[engineering materials] nanocoatings volume 27 || size effect in mechanical properties of...

Chapter 5Size Effect in Mechanical Propertiesof Nanostructured Coatings

5.1 Introduction

Research studies have shown that when particles’ size reaches to the dimensionsof nanometer, remarkable improvement will be observed in strength of com-posite. For example, remarkable increase was observed in hardness of nickel-alumina composite when size of improving particles was decreased from 10 lmto 10 nm. Shape, size and surface of nanoparticles play important role inproperties of nanocomposite. In recent years, nanocomposites have been usedwidely due to their better magnetic, mechanical, optical and physical properties.Interface volume, layer thickness, superficial energy and interface are theparameters which have noticeable effects on nanostructure thin films [1–12].Figure 5.1 shows schematics of particles used in nanocomposites and degree ofproportion of surface area to their volume. As an example of particulatenanomaterials, Ohno et al. [13] studied the size effect of TiO2-SiO2 nano-hybridparticles. The well-dispersed primary TiO2-SiO2 nano-hybrid particles weresuccessfully prepared by using the super critical drying of the molecular-designed nano-hybrid precursor. The particle diameter of the resultant hybridparticles was about 140 nm. The crystal size of titania on the surface of the silicacore particle was determined to be 7 nm from the result of TEM and XRDanalysis. The crystal structure was anatase. The band gap energy was measuredform the ultraviolet–visible spectrum. As a result, the band gap energy of thenano-hybrid particles were 0.13 eV blue shifted compared with that of theanatase crystal without the quantum size effect. Therefore, we concluded thatnano-hybrid particles has the possibility to control the quantum size effect, if wecan successfully develop the well handling method for nano-materials. Figure 5.2illustrates the surface morphology of the obtained TiO2-SiO2 nano-hybrid par-ticles with super critical drying process [13].

M. Aliofkhazraei, Nanocoatings, Engineering Materials,DOI: 10.1007/978-3-642-17966-2_5, � Springer-Verlag Berlin Heidelberg 2011

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5.2 Nanocomposite Coating Production Method

Extant physical vapor deposition (PVD) and chemical vapor deposition (CVD)processes for the provision of nanocomposite coatings can be applied for theproduction of nanocomposite coatings through improvement of process parametersor through the application of initial powder with nanostructure. Application ofvarious nanoparticles provided from steam, liquid and/or solid methods haveaccelerated development of nanocomposite coatings with resistant against frictionand oxidation. Under various PVD accessible processes, deposition through ionray is effective especially for making metallic nitride nano-crystallization coatingswith higher stick and controlled microstructure. Process with less dependencyparameters than PVD is salient advantage of deposition through ion ray. Energyand Flux can improve ion bombardment, size and direction of crystallography ofgrain as well. Ever increasing requirement for advanced materials with the aim oftolerating practical conditions has caused that many research works carried out inthe field of very hard coatings. Recently, special research activities have beencarried out on designing very hard coatings with superior strength, high stiffnessand toughness. Formation of multilayer or super elasticity structures with variouselasticity modules between layers is one of designing principles.

Thickness of each layer should be in nano dimension, aimed at preventing fromoperation of unwanted supply source between layers. Production of films withpleasant stiffness amounts will be possible through layer to layer sit. Formation ofnanocomposite of a nanocomposite layer with microstructures, including crystal-line network, is the other method of design with grains in nano dimensionsenvisioned in an amorphous background. Plating method is the other methods ofproduction of nanocomposite coatings. Since this method is economical, it enjoys

Fig. 5.1 Various types ofnanoparticles and degree ofsurface area to their volume

150 5 Size Effect in Mechanical Properties of Nanostructured Coatings

many capabilities for production of nanocomposite coatings in industrial scale.This method is carried out based on accepted composite plating principles in a waythat process parameters in this method are not much more complicate; rather, theircontrols are made easily [14–26].

5.3 Provision of Nanocomposite Coatings with Plating Method

During recent years, successful coexistence of very minute particles, likemetallic powders, silicon carbide, oxides, diamonds and polymers, has beenreported with metallic or alloy field and their accordance structures and

Fig. 5.2 The surface morphology of the obtained TiO2-SiO2 nano-hybrid particle with supercritical drying process: a agglomerate state, b primary TiO2-SiO2 nano-hybrid particle, c titaniacrystals on the surface of the silica core particle and d the first Fourier transform (FFT) imageobtained from the titania crystals on the silica particle, reprinted with kind permission fromOhno [13]

5.2 Nanocomposite Coating Production Method 151

properties have been studied by various researchers. Not only structure andproperties of nanocomposite coatings depends on density, size, distribution andnature of improved particles nature, but also it depends on type of used solution,current density plating parameters, temperature, and degree of pH, etc. Nano-composite plating includes revival of metallic ions from suspension electrolyteand insoluble powders like oxides (SiO2, TiO2, Al2O3), carbides (SiC), nitrides(Si3N4), polymers (PTFE, Polytetrafluoroethylene). This activity will result inentering very minute particle to the growing metallic or alloy substrate. Highsuperficial energy and inclination of nanoparticle to agglomeration in highconductor metallic electrolyte will bar congruousness of distribution of particles.For this reason, many research activities have been done in the field of nano-composite coatings entitled ‘‘Congruous Distribution of Very Minute Particles inMetallic Substrate and Avoiding their Agglomeration in Electrolyte’’, aimed atboosting volumetric percentage of nanoparticle in coating. Table 5.1 showsschematic of various types of nanocomposite coatings which have been preparedthrough plating.

5.4 Plating of Nickel-Alumina Nanocomposite Coating

Due to the application of nickel as protective coating, nickel nanocompositecoatings, containing ceramic nanoparticle with high hardness and resistance toerosion, have been taken into consideration seriously. Hardness and resistance ofdeposited coatings of nickel nanocomposite strictly depends on degree of ceramicparticles extant in nickel background. Al2O3, WC, MoS2, TiO2, Cr2O3, ZrO2 anddiamond are of ceramic powders which have been used in manufacturing ofnanocomposites with nickel background. Up to the present time, more researchactivities have been made on nanocomposite coatings of Ni/Al2O3 and Ni/SiC[27–37].

Table 5.1 Various types of nanocomposite coatings obtained through electro-deposition method

Methods ofelectrodeposition

Type of nanostructured materials

Nanoparticles in ametallic matrix

Nano-multilayer

Nanotubes/nanowires

Nanocrystallinematerials

Direct current (DC) Ni/Alumina Ni–Cr Co nanowires Ni-W alloyPulsed direct current

(PDC)Ni-W/CNT

Pulsed reversecurrent (PRC)

Multilayeredcomposites

Potentiostatic (P)Pulsed potetiostatic

(PP)


5.5 Effects of Participation of Alumina Nanoparticlein Nickel Coating

Participation of alumina nanoparticle will improve hardness and resistance ofcoating against friction remarkably. Final tension will boost traction and tension ofsubmit in comparison with nickel, depending on degree of participation of parti-cles in coating, partly twofold or more than two fold. Resistance against friction isboosted upon increase of density of alumina in coating. At any rate, ductility ofnickel-alumina nanocomposite coatings is less than sole nickel. Annealing ofnanocomposite in high temperature will increase ductility but will reduce theirstrength. Resistant against corrosion of nickel is improved with alumina nano-particle. Increase of hardness is made based on preventing grains from growingand according to Hall-Petch Law.

