Influence of Vacuum Heat Treatment on the Residual Stress of Thermal Spray Cermet Coatings (original) (raw)
Related papers
Fine lamellar microstructure of thermal spray coatings results in their unique properties when compared to conventional materials, e.g. enhanced strain tolerance of the plasma sprayed ceramics when compared to their bulk counterparts. However, detectable changes in the coating microstructure take place already since very low strains. Understanding of the microstructural changes in the coatings is commonly based on the macroscopic stress-strain evaluation e.g. from the four-point bending test or fractographic analysis of failed samples. Some of the advanced techniques such as tomography are not commonly available and their resolution may be insufficient, as small microcracks have a crucial role in the coating properties and failure mechanisms. In this study, advantages of relatively simple direct in-situ microstructural observation of polished cross-sections of thermal spray coatings during mechanical loading by means of scanning electron microscopy and subsequent strain-mapping anal...
Materials Science and Engineering: A, 2004
Thermal spray offers a variety of subsets of processing approaches to produce coatings. The various processes are classified based on the thermal spray source (from low velocity combustion spray to high temperature plasma jets) and method of material injection (in the form of powder, wire or rod). However, it is this intrinsic versatility which sets-up variations in characteristics of the applied coatings. Properties of thermally sprayed coatings, including process induced residual stress, are controlled by various parameters of the spraying process. This study examines three thermal spraying techniques with significantly different particle temperatures and velocities. They are air plasma spraying (APS), twin wire-arc spraying (TWA) and high velocity oxy-fuel (HVOF) spraying. For comparison purposes the recently developed cold spray processed materials were included in the study. For each method, in-flight particle diagnostics was performed; Ni-5 wt.%Al splats and deposits were fabricated and analyzed. Porosity, elastic modulus and thermal conductivity of the deposits were evaluated and correlated to the process variables. Using indentation at different loads and analysis of the indented region, stress-strain relationships for these coatings were obtained. Surprising differences in the properties were observed and were explained based on the fundamental variations in microstructure development. Through-thickness residual stress profiles in Ni-5 wt.%Al coatings on steel substrates were determined non-destructively by neutron diffraction. The stresses range from highly tensile in the APS coating to compressive in the HVOF coating. Various stress generation mechanisms-splat quenching, peening and thermal mismatch-are discussed with respect to process parameters and material properties.
Surface Engineering, 2001
In this work, the influence of the thermal spraying in traction in the APS, V PS, and HVOF coatings: the residual stress level in the HV OF and APS coatings is processes, atmospheric plasma spraying (APS), high less important than that in the VPS coating. T he velocity oxygen fuel (HV OF), and vacuum plasma microstructure observations show that the VPS copper spraying (V PS), on the microstructure and properties coating has a recrystallised structure whereas the APS of copper coatings is discussed. T he diVerences in and HV OF copper coatings have a splat type structure. microstructure, microhardness, and residual stress for As a consequence, the VPS copper coating has a high each type of coating are shown. T he X-ray diVraction Young's modulus, an important mechanical resistance, (XRD) method is used to evaluate the mechanical and a high elongation as compared with the other anisotropical characteristics of the materials and the copper coatings. SE/S185 residual stress distribution. T he particularity of this study is that the thickness of the coating is of the Dr Gassot and Dr Junquera are at the Institut de millimetre scale; the massive coating specimens without Physique Nucléaire d'Orsay,
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.
In situ measurement of residual stresses and elastic moduli in thermal sprayed coatings
Acta Materialia, 2003
Mechanical properties, such as residual stress and Young's modulus, play a critical role in the synthesis and performance of thermally sprayed coatings. Thus, it is important to understand their evolution, the influence of processing parameters, and to be able to determine them accurately. The first part of this two-part paper presents a novel in situ curvature method for determination of stresses and Young's modulus of plasma sprayed coatings. The principle of the method is explained, details of the instrument are provided and the analytical procedure is described. The capabilities of the method are discussed in detail, namely the ability to observe the stress evolution during the entire spraying process, to separate the quenching and thermal stress contributions to final residual stress and to determine the Young's modulus of the coating. Brief examples of application are also included, and the potential for use of this method for process control of coating quality is addressed. In the second part, a case study for plasma sprayed molybdenum will be presented, focusing on the influence of the key processing parameters.
Analysis of Thermal History and Residual Stress in Cold-Sprayed Coatings
Journal of Thermal Spray Technology, 2014
Residual stress in coatings has significant effect on their performance. In cold-sprayed coatings, in which particles impact the substrate at high velocity in solid state, in-plane residual stresses are usually conceived to be compressive. In this research, analysis of residual stresses in cold-sprayed deposits is performed by analytical and numerical modeling. The influence of various parameters such as the dimensions and elastic properties of the coating and the substrate on the residual stress are analyzed. In addition, the amount of heat input as a key parameter in the build-up of the residual stress is examined. It has been found that the heat input and the associated thermal history have a major influence on the final distortion and the residual stress, to an extent that the in-plane stress can in some cases change from compressive to tensile. Based on these results, a simple model is put forward for the prediction of the final state of the stress and distortion in cold-sprayed flat components.
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...
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.
Coatings
Residual stresses’ magnitude generated by deposition, quenching stress, thermal stress operation temperature, and infiltration in the thermal barrier coating (TBC) of gas turbines was determined. A thermal barrier coating was manufactured by the deposition of two layers, CoNiCrAlY and yttria-stabilized zirconia (YSZ), on an AISI 304 stainless steel substrate. The CoNiCrAlY was deposited by using an HVOF gun and the YSZ by an atmospheric plasma spray (APS). The TBCs were heat-treated at 1250 °C, with a CMAS (CaO, MgO, Al2O3, and SiO2) attack with a concentration of 10 mg/cm2 for 6 h in order to evaluate the evolution of the state of residual stresses in the coating at a high temperature. Residual stresses were determined by employing the modified layer removal method for duplex coatings (MLRMDC), ANSYS Version R19.2, and the equations proposed by Noda et al. In the YSZ, the total maximum residual stresses were 139 MPa in compression, and in the CoNiCrAlY, the maximum residual stress ...
Acta Materialia, 1997
An experimental method is proposed which enables the determination of processing-induced intrinsic stresses, elastic modulus, and coefficients of thermal expansion of surface coatings ol homogeneous and graded compositions. In this method, a number of identical substrate specimens are coated simultaneously with surface layers of fixed or graded compositions, and specimens with different layer thicknesses are periodically removed from the deposition chamber. It is shown that the following results can be obtained from a knowledge of the strain or curvature and thermal history of the coated specimens, in conjunction with simple four-point bend tests and thermal loading/cycling at different temperatures: (i) the magnitude of the processing-induced intrinsic stresses through the thickness of the coating, (ii) the in-plane Young's modulus, E, as a function of the coating thickness. (iii) the coefficient of thermal expansion, a, as a function of the coating thickness, (iv) the variation of E and c(as a function of temperature at any thickness location within the coating, and (v) the separation of internal stresses arising from thermal expansion mismatch between different constituent phases or layers from those arising from the deposition process (so-called "intrinsic" or "quench" stresses). The thermomechanical analyses underlying this method are discussed in detail, and its significance and limitations are addressed. The proposed method is used to determine the evolution of processing-induced stresses during the successive build-up of plasma-sprayed Ni-Al?03 coatings of homogeneous and graded compositions. KT' 1997 Acttr Metallurgica Inc.