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7.1.1 Fundamentals of Blade Tip Rubbing

Causes of rubbing and special demands on the rubbing system:

Blade tip rubbing can have several very different causes. Because of this, rubbing systems must control various operating conditions and demands, as well as minimizing clearance. Therefore, there are special requirements for rubbing systems. For example, rubbing must only occur in such a way that consequential damages are kept to a minimum. The turbine-side rotor must be braked in case of shaft separation in order to avoid dangerous overspeed. Blade failure must not result in the entire blading failing. Rubbing should not be self-increasing and must not overstress the housing (thermally, mechanically, friction). This is especially dangerous if blade fragments are “run-over” by the remaining blade tips. There must be no (titanium) fires and/or (dust) explosions.

The rub-tolerant/abradable behavior of tribo-systems used to minimize clearance

The many operating influences (Fig. "Rub tolerant blade tip systems") on clearance-maintaining systems make their development and optimization extremely costly in terms of money and time (Fig. "Properties and tests of rub systems"). Characterizing, researching, and testing the qualities of rub-tolerant and abradable systems in conditions sufficiently close to those during operation is especially problematic. This begins with quality control during the production process, and continues during assessment in technical trials and proving in the engine. Evaluation of the test results occurs while comparing them with experience from systems already operating in engines. Testing should be done on original parts or samples that are sufficiently similar to these, since characteristics unique to those parts (e.g. fatigue behavior, stiffness, cross-sections, and thus heat removal capacity) are determining factors for the test results. In the end, the long term performance in the engine during typical operation is the deciding factor (“the engine will tell us!”). The selection of materials for the tribo-system must take into account not only rubbing behavior but many other factors. These include:

  • No impermissible influence of components (e.g. through dynamic fatigue or overheating).
  • No damaging of other parts through particles that have been rubbed off (e.g. clogging hot parts, reacting with hot part surfaces, erosion).

Rubbing parameters

The size and shape of a gap between parts with different movement speeds depends on several parameters and can vary. Rubbing parameters include infeed, rubbing speed, and time duration. The important factor for infeed is the type of infeed movement. This can be continual or oscillating, permanent (e.g. creeping) plastic deformation or reversible (e.g. heat strain or elastic deformation). During rubbing, the contact area is very important (e.g. for heat creation and oscillation incitement). Blade tips can rub against the opposing surface along the entire circumference, along a short section, or only periodically. Typical causes of uneven housing distortion are stiffness jumps in the areas around axial flange.

Chronology and Infeed:

The stressing of the parts during rubbing is dependent upon the rubbing conditions. These include
oscillation incitation, static deformation, and heating-up of blades, rotor hubs, and housings.
Naturally, the infeed movements that cause clearance to be lost are also very different. Impact-like rotor bending can cause a rubbing to transpire in a fraction of a second, whereas heat strain in the housing may take minutes.
In Ref. 7.1.1-3 it is explained that slow infeed movements at high operating temperatures cause serious, especially damaging rubbing. On the other hand, high infeed speeds are more dangerous at low operating temperatures.
Loss of clearance between rotating and static parts depends of parameters specific to the parts involved, such as rubbing time, infeed, contact path/contact surface:

  • Large infeed in fractions of a second after large imbalances as consequential damages (compressor surges, bird strikes, imbalances)
  • Large infeed in fractions of a second due to deformations of the housing and stator assembly resulting from damages with extreme force (e.g. bird strike, containment of a blade failure)
  • Infeed in fractions of a second resulting from a foreign object in the clearance gap
  • Large and fast axial rubbing, especially with axial movements caused by shaft separation
  • Sudden large infeed due to self-increasing rubbing processes such as a fan blade catching and twisting in the housing or material buildup from lubricants
  • Small, fast, radial infeed in fractions of a second due to high-frequency vibrations of housings and/or rotor components
  • Large infeed increasing over several seconds (e.g. after rotor bending)
  • Elastic deformations in the minute-region of housings and/or rotors (e.g. due to vis inertia and gyroscopic force during spiraling)
  • Slow, relatively small infeed over several minutes (e.g. due to varying heat strain rates, centrifugal force, gas pressure)

Figure "Rub tolerant blade tip systems": The multitude of often conflicting requirements for rubbing systems necessitates compromises.

