Abstract This present work investigates the effect of electromagnetic fields on cold spray processes by means of an induction-heating cold spray (IHCS) system. Aluminum powder was cold sprayed onto inductively heated Ti6Al-4V (Ti64) substrates. These materials were selected to minimize the mechanical contribution to coating adhesion. As a result, changes in coating adhesion strength can be attributed to improved metallic bond formation due to the effect of the electromagnetic field. Four different initial substrate surface temperatures were used in the study to assess the role of initial temperature as well. Deposition efficiency and adhesion and tensile strength measurements were recorded and are used to characterize the hybrid coating process and compare it with traditional techniques.

Abstract Segregating the convoluted effects of particle size, impact temperature and velocity on deposition behavior and adhesion is of utmost interest to the cold spray field. The current study aims to associate the particle impact behavior and adhesion to its in-flight characteristics by studying and decoupling the influence of particle size, temperature and velocity for single particle impacts and full coatings. Experimental results reveal that in-situ peening processes contribute to the adhesion at low impact temperature while particle velocity controls the adhesion/cohesion at increased particle impact temperatures. The benefits of both bonding mechanisms are discussed in terms of measured adhesion/cohesion, bend-to-break fracture surfaces, pseudoplasticity, deposition efficiency and critical velocity. Computational fluid dynamics (CFD) results provide individual particle trajectory, size, temperature and velocity, of successfully deposited particles, which have led to the observed signs of metallurgical bonding.

Abstract Low pressure cold spraying is an attractive technique for onsite metal coating fabrication due to its compactness and portability. However, the bonding strength of the coating prepared by low pressure cold spraying is generally low, which restricts the further applications in engineering and industrial fields. To improve the bonding strength, pre-treatment on substrate surface can be an effective procedure. In this study, a low-temperature plasma treatment was applied to a pretreatment technique, and the effect of the treatment on particle bonding was compared with that of a laser treatment. Copper coatings on aluminum and copper substrates were selected and studied as basic metal materials. The SEM observation results show that the particle adhesion rate significantly increases by the laser and plasma treatments, due to the removal of the native oxide films on the substrates. The particle bonding on the plasma-treated substrate reveals better interfacial adhesion with less gap compared with the laser-treated one. The pre-treatment by low-temperature plasma can be an attractive technique to assist the cold spraying process due to the oxide removal ability and no thermal effect which can apply a wide range of materials.

Abstract In this study, a new physically-based finite element approach is proposed to model and predict the superficial oxide layer removal and the occurrence of localized metallic bonding during particle impacts. The process physics, based on explosive welding theory and experiments, and method implementation is presented. Prediction of critical velocity of copper is obtained and compared to experimental data to validate the model. Moreover, the model is also able to show the bonding locations at the interface between particles and substrate. The predicted bonding locations are consistent with experimental data from literature for several metals.

Abstract 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.

Abstract Recently, cold spray (CS) technology has attracted extensive interest as an alternative to thermal spray methods to build a coating, which uses high kinetic energy solid particles to impact and adhere to the substrate. To date, numerous numerical studies have been carried out to investigate the deposition processes and associated mechanisms during multiple particle impact in CS. However, in the commonly used numerical techniques, the individual powder particles are often treated separately from one another, thus fail to properly consider the adhesion mechanisms during deposition. In this study, we propose a new numerical approach on base of peridynamics (PD), which incorporates interfacial interactions as a part of constitutive model to capture deformation, bonding and rebound of impacting particles in one unified framework. Two models are proposed to characterize the adhesive contacts: a) a long-range Lenard-Johns type potential that reproduce the mode I fracture energy by suitable calibrations, and b) a force - stretch relation of interface directly derived from the bulk materials mode I fracture simulations. The particle deformation behavior modeled by the peridynamic method compares well with the benchmark finite element method results, which indicates the applicability of the peridynamic model for CS simulation. Furthermore, it is shown that the adhesive contact models can accurately describe interfacial bonding between the powder particles and substrate.

Abstract Thermal spray coatings are widely used to protect materials from corrosion, wear, and oxidation, but they have yet to reach their full potential because of porosity limitations and the detrimental effects of oxidation on interlamellar bonding. This paper investigates an atmospheric plasma spraying process that deposits oxide-free dense metallic coatings with well bonded lamellae. The process produces ultrahigh temperature metallic droplets, up to 2650 °C, using specially designed powders that are deoxidized in-flight through the evaporation or gasification of oxides. The impact of these oxide-free ultrahigh temperature droplets has a spreading-fusing, self-metallurgical bonding effect resulting in fully dense bulk-like metallic coatings. Various coating materials, including NiCrMo, 304SS-Mo, NiCrBSi, and Al, are investigated, demonstrating the versatility of the new technique.

