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Effects of microarc oxidation surface treatment on the mechanical properties of Mg alloy and Mg matrix composites
Materials Science and Engineering A 447 (2007) 227–232
Effects of microarc oxidation surface treatment on the mechanical properties of Mg alloy and Mg matrix composites K. Wu, Y.Q. Wang ∗ , M.Y. Zheng School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Received 5 July 2006; received in revised form 14 September 2006; accepted 26 October 2006
Abstract Mechanical properties of coated AZ91 Mg alloy, Al18 B4 O33 w/AZ91 and SiCw/AZ91 composites by microarc oxidation (MAO) surface treatment technique were measured and effects of MAO treatment on the mechanical properties were investigated. Tensile test shows that MAO treatment just slightly decreases ultimate tensile strength (UTS) and elongation of the Mg-based materials, while enhances their elastic modulus to some extent. But the effects of MAO treatment on the mechanical properties will be influenced by the energy parameters applied during surface treatment. The three materials coated under the same process conditions have exhibited differential surface morphology and fracture after tensile test, which indirectly indicates that the qualities of MAO coatings are also related with the substrate materials. © 2006 Elsevier B.V. All rights reserved. Keywords: Mg alloy; Composite; Microarc oxidation; Coatings; Mechanical properties
1. Introduction Mg-based materials are considered to have great applied potential since their low densities can meet the need of lightweight structural materials. But their poor corrosion resistance resulting from chemically active nature of Mg has become one of the most serious bottlenecks which greatly limit the application of Mg-based materials [1]. Fortunately, many surface treatment techniques have been applied for protecting Mg alloys against corrosion attack [2,3], with microarc oxidation (MAO) being one of the most popular methods in recent years [4]. MAO technique can provide a protective ceramic coating on surface of valve metals by a relatively simple and cost-effective process. Essentially, MAO is an advancement of the conventional anodization. MAO coating is mainly composed of oxide of the substrate itself and usually exhibits a uniform porous morphology because it combines conventional anodization with a high-voltage sparking discharge process. Early work has demonstrated that MAO coating can protect Mg alloys and Mg matrix composites against corrosion attack largely [5]. But during MAO, dielectric breakdown of the coating and subsequent sparking discharge would occur repeatedly, which will lead to the
∗
Corresponding author. Tel.: +86 451 86402291; fax: +86 451 86413922. E-mail address: [email protected] (Y.Q. Wang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.133
local instantaneous high temperature and high pressure inside the discharge channels [6] and may induce flaws on the substrate surface. And that the residual stress in the coating arising during the deposition process also may result in some micro-cracks [7]. These structural defects and inherent porous microstructure of the MAO coating are like to influence the mechanical properties of the substrate. Therefore, as a promising surface treatment technique, whether MAO treatment would weaken the mechanical properties of the substrate will be an interesting and also important problem in practice. Some research work [7–9] has investigated the mechanical properties variation of Mg and Al alloys after undergoing surface treatment by anodization or MAO, but consistent conclusions and explanations have not been obtained till now. One of the main purposes of this study is to investigate the effect of MAO surface treatment on mechanical properties of Mg-based materials using tensile test and also analyze the effect mechanism; another purpose is to investigate the difference in coating characteristics between different substrate materials by observation of fracture and morphology of the coatings after undergoing tensile test. 2. Experimental details The materials used in this study were AZ91 alloy with nominal composition of Mg–9%Al–0.9%Zn in mass, 22 vol.% Al18 B4 O33 whisker reinforced AZ91 composite and 22 vol.%
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Table 1 Electrical parameters for MAO treatment and data of coating thickness Electrical parameters (+/−) Current density
6 12
(A/dm2 )
Thickness of coatings Duty cycle (%)
8/1 8/1
Frequency (Hz)
500/300 500/300
A
B
C
TC (m)
PC (%)
TC (m)
PC (%)
TC (m)
PC (%)
28 50
3.02 5.26
20 43
2.17 4.56
14.7 28
1.61 3.02
Note: A, B and C represent the used substrates of AZ91 alloy, Al18 B4 O33 w/AZ91 and SiCw/AZ91, respectively. TC and PC represent thickness of MAO coating and percent of coating thickness to total thickness of coated sample, respectively.
