functionally gradient nanocomposite coatings

Applied Surface Science 258 (2012) 2093–2097Contents lists available at ScienceDirectApplied SurfaceSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p s u scFabrication of functionally gradient nanocomposite coatings by plasma electrolytic oxidation based on variable duty cycleM.Aliofkhazraei,A.Sabour Rouhaghdam ∗Department of Materials Science,Faculty of Engineering,Tarbiat Modares University,P.O.Box:14115-143,Tehran,Irana r t i c l ei n f oArticle history:Available online 15 April 2011Keywords:Plasma electrolytic oxidation Functionally gradient coatings Nanocomposite Pulse currenta b s t r a c tPlasma electrolytic oxidation (PEO)was applied on the surface of commercially pure titanium substrates in a mixed aluminate-phosphate electrolyte in the presence of silicon nitride nanoparticles as suspension in the electrolyte in order to fabricate nanocomposite coatings.Pulsed current was applied based on vari-able duty cycle in order to synthesize functionally gradient coatings (FGC).Different rates of variable duty cycle (3,1.5and 1%/min),applied current densities (0.06–0.14A/cm 2)and concentrations of nanoparti-cles in the electrolyte (2,4,6,8and 10g l −1)were investigated.The nanopowder and coated samples were analyzed by atomic force microscope,scanning electron microscope and transmission electron micro-scope.The influence of different rates of variable duty cycle (or treatment times)on the growth rate of nanocomposite coatings and their microhardness values was investigated.The experimental results revealed that the content of Si 3N 4nanoparticulates in the layer increases with the increase of its concen-tration in the plasma electrolysis bath.Nanocomposite coatings fabricated with lower rate of variable duty cycle have higher microhardness with smoother microhardness profile.© 2011 Elsevier B.V. All rights reserved.1.IntroductionSpecial properties of micro-and nanocomposite materials [1–3]including dispersion hardening [4],self-lubrication [5],hot oxi-dation resistance [6],corrosion [7]and wear resistances [8]are different.Plasma electrolysis [9]is one of the most important methods for fabrication of micro-and nanocomposites due to precise control of the process,high deposition rate and low cost [10].Recently,a number of reports showed Si 3N 4,WC and ZrO 2nanoparticulates,which were successfully added into the plasma electrolysis bath and could be embedded with the layer to fabricate nanocomposite coatings [11].Oxide based coatings were performed as one of the surface coatings to replace hard chromium coatings for their superior properties [12],and the process has advantage of considering environmental protection [13].Among the hard ceramic based nanoparticulates used for reinforcement,silicon nitride (Si 3N 4)has been frequently studied and used for its unique properties such as high hardness,high wear resistance,good inert chemical behav-ior and high temperature properties [14].However,little work has been done about the incorporation of Si 3N 4nanoparticulates in oxide based matrix,especially with high concentration.Just one∗Corresponding author.Tel.:+989126905626;fax:+982166960664.E-mail addresses:sabour@modares.ac.ir ,sabour01@ (A.S.Rouhaghdam).report [15]has been published on the fabrication,wear and rough-ness of high concentrated Si 3N 4nanoparticulates in oxide based ceramic matrix nanocomposite layer.In this paper,Si 3N 4/TiO 2gradient nanocomposite coatings [16]were fabricated by the novel method of anodic plasma electrolysis in an aqueous based electrolyte containing Si 3N 4nanoparticulates for achieving layers with high and a gradient of concentration for nanoparticulates based on the variable duty cycle of the applied pulsed current [17].The effects of the concentration of Si 3N 4nanoparticulates in the plasma electrolysis bath and effective parameters of coating process such as current density and the rate of variable duty cycle on the amount and gradient of Si 3N 4nanoparticulates co-deposited in the nanocomposite layers were evaluated.The surface morphologies,microhardness and incorpo-ration of nanoparticles for high concentrated Si 3N 4/TiO 2gradient nanocomposite coatings were studied.2.Experimental procedureThe plasma electrolysis bath is composed of an aqueous elec-trolyte containing chemically pure sodium aluminate (15g l −1),sodium-phosphate (2g l −1)(for denser coatings)and dissolved potassium hydroxide (for reaching a desirable electrical conduc-tivity of electrolyte)mixed with Si 3N 4fine nanoparticulates.Si 3N 4nanoparticulates with the concentration range of 2–10g l −1in the electrolyte were used.A 316stainless steel electrode was used as the anode mercially pure titanium (CP-Ti)discs0169-4332/$–see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2011.04.0482094M.Aliofkhazraei,A.S.Rouhaghdam /Applied Surface Science 258 (2012) 2093–2097Fig.1.BF TEM image of used Si 3N 4nanopowder.