Table of Contents
5.3.1 Erosion Mechanisms
The erosion mechanisms in an engine can be influenced both microscopically and macroscopically in a characteristic manner by the locally effective parameters. For example, depending on the angle of attack of erosive particles, erosive wear can occur primarily through cutting or splintering mechanisms (Fig. "Erosion mechanisms").
The damage symptoms understandably depend on the many parameters of the erosion process. These include the particle size and type, impact speed, impact angle, number of particles, and the duration of the particle influence. In addition, the extent of the damage is also dependent on the properties of the erosion-stressed part, such as material properties (strength, ductility, oxidation behavior, etc.), surface topography (size and geometry of roughness), and geometry (such as the leading edge radius of blades), to name a few important factors.
It is easy to see that an erosion process is a combination of extremely complex single processes that must be understood.
The dominant erosion mechanism in engines is abrasion through impacting particles. This abrasion mechanism can occur in various ways, depending on the particle energy and the properties of the part surface. With ductile materials micro-level chip removal can be expected. Hard, brittle materials can suffer micro-level fatigue processes resulting nicks and disruptions, and brittle materials and coatings can spontaneously splinter out.
Other types of wear in which the dominant wear mechanism is surface disruption, such as rain erosion (droplet impact) or cavitation (erosion in liquids due to vapor bubble implosion), are briefly covered here (Fig. "Types of erosion"). Another type of erosion that has not been recognized as being damaging until now should also be mentioned (referred to here as “fine erosion”). It is usually observed at the micro-level on the back sides of blades and lays bare the microstructure (especially in titanium alloys) in a manner similar to etching (sputter effect of the air molecules?).
With erosion processes in hot parts, additional mechanisms are involved that are based on the oxidation of blank metal surfaces (erosion surfaces) or on reactions with the base material or the oxide layers (such as sulfur particles in gypsum dust). These mechanisms can significantly increase the wear rate and damage caused by otherwise relatively weak abrasive erosion processes.
Figure "Types of erosion": If erosion is understood to mean surface wear caused by the mechanical action of a media, then there are many different mechanisms which fit this definition (bottom diagram).
Erosion through solid particles is generally referred to as peening wear. Other terms that are frequently found especially in older literature, but are not in DIN 50320, include mineral wear, sand and dust erosion, and abrasive wear. The material being eroded through the impact of hard particles is experiences slice/furrow-like chip removal. With hard materials, and especially hard coatings on softer, the wear occurs primarily as splintering and/or fatigue crack formation, as well as the breaking out of particles. This type of erosion process occurs primarily on the aerodynamic surfaces of the compressor and turbine bladings. Abrasive wear does not occur solely through the air flow, but can also be caused by hard particles carried in liquids. A typical example is coke particles in an overheated fuel flow that can cause extensive erosion damage to the injection system.
Droplet impact always occurs as an erosive load when droplets of liquid strike a surface at high speeds. If the liquid is rain, then it is referred to as rain erosion. This process creates shockwaves and deformations in the material which, in case this process is frequently repeated, lead to fatigue and breaking-out of the material. This type of wear can occur in rotor blades and fan outlet guide vanes.
Rain erosion can be a serious problem for compressor blades (fan blades) made from fiber-reinforced plastics/synthetics (Ref. 5.3.1-19).
With cavitation, the erosion process takes place in a flowing liquid. If, in accordance with Bernoulli`s law, pronounced underpressure zones occur in a liquid that result in the formation of vapor bubbles, the latter can implode if the flow carries them into an area with higher pressure. The damage model is a “liquid stinger” that is created in the imploding vapor bubble and strikes the part surface at high speed, creating a damage mechanism similar to droplet impact (Figs. "The damage mechanism of cavitation I" and "The damage mechanism of cavitation II"). This type of wear, which is primarily based on fatigue of the surface, is also referred to as “cavitation erosion”. In watery media, the cavitation process is often increased by simultaneous corrosion (cavitation corrosion).
Cavitation damages have been recorded on components of the fuel system such as injection nozzles and regulators as well as on the pump gears of turbo pumps.
Cavitation has also been blamed for damages to gear pumps and transmission gears in high altitude flights (easier vapor bubble formation due to the low air pressure).
Rain can cause considerably heavier erosion damage than cavitation (top right diagram). The extreme sensitivity of polycarbonate and glass to droplet impact is especially conspicuous.
