Table of Contents
184.108.40.206 Bond Strength Problems in Coatings and Connections
Parts are taking over more and more functions. For example, turbine blades must have sufficient life spans (creep, thermal fatigue, oxidation), a minimal tip clearance gap (sealing effect) and good rubbing behavior at extreme gas temperatures, as well as very high LCF strength in the roots. In order to meet all of these demands, an ever greater number of coatings are being used. This makes the connections of the coatings to the surfaces extremely important.
Two concepts characterize this bond (Ref. 220.127.116.11-5). Bond quality refers to the quality of the connection between the base material and the coating. Bond strength is a measured value in the dimension of stress/strength. It is determined with the aid of tensile tests on specimens (Fig. "Measurement of coating bond strength"). However, the loads on connections in operation are often not tensile, but thrust/shear and peeling. This means that the strength values determined in tensile tests have limited applicability to the part behavior during operation. It is more of a quality characteristic than a characteristic design value.
In engines, coatings are the primary systems for which bond strength has a decisive influence on the risk of damage during operation. Coatings are usually thermal spray coatings, galvanic coatings, and lacquers (Fig. "Insufficient bond strength of coatings"). Insufficient bond strength can cause coatings to fail, leading to unallowable changes in the operating behavior of the directly affected part (primary damage). However, consequential damages are often the most dangerous. For example, particles breaking off of a protective labyrinth coating can damage bearings.
In general, forces are transmitted at the bond layer through adhesion (mechanical interlocking) and cohesion (material strength) (Fig. "Interactions of bond mechanisms"). There are other effects such as residual stresses and gas bubbles from the diffusion of hydrogen. The influencing of the bond strength by a combination of these properties and effects is not completely understood even today (Ref. 18.104.22.168-2).
In finishing, there are a large variety of factors that affect bond strength (Ills. 22.214.171.124-3.1, 126.96.36.199-3.1-3.2, 188.8.131.52-4.1 and 184.108.40.206-4.2). These include parameters of the bonding process and pretreatment, as well as effects related to handling and any cleaning processes that occur after the bond is created.
Figure "Insufficient bond strength of coatings": Engines contain a large number of coatings, the bond strength of which makes the required operating behavior possible. Failure of a bond between a coating and base material (substrate) presents a danger of failure of the part with extensive consequential damages to other parts. The following text deals with the influence of bond quality and bond strength on the behavior of typical engine parts, as well as possible effects:
“A” Thermal barrier coatings: The determining value is the bond strength. This is influenced by high thermal stresses between the metallic substrate and the ceramic coating.
If the bond strength of the coatings and connections is included in the strength design of the part, failure will have especially serious consequences (Volume 3, Ills. 12.1-5 and 14-24).
If the bond strength of a thermal barrier on a turbine blade is insufficient, after the coating separates there will be a dangerously increased heat flow from the hot gas into the base material. If the coating was incorporated into the design of the part, it will result in rapid, practically unpreventable destruction of the part.
In older engine types, abradable coatings are often made from thick layers of soft metal. Examples include aluminum in compressor housing and silver in labyrinths, either thermal-sprayed or soldered. In modern engines, porous (soft) thermal spray coatings (Ni graphite, etc.) and glued or soldered porous metal felts are the preferred systems.
“B”: There are wetting problems with the typical soft metallic soldered coatings made from silver or copper alloys. Thermal stresses between the substrate (usually steel) and the coating promote separation at these weak points. This causes the coating to lift up, and this process is accelerated by heat development during rubbing. This can result in serious damages in the rotor area (Fig. "Separation of brazed coatings").
“C”: Abradable layers of metal felts are primarily used as coatings in labyrinth seals. Due to their absorptive capacity, they present a special challenge with regard to creating a bond with the base material (Fig. "Brazing porous coatings"). They are applied using adhesive or solder. Secure bonding can only be expected from sufficiently tested and monitored procedures. If these conditions are not met, there is a risk that large sections or even the entire coating ring could separate, causing catastrophic failure of the labyrinth and extensive consequential damages (Fig. "Adhesive bond failure of a metal felt").