-SiC whisker reinforced AZ91 composite. The composites were fabricated by a squeeze casting method. The properties of Al18 B4 O33 whisker and -SiC whisker and the details of composites fabrication have been described in elsewhere [10,11]. The specimens were polished and degreased ultrasonically in acetone before MAO treatment. During MAO, the specimen was always used as the anode, while a stainless steel plate was used as the cathode. The electrolyte was prepared from a solution of sodium silicate in distilled water with other additives. A MAO device with AC pulse power supply was adopted. For 1 pulse output cycle, 20 positive pulses were followed by 2 negative pulses. The cell voltage varies with treatment time when a constant current mode is applied during MAO treatment. The detailed electrical parameters for MAO treatment including both positive and negative parameters are listed in Table 1. The same process conditions are selected for the three substrates. The thickness of MAO coating was measured by an eddy current thickness gauge. The percent of coating thickness to total thickness of coated sample (PC ) is approximately calculated as 2TC × 100/(2TC + TS ), namely 2TC × 100/T, where TC , TS and T represent thickness of MAO coating, thickness of uncoated sample and total thickness of coated sample, respectively. The mechanical properties of the materials before and after MAO treatment were evaluated by tensile test using an Instron 5569 universal test machine. Sheet tensile specimens with gauge length of 15 mm and cross-sectional area of 6 mm (in width) × 1.8 mm (in thickness) were subjected to tensile test at a tension speed of 0.083 mm/s. Every series experiment had three same specimens in order to obtain mean value and corresponding error. After tensile test, the fracture and surface morphology of the coatings were observed by scanning electron microscope (SEM).
3. Results 3.1. Changes of the mechanical properties after MAO treatment Changes of the mechanical properties including UTS, elongation to failure and elastic modulus after the three substrates are coated by MAO at different current density are shown in Fig. 1(a–c), respectively. The results show that UTS and elongation hardly change or decrease slightly when the substrates are coated at 6 A/dm2 . But when the applied current density is increased to 12 A/dm2 , UTS and elongation of all the three
coated materials are distinctly lower than those of their corresponding substrate materials. However, in the case of elastic modulus, a converse variation tendency is obtained. When the substrates are coated at 6 A/dm2 , elastic modulus of all the three coated materials is remarkably increased. Especially for coated Al18 B4 O33 w/AZ91 composite, the elastic modulus is even 10 GPa higher than that of the substrate. With the increasing current density, the elastic modulus decreases, but still is higher than that of the substrate materials. 3.2. Morphology and fracture of the MAO coatings after suffering tensile test The typical surface morphologies of the three coated materials after suffering tensile test are shown in Fig. 2. It is observed that the ceramic coating formed on AZ91 alloy (shown in Fig. 2(a and b)) is not been stripped from the substrate in a large area during the process of tensile test, which indicates high bonding strength between the coating and substrate. But the coating is split uniformly due to the great difference in plasticity between the ceramic coating and alloy substrate, and then some coating chipping and micro-cracks are created. Fig. 2(c and d) shows that the MAO coating formed on Al18 B4 O33 w/AZ91 composite can hardly been destroyed after suffering tensile test. Except some micro-cracks near the fracture, any damages to the coating, such as delamination or spalling of coating, do not occur. But the coating formed on SiCw/AZ91 composite has exhibited severe damage after tensile test. Close to the fracture, the coating scales off from the composite substrate (shown in Fig. 2(e)). This indicates the relatively poor bonding strength between the coating and substrate. Moreover, many cracks originate in the coating near the facture (shown in Fig. 2(f)). The tensile fractographs of the coated AZ91 alloy, Al18 B4 O33 w/AZ91 composite and SiCw/AZ91 composite are shown in Fig. 3(a–c), respectively. The fracture of the coated AZ91 alloy reveals that the rupture mechanism of the ceramic coating is distinctly different from that of the substrate. The MAO coating formed on Al18 B4 O33 w/AZ91 composite appears to be well bonded with the substrate since there is not microcrack or other flaw in the cross-sectional fracture and that there is even no clear and distinct interface between the substrate and the coating. This intuitively demonstrates the excellent bonding strength. But for the coated SiCw/AZ91 composite, there is a distinct interface between the substrate and coating, and that a crevice appears in the interface, which may occur during the process of tensile test.
