.-Q. Huang, S.-R. Yu, and M.-Q. Li, Transactions of Nonferrous Metals Society of China, 20 (Supplement 2) (2010), pp. s458–s462.
34. P.K. Rohatgi, A. Daoud, B.F. Schultz, and T. Puri, Composites Part A: Applied Science and Manufacturing, 40 (6-7) (2009), pp. 883–896.
35. X.B. Xue, Y.Y. Zhao, V. Kearns, and R.L. Williams. Supplemental Proceedings: Volume 2: Materials Characterization, Computation, Modeling and Energy (Warrrendale, PA: TMS, 2010), pp. 129–136.
36. X. Xue and Y. Zhao, JOM, 63 (2) (2011), pp. 36–40.
37. A. Daoud, Materials Science and Engineering: A, 525 (1-2) (2009), pp. 7–17.
38. Z.Y. Dou, L.T. Jiang, G.H. Wu, Q. Zhang, Z.Y. Xiu, and G.Q. Chen, Scripta Materialia, 57 (10) (2007), pp. 945–948.
39. D.D. Luong, N. Gupta, and P.K. Rohatgi, JOM, 63 (2) (2011), pp. 46–49.
40. V.C. Shunmugasamy, N. Gupta, N.Q. Nguyen, and P.G. Coelho, Materials Science and Engineering: A, 527 (23) (2010), pp. 6166–6177.
41. G.H. Wu, Z.Y. Dou, L.T. Jiang, and J.H. Cao, Materials Letters, 60 (24) (2006), pp. 2945–2948.
42. Y. Mu, G. Yao, and H. Luo, Materials & Design, 31 (2) (2010), pp. 1007–1009.
43. Q. Zhang, P.D. Lee, R. Singh, G. Wu, and T.C. Lindley, Acta Materialia, 57 (10) (2009), pp. 3003– 3011.
44. S. Mohanty and Y.P. Chugh, Tribology International, 40 (7) (2007), pp. 1217–1224.
45. Sudarshan and M.K. Surappa, Wear, 265 (3-4) (2008), pp. 349–360.
46. P.R.S. Kumar, S. Kumaran, T.S. Rao, and S. Natarajan, Materials Science and Engineering: A, 527 (6) (2010), pp. 1501–1509.
47. C.N. Panagopoulos, E.P. Georgiou, A. Tsopani, and L. Piperi, "Composite Ni-Co-Fly Ash Coatings on 5083 Aluminium Alloy," Applied Surface Science, doi:10.1016/j.apsusc.2010.10.130.
48. S.P. Sahu, A. Satapathy, A. Patnaik, K.P. Sreekumar, and P.V. Ananthapadmanabhan, Materials & Design, 31 (3) (2010), pp. 1165–1173.
49. Z. Dou, G. Wu, X. Huang, D. Sun, and L. Jiang, Composites Part A: Applied Science and Manufacturing, 38 (1) (2007), pp. 186–191.
50. X.-F. Meng, D.-H. Li, X.-Q. Shen, and W. Liu, Applied Surface Science, 256 (12) (2010), pp. 3753–3756.

Pradeep K. Rohatgi, Wisconsin and UWM Distinguished Professor and Director of UWM Centers of Composites and Advanced Materials Manufacture, and Benjamin F. Schultz, postdoctoral research fellow, are with the Center for Composite Materials, Materials Engineering Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53201 USA. Nikhil Gupta, associate professor, and Dung D. Luong, Ph.D. candidate, are with the Composite Materials and Mechanics Laboratory, Mechanical and Aerospace Engineering Department, Polytechnic Institute of New York University, Brooklyn, NY 11201 USA. Dr. Gupta can be reached at (718) 260- 3080; fax (718) 260-3532; e-mail ngupta@poly.edu.