nano-fabrication by cathodic plasma ... - aliofkhazraei · m. aliofkhazraei, 1, a. sabour...

Critical Reviews in Solid State and Materials Sciences, 36:174–190, 2011Copyright c© Taylor and Francis Group, LLCISSN: 1040-8436 print / 1547-6561 onlineDOI: 10.1080/10408436.2011.593269

Nano-Fabrication by Cathodic Plasma Electrolysis

M. Aliofkhazraei,1,∗ A. Sabour Rouhaghdam,1,∗∗ and P. Gupta2

1Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University, Tehran, Iran2Boston Scientific, Three Scimed Place, Maple Grove, Minnesota, USA

Cathodic plasma electrolytic (CPE) techniques are new groups of coating processes, which canbe used for fabrication of nanostructured layers on surface of a wide range of metallic substrates.The most exciting visible feature of these atmospheric-based plasma techniques is continuoussparking on processed surface inside an electrolyte. Unlike the anodic part of plasma electrolysis(usually known as plasma electrolytic oxidation (PEO) or micro arc oxidation (MAO)), which iscommonly used for oxidation of light metals/alloys such as aluminum, titanium and magnesium,CPE techniques can clean and coat different metals and alloys such as steel, copper, and lightmetals/alloys with formation of wide range of nanostructures including complex carbides,carbonitrides, intermetallics, and even oxides. It has been observed that the properties ofobtained layers depend on the characteristics of achieved nanostructures such as average size,distribution and average coordination number of nanocrystallites. Furthermore, the propertiesof the processed surface can be tailored by tailoring the nanostructure characteristics. Thereis limited literature available on the mechanism of CPE and its connection to the morphologyof nanostructured layers. This article addresses the two important aspects of CPE, namelycharacterization of nanostructured layers and mechanism of cathodic plasma electrolysis, whichare reviewed in accordance to the morphology of fabricated nanostructures.

Keywords average coordination number, cathodic plasma electrolysis, coating, complex com-pounds, nanostructure, morphology

Table of Contents

1. INTRODUCTION .............................................................................................................................................. 175

2. PERFORMANCE OF CATHODIC PLASMA ELECTROLYSIS ....................................................................... 1762.1. Electrolyte ................................................................................................................................................... 1762.2. Applied Current ........................................................................................................................................... 1782.3. Cell Design .................................................................................................................................................. 179

3. MECHANISM OF CATHODIC PLASMA ELECTROLYSIS ............................................................................. 180

4. AVERAGE COORDINATION NUMBER ........................................................................................................... 1814.1. Simulation of ACN for Nanostructured Layer ................................................................................................. 1814.2. ACN Changing Trend ................................................................................................................................... 181

5. MORPHOLOGICAL ASPECTS OF ACHIEVED NANOSTRUCTURES .......................................................... 1825.1. Growth Kinetics and Size Effect .................................................................................................................... 1825.2. Correlation among Nanostructure and Properties of Layers .............................................................................. 1835.3. Electrochemical Properties of Nanostructured Layers ...................................................................................... 1835.4. Mechanical Properties of Nanostructured Layers ............................................................................................. 184

∗E-mail: [email protected], [email protected]∗∗E-mail: [email protected], [email protected]

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NANO-FABRICATION ELECTROLYSIS 175

6. CONCLUSION .................................................................................................................................................. 185

ACKNOWLEDGMENTS .......................................................................................................................................... 186

REFERENCES ......................................................................................................................................................... 186

1. INTRODUCTIONDifferent plasma treatments1–8 are increasingly used for sur-

face modification9-11 of metallic components, as most of themethods are environmentally friendly.12; 13 One that has re-cently attracted the attention of researchers is plasma electrol-ysis,14;15 which can be classified under atmospheric pressureplasma treatments.16–21 Like most of the electrochemical treat-ment methods,22–24 this process consists of immersion of sub-strate in an electrolyte with a relatively high applied potential(from 200 to 2000 volts).25–36 During this process surface ofthe processed substrate is subjected to continuous sparking37, 38

and based on chemistry of the electrolyte and applied modeof voltage, different coatings such as oxides,39–41 carbon-basedmaterials, e.g., carbides and carbonitrides,42–46 etc.47 can be pro-duced. Typical applications of this method are summarized inTable 1.

The processed substrate can behave as the anodic48 or ca-thodic49 electrode based on the polarity of applied voltage andhence this method is usually divided into anodic or cathodicelectrolytic plasma. Figure 1 shows a schematic division for theprocess. Pulsed current with bipolar mode50, 51 has been usedfor electrolytic plasma treatment, in which the polarity of elec-trodes continuously changes. Thus, the type of method (anodicor cathodic) is usually determined by the polarity in which theintensity of applied bipolar current is greater. For example, if thepulsed current with 40% of duty cycle (ton/toff = 40/60 = 0.66)and applied cathodic voltage of 600 volts and applied anodicvoltage of 300 volts, has been used, the cathodic plasma elec-

FIG. 1. Typical classification of plasma electrolysis and its applications. (Reprinted with permission from Aliofkhazraei andSabour Rouhaghdam,143 Copyright 2010: Wiley-VCH Verlag GmbH & Co. KGaA.)

trolysis is being used, as the intensity of cathodic side (600 ×0.4) is higher than anodic one (300 × 0.6).

The anodic part of plasma electrolysis (frequently recognizedas plasma electrolytic oxidation (PEO)52–54 or micro arc oxida-tion (MAO)55–58) is generally used for deposition of an oxidecoating on surface of light metals/alloys such as aluminum, ti-tanium and magnesium. The oxide coatings are generally usedfor enhancing properties of light metals/alloys such as wear,59, 60

corrosion,61–63 thermal barrier,64 etc. Based on the parametersfor coating process, the size of formed coating structure vary inthe range of few nanometers26,65 to several micrometers.33 Theresearchers are interested in the structure of coatings formed byelectrolytic plasma process and are actively working towardsincreasing their understanding of the mechanism for observedstructure. Furthermore, formation of nanostructure has openedavenues to produce and study nanocomposite66–68 layers by thismethod.

