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AC plasma electrolytic oxidation of magnesium with zirconia nanoparticles
Applied Surface Science 254 (2008) 6937–6942
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
AC plasma electrolytic oxidation of magnesium with zirconia nanoparticles R. Arrabal a, E. Matykina a, F. Viejo a, P. Skeldon a,*, G.E. Thompson a, M.C. Merino b a b
Corrosion and Protection Centre, School of Materials, The University of Manchester, Sackville Street, P.O. Box 88, Manchester M60 1QD, UK Departamento de Ciencia de Materiales, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 April 2008 Received in revised form 29 April 2008 Accepted 29 April 2008 Available online 7 May 2008
The incorporation of monoclinic zirconia nanoparticles and their subsequent transformation is examined for coatings formed on magnesium by plasma electrolytic oxidation under AC conditions in silicate electrolyte. The coatings are shown to comprise two main layers, with nanoparticles entering the coating at the coating surface and through short-circuit paths to the region of the interface between the inner and outer coating layers. Under local heating of microdischarges, the zirconia reacts with magnesium species to form Mg2Zr5O12 in the outer coating layer. Relatively little zirconium is present in the inner coating layer. In contrast, silicon species are present in both coating layers, with reduced amounts in the inner layer. ß 2008 Elsevier B.V. All rights reserved.
PACS: 81.65. b 81.65.Mq Keywords: Magnesium Coatings Plasma electrolytic oxidation Zirconium oxide Anodizing
1. Introduction Plasma electrolytic oxidation (PEO) is used for the surface protection of magnesium alloys; several commercial processes are available [1–3]. Processes are also being developed that provide coatings with improved properties. The coatings are formed at high voltages, in various, usually aqueous, electrolytes, when microdischarges are present on the alloy surface [4]. The mechanism of formation of the coating potentially involves anodic oxidation, thermal oxidation and plasma-chemical reactions [5]. The growth of the coating is accompanied by relatively profuse generation of oxygen gas. Coatings can be grown under DC and AC conditions, with a range of waveforms, leading to coatings that generally thicken roughly in proportion to the charge passed [6]. Thicknesses of tens of microns are readily produced, containing amorphous and crystalline constituents dependent on the particular alloy and electrolyte [7]. Magnesium oxide is generally present in the coatings, often accompanied by phases related to the electrolyte anions, for instance magnesium silicate in silicate electrolyte. In cross-section, two main layers of DC coatings may be discerned; the inner layer has a finer texture than the outer layer. At the base
* Corresponding author. Tel.: +44 161 306 4872; fax: +44 161 306 4865. E-mail address: [email protected] (P. Skeldon). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.100
of the inner layer, a relatively thin, compact barrier region is located at the alloy/coating interface, with a thickness of up to l mm. Short-circuit transport of electrolyte components through the outer layer has been shown to occur [7–9]. This allows anion species of the electrolyte, or possibly derivatives formed in the microdischarge, ready access to the inner coating. Cation species can also be transported across the outer coating, although in greatly reduced amounts compared with the anion species [10]. In addition, nanoparticles in suspension in the electrolyte can reach the inner layer via the short-circuit paths, which has been demonstrated using monoclinic zirconia [11]. The nanoparticles are also incorporated into the outer part of the coating, which reveals a modified microstructure compared with coating formed in the absence of nanoparticles. Tetragonal zirconia is detected in significant amounts in the final coating, due to the high temperatures generated at microdischarge sites. In the present study, the influence of AC treatment on the incorporation of zirconia is investigated for magnesium treated in silicate electrolyte. The incorporation of nanoparticles is relevant to understand the mechanism of coating growth and for the potential to improve coating properties through modification of the coating microstructure [12]. The main interest of the work concerned the microstructure and morphology of the coatings, especially the distribution of zirconium and the types of zirconia-containing phases.
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2. Experimental Specimens of 99.9% magnesium sheet were embedded in resin, with electrical connection provided through a shielded copper wire. The exposed surface, of area 1 cm2, was ground to 1200 grit SiC, using water as a lubricant, degreased in ethanol, rinsed in deionized water and dried in warm air. PEO treatment was carried out at 200 mA cm 2 (rms) at 50 Hz with a sinusoidal waveform in 0.025 M Na2SiO35H2O/0.05 M KOH for up to 2400 s. The electrolyte was prepared from deionized water and high-purity chemicals. Additions of 10 g/l of monoclinic zirconia nanoparticles, of size 150–300 nm, were made as required. The anodizing condition was selected following evaluation of a range of current densities and electrolytes compositions for incorporation of zirconia into the coating. The coatings were formed in a 1 l double-walled glass cell through which a cooled water/glycol mixture was pumped in order to keep the electrolyte temperature close to 293 K. During the treatment, the electrolyte was stirred continuously, which kept the zirconia nanoparticles in suspension. A sheet of type 304 stainless steel, of size 7.5 cm 15 cm, was used as the cathode. Voltage responses were recorded electronically, with a sampling time of 20 ms, employing an SCXI data acquisition system (National Instruments) with data analysis by Igor Pro (Wavemetrics). After coating, specimens were rinsed with deionized water and dried in warm air. PEO-treated specimens were examined by field emission gun scanning electron microscopy, using a Philips XL30 instrument equipped with energy-dispersive X-ray (EDX) analysis facilities. Cross-sections were ground through successive grades of SiC paper, followed by finishing to 1 mm diamond. Phase composition was investigated by X-ray diffraction (XRD), using a Philips X’PertMPD (PW 3040) instrument with a step size 0.0058 and a scan range from 5 to 858 (in 2u). 3. Results Voltage–time responses were practically the same during PEO without and with zirconia in the electrolyte, with commencement of sparking at 250 V coinciding with the major change in slope between the initial period of relatively rapid voltage rise and the start of the main period of slow voltage increase (Fig. 1). Without zirconia in the electrolyte, the surface of the resultant coating comprises apparently overlapping, roughly circular features (Fig. 2(a)). The shapes of the features suggest outward flow of material along a channel through the underlying coating to the coating surface, followed by its spreading over the coating surface and solidification. The largest of the features have dimensions of up to 50 mm. The surface is significantly rough and pores and cracks are evident. In addition, micron-sized silicon-rich particles, with
Fig. 1. Voltage–time responses during AC PEO treatment of magnesium at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH, without and with addition of 10 g/l of monoclinic zirconia.