K. Wu et al. / Materials Science and Engineering A 447 (2007) 227–232
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Fig. 1. Mechanical properties including (a) UTS, (b) elongation and (c) elastic modulus of the uncoated and coated Mg alloy and Mg matrix composites. It should be noted that AZ91 alloy is solution and aging heat treated (T6 state), Al18 B4 O33 w/AZ91 composite is as-cast state and SiCw/AZ91 composite is as-extruded state, therefore the mechanical properties between the three substrates have not comparability.
4. Discussion MAO technique, as an efficient and potential surface protection method, is expected to provide corrosion and/or wear protection for valve metals, and at the same time not to greatly degrade the mechanical properties of substrates used as structural materials. It is known that MAO treatment is a repeated dielectric breakdown and sparking discharge process, and that intense arcing discharge even occurs at the late stage of MAO. Moreover, residual stresses may arise in the MAO coating during the deposition process due to the growth of oxide film on the substrate surface or thermal effects from microdischarge [12]. The intense discharge process and the residual stress in the coating will induce flaws in the substrate–coating system, such as microcracks [13]. These flaws will become possible originations of damage to the coated material when the load is applied on it. In this work, it has been found that MAO treatment just decreases UTS and elongation of the Mg-based materials slightly when relatively lower current density is selected during MAO. It should be noted that the phenomena of discharge and microstructure of the MAO coating are closely related to the electrical param-
eters. The intensity of discharge depends on the magnitude of energy density distributing on the sample, which depends on the applied current density under constant current mode when other electrical parameters (frequency and duty cycle) remain constant. Therefore, relatively high energy parameters applied during MAO will result in intenser discharge process and then induce higher porosity or more serious structural flaws than relatively low energy parameters. Thus, the mechanical properties of the Mg-based materials decrease evidently when they are microarc oxidized at relatively high current density. Consequently, if MAO technique is adopted to protect Mg-based structural materials against corrosion or wear attacks, appropriate electrical parameters should be selected to avoid or decrease the damage to their mechanical properties as far as possible. But it is found that elastic modulus of the substrate–coating structure is enhanced comparing with that of the substrate. MAO coating is an oxide ceramic coating which usually is mainly composed of oxide of the substrate itself. Previous study has demonstrated that MAO coatings formed on AZ91 Mg alloy and Mg matrix composites almost have the same phase composition, with cubic MgO as the main phase [5]. The elastic
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Fig. 2. Surface morphology of the MAO coatings obtained at current density of 12 A/dm2 on (a and b) AZ91 alloy, (c and d) Al18 B4 O33 w/AZ91 composite and (e and f) SiCw/AZ91 composite after undergoing tensile test.
modulus of bulk MgO ceramic material is 250 GPa, which is far more than that of AZ91 alloy (43 GPa). If just considering an ideal condition, namely excluding the effect of porosity or other structural defects in the coating on elastic modulus of MgO, and that the percent of coating thickness to total thickness of the coated sample is not negligible (the corresponding data are listed in Table 1), it is very likely to improve elastic modulus of lamellar Mg-based materials to a certain extent by MAO surface treatment due to the high elastic modulus of MgO ceramic coating and high bonding strength between the coating and sub-
strate resulting from the inherent characteristic of in-situ-growth of MAO coating on substrate surface. But if coating thickness is negligible comparing with the total thickness of coated sample, the contribution of MAO coating to elastic modulus of the substrate–coating structure will be slight. In addition, as mentioned above, relatively high energy parameters applied during MAO will bring higher porosity or more serious structural flaws, which will further decrease elastic modulus of the MgO coating. Therefore, the contribution of MAO coating to the elastic modulus of substrate–coating system not only depends on the
K. Wu et al. / Materials Science and Engineering A 447 (2007) 227–232
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Fig. 3. Tensile fracture of coated (a) AZ91 alloy, (b) Al18 B4 O33 w/AZ91 composite and (c) SiCw/AZ91 composite by MAO at current density of 12 A/dm2 .
