Plasma Electrolytic Oxidation of Zr-1%Nb Alloy: Effect of Sodium Silicate and Boric Acid Addition to Calcium Acetate-Based Electrolyte

Abstract

This work aimed at the development of wear and corrosion resistant oxide coatings for medical implants made of zirconium alloy, by plasma electrolytic oxidation (PEO). The effect of sodium silicate and boric acid addition to calcium acetate electrolyte on the coating properties was studied. Different aspects of the PEO coating were investigated: microstructure, electrochemical and wear behavior, wettability and apatite-forming ability. The resultant coatings consist of a dense inner layer 1.4–2.2 µm thick and a porous outer layer. The total thickness of the coating is 12–20 µm. It was found that the coating contains the tetragonal zirconia (70–95%). The obtained coatings show high corrosion resistance and reduce the surface corrosion current by 1–3 orders of magnitude, depending on the electrolyte additive, compared to the uncoated surface. The addition of boric acid to the electrolyte significantly increases the wear resistance of the coating and reduces the coefficient of friction. In terms of the combination of the coating characteristics, the electrolyte with the addition of the alkali and boric acid is recommended as the most effective.

The Effect of Plasma–Electrolytic Nitrocarburizing of a Medium Carbon Steel Surface on Friction and Wear in Pair with Tin–Lead Bronze

“…Based on the XRD analysis, the surface layer of samples after PENC contains mainly iron oxides Fe3O4, Fe2O3, and FeO (Figure 27a-e), which is typical for this kind of processing [3,7,10,11]. It has been shown that, with an increase in the PENC temperature, the relative amount of oxide Fe3O4 decreases, while FeO increases.…”

“…Thus, the contact is considered plastic and the stress on the contact is taken equal to the hardness, i.e., P r ≈ HB. The surfaces of the samples and the counterbody had no waviness, and the contour pressure was determined by the applied load N. Equating the equation approximating the reference curve in the form (3) to the ratio of the contour and actual pressures (7) at the level y = h, the following expression for the absolute convergence of the contacting surfaces is obtained:…”

“…Technologies for plasma-electrolytic treatment have found applications in applying protective ceramic coatings onto the surfaces of metals of the valve group, polishing metal surfaces, and forming hardened diffusion layers on steel and titanium alloys [2][3][4][5][6][7][8][9].…”

Influence of PEO Electrolyzer Geometry on Current Density Distribution and Resultant Coating Properties on Zr-1Nb Alloy

“…The samples were PEO treated in a 10 L tank with the electrolyte having a temperature of 20 ± 1 • C. An aqueous solution of 15 g/L sodium phosphate, 25 g/L calcium acetate, 1 g/L sodium hydroxide and 1 g/L boric acid was selected as the electrolyte composition, as it previously demonstrated the best coating properties [14]. The electrolyte was stirred with an electromagnetic stirrer located under the tank.…”

“…PEO process can produce biocompatible coatings on zirconium implants due to the porous morphology that imitate the bone surface [12][13][14]. For medical implants, it is crucial that the surface treatment procedure provides coatings of the required quality on devices that typically have complicated shapes, with threaded connections and holes.…”