Surface Engineering
Surface Engineering
Surface engineering is defined as the design of a surface/substrate composite system to achieve performance that could not be achieved by either the surface composition or the substrate alone, through engineering the substrate surface to improve the appearance, to provide protection from environmental damage or to enhance the mechanical or physical performance of the surface.
Aerospace Thermal Coatings offers a full range of surface protection coatings from general protection to superior performance coatings, for unparalleled corrosion and oxidation protection.
These coatings must withstand extreme operating environment and can be applied to both the exterior and interior surfaces of the aircraft and engine components.
High performance coatings are needed to protect surface from extreme weather conditions and enhance dirt resistance and reduced drag resistance. The requirement of coatings in aerospace industry is highly specific due to the changes in environment regulations.
Sealed aluminide coating is an inorganic sacrificial type multi layer surface coating system consisting of a basecoat and a topcoat, providing excellent corrosion, oxidation, erosion and abrasion protection for various types of metallic substrates. These coatings are cured at low temperature and has a proven track record in the aerospace and marine industries for yielding cost savings from extended component life and reduced maintenance costs.
These coatings are used as a basecoat for sealed aluminide coatings, but also excel as a standalone surface treatment process producing an excellent aesthetic shiny finish when mechanically burnished. It contains a dense matrix of aluminium particles in a liquid binder to form a conductive aluminium structure for sacrificial protection of substrates from corrosion. Typical applications are compressor vanes and external surfaces of engines operating in corrosive environment.
Corrosion Control Coatings - eg: Sermetal, Alseal, Ipseal
A polyurethane coating is a polyurethane layer applied to the surface of a substrate for the purpose of protecting it. These coatings help protect substrates from various types of defects such as corrosion, weathering, abrasion and other deteriorating processes. Typical applications are for general aviation applications where excellent appearance, durability, sag resistance and ease of use are required.
Dry Powder coating is a type of coating that are applied as a dry powder usually to metal substrates such as aluminium or steel which adhered electrostatically onto the charged surfaces, followed by oven curing to form a layer of protective and decorative coating. The curing process heats the dry powder allowing it to flow and form a tough, solid film. The main difference between the conventional liquid paint and a powder coating is that the powder coat do not require a solvent to keep the binder and filler parts in a liquid suspension form. The powder may be thermoplastic or a thermoset polymer and usually used to create a hard finish that is tougher, thick and dense finish having a much longer life than a conventional paint.
Thermal spraying is a technology which improves or restores the surface of a solid material. The process can be used to apply coatings to a wide range of materials and Component , to provide resistance to: Wear, erosion, cavitation, corrosion, abrasion or heat. Thermal spraying is also used to provide electrical conductivity or insulation, lubricity, high or low friction, sacrificial wear, chemical resistance and many other desirable surface properties.
All methods of thermal spraying involve the projection of small softened particles onto a cleaned and prepared surface where they adhere to form a continuous coating. Combined thermal and kinetic energy causes the particles to flatten or ’splat’ onto the surface, and onto each other, to produce a cohesive coating of successive layers.
Arc spray is a form of thermal spraying where two consumable metal wires are fed independently into the spray gun. These wires are then charged and an arc is generated between them. The heat from this arc melts the incoming wire, which is then entrained in an air jet from the gun. This entrained molten feedstock is then deposited onto a substrate with the help of compressed air. This process is mainly used for the application of metal coatings ie, carbon steel, stainless steels, bronzes, copper, zinc, etc.
In plasma spraying process, the material to be deposited — typically as a powder, sometimes as a liquid, suspension or wire — is introduced into the plasma jet, emanating from a plasma torch. In the jet, where the temperature is on the order of 10,000 K, the material is melted and propelled towards a substrate. There, the molten droplets flatten, rapidly solidify and form a deposit. Commonly, the deposits remain adherent to the substrate as coatings; free-standing parts can also be produced by removing the substrate. It can be used for the application of both metal and ceramic coatings. It has the advantage of a high ‘melt’ temperature (up to 10,000oC) when applying ceramic coatings.
Flame spraying is an excellent option for surfaces which aren’t designed to handle extreme stress. The coating which results from this process is not strongly attached to the surface since the spraying mechanism is powered by a lower flame velocity. The flame will be generated via oxygen which has been combined with fuel, and this will melt the mixture. Combustion flame spraying is popular for low intensity applications due to its low cost and used for the application of abradable seal coatings (seal control) and spray/fuse applications.
