Aeroengine Safety

- 5.3.1 Erosion Mechanisms
- 5.3.2 Erosion Damage
- 5.3.3 Measures Against Erosion Damage

Erosion is a type of tribological stress in which a solid body in flowing liquid or gas wears engine parts in an abrasive, shattering, and tribochemical process. All of these erosion mechanisms can occur in turbine engines. This chapter focuses mainly on wear caused by particles in the gas flow.
Aside from erosive particles in the air and gas flows, particles in the fuel can also damage the engines. Even fuel droplets themselves can have an erosive effect when they strike a surface (Figs. "Types of erosion" and "The damage mechanism of cavitation I"). Vapor lock in the fuel flow can cause cavitation (Fig. "The damage mechanism of cavitation II"). Overheated fuels can create microscopic coke particles that damage fuel system components such as fuel nozzles and can make them fail.

With regard to their ability to ingest particles along with the inlet air flow, engines are essentially large vacuum cleaners. However, other erosive particles may originate in the engine itself (Fig. "Erosive particles") and cause erosion damage. These particles can damage engine parts in many different ways, especially through their abrasive qualities. Wherever there is an unfiltered air flow, erosion damage can occur. One of the components most heavily stressed by erosion is the compressor with its blading (Fig. "Erosion damage on suction an pressure side").

Ingested particles that can erode the blading are primarily dust, rain drops (see Chapter 5.1.1), and hailstones (Chapter 5.1.2). Among these, dust erosion damage is the most frequent. It occurs in all stages, decreasing towards the rear of the engine. Rain and hail damage is only to be expected in the front stages, especially in fan engines with large bypass ratios, which usually do not have long air ducts ahead of the fan. Intense erosion conditions are present with engines on cargo aircraft when using thrust reversers, which throw up dust from the runway. A critical erosion problem in fan blades is likely to occur especially when fiber-reinforced materials with a synthetic matrix are in serial production. In this case, suitable protective coatings are a prerequisite for successful long-term serial use.

Engines in aircraft that frequently operate at low altitudes, such as helicopters, are at especially high risk. Large amounts of dust may be thrown up and affect the engines of VTOL aircraft during takeoff and landing. Heavy dust and rain erosion can also be expected in the engines of military aircraft that are frequently operated at high speeds near the ground.

As mentioned above, the erosive effect of ingested particles decreases in the rear compressor stages. This can be explained by the bursting of the particles when they strike the blades of the front stages. Therefore, no significant direct erosion of those dust particles is to be expected in the turbine area. However, this is not the case when the erosive particles are created in the rear part of the engine. These particles are usually spalled ceramic thermal barriers and coke particles from the combustion chamber. A special sort of 'erosion' is triggered by dust melts (in desert environment or volcano ash) which stuck and react at the hot parts and accelerate the wear by spalling markedly (Fig. "Dust particle melting" and Fig. "Danger of volcanoes").