the appearance of a deposit, are also present; such particles are often found following PEO in silicate electrolyte [13]. The coating roughness and pores are clear in cross-sections of the coating (Fig. 2(b)). The thickness of the coating at the thickest regions is about twice that at the thinnest regions, with inclusion of porosity in the thickness measurements. Two main layers are also revealed, with the inner layer being finely porous, whereas the outer layer has relatively large pores, cavities and channels. There are also regions of finer pores in the outer layer and of larger pores in the inner layer although these are of relatively minor extent. Cracks are mostly evident in the outer layer. Magnesium, oxygen and silicon are detected in both layers by EDX point analyses; the atomic ratios of Si:Mg were 0.28 0.08 and 0.10 0.04 in the outer and inner layers, respectively (points A and B, Fig. 2(b)). The corresponding O:Mg ratios were 3.1 0.4 and 2.6 0.3. The surface of the coating formed in electrolyte containing zirconia nanoparticles revealed two distinct types of region in backscattered electron images (Fig. 3(a)). An extensive, irregular network of light regions corresponded to coating decorated by numerous zirconia nanoparticles. In contrast, relatively dark, roughly circular zones, of size 50–100 mm, are mainly free of nanoparticles, but disclose dendrites of size 1–2 mm (Fig. 3(b)). Non-uniformity in the distribution of zirconia particles at the coating surface was also observed following DC treatment in an alkaline phosphate electrolyte [11]. Cracks and pores are found in both types of region of the coating. Further, EDX point analyses revealed the presence of silicon and zirconium, with Si:Mg and
Fig. 2. Scanning electron micrographs (secondary electrons) of magnesium following AC PEO treatment for 2400 s at 200 mA cm KOH. (a) Plan. (b) Cross-section.
2
(rms) in 0.025 M Na2SiO35H2O/0.05 M
R. Arrabal et al. / Applied Surface Science 254 (2008) 6937–6942
Fig. 3. Scanning electron micrographs (backscattered electrons) of the surface of magnesium following AC PEO treatment for 2400 s at 200 mA cm Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (a) General view. (b) Region with relatively few nanoparticles.
Zr:Mg ratios of 0.26 0.04 and 0.26 0.06, respectively in the region containing micron-sized dendrites (point C, Fig. 3(a)) and 0.33 0.08 and 0.55 0.14, respectively, in the region covered by zirconia nanoparticles (point D, Fig. 3(a)). In cross-section, the coating formed with zirconia in suspension in the electrolyte comprises two main layers (Fig. 4(a and b)). The
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2
(rms) in 0.025 M
inner layer has a relatively fine porosity. The outer layer is more compact, and with reduced porosity compared with the outer layer of coatings formed in the absence of zirconia in the electrolyte. Cavities appear to be prevalent near the interface between the inner and outer layers. The X-ray elemental maps show the presence of magnesium, oxygen and silicon in most coating
Fig. 4. (a) Scanning electron micrograph (backscattered electrons) of a cross-section of magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (b) Detail of region between the inner and outer coating layers. EDX elemental maps are shown for magnesium, oxygen, silicon and zirconium of the cross-section of (a).
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Fig. 5. Scanning electron micrographs (backscattered electrons) of cross-sections of magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (a) Surface region with relatively few nanoparticles. (b) Surface region with nanoparticle layer. (c) Detail of (a). (d) Detail of (b).
regions, with a reduced amount of silicon in the inner layer (Fig. 4). Zirconium was mainly a component of the outer layer. EDX point analyses disclosed Si:Mg and Zr:Mg ratios of 0.26 0.07 and 0.16 0.05, respectively, in the outer layer (point E, Fig. 4(a)) and 0.10 0.03 and 0.02 0.01, respectively, in the inner layer (point F, Fig. 4(a)), confirming the moderately reduced concentration of silicon and greatly reduced concentration of zirconium in the inner layer. The Si:Mg ratios in the two layers were similar to those determined for the coating formed in zirconia-free electrolyte. Zirconia nanoparticles, similar in size to those present in the electrolyte, are prevalent near the interface between the inner and outer layers and within larger cavities and channels in the outer layer (Fig. 4(b)). Zirconium is also generally distributed throughout the outer coating at locations largely free of nanoparticles. The outer coating in these latter regions contains numerous dendrites roughly similar in size and shape to those observed at the coating surface (Fig. 5(a and c)). Observation at another region of the cross-section, corresponding to the network of nanoparticles observed at the film surface, revealed that the nanoparticles were incorporated into a layer of thickness 1–2 mm (Fig. 5(b and d)). EDX point analysis indicated Si:Mg and Zr:Mg ratios of 1.33 0.31 and 1.09 0.46, respectively, suggesting trapping of nanoparticles within silicon-rich material formed at the coating surface (point G, Fig. 5(b)). The underlying coating microstructure disclosed discrete light particles, of size suggestive of incorporated nanoparticles, mainly within a relatively light network of matrix material (Fig. 5(d)). The latter surrounded micron-sized islands of relatively dark material. EDX point analyses indicated Zr:Mg ratios of 0.16 0.06 and 0.18 0.05 and Si:Mg ratios of 0.16 0.03 and 0.17 0.03 in the darker and lighter materials, respectively (points H and I, Fig. 5(d)), although the results are probably affected by the large size of the analysis volume relative to the dimension of the zones and hence, represent average compositions for the microstructure. The kinetics of coating growth were approximately linear, with average rates of 15.8 and 18.0 nm s 1 without and with zirconia, respectively (Fig. 6). The thickness of both the inner and
outer coating layers increased with increase of treatment time. The inner layer thickness appeared to increase relative to the total film thickness, from 20% of the film at 10 min to 40% at later times. XRD revealed strong peaks for MgO and weaker peaks for Mg2SiO4 in the coating formed in electrolyte without nanoparticles (Fig. 7). These peaks were also found for the coating produced with nanoparticles, with relatively reduced peaks for Mg2SiO4 and the additional presence of peaks for monoclinic zirconia and Mg2Zr5O12. Coatings formed for 10 min in electrolyte with nanoparticles followed by 10 min in electrolyte free of nanoparticles, and vice versa, also resulted in two-layered coatings (Fig. 8). For the
Fig. 6. Dependence of coating thickness on time for AC PEO treatment of magnesium at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH without and with addition of 10 g/l of monoclinic zirconia.
