Method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation

Abstract

A method for producing a ceramic coating on the surface of an aluminum alloy substrate by means of plasma electrolytic oxidation may include immersing the substrate as an electrode together with a counter-electrode in an alkaline electrolytic aqueous solution. The method may also include the step of applying an electrical potential sufficient to generate spark discharges on the surface of the substrate for a predefined period of treatment time so as to lead to the formation of the coating. The coating may be aluminum oxides and oxides of any alloying agents of the alloy. The electrolytic aqueous solution may have from 9 to 14 g/l of Na2SiO3, from 2.3 to 2.8 g/l of K3PO4, not less than 5 g/l of Na2WO4·2H20, from 0.4 to 1.5 g/l of Na3AlF6, and NaOH at a concentration such that the electrolytic solution has a pH between 11.8 and 12.0, and a conductivity between 9.5 and 10.5 mS/cm.

1 - 25 . (canceled) 
     
     
         26 . Method for producing a ceramic coating on the surface of an aluminium alloy substrate by means of electrolytic plasma oxidation, comprising the following steps:
 a) immerging the substrate as an electrode together with a counter-electrode in an alkaline electrolytic aqueous solution;   b) applying an electrical potential sufficient to generate spark discharges on the surface of the substrate for a predefined period of treatment time so as to lead to the formation of said coating, consisting mainly of aluminium oxides and oxides of any alloying elements of said alloy,   wherein the electrolytic aqueous solution comprises:
 from 9 to 14 g/l of Na2SiO3; 
 from 2.3 to 2.8 g/l of K3PO4; 
 not less than 5 g/l of Na2WO4·2H20; 
 from 0.4 to 1.5 g/l of Na3AlF6; 
 NaOH at a concentration such that the electrolytic solution has a pH between 11.8 and 12.0, and a conductivity between 9.5 and 10.5 mS/cm. 
   
     
     
         27 . The method according to  claim 26 , wherein the electrolytic solution comprises from 9 to 11 g/l of Na2SiO3, preferably 10 g/I. 
     
     
         28 . The method according to  claim 26 , wherein the electrolytic solution comprises from 2.4 to 2.6 g/l of K3PO4, preferably 2.5 g/I. 
     
     
         29 . The method according to  claim 26 , wherein the electrolytic solution comprises 5 g/l of Na2WO4·2H20. 
     
     
         30 . The method according to  claim 26 , wherein the electrolytic solution comprises 0.4 to 0.6 g/I Na3AlF6, preferably 0.5 g/I. 
     
     
         31 . The method according to  claim 26 , wherein the electrolytic solution comprises NaOH at a concentration such that the electrolytic solution has a pH of 11.9, and a conductivity of 10.0 mS/cm. 
     
     
         32 . The method according to  claim 26 , wherein the electrolytic solution comprises 0.8 to 1.2 g/I NaOH, preferably 0.9 to 1.1 g/I, even more preferably 1.0 g/I. 
     
     
         33 . The method according to  claim 26 , wherein the electrolytic aqueous solution comprises:
 10 g/l of Na2SiO3;   2.5 g/l of K3PO4;   5 g/l of Na2WO4·2H20;   0.5 g/l of Na3AlF6;   1.0 g/I NaOH,   
       and wherein the electrolytic solution has a pH of 11.9 and a conductivity of 10.0 mS/cm. 
     
     
         34 . The method according to  claim 26 , wherein the alkaline electrolytic aqueous solution is cooled by means of a cooling system, preferably to maintain said alkaline electrolytic aqueous solution at a temperature between 25° C. and 45° C. during said predefined period of treatment time. 
     
     
         35 . The method according to  claim 26 , wherein the electrical potential is kept substantially constant for said predefined period of time, preferably at a value between 300 and 400 V, more preferably 350 V. 
     
     
         36 . The method according to  claim 26 , wherein during said predefined period of treatment time an electrical current with a current density between 20 and 25 A/dm3, preferably 25 A/dm3, is applied to the substrate. 
     
     
         37 . The method according to  claim 26 , wherein an electrical current with a frequency of at least 50 Hz, preferably 50 Hz is applied to the substrate. 
     
     
         38 . The method according to  claim 36 , wherein the electrical current can be applied continuously or in a pulsed mode. 
     
     
         39 . The method according to  claim 26 , wherein the predefined period of treatment time is between 20 and 40 min, preferably 30 min. 
     
     
         40 . The method according to  claim 26 , wherein the electrical potential is kept substantially constant for said predefined period of treatment time at a value of 350V and wherein during said predefined period of treatment time an electrical current with a current density equal to 25 A/dm3 and a frequency equal to 50 Hz is applied continuously to the substrate, said predefined period of treatment time being 30 min. 
     
     
         41 . The method according to  claim 26 , comprising a pre-treatment step (c) of the substrate, to be carried out before said steps (a) and (b), wherein the pre-treatment consists of subjecting said substrate to caustic attack and then washing said substrate with distilled water. 
     
     
         42 . The method according to  claim 41 , wherein said caustic attack is obtained by immerging said substrate for a predefined period of time in an aqueous solution of NaOH, preferably containing 50 g/l of NaOH, maintained at a temperature between 60° C. and 70° C., preferably at 60° C., said predefined immersion time being between 5 and 15 min, preferably 10 min. 
     
     
         43 . The method according to  claim 41 , wherein in said step (c) of pre-treatment of the substrate, after the caustic attack and subsequent washing with distilled water, the substrate is immersed in an acid bath for a predefined period of time and then washed with distilled water. 
     
     
         44 . The method according to  claim 43 , wherein said acid bath consists of an aqueous solution of nitric acid, said predefined immersion time in said acid bath being between 5 and 15 s, preferably 10 s. 
     
     
         45 . The method according to  claim 26 , comprising a post-treatment step (d) of the substrate, to be carried out after said steps (a) and (b), wherein said post-treatment consists of:
 washing said substrate with distilled water;   cleaning the surface of said substrate with alcohol; and   allowing it to dry at room temperature.   
     
     
         46 . The method according to  claim 26 , wherein said ceramic coating consists essentially of a non-porous, compact layer, which may possibly have on the surface, a porous layer with a thickness not exceeding 5% of the total thickness of said coating. 
     
     
         47 . The method according to  claim 46 , wherein said ceramic coating consists only of said non-porous, compact layer. 
     
     
         48 . The method according to  claim 46 , wherein said ceramic coating has a roughness Ra≤2 μm and a hardness HV0.01≥1.400. 
     
     
         49 . The method according to  claim 26 , wherein said substrate is in aluminium-silicon alloy and wherein the ceramic coating obtained is a layer consisting mainly of a mixture of aluminium oxides, silicon oxides and mixed aluminium-silicon oxides. 
     
     
         50 . The method according to  claim 26 , wherein said substrate consists of a component of a braking system, preferably a disc braking system, in particular a brake caliper, a brake caliper piston or a brake disc bell.