Advances in Chemical Engineering & Technology

Biography

Biography: Dah-Shyang Tsai

Abstract

Plasma electrolytic oxidation (PEO) of an aluminum alloy workpiece enables its surface protection beyond anodizing. One salient attribute of PEO is the micro discharges, which accelerate the growth rate, densify the coating, meanwhile damage the coating microstructure and leave pinholes and cracks which are viewed as a major weakness of this technology. Entering the soft sparking regime may produce a coating with the feature of a dense inner layer and lessens the intrinsic porosity problem. The transition of soft sparking is typically identified through the anodic voltage drop under a constant current mode. Fig. 1a shows three soft sparking transitions with anodic current 1.2 A and negative current 1.4 A (1.5 or 1.6 A) of bipolar pulsed DC. The transitions occur at 66 (1.4 A), 54 (1.5 A), 45 (1.6 A) mins. We long suspected that the output voltage drop of power supply is a belated signal, since the micro arc state has been softened before voltage drop. Indeed, our analysis of V-I transients of positive pulse, points out that the fitted time constants R₁C₁, R₂C₂ are transition precursors, since they plunge before the anodic voltage drops. Figs. 1b and 1c show R₁C₁ and R₂C₂ fall simultaneously at 40 mins for three negative currents 1.4, 1.5, 1.6 A. On the study of frequency dependence, Fig. 2a indicates that the soft sparking transition moves forward with increasing frequency of applied current, 54 (50 Hz), 44 (100 Hz), and 29 (500 Hz) min. Fig. 2b shows R₁C₁ also displays a consistent fall at an earlier time, 40, 38, 28 min. The only exception is R₂C₂ at 500 Hz, Fig. 2c, which falls three times, instead of one time. We have thus found a stratified microstructure of the 500 Hz coating, suggesting that the three falls in R₂C₂ correspond to three intensity decreases in local plasma, an indication of sensitive diagnosis.

Recent Publications

1. Tsai D S and Chou C C (2018) Review of the soft sparking issues in plasma electrolytic oxidation. Metals 8:105.

2. Rogov A B, Yerokhin A and Matthews A (2017) The role of cathodic current in PEO of aluminum. Langmuir 33:11059–11069.

3. Fatkullin A R, Parfenov E V, Yerokhin A, Lazarev D M and Matthews A (2015) Effect of positive and negative pulse voltages on surface properties and equivalent circuit of the plasma electrolytic oxidation process. Surf Coat Technol. 284:427–437.

4. Liu C Y, Tsai D S, Wang J M, Tsai J T J and Chou C C (2017) Particle size influences on the coating microstructure through green chromia inclusion in PEO. ACS Appl Mater Interfaces 9:21864–21871.

5. Matykina E, Arrabal R, Skeldon P, Thompson G E and Belenguer P (2010) AC PEO of aluminum with porous alumina precursor films. Surf Coat Technol. 205:1668–1678.