The adhesion of copper on aluminum depends on the presence of intermetallic phases. Such phases can form during spraying at the interface between the layer and substrate. This paper deals with the formation mechanism of the intermetallic phases and their influence on adhesion. The type, size, and distribution of the intermetallic phases are investigated as a function of spray parameters and bonding strength is determined by laser shock adhesion testing. Paper includes a German-language abstract.

Fiber-reinforced polymer composites (CFRP) are increasingly used in aerospace for weight-sensitive applications. However, they are subjected to degradation from the erosive forces of solid particles and water droplets. This degradation results in a decreased service life of composite components and increased repair costs. A coating can protect the CFRP surface against wear and plasma spraying could be a candidate technique to achieve this coating. However, an issue is the thermal and mechanical damage to the composite surface by the plasma-sprayed particles. Another issue is the coating adhesion, because of the low wettability of polymer surface to liquid metal and ceramic and different atomistic properties between substrate and coating material. A possible solution to both issues is the use of a primary layer deposited by a “softer” technique than thermal spraying. This study deals with the deposition of this primary layer by three methods (magnetron sputtering, air gun spraying and sol-gel) and the deposition of topcoat layer by plasma spraying. The effectiveness of the protection of the CRFP by the primary layer during topcoat plasma spraying is investigated as well as the interfaces of the duplex coatings.

The application of aluminum coatings onto steel for corrosion mitigation is governed by standards specifying surface cleanliness and roughness prior to coating, and minimum coating bond strength. Controlling the surface preparation and spray parameters to achieve the specified surface condition and coating bond strength is challenging, particularly for manual on-site work. In this research, the process parameters were varied and the effect on surface quality and coating adhesion determined. It was found that blasting at angles as low as 30° from the surface, and varying stand-off distances up to 100 mm from the optimum, produced an acceptable surface; whilst spray angles of 60° to 90° and stand-off distances up to 50 mm from the optimum produced acceptable coatings with adhesion above 20 MPa. Adhesion appeared unaffected by a limited amount of remaining mill scale, but was reduced to ≈15 MPa when the surface chloride content was increased from 2 to 20 μg cm-2.

Surface preparation is very important for reliable adhesive bonding of cold sprayed coatings to the substrate. In this work, the grit blasting of low-carbon A516 steel substrates with Al2O3 particles was studied and the roughness parameters Ra and Rt of the grit blasted surfaces were then measured. The influence of alumina grit size on the roughening of the A516 steel substrate, and the resulting effect on the roughness of the Cu coating – steel interface were studied. The results showed that variations of the grit blast size had significantly affected the resultant surface roughness of the substrate. The adhesive strength of the formed copper coatings on A516 steel substrates depends on the surface roughness and hardness of the base material. The adhesive strength about 110-200MPa was achieved. The specific features of the Cu coating-A516 steel interface topography were examined and discussed.

To achieve sufficient adhesion strength within thermal spraying, the surface to be coated has to be modified. Grit blasting is the most common way to generate a clean and roughened surface. The bonding mechanism between the grit-blasted substrate and the coating is assumed to be due to mechanical anchoring, why an optimal surface roughness is essential. The surface roughness is usually evaluated using Ra which cannot fully characterize the complex nature of the chaotic substrate topography. This study was performed in order to evaluate if Ra can be replaced by other surface characteristic parameters such us RΔq, Rpk, Rpv, Rk…with higher correlation to adhesion strength. Average roughness was measured by a perthometer and with white light interferometry to get 3D images of the surface topography. Disc shaped substrate samples of Ti6Al4V (AMS 4928) were grit blasted with aluminium oxide grit and plasma sprayed with a Ni5%Al coating. Adhesion strength was determined according to the ASTM C633 standard. The correlation between a number of different surface-parameters and adhesion strength were evaluated and compared with Ra.

Alumina coatings plasma-sprayed on alumina substrate with metallic bond coating (50Ni-50Cr) and on metal (Ni) substrate have been investigated in terms of adhesion strength and a veined structure formed in alumina coating. The veined structure is formed to heal up cracks and pores in sprayed alumina and substrate alumina after heat-treatment in air. The veined structure consists of oxides of NiAl 2 O 4 (spinel-type) and NiO (NaCl-type). This indicates that the metallic elements in the bond coating or the metallic substrate diffuse along the cracks and pores in alumina and react with alumina. The alumina coating with veined structure shows high strength due to the mechanical anchoring of veined oxide and chemical bonding.

