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Growth, microstructure and mechanical properties of microarc oxidation coatings on titanium alloy in phosphate-containing solution
Applied Surface Science 233 (2004) 258–267
Growth, microstructure and mechanical properties of microarc oxidation coatings on titanium alloy in phosphate-containing solution Yaming Wanga,*, Tingquan Leia, Bailing Jiangb, Lixin Guoa a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China
b
Received in revised form 17 March 2004; accepted 17 March 2004 Available online 30 April 2004
Abstract Ceramic coatings were fabricated by microarc oxidation in galvanostatic regime on Ti6Al4Valloy in (NaPO3)6–NaF–NaAlO2 solution. The growth, microstructure and phase composition of coatings were investigated using scanning electron microscope and X-ray diffraction. With increasing treatment duration, coating growth varies from rapidness to tardiness accompanied by gradually roughening in appearance. Meanwhile, phase transformation of anatase to rutile occurs. The crystalline AlPO4 is involved in the coatings via high-temperature thermolysis of hydrated aluminium polyphosphates in the nearby discharging channels. The stepped current regime enables coating structure to be controllable. The mechanical properties distribution across the coating thickness and the adhesion strength were determined by nanoindentation and shear test, respectively. A similar evolution profile of hardness and elastic modulus across the coating thickness is found: remaining high values (5.5 and 69.1 GPa) in the compact region before finally declining to low values (5.1 and 65.6 GPa) in the looser region. The adhesion strength of substrate/coating interface is about 40 MPa. # 2004 Elsevier B.V. All rights reserved. Keywords: Microarc oxidation; Coating; Microstructure; Mechanical properties; Functional applications
1. Introduction A relatively novel surface modifying technique, conventionally called ‘‘microarc oxidation (MAO)’’, also commonly called ‘‘plasma electrolytic oxidation (PEO)’’, ‘‘microplasma oxidation (MPO)’’ and ‘‘anodic spark deposition (ASD)’’ in modern scientific literatures, is attracting increasing interest in fabricat*
Corresponding author. Tel.: þ86-451-86413910; fax: þ86-451-86413922. E-mail address: [email protected] (Y. Wang).
ing ceramic-like coatings on titanium alloys, with the purpose of providing corrosion- and wear-resistance or various functional properties. In addition, this technique is economic efficiency, ecological friendly and characterized by high productivity. MAO is based on the conventional anodic oxidation of processing metals and alloys in aqueous electrolyte solutions under the additional condition of plasma discharge at exceeding the critical values of the polarization potential, and the fact that discharge leading to localized high temperature and high pressure, allows forming coatings composed of not only predominant
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.231
Y. Wang et al. / Applied Surface Science 233 (2004) 258–267
substrate oxides but of more complex oxides containing compounds which involve the components presented in the electrolyte. Both intrinsic factors (electrolyte compositions and pH) and extrinsic factors (electrical parameters, and electrolyte temperature) affect the formation and microstructure of microarc oxidation coatings. Therein, composition and concentration of electrolyte and electrical parameters during the process play a crucial role in obtaining the desired coatings of special phase component and microstructure. Among them, it is assumed that the intrinsical effects of electrolytes may be summarized as follows: firstly, as the medium of current conduct, transmitting the essential energy needed for anode oxidizing occurring in the interface of metal/electrolyte; secondly, providing the oxygen source in the form of oxysalt needed for oxidation; finally and also more interesting, components presenting in the electrolyte incorporated into the coatings can further modify or improve the properties of microarc oxidation coatings. Various specially selected electrolytes and their combinations have been successfully developed in order to provide protective coatings of corrosion- and wear-resistance [1–7] and diverse functional coatings, such as biocidal or catalytic [8,9], biomedical [10,11], ferroelectric [12], and semiconducting [13] properties, on titanium alloys. It has been found that coatings of phosphorus-containing compounds, introduced by the phosphate-containing electrolytes during the MAO process, can find applications in biocidal effect, catalysis, and corrosion protection. The functional actions mentioned above are attributed to the polyphosphates, elemental phosphorus, or phosphides incorporated into the MAO coatings [8]. The earlier works are focused on optimizing electrolytes composition and concentration in order to obtain the coatings with special phase composition [1–13]. However, no data at present are available
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concerning the growth, microstructure and mechanical properties of such coatings. The primary objectives of this paper are: (1) a new aqueous electrolyte containing (NaPO3)6 was developed, then the growth process and corresponding microstructure evolution of the coatings were investigated; (2) in order to obtain the improved coatings, a stepped adjusting regime of current density was proposed; (3) subsequently, the mechanical hardness and elastic modulus, as well as the adhesion strength between coating and substrate were evaluated.
2. Experimental details 2.1. Specimen fabrication Ti6Al4V discs with a diameter of 30 mm and thickness of 3 mm were used as the substrate. The surface of the discs was polished with 600# abrasive papers to a roughness of Ra 0:12 mm, and then ultrasonically cleaned in distilled water followed in acetone. The discs were used as anodes, while stainless steel plates were used as cathodes in the electrolytic bath. The samples carefully pretreated as above were anodized in accordance with process conditions detailed in Table 1. An aqueous electrolyte was used, containing mainly (NaPO3)6 and some other proprietary substances (namely NaF and NaAlO2) to adjust its alkalinity and increase conductivity, as well as addition of NaAlO2 increased growth rate of the microarc oxidation coatings. A 65 kW AC microarc oxidation device provided constant current which can be set variably depending on the requirements, and the voltage varied with the duration of anodizing time. Voltage magnitude of the negative pulse equals to that of the positive pulse. From our practice, it is the positive pulse rather than the negative pulse plays a decisive role in the microstructure formation of
Table 1 Process conditions for ceramic coatings by the microarc oxidation method Electrolyte composite and concentration
(NaPO3)6–NaF–NaAlO2 (15–6–6 g/L)
Electrical parameters (positive/negative) Current density (mA cm2)
Frequency (Hz)
Duty cycle (%)
60
600/100
8.0/1.0
Electrolyte temperature (8C)
Oxidizing time (min)
<50
2, 5, 10, 15, 30
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coatings. The negative pulse is only interspersed within the positive pulses as a mean to interrupt the spark discharges, permitting the surface to cool and inducing the re-conversion of soluble components into metal oxide [14]. Therefore, in order to obtain coatings with optimum surface morphology, a stepped adjustment regime in positive pulse current was applied. The electrolyte was cooled by a cooling system to keep its temperature below 50 8C. The as-prepared samples were washed with distilled water and dried in an air oven for testing.
modulus were determined using the Oliver and Pharr method [15] for a constant depth of 1000 nm.
2.2. Microstructure characterization
2.4. Adhesion strength test
The thickness of the resulting coatings was measured using a coating thickness gauge (TT230, Time Group Co.). Ten measurements were carried out evenly on each whole test surface of both sides of the samples. After removing maximum and minimum readings, the rest were averaged to obtain the value of the coating thickness. The surface morphology of the coatings was observed using scanning electron microscopy (SEM, S-570, Hitachi Co., Japan). At a number of randomly selected locations, the scanned regions were photographed and these micrographs were subjected to image analysis to measure the number of micropores and their sizes. The phase composition of the coatings was analysed by X-ray diffraction (XRD, Philips X’Pert, Holland) using a Cu Ka radiation at a grazing angle of 38.
