Microplasma Oxidation (MPO)
A promising method of surface hardening of details made of aluminum (titanium, magnesium) alloy. The method is one of the various electrochemical microplasma processes. When using special nanopowder additives it is possible to obtain nano-programmed properties of the coating.
The essence of the process: in a special electrolyte solution under certain electro modes on the surface of a solid product quite dense ceramic layer, containing alpha – Al2O3 (corundum) with thickness up to 200 microns for aluminum alloys, is formed. In this case, the geometric dimensions of parts of any shape remain virtually unchanged (the coating is built up “inside” the product).
Technical characteristics of the coating on aluminum alloys:
- Adhesion strength is comparable to the strength characteristics of the substrate layer;
- Vickers microhardness 20-22 GPa;
- Durability is not less than of tungsten carbide;
- The coefficient of friction – 0.04-0.08;
- High hydro-and gas-abrasive wear resistance (use of the food industry is possible);
- Withstands thermal cycling up to 1000°C;
- An excellent insulator.
The composition of coating for aluminum alloys:
Coating thickness has three zones:
- Transition barrier layer comprising the phases typical for the substrate layer material, with good adhesion (350 MPa);
- The main working layer consisting of alpha – Al2O3 and elements of the substrate layer;
- Technological porous surface layer with the maximum number of micro-and macro-defects of less than one third of the total thickness of the coating. This layer is removed by machining (grinding, polishing).
Cost of production of MPO coating.
Energy makes up the main costs. The coating with thickness of 60 microns (about an hour of unit operation) on an area of 1 square decimeter takes about 8 kW.
The cost of consumables and components make up approximately 5% of the energy costs.
The operator of the unit in obtaining thick (over 30 microns) coatings is involved by 25%.
Areas of application.
It is appropriate to be applied in friction joints of processing industry equipment, in the chemical, petroleum, automotive and agricultural machinery, in the textile industry.
- Face seals in pumps;
- Friction pairs in the sleeve bearings;
- Electric and thermal insulation coatings;
- Corrosion and erosion wear resistant coatings on the blades of gas and steam turbines.
- Friction joints with abrasive wear.
- Hydraulic control valve spools.
- Yarn thread guides, pneumatic turbines, friction joints in textile industry etc.
Obtaining a protective corrosion wear resistant coating with thickness up to 20 microns:
in fact, it is the replacement of traditional anodizing by environmentally friendly technology, but which is 50% more resistant to corrosion and 80% harder. There are no ceramic inclusions in the obtained film, it has a small porosity. The coating can be scratched with a steel object. When using certain additives to the electrolyte (for each alloy there are separate additives) colored coatings may be obtained. Examples of color coatings – aluminum housings of smartphones. It is not necessary to take up aluminum road wheels and parts of large size (at area of over 20 square decimeters).
Preparation of heat-resistant coatings with thickness of 50 ... 70 microns:
Very promising for automotive pistons, especially for high-powered engines: coatings on the bottoms to protect against burnout. Piston life is increased by 80%. If you take 15% of the cost of coverage for the piston, then the installation will pay for itself very quickly. It is necessary to consider the structures experiencing thermal shocks and in which aluminum alloy is used or may be used. For example, casting molds for plastics etc.
Obtaining wear-resistant coatings with thickness of 100 ... 200 microns.
These coatings have a very high hardness, since they consist essentially of corundum (second in hardness only to diamond). The following conclusion can be made: if there is heavy wear, there should be this coating. Examples: textile industry - a thread wipes off everything, it means it is necessary to change for the aluminum parts with MPO - coating , the tobacco industry - the same problem; pumps, including fuel ones. It is very promising to have a coating on cylinder blocks in fuel combustion area and stroke of piston. There is a double effect: the coating is heat and wear resistant. Cable industry - rollers, along which the wire moves, wear a lot. Rollers made of aluminum with MPO coating make the service life 3 times longer.
The specific advantages of MPO:
- The possibility of coating on both external and internal surfaces of the parts of any complex configuration, ease of fitting;
- By varying the processing modes and characteristics of the electrolyte, using one substrate layer material, it is possible to obtain coatings with different properties;
- High efficiency of the process.
Compared with conventional anodizing MPO performance increases by 20 times.