Alumina hard nanoparticle will generally improve trio biological properties andhardness of nickel composite layers by dispersant hardness mechanism. Aluminananoparticle can prevent nickel grain boundary from movement and also canprevent grains from growing while conducting heat treatment operations. Whenmaterial is exposed foreign (external) tension, alumina particles can prevent fromunwanted movement in metal substrate. Consequently, plastic shape change willbe more complicated. Hence, hardness of nanocomposite layers is increased whileresistance against friction is improved. Also, resistance against friction isimproved due to the reduction of grains size of metal substrate, containing aluminananoparticle at grain boundary.

5.6 Plating of Nickel-Alumina Nanocomposite Coating

In systems with simple counting ions, zeta potential is regarded as a criterion forgradient of electrical potential, when surface potential is fixed. The pH, which itszeta potential is equal to zero, is called Iso Electric Point. To enrich loading ofhydrated surface by OH– and H3O+, increase or decrease of pH from Iso ElectricPoint will first boost absolute fraction of zeta potential. Iso electric point foralumina is pH = 9 i.e. alumina particles will have negative superficial load in theelectrolyte with pH more than 9. In the same direction, alumina particles will havepositive superficial load at the pH with less than 9. Consequently, aluminananoparticle can be seeped simultaneously with nickel for formation of compositelayers without needing to specific additives. Because, pH of all composite platingsolutions of nickel is smaller than IEP for alumina and alumina particles at thesebaths have positive superficial load. But, alumina nanoparticles are agglomeratedeasily in electrochemical electrolyte due to their high superficial energy and thisactivity will cause weak mechanical properties in nanocomposite coatings, for, itprevents particles from being distributed equally. After carrying out operations,physical distribution of nanoparticles at electrolyte solution by mixing and

5.4 Plating of Nickel-Alumina Nanocomposite Coating 153

ultrasonic operation and/or through distribution of chemical dispersants in elec-trolyte is a mandatory activity. The more volumetric percentage of aluminananoparticles can be boosted in Ni/Al2O3 nanocomposite coating the more pro-vided hardness of nanocomposite coat can be expected. Hence, this activityrequires getting familiarity with effective parameters in simultaneous electricaldeposition process of alumina and nickel [38–41]. For this reason, electroplatingvariables and their effects in amount of participating alumina nanoparticles in coatwill be studied in next part.

5.7 Size Effect in Mechanical Properties of Two DimensionalNano-Films

He et al. [42] analyzed ultra-thin elastic films of nano-scale thickness with anarbitrary geometry and edge boundary conditions. An analytical model is proposedto study the size-dependent mechanical response of the film based on continuumsurface elasticity. By using the transfer-matrix method along with an asymptoticexpansion technique of small parameter, closed-form solutions for the mechanicalfield in the film is presented in terms of the displacements on the mid-plane. Theasymptotic expansion terminates after a few terms and exact solutions areobtained. The mid-plane displacements are governed by three two-dimensionalequations, and the associated edge boundary conditions can be prescribed onaverage. Solving the two-dimensional boundary value problem yields the three-dimensional response of the film. The solution is exact throughout the interior ofthe film with the exception of a thin boundary layer having an order of thickness asthe film in accordance with the Saint–Venant’s principle.

The surface of a solid is a region with small thickness which has its own atomarrangement and property differing from the bulk. For a solid with a large size, thesurface effects can be ignored because the volume ratio of the surface region tothe bulk is very small. However, for small solids with large surface-to-bulk ratiothe significance of surfaces is likely to be important. This is extremely true fornano-scale materials or structures. Recently, mechanical experiments of nano-scale bars and plates indicate that the effective elastic properties of these minutestructural elements strongly depend on their size. The understanding and modelingof such a size-dependent phenomenon has become an active subject of muchresearch. Classical elasticity lacks an intrinsic length scale, and thus cannot beused to model the size effect. Atomistic simulation, though very powerful inpursuing the details at microscopic level, seems too complex for practical appli-cations as it needs tremendous computation. An efficient approach has beendeveloped by upon the continuum concept of surface stress. They examined uni-directional tension and pure bending of nano-scale bars and plates, and found moreremarkable size effect in bending than in tension. The results are in excellentagreement with their atomistic simulation by embedded atom method for face-centered cubic aluminum and the Stillinger–Weber model for silicon.


From the continuum point of view, a surface is regarded as a negligibly thinobject adhering to the underlying material without slipping, and the materialconstants for both are different. A generic and mathematical exposition on surfaceelasticity has been presented by some researchers. In their work, surface stressdepends on deformation. The equilibrium and constitutive equations of the bulksolid are the same as those in the classical elasticity, but the boundary conditionsmust ensure the force balance of the surface object. This model has been appliedby several authors.

He et al. [42] concluded that a continuum model based on surface elasticity isproposed to analyze the size-dependent mechanical response of ultra-thin elasticfilms of nano-scale thickness. Being expressed in terms of displacements of themid-plane, the governing equations are two-dimensional and the associatedboundary conditions are specified at the edge of the film in an average manner asin the classical plate theory. Once the two-dimensional equations are solved, thethree-dimensional mechanical field that is exact in Saint–Venant’s sense isgenerated directly. The asymptotic analysis developed for solving the two-dimensional equations can be regarded to yield exact solutions because theexpansion terminates after a few terms. The solution procedure is illustrated byanalyzing a clamped circular film under a concentrated force. The result isconsistent with the other existing studies and it approaches the classical platesolution without surface stress effects. It is concluded that the size-dependence isdue to the dependence of surface stress on strain. Ignoring this strain-dependenceof surface stress will lead to the disappearance of size effect. The presence ofsurface Lamé constants and residual surface tension under unconstrained con-ditions increases and decreases the film stiffness, respectively. Figure 5.3 shows

Fig. 5.3 Through-the-thickness distribution of thedimensionless transverseshear stress at r = R/2,reprinted with kindpermission from Lim [42]

5.7 Size Effect in Mechanical Properties of Two Dimensional Nano-Films 155

through-the-thickness distribution of the dimensionless transverse shear stress atr = R/2.

The effect of the material micro-structural interfaces increases as the surface-to-volume ratio increases. Abu Al-Rub [43] showed that interfacial effects have aprofound impact on the scale-dependent yield strength and strain hardening ofmicro/nano-systems even under uniform stressing. This is achieved by adopting ahigher-order gradient-dependent plasticity theory that enforces microscopicboundary conditions at interfaces and free surfaces. Those nonstandard boundaryconditions relate a microtraction stress to the interfacial energy at the interface. Inaddition to the nonlocal yield condition for the material’s bulk, a microscopic yieldcondition for the interface is presented, which determines the stress at which theinterface begins to deform plastically and harden. Hence, two material lengthscales are incorporated: one for the bulk and the other for the interface. Differentexpressions for the interfacial energy are investigated. The effect of the interfacialyield strength and interfacial hardening are studied by analytically solving a one-dimensional Hall–Petch-type size effect problem. It is found that when assumingcompliant interfaces the interface properties control both the material’s globalyield strength and rates of strain hardening such that the interfacial strengthcontrols the global yield strength whereas the interfacial hardening controls boththe global yield strength and strain hardening rates. On the other hand, whenassuming a stiff interface, the bulk length scale controls both the global yieldstrength and strain hardening rates. Moreover, it is found that in order to correctlypredict the increase in the yield strength with decreasing size, the interfacial lengthscale should scale the magnitude of both the interfacial yield strength and inter-facial hardening.

