Cold spray (CS) is a solid-state process for depositing metal powder, accelerated by a high-velocity gas such that it bonds to the substrate metal through kinetic impact energy. Although the technology is finding applications in non-load bearing repair and coating applications, work is needed in the quality control procedures for CS for its use in load bearing structural applications. in this study, the viability of electrical conductivity and through thickness ultrasound wave velocity measurement methods are studied to serve as a means for nondestructive quantitative measurement methods for quality control in CS and potentially other additive manufacturing (AM) methods. Eddy current, ultrasound, porosity, hardness, and uniaxial tensile strength tests were conducted on copper and aluminum samples that were manufactured using CS. Ultrasound measurements of longitudinal wave velocity and eddy current electrical conductivity measurements showed good correlation with process conditions that were varied to control particle velocity to intentionally produce samples with varying deposition quality. Influence of process conditions on particle velocity was confirmed via particle image velocimetry. Porosity, hardness, and tensile test results were further correlated to ultrasound wave velocity and electrical conductivity measurements. The results of this work show that nondestructive testing methods can be effectively used to quantitatively assess the cold spray products for quality control purposes.

This work provides a new in-situ measurement method for the analysis of the spray-spot geometry and the thermal properties of the coating. The new approach is based on infrared detection of the thermal radiation from the coating surface combined with a subsequent automated spray-spot characterization. With this method it is possible to describe the geometry, the axis-position of the torch, the powder injection properties, and the temperature distribution in of the spray-spot. Especially for the automated production in high quantity the spray-spot analysis is a useful assistance for the operator because the detector reacts very sensitive on small changes of the process conditions. With regard on important fields of application (e.g., gas turbine production) the sensor is suitable to detect drifting spray system parameters. Also, the progression of wear at the nozzle, injector and electrode can easily be estimated. In recent research the in-situ spray spot analysis is being developed further for the characterization of multipair electrode plasma generators.

Boundary layers on surfaces will change from laminar to turbulent flow after a critical length. Due to the differing heat transfer coefficients of laminar and turbulent flow, the point of transition can be detected by heating the surface and measuring surface temperature by thermographic imaging. Locating the transition point is crucial for the aerodynamic optimization of components. In this study, fiber reinforced polymer composites (FRPCs) were chosen as the test substrate. Experiments were conducted using the flame spray process and NiCrAlY coatings. Multilayered coatings consisting of an aluminum bond coat, a layer of alumina as electrical insulation, and a heating layer of titania were fabricated by atmospheric plasma spraying. Free-flight tests were conducted with a functionalized winglet in order to assess the ability of thermally-sprayed heating elements to detect the location of transition of the flow regime. The results showed that the thermally-sprayed elements heat surfaces uniformly, with sufficient radiation losses for thermographic imaging. It was also shown that the change in temperature at the point of transition was readily observable using thermography.

YSZ coatings were deposited by suspension plasma spraying and a parametric study was performed with different process parameters. Afterward, the porosity of the as-prepared coatings was investigated by SEM imaging and X-ray transmission and a multivariate analysis of the collected data was carried out. The results show that total porosity correlates negatively with suspension mass load, but positively with original powder size, spray step, substrate roughness, and spray distance, which was found to have the greatest impact. A porosity prediction model was also developed and its practical use is discussed.

In this study, hydroxyapatite, titania, and HA-TiO 2 composite layers are deposited by suspension plasma spraying on titanium substrates and assessed by means of SEM and XRD analysis, Raman spectroscopy, and acoustic emission testing. The coatings exhibited dense microstructures with low porosity and good interfacial bond strength. The main phase in the HA and composite coatings was found to be similar to the peak of the feedstock powder. In the composite and titania coatings, besides rutile and anatase, a significant percentage of thermally stable Ti 3 O 5 was observed, which is favorable for photocatalytic performance.

This study focuses on the relationship between porosity and leak tightness in cold-sprayed aluminum. Aluminum coatings with 0.2-9% porosity were produced by cold spraying and evaluated via helium leak testing. Multiscale porosity was determined through SEM and TEM analyses and shows good correlation with leak test results. The mechanisms involved in the creation of porosity were investigated as well through finite element analysis of single and multi-particle impacts.

