• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.429-432
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• 2009

The influence of the subsequent-annealing (SA) treatment on plasma electrolytic oxidation (PEO)-treated Mg-based alloy was investigated and the dependence of the dehydration reaction on the SA temperature was also studied. For this purpose, a series of the SA treatments were carried out on the coated samples at two different temperatures, i.e. 423 and 523 K for 10 h. In contrast to the sample without SA treatment, the sample annealed at 523 K exhibited a significant difference in term of surface morphology since the MgO content in the oxide layer increased with increasing SA temperature. With increasing SA temperature, the dehydration of $Mg(OH)_2$ led to the increase in the relative amount of the MgO, which was a hard phase. From the nano-indentation results, the applied loads of the samples were seen to increase as SA temperature increased. However, the corrosion resistance of the sample annealed at 423 K was higher than that of the samples annealed at 523 K.

• Transactions of Materials Processing
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• v.18 no.8
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• pp.625-632
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• 2009

The influence of the subsequent-annealing(SA) temperature on the plasma electrolytic oxidation(PEO)-treated Mgbased alloy was investigated in terms of surface properties associated with hardness and corrosion. For this purpose, a series of the SA treatments were performed on the PEO-treated samples at four different temperatures, i.e., 100, 150, 200, and $250^{\circ}C$ for 10 hrs. When compared to the sample without SA, the samples annealed at temperatures higher than $200^{\circ}C$ showed a difference in surface morphology due to the volume expansion accompanied by the dehydration reaction where the part of $Mg(OH)_2$ changed into MgO, working as harder phase. From the results of nano-indentation tests, the applied loads of the samples were seen to increase with increasing SA temperatures. However, the electro-chemical and corrosion properties of the sample annealed at $150^{\circ}C$ were higher than those of the samples annealed at three temperatures.

• Journal of the Korean institute of surface engineering
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• v.41 no.6
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• pp.308-311
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• 2008

Oxide layers were prepared on Al2021 alloys substrate under a hybrid voltage of AC 200 V (60 Hz) combined with DC 260 V value at room temperature within $5{\sim}60\;min$ by plasma electrolytic oxidation (PEO). An optimized aluminate-fluorosilicate solution was used as the electrolytes. The surface morphology, thickness and composition of layers on Al2021 alloys at different reaction times were studied. The results showed that it is possible to generate oxide layers of good properties on Al2021 alloys in aluminate-fluorosilicate electrolytes. Analysis show that the double-layer structure oxide layers consist of different states such as ${\alpha}-{Al_2}{O_3}$ and ${\gamma}-{Al_2}{O_3}$. For short treatment times, the formation process of oxide layers follows a linear kinetics, while for longer times the formation process slows down and becomes a steady stage. During the PEO processes, the average size of the discharge channels increased gradually as the PEO treatment time increased.

• Journal of the Korean institute of surface engineering
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• v.47 no.4
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• pp.147-154
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• 2014

Formation behavior of PEO (Plasma Electrolytic Oxidation) films on AZ31 Mg alloy was studied in aqueous solutions containing various concentrations of hydroxide ion ($OH^-$) and silicate ion ($SiO_3{^{2-}}$) by voltage-time curves, and corrosion resistance of the PEO film-covered specimen was investigated by immersion test in 0.5 M NaCl solution. From the analyses of the voltage-time curves, it is suggested that two different types of anions are essentially needed for the formation of PEO films on AZ31 Mg alloy: film formation agent and local film breakdown agent. $SiO_3{^{2-}}$ ion acts only as a film formation agent but $OH^-$ ion acts not only as a film formation agent but also film breakdown agent. The PEO films prepared on AZ31 Mg alloy in alkaline silicate solution showed very good corrosion resistance without any pitting or filiform corrosions up to 480 h of immersion in 0.5 M NaCl.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2017.05a
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• pp.151.2-151.2
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• 2017

본 연구에서는 정전류 조건에서 알루미늄 합금의 PEO(Plasma Electrolytic Oxidation) 피막 형성 거동에 미치는 전해질 내 cation($K^+$, $Na^+$)의 영향을 아크 발생 양상, 전압-시간 곡선 및 형성된 표면피막 구조를 관찰하여 연구하였다. 전해질은 0.5 M NaOH + 1 M $Na_2SiO_3$ 수용액과 0.5 M KOH + 1 M $Na_2SiO_3$ 수용액이 사용되었다. 아크 발생은 cation의 종류에 상관없이 동일하게 가장자리부터 시작되어 내부로 이동함으로써 전 표면에 걸쳐서 일어났다. 전 표면에서 PEO 피막이 형성된 이후에는 한 지점에서 지속적으로 아크가 발생하는 로컬버닝 현상이 두 용액에서 모두 관찰되었으나 $K^+$이온이 포함된 용액에서 로컬버닝이 빠르게 일어났다. 시편표면에서 아크가 발생하는 동안 중 각 전해액에서의 전압-시간 곡선에서 전압의 상승과 하강이 반복되는 거동이 동일하게 관찰되었다. 이러한 전압 등락의 크기는 $K^+$ 이온이 포함된 수용액에서 더 크게 나타났으며, 그 결과 표면 거칠기가 상대적으로 더 높은 PEO 피막이 형성되었다.