The emerging areas of micro and nanotechnologies exhibit important strengthdifferences that result from continuous modification of the material micro-struc-tural characteristics with changing size, whereby the smaller is the size thestronger is the response. For example, experimental works have shown increase instrength by decreasing: (a) the particle size of particle-reinforced composites whilekeeping the volume fraction constant; (b) the diameter of micro-wires under tor-sion; (c) the thickness of thin films under bending or uniaxial tension; (d) theindentation depth in micro/nano-indentation tests; (e) the grain size of nano-crystalline materials (the well-known Hall–Petch effect); the void size in nano-porous media; and several others. Therefore, accurate identification of themechanical properties of micro/nano-systems (e.g. micro/nano thin films, micro/nano wires, micro/nano-composites) is essential for the design, performance, anddevelopment of, for example, micro/nano electronics and micro/nanoelectrome-chanical systems (MEMS/NEMS) to be used, for example, as actuators or sensors(e.g. pressure, inertial, thermal, and chemical sensors, position detectors, accel-erometers, magnetometers, micromirrors, etc.). The mechanical properties ofsmall-scale structures are different from those of the conventional or bulk coun-terparts because they are very sensitive to the micro-structural features of thematerial such as the grain size, the finite number of grains, the boundary layerthickness, texture, and dislocation structure. Therefore, when one or more of the


dimensions of these systems begin to approach that of their microstructural fea-tures, the material mechanical properties (e.g. yield strength, strain hardening,fracture toughness) begin to exhibit a dependence on the structure size as sche-matically shown in Fig. 5.4. In metallic systems this translates to plastic yieldingoccurring at increased stresses over their bulk counterparts. The small sizesinvolved limit the conventional operation of dislocations and the application ofclassical continuum mechanics concepts; thus, a fundamental question arises:since the initial yield stress (i.e. onset of plasticity) in micro/nano-systems is size-dependent, a question that needs to be addressed, what yield strength should beused in the design of these systems?

Size effects in micro/nano-systems could not be explained by the classicalcontinuum mechanics since no length scale enters the constitutive description.A multiscale continuum theory, therefore, is needed to bridge the gap between theclassical continuum theories and micromechanical theories. Since the increase instrength with decreasing scale can be related to proportional increase in the straingradients, which accommodate the evolution of geometrically necessary disloca-tions (GNDs), the gradient plasticity theory has been successful in addressing thesize effect problem. This success stems out from the incorporation of a micro-structural length scale parameter through functional dependencies on the plasticstrain gradient of nonlocal media. Furthermore, for mathematical consistency, inthe gradient-dependent framework, additional boundary conditions have to bespecified at interfaces and free surfaces allowing one to include interfacial effects.However, recently many researchers who are engaged in nano/micro character-ization have questioned the ability of the gradient plasticity theory in predictingthe Hall–Petch-like size effect; i.e. the increase in the yield strength withdecreasing the grain size under macroscopically homogeneous stressing orstraining (i.e. under uniaxial tension or compression). This is attributed to lack ofthe physical understanding of the nature of the non-classical boundary conditionsthat the gradient plasticity theory enforces at the material free surfaces andinterfaces. Free surfaces and interfaces of a material confined in a small volumecan strongly affect the mechanical properties of the material. Free surfaces insubmicron and nano-systems can be sources for development of defects and itspropagation towards the interior. Hard, soft, or intermediate interfaces between

Fig. 5.4 Illustration ofstrengthening in micro/nano-systems. a Stress–straindiagrams for various sizes,and b increase in the yieldstrength and/or the rate ofstrain hardening as sizedecreases, reprinted with kindpermission from Abu Al-Rub[43]


distinct phase regions can also be locations for dislocations’ blocking and pile-upsthat give rise to strain gradients to accommodate the GNDs. The increase in theinitial yield stress with decreasing thickness observed in tensile tests of variousthin films in the size range of 100–500 nm may be taken as a hint in this direction.The free surfaces of the thin film and the interface between the film and substrate,therefore, can have a significant effect on the strength of the thin film. Lower-orderstrain gradient plasticity theories which neglect the application of the corre-sponding higher-order boundary conditions at interfaces and free surfaces indeedfail to predict boundary layer effects. Therefore, the focus of this paper is laid onthe effect of dimensional constraints on the yield strength and plastic flow and toshow that higher-order gradient plasticity theories (as opposed to lower-ordertheories) can be used successfully to interpret size effects under macroscopicallyhomogeneous stressing or straining conditions.

Dislocation pile-ups, which result in local plastic strain gradients, could beencountered at free surfaces and interface depending on the level of surface/interfacial energy which increases as the surface-to-volume ratio increases. Inother words, it is expected that as the characteristic size decreases, the higher isthe surface/interfacial energy and the more significant is the effect of theboundary layer thickness on the strength of the system. Therefore, size effect canbe explained by constrained plastic slipping due to grain boundaries and inter-faces which result in a nonuniform straining, thereby setting up strong gradientsof strain. Plastic deformation in small-scale structures, accommodated by dislo-cation nucleation and movement, is therefore strongly affected by interfaces.Until now, little attention is devoted to interfacial strengthening effects (e.g. film-substrate interface, phase or grain boundaries, inclusion’s interface, void freesurface, nano-wires free surfaces, etc.) on the scale-dependent plasticity in small-scale systems. Interface and boundary conditions for higher-order variables aregenerally modeled as infinitely stiff or completely free; and the references quotedtherein). These conditions are very difficult to be satisfied in reality, particularly,for systems with large surface-to-volume ratios. However, recently there havebeen few attempts to model intermediate (i.e. not free and not stiff) boundaryconditions for higher-order variables within the higher-order strain gradientplasticity framework.

Abu Al-Rub [43] studied the effect of interface properties (yield strength andhardening) on the scale-dependent behavior of small-scale systems within theframework of higher-order gradient plasticity theory. It is shown that the addi-tional microscopic boundary conditions, which are supplemented by the gradientapproach, allows one to predict size effects under uniform stressing. This isachieved by relating the microtraction stress at interfaces to an interfacial energythat depends on the plastic strain at the interface. Furthermore, by examining linearand nonlinear expressions for this interfacial energy, it is shown that an interfacialyield condition, besides the nonlocal yield condition for the bulk, can be formu-lated. This condition governs the emission/transmission of dislocations across theinterface and is expressed in terms of the microtraction stress, the interfacial yieldstrength, the interfacial hardening, and the interfacial length scale. Therefore, two


internal length scales are incorporated in the present formalism, one for the bulk, ‘,and the other for the interface, ‘I.

It is shown that the higher-order gradient plasticity theory when supplementedby interfacial energy effects, at least for the one-dimensional example presentedhere, can qualitatively describe many features of the size effect due to GNDs,including the strengthening, the development of boundary layers, and the strainhardening. The qualitative modeling of the strengthening is explained by theinterfacial yield strength, whereas the strain hardening is described by accountingfor the interfacial hardening effect. Four different forms for the interfacial energy(or equivalently the interfacial yield condition) in terms of the plastic strain at theinterface are examined: (a) a linear one which allows the interface to yield in aperfectly plastic manner without hardening; (b) a quadratic form which allows theinterface to harden but yields at the same time as the bulk; (c) a combination of (a)and (b) such that the interfacial yield strength and interfacial hardening can bealtered independently; and (d) a combination of (a) and (b) such that the interfacialyield strength and interfacial hardening are both scaled with the interfacial lengthscale. It is found through (a) that that interfacial yield strength controls the overallyield strength (i.e. onset of plasticity) of the specimen. Moreover, an analyticalexpression for the interfacial yield stress at which interface deforms plastically isderived. This is one of the most interesting features of the present formulation.From this expression, it is concluded that the yield strength of ultra-fine grainedmaterials is controlled by the interfacial strength of the grain boundary. Moreover,it is found through (b) that interfacial hardening controls the increase in theplasticity tangent hardening modulus and in the flow stress with decreasing size.The expression in (c) shows that the interfacial hardening contributes to the globalyield strength as well as to the strain hardening rates (i.e. flow stress). However, itis shown that the expression in (c) yields incorrect decrease in the yield strengthwhen increasing the interface stiffness. This is corrected by adapting the expres-sion in (d) which shows that by increasing the interfacial hardening, stifferinterfaces are formed that in turn increases the yield strength of the material due todislocation networking at the interface which obstructing further emission/trans-mission of dislocations across the interface. Therefore, it is concluded that theinterfacial length scale should scale the effect of both the interfacial yield strengthand interfacial hardening. Moreover, one should be careful when choosing aproper form for the interfacial energy such that it should at least qualitativelyconfirms with the experimental observations of size effect behavior.