(20mm Ø×5mm)were used as the cathode materials and the substrates were mechanically polished and ultrasonically cleaned in acetone for 10min.Then they were immersed without delay in the plasma electrolysis bath.Prior to the coating process,the Si 3N 4nanoparticulates (with the average size of 37nm)were immersed in the electrolyte.Fig.1illustrates the TEM bright field image of the used nanopowder.The operating plasma electrolysis condi-tions were similar to other work [15].The bath temperature was maintained constant at 25◦C by heat exchanger.The samples were biased to the rectangular-shape pulsed current generated from two power supplies [18].The frequency was maintained constant at 10kHz while the duty cycle was changed from 20%to 80%with the steps equal to 1%during the total coating time.Three differ-ent rates (3,1.5and 1%/min)were applied for changing the duty cycle.Fig.2illustrates an example of the cross-section for obtained nanocomposite coating.Phase analysis of the coatings has been done in another report [15].The surface nanostructure and the composition of Si 3N 4/TiO 2-nanocomposite layers were studied by a scanning electron microscope (SEM,Phillips XL30)equipped with energy dispersive spectrometer (EDS).Vickers microhardness tests were done on Buhler Micromet 1microhardness tester using a load of 200g,applied for 20s.The average of five measurements was reported for each microhardness value.Atomic force microscope (AFM,NanoScope II)and transmission electron microscope (TEM,CM-120FEG Philips)were used for determining the surface char-acteristics and nanostructure of the coatings and nanopowder,respectively.Fig.2.SEM micrograph of cross section for obtained nanocomposite coating.3.Results and discussionFig.3illustrates the relationship between the weight percent-age (wt.%)of the embedded Si 3N 4nanoparticulates in the surface (top layer)of the nanocomposite coatings and the concentration of the nanoparticulates in the plasma electrolysis bath at the cur-rent density of 0.045A/cm 2and the stirring rate of 100rpm.It can be said that the weight percentage of the Si 3N 4nanopartic-ulates in the nanocomposite layers increases with the increase of Si 3N 4nanoparticulates content in the plasma electrolysis bath.The embedding of Si 3N 4nanoparticulates by the plasma electrolysis method may be recognized by the adsorption of Si 3N 4nanopar-ticulates on the surface of the specimen,as studied and reported by our earlier reports [15].Variable duty cycle caused more satu-rated layer and longer coating times (lower rates for variable duty cycle)caused higher silicon concentration in the layer.Higher con-centrations of Si 3N 4nanoparticulates in the plasma electrolysis bath enhanced the adsorption rate,thus leading to a higher weight percentage of the embedded Si 3N 4nanoparticulates [19].Fig.4illustrates the effect of current density on the embedded Si 3N 4content from a bath containing 10g l −1of Si 3N 4nanopar-ticulates being stirred at 300rpm.As shown,the content of the embedded Si 3N 4nanoparticulates increases with the current density.The weight percentage of the embedded Si 3N 4nanopar-ticulates becomes approximately constant for the variable duty cycle rates higher than 1.5%/min.The increasing content of Si 3N 4nanoparticulates can be related to the increasing of embedded nanoparticulates to arrive in the surface of sample.The whole coat-ing is restricted by the adsorption of the nanoparticulates.Since the rates of variable duty cycle are higher than 1.5%/min,nanoparticu-lates have lower times to move from the electrolyte to the surface of the sample.In this case,faster plasma electrolysis process leads to fewer embedded nanoparticulates in the nanocomposite coatings [20].Fig.5illustrates the effect of different rates of variable duty cycle on the content of the embedded Si 3N 4nanoparticulates from two different baths containing 2and 6g l −1Si 3N 4nanoparticulates at current density of 0.045A/cm 2.It is shown that the rates of variable duty cycle strongly affect the weight percentage of the embedded Si 3N 4nanoparticulates,because Si 3N 4should be transported to the surface of the sample for embedding with titanium oxide ceramic matrix.The weight percentage decreases