Coating thickness is considered to be one of the most important characteristics of thermally sprayed coatings. Therefore, it has long been the goal to be able to control it. This could be achieved by implementing an online, closed-loop control. A prerequisite for such a control mechanism is a feedback signal of the coating thickness with sufficiently small measurement uncertainty. Optical distance measurement techniques have been demonstrated in the past to produce promising results for such applications. This paper analyses the measurement uncertainty of an optical distance measurement technique based on confocal distance sensors used for in situ coating thickness evaluation. As an alternative, pneumatically actuated length gauges are also used for the in situ measurement. Both techniques are applied during atmospheric plasma spraying of samples in a carousel setup. The two sensing techniques are compared with a reference, destructive coating thickness measurement method. Pros and cons of using different in situ coating thickness measurement techniques for process control applications are discussed.

The piezoresistivity of flame-sprayed NiCoCrAlTaY on an electrically insulated surface of a steel substrate was investigated through cyclic extension and compression cycles between 0 and 0.4 mm for 1000 cycles and uniaxial tensile test. The sprayed NiCoCrAlTaY was in grid form with grid thickness of 3 mm and grid length of 30 mm while the electrical insulation was fabricated by flame spraying alumina on the surface of the steel. During mechanical loading, instantaneous electrical resistance measurements were conducted to evaluate the corresponding relative resistance change. Images of the loaded samples were captured for strain calculations through Digital Image Correlation (DIC) technique. After consolidation of the pores within the coating, the behavior of the flame-sprayed NiCoCrAlTaY was consistent and linear within the cyclic compression and extension limits, with strain values of approximately -1000 με and +1700 με, respectively. The coating had a consistent and steady maximum relative resistance change of approximately 5% within both limits. The tensile test revealed that the coating has two gauge factors due to the bi-linearity of the plot of relative resistance change against strain. The progression of damage within the coating layers was analyzed from its piezoresistive response and through back-scattered scanning electron microscopy images. Based on the results, the nickel alloy showed high piezoresistive sensitivity for the duration of the loading cycles, with little or no damage to the coating layers. These results suggest that the flame-sprayed nickel alloy coating has great potential as a surface damage detection sensor.

When testing the thermal cycling resistance of thermal barrier coatings, the surface temperature of the materials must be controlled so that test results can be used for coating life prediction. In this study, the temperature at the surface of plasma-sprayed TBCs was controlled during thermal shock testing using feedback from a double-color IR thermometer and high-rate cooling. The results are presented and discussed, highlighting the capability of the recently designed thermal shock test.

The residual stresses within plasma sprayed coatings are an important factor which can influence the lifetime of the coatings. The investigation of the evolving stresses during deposition and post-deposition cooling of YSZ coatings by measuring in-situ the specimen’s curvature with the so-called ICP sensor is a powerful tool to identify the different stress generation factors. Under certain spray conditions one can observe that the first torch pass leads to a significantly higher increase in specimen´s curvature than the following deposition passes, which indicates significantly higher stresses within the interface coating region. The reason for this steep curvature increase was investigated. It is suggested to be a combination of a stronger bonding of those splats being connected directly to the substrate and the relief of compressive stress within the substrate. The slope of this increase depends on the spray parameters as well as on the substrate conditions, which was investigated also.

This paper discusses the different ways plume sensors can be used in thermal spraying and presents examples highlighting the use of a commercial plume sensor in actual production situations.

In this paper we proposed a rapid and continuous process for the production of nanoporous coatings for functional applications. Experiments following two statistical designs were implemented to screen and investigate the spraying parameters’ effects on coating crystallinity and porosity to gain a better understanding. The spraying stand-off distance, solution flow rate and arc current were identified as having significant effects on coating porosity and crystallinity. The investigation yielded a microstructure comprised of interpenetrating pores and layered structures with embedded nanopores. A deposition mechanism was postulated to explain this peculiar microstructure. Gas sensors constructed from the coatings had ethanol sensitivities at room temperature comparable to those reported in the literature for conventional thick-film coatings and a maximum sensitivity near 200°C.

The University of Limoges developed a compact sensing module that measures particle beam properties and substrate surface temperatures during plasma spraying. In this paper, the authors explain how they built and tested a closed-loop controller for APS processes using the sensing module. One of the key elements in the online control system is an empirical model that relates process inputs, in-flight particle parameters, substrate temperatures, and coating properties. The paper discusses the development and implementation of the model and the determination of the main input parameters. Paper includes a German-language abstract.

