Table of Contents
16.2.1.8.2 Thermal Spray Coatings
Thermal spray coatings are applied through a process in which the coating material, in the form of a powder or wire, is brought into contact with an intense heat source (e.g. plasma torch), melted, and carried onto the work surface by a continual or pulsating gas flow (Refs. 16.2.1.8-1 and 16.2.1.8-2). The coating process can occur in regular atmosphere, in special gases (e.g. cover gas), or at low pressure. These processes can be used to create metallic (e.g. Al, Mo, Ni, MCrAlY) and/or nonmetallic (e.g. A12O3, WC) coatings. If heated (usually melting) particles strike the part surface at high speeds, it usually creates porous coatings. Their adhesive strength is based on mechanical interlocking and/or diffusion processes (especially during subsequent heat treatments, Fig. "Influences of thermal spray coating structures"). Depending on the type of heat source, the type of additive material, and the acceleration process, thermal spraying methods include flame spraying (powder, wire, or rod), plasma spraying (powder, wire, rod), detonation spraying (powder spraying, used by Union Carbide Corporation), and arc spraying (wire spraying).
The various spraying processes have a major influence on the base material and the sprayed coating. Depending on the process, the base material is heated in different ways, which can change its strength properties. This heating is primarily the result of the application of the heated spray material and, in some cases, preheating.
Differences in the thermal strain of the spray coating and the base material can result in considerable residual stresses, depending on the temperature levels and their progression during spraying. The levels of the residual stresses in the coating also depend on the strength and modulus of elasticity of the spray coating. They affect the cyclical strength and adhesive strength of the spray coating. If brittle spray coatings such as thermal barrier coatings experience high cyclical strain during operation, one can attempt to use multi-layer and/or gradiated coatings to create a soft transition and to improve the stress patterns (Ref. 16.2.1.8-13). The buildup (e.g. porosity, structure) and composition (e.g. oxide content or proportion of various phases such as nickel and graphite) of spray coatings depend largely on the spraying method. Fatigue cracks in the spray coating can spread into the base material if they are aided by good adhesive strength. However, it has also been observed that fatigue cracks form in the base material below the spray coating, but do not break through the coating to the surface. This behavior can be explained by a very low modulus of elasticity in the coating (especially likely in porous coatings) relative to the base material. This makes the non-destructive testing of parts with spray coatings especially problematic.
The principle of metal spraying means that tensile stresses will be induced in the spray coating during the cooling process, since it is hotter than the base material. These tensile stresses lead to the deformation of thin parts (warping), or cause a worsening of the adhesive strength. Tensile stresses can worsen the dynamic fatigue strength of the adhesive zone and cause the coating to delaminate prematurely during operation. Excessively high compressive stresses are also a problem. On convex curved surfaces such as blade edges, these stresses promote separation of thermal barrier coatings (Volume 3, Ills. 12.4-13, Ill. 16.2.2.8-4.3).
If the base material is a Ti alloy, there is a special danger of gas absorption during spraying, which results in pronounced embrittlement. Gases that have an embrittling effect on titanium in this context include primarily oxygen (oxide formation, stabilization of certain structural components, a proportion is increased) and hydrogen. Oxygen absorption cannot be reversed through aftertreatment. This is also true of hydrogen embrittlement if titanium hydrides have already formed.
Figure "Applications of thermal spray coatings in engines" Thermal spray coatings are used in many different engine areas for various purposes:
- Wear protection
- Thermal barriers
- Oxidation protection
- Protection against hot gas corrosion (HGC)
- Abradable coatings
- Armoring
The characteristics of the coatings are selected to match the specific applications. Characteristics include:
- Wear behavior/abradability
- Resistance to fretting wear
- Erosion resistance
- Thermal insulation
- Cutting effect
- Corrosion protection
- Resistance to metal fires/metal melts
- Vibration damping
One can easily recognize that modern engines cannot be realized without thermal spray coatings. This is especially true for requirements for minimal deterioration (increase in fuel consumption over engine running time). Deterioration is decisively dependent on leakages at moving seals (blade tips, labyrinths; Volume 2, Chapter 7.1.1).