R. Arrabal et al. / Applied Surface Science 254 (2008) 6937–6942
Fig. 7. XRD data for magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH without (bottom) and with (top) addition of 10 g/l of monoclinic zirconia.
former sequence, occasional regions of material with incorporated nanoparticles were present in the final coating, usually at the coating surface, within cavities and near the inner layer/ outer layer interface (Fig. 8(a)). EDX analyses indicated the presence of zirconium species in the outer layer material, with a Zr:Mg ratio of 0.09 0.05 (point J, Fig. 8(a)); no significant amount of zirconium was detected in the inner layer (point K, Fig. 8(a)). The respective Si:Mg ratio were 0.21 0.04 and 0.19 0.05, indicating less silicon in the inner layers. For the reverse sequence, increased amounts of zirconia nanoparticles were present at the coating surface, within cavities and near the inner layer/outer layer interface (Fig. 8(b)). Surface regions with much reduced numbers of nanoparticles were also identified, similar to the findings for the previous single PEO treatment with nanoparticles. Point analyses through the coating thickness disclosed reducing Zr:Mg ratios of 0.07 0.05, 0.06 0.05 and 0.03 0.01 with increasing depth in the coating (points L–N, Fig. 8(b)). A similarly reducing trend was found for Si:Mg ratios, namely 0.23 0.03, 0.20 0.05 and 0.14 0.05. 4. Discussion The two-layered coatings observed previously following DC PEO treatment of magnesium in silicate electrolyte in the presence and
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absence of a zirconia nanoparticle suspension [11] are also formed under the present AC conditions. Similarly, the transport of zirconia nanoparticles along short-circuit paths, through the outer layer to the inner layer, is also revealed. Irrespective of the presence of zirconia, the coatings contained magnesium species from the substrate and silicon species from the electrolyte, with detection of crystalline MgO and Mg2SiO4. Further, the inner layer contained a reduced amount of silicon, with typical Si:Mg ratios from EDX point analyses of 0.10 0.04 compared with 0.26 0.04 in the outer layer. For coating containing zirconium species, the local high temperatures of coating growth result in formation of dendrites in the outer coating layer, and generation of Mg2Zr5O12, which is reported to be stable in the temperature range 2123–2373 K [14]. This phase is presumed to be a constituent of the outer coating, since, relatively little zirconium is found within the inner layer, for which EDX point analyses indicated a Zr:Mg ratio of 0.02, compared with 0.09–0.16 in the outer layer. Mg2Zr5O12 was not detected by XRD in coatings formed in silicate electrolyte under DC conditions with a current density of 30 mA cm 2 and a reduced concentration of zirconia in the electrolyte, 2 g/l [11]. However, generation of tetragonal zirconia occurred under DC treatments with silicate electrolyte and both tetragonal zirconia and Mg2Zr5O12 in phosphate electrolyte. The absence of tetragonal zirconia in the AC coatings, contrary to findings for a DC coating, is due to the different thermal conditions in the coatings, related to the energies dissipated in the individual discharge events. The latter will depend on the local current density of the discharge sites and the size and lifetime of the discharge. The presence of tetragonal zirconia under the DC conditions compared with the presence of Mg2Zr5O12 under AC conditions suggests higher temperature in the discharges for the latter. Monoclinic zirconia is also detected in the present coatings, and those formed previously under DC conditions, due to incorporation of zirconia that either does not transform under the local conditions of temperature and pressure, or reverts to the monoclinic form on cooling. The occurrence of dendrites indicates local melting in the outer layer. The melting point of the outer layer material will depend upon the precise composition of the coating. The melting points of relatively pure MgO and Mg2SiO4 are 3073 and 2163 K, respectively. The extensive cracking of the coatings is presumed to arise from stresses in the coating associated with volume changes due to coating growth and phase transformation, differential thermal expansion within the coating and substrate, and the pressure of oxygen and hydrogen generated during anodic and cathodic half-cycles, respectively [15]. Zirconia nanoparticles were incorporated into the coating surface. In some regions of the surface, the particles are entrained within a thin layer of silicon-rich material, which possibly contains silica formed either by precipitation, due to reduction in pH of the surface electrolyte, or thermolysis. The particles appear to be
Fig. 8. Scanning electron micrographs (backscattered electrons) of cross-sections of magnesium following AC PEO treatment for at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH. (a) Treatment for 600 s in electrolyte with addition of 10 g/l of monoclinic zirconia then for 600 s in zirconia-free electrolyte. (b) Treatment for 600 s in zirconia-free electrolyte then for 600 s in electrolyte with addition of 10 g/l of monoclinic zirconia.
