African Fusion March 2015

SAIW: Thermal spray coatings seminar

With a nozzle design based on jet engine technology, HVOF dif- fers from conventional flame spraying in that combustion is internal and the gas flow rates and delivery pressures are much higher. HVOF torches generate a supersonic flame with characteristic shock dia- monds. “Very high particle velocities are achieved, but at the relatively low temperatures of a combustion process. This combination makes it the ideal process for modern wear resistant carbide coatings,” says Lovelock. The HVOF process produces exceptionally high quality cermet coatings (e.g., WC-Co), but it is also used to produce coatings ofmetals, alloys and (less often) oxide ceramics. Although similar in principle, significant details, such as powder feed position, gas flow rates and oxygen to fuel ratios, are apparent between systems from different manufacturers. The fuel can be propylene, propane, ethylene, acety- lene, hydrogen, natural gas, MAPP, kerosene, and others, depending on the system design. Combustion parameters are the most important variable in HVOF spraying: the ratio of oxygen to fuel; volume flows of oxygen and fuel; and combustion pressure. Powder size and shape are also important and eachHVOF gun is optimised for a specific powder size range. Pow- der feed rate, standoff distance and traverse speeds also need to be taken into consideration. HVOF coatings, because of the high velocities and moderate temperatures, result in less oxidation, less thermal decomposition and high impact energy. This makes them ideal for depositing ther- mally sensitivematerials such as carbides, without degrading particle properties. HVOF is ideal for depositing dense, highly adherent wear resistant coatings with very low levels of oxide, porosity and unmelted particles. Bond strengths are among the highest available, particular for the tungsten carbide-cobalt (WC- Co) and tungsten carbide-cobalt chrome (WC-CoCr) coatings, whichare fast becominga replacement for hardchrome coatings. HVOF-depositedcarbide coatings have replaced hard chrome on the landing gear cylinders of the A380 and the Boeing Dreamliner, which have to sustain very high stresses when landing. On the downside, it is a process suited only to automation – HVOF guns are often manipulated by a robot – and while some internal diameter guns are available, internal bores are usually problematic. It is also not well suited to onsite or in-situ work. Cold Spray coatings Cold spray, or gas dynamic cold spray (GDCS), is a modern coating deposition method based on accelerating solid particles in a super- sonic gas jet at velocities up to 1 000 m/s. The particle temperatures remain below the melting point (typically below 800 °C) while being projected to the substrate and, on impact, are subjected to extreme local deformation with high transient temperatures at the interface. There are twobroad variants of the process, lowpressure andhigh- pressure cold spray. “In low pressure systems, the powder is injected downstream of the nozzle throat, so the feeder itself does no have to be pressurised. These systems areavailableas portable systems for the surface repair of aluminiumandmagnesiumor copper components,” says Lovelock. “High pressure cold spray systems inject the powder upstream of nozzle throat, which requires that the powder feeder be classified as a pressure vessel,” she adds. While the full potential of this relatively new thermal spray process is still being developed and explored, the process is already used for high value repairs of aeronautical components made of aluminium and magnesium alloys; the coating of dense oxygen-free copper for electrical conductors, additive manufacturing, titanium coating of medical implants and the application of zinc and zinc-magnesium alloys onto laser engravable print rolls.

Courtesy of Kennametal Stellite.

The as ground finish of an HVOF applied WC 10% Co 4% Cr coating.

Courtesy of TWI Limited. Courtesy of TWI Limited.

The cross section of an HVOF-applied WC-Co-Cr coating. The coating offers resistance to sliding, abrasive and low angle erosive wear at temperatures of up to 540 °C and chromium in the matrix gives better corrosion resistance compared to WC-Co coatings.

The cross section of an HVOF-applied Alloy 625 coating, a barrier coating for corrosion resistance. Conclusions “While there aremany thermal spray processes, it is impor- tant to select the right coating for thewear conditions, wear mechanism, corrosiveenvironment, operating temperature and loading requirements of the component. Tight control of process parameters is crucial, and standards and cus- tomer specifications are becoming increasingly stringent as the global thermal spray industry is required to service ever more critical applications,” Lovelock concludes.

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March 2015

AFRICAN FUSION