Fig. 1 gives the schematic of shear strength test for the microarc oxidation coatings. For the shear sample, the coated side by microarc oxidation treatment and the grit blasted side of two pieces of Ti6Al4V cylinder samples in the same size were glued together by epoxy. The as-prepared shear samples were mounted rightly in a specially designed holding device. A load parallel to the coating was continually applied on the Ti6Al4V cylinder at a rate of 0.5 mm/min using the Instron-1186 stretcher until adhesion failure was found. The instant maximum load was recorded. The shear strength was determined by the following equation:
2.3. Nanoindentation test A nanoindentation testing system (Nanoindenter XP, MTS, America) with a well-calibrated berkovich diamond indenter was employed to determine the mechanical properties. The maximum indentation depth and the maximum load are 500 mm and 500 mN, respectively. The load resolution is about 50 nm and displacement resolution is approximately 0.01 nm. Temperature changes were kept below 1 8C and diamond tip drift limit of 0.25 nm/s was preset for the test to start. All continuous stiffness measurements (CSM) were carried out under displacement control mode with tip-displacement rate of 10 nm/s. Load–displacement curves were recorded. After indenting, indentation morphology was observed using optical microscope (OM) attached to the system. Hardness and elastic
Load Coating Ti Cylinder
Adhesive Ti Cylinder
Fig. 1. Schematic representation of shear test setting.
t¼
Pmax S
(1)
where Pmax is the maximum load, S the area of the coating surface, and t the shear strength. To acquire good statistical results, six identical measurements were carried out. After removing the maximum and minimum calculating values, the rest were averaged to obtain the value of the shear strength. Here, the shear strength calculated is regarded as the adhesion strength of coating to the substrate.
3. Results and discussion 3.1. Growth and microstructure of coatings Fig. 2 presents the variation of coating growth rate versus voltage microarc oxidized at a constant current density mode (j ¼ 60 mA cm2). Clearly, an approximately linear growth of coatings is found within the
Y. Wang et al. / Applied Surface Science 233 (2004) 258–267
600
40
Voltage (V)
500 30 400 20
300 200
10
Coating thickness (µm)
50
700
100 0
5
10
15
20
25
30
0
Treatment time (min) Fig. 2. Variation of coating thickness and voltage during microarc oxidation treatment of Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution.
beginning 10 min. The linear rate constant calculated is 3.0, i.e., the coating growth rate is defined as 3.0 mm/ min. The coating growth rate begins to slow down beyond 10 min. At about 15 min a critical transitional point is observed, from which to the end of the process, the coating growth is not obvious. On the other hand, when considering the oxidizing voltage, at a voltage of about 160 V, the first visible sparking occurs. During the initial oxidizing stage (as low as 2 min), the voltage quickly rises up to 470 V. With the proceeding of the oxidation, the voltage increases slowly. The 15 min later, the voltage reaches, and retains at a higher value of about 660 V. Dielectric breakdown occurs when the applied voltage exceeds the critical voltage of originally naturalformed insulating film on Ti6Al4V alloy, which is the prerequisite for microarc oxidation being able to proceed in consideration of electrical aspect. Since current density defines the intensity of sparking on the surface, when microarc oxidizing at a constant current density of 60 mA cm2, i.e., the sparking intensity is kept at the same level in all the experiments, coatings thickness increases approximately linearly. In order to maintain the constant current density throughout the MAO process, voltage value has to response and increases with the increasing self-resistance in the coating, which leads to a rise in discharging energy of a single pulse, in other words, the discharging intensity of a single pulse is enhanced. As is known from phase analysis of coatings, coating products contains components of both the anode Ti6Al4V and electrolyte, and coating thickens as a result of
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sparking-anodic actions and thermolysis of hydrated aluminium polyphosphates precipitated from the electrolyte [16]. However, the thermolysis product (AlPO4) from the hydrated aluminium polyphosphates has to be subjected to dissolution in so strong an alkaline solution ðpH 12:5Þ. With the increasing precipitation, the coating growth is gradually dominated by the dissolution of thermolysis products, which is responsible for the gradually decreasing growth rate of the coatings. The surface morphology evolutions of the coatings formed on Ti6Al4V alloy during the microarc oxidation process are shown in Fig. 3. The surface is characterized by micropores in different sizes and shapes. The porous feature strongly depends on discharging nature involved in the microarc oxidation mechanism [17]. Variations in number and porosity of micropores are also presented in Fig. 4. It can be seen clearly that at the initial oxidation stage (bellow 2 min), fine and uniform micropores with sizes less than 1 mm distribute randomly on the surface of the coatings. With proceeding of the oxidation, the micropores number quickly decreases in an exponential function, while micropores sizes increases strikingly. As a result, the porosity increases from 3.8% for 2 min to 11.6% for 30 min. The evolution of surface morphology clearly demonstrates the visible decrease in micropores number and increase in micropores size. At the beginning of microarc oxidation, Ti6Al4V alloy with uniform and fewest defects surface facilitates the production of fine, uniform and dispersed microdischarge in considerable numbers. In this case, the number of micropores remained after microdischarge had decayed is rather amazing. The size of the micropores is significantly small, typically less than 2 mm (Fig. 3a). With oxidizing time increasing, the voltage value increases, accompanied by remarkably reducing number of microdischarge channels. The increasing voltage leads to an enhanced discharging energy, hence the increased product mass by a single pulse, which contributes to the enlarged pore sizes after the discharge channels are cooled. Moreover, continually multiple discharging occurs at a local region of relative weakness in the coating, or several discharging channels merge to form a single larger channel, which also leads to the increased pores size. The surface of MAO coatings is gradually roughened
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Fig. 3. Surface morphology of microarc oxidation coatings formed on Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution at different treatment time periods: (a) 2 min; (b) 5 min; (c) 10 min; (d) 15 min; (e) 30 min.
180
ment period. The coatings are mainly composed of two modifiers of titanium oxide, i.e., metastable anatase at low temperature and stable rutile at higher temperature, as well as crystalline AlPO4 is incorporated into the
Anatase Rutile AlPO4 Titanium
12 10
120 100
8
80 60
6
40 20 0
30min 15min
Intensity
140
(A.U.)
160
Porosity (%)
Microporres number (×103⋅mm-2)
as the result of the reduced pores number, as well as the increased pores size (Fig. 3). Fig. 5 gives XRD patterns from the surface of microarc oxidation coatings formed in different treat-
10min
♦
5min
♦ ♦
4 0
5
10
15
20
25
30
Treatment time (min) Fig. 4. Variation of number of micropores and porosity on the surface of the coating during microarc oxidation treatment of Ti6Al4V alloy.
