Influence of surface etching pretreatment on PEO process of eutectic Al–Si alloy☆

Kang Li ,Wenfang Li ,*,Guoge Zhang ,Min Wang ,2,Peng Tang ,3

1 School of Materials Science and Engineering,South China University of Technology,Guangzhou 510641,China

2 College of Mechanical Engineering,Guangdong Polytechnic Normal University,Guangzhou 510635,China

3 School of Materials Science and Engineering,Guangxi University,Nanning 530004,China

Keywords:Plasma electrolytic oxidation Eutectic Al–Si alloy Pretreatment Etching

A B S T R A C T To solve the problems generally encountered during the plasma electrolytic oxidation(PEO)of Al alloys with high Si content,a pretreatment of chemical etching was applied before the process.The influence of such pretreatment was studied by SEM,EDS and XRD.The pretreatment presents a significant effect on positive voltage at the beginning stage of PEO,leading to higher voltage over the whole process.The difference between the positive voltages of non-etched and etched specimens decreases gradually with the increase of processing time.The pretreatment exhibits much less influence on the negative voltage.For the sample with surface pretreatment,the average growth rate of PEO coating is increased from 0.50 to 0.84 μm·min?1 and the energy consumption is decreased from 6.30 to 4.36 kW·h·μm?1·m?2.At the same time,both mullite and amorphous SiO2 contents are decreased in the coating.

1.Introduction

Eutectic Al–Si alloys are widely used in the manufacturing industry owing to the advantages such as excellent stability,favorable mechanical properties and low cost[1,2].However,the surface property of Al–Si alloys(hardness,abrasion resistance,etc.)is not satisfactory.It is necessary to enhance their surface property to meet some harsh operating demands.Although anodic oxidation is a conventional surface modification technique,it is difficult to be applied on the Al alloys with high Si content[3,4].Due to the semiconducting nature of Si,it is difficult to fabricate a uniform anodic film[5].

Plasma electrolytic oxidation(PEO)is a fast method to synthesize insitu ceramic coatings on lightweight alloys(Al,Mg and Ti)[6].The PEO layers possess excellent properties,such as strong adhesion strength,low thermal conductivity,high hardness,good abrasion and excellent corrosion resistance[7,8].PEO has advantages in the surface strengthening of casting Al–Si alloys owing to the high current density and voltage,which will benefit the oxidation of Si[9].Nevertheless,there are some problems in the PEO processing of eutectic Al–Si alloy.Amount of eutectic Si greatly limits the growth rate of PEO layer[10].It is also the cause of a porous inner layer structure[11],owing to the evolution of large amount of gas(O2)during the oxidation of Si phase[5].The PEO coating on Al–Si alloys is rich in mullite and demonstrates inferior property[11].High Si content is also responsible for high current leakage and significantly reduces the PEO energy efficiency[11–14].Although extensive studies have been carried out to improve properties and energy efficiency of PEO coatings on Al alloys by the optimization of electrical parameters or electrolyte[12,15–17],little progress has been made in the PEO of Al alloys with high Si content.

Since the surface condition of alloy is the key to a PEO process and Si can be selectively removed by certain surface treatment[12,18,19],a pretreatment technique will be applied on eutectic Al–Si alloy before the PEO process.The effects of pretreatment on the formation,characteristics and energy consumption of the PEO coating will be investigated in this study.The objective is to develop a facile and effective chemical etching technique to increase the growth rate of PEO coating on eutectic Al–Si alloy and reduce the energy consumption.

2.Experiments

2.1.Coating preparation

The experimental materials were pure Al and a binary Al–Si casting alloy with Si mass content of about 12%.The size of the samples was 20 mm×20 mm×4 mm.They were polished by 1000 grit water proof sand paper,and then part of the eutectic Al–Si samples were dipped in a mixed acid for 30 s,which was composed of HNO3(65%,by mass)and HF(40%,by mass)with a volume ratio of 4 to 1.Before the PEO treatment,all the specimens were ultrasonic degreased in acetone for 15 min,then rinsed with distilled water followed by hot air drying.