In the same way that strong wind throws up sand, an extremely intense gas flow, especially in the rear compressor area, can cause particles to break free from soft coatings such as the abradable coatings in compressor casings (e.g. porous nickel-graphite spray coatings). This process can be self-increasing, since these particles, in turn, have an erosive effect.
The top left diagram shows that materials can behave considerably differently during erosion through solid particles than they do during erosion through drops of liquid (Refs. 5.3.1-1 and 5.3.1-2). For example, it can be seen that soft, elastic, very ductile polyurethane has a high erosion resistance against sand and rain (compared with Plexiglas or aluminum, etc.), which makes its use as a protective coating against erosion on fan parts (rotating nose cone, fiber-reinforced synthetic outlet guide vanes, etc.) understandable. The great resistance to rain erosion relative to the resistance to sand erosion cannot be observed in any other (harder) materials.
Figure "Erosive particles": Many different particles can have an erosive effect in engines. These particles may be ingested with the inlet air, be created indirectly by erosion, or form inside the engine in some other way.
Erosion particles in the inlet air “E1”: These are usually dust from the surrounding atmosphere. There is a correspondingly large variation in the size, consistency, and composition of the dust particle.
Wear products from housing/casing coatings “E2” and “E3”: In the fan and compressor area, tight tip clearance is maintained through the use of rubbing systems. In order to prevent the blade tips from being unallowably damaged or worn, relatively soft abradable coatings are used on the housing side. However, the macroscopically soft behavior during rubbing does not mean that all of the coating particles are also soft. The coating often consists of hard particles bonded into a soft matrix, and the abradable behavior occurs by these particles breaking out at the micro-level. Ingested particles that blow over these coatings in concentrated amounts can erode these coatings, and the erosion products can further increase the total process in the following stages.
Wear products from the blading “E4”: With compressor blades with hard coatings (e.g. TiN erosion protection), if the kinetic energy of the particles is too great, the thin coatings may splinter. In turn, the extremely hard coating particles can have an abrasive effect. If blades with tip armor are used, then particles of the armor can break out and increase the erosion stress.
Wear products from rotor spacer rings “E5”: In order to minimize the tip clearance between the stator vanes and the rotor, hard ceramic rub-tolerant coatings are used on the spacers (Fig. "Singularities in the gas flow"). If these coatings spall, the resulting particles can even damage the blading near the rotor.
Soot and coke particles from the combustion chamber “E6”: Coke and soot feel so soft, that dangerous erosive effects are not usually expected. However, experience has shown that these particles can cause unallowably serious wear in the hot part area in a relatively short time (Example "Improved combustion liner", Fig. "Low coke combustor"). In the combustion chamber, coke can collect on the surfaces of the walls and the injection system and break off as erosive particles. The formation of soot during the combustion process in the gas flow can also lead to heavy erosion of the turbine blading. Erosion processes on hot parts create fresh metal surfaces and alternatingly accelerate oxidation.
Particles of combustion chamber coatings “E7”: The walls of modern combustion chambers are increasingly being coated on the inside with ceramic thermal insulating layers and thermal barriers in order to minimize the required cooling air. These coatings can spall or their surfaces may crumble out, causing the abrasive particles to threaten the turbine blading, especially.
Particles of the rub coatings of the outer seal ring of the HPT “E8”: In order to minimize the tip clearance at the high-pressure turbine rotor blades, the housings are outfitted with seal segments that are coated with a hard ceramic rub coating (usually zirconium oxide) that the blades can “grind” into. Abrasive particles of these coatings can be created in a short time due to rubbing or over longer periods through “crumbling out”, causing damage to the rear hot parts.
Labyrinth wear products “E9”: In labyrinths, rub coatings can create particles either through rubbing or erosion. Even the armor on labyrinth tips can spall and allow the base material to be worn. These particles can have erosive effects in the labyrinth itself or in other areas of the engine (Fig. "Damage mechanism in labyrinths")
Coke particles in fuel “E10”: If fuel temperatures are too high, the fuel may decompose, creating fine, erosive carbon particles. These particles can damage fuel nozzles and change the angle of the fuel stream so it burns through the combustion chamber and the combustion chamber liner.