Abstract Assemblies containing fiber-reinforced plastic (FRP) and metal parts are typically fastened together via mechanical joining or adhesive bonding. Mechanical joining processes tend to weaken FRP parts by cutting fibers, while adhesives require long cures and often lead to inseparable material compounds. This paper evaluates a new joining method in which plastic parts are laser treated, then metallized via wire-arc spraying, and finally soldered to mating metal parts using a low-temperature process. Due to the effective increase in interface area resulting from laser structuring, bond strengths of up to 15.5 MPa can be achieved.

Abstract In cold spray, high adhesion of soft materials on hard substrates is commonly achieved by using helium as the propelling gas. This is the case of copper coatings on steel where adhesion may reach values as high as 60 to 80 MPa (glue failure), however, helium is a limited, expensive natural resource, and the use of more abundant nitrogen gas is preferred in an industrial setting. Unfortunately, when using nitrogen gas, little to no adhesion is obtained. In order to eliminate the use of helium gas we studied how laser assisted cold spray could lead to an improvement in adhesion of nitrogen sprayed copper coatings. In this work, several laser parameters (e.g., power and spot size) and process parameters (traverse speed, relative position laser spot vs. gas jet) were varied at a coupon level. Upon optimization, an equivalent adhesion to the coatings prepared with helium was obtained. Furthermore, the cross section of the coatings showed that the copper particles penetrated the steel, similar to what is observed when using helium gas. Optimization of these parameters for application to large diameter (~559 mm) cylinders was also performed. A discussion on the mechanisms which contribute to achieving high adhesion considering the use of helium versus laser assistance is provided.

Abstract Metals often require coatings with wear resistant metals like ceramics or WC-Co for applications in mechanical engineering. Due to the different classes of materials of substrate and coating in these systems, the mechanisms of adhesion available are mainly mechanical interlocking bonding. In this paper a process is described, which makes use of these bonding mechanisms to apply mainly WC-Co coatings by a thermal sprayed process (HVOF). In order to assess the adhesive strength of the coatings a fracture mechanical test was developed. Furthermore it is shown that some reliable and numerical effective local energy method (crack closure integral method), which offer special advantages for mixed-mode fracture analysis of inhomogeneous materials, like WC-Co coated metals, work well successful. This is done by the finite element analysis of a modified compact tension specimen, the MOCH- specimen. In addition to that, the adhesive strength of the WC-Co layers is optimized by a variation the parameter of the thermal spray process. The results show different criteria of failure, as there is premature crack propagation depending on spread parameters and the damaging of the composite material accompanied by low energy release rates and low adhesive strength. Paper text in German.

Abstract The adhesion of the copper layer to the Al substrate depends on the presence of intermetallic phases. Such intermetallic phases can form during spraying at the boundary between the layer and the substrate. This paper deals with the formation mechanism of the intermetallic phases and their influence on the adhesion. The type, size, and distribution of the intermetallic phases are investigated as a function of the spray parameters. The adhesive strength of the layers is determined using the "Laser Shock Adhesion Test.". Paper includes a German-language abstract.

Abstract This paper aims to evaluate various measurement methods that characterize the elastic layer behavior. The use of thermally sprayed multilayer composite materials for mechanically and/or thermally highly stressed components has proven itself in a wide variety of areas of modern mechanical and plant engineering. The mechanical operating behavior of layered composite materials is determined by the interplay of residual stresses and load stresses as well as the existing bond strength. The determination of the Young's modulus in thermally sprayed coatings is, however, a difficult task because of the limited coating thickness, their brittleness and the very inhomogeneous microstructure consisting of nonequilibrium phases, oxidation products, cracks, and pores. Two different investigations routines were performed, first the investigation of the influence of the indentation load on the determined Young's modulus and second the influence of the analysis program. Paper includes a German-language abstract.

Abstract This article reports on a method for densifying thermally sprayed layers by arc plasma sintering (SPS). Thermal spray ceramic coatings consist from relative high percentage of porosity compared than that of metallic coatings. An investigation of the metallurgical and mechanical properties of flame-sprayed zirconia coatings after SPS at various temperatures and loadings condition was performed. The results obtained in the present work can be summarized as follows: bond strength and microhardness of coating after SPS is increased by three times compared than as-sprayed coating, and X-ray diffraction analysis indicates that the phase of zirconium dioxide-25MgO coating was influenced the physical properties of coating. Paper includes a German-language abstract.