The cathodic part of plasma electrolysis can be used to pro-cess different types of metals/alloys and can be applied to fabri-cate complex coatings such as graphite69 and oxides.70 Figure 2illustrates schematic diffusion of small elements from the elec-trolyte into metallic lattice through nanostructured compoundlayer and stressed under layer formed by cathodic plasma elec-trolysis (CPE). It has been observed that the sizes of structuresproduced by this method are, mostly, in the range of 1 to 100 nm,which is in domain of nanostructures.14,71–73 The key aspect ofthe nanostructures obtained by CPE is that the properties of theprocessed surface can be tailored by changing characteristics of

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176 M. ALIOFKHAZRAEI ET AL.

TABLE 1Typical application and potential usage of the plasma electrolysis

Target industry Industrial component Plasma Electrolysis Improvement

Oil industry Valves, casings, pumps Surface hardening,nanocomposite coatings

Hardness improvement,distribution control fornanocomposite coatings

Aerospace industry Light alloys, turbines Nanocomposite coatings Thermal barrier applications,lifetime increasing

Electrical industry Wires Cleaning Surface characteristics forpost coatings

Tool industry Industrial components underhigh wear conditions

Nanocrystalline plasmaelectrolytic saturation andnanocomposite coatings

Hardness, wear resistance,life time and in some casescorrosion resistance

Note: Reprinted with permission from Aliofkhazraei and Sabour Rouhaghdam,143 Copyright 2010: Wiley-VCH Verlag GmbH & Co. KGaA.

these nanostructures (such as shape of nanocrystallites).71, 72 Ithas been observed that by reducing average size of the nanos-tructures, properties of the surface (such as corrosion and wearresistances) will improve significantly.74, 75 This method canalso be used for surface cleaning of different metallic sub-strates.73,76–79 Table 2 compares routine carburizing processesand plasma electrolysis.

CPE has shown promise to produce nanostructured surfaceswith desired properties, the mechanism and its relation to ef-fective factors of process80 and morphology of nanostructuredlayers is an important aspect. This article reviews the nanostruc-tures formed by CPE and current understanding of the involvedmechanism studied by the authors and others researchers. Thisreview comprises four sections (except this section), which fo-cus on the cathodic part of plasma electrolysis for formationof nanostructured layers (primarily coating aspect and not forcleaning). Cleaning procedure by this method has been wellstudied previously.14,76–79 The authors intend to review open lit-

FIG. 2. Schematic diffusion of elements into metallic lat-tice through nanostructured compound layer and stressed un-der layer. (Reprinted with permission from Aliofkhazraei andSabour Rouhaghdam,143 Copyright 2010: Wiley-VCH VerlagGmbH & Co. KGaA.)

erature in nano-coatings deposited by cathodic plasma processand the findings from their own research.

It is notable here that the term “applied potential” or “appliedvoltage” which has been used through this paper means thehighest applied potential during CPE. In the case of pulsedvoltage, it means the peak of applied voltage on the cathodicdirection. Exceptions have been determined in the case of usedanodic potentials.

2. PERFORMANCE OF CATHODIC PLASMAELECTROLYSIS

2.1. ElectrolyteThere are two effective factors on the process: (a) compo-

sition of electrolyte (b) mode of applied current.15,81,82 Aque-ous based electrolytes can be used for deposition of coatingsby this process,83 but most of the used electrolytes have beenwith organic based electrolytes for fabrication of nanostructuredcarbide-based layers. CPE is characterized by diffusion of high-energy atoms from electrolyte (such as carbon, nitrogen, etc.)under a relatively strong electrical field toward the surface ofthe sample. During processing, the atoms continuously diffuseinto the lattice of metallic substrate, thereby creating intrinsicstresses in the affected lattice. This results in increase in hard-ness of the processed surface and simultaneously (in combina-tion with localized elevated temperature of surface) in formationof a compound layer consisting of nanocrystalline carbides,carbonitrides, etc.84,85 The primary constituent of an organicbased electrolyte is carbon, so the formed layer will consistof carbide-based compounds. Carbamide (Urea), Glycerol, Tri-ethanolamine, Formamide, etc.82 are examples of organic elec-trolytes that can be used for fabrication of nanostructured layers.

Some additives are usually added to these electrolytes for en-hancement of electrical conductivity of the electrolyte. As seenin Figure 3, electrical conductivity of electrolyte has an impor-tant role in energy consumption of CPE process. These addi-tives can be sodium carbonate, sodium nitrate, sodium chloride,

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FIG. 3. Current-voltage changes during cathodic plasma elec-trolysis (vector shows decreasing for electrical conductivity ofelectrolyte and hence lowering energy consumption).

etc.86 It is important to select an appropriate additive in orderto achieve a uniform layer with desired chemistry. For purecarburizing, the additives containing nitrogen (such as sodiumnitrate) should not be used as it can form nitrides or carboni-trides. Additives that are more efficient in increasing of electricalconductivity of the electrolyte are preferable due to lower watercontent in an organic electrolyte.

Water content plays an important role while using organicelectrolyte with CPE. Water dissolves the additives in the elec-trolyte and helps chemical reactions that occur at the processedsurface. CPE process results in evaporation of water from theelectrolyte, so it is necessary to add water at regular interval tomaintain chemistry of the electrolytic bath. But it is critical thatthe water content does not exceed a certain amount in the organicelectrolytic bath, generally not more than 5%. Higher amountsof water content in electrolyte can limit capability of coatingprocess to form nanostructured carbide based layers, due to ox-idation of the surface.87 The oxide layer also has a nanometricstructure and may be useful for some other purposes rather thanincreasing the hardness for surface of different metals/alloys.

Usage of aqueous based electrolytes for cathodic plasmaprocesses with different applied potentials has been reportedin literature.83,88 These electrolytes can be used to fabricatenanostructured layers as well as nanoparticles by use of CPE.89

Aqueous electrolytes consist of appropriate dissolved mineralcomponents (e.g., aluminum chloride) for fabrication of desirednanostructured coating (e.g., intermetallic compounds of alu-minide containing layer).