♦
♦
20
30
40
50
60
♦ 2min ♦♦
70
80
2q (°) Fig. 5. Effect of different treatment time periods on phase composition of microarc oxidation coatings on Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution.
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3.2. Structure control by adjustment of current According to our experimental practices, microarc oxidation using the constant current density regime is time saving compared with the constant voltage regime. This is not surprising because the current density, which defines the intensity of sparking on the surface, is kept at the same high level through the microarc oxidation process, which facilitates the coating growth. While upon constant voltage oxidation, current density gradually decays, i.e., sparking intensity decreases in the process, which leads to a relatively low coatings growth rate [19]. However, coatings formed using the constant current density regime tends to have a loose and rough microstructure. For obtaining the optimum microstructure of relatively
700
a’
600
b’ c’ a
500
80 60
400
b c
300 200
40
20
100 0
Current density (mA·cm-2)
100
800
Mean voltage (V)
coatings. As the treatment time increases, the relative amount of anatase phase decreases, while that of rutile increases, which indicates that phase transformation of anatase to rutile occurs during the oxidizing process. The crystalline AlPO4 is involved in the coatings via high-temperature thermolysis of hydrated aluminium polyphosphates in the nearby channel zone of microdischarging [16], and its content also shows slight increase. Noticeably, diffraction peaks of titanium substrate are rather strong in coatings at the oxidation time of 2 min, which is attributed to the very thin film of about 5 mm, and the thickness is bellow the maximum X-ray penetrating depth. During microarc oxidation, the temperature and pressure in the discharge channels can reach about 2 104 K and 102 MPa [18], which promote the conversion of Ti substrate into titanium oxides. Meanwhile, the incorporation of electrolyte components into coatings results from the high-temperature reactions proceeding just in discharges channels or adjacent areas. As the coatings thickness increases, the previously formed products are calcined repeatedly under the effect of the frequent microdischarge, which facilitates the anatase to rutile transformation, as well as the thermolysis process. Furthermore, the low thermal conductivity of titanium dioxide causes the underlying layer of the coatings to become heated, which also promotes the further transformation of the initially formed metastable anatase to the stable rutile phase. Thereby, the amount of rutile phase in the coatings increases with increasing oxidizing time.
263
0 0
2
4
6
8
10
12
14
16
Treat time (min) Fig. 6. The stepped adjusting regime of current density and associated to variation of voltage during microarc oxidation process: a,a0 – single-stepped; b,b0 – two-stepped; c,c0 – three-stepped.
compact coatings, a stepped adjusting of current density during microarc oxidation process was proposed. Variation of mean voltage responding to the different stepped adjusting regime of current density during microarc oxidation process is shown in Fig. 6, and the corresponding surface morphology evolution is presented in Fig. 7. Evidently, using the single-stepped adjusting mode of current density (constant current density), the coating surface is loose, with a lot of big and long pores of different sizes accompanied by many visual cracks (shown in arrows). In the case of twostepped mode, though no visual cracks are found in the surface, the pore sizes still maintain big. While, for three-stepped mode, both the sizes and the number of the pores decrease, and no cracks are observed in the surface. In contrast with the singleand two-stepped mode, the coating microstructure is improved evidently. Varying current density can regulate the surface discharge characteristics. High current density leads to increased sparking discharge intensity caused by the highly energy pulse (high pulse voltage), which contributes to the increasing accumulation of coating products, and readily forms the coarse grains structure of the coatings. In contrast, low current density readily tends to form the fine grain structure. The mechanism of the stepped adjusting in current density is attributed to the following process: at the first stage, the coating grows quickly at a relatively high current density (j ¼ 60 mA cm2); while, at the later stages, fine grains produced because the stepwise decrease in
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Fig. 7. Surface morphology evolution of microarc oxidation coatings on Ti6Al4V alloy processed at different electrical control mode of current density: (a) single-step mode; (b) two-step mode; (c) three-step mode. (note: arrows indicate the cracks existing in the surface).
3.3. Mechanical properties by nanoindentation Typical loading–displacement curves in an identical indentation depth of 1000 nm are presented in Fig. 8, indicating that both loading and unloading curves are nonlinear. The maximal displacement reached for the indenter is from contributes of elastic and plastic
deformation, and the elastic recovery is produced during the unloading duration. A comparative result of mechanical properties by indentation between substrate Ti6Al4V and the compact region of a MAO coating is shown in Table 2. The elastic recovery in the MAO coating shows a relative high value compared with that of the substrate and reaches a value of 28.6%. In this case, the residual displacement is determined by the elastic deformation, which is characterized by the features of ceramic materials. While, the elastic recovery in substrate Ti6Al4V is 18.5%, and the residual displacement mainly depends on the plastic deformation. 10
80
85
60
loading
40
20
(b) (a) 0
H E
8
Hardness (GPa)
Applied load (mN)
9
0
200
400
600
unloading
800
7
Displacement (nm) Fig. 8. Typical indentation load–displacement curves of Ti6Al4V substrate and microarc oxidation coating: (a) Ti6Al4V substrate; (b) microarc oxidation coating.
75 70
6
65
5 60
4
55
3
50
2 1
1000
80
0 -20
coating
Substrate -10
0
10
Distance (µm)
Elastic Modulus (GPa)
current density (for example 45 and 35 mA cm2) seal the originally formed bigger pores and cracks, which makes the coating more dense and homogeneous in structure, though the pores in the surface is inevitable depending on the sparking discharge nature.
45 20
40 30
Fig. 9. Distribution of hardness and elastic modulus along the cross-section of microarc oxidation coating on Ti6Al4V.
Y. Wang et al. / Applied Surface Science 233 (2004) 258–267
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Table 2 Comparison of mechanical properties for Ti6Al4V substrate and microarc oxidation coating Indentation position
Hardness (GPa)
Elastic modulus (GPa)
Elastic recovery (%)
Ti6Al4V substrate (5 mm from interface) Microarc oxidation coating (6 mm from interface)
3.4 5.5
74.2 69.1
18.5 28.6
Fig. 9 demonstrates the depth distribution profile of hardness and elastic modulus ranging from substrate through interface to the outmost coating. It can be seen that the substrate and MAO coatings at both sides of the interface differ significantly in hardness and elastic modulus. The corresponding indentation morphology is shown in Fig. 10. Hardness and elastic modulus value for the indentation point in the coating off 2 mm from the interface is 5.2 and 70.0 GPa, respectively. Owing to the fact that with indentation depth of 1000 nm, one angle of the Berkovich indenter applied has interacted with the substrate, so test value is influenced by the substrate to some degree. In the region ranging from the interface to the depth of 26 mm across the coating, hardness and elastic modulus maintain relatively high (above 5.0 and 65.0 GPa, respectively) before reaching the outmost porous layer, which also indicates that the inner region in the MAO coating is relatively compact, uniform and defects free. It is noticeable that hardness in the compact region is higher, whereas the elastic modulus is a bit lower than that of the substrate. Hardness and elastic modulus depend on the distribution of the various phases and the uniformity in microstructure of the MAO coatings. It has been confirmed by XRD analysis that titanium oxide is predominant in the internal layer of the coating, while crystalline AlPO4 is dominant in the looser outer layer, which is respon-
sible for the distributing characterization of mechanical parameters across the coating. 3.4. Adhesion strength Many researchers proposed that a higher adhesion strength of coatings can be achieved using MAO process. However, such a conclusion is mostly based on the observations of experimental phenomena rather than on the support of direct experimental data. Recently, scratch test has been applied to evaluate the adhesion strength of MAO coatings formed on titanium alloys. However, controversial conclusions were drawn. Yerokhin et al. measured the adhesion strength of coatings formed in different electrolytes, and found that the highest critical load Lc2 (96 N) was obtained in coatings formed in KAlO2/Na3PO4 electrolyte [20]. However, using the same method, investigators of literatures [21,22] and this author received no effective data owing to free of the distinct acoustic launching signals caused by coatings failure. Possibly, this is attributed to the higher adhesion strength of coating and substrate interface compared to the internal strength in the coatings. Therefore, in this work, a shear test method was attempted to evaluate the adhesion strength of coatings. As a result, the shear strength determined by Eq. (1), 40 MPa, is regarded as the adhesion strength between coating and substrate. Coatings produced by MAO
Fig. 10. Indentation morphologies of Ti6Al4V substrate and microarc oxidation coating. (Indentations in the coating are marked by numbers).