percent of coating thickness to total thickness but also on the quality of the coating, such as its porosity or density. It also has been found that the coatings formed on the three substrates under the same process conditions have exhibited different morphologies and fractures after tensile test. Early study [5] has revealed that great differences in growth, microstructures and performances exist between the MAO coatings on the three substrates due to the presence of reinforcement phases (whiskers) with different properties in Mg alloy matrix, and that the coatings formed on AZ91 alloy and Al18 B4 O33 w/AZ91 composite present more compact microstructure and better cor-
rosion resistance than that on SiCw/AZ91 composite. This is consistent with the phenomena observed during the process of tensile test. From surface morphologies and fractures after tension, it can be seen that the coatings on AZ91 alloy and Al18 B4 O33 w/AZ91 composite exhibit excellent bonding state with the corresponding substrates. Especially, the coating obtained on Al18 B4 O33 w/AZ91 presents relatively great contribution to the total elastic modulus of the substrate–coating structure, which should be closely related with its compact microstructure and perfect substrate–coating interface (shown in Fig. 3(b)). Therefore, effects of MAO surface treatment on
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mechanical properties of the Mg-based materials not only are related with the process conditions applied during MAO but also related with substrate itself since the quality of MAO coating will be partially influenced by the substrate material which the coating forms on. 5. Conclusions MAO surface treatment just decreases UTS and elongation of the Mg-based materials slightly when relatively lower current density is selected during MAO. But UTS and elongation will decrease evidently when high current density is applied because high energy parameters applied during MAO will result in intenser discharge process and may induce more structural flaws than low energy parameters, and then increase possible originations of damage to the substrate–coating structure during load-bearing. However, elastic modulus of the substrate–coating structure is enhanced comparing with that of the corresponding substrate due to the high elastic modulus of MgO ceramic coating and good bonding state between the coating and substrate. Therefore, MAO treatment will not greatly decrease the mechanical properties of the Mg-based materials when appropriate electrical parameters are selected, especially it will enhance elastic modulus to some extent if the percent of coating thickness to total thickness of coated sample is not negligible. The effects
of MAO treatment on mechanical properties are also related with the substrate materials. References [1] E.F. Emley, Principles of Magnesium Technology, Pergamon Press Ltd., 1966. [2] Y.J. Zhang, C.W. Yan, F.H. Wang, H.Y. Lou, C.N. Cao, Surf. Coat. Technol. 161 (2002) 36–43. [3] E.H. Han, W.Q. Zhou, D.Y. Shan, W. Ke, Mater. Sci. Forum 419–422 (2003) 879–882. [4] A.L. Yerokhin, A. Shatrov, V. Samsonov, P. Shashkov, A. Leyland, A. Matthews, Surf. Coat. Technol. 182 (2004) 78–84. [5] Y.Q. Wang, K. Wu, M.Y. Zheng, Surf. Coat. Technol. 201 (2006) 353. [6] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195–206. [7] X.S. Wang, X.Q. Feng, X.W. Guo, Key Eng. Mater. 261–263 (2004) 363–368. [8] P. Ross, J. MacCulloch, C. Clapp, R. Esdaile, SAE Technical Paper Series, 1999-01-0925. [9] W. Xue, C. Wang, R. Chen, Z. Deng, Mater. Lett. 52 (2002) 435–441. [10] M.Y. Zheng, W.C. Zhang, K. Wu, C.K. Yao, J. Mater. Sci. 38 (2003) 2647–2654. [11] M.Y. Zheng, K. Wu, M. Liang, S. Kamado, Y. Kojima, Mater. Sci. Eng. A 372 (2004) 66–74. [12] R.H.U. Khan, A.L. Yerokhin, T. Pilkington, A. Leyland, A. Matthews, Surf. Coat. Technol. 200 (2005) 1580–1586. [13] Y.Q. Wang, M.Y. Zheng, K. Wu, Mater. Lett. 59 (2005) 1727–1731.