These demands can be met with the aid of suitable spraying powders and optimized, tested, and specified process parameters, which are often part-specific. Even minor deviations from the prescribed process parameters can have an unallowable effect on the coating properties (Ills. 16.2.1.8.2-2 and 16.2.1.8.2-3).
Figure "Influences of thermal spray coating structures": Knowledge of typical characteristics and properties of thermal spray coatings should not be limited to specialists.
There are a wide variety of thermal spraying processes (e.g. plasma and flame spraying in air or in cover gas, at atmospheric pressure or at low pressure). The typical thickness of spray coatings is in the range of several tenths of a millimeter. In accordance with the principles of thermal spraying, the structure of these coatings (Ref. 16.2.1.8-1) is lamellar (“A”), and is created by the soft of fluid particles striking the surface (Fig. "Bond strength of thermal spray coating structures"). Processes that occur in air generate oxides (“B”) and particles with oxide skins. Frequently, a cross-section or fracture surface will reveal both oxidized particles that formed in the spray jet upon contact with oxygen (“C”), as well as particles that were not sufficiently melted (“D”).
Noticeable oxidation of the spraying surface due to exposure to oxygen at high processing temperatures worsens the adhesive strength of the coating. Abrasive blasting as a method of surface preparation leaves behind mineral blasting particles (“E”, loading effect, e.g. Al2O3 or SiO2).
Large numbers of sprayed particles ricocheting (“F”) off the sprayed surface or spalled coatings are indicative of poor adhesive strength. A comparable phenomenon is a poorly adhering strip of tape on a dusty surface (Volume 2, Ills. 7.2.2-34 and 16.2.1.8.2-4).
Pores (“G”) in spray coatings are typical and also necessary to attain certain qualities. These include abradability, resistance to thermal cycles, and heat insulation. Therefore, evaluation must be in line with the relevant specifications, which are based on operation-relevant tests and experiences. Excessive porosity in the bonding layer will have a negative effect on the adhesive strength. Unmelted inclusions (“H”) with high melting points are also recognizable in coating structures, and the presence of a large number of these will indicate unallowable contamination of the spray powder.
Figure "Bond strength of thermal spray coating structures": The adhesive strength and its behavior over the operating period are of decisive importance for the life span of the part. The greater the adhesive strength, the less likely coating separation will be. On the other hand, high adhesive strength also promotes the spreading of coating cracks into the base material (Fig. "Fatigue cracks in parts with thermal spray coating"). Adhesive strength is determined by a
combination of different influences. They can be categorized as relating to process parameters, material properties, and design. These are important factors (Ills. 16.2.2.8-4.1, 16.2.2.8-4.2, 16.2.2.8-4.3):
- Positive fitting as a mechanical interlock
- Residual stresses in the coating
- Material bonding with the base material
The complex relationships between all these influences make exact predetermination impossible. The optimal process conditions for the required operating behavior must be determined with the aid of empirical tests and documented in sufficient detail. The finishing process must adhere to these prescriptions exactly. This is especially important because there is no satisfactory, serially implementable procedure for non-destructive testing of the adhesive strength of thermal spray coatings on parts (Fig. "Methods of qualitative bond strength testing"). Therefore, the only option is to use specified technological tests on statistically selected parts to conduct comparative verification tests. On the other hand, quantitative analysis of the adhesive strength on specimens is entirely feasible (Fig. "Measurement of coating bond strength").
Positive fitting is primarily dependent on the pretreatment of the surface to be sprayed:
In order to ensure good adhesion, spray surfaces undergo suitable treatments. Typical processes include turning, milling, electrical roughing with nickel electrodes, blasting with aluminum oxide abrasive or wire shot. There are limits to attainable roughness if the notch effect influences the dynamic fatigue strength of the part (Ills. 16.2.1.8.2-5 and 16.2.2.1-8; Ref. 16.2.1.8-10).
