Search Results for thermal radiation

In-flight particle analysis via infrared sensing has proven helpful in the development and control of thermal spray processes. The aim of this work is to assess the effect of optical system design on thermal measurement accuracy. Through experimental testing and theoretical analyses, investigators show how variations in optical component sizes, alignments, and arrangements affect the amplitude and shape of sensor voltage waveforms and signal peaks. It is observed that only when photodetectors are in saturation, marked by the trapezoidal shape of their output signals, can they provide information about particle velocity, temperature, and size, and in some cases, even volume distribution. Correlation tests are carried out by means of plasma spraying molybdenum powder. Paper includes a German-language abstract.

Characteristics of in flight particles before they impact on the substrate influence strongly the quality of coating obtained by plasma spraying. Various optical techniques can be used to measure the in-flight particle characteristics; some of these techniques require the use of high- speed two-color pyrometers to collect the light emitted by the particle during the in-flight period when they pass through the measurement volume. However, the intense radiation coming from the plasma can affect the particle thermal radiation and lead to erroneous measurements. This work was dedicated to the study of reflected light coming from the plasma and scattered by the injected particles. To achieve this goal, sprayed particles were analyzed by optical emission spectrometry. The light scattered by the particles was found to influence significantly the measured temperature. This work allows thus the estimation of the accuracy of temperature measurements on particle surface for the thermal spraying process. Abstract only; no full-text paper available.

The impact of plasma-sprayed molybdenum particles on glass surfaces held at 25 and 400°C was photographed. A two-color pyrometer was used to collect thermal radiation from the particles to follow their temperature evolution and to calculate the splat cooling rate. Significant fragmentation of the splat on the surface at 25°C was observed. A 3D model of droplet impact and solidification was used to estimate the thermal contact resistances between the splat and glass. It was found that the thermal contact resistance was approximately two orders of magnitude smaller on the surface at 400°C, indicating faster solidification, which reduced splashing. The larger thermal contact resistance between the non-heated glass and splat was attributed to the presence of a gas barrier at the surface.

Plasma-sprayed, molten nickel particles (60 µm diameter) were photographed during impact on oxidized 304L stainless steel surfaces that were maintained at room temperature or at 350oC. The steel samples were oxidized at different temperatures. Droplets approaching the surface were sensed using a photo detector and after a known delay, a fast charge-coupled device (CCD) camera was triggered to capture time-integrated images of the spreading splat from the substrate front surface. A two-color pyrometer was used to collect the thermal radiation from the particles to follow the evolution of their temperature after impact. Molten nickel particles impacting on oxidized steel at room temperature fragmented significantly, while heating the surfaces produced splats with disk-like morphologies. Impact on steel that was highly oxidized induced the formation of finger-like splash projections at the splat periphery. The splat cooling rate and thermal contact resistance between the splat and non-heated oxidized steel varied significantly as the degree of oxidation increased; heating the oxidized steel greatly reduced the variations. It was suggested that the large variations in splat cooling rates and thermal contact resistances on the non-heated oxidized steel was due primarily to the presence of adsorbates on the steel surface.

Plasma-sprayed, molten molybdenum particles (~55 µm diameter) were photographed during impact on grit-blasted glass surfaces that were maintained at either room temperature or at 350°C. Droplets approaching the surface were sensed using a photodetector and after a known delay, a fast charge-coupled device (CCD) camera was triggered to capture time-integrated images of the spreading splat from behind the glass. A rapid two-color pyrometer was used to collect the thermal radiation from the spreading droplets to follow the evolution of their temperature and calculate the splat cooling rates. It was found that as the surface roughness increased, the maximum spread diameters of the molten molybdenum droplets decreased, while the splat cooling rates increased. Impact on non-heated and heated roughened glass with similar roughness values produced splats with approximately the same maximum spread diameters, skewed morphologies, and cooling rates. On smooth glass, the splat morphologies were circular, with larger maximum spread diameters and smaller cooling rates on non-heated smooth glass. An established model was used to estimate the splat-substrate thermal contact resistances. On highly roughened glass, the thermal contact resistance decreased as the glass roughness increased, suggesting that splat-substrate contact was improved as the molten metal penetrated the spaces between the large asperities.

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.

The internal plasma spraying for the deposition of thermal barrier coating already was introduced about 20 years ago. During the last 8 years a new generation of internal plasma torch was developed and introduced in the industry. The new generation is characterized by an improved resistance against the thermal radiation and a significantly improvement of the time life of the guns. The performances of the plasma torches regarding powder feed rate and deposition efficiency also were increased to allow the using in mass production in the automotive industry. The improvement of the time life of cathode and anode was realized by using CFD program to optimize the water cooling of the plasma torch. By the production line for cylinder bore the time life of cathode and anode was increased from 50 to more than 100 hours by the modification of the cooling geometry. Several application of internal spraying for mass production in the automotive, aerospace and the electrical industry will be discussed in details. Especially the internal spraying of the cylinder bore will be shown as typical application of high volume. The advantages of the plasma spraying in comparison with alternative technology will be demonstrated. The further development and future potential applications will be discussed at the end as outlook.