Effects of microarc oxidation surface treatment on the mechanical properties of Mg alloy and Mg matrix composites K. Wu, Y.Q. Wang ∗ , M.Y. Zheng School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Received 5 July 2006; received in revised form 14 September 2006; accepted 26 October 2006
Abstract Mechanical properties of coated AZ91 Mg alloy, Al18 B4 O33 w/AZ91 and SiCw/AZ91 composites by microarc oxidation (MAO) surface treatment technique were measured and effects of MAO treatment on the mechanical properties were investigated. Tensile test shows that MAO treatment just slightly decreases ultimate tensile strength (UTS) and elongation of the Mg-based materials, while enhances their elastic modulus to some extent. But the effects of MAO treatment on the mechanical properties will be influenced by the energy parameters applied during surface treatment. The three materials coated under the same process conditions have exhibited differential surface morphology and fracture after tensile test, which indirectly indicates that the qualities of MAO coatings are also related with the substrate materials. © 2006 Elsevier B.V. All rights reserved. Keywords: Mg alloy; Composite; Microarc oxidation; Coatings; Mechanical properties
1. Introduction Mg-based materials are considered to have great applied potential since their low densities can meet the need of lightweight structural materials. But their poor corrosion resistance resulting from chemically active nature of Mg has become one of the most serious bottlenecks which greatly limit the application of Mg-based materials [1]. Fortunately, many surface treatment techniques have been applied for protecting Mg alloys against corrosion attack [2,3], with microarc oxidation (MAO) being one of the most popular methods in recent years [4]. MAO technique can provide a protective ceramic coating on surface of valve metals by a relatively simple and cost-effective process. Essentially, MAO is an advancement of the conventional anodization. MAO coating is mainly composed of oxide of the substrate itself and usually exhibits a uniform porous morphology because it combines conventional anodization with a high-voltage sparking discharge process. Early work has demonstrated that MAO coating can protect Mg alloys and Mg matrix composites against corrosion attack largely [5]. But during MAO, dielectric breakdown of the coating and subsequent sparking discharge would occur repeatedly, which will lead to the
∗
Corresponding author. Tel.: +86 451 86402291; fax: +86 451 86413922. E-mail address: [email protected] (Y.Q. Wang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.133
local instantaneous high temperature and high pressure inside the discharge channels [6] and may induce flaws on the substrate surface. And that the residual stress in the coating arising during the deposition process also may result in some micro-cracks [7]. These structural defects and inherent porous microstructure of the MAO coating are like to influence the mechanical properties of the substrate. Therefore, as a promising surface treatment technique, whether MAO treatment would weaken the mechanical properties of the substrate will be an interesting and also important problem in practice. Some research work [7–9] has investigated the mechanical properties variation of Mg and Al alloys after undergoing surface treatment by anodization or MAO, but consistent conclusions and explanations have not been obtained till now. One of the main purposes of this study is to investigate the effect of MAO surface treatment on mechanical properties of Mg-based materials using tensile test and also analyze the effect mechanism; another purpose is to investigate the difference in coating characteristics between different substrate materials by observation of fracture and morphology of the coatings after undergoing tensile test. 2. Experimental details The materials used in this study were AZ91 alloy with nominal composition of Mg–9%Al–0.9%Zn in mass, 22 vol.% Al18 B4 O33 whisker reinforced AZ91 composite and 22 vol.%
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Table 1 Electrical parameters for MAO treatment and data of coating thickness Electrical parameters (+/−) Current density
6 12
(A/dm2 )
Thickness of coatings Duty cycle (%)
8/1 8/1
Frequency (Hz)
500/300 500/300
A
B
C
TC (m)
PC (%)
TC (m)
PC (%)
TC (m)
PC (%)
28 50
3.02 5.26
20 43
2.17 4.56
14.7 28
1.61 3.02
Note: A, B and C represent the used substrates of AZ91 alloy, Al18 B4 O33 w/AZ91 and SiCw/AZ91, respectively. TC and PC represent thickness of MAO coating and percent of coating thickness to total thickness of coated sample, respectively.
