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Corrosion resistance and wear resistance of plasma electrolytic oxidation coatings on metal matrix composites Shihai Cui , Jianmin Han, Yongping Du, Weijing Li School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China Available online 1 September 2006 Abstract Protective coating was successfully prepared on SiC p /A356 composites by plasma electrolytic oxidation(PEO) technique. XRD analysis showed that the PEO coating was mainly composed of α-Al 2 O 3 γ-Al 2 O 3 and mullite. The electrochemical corrosion behavior of coated and uncoated SiC p /A356 composites was tested in 3.5 wt.% NaCl solution by potentiodynamic polarization technique with low scanning rates. It was found that the corrosion potential was shifted significantly in the anodic direction and the corrosion current density was much lower due to the presence of the PEO coating. Additionally, the wear behavior of the PEO coating was also tested using a ring-on-ring wear tester. The PEO coating showed a low specific wear of 4.76 × 10 6 mm 3 /(N·m) against bearing steel at a speed of 0.52 m/s and a contact pressure of 0.75 MPa. © 2006 Elsevier B.V. All rights reserved. PACS: 52.80.wq Keywords: Metal matrix composites; Plasma electrolytic oxidation; Coating; Corrosion; Wear 1. Introduction Metal matrix composites (MMCs) are capable of providing excellent properties such as high specific strength and modulus, good wear resistance etc., which expands the range of potential application of the unreinforced metals. However, the addition of reinforcement to metal matrix creates its corrosion sensitivity because of the existence of two phases and their interface. Therefore, MMCs are very susceptible to corrosion. A suitable protective surface layer may act as a barrier between the MMC and the corrosive environment and improve their corrosion resistance. Anodic oxidation coatings and chemical conversion coatings can improve the corrosion resistance of MMCs to some extent. However, environmental problem and lower hardness and poor wear resistance confine their applications. A new anodizing technology named plasma electrolytic ox- idation (PEO) has been developed to treat nonferrous alloy to improve their corrosion resistance and wear behavior in recent years [15]. The PEO method has unique advantages over con- ventional anodizing processes. Pretreatments except for water rinses are not strictly necessary for PEO. Coatings obtained by this method are crystalline and have good corrosion resistance, wear resistance and excellent adhesion property to substrate, etc. Furthermore, electrolytes used in PEO process are weak alkaline solutions and are friendly to environment. Although there have been many investigations on the treatment of aluminum, magnesium and titanium alloy by PEO method [49], few inves- tigations have been done on MMCs using this method. In this work reported here, protective coating was successfully prepared on SiC p /A356 composites by PEO method. 2. Experimental details 20 vol.% SiC p /A356 (7.0% Si, 0.42% Mg, 0.20% Ti, b 0.12% Fe, b 0.10% Cu, b 0.05% Mn, b 0.05% Zn, Al re- mainder) composites manufactured by vacuum stir casing method were used as substrate material. Column samples (Φ32 × 7 mm) were anodized with a 75 kW PEO equipment mentioned in Ref. [7]. In the experiment, the samples and the stainless steel wall of the bath were used as two electrodes, respectively. The electrolyte was prepared from a solution of sodium aluminate and sodium silicate in de-ionized water with other additives. The electrolyte temperature was controlled to Surface & Coatings Technology 201 (2007) 5306 5309 www.elsevier.com/locate/surfcoat Corresponding author. Tel./fax: +86 10 5168 3300. E-mail address: [email protected] (S. Cui). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.126

Corrosion resistance and wear resistance of plasma electrolytic oxidation coatings on metal matrix composites

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Page 1: Corrosion resistance and wear resistance of plasma electrolytic oxidation coatings on metal matrix composites

201 (2007) 5306–5309www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Corrosion resistance and wear resistance of plasma electrolyticoxidation coatings on metal matrix composites

Shihai Cui ⁎, Jianmin Han, Yongping Du, Weijing Li

School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China

Available online 1 September 2006

Abstract

Protective coating was successfully prepared on SiCp/A356 composites by plasma electrolytic oxidation(PEO) technique. XRD analysisshowed that the PEO coating was mainly composed of α-Al2O3 γ-Al2O3 and mullite. The electrochemical corrosion behavior of coated anduncoated SiCp/A356 composites was tested in 3.5 wt.% NaCl solution by potentiodynamic polarization technique with low scanning rates. It wasfound that the corrosion potential was shifted significantly in the anodic direction and the corrosion current density was much lower due to thepresence of the PEO coating. Additionally, the wear behavior of the PEO coating was also tested using a ring-on-ring wear tester. The PEO coatingshowed a low specific wear of 4.76×10−6 mm3/(N·m) against bearing steel at a speed of 0.52 m/s and a contact pressure of 0.75 MPa.© 2006 Elsevier B.V. All rights reserved.

