Adhesion Strength of HVOF Sprayed IN718 Coatings

The adhesion strength of high-velocity oxyfuel thermally sprayed coatings is of prime importance when thick coatings are to be sprayed in repair applications. In this study, relationships between process parameters, particle in-flight characteristics, residual stresses, and adhesion strength were explored. The most important process parameters that influence HVOF sprayed IN718 coating adhesion strength on IN718 substrate material were identified. Residual stress distributions were determined using the modified layer removal method, and adhesion strength was measured using an in-house-developed tensile test. Relationships between process parameters, particle in-flight characteristics, coating microstructure, and adhesion strength were established. Particle temperature, particle velocity, substrate preparation, and deposition temperature were identified as critical parameters to attain high adhesion strength. Controlling these parameters can significantly improve the adhesion strength, thus enabling thick coatings to be sprayed for repair applications.     

Measurement of the Young’s Modulus of Thermal Spray Coatings by Means of Several Methods

Thermally sprayed coatings are usually defined by their hardness, porosity, roughness, and wear resistance. Even though the Young’s modulus is an essential property, which describes the mechanical behavior of the coated components during their use, only few efforts have been made in the past to determine this property. The most common measurement methods of the Young’s modulus of thermally sprayed coatings are tensile tests, bending tests, and nanoindentations. During the tensile and bending tests a sliding of the splats can occur due to the laminar structure of the thermally sprayed coatings, influencing the measurement value. When using the nanoindentation test, only the elastic behavior of some splats can be determined because of a minimal measuring volume. However, the Young’s modulus of thermally sprayed coatings can also be determined by means of a resonant method, called impulse excitation technique. In this paper, the values of the Young’s moduli of thermally sprayed coatings, measured by several methods, are compared with each other and correlated to the microstructure of the coatings, investigated by means of scanning electron microscopy.     

Comparison of the Thermal Transport Property Measurements of Thermally Sprayed Coatings by the Laser and Xenon Flash Techniques

Development of precise and reliable measurement methods is an important step in the study of the thermal transport property of inherently complex systems such as thermally sprayed coatings. In this study, the applicability and repeatability of both the laser and xenon flash techniques have been investigated through the measurements on the coatings made from ceramics, metals, alloys, and composites. For the four series of material systems, issues such as the effect of powder morphology, coating thickness, spraying technique, chemical composition, and oxidation on the thermal diffusivity as well as the corresponding measurement repeatability are assessed. This investigation provides information necessary for precise and reliable characterization of the thermal transport property of various thermally sprayed coatings.     

An ultrasonic testing method for detecting delamination of sprayed ceramic coating

The adhesion strength of thermal sprayed coatings is relatively low, and they sometimes tend to delaminate from the substrate during operation. In particular, sprayed ceramic coatings for thermal barriers, such as ZrO₂, often delaminate because of thermal shock; therefore, ceramic coatings are often submitted to thermal shock tests. A nondestructive inspection method using ultrasound to detect the delamination of sprayed coating was proposed. In this study, a coating model was made with acrylic plates, and an ultrasonic test was applied to investigate the precision of detecting delamination by the ultrasonic testing method. Results indicate that delamination more than 1mm in diameter can be detected by the ultrasonic testing method. Moreover, the delaminating process of sprayed coatings under thermal shock tests can be detected by this method.     

Mechanical properties of cold-sprayed and thermally sprayed copper coatings

The present investigation compares the mechanical properties of cold-sprayed and thermally sprayed copper coatings. The mechanical properties of the Cu-coatings are determined by in plane tensile test using micro-flat tensile specimen technique. A deeper view into the type of obtained defects, their stability and their influence on coating performance, is supplied by subsequent failure analyses and the comparison to annealed copper coatings. The results demonstrate that cold-sprayed coatings, processed with helium as propellant gas, show similar performance as highly deformed bulk copper sheets and respective changes in properties after annealing. In the as-sprayed condition, cold-sprayed coatings processed with nitrogen and thermally sprayed coatings show rather brittle behavior. Whereas subsequent annealing can improve the properties of the cold-sprayed coating, processed with nitrogen, such heat treatments have only minor influence on the tensile properties of thermally sprayed copper coatings. The investigation of failure modes for the as-sprayed states and after different heat treatments provided further information concerning particle–particle bonding and the effect of oxides on mechanical properties.     