-SiC whisker reinforced AZ91 composite. The composites were fabricated by a squeeze casting method. The properties of Al18 B4 O33 whisker and -SiC whisker and the details of composites fabrication have been described in elsewhere [10,11]. The specimens were polished and degreased ultrasonically in acetone before MAO treatment. During MAO, the specimen was always used as the anode, while a stainless steel plate was used as the cathode. The electrolyte was prepared from a solution of sodium silicate in distilled water with other additives. A MAO device with AC pulse power supply was adopted. For 1 pulse output cycle, 20 positive pulses were followed by 2 negative pulses. The cell voltage varies with treatment time when a constant current mode is applied during MAO treatment. The detailed electrical parameters for MAO treatment including both positive and negative parameters are listed in Table 1. The same process conditions are selected for the three substrates. The thickness of MAO coating was measured by an eddy current thickness gauge. The percent of coating thickness to total thickness of coated sample (PC ) is approximately calculated as 2TC × 100/(2TC + TS ), namely 2TC × 100/T, where TC , TS and T represent thickness of MAO coating, thickness of uncoated sample and total thickness of coated sample, respectively. The mechanical properties of the materials before and after MAO treatment were evaluated by tensile test using an Instron 5569 universal test machine. Sheet tensile specimens with gauge length of 15 mm and cross-sectional area of 6 mm (in width) × 1.8 mm (in thickness) were subjected to tensile test at a tension speed of 0.083 mm/s. Every series experiment had three same specimens in order to obtain mean value and corresponding error. After tensile test, the fracture and surface morphology of the coatings were observed by scanning electron microscope (SEM).
3. Results 3.1. Changes of the mechanical properties after MAO treatment Changes of the mechanical properties including UTS, elongation to failure and elastic modulus after the three substrates are coated by MAO at different current density are shown in Fig. 1(a–c), respectively. The results show that UTS and elongation hardly change or decrease slightly when the substrates are coated at 6 A/dm2 . But when the applied current density is increased to 12 A/dm2 , UTS and elongation of all the three
coated materials are distinctly lower than those of their corresponding substrate materials. However, in the case of elastic modulus, a converse variation tendency is obtained. When the substrates are coated at 6 A/dm2 , elastic modulus of all the three coated materials is remarkably increased. Especially for coated Al18 B4 O33 w/AZ91 composite, the elastic modulus is even 10 GPa higher than that of the substrate. With the increasing current density, the elastic modulus decreases, but still is higher than that of the substrate materials. 3.2. Morphology and fracture of the MAO coatings after suffering tensile test The typical surface morphologies of the three coated materials after suffering tensile test are shown in Fig. 2. It is observed that the ceramic coating formed on AZ91 alloy (shown in Fig. 2(a and b)) is not been stripped from the substrate in a large area during the process of tensile test, which indicates high bonding strength between the coating and substrate. But the coating is split uniformly due to the great difference in plasticity between the ceramic coating and alloy substrate, and then some coating chipping and micro-cracks are created. Fig. 2(c and d) shows that the MAO coating formed on Al18 B4 O33 w/AZ91 composite can hardly been destroyed after suffering tensile test. Except some micro-cracks near the fracture, any damages to the coating, such as delamination or spalling of coating, do not occur. But the coating formed on SiCw/AZ91 composite has exhibited severe damage after tensile test. Close to the fracture, the coating scales off from the composite substrate (shown in Fig. 2(e)). This indicates the relatively poor bonding strength between the coating and substrate. Moreover, many cracks originate in the coating near the facture (shown in Fig. 2(f)). The tensile fractographs of the coated AZ91 alloy, Al18 B4 O33 w/AZ91 composite and SiCw/AZ91 composite are shown in Fig. 3(a–c), respectively. The fracture of the coated AZ91 alloy reveals that the rupture mechanism of the ceramic coating is distinctly different from that of the substrate. The MAO coating formed on Al18 B4 O33 w/AZ91 composite appears to be well bonded with the substrate since there is not microcrack or other flaw in the cross-sectional fracture and that there is even no clear and distinct interface between the substrate and the coating. This intuitively demonstrates the excellent bonding strength. But for the coated SiCw/AZ91 composite, there is a distinct interface between the substrate and coating, and that a crevice appears in the interface, which may occur during the process of tensile test.
