Atmospheric plasma spraying (APS) is characterized by complex interactions between input, process and output variables. Process control relies heavily on human expert knowledge and experience. Process diagnostics can provide additional information to the operator and support cognitive processes in task execution. When using non-cascaded torch systems, significant plasma fluctuations occur, affecting the coating quality. High-frequency fluctuations can only be detected by suitable diagnostic systems and interpreted by experienced APS operators. In this study, the state of the plasma jet (area, fluctuation) is investigated depending on total plasma gas flow rate (50 vs. 65 l/min) and the H 2 content of plasma gases (17, 20 and 23 vol. %) using high-speed camera pictures. To evaluate plasma fluctuation effects, particle temperature and velocity as well as resulting coating properties (thickness and porosity) are determined for two ceramic systems. The results show that fluctuations of the plasma jet have a significant effect on the particle state and coating quality. The use of a high-speed camera to evaluate the stability of the plasma jet is an attractive method that, when properly integrated, has the potential to provide the human operator with important information to allow rapid assessment of input parameters or the condition of the plasma torch.

In order to improve the quality of coating, the causes of defect are analyzed in this paper. The research innovative uses single factor experiment combined with MATLAB to fit out the relationship between porosity and laser power, scanning speed, thickness, overlap rate. The multivariate quadratic equation is derived in this paper. It provides a solution to avoid defects in the aspect of laser cladding process.

The scanning acoustic microscopy (SAM) technique is used for studying the character and interface quality of cold sprayed Fe coatings deposited onto notched Al-based substrates. Three notch geometries were used: a rectangular notch, a trapezoidal notch with a flat bed, and a trapezoidal notch with a cylindrical bed. Scanning electron micrographs demonstrated an increased porosity and cracks at the areas where the spraying direction was not perpendicular to the surface of the substrate. The SAM measurements were then performed on thin plates cut vertically across the notches such that the scanned area included the locations of the increased porosity and their surroundings. The resulting distributions of longitudinal wave velocities and their attenuation revealed that the affected area is more complex and the mechanical response of the coatings could be limited not only at areas of the visible porosity, but also in their vicinity.

Additive Manufacturing (AM) processes offer geometrical freedom to design complex shaped parts that cannot be manufactured with conventional processes. This leads to new applications including aerospace propulsion systems where the Ni-superalloy based material has to withstand high operating temperatures. In this contribution, the influence of heat treatment and surface conditioning of the additively manufactured Inconel 718 substrates on the thermocycling performance of suspension sprayed YSZ coatings was investigated. The different surface conditions included as-built, sandblasted and milled substrate surfaces with and without heat treatment. YSZ coatings were applied using suspension plasma spraying (SPS) with commercial available suspensions. Thermal cycling tests (FCT) at 1100°C, 1300 °C, and 1500 °C were applied to coating systems until failure occurred. The microstructures of the samples were characterized before and after thermal cycling. The performance of the coatings was mainly influenced by the coating morphology and FCT test conditions and less by the state of the AM substrates. Columnar-like YSZ SPS sprayed coatings on AM Inconel 718 substrates seemed to be a promising candidate for rocket engine applications.

The development of thermal spray processes usually requires an analysis of the complex coating microstructure. In order to inspect critical areas of a coating, destructive testing methods such as the preparation of cross-sections are commonly used. In this work, the suitability of largely non-destructively measured polarisation curves for the quality assessment of thermal sprayed AISI 316L coatings is investigated. Therefore, a 3.5 % NaCl gel electrolyte was developed to prevent the corrosive medium from infiltrating the porous and micro-cracked microstructure characteristic for thermal sprayed coatings. In addition, a measuring cell based on the 3-electrode arrangement was designed to simplify the setup, reduce the measurement time and enable mobile measurements directly on the component surface at a later stage of development. The effects of process-related differences in the microstructure of HVAF and APS AISI 316L coatings on the polarisation curve was investigated by determining the corrosion current density. The microstructure of the AISI 316L coatings was analysed by optical microscope, SEM and EDS, focussing on the porosity and oxide content. The results clearly show that the potentiodynamic polarisation curves of the AISI 316L coatings differ significantly depending on the spray process used and microstructure created. Even small changes in the oxide content within a coating can be detected. Therefore, electrochemical measurement methods using gel electrolyte offer an interesting opportunity to evaluate the quality of thermal sprayed AISI 316L coatings in a largely non-destructive manner.