The alloys CuAl9Ni5Fe4Mn and CuMn13Al8Fe3Ni2 were arc-sprayed with a spiral-shaped pattern in this work, using both pressurized air and a mixture of nitrogen and hydrogen. Process temperatures were recorded by thermographic imaging and residual stresses were measured by modified hole-drilling method. Moreover, analyses of the cavitation erosion behavior and other properties were carried out. It was found that a change in the spray pattern can strongly reduce residual stresses and material loss by cavitation erosion.

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.

In light weight constructions, research focuses more and more on ecological aspects of materials. In this way vulcanized fiber relive a renaissance because it is produced from renewable raw materials. Vulcanized fiber is a composite material, which was approximately discovered in 1855. It is manufactured by parchmentizing of pulp with the use of a zinc chloride solution. This material is well known for its good strength to weight relationship, but it is susceptible to the contact with aqueous fluids and also shows only a low resistance to wear. Therefore, a functionalization of the surface is needed. The functionalization can be realized by thermal spray processes to increase the potential of vulcanized fiber. Especially the pretreatment of the specimen to enable the coating application is one of the key topics of this paper. Vulcanized fiber is a thermal sensitive material. The impact of hot spray materials can cause undesired reactions such as the combustion of fibers on the substrate’s surface. To reduce this effect, twin wire arc spraying and low pressure cold gas spraying processes were used to apply low-melting materials (copper, zinc) onto the specimen. Thereby, the influence of the coating on the vulcanized fiber surface was investigated. In addition, non-destructive testing methods were evaluated to gain information about the vulcanized fiber.

In order to guarantee their protective function, thermal sprayings must be free from cracks, which expose the substrate surface to e.g. corrosive media. Cracks in thermal sprayings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross-sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology as well as coating defects were inspected using light microscopy on metallographic cross-sections. Additionally, the resulting crack patterns were imaged in 3D by means of X-ray micro-tomography.

Ceramic coatings are applied in mechanical components subject to high temperature conditions, normally are deposited by plasma thermal spraying process. In this work, the porosity of YSZ ceramic coatings, deposited with different parameters conditions were analyzed by optical microscopy, scanning electron microscopy using back-scatter electron (SEM-BSE) detector and ultrasonic technique. It was verified that porosity measurement by optical and scanning electron microscopy is very sensitive with respect to metallographic preparation, mainly cutting process, and gray level adjustment. SEM-BSE technique showed less scatter results with easier porosity visualization, compared with optical microscopy. The porosity of the coatings was also measured by ultrasonic technique. It was observed that the ultrasonic velocity increase with porosity reduction. Ultrasound technique showed a good correlation with OM and SEM porosity measurement.

The aim of this study is to determine how various factors, including process parameters and nozzle configurations, affect the shape and size of the spray jet in twin wire arc spraying. In the experiments, steel specimens were sprayed using an iron-based cored wire with a fused tungsten carbide filling. In-flight particle temperature and velocity and fluctuations in voltage and current were measured during spraying. The shape of the thermal spray spot and the 3D footprint of the plume were determined by means of image analysis and tactile surface profiling methods. The results obtained show that spray plume characteristics, and thus particle distribution, are heavily influenced by secondary gas flow, particularly the number, location, and angle of atomization outlet holes in the secondary gas nozzle.

This paper presents a thickness measurement method that can be used during thermal spraying. The new method is based on photogrammetry and image reconstruction and is able to measure complex 3D shapes with continuous contours. Initial results demonstrate the nondestructive nature of the method as well as its accuracy, versatility, and speed.

In this study, acoustic emission sensing is used to monitor interfacial cracking in thermal barrier coatings during uniaxial tensile adhesion testing. The TBCs consist of a ZrO 2 topcoat and a NiCrAl bond coat, both of which are applied by atmospheric plasma spraying. Tensile testing was performed to failure and the resulting fracture surfaces were examined by SEM and XRD analysis. Experimental results show that cracks usually initiate in the ceramic layer then propagate toward the metallic-ceramic interface where failure occurs. Finite element simulations were also conducted, confirming the experimental findings.

This study demonstrates an experimental setup in which acoustic emission sensing is used to monitor a twin wire arc spraying (TWAS) process. Emitted acoustic signals were recorded by broadband sensors attached to the spray nozzle and mounted under the substrate. Sensor outputs were converted from the time domain to the frequency domain by fast Fourier analysis. Acoustic emission amplitude plots were produced and are correlated with gas pressure, arc voltage, in-flight particle velocity and temperature, coating thickness, and crack formation due to cooling.