Figure "Erosion mechanisms": The angle of attack is especially important for the wear effect of peening wear. The extent of damage depends largely on the angle of attack and the ductile or brittle behavior of the material (top diagram). Brittle materials such as glass (curve C) exhibit a wear sensitivity that increases almost linearly with the angle of attack. Due to the large energy transmitted (shockwaves), these materials fail by shattering if the erosion particles strike them at a steep angle. The wear process includes micro-crack initiation (middle detail). At flatter angles and high hardness, these materials are highly resistant to slicing mechanisms.
Ductile materials (curve A) such as titanium alloys, martensite heat-treated steels, and aluminum alloys exhibit a steep increase in wear until 25°, but then the wear rate drops almost continuously until the angle of attack is roughly 90°. This is understandable from the chipping wear mechanism (top diagrams) at the micro-level: when the angle of attack is flat, the cutting process is especially effective on the soft materials, while particles striking the surface at steep angles bounce off without cutting. This characteristic is used, for example, when lining blasting cubicles with rubber coatings. For this reason, soft erosion protection coatings, such as PU coatings on the rotating nose cone, are especially suitable for surfaces with relatively steep angles of attack.
Curve “B” is a material system with a hard, brittle coating (e.g. TiN, see Ref. 5.3.1-3) on a relatively soft base material (titanium alloy, etc.). In this case a sufficiently large energy transfer between the particle and surface will shatter and break through the surface. This causes increase erosion wear. Because the energy transfer in extremely hard surface coatings (no cutting) increases with the angle of attack, the destructive process mentioned above occurs relatively suddenly above a discrete threshold value (sudden rise in the curve).
Because the erosion process is influenced by the energy transfer, it can be concluded that curve “B” is also determined by the size of the erosion particles and their impact speed.
If mineral particles (larger than 0.010 mm , Fig. "Particle size") strike a surface, they may burst. The low energy of the smaller resulting particles (due to their small mass) means that thin, hard coatings can be used as erosion protection in the rear area of compressors. These coatings will not be destroyed by the low-energy particle fragments.
Example "Improved combustion liner" (Ref. 5.3.1-16, Fig. "Low coke combustor"):
Excerpt: “…To further reduce the… unscheduled removal rate,…engineers are examining at least two additional improvements- a low coke combustor and…
Officials expect the new combustion liner to eliminate blade erosion caused by carbon buildup, increasing blade longevity. The new liner also will cut visible smoke and increase liner life, since the effusion cooled liner eliminates hot spots and improves the distribution of temperatures in the component. Company engineers have not yet fully quantified the improvement in component lives, but they believe blade live will be doubled or tripled.”
Comments: This concerns a helicopter engine with a reverse-flow combustion chamber. The considerable decrease in the life span of the rotor blading on the first turbine stage caused by the coke deposits is interesting (Ill. 22.214.171.124-6.2).
Figure "Influence of hardness of particles": The strength and hardness of the particle material and wear-stressed metallic engine part, as well as the relationship of the hardness of the part surface to the hardness of the particles, have a decisive influence on the wear rate (top diagram; schematic). Up to a certain hardness of the impacting particles, relatively little erosion wear occurs (low area), depending on the hardness of the engine part. When this hardness is exceeded at point T, the erosion rate increases rapidly (high area). This behavior explains why apparently small differences in the hardness of the attacking minerals and/or the erosion-stressed material can lead to large differences in the erosion behavior. However, in the middle parts of the high and low areas, even relatively large differences in hardness may only have a minor influence on the wear rates.
The erosion rate then remains relatively constant at this level.
Relatively small differences in the chemical composition of the ingested dusts (such as proportion of SiO2 -, Al2O3) strongly influence the wear rate through the proportion of hard particles. This behavior must be considered when selecting erosion particles for tests.
With soft ductile materials such as rubber or PU coatings the described effect does not apply, since the wear rate may even decrease with increasing particle hardness.
In the right diagram the described erosion behavior of steel plates with various hardnesses (numbers in the circles are the Vickers hardness of the test plate) at right-angle (pure) impact of a metallic peening material. It is easy to recognize the shifting of the steep increases, depending on the hardness of the test plate.
