Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Controlling current distribution within bath
Reexamination Certificate
1999-07-09
2001-05-29
Mayekar, Kishor (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Controlling current distribution within bath
C205S106000, C205S107000, C205S316000, C205S326000
Reexamination Certificate
active
06238540
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of microplasma electrolytic processing of surfaces of electroconductive materials being metals, alloys and carbonic composites in order to form on their surfaces corrosion-resistant, heat-resistant, and wear-resistant dielectrical coatings. The invention may be applied in such fields as mechanical engineering, aircraft construction, in petrochemical and oil industry. The invention can in particular be used in the manufacturing of large and intricate workpieces the surfaces of which are exposed to aggressive mediums, high temperature, and abrasion; thus, the invention can e.g. be used in the manufacturing of valves of pneumatic devices, and con-rods and cylinders of engines.
BACKGROUND OF THE INVENTION
Known is a method of electrolytic plating the surface of a material, the method including immersion of the treated material, which serves as a first electrode, and of the second electrode in an electrolyte, application of a voltage between them until a plurality of microplasma discharges appears, the micorplasma discharges being evenly spaced across the surface of the treated material, and maintenance of the voltage until a coating of desired thickness is formed. The voltage is increased up to 400 V for basic valve metals and up to 600 V for induced valve metals, the temperature of the electrolyte is maintained in a range of 45-60° C., current density is 250-500 Ma/dm
2
[
1
].
However, this method has some substantial disadvantages:
low current density entails difficulties in ignition and maintenance of a stable microplasma discharge on the surface of the treated material, in particular for induced valve metals and their alloys, this lowers the quality of the process;
it is not possible to process intricate workpieces or workpieces having a large surface in the suggested electrical regimes;
it is not possible to process workpieces made from carbonic materials (graphite or composites made from it).
Known is also a method of electrolytic micro-arc plating of a silicate coating onto a aluminium workpiece [
2
]. The method comprises steps of forming a coating by preliminary dipping a part into the electrolyte by 5-10% of its surface area at initial current density of anode current, equal to 5-25 A/dm
2
and performing further dipping uniformly with a rate, determined by the relation S/T=0.38+1.93 i,
wherein
S—total surface of the workpiece, dm
2
;
T—immersion time, min;
i—initial density of the anode current, A/dm
2
.
This method has some substantial disadvantages:
the great thickness of the peripheral technological layer having a relatively porous structure of silicon oxide and aluminium oxide makes it hard to remove it;
the dependence of the immersion speed of the workpiece on the value of the initial density of the anode current applied works only effectively, if the power values (N) of the power sources used are very low (because N=I·U). In this case only workpieces with limited surface can be coated, so that the preliminary immersion by 5-10% still ensures the ignition and stable burning of microplasma discharges. Due to this fact the possibility of coating large workpieces is limited.
The most similar method in terms of the main features is a method for forming coatings by electrolyte discharge[
3
]. This method for forming relatively thick composite coatings on a region of the surface of a metallic workpiece comprises exposing the surface region to an electrolyte fluid, either by immersion or by spraying the electrolyte against the surface region. A preferred electrolyte fluid is an aqueous solution including an electrolytic agent, a passivating agent and a modifying agent in the form of a solute or a powder suspended in the solution. A voltage signal is applied to induce a current flow of constant magnitude between the metallic member and the electrolyte fluid so that the metallic member interacts with the passivating agent to form a passive oxide layer on the surface region. The voltage signal increases in magnitude until local voltage reaches a breakthrough level across separate highly localized discharge channels along the surface region of the metallic member. At this breakthrough level, localized plasmas including components of the oxide layer and the modifying agent form near the discharge channel and react to form the coating. At some point after the discharges appear, the signal is changed to a series of unipolar anodic pules interspersed with the cathodic pulses which serve to stabilize the growth of the coating.
Thus, the known method comprises the steps of establishing a contact of the material serving as first electrode and of the second electrode with the electrolyte; applying a voltage between the electrodes in the regime of ignition of a plurality of microplasma discharges and maintaining the material in the electrolyte at given electrical parameters, thus, generating a coating of desired thickness.
The substantial disadvantages of this methods are:
difficulties arise when igniting and maintaining stable microplasma discharges at the same time on large surfaces of a bulky workpiece to be processed or on the surfaces of many small workpieces. Due to this fact the coatings generated do not have uniform thickness and characteristics across the total surface of the workpiece to be processed;
it is necessary to provide a current source of big power in order to maintain a stable discharge on large surfaces of a bulky workpiece to be treated or on the surfaces of many small workpieces, this entails increased energy expenditure during the process;
it is not possible to generate coatings of uniform thickness and characteristics across the whole surface of a workpiece having holes or voids or notches with relation of diameter to length being less than 0.3;
it is not possible to use the method for other non-metallic materials, e.g. graphite or composites made from it.
SUMMARY OF THE INVENTION
The technical objects solved by the present invention are:
generating a high-quality coating on large surfaces of one bulky workpiece to be treated or on the surfaces of many small workpieces by simplifying the process of ignition of microplasma discharges and maintenance of their stable burning on the surface to be treated during the whole process while using current sources of moderate power;
obtaining heat-resistant, corrosion-resistant, and wear-resistant dielectric coatings of uniform thickness and characteristics across the total surface to be treated of workpieces and of workpieces with intricate shape, including inner surfaces of holes:
forming a uniform protective coating with a thickness up to 700 &mgr;m on workpieces made from aluminium and its alloys with alloying additions or other valve metals, like zirconium, titan, hafnium and their alloys, but also on those materials like graphite and composites thereof. The coating formed comprises the above mentioned properties.
The above mentioned technical result is achieved by treating the surface of a electroconductive material with a known method of microplasma electrolytic processing, the method including
immersion of the surface of the electroconductive material being the anode in the electrolyte fluid or establishing a contact of said first electrode with the electrolyte;
positioning of the second electrode by either immersing it in the electrolytic bath or by using the wall of the bath's body made of electroconductive material as a counterelectrode;
applying an electric regime in the circuit (anode—electrolyte—counterelectrode), including the application of an initial amperage of the polarizing current, maintenance until formation of a coating of required thickness on the surface of the workpiece to be treated, switching off the forming voltage;
taking out the workpiece;
depending on the chemical composition of the electroconductive material and the size of the surface to be treated the contact is established by immersing a portion of the material in the electrolyte, the portion being determined by the equation
Rakoch Aleksandr Grigorevich
Timoshenko Aleksandr Vladimirovich
Birch & Stewart Kolasch & Birch, LLP
Mayekar Kishor
R-Amtech International, Inc.
LandOfFree
Method for microplasma electrolytic processing of surfaces... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for microplasma electrolytic processing of surfaces..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for microplasma electrolytic processing of surfaces... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2437727