The authors have used statistical methods for optimization ofelectrolyte composition with desired target of uniform coatingand smaller nanocrystal size.71,72 It was seen that the coatingwith lower average size of nanocrystallites, exhibits better elec-trochemical and mechanical properties. This is consistent withnano-structured coatings that have been reported in literaturedeposited by using various technologies.90–94 The distributionand its deviation from normal status is also an important is-sue for this kind of nanostructured coatings.95,96 It was revealed

that the best properties achieved by the normal distribution ofnanocrystallites and this kind of distribution is fully accessibleby precise controlling of CPE effective factors.97,98

A study was conducted by use of DC current to optimizeprocess parameters for plasma electrolytic nitrocarburising (thecontent of nitrogen is higher than the content of carbon in com-pound layer) with the goal of improving corrosion resistance of316L austenitic stainless steel.86 Applied voltage was the pri-mary factor to achieve higher corrosion resistance of the 316Lsubstrate, due to formation of thicker modified layer. Treatmenttime was the second effective factor in ranking, while electri-cal conductivity of the electrolyte and carbamide concentrationhad similar effects. The study showed that optimal processingparameters for the CPE system were 1050 g/L for carbamideconcentration, 360 mS/cm for electrical conductivity of elec-trolyte, 260 volts for applied direct voltage and 6 minutes oftreatment time. The corrosion resistance increased by ∼50% ascompared with the unprocessed substrate.

Plasma electrolytic carbonitriding process (the content of ni-trogen is lower than the content of carbon in compound layer)was studied by using pulsed current,99 with a target of achiev-ing small size of nanocrystalline carbonitrides on the surface ofcommercially pure titanium (CP-Ti). The process parameters inthe order of their significance can be ranked in following order;(1) applied pulsed voltage (2) frequency (3) treatment time and(4) temperature of electrolyte. The estimated optimized condi-tions are 15 KHz for applied frequency, 50 C for temperatureof electrolyte, 500 volts for applied voltage and 30 minutes fortreatment time. The average size of nanocrystalline carboni-trides was ∼67 nm, which resulted in improvement of corrosionand wear resistance as compared to the unprocessed substrate.99

Temperature of electrolyte must be controlled by a coolingcircuit.100,101 It has been observed that it is important to con-trol the temperature of the electrolyte. First, higher electrolytetemperature can lead to decomposition of organic constituentssuch as carbamide. Second, electrolyte temperature also affectsmicro quenching of nanocrystallites on the processed substrate,thereby affecting its size and properties. Micro-quenching as-pect is discussed in more detail in later section.

2.2. Applied CurrentMode of applied current is a key factor for CPE process. The

applied current or potential is directly related to the localizedtemperature of surface,87 which is important to control the prop-erties of phases formed on the substrate. It is necessary to controlthe applied current to tailor surface elemental composition andto obtain desired nanostructured layer. In general, applying anadjusted pulsed current can lead to more uniform coating withdesirable properties.102 Although there are studies on exotic useof pulsed current for CPE such as pulse on pulsed currents,99 butthe most effective factors of pulsed current mode are frequency,duty cycle (ton/(ton+toff)) and polarity of processed substrate(monopolar or bipolar).

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Plasma electrolytic saturation process used in bipolar pulsedcurrent mode with glycerol-based electrolyte showed that higherapplied cathodic voltages accompanied with lower anodic volt-ages, and in combination with lower duty cycle ratio (cathodicdirection to duty cycle of anodic direction) was more appropri-ate to achieve nanocrystallites with smaller size.81 Distributionof nanocrystallites exhibited a Gaussian-shape (near normal dis-tribution). Samples with high height to width ratio on the dis-tribution curves, exhibited nanocrystallites with lower averagesize and lower aspect ratio (length-to-diameter ratio).96

The surface composition of substrate was not an impor-tant factor for CPE; however, substrates with higher meltingpoints act better in the process.102 One of the most interestingfacts related to CPE, is that the morphology and distribution ofnanocrystallites is not only directly proportional to mechanicalproperties of deposited nanolayers (also reported for other meth-ods for deposition of nanostructured layers), but also is directlyproportional to electrochemical properties of the nanolayers,which is rarely observed in other methods.98

Use of plasma electrolysis on γ -TiAl alloy substrates forfabrication of nanostructured coating by bipolar pulsed currentshowed that certain factors have strong effect on the propertiesof achieved layer.96 The factors include applied current (ratioof duty cycle for cathodic direction to anodic direction between0.2 to 0.4), frequency of pulsed current (range between 5 and15 kHz), and treatment time (10 to 30 minutes). The studyshowed that higher frequencies in combination with lower dutycycle ratio (cathodic direction to anodic direction) and treat-ment time resulted in complex nanocrystallites with smallersize.

A study on skewness and kurtosis of Gussian distributioncurves of nanocrystallite size revealed that current density andduty cycle of the pulsed current significantly affect the averagesize of hard nanocrystallites.95 Furthermore, higher current den-sities and lower duty cycles of the pulsed current are, in general,better for deposition of hard nanocrystallites with smaller sizes.These two factors also affect the surface roughness of coatedsubstrate.

Different pulsed current modes can be used during CPE pro-cessing. Figure 4 illustrates the pulsed currents which have beenused for CPE; namely triangular,103 rectangular104 and sinu-soidal.105 The studies revealed that nanocrystallites had somedefects (such as pores and cracks) were deposited by use oftriangular and rectangular modes. Triangular mode resulted indiffused layer with higher hardness as compared to rectangularand sinusoidal modes, which was related to higher electricalfield around the substrate in triangular mode. Sinusoidal moderesulted in better mechanical and electrochemical propertiesthan triangular and rectangular modes, which was related todenser and more uniform compound layer formed on the pro-cessed surface. Also, sinusoidal mode resulted in smallest aver-age size of nanocrystalline carbides and minimum percentage ofaverage cracks density than the other modes of applied pulsedcurrent.

FIG. 4. Schematic diagram of different (a) triangular (b) rect-angular (c) sinusoidal pulsed currents. (Reprinted with permis-sion from Aliofkhazraei and Sabour Rouhaghdam,143 Copyright2010: Wiley-VCH Verlag GmbH & Co. KGaA.)

2.3. Cell DesignDesigning a suitable cell for holding the electrolyte and elec-

trodes with a high-efficiency cooling circuit is also a key stepfor successful performance of CPE. The mechanism of CPE isbased on the evaporation or reaction of electrolyte and then elec-trical break down of the gaseous envelope around the sample,which results in formation of sparks around the processed sub-strate.14,43,100 Therefore, the cell configuration must prevent highagitation of electrolyte around the substrate to prevent disrup-tion of the gaseous envelope around the substrate. Furthermore,it is recommended for the CPE systems in which a substrate isimmersed in the electrolyte, to avoid high hydrodynamic flowof electrolyte toward the substrate. Figure 5 shows a schematicexample of cell design.