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Fig. 11. Fractured morphologies of the as-fabricated coatings, the substrate and the interface after shear test. (Arrows indicate the cohesive failure of the coating, region A indicates coating failure, and region B is the glue on the top of the coating).
grow as a result of plasma chemical oxidation reaction of the substrate near the electrolyte, and do not contain an ‘‘artificial’’ interface as deposited coatings do. Therefore, the adhesion strength is expected to be relatively high. However, the measured adhesion strength (nominal strength) is determined not only by the adhesion of the coating to substrate, but also partly by the cohesive strength inside the coating. Fig. 11 shows the fractured morphologies of an asfabricated coating, the substrate and the interface after shear test. It can be seen from Fig. 11(a), the pores of irregular shapes (in arrows), remained in the fractured surface, derives from the partly cohesive nature of the coating failure. From Fig. 11(a) and (b), the fractured surface is rough, with rather small sizes of ceramic grains compared with those existing on the surface of the coatings (Fig. 2). Fig. 11(c) gives the interface of the failed coatings, being a bit zigzag, which also indicates that the adhesion is high. In Fig. 11(c), region A shows the substrate surface after the coating had failed and B shows the coating with a layer of glue on the top (glue was used to bond the coating with the counter cylinder). The polished traces on the surface of the pre-treated sample are not found, which implies that the coating grows toward the inside of the substrate at the cost of consuming substrate metal.
4. Conclusion Ceramic coatings were prepared by microarc oxidation method using a galvanostatic regime on
Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution. The growth kinetics, microstructure and phase composition of coatings were investigated. The results show that with increasing treatment duration, the coating growth varies from rapidness to tardiness accompanied by gradual roughening in appearance. The coating is composed of anatase, rutile and AlPO4 phases. During oxidizing process, phase transformation of anatase to rutile occurs, and crystalline AlPO4 is involved in the coatings via high-temperature thermolysis of hydrated aluminium polyphosphates in the near discharging channel. Using the stepped current regime, the coating microstructure is improved evidently. A similar evolution profile of hardness and elastic modulus across the coating thickness is found, remaining high (5.5 and 69.1 GPa) in the compact region before final declining to low values (5.1 and 65.6 GPa) in the looser region. The distributing characterization of mechanical parameters across the coating depends on the distribution of various phases and the uniformity in microstructure of the MAO coating. The adhesion strength of substrate/coating interface is about 40 MPa, in fact, which is determined by the mixing results of the adhesion of the coating to substrate and the cohesive strength inside the coating. References [1] P.S. Gordienko, S.V. Gnedenkov, S.L. Sinebryukhov, O.A. Khrisanfova, T.M. Skorobogatova, Elektron. Obrab. Mater. (1) (1993) 21 (in Russian).
Y. Wang et al. / Applied Surface Science 233 (2004) 258–267 [2] S.V. Gnedenkov, P.S. Gordienko, S.L. Sinebryukhov, A.N. Kovryanov, O.A. Khrisanfova, A.I. Cherednichenko, S.V. Korkosh, K.D. Khromushkin, Russ. J. Appl. Chem. 73 (2000) 6. [3] G.A. Lavrushin, S.V. Gnedenkov, P.S. Gordienko, S.L. Sinebryukhov, Russ. Prot. Met. 38 (2002) 363. [4] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195. [5] A.L. Yerokhin, A. Leyland, A. Matthews, Appl. Surf. Sci. 200 (2002) 172. [6] W.B. Xue, C. Wang, R.Y. Chen, T.H. Zhang, Mater. Lett. 52 (2002) 435. [7] W.B. Xue, Z.W. Deng, H. Ma, R.Y. Chen, Surf. Eng. 17 (2001) 323. [8] V.S. Rudnev, L.M. Tyrina, V.M. Nikitin, N.V. Speshneva, P.S. Gordienko, Russ. Prot. Met. 39 (2003) 334. [9] V.S. Rudnev, I.V. Lukiyanchuk, D.L. Boguta, V.V. Kon’shin, A.S. Rudnev, P.S. Gordienko, Russ. Prot. Met. 38 (2003) 191. [10] Y. Han, S.H. Hong, K.W. Xu, Surf. Coat. Technol. 154 (2002) 314. [11] J.P. Schreckenbach, G. Marx, F. Schlottig, M. Textor, N.D. Spencer, J. Mater. Sci. Mater. M 10 (1999) 453.
267
[12] S.V. Gnedenkov, P.S. Gordienko, O.A. Khrisanfova, T.M. Scorobogatova, S.L. Sinebrukhov, J. Mater. Sci. 37 (2002) 2263. [13] P.S. Gordienko, S.V. Gnedenkov, S.L. Sinebryukhov, O.A. Khrisanfova, T.M. Skorobogatova, Russ. Electrochem. 29 (1993) 1232. [14] Patent US No. 5,720,866. [15] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [16] V.S. Rudnev, T.P. Yarovaya, V.V. Kon’shin, P.S. Gordienko, Russ. Prot. Met. 39 (2003) 182. [17] A.L. Yerokhin, V.V. Lyubimov, R.V. Ashitkov, Ceram. Int. 24 (1998) 1. [18] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [19] Y.M. Wang, T.Q. Lei, B.L. Jiang, D.C. Jia, Y. Zhou, Mater. Sci. Technol. 11 (2003) 244 (in Chinese). [20] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195. [21] S.V. Gnedenkov, O.A. Khrisanfova, A.G. Zavidnaya, S.L. Sinebrukhov, P.S. Gordienko, S. Iwatsubo, A. Matsui, Surf. Coat. Technol. 145 (2001) 146. [22] P. Huan, K.W. Xu, Y. Han, Rare Met. Mater. Eng. 32 (2003) 272 (in Chinese).