PACS: 52.80.wqKeywords: Metal matrix composites; Plasma electrolytic oxidation; Coating; Corrosion; Wear

1. Introduction

Metal matrix composites (MMCs) are capable of providingexcellent properties such as high specific strength andmodulus, good wear resistance etc., which expands the rangeof potential application of the unreinforced metals. However,the addition of reinforcement to metal matrix creates itscorrosion sensitivity because of the existence of two phasesand their interface. Therefore, MMCs are very susceptible tocorrosion. A suitable protective surface layer may act as abarrier between the MMC and the corrosive environment andimprove their corrosion resistance. Anodic oxidation coatingsand chemical conversion coatings can improve the corrosionresistance of MMCs to some extent. However, environmentalproblem and lower hardness and poor wear resistance confinetheir applications.

A new anodizing technology named plasma electrolytic ox-idation (PEO) has been developed to treat nonferrous alloy toimprove their corrosion resistance and wear behavior in recentyears [1–5]. The PEO method has unique advantages over con-

⁎ Corresponding author. Tel./fax: +86 10 5168 3300.E-mail address: [email protected] (S. Cui).

0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2006.07.126

ventional anodizing processes. Pretreatments except for waterrinses are not strictly necessary for PEO. Coatings obtained bythis method are crystalline and have good corrosion resistance,wear resistance and excellent adhesion property to substrate, etc.Furthermore, electrolytes used in PEO process are weak alkalinesolutions and are friendly to environment. Although there havebeen many investigations on the treatment of aluminum,magnesium and titanium alloy by PEO method [4–9], few inves-tigations have been done on MMCs using this method. In thiswork reported here, protective coating was successfully preparedon SiCp/A356 composites by PEO method.

2. Experimental details

20 vol.% SiCp/A356 (7.0% Si, 0.42% Mg, 0.20% Ti,b0.12% Fe, b0.10% Cu, b0.05% Mn, b0.05% Zn, Al re-mainder) composites manufactured by vacuum stir casingmethod were used as substrate material. Column samples(Φ32×7 mm) were anodized with a 75 kW PEO equipmentmentioned in Ref. [7]. In the experiment, the samples and thestainless steel wall of the bath were used as two electrodes,respectively. The electrolyte was prepared from a solution ofsodium aluminate and sodium silicate in de-ionized water withother additives. The electrolyte temperature was controlled to

Page 2: Corrosion resistance and wear resistance of plasma electrolytic oxidation coatings on metal matrix composites

Fig. 1. Cross-section morphology of PEO coating on SiCp/A356 composites.

Table 1The results of the potentiodynamic corrosion tests in 3.5 wt% NaCl solution

Samples Ecorr/V Icorr/(uA/cm2)

Anodic Tafelslope

Cathode Tafelslope

UncoatedSiCp/A356

−0.720 3.09 3.8 2.108

CoatedSiCp/A356

−0.681 1.65 4.173 4.265

5307S. Cui et al. / Surface & Coatings Technology 201 (2007) 5306–5309

remain lower than 45 °C during PEO process. The currentdensity varied in the range of 1000–1200 A/m2.

The morphologies and microstructures of PEO coatings wereobserved by a JSM-5800 scanning electron microscope (SEM).The phase of PEO coatings was investigated by using a RigakuD/Max2200 X-ray diffractometer (Cu target, 40 KV, 40 mA).

Fig. 2. XRD patterns of PEO coating on SiCp/A356 composites.

The corrosion behavior of coated and uncoated SiCp/A356composite was evaluated by both a salt spray corrosion test andelectrochemical corrosion test. The salt spray test was performedat 35 °C for 120 h under a neutral 5 wt.% NaCl aqueous sprayaccording to GB/T 10125-1997 standard. The electrochemicalcorrosion tests were carried out in a 3.5 wt.% NaCl solution in athree-electrode cell using a M283 model potentiostat. Acommercial Pt electrode was used as the counter electrode anda saturated calomel electrode (SCE) as the reference electrode.The samples were kept in NaCl solution for at least 1 h to stabilizeat their corrosion potential before the potentiodynamic test. Then,the polarization scan was started below the steady open-circuitpotential in the cathodic direction at a scanning rate of 1 mV/s.

The wear resistance of PEO coating was evaluated with aring-on-ring wear tester. Coated and uncoated SiCp/A356 sam-ples were used as a lower ring specimen and quenched bearingsteel GCr15(0.95∼1.05% C, 0.15∼0.35% Si, 0.25∼0.45%Mn, 1.4∼1.65% Cr, Fe remainder) sample with a hardness ofHRC60-62 was utilized as a counter ring. The test was carriedout in air without lubricant with a total sliding distance of1248 m at a sliding speed of 0.52 m/s and contact pressure0.75 MPa. The specific wear volume was estimated from themeasured weight loss.

3. Results and discussion

3.1. Coating microstructure and phase analysis

The PEO coating on SiCp/A356 is about 90 μm thick asshown in Fig. 1(a). Fig. 1(b) shows that the coating is mainlycomposed of two layers, the dense layer of 60 μm and the porouslayer of 30 μm. Except for some SiC particles on the interface,the interface between the coating and the substrate is clear. Fig. 2is a XRD pattern showing that the PEO coatings consist mainlyofα-Al2O3 γ-Al2O3 andmullite. It should be noted thatα-Al2O3

and mullite are the main phase in the coating. The mullite phase

Fig. 3. Variation of friction coefficient with the sliding distance.

