Influence of Applied Frequency on Thermal Physical Properties of Coatings Prepared on Al and AlSi Alloys by Plasma Electrolytic Oxidation

Abstract

In this study, plasma electrolytic oxidation (PEO) was performed on Al and AlSi substrates using a pulsed direct current (DC) power source. The coating process was carried out in a Na2SiO3 electrolyte with the systematic change of pulse frequency (50–1400 Hz). The surface characteristics of the coatings were examined using scanning electron microscopy (SEM). The phase structure was characterized using X-ray diffraction (XRD). A differential scanning calorimeter (DSC) and a laser flash apparatus (LFA) were employed to test heat capacity and heat conductivity, respectively. Results showed that as the discharge frequency increased, the thermal physical properties of Al-PEO and AlSi-PEO coatings changed in different ways. At a high frequency, Al-PEO coatings had low porosity and were closed-pore structured whereas AlSi-PEO coatings had high porosity and large-size open-pore structures could be observed on their surfaces due to concentrated discharges. Based on these findings, it was found that the thermal productivity of coatings is closely correlated with the open-/closed-pore structure instead of porosity. PEO coatings with low heat capacity or low heat conductivity could be obtained with a controlled frequency.

Study on Discharge Characteristics and Microstructural Evolution of PEO Coatings Based on an Al/Ti Tracer Substrate

“…However, as discharge voltage and energy increase, the discharge temperature of the partial region gradually rises, leading to the appearance of α-Al 2 O 3 within the coating. This finding is consistent with the results of our previous research [27]. The anatase phase was observed in the 20th min of discharge, indicating that the partial temperature within the coating had increased above 700 • C. The crystallinity of the rutile TiO 2 phase and the rutile phase gradually increased with prolonged discharge time.…”

“…The duty cycle for positive and negative pulses was 50%, with a frequency of 1400 Hz. Our previous work [27] on discharged pure Al substrate indicated that closed pore structures were obtained during the later period of discharge at a current density of 0.12 A/cm 2 and a frequency of 1400 Hz. This phenomenon aligns with the desired microstructural structure for enhancing the thermal insulation property of coatings.…”

Influence of Plasma Electrolytic Oxidation of Cast Al-Si Alloys on Their Phase Composition and Abrasive Wear Resistance

“…This leads to the formation of layers with a large number of pores and cracks. The structure of such coatings is often given in the literature [33][34][35][36][37]. On the basis of the research carried out, simple electrolytes have been developed that can significantly increase the microhardness and wear resistance of AK9 and AK12 alloys.…”