Growth, microstructure and mechanical properties of microarc oxidation coatings on titanium alloy in phosphate-containing solution Yaming Wanga,*, Tingquan Leia, Bailing Jiangb, Lixin Guoa a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China
b
Received in revised form 17 March 2004; accepted 17 March 2004 Available online 30 April 2004
Abstract Ceramic coatings were fabricated by microarc oxidation in galvanostatic regime on Ti6Al4Valloy in (NaPO3)6–NaF–NaAlO2 solution. The growth, microstructure and phase composition of coatings were investigated using scanning electron microscope and X-ray diffraction. With increasing treatment duration, coating growth varies from rapidness to tardiness accompanied by gradually roughening in appearance. Meanwhile, phase transformation of anatase to rutile occurs. The crystalline AlPO4 is involved in the coatings via high-temperature thermolysis of hydrated aluminium polyphosphates in the nearby discharging channels. The stepped current regime enables coating structure to be controllable. The mechanical properties distribution across the coating thickness and the adhesion strength were determined by nanoindentation and shear test, respectively. A similar evolution profile of hardness and elastic modulus across the coating thickness is found: remaining high values (5.5 and 69.1 GPa) in the compact region before finally declining to low values (5.1 and 65.6 GPa) in the looser region. The adhesion strength of substrate/coating interface is about 40 MPa. # 2004 Elsevier B.V. All rights reserved. Keywords: Microarc oxidation; Coating; Microstructure; Mechanical properties; Functional applications
1. Introduction A relatively novel surface modifying technique, conventionally called ‘‘microarc oxidation (MAO)’’, also commonly called ‘‘plasma electrolytic oxidation (PEO)’’, ‘‘microplasma oxidation (MPO)’’ and ‘‘anodic spark deposition (ASD)’’ in modern scientific literatures, is attracting increasing interest in fabricat*
Corresponding author. Tel.: þ86-451-86413910; fax: þ86-451-86413922. E-mail address: [email protected] (Y. Wang).
ing ceramic-like coatings on titanium alloys, with the purpose of providing corrosion- and wear-resistance or various functional properties. In addition, this technique is economic efficiency, ecological friendly and characterized by high productivity. MAO is based on the conventional anodic oxidation of processing metals and alloys in aqueous electrolyte solutions under the additional condition of plasma discharge at exceeding the critical values of the polarization potential, and the fact that discharge leading to localized high temperature and high pressure, allows forming coatings composed of not only predominant
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.231
Y. Wang et al. / Applied Surface Science 233 (2004) 258–267
substrate oxides but of more complex oxides containing compounds which involve the components presented in the electrolyte. Both intrinsic factors (electrolyte compositions and pH) and extrinsic factors (electrical parameters, and electrolyte temperature) affect the formation and microstructure of microarc oxidation coatings. Therein, composition and concentration of electrolyte and electrical parameters during the process play a crucial role in obtaining the desired coatings of special phase component and microstructure. Among them, it is assumed that the intrinsical effects of electrolytes may be summarized as follows: firstly, as the medium of current conduct, transmitting the essential energy needed for anode oxidizing occurring in the interface of metal/electrolyte; secondly, providing the oxygen source in the form of oxysalt needed for oxidation; finally and also more interesting, components presenting in the electrolyte incorporated into the coatings can further modify or improve the properties of microarc oxidation coatings. Various specially selected electrolytes and their combinations have been successfully developed in order to provide protective coatings of corrosion- and wear-resistance [1–7] and diverse functional coatings, such as biocidal or catalytic [8,9], biomedical [10,11], ferroelectric [12], and semiconducting [13] properties, on titanium alloys. It has been found that coatings of phosphorus-containing compounds, introduced by the phosphate-containing electrolytes during the MAO process, can find applications in biocidal effect, catalysis, and corrosion protection. The functional actions mentioned above are attributed to the polyphosphates, elemental phosphorus, or phosphides incorporated into the MAO coatings [8]. The earlier works are focused on optimizing electrolytes composition and concentration in order to obtain the coatings with special phase composition [1–13]. However, no data at present are available
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concerning the growth, microstructure and mechanical properties of such coatings. The primary objectives of this paper are: (1) a new aqueous electrolyte containing (NaPO3)6 was developed, then the growth process and corresponding microstructure evolution of the coatings were investigated; (2) in order to obtain the improved coatings, a stepped adjusting regime of current density was proposed; (3) subsequently, the mechanical hardness and elastic modulus, as well as the adhesion strength between coating and substrate were evaluated.
2. Experimental details 2.1. Specimen fabrication Ti6Al4V discs with a diameter of 30 mm and thickness of 3 mm were used as the substrate. The surface of the discs was polished with 600# abrasive papers to a roughness of Ra 0:12 mm, and then ultrasonically cleaned in distilled water followed in acetone. The discs were used as anodes, while stainless steel plates were used as cathodes in the electrolytic bath. The samples carefully pretreated as above were anodized in accordance with process conditions detailed in Table 1. An aqueous electrolyte was used, containing mainly (NaPO3)6 and some other proprietary substances (namely NaF and NaAlO2) to adjust its alkalinity and increase conductivity, as well as addition of NaAlO2 increased growth rate of the microarc oxidation coatings. A 65 kW AC microarc oxidation device provided constant current which can be set variably depending on the requirements, and the voltage varied with the duration of anodizing time. Voltage magnitude of the negative pulse equals to that of the positive pulse. From our practice, it is the positive pulse rather than the negative pulse plays a decisive role in the microstructure formation of
Table 1 Process conditions for ceramic coatings by the microarc oxidation method Electrolyte composite and concentration
(NaPO3)6–NaF–NaAlO2 (15–6–6 g/L)
Electrical parameters (positive/negative) Current density (mA cm2)
Frequency (Hz)
Duty cycle (%)
60
600/100
8.0/1.0
Electrolyte temperature (8C)
Oxidizing time (min)
<50
2, 5, 10, 15, 30
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coatings. The negative pulse is only interspersed within the positive pulses as a mean to interrupt the spark discharges, permitting the surface to cool and inducing the re-conversion of soluble components into metal oxide [14]. Therefore, in order to obtain coatings with optimum surface morphology, a stepped adjustment regime in positive pulse current was applied. The electrolyte was cooled by a cooling system to keep its temperature below 50 8C. The as-prepared samples were washed with distilled water and dried in an air oven for testing.
modulus were determined using the Oliver and Pharr method [15] for a constant depth of 1000 nm.
2.2. Microstructure characterization
2.4. Adhesion strength test
The thickness of the resulting coatings was measured using a coating thickness gauge (TT230, Time Group Co.). Ten measurements were carried out evenly on each whole test surface of both sides of the samples. After removing maximum and minimum readings, the rest were averaged to obtain the value of the coating thickness. The surface morphology of the coatings was observed using scanning electron microscopy (SEM, S-570, Hitachi Co., Japan). At a number of randomly selected locations, the scanned regions were photographed and these micrographs were subjected to image analysis to measure the number of micropores and their sizes. The phase composition of the coatings was analysed by X-ray diffraction (XRD, Philips X’Pert, Holland) using a Cu Ka radiation at a grazing angle of 38.
