African Fusion March 2015

SAIW: Thermal spray coatings seminar

Overview of thermal spray coating technologies Flame spray processes Powder flame spray is a subsonic flame-based process. Powder is blown or drawn into an annular combustion chamber where an aspirating gas and a fuel gas are mixed and combusted, creating a high velocity (subsonic) ex- haust stream. A second outer annular gas nozzle feeds a streamof compressed air around the combustion flame, which accelerates the spray particles towards the substrate and focuses the flame. The powder can be fed through a small hopper on the gun, or via larger freestanding powder feeders. There are also several fuel gas options, depending on the application and temperatures required. “It is a low velocity, low tem- perature process and, because the par- ticles are projected through air, a surface layer with relatively high proportions of oxides, porosity and unmelted particles is produced,” says Lovelock. Wire flame spraying is a similar process, except that the coating mate- rial is fed into the flame spray gun as solid wire. The flame melts the wire, which is then atomised and accelerated towards the substrate by the annular compressed air flow. Also a subsonic process, Lovelock says that the wire feed rate and flame settings have to be balanced toproduce continuousmelting of thewire anda continuous spray of fine particulate. Showing amicrograph of an aluminium coated sample she points out the black lines of oxide stringers and says, “If you consider how easily aluminiumoxidises, this is not too bad”. The flame spray processes have low capital and running costs. They can be done on-site and by hand, if appropriate safety precautions are adopted, and they offer adequate surface properties for low-stress build-up applications and sacrificial coatings. “They are suitable for non-demanding applications such as repairing wear or corrosion damage on non critical parts that are not intended for demanding engineering applica- tions. A variety of iron- and nickel-based powders are available for repair and salvage,” she adds. Avariationof theseprocesses are the spray and fuse processes, also known as spray-brazing, which involve remelt- ing the fused layer – with a flame, in a furnace in by induction heating – after it has been deposited. “The fusingmelts and seals the surface and can produce

up to 300 to 800 m/s; and the modern cold spray process, which, at tempera- tures below300 °C, accelerates particles towards a substrate at up to 1 000 m/s. Each process type has its own niche in the market. “From left to right, we can see these processes as first, second, third and fourth generation thermal spray developments. The higher the particle velocity, the better the layer quality and bond strengths. And the lower the temperature can be kept, the less the chance of oxidisation occurring while a particle is travelling to the surface. The combination of high velocity and low temperature characteristics make cold spray processes very attractive for sensitive and expensive coatings,” she adds. In terms of the coatings themselves, since substrate temperature can be maintained below 200 °C with good thermal management, there is no heat- affected zone (HAZ) or dilution between the substrate and coating. Due to the mechanical bondand the layerednature of the coating structure, the material properties of the coating are not the same as bulk cast/wrought coating ma- terial properties would be. And while a vast range of coating properties can be achieved, it is generally not a good idea to subject coatings to high point or line impact loading. “Components can be coated with metals, alloys, carbides/cermets, oxides, polymers, blends and graded coatings. Deposition rates of between 1.0 and 20 kg/hr can be achieved and, with the addition of the cold spray process, the thickness range is almost unlimited – from 20  m to > 30 mm. “The greatest limitation is that it is a line of sight process. If you have a thin narrowborewith limitedaccess, it is very difficult to coat the inside of it,” Lovelock informs. She also advises caution in some aqueous corrosive environments, because “thermal spray coatings are not generally recommended for corrosion barrier protection in the as-sprayed condition, because they do not seal. While penetration could take years, the coating itself cannot be guaranteed to last forever, even if it has been success- fully subjected to laboratory corrosion tests and appears to be fully dense. This does not apply to sacrificial corrosion protection coatings, however, because this protection does not depend on a sealed barrier,” she explains.

Void Oxide inclusion Unmelted particle Substrate

Features of thermal spray coatings: “The bonding mechanisms at the interface and between the coating ‘splats’ is still subject to some speculation, but while both mechanical interlocking and diffusion bonding may occur, mechanical bonding predominates,” says Lovelock. Image ©ASM International.

ametallurgical bondwith the substrate. The process is commonly used to coat glass formingplungers andglassmoulds with a NiCrBSiFe alloy, a ‘self-fluxing’ alloy with low melting point. This coat- ing is known to produce a favourable surface interaction with molten glass (silica) which at 700 °C is very abrasive,” says Lovelock. Arc wire spray The key difference between these pro- cesses and flame-based process is that an electric arc is used to generate the requiredheat. Themost commonly used variation is twinwire arc spraying, where a dc electric arc is struck between two continuously fedwires. Thewire speeds are set to balance the melt-off rate of the wires and to keep the arc stable. On melting, droplets are propelled by compressed air or an inert gas jet onto the surface being coated. Arc spraying is a very high produc- tivity thermal spraying process with de- position rates for steel at 10 to 14 kg/hr and up to 5.0 to 8.0 kg/hr for aluminium. “With flame spray, it is possible to spray unmelted powders onto a surface, but with twin wire arc spray, the coating material has to be melted before it is sprayed,” Lovelock notes. The process is simple to operate; canbe usedmanually or automated and a wide range of met- als, alloys andmetal matrix composites (MMCs) can be deposited, including a limited rangeof carbide-basedmaterials that are available in cored-wire form. Generally speaking, the coating quality is better than flame sprayed coatings, with less porosity, fewer ox- ides and higher bond strengths. The

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March 2015

AFRICAN FUSION