Aeroengine Safety

Figure "Applications of diffusion coatings in engines i": Very different diffusion coatings with specific characteristics are used in engines.

Corrosion protection: Parts made from heat-treated steel, such as integral housings (“F”) and rings (“B”), are common in older engine types. Rings made from corrosion-sensitive steels (13% Cr steels) are still used because they have low thermal strain and high thermal strength. Therefore, they are useful for the transition between parts made from titanium alloys and nickel alloys. In addition, they have good containment behavior in case of titanium fires in compressors. Problems include their tendency to corrosion (pitting corrosion) and oxidation at the relatively high operating tempeartures. To protect against this, Cr diffusion coatings are used (chromizing). These offer limited corrosion protection, and in case of cracking in the brittle coating, element formation is more likely to occur than a protective cathodic effect. The advantage of chromizing is that it does not result in significant dimensional changes, as the application of a lacquer would. However, the chromizing process can be more expensive and elaborate than the application of an inorganic lacquer. The coating process is demanding, and must be designed for the specific part. This process also tends to form small Cr warts that are unacceptable on sealing surfaces. Unfortunately, these cannot be easily mechanically removed because this often causes sections of the coating to break out, resulting in areas suscpetible to corrosive attack. In case of deformation, subsequent material-removing work is hardly possible due to the minimal thickness of the coating (in the 10 mm range). Another problem is the pronounced brittleness of the coating. Plastic deformations during finishing processes, such as clamping, straightening, and thermal strain, can lead to cracking in areas with
stress concentrations. During later operation, corrosion can then occur in these part zones, which are often difficult to examine visually. For these reasons, if it is possible to use them, inorganic lacquers are generally favored on complex structures (“F”).

Erosion protection: In combination with erosion-resistant properties, Cr diffusion is also used as erosion protection (“F”) on compressor blades made from martensitic steels (e.g. Cr steels). However, experience has shown that this latter protection is very limited due to the very small thickness of the coating (in the 10 mm range). On the other hand, the advantage of this coating is that it can be applied to parts with very tight tolerances (e.g. blade edges). In addition, it will not result in a noticeable detuning of the resonance frequencies, as may be the case with relatively thick, thermal sprayed WC-Co coatings.
One problem is brittleness, which can cause cracking in part zones under LCF loads due to plastic deformations. In case of small FOD, such as impact damage from sand grains and small pebbles, there is a danger of cracking in the coating and brittle coating fractures, resulting in a loss of dynamic fatigue strength. Coating temperatures in the finishing process must naturally be determined by the heat-treatment state of the part. A loss of strength due to excessively high tempering temperatures must be avoided. In addition, the high aerodynamic surface qualities of compressor blades must be maintained.

Oxidation protection coatings: These are usually Al diffusion coatings, often in combination with other coating components such as platinum (Fig. "Powder pack Al diffusion coatings"). They are used in hot parts such as turbine blades (“A”). The coating (Ref. 16.2.1.8-1) is applied in a CVD process (Fig. "Powder pack Al diffusion coatings") using a powder pack (powder pack aluminizing) or using the gaseous phase (above-pack process). In order to prevent flaws, the coating process requires a very demanding setup (Fig. "Oxidation protecting Al diffusion coating problems").

Increasing dynamic fatigue strength: Diffusion coatings such as case-hardening coatings (“E”) and nitriding (“C”) can decisively increase the dynamic fatigue strength of parts made from martensitic steels. One area they are used in is power-transfer shafts, especially around pre-determined breaking points (“C”, Volume 1, Ill. 4.5-12). The high dynamic fatigue strength of case-hardened coatings (“E”) is utilized on gear teeth. One difficulty in finishing is the prevention of soft spots. These are areas that are unintentionally covered and protected from the diffusion process, such as areas under fouling. These may be revealed by suitable subsequent etching processes as “stains”.
Experience has shown that, when using lacquer-type masking systems, improper positioning of the parts on the charging rack, or fouling of the rack with lacquer will create a risk of soft spots. If the masking lacquer temporarily softens or liquifies during heating, it can drip on parts below and unintentionally mask them. If these damages are only detected at a late stage, and a large number of almost completed parts are affected, these flaws, which cannot be reworked, will be especially cost-intensive.
Masking systems should be used to protect areas that are not intended to be coated. Masking systems often use galvanic Cu coatings, which are later removed. If these protective coatings fail, the case-hardened coating will penetrate through them and create local hardened areas, which can act as dangerous notches. These hardened areas can reveal themselves during subsequent peening processes (Fig. "Shot peening as testing method").
Case hardening can also increase the fatigue resistance of the running tracks of high-RPM roller bearings (“D”), also in the form of integral bearing rings on toothed gear shafts (“E”).

Friction and vibration wear: Case hardening and nitriding have a high resistance to friction wear. This effect can be traced back to their high strength, as well as special chemical characteristics. This effect results from the non-metallic thin epsilon coating (few mm) of bath nitriding coatings. Unlike the thicker, brittle epsilon coating of gas nitriding coatings, it does not have to be worn down by sliding surfaces. On the contrary, in cases with lubrication problems, such as sliding surfaces with high relative movement speeds and tight tolerances in fuel (control mechanisms in fuel regulators), the thin epsilon coating makes reliable functioning possible. Even minor work to correct possible deformations can catastrophically compromise the functioning capabilities of the part after short run times (danger of cold welding/galling).
Multi-spline connections (“C” and “E”) on steel parts, usually in auxiliary transmissions, are case-hardened to minimize friction wear, as well as to increase dynamic fatigue strength (fretting, Volume 2, Ill. 6.2-18).

Figure "Powder pack Al diffusion coatings": In order to ensure the maximum oxidation protection from an Al diffusion coating (alitizing), the highest possible concentration of aluminum is desirable. At operating temperatures, the aluminum, combined with the chrome in the base material, forms a thin, dense, protective oxide layer. Alitized coatings behave brittly, at least in lower temperature ranges. The brittleness increases with the Al concentration. This negatively influences the thermal fatigue behavior (Volume 3, Ill. 12.6.2-15). During handling of the coated parts during the finishing process, it must be ensured that no plastic deformation occurs (e.g. through striking or clamping, Fig. "Oxidation protecting Al diffusion coating problems"). In order to minimize the diffusion process at operating temperatures, even if it is gradual, diffusion-retarding coating components (platinum, Pt-Al coatings) are used at the transition to the base material (Ref. 16.2.1.8-1).

Al and Cr must be distributed as evenly as possible across the surface not only for the oxidation behavior, but also in order to prevent local weak points or areas of excessively pronounced brittleness.
In all cases, the coating process occurs using the gaseous phase, and can be seen as a type of chemical vapor deposition (CVD). However, the literature classifies CVD as a process in which the retort is exposed to reactive gas that has already been generated.
In the case of pack cementation, the parts are placed in a powder pack consisting of a mixture of the donor (Al powder, inter-metallic phases) and the activator (NH₄Cl, NH₄F). The gas is generated directly next to the part surface. Understandably, the coating quality is decisively dependent on proper mixing of the powder and strict adherence to the tested optimal process parameters (e.g. temperature control, adding of gases). The desired temperature progression must be provided for all parts in the retort (Ref. 16.2.1.8-3). This is a complex task, since convection can be expected and the parts will also have an influence on the heat flow. In order to attain a defined coating atmosphere, a minimum gas density in the retorts is necessary. These are exposed to extreme
thermal fatigue by the recurring process cycles, which limits their life spans significantly. This means that the retorts are a cost factor that should not be underestimated. Their optimal design is a challenging task for the production design engineer, in spite of their complex internal structure.
In order to prevent the coating of part zones under high LCF loads (e.g. blade roots), they must be masked with suitable coating combinations (Fig. "Oxidation protecting Al diffusion coating problems"). Masking coatings can be made from inert materials or reactive ones that are specifically used as getters. The part-specific development of these masking systems requires a great deal of technical expertise and experience. Often, elaborate and extensive production testing is necessary.
There are two types of powder pack coating processes (Ref. 16.2.1.8-3): low-activity coating (left diagram) and high-activity coating (right diagram). Both variants consist of several zones.

High-activity coating occurs at relatively low temperatures between 760°C and 980°C in a large aluminum-releasing source. Under these conditions, very active Al combinations are created. A coating system will form on Ni alloys within a few hours. In order to reduce the high brittleness of the coating components, a subsequent heat-treatment of several hours is done at about 1100°C. This creates three zones, resulting from the inward diffusion of the Al and the outward diffusion of the Ni.

