Functional Metal Matrix Composites: Self-lubricating,Self-healing, and Nanocomposites-An Outlook

AFSANEH DORRI MOGHADAM,1,3 BENJAMIN F. SCHULTZ,1

J.B. FERGUSON,1 EMAD OMRANI,1 PRADEEP K. ROHATGI,1

and NIKHIL GUPTA2

1.Department of Materials Science and Engineering, University of Wisconsin-Milwaukee,3200 N. Cramer St, Milwaukee, WI 53211, USA. 2.Mechanical and Aerospace EngineeringDepartment, Polytechnic School of Engineering, New York University, Brooklyn, NY 11201, USA.3.e-mail: afsaneh@uwm.edu

Many different types of advanced metal matrix composites are now available,some of which possess functional properties. Recent work on particle-rein-forced, self-lubricating and self-healing metals and metal matrix nanocom-posites (MMNCs) synthesized by solidification synthesis is reviewed. Particle-based MMNCs have been developed by several modern processing tools basedon either solid- or liquid-phase synthesis techniques that are claimed to ex-hibit exciting mechanical properties including improvements of modulus, yieldstrength, and ultimate tensile strength. This article presents a brief andobjective review of the work done over the last decade to identify the chal-lenges and future opportunities in the area of functional nanocomposites.Increasing interest in lightweight materials has resulted in studies on hollowparticle-filled metal matrix syntactic foams. Syntactic foams seem especiallysuitable for development with functional properties such as self-healing andself-lubrication. The metal matrix micro and nanocomposites, and syntacticfoams having combinations of ultrahigh strength and wear resistance, self-lubricating, and/or self-healing properties can lead to increased energy effi-ciency, reliability, comfort of operation, reparability, and safety of vehicles.The focus of the present review is aluminum and magnesium matrix func-tional materials.

INTRODUCTION

A metal matrix composite (MMC) is synthesizedby dispersing particles, whiskers, or fibers in ametal or alloy matrix to form a multiconstituentsmaterial having superior properties. In the auto-motive, aerospace, and sports industries, therequirement for high-performance materials tomeet challenging demands keeps increasing. Themost important advantage of MMCs compared withmonolithic alloys is their ability to be tailored tospecific design needs. Density, specific strength,specific stiffness, wear resistance, coefficient offriction (COF), hardness, thermal conductivity,coefficient of thermal expansion, energy absorption,and damping capacity of the composite materialscan be easily tailored by means of reinforcementshape, size, content, and distribution. In recent

years, research on MMCs has been extended tomore advanced systems including metal matrixnanocomposites (MMNCs) and hollow particle-filledsyntactic foams as well as functional materialsincluding self-lubricating, self-cleaning, and self-healing composites. Innovative use of nanosizedparticles, hollow balloons, shape memory alloy(SMA) fibers, and other engineered reinforcementshas made it possible to develop such next-genera-tion MMCs.

The size of reinforcement particles, especially in thenanoscale, is found to have a very strong influence onthe properties of MMCs. With recent advances inproducing particles below 100 nm, it has become pos-sible to synthesize MMNCs with significantly im-proved properties. It is expected that MMNCs canovercome some of the shortcomings of MMCs such aspoor ductility, machinability, and fracture toughness.

JOM, Vol. 66, No. 6, 2014

DOI: 10.1007/s11837-014-0948-5 2014 The Minerals, Metals & Materials Society

872 (Published online April 5, 2014)

Significantly improved mechanical properties result-ing from a combination of Orowan strengthening andgrain refinement coupled with high-temperaturecreep resistance make MMNCs very attractive espe-cially when lightweight metals such as Al or Mg areused as the matrix.1,2 Although polymer matrixnanocomposites have received much more attention inthe recent past and are now used in selected applica-tions,3,4 MMNCs have received relatively little atten-tion.5

In addition to conventional uses of MMCs, newMMCs are being envisioned and developed withself-healing and self-cleaning capabilities similar inconcept to the capabilities of biological systems. Thedevelopment of these materials stems from designcriteria and the wish lists of design engineers whoserequirements call for a material with propertiesunattainable by conventional materials and alloys.

Several functionalities are currently being ex-plored in MMCs including self-lubricating, self-healing, and self-cleaning composites. In addition,hollow balloon-filled syntactic foams are also beingaggressively studied because of the requirement forweight reduction in modern engineering applica-tions. A combination of functional composites and asyntactic foam microstructure can provide advancedlightweight functional materials that can be veryattractive to numerous applications.