Electrolyte can be cooled by two methods, (a) passing elec-trolyte that is exiting the cell through a cooling system andthen pumping it back in the cell (b) circulating cooling mediainto/around the CPE cell. First method is not appropriate forthe electrolytic processes that use organic-based electrolytes.The organic liquid has a tendency to crystallize, while passingthrough the refrigeration system and thereby posing difficulty inprocessing. For any reason, however if this method is preferredway to cool the electrolyte, then there are some considerationsto circumventing a processing challenge, such as prevention ofhigher agitation. Agitation problem can be solved by protectingexiting and entering electrolyte (colder electrolyte) with dams/barriers of material compatible with electrolytic bath.

Second method can be divided in two different types, (a)circulating cooling media (e.g., cold water) in the CPE cellthrough the electrolyte by an appropriate coil, and (b) circulatingit around the CPE cell to cool the walls of CPE cell. Type(a) offers advantage of better controlling the cooling rate by

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FIG. 5. Schematic setup for cathodic plasma electrolysis: (1)cooling water outlet (2) window for observation sparking sur-face (3) treated sample (4) magnetic stirring bar (5) electrolyte(6) cooling water inlet (7) stainless rod for holding sampleand electrical connection (8) wires for electrical connectionto power supply (9) power supply for applying pulsed cur-rent. (Reprinted with permission from Aliofkhazraei and SabourRouhaghdam,144 Copyright 2010: Elsevier.)

adjusting the flow of cold media than type (b); however, in type(a) designing of coil, especially for large industrial units canadd significant cost. In both types it is recommended to designan appropriate window on the wall of the cell for observation ofsparking sample.

In both types of cooling systems, the electrolyte bath can beset as the positive electrode for electrochemical circuit of coatingprocess. Also an external plate can be used as anode materialfor CPE processes. The positive electrode is usually made ofstainless steel and exhibits corroded zones after long periodusage, which is dependent on properties of electrolytic bath,such as water content, pH, etc. It is better to design the anode asan external anode plate or a coil (especially for laboratory scalebaths) in order to ease its inspection and replacement.

As discussed in earlier section, CPE is characterized by adistinctive sparking on the processed surface, it is recommendedto make the processing chamber from the transparent materials(preferably with high thermal shock resistance) for observing thetreating sample during coating process. It has been observed thatthe symmetrical design of most of the components in CPE cellcan facilitate deposition of uniform nanocrystallites, especiallyon the sharp edges of the substrate.

3. MECHANISM OF CATHODIC PLASMA ELECTROLYSISThe break down voltage of gaseous envelope around the sub-

strate is directly related to the applied voltage.73 Hence, CPE be-haves similar to conventional electrolytic process up to a certainvoltage, which obeys ohmic law by linear resistance electrical

circuits. Gupta et al,14 have shown theoretical and experimentalbehavior of cathodic electrolytic process in aqueous electrolyteby use of DC current. Deviation from Faraday’s law is observedafter a certain critical voltage and at a breakdown voltage sparkis seen on the surface of the substrate. In fact critical voltage isa critical point that the current-voltage diagram shows deviationform a linear trend of increasing. In case of pulsed current thecritical voltage for a given system may increase as comparedto the DC current (but the necessary power for reaching sparkson the surface of cathode remains approximately constant). Forexample, by applying pulsed voltage with 50% of duty cycle,the critical applied voltage for sparking can be two times higherwith respect to similar conditions without pulsed current. It isworth while mentioning that the anodic part of plasma elec-trolysis (e.g., PEO treatment) does not show this behavior, andsparking begins at a constant value of applied voltage and isnot dependent on DC or AC source for a given electrolytic sys-tem.106-109

The mechanism of CPE can be described by linear increase ofcurrent due to an increase in applied voltage. After reaching spe-cific value of applied potential (break down potential), randomsparks will form on the surface of substrate. Increasing appliedpotential after this level (approximately 10 to 30 volts) will leadto continuous sparking on the substrate. In this region, currentwill drop linearly with applied potential as the impedance ofnonlinear system of sparking is lower than total resistivity ofused electrolyte. In other words, the slope of current-voltagediagram in this region is lower than the linear ohmic region(first step) of CPE. Similar behavior has been reported in litera-ture for electrolytic plasma processes.14,15,43,100 The breakdowncurrent density and treatment time are also dependent on thecomposition of the used electrolyte, but its effects are negligibleas compared with the applied potential.

Composition of the gaseous envelope is dependent on thechemistry of the electrolytic bath. In aqueous solutions, watervapor is major component, where as in organic electrolyte, suchas for nitro-carburizing chemical reactions affect the composi-tion of gaseous vapor. Some of the possible reactions in organicelectrolyte include:15,43

2H+ + 2e− → H2 [1]

HCONH2 → NH3 + CO [2]

HCONH2 → HCN + H2O [3]

(C2H4OH)3N → 2CH4 + 3CO + HCN + 3H2 [4]

Increasing the applied potential will not only affect chemicalreactions on the surface of the cathode, but will also affect localevaporation rate of the used electrolyte due to localized heatingof electrolyte in vicinity of the cathode.

The regime of potentials which forms stable gas envelopeand spark around the substrate has been exploited by CPE fornumerous surface treatments by use of organic and inorganicelectrolytes. Formation of sparks on the surface will lead to syn-thesis of nanocrystallites, which will remain in the nanometric

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range due to rapid quenching in electrolyte at near room temper-ature. Higher applied potentials or higher electrical fields aroundthe substrate provide higher powers for ion bombardment on theprocessed surface and can lead to deeper diffusion zone into thedepth of the substrate. In lower applied potentials, only smalldiffusive atoms can become accelerated in plasma envelope anddiffuse into the surface of cathode. Higher applied potentialscan provide sufficient energy to bigger diffusive atoms, such asAluminum and Silicon for bombardment onto the substrate.83

It is proposed that these atoms become ionized in a very shorttime of sparking and accelerate toward the cathodic surface un-der strong electrical field. Similar mechanism has been reportedby other researchers.14,15,43