Figure "Influence of angle of attack": (Fig. "Erosion mechanisms") Depending on the angle of attack of the particles, the erosion behavior of an engine part made from soft and ductile materials is clearly and characteristically different from the wear behavior of brittle and hard materials. Typical “soft” blade materials in the compressor (top left diagram) are heat-treated steels (typically 13% Cr steels in older engines) and titanium alloys. Titanium materials are almost twice as sensitive to abrasive erosion as are steels. The erosion maximum seems to be slightly shifted towards greater angles of impact. Considering that the blade profiles of modern compressors have considerably sharper edges than the steel blades in earlier engines, then it must be assumed that the erosion sensitivity has also increased accordingly.
Because the impact angles of particles on the blade profile vary greatly (bottom diagram), the erosion behavior of blades with the same shape but different materials can be expected to be different. Blades made from fiber-reinforced synthetics/plastics or with lacquer coatings, especially, should have considerably different erosion behavior than titanium alloys and coatings of brittle hard materials (e.g. TiN) on titanium alloy bases. The variation in behavior must be considered when developing and verifying the operating properties of bladings.
Of course, the particle-carrying air/gas flow also has an important influence on the particle trajectories and therefore also the impact angles. It is usually assumed that particles smaller than 0.010 mm will largely follow the air flow. For this reason, the leading blade edge should exhibit large impact angles in a small area, while the blade surfaces will be struck at very flat angles (dotted curve in bottom left diagram). However, larger dust particles are less and less likely to be deflected by the air flow, making them more likely to damage the trailing edge (bottom right diagram). In the rear compressor area, in which only small, already shattered particles occur, hard, thin coatings can successfully protect against erosion.
According to the depicted erosion model, it can be expected that the trailing edge of the blade and the suction side of the blade are not subject to erosion stress. However, this is not always true. There have been many cases in which especially the trailing edges of stator vanes show considerable damage from particle impacts or the suction side has significant erosion damage. This can be explained through ricocheting particles, such as sand grains that bounce off of neighboring blades and/or the blades of the following stage and are thrown forward (Fig. "Particle size").
Figure "Influence of Material" (Ref. 5.3.1-4): The particle size is important for the wear/erosion rate and the damage to the blades for several reasons (such as notch effect, dynamic strength, geometric changes to the edges):
- At the same impact speed, the effective energy of a heavier is correspondingly greater
- The particle experiences less deflection from the flow and has different impact angles than smaller particles (Fig. "Influence of angle of attack").
- The relative speed perpendicular to the blade surface (circumferential speed of the blade), i.e. the impact speed, should be higher than with smaller particles.
- The impact craters are considerably larger and change the blade profile (burr formation, craters), especially at the leading edge (Fig. "Erosion damage on suction an pressure side").
The impact speed of the particles on compressor parts is roughly that of the gas- and circumferential speeds, i.e. between 100 m/s and over 300 m/s.
According to the bottom right diagram and the relationship described above, at the same impact speed and a 90° impact angle, the erosion rate increases exponentially with the particle size (particle material Al2O3).
The particle speed goes into the erosion wear exponentially (see top diagram and bottom left diagram). This dependency can be far greater in brittle materials than it is in ductile materials. The increase of the wear rate with increasing particle impact speed is independent of the impact angle. However, the absolute erosion rates clearly show the angle-dependency described in Figs. "Erosion mechanisms" and "Influence of angle of attack".
Figure "Unexpected behavior" (Ref. 5.3.1-1): Materials with apparently very similar material parameters (strength, hardness, ductility, etc.), can have very different erosion behaviors. For example, under the erosion conditions given in the top left diagram, the performance of window glass is extremely poor compared with that of sapphire, and also considerably worse than Plexiglas® (acrylic glass) erosion. Therefore, it is understandable that enamel and glass coatings (enamel) on compressors were unsuccessful. On the other hand, pure nickel performs remarkably well under erosion stress, which explains why it is used as erosion protection on the edges of fan rotor blades made from fiber-reinforced plastics.
At the beginning of an erosion process at certain impact speeds, a slight weight increase in the erosion-stressed part can be observed (right diagram). This “loading effect” is due to erosion particles sticking in the part and seems to be less pronounced at higher impact speeds.
Figure "Damage mechanism in labyrinths": Erosion processes in and around labyrinths are not as well known, but can certainly be damaging.
Erosive labyrinth wear products from fin armor or abradable coatings can be blown out of the labyrinth (C2) and become caught in neighboring housing areas (A). This whirling dust film can wear its way through metal/steel cross-sections that are several millimeters thick.