In order to ensure good adhesive strength even under extreme operating conditions (long times, high temperatures, cyclical loads), adhesive layers are used between the base material and spray coating.
Residual stresses in the coating: When the hot sprayed coating cools, thermal shrinkage creates tensile stresses in the coating. These stresses primarily act parallel to the spray surface and must be absorbed by the base material. The resulting shearing stress can weaken the adhesion and cause separation during later operation. Depending on the temperature of the part, cooling of the system after the coating process will change these residual stress conditions. In the cooled part, the residual stresses are influenced by the temperature differences and also by the different thermal strain behaviors of the coating and base material. In extreme cases in which a coating has very minimal thermal expansion relative to the substrate, high compressive stresses will be created in the coating after cooling. This effect is especially pronounced in ceramic coatings (thermal barriers) on metal substrates. Coatings with high compressive stresses on convex surfaces are especially prone to separation and spalling (Ill. 16.2.2.8-4.3).
Material bonding occurs through the fusion and diffusion of metallic sprayed particles during contact upon striking the surface. This requires sufficiently high surface temperatures during spraying and/or during subsequent heat treatments. This cohesive bonding of the coating and substrate is especially likely between metallic coatings and metallic substrates. It results in very high adhesive strength. This type of bond occurs locally in micro-zones in which there is fusion and/or diffusion of a droplet with the substrate. Diffusion can also occur during subsequent heat treatments. The higher the spraying temperature and the temperature of the sprayed surface, and the greater the reactivity (i.e. wettable) of the latter (bright metal), the more likely it is that material bonding will occur.
Figure "Properties of thermal spray coatings by production": (also see Ills. 16.2.2.8-5 and 16.2.2.8-6; and Volume 2, Ill. 7.1.3-10.1). There are finishing-related influences that are not known or recognized, but that can have a decisive negative effect on the properties of thermal spray coatings:
Dust deposits on the part surface being coated: This problem occurs during the spraying process, and microscopic examination (SEM) reveals typical results. On separated surfaces, both on the coating and substrate, one can recognize collections of small, usually spherical particles. These are usually particles from the spraying process that ricocheted off the surface and were able to settle on the surface nearby due to insufficient shielding. The spraying process then passed over the surface on which they settled. In case of multiple coating passes, these ricocheted particles can be lamellarly arranged in the coating, where they can compromise its internal strength (microsphere effect). This effect can be compared with the widely known poor adhesion of adhesive tape on a dusty surface (bottom left diagram). This worsening of the adhesive strength is observed especially in porous abradable coatings (e.g. Ni/graphite) and ceramic thermal barriers.
If damp spray powder is used, it can result in considerably worsened thermal fatigue behavior in ceramic thermal barriers (zirconium oxide; top right diagram, Ref. 16.2.1.8-9). This effect is evidently attributed to the poorer flowability and/or mixability of the powder into the plasma jet. This results in uneven particle distribution and stuttering delivery of the particles in the jet (bottom right diagram).
Figure "Fatigue cracks in parts with thermal spray coating": The finishing process, as well as the thermal spray coating itself, can decisively affect the dynamic fatigue strength of a part. The effects described below were generally confirmed by tests on specimens consisting of 12% Cr steel with tungsten carbide coatings (WC-17%Co), chromium carbide (Cr2C3-20% NiCr) or Inconel 625 (Ref. 16.2.1.8-15). The importance of this influence depends largely on the type of vibrational load. The high stress gradients during flexure significantly lower the stresses to the base material (middle diagram). The adhesive surface is already being strained slightly less than the coating. If the cross-section is subjected to even tensile stress, the coating, adhesive layer, and substrate will experience the same strain. The likely cracks due to dynamic fatigue will also be determined by the specific conditions (details). However, it must be noted that, in addition to the elasticity modulus and load type, there are a large number of other factors that affect location of the crack origin and crack growth (tables). Therefore, this depiction is merely intended to indicate the problems involved.