Korrosionsverhalten gespritzter Zink‐Aluminium‐Überzüge in Kurzzeitbewitterungstests

Corrosion behaviour of thermally sprayed zinc‐aluminium‐coatings in short‐term corrosion tests By alloying aluminium to zinc the corrosion resistance can be increased in a multiplicity of media. Thermally sprayed ZnAl 15‐coatings show a higher corrosion resistance than thermally sprayed zinc‐ or hot dip galvanized coatings. This paper presents the results of short term corrosion tests (salt spray test, Kesternich test) with sprayed ZnAl‐coatings with an aluminium content of 2 to 50 M.‐%. The investigations verify very clearly the influence of aluminium on the corrosion behaviour. Depending on the aluminium content the corrosion resistance shows a maximium with a medium content of about 15 or 22 M.‐%.     

Anti-icing Behavior of Thermally Sprayed Polymer Coatings

Surface engineering shows an increasing potential to provide a sustainable approach to icing problems. Currently, several passive anti-ice properties adoptable to coatings are known, but further research is required to proceed for practical applications. This is due to the fact that icing reduces safety, operational tempo, productivity and reliability of logistics, industry and infrastructure. An icing wind tunnel and a centrifugal ice adhesion test equipment can be used to evaluate and develop anti-icing and icephobic coatings for a potential use in various arctic environments, e.g., in wind power generation, oil drilling, mining and logistic industries. The present study deals with evaluation of icing properties of flame-sprayed polyethylene (PE)-based polymer coatings. In the laboratory-scale icing tests, thermally sprayed polymer coatings showed low ice adhesion compared with metals such as aluminum and stainless steel. The ice adhesion strength of the flame-sprayed PE coating was found to have approximately seven times lower ice adhesion values compared with metallic aluminum, indicating a very promising anti-icing behavior.     

Modified tensile adhesion test for evaluation of interfacial toughness of HVOF sprayed coatings

Tensile adhesion test is widely used to evaluate the adhesion strength of coatings sprayed by High Velocity Oxy-Fuel (HVOF) technique. But there are two issues to be improved. Firstly, when the coatings have high adhesion strength, failure occurs in an adhesive layer, and secondary, the edge of a substrate is heavily deformed and rounded due to the high impact energy of sprayed particles. This deformation causes large scatter of adhesion test results. In this paper, a new technique to evaluate the interfacial fracture toughness has been proposed by introducing pre-crack at the interface of a conventional tensile adhesion test specimen. The asymptotic analytical formula was derived for interfacial toughness evaluation. Numerical analysis was also carried out for comparison. The difference between the numerical and the theoretical data was less than 5%. The developed procedure was applied for the SUS316 L steel coatings and the significant effects of the surface roughness and preheating temperature on adhesion strength were reconfirmed quantitatively.     

Modeling Thermal Conductivity of Thermally Sprayed Coatings with Intrasplat Cracks

Thermal spraying is one of the most important approaches for depositing thermally insulating ceramic top coatings for advanced gas turbines due to the low thermal conductivity of the coating resulting from its lamellar structure. The thermal conductivity of the coating has been explained based on the concept of thermal contact resistance and correlated to microstructural aspects such as splat bonding ratio, splat thickness, and the size of the bonded areas. However, the effect of intrasplat cracks on the thermal conductivity was usually neglected, despite the fact that intrasplat cracking is an intrinsic characteristic of thermally sprayed ceramic coatings. In this study, a model for the thermal conductivity of a thermally sprayed coating taking account of the effect of intrasplat cracks besides intersplat thermal contact resistance is proposed for further understanding of the thermal conduction behavior of thermally sprayed coatings. The effect of the intersplat bonding ratio on the thermal conductivity of the coating is examined by using the model. Results show that intrasplat cracks significantly decrease the thermal conductivity by cutting off some heat flux paths within individual splats. This leads to a deviation from the typical ideal thermal contact resistance model which presents cylindrical symmetry. Based on the modified model proposed in this study, the contribution of intrasplat cracks to the thermal resistivity can be estimated to be 42–57 % for a typical thermally sprayed ceramic coating. The results provide an additional approach to tailor the thermal conductivity of thermally sprayed coatings by controlling the coating microstructure.     