The electrolyte,composed of Na2SiO3,NaOH and C6H12N4,was stirred during the PEO by a magnetic stirring apparatus,and it was cooled by a water circulating device to keep the temperature below 40°C.A stainless steel sheet was served as the cathode.The power supply for oxidation was a pulsed bipolar current supply(60 kW),equipped with a data logging system to record the electrical parameters.The applied positive and negative current density was fixed at 10 and 4 A·dm?2,respectively.The impulse frequency was set at 400 Hz and the duty cycle was 25%.

2.2.Coating characterization

The thickness of the PEO layers was the average value of ten points measured by a Surfix N eddy current thickness meter.The surface roughness was the mean value of four positions tested by a Surface Plasmon Resonance Analyzer(D-150),with a scan length of 6 mm.A field emission scanning electron microscope(Nano 430),equipped with an energy dispersive spectrometer,was used to study the morphology and element composition of samples.The crystalline structure of PEO coatings was determined by an X-ray diffraction(Rigaku Miniflex 600),with a step size of 0.02°and a scan range from 20°to 70°(in 2θ).

3.Results and Discussion

3.1.Voltage variation

The variation of PEO voltage of specimens is shown in Fig.1.During the anodic oxidation stage,the voltage ascent rate of the etched sample was much faster than that of the non-etched one,but a little slower than that of pure Al.With the increase of positive voltage,more and more gas bubbles generated on the surface of all specimens.Some visible discharge sparks began to emerge on the etched sample about 2 min earlier than that on the non-etched one.Several minutes later,the positive voltage increasing rate of all samples slowed down gradually.The arcs on the etched specimen were more drastic than that on the non-etched one at the same time and part of the arcs on the etched specimen were much larger and brighter than the rest(Fig.2).Over the whole PEO,the positive voltage of the etched sample was always higher than that of the nonetched one at the same moment,but their value difference decreased gradually with the PEO.Finally,the positive voltage of the etched sample reached 470 V,about 20 V higher than that of the nonetched one.As to the negative voltage,the difference between the non-etched and etched specimens was small.The negative voltage value of the etched specimen was a little higher during the discharge oxidation period.It finally reached 33 V and was about 10 V higher than that of the non-etched sample.

Fig.1.Positive and negative voltage variation with PEO time.

3.2.Characteristics of PEO coatings

3.2.1.Surface morphology

The surface morphology of the layers on the non-etched and etched Al–Si alloys with different PEO time is shown in Fig.3.After 2 min oxidation,an anodic oxide film formed on the non-etched sample[Fig.3(a)].On the other hand,both the tiny holes caused by discharge channels and the stripe-like erosion marks were observed on the etched sample[Fig.3(b)].When treated for 5 min,lots of discharge holes appeared on the non-etched specimen[Fig.3(c)]but their distribution was non-uniform.The surface of the etched sample became much smoother as the etching pits disappeared mostly[Fig.3(d)].After 10 min processing,the holes in the PEO coating of the non-etched sample became much more uniform due to complete coverage of the sparks on its surface[Fig.3(e)].No etching trace was found on the etched sample and the PEO coating porosity was prominently decreased[Fig.3(f)].When the PEO processing was extended to 30 min,the holes were bigger and much lesser for both etched and un-etched specimens[Fig.3(g)and(h)].Some debris was left around the discharge channels and it was larger for the etched sample.

3.2.2.Element on the PEO coating surface

Variation of the coating composition with PEO processing time is shown in Fig.4.The main elements of the PEO coating were Al,O and Si for both non-etched and etched specimens.Al content decreased rapidly with the extension of PEO treatment time,although it was stable over the last 10 min for the non-etched sample.O content in the coating of both specimens increased gradually during the first 5 min treatment,followed by a more stable state up to 30 min treatment.The Si content in the coating of the non-etched sample showed a small variation during the first 5 min,followed by a linear increase to 20 min and decreased a little over the last 10 min treatment.On the contrary,the Si content in the coating of the etched sample kept increasing during the 30 min PEO process.It is noted that the element content in PEO coating is not remarkably influenced by the surface pretreatment after the 30 min PEO process.However,for the sample treated by shorter time,the Si content is always lower in the coating of the etched sample than that in the coating of the non-etched one.