HVOF (High Velocity Oxy-Fuel Spraying) is a process which makes use of a torch that allows the flame to spread whenever the nozzle is used. This creates rapid acceleration which speeds up the particles in the mixture. The end result is an exceptionally thin coating which is evenly applied. Despite being thin, this coating is strong and adheres well. Its resistance to corrosion is better than plasma coatings, but it is not well suited for high temperatures.HVOF primarily used for the application of very dense (<1% porosity) tungsten carbide and chrome carbide coatings. It can also be used to apply very dense metal coatings for special applications. These dense coatings are achieved by the combination of a relatively low melt temperature and the high particle velocity (up to 1000 metres per second), which is a result of the kerosene/oxygen combustion within the torch.
Hardfacing/cladding may be applied to a new part during production to increase its wear resistance, or it may be used to restore a worn-down surface. Hardfacing by arc welding is a surfacing operation to extend the service life of industrial Component , preemptively on new Component , or as part of a maintenance program.
According to the results of an experimental study, the shielded metal arc welding and the gas metal arc welding hardfacing processes were effective in reducing the wear on the mouldboard ploughshare. With the shielded metal arc welding and gas metal arc welding hardfacing processes, the life span of the ploughshare was increased approximately two times
Hardfacing/Cladding can be applied to a new part during production to increase the wear resistance, hence increasing the projected life, or used to restore a worn surface and extending the lifecycle. This can be achieved by:
The spray and fuse method is by far the most commonly used technique for hardfacing. It is a two step process:
First the spraying / application of the coating material with either low velocity combustion thermal spray or HVOF thermal spray. With either of these, the coating material ( in powder form ) is fed into the flame and projected onto the substrate.
In the second step the coating is fused to the substrate by applying a very high heat, normally by torch ( usually either oxy/acetylene or oxy/propane, with a slightly reducing flame) to achieve a temperature in the 1000C to 1150C range. The temperature required is determined by the material being applied. The fusing process densifies the coating and produces a full metallurgical bond.
The spray and fuse process results in a coating with no interconnected pores and a very high bond strength.
The major disadvantage of spray and fuse coatings is the risk of distortion of the component during the fusing stage of the process by the use of excessively high heat.
Also because of this high heat requirement for the fusing, the physical size of the item that can be repaired can also be a constraint. For these larger items that require hardfacing the PTA method could well be the solution.
The “spray and fuse” and the “spray / vacuum furnace fuse” methods are essentially the same process, except that the fusing process in this case is carried out in a vacuum furnace. The main advantage with this method being the control of the fusing temperature that can be achieved within the vacuum furnace. With some overlays, variations in the fusing temperature can greatly affect the hardness of the final coating. The vacuum furnace method is normally only used for production runs of many similar items with the same overlay material. This is due to the development required to perfect the processing for each item / coating application.
Plasma Transferred Arc (PTA) hardfacing is a versatile method of depositing high-quality metallurgically fused deposits on low cost surfaces.Soft alloys, medium and high hardness materials, and carbide composites can be deposited on a variety of substrates to achieve diverse properties such as mechanical strength, wear and corrosion resistance, and creep. PTA hardfacing has significant advantages over traditional welding processes such as oxyfuel (OFW) and gas tungsten arc welding (GTAW).
Plasma arc welding with semi transferred arc is more concentrated than a TIG welding process, leading to deeper localized penetration in which the joining of materials is produced by the heat of a constricted arc between an electrode and a base metal. In PTA welding, a shielded arc is struck between a non-consumable electrode (Tungsten) and the torch body, and this arc transforms an inert gas (Argon) into a plasma by heating it to a high temperature. The PTA welding process uses this plasma to transfer an electric arc to a work piece. Metal powder is metered, under a positive pressure of Argo flow, from the bottom of the torch into a pool of molten metal on the workpiece surface. Coating purity is extremely high, with very little dilution, typically approximately 5% into the base material. When this process is applied using fully automated controls, combined with robotic work handling equipment a very high level of control of heat input is attainable. This results in an extremely reliable, and repeatable output during processing being achieved.