Manufacture:

Coatings:
The structure of a coating influences chipping behavior and aging processes (e.g. oxidation, corrosion). It is determined by characteristics such as porosity, orientation (e.g. layering in spray coatings), composition, size, and form of various phases (e.g. nickel and graphite). Coating qualities can be influenced to varying degrees through the parameters of the coating process (e.g. thermal spraying).
The hardness of a coating affects the both its own behavior during rubbing as well as the wear behavior of the rubbing partner. Measuring hardness of porous, multi-phase brittle coatings is very problematic and usually results in scattered results that have only limited suitability for adequately safe quality control.
Strength: structure and hardness determine the inner strength of a coating. Strength, along with deformation behavior (brittle or ductile), is especially important for chipping behavior and therefore for the rubbing forces which affect blade tips. It also affects the erosion sensitivity of the coating and the susceptibility to fatigue fractures.
Bond strength: coatings must stay firmly bonded to the substrate/part throughout the entire operating life, especially operating cycles. Varying heat strain rates and mechanical loads (e.g. centrifugal force) must be absorbed without the bond being weakened to a dangerous degree. Usually, in order to ensure sufficient bond strength, a bond layer is affixed to the mechanically roughened part surface (abrasive blasted). A suitable mechanical grouting (roughness and topography) and sufficient cohesion forces (diffusion, cold welding) are prerequisites for good bond strength. Bond strength is also dependant on inner strength of the coating and internal stress.
Internal stress overlay with the operational loads. This means that, depending on the size and type of internal stress, the life of the coating can be lengthened or shortened by the bond strength. The formation of internal stress is guided by the manufacturing process. The temperature control of the coating and substrate during the coating process is especially important for this.
The thickness of an abradable coating determines its possible radial infeed. However, it also influences the heat transfer into the part and can act as a thermal insulation layer and make the housing thermally inert. On the other hand, dense Al spray coatings improve heat transfer. This allows the coating to make a decisive contribution to clearance control. The thicker the coating, the higher the internal stresses in the coating/substrate system usually are.
Allowable voids must be findable and utilizable by serially applicable methods. These voids demand a sufficient degree of experience.
In relation to this, the testability of coatings is an important prerequisite to their serial implementation. It is not rare for necessary procedures (e.g. thermography, special penetration methods) to be specifically designed for the coating type. An ultra-sound test, for example, can not be effectively used with lamellar coating and/or porous coating structures.
The costs of a rubbing system are created not only by the manufacturing process, but also through necessary repairs during overhaul. Coatings that can only be renewed a few times after perceptible damage can become very expensive indirectly due to the part costs.

Opposite surfaces (blade and labyrinth tips):

The manufacture of the geometry of the partner surface must not damage it by lowering its dynamic strength. Overheating during a grinding process or dynamic fatigue cracks due to overgrinding the tips must be avoided.

Armor must not detract from the aerodynamic effectiveness of blade tips. Therefore, it must be
ensured that the prescribed coating thickness and roughness are maintained. With armoring, special attention must be given to possible changes in the blade`s dynamic strength. Reactions between the coating (e.g. solder for embedding hard particles) and the base material can have an embrittling effect. In any case, during rubbing the opposing surface also experiences wear.
During overhaul and/or repair, the geometry of the original new part must be restored. This can be done, for example, through plastic deformations of the blade tip (feather edging) or deposit welding and mechanical reworking. Labyrinth tips are also deposit welded and reworked in a special procedure. In any case, armor must be removed before beginning repairs. Therefore, armor that cannot be removed during overhaul without too much difficulty is, in practice, unsuitable for use. These include chemically and abrasively resistant ceramic layers, the removal of which in an etching bath also dissolves the natural carbides of the base material. Materials that cannot be deposit welded limit the practical usability of the parts due to their limited capacity for repair.
Additionally, armoring must have a sufficiently cuttable surface in order to ensure low rubbing forces and low friction heat. Therefore, coatings should not have a smooth, reworked surface, but instead be somewhat rough.

