Table of Contents
16.2.1.8.4 Synthetic Resins, Lacquers, Elastomers
These are often coating systems that consist of a pretreatment (primer) and an aftertreatment such as sealing. These coatings can be applied in various ways, such as spraying (liquid, solid), dipping in powder (fluidized bed), dipping in lacquer, brushing, etc. They have very different mechanisms (e.g. single- and multi-component lacquers) and compositions (e.g. epoxide, PU, polyester, silicon, metal-filled ceramics, PTFE). Lacquers are often used as anti-friction coatings, in which case the filler materials (e.g. graphite, MoS2) are decisively important.
Fig. "Applications of lacquers and organic coatings" shows typical applications of lacquers and organic coatings. In older engine types that contained a large proportion of corrosion-sensitive materials such as martensitic steels (housings, compressor blading, rotor disks), magnesium alloys (gear box housings, front compressor housing), and aluminum alloys (compressor blades, compressor housing, gear box housing), protective lacquers were a necessity. The compressors of modern engines use corrosion-resistant titanium alloys that do not require additional protection. These materials are now being increasingly used in fiber-reinforced polymers (FRP) in fan housings (Fig. "Adhesive applications in machines") and as filled elastomers (silicon rubber) in abradable coatings opposite labyrinth and blade tips.
A special problem is the preparation of the surface to be coated, and can be compared to the surfaces of adhesive joints (Ill. 16.2.1.5). Chaper 16.2.1.7 discusses problems with cleaning, etching, and rinsing.
The testing of adhesive strength is a special task that is also dealt with in Ills. 16.2.1.8-4, 16.2.2.8-5, and 16.2.2.8-6. The development and testing of coating processes and their quality assurance requires non-destructive verification of coating thickness. There are several available processes for this.
Figure "Problems with lacquers": With the exception of electrostatic application processes, the thinner coating on outer edges is a latent lacquering problem (top diagram). This is exacerbated by the fact that outer edges are especially exposed and sensitive to mechanical damages. Electrostatic processes can be used to generate strong fields at the edges, resulting in especially intensive precipitation at these points.
Another problem is the blocking of bores. Suitable masking measures are also necessary in areas that are not to be coated, such as sealing and fitting surfaces.
Figure "Applications of lacquers and organic coatings": Organic and inorganic lacquers, as well as organic coatings, are being used less and less for corrosion protection in modern engines thanks to the introduction of corrosion-resistant materials (Ti and Ni alloys). Instead, other applications are becoming more important. Also see Fig. "Adhesive applications in machines", which shows the applications of adhesives.
Corrosion protection: Light metals (Al and Mg) in housings and blades in older engine types must be protected with organic lacquers.
Steel parts such as shafts and spacers (transition between Ti alloys and Ni alloys) in compressor disks of older engine types are coated with inorganic, Al-filled lacquers due to the high operating temperatures.
Abradable coatings are used in order to maintain high efficiency, i.e. low fuel consumption. Examples of applications in fan housings are filled honeycomb seals, Al-polyester coatings (Volume 1, Ill. 5.4.1.2-3), synthetic resins (Volume 2, Ill. 7.1.4-16), and filled elastomers (Volume 2, Ill. 7.2.2-32).
Low weight: Their use is especially effective in fans with large diameters.
Containment: Bands made from aramid fibers have especially good behavior with low weight (nesting, Volume 2, Ill. 8.2-16.2)
Sound absorption: This is becoming increasingly important. Filled honeycomb seals made from organic materials are used in fans and bypass ducts.
Erosion protection: PU coatings on fan outlet guide vanes and spinners. Lacquers on Al blades in older engine types (Volume 1, Ill. 5.3.2-6)
Vibration damping: Fastening blades with silicon adhesives (Volume 3, Ill. 12.6.3.4-6). Adhesive application of metallic damping foils (Volume 1, Ill. 5.3.2-6).
Anti-friction coatings: Graphite on the contact surfaces of compressor rotor blades. PTFE on sliding surfaces and as a sealant to prevent fouling.
Infiltration and sealing: Porous housings of auxiliary components such as gear boxes made from cast Al or Mg are sealed against oil leaks with the aid of synthetic resins or infiltrated sodium silicate (Fig. "Flaws in light metal castings").
Remedies for Damages and Problems in Lacquered or Organically Coated Parts
Based on experience, a few fundamental guidelines can be outlined that can help prevent typical problems in lacquers and organic coatings. There is no claim to completeness. These guidelines should especially be known and understood by the design engineer and the responsible technicians in the finishing process, particularly in the areas of work preparation, quality assurance, and process monitoring.
When selecting coatings, the design engineer must pay special attention to possible changes in the required characteristics during operation. If sufficient long-term experience is not available, part tests must be undertaken using original parts and sufficiently realistic conditions (time, temperature, environmental factors, tribological effects, etc.) in order to verify suitable behavior.
- Over time, characteristics of organic lacquers and coatings (hardness, strength, deformability, volume) can be reversibly or irreversibly changed by environmental influences such as moisture, fuel, oil, or cleaning agents. Typical signs of aging include swelling, shrinking, embrittlement, and cracking. Swelling of bearing housings can cause variable compressor stator vanes to lock. Shrinking can cause positive-fit parts to separate or coatings to delaminate. Characteristics such as erosion-resistance and FOD behavior can unacceptably worsen. Near their operational limits, the life spans of organic lacquers and coatings react very sensitively to apparently minor temperature increases of only a few °C. This can cause synthetic materials to creep and delaminate (Volume 1, Ill. 5.3.2-6).
- The realizability of a coating must be confirmed by the finishing departments. This includes the process-specific accessibility and the finishing steps. The possibility of improvements and repairability through re-coating is also an important consideration. The prerequisite is that the system can be removed with an acceptable amount of work and without damaging the part. This must also take into account environmental regulations (toxicity of solvents, disposal). In addition, the realizability depends on the availability of the system (patents, licenses, suppliers, deadlines) and its approval status (authorities, OEM).
- Quality assurance: Determining testability and documenting the specifications and definitions of the testing characteristics. Determining the characteristics of the surface to be coated (e.g. roughness).
- The finishing department must have access to the process and have sufficient experience with the part-specific coating process. If this is not the case, then process testing is required.
- Storage conditions (temperature, containers, air exposure, etc.) and the treatment of remnants must be specified and observed.
- Relevant receiving inspections of the systems and components.
- Preparation of the surface to be coated: cleaning, work/treatment conditions (e.g. etching, abrasive blasting).
- Sufficient mixing and reaction times. Use within the flow times (pot life). Working conditions: room, temperature, air humidity, air purity, must be determined, specified, and strictly observed.
- Workability must be tested under serial conditions in suitable rooms and environmental conditions (temperature, atmosphere).
= References =
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