It is concluded that the increase in the material’s yield strength and strainhardening rates with decreasing size is determined by the weakest link of bulkand interface. If the interface is compliant then the properties of the interfacecontrol the yield strength and hardening rates of the material (i.e. controlled bythe interfacial length scale ‘I). On the other hand, if the interface is rigid, theyield strength and hardening rates are controlled by the bulk behavior (i.e.controlled by the bulk length scale ‘). Therefore, for intermediate interfaces, acompetition between those two mechanisms exists. Interfacial effect is animportant aspect for further development of gradient-dependent plasticity that is


capable of modeling size effects in micro/nano-systems that are initially sub-jected to macroscopically uniform stresses or strains. It is shown that the exis-tence of both gradients and interfacial energies contribute to the observed sizeeffects. Moreover, it is emphasized that in the absence of the interfacial energy,the material would support uniform fields and hence the constitutive gradient-dependence would have no influence. Therefore, strain gradients come into playif the boundaries are assumed to constrain the plastic flow. Therefore, if con-tinuum theories are to be used to predict plastic behavior at the micron orsubmicron length scales, a higher-order theory with interfacial energies is likelyto be required.

Also, it would be interesting to compare the results provided by the presenttheory and its rate form counterpart obtained in two- or three-dimensional appli-cations. In a forthcoming work, a detailed Finite Element implementation of theproposed model will be presented and used to simulate size effect in small-scalestructures under various loading conditions (e.g. bending, torsion, cyclic loading).Moreover, it is interesting to validate the present conclusions by performingdetailed discrete dislocation dynamics. It is noteworthy that several researchershave questioned the ability of strain gradient plasticity theory to explain theobserved size effect in nano/micro pillars or columns when subjected to

Fig. 5.5 Size effects due to interfacial yield strength only without interfacial hardening. Theinterfacial yield strength is varying according to a and c, d1 = 0.1 and b and d, d1 = 0.45. a,b Normalized plastic strain distribution along d for �r0 ¼ 2: c, d Normalized stress–strainrelations. Different sizes are represented by ‘/d = 0.1, 0.5, 1, 1.5, 2, reprinted with kindpermission from Abu Al-Rub [43]


macroscopically homogenous deformation. These debates are attributed to theabsence of strain gradients in these systems when subjected to uniform straining orstressing. Moreover, it has been argued that this type of size effect is due todislocation starvation; i.e. the rate at which dislocations multiply is less than thatrate at which dislocations escape and annihilate from the pillar surface as the sizedecreasing to hundreds of nanometers.

Finally, more than 50 years of research on grain boundaries has establishedtheir impact on the overall strength of materials, yet experimental studies ontheir yield strength or Young’s modulus are rare in the literature. This is becausegrain boundaries are random networks of interfaces that are only a few nano-meters wide and cannot be isolated and characterized by conventional tensile,bending, indentation tools. Therefore, the fundamental understanding on theinterfaces in materials will impact grain boundary engineering, an evolvingresearch direction towards optimized materials design. Figure 5.5 shows sizeeffects due to interfacial yield strength only without interfacial hardening whileFig. 5.6 illustrates size effects due to interfacial yield strength and interfacialhardening for d1 = d2 = d.

Fig. 5.6 Size effects due to interfacial yield strength and interfacial hardening for d1 = d2 = d.Both the interfacial yield strength and hardening is varying simultaneously according to a and c,d = 0.45 and b and d, d = 1. a, b Normalized plastic strain distribution along d for �r0 ¼ 2: c,d Normalized stress–strain relations. Different sizes are represented by ‘/d = 0.1, 0.5, 1, 1.5, 2,reprinted with kind permission from Abu Al-Rub [43]


5.8 Studying Effective Factors on Simultaneous Depositionof Alumina Nanoparticles with Nickel

5.8.1 Effect of Density of Alumina Nanoparticles in ElectrolyteBath

Guglielmi Model has specified that density of particles in bath affects on degree ofparticipation of these particles in coat. With the increase of their density inelectrolyte, the degree of their attraction will be increased on cathode surface andconsequently, it will cause increase of participation of these nanoparticles innickel-based coating. In Celice model, which has been posed based on possibilityof passage of particle from penetrated layer, with the increase of density of alu-mina nanoparticles in electrolyte solution, possibility of passing them from pen-etrated layer is increased and consequently, degree of participation ofnanoparticles will be increased in nanocomposite coat. Some researches have beenmade on effect of pulse current variables on hardness and resistance of friction ofnickel-alumina composite coating he attained similar results. These results havebeen shown in Fig. 5.7. It is observed that alumina density increase at bath hasboosted hardness of composite coating which is related to more participation ofalumina particles in coating. In another research on electrical deposition of Ni/Al2O3 with revolving multidimensional electrode, effect of density of aluminaparticles was studied on particles volumetric percentage in coating [39, 44–57].

Fig. 5.7 Effect of alumina concentration in bath on hardness of nickel-alumina compositecoatings electroplated with a direct current (DC) with 5 A/dm2, b pulse current 5 A/dm2 and dutycycle 20% and frequency 75 Hz


5.8.2 Effect of Electroplating Current Density

According to Guglielmi Model, it was specified that degree of participation ofneutral particles in coating will be decreased in tandem with increase of densityof plating current. This subject was studied by Guglielmi in simultaneous leakageof Ni/TiO2 and Ni/SiC particles. This proportion has been increased in tandemwith increase of current density, i.e. degree of participation of particles has beenreduced in composite coating. But, effect of current density has been studied ondegree of participation of alumina particles in nickel-alumina composite coating.The obtained results accord with Guglielmi Model, indicating reduction of degreeof alumina particles in coating due to the increase of current density. Reduction onparticipation of alumina particles with current density increase can be correlated toelectrochemical potential which affects on attraction of alumina particles onsubstrate surface. Higher current will create higher cathodic deposition potentialand cause double layer more negative coupled with repulsion of alumina particles.Consequently, less alumina will participate in nickel base. Figure 5.3 indicatesresults on effect of current on degree of participating alumina in Ni/Al2O3 nano-composite coating. According to Fig. 5.3, it is specified that hardness of nickel-alumina composite coatings is reduced with the increase of density of current.

In fact, density of current can have two various impacts: density of current canaffect on degree of alumina entered nickel base which will increase hardness ofcomposite coatings and also can change in microstructure. Change of micro-structure will cause hardness change. Figure 5.8 indicates that composite coatingsdeposited in density of lower currents contain more alumina particles than coatingsdeposited in density of higher currents. This issue accords with many of reportsdeveloped at different papers.