Function:

Rubbing behavior:
During the rubbing process, no element of the system should sustain unallowable damage. Overheating of the blade tips, rub-tolerant coatings, or rotor and housing wall must be avoided. Materials of the partner surfaces that tend to thermal fatigue cracks or heat cracks during rubbing are just as unsuitable as coatings that cause the blading to oscillate dangerously during rubbing.
The wear behavior of a rubbing system`s contact materials must be tuned to one another by means of a rub-tolerant or abradable function. Usually, the opposing surfaces (e.g. blade tips) are allowed to wear as little as possible, since repairs are more expensive, full of risk, and more complex than renewing the coating is.
Catastrophic failures caused by progressive buildup of worn-off material on the partner surface should be avoided through use of armor and design measures.
Rubbing must not result in fires (titanium fires, oil fires). Therefore, the heat created by the rubbing process must not reach the ignition temperature of the base material in the air flow.
Extreme rubbing, such as after large imbalances, must not create large amounts of removed material that results in an uncontrollable dust explosion (Chapter 9.4). Especially dangerous are, for example, finely dispersed metallic dusts (aluminum, magnesium, titanium) and/or synthetic resin or graphite dust (graphite coatings, carbon fiber reinforced synthetics). This must be considered when choosing a coating material.

Sealing effect:
The sealing effect of a rubbing system depends on more than just the size and geometry of the clearance gap. It is also dependant upon the surface structure of the coating (e.g. honeycomb) or an open inner porosity (e.g. metal felt). This problem can be solved through a suitable contouring of the opposing surface.
Erosion or outbreaks due to operating influences such as dynamic fatigue and thermal stress can increase the performance loss through leakages considerably throughout the operating time (deterioration). It is important, therefore, that coatings, especially relatively soft abradables, are sufficiently durable. The only real way of determining this is experience gained from operation and/or results of realistic trials.
Coatings affect the heat transfer and heat transition. Therefore, depending on the type of coating, they can accelerate or decelerate the heating-up of the housing walls or rotor components, leading to clearance changes during operation. This can and must be considered and made use of in high-pressure compressor housings, for example. Changes in the roughness or thickness of the coating also affect the clearance size, especially in transient operation.

Operational influences:

On the tribo-system:
The relatively soft abradables, especially, are affected by operational influences such as oxidation, corrosion, erosion, and oscillating loads. Between 300 and 400°C, a noticeable oxidation of graphite components in Ni/graphite spray coatings begins.
Pressure waves (also sound charges) can cause fatigue and outbreaks of coatings. Also The so-called “Blade Passing Frequency”, is also noteworthy, and can be traced back to the pressure distribution around the blade tips. The pressure peak before and the pressure valley behind a compressor rotor blade tip create high-frequency gas vibrations on the coating when the blades run by.
During standstill, condensation water forming in an ocean atmosphere causes corrosion damage with coating outbreaks especially in Al-powder filled synthetic coatings in the front compressor region. If condensation water penetrates through cracks, tears, or open porosity and reaches a corrosion sensitive base material or a sensitive bond layer, even corrosion resistant coatings become detached. This effect must be taken into account with, for example, coated spacer rings made of steel.
The gas flow and the particles it carries with it noticeably affect anti-wear coatings, especially the soft abradable coatings in housings. If their strength is reduced due to aging, erosion can cause pronounced increases in the clearance gap and lower engine performance.
Erosion at hot parts is often observed as the result of crumbling ceramic coatings. If thin anti-oxidation coatings are damaged by erosion, oxidation and material removal are accelerated.

Heat strain and strain due to mechanical loads (in rotor components, due to centrifugal force; in compressor housings, due to pressure buildup) can lower the bond strength of coatings to base parts and lead to outbreaks. Experience has shown that ceramic intermediate ring coatings with a poor internal stress state are endangered by the LCF loads created by start-up/shut-down cycles.

Consequential damages after wear coating failure:
Rubbing systems can also noticeably affect other engine components. Coating particles created by the rubbing process or erosion are carried in the gas flow and have an erosive or corrosive effect on other components. Reactions with hot part surfaces must be avoided, as well as internal or external blockage of the hot parts. The tendency for blocking up hot parts depends, among other factors, upon the toughness and melting point of the melted particles that are created in the combustion chamber area. Fused material that reacts with the base material (i.e. oxide coatings) can break off when it cools (different heat strain rate than the base material). This accelerates the emaciation process.

Figure "Properties and tests of rub systems": Many specific values of the material affect the rubbing behavior of a housing coating (Ref. 7.1.1-3). These material characteristics include structure, density, composition, and mechanical properties. Additionally, thermo-physical properties such as melting point, specific heat, heat conductivity, and heat strain are also vital. Due to the complexity of the rubbing process, selecting potential coating materials on basis of these values cannot replace a practical verification of suitability. However, specific values are helpful for explaining and interpreting test results. It must be remembered that:

The development and testing of rubbing systems demands many varied technological tests, suitable to the load spectrum. These include:

A and F: Measuring the behavior during material removal. In assembly A, the energy loss from an oscillating blade is measured for a predetermined amount of material to be removed from the coating sample being tested. Measured values from this assembly scatter considerably and do not sufficiently reflect engine operation with considerably higher cutting speeds and variations in elasticity. More relevant results are gained by testing original parts in a rubbing rig (F), but this is considerably more elaborate.