The HVOF process with reduced heat effect on the substrate and therefore minimal degradation of fatigue properties is now finding wide application in fatigue critical applications. The critical parameters for process control are residual stress in the deposit and maximum substrate temperature. Quality control tools for these parameters are deflection of Almen Strips (similar to shot peening) for simulating residual stress and the use of infared pyrometry for temperature measurement. Both of these methods are technically sensitive particularly in spraying of coupons to evaluate the effect of coating on material properties. Lessons learned will be presented and recommendations made for applications of these tools in controlling the HVOF process.

In this paper, we describe a new sensor for monitoring inflight particles in thermal spray processes. The sensor can measure simultaneously and in real-time, the mean velocity and mean temperature of the particle jet for a very broad range of powder feed rates. The thermal radiation emitted by the hot particles is collected by a lens and focused on two optical fibers. Knowing the distance between the optical fibers and the magnification of the optics, the mean particle velocity is computed by measuring the time delay between the signals collected in the two fibers by cross-correlation. The signals are band-pass filtered to prevent spurious reflection, equipment movement and noise from disturbing the measurement. Using the same signals filtered at two specific wavelengths, the mean temperature of the particle jet is obtained by the two-color pyrometry technique. In this technique, the temperature is computed from the ratio of the light intensity detected at two different wavelengths.

A two-wavelength particle imaging pyrometer has been developed to measure temperature, velocity and size of individual particles within a field of view and a depth of field that spans the entire particle stream in most thermal spray devices. The pyrometer provides continuous updates to particle condition profiles, histograms and correlations. The software locates particle streaks, determines the intensity ratio and dimensions of each streak, and calculates the particle temperature, velocity and size. Many forms of advanced materials processing technologies, such as thermal spray, spray-forming and atomization processes, have considerable need of process control sensor technology. These measurements provide the basis for application of the pyrometer to many of these processes. Particle temperature measurements of plasma-sprayed ceramic powder were obtained using a spectrometer and the pyrometer. Comparisons of the measurements show that the vision-based pyrometer has excellent accuracy. The standard deviation of the measurements was 40 K or about 1.3 %. Additional pyrometer measurements were used to determine its minimum detectable temperature and velocity change, which were 12.4 K and 2.77 m/s, or 0.4 % and 1.5 %, respectively. The vision-based particle sensor can now be applied to high performance control strategies to provide stable particle temperatures and velocities over long duration plasma spray processes.

A novel technique and an instrument for on-line, quantitative imaging diagnostics and process control in thermal spraying have been developed and tested in laboratory and industrial conditions. In-flight spray particles are imaged by their natural luminosity with a short-exposure, digital CCD camera system. Particle images are processed using digital image processing techniques in a PC computer. The number of particles per frame and the spatially resolved particle velocities are calculated from the images. Spectrally resolved image information is further used to determine pyrometric two-color particle temperature. A number of different modes of data presentation have been developed. The developed instrument can be used to determine particle number, velocity and temperature distributions of individual in-flight particles from the imaged region of interest of the plume. Dividing the imaged area into smaller sections, spatial distributions of these parameters can be studied. SprayWatch system provides a technically simple, easy to operate, single imaging instrument, which can provide a visual overview of the spray plume in combination with quantitative evaluation of the most important spray particle parameters. In this paper examples of using the monitoring system with plasma and HVOF spraying are presented. Preliminary test results of using a semiconductor laser generated light sheet to detect cold particles is also demonstrated.

Three actual applications of spray plume sensors for industrial coating operations are presented. Two of these are in the aeronautic industry and one in the automotive industry. Depending on the application, different types of sensors were used ranging from plume shape monitoring (particle trajectories) to full plume characterization including in-flight measurement of individual particle properties. In each case, the process monitoring scheme developed depended on the manufacturer's requirements as well as the nature of the information provided by the sensor. The significant differences in usage and working conditions compared to an R&D environment are examined. Advantages and drawbacks in each of the three situations are detailed. A tentative forecast of coming developments in this field is also given.

A high-speed video recording system was developed to measure the velocity and temperature of thermal spray particles in flight. The system consists of three image sensors and a cubic beam-split prism that directs light, based on color or wavelength, to three imaging planes. In this study, one of sensors is used to measure particle velocity while the other two measure particle temperature. The prospect of simultaneously measuring the temperature and velocity of individual particles in flight is discussed along with the insight it provides on the formation of spray coatings.