Microstructure and adhesion of 100Cr6 steel coatings thermally sprayed on a 35CrMo4 steel substrate

Thermally sprayed of 100Cr6 steel coatings are widely used to combat degradation of components and structures due to mechanical wear. In this paper, the microstructure and adhesion energy of 100Cr6 steel coatings thermally sprayed on a 35CrMo4 steel substrate are investigated. The microstructure characteristics of the deposits are studied using the combined techniques of X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) including energy-dispersive spectroscopy (EDS). The practical work of adhesion of flame-sprayed 100Cr6 on steel substrate is determined using a four-point delamination bending test. The influence of a molybdenum bond coat on the adhesion is also studied. Microstructure suggests that the coating is mainly constructed by splats of γ-phase (fcc) and FeO. Phase analysis also confirms that during spraying process, a stable α-phase (bcc) was transformed into a new γ-phase (fcc). The highest values of the fracture energy are obtained for the 35CrMo4 substrate/100Cr6 steel deposit type samples. On the contrary, when a molybdenum bond coat is introduced (composite system 35CrMo4 substrate/Mo bond coat/100Cr6 steel deposit), the fracture energy decreases in a ratio of approximately three. So, the presence of a Mo bond coat as a barrier between the coating and the substrate has a negative role on the adhesion.     

New Approach to Ceramic/Metal-Polymer Multilayered Coatings for High Performance Dry Sliding Applications

The combination of thermally sprayed hard coatings with a polymer based top coat leads to multilayered coating systems with tailored functionalities concerning wear resistance, friction, adhesion, wettability or specific electrical properties. The basic concept is to combine the mechanical properties of the hard base coating with the tribological or chemical abilities of the polymer top coat suitable for the respective application. This paper gives an overview of different types of recently developed multilayer coatings and their application in power transmission under dry sliding conditions. State of the art coatings for dry sliding applications in power transmission are mostly based on thin film coatings like diamond-like carbon or solid lubricants, e.g. MoS₂. A new approach is the combination of thin film coatings with combined multilayer coatings. To evaluate the capability of these tribological systems, a multi-stage investigation has been carried out. In the first stage the performance of the sliding lacquers and surface topography of the steel substrate has been evaluated. In the following stage thermally sprayed hard coatings were tested in combination with different sliding lacquers. Wear resistance and friction coefficients of combined coatings were determined using a twin disc test-bed.     

Adhesion evaluation of multilayered based WC-Co-Cr thermally sprayed coatings

This paper describes the adhesion evaluation of different interlayers such as Co-Cr, Ni-Cr 80-20 HVOF (High Velocity Oxy-Fuel) thermally sprayed coatings and Ni-plating between the cermet based WC-Co-Cr coatings. Three adhesion measurement methods for these different multilayered based thermal spray coatings, namely tensile adhesive strength (according to EN 582), interfacial indentation and solid impact tests were conducted. The distinguished coating properties include: i) the adhesive strength, ii) the interfacial toughness, iii) the depth of impact. The metallographic and experimental results show that the electrochemically deposited interlayer Ni-plating provides the highest adhesion to cermet coating within the multilayered structured coatings. This is not only due to the chemical affinity between the Ni-plating and the cermet coating, but also to its homogeneous microstructure, since the electrochemically deposition does not provide splat formation.     

Fatigue Behavior of Bodies with Thermally Sprayed Metallic and Ceramic Deposits

This paper summarizes the basic results of fatigue testing of bodies with both metallic and ceramic thermally sprayed coatings. Three kinds of ceramic coatings (Al₂O₃, Cr₂O₃, and olivine) sprayed with DC plasma under identical conditions were investigated together with metallic Ni-5wt.%Al coatings sprayed by wire arc, DC plasma, and HVOF. The elastic modulus of the deposited coatings was investigated using four point bending and resonance method. Bending fatigue tests at resonance frequency were performed with cantilever beam specimens. The processes taking place during the fatigue test are identified and discussed. The morphology of the fracture surfaces was investigated together with microstructure and porosity of the coatings.     

Recent Developments in Metal Spraying by the Powder Process

General principles of metal spraying by the powder process are described, and an account is given of an improved powder-spraying pistol. In this apparatus, powder is conveyed into an oxy-gas flame by any desired conveying gas under pressure: this allows the application of denser, less oxidized and better adherent coatings of zinc, aluminium, etc., than does the older spraying pistol wherein the powder is sucked into the pistol in an air stream, and also the spraying operation is faster and more efficient.

A simple quantitative adhesion test for sprayed metal coatings is described.

Some recent specialized techniques and applications are outlined, including the production of sprayed coatings of very low and of very high resistivity, the spraying of steels, the production of “aluminized” coatings by powder spraying followed by heat treatment, the production of hard, non-porous surfaces by “flaming” special sprayed coatings, and the improvement of the adhesion of zinc and aluminium coatings to be had either by an undercoat of sprayed steel or by preheating the base metal.     