B: In this assembly for determining dynamic strength of a coating and base surface, an ultrasonic tool is pressed against the substrate (housing wall). This simulates high-frequency vibrations in the air (blade pressing frequency) and the housing fairly well. These tests are used primarily to simulate damage in order to ensure improvements in comparisons.

C: Bond strength is tested on knob samples that are affixed between two clamped supports for a tenacity test. Bond/coating strength above the strength of the synthetic resin adhesive cannot be determined in this way. This is especially true for temperatures at which the strength of the adhesive drops considerably (about 100°C ).

D: Resistance to thermal fatigue can be comparatively estimated by use of cyclical hot gas and cold air flow. Evaluating the test results requires experience with the operating behavior of the components in the engine. Criteria are crack initiation, separation, and aging.

E: A hot gas test can be used to estimate the damaging effects of removed coating particles on the hot parts, such as blocking of cooling-air vents and canals or unallowable reactions with the surface. In this test, coating particles are blown into a combustion chamber and carried by the gas flow onto or into the hot part. Molten particles can deposit themselves on the blade, block bores for the shroud cooling system, and react with the protective oxide layer. In some cases, the brittle coating breaks off along with the oxide coating upon cooling. The newly exposed metallic surfaces oxidize heavily. Inner cooling-air bores can be blocked by semi-fluid dust.

F: Rubbing rig for determining the behavior of a system under realistic rubbing conditions. The thermal and dynamic blade stressing during the rubbing process is especially important. To measure this, original blades outfitted with special sensors (temperature, strain) are made to rub under controlled infeed conditions with original rubbing partners (housing, rotor hub) with circumferential speeds and temperatures typical for those during operation (Fig. "Chipping at abradable coatings").

G: Measuring the permeability of porous rub-coatings. This determines the tendency of the coating to allow air to leak around the blade- or labyrinth tip.

H: This erosion test should be compared with experience from operation. Dust is blown onto the sample with injected air. This test can estimate the influence of various parameters on the erosion behavior of the rub-coating, such as particle speed and impact angle.

Figure "Tribo-conditions at blade tips": At top left is a wear diagram for the rubbing system of a blade (TiAl 6-4) against a plasma-sprayed nickel/graphite abradable coating. It is interesting to note that the material removal from the blade tip decreases as rubbing speed increases. The material removal at first increases along with the infeed speed, then decreases.
At top right is a typical wear diagram (schematic) of a wear mechanism for a rubbing system as created by Sulzer Co. (also see Fig. "Rub coating and blade stresses"). The individual measured values, which are not given here, indicate the rubbing parameters: weighted coating roughness and weight changes of the blade. This can be caused by material removal or deposits. The behavior of the blade tip (depositing) overlaps with various coating behaviors (cutting, coating outbreaks). Large coating roughness is created when some of the coating is smeared (deposited) onto the blade tip. Cutting causes little material removal at the blade tip, but the resulting coating roughness can lie anywhere in a wide range between smooth and very rough.
The bottom diagram is a wear diagram of a real system. Evidently, the wear mechanism partly corresponds to the rubbing speeds. At low rubbing speeds (150 m/s), primarily blade material removal occurs, at middle speeds around 300 m/s coating outbreaks occur, and at high speeds there is a danger of material being deposited on the blade tips. This results in a self-reinforcing wear process.
Therefore, a rubbing system must be tuned exactly to the parameters applicable to the specific situation.