Fig. 1 gives the schematic of shear strength test for the microarc oxidation coatings. For the shear sample, the coated side by microarc oxidation treatment and the grit blasted side of two pieces of Ti6Al4V cylinder samples in the same size were glued together by epoxy. The as-prepared shear samples were mounted rightly in a specially designed holding device. A load parallel to the coating was continually applied on the Ti6Al4V cylinder at a rate of 0.5 mm/min using the Instron-1186 stretcher until adhesion failure was found. The instant maximum load was recorded. The shear strength was determined by the following equation:
2.3. Nanoindentation test A nanoindentation testing system (Nanoindenter XP, MTS, America) with a well-calibrated berkovich diamond indenter was employed to determine the mechanical properties. The maximum indentation depth and the maximum load are 500 mm and 500 mN, respectively. The load resolution is about 50 nm and displacement resolution is approximately 0.01 nm. Temperature changes were kept below 1 8C and diamond tip drift limit of 0.25 nm/s was preset for the test to start. All continuous stiffness measurements (CSM) were carried out under displacement control mode with tip-displacement rate of 10 nm/s. Load–displacement curves were recorded. After indenting, indentation morphology was observed using optical microscope (OM) attached to the system. Hardness and elastic
Load Coating Ti Cylinder
Adhesive Ti Cylinder
Fig. 1. Schematic representation of shear test setting.
t¼
Pmax S
(1)
where Pmax is the maximum load, S the area of the coating surface, and t the shear strength. To acquire good statistical results, six identical measurements were carried out. After removing the maximum and minimum calculating values, the rest were averaged to obtain the value of the shear strength. Here, the shear strength calculated is regarded as the adhesion strength of coating to the substrate.
3. Results and discussion 3.1. Growth and microstructure of coatings Fig. 2 presents the variation of coating growth rate versus voltage microarc oxidized at a constant current density mode (j ¼ 60 mA cm2). Clearly, an approximately linear growth of coatings is found within the
Y. Wang et al. / Applied Surface Science 233 (2004) 258–267
600
40
Voltage (V)
500 30 400 20
300 200
10
Coating thickness (µm)
50
700
100 0
5
10
15
20
25
30
0
Treatment time (min) Fig. 2. Variation of coating thickness and voltage during microarc oxidation treatment of Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution.
beginning 10 min. The linear rate constant calculated is 3.0, i.e., the coating growth rate is defined as 3.0 mm/ min. The coating growth rate begins to slow down beyond 10 min. At about 15 min a critical transitional point is observed, from which to the end of the process, the coating growth is not obvious. On the other hand, when considering the oxidizing voltage, at a voltage of about 160 V, the first visible sparking occurs. During the initial oxidizing stage (as low as 2 min), the voltage quickly rises up to 470 V. With the proceeding of the oxidation, the voltage increases slowly. The 15 min later, the voltage reaches, and retains at a higher value of about 660 V. Dielectric breakdown occurs when the applied voltage exceeds the critical voltage of originally naturalformed insulating film on Ti6Al4V alloy, which is the prerequisite for microarc oxidation being able to proceed in consideration of electrical aspect. Since current density defines the intensity of sparking on the surface, when microarc oxidizing at a constant current density of 60 mA cm2, i.e., the sparking intensity is kept at the same level in all the experiments, coatings thickness increases approximately linearly. In order to maintain the constant current density throughout the MAO process, voltage value has to response and increases with the increasing self-resistance in the coating, which leads to a rise in discharging energy of a single pulse, in other words, the discharging intensity of a single pulse is enhanced. As is known from phase analysis of coatings, coating products contains components of both the anode Ti6Al4V and electrolyte, and coating thickens as a result of
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sparking-anodic actions and thermolysis of hydrated aluminium polyphosphates precipitated from the electrolyte [16]. However, the thermolysis product (AlPO4) from the hydrated aluminium polyphosphates has to be subjected to dissolution in so strong an alkaline solution ðpH 12:5Þ. With the increasing precipitation, the coating growth is gradually dominated by the dissolution of thermolysis products, which is responsible for the gradually decreasing growth rate of the coatings. The surface morphology evolutions of the coatings formed on Ti6Al4V alloy during the microarc oxidation process are shown in Fig. 3. The surface is characterized by micropores in different sizes and shapes. The porous feature strongly depends on discharging nature involved in the microarc oxidation mechanism [17]. Variations in number and porosity of micropores are also presented in Fig. 4. It can be seen clearly that at the initial oxidation stage (bellow 2 min), fine and uniform micropores with sizes less than 1 mm distribute randomly on the surface of the coatings. With proceeding of the oxidation, the micropores number quickly decreases in an exponential function, while micropores sizes increases strikingly. As a result, the porosity increases from 3.8% for 2 min to 11.6% for 30 min. The evolution of surface morphology clearly demonstrates the visible decrease in micropores number and increase in micropores size. At the beginning of microarc oxidation, Ti6Al4V alloy with uniform and fewest defects surface facilitates the production of fine, uniform and dispersed microdischarge in considerable numbers. In this case, the number of micropores remained after microdischarge had decayed is rather amazing. The size of the micropores is significantly small, typically less than 2 mm (Fig. 3a). With oxidizing time increasing, the voltage value increases, accompanied by remarkably reducing number of microdischarge channels. The increasing voltage leads to an enhanced discharging energy, hence the increased product mass by a single pulse, which contributes to the enlarged pore sizes after the discharge channels are cooled. Moreover, continually multiple discharging occurs at a local region of relative weakness in the coating, or several discharging channels merge to form a single larger channel, which also leads to the increased pores size. The surface of MAO coatings is gradually roughened
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Fig. 3. Surface morphology of microarc oxidation coatings formed on Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution at different treatment time periods: (a) 2 min; (b) 5 min; (c) 10 min; (d) 15 min; (e) 30 min.
180
ment period. The coatings are mainly composed of two modifiers of titanium oxide, i.e., metastable anatase at low temperature and stable rutile at higher temperature, as well as crystalline AlPO4 is incorporated into the
Anatase Rutile AlPO4 Titanium
12 10
120 100
8
80 60
6
40 20 0
30min 15min
Intensity
140
(A.U.)
160
Porosity (%)
Microporres number (×103⋅mm-2)
as the result of the reduced pores number, as well as the increased pores size (Fig. 3). Fig. 5 gives XRD patterns from the surface of microarc oxidation coatings formed in different treat-
10min
♦
5min
♦ ♦
4 0
5
10
15
20
25
30
Treatment time (min) Fig. 4. Variation of number of micropores and porosity on the surface of the coating during microarc oxidation treatment of Ti6Al4V alloy.
♦
♦
20
30
40
50
60
♦ 2min ♦♦
70
80
2q (°) Fig. 5. Effect of different treatment time periods on phase composition of microarc oxidation coatings on Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution.
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3.2. Structure control by adjustment of current According to our experimental practices, microarc oxidation using the constant current density regime is time saving compared with the constant voltage regime. This is not surprising because the current density, which defines the intensity of sparking on the surface, is kept at the same high level through the microarc oxidation process, which facilitates the coating growth. While upon constant voltage oxidation, current density gradually decays, i.e., sparking intensity decreases in the process, which leads to a relatively low coatings growth rate [19]. However, coatings formed using the constant current density regime tends to have a loose and rough microstructure. For obtaining the optimum microstructure of relatively
700
a’
600
b’ c’ a
500
80 60
400
b c
300 200
40
20
100 0
Current density (mA·cm-2)
100
800
Mean voltage (V)
coatings. As the treatment time increases, the relative amount of anatase phase decreases, while that of rutile increases, which indicates that phase transformation of anatase to rutile occurs during the oxidizing process. The crystalline AlPO4 is involved in the coatings via high-temperature thermolysis of hydrated aluminium polyphosphates in the nearby channel zone of microdischarging [16], and its content also shows slight increase. Noticeably, diffraction peaks of titanium substrate are rather strong in coatings at the oxidation time of 2 min, which is attributed to the very thin film of about 5 mm, and the thickness is bellow the maximum X-ray penetrating depth. During microarc oxidation, the temperature and pressure in the discharge channels can reach about 2 104 K and 102 MPa [18], which promote the conversion of Ti substrate into titanium oxides. Meanwhile, the incorporation of electrolyte components into coatings results from the high-temperature reactions proceeding just in discharges channels or adjacent areas. As the coatings thickness increases, the previously formed products are calcined repeatedly under the effect of the frequent microdischarge, which facilitates the anatase to rutile transformation, as well as the thermolysis process. Furthermore, the low thermal conductivity of titanium dioxide causes the underlying layer of the coatings to become heated, which also promotes the further transformation of the initially formed metastable anatase to the stable rutile phase. Thereby, the amount of rutile phase in the coatings increases with increasing oxidizing time.
263
0 0
2
4
6
8
10
12
14
16
Treat time (min) Fig. 6. The stepped adjusting regime of current density and associated to variation of voltage during microarc oxidation process: a,a0 – single-stepped; b,b0 – two-stepped; c,c0 – three-stepped.
compact coatings, a stepped adjusting of current density during microarc oxidation process was proposed. Variation of mean voltage responding to the different stepped adjusting regime of current density during microarc oxidation process is shown in Fig. 6, and the corresponding surface morphology evolution is presented in Fig. 7. Evidently, using the single-stepped adjusting mode of current density (constant current density), the coating surface is loose, with a lot of big and long pores of different sizes accompanied by many visual cracks (shown in arrows). In the case of twostepped mode, though no visual cracks are found in the surface, the pore sizes still maintain big. While, for three-stepped mode, both the sizes and the number of the pores decrease, and no cracks are observed in the surface. In contrast with the singleand two-stepped mode, the coating microstructure is improved evidently. Varying current density can regulate the surface discharge characteristics. High current density leads to increased sparking discharge intensity caused by the highly energy pulse (high pulse voltage), which contributes to the increasing accumulation of coating products, and readily forms the coarse grains structure of the coatings. In contrast, low current density readily tends to form the fine grain structure. The mechanism of the stepped adjusting in current density is attributed to the following process: at the first stage, the coating grows quickly at a relatively high current density (j ¼ 60 mA cm2); while, at the later stages, fine grains produced because the stepwise decrease in
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Fig. 7. Surface morphology evolution of microarc oxidation coatings on Ti6Al4V alloy processed at different electrical control mode of current density: (a) single-step mode; (b) two-step mode; (c) three-step mode. (note: arrows indicate the cracks existing in the surface).
3.3. Mechanical properties by nanoindentation Typical loading–displacement curves in an identical indentation depth of 1000 nm are presented in Fig. 8, indicating that both loading and unloading curves are nonlinear. The maximal displacement reached for the indenter is from contributes of elastic and plastic
deformation, and the elastic recovery is produced during the unloading duration. A comparative result of mechanical properties by indentation between substrate Ti6Al4V and the compact region of a MAO coating is shown in Table 2. The elastic recovery in the MAO coating shows a relative high value compared with that of the substrate and reaches a value of 28.6%. In this case, the residual displacement is determined by the elastic deformation, which is characterized by the features of ceramic materials. While, the elastic recovery in substrate Ti6Al4V is 18.5%, and the residual displacement mainly depends on the plastic deformation. 10
80
85
60
loading
40
20
(b) (a) 0
H E
8
Hardness (GPa)
Applied load (mN)
9
0
200
400
600
unloading
800
7
Displacement (nm) Fig. 8. Typical indentation load–displacement curves of Ti6Al4V substrate and microarc oxidation coating: (a) Ti6Al4V substrate; (b) microarc oxidation coating.
75 70
6
65
5 60
4
55
3
50
2 1
1000
80
0 -20
coating
Substrate -10
0
10
Distance (µm)
Elastic Modulus (GPa)
current density (for example 45 and 35 mA cm2) seal the originally formed bigger pores and cracks, which makes the coating more dense and homogeneous in structure, though the pores in the surface is inevitable depending on the sparking discharge nature.
45 20
40 30
Fig. 9. Distribution of hardness and elastic modulus along the cross-section of microarc oxidation coating on Ti6Al4V.
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Table 2 Comparison of mechanical properties for Ti6Al4V substrate and microarc oxidation coating Indentation position
Hardness (GPa)
Elastic modulus (GPa)
Elastic recovery (%)
Ti6Al4V substrate (5 mm from interface) Microarc oxidation coating (6 mm from interface)
3.4 5.5
74.2 69.1
18.5 28.6
Fig. 9 demonstrates the depth distribution profile of hardness and elastic modulus ranging from substrate through interface to the outmost coating. It can be seen that the substrate and MAO coatings at both sides of the interface differ significantly in hardness and elastic modulus. The corresponding indentation morphology is shown in Fig. 10. Hardness and elastic modulus value for the indentation point in the coating off 2 mm from the interface is 5.2 and 70.0 GPa, respectively. Owing to the fact that with indentation depth of 1000 nm, one angle of the Berkovich indenter applied has interacted with the substrate, so test value is influenced by the substrate to some degree. In the region ranging from the interface to the depth of 26 mm across the coating, hardness and elastic modulus maintain relatively high (above 5.0 and 65.0 GPa, respectively) before reaching the outmost porous layer, which also indicates that the inner region in the MAO coating is relatively compact, uniform and defects free. It is noticeable that hardness in the compact region is higher, whereas the elastic modulus is a bit lower than that of the substrate. Hardness and elastic modulus depend on the distribution of the various phases and the uniformity in microstructure of the MAO coatings. It has been confirmed by XRD analysis that titanium oxide is predominant in the internal layer of the coating, while crystalline AlPO4 is dominant in the looser outer layer, which is respon-
sible for the distributing characterization of mechanical parameters across the coating. 3.4. Adhesion strength Many researchers proposed that a higher adhesion strength of coatings can be achieved using MAO process. However, such a conclusion is mostly based on the observations of experimental phenomena rather than on the support of direct experimental data. Recently, scratch test has been applied to evaluate the adhesion strength of MAO coatings formed on titanium alloys. However, controversial conclusions were drawn. Yerokhin et al. measured the adhesion strength of coatings formed in different electrolytes, and found that the highest critical load Lc2 (96 N) was obtained in coatings formed in KAlO2/Na3PO4 electrolyte [20]. However, using the same method, investigators of literatures [21,22] and this author received no effective data owing to free of the distinct acoustic launching signals caused by coatings failure. Possibly, this is attributed to the higher adhesion strength of coating and substrate interface compared to the internal strength in the coatings. Therefore, in this work, a shear test method was attempted to evaluate the adhesion strength of coatings. As a result, the shear strength determined by Eq. (1), 40 MPa, is regarded as the adhesion strength between coating and substrate. Coatings produced by MAO
Fig. 10. Indentation morphologies of Ti6Al4V substrate and microarc oxidation coating. (Indentations in the coating are marked by numbers).