Fig. 5.8 Effect of density of direct current on hardness degree of electroplated nickel-aluminacomposite coating from bath containing 80 g/1 alumina

5.8 Studying Effective Factors on Simultaneous Deposition 163

5.8.3 Distribution of Alumina Nanoparticles

Coating neutral nanoparticles in deposited layer is hard due to agglomeration ofnanoparticles in electrolyte than particles with micro dimensions, because, whenceramic particles turn into fine particles, superficial forces will be increased andconsequently they will increase agglomeration activities. The particles distributedin an electrolyte solution are based on Brownian motion. When two particlesapproach or near each other, the energies, which exist between two particles, willdetermine whether particles will be isolated or agglomerated from each other.Generally, agglomeration of particle is occurred due to the gravity energy largerthan repulsion energy between particles. Size of net involved forces in creation ofagglomerated structure depends on condition and nature of system clearly.Knowledge of structure of interface region is regarded as a very important factor incomprehension of sustainability of distribution of sold particles with electrolyte.For fair and appropriate distribution of alumina in nickel-sulfamate bath, a changein inner-particle interface region is required by physical or chemical methods.

5.8.3.1 Physical Distribution of Alumina Nanoparticle with UltrasonicOperations

Physical effect is occurred when absorbs destructive energy particles like ultra-sonic waves. Disperse of ultrasonic waves in liquid environment produces veryhigh pressure (over thousand fold of atmosphere pressure), causing exertion of ahuge tension, which entangle bonding energy between particles. Air bubbles enterinter-particle grooves from holes, aimed at reducing diameter of agglomeratedalumina particles.

5.8.3.2 Chemical Distribution of Alumina Nanoparticle with Dispersants Aid

Chemical effect happens when electrolyte, containing surfactants or molecules,turns macro with the aim of forming electrostatic or Setric interface betweenparticles. Under such specific condition, these interfaces will result in repulsionthanks to amalgamation of absorbed layer and reduction of entropy. When col-loidal two-particle double layers start overlapping each other with the same load,repulsion forces between them will confront with Van der Waals force. If repulsivepotential turns large enough for overcoming Van der Waals potential, it will resultin sustainability of electrical double layer [58–71].

A. Setric sustainabilityPrincipally, Setric sustainability has been developed based on two mechanisms.

When two particles near or approach each other with attracted polymers, thenumber of situations, which polymer creates, is reduced due to the participation ofother particles. This activity will result in reduction of entropy. Secondly, density


of polymer is increased at overlapping region which will result in OsmoticRepulsion between particles in suspension.

B. Electrostatic sustainabilitySuspension sustainability can be obtained with combination of electrostatic and

Setric forces. This activity will be obtained through the following three methods:

1. With the participation of particles with electric load and one neutral polymermolecule,

2. With the participation of neutral particles and one polymer molecule withelectric load,

3. With the participation of particles with electric load and one loaded polymermolecule,

Presence of particles with electric load creates repulsive potential with longrange and presence of polymer will create repulsive potential with short rangewhen particles near or approach each other.

C. Getting familiarity with chemical dispersantsIn the processes followed with suspensions, a number of fundamental and basic

interactions can be applied for affecting inter-particle forces. These forces includeVan der Waals attraction forces and electrical double-layer repulsive forces. Thetwo forces emerge when solution ions with loaded agent group attracted surface ofparticles and/or isolation of solution ions is made from surfaces of particles. Setricforces are created by large-size molecules which have been struck to surface ofparticles. In the same direction, large loaded molecules, which are polyelectrolyte,will create repulsive electro-setric forces. Combination of interaction of Van derWaals and double-layer repulsion is basic of renowned DLVO theory which willprovide network interaction energy. In addition, attraction forces are affected bychange of dispersant environment. The largest effect is made on Van der Waalsattraction through change of solution for solvability with up-to-down dielectriccoefficient. General explanation for mechanism of particles superficial load inpolar solutions depends on congregation of ion species in interface. Dispersantscan be organic chemicals with low molecular weight, carrying agent groups forstabilizing on surface and marking specific part. That part of specific section isbetter to be severely polar additional group especially the groups like acid car-boxylic and those groups which increase negative load of surface in proton dis-persant environment in pH above 3.

Citric acid or Maleic acid is regarded as examples of the said group. Otherorganic dispersants include polymer and polyelectrolyte with molecular weightless than 2,000 for polyelectrolyte containing high load and 10,000 for polymers.The separable agent or typical polar groups for dispersants include as follows:Hydroxyl (–OH), carboxyl (–COOH), sulfanate (–SO3

-), sulfate 9-OSO3-),

ammonium (–NH4+), ammonia (–NH2), amino (–NH-) and poly oxy ethylene

(–CH2CH2O-). Setric sustainability enjoys major share in non-ion large moleculesin solid/solvable system. Better output can be observed or Setric sustainability innon-ion large molecules in comparison with the compounds having low molecularweight. Some of common dispersants have been shown in Table 5.2.


5.8.4 Effect of Density of Nickel Ions in Plating Bath

Nickel coating baths include as follows: (1) Watts bath, (2) sulfate bath, (3) chloridebath, (4) nickel-sulfamate, (5) nickel fluorobrate. Nickel is usually deposited inwatts bath type. This method is yet used for most nickel plating whether as sub-layer or thick geometrical sections. Hereunder are regarded as main components offormation of bath: nickel-sulfamate as noncomplex (Ni2+), chloral nickel forimprovement of solvability of anode and increase of density of currents throughfracturing penetrated layer, boric acid for fixation of pH approximately 4 andreducing inclination to hydrolysis due to reduction of establishing non-acid salts,various additives for increasing of leveling surface, reduction of tension andcrystallization of coating. Full-chloride baths can be used for delicacy of grains andgranules in hard coatings and full sulfate baths can be applied while using lead non-solvable anodes. Fluorobrate and sulfamate solutions have specific applications.Using sulfamate baths in electroforming, which speed of plating is of paramountimportance, has been developed more.

Comparison of output of cathode current for various baths of nickel is asfollows:

• Watts nickel bath 98–90%• Full chloride 98–99%• Sulfamate 97–99%• Fluorobrate 90–95%

The baths used for nickel-alumina nanocomposite coating are principally are asfollows: Watts baths, sulfamate and rarely chloride bath. The degree of effect ofnickel ion on agglomeration of alumina nanoparticles in electrolyte and alsodegree of participation of alumina nanoparticles in nanocomposite coating is thesalient and the most significant point at these baths. Increase of metal ions inelectrolyte will cause reduction of distance of alumina nanoparticle electricaldouble layers.

Table 5.2 Common dispersants used in ceramic processing

Low molecular weight High molecular weight

Sodium borate Poly(acrylic acid) PAASodium carbonate Poly(methacrylic acid) (PMAA)Sodium pyrophosphate Ammonium polyacrylateSodium silicate Sodium polyacrylateCitric acid PolyisobuteneAmmonium citrate Menhaden fish oilSodium citrate Phosphate esterSodium tartrate Sodium polysulfonateSodium succinateGlycerol trioleate


Consequently, repulsion force is reduced between particles. This activity willcause facilitation of agglomeration of alumina nanoparticles in high-conductormetal electrolyte. Using chemical and physical distribution has limited effect inpreventing from agglomeration of alumina nanoparticles, rather, using bath withlow nickel ion density will cause reduction of degree of agglomeration of aluminananoparticles. In some researches, it was specified that average diameter ofagglomerated alumina has been de-ionized in water without application of usingultrasonic energy and nickel-sulfamate bath will be 183 and 1109 nm respectively.That is to say that effect of solution ion power on agglomeration of particles cannot be ignored. They reduced the average diameter of agglomerated aluminaparticles through the application of physical distribution (with imposed ultrasonicenergy) and chemical distribution (with adding surfactants to nickel bath) 280 and448 nm respectively.