3.2.3.Thickness

The relationship between the coating thickness and PEO time for non-etched/etched Al–Si alloy is shown in Fig.5.The coatings on both samples became thicker and thicker as the PEO went on.The growth rate of the coating was much slower for the non-etched sample after 20 min.With the same processing time,the coating on the etched sample was always thicker than that on the non-etched one.The coating thickness of the etched sample reached 25.2 μm after 30 min processing,about 10 μm thicker than that of the non-etched sample.

3.2.4.Surface roughness

Fig.6 shows the variation of surface roughness of PEO coatings with processing time.The variation is similar to that of the thickness.With the same treatment time,the coating on the etched specimen is always rougher than that on the non-etched one.After the 30 min PEO,the surface roughness of the layer on the etched specimen increases to 2.29 μm,about 1.4 times of that on the non-etched one.

3.3.Analysis of the coatings

3.3.1.Phase constituent

Fig.7 shows the phase constituents of the PEO layers on the nonetched and etched Al–Si alloys with 30 min treatment.Besides Al and Si from the matrix,γ-Al2O3,α-Al2O3and mullite are clearly revealed[20].Some amorphous phase has been detected and attributed to SiO2[9].It is evident that the layer on the etched sample is composed of a much higher Al2O3but a lower SiO2content than the layer on the nonetched one.

Fig.2.Discharge arcs on the non-etched(a)and etched(b)alloys at the 5th min of the PEO,and arcs on the non-etched(c)and etched(d)alloys at the 10th min of the PEO.

3.3.2.EDS analysis

EDS analysis for the PEO coatings is shown in Fig.8.The substrate/coating interfaces are indiscernible for both samples,indicating good adhesion.From the substrate to the substrate/coating interface,Al content decreases rapidly while O and Si contents increase.From the inner to the outer part of the coating,O and Si contents decrease first and then increase slowly,while Al content increases first and then keeps stable.For the layer on the etched sample,its Al,O and Si contents fluctuate from the 25th to the 30th μm of the scanning distance[Fig.8(d)].This may be resulted from the polish of the coated sample.Close to the surface of both layers,the Al content decreases obviously while O and Si contents increase.

3.4.Energy consumption

The electric energy consumed by the PEO was determined by the electrical parameter acquisition system.After 30 min processing,the energy consumed was 0.105 and 0.123 kW·h for the non-etched and etched samples,respectively.Considering the thickness and surface area(～1.12×10?3m2)of the coating,the specific energy consumption is calculated by

where q is the specific energy,Q is the total energy consumed,S is the surface area,and δ is the thickness.The specific energy consumption for the non-etched sample is 6.30 kW·h·μm?1·m?2,while that for the etched one is 4.36 kW·h·μm?1·m?2.Such result indicates that the pretreatment is an effective approach reducing the energy consumption during the PEO treatment of eutectic Al–Si alloy.

3.5.Mechanism of etching

3.5.1.Influences of etching on the substrate

After the mechanical polishing,the Al–Si alloy appears relatively flat[Fig.9(a)]with the surface roughness about 0.54 μm.The Si content near the surface is about 18.54 wt%[Fig.9(d)]and higher than the average value of the matrix.The reason can be explained from the fact that Al is softer and easier to be ground than Si during the polishing.

The surface state of the polished eutectic Al–Si substrate is remarkably altered after the mixed acid treatment.The surface becomes much rougher with a roughness of about 1.01 μm.Numerous stripelike pits about 1–1.5 μm deep appear[Fig.9(b)and(c)]due to the dissolution of Si by HF[Fig.9(e)and(f)].

At the same time,Al is protected by a thin Al2O3layer resulted from the passivation in concentrated HNO3[21].

No O element is detected on the surface of the etched sample[Fig.9(e)]as the result of small passive film thickness(several nm)[22].The existence of the nano-structured Al2O3film is confirmed by the composition analysis near the surface of the etched sample[Fig.9(c)and(f)].