“D”: Soft, porous thermal spray coatings are primarily used in compressor housings. The risk of damage is especially high at the edges of unenclosed coatings because these areas tend to break out. If the bond strength is insufficient, high-frequency vibrations can cause large separations.
Coated inner and outer corners are predestined to be the origin points of delaminations. This is due to the problematic spraying conditions that result in poorer coating quality and bonding (Fig. "Finishing related flaws of thermal spray coatings").
“E”: Abradable coatings made from elastomers (usually filled silicon rubber) are used in the front compressor area as sealing surfaces opposite blade tips or interstage labyrinths. They are poured or glued directly into the metallic housing rings or inner shrouds of the stators. Especially the seemingly simple pouring process can lead to insufficient bond strength if there are minor deviations in the procedure (also see Fig. "Effects at adhesive joint properties"). Typical problems occur during pretreatment of the substrate surface and variations of the process parameters. Surrounding conditions such as humidity and temperature are especially important during this process. Flaws usually remain undetected until separation occurs during operation. If large sections of the coating break out, there is a danger that the blades could be overstressed.
“F”: Ceramic thermal barriers protect many hot parts such as housing segments, combustion chambers, and blades (see “A”). The specific problem with these is thermal stresses resulting from thermal strain differences between the coating and the metallic substrate. If the coating separates, the base material can be overheated, and other parts can also be damaged. This applies especially to turbine rotor blades that are struck by large chunks of separated coating. If these cause plastic deformation of the blade wall, it can disrupt the internal cooling air flow and result in local overheating. If the impact occurs in an area with a diffusion coating for oxidation protection, it can cause cracking that in turn causes other consequential damages (e.g. dynamic fatigue fractures).
“G”: Ceramic coatings such as Al oxide on rotating spacers with insufficient bond strength can tend to separate under cyclical loads after longer operating times. Separated coating fragments can cause dangerous damage to filigreed compressor blades. The impact notches can become the origin point of dynamic fatigue cracks and blade fractures.
“H”: Ceramic wear-resistant coatings are applied to labyrinth tips in order to improve rubbing behavior (less heat development and conduction). Spalling of these coatings, especially during rubbing, can promote hot cracking in the labyrinth tips (Volume 2, Ill. 7.2.2-9). If the labyrinth is located in an oil-carrying area, such as a bearing chamber, in some cases it is possible that hard coating particles could enter the bearings and cause damage to the races.
If the abrasively acting particles become trapped in the shaft system, they can run around for long periods and dangerously weaken or even separate cross-sections (Volume 1, Ill. 5.3.1-8).
Figure "Interactions of bond mechanisms": The bond strength of a coating or connection is based on combined action of different mechanisms. The following is an overview of some of the terminology:
Bond strength: This technical term should only be used when a measured value in the dimension of stress or strength can be determined (Ref. 220.127.116.11-5, Fig. "Measurement of coating bond strength").
If this is not possible, the literature recommends using the term bond quality (Fig. "Methods of qualitative bond strength testing"). This describes the quality of the connection between the coating and the base material.
Cohesion describes the forces on the basis of chemical bonding (non-metallic bonding). It is responsible for the inherent strength of a material. The forces act within the material. Adhesives use cohesion during hardening, when they “felt” long chains of molecules (Ref. 18.104.22.168-8).
Adhesion refers to the process of adhering to a substrate surface on the basis of chemical bonding. The bonding forces between molecules are balanced within a material. In contrast, there are open bonds at the edges, which make themselves known when another material comes sufficiently close. Because adhesion forces only act over very short distances, the task of adhesives and glues is to bridge the roughness-specific gaps. To accomplish this, they must have sufficient wetting properties.
Chemical bonds combine both cohesion and adhesion. They are also present in the case of oxides, which are used for ceramic thermal barriers, etc. This also applies to oxides that form around a metal droplet during its flight through the atmosphere and are built into spray coatings, or are created on the hot base material.
Metallic bonding occurs through the metal lattice. It occurs during metal coating, soldering, plating, and diffusion processes.
Interlocking of roughness tips (positive fitting) is purely mechanical. Its contribution to the bond strength is influenced by strain differences (residual stresses).