In this paper, a diffusion kinetic model was applied to simulate the microstructure development in a MCrAlY-superalloy system at high temperatures. Both simulation and experimental results showed that γ+γ’ microstructure was obtained in the coatings due to Al depletion after oxidation. With the help of the modelling, the mechanism of the formation of the diffusion zones in the single crystal (SC) superalloy can be also analyzed. The results revealed that the inward diffusion of Al from coating affected the depth of secondary reaction zone (SRZ) with the precipitation of TCP phases while the depth of inter-diffusion zone (IDZ) was decided by the inward diffusion of Cr.

Thermally sprayed ceramic coatings can be used for wear protection as well as thermal and electrical insulation. When exposed to environments with high humidity, the water absorption of the ceramic coating has a tremendous impact on the electrical insulation. In thermally sprayed ceramic coatings, water can easily be absorbed by the porous microstructure of the coating. A general result of the water absorption is the reduction of the dc resistivity. However, in the high frequency regime of ac loads, contrary results were observed for sealed Al 2 O 3 coatings on steel substrates. Specimens exposed to high air humidity have shown an increased ac resistance compared to dry specimens if frequencies above 1 MHz are considered. To analyse this phenomenon, a novel measurement technique was developed to investigate the influence of the water absorption of detached ceramic coatings on the ac resistivity at high frequencies. Moreover, the water absorption of the ceramic is measured gravimetrically. To ensure the results are also applicable to ceramic coatings on substrates, the morphology of the coating was analysed using electron microscopy and compared to reference specimens deposited on steel substrates from [1].

The advantages of UV-curing polymers are well known and used in various coating and adhesive applications. Curing times of a few seconds and long application windows allowing an increased throughput in series production. The use of UV-curing polymers in sealers is beneficial, but so far insufficient due to only surface curing. With a newly developed dual-cure mechanism in sealers, it is now possible to combine deep penetration curing and surface curing. The hybrid sealers combine radical polymerization with subsequent polyaddition or polycondensation. The development of sealers for thermal sprayed (TS) coatings involves an extensive requirement profile. This includes properties such as corrosion protection, penetration depth and processing times. High penetration depths of the sealant into the coating system are important to ensure a protection over the full lifetime of the TS coatings. The depth of penetration of the developed sealers into various TS coatings was determined by measuring the gas permeability in a specially developed test procedure. The corrosion protection effect in combination with TS coatings was determined by measuring the cell voltage. In summary, two UV dual-cure sealers have been developed to seal TS coatings with deep penetration and corrosion protection.

Thermally sprayed coatings can be used in structural health monitoring devices where the coatings can reveal defects in the real-time integrity of the component through changes in mechanical, thermal or modal properties during service. In this emerging application, the mechanical properties of the coating are strongly affected by the interfacial bond between the coating and the substrate. This paper presents an analytical study of the interfacial stress distribution based on piezoresistive-stress constitutive relation of a coating layer. Both a single layer coating- and a bilayer coating-substrate system were considered. An analytical solution of the interfacial stress was developed by solving a Fredholm-Volterra singular integro-differential equation of a coating-substrate model using Chebyshev polynomials. Numerical simulation was conducted to analyze the effects of geometric and effective material properties of the coating-substrate system on the interfacial stress distribution. It was found that the susceptibility of the piezoresistive layer to delamination primarily relies on thicknesses of the coating layers and the stiffness of the intermediary insulating layer and substrate.