4. AVERAGE COORDINATION NUMBER

4.1. Simulation of ACN for Nanostructured LayerThe average coordination number (ACN) of atoms depends

on the size and shape of fabricated nanoclusters. Large amountof surface atoms will decrease the ACN of nanostructured layerwith simultaneous increase of dangling bonds. Hence, the ACNcan be estimated from the amount and size of the surface atoms.Direct relation of the ACN to physical and chemical prop-erties of materials, such as catalytic properties of nanostruc-tures,110 metallic manners of materials,111 photoemission prop-erties,112 ionization potentials,113 electronic states concentra-tion,114 atomic packing factor and crystalline structure115 havebeen extensively studied. Evaluation of distribution, averagesize and geometrical shape of nanostructures from their ACNcan help in estimation of catalytic activity of nanostructures.116

As we have discussed in the previous sections, CPE nanos-tructured layer consist of different nanocrystallites and nan-oclusters.81,83 Their atoms could either be at the surface or ininternal layers. The coordination number of different atoms canbe assigned as ZS and ZI for surface and internal atoms, respec-tively. ZS describes only surface bonds and is not related to theinternal bonds of surface atoms. nS, nI and nT denote the numberof surface, internal and total atoms, respectively (nT = nS + nI).Surface, internal and total ACN of nanostructured layer can bedescribed as:

ZS =( nS∑

1

ZS

)/nS [5]

ZI =( nI∑

1

ZI

)/nI [6]

ZT =( nS∑

1

ZS +nI∑1

ZI

)/nT [7]

In a CPE system, there are three primary sources for atoms toform a nanostructured layer, namely substrate, electrolyte andchemical reactions in sparking region. The proportional changesin coating thickness with bombarded atoms from electrolyte83 inCPE will lead (nT/nI) to be constant. Further more, nT and nI will

change unidirectional, i.e., nT will increase with increase in nI

(in the case of changing one effective factor of coating process).For example, if all of the processing parameters assumed to beconstant and just duty cycle of pulsed current is changed, byincreasing the duty cycle, the total (nT) and hence internal (nI)atoms of the layer will increase simultaneously, but the surfaceatoms (nS) of the layer will also depend on the distribution ofporosities and their size over the nanostructured layer.

4.2. ACN Changing TrendThe strong relation among different properties of CPE nanos-

tructured layers with their characteristics of nanocrystallites98

can be studied by studying distribution of ACN. A correlationbetween processing parameters with properties of nanostruc-ture can be made by estimating the changing trend for differentexpressions of Equ. 7 with change in processing parameters.The changing trend of total and internal atoms due to changein processing parameters has been extensively studied and re-ported by the authors.75,81,83,87,117 In accordance with Equ. 5to 7, as nT and nI are proportional, changing trend of ACNcan be determined by nSZS/nT . Distribution of surface atomscan be determined by analyzing the surface porosities. Authorshave done investigations on distribution and size of porositiesby SEM analysis of micro/nanostructures on the surfaces andthe cross sections (with enough measurements for minimiz-ing statistical errors).65,82,83 Image analysis measurements wereused to determine relationship of size and distribution of porosi-ties with processing parameters and results are summarized inTable 3. It is necessary to emphasize that change in porositywith processing parameter are in agreement with experimen-tal observations on properties of the deposited nanostructures.For example, frequency and duty cycle of pulsed current areinversely proportional to the properties of nanolayers, espe-cially corrosion resistance.72,117 It is well known that corrosionresistance is affected by the surface activity of porosities orthe amounts of dangling bonds which is related to the aver-age coordination number of the nanostructured layer. Table 3shows that ns decreases with increase in duty cycle, i.e., in-crease in surface porosity and hence leads to decreased corrosionresistance.

TABLE 3Estimation of changing trend for ACN of total nanocrystallites∗

Increasing () ofprocessing parameter nT nS ZT

Frequency Duty cycle Treatment time Current density Applied potential probably

∗) = increase, = decrease, = approximately constant

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FIG. 6. FESEM (a) and TEM (b) images of the plasma electrolytic carbonitrided sample for the discharge time of 2.5 hours.(Reprinted with permission from Li and Han,43 Copyright 2006: Elsevier.)

5. MORPHOLOGICAL ASPECTS OF ACHIEVEDNANOSTRUCTURES

5.1. Growth Kinetics and Size EffectPhase analysis, X-ray diffraction patterns and growth kinet-

ics for nanocrystalline coatings have been reported in someresearches.75,87,118 Based on the composition of electrolyte, dif-ferent hard layers can be deposited. Usage of nitrogen contain-ing organic electrolyte causes the formation of complex car-bonitride with the general formula of MCxN1-x (M = substratemetal). Based on the experiments, the amount of x in this gen-eral formula is mainly related to the factors affecting the coatingprocess such as the peak of applied voltage and the frequencyof pulsed current rather than the ratio of carbon to nitrogen(C/N) in electrolyte. These factors affect the plasma mediumand its containing species differently and thus causes differentamounts of carbon and nitrogen atoms to accelerate toward thesample. It was revealed that average MCxN1-x nanocrystallitessizes start from near 4 nm to higher values. Presence of densenanocrystalline coatings has been confirmed by field emissionscanning electron microscope (FESEM) and transmission elec-tron microscope (TEM) images in Figure 6.43 Direct current hashigh growth rate while its average size of nanocrystallites (ASN)is relatively bigger and its layer has more porosities. Applyingpulsed current will cause the growth rate to decrease and denserlayers can be achieved. Bipolar pulsed currents will decreasethe growth rate more than monopolar pulsed currents do. As thevector shown in Figure 7 illustrates, increasing the frequency ofapplied pulsed current is similar to decreasing its duty cycle forlowering the growth rate and hence, decreasing the ASN andthe amount of porosities. Unlike the direct current, monopolarpulsed current (generally pulsed current) will affect more on thegrowth rate rather than bipolar pulsed current.