Figure "Rub coating and blade stresses": The top left diagram (Ref. 7.1.1-1) shows the rubbing temperatures for typical guide vane tip coatings against a rotor spacer ring of Cr steel as determined in a testing rig. The coating with the lowest friction temperature was applied galvanically with embedded hard particles. The low heat creation can be explained by the galvanic bond, which is intended to prevent that the abrasive particles are not rounded off by melting-on, as happens with thermal spraying processes. Therefore, the chip removal of the rub-coating is improved by the sharp-edged particles. The lower the rubbing temperatures, the better the coating should be rated with regard to damage to the rotor/and or blade tip. For example, it lowers the danger of the rotor burning through (Fig. "Compressor rotor perforated by rubbing" ). Low rubbing temperatures in hard tip coatings indicate chip removal in the micro-region or a very low coefficient of friction of the tribo-system with correspondingly low heat creation. It is interesting to note that friction increased along with the roughness of hard coatings (top right diagram). This can be explained by the fact that this tribo-system experienced less chip removal and more material removal. This behavior should not be seen as universal. Data in Ref. 7.1.1-3 shows that the friction energy of a rubbing process is not suitable as criteria for an abradable system. “Rub energy does not correlate with blade wear and cannot be used as a screening test for coating materials”.
The behavior of a tribo-system can be estimated to a limited degree from the appearance of the worn blade tips (bottom left diagram). With soft abradable coatings and/or blade tips with cutting capacity with relatively little heat creation, desirable material removal without burring can be expected (“A”). Poor chip removal with large heat production results primarily in one-sided burrs (“B”). In extreme cases, two-sided burrs are created (“C”). Two-sided burrs are promoted by compression forces, which can be expected from fast infeed speeds.
The bottom right diagram shows typical rubbing appearances of coatings of rub-tolerant systems with hard coatings (e.g. spacers). It is possible, that the three main types depicted also occur in combination. An even, light, rubbing scar is normal. Heavier material application with signs of high heat development (Ref. 7.1.1-5) is often observed along with heavy tarnishing on ceramic coatings, such as in high-pressure turbine ring segments. In compressors, these rubbing signs indicate a potentially dangerous accelerated rubbing process that can burn through thin cross-sections, such as rotor spacer rings.
Weak, periodical rubbing traces are often observed (Ref. 7.1.1-1). The cause of these is not clear. It may indicate oscillations of a rubbing blade. It may also be that they are simply caused by steps in the coating (chatter marks from the grinding process) from manufacture that become apparent during the rubbing process.



Figure "Temperature problems at rub systems" and Figure "Operation behaviour of rub-system": There are rub-coatings specific to the demands of the components throughout the entire wide temperature spectrum of an engine. In the high-temperature areas, ceramic and metallic coatings are used, whereas in the low-temperature compressor, softer coatings (e.g. Ni/graphite, filled synthetic resins, filled elastomers) are more suitable.

Whether a coating is classified as a rub-tolerant or abradable coating depends primarily on the hardness of the blade tip relative to the coating hardness. In the following text, if a coating is classified as an abradable one, then it is assumed that the blade tip is considerably harder than the coating.

Elastomers: Abradable coatings made from filled silicon rubber are used in the fan area. They are poured or glued in as strips several millimeters thick. Their rubbing behavior is similar to that of an eraser. They are easily chipped by the blade tip. These coatings can dampen the housing and thus protect against oscillations. The strength of the adhesive must be secure for the demands of the rubbing process (forces, temperature) for the duration of its operating life.

Filled synthetic resins, such as epoxide coatings with graphite powder, are stopped onto or (in a “flinging process”) poured into the rotating, already bladed housing. This makes these houses have an vibration-damping effect on the guide vanes. Long operating times can lead to the coatings cracking and separating due to aging and shrinking processes.

Thermal-sprayed abradable coatings: The usual coating process for these coatings is flame- or plasma spraying.

Aluminum/Polyester: These coatings, which are up to several millimeters thick, are a mixture of flame-sprayed Al-alloy powder and an especially thermally resistant polyester resin. Their maximum operating temperature is about 340°C. Because these coatings are very sensitive to corrosion in ocean atmosphere, an alternative is Al-bronze powder (AlCu alloy) or an anodized powder. Here, however, there is the problem of availability. For repairs and supplementary sealing of the rubbing surface as well as the edges (shrinkage gaps!), organic coating or, for high temperatures, inorganic coating (e.g. ceramic binders filled with Al powder) are used. This corrosion protection is understandably very limited in case of abrasion or crack initiation in the blade. There are known cases where the use of engines was noticeably impeded by the corrosion of these types of coating. These coatings have a relatively low porosity, depending on the polyester content, which allows a good compromise between abradability (chip removability) and erosion resistance (Fig. "Chipping at abradable coatings"). Heavy material removal of large amounts of coating (e.g. after a bird strike or blade damage) there is a risk of dust explosions. Reactions of molten Al particles with hot part surfaces can lead to local deposits and/or changes (e.g. embrittlement) through diffusion.