The present article summarizes the propertiesobtainable in MMCs and describes highlights ofresearch directions and applications that are envi-sioned for these technologies (summarized inTable I).

The use of these novel materials can improveenergy efficiency, safety, reliability and comfort oftransportation applications.

FUNCTIONAL METAL MATRIXCOMPOSITES

Self-lubricating MMCs

In recent decades, advanced tribological materialshave been designed as an integral part of strategiesto reduce energy and to improve heat dissipationbetween two moving bodies. Self-lubricating MMCsoffer the advantage of not only improved wearresistance and reduced COF, but also they eliminatethe need for external lubricants to avoid seizing.There are several methods to enhance the tribolog-ical behavior of aluminum alloys including mor-phology modification, reinforcement modification,alloying, and surface modification.6 Self-lubricatingsystems are characterized by their ability to trans-fer microscopic amounts of lubricant material,which creates a film that provides lubrication andreduces friction over the contact surface. MMCsreinforced with graphite particles are self-lubricat-ing and have superior wear resistance and reducedCOF because the graphite particles act as a solidlubricant between contact surfaces.7,8

A limitation of aluminum/graphite (Al/Gr) com-posites reinforced with micron-size graphite parti-cles is their low mechanical properties, which cansometimes be less than unreinforced alloy and tendto limit their applications. Changes in the hardness,wear rate, and COF for Al/Gr composites are shownin Fig. 1 as a function of graphite content. Althoughan increase in graphite content results in reductionof both wear and COF, it leads to the decrease inmechanical properties of the composite as repre-sented here by hardness. Adding hard ceramicparticles to Al/Gr and making a hybrid MMC aresuggested to overcome such deficiencies. The hybrid

Table I. MMCs being developed for use in automotive applications

Property Materials Application

Wear resistance SiC-, Al2O3-, and/or graphite-reinforced micro andnano MMCs

Bearing surfaces, cylinder liners, pistons, camshafts, tappets, lifters, rockers, brake compo-

nentsLight weight,

energyabsorption

Fly ash cenosphere- and low-density ceramicmicroballoon-reinforced syntactic foam MMCs

Crumple zones, frame members and reinforce-ments, pedestrian impact zones, batteries

Self-cleaning MMCs with hydrophobic reinforcements, biomi-metic coatings, and surface finishes

Water pumps, water jackets, exposed metalliccomponents

Self-lubricating Micro and nano MMCs incorporating graphite,MoS2, TiB2, hexagonal BN, or other solid lubricants

Bearing journals, cylinder liners, pistons, cvjoints, gear surfaces

High thermalconductivity

Micro and nano MMCs reinforced with high-con-ductivity carbon, diamond, or cubic boron nitride

(cBN) powder

Cylinder liners, water passages, brake compo-nents, turbo/supercharger components, catalytic

converters, electronics packagingHigh strength Micro and nano MMCs reinforced with SiC or Al2O3

particles, CNTs, carbon or Nextel fibers, and in situceramics

Connecting rods, brake calipers, brake rotors,brake calipers

Low cost MMCs containing fly ash or waste sand as fillers Intake manifolds, accessory brackets, low-loadbrackets, oil pans, valve covers, alternator cov-

ers, water pumps

Functional Metal Matrix Composites: Self-lubricating, Self-healing, and Nanocomposites-An Outlook 873

MMCs possess better mechanical and tribologicalproperties than the Al/Gr composite because theload-bearing capacity of ceramic particles increasesthe mechanical properties and the graphite parti-cles act as solid lubricants. Adding 10 wt.% SiC and1 wt.% graphite keep the mechanical properties inthe range of unreinforced aluminum while the wearrate decreases to half of that of unreinforced alu-minum and one third that of the Al/SiC composite asa result of the presence of graphite.9

Another approach proposed to achieve promisingmechanical and tribological properties simulta-neously in a single reinforced system is by addingcarbon-based nanomaterials such as graphene andcarbon nanotubes (CNT) as reinforcement in metalmatrices.10,11 Previous research reveals that themechanical properties of Al/CNT and Al/graphenecomposites are significantly higher than Al/graphitecomposites.12,13 Figure 2 demonstrates how theaddition of CNT and graphene increases themechanical properties by a considerable extent. Acomparison of the Cu/Gr and Cu/graphene self-

lubricating nanocomposites shows improved tribo-logical properties as shown in Fig. 3. However, theCNT and graphene reinforced composites are usu-ally difficult to produce by conve