Cool Coatings Let Engines Run Hotter

Dec. 12, 2002
CVD emerges as coating of choice for high-temperature gas turbine components.
CVD coatings can be applied with such precision and consistency that designers can tighten tolerances even on intricate internal passages on components such as this turbine blade.

Bruce M. Warnes
Director, Development Engineering
Howmet Thermatech Coatings, and Alcoa business
Whiteall, Mich

Design engineers striving to improve the thrust-to-weight ratio, power, and fuel efficiency of gas turbine engines continue to demand higher engine-operating temperatures. The reason is simple: An engine that runs hotter is more fuel efficient and powerful. Thus, it's vital to keep hot-section components as cool as possible. It is also key to attaining acceptable service cycles for engine parts.

Strategies for getting engine hardware to run cool in hotter operating environments include serpentine interior-cooling passageways and surface-film cooling schemes. However, many hot-section components now perform in environments where operating temperatures exceed the theoretical melting point of the component material, typically a superalloy. Here there's a critical need for effective coatings on both the external and internal turbine-component surfaces.

For today's engine designs, the complex internal passageways of turbine airfoils can no longer be coated by conventional technologies such as pack cementation or above-the-pack techniques. Complex airfoils with these processes will oxidize to unacceptable levels fairly quickly. The new generation of internal design schemes calls for a coating process that meets two stringent requirements. Namely, the complete coverage of the serpentine interior passageways and surface-film cooling holes and undeviating consistency in both the coating thickness and composition.

Test results underscore the superiority of the CVD coating, when compared to other coatings and processes. Samples cast from Rene 80 illustrate the ability of advanced metallic coating to protect parts from hot corrosion. Shown from left to right are uncoated, aluminide-coated, and platinum-aluminide-coated samples.

CVD for gas-turbine applications

Chemical-vapor-deposition (CVD) coating technology has proven effective at forming coatings on both internal and external surfaces of turbine airfoils. Furthermore, thanks to computer-control of all process variables, the quality of CVD diffusion coatings is often beyond that possible with other coating technologies -- even when the most reactive materials are deposited. This is especially important for the design of surface-film cooling airflow patterns. Here, designers must take into account cooling hole "coat down" when determining component airflow tolerances. Coat down is the narrowing of an individual passageway when a coating layer is deposited on the interior diameters.

Accurately controlling this aspect of the coating process eliminates the possibility of costly rework such as redrilling cooling holes to correct an inconsistent coating thickness. The CVD piping system design and related tooling mechanisms let this technology completely coat even the most complex internal passage with precise consistency.

It is known that the ductility of diffusion aluminide coatings is inversely related to the aluminum concentration in the coating. Low-temperature, high-activity aluminide coatings made by pack cementation or above-the-pack processes typically have high average aluminum concentrations (> 28 wt%). Consequently, coatings made by pack and above-the-pack processes are typically brittle, and chipping of the coating during postcoat handling and engine assembly operations is a major cause of rework.

In contrast, the CVD low-activity aluminizing process yields much lower aluminum concentrations in the coating (

Cast-coat production lets gas-turbine engine makers shorten manufacturing lead times and lower costs by reducing component-routing steps. Cast-coat processing also eliminates the time and expense of masking operations required of other coating strategies that must be machined prior to coating because of chipping and cracking of the brittle aluminide.

Extensive experimental work during the last decade has also demonstrated that CVD low-activity aluminizing removes elements detrimental to oxidation resistance during coating deposition. Furthermore, the removal of harmful tramp-coating impurities significantly boosts the oxidation resistance of the coating. With CVD low-activity platinum aluminides, turbine-engine manufacturers can obtain the benefits of both cast-coat processing and superior resistance to high-temperature degradation.

During cyclic oxidation testing at 2,000°F, a CVD low-activity platinum-aluminide-coated part survived 22% longer compared to its platinum-aluminide pack-coated counterpart.

Recent advances

The most recent turbine designs cut weight by using thinner airfoil walls. During engine service, the thin-walled airfoils undergo more elastic deformation than in previous engine designs. Thus, fatigue cracking of protective coating can be a more serious problem than in the past. The most oxidation and corrosion-resistant coating is worthless if it cracks, exposing the super alloy substrate to the chemically harsh engine environment.

In general, a coating better resists fatigue cracking as it gets thinner and its ductility increases. CVD low-activity coatings are more ductile than high-activity diffusion aluminides made by either pack cementation or above-the-pack processes. Also, because CVD coatings have better oxidation and corrosion resistance, a thinner coating can give the same component life. Hence, the fatigue-crack resistance of CVD low-activity coatings far exceeds that of high-activity products. But the extreme performance demands of the most advanced engine designs are rapidly approaching the fatigue-crack resistance limits of CVD low-activity aluminide coatings.

Grit Blasting

Grit blasting is an inexpensive and effective means of cleaning the surface of precision-cast turbine components. However, grit blasting embeds aluminum oxide particles in the surface of the component. When coatings are applied to grit-blasted turbine hardware, the embedded oxide particles are incorporated into the coating. It has been shown that voids or particles in a continuous matrix can result in up to triple the concentration of applied stresses (thermal and/or mechanical). Thus, grit blast particles contained in a protective coating can make the coating more prone to fatigue cracks.

CVD coatings on turbine blades can be applied with enough precision and consistency to let OEMs tighten design tolerances and simplify fabrication, assembly, and inspection requirements.

But the situation is improving thanks to a new process that rapidly removes embedded grit from the surface of turbine components without damaging the casting. High-cycle and thermomechanical fatigue testing of coatings made with the new process showed the influence of grit removal on coating performance was significant. For example, selected thermomechanical fatigue tests took place on a high-activity platinum aluminide, a CVD low-activity platinum aluminide, and a CVD low-activity platinum aluminide without entrapped grit particles. The observed improvement of the CVD low-activity platinum aluminide compared to the high-activity product is due to the improved ductility of the coating. The CVD low-activity platinum aluminide performed better than the high-activity product because of the improved ductility of the coating. CVD platinum aluminides also boosted performance over that of the nontreated sample.

Tests demonstrated that entrapped grit particles had only minimal effects on the low-cycle, fatigue-crack resistance of the various coatings tested. The test conditions in these experiments were far more aggressive than those in typical gas turbine engines. Thus actual turbine hardware in service would have a much larger thermomechanical fatigue life.

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