3.5.2.Influences of etching on the formation of PEO coating

The etching treatment removes most of Si in the skin layer of the eutectic Al–Si alloy.At the beginning stage of PEO,Al is the main component to be oxidized.The passive layer on the Al phase is the reason for faster increase of positive voltage for the etched sample.At the same time,the surface area of etched substrate increases due to the existence of etching pits.The actual current density decreases as the result,leading to a slightly lower growth rate of PEO coating in comparison to the pure Al sample.Consequently the positive voltage increases slower for the etched specimen compared with the pure Al as shown in Fig.1.As to the specimen without etching,large amounts of Si at the surface prevent the formation of a uniform film with high resistance because only a thin passivation layer(～10 nm)can be deposited on Si by chemisorption[10].Another cause to the smaller resistance of the PEO coating on the non-etched sample is the structural defect such as grain boundaries between alumina and Si.Therefore,a non-uniform layer forms on the surface of the non-etched sample and the positive anodizing voltage increases much slower.

Fig.4.Variation of element content in the PEO coating with processing time for non-etched(a)and etched(b)specimens.

Fig.5.Variation of coating thickness with PEO time.

Fig.6.Variation of coating surface roughness with PEO time.

At the beginning of the discharge oxidation stage,the distribution of discharge pores on the non-etched sample is not as uniform as that on the etched sample,which is caused by the preferential dielectric breakdown at the boundary between Al and Si phases[23].The positive voltage for the etched specimen is higher and the discharge sparks are consequently intenser.Many tiny gas bubbles generate in the etching pits and develop to much larger bubbles during their rise.Occasionally large sparks appear when bubbles are broken by the high electric field[24].Such intense sparks bring a high temperature and result in large amounts of reaction product depositing around the discharge channels,leading to a high coating growth rate.And the erosion pits on the etched substrate are gradually filled by the chilled melt.

Fig.8.Cross-sectional image of the coatings by 30 min PEO treatment on the non-etched(a)and etched(b)specimens,and EDS line scanning of the non-etched(c)and etched(d)specimens.

Fig.9.Surface morphology of the non-etched(a)and etched(b)substrates,cross section of the etched substrate(c),and EDS analysis from the surface of the non-etched(d),etched(e)substrates and region 1 of the profile of etched substrate(f).

With the increase of positive voltage,the sparks become larger and intenser little by little.The mean dimension of discharge sparks is about 1–2 mm[25],which greatly exceeds the coating thickness.The temperature of the discharge events may be higher than 4000 K[24],and cause not only the local coating at the discharge sites but also the melting of Al and Si matrix below the coating.Then these substances sputter out of the discharge channels and react with O2(from H2O electrolysis)and(from the electrolyte)[9],and most of their resultants will be chilled by the electrolyte and deposit on the sample surface.Hence,the PEO coatings on both samples grow thicker with the treatment time,with a rougher surface.However,SiO2has a low critical cooling rate to form amorphous phase during the quenching of the melt[9].This may be the main reason why SiO2amorphous exists in both PEO coatings.Since the Si proportion in the skin layer(several micron deep)of the etched sample is greatly decreased,less Si is oxidized during its PEO treatment,which eventually results in a layer with much less mullite and SiO2amorphous compared with that on the non-etched one.Besides,the leakage current during the PEO processing of the etched sample will be decreased,and more positive current can be utilized for the formation of oxide layer[26].Consequently,the specific energy consumption of the PEO layer on the etched Al–Si alloy is much less,while its average growth rate is much higher.

4.Conclusions

In this study,the effect of an acid etching on the PEO of eutectic Al–Si alloy was investigated.The pretreatment presents a significant effect on positive voltage at the beginning stage of the PEO,leading to a higher voltage and a faster coating growth rate(from 0.50 to 0.84 μm·min?1)during the whole process.The energy consumption is successfully decreased from 6.30 to 4.36 kW·h·μm?1·m?2.At the same time,both mullite and amorphous SiO2contents are decreased in the coating.As a facile and effective approach,the surface etching technique is helpful in solving the problems encountered during the PEO processing of eutectic Al–Si alloy.