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.

Plasma-sprayed, molten molybdenum particles (~50 µm diameter) were photographed during impact (with velocity ~135 m/s) on a glass surface that was maintained at either room temperature or 400°C. A droplet approaching the surface was sensed using a photodetector and after a known delay, a laser was triggered to illuminate the spreading splat and photograph it with a CCD camera. A rapid two-color pyrometer was used to collect the thermal radiation from the impacting particles to follow the evolution of their temperature and size after impact. Molten molybdenum particles impacting on a surface at room temperature splashed and broke up after impact leaving only a small portion adhering to the substrate. On a surface held at 400°C, there was no splashing and a circular splat remained on the surface. Splats on a glass surface held at room temperature had a large maximum spread diameter, approximately 2.7 times that on a hot surface. The cooling rate on a cold surface was an order-of-magnitude lower than that on a hot surface, suggesting that thermal contact resistance was much greater.

The velocity of cold spray particles was measured by a diagnostic system for thermal spray particles based on thermal radiation. A laser beam was employed to illuminate the cold sprayed particles in cold spraying for obtaining a sufficient radiant energy intensity for detection. The measurement was carried out for Cu particles of different mean particle sizes. The particle velocity was also estimated using the previously developed two-dimensional axisymmetric model. It was found that the measured results agreed well with the calculated ones. The proposed measurement method in this paper is reliable. On the other hand, it is confirmed that the particle acceleration behavior in cold spraying can be accurately predicted through the simulation method developed previously. The optimization of cold spray process can be conducted following the simulation method.

The conditions of particle injection into the side of plasma jets play an important role in determining the microstructure and properties of sprayed deposits. However, few investigations have been carried out on this topic. The current work presents the results of an experimental and computational study of the influence of injector geometry and gas mass flow rate on particle dynamics at injector exit and in the plasma jet. Two injector geometries were tested: a straight tube and a curved tube with various radii of curvature. Zirconia powders with different particle size range and morphology were used. A possible size segregation effect in the injector was analyzed from the space distribution of particles collected on a stick tape. The spray pattern in the plasma jet was monitored from the thermal radiation emitted by particles. An analysis of the particle behavior in the injector and mixing of the carrier-gas flow with the plasma jet was carried out using a 3-D computational fluids dynamics code.

A microplasma spraying torch with a hollow cathode electrode is designed to melt completely the refractory materials and deposit coatings at plasma power level up to several kilowatts. The designed torch permits spray material to be fed into plasma arc jet through axial powder injection. In the present study, molybdenum is used as a typical refractory spray material. The effects of the main processing parameters including plasma arc power, plasma gas flow and spray distance on the particle velocity during spraying, and the microstructure and properties of the coatings are investigated. The microstructure of coating is characterized with optical microscopy and scanning electron microscopy. The properties of the coating are characterized by microhardness and abrasive wear tests. The particle velocity during in-flight is carried out using a particle velocity/temperature measurement system based on thermal radiation. The comparison of the microstructure and property of micro-plasma sprayed Mo coatings with those of the coating deposited by the conventional plasma spraying operated at a power of 42 kW is performed. The results show that the abrasive wear loss of the Mo coatings deposited by the micro-plasma spray torch is comparable to that of the coating deposited by the conventional plasma spraying disregarding the one order difference in the plasma operating power.

Plasma-sprayed, molten molybdenum particles (~40 µm diameter) were photographed during impact (with velocity ~110 m/s) on Inconel surfaces that were preheated or maintained at room temperature or 400oC. A droplet approaching the surface was sensed using a photodetector and after a known delay, a fast CCD camera was triggered to capture images of the spreading splat from the substrate front surface. A rapid two-color pyrometer was used to collect the thermal radiation from the impacting particles to follow the evolution of their temperature and size after impact. Molten molybdenum particles impacting on surfaces at room temperature disintegrated and splashed, after achieving a maximum diameter larger than 400 µm. Impact on preheated and heated Inconel produced splats with maximum diameters between 200 µm and 300 µm and with less splashing. The cooling rate of splats on the preheated Inconel was larger than that of splats on non-heated Inconel, suggesting that the splat-substrate contact was improved.

Plasma-sprayed yttria-stabilized zirconia particles (~40 µm diameter) were photographed during impact (velocity ~200 m/s) on a glass surface that was maintained at either room temperature or 400°C. A droplet that approached the surface was sensed using a photodetector and after a known delay, a light source was triggered to illuminate the particle in order to photograph it with a charge-coupled device (CCD) camera. A rapid two-color pyrometer was used to collect the thermal radiation from the particles to follow the evolution of their temperature and size, in-flight and after impact. The fully molten particles spread into a thin liquid splat after impacting the surfaces. The partially molten particles disintegrated into small satellite fragments immediately upon impact. The surface area, as indicated by the pyrometric signals, of the partially molten particles during spreading were almost an order of magnitude smaller than that of the fully molten particles. The pyrometric signals, characteristic of the impact of partially molten zirconia, provide a novel method of identifying partially molten ceramic particles after impact on a flat surface.

