diagnostics in plasma spraying techniques

201 (2006) 1948–1954www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Diagnostics in plasma spraying techniques

K. Landes

University BW Munich, Germany

Available online 5 July 2006

Abstract

The paper presents three innovative diagnostics for application to the plasma spraying process. With plasma computer tomography, the plasmajet is investigated by recording its radiation under several directions. The laser-based particle shape imaging technique is applied for determiningdynamic and geometric parameters of spray particles within the plasma jet. The particle flux imaging method, which allows the characterisation ofthe total process except the formation of the coating, can be used for process monitoring and quality control.© 2006 Published by Elsevier B.V.

Keywords: Plasma spraying; Tomographic plasma jet diagnostics; Particle flux imaging; Particle shape imaging

1. Introduction

The main sections of the plasma spraying process are thegeneration of the plasma jet, the injection and treatment ofparticles within the plasma jet and finally the formation of thecoating. As a first group of diagnostic techniques for eachsection, individual and powerful methods have been developedin the past. For use in industrial plasma spraying applications,there is additionally a need of another type of diagnostics, whichallows proving and monitoring the total process. Among thethree innovative diagnostics presented in the paper, plasmacomputer tomography (PCT) [1,2] and particle shape imaging(PSI) [3,4] belong to the first and particle flux imaging (PFI) [5]belongs to the second group of diagnostics.

2. Plasma computer tomography (PCT)

In usual emission spectroscopy the radiating object isobserved only under a single angle. In the plasma computertomography (PCT) presented in this paper, the emitted radiationof the plasma jet is detected under several directions one afterthe other and distributed over a sector of 180° in a planeperpendicular to the torch axis (Fig. 1).

Via a lens system a cross-section of the plasma jet is imagedon the entrance slit of a spectrograph. In its exit plane a CCDcamera detects the radiation of the plasma depending on the

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0257-8972/$ - see front matter © 2006 Published by Elsevier B.V.doi:10.1016/j.surfcoat.2006.04.036

wavelength. The real two-dimensional plasma emissivitydistribution ε in a cross-section plane perpendicular to thetorch axis is reduced to a one-dimensional distribution along thevertical pixel line of the CCD camera. Without a preliminaryknowledge of the plasma jet geometry, starting from this one-dimensional result the local two-dimensional distribution of theplasma emissivity cannot be achieved. This principal

Fig. 1. Schematic emission spectroscopy with rotating optical system fortomographic side-on data acquisition.

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Fig. 2. Principle of tomographic data acquisition and evaluation.

Fig. 3. Influence of arc current on TRIPLEX II plasma jet rotation.

1949K. Landes / Surface & Coatings Technology 201 (2006) 1948–1954

disadvantage of usual emission spectroscopy can be eliminatedby applying a tomographic procedure detecting the emittedradiation under several angles of observation. A necessarycondition for this method to be applied successfully is a highdegree of plasma jet stationarity. Additionally a definite axismust exist, around which the total detecting optical system isrotated.

The successive tomographic acquisition of data as well astheir evaluation is schematically demonstrated in Fig. 2.

For simplicity reason the explanation is given only for asingle wavelength λ with the consequence, that only a singlevertical column of pixels on the CCD camera in Fig. 1 isconsidered. The pixels of this column are numbered by thechannel index k with 1≤k≤K. When the radiation of a plasmajet cross-section with an emissivity depending on the coordi-nates x and y (Figs. 1 and 2) is detected under differentobservation angles 1≤w≤Wone after the other, different signaldistributions result on the considered vertical CCD camera line.In this way the data acquisition results in a matrix withhorizontal rows corresponding to the different angles w ofdetection and with vertical columns corresponding to the pixelsk in the considered CCD camera pixel line. The resultingmeasured valueM in a channel k at an detection angle w is givenas a function f of the local emission ε weighted with thesensitivity E of the detecting system:

Mðw;k;kÞ ¼ EðkÞd f ðeðx;v;kÞÞ

The function f is a double integral over the coordinates x and ywith integration limits depending on k and w. An inversefunction f − 1 to determine the local emission ε cannot be foundanalytically. But there exist numerical recursive iterativealgorithms to reconstruct the local emission ε from tomogra-phically measured data M(w,k,λ). The result of such acalculation is depicted in Fig. 2. The reconstructed distributionof the emission ε of the measuring object is a discrete one. Theresolution of the calculated cross-section image depends on thenumber K of detection channels and on the number W ofdetection angles (usually K=W).

The calculation and storage capacity, necessary for thereconstruction of cross-section images from tomographic data isvery high and increases with the power three of cross-sectionresolution. Nevertheless, nowadays high-resolution cross-section images can be reconstructed with customary PCs inan acceptable time.