Higher frequencies will lead to higher densities of sparks onthe surface, which cause lower dimensions of volcano-like struc-tures and thus decrease the roughness of the coating. One of theimportant applications of these coatings is in anti-abrasive in-dustrial parts and hence, lower roughness values are among their

advantages. As indicated in other research, achieved nanocrys-tallites with lower length-to-diameter (L/D) ratio also showedlower ASN and narrower distribution of nanocrystallites. It wasrevealed that lower ASN of nanocrystallites can lead to higherband gap energy of the fabricated layer (Figure 8). This kind ofsize dependency has been observed also for some other kinds ofnanocrystallites.119–121

In comparison with used direct current (DC), the pulsed CPEmethod effectively assists MCxN1-x nucleation. Nucleation den-sities of the coatings fabricated by the pulsed CPE were around100 times higher than those fabricated by direct current. The dif-ference in MCxN1-x growth characteristics by two kinds of CPEmethod could outcome from the difference in the distribution ofsparks and the amount of diffusive elements. As the power ofplasma for bombardment of diffusive elements is directly relatedto the peak of applied voltage, the pulsed current can improvethe concentration of saturated layer on the surface of the sam-ple. The concentration of diffusive elements will be changedalong the compound layer gradually while it has been observedthat the slope of their change in diffusive layer is higher than

FIG. 7. Variation of aluminum carbonitride thickness versustreatment time for (♦) direct current ( ) monopolar pulsed cur-rent () bipolar pulsed current.

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FIG. 8. Variation of band gap energy versus length to diam-eter for fabricated aluminum carbonitride nanocrystallites bydifferent kinds of applied current.

in the compound layer. It has been shown that concentration ofdiffused atoms decrease toward the core of the substrate (Figure9).45

5.2. Correlation among Nanostructure and Properties ofLayers

It has been proved that the properties of obtained nano-layers are in close relation with their morphological character-istics.122–124 These morphological characteristics can be char-acterized by the shape of nanocrystallites and size distributionof nanostructured layers. It has been discussed in previous sec-tions that the morphological characteristics depend on the pro-cessing parameters such as applied potential and current den-sity; hence tailoring these factors can lead to nanostructuredlayers with desired properties. The average sizes of nanocrys-tallites for CPE treated surfaces can vary from few nanometersto 100 nm; but usually it is less than 70 nm.43,81,98 Distribution ofnanocrystallites usually shows a Gaussian shape, and a Gussiandistribution with no kurtosis and skewness can enhance electro-chemical and mechanical properties of deposited nanostructuredlayers to a high degree.95 Figures 10 and 11 show different ex-amples of deposited nanostructures by CPE and their relativedistributions.

Statistical tools were used to study relation between proper-ties of the nanostructured layers, and length-to-diameter ratio ofnanocrystallites (L/D) and height-to-width (H/W) of the Gussiandistribution curve.96,98 It was observed that the nanocrystalliteswith high H/W ratios have lower average nanocrystallite sizeand lower L/D ratios. Higher H/W ratios indicate tighter dis-tribution of the nanocrystallites around a specific average size,which have exhibited better corrosion and wear resistances.96,98

5.3. Electrochemical Properties of Nanostructured LayersAs per the research done by the authors on CPE, applied

voltage and treatment time are the most significant factors

FIG. 9. Cross-sectional morphology (a) and concentration ofTi, C and N (b) of the plasma electrolytic carbonitrided sampletreated for 2.5 h, and subsequently vacuum annealed for 1 h.(Reprinted with permission from Li et al.45 Copyright 2007:Elsevier.)

that have effect on electrochemical properties of nanostruc-tured layers.98,100 The electrochemical properties of the CPEprocessed substrate have been studied by potentiodynamic po-larization (PDS) and electrochemical impedance spectroscopy(EIS). The polarization resistances of carburized 316 stainlesssteel substrate98 with CPE at different conditions varied between153.4 KΩ.cm2 and 635 KΩ.cm2 as compared to an unprocessedsubstrate with maximum polarization resistance of around10 KΩ.cm2.125,126 Obtained results indicate that the corrosionresistances of obtained layers are in a inverse relation of theirASN, e.g., a treated substrate (316 austenitic stainless steel) withASN ∼ 73.5 nm exhibited corrosion resistance ∼ 218.6 KΩ.cm2

as compared with a treated substrate with ASN ∼ 47.7 nm withcorrosion resistance ∼ 548.4 KΩ.cm2.98 The research revealed

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FIG. 10. SEM nanostructure for carburized samples by CPE at(a) 700 volts for peak of applied cathodic voltage, 200 volts forpeak of applied anodic voltage and 0.25 for ratio of duty cycleof cathodic direction to duty cycle of anodic direction (b) 500volts for peak of applied cathodic voltage, 400 volts for peakof applied anodic voltage and 0.35 for ratio of duty cycle ofcathodic direction to duty cycle of anodic direction. (Reprintedwith permission from Aliofkhazraei et al.81 Copyright 2009:Elsevier.)

that the CPE conditions for achieving maximum polarizationresistance were 700 volts for the peak of applied cathodic volt-age, 15 kHz for the frequency of pulsed current, 40 C for thetemperature of electrolyte and 10 minutes for the treatment time.The polarization resistance of the 316 SS substrate increased to930 KΩ.cm2 by applying these parameters.

It has been revealed that the nitrogen diffusion by CPE willpostpone the pitting phenomenon of stainless steels in corro-sive media.127 On AISI 304, applied DC voltage of 260 voltscompletely increased the corrosion resistance, although appliedvoltage of 245 volts is sufficient for protection against pitting.127

For AISI 430, nitrogen diffusion by applied voltages near 230volts can completely increase the corrosion potential and corro-sion current density. On AISI 316L, nitrogen diffusion producedby applied voltages among 230 to 260 volts decreases the cor-

rosion rate to some extent and does not improve in corrosionresistance. More than likely, higher amounts of alloying ele-ments (mainly Mo), hinder the effect of nitrogen diffusion in316L stainless steel.

Carbon diffusion in AISI 304 by CPE increases the localizedcorrosion resistance. Diffusions produced by applied voltagesabove 260 volts have shown to enhance the corrosion behaviorby fabrication of a surface layer, which could increase protec-tion of passive layer.127 The authors are investigating the mi-crostructure of the diffused layer to understand the mechanismof observed improvement in corrosion behavior. In contrast,carbon diffusion by CPE can be harmful for AISI 430 and willcause low pitting resistance. Comparing with other methods forfabrication of similar coatings,128–132 CPE strongly shows betterelectrochemical properties.