Labyrinth wear products or dust-laden barrier air can become trapped between the labyrinth fins and be whirled around (C1). This can seriously erode the relatively soft abradable coatings across from the labyrinth fins (B).
Figure "Particle size" (Ref. 5.3.1-14): The tests with mineral particles in the air flow have shown that particles smaller than 0.010 mm largely follow the air flow and therefore put relatively little erosion stress on the blading. However, larger particles can ricochet and be reflected around the air duct. For this reason erosion damages can, specific to the engine type, occur in unexpected areas such as on the rear edges of stator vanes. Typical signs of these ricochets are impact marks on trailing edges, which indicate movement of the particles against the direction of the air flow.
It should also be considered that larger mineral particles burst upon impact and the erosion stress in the rear compressor area is correspondingly lower.
Figure "Erosion sensitivity of different compressor designs" (Ref. 5.3.1-5, 5.3.1-6, 5.3.1-7): Depending on the configuration of a compressor, characteristic zones in the gas flow are especially erosion sensitive. The top diagram shows an axial-radial compressor (Ref. 5.3.1-18) of the type common in smaller helicopter engines. The dust is centrifuged outward and puts erosion stress especially on the tips of the rotor blades and the casing wall: This increases the corresponding tip clearances (arrows). The dust is concentrated outside on the air flow and also erodes the outer area of the radial compressor blading (Fig. "Erosion patterns on radial compressor disk").
The bottom diagram shows a two-stage radial compressor. Here the centrifugal forces from the air flow deflection are greater than the outwardly acting centrifugal forces, causing the dust to concentrate in the hub area. This causes the blades in the radial compressor to be especially erosion-stressed in the less sensitive hub area. The erosion behavior of this type of compressor can be expected to be more robust.
Wear/Erosion Under the Influence of Liquids: Droplet Strikes, Rain Erosion
Rain erosion (Fig. "Types of erosion") is a type of damage that has been known for a long time in helicopter rotor blades. Even in the 1970s extensive investigations regarding this were conducted. Additionally, rain erosion on cockpit glass of military aircraft designed for high speed low-altitude flight was also researched. The available literature therefore primarily concerns the typical materials and erosion parameters from these operating fields.
Erosive damaging rain can be expected in the front compressor changes or fan when there is no long air inlet duct. Rain erosion should be considered in cases where fiber-reinforced plastics (especially carbon fiber and aramdid fibers, see Ref. 5.3.1-19) are used in the compressor blading. Conditions for rain erosion are more likely to occur with engines of aircraft that operate at low altitudes, such as helicopters.
In soft materials, the wear mechanism is primarily plastic deformation, constriction, and breaking out of small surface areas (Fig. "The damage mechanism of cavitation I"). Hard materials may break out due to hardening and crack initiation (exhaustion of the deformability) or fatigue. Weight loss is often noticeable only after a certain incubation period. Rain erosion causes material removal. However, the geometric changes in, for example, a blade leaf, is not necessarily the most serious damage. The micro-crack initiation and/or pitting material removal can cause the fatigue strength of the part to decrease considerably.
The following example is intended to make clear the expected material removal due to rain erosion. If rain drops of 1.2 mm diameter (drop density of 0.01 drops per cm3, temperature 25 °C) strike a surface at a right angle at nearly the speed of sound, then the amount of aluminum removed would be about 0.01 mm/s, but with titanium alloys it would be far less (0.01mm/101-102h). Typical rain densities have been measured to be between 0.001 to 0.1 drops per cm3 (see Chapter 5.1.1, Fig. "Drop size distribution").
Drop impact damage must not always be related to rain. For example, if droplets of liquid from a water injection into the compressor strike the blading or from a water injection into combustion chamber area (e.g. in industrial turbines) strike the turbine blading, it can cause erosion damage to coatings. In the temperature range between 10°C and 50°C, rain erosion is virtually unaffected by the temperature. Above 50°C the erosion effect decreases.
Cavitation (5.3.1-11.2) is a type of damage related to droplet impact. In this case, as well, the material is stressed by the impact of a liquid. However, with cavitation a “liquid spike” forms in an imploding vapor bubble in a liquid flow. Cavitation damage is especially found in components of fuel systems.
Figure "The damage mechanism of cavitation I" (Ref. 5.3.1-8): A droplet of liquid that strikes a solid surface at high speed create shockwaves that cause large local elastic and/or plastic deformations, depending on the material. This can remove protecting oxide coatings and result in increase corrosion attacks.