Stresses in the coating or substrate must be transferred by the connection between the two. The harder the coating, i.e. the greater the cohesion, the greater the stresses that can be transferred to the bond surface. This must absorb them through adhesion, metallic bonding, and mechanical interlocking (positive fitting).
The cohesion of soft, elastic transition layers such as organic adhesives can even out stress peaks between the coating and base material. The force-transmitting influences can act very differently in different coatings and bond types.
Thermal spray coatings (top detail): All bonding forms are present, albeit in various proportions, in the coating and bonding zone (see frame, Ref. 22.214.171.124-4). An optimal roughness (depth, form) improves the bond strength. Interlocking and adhesion are active in the large reactive substrate surface.
Galvanically deposited coatings (second detail from top): The bond strength is primarily determined by metallic bonding within the coating and to the base material (Ref. 126.96.36.199-5). This is the reason why these coatings adhere better to smooth surfaces, unlike thermal spray coatings. There are often high tensile residual stresses in galvanic coatings, and the good bond strength transfers these to the base material and reduces its dynamic fatigue strength. Heat-treated steels, especially, tend to absorb hydrogen during the coating process or pretreatment in an etching bath. After coating, preferably during disembrittling heat-treatments, the hydrogen can diffuse out. Hydrogen pressure can build up under thick coatings and exceed the bond strength of the coating. In thin, tough coatings (silver, copper, nickel) this can lead to delamination and blistering (also see Fig. "Shot peening as testing method").
Chemically deposited coatings: These include electroless nickel plating. In addition to the process parameters, the cleanliness of the surface being coated is decisive for the bond strength (Ref. 188.8.131.52-10).
Adhesive connections (second detail from bottom) act on the basis of chemical bonds. Cohesion acts in the adhesive layer, and adhesion acts on the connected surfaces. If the adhesive layer exceeds an optimal thickness, its bond strength will be reduced. This effect can also be seen in soldered joints (Fig. "Influences at the thrength of brazings") and is due to the supporting action of the harder and stiffer base material. An increase in roughness, with a correspondingly larger reactive bonding surface, should increase the bond strength. Adhesive connections are especially sensitive to peeling stress (Fig. "Design of adhesive bonds").
Soldered connections (bottom detail): The bond strength is determined by metallic bonds, in which diffusion plays an important part. In this case, as well, the bond strength of thin layers is better (Fig. "Influences at the thrength of brazings"). It increases with the base material strength of the soldered parts. Similar to adhesive connections, soldered connections are very sensitive to peeling loads (used in tin can lids; Fig. "Peeling of solders and brazings").
Illustrations 184.108.40.206-3.1 and 220.127.116.11-3.2: The finishing process affects the bond strength of organic coatings and connections in many very different ways. The chart is intended to provide an overview of this, with no claim to completeness. The influences can be categorized into 4 groups:
Bond surface: Because the bond strength of organic materials relative to metals is primarily based on adhesion, any factor that influences the reactive surface will play an important part. This includes roughness, topography, cleanliness, reaction coatings, and wetting. Controlled processing times must ensure that influences such as corrosion or oxidation do not negatively affect the results. In order to attain a solidly-bonding chemical state in the surface, it can be suitably pretreated before coating. The application of a primer is a typical method.
Temperature control must ensure that the viscosity is sufficiently low to fill the roughnesses and ensure strength during solidification.
Bending of the bonding surface can affect bond strength, depending on tensile or compressive residual stresses as a result of shrinkage or swelling in the coating. Unprotected edges at which peeling occurs are especially sensitive to this, especially since their bond strength can be reduced through contact with surrounding media (fuel, oil, cleaning agents, water).
Base material: As the carrier of the bond layer, the base material has an important influence on the attainable bond strength. The low modulus of elasticity of organic coatings results in much lower residual stresses than is the case with ceramic coatings. In contrast, the heat conductivity is a more significant issue. By importing or removing heat, it influences the temperature, viscosity, and solidification processes.
If the base material reacts with the coating or influences the chemical processes in the coating, it will affect the bond strength. If materials oxidize or corrode before coating, the bond strength will be reduced either immediately or over longer periods (e.g. undercorrosion).
