Residual stresses and adhesion of thermal spray coatings (original) (raw)
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Journal of Thermal Spray Technology, 1996
An overview is presented of the development of residual stresses in thermal spray coatings and their effects on interfacial debonding. The main experimental techniques for measurement of residual stresses are briefly described, with particular attention given to the method of continuous curvature monitoring. Boundary conditions satisfied by all residual stress distributions are identified and expressions derived for the curvatures and stress distributions arising from a uniform misfit strain between coating and substrate. It is noted that stress distributions in thick coatings rarely correspond to the imposition of such a uniform misfit strain, so that recourse to numerical methods becomes essential for quantitative prediction of stress distributions. Relationships are presented between residual stresses and corresponding strain energy release rates during interfacial debonding. The effect on this of superimposing stresses from an externally applied load is outlined. The initiation of debonding is then considered, covering edge effects and other geometrical considerations. Finally, some specific case histories are briefly outlined to illustrate how the various theoretical concepts involved relate to industrial practice.
Adhesion and residual stress evaluation of thermally sprayed coatings
2010
Pour les revêtements obtenus par projection thermique, c'est-à-dire entrant dans la catégorie des revêtements épais, l’adhérence sur le support et les contraintes résiduelles sont les paramètres principaux déterminant leur performance en service. Bien que de nombreuses méthodes aient été essayées pour évaluer l'adhérence, il n'existe pas de test satisfaisant toutes les exigences tant techniques que théoriques nécessaires pour représenter valablement l’adhérence d’un revêtement. L’idée essentielle est de comparer plusieurs méthodes d’essais capables d’aboutir à une ténacité d’interface ou une énergie de fissuration interfaciale représentatives de l’adhérence de revêtements préparés dans des conditions d’élaboration les plus variées possibles. En dehors de l’essai normalisé EN582, l’indentation interfaciale, l’essai de cisaillement et l’indentation Rockwell-C associée à une modélisation par éléments finis ont été utilisés. Les contraintes résiduelles ont été estimées par l...
Interfacial indentation and shear tests to determine the adhesion of thermal spray coatings
Surface and Coatings Technology, 2006
Adhesion is one of the most important parameters which influences the development of thermal spray coatings. Therefore, the level of adhesion should be known for a given application. Apart from the standardized Tensile Adhesive Test (TAT), more than 80 methods are reported to measure the coating adhesion. Most of them are energy consuming in terms of time, cost and equipment. Moreover, they do not fulfil the necessary requirements of accuracy, confidence and representation of the real delamination process observed in service. To address this problem, the interfacial indentation test is used here to initiate and propagate a crack at the interface between the substrate and the coating. Studying the extension of the crack, an interfacial toughness is defined and deduced analytically from the experimental results. The new shear test, developed in the frame of the EU-CRAFT-project "Shear Test for Thermally Sprayed Coatings", is also employed to assess the coating adhesion. Both tests are compared to the standardized TAT for various spraying systems, materials, substrate roughness and coating thickness. Advantages and disadvantages of the three tests are discussed. Correlations between the tests results obtained for different coating-substrate combinations are presented and general trends are described.
Three adhesion measurement methods for thermal spray coatings, namely tensile adhesive strength (according to EN 582), interfacial indentation and in-plane tensile tests were investigated in terms of accuracy of the results and application potential for different coating / substrate conditions. Whereas the tensile adhesive strength test is widely used in industry, the other two methods are still under development in research laboratories and therefore only few experimental data on the accuracy of the methods and on the potential in an industrial context are available. For that reason, dissimilar coating-substrate combinations covering a wide range of types of thermal spray coatingsubstrate systems were tested using all these methods. Ceramic (Al 2 O 3) and metallic (NiCr 80-20) coatings were thermally sprayed by flame spraying with two different thickness on titanium alloy and steel substrates exhibiting each two distinct roughness levels. The distinguished coating properties include the coating toughness, shear strength, interfacial toughness, and adhesive strength. Thermally sprayed coatings do not only show an interfacial complexity, but also the integrity of the interface of substrate and coating has to be considered, as well as porosity, cracks and residual stresses. In this paper, each measurement method was found to be related to certain type of loading conditions and fracture mode. The results of the different methods are compared and the limits of applicability of the different methods are discussed.