To fully understand particle interactions with both substrate and neighboring particles in cold spray, ultrasound wave generation and ultrasonic monitoring of particle impacts were studied. The multi–channel ultrasonic system works in pulse– echo and passive modes. In pauses between pulse–echo data acquisition frames the system is in the passive mode receiving the signals generated by the particle impacts. The particles being deposited generate the ultrasonic signals in a very wideband frequency range at more than 40 dB signal-to-noise ratio. The particle impact signals are considered as a sum of the background wideband noise and the sparse strong pulses observed when the nozzle passes over the location of the transducer. It was shown that these components of the particle impact signal can be separated by threshold processing. The results of the passive monitoring are confirmed by the ultrasonic pulse–echo and direct measurements of a deposited coating geometry.

One of the greatest obstacles for a wide distribution of thermal spraying techniques is the lack of online control over the spraying process. The thermally sprayed coatings are optimized by an empirical modification of the spraying parameters and the subsequent correlation of these parameters to the obtained coatings. Some intrinsic parameters, such as the fluctuations in twin wire arc spraying and wear in the atomization nozzle, are not adjustable. Even though they have an enormous impact on the obtained coating quality, they are often scientifically neglected for reasons of simplification. In this work, acoustic emission analysis is utilized to study the effect of uncontrollable parameters on acoustic signals. In order to enable an easy determination of the changes in the acoustic signals, the acoustic sensors were mounted on the spraying nozzle as well as on the substrate. At increased current, a lower acoustic emission is recorded. A correlation between uncontrollable parameters, the acoustic signals, and the obtained coating quality was observed. This research contributes to the online control of the spraying process.

For the development and quality control of highly electrically conductive coatings, a device is required by which the electric conductivity can be measured. For this purpose a handheld device for measuring the electric conductivity of nonferrous metals in a nondestructive manner was tested. The measurement principle is based on an eddy current sensor which allows determining the electric conductivity within seconds. The method fulfills the demands for using it in the environment of a job shop for thermal spraying. Coatings applied with different thermal spraying methods like cold gas, HVOF, electric arc or flame spraying have been examined. Thus, it will be presented a comparison of the electric conductivity dependent on different spraying methods. Additionally, important edge conditions for spraying and measuring the conductivity of highly electrically conductive coatings like the influence of the oxygen content of the powder, the minimal coating thickness measurable with the device and the influence of the surface roughness onto the measurement were analyzed.

This work assesses the potential of using an ultrasonic probe attached to the back of the substrate to monitor the cold spraying process. While this is only a preliminary study, focusing more on presenting the results than analyzing them, a few conclusions may be drawn. With acoustic sensing, not only can the final value of thickness be estimated, it is also possible to see the dynamics of how the buildup takes place in real time. As shown in the data plots, the buildup process for aluminum-alumina composites is fairly universal across the spray with slower buildup at the outer edges of the coating. More importantly, it is shown that nozzle speed, spray diameter, and thickness estimates fit well with measured values.

Near net shape coating is a trend in thermal spraying being aimed at for several years to lower costs through shortened spray time and reduced after treatment work. Because of better microstructure compared to conventional coatings, the thickness is often also reduced. To characterise the quality of those thin layer systems the standardized tensile adhesive test is not suitable, as the adhesion outweighs the cohesion by far due to reduced quantities of coating defects. To characterize the coatings behaviour under tensile stress, three-point bending tests were performed. As especially thin cermet coatings do not show abrupt catastrophic failure in bending tests, ultrasonic signals being emitted from cracks propagating through the coatings were taken during the tests. Three coating systems (Cr 2 O 3 , Cr 3 C 2 -NiCr and WCCoCr) were investigated in this work using fine grained feedstock powders with three different size distributions per system. The tests showed the negative influence of stress moments in case of the oxide ceramic and the embrittlement of the cermets, especially the Cr 3 C 2 -NiCr, with increasing spraying temperature regime due to increased formation of solid solutions and decarburization. It can be said, that the analysis of acoustic emission during bending tests gives valuable information about how to achieve thin wear and corrosive protecting coatings being qualified for high operational demands.