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subsequently incorporated into the underlying melted coating material. At other locations, the surface appeared relatively-free of particles and silicon-rich material, which may be due to instability of silica at the locally high pH generated during cathodic polarization. Alternatively, the silicon-rich material and zirconia nanoparticles may have been incorporated into the coating at these sites, without sufficient time being subsequently available to reestablish their presence. In addition to incorporation at the coating surface, nanoparticles of zirconia were incorporated following transport to the inner layer/outer layer interface region. Further, cavities within the coating were often lined with nanoparticles, suggesting that cavities may be sections of channels that provide access to the inner coating regions. However, cavities may also arise from bridging of open pores that were located at the coating surface at an earlier stage of coating growth. The sequentially treated specimens reveal fewer zirconia particles at the coating surface when the second stage of treatment uses the zirconia-free electrolyte. In contrast, a thin layer of zirconia particles is established during the second stage of treatment with the zirconia suspension. It appears that new coating material may be formed at the coating surface on top of the zirconia-rich layer, which is suggested by the lighter, zirconiumcontaining material at location L of Fig. 8(b) that is separated from the slightly darker material M, by a line of zirconia nanoparticles connecting to the adjacent zirconia-rich layer. Zirconium-containing material also appears to form at the inner/outer coating interface, which is suggested by the presence of lighter material near the interface. However, some zirconia nanoparticles remain near the interface following the second stage treatment in zirconia-free electrolyte (Fig. 8(a)), suggesting that lower temperatures are achieved in this region. The coating kinetics of Fig. 6 indicate that both the outer and inner coating layers continue to grow during the PEO process, with incorporation of silicon, but little zirconium, into the inner layer. Early studies of PEO using sequential treatments in DC conditions in nanoparticle-free electrolytes with different anion species revealed transport of electrolyte anions, or derivatives produced by the microdischarge, to the inner coating regions, paralleling the transport of zirconia nanoparticles to the inner coating/outer coating interface of the present films [7]. Further, the inner coating material appeared to be readily substituted by new material following change of the electrolyte, whereas the outer coating underwent relatively slow change in composition. Components of coating material formed in the first electrolyte, i.e., containing anion or derivative species of the first electrolyte, were also detected in the second electrolyte, consistent with an accelerated dissolution of prior formed coating [16]. Studies are in progress to determine whether similar loss of inner coating material occurs during AC PEO, when the lifetimes of discharges may be restricted compared with DC conditions. The inability to incorporate significant amounts of zirconium into the inner coating layer under the present conditions of PEO contrasts with the ready incorporation of silicon species into the inner coating material. Further, work is required on the nature and growth mechanism of the inner layer in order to understand the differing behaviours. However, the difference in distributions of silicon and zirconium may relate to the transport of the zirconium in particle form, rather than atomic, ionic or molecular form, to the inner layer/outer layer interface region. Such relatively large particles appear not to be melted readily. Further, the electric field will oppose the migration of zirconium ions into the inner layer during anodic polarization.
A thin region of compact material, with relatively uniform thickness, is present at the base of the inner coating layer. The thickness is consistent with development of a barrier film, with a formation ratio of the order 1 nm V 1, at the final anodic voltage of 430 V achieved at the particular microdischarge site. The material is formed at the termination of coating growth, during cooling of the overlying main coating material. The growth of the barrier film indicates that electrolyte species can penetrate the inner coating layer, enabling film formation at the metal/coating interface. 5. Conclusions 1. Zirconia nanoparticles can be incorporated into PEO coatings formed on magnesium under AC conditions in silicate electrolyte with monoclinic zirconia in suspension. 2. The coatings comprise two main layers, with the inner layers revealing a relatively fine porosity. Both layers thicken as the coating grows, with the inner layer representing about 40% of the coating for a total coating thickness of 40 mm. 3. Magnesium, oxygen and silicon are present within the coatings, which contain crystalline MgO and Mg2SiO4. The concentration of silicon is less in the inner layer compared with the outer layer. 4. Zirconia nanoparticles are incorporated into the coating at the coating surface and following transport along short-circuit paths to the inner coating region. Zirconium species are subsequently incorporated into the coating material, resulting in a modified microstructure of the outer coating layer. Relatively little zirconium is incorporated into the inner coating layer. 5. Due to locally high temperatures at sites of microdischarges Mg2ZrO5 is formed within the coatings following incorporation of monoclinic zirconia from the electrolyte. Acknowledgements The authors are grateful to the Engineering and Physical Sciences Research Council (U.K.) and the Spanish Ministry of Education (grant no. EX2006-1371) for support of this work. References [1] Technol. Appl. Group, Inc. http://www.tagnite.com/. [2] S. Hutchins, P. Shashkov, V. Samsonov, A. Shatrov, in: K.U. Kainer (Ed.), Proceedings of the 6th International Conference Magnesium Alloys and Their Applications, WILEY-VCH, Weinhem, 2004, p. 553. [3] B. Olbertz, A.T. Haug, Metalloberfla¨che 43 (1989) 174. [4] A. Kuhn, Met. Finish. 101 (2003) 44. [5] P. Kurze, in: H.E. Friedrich, B.L. Mordike (Eds.), Magnesium Technology, Metallurgy, Design Data, Applications, Springer, Berlin, 2006, p. 431. [6] C. Blawert, W. Dietzel, E. Ghali, G. Song, Adv. Eng. Mater. 8 (2006) 511. [7] R. Arrabal, E. Matykina, P. Skeldon, G.E. Thompson, A. Pardo, J. Electrochem. Soc. 155 (2008) C101. [8] F. Monfort, A. Berkani, E. Matykina, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, J. Electrochem. Soc. 152 (2005) C382. [9] E. Matykina, F. Monfort, A. Berkani, P. Skeldon, G.E. Thompson, P. Chapon, Philos. Mag. 86 (2006) 49. [10] F. Monfort, A. Berkani, E. Matykina, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, Corros. Sci. 49 (2007) 672. [11] R. Arrabal, E. Matykina, P. Skeldon, G.E. Thompson, J. Mater. Sci. 43 (2008) 1532. [12] L. Besra, M. Liu, Prog. Mater. Sci. 52 (2007) 1. [13] H.F. Guo, M.Z. An, H.B. Huo, S. Xu, L.J. Wu, Appl. Surf. Sci. 252 (2006) 7911. [14] M. Ray, D.R. Sahu, S.K. Singh, S. Verma, B.K. Roul, Mater. Chem. Phys. 107 (2008) 443. [15] A.G. Rakoch, V.V. Khokhlov, V.A. Bautin, N.A. Lebedeva, Y.V. Magurova, I.V. Bardin, Prot. Met. 42 (2006) 158. [16] E. Matykina, G. Doucet, F. Monfort, A. Berkani, P. Skeldon, G.E. Thompson, Electrochim. Acta 51 (2006) 4709.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
AC plasma electrolytic oxidation of magnesium with zirconia nanoparticles R. Arrabal a, E. Matykina a, F. Viejo a, P. Skeldon a,*, G.E. Thompson a, M.C. Merino b a b
Corrosion and Protection Centre, School of Materials, The University of Manchester, Sackville Street, P.O. Box 88, Manchester M60 1QD, UK Departamento de Ciencia de Materiales, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 April 2008 Received in revised form 29 April 2008 Accepted 29 April 2008 Available online 7 May 2008
The incorporation of monoclinic zirconia nanoparticles and their subsequent transformation is examined for coatings formed on magnesium by plasma electrolytic oxidation under AC conditions in silicate electrolyte. The coatings are shown to comprise two main layers, with nanoparticles entering the coating at the coating surface and through short-circuit paths to the region of the interface between the inner and outer coating layers. Under local heating of microdischarges, the zirconia reacts with magnesium species to form Mg2Zr5O12 in the outer coating layer. Relatively little zirconium is present in the inner coating layer. In contrast, silicon species are present in both coating layers, with reduced amounts in the inner layer. ß 2008 Elsevier B.V. All rights reserved.