Application: In the case of organic coatings, this process reacts extremely sensitively to the surrounding conditions (Fig. "Effects at adhesive joint properties"). Apparently minor changes in temperature and air humidity can have an unexpectedly pronounced effect on the bond strength. Process parameters such as time durations or the type of application can also be decisive for the final result. Environmental conditions affect both the substrate surface and the coating material before and during application. For example, excessive humidity can condense on the surface to be coated as a thin film of water. Water absorption can change the viscosity and hardening properties of synthetic resins and influence their bond strength.
The effects on the coating mass before application should not be underestimated. This begins with storage beyond the product life or in opened containers without sufficient protection against contact with the atmosphere. The mixing of remnants from partially filled containers is problematic because this process can be influenced by the atmosphere. Naturally, specified storage requirements such as temperatures (e.g. in a refrigerator) must be adhered to. Power outages that cause temporary temperature increases can have very negative effects.
The prescribed, usually intensive, mixing process before working with the coating mass must be strictly adhered to. It determines the coating and bond strength that is attainable through hardening. Systems with multiple components require exact adherence to the mixing conditions as a prerequisite for optimal bond strength.
Coating in its finished state: Physical and chemical properties determine the bond strength of the coating to a considerable degree (Fig. "Finishing proess affecting bondstrength"). Transmittable forces and stresses, and therefore the loads on the bond zone, are dependent on the coating strength, elasticity, and fracture strain. The greater the thermal strain of the coating relative to the substrate, and the stiffer the coating, the greater the possible residual stresses. Brittle coatings or those that have been embrittled by media and/or temperatures tend to cracking and separation.
Shrinking or swelling of coatings induces residual stresses. Media that cause this include unsuitable cleaning and degreasing baths or preservative oil. At the same time, the bond strength of the coating is reduced to the point of separation.
The part contour that the coating must follow also affects the bond strength (Fig. "Coating influencing bond strength"). The way this works depends on the residual stresses and the coating properties discussed above. The sharper the curvatures or outer corner, the more likely that compressive residual stresses will cause separation (Ill. 18.104.22.168-4.3). On the other hand, the bond strength on inner corners is weakened by tensile residual stresses. Edges also influence the coating thickness of lacquers. Unless special measures are taken, lacquer coatings will be thinner at the edges.
For the sake of completeness, it must be mentioned that brittle coatings such as ceramic sprayed coatings (thermal barriers) behave similarly (Fig. "Finishing influencing thermal spray coating"). They tend to separate at outer edges under compressive stresses. For technological reasons, at inner corners the bond strength will be reduced and coating flaws more likely (Fig. "Finishing related flaws of thermal spray coatings").
In contrast to lacquers, galvanic coatings build up thickly at outer edges due to the electric field (Fig. "Typical flaws on galvanic coatinga"). In this case, inner areas will often have a significantly thinner coating, and extreme cases will not be coated at all.
Illustrations 22.214.171.124-4.1, 126.96.36.199-4.2 and 188.8.131.52-4.3): The bond strength of thermal spray coatings reacts highly sensitively to changes in the coating process. The very different physical properties of strength and fracture toughness react in a coating-specific way.
Bond surface of the base material or bond coat: In order to attain special characteristics in the bond surface, such as increased roughness or oxidation resistance, a bond coat is applied to the base material. The bond coat is usually also a thermal spray coating. However, it is also possible to use other coating techniques such as sintering an inorganic slurry.
Roughness and topography are important characteristics in a bonding surface. The tensile bond strength usually increases from a minimum roughness almost linearly with a mean roughness “Ra” (Ref. 184.108.40.206-2). However, topography has proven to be a better indicator. The effect is evidently not only due to mechanic interlocking, but also due to the effective reactive bond surface area. Deposits such as “loading” with embedded blasting media can have a major influence on the bond strength. Spray dust is a very common and dangerous influence on the bond strength of thermal
spray coatings. This is referred to as the “microsphere effect” due to the round particles (Fig. "Properties of thermal spray coatings by production").
Reactive surfaces can be expected to result in close chemical and metallic bonds, and therefore good bond strength. However, reactivity can also have a negative effect if disruptive oxide coatings form. For this reason, the coating process may have to take place in a suitable atmosphere (cover gas, low pressure).
Temperature control especially influences the bond strength of brittle coatings such as ceramic thermal barriers. Active factors include thermal stresses and residual stresses.
The influence of the geometry of a coated surface (Ill. 220.127.116.11-4.3) should not be underestimated. It is important in several ways: especially in brittle coatings, compressive stresses on convex radii promote separation. Tensile stresses tend to cause cracking in the coating. In concave radii and inner corners, the coating is pressed down by compressive stresses. However, due to turbulence in the spray jet and ricocheting particles, the coating quality and bond strength in these areas will usually be inferior (Fig. "Problems of thermal spray coatings").
Base material (with no bond coat): In thermal spray coatings with a high modulus of elasticity, thermal strain especially affects the bond strength. The thermal strain influences thermal stresses and residual stresses, and therefore determines the optimal temperature progression during the coating process. Reactivity, including oxidation and corrosion behavior, must be considered during pretreatment of the surface to be coated. This means that any intermediate storage can play a significant role.
Application: The principle-specific relatively high processing temperatures with thermal gradients, heat stress, and oxidation make it difficult to ensure good bond strength. For this reason, temperature control in the coating and part is an especially important factor. It is closely related to process-specific spraying parameters such as prewarming and number of passes. The atmosphere of the spraying process must take into consideration properties of the spray powder and base material. It may be necessary to use cover gas or a partial vacuum (e.g. low-pressure plasma spraying). This can prevent unallowable oxidation of the spray particles and bond coating.
Coating (in its finished state): Physical coating properties such as stiffness, strength, fracture strain, thermal strain, and heat conductivity determine the thermal stresses. These form during the coating process and induce residual stresses afterwards. If tensile stresses exceed the strength of the coating, it will result in desired segmentation cracks that protect thermal barrier coatings from separating. In contrast, compressive stresses promote cracking along the bond layer, causing the coating to spall (Ill. 18.104.22.168-4.3).
Figure "Measurement of coating bond strength": The tests depicted here are used for the quantitative determination of bond strength. Failure criteria are cracking and signs of separation of the coating. These are usually specimen tests (top three diagrams) in which the coating is destroyed and must afterward be reapplied.
The bottom diagram shows an essentially non-destructive integral test in which a section of the coating is cut out and pulled off. If there is sufficient bond strength, the removed section can be replaced in its original position, assuming a suitable adhesive is available.
Figure "Methods of qualitative bond strength testing": There are several testing possibilities for bond quality, with no quantitative strength value. For this reason, experience or possibilities for comparison are prerequisites for assessments.
Blasting with glass beads can be used to induce compressive stresses in ductile metallic coatings, which will blister in areas with local bonding flaws (top left diagram; Fig. "Shot peening as testing method").
Thermography uses the increased local temperatures resulting from poor heat conductivity in separated coating areas.
Ultrasonic methods require suitable coating materials with sufficient homogeneity and thickness. The sonic penetration in areas with good bonding must be different than in flawed zones in order to provide information regarding the quality of the bonds. Soldering can meet these requirements.
With cutting procedures such as cross cut testing it is possible to test coatings and lacquers, but not most thermal spray coatings and elastomers. The criterion is separation in the rhombi that are cut free in the coating.
Thermocycles are primarily used for quality testing of thermal barrier coatings on hot parts. The failure criterion is signs of separation in the coating. The characteristic value is the number of cycles that were reached. This test has proven successful in process development and as a type of sampling inspection in serial production.
In impact bending tests on specimens with tough coatings (Ref. 22.214.171.124-5), analysis of the broken coating surface, any cracking, and signs of separation are used to draw conclusions regarding bonding behavior.
A section of thick elastomer coatings can be cut free and peeled off. The subjective resistance to separation (tension or peeling) allows experience-based assessments. A similar test can also be done on accessible edges or corners of coatings. The advantage of this type of inspection is that, following a satisfactory result, the removed coating section can be glued back on.
Figure "Verifying of bond strength by process monitoring": There are finishing processes such as coating technology, the products of which are almost impossible to non-destructively test in a satisfactory and serially-implementable manner. In these cases, quality assurance must be done through monitoring and documentation of relevant process parameters. This must be determined and documented in advance during process development.
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