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Dr. Dmitri Kopeliovich
Wear is the removal of the material from the surface of a solid body as a result of mechanical action of the counterbody.
Wear is one of the main causes of failing mechanical systems.
Wear of a metallic, polymeric or ceramic part may be considerably reduced by applying a wear resistant coating over the part surface.
Alumina ceramics have an excellent resistance to different types of wear due to their high strength and hardness, which are determined by the strong ionic bonding between the atoms.
Besides to high wear resistance and hardness alumina coatings impart the substrate corrosion and thermal protection.
The following techniques are used for deposition of wear resistant alumina coatings:
Plasma Spraying is the most widely used method for applying wear resistant alumina coatings.
Plasma spraying uses a DC arc struck between two non-consumable electrodes for ionization of an inert gas delivered to the arc region.
Ionized gas (plasma) heats, melts and accelerates the coating material (alumina) fed to the plasma torch in form of powder.
The wear resistance and other properties of the alumina coatings deposited by Plasma Spraying are primarily determined by the porosity of the resulted coating and the alumina grains size.
Plasma Spraying permits to manufacture coatings with sub-micrometer or even nanometer scale structure[1]. Depending on the spraying process parameters the coatings structures from dense to 20% porosity may be obtained.
The alumina coating properties (wear resistance, coefficient of friction, fracture toughness) may be improved by incorporating other ceramic particles into the alumina matrix.
Addition of SiC in alumina matrix results in a decrease of the coating friction coefficient.
Al2O3-ZrO2 exhibits a better wear resistance than pure Al2O3 [1].
Effect of temperature on the wear resistance and the coefficient of friction of alumina-3%titania coating deposited by plasma spray process was studied in [2]. The results show that above 300ºC the friction coefficient decreases due to softening of coating material. The wear rate increases with increase in temperature. The coating showed brittle fracture at higher temperature.
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High Velocity Oxygen Fuel (HVOF) involves a combustion heat of the mixture of a high pressure fuel gas (propane or acetylene) with Oxygen burning of which produces a flame of supersonic velocity.
The coating material (alumina) in form of powder is fed to the spraying gun where it is melted, atomized by the compressed air and ejected to the substrate surface.
According to [3] HVOF has the following advantages compared to the Plasma Spraying (PS) coating:
1)high particle velocity (above 5 Mach) renders a dense coating with higher adhesive and cohesive strength;
2)low surface roughness;
3)less thermally induced changes are generated in the coating material.
The disadvantage of HVOF alumina coating is low density due to incomplete melting of the alumina particles.
However, as shown in [4], when temperature and normal load increase, making brittle cracking a significant wear mechanism, HVOF coatings become superior to Atmospheric Plasma Spraying (APS) ones, thanks to higher toughness. In dry particles abrasion, brittle fracture prevails; therefore, the tougher HVOF coatings outperform APS ones.
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Physical Vapor Deposition (PVD) is the process involving vaporization of the coating material in vacuum, transportation of the vapor to the substrate and condensation of the vapor on the substrate (part) surface.
Two PVD techniques are used for depositing wear resistant alumina coatings: Sputtering and Electron Beam Physical Vapor Deposition (EB-PVD).
Sputtering is a Physical Vapor Deposition method utilizing argon ions for bombarding a cathodically connected target made of the coating material. Atoms of the target are knocked out by the high energy ions and deposit on the substrate surface.
In the EB-PVD method the target anode is bombarded in high vacuum with an electron beam generated by a charged tungsten filament. Electron Beam evaporation method is much faster than sputtering. According to [5] the sputtering process takes approximately 50 hours to prepare a 6-8 µm thick alumina film compared to only 20 minutes needed for the E-beam evaporation.
Typical alumina coating obtained by the Electron Beam Physical Vapor Deposition methods has a columnar structure.
Dense fine grain crack-free structure of PVD deposited alumina coatings does not requires post deposition polishing and provides low wear rate. It is shown in [6] that the wear characteristics of the foil air bearings are greatly improved by applying a protective sputter deposited Al2O3 coating.
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Chemical Vapor Deposition (CVD) is the process in which the coating is formed on the hot substrate surface placed in an atmosphere of a mixture of gases as a result of a chemical reaction or decomposition of the gases on the substrate material.
Alumina CVD coatings are used in multilayered coatings on cemented carbide cutting tools.
Alumina has an excellent resistance to the diffusion and oxidation wear, which is particularly important for high speed cutting tools.
The most common crystallographic variations of CVD alumina are the stable alpha-Al2O3 and the metastable (i.e. stable at room temperature, but not so at machining temperatures) kappa-Al2O3. The grain shape of alpha-Al2O3 is columnar in the growth direction; the density of dislocations and pores is large and voids are frequent in grain boundaries in comparison to kappa-alumina. Kappa-alumina coatings are fine-grained and mainly dislocation free [7].
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Slurry is a stable suspension consisting of ceramic (alumina) powders, processing additives and 20-35% of liquid (water or solvent).
The slurry is spread (sprayed) over the substrate surface and then cured at 250-900ºF (121-482ºC).
Alumina slurry coatings are widely used as a thermal barrier on the gas turbine surfaces.
According to [8] alumina slurry coating has performed successfully on an aluminum internal combustion engine. The cylinder liners coated with alumina possess good thermal and tribological properties providing reduction in cooling requirements, lower friction and wear, reduced emissions and higher durability.
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Sol-Gel process is a synthesis of an oxide ceramic (alumina) in a liquid solution of an alkoxide based precursor in which submicron particles of a solid phase suspended in the liquid (Sol) are obtained and then condensed forming Gel - a 3-D network of polymerized macromolecules surrounded with the solution.
Easy and cheap sol-gel method is commonly used for preparation of alumina coatings.
Wear and friction of alumina films on silicon wafers prepared by the sol-gel method using dip-coating technique were investigated in [9]. It was shown that the friction coefficients in the range of temperatures 300°C-900°C were on the level of 0.18-0.23 while at the temperature of 1000°C the friction coefficient increased up to 0.39.
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Anodizing is the electrochemical process of growing conversion oxide coating as a result of oxidation of an anodically connected metal in an acidic electrolyte solution.
Aluminum and Aluminum alloys form an alumina film when exposed to air. The film is extremely thin and poorly adherent to the aluminum surface therefore it can not serve as a protective and wear resistant coating.
Alumina films prepared by the anodic oxidation of aluminum have excellent adhesion to the substrate surface. Anodized alumina coatings have high hardness, which provides good wear resistance. Columnar porosity characteristic for the anodized oxide film helps in retaining Lubricants and leads to reducing coefficient of friction.
The authors of [10] state that the main application of such an oxide layer is cylinder bearing surfaces in non-lubricated air-compressors. The oxide layer sliding over rings made of polytetrafluoroethylene (PTFE) with a graphite filler demonstrated very low friction coefficient μ = 0.08.
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