The size of alumina nanoparticles used by them stood at 80 nm whichelectrical deposition was carried out in three nickel baths, (1) without surfactantand ultrasonic operation, (2) with ultrasonic operation with 5 W/l for a periodof 40 min and (3) with adding Cetytrimethyl Ammonium Bromide (CTAB) assurfactant. The diameter of agglomerated particles of alumina nanoparticles inNi bath without adding surfactant and conducting ultrasonic operation stands at1109 nm. It will cause presence of very low degree of alumina 1.42% vol, withagglomerated structure in coating. The diameter of alumina agglomerated par-ticles was reduced to 280 and 448 nm in order through the application ofphysical distribution with imposed ultrasonic energy and chemical distributionthrough surfactant added to nickel bath. They used four baths with variousnickel densities for studying effect of nickel ion density on degree ofagglomeration. It is observed that average diameter of agglomerated alumina isreduced in tandem with reduction of density of nickel ion in bath. Volumetricpercentage of alumina at this coat has reached from 8.37%, in a bath with highnickel density, to maximum amount of 26.78%, in a bath with low nickeldensity and has reduced slowly to 24.65% through more reduction of nickeldensity in bath. Distribution of alumina particles in nickel-sulfamate bath willbe improved when density of electrolyte is low. It is not unnatural that volu-metric percentage of alumina in nanocomposite coating (Ni/Al2O3) is increasedthrough reduction of nickel ion at this electrochemical reaction. But whennickel ion density is very low, output of density of low current will result inrevival of hydrogen ion. Therefore, volumetric percentage of alumina in Ni/Al2O3 coating has maximum amount i.e. approx. 26.78%. At this figure, dis-tribution of particles in composite coating with low ion density is more con-gruous than its distribution in dense solution. The average diameter of aluminaagglomerated in composite coating is reduced with the reduction of low nickelion density. The degree of fine and minute particles in coating will be increasedwith dilution of solution. Figure 5.9 indicates effect of nickel electrolyte iondensity on degree of nanoparticles entered in coating. It is observed thatmaximum of weight and volumetric percentage of alumina in coating happensin [Ni] = 3% M density.


5.8.5 Effect of pH

Low pH is usually used in nickel baths. At the baths with low pH, nickel density iskept constant with solving anode. With regard to the baths with low pH (highsulfate), cathode has more inclination to be turned into a hole. Unlike baths withhigh pH type, baths with low pH does not need more care. pH used in papers forplating nickel-alumina nanocomposite stands at 3.5–4.5 output. The degree of pHhas salient effect on viscosity of suspension. With regard to oxides, viscosity ischanged with zeta potential. Alumina suspension viscosity is increased with theincrease of pH up by specified amount. After pH, viscosity will be reduced withpH increase. An amount of pH, in which its viscosity is reached to maximumamount, nears iso-electric point for suspension. Figure 5.10 shows viscositychanges of alumina suspension based on pH.

Fig. 5.9 Volumetric percentage of Al2O3 by image analysis and weight percentage by EDS

Fig. 5.10 Schematic ofeffect of pH on viscosity ofalumina suspension


With due observance to Fig. 5.10, it is specified that the more pH rate isreduced, the more alumina suspension viscosity will be reduced. In nanocompositeplating, the more rate of bath viscosity is less, the more neutral particles can betransferred to surface of cathode. Hence, pH reduction will help greatly to morepresence of alumina nanoparticles in nanocomposite coating (Ni/Al2O3). Sinceiso-electric point of alumina in pH is 9 (pH = 9), with more reduction of pH,alumina particles surface load in electrolyte will turn more positive. This activitywill cause enlargement of electrical double layer. Consequently, double layerrepulsion forces will be increased. Moreover boosting electrolyte sustainability,alumina particles inclination for agglomeration will be reduced. This subject willresult in more presence of alumina nanoparticles in nickel-alumina nanocompositecoating. We should bear in mind that cathode has more inclination to be turned ashole at the baths with low pH degree. For this reason, neither pH degree should belessened nor increased.

5.8.6 Pulse Current Effect

Imposed current is regarded as one of the other parameters which has major effecton microstructure and morphology of deposited composite coatings. This subjecthas shown that more and better control can be imposed on properties of coatingsby improvement of their microstructure through the application of pulse current forelectrical deposition of metals and alloys. Consequently, metals and alloys’ pulsedeposition are of paramount significance due to the possibility of change of theirproperties by accurate setting of pulse parameters. Nickel pulse deposition hasattracted the attention of many to itself. Some researchers have recently studiedeffect of pulse plating on roughness of deposited nickel thin films surface. Pulseplating of metals and alloys has been also studied. Reports show that selection ofimposed pulse parameters affects alloy deposition compound tremendously. It hasbeen reported that remarkable reduction is appeared in internal tension of electricaldepositions while using imposed current as compared with common direct currentat the same density. Some other researchers studied pulse plating parameters onresistance against corrosion of nickel depositions. Their results show that nickeldeposition by pulse current imposition can produce nickel coatings with lessporosity and resistant against corrosion better in comparison with direct currentplating.

Using TEM has shown that distribution of nanoparticles size in deposited filmsunder Direct Current (DC) condition is naturally the same distribution of size ofnano powders used in deposition. This subject is of paramount significance withrelation to obtaining composite films. When films deposited from the same bathunder pulse direct current (PDC), large nanoparticles or part of particle which hasbeen agglomerated, may not coat by growth of sprout during a pulse on a film.Therefore, it may exclude before any pulse is occurred which does not necessarilycause continuous growth of juvenile. The initial results on electric deposition


(PDC) of nickel-alumina composite films show that smaller nanoparticles arecoated as larger particles while PDC plating. Hence, a selection of size of nano-particle is occurred.

Some researchers studied effect of pulse deposition parameters on hardness andresistance against nickel-alumina composite friction. They indicate that degree ofhigher hardness is obtained at duty cycle and lower frequencies. This subjectpoints this matter that reduction of duty cycle and frequency will cause presence ofmore particles of alumina particles, and consequently, hard coat is obtained. Lowduty cycle is meant an off-time more when not current passes from electrolyte andsuch issue will create more chance for ceramic particles for attaining doublelayers. Hence, more alumina will be participated in coat. Frequency of pulsecurrent also has similar effect. In low frequencies, there is low number of cycles,resulted in creation of a better situation for alumina particles. Finally, hard coat isdeposited in low-frequency pulses. Effect of duty cycle and pulse frequency hasbeen studied on degree of participating alumina particles [28, 72–85].

In study of reverse pulse plating of nanocomposite thin film, some researchershave studied effect of duty cycle on presence of alumina particles in coppernanocomposite thin films. Although they have not studied effect of pulse frequencyon degree of participation of alumina, they showed that reduction of duty cyclewill increase presence of alumina nanoparticle in nanocomposite coating. Metal-lographic test of nickel-alumina composite coating deposited in duty cycle andvarious frequencies specify that morphology of these coatings is severely affectedwith the selection of pulse parameters. In Table 5.3, effect of pulse parameters hasbeen shown on rate of participating alumina particles and hardness of compositecoatings.