For example, the method has been applied to the plasma jetof a TRIPLEX II torch (Fig. 3).

A strong influence of the arc current on the plasma jetrotation was found. Using the tomographic emission spectros-copy, the influence of other operation parameters as gas flowand gas composition on the geometry of plasma jets generatedby a TRIPLEX torch can also be investigated.

Fig. 4. Single camera (left) and multi camera (right) PCT set up.

Fig. 5. Principle of particle shape imaging.

1950 K. Landes / Surface & Coatings Technology 201 (2006) 1948–1954

Under current development is a new PCTset up which can beused for the investigation not only of stationary plasma jet asgenerated for example by plasma guns of type TRIPLEX, butalso for nonstationary plasma jets as they are generated by manygun types used in the industry today. For this purpose theconventional PCT system where a single camera rotates around

Fig. 6. Schematic set up of PSI wi

the plasma jet has to be replaced by a set of cameras positionedon a half circle around the plasma jet (Fig. 4).

Using the relevant parts of a cheap commercial small sizedcamera with sufficient resolution the costs of the system can bekept very low. The camera units are triggered simultaneouslyand the stored data are recorded then in a serial mode by the PC.For the evaluation of the data the same mathematical procedureis applied as for the former conventional PCT.

3. Particle shape imaging (PSI)

Particle shape imaging is on principle a laser based shadowtechnique (Fig. 5). The beam of a linear polarized Nd–YAGLaser (100 mW cw) is split into two parts of equal intensitywhich are crossed by mirrors in the focal plane of a Long-Distance-Microscope (Optics).

The approximately round diameter of the laser beamintersection in the focal plane is projected into the imageplane. From the resulting image a rectangular region is

th an integrated LDA system.

Fig. 7. Triggered snapshots of Al2O3–TiO2 particles within the plasma spray jet.


monitored by an ICCD camera (4 Picos, Stanford ComputerOptics). Thus the maximum size of the measuring volume isdefined by the field of view within the laser beam intersectionarea and the depth of focus of the Long-Distance-Microscope.Particles being outside the depth of focus (940 μm) areprojected blurred or with interference effects. A particlecrossing the measuring volume creates on principle twoshades in the image plane because the particle is located inthe two laser beams. The partial shades are separatedproportional to the distance of the particle from the focalplane. With this effect the particle distance relative to thefocal plane can be determined. The trigger system (Fig. 6)starts an image acquisition when a particle enters themeasurement volume. The image acquisition is started afteran adjustable delay time so that in the resulting picture theparticle distance from the trigger line corresponds to itsvelocity. By image processing, particle size can be measuredby the number of pixels which represent the particle in theimage. Laser Doppler Anemometry (LDA) is integrated into

Fig. 8. Comparison between System PS

the diagnostic system to calibrate the PSI method and tocompare the velocity results.

The qualification of the PSI System for in-flight particlediagnostics inside the hot core of a plasma jet was tested.For this purpose a F4 plasma torch (Sulzer Metco) wasoperated at 340 A with a plasma gas mixture of 35 SLM Arand 14 SLM N2. The used powder was Al2O3 with diameterrange 3 μm–97 μm. Under these conditions, the system hadto cope with a very low particle density in the range of0.005 particles/mm3.

In Fig. 7 typical PSI pictures taken in a plane 3 cm fromthe nozzle exit of the torch are depicted. Only sharp particlesimages with a full shaded region are selected by the triggersystem. Different particle shapes and orientations can beresolved. Neither image quality nor trigger operation has beenimpaired by plasma radiation, heat or electromagneticradiation. This can be explained by the small solid angledefined by the aperture of the Long-Distance-Microscope andby the laser beam intensity which exceeds the plasmaradiation. The black stripe on the right edge (Fig. 7) is thetrigger area. When the shape of a particle passes this area fromright to left a image acquisition is triggered with an adjustabledelay time. The distance of a particle image from this triggerarea is therefore directly proportional to the particle velocity.With trigger response time and chosen delay time for imageexposure, a velocity scale (bottom of Fig. 7) in the Long-Distance-Microscope produce a single image on the ICCDcamera. In PSI 2, two identical optical systems (upper andlower beam systems) produce two separated particle imagescan be calculated. The exposure time of 5 ns guarantees thatparticles with velocities below 500 m/s are sharply imaged.Using an innovative camera exposure time can be decreasedfurther down to 0.1 ns when sufficient laser power isavailable.

Compared to the former experimental set up PSI 1 describedabove the PSI diagnostic technique has been improved by thenew system PSI 2. It allows measurements with high imageacquisition efficiency and unambiguous particle location. Thetransmitting sections of the LDA, PSI 1 and PSI 2 set upsincluding a cw-laser are very similar and can be used identically,

I 1 (left) and System PSI 2 (right).

Fig. 9. PSI 2: Original (left) and processed (right) upper and lower images.

Fig. 10. Particle in front of (left), particle in (center) and particle behind focal plane (right).

Fig. 11. Simultaneous detection of plasma jet and particle flux with PFI.


whereas the function principles and the receiving parts are verydifferent. The combination of LDA and PSI allows in a non-intrusive and online procedure a reliable and partially redundantdetermination of a set of relevant particle parameters as velocity,size, shape and number density.

The experimental setup PSI 1 and the improved experi-mental setup PSI 2 are compared in Fig. 8. In PSI 1, the laserbeam is split into two laser beams of equal intensity crossingin the focal plane of the long distance microscope. The twolaser beams superpose upper and lower image) on the ICCDcamera. An important advantage of the new modular opticalSystem PSI 2 compared to PSI 1 is the transition from threegradation levels (full shadow, half shadow, white) to twogradation levels (full shadow and white) (Fig. 9). This allowsa more simple and rapid image analysis and particlecharacterization [3,4]. As another advantage of PSI 2 theposition of the upper particle image relative to the position ofthe lower image of the same particle gives unambiguousinformation about the particle position relative to the focalplane. This is shown in Fig. 10 for a spherical 100 μm testparticle. The absolute amount of the relative shift of theparticle images is proportional to the particle distance fromthe focal plane. With the PSI diagnostic technique thevelocity, size, shape and position of individual particles can

be determined. Regarding the shape this can lead to anenormous quantity of data, if for example particles of acrushed powder are investigated, where all particles havedifferent shapes. In this case a reduction of data is necessary.This is achieved by an ellipse fit procedure where the shapeprojection of each particle is approximated with an ellipse ofidentical area.

Fig. 12. Ellipse characteristics.

Fig. 13. Semi-axis a of a particle flux ellipse versus secondary gas flow rate.

Fig. 14. Particle ellipse centre sx versus plasma current.

Fig. 15. PFI-S image of a running process (left) and


4. Particle flux imaging (PFI)

The particle flux imaging (PFI) methods (Linspray® PFI andPFI-S) belong to the group of diagnostics used for monitoringthe total process. To reduce the multitude of measurement datato a few significant criteria for process assessment the PFImethod records phenomenologically the plasma jet and theparticle flux and in a new version also the spray spot. The goalof the method is to provide on-line relevant data for processmonitoring. Additionally it meets the requirements of a harshindustrial environment.

PFI is on principle an optical volume diagnostic methode [3].Using a CCD camera the very luminous plasma jet close to thetorch and the less luminous particle flux in the downstream zoneare imaged simultaneously. A PC reduces the information byfinding lines of constant radiation intensity in the images of thehot plasma jet and of the particle flux. These lines are closedcurves and can be approximated by ellipses. Their character-istics are typical for the state of the coating process. In this wayvariations in the hot plasma jet as well as in the particle flux canbe detected without an exact knowledge of exact physicalplasma jet or particle parameters.

A stationary CCD camera images the area between the torchand the substrate surface. The radiation of the plasma jet and ofthe particles is recorded over the whole visible spectrum.Because of the exposure time up to 2000 ms the plasma jet aswell as the particle flux is imaged as diffusive luminous areas.Single particle paths as well as short variations of the hot plasmajet and of the particle flux are not detectable. An overexposureof the camera can be avoided with neutral filters.

In principle the PFI method can be applied to all kinds ofthermal coating methods. In the case of plasma spraying twoneutral filters with different transmissivities τ1 and τ2 arenecessary. For imaging the intensively radiating plasma jet thefilter with the lower transmissivity (τ1=10

−6) and for the lessluminous particle flux a filter with higher transmissivity(τ2=10

−4) is used (Fig. 11).

PFI-S image with two calculated ellipses (right).


The combined resulting image of the plasma jet and of theparticle flux represents the actual state of the coating process.The plasma jet with its shape depending on the state of theplasma torch is characterized by the left and the luminousparticle flux by the right ellipse in Fig. 12. The ellipses arefound by a very fast, patented approximation algorithm. Fivecharacteristics exist for each ellipse.

Under the many quantities influencing the plasma sprayprocess the most important ones are: current, powder feed rateand flow rates of primary gas, secondary gas and powder carriergas. For example Figs. 13 and 14 show the influence of theseparameters on ellipse characteristics.

The modified PFI-S system is moved attached to the gun andimages continuously the whole area between the gun and thesubstrate surface. The radