A Study On The Microstructure And Performance Of The Plasma Electrolytic Oxidation Coatings On Zr Alloy

Zirconium alloy is an important structural material. As it is often used undercomplicated and harsh conditions of nuclear industry, the researches of zirconiumalloy’s corrosion and wear-resistance problem are very meaningful. This paperstudies the microstructures and performance characteristics of Plasma electrolyticoxidation (PEO) coatings on zirconium alloys formed in aluminate and silicateelectrolytes. It researches differing discharging behaviors, potential-timeresponses and growth rates, phase compositions, morphology of the surface andcross section, polarization curves, wear-resistance of the coating formed in twodifferent electrolytes.A pulsed waveform is used, with a duty cycle of20%at1KHz, averagepositive and negative current densities of150mA/cm2and100mA/cm2respectively.Coatings prepared in the dilute aluminate electrolyte (8g/L NaAlO2+1g/L KOH)for different time are studied. The results show that the coating formed for5min,which consists of a compact outer layer, a much thinner inner layer with littlecracks and a barrier layer at the coating/substrate interface. Lateral pores existbetween the outer layer and inner layer which severely reduce the wear resistanceof the coating. A similar coating was formed after10min, however the lateralpores had developed into wider gap between the outer and inner layers. This makesthe wear resistance worse. Phase compositions comprised of quartet zirconia weredetermined by X-ray diffraction (XRD) which indicates that aluminum oxideenables t-ZrO2stable at room temperature. After PEO for30min, a significantchange occurs in the coating, namely the development of the inner layer whichcoincides with a reduced width of the gap, and an enrichment of substancecontaining aluminium below the arch of outer layer. Because of the disappearanceof the gap, wear resistance increases significantly. The increase of the inner layerand fulfillment of the gap is due to the “soft” sparking which lasts100s duringthe reaction. In this period, mass of the electrolyte component pours into the innerlayer through the pores and fulfills the gap, thus greatly enhance the wearresistance of the coatings.Coating forms more rapidly in32g/L NaAlO2+1g/L KOH. A~10μm thickcoating forms for only2min and a~70μm thick coating forms for only10min. Adifferent coating structure of compact single layer is formed which has much better wear resistance than the coatings formed in dilute electrolyte. X RD showsthe phase compositions comprised of quartet zirconia and γ-Al2O3, the latterattributes to the outstanding wear resistance.A pulsed bipolar waveform with the identical electrical parameters is used insilicate electrolyte. The results show that increasing concentration of electrolyteraise the formation rate of the coatings, and render the surface coarser.Examination of coating formed in8g/L NaSiO3+1g/L KOH electrolyte after30minshows smooth surface while white lump on the coating formed in56g/LNaSiO3+1g/L KOH electrolyte after30min can be observed with naked eyes. Thephase compositions also change a lot as the variation of concentration. It mainlyconsists of m-ZiO2from the dilute aluminate electrolyte but t-ZiO2and amorphoussolid from the concentrated electrolyte. This approves that SiO2can also stabilizet-SiO2but the stability is weaker than that held by Al2O3. Corrosion test shows thatthe coating forms in32g/L NaSiO3+1g/L KOH electrolyte is the mostanti-corrosive.