Surface Engineering, 2006
Interfacial indentation is used to create and propagate a crack in the interface plane between a coating and its substrate. It has been shown earlier that this methodology allows the measurement of an apparent interface toughness, which can represent the adhesion of the coating to its substrate. In service thermal spray coatings are subjected to various external influences, which could be detrimental to their adhesion, e.g. thermal shock, fatigue, wear, corrosion and combinations of these. The objective of the present work is to study some of these effects and their influence on the interface adhesion toughness of various substrate/coating systems based on new phenomenological approaches to the residual stress state existing in the coating: (a) annealing treatment -for as sprayed specimen, it was first shown that the interface toughness is directly proportional to the reciprocal of the squared coating thickness. Extrapolated to an infinite thickness, the toughness could be assumed then to represent the adhesive properties of the coating. The same tests, performed on annealed samples, for which, it was expected to see significant modification of the residual stresses state, led to an apparent interface toughness independent of the coating thickness. Moreover, this value corresponds to the value extrapolated for the as sprayed specimen. The difference between the calculated values of K ca on both as sprayed samples and on annealed ones is discussed in terms of a stress amplitude factor defined as the difference between the residual stresses in the coating and in the substrate. The obtained result is very important since it confirms that there exists a value which, being independent on the thickness, can represent the adhesion of the coating; (b) hydrogen contamination -using this methodology, it was found that, besides the embrittlement of the coating, adhesion was also affected because the critical load necessary to initiate a crack at the interface was reduced in the presence of hydrogen. Using apparent interface toughness, it was also possible to compare the effect of hydrogen with that of the residual stresses (thickness) effect, according to the stress amplitude factor at the interface and to the capability of the substrate and/or the coating to trap atoms of hydrogen in the neighbourhood of the interface; (c) thermal treatments -in addition, it is demonstrated here that the methodology allows quantification of the effects of the thermal treatments such as thermal shock or thermal cycling on the adhesion properties of the coatings. It was shown that thermal treatments for particular conditions of time or temperature lead to a reinforcement of the metallurgical bond and, consequently, to an increase in the adhesion of the coating on its substrate.
AQHESION OF THERMALLY SPRAYED COATINGS-AWRA CONTRACT 81
A fracture mechanics approach to the adhesion of plasma sprayed coatings provides insights into the mechanism of adhesion. Thus the strain energy release rate for adhesive or cohesive failure may be related to microstructural features of coatings. Plastic deformation of coating lamellae and the effecriveness of contact between impinging droplets and the substrate or previously solidified material are important factors in determining coating properties.
Adhesion prediction on metal thermal spray coatings
Surface Engineering, 2005
Bonding prediction (presence or absence of adhesion) of metal thermal spray coatings is of theoretical and practical interest. The present paper introduces a simple method for estimating the adhesion performance of metal thermal spray coatings. To improve the conditions expected in the case of poor adhesion, the proposed method can also provide and anticipate suitable types of metal thermal spray coatings to overcome the problem. The expansion coefficient is inventively used as a quality indication for bonding performance of metal thermal spray coatings.
Thermal Sprayed Coatings Adherence - Influencing Parameters
2008
Thermal spraying represents the process of obtaining new special multilayer structure materials, with good mechanical and chemical characteristics. These materials are often used in solving real important problems, like repairing worn parts working under severe wearing conditions or, ensuring efficient corrosion protection of parts used in sea, as platform, bridges, or obtaining high refractory surfaces. Most of the times, once obtained, these coatings need additional machining and, it is of interest to study how, and, if, any of the machining parameters do influence one of their very important characteristic, meaning, adherence to the basic substrate. The paper presents a study on the adherence of thermal sprayed coatings, obtained from some Romanian thermal sprayed materials and submitted to exterior cylindrical turning.
Journal of Thermal Spray Technology, 1994
Wire-arc-sprayed nickel-aluminum is widely used in the aircraft industry for dimensional restoration of worn parts and as a bond coat for thermal barrier coatings and other top coats. Some repair applications require thick coatings, which often result in lower bond strength. A mechanism being investigated to explain this decrease in bond strength is the free edge effect, which includes both coating residual stresses and coating thickness. The layer-removal method was used to determine experimentally the residual stresses in wire-arc-sprayed nickel-aluminum coatings of different thicknesses. Bond strength evaluations were performed using an improved ASTM C 633-79 test specimen. Finite-element analysis and fracture mechanics were used to investigate the effects of coating thickness and residual stress state on coating bond strength. 2. Experimental Program The coating material selected for this study was 95% Ni and 5% A1 by weight. This coating and variations of it are frequently
Effect of thermal treatments on adhesive properties of a NiCr thermal sprayed coating
Thin Solid Films, 2000
Thermal sprayed coatings are most often used to resist wear or as thermal barriers. In some situations they may have to resist to the combined effects of corrosion and wear at high temperature. Ni-base thermal coatings are being used successfully in this case. It has been demonstrated that adhesion may be modified after a thermal treatment. For example, a substantial increase in adhesive properties was obtained after annealing of a chromium carbide thermal sprayed coating. Using the interfacial indentation method we have studied the influence of an annealing treatment upon adhesion of NiCr coatings for different thicknesses. It was confirmed that annealing improves adhesion to a great extent. Thermal cycling between room temperature to Ž . 900ЊC =5 times and thermal shocks consisting in heating the samples to 900ЊC, then quenching them in oil at room Ž . temperature =5 times , were also studied after the annealing treatment. It was found that a beneficial effect is obtained after such treatments since the critical load necessary to initiate a crack at the interface was increased by thermal shock and even more by thermal cycling. In addition, crack propagation at the interface was slowed down after the heat treatments. This behaviour is discussed and related to the role played by the residual stresses in the coating. ᮊ