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Fig. 11. Fractured morphologies of the as-fabricated coatings, the substrate and the interface after shear test. (Arrows indicate the cohesive failure of the coating, region A indicates coating failure, and region B is the glue on the top of the coating).
grow as a result of plasma chemical oxidation reaction of the substrate near the electrolyte, and do not contain an ‘‘artificial’’ interface as deposited coatings do. Therefore, the adhesion strength is expected to be relatively high. However, the measured adhesion strength (nominal strength) is determined not only by the adhesion of the coating to substrate, but also partly by the cohesive strength inside the coating. Fig. 11 shows the fractured morphologies of an asfabricated coating, the substrate and the interface after shear test. It can be seen from Fig. 11(a), the pores of irregular shapes (in arrows), remained in the fractured surface, derives from the partly cohesive nature of the coating failure. From Fig. 11(a) and (b), the fractured surface is rough, with rather small sizes of ceramic grains compared with those existing on the surface of the coatings (Fig. 2). Fig. 11(c) gives the interface of the failed coatings, being a bit zigzag, which also indicates that the adhesion is high. In Fig. 11(c), region A shows the substrate surface after the coating had failed and B shows the coating with a layer of glue on the top (glue was used to bond the coating with the counter cylinder). The polished traces on the surface of the pre-treated sample are not found, which implies that the coating grows toward the inside of the substrate at the cost of consuming substrate metal.
4. Conclusion Ceramic coatings were prepared by microarc oxidation method using a galvanostatic regime on
Ti6Al4V alloy in (NaPO3)6–NaF–NaAlO2 solution. The growth kinetics, microstructure and phase composition of coatings were investigated. The results show that with increasing treatment duration, the coating growth varies from rapidness to tardiness accompanied by gradual roughening in appearance. The coating is composed of anatase, rutile and AlPO4 phases. During oxidizing process, phase transformation of anatase to rutile occurs, and crystalline AlPO4 is involved in the coatings via high-temperature thermolysis of hydrated aluminium polyphosphates in the near discharging channel. Using the stepped current regime, the coating microstructure is improved evidently. A similar evolution profile of hardness and elastic modulus across the coating thickness is found, remaining high (5.5 and 69.1 GPa) in the compact region before final declining to low values (5.1 and 65.6 GPa) in the looser region. The distributing characterization of mechanical parameters across the coating depends on the distribution of various phases and the uniformity in microstructure of the MAO coating. The adhesion strength of substrate/coating interface is about 40 MPa, in fact, which is determined by the mixing results of the adhesion of the coating to substrate and the cohesive strength inside the coating. References [1] P.S. Gordienko, S.V. Gnedenkov, S.L. Sinebryukhov, O.A. Khrisanfova, T.M. Skorobogatova, Elektron. Obrab. Mater. (1) (1993) 21 (in Russian).
Y. Wang et al. / Applied Surface Science 233 (2004) 258–267 [2] S.V. Gnedenkov, P.S. Gordienko, S.L. Sinebryukhov, A.N. Kovryanov, O.A. Khrisanfova, A.I. Cherednichenko, S.V. Korkosh, K.D. Khromushkin, Russ. J. Appl. Chem. 73 (2000) 6. [3] G.A. Lavrushin, S.V. Gnedenkov, P.S. Gordienko, S.L. Sinebryukhov, Russ. Prot. Met. 38 (2002) 363. [4] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195. [5] A.L. Yerokhin, A. Leyland, A. Matthews, Appl. Surf. Sci. 200 (2002) 172. [6] W.B. Xue, C. Wang, R.Y. Chen, T.H. Zhang, Mater. Lett. 52 (2002) 435. [7] W.B. Xue, Z.W. Deng, H. Ma, R.Y. Chen, Surf. Eng. 17 (2001) 323. [8] V.S. Rudnev, L.M. Tyrina, V.M. Nikitin, N.V. Speshneva, P.S. Gordienko, Russ. Prot. Met. 39 (2003) 334. [9] V.S. Rudnev, I.V. Lukiyanchuk, D.L. Boguta, V.V. Kon’shin, A.S. Rudnev, P.S. Gordienko, Russ. Prot. Met. 38 (2003) 191. [10] Y. Han, S.H. Hong, K.W. Xu, Surf. Coat. Technol. 154 (2002) 314. [11] J.P. Schreckenbach, G. Marx, F. Schlottig, M. Textor, N.D. Spencer, J. Mater. Sci. Mater. M 10 (1999) 453.
267
[12] S.V. Gnedenkov, P.S. Gordienko, O.A. Khrisanfova, T.M. Scorobogatova, S.L. Sinebrukhov, J. Mater. Sci. 37 (2002) 2263. [13] P.S. Gordienko, S.V. Gnedenkov, S.L. Sinebryukhov, O.A. Khrisanfova, T.M. Skorobogatova, Russ. Electrochem. 29 (1993) 1232. [14] Patent US No. 5,720,866. [15] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [16] V.S. Rudnev, T.P. Yarovaya, V.V. Kon’shin, P.S. Gordienko, Russ. Prot. Met. 39 (2003) 182. [17] A.L. Yerokhin, V.V. Lyubimov, R.V. Ashitkov, Ceram. Int. 24 (1998) 1. [18] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [19] Y.M. Wang, T.Q. Lei, B.L. Jiang, D.C. Jia, Y. Zhou, Mater. Sci. Technol. 11 (2003) 244 (in Chinese). [20] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195. [21] S.V. Gnedenkov, O.A. Khrisanfova, A.G. Zavidnaya, S.L. Sinebrukhov, P.S. Gordienko, S. Iwatsubo, A. Matsui, Surf. Coat. Technol. 145 (2001) 146. [22] P. Huan, K.W. Xu, Y. Han, Rare Met. Mater. Eng. 32 (2003) 272 (in Chinese).