Cu coating was deposited by microplasma spraying system under a low power of 2.8 to 4.2kW. The effects of the main processing parameters including plasma arc power, operating gas flow and spray distance on particle velocity during spraying, and the microstructure and properties of the coating were investigated. The coating microstructure was examined with optical microscopy. The coating properties were characterized by cross sectional microhardness. The particle velocity during in-flight was examined using a particle velocity/temperature measurement system based on thermal radiation. The experiment results showed that particle velocity was increased with the increase in operating gas flow, and was not influenced significantly by plasma arc power and spray distance. Moreover, the microhardness of the coating was increased with the increase in arc power and with the decrease in spray distance. The operating gas flow showed no significant influence on the microhardness of the coating. The analysis suggested that the microhardness of the coating is influenced significantly by particle temperature. The comparison showed that the microhardness of the Cu coating deposited by microplasma spray is comparable to that of the coating deposited by conventional plasma spray system at power level of 30kW.

The temperature of in-flight particles in plasma spraying is one of the main parameters affecting the coating microstructure. Temperature measurement has been carried out before for inflight particles in air plasma spray (APS) and other thermal spraying processes. Suspension plasma spray (SPS) is an emerging coating deposition technology that permits the deposition of nanostructured coatings with unique structural characteristics. The aim of this work is to evaluate the influence of radiation emitted by the plasma and metallic vapors on temperature measurement of SPS particles performed by two-color pyrometry. To do so, spectroscopic analysis in the visible to near-infrared range is carried out on the jet stream when suspension of 20wt% YSZ particles in ethanol is sprayed. The analysis takes into account the radiation scattered by the particles (Mie scattering) as well as the radiation directly detected from the jet stream, and it was found that the effect of the scattered radiation by the particles on temperature measurement is 1 degree at its melting point (2700°C) and 16 degrees at 2500°C.

In thermal spray processes, the coating structure is the result of flattening and cooling of molten droplets on the substrate. The study of the cooling time and evolution of the splat size during impact is then of the highest importance to understand the influence of the spray parameters and substrate characteristics on the coating structure. Measurement of particle temperature during impact requires the use of a high-speed 2-color pyrometer to collect the thermal emission of the particle during flattening. Simultaneous measurement of the splat size with this pyrometer is difficult since the size of the particle can change as it cools down. To measure the splat size independently, a new measurement technique has been developed. In this technique the splat size is measured from the attenuation of the radiation of a laser beam illuminating the particle during impact. Results are presented for plasma sprayed molybdenum particles impacting on a glass substrate at room temperature. It is shown that the molybdenum splat reaches its maximum extent about 2 microseconds after the impact. In this work, we show that this increase of the splat surface is followed by a phase during which the splat size decreases significantly during 2 to 3 microseconds.

This paper describes the development of a diagnostic system that monitors in-flight particle diameters, velocities, and temperatures during thermal spraying. The system is based on a low-cost CCD camera and user-developed software. The camera incorporates a 732 x 282 pixel sensor with high sensitivity in the near IR range where the only radiation is that of the particles. User-developed software modules handle signal processing, image analysis, calibration, and data visualization. In video images, particles appear as light tracks of varying length, width, and intensity, corresponding (respectively) to velocity, diameter, and temperature. A test case in which Cr 2 O 3 powder is sprayed in a plasma jet demonstrates the capabilities of the diagnostic system. Paper includes a German-language abstract.

Substrate temperature is nowadays recognized as a key parameter to optimise the coating quality in the thermal spraying process. Generally parts being processed are in motion and therefore non contact temperature measurement devices are appropriate. In contrast to thermocouples, optical pyrometers have several advantages. First, they are easy to install and second they do not bring any disturbance to the measured system. Meanwhile, several problems may arise with those devices which are not always considered as they should be and in particular the variation of material emissivity temperature, the effect of the reflection of the external radiation or the attenuation of the optical signal due to the variable transmissivity of the optical path. The aim of this work was to develop algorithms for correcting optical pyrometer temperature measurements during thermal spraying by taking into account emissivity variations and radiation reflexion on the components. Emissivity of some materials with respect to the specific spectral band of the pyrometer and the influence of reflected radiations were measured. Results are discussed in order to point out the influence of each parameter on the temperature value.

Single-wavelength pyrometers are widely used as a noncontact temperature measurement tool in material processing, petrochemical and laser-machining industries. In addition, these intensity-based IR sensors are used extensively as a diagnostic and health monitoring in the development and research of advanced high-temperature military and commercial gas turbine engines. In contrast to thermocouples, optical pyrometers have several advantages. First, they are easy to install and second they do not bring any disturbance to the measured system. However, they suffer from some problems, in particular the variation of the material emissivity and perturbations introduced by extraneous radiations. Yttria stabilised Zirconia (YSZ) thermal barriers are known to be more emissive and opaque in the 8-14 µm spectral band, therefore we can take advantage using this spectral band in temperature measurement. Spectral emissivities of an YSZ sample were measured using two commercial pyrometers. And a method for adapting commercial wide-band pyrometers (generally used for low temperature measurements) for high temperature measurements of thermal barrier coatings was tested.