EFFECTS OF SECOND PHASES ON MICROARC OXIDATION PROCESS OF MAGNESIUM BASE MATERIALS
Yanqiu WANG1(),Kun WU2,Fuhui WANG1,3
1) Education Ministry Key Laboratory of Superlight Materials and Surface Technology, Harbin Engineering University, Harbin 150001, China 2) School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China 3) Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
Yanqiu WANG,Kun WU,Fuhui WANG. EFFECTS OF SECOND PHASES ON MICROARC OXIDATION PROCESS OF MAGNESIUM BASE MATERIALS. Acta Metall Sin, 2016, 52(6): 689-697.
The effects of second phases on microarc oxidation (MAO, also named plasma electrolytic oxidation-PEO) behavior of Mg base materials were investigated and the related mechanism was discussed. The formation of barrier layer and its influence on sparking discharge behavior were characterized and analyzed on the base of systematic selecting and designing substrate materials. The variation of second phases at the early MAO stage was observed and analyzed by SEM and EDS, and then the effect mechanism of second phases on MAO behaviors was revealed. Voltage evolution trend during MAO were recorded to study the formation state of the barrier layer on the different Mg base materials. According to the growth mechanism of MAO film, the film growth process can be simplistically considered as a repeated breakdown and reconstruction process of a capacitor. Accordingly, the growth process of MAO film on multiphase metal materials and the effects of second phases were discussed. The results show that different second phases in substrate materials have different effects on formation process of MAO films, depending on their own characteristics. For the second phases which have the characteristics of valve metals, although selective sparking discharge occurs at the early stage of MAO, the second phases will not hinder the growth of MAO film since barrier layer can form on the second phases, and they will not induce structural defects into the film-substrate interface. If the second phases have not the characteristics of valve metals, their conductivity property will be an important influencing factor to affect the MAO behaviors. For the elecinsulating second phases which have not the characteristics of valve metals, sparking discharge just occurs on Mg matrix in the substrate, while doesn't occur on the second phases; the second phases exist in the MAO film as heterogeneous phases, do not react in MAO process, and will not hinder the growth of MAO film. For the semi-conductive second phases which have not the characteristics of valve metals, they delay the growth of MAO film because they destroy the integrity of barrier layer. For the electroconductive second phases which have not the characteristics of valve metals, they seriously hinder the growth of MAO film.
Fig.2 SEM images of AZ91D Mg alloy at early stage of microarc oxidation (MAO) treatment for 45 s (a) and 60 s (b)
Fig.3 SEM images of Al18B4O33w/AZ91D composite at early stage of MAO treatment for 30 s (a) and 60 s[23] (b) (W—Al18B4O33 whisker)
Fig.4 SEM images of SiCw/AZ91D composite at the early stage of MAO treatment for 60 s (a), 75 s (b) and 90 s (c)
Fig.5 EDS results of SiCw/AZ91D composite after 60 s MAO treatment for areas A (a) and B (b) in Fig.4a
Fig.6 SEM images of (Cf+Al18B4O33w)/AZ91D composite before (a) and after MAO treatment for 60 s (b) and 300 s (c)
Fig.7 Voltage-time curves during MAO treatment under constant current mode for different Mg base materials (MMC—metal matrix composite)
Fig.8 Schematic for microarc oxidation film growth
Substrate material
20 mAcm-2
40 mAcm-2
60 mAcm-2
80 mAcm-2
R / (Vs-1)
U / V
R / (Vs-1)
U / V
R / (Vs-1)
U / V
R / (Vs-1)
U / V
AZ91D
3.1
219
6.0
369
9.2
407
10.2
416
Al18B4O33w/AZ91D
3.6
207
6.2
358
8.3
408
9.2
426
SiCw/AZ91D
1.2
188
2.4
256
3.8
312
5.8
374
(Cf+Al18B4O33w)/AZ91D
-
33
-
74
-
166
-
155
Table 1 Voltage increasing rate at the initial stage of MAO treatment (R) and ultimate steady-state voltage (U) under different applied current densities for various substrate materials
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