Low-activity coating occurs at higher processing temperatures (about 1100 °C) and lower Al activity. In addition to Al, Cr may also be involved in the diffusion process. The coating structure depends decisively on the temperature management, the concentration and composition of the activator, and the base material. The result is a two-layer coating.
In order to locally repair coatings on new parts, the coating is first removed from the flawed area (abrasive blasting). Then, a ceramic slurry containing Al is applied and the part is annealed.
The testing of parts to determine whether undesirable coatings (e.g. creeping coatings) are present is done through specific oxidation which results in tarnishing (heat tint test).

Figure "Aftertreatment cracks on Cr-coating": Al diffusion coatings (aluminizing) can have various finishing-specific flaws and deviations from optimal structure. During the coating process, damages can occur even outside of the coated area.
Coating composition: Deviation from the optimal prescribed process parameters can affect the coating properties, such as thickness, composition (detail “1”), and the concentration patterns of the alloy components, especially aluminum. This, in turn, influences the oxidation life of the parts. Interior coatings (“2”) are especially affected by this type of deviation. This is related to the problematic gas exchange/gas inflow of the reactive gas in tight cooling air ducts. It requires extensive process development to achieve a composition in a retort that ensures sufficient coating of all interior surfaces.

Surface preparation: The preparation of the surface must ensure even coating. Fouling or oxidation can lead to problems. This includes stuck shot particles (“3”, loading effect, Fig. "Loading effect by blasting processes"). They lead to coating flaws and can decisively shorten the oxidation life of the part.
If coating powder enters cooling air ducts and remains there (“4”), the creep life of the hot parts during operation can be reduced considerably due to a lack of cooling air. It is possible to detect deposits that impede through-flow with the aid of flow measurements. In some cases, these deposits must be removed, requiring extensive rework.

Coating cracks: At room temperature, aluminized coatings usually behave brittly. If a coated part zone is plastically deformed through carelessness (bumping, excessively tight clamping, straightening) during finishing, cracking can be expected to occur in the coating (“5”). This cracking is especially a cause for concern if it spreads into the base material. Unfortunately, it is evidently not possible to non-destructively test whether the cracks are limited to the coating with a sufficiently high degree of reliability. If the sensitive coated parts are not stored and transported with sufficient care, their edges are especially prone to spalling (“9”). If sharp-edged notches form, their effect on the oxidation resistance must be determined, as well as whether or not they are located in a part zone under high dynamic stress.

Creeping coatings: Coatings make it possible to obtain locally limited, special necessary operating characteristics. However, these characteristics are not always desirable across the entire part surface. Their brittleness at low temperatures can even cause them to have a damaging effect. Aluminized coatings are then unable to tolerate the plastic deformation of the base material in areas under high LCF loads. In addition to cyclical centrifugal force, thermal fatigue should also be seen as LCF stress. Therefore, it is important that there are no creeping coatings from neighboring zones (“6”). Suitable masking and coverings are necessary in order to prevent the coatings from creeping, and the development of these may require testing.

Successful masking can be non-destructively tested with the aid of tarnishing in connection with special material-specific, rated etching processes or heating in air (heat tint test).
In the transition zone between coated and uncoated areas, there will be a notch effect that may affect dynamic fatigue strength. Therefore, the coating boundary must conform to the specified drawing requirements.

Caking: If, after coating, the part has noticeable caking, it may be due to coating powder. In this case, there is a danger that, when the caking is removed, the coating may spall and/or suffer brittle cracking (“7”). For this reason, caking should be minimized (also see Fig. "Surface fouling by soldering paste").

Reactions on uncoated surfaces: This usually concerns coating powder (“8”) that penetrated underneath the masking layer (e.g. before masking). A diffusion zone, including melting, can form around this type of contamination. The same effect occurs when coating particles settle on a part as dust during storage before a heat-treatment process (Ills. 16.2.1.4-14 and 16.2.2.3-2). For this reason, the area around the coating process must be kept especially clean, and the disrtibution of the powder into other finishing areas must absolutely be avoided (Ill. 16.2.2.3-2).
These flaws pose a risk that life span-determining characteristics such as toughness/LCF strength and creep strength of the base material will deviate unallowably from the design specifications. Additionally, on contact surfaces, stress concentrations can result from unevenness. This may exacerbate the problem.

Reactions with solder: If aluminizing is done on soldered parts, especially with a subsequent heat treatment at high temperatures, it may damage the solder (“10”). This damage can take the form of cauliflower-like foaming (Fig. "Braze damaged by diffusion coaating"). If the soldered joint is also supposed to be coated to prevent oxidation, when the process is introduced, it must be ensured that the difference in composition relative to the base material will allow this to be done.