Self-fluxing nickel based materials are widely used as hard coating materials. HVOF process is promising to produce dense coating with high adhesion. In present study the effects of HVOF spray conditions, and WC-Co addition into self-fluxing nickel based material (NiCrBSi) and the introduction of HVOF WC-Co bond coat on the adhesion of HVOF NiCrBSi coatings are investigated. With regarding to NiCrBSi material, the spray conditions are optimized by orthogonal regression experimental design method with adhesive strength. The adhesive strength is estimated by tensile test. The results show that the adhesive strength of the NiCrBSi coating sprayed at the optimized conditions reaches to about 40MPa. It is found that the addition of WC-Co material through mechanical blending can improve the adhesion of HVOF NiCrBSi coating. The adhesive strength is increased with the increase in WC-Co content in the composite powder. The introduction of HVOF WC-Co bond coat between NiCrBSi coating and substrate can improve the adhesive strength. The multiply enhancing effect on the adhesive strength of HVOF nickel based coating is recognized by applying HVOF WC-Co bond coat to NiCrBSi-WC-Co composite coating.

This study evaluates a nondestructive method for determining the adhesion strength and porosity of thermally sprayed coatings. The method is based on ultrasonic surface echo measurements. Porosity is surmised by the impedance of the coating and adhesion by the transmission coefficient. Experimental results confirmed that the method is effective for its intended use.

Metal-ceramic coatings have been widely used for industrial applications, mainly in the thermal barrier coating technology (TBC). Plasma spraying is the common manufacturing process of TBC's. Conventional thermal barrier coatings consist of a metallic bond coat layer and an insulating ceramic overlay. Graded coatings or functionally gradient coatings have also been applied in order to solve the problems associated with the early spallation of plasma-sprayed conventional TBCs. Temperatures and gradients during plasma spraying have and important influence on the coating quality, specially the temperature of the particles just hitting the substrate surface. When applying so distinct materials like metals and ceramics this fact has an increased importance. In this work metal-ceramic coatings have been applied on metallic substrates. The interfacial temperature measurements were performed by optical pyrometry. The substrate temperature was measured by thermocouples. The adhesion of the coatings was determined by standard ASTM tests and correlated with the measured temperatures. In a general way, results show that the coatings with lower adhesion values were that with lower interfacial measured temperatures.

Cold spray process was chosen as a good candidate for dimensional restoration and protection of components. Commercially pure aluminum, aluminum-alloy or titanium were recommended for different applications. This paper investigates laser surface texturing association to enhance durability of sprayed coatings. Laser is easy automated, localized and reliable process. It was applied for prior-surface treatment. Textured surfaces were produced and compared to conventional treatments, such as grit-blasting, in terms of deposition efficiency and adhesion bond strength. Patterns promoted direct particle embedment. Particle-substrate interface exhibited significant temperature rate and strain in cavities. Intimate contacts and particle compressive states were assumed responsible for improvement. The particle deformation and bonding behaviors were evaluated and discussed for the different configurations. Thus, window of deposition was increased with laser surface texturing. Anchoring mechanisms increased two fold the adhesion strength compared to conventional pre-treatments. In one case, the interface was stronger than the coating cohesive strength.

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 coating-substrate 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.

The adhesion strength of a ceramic coating deposited through direct spraying on a roughened substrate is a key issue in the manufacture of high-quality coatings on industrial components. The purpose of this work was to develop a rapid and discerning procedure for establishing adhesion level of a ceramic coating on a metallic substrate. The Laser Shock Adhesion Test, namely LASAT, was successfully applied to ceramic coatings with irradiation impact on the metallic side. Suitable parameters were found to determine the LASAT adhesion threshold using a standard Nd:YAG laser source. With a laser-irradiated area of several millimetres in diameter, it allowed assessment of the coating threshold on several areas of a coated plate sample. A control procedure for a qualitative assessment of coating adhesion was developed. This testing procedure could be easily used in industry, with possible location of the LASAT unit near to the spraying booth, for a direct production control on coated sample to improve the tracability of manufactured parts. Additional work was carried out to investigate a quantitative approach of the LASAT test to ceramic coating. The purpose was to simulate the shock wave propagation with the RADIOSS® code (a 3D software originally developed for car crash simulation). This code was implemented to calculate the velocity of the material and corresponding pressure throughout the substrate and the coating during the shock wave release (less than 2 ms). Experimental VISAR profiles ('Velocity Interferometer System for Any Reflector') were monitored in the straight direction of the laser-irradiated area on the rear side. These experimental signals (velocity measures) of the ceramic coating could be fitted and compared with a fairly good agreement with simulated profiles obtained by RADIOSS®. This modelling work was the first step towards a more comprehensive coating adhesion strength calculation in the future.