Analysis of tantalum coatings produced by the kinetic spray process

Tantalum (Ta) coatings have been produced using a relatively new process, kinetic spray. Ta starting powders having particle diameters greater than 65 μm are injected into a de Laval-type nozzle, entrained in a supersonic gas stream, and accelerated to high velocities due to drag effects. The particles’ kinetic energy is transformed via plastic deformation into strain and heat on impact with the substrate surface. Particles are not thermally softened or melted, producing relatively low oxide, reduced residual stress, high adhesion and low porosity coatings. Analysis of the mechanical and physical properties of these Ta coatings demonstrated increasing hardness, cohesive adhesion, and decreasing porosity as a function of particle velocity. Comparison between kinetically sprayed coatings and coatings produced using conventional coating methods will be discussed.     

Metallization of Thin Al2O3 Layers in Power Electronics Using Cold Gas Spraying

A successful combination of insulating substrates with conducting metal coatings produced by cold spraying could open new industrial application areas like the fabrication of power electronic components. For minimizing the number of industrial process steps, insulating ceramic layers should ideally be processed by thermal spray techniques. Thus, this study investigates the impact behavior and coating formation of ductile metallic feedstock powders onto brittle ceramic coatings. With respect to high electrical conductivity of the metallic lines and good electrical insulation of the ceramic interlayer, copper was cold gas sprayed on previously thermally sprayed Al₂O₃ coatings. Successful cold coating formation requires different strategies for the activation of the ceramic layer to increase adhesion and to avoid brittle failure. These both can be achieved either by applying a bondcoat on the ceramic layer or using heated substrates during the cold spray process.     

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.     

Effect of Heat Treatments on HVOF Hydroxyapatite Coatings

Highly crystalline hydroxyapatite (HAp) powder was thermally sprayed onto Ti-6Al-4V substrates using the High-Velocity Oxy-Fuel (HVOF) process. Coatings were heat treated for 60 min at 700 °C to study the influence of the crystallization on chemical and mechanical properties. Characterization of the HAp coatings was carried out by Fourier Transform Infrared Spectroscopy (FTIR) and x-ray diffraction (XRD) using Rietveld analysis. The results showed that the coatings were highly crystalline (82%) and no other phases of calcium phosphate were present. Coatings were 100% crystalline after the heat treatment. Bioactivity of the coatings was investigated by immersion in Kokubo’s simulated body fluid. The dissolution/precipitation behavior was studied and the degradation of HAp coatings caused by the immersion test was studied by measuring the adhesion strength of the coatings. After immersion in SBF bond strength decreased for the as-sprayed coatings, without any thermal treatment, but it was constant for the heat-treated coatings. This phenomenon was related to the dissolution of the amorphous phase in the interface substrate-coating in the as-sprayed coatings.     

Fractal Evaluation on Effective Dielectric Properties for High-Velocity Oxygen Fuel Sprayed LBS-SiCβ Composite Absorber Coatings

An evaluation of microstructure-property relationship is important for thermally sprayed composite absorber coatings, because self-similarity in the microstructure is a key characteristic that can affect coating properties, such as flattened particle shape, pores, absorbent phase, and coating thickness. In this paper, a multiscale effective fractal model is reported to characterize the microstructure-property relationship for high-velocity oxygen fuel (HVOF) sprayed composite coatings. It shows the fractal of coating structure was presented to calculate the volume fraction of thermally sprayed absorber coatings with wavelet-fractal algorithm. As an example, the flattened particle shape, porosity, and thickness were researched for the microwave reflectivity coefficient of HVOF sprayed nanometer LBS (Li₂O-B₂O₃-SiO₂)-SiC_β composite coatings. The modeling approaches to establishing the relationships between coating microstructure and absorbing property was checked.     

Metallurgical characterization and determination of residual stresses of coatings formed by thermal spraying

This work presents an experimental determination of residual stresses in 35CrMo4 (Euronorm) low alloyed steel substrates with thermally sprayed coatings. Two different materials were separately deposited. The first one consisted of a blend of two superalloys: Cr-Ni steel and Cr-Mn steel, designated 55E and 65E, respectively. The second material was molybdenum. In a first part, basic characteristics of the deposited layers (metallographic analysis, hardness, and adhesion) are presented. In a second part, the determination of the residual stresses, in both substrate and thermal sprayed layers is performed using an extensometric method in combination with a simultaneous progressive electrolytic polishing. The influence of a nickel-aluminum (80:20%) bond-coat and/or a post-annealing at 850 °C in air for 1 h is studied.