Deposition in low duty cycle will cause increase of proportion of large particlesin coating, as the pulse off-time is more in low duty cycle. Consequently, largeparticles can reach cathode surface through effect of mass transfer in pulse off-timeperiod. Increase of duty cycle of two various effects is on presence of particles incoating:

1. Remarkable reduction of degree of participation of particles in coating2. Participation of smaller particles more than large-size particles

Some researchers have recently explained that selection of particles size isoccurred while PDC plating clearly. In deposited samples with DC, ceramic

Table 5.3 Laboratory data related to obtained results for nanocomposite Ni/aluminaelectroplating

Coating thickness(microns)

Duty cycle(%)

Frequency(Hz)

Vol.% ofalumina

Hardness VHN(kg/mm2)

15 20 50 35 36420 80 50 13 25010 60 20 15 28012 60 80 11 238


particles larger than 100 nm are observed clearly and simultaneous depositedparticles have been agglomerated. For the sample provided under PDC condition(right hand), any particle larger than 100 nm and agglomerated particle similar towhat is observed in DC, is not found. In addition, the average base grain sizestands at 20 nm. The research activities on simultaneous deposition of aluminanano-wisckers in nanocomposite coating in pulse method specified that rate ofalumina nano-wisckers coated in composite coating is increased with reduction offrequency and more congruity is obtained in distribution of nano-wisckers incoating with lower frequencies.

5.9 Size Dependency of Tensile and Fatigue Strengthin Ultra-Thin Films

Tensile and fatigue tests of ultra-thin copper films were conducted using a micro-force testing system by Zhang et al. [86]. Fatigue strength as a function of filmthickness was measured under the constant total strain range control at a frequencyof 10 Hz. The experimental results exhibit that both yield strength and fatiguelifetime are dependent on film thickness. Fatigue damage behavior in the 100 nmthick Cu films with nanometer-sized grains is different from that in the microm-eter-thick copper films with large grains observed before. A comparison of thepresent results with those reported in literatures is conducted. Possible fatiguestrengthening mechanism in the ultra-thin copper films is discussed.

Fatigue of thin metal films is a key issue for the long-term service of mic-rodevices. Previous investigations of fatigue of thin metal foils show a tendency ofthe improved fatigue strength with decreasing foil thickness. Especially, severalstudies on thin metal films, such as thin Ag films and Cu films, have demonstratedthat fatigue properties of these metal films are significantly different from those ofthe bulk materials. When the film thickness approaches 200 nm, interface-inducedfatigue damage becomes more prevalent. In these studies, the film thickness andgrain size are usually ranged from several micrometers to sub-micrometers.However, little is known about fatigue damage and strength of metal films withnanometer-scale thickness and grain size. Zhang et al. [86] present the evaluationof tensile yield strength and fatigue lifetime of ultrathin Cu films with a thicknessof about 100 nm or less and nanometer-sized grains. Figure 5.11 shows tensileyield strength of the ultrathin Cu films as a function of film thickness and Fig. 5.12shows the mechanical energy loss versus the number of cycles of the 60 nm thickCu films and the number of cycles to fatigue damage (Nf) as a function of filmthickness.

Zhang et al. [86] concluded that the yield strength of the ultra-thin Cu filmsfurther increases with decreasing film thickness down to several tens of nano-meters. A comparison of fatigue lifetimes between the ultra-thin films and thosein the literature indicates that the fatigue resistance of the ultra-thin Cu films is


Fig. 5.11 a Tensile yieldstrength of the ultrathin Cufilms as a function of filmthickness, reprinted with kindpermission from Zhang [86]

Fig. 5.12 a The mechanicalenergy loss vs. the number ofcycles of the 60 nm thick Cufilms; b the number of cyclesto fatigue damage (Nf) as afunction of film thickness,reprinted with kindpermission from Zhang [86]


higher than those of the micrometer-thick films, but somewhat less than that ofthe sub-micrometer-thick films. It is suggested that the activated GB-relateddeformation mechanism is responsible for the potential decrease in the fatigueresistance in the nanometers thick films compared with the sub-micrometer-thickfilms.

5.10 Application of Coating for Strength Enhancement

Researchers produced nano-composite materials made of steel alloy, with very fewmolecules in their particles which can be used in buildings to increase strength andother similar cases. There existed a common physical mechanism which con-tributes to control alloy hardness. Hardness increase causes malleability, foliating,and tabularization decrease. Using nano-composites in these alloys it is possible todecrease these shortages to a high extent. This is caused by an increase in con-trolling mechanisms for each material property in nano scales.

This method involves creating an alloy in frozen glass structure. Grindingobtained product make it possible to produce a particular powder which makebonds with other materials and create a very dense coating during heating process.Under this conditions particles diameter is about 50 nm. The process can generatevery strong bonds in substances. Available steels are of strength about 10% ofthose calculated through theoretical methods, once using this method enables us toreach strengths about 40–45% of calculated one. This method also contributes toobtain better corrosion resistant properties.

Experiments show that steel nano-coating is harder than traditional steel. Thiscoating can be performed either construction of main data or after that. Themethod is very cheap compared with other conventional ones. This material canalso be used for aluminum coating which significantly enhances its strength, whileadds no mentionable weight to that. Also, empirical observations show that thistype of coatings makes bonds with aluminum, while conventional Fe–Al coating isnot easily performed. It is expected this method be of frequent applications[87–101].

5.11 Nano-Coating Use in Dressing Industry

Some clothes producers have used a nano-protector coating to coat clothes’ sur-face. These include coatings resistant against pollution, decay, abrasion, and fire;however, they do not bring a good feeling to customers while wearing them. LLCNano-Tex uses these coatings in United States. Also, U-Right uses this kind ofcoating, developed by Sweden researchers, in its products. The coating is used inclothes fabric as well as its other parts.

5.9 Size Dependency of Tensile and Fatigue Strength in Ultra-Thin Films 173

5.11.1 Making WC/Co–Ni Nano-Coating Using Electrodeposition

WC/Co–Ni coating is broadly used for its high hardness and low friction ratio incoating process of variant pieces; while in conventional methods thermo-aerosolmethod is applied. Through this method a bid deal of WC is used which leads toincrease of piece weight and decrease of its prices. Inframat Company, withcontribution of National Science Foundation (NSF), has developed coating withWC/Co–Ni nano-particles. Cr–C/W coating is used in some similar methods.

Cr compounds are convenient to create hardness but are environmentallyhazardous due to CRVI release. New substitute material for mentioned materialsshould have their high hardness and the other coating properties. In new methods,WC nano-coatings are coated via electrolyte method with Ni-Co matrix. Coatingwith this method creates an equal thickness and does not involve any high costmechanical methods for compellation of coating process. Through new method alower ratio of WC is consumed. This considerably decreases pieces’ weight andcost, as well as friction ratio and surface hardness.