Corrosion tests also showed that CPE fabricated coatingshave enhanced bio-compatibility (better than their titanium sub-strate) and hence CPE can used to develop coating for biomed-ical applications.42 Titanium and its alloys are widely used bio-materials133-135 were processed by CPE and corrosion results insimulated body fluids such as Ringer’s electrolyte136–138 showedenhanced corrosion resistance of the coated substrate than un-treated substrates.65 It has been revealed that CPE processedsubstrates have more noble corrosion potential (Ecorr), lowercorrosion current density (icorr) and higher polarization resis-tance (PR) than untreated samples, which shows that CPE hasgreat potential for biomedical applications.42,65

5.4. Mechanical Properties of Nanostructured LayersCPE significantly reduced the friction coefficients and wear

volume loss of the carbonitrided titanium alloys as comparedto the unprocessed substrates. These deposited layers also ex-hibited high surface hardness, strong adhesion to substrate, andhigh fracture toughness.44,45,139 Mechanical properties of CPEformed layers are affected by applied voltage, treatment timeand current density.96,139 The roughness of the nanostructuredlayers increase by increasing the applied voltage of CPE, but therate of increase in roughness decreases up to 600V, and increasesthereafter.97 An increase in applied voltage leads to a decreasein ASN and at the same time increases impact of micro andnano sparks on the processed substrate. As per earlier researchsparks imploding of the processed surface enhances the surfaceroughness.14 But, smaller crystal size will have a tendency toreduce the surface roughness. More than likely, after 600 Vboth the impact of sparks and crystal size increase due to highertemperature at processed substrate leading to increase in rateof surface roughness. Statistically, the increase in skewness andkurtosis of distribution of nanocrystallites after its normal modewill lead to increase in roughness,97 which may not be suitablefor certain industrial application due higher friction coefficientof rougher surfaces.

It has been observed that thickness of the deposited nanolayer changes linearly with the square root of the treatment timefor CPE.83,87 It is well known that the thickness of nanolayer

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FIG. 11. Distribution curves of nanocrystallites for relative (a) and (b) treated samples which their nanostructures have been shownin figure 10. (Reprinted with permission from Aliofkhazraei et al.81 Copyright 2009: Elsevier.)

layer has an affect on mechanical properties of the substrate.Equ. 8 shows the relationship between thickness of depositedlayer to the applied voltage for CPE.

Th = αV + β(VSparking < V < VInterrupt) [8]

Where Th and V are thickness of influenced layer and appliedvoltage, respectively. α and β are constants. α is related to thetreatment time and β is related to the modified transmissioncoefficient of treated media.

The hardness of the nanostructured layer usually decreasesgradually with depth of the processed layer into surface of thesubstrate. The thickness of the obtained layer will not increaseafter an optimum level of treatment time, which might be re-lated to diffusion limited system after a certain thickness is

reached.43,83 Comparing with other methods for applying ofsimilar coating,140–142 CPE can be useful for special industrialparts, which have problems for surface hardening and coatingwith other methods.

6. CONCLUSIONThe setup, performance and properties of CPE have been

reviewed with a focus on its use in deposition of nanostructuredlayers. The importance of design of CPE cell, along with theprocessing parameters, has been discussed with respect to theirinfluence on properties of nanolayers. The mechanism of CPEhas been reviewed; however, authors feel that tremendous op-portunity exists in fully understanding the mechanism. Authorshave discussed the influence nanostructural characteristics such

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as average size, distribution and average coordination numberof nanocrystallites on mechanical and electrochemical proper-ties of CPE deposited nanostructures layers. CPE presents astrong potential for various industrial applications, either re-placing conventional processes or solving challenging posed bycurrent processes.

ACKNOWLEDGMENTSThe authors would like to acknowledge the financial support

for investigations on CPE by Arvandan oil and gas ProductionCompany and Iranian nanotechnology initiative council. Theauthors would also like to thank Dr. J.A. Curran from CambridgeUniversity for the useful discussions during different aspects ofplasma electrolysis.

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67. E. Matykina, R. Arrabal, F. Monfort, P. Skeldon, G. E. Thompson,Incorporation of zirconia into coatings formed by DC plasmaelectrolytic oxidation of aluminium in nanoparticle suspensions,Appl. Surf. Sci., 255(5, Part 2), 2830 (2008).

68. X. Nie, L. Wang, E. Konca, A. T. Alpas, Tribological behaviourof oxide/graphite composite coatings deposited using electrolyticplasma process, Surf. Coat. Technol., 188-189, 207 (2004).

69. T. Paulmier, J. M. Bell, P. M. Fredericks, Deposition of nano-crystalline graphite films by cathodic plasma electrolysis, ThinSolid Film., 515(5), 2926 (2007).

70. T. Paulmier, J. M. Bell, P. M. Fredericks, Development of anovel cathodic plasma/electrolytic deposition technique part 1:Production of titanium dioxide coatings, Surf. Coating. Technol.,201(21), 8761 (2007).

71. M. Aliofkhazraei, A. S. Rouhaghdam, A. Denshmaslak, H.Jafarian, M. Sabouri, Study of bipolar pulsed nanocrystallineplasma electrolytic carbonitriding on nanostructure of compoundlayer for CP-Ti, J. Coat. Technol. Res., 5(4), 497 (2008).

72. M. Aliofkhazraei, A. S. Rouhaghdam, M. Sabouri, Effect of fre-quency and duty cycle on corrosion behavior of pulsed nanocrys-talline plasma electrolytic carbonitrided CP-Ti, J. Mater. Sci.,43(5), 1624 (2008).

73. E. I. Meletis, X. Nie, F. L. Wang, J. C. Jiang, Electrolytic plasmaprocessing for cleaning and metal-coating of steel surfaces, Surf.Coat. Technol., 150(2-3), 246 (2002).

74. M. Aliev, A. Saboor, Pulsed nanocrystalline plasma electrolyticboriding as a novel method for corrosion protection of CP-Ti (Part1: Different frequency and duty cycle), Bull. Mater. Sci., 30(6),601 (2007).

75. M. Aliofkhazraei, S. H. H. Mofidi, A. Sabour Rouhaghdam,E. Mohsenian, Duplex Surface Treatment of Pre-Electroplatingand Pulsed Nanocrystalline Plasma Electrolytic Carbonitridingof Mild Steel, J. Thermal Spray Technol., 17(3), 323 (2008).