with the increasing rate of the variable duty cycle and meets a minimum value at 3%/min in this research.When the rates of variable duty cycle are high (lower coating time),the embedding behavior of Si 3N 4nanoparticulates is apparently limited by the coating time.Fig.6illustrates the AFM image of both the pure TiO 2coat-ing and the Si 3N 4/TiO 2nanocomposite layer fabricated from the plasma electrolysis bath containing 8g l −1of Si 3N 4nanoparticu-lates.It can be seen that the Si 3N 4/TiO 2nanocomposite layer has lower roughness than the pure TiO 2coating.It is evident that the embedded Si 3N 4nanoparticulates are uniformly distributed in the TiO 2ceramic based matrix by plasma electrolysis.Therefore,in order to obtain nanocomposite layers containing homogenous dis-tributed Si 3N 4nanoparticulates,it is important to well immerse the Si 3N 4nanoparticulates in the electrolyte [21].As long as the Si 3N 4nanoparticulates are uniformly distributed in the layers,the Si 3N 4/TiO 2nanocomposite coatings could show excellent mechan-ical and electrochemical properties [22].Fig.7illustrates the relationship between the microhardness profile of the gradient Si 3N 4/TiO 2nanocomposite layers and the rate of variable duty cycle.It is seen that the Si 3N 4/TiO 2nanocom-posite coating fabricated with lower rate has higher microhardness.The reason of higher microhardness of the Si 3N 4/TiO 2nanocompos-ite coatings than the TiO 2coating is that the Si 3N 4nanoparticulates,uniformly distributed in the titania based matrix,could limit theM.Aliofkhazraei,A.S.Rouhaghdam/Applied Surface Science258 (2012) 2093–20972095Fig.3.Effect of the Si3N4nanoparticulates content in the plasma electrolysis bath on the weight percentage of Si3N4in the surface of nanocomposite layers at different rates of variable duty cycle.Fig.4.Effect of the current density on the weight percentage of Si3N4nanoparticulates in the nanocomposite coatings for different rates of variable duty cycle.Fig.5.Effect of the different rates of variable duty cycle on the weight percentage of Si3N4nanoparticulates in the nanocomposite coatings.2096M.Aliofkhazraei,A.S.Rouhaghdam /Applied Surface Science 258 (2012) 2093–2097Fig.6.AFM image of the surface morphology of (a)TiO 2coating,(b)Si 3N 4/TiO 2nanocompositelayer.Fig.7.Relation between microhardness and weight percentage of embedded Si 3N 4nanoparticulates in the nanocomposite layers.growth of the TiO 2grains and the deformation of the ceramic matrix under a loading through grain polishing [23]and dispersive strengthening mechanisms [24].The grain polishing and dispersive strengthening effects get stronger as the content of Si 3N 4nanopar-ticulates in the nanocomposite layers increases,thus leading to the increased microhardness of the Si 3N 4/TiO 2nanocomposite layers.As shown in Fig.7,the microhardness profile shows three dif-ferent zones which can be studied by their microstructures and phases.As described in other paper [15],coatings show agglom-erated hard particles in the surface (top coat)while have a barrier dense layer near the coating/substrate interface.Smooth micro-hardness profile continues in the first zone with approximately a sharp decrease of microhardness in the second zone and finally shows a flat profile near the substrate [25].It is also evident that lower rates of variable duty cycle cause more smooth microhard-ness profile,which is clearly better for the load bearing ability of the coating [26].4.ConclusionsPlasma electrolysis was done successfully for fabrication of high concentrated gradient Si 3N 4/TiO 2nanocomposite layer.Embed-ding of Si 3N 4nanoparticulates in TiO 2matrix depends on the concentration of Si 3N 4nanoparticulates in the plasma electroly-sis bath,the current density and the stirring rate.The content of Si 3N 4nanoparticulates in the layer increases with the increase of its concentration in the plasma electrolysis bath.The microhard-ness of Si 3N 4/TiO 2nanocomposite layers increases with the weight percentage of the embedded Si 3N 4nanoparticulates.Si 3N 4/TiO 2nanocomposite coatings fabricated with lower rate of variable duty cycle (longer treatment times)has higher microhardness with smoother microhardness profile.The microhardness profile showed three different zones,which are related to the structure and phase distribution of the nanocomposite coatings.AcknowledgementFinancially support of Iranian nanotechnology initiative is acknowledged.References[1]V.G.Ponomareva,vrova,L.G.Simonova,Effect of SiO 2morphology andpores size 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