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Fig. 1. Mechanical properties including (a) UTS, (b) elongation and (c) elastic modulus of the uncoated and coated Mg alloy and Mg matrix composites. It should be noted that AZ91 alloy is solution and aging heat treated (T6 state), Al18 B4 O33 w/AZ91 composite is as-cast state and SiCw/AZ91 composite is as-extruded state, therefore the mechanical properties between the three substrates have not comparability.
4. Discussion MAO technique, as an efficient and potential surface protection method, is expected to provide corrosion and/or wear protection for valve metals, and at the same time not to greatly degrade the mechanical properties of substrates used as structural materials. It is known that MAO treatment is a repeated dielectric breakdown and sparking discharge process, and that intense arcing discharge even occurs at the late stage of MAO. Moreover, residual stresses may arise in the MAO coating during the deposition process due to the growth of oxide film on the substrate surface or thermal effects from microdischarge [12]. The intense discharge process and the residual stress in the coating will induce flaws in the substrate–coating system, such as microcracks [13]. These flaws will become possible originations of damage to the coated material when the load is applied on it. In this work, it has been found that MAO treatment just decreases UTS and elongation of the Mg-based materials slightly when relatively lower current density is selected during MAO. It should be noted that the phenomena of discharge and microstructure of the MAO coating are closely related to the electrical param-
eters. The intensity of discharge depends on the magnitude of energy density distributing on the sample, which depends on the applied current density under constant current mode when other electrical parameters (frequency and duty cycle) remain constant. Therefore, relatively high energy parameters applied during MAO will result in intenser discharge process and then induce higher porosity or more serious structural flaws than relatively low energy parameters. Thus, the mechanical properties of the Mg-based materials decrease evidently when they are microarc oxidized at relatively high current density. Consequently, if MAO technique is adopted to protect Mg-based structural materials against corrosion or wear attacks, appropriate electrical parameters should be selected to avoid or decrease the damage to their mechanical properties as far as possible. But it is found that elastic modulus of the substrate–coating structure is enhanced comparing with that of the substrate. MAO coating is an oxide ceramic coating which usually is mainly composed of oxide of the substrate itself. Previous study has demonstrated that MAO coatings formed on AZ91 Mg alloy and Mg matrix composites almost have the same phase composition, with cubic MgO as the main phase [5]. The elastic
230
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Fig. 2. Surface morphology of the MAO coatings obtained at current density of 12 A/dm2 on (a and b) AZ91 alloy, (c and d) Al18 B4 O33 w/AZ91 composite and (e and f) SiCw/AZ91 composite after undergoing tensile test.
modulus of bulk MgO ceramic material is 250 GPa, which is far more than that of AZ91 alloy (43 GPa). If just considering an ideal condition, namely excluding the effect of porosity or other structural defects in the coating on elastic modulus of MgO, and that the percent of coating thickness to total thickness of the coated sample is not negligible (the corresponding data are listed in Table 1), it is very likely to improve elastic modulus of lamellar Mg-based materials to a certain extent by MAO surface treatment due to the high elastic modulus of MgO ceramic coating and high bonding strength between the coating and sub-
strate resulting from the inherent characteristic of in-situ-growth of MAO coating on substrate surface. But if coating thickness is negligible comparing with the total thickness of coated sample, the contribution of MAO coating to elastic modulus of the substrate–coating structure will be slight. In addition, as mentioned above, relatively high energy parameters applied during MAO will bring higher porosity or more serious structural flaws, which will further decrease elastic modulus of the MgO coating. Therefore, the contribution of MAO coating to the elastic modulus of substrate–coating system not only depends on the
K. Wu et al. / Materials Science and Engineering A 447 (2007) 227–232
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Fig. 3. Tensile fracture of coated (a) AZ91 alloy, (b) Al18 B4 O33 w/AZ91 composite and (c) SiCw/AZ91 composite by MAO at current density of 12 A/dm2 .