PACS: 81.65. b 81.65.Mq Keywords: Magnesium Coatings Plasma electrolytic oxidation Zirconium oxide Anodizing
1. Introduction Plasma electrolytic oxidation (PEO) is used for the surface protection of magnesium alloys; several commercial processes are available [1–3]. Processes are also being developed that provide coatings with improved properties. The coatings are formed at high voltages, in various, usually aqueous, electrolytes, when microdischarges are present on the alloy surface [4]. The mechanism of formation of the coating potentially involves anodic oxidation, thermal oxidation and plasma-chemical reactions [5]. The growth of the coating is accompanied by relatively profuse generation of oxygen gas. Coatings can be grown under DC and AC conditions, with a range of waveforms, leading to coatings that generally thicken roughly in proportion to the charge passed [6]. Thicknesses of tens of microns are readily produced, containing amorphous and crystalline constituents dependent on the particular alloy and electrolyte [7]. Magnesium oxide is generally present in the coatings, often accompanied by phases related to the electrolyte anions, for instance magnesium silicate in silicate electrolyte. In cross-section, two main layers of DC coatings may be discerned; the inner layer has a finer texture than the outer layer. At the base
* Corresponding author. Tel.: +44 161 306 4872; fax: +44 161 306 4865. E-mail address: [email protected] (P. Skeldon). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.04.100
of the inner layer, a relatively thin, compact barrier region is located at the alloy/coating interface, with a thickness of up to l mm. Short-circuit transport of electrolyte components through the outer layer has been shown to occur [7–9]. This allows anion species of the electrolyte, or possibly derivatives formed in the microdischarge, ready access to the inner coating. Cation species can also be transported across the outer coating, although in greatly reduced amounts compared with the anion species [10]. In addition, nanoparticles in suspension in the electrolyte can reach the inner layer via the short-circuit paths, which has been demonstrated using monoclinic zirconia [11]. The nanoparticles are also incorporated into the outer part of the coating, which reveals a modified microstructure compared with coating formed in the absence of nanoparticles. Tetragonal zirconia is detected in significant amounts in the final coating, due to the high temperatures generated at microdischarge sites. In the present study, the influence of AC treatment on the incorporation of zirconia is investigated for magnesium treated in silicate electrolyte. The incorporation of nanoparticles is relevant to understand the mechanism of coating growth and for the potential to improve coating properties through modification of the coating microstructure [12]. The main interest of the work concerned the microstructure and morphology of the coatings, especially the distribution of zirconium and the types of zirconia-containing phases.
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2. Experimental Specimens of 99.9% magnesium sheet were embedded in resin, with electrical connection provided through a shielded copper wire. The exposed surface, of area 1 cm2, was ground to 1200 grit SiC, using water as a lubricant, degreased in ethanol, rinsed in deionized water and dried in warm air. PEO treatment was carried out at 200 mA cm 2 (rms) at 50 Hz with a sinusoidal waveform in 0.025 M Na2SiO35H2O/0.05 M KOH for up to 2400 s. The electrolyte was prepared from deionized water and high-purity chemicals. Additions of 10 g/l of monoclinic zirconia nanoparticles, of size 150–300 nm, were made as required. The anodizing condition was selected following evaluation of a range of current densities and electrolytes compositions for incorporation of zirconia into the coating. The coatings were formed in a 1 l double-walled glass cell through which a cooled water/glycol mixture was pumped in order to keep the electrolyte temperature close to 293 K. During the treatment, the electrolyte was stirred continuously, which kept the zirconia nanoparticles in suspension. A sheet of type 304 stainless steel, of size 7.5 cm 15 cm, was used as the cathode. Voltage responses were recorded electronically, with a sampling time of 20 ms, employing an SCXI data acquisition system (National Instruments) with data analysis by Igor Pro (Wavemetrics). After coating, specimens were rinsed with deionized water and dried in warm air. PEO-treated specimens were examined by field emission gun scanning electron microscopy, using a Philips XL30 instrument equipped with energy-dispersive X-ray (EDX) analysis facilities. Cross-sections were ground through successive grades of SiC paper, followed by finishing to 1 mm diamond. Phase composition was investigated by X-ray diffraction (XRD), using a Philips X’PertMPD (PW 3040) instrument with a step size 0.0058 and a scan range from 5 to 858 (in 2u). 3. Results Voltage–time responses were practically the same during PEO without and with zirconia in the electrolyte, with commencement of sparking at 250 V coinciding with the major change in slope between the initial period of relatively rapid voltage rise and the start of the main period of slow voltage increase (Fig. 1). Without zirconia in the electrolyte, the surface of the resultant coating comprises apparently overlapping, roughly circular features (Fig. 2(a)). The shapes of the features suggest outward flow of material along a channel through the underlying coating to the coating surface, followed by its spreading over the coating surface and solidification. The largest of the features have dimensions of up to 50 mm. The surface is significantly rough and pores and cracks are evident. In addition, micron-sized silicon-rich particles, with
Fig. 1. Voltage–time responses during AC PEO treatment of magnesium at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH, without and with addition of 10 g/l of monoclinic zirconia.