Many impacts one after the other with elastic deformations can cause dynamic fatigue at the micro-level. This leads to small break-outs of material that accumulate into macroscopic wear.
In case of plastic deformations, craters can form in the impact, and in these craters overexpansion can initiate concentric micro-cracks that lead to material breaking out when struck by further droplets. If micro-breakouts have occurred, then droplets can create “liquid spikes” similar to those during cavitation (Fig. "The damage mechanism of cavitation II") in the hollow space, accelerating the erosion process.
On the surface the liquid is distributed concentrically to the droplet. This liquid disk can build up on rough areas of the surface and cause micro-cracks there, as well.
The dynamic cracks (LCF) cause pitting-like material breakouts up to 0.1 mm deep on the surfaces of titanium parts. This damage form was observed on part surfaces on which coating remnants were removed with a high-pressure water jet after oxide peening (creates relatively high roughness).
Figure "The damage mechanism of cavitation II" (Ref. 5.3.1-15): This diagram is based on a high-speed photograph and shows the implosion of a vapor bubble in a liquid during supersonic vibrations. One can clearly recognize the liquid spike that forms during the implosion process and damages the material surface in a manner similar to the damage mechanism of droplet impact (Fig. "The damage mechanism of cavitation I"). If there are many of these impulses after one another, they can destroy protective coatings and additionally increase corrosion. This damage/wear type is referred to as cavitation corrosion. Continued stressing results in local deformation and fatigue of the surface.
Damage-causing vapor bubbles form in underpressure zones like those that can occur in fast-flowing or vibrating liquids (Bernoulli`s law).
Thereby it is not necessary that only vapor bubbles of the flowing liquid itself are concerned. Also dissolved gases and/or earlier evaporating liquids like water (e.g., condensate that during unsuitable storage was introduced into the oil) can during elevated temperature and/or pressure drop similar to a bottle of fizzy water trigger the formation of vapor bubbles and so cavitation.
It is interesting that with increasing roughness, different to rain erosion (Fig. "Influences on rain erosion II") the erosion attack gets slower (Ref. 5.3.1-25).
The sketch above right shows areas of gear pumps in an oil system, where damage by cavitation was observed (sketch above right, Fig. "Parts effected by erosion", Ref. 5.3.1-21; Ill. 23.2.1-18). Especially affected are the suction side of scavenge pumps and the front side of the gears as a result of the expansion of the squeeze oil.
Figure "Axial face seals" (Ref. 5.3.1-22): Also axial face seals (Ill. 126.96.36.199-1), as they are especially used in the region of bearing chambers, are endangered by cavitation caused failures (Ill. 188.8.131.52-2 and Ill. 184.108.40.206-3).
Figure "Insufficient oil pressure" (Ref. 5.3.1-24): In the region of the contact surfaces from oil dampened anti friction bearings also cavitation was observed (Ill. 23.1.1-2). The typical high frequency vibrations of the antifriction bearings (outer ring) during unsufficient oil pressure and/or high oil temperature can promote the formation of vapor bubbles.
Figure "Erosion parameters": If the erosion rate “E” refers to the volume of material eroded during a specific time from a certain material, divided by the volume of the impacting liquid, then a typical pronounced erosion behavior during rain erosion can be determined:
With a certain incubation period (corresponding to an incubation volume of droplets), the erosion rate increases quickly at first, reaches a maximum, and then decreases. After a longer period the erosion rate is virtually constant. The incubation time depends both on the material and on the kinetic energy of the droplets during impact. During the incubation period, micro-crack-initiating fatigue processes occur, which cause the surface material to break off or break out after the incubation period. The steep erosion increase at first is accompanied by a clear increase in roughness.