Hydraulic turbines, valves, and pumps operate in environments where they are exposed to cavitation phenomena and corrosion, which can result in mass loss, leading to reduced performance and failure. HVOF spraying has been used to repair eroded surfaces on such components and new alloys are being developed to reduce repair costs. This investigation assesses the cavitation resistance of FeMnCrSiNiB alloy coatings deposited by HVOF spraying. Corrosion rates and oxidation potentials are measured under different conditions and compared to stainless steel coatings normally used on water turbine runners.

Aluminum castings have limited strength and stiffness and tend to exhibit brittle fracture behavior under fatigue loading. These properties can be significantly improved, however, as this study shows, by reinforcing cast aluminum parts with magnesium metal-matrix composite structures. In order to obtain a bond between the cast Al and fiber-reinforced Mg composite surfaces, Al alloy (Al 99 and AlSi 12 ) layers were deposited on the Mg structures by thermal spraying. The mechanical properties of the bonding were assessed via single-lap shear and adhesive tensile tests along with optical microsection analysis. Hybrid aluminum alloy AlSi 10 Mg castings incorporating coated Mg-MMC inserts were also produced and examined, validating the general approach, while revealing that heat input to the MMC structure must be reduced through design or process adaptations.

Nondestructive Evaluation and Testing (NDE&T) techniques have been played vital roles in property characterization, process development and quality control of various thermal spray coatings. Besides conventional NDE&T lab methods such as eddy current test (ECT) for thickness measurement and fluorescent penetrant inspection (FPI) for cracking detection, some latest NDE techniques have been developed, demonstrated and applied to evaluate and characterize thermal sprayed coatings recently. The improved and innovative NDE methods provide more capable and accurate measurement to inspect on surface morphology, 2D and 3D coating porosity, oxide content, interface debonding, as well as other types of coating features, defects or specific properties. In this work, some non-contact NDE techniques and their applications were investigated and discussed based on several case studies of thermal sprayed coatings. Laser confocal microscopy had been used for characterizing surface morphologies and roughness profiles of HVOF WC-based coatings with 2D and 3D mapping methods. In particular, thermal wave imaging and ultrasonic micro imaging methods were used to detect the suspicious existence of lateral coating separation within or at the MCrAlY coating-substrate interfaces. Laser dimension sensoring method exhibited the extended capability of in-situ coating thickening measurements on turbine blade and vane. The latest non-contact NDE techniques demonstrated their unique and strong capability for in-situ and ex-situ coating characterization, process and quality control and coating failure analysis.

This paper reports on the performance evaluation of stainless steel (SS) thermal spray coatings aimed at shielding lightweight aluminum (Al) brake rotor disks from excessive heat and providing an adequate tribological surface in contact with brake pads. Coating wear, corrosion and heat resistance performances were evaluated using pin-on-disk, cyclic corrosion tests and thermal cycling using a custom laser rig, respectively. Arc spray optimized coatings displayed lower or equivalent wear rates when compared with the baseline gray cast iron disks, with similar frictional behavior. However, arc spray coating exhibited low adhesion which limits the maximum coating thicknesses achievable and leads to early coating spalling after about 1000 thermal cycles. Arc sprayed coatings also corroded and delaminated under corrosion tests. Optimized cold spray coatings present high corrosion resistance and could resist above 10,000 thermal cycles without spalling. However, cold spray coatings exhibit wear rates at least 4 times those of the cast iron. Taking advantage of both types of coatings, it was found that the production of a duplex coating made of a cold spray bond coat and an arc spray top coat could meet the requirements for protecting Al disks, with near 50% weight reduction.

Plasma sprayed coatings of Raney nickel alloys developed as electrodes for hydrogen evolution electrodes in alkaline media, exhibit poor resistance to electrochemical erosion. The aim of this work is to develop an understanding of the correlation between plasma spray process parameters and coating quality and with that improve the electrochemical performance of the coatings. Air plasma spraying with TriplexPro gun was performed using NiAlMo powders. Plasma parameters were varied and particle inflight velocity and temperature was measured by Accuraspray. Coatings were developed for conditions in which particles in-flight temperatures were comparable but in-flight velocities differed. Electrochemical tests were performed for evaluating the effect of different velocities on electrode performance. Coating attained with particles having higher velocity exhibited better electrochemical performance and durability. The microstructure and elements map before and after the electrochemical test performed by SEM and EDX show that the coatings with higher velocity particles led to microstructure that enabled better activation of the electrodes and higher surface area for reactions.

High manufacturing costs and long-term degradation are the main problems that have become a “bottleneck” and impeded SOFC’s further development. It is well known that a high operating temperature is the major cause that leads to these problems. As such, reducing the operating temperature becomes a hotspot of research. It has been reported that a uniform and dense coating can be prepared by using very low pressure plasma spraying (VLPPS) technology. The current study focuses on VLPPS for application in large-area (~100 × 100 mm) porous metal supported solid oxide fuel cell (MSSOFC), especially for the preparation of the electrolyte. It was found that the densification of the electrolyte was very good, as confirmed by the open-circuit voltage (OCV) of the cell. In the temperature range of 550~750°C, the OCV of the cell stabilized between 1.05 V and 1.1 V. The power density of the cell was also measured.

Boiler tube failure is the number one source of forced outages in all coal-fired and biomass-fired power generation plants. It is estimated that plants lose approximately 6% of their power generation annually, due to boiler tube leaks. The major causes for premature tube failure are excessive fireside boiler tube erosion and corrosion caused by impact, abrasive wear, oxidation and molten corrosion of low eutectic alloys.

Well-trained personnel is decisive for a successful company. International business relations are increasingly demanding internationally recognised qualification and certification measures. The requirements are stringent particularly in sensitive fields, e.g. aviation and rail vehicles. No matter whether in aircraft engines, power station engineering or the printing and paper industries, numerous requirements on coatings are solved with the aid of thermal spraying. However, in order to be able to master what the language in standards calls a "special process" (i.e. a process whose final result can be checked by means of non-destructive testing to a limited extent only), there is a need for calibrated and monitored installations and parameters as well as for correspondingly experienced, trained and certified personnel. Frequently, ETSSs, ETSs as well as the qualification tests according to ISO 14918 are already integral constituents of contracts relating to thermal spraying (requirement of the German Railways according to DIN 27201-10). The article provides detailed information about training and qualification as well as about the certification of plants and personnel in the field of thermal spraying. Not only the significance of a quality management system but also the particular advantages of the GTS certification are highlighted in this respect.

The HVAF process provides a slightly different (to HVOF and other thermal spray processes) and unique combination of thermal and kinetic energy, which presents a technical challenge to material manufacturers who have over the past years optimized their material offerings to the demands and characteristics of plasma, combustion and more recently the various HVOF processes. The recent development of HVAF ID torches, capable of applying the highest quality coatings into dimensions of less than 100mm (4”), has provided a further need to optimize spray materials for the best possible interaction between the spray device and the feedstock in order to meet developing coating specifications from various industries. This paper will discuss the methodologies utilized from the points of view of torch design, material manufacture, spray parameter and procedure optimization, as a means of producing the highest performing coatings to meet specific industry aspirations.

Experience has shown that qualified supervisors and skilled workers are essential for manufacturing premium-quality thermal spray products. This paper describes the training defined by EWF guidelines and reviews prevailing curricula. Changes and reforms associated with DIN EN ISO 14922 and ISO Norm 14918 are also presented along with emerging trends in thermal spray training and certification.

Coating adhesion has a major effect on the performance of thermal spray coatings and is in large part determined by the chemical, physical, and mechanical properties of the substrate surface. This paper reviews some of the work that has been done to optimize the surface of different materials for thermal spraying. It covers a number of substrate preparation methods, including grit blasting, laser ablation, laser surface texturing, and dry ice blasting, and explains how different process variables affect surface roughness and bond strength.