the appearance of a deposit, are also present; such particles are often found following PEO in silicate electrolyte [13]. The coating roughness and pores are clear in cross-sections of the coating (Fig. 2(b)). The thickness of the coating at the thickest regions is about twice that at the thinnest regions, with inclusion of porosity in the thickness measurements. Two main layers are also revealed, with the inner layer being finely porous, whereas the outer layer has relatively large pores, cavities and channels. There are also regions of finer pores in the outer layer and of larger pores in the inner layer although these are of relatively minor extent. Cracks are mostly evident in the outer layer. Magnesium, oxygen and silicon are detected in both layers by EDX point analyses; the atomic ratios of Si:Mg were 0.28 0.08 and 0.10 0.04 in the outer and inner layers, respectively (points A and B, Fig. 2(b)). The corresponding O:Mg ratios were 3.1 0.4 and 2.6 0.3. The surface of the coating formed in electrolyte containing zirconia nanoparticles revealed two distinct types of region in backscattered electron images (Fig. 3(a)). An extensive, irregular network of light regions corresponded to coating decorated by numerous zirconia nanoparticles. In contrast, relatively dark, roughly circular zones, of size 50–100 mm, are mainly free of nanoparticles, but disclose dendrites of size 1–2 mm (Fig. 3(b)). Non-uniformity in the distribution of zirconia particles at the coating surface was also observed following DC treatment in an alkaline phosphate electrolyte [11]. Cracks and pores are found in both types of region of the coating. Further, EDX point analyses revealed the presence of silicon and zirconium, with Si:Mg and
Fig. 2. Scanning electron micrographs (secondary electrons) of magnesium following AC PEO treatment for 2400 s at 200 mA cm KOH. (a) Plan. (b) Cross-section.
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Fig. 3. Scanning electron micrographs (backscattered electrons) of the surface of magnesium following AC PEO treatment for 2400 s at 200 mA cm Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (a) General view. (b) Region with relatively few nanoparticles.
Zr:Mg ratios of 0.26 0.04 and 0.26 0.06, respectively in the region containing micron-sized dendrites (point C, Fig. 3(a)) and 0.33 0.08 and 0.55 0.14, respectively, in the region covered by zirconia nanoparticles (point D, Fig. 3(a)). In cross-section, the coating formed with zirconia in suspension in the electrolyte comprises two main layers (Fig. 4(a and b)). The
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inner layer has a relatively fine porosity. The outer layer is more compact, and with reduced porosity compared with the outer layer of coatings formed in the absence of zirconia in the electrolyte. Cavities appear to be prevalent near the interface between the inner and outer layers. The X-ray elemental maps show the presence of magnesium, oxygen and silicon in most coating
Fig. 4. (a) Scanning electron micrograph (backscattered electrons) of a cross-section of magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (b) Detail of region between the inner and outer coating layers. EDX elemental maps are shown for magnesium, oxygen, silicon and zirconium of the cross-section of (a).
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Fig. 5. Scanning electron micrographs (backscattered electrons) of cross-sections of magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (a) Surface region with relatively few nanoparticles. (b) Surface region with nanoparticle layer. (c) Detail of (a). (d) Detail of (b).
regions, with a reduced amount of silicon in the inner layer (Fig. 4). Zirconium was mainly a component of the outer layer. EDX point analyses disclosed Si:Mg and Zr:Mg ratios of 0.26 0.07 and 0.16 0.05, respectively, in the outer layer (point E, Fig. 4(a)) and 0.10 0.03 and 0.02 0.01, respectively, in the inner layer (point F, Fig. 4(a)), confirming the moderately reduced concentration of silicon and greatly reduced concentration of zirconium in the inner layer. The Si:Mg ratios in the two layers were similar to those determined for the coating formed in zirconia-free electrolyte. Zirconia nanoparticles, similar in size to those present in the electrolyte, are prevalent near the interface between the inner and outer layers and within larger cavities and channels in the outer layer (Fig. 4(b)). Zirconium is also generally distributed throughout the outer coating at locations largely free of nanoparticles. The outer coating in these latter regions contains numerous dendrites roughly similar in size and shape to those observed at the coating surface (Fig. 5(a and c)). Observation at another region of the cross-section, corresponding to the network of nanoparticles observed at the film surface, revealed that the nanoparticles were incorporated into a layer of thickness 1–2 mm (Fig. 5(b and d)). EDX point analysis indicated Si:Mg and Zr:Mg ratios of 1.33 0.31 and 1.09 0.46, respectively, suggesting trapping of nanoparticles within silicon-rich material formed at the coating surface (point G, Fig. 5(b)). The underlying coating microstructure disclosed discrete light particles, of size suggestive of incorporated nanoparticles, mainly within a relatively light network of matrix material (Fig. 5(d)). The latter surrounded micron-sized islands of relatively dark material. EDX point analyses indicated Zr:Mg ratios of 0.16 0.06 and 0.18 0.05 and Si:Mg ratios of 0.16 0.03 and 0.17 0.03 in the darker and lighter materials, respectively (points H and I, Fig. 5(d)), although the results are probably affected by the large size of the analysis volume relative to the dimension of the zones and hence, represent average compositions for the microstructure. The kinetics of coating growth were approximately linear, with average rates of 15.8 and 18.0 nm s 1 without and with zirconia, respectively (Fig. 6). The thickness of both the inner and
outer coating layers increased with increase of treatment time. The inner layer thickness appeared to increase relative to the total film thickness, from 20% of the film at 10 min to 40% at later times. XRD revealed strong peaks for MgO and weaker peaks for Mg2SiO4 in the coating formed in electrolyte without nanoparticles (Fig. 7). These peaks were also found for the coating produced with nanoparticles, with relatively reduced peaks for Mg2SiO4 and the additional presence of peaks for monoclinic zirconia and Mg2Zr5O12. Coatings formed for 10 min in electrolyte with nanoparticles followed by 10 min in electrolyte free of nanoparticles, and vice versa, also resulted in two-layered coatings (Fig. 8). For the
Fig. 6. Dependence of coating thickness on time for AC PEO treatment of magnesium at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH without and with addition of 10 g/l of monoclinic zirconia.