Aluminum- and aluminum/graphite (Al/Cg, Cg being carbon in graphite form): Spray coatings of Al alloys have long proven themselves as abradable coatings in compressors at operating temperatures of 480 °C. The have good corrosion and erosion resistant with sufficient abradable behavior. The material removed from these Al spray coatings can be optimized by the proportion of graphite (between 25% and 50%). These coatings are a mixture of Al- and graphite coatings. There is a danger of dust explosions in case a large amount of material is removed. The diffusion of splattered coating particles can damage hot parts.

Cu/Ni/sodium silicate: These coatings are made of a porous structure of flame-sprayed Cu- and Ni-particles infiltrated with sodium silicate (watery solution of alkali silicates). The corrosion- and erosion resistant of these coatings is acceptable, but due to the sodium silicate, they tend to increased material removal and heat creation at the blade tips.

Nickel/graphite (Ni/Cg): These flame-sprayed coatings (plasma-sprayed coatings are usually too hard), are the most commonly used abradable coatings in compressors. The weight proportion of graphite is usually between 15% and 25%, through which the porosity and abradable behavior can be adjusted. The powder used for the spraying process is usually made of graphite particles galvanically coated with nickel. If these particles are primarily round, they are referred to as globular powder and often confusingly as globular coatings, although the spraying process creates a typical lamellar spray-coating structure (Fig. "Operation behaviour of rub-system"). The abradable behavior is not as good as that of Al- or Al/Polyester coatings. The material removal at the blade tips and the heat creation are relatively high if the coating is sprayed densely to ensure sufficient erosion resistance. The maximum temperature for continual operation is about 480 °C. However, even at this temperature a noticeable altering of the coating due to oxidation is to be expected after only a few hundred hours of run time, which clearly worsens abradable- and erosion behavior. The material removed from these coatings can block up cooled hot parts.
Long-term durability can be improved by sintering the globular Ni/Cg powder into a coating with a mostly closed cell structure. In this case, it can safely referred to as a globular coating structure. However, these coatings are difficult to produce and have not found serial use.

NiCrAl coatings with reservoirs:
Flame-sprayed NiCrFeAl/Bn coatings are typical of this type, which are used in many parts of the turbine region. Their maximum operating temperature is about 800 °C. The metallic matrix contains roughly 15% hexagonal boron nitride, which gives it gliding properties comparable to graphite with higher oxidation resistance.

NiCrAl/Bentonite flame-sprayed coatings are an alternative where the bentonite determines the glide properties. These coatings are porous and have a thermal insulating effect. This can be used to control clearance changes due to thermal expansion. Because of their thermal insulating properties, these coatings have also been used to fill honeycombs in order to keep the fastening solder sufficiently cool.

Unfilled metallic spray coatings: These include coatings made of intermetallic phases (IP) such as NiAl. They are used at high operating temperatures (around 800°C, e.g. on turbine rings or turbine segments). With good erosion- and oxidation resistance, their abradability is poor.
Due to their thermal strength, hard coatings are used primarily on spacers in compressor rotors or on seal segments of shroudless high-pressure turbines.

Metal felts and webbing: These materials are used in coating thicknesses of up to several millimeters, and can be used as semi-finished products (the commonly used term for these coating types comes from the product name “Feltmetal®” ). The coating structure consists either of nickeled carbon fibers or fine shavings of heat resistant metal alloys. Application is done by use of adhesives or solder. This results in a special problem (Fig. "Production caused delamination of rubcoating"). If the coating is infiltrated to strongly by the adhesive media, the rubbing behavior can be unallowably worsened. If the amount of adhesive media is insufficient, the bond may be too weak and the coating may separate during later operation (Fig. "Delamination of rubcoating by poor bond"). Due to oxidation, the boundary for use of galvanically manufactured felts is at about 500 °C, for felts of heat-resistant alloys, it is at about 800°C. An Al-diffusion procedure can give these felts a higher oxidation resistance. The abradable behavior of metal felts is problematic. High heat development, material buildup, and wear of the blade tip have been observed.