Residual stresses are due to thermal strain differences (temperature gradients) in the coating itself, as well as between the coating and base material. In general, tensile stresses act as a mean stress increase and reduce the dynamic fatigue strength, while compressive stresses have the opposite effect. The residual stresses in the coating and base material are created first in the coating when the droplets solidify after striking the surface. This process is complex, since every droplet has a heating effect, and perhaps even a forging effect, that changes the residual stresses in the layers beneath. Depending on the temperature of the base material, the residual stresses will change during cooling following the coating process. They may change again during subsequent heat treatment. Usually, the stresses are broken down by creep/relaxation (Fig. "Annealing time effect on residual stress level").
If one observes/tests the behavior of coatings at operating temperatures, it can be seen that in coatings with higher thermal strain coefficients than the base material, compressive stresses will be induced that give these coatings a considerably greater dynamic fatigue strength than would be expected in coatings with smaller thermal strain coefficients. This will also be affected by the coating strength and stress patterns. If the strength of the coating is comparable to that of the base material, and the coating has a high elasticity module, it is possible that powerful tensile stresses will be transferred to the substrate. If the stress gradient is flat, i.e. there are strong vibrational stresses on the adhesive layer of the base material, dangerous cracking may occur.
The lower the modulus of elasticity of the coating relative to the base material (mismatch), the lower the stresses will be in the coating at the same strain levels. In this contact area of the coating, the maximum stress levels in the specimen are exceeded, and the location of this stress maximum is changed (middle diagram). This is easy to observe in the example of a rubber coating on a steel specimen. The level of residual stresses during strain-controlled processes, such as restricted thermal strain (thermal fatigue), also behaves in accordance with the modulus of elasticity.
Adhesive strength: The greater the adhesive strength of a coating, the easier it is for cracks to spread from the coating into the substrate (top details, Fig. "Damage potential of a coating") and reduce its dynamic fatigue strength. However, if the coating delaminates, this separation will stop the crack from spreading.
Coating strength: The greater the tensile strength of the coating, the greater the proportion of the loads it can bear in case of external stressing. The base material can be more highly stressed by the residual stresses in the coating. Therefore, depending on the thermal strain behavior, specific operating temperatures may cause the dynamic fatigue strength to be increased or reduced.
Strength changes in the base material: Hardness/strength can decrease significantly as a result of warming during the spraying process or in areas of diffusion with the coating. This is especially true of heat-treated steels. The pre-heating temperature of the part and the spraying temperature must therefore be properly matched. If necessary, the process parameters should be verified using part-relevant specimens.
Topography of the adhesive surface: In specimen tests, dynamic fatigue cracks were observed in the base material beneath spray coatings. The cracks originated in notches that were created by the impact of sharp-edged blasting shot (top details, Ref. 16.2.1.8-22). This is understandable if the stiffness (modulus of elasticity) of the coating is considerably lower than that of the base material. A typical example of this is porous abradable coatings made from Ni-graphite.
Figure "Finishing problems by powder segregation": Depending on their flowability, vibrations can cause powders to segregate (also see Fig. "Safety of parts by manufacturing"). Segregation does not necessarily require powerful vibrations such as those that occur during transport. It can also occur due to the seemingly harmless vibrations of the floor where the powder container is stored.
Segregation of the powder can be influenced by grain geometries, sizes, or specific weights. Even minor alloy differences such as increased aluminum content can promote segregation. Powder mixtures are especially prone to segregation.
If powder is taken directly from the top of the container without prior mixing, grain sizes, grain structures, and composition can deviate from the tested process data, which will have unexpected effects on coating properties such as porosity, strength/hardness, abradable behavior, and thermal fatigue.
Note:
Powder must be thoroughly mixed before it is removed from a container and used in a thermal spray process.
Figure "Finishing related flaws of thermal spray coatings": If a thermal spray coating process is less than optimal, it can result in various coating problems. This is especially problematic because, due to factors such as porosity and coating characteristics (e.g. non-magnetic, non-metallic), these coating flaws can often not be reliably detected on the part using non-destructive methods (Fig. "Testing bond strength of coatings" and Chapter 16.2.2.8). In these cases, the only option is constant documented monitoring of the relevant process parameters.