percent of coating thickness to total thickness but also on the quality of the coating, such as its porosity or density. It also has been found that the coatings formed on the three substrates under the same process conditions have exhibited different morphologies and fractures after tensile test. Early study [5] has revealed that great differences in growth, microstructures and performances exist between the MAO coatings on the three substrates due to the presence of reinforcement phases (whiskers) with different properties in Mg alloy matrix, and that the coatings formed on AZ91 alloy and Al18 B4 O33 w/AZ91 composite present more compact microstructure and better cor-
rosion resistance than that on SiCw/AZ91 composite. This is consistent with the phenomena observed during the process of tensile test. From surface morphologies and fractures after tension, it can be seen that the coatings on AZ91 alloy and Al18 B4 O33 w/AZ91 composite exhibit excellent bonding state with the corresponding substrates. Especially, the coating obtained on Al18 B4 O33 w/AZ91 presents relatively great contribution to the total elastic modulus of the substrate–coating structure, which should be closely related with its compact microstructure and perfect substrate–coating interface (shown in Fig. 3(b)). Therefore, effects of MAO surface treatment on
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mechanical properties of the Mg-based materials not only are related with the process conditions applied during MAO but also related with substrate itself since the quality of MAO coating will be partially influenced by the substrate material which the coating forms on. 5. Conclusions MAO surface treatment just decreases UTS and elongation of the Mg-based materials slightly when relatively lower current density is selected during MAO. But UTS and elongation will decrease evidently when high current density is applied because high energy parameters applied during MAO will result in intenser discharge process and may induce more structural flaws than low energy parameters, and then increase possible originations of damage to the substrate–coating structure during load-bearing. However, elastic modulus of the substrate–coating structure is enhanced comparing with that of the corresponding substrate due to the high elastic modulus of MgO ceramic coating and good bonding state between the coating and substrate. Therefore, MAO treatment will not greatly decrease the mechanical properties of the Mg-based materials when appropriate electrical parameters are selected, especially it will enhance elastic modulus to some extent if the percent of coating thickness to total thickness of coated sample is not negligible. The effects
of MAO treatment on mechanical properties are also related with the substrate materials. References [1] E.F. Emley, Principles of Magnesium Technology, Pergamon Press Ltd., 1966. [2] Y.J. Zhang, C.W. Yan, F.H. Wang, H.Y. Lou, C.N. Cao, Surf. Coat. Technol. 161 (2002) 36–43. [3] E.H. Han, W.Q. Zhou, D.Y. Shan, W. Ke, Mater. Sci. Forum 419–422 (2003) 879–882. [4] A.L. Yerokhin, A. Shatrov, V. Samsonov, P. Shashkov, A. Leyland, A. Matthews, Surf. Coat. Technol. 182 (2004) 78–84. [5] Y.Q. Wang, K. Wu, M.Y. Zheng, Surf. Coat. Technol. 201 (2006) 353. [6] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195–206. [7] X.S. Wang, X.Q. Feng, X.W. Guo, Key Eng. Mater. 261–263 (2004) 363–368. [8] P. Ross, J. MacCulloch, C. Clapp, R. Esdaile, SAE Technical Paper Series, 1999-01-0925. [9] W. Xue, C. Wang, R. Chen, Z. Deng, Mater. Lett. 52 (2002) 435–441. [10] M.Y. Zheng, W.C. Zhang, K. Wu, C.K. Yao, J. Mater. Sci. 38 (2003) 2647–2654. [11] M.Y. Zheng, K. Wu, M. Liang, S. Kamado, Y. Kojima, Mater. Sci. Eng. A 372 (2004) 66–74. [12] R.H.U. Khan, A.L. Yerokhin, T. Pilkington, A. Leyland, A. Matthews, Surf. Coat. Technol. 200 (2005) 1580–1586. [13] Y.Q. Wang, M.Y. Zheng, K. Wu, Mater. Lett. 59 (2005) 1727–1731.