Abstract Within this project two types of cored wires, AI+AI2O3 and Al+Cr2O3, were manufactured. Both wire types consist of an Al-sheath (tube) and mechanically alloyed Al+Oxide powder core. The mechanically alloyed powder Al+Cr2O3 forms a self propagating high temperature synthesis (SHS) during the spraying process. The very high reaction enthalpy of the exothermal process increases the temperature of sprayed particles what, in consequence, can positively influence coating properties such as bonding, cohesion and wear resistance. In consideration of these circumstances special tube cored wires with an Al-sheath and Al+Al2O3 and Al+Cr2O3 mechanically alloyed powders were manufactured. As spray techniques, a conventional, high velocity flame and plasma wire spray process were used. Coatings constitution was characterised using optical and scanning electron microscopy. The structure of coatings was described using X-ray examinations. Paper text in German.

Abstract For an industrial application is a coating for a bearing to define. In this paper it will be reported about the choice of the coating material, the selection and qualification of the thermal spraying method. The recommended coating will be characterized by metallurgical cross section pictures. Additional evaluation will be offered such as bond stregth mesurements and corrosion resistance tests. Paper text in German.

Abstract This paper discusses the relationships between the splat structure and the cohesive and adhesive forces of the layers. The shape and size of splats were examined with a quantitative scanning electron microscope (SEM) examination of the layer surface. Surface morphology was characterized by the distribution of shape and dimension of each splat from the SEM images. The morphologies were quite different among the splats at the center of the spray pattern and those at the peripheral area. The splat morphology at the center of the spray pattern was consisted of the wide spread irregular shaped splats. Cohesive-adhesive strength of coatings sprayed with ordinary process and with the particles passing through the central area of spray pattern were compared. Results obtained show us the splats sprayed with the slit have superior cohesive-adhesive properties compared with those passing through peripheral area of the spray pattern, as expected. Paper includes a German-language abstract.

Abstract Thermally grown oxides (TGO) form at interfaces between the upper thermal insulation layers (TBCs) and the bond coats during operation. Macro cracks can be observed in many places in the TGO. The formation of macrocracks would reduce adhesion or lead to areas of stress peaks. To improve the adhesion, a modified powder for the boncoat was developed by adding cerium and silicon to the conventional MCrAlY. This paper investigates two kinds of TBC, which are a conventional CoNiCrAlY and a developed CoNiCrAlYCeSi bond coats. These TBCs are compared with regard to the oxidation behavior and the bond strength of the interface. To measure the adhesive strength, four-point bending tests are carried out with the normal TBC and the TBC with a modified bond coat. It is observed that the modified variant has a higher adhesive strength. The reason for this could be a reduction in the formation of mixed oxides. Paper includes a German-language abstract.

Abstract This paper examines the bonding mechanisms of high-speed flame spraying (HVOF) sprayed coatings on the basis of the experiments with the substrate at different surface roughness. In order to determine the dominant adhesion mechanisms of HVOF layers, the layers are applied to substrates of different roughness with completely melted powder particles as well as with powder particles with solid and liquid phases. The spray materials are NiCrBSi and WC-Co. The results of adhesion tests showed that the adhesion of the NiCrBSi layer on the roughened substrate was approx. 40 MPa, while there was no adhesion on the polished substrate. In contrast, the adhesive strength of the WC-Co layer on the polished substrate was already between 20 and 40 MPa. The adhesive strength of the WC-Co layers on the substrate with a roughness greater than 5.8 micrometer already exceeded that of the binder materials. Paper includes a German-language abstract.

Abstract Within the atmospheric plasma spray process the main spray parameters which determine coating properties are the thermal and kinetic energy of the spray particles at impact on the surface of the substrate. Especially the thermal energy of the particles is influenced by plasma flame properties and the interaction between the plasma and particles. In general Ar-H2- and Ar-He-mixtures are used in the atmospheric plasma spraying process. Varying the composition of the plasma gas by adding He, respectively H2 leads to different behaviour of the plasma flame especially in terms of viscosity, thermal conductivity and enthalpy. Using ternary mixtures it is possible to obtain improved coating properties for certain oxide spray materials. Al2O3-TiO2- and ZrO2-8Y2O3-coatings were examined using Ar-H2-mixtures in comparison to SPRAL 22 (Ar, H2, He) gas from Air Liquide. For the materials improved coating properties regarding bond strength and porosity were verified and an increased deposition efficiency was determined at the same time.

Abstract HVOF cermet layers are increasingly viewed as an alternative to galvanic hard chrome layers for applications with high demands on wear and corrosion protection. The WC-Co-Cr layer is one of them. The WC-Co-Cr layers were produced with different HVOF systems: Jet-Kote, Diamond Jet, and JP5000. With all three systems, dense and well-adhering layers could be produced. This article considers HVOF sprayed WC-10Co- 4Cr as a generic coating type for replacing hard chrome plating. The friction, wear, adhesion, and corrosion performance of the HVOF WC-10Co-4Cr coatings are compared against the performance of a commercial hard chrome coating system. The article also evaluates effect on the coating adhesion of corrosion through the coating and penetrating along the interface at the coating edge. Paper includes a German-language abstract.