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Fig. 7. XRD data for magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH without (bottom) and with (top) addition of 10 g/l of monoclinic zirconia.
former sequence, occasional regions of material with incorporated nanoparticles were present in the final coating, usually at the coating surface, within cavities and near the inner layer/ outer layer interface (Fig. 8(a)). EDX analyses indicated the presence of zirconium species in the outer layer material, with a Zr:Mg ratio of 0.09 0.05 (point J, Fig. 8(a)); no significant amount of zirconium was detected in the inner layer (point K, Fig. 8(a)). The respective Si:Mg ratio were 0.21 0.04 and 0.19 0.05, indicating less silicon in the inner layers. For the reverse sequence, increased amounts of zirconia nanoparticles were present at the coating surface, within cavities and near the inner layer/outer layer interface (Fig. 8(b)). Surface regions with much reduced numbers of nanoparticles were also identified, similar to the findings for the previous single PEO treatment with nanoparticles. Point analyses through the coating thickness disclosed reducing Zr:Mg ratios of 0.07 0.05, 0.06 0.05 and 0.03 0.01 with increasing depth in the coating (points L–N, Fig. 8(b)). A similarly reducing trend was found for Si:Mg ratios, namely 0.23 0.03, 0.20 0.05 and 0.14 0.05. 4. Discussion The two-layered coatings observed previously following DC PEO treatment of magnesium in silicate electrolyte in the presence and
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absence of a zirconia nanoparticle suspension [11] are also formed under the present AC conditions. Similarly, the transport of zirconia nanoparticles along short-circuit paths, through the outer layer to the inner layer, is also revealed. Irrespective of the presence of zirconia, the coatings contained magnesium species from the substrate and silicon species from the electrolyte, with detection of crystalline MgO and Mg2SiO4. Further, the inner layer contained a reduced amount of silicon, with typical Si:Mg ratios from EDX point analyses of 0.10 0.04 compared with 0.26 0.04 in the outer layer. For coating containing zirconium species, the local high temperatures of coating growth result in formation of dendrites in the outer coating layer, and generation of Mg2Zr5O12, which is reported to be stable in the temperature range 2123–2373 K [14]. This phase is presumed to be a constituent of the outer coating, since, relatively little zirconium is found within the inner layer, for which EDX point analyses indicated a Zr:Mg ratio of 0.02, compared with 0.09–0.16 in the outer layer. Mg2Zr5O12 was not detected by XRD in coatings formed in silicate electrolyte under DC conditions with a current density of 30 mA cm 2 and a reduced concentration of zirconia in the electrolyte, 2 g/l [11]. However, generation of tetragonal zirconia occurred under DC treatments with silicate electrolyte and both tetragonal zirconia and Mg2Zr5O12 in phosphate electrolyte. The absence of tetragonal zirconia in the AC coatings, contrary to findings for a DC coating, is due to the different thermal conditions in the coatings, related to the energies dissipated in the individual discharge events. The latter will depend on the local current density of the discharge sites and the size and lifetime of the discharge. The presence of tetragonal zirconia under the DC conditions compared with the presence of Mg2Zr5O12 under AC conditions suggests higher temperature in the discharges for the latter. Monoclinic zirconia is also detected in the present coatings, and those formed previously under DC conditions, due to incorporation of zirconia that either does not transform under the local conditions of temperature and pressure, or reverts to the monoclinic form on cooling. The occurrence of dendrites indicates local melting in the outer layer. The melting point of the outer layer material will depend upon the precise composition of the coating. The melting points of relatively pure MgO and Mg2SiO4 are 3073 and 2163 K, respectively. The extensive cracking of the coatings is presumed to arise from stresses in the coating associated with volume changes due to coating growth and phase transformation, differential thermal expansion within the coating and substrate, and the pressure of oxygen and hydrogen generated during anodic and cathodic half-cycles, respectively [15]. Zirconia nanoparticles were incorporated into the coating surface. In some regions of the surface, the particles are entrained within a thin layer of silicon-rich material, which possibly contains silica formed either by precipitation, due to reduction in pH of the surface electrolyte, or thermolysis. The particles appear to be
Fig. 8. Scanning electron micrographs (backscattered electrons) of cross-sections of magnesium following AC PEO treatment for at 200 mA cm 2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH. (a) Treatment for 600 s in electrolyte with addition of 10 g/l of monoclinic zirconia then for 600 s in zirconia-free electrolyte. (b) Treatment for 600 s in zirconia-free electrolyte then for 600 s in electrolyte with addition of 10 g/l of monoclinic zirconia.