Page 3: Corrosion resistance and wear resistance of plasma electrolytic oxidation coatings on metal matrix composites

Fig. 4. Specific wear rate of coated and uncoated SiCp/A356 against Gr15 aftersliding 1248 m.

Fig. 5. EDS results of worn surface of coated and uncoated SiCp/A356 againstGr15 after wear test.

Fig. 6. Micro-morphology of worn surfaces observed by SEM (a) uncoatedSiCp/A356 (b) coated SiCp/A356.

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was created from the electrolyte constituents (i.e. sodiumsilicate) during the plasma chemical reaction.

3.2. Corrosion resistance

The time to the first appearance of corrosion spots on thecoating surface in the salt spray test are recorded. It is found thatthe coated SiCp/A356 composite gives a near impenetrablecoating with no sign of corrosion after 120 hours exposurewhile corrosion spots appeared on uncoated SiCp/A356 com-posite after only about 12 h.

The corrosion potentials, corrosion rates and anodic, cathodicTafel slopes calculated from the electrochemical corrosion testare given in Table 1. The coated SiCp/A356 possesses muchhigher corrosion potential and lower corrosion rate than un-coated SiCp/A356 alloy. The corrosion potential Ecorr increasedfrom − 720 mV for the SiCp/A356 substrate to −681 mV for thePEO coating. And the corrosion current density of the coatedMMC decreased from 3.09 uA/cm2 to 0.165 uA/cm2. Thoughthe out layer of the coating is porous, the dense layer mainlycomposed of α-Al2O3 and mullite can act as a barrier betweenthe corrosion environment and substrate.

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3.3. Tribological behavior

Variation of the friction coefficient for coated and uncoatedSiCp/A356 against bearing steel GCr15 with the sliding distancewas plotted in Fig. 3 .The friction coefficient of coated SiCp/A356 against GCr15 ranges from 0.6 to 0.9 while the frictioncoefficient of uncoated SiCp/A356 ranges from 0.38 to 0.5.After sliding about 400 m, the coated SiCp/A356 gave a rela-tively low friction coefficient which ranges from 0.6 to 0.7. Thisis resulted by the worn of porous layer of the PEO coating afterabout 400 m.

Fig. 4 shows specific wear rate of coated and uncoated SiCp/A356 against GCr15 after 1248 m at a sliding speed of 0.52 m/sand a normal load of 300 N .Though the friction coefficient ofcoated SiCp/A356 against GCr15 is higher than that of uncoatedSiCp/A356, the specific wear of coated SiCp/A356 with4.76×10−6 mm3/(N·m) is about fourteen times lower thanthat of uncoated SiCp/A356 with 68.3×10−6 mm3/(N·m). Inpractical use, the wear rate of the counter material is alsoimportant. As shown in Fig. 4, the specific wear of bearing steelagainst coated SiCp/A356 is a little larger than that againstuncoated SiCp/A356. EDS analysis were performed to deter-mine the composition of worn surface of coated and uncoatedSiCp/A356. Results as shown in Fig. 5 reveal that the relativecontent of Fe and Cr on coated SiCp/A356 (Fig. 5(b)) is largerthan that on uncoated SiCp/A356 (Fig. 5(a)). This implies thatthe transfer of element Fe and Cr to the surface of coated SiCp/A356 increased because of the presence of PEO coating. This isresulted by the higher friction coefficient and microhardness ofcoated SiCp/A356 against GCr15.

The micro-morphology of worn surfaces of coated anduncoated SiCp/A356 observed by SEM is shown in Fig. 6. Itcould be observed that the coated SiCp/A356 presents a sig-nificantly smoother track than uncoated SiCp/A356. Obviousgroove marks appeared on the uncoated MMC. Only smallgrooves on the coated SiCp/A356 can sometimes be foundparallel to the sliding direction indicating that some plasticdeformation of the coating has occurred. Except this, no fracture,crack and collapse of coated SiCp/A356 took place during thewear test.

4. Conclusion

PEO coating was successfully fabricated on SiCp/A356composites in spite of the existence of nonconductive SiCparticles. The coating was mainly composed of two layers, thedense layer of 60 μm thickness and the porous layer of 30 μmthickness. Both salt spray test and electrochemical measurementshowed that PEO coating could improve the corrosion resis-tance of SiCp/A356 significantly. The friction coefficient ofcoated SiCp/A356 against GCr15 which ranged from 0.6 to 0.9was higher than that of uncoated SiCp/A356 which ranged from0.38 to 0.5. The specific wear of coated SiCp/A356 was aboutfourteen times lower than that of uncoated SiCp/A356. Nofracture, crack and collapse of coated SiCp/A356 took placeduring the wear test.

Acknowledgements

This work is supported by the National High TechnologyResearch and Development Program of China (863 Program)under Grant No. 2003AA331190, and by the fund of BeijingJiaotong university and by the open fund of state key laboratoryof tribology of Tsinghua University under Grant No. SKLT02-4.

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