Effect of Na2B4O7 on Wear and Corrosion Resistance of Micro-arc Oxidation Coating on Titanium Alloy
Micro-arc oxidation(MAO)technology is often used in the surface treatment of medical titanium alloys,giving them abrasion-resistant,corrosion-resistant,or osteogenic properties.The performance of the MAO coating is determined by a combination of factors,such as power supply mode,voltage/current size,electrolyte type,oxidation time,additives,etc.The composition of the electrolyte is one of the key factors affecting the structure and performance of the MAO coating.Sodium tetraborate is used as a single electrolyte for micro-arc oxidation,and due to its special discharge method,the coating often results in high corrosion resistance.However,when sodium tetraborate is used as an additive,it not only plays a discharge role,but also may react with the coating,and the effect of adding sodium tetraborate to different electrolytes is worth exploring.The work aims to improve the wear resistance and corrosion resistance of micro-arc oxidation coatings on titanium alloy and develop better composite electrolytes.In phosphate and silicate electrolytes,by adding varying amounts of tetraborate sodium,the chemical reactions and micro-arc discharge methods during the oxidation process were altered to reduce coating surface roughness,modify internal structure,thickness,and titanium dioxide phase transformation,ultimately improving the wear and corrosion resistance of the coating.Titanium alloy were made into 30 mm×20 mm×5 mm samples,and sandpaper was used to grind to 1 200 mesh.In the MAO process,the unipolar pulse was used,the anode current density was set to 2.6 A,the frequency was set to 900 Hz,and the duty cycle was 15％.The silicate electrolyte contained 15.0 g/L Na2SiO3·9H2O and 2.0 g/L NaOH,and the phosphate electrolyte contained 15.0 g/L Na3PO4·12H2O and 2.0 g/L NaOH.Different amounts of sodium tetraborate were added to the electrolytes in the two groups,and the duration of the MAO process was 12 min.The surface and side morphology of the MAO coating were analyzed by scanning electron microscope,and the elemental composition of the coating before and after the friction test was characterized by energy spectrometer,the phase composition of the MAO coating was analyzed by X-ray diffractometer,and finally the corrosion resistance and wear resistance of the coating were tested with an electrochemical workstation and a multifunctional friction and wear testing machine.In the silicate electrolyte,the addition of tetraborate sodium reduced the coating surface roughness(Sa)from 5.408 μm to 1.089 μm.Compared to the self-corrosion current density of TC4 titanium alloy,it reduced by 2-3 orders of magnitude,and the wear volume significantly decreased.The addition of tetraborate sodium in the phosphate electrolyte showed a similar trend.Tetraborate sodium not only reduced the coating surface roughness by dissolving surface oxide but also introduced a new breakdown discharge method,increasing the coating thickness and altering its internal structure.Adding 5 g/L of tetraborate sodium in both electrolytes resulted in optimal performance for the respective groups.It was observed that during friction experiments in simulated body fluid,MAO coatings induced the formation of hydroxyapatite lubrication layers to protect the substrate.The addition of sodium tetraborate increases the hydrophilicity of the coating and induces the formation of more hydroxyapatite in the friction experiment.Among them,the coating formed by phosphate-based electrolyte is more hydrophilic,and the content of apatite induced by the friction process is several times that of the silicate coating.
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