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subsequently incorporated into the underlying melted coating material. At other locations, the surface appeared relatively-free of particles and silicon-rich material, which may be due to instability of silica at the locally high pH generated during cathodic polarization. Alternatively, the silicon-rich material and zirconia nanoparticles may have been incorporated into the coating at these sites, without sufficient time being subsequently available to reestablish their presence. In addition to incorporation at the coating surface, nanoparticles of zirconia were incorporated following transport to the inner layer/outer layer interface region. Further, cavities within the coating were often lined with nanoparticles, suggesting that cavities may be sections of channels that provide access to the inner coating regions. However, cavities may also arise from bridging of open pores that were located at the coating surface at an earlier stage of coating growth. The sequentially treated specimens reveal fewer zirconia particles at the coating surface when the second stage of treatment uses the zirconia-free electrolyte. In contrast, a thin layer of zirconia particles is established during the second stage of treatment with the zirconia suspension. It appears that new coating material may be formed at the coating surface on top of the zirconia-rich layer, which is suggested by the lighter, zirconiumcontaining material at location L of Fig. 8(b) that is separated from the slightly darker material M, by a line of zirconia nanoparticles connecting to the adjacent zirconia-rich layer. Zirconium-containing material also appears to form at the inner/outer coating interface, which is suggested by the presence of lighter material near the interface. However, some zirconia nanoparticles remain near the interface following the second stage treatment in zirconia-free electrolyte (Fig. 8(a)), suggesting that lower temperatures are achieved in this region. The coating kinetics of Fig. 6 indicate that both the outer and inner coating layers continue to grow during the PEO process, with incorporation of silicon, but little zirconium, into the inner layer. Early studies of PEO using sequential treatments in DC conditions in nanoparticle-free electrolytes with different anion species revealed transport of electrolyte anions, or derivatives produced by the microdischarge, to the inner coating regions, paralleling the transport of zirconia nanoparticles to the inner coating/outer coating interface of the present films [7]. Further, the inner coating material appeared to be readily substituted by new material following change of the electrolyte, whereas the outer coating underwent relatively slow change in composition. Components of coating material formed in the first electrolyte, i.e., containing anion or derivative species of the first electrolyte, were also detected in the second electrolyte, consistent with an accelerated dissolution of prior formed coating [16]. Studies are in progress to determine whether similar loss of inner coating material occurs during AC PEO, when the lifetimes of discharges may be restricted compared with DC conditions. The inability to incorporate significant amounts of zirconium into the inner coating layer under the present conditions of PEO contrasts with the ready incorporation of silicon species into the inner coating material. Further, work is required on the nature and growth mechanism of the inner layer in order to understand the differing behaviours. However, the difference in distributions of silicon and zirconium may relate to the transport of the zirconium in particle form, rather than atomic, ionic or molecular form, to the inner layer/outer layer interface region. Such relatively large particles appear not to be melted readily. Further, the electric field will oppose the migration of zirconium ions into the inner layer during anodic polarization.
A thin region of compact material, with relatively uniform thickness, is present at the base of the inner coating layer. The thickness is consistent with development of a barrier film, with a formation ratio of the order 1 nm V 1, at the final anodic voltage of 430 V achieved at the particular microdischarge site. The material is formed at the termination of coating growth, during cooling of the overlying main coating material. The growth of the barrier film indicates that electrolyte species can penetrate the inner coating layer, enabling film formation at the metal/coating interface. 5. Conclusions 1. Zirconia nanoparticles can be incorporated into PEO coatings formed on magnesium under AC conditions in silicate electrolyte with monoclinic zirconia in suspension. 2. The coatings comprise two main layers, with the inner layers revealing a relatively fine porosity. Both layers thicken as the coating grows, with the inner layer representing about 40% of the coating for a total coating thickness of 40 mm. 3. Magnesium, oxygen and silicon are present within the coatings, which contain crystalline MgO and Mg2SiO4. The concentration of silicon is less in the inner layer compared with the outer layer. 4. Zirconia nanoparticles are incorporated into the coating at the coating surface and following transport along short-circuit paths to the inner coating region. Zirconium species are subsequently incorporated into the coating material, resulting in a modified microstructure of the outer coating layer. Relatively little zirconium is incorporated into the inner coating layer. 5. Due to locally high temperatures at sites of microdischarges Mg2ZrO5 is formed within the coatings following incorporation of monoclinic zirconia from the electrolyte. Acknowledgements The authors are grateful to the Engineering and Physical Sciences Research Council (U.K.) and the Spanish Ministry of Education (grant no. EX2006-1371) for support of this work. References [1] Technol. Appl. Group, Inc. http://www.tagnite.com/. [2] S. Hutchins, P. Shashkov, V. Samsonov, A. Shatrov, in: K.U. Kainer (Ed.), Proceedings of the 6th International Conference Magnesium Alloys and Their Applications, WILEY-VCH, Weinhem, 2004, p. 553. [3] B. Olbertz, A.T. Haug, Metalloberfla¨che 43 (1989) 174. [4] A. Kuhn, Met. Finish. 101 (2003) 44. [5] P. Kurze, in: H.E. Friedrich, B.L. Mordike (Eds.), Magnesium Technology, Metallurgy, Design Data, Applications, Springer, Berlin, 2006, p. 431. [6] C. Blawert, W. Dietzel, E. Ghali, G. Song, Adv. Eng. Mater. 8 (2006) 511. [7] R. Arrabal, E. Matykina, P. Skeldon, G.E. Thompson, A. Pardo, J. Electrochem. Soc. 155 (2008) C101. [8] F. Monfort, A. Berkani, E. Matykina, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, J. Electrochem. Soc. 152 (2005) C382. [9] E. Matykina, F. Monfort, A. Berkani, P. Skeldon, G.E. Thompson, P. Chapon, Philos. Mag. 86 (2006) 49. [10] F. Monfort, A. Berkani, E. Matykina, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, Corros. Sci. 49 (2007) 672. [11] R. Arrabal, E. Matykina, P. Skeldon, G.E. Thompson, J. Mater. Sci. 43 (2008) 1532. [12] L. Besra, M. Liu, Prog. Mater. Sci. 52 (2007) 1. [13] H.F. Guo, M.Z. An, H.B. Huo, S. Xu, L.J. Wu, Appl. Surf. Sci. 252 (2006) 7911. [14] M. Ray, D.R. Sahu, S.K. Singh, S. Verma, B.K. Roul, Mater. Chem. Phys. 107 (2008) 443. [15] A.G. Rakoch, V.V. Khokhlov, V.A. Bautin, N.A. Lebedeva, Y.V. Magurova, I.V. Bardin, Prot. Met. 42 (2006) 158. [16] E. Matykina, G. Doucet, F. Monfort, A. Berkani, P. Skeldon, G.E. Thompson, Electrochim. Acta 51 (2006) 4709.