76. P. Gupta and G. Tenhundfeld, Surface and mechanical propertiesof steel processed by electro-plasma technology, Plating Surf.Finishing, 92(2), 48 (2005).

77. P. Gupta, G. Tenhundfeld, E. O. Daigle, Surface modification ofbrass-coated steel cord by electro plasma technology, Wire J. Int.,38(2), 50 (2005).

78. Y. Cheng, P. Gupta, E. Meletis, Surface characteristics of 4340steel treated by electrolytic plasma processing, J. Mater. Sci.,45(2), 562 (2010).

79. A. Yerokhin, A. Pilkington, A. Matthews, Pulse current plasmaassisted electrolytic cleaning of AISI 4340 steel, J. Mater. Pro-cess. Technol., 210(1), 54 (2010).

80. D. I. Slovetskii and S. D. Terent’ev, Parameters of an electricdischarge in electrolytes and physicochemical processes in anelectrolyte plasma, High Energy Chem., 37(5), 310 (2003).

81. M. Aliofkhazraei, A. Sabour Rouhaghdam, A. Heydarzadeh, H.Elmkhah, Nanostructured layer formed on CP-Ti by plasma elec-

trolysis (effect of voltage and duty cycle of cathodic/anodic di-rection), Mater. Chem. Phys., 113(2-3), 607 (2009).

82. M. Aliofkhazraei, M. Salasi, A. S. Rouhaghdam, P. Taheri, Elec-trochemical study of nanocrystalline plasma electrolytic car-bonitriding of CP-Ti, Anti-Corr. Method. Mater., 54(6), 367(2007).

83. M. Aliofkhazraei and A. Sabour Roohaghdam, A novel methodfor preparing aluminum diffusion coating by nanocrystallineplasma electrolysis, Electrochem. Commun., 9(11), 2686 (2007).

84. P. Taheri, C. Dehghanian, M. Aliofkhazraei, A. S. Rouhaghdam,Evaluation of nanocrystalline microstructure, abrasion, and cor-rosion properties of carbon steel treated by plasma electrolyticboriding, Plasma Process. Polymer., 4(S1), S711 (2007).

85. P. Taheri, C. Dehghanian, M. Aliofkhazraei, A. S.Rouhaghdam, Nanocrystalline Structure Produced by Com-plex Surface Treatments: Plasma Electrolytic Nitrocarburizing,Boronitriding, Borocarburizing, and Borocarbonitriding, PlasmaProcess. Polymer., 4(S1), S721 (2007).

86. M. Aliofkhazraei, P. Taheri, A. S. Rouhaghdam, C. Dehghanian,Systematic study of nanocrystalline plasma electrolytic nitrocar-burising of 316L austenitic stainless steel for corrosion protection,J. Mater. Sci. Technol., 23(5), 665 (2007).

87. M. Aliofkhazraei, P. Taheri, A. Sabour Rouhaghdam, C.Dehghanian, Study of nanocrystalline plasma electrolytic car-bonitriding for CP-Ti, Mater. Sci., 43(6), 791 (2007).

88. M. A. Bejar, and R. Henriquez, Surface hardening of steelby plasma-electrolysis boronizing, Mater. Design, 30(5), 1726(2009).

89. J. Guo, H. Wang, J. Zhu, K. Zheng, M. Zhu, H. Yan, M.Yoshimura, Boron nitride synthesized at ambient pressure androom temperature by plasma electrolysis, Electrochem. Com-mun., 9(7), 1824 (2007).

90. A. Goncharov, V. Konovalov, G. Volkova, V. Stupak, Size effecton the structure of nanocrystalline and cluster films of hafniumdiboride, Phys. Metal. Metallography, 108(4), 368 (2009).

91. F. Kauffmann, G. Dehm, V. Schier, A. Schattke, T. Beck, S.Lang, E. Arzt, Microstructural size effects on the hardness ofnanocrystalline TiN/amorphous-SiNx coatings prepared by mag-netron sputtering, Thin Solid Films., 473(1), 114 (2005).

92. B. Moser, R. Schwaiger, M. Dao, Size effects on deformationand fracture of nanostructured metals, Nanostructured Coat., 27(2006).

93. H. Swygenhoven, A. Hasnaoui, P. Derlet, Nanoindentation innanocrystalline metallic layers: A molecular dynamics study onsize effects, Nanostructured Coat., 109 (2006).

94. L. Wang, J. Zhang, Y. Gao, Q. Xue, L. Hu, T. Xu, Grain sizeeffect in corrosion behavior of electrodeposited nanocrystallineNi coatings in alkaline solution, Scripta Mater., 55(7), 657 (2006).

95. M. Aliofkhazraei and A. S. Rouhaghdam, EFfect of pulse dutycycle on properties of hard nanocrystalline surface fabricated byDuplex treatments, Surf. Review Let., 16(3), 441 (2009).

96. M. Aliofkhazraei, S. A. Hassanzadeh-Tabrizi, A. SabourRouhaghdam, A. Heydarzadeh, Nanocrystalline ceramic coat-ing on [gamma]-TiAl by bipolar plasma electrolysis (effect offrequency, time and cathodic/anodic duty cycle), Cer. Int., 35(5),2053 (2009).

97. M. Aliofkhazraei and A. Sabour Rouhaghdam, Relationshipstudy among nanocrystallite distribution and roughness of a

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99. M. Aliofkhazraei, S. Ahangarani, A. S. Rouhaghdam, Study ofpulse on pulsed nanocrystalline plasma electrolytic carbonitridingon the nanostructure of compound layer, Surf. Review Let., 15(4),427 (2008).

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118. M. Aliofkhazraei, M. Yousefi, S. Ahangarani, A. S.Rouhaghdam, Synthesis and properties of ceramic-basednanocomposite layer of aluminum carbide embedded with ori-ented carbon nanotubes, Cer. Int., 37(7), 2151 (2011).

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125. H. Hasannejad, T. Shahrabi, A. S. Rouhaghdam, M.Aliofkhazraei, Effect of temperature on pitting corrosion resis-tance of 316 stainless steel coated by cerium oxide film in 3.5%NaCl solution, J. Mater. Sci. Technol., 24(5), 715 (2008).

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nano-fabrication by cathodic plasma ... - aliofkhazraei · m. aliofkhazraei, 1, a. sabour...

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