The top diagram shows typical flaws and weak points in thermal spray coatings. These flaws occur in various combinations, depending on coating process characteristics, part geometry, and coating properties. For more information regarding the causes of these flaws see Ills. 16.2.1.8.2-3 and 16.2.1.8-4. The bottom frame shows two typical problems related to the geometry of the part. If the spray jet is blocked by edges of the part or equipment, it will create “shaded” areas in which the coating thickness and/or quality (e.g. increased porosity, poor adhesion) is insufficient (left diagram). This type of problem indicates insufficient testing of the coating process.
If one attempts to spray into a hollow space or concave contour, it will understandably result in interactions of the spray jet with ricocheting particles and turbulence in the carrying gas. This results in typical spraying flaws (top right detail) that can be identified by the unique orientation of their porosity. A typical example is an attempt to fill a soldered honeycomb structure (turbine segment, bottom left) with a spray coating (bottom right detail).
Figure "Influence of finishing at coating properties" (also see Fig. "Aftertreatment effects on coatings"): Thermal spray coatings can be specifically altered through targeted reworking. However, they can also be damaged by subsequent finishing steps.
Common targeted aftertreatments for spray coatings:
Machining: Depending on the coating properties, grinding or turning is used to ensure dimensional accuracy. This is necessary, for example, to minimize the clearance gaps between sealing surfaces such as abradable coatings. Machining is also used to improve surface quality (reduced roughness). Machining can open pores, but may also close them. In the case of soft abradable coatings, this influences their hardness and rubbing behavior. Densified coatings have higher hardness/strength, which increases undesirable blade tip wear. In addition, they will cause greater heat development in the blade tip during rubbing, and excited dangerous vibrations in the blades (Volume 2, Ill. 7.1.3-4).
In brittle coatings such as ceramic thermal barriers, micro-cracks (not segmentation cracks!) will be created in the surface area, reducing the erosion resistance of the coating. The erosion of ceramic rubbing surfaces on seal segments in the high-pressure turbine is especially likely to increase the leakage losses in an engine considerably (Volume 2, Ill. 7.1.3-11).
If dust created by the machining process settles on the coating surface, it can enter into the cooling air or oil flow during operation. This increases the risk of dangerous consequential damages such as race fatigue in roller bearings or the overheating of hot parts due to blockages. If machining creates deformations and residual stresses in the coating, it can worsen the fatigue behavior of the coating and/or adhesive layer, reducing the life span of the coating.
Targeted heat treatments are used on thermal spray coatings to break down undesirable residual stresses and to improve ductility. An improvement in the adhesive strength can also be attained through residual stress reduction, diffusion annealing, and possibly melting. Heat treatments that are not specific to the coating and are not sufficiently matched to the coating characteristics can have negative effects. For example, porous coatings can oxidize across their entire volume if they are heated excessively in air. This may only affect certain coating components, such as the graphite in porous Ni-graphite abradable coatings. In this way, the coating hardness and/or tribological characteristics may be unallowably altered (for example, the oxidation of graphite may reduce the required lubricating effect).
With the aid of optimized temperature cycles, desired crack patterns (segmentation cracks) can be created in thermal barrier coatings.
Etching and cleaning: If these baths are absorbed by porous coatings and not subsequently removed, when the coating reaches operating temperatures, it may result in concentration of the media and corrosive attack on the coating and/or base material. This can in turn worsen the adhesive strength of the coating and cause it to separate. Even dried bath residue is capable of combining with condensation water to form corrosive electrolytes.
Penetrant testing: The more porous the coating, the less suitable it is for penetrant testing. It is possible that penetrant residue may remain in open porosity and hinder subsequent penetrant tests.
Abrasive blasting: Especially in brittle and/or soft coatings, damage due to material removal or micro-cracking is possible (e.g. in ceramic coatings). Micro-cracking will especially affect the erosion resistance of the coating. Lateral coating transitions, which necessarily have especially porous structures (Fig. "Finishing related flaws of thermal spray coatings"), may be “washed out”.