Method of forming a protective oxide ceramic coating on the surface of valve metals and alloys

Abstract

The invention relates to the field of electroplating. A method of plasma electrolytic oxidation (PEO) includes submerging an article as an electrode, together with a counter electrode, in a bath filled with an aqueous alkaline electrolyte, and supplying bipolar rectangular voltage pulses to the electrodes using a pulsed power supply. The ratio of the duration of an anodic pulse to dead time is selected in a range of 1:5-1:6, the duration of an anodic pulse is 3-30 μs, and the total duration of a period is 30-300 μs. PEO is carried out at the following amplitude values: 600-1200 V for the anodic voltage pulses, and 150-400 V for the cathodic voltage pulses. The invention provides for the production of fully melted, homogeneous oxide ceramic coatings having a high microhardness, a high elastic coefficient, high adhesive and cohesive strength, and a high density.

What is claimed is: 
     
         1 . A method of forming a protective oxide ceramic coating on the surface of articles made of valve metals and alloys by using plasma electrolytic oxidation (PEO), wherein the method comprises:
 submerging an article as an electrode, together with a counter electrode, in a bath filled with an aqueous alkaline electrolyte, and   supplying, by a pulsed power supply, bipolar voltage pulses to the electrodes, wherein the method is characterized in that   bipolar square voltage pulses are supplied to the electrodes, an anodic pulse being followed by dead time and then a cathodic pulse, wherein a ratio of durations of the anodic pulse and the dead time (Ta/To) is selected in a range of from 1:5 to 1:6, the duration of the anodic pulse is 3-30 microsec, and a total duration of a period (T=Ta+To+Tc) is 30-300 microsec;   a relationship Ua*Ta=Uc*Tc is observed between amplitude values and durations of the anodic and cathodic voltage pulses, and the PEO is performed at the following amplitude values of the voltage pulses: anodic voltage pulses 600-1200 V and cathodic voltage pulses 150-450 V, depending on the nature of a material to be oxidized.   
     
     
         2 . The method of  claim 1 , wherein the protective oxide ceramic coating is formed on metals and aluminum, titanium, magnesium, zirconium, tantalum, niobium, beryllium alloys. 
     
     
         3 . The method of  claim 1 , wherein the PEO of aluminum and its alloys is performed at the amplitude values of the anodic voltage pulses 900-1200 V and the cathodic voltage pulses 250-400 V, and the PEO of titanium, magnesium, zirconium, tantalum, niobium, beryllium and their alloys is performed at the amplitude values of the anodic voltage pulses 600-800 V and the cathodic voltage pulses 150-200 V. 
     
     
         4 . The method of  claim 1 , wherein the PEO is performed at effective current densities 5-20 A/dm 2  in an anodic circuit and 6-25 A/dm 2  in a cathodic circuit, depending on the nature of the material to be processed. 
     
     
         5 . The method of  claim 4 , wherein the PEO is performed in a pulsed potentiostatic or pulsed galvanostatic mode in the anodic circuit, and in the cathodic circuit—in a pulsed potentiodynamic mode with a uniform rate of increase of the amplitude of the cathodic voltage pulses 1-3 V/min or in a pulsed galvanodynamic mode with a uniform rate of decrease of the amplitude of the cathodic current pulses 0.2-0.5 A/min.