5.11.2 Using (Me-Ti1-xAlxN)/(a-Si3N4) Nanocomposite Coatings

Aluminum and titanium alloys and (Me-Ti1-xAlxN)/(a-Si3N4) nano-compositecoatings are used for cutting tools coating. These coatings are of unique featuresmake them suitable for these tools, including:

• High rate of hardness (25–38 GPa)• Hardness in high temperatures (in 800�C about 30–40%)• Resistance against oxidization (15-20 lg/cm2), TiCN, and TiN, respectively, at

800, 400, and 550�C• Thermal conductivity

There are some other issues while using these alloys which must be dealt,including:

• Optimization of fuel processes• Optimization of crystalline structure to prevent creation of columnar structure

for improvement of pieces resistant against corrosion• Making multi-layers• Adding other materials such as

– Cd and Y to enhancement of strength against oxidization– Zr, V, and B to enhancement of strength against wear and corrosion– Si for increase of hardness and resistance against chemical agents

Among most important achievements of TiAlCN detecting layer one can namenano-coating and increase of Al ratio in coatings. Using nano-coating of this alloy,as well as multi-stage coating with nano thickness, decreases different features of


the surface including hardness, scratch resistance, and oxidization. Through thisnew method, nano-composite coating is used for multi-stage coatings. During thismethod different materials such as Al, Si, and Ti, which cannot be mixed with eachother, serve as detector layer. They mix one another in plasma state and placed inamorphous Si3N4 matrix. At high temperatures (up to 1100�C), this obtained nano-composite is of high hardness (40–50 GPa). This kind of nano-composite coatingis required for nano-composite coating of highly efficient melting pieces.Regarding this coating’s high efficiency compared with other materials andmethods, their application is persistently increasing.

5.12 Size Dependency in Nanocomposite Layers

Properties of Si3N4/Ni electroplated nanocomposite layer such as roughness ofobtained layer and distribution of nanometric particulates have been studied [102].All of the other effective factors for fabrication of nanocomposite coatings havebeen fixed for better studying the effect of the average size of nanoparticulates.The effects of the different average size of nanometric particulates (ASNP) fromsubmicron scale (less than 1 lm) to nanometric scale (less than 10 nm) have beenstudied. The roughness illustrated a minimum level while the distribution ofnanometric particulates will be more uniform by decreasing the ASNP. The effectsof pulsed current on electrodeposition (frequency, duty cycle) and concentration ofnanoparticulates on electrodeposition bath on trend of obtained curves have beendiscussed. Response Surface Methodology was applied for optimizing the effectiveoperating conditions of coatings. The levels studied were frequency range between1,000 and 9,000 Hz, duty cycle between 10 and 90% and concentration ofnanoparticulates among 10–90 g l-1.

Figure 5.13 illustrates the effect of different ASNPs on the Ra of coatings.Interpolated equation shows that there is a quadratic relation among the roughnessof obtained layer and ASNP. It can be concluded that the interaction among

Fig. 5.13 Effect of differentASNPs on the Ra ofelectroplated nanocompositeSi3N4/Ni coatings [102]

5.11 Nano-Coating Use in Dressing Industry 175

nanoparticulates with low ASNP (approximately less than 90 nm) will increase theroughness of obtained layer. The minimum roughness has been obtained forthe nanocomposite layer with ASNP equal to 93 nm. The effect of ASPN on thedistribution of nanometric particulates has been illustrated in Fig. 5.14. It can beeasily concluded that the gaussian shape of distribution curves are narrower forlower amounts of ASPN. Also it can be seen that the distribution curves ofobtained layer for higher amount of ASPN are wider which means that althoughthe nanometric powders with narrow distribution of particulates around the spe-cific ASPN have been used but the distribution of nanometric particulates inobtained nanocomposite layer is not as same as the distribution of used nanometricparticulates for large amounts of ASPN. So, in this point of view, it is better to usethe nanometric particulates for fabrication of nanocomposite layer, with loweramounts of ASPN.

In another study, hard silica/epoxy nanocomposite coatings were prepared byspinning method on the surface of AA6082 aluminum alloy with addition of CdTequantum dots as the second phase in hard nanocomposite coating with differentratios in respect to main phase (silica nanoparticulates). Wear tests have been doneon the coatings for investigation of the possible enhanced or inverse effects ofaddition QDs on properties of hard nanocomposite. It has been shown that byadding QD nanoparticulates the electrical conductivity of layers is completelycontrollable without adverse effect on wear resistance. Figure 5.15 shows theeffect of different SiO2/QD ratios on the wear rate of obtained layers. Wear rateillustrates an optimum level and increasing the SiO2/QD ratio after this level willdecrease wear rate significantly. QD nanoparticulates are softer than SiO2

-

nanoparticulates and this behavior in wear rate was predictable, somehow

Fig. 5.14 Distribution curve of electroplated nanocomposite Si3N4/Ni coatings with ASPN equalto a 9 nm, b 72 nm, c 168 nm, d 499 nm [102]


determining the optimum level will affect considerations for possible industrialusage especially for achieving desirable wear rate and electrical conductivitytogether, in economical point of view [103].

In another study, ultra hard ceramic based matrix nanocomposite layers oftungsten carbide (WC) on matrix of titanium carbide were fabricated in an organicelectrolyte. The dependence of WC amount in nanocomposite coatings wasinvestigated in relation to the temperature of electrolyte, WC concentration inbath, current density and stirring rate. It was shown that volume percentage of WCin the layer can be affected by these parameters. Increasing of the WC nanopar-ticles concentration in the electrolyte in a constant stirring rate will lead to anincrease in content of nanoparticles in the nanocomposite layers. Concentration ofWC nanoparticles in the bath illustrated specific level for increasing of tungstenpercentage in the nanocomposite layers [104].

Fig. 5.15 Effect of SiO2/QDratio on wear rate of differentsilica/epoxy nanocompositelayers [103]

Fig. 5.16 X-ray diffraction pattern of WC/TiC nanocomposite layer fabricated by plasmaelectrolysis [104]

5.12 Size Dependency in Nanocomposite Layers 177

The XRD and GAXRD pattern (Fig. 5.16) confirms the formation of a TiC/WCnanocomposite layer. The concentration of WC nanoparticles was increasedslightly by decreasing GAXRD angle which means that the amount of WC nano-particles was increased toward the top surface of nanocomposite layer. Averagegrain sizes was determined by Scherer equation around 51, 58, 72 and 89 nm for thelayers by 1�, 5� and 10� of glancing angle and also simple XRD, relatively.Tungsten carbide nanoparticles probably act as new sites for grain growth andhence decrease the final size of grains. Roughness values of the nanocompositelayers were calculated to be approximately between 1.6 and 4.9l. Figure 5.17illustrates the changes of roughness with the change in the concentration of WCnanoparticles in the electrolyte. The increase of roughness is due to the agglom-eration of WC nanoparticles on the surface of the treated sample. Concentration ofWC nanoparticles in the electrolyte has an optimum level for achieving the mini-mum roughness on the surface of the nanocomposite layer at higher current den-sities. In fact, the increase of nanoparticles concentration in the electrolyte and theincrease of the current densities have similar effects on surface roughness. Highercurrent densities will lead to big sparks with more damaging effects and their effectswill show themselves on low concentrations of nanoparticles in the electrolyte.

Electrodeposition of tertiary Alumina/Yitria/carbon nanotube (Al2O3/Y2O3/CNT) nanocomposite layer by using pulsed current has been also studied. The

Fig. 5.17 Relation amongsurface roughness of coatingand WC nanoparticleconcentration in electrolyte indifferent current densities[104]

Fig. 5.18 Relation betweenaverage size of nanoparticlesand wear rates forelectrodeposited tertiarynanocomposite layer [105]


effects of some process variables have been experimentally studied and statisticalmethods were used to achieve the minimum wear rate and average size of nano-particles. It has been revealed that by changing the size of nanoparticles, wearproperties of coatings will change significantly. In the case of average size ofnanoparticles ranking of effective factors by their relative contributions is the sameas for wear rate which shows strong relation between these two measured prop-erties of coatings [105]. This relation can be seen in Fig. 5.18.

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