Figure "Influences on rain erosion I" (Refs. 5.3.1-9 and 5.3.1-10):
Influence of materials on rain erosion:
In metals with pronounced strain hardening (increase of the yield strength during plastic deformation), the mechanical stress during erosion can increase the erosion resistance. In general the erosion resistance of metals increases with hardness (top left diagram), whereas for minerals and synthetics this is often not the case. Aside from the macro-hardness, the micro-hardness is also important, i.e. whether or not there are soft or hard phases. In areas with large variations in hardness, such as along grain boundaries (carbide fortification, etc.), increased erosion attacks occur. Because fine-grained metals have more grain boundaries than coarse-grained metals, the former are more sensitive to rain erosion. Depending on the kinetic energy of the rain drops, the order of the erosion resistance of different materials may change. For example, soft PU lacquers perform very well with droplets at low impact speeds, i.e. with kinetic energy that can be absorbed elastically. However, if the impact speed is above a certain limit, the coating is quickly destroyed. It is has been conclusively determined that materials behave very differently during rain erosion than they do during sand erosion, and the performance of a material during one of these does not indicate its performance during the other.
Influence of droplet size on rain erosion:
The influence of droplet size on erosion behavior can vary greatly between materials. There have even been observations of contradictory dependencies: the erosion rate of pure aluminum decreases with the droplet diameter while it increases with droplet diameter in PU surfaces. This can be explained with the overlaying of several different influences such as material properties, number of impulses, and surface topography.
The most common rain drop diameter is between roughly 1 mm and 3 mm (see Fig. "Drop size distribution"), which corresponds to a mass of about 1 mg to 10 mg. Both the volume and shape of droplets influence the erosion rate. The critical droplet speed (v), above which noticeable erosion begins to occur after longer periods, is dependent on the droplet size (top right diagram). The influence of the droplet size is considerable near the critical speed and decreases at higher impact speeds (Fig. "Erosion parameters"). These threshold conditions, above which considerable erosion occurs, are given approximately for metals by the relationship vc2 . d = 107 (vc in m/s, d in 0.001 mm ).
Figure "Influences on rain erosion II" (Ref. 5.3.1-9, 5.3.1-10, 5.3.1-11):
Influence of the droplet impact speed on rain erosion:
The impact speed v (m/s) determines the kinetic energy released by the droplet and therefore its erosive effect (bottom right diagram). If vc is the critical droplet speed (Fig. "Influences on rain erosion I"), below which no noticeable erosion occurs even over long periods of time, then the following relationship applies to the erosion rate:
E = a (v-vc)n. “a”, “n” and “vc” depend on the material and erosion parameters such as he droplet diameter and shape. For example, with steels, E = a (v-125)2 to 2,16.
Influence of the impact angle on rain erosion:
The erosion rate increases rapidly with the impact angle (bottom left diagram). This shows that (with smooth surfaces) the normal component of the impact speed is decisive. The tangential component only plays a minor role in droplet strikes on smooth surfaces. However, the rougher the surface, the greater the influence of the tangential component. For this reason, only the inlet edges of compressor blades are noticeably affected by rain erosion. The advantage of blades with sharp inlet edges is that they slice up the drops and prevent stressing pressure waves from forming.
Influence of surface roughness on rain erosion:
The top diagram shows that with decreasing roughness, the wear due to rain erosion decreases considerably. High erosion resistance in the shape of considerably longer incubation times can therefore be expected primarily from smooth surfaces. As soon as the incubation period has passed, the roughness of the erosion surface increases and the erosion attack accelerates correspondingly.
Also for cavitation (cavitation erosion) a higher starting roughness increases the attack. However during progression of the deterioration with the jaggedness a slower erosion speed can be observed (Ref. 5.3.1-25).
5.3.1-1 W. Kayser, “Erosion durch Festkörper”, proceedings of the “2. Forschungskonferenz Regenerosion”, 16.-18. August 1967, Meersburg Germany, pages 278-290.
5.3.1-2 H.Rieger, “Vergleichende Untersuchungen zur Werkstoffzerstörung beim Tropfenschlag und bei der Kavitation”, proceedings of the “2. Forschungskonferenz Regenerosion”, 16.-18. August 1967, Meersburg Germany, pages 260-277.
5.3.1-3 D.R. Nagy, V.R. Parameswaran, J.D. Mac Leod, J.P. Immarigeon, “Protective Coatings for Compressor Gas Path Components”, proceedings AGARD-CP558 of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Rotterdam, The Netherlands, 25-28 April 1994, chapter 27.
5.3.1-4 R.Ball, W. Tabakoff, “An experimental Investigation of the erosive Characteristic of 410 Stainless Steel and 6-Al-4V Titanium”, Report No. 73-40, Department Aerospace Engineering - University of Cinncinati, US-Army Research Office Nr. DA-ARO-D-124-G-154, 1973.