• Journal of the Korean Society for Heat Treatment
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• v.24 no.4
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• pp.209-216
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• 2011

Surface coatings were prepared on SKD11 core mold steel by plasma electrolytic oxidation (PEO). The coatings were investigated about the formation condition of core mold steel. SKD11 were coated by PEO in a mix solution of Sodium Aluminate $NaAlO_2$ (10 g/l), Sodium Silicate powder $Na_2SiO_3$ (0.5 g/l), Sodium tungstate dihydrate $Na_2WO_42H_2O$ (0.5 g/l) at less than $30^{\circ}C$. The electrical condition were voltage : 500~600 V; Pulse : 600~1800 Hz; current density 15~20 $A/dm^2$ various time : 3 min~40 min. The coatings surface morphology, cross-section, friction coefficient, hardness were investigated. The PEO coatings on SKD11 core mold steel showed the extended service life.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2018.06a
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• pp.29.1-29.1
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• 2018

양극산화 표면처리 방법의 일종인 플라즈마 전해산화(PEO, Plasma electrolytic oxidation)는 금속 소재에 양극 전압을 인가하여 고경도의 산화 피막을 금속 표면에 형성시키는 표면처리 기술이다. PEO 공정은 피막의 국부적 유전체 파괴에 의한 아크의 발생을 동반하며, 형성된 산화 피막이 아크 발생에 의한 높은 열에 의해 결정화 되어 일반적인 양극산화 피막보다 우수한 경도와 내마모성을 가진다. 하지만 PEO 공정은 고전압을 필요로 하여 일반적인 양극산화 처리보다 소모되는 전력량이 많으며, 아크 발생에 의해 형성된 피막의 표면 거칠기가 높기 때문에 활용 분야가 제한되거나 후속 연마 공정을 필요로 하는 단점이 존재한다. 본 연구에서는 전류 파형이 알루미늄 합금의 플라즈마 전해산화 피막의 형성 거동에 미치는 영향을 직류 및 펄스전류를 사용하여 연구하였다. NaOH 및 $Na_2SiO_3$가 혼합된 전해액에서 직류 전류 밀도, 전압, 펄스폭을 달리하여 알루미늄 합금에 전류를 인가할 때 발생되는 아크의 거동, 형성된 산화 피막의 두께, 거칠기, 경도, 표면 및 단면 구조를 비교 분석하였다.

• Journal of Korea Foundry Society
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• v.29 no.5
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• pp.233-237
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• 2009

The effect of electrolyte on mechanical and corrosion properties of AZ91 magnesium alloy by plasma electrolytic oxidation (PEO) method was investigated. The coating layers formed in the silicate and the aluminate electrolytes showed porous structures. The small pores were randomly distributed on the coatings formed in aluminate electrolyte while the coatings formed in silicate electrolyte showed much bigger pores. In the aluminate electrolyte, the coatings were composed of Mg, MgO and $MgAl_2O_4$, whereas Mg, MgO, $MgAl_2O_4$ and $Mg_2SiO_4$ were identified in the coatings formed in silicate electrolyte. The hardness of coatings in the silicate electrolyte was higher than that of coating grown in the aluminate electrolyte. The AZ91 alloy coated in the silicate electrolyte had higher tensile strength and elongation than that coated in the aluminate electrolyte. In addition, the coatings formed in the silicate electrolyte showed much better corrosion resistance compared to the coatings formed in the aluminate electrolyte.

• Journal of the Korean institute of surface engineering
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• v.49 no.3
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• pp.225-230
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• 2016

The present work was conducted to investigate the effects of NaF concentration in phosphate and silicate-containing alkaline electrolyte on the morphology, thickness, surface roughness and hardness of anodic oxide films formed on AZ31 Mg alloy by plasma electrolytic oxidation (PEO) method. The PEO films showed flat surface morphology with pores in the absence of NaF in the electrolyte, but nodular features appeared on the PEO film surface prepared in NaF-containing electrolyte. Numerous pores ranging from 1 to $20{\mu}m$ in size were observed in the PEO films and the size of pores decreased with increasing NaF concentration in the electrolyte. Surface roughness and thickness of PEO films showed increases with increasing NaF concentration. Hardness of the PEO films also increased with increasing NaF concentration. It was noticed that hardness of inner part of the PEO films is lower than that of outer part of them, irrespective of the concentration of NaF. The low hardness of PEO films was explained by the presence of a number of small size pores less than $2{\mu}m$ near the PEO film/substrate interface.

• Journal of the Korean institute of surface engineering
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• v.51 no.6
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• pp.344-353
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• 2018

Plasma electrolytic oxidation of Ti-25Ta-xHf alloy in electrolyte containing Ca and P for dental implants was investigated using various experimental techniques. Ti-25Ta-xHf (x=0 and 15 wt.%) alloys were manufactured in an arc-melting vacuum furnace. Micropores were formed in PEO films on Ti-25Ta-xHf alloys in 0.15 M calcium acetate monohydrate + 0.02 M calcium glycerophosphate at 240 V, 270 V and 300 V for 3 min, respectively. The microstructure of Ti-25Ta-xHf alloys changed from (${\alpha}^{\prime}+{\alpha}^{{\prime}{\prime}}$) phase to (${\alpha}^{{\prime}{\prime}}+{\beta}$) phase by addition of Hf. As the applied potential increased, the number of pore and the area ratio of occupied by micro-pore decreased, whereas the pore size increased. The anatase phase increase as the applied potential increased. Also, the crystallite size of anatase-$TiO_2$ can be controlled by applied voltage.