The PROTAL process combines a laser surface preparation and the thermal spraying stage. This laser surface preparation avoids some of the traditional drawbacks of the degreasing and sand-blasting stages which is an important factor especially for the notch-sensitive materials. Previous studies showed that deposit adhesion obtained with the PROTAL process is similar to that produced by traditional surface preparation despite the absence mechanical anchorage offered by surface roughness. In order to get a better knowledge of the effects of such a laser treatment, a Ni-5%Al coating was plasma sprayed using the PROTAL process under different surface conditions. The morphology of the impinging splats and adhesion of the deposits were then examined. Removal of the surface contaminants, adsorbates and oxides is confirmed and the role of the laser irradiation on the coating adhesion is discussed.

A good adhesion of plasma sprayed hydroxyapatite (HA) coating on Ti-based alloy is crucial for ensuring highly-reliable non cemented implants in the biomedical industry. In the present work, the laser shock adhesion test, namely LASAT, has been applied to investigate the interface strength of plasma sprayed HA coatings. This contact less method allowed a rapid assessment of the HA coating adhesion on simple coated plates. Varying the laser energy to impact the substrate and to generate the interface decohesion, a LASAT adhesion threshold can be determined for the highest laser fluence (J/m²) for which no debonding of the coating occurred. This qualitative and discerning LASAT procedure has been carried out on HA coatings to investigate the role of various interfaces on the adhesive property of the HA/Ti bond. According to the LASAT analysis, a surface roughness prepared with medium or coarse grit-blasting did not influence drastically the adhesion threshold while smooth pre-oxidized specimens LASAT threshold were near to those obtained with a Ti bond-coat. These thresholds also corresponded with the highest adhesion measured in this study. In addition, pre-heating treatment of substrates just prior to spraying up to 270°C did not exhibit a significant difference with grit-blasted HA/Ti interface. Further investigations (SEM, XRD) was also achieved to investigate the interface characteristics before and after the laser treatment. Sample cross-sections of laser shocked specimens were examined in detail, right at the impact location and within the debonding area to assess the fracture feature. This complementary materials analysis permitted to establish the relevance of the LASAT test as a fast and easy-to-use method devoted to the design or the control of highly adhesive HA coatings. Preliminary experiments to apply the LASAT method in liquid environment is described. Further work is on progress to implement an in situ adhesion testing of HA coating in simulated body fluid.

Excepted in a few cases where metallurgical bonding occurs between deposit and substrate, thermal spray deposit adhesion generally results from a mechanical anchoring. In this case, the very first impinging particles forming the first deposited layer spread and solidify into and around the cavities of the grit-blasted surface. A palliative process to degreasing and grit-blasting prior to thermal spraying is simultaneous laser ablation; i.e., the PROTAL process. In such a case, little topographic change results from the laser-matter interaction: deposit adhesion does not derive anymore mainly from mechanical anchoring but from other types of bonding such as chemical bonding. This paper aims to clarify the bonding mechanisms of thermal spray coatings manufactured implementing the PROTAL process. The case of metallic coatings deposited on metallic substrates is especially discussed. At first, laser ablation effects on various metallic substrates are presented, from the topographic and energetic points of view. Then, the induced effects on impinged particle morphologies are discussed. The results are correlated to thick deposit adhesion.

Importance of coating adhesion in a corrosive environment was studied experimentally. Tensile adhesion strength of HVOF sprayed 316L stainless steel and Hastelloy C coatings were tested in as-sprayed condition as well as after immersion in seawater. It was found that the adhesion strength of the stainless steel coatings degraded rapidly whereas that of the Hastelloy coatings remained almost intact. Specimens with an artificial defect were also immersed in seawater. The cross sectional observation after the test revealed that the corrosion at the coating-substrate interface proceeded much faster with the stainless steel coating as compared to the Ni-base alloy coating. A model experiment to simulate the galvanic corrosion of a coating-substrate couple was carried out and no significant difference in the galvanic current density was found between the two coatings when coupled with the steel substrate. The tightness of the coating-substrate interface was then tested with a fluorescent dye penetration test. The dye could penetrate the boundary between the stainless steel coating and the substrate whereas the boundary between the Ni-base alloy coating and the substrate was so tight that no penetration occurred. The size of the micro-gaps at the coating-substrate boundary was discussed from the viewpoint of classical Washburn-Ridiel theory. It was concluded that such micro-gaps between the coating and substrate must be eliminated for these barrier-type coatings to be used in corrosive environments. Heat treatment was highly effective for suppressing the preferential corrosion at the coating-substrate boundary.

In this study, a new laser based technique was evaluated for the characterization of plasma-sprayed oxide coatings. It uses the contactless laser generation and detection of ultrasonic waves in the bi-layered systems. For this purpose, a nanosecond pulsed Nd:YAG laser (λ : 1064 nm, τ =14 ns) was used for irradiating the ceramic coating, whilst the longitudinal displacements of the rear surface of the metallic substrate were detected at the epicenter using a laser heterodyne interferometer. The acoustic signal recorded at the rear surface of the substrate was found to be characteristic of the different events taking place within the irradiated system. In this way, the longitudinal wave velocity, the porosity, as well as the Young's modulus of the coatings can be easily determined, whilst the coating/ substrate adhesion strength can be calculated, taking into account both the thermal, as well as the acoustic effects of the laser radiation. The proposed technique was applied to alumina coatings deposited onto stainless steel coupons by Atmospheric Plasma Spraying and the results were found to be in accordance with those obtained by the techniques commonly used for testing thermal spray coatings (interfacial indentation test, porosity measurement, etc.).

High velocity oxyfuel flame spraying (HVOF) has a great potential for replacing the low pressure plasma spraying (LPPS) process in most applications for hot corrosion protective MCrAlY coatings for turbine hot gas path parts. Compared to LPPS coatings, state-of-the-art HVOF sprayed MCrAlY coating systems feature competitive hot corrosion protective properties. Low HVOF facilities investment costs and a stable, easy process controlling are major advantages in terms of application. Besides hot corrosion protective single layers, MCrAlY coating systems are used as bond coats for ceramic thermal barrier coatings (TBC). But HVOF sprayed MCrAlY coatings show a comparatively low surface roughness which leads to a poor adhesion of the ceramic top layer, restricting the application area of HVOF. This paper deals with a development project which aims on roughness enhancement of HVOF sprayed MCrAlY coatings in order to improve the bonding properties of the TBC. In the project’s framework, several HVOF systems and different powders were investigated. Parameter sets were developed considering both a high surface roughness and a low level of defects in the coating.

The purposes of the present study are firstly to establish a reliable experimental method to evaluate the adhesion strength and secondly to reveal the relationship between the adhesion strength and the residual stress in the coatings. SUS304 stainless steel coatings were deposited on low carbon steel substrates by HVOF spraying. Two types of samples were fabricated. One is a cylindrical shape for the adhesion test and the other has a rectangular dimension for residual stress evaluation. The tensile adhesion tests were carried out by modifying the conventional test method. Circumferential interface cracks were introduced by putting carbon thin layer on the substrate before spraying, and the edge of the substrate was masked to prevent undesirable deformation during spraying. The interface fracture toughness was estimated from the load at failure and the width of carbon area in radial direction (corresponding to the crack length) by finite element analysis. The residual stress was evaluated by measuring the relaxed strain at the substrate surface for reduction of the coating thickness. Thinning of the coatings was performed by polishing. It was possible to evaluate the adhesion strength of SUS304 steel coatings with good reproducibility for the samples deposited under various spray conditions. The correlations between adhesion strength and residual stress will be discussed.

The adhesion at the interface between coating and substrate is a key factor for the reliability and performance since the main problem in the application of coating systems is the delamination at the interface. Finite element models are developed to find a way to predict the adhesion strength of a thermal sprayed coating system from simply designed indentation test. Large depth indentation behavior is simulated to study shear induced delamination beneath the indenter. The interface between the coating and the substrate is modeled by three different bonding characters to investigate the effect of interfacial bonding on indentation test. Pressure-strain relations based on Tabor’s suggestion are observed for various combinations of material properties and interfacial bonding characteristics. The growth of a crack in the interface plane leaves a clear imprint on indentation load-depth curve in case of soft coating on hard substrate. Tabor curve also shows potential ability to detect interface bonding strength under indentation test.