5.3.1-5 D.L. Mann, G.D. Warnes, “Future Directions in Helicopter Engine Protection System Configuration”, proceedings AGARD-CP558 of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Rotterdam, The Netherlands, 25-28 April 1994, chapter 4, page 4-6
5.3.1-6 S.C. Tan, R.L. Elder, P.K. Harris, “Particle Trajectories in Gas Turbine Engines”, proceedings AGARD-CP558 of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Rotterdam, The Netherlands, 25-28 April 1994, chapter 14, page14-12.
5.3.1-7 C.G. Horton, H.Vignau, G. Leroy, “The Calculation of Erosion in a Gas Turbine Compressor Rotor”, proceedings AGARD-CP558 of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Rotterdam, The Netherlands, 25-28 April 1994, chapter 15, page 15-14.
5.3.1-8 “Failure Analysis and Prevention”, Metals Handbook Ninth Edition, Volume 11, American Society for Metals, Metals Park, Ohio 44073, page 164.
5.3.1-9 W. Herbst, “Tropfenschlagverhalten von Eisen und Vergütungsstählen”, Proceedings der “2. Forschungskonferenz Regenerosion”, 16.-18. August 1967, Meersburg, Germany.
5.3.1-10 Baker, Jolliffe, Pearson, contribution to the proceedings of the “2. Forschungskonferenz Regenerosion”, 16.-18. August 1967, Meersburg, Germany.
5.3.1-11 J.M. Hobbs, Beitrag in den Proceedings der “2. Forschungskonferenz Regenerosion”, 16.-18. August 1967, Meersburg, Germany.
5.3.1-12 “V2500 designed for maximum core protection”, diagram, periodical “Aircraft Engineering”, May 1993, page 3.
5.3.1-13 “Delta Weighs Changes to CFM56-2”, periodical “Aviation Week & Space Technology”, December 20, 1982, page 31.
5.3.1-14 A.Hamel, “Solid Particle Dynamic Behaviour Through Twisted Blade Rows”, proceedings of the “2. Forschungskonferenz Regenerosion”, 16.-18. August 1967, Meersburg, Germany.
5.3.1-15 J. Hancock, “Ultrasonic Cleaning”, ASM Handbook Volume 5 Surface Engineering.
5.3.1-16 S.W.Kandebo, “Alliedsignal Commits to LT101 Improvements”, periodical “Aviation Week & Space Technology”, March 11, 1996, page 70.
5.3.1-18 J. Pugnale, “Dual Cebtrifugal Compressor: The Helicopter Solution to Sand and Ice Ingestion”, “American Helicopter Society”, Washington DC, 44th Annual Forum Proceedings, June 16-18, 1988, pages 673-681.
5.3.1-19 J.M.S. Keen, “Development of the Rolls-Royce RB.211 turbofan for airline operation”, proceeding paper 700292 of the ASE “National Transportation Meeting” New York,N.Y., April 20-23, 1970.
5.3.1-20 D.I.G. Jones, C.M. Cannon, „Control of Gas Turbine Stator Blade Vibrations by Means of Enamel Coatings“, Zeitschrift „Journal of Aircraft”, Vol. 12, No. 4, April 1975, page 226 -230. (2497)
5.3.1-21 Z.S. Palley, I.M. Korolev, E.V. Rovinsky, „Structure and Strength of Aircraft Gas-Turbine Engines“, Übersetzung FTD-HT-23-903-68 aus dem Russischen von „Foreign Tecnology Division”, 1968, page 372.
5.3.1-22 B.S. Nau, „Film Cavitation Observationw in Face Seals“, Proceeding der „Fourth International Conference on Fluid Sealing”, Philadelphia, Pa., 1969, page 190-198.
5.3.1-23 K. Steffens, „Technik der Luftfahrtantriebe“, Vorlesung an der TU-Aachen, März 2003.
5.3.1-24 H.R.Carr, G.J.Ives, P.Jenkins, „A Joint Study on the Computerisation of In-field Aero Engine Vibration Diagnosis”, Proceedings AGARD-CP448 der Konferenz „Engine Condition Monitoring-Technology and Experience“, Quebec, Canada, 30May-3.June 1988, Kapitel 31, page 31-5.
5.3.1-25 „Kavitation von Stellventilen”, Technische Information V74 / Schulung der Fa. SAMSON AG page 3-58.