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COATINGS PROVIDE A BARRIER AGAINST EROSION, CORROSION AND HEAT IN THE FIGHT AGAINST WEAR AND TEAR
BY MATTHEW WATSON
Caption: An employee at HFW Industries finish-grinds a shaft for a power industry customer
If you operate turbomachinery, the best way to extend the service life is a barrier against erosion, corrosion and heat. Getting the right barrier, or coating, can extend the time between scheduled maintenance and reduce the number of unscheduled events. It can also improve fuel consumption by up to 1.7%.
Hard-chrome plating has been used for many power generation applications, including gas turbine shafts. However, heat and abrasives can dull the surface of turbine shafts over time. It is not uncommon for a low-amplitude, high-frequency application to wear through chrome plating. Similarly, impurities deposit on steam turbine surfaces and cause erosion. Hard-chrome plating ranges in hardness from 700HV to 1,000HV (Vickers Pyramid Number).
A chemical nickel coating on a component can achieve about 600HV. A better option is a carbide coating, such as tungsten carbide (1,200HV) or chrome carbide (900HV). OEMs and machine shops apply carbide coatings using a high velocity oxygen fuel (HVOF) spray process. An HVOF gun or torch takes feedstock and heats it to a semi-molten state, then applies the coating to a machine component using kinetic energy from a hot-gas stream.
The process makes for a coating that is dense, has low porosity, and resistance to wear and abrasion. A tungsten carbide coating on a turbine shaft or fan blade can improve lifespan by as much as 45%. Another option is abradable coatings (e.g., aluminum silicon, nickel graphite) applied via a plasma- or thermal-spray gun. Abradable coatings are softer than steel, titanium and nickel alloys. Applying an abradable coating to an engine shroud makes it possible to reduce the clearance between the blade tips and casing. They also work well on steam turbine shaft seals and help to reduce leakage.
Nickel graphite is a good coating to apply to compressor shrouds and labyrinth seals. It can withstand operating temperatures of 900°F. Cobalt-based abradables are better choice for components with service temperatures of up to 1,300°F. Zirconia- based ceramic abradables can protect components against thermal shock when operating at more than 2,000°F.
Once an OEM or machine shop applies a coating, it takes one of three steps to achieve the desired finish: Leave the coating as applied; machine the coating; and grind the coating with a diamond-wheel grinder to a specified finish, measured in roughness average (Ra). Power generation applications generally require machining or grinding to achieve finishes ranging from 8 to 16 Ra or finer.
Grinding HVOF-applied metal compounds to the appropriate tolerances is a skilled activity. Cobalt- or zirconia-based abradables can require machining too, but not grinding, to attain the desired finish. During the spraying process, OEMs and coating facilities scrutinize the process parameters of the spray torch to obtain the correct hardness or porosity. This is best done by metallurgical analysis in a materials lab on the premises.
Projects often require machine shops to undertake pre- and post-machining to prepare components. Operators must carefully monitor the speeds and feeds of machining or grinding tools when finishing coatings, otherwise they can de-bond or tear the coating. Done right, however, abradable coatings enable turbomachinery users to reach higher operating temperatures and increased efficiency and reliability.
■ Matthew Watson, Vice President of Operations for HFW Industries, a Buffalo, N.Y.-based machine shop serving industries such as power generation and chemicals. For more information, visit hfwindustries.com