Unfilled and filled honeycombs: Seals made of thin-walled honeycomb structures are some of the oldest seal systems in use in engines. They are used on hot parts such as turbine rings and turbine segments in combination with blades with and without shrouds. The thin sheets are made usually made of the Ni-base alloy “Hastelloy X”. Modern civilian engines have overhaul intervals of several 10,000 hours. Over this long operating time, there is a danger of oxidation and hot gas corrosion damage to the seal carriers. This results in large surface outbreaks of the oxidized cell structure. It is recommended that the blank new part surfaces are pre-oxidized in order to gradually improve their oxidation resistance. A considerably improvement can be achieved by giving the honeycomb structure an Al-diffusion treatment. If the honeycombs are filled with a spray coating or sintered material (e.g. Ni or NiAl), then oxidation resistance is improved, as well as creating a thermal insulation effect. This type of honeycomb has a very limited abradability, which causes heavy wear and thermal damage to the blade tips. This behavior is even worse with ceramic fillers. The maximum long-term operating temperature of honeycomb seals is 1000 °C .

Ceramic coatings:
This usually involves thermal spray coatings on a zirconium oxide (ZrO2) or aluminum oxide (Al2O3) base. These coatings are used in compressors on rotor spacers and in turbines on housing-side turbine rings/turbine ring segments (Fig. "Ceramic rub coatings").

Coatings with cutting particles: Trials have shown (Fig. "Rub coating and blade stresses", Ref. 7.1.1-1) that the installation of hard particles with cutting capacity, such as SiC that has been galvanically bonded into the coating, considerably decreases the heat development and bending stress of blade leaves. The “mounting” of the grains is the deciding factor for the cutting effect (Fig. "Optimizing blade tip coatings").

Figure "Ceramic rub coatings": Even though material removal from guide vane tips is undesirable during overhaul because of the high repair costs, it cannot be avoided in the case of spacer rings in compressor rotors (top diagram). The operating loads on the coating (centrifugal force, thermal expansion, centrifugally-induced elastic expansions) put high amounts of stress on the strength and bond strength of coatings. For this reason, the coating materials are usually selected on basis of a good grinding effect (Fig. "Rub coating and blade stresses"). Criteria for good rub-tolerant behavior and therefore the suitability of these coatings are:

  • low blade tip temperature
  • low blade bending stress
  • low dynamic loads on the blade
  • low heating-up of the coating and base material
  • low tendency to deposit on the opposing surface, i.e. good “self-cleaning”
  • secure bonding of the coating over the entire time of operation under LCF loads typical for rotors
  • thermal stability
  • optimized roughness with regard to grinding behavior and influence on the flow

Another important area where ceramic rub-coatings are used is the high-pressure turbine (bottom diagram). The use of these coatings is necessary due to the high operating temperatures (about 1000°C). Even the thermal insulation effect of these coatings is essential for protecting the supporting metallic structure of the seal ring and to control thermal expansion with as little cooling air as possible. Coating the shroudless blade tips with hard particles such as SiC turns a rub-tolerant system into an abradable system (Fig. "Optimizing blade tip coatings").

Figure "Chipping at abradable coatings" (Ref. 7.1.1-4): In order to develop and test suitable rubbing behavior, realistic rubbing trials in suitable testing rigs are essential (Fig. "Properties and tests of rub systems"). The rub-tolerant behavior of porous coatings is fundamentally different from dense, thermal-sprayed abradable coatings (Ref. 7.1.1-6).
Porous coatings (15-45 Vol% porosity) with low density release individual particles that break out of the coating during the rubbing process (left diagram). The coating seems macroscopically brittle. However, the outbreaks only occur in intentionally weakly bonded zones. In the micro-region, these separations can be very tough.
“Dense” coatings (2-15 Vol % porosity) exhibit a more complex chip removal process that is primarily influenced by three effects:

  • plastic deformation of the coating
  • yielding or compression of the coating
  • micro chip removal by the blade tip

Therefore, the rubbing behavior of thermal spray coatings is determined by the porosity to a large degree. The micro-hardness of the coating is an indirect and limited indicator. The hardness is usually measured with a taper and low force (Rockwell R15Y). However, the coating hardness is usually between R15Y 30 and R15Y 80. Coatings with low hardness are too erosion sensitive, hard coatings wear down the blades too much (Figs. "Factors for tip coating operation behaviour" and "Rubbing behaviour of elastomer coatings").

Figure "Factors for tip coating operation behaviour" (Ref. 7.1.1-3): In a comprehensive program, the rubbing behavior of various metallic thermal spray coatings was tested against rotor blade tips made from the titanium alloy TiAl6V4. It became clear that small changes in the spray parameters and the grain size dispersion of the spray powders have a deciding effect on the suitability of rubbing systems. This underlines the importance of stable manufacturing processes. It was also observed that small changes of the rubbing conditions (e.g. infeed speed and rubbing speed) noticeably change the behavior of the tribo-system. This underlines the importance of sufficiently realistic suitability tests.
It also became clear that, compared to pure copper coatings, Al-bronzes not only reduce blade tip wear, but also reduce the roughening of the coating surface. It remains to be determined to what degree the compatibility of the material removed from these coatings with hot part surfaces can be ensured. It is not predictable from the material data whether material will be deposited on the blade tips or the coating will be worn down. Reactions between the blade material and coating material play an important role. The testing temperature also clearly has an effect, depending on the material (top left diagram).
Dense coatings (low porosity) evidently tend to increased material deposits on the blade tips and therefore increased rubbing. This behavior is undesirable and can cause uncontrollable rubbing. This increases the mechanical loads on the blade and thus the danger of static or dynamic overstress (Fig. "Compressor blade cracks by rubbing").

Conclusion: The porosity of a rub-coating is important for a positive rubbing behavior without material deposits.
The heating-up of the blade tips through the released friction energy leads to damaging of this part area, which is subject to high dynamic stress (Fig. "Blade tip rubbing damage symptoms", embrittlement through oxidation, loss of strength, thermal stress/internal stress). Increasing the test temperature usually, but not always, results in a deeper heating-up zone (bottom left diagram). This can be explained by the fact that some coatings have better rubbing behavior at higher testing temperatures.
If possible, the material removed from the blade tips should be minimized. Large amounts of material removal cause larger clearance gap cross sections than coating run-in (see Figs. "Maintaining tip clearances" and "Leakage losses and rub wear behaviour") and also higher repair costs.
In order to minimize blade tip material removal, the tip can be armored (bottom right diagram). Uncoated blade tips show greater material removal opposite both metal felt coatings as well as flame-sprayed coatings. Armoring the tip with tungsten carbide particles embedded in a cobalt matrix reduces the tip material removal considerably under the given parameters, at least opposite flame-sprayed coatings. Mixing CrMo into the armoring results in a significant improvement over both coating materials. This example shows how small changes in the tribo-system can have a large effect.
Today, compressor blades and shroudless turbine rotor blades are also successfully armored with hard particles.

References

7.1.1-1 Stetson A.R., Vogan J.W., Compton W.A. (Solar Turbine Int.) “Abrasive Coatings as Self Cleaning Gas Turbine Compressor Vane Tip Seals” AGARD-CP-237 Conference Proceedings 1973 pages 3-1 to 3-14.

7.1.1-2 Patent application EP 0254 667 B1 of the 4.7.87, United Technologies Corporation, “Improved Method for Adhesion of Grit to Blade Tips”.

7.1.1-3 Schwab R.C., (G.E. Company) “Program to Develop Sprayed, Plastically Deformable Compressor Shroud Materials”, NASA-Cr-159741, Progress Report 1976 - 1979, page 1 - 65.

7.1.1-4 T.J.Uihlein, “Airseals for Advanced Military Jet Engines”, AGARD Conference Proceedings 589, AGARD Structures and Materials Panel, Sesimbra, Portugal, 6-7 May 1996, pages 17-1 to 17-19.

7.1.1-5 L.T. Shiembob, O.L. Stewart, R.C. Bill, “Developing of Sprayed Ceramic Seal System for Turbine Gas Path Sealing”, ASME Publication Paper 78-WA/GT-7, Transactions of ASME, Jourrnal of Engineering for Power 1979, page 1-7.

7.1.1-6 E.R. Novinski, “The Design of Thermal Sprayed Abradable Seal Coatings for Gas Turbine Engines”, Proceedings of the Fourth National Thermospray Conference, Pittsburgh, PA, 4-10 May 1991, page 451-456.

7.1.1-7 F.E. Kennedy, “Thermomechanical Phenomena in High Speed Rubbing”, Paper presented at the Workshop on Thermal Deformation, Annapolis, Maryland, June 1979.

7.1.1-8 R.C. Bill, L.T. Shiembob, “Some Considerations of the Performance of Two Honeycomb Gas-Path Seal Material Systems”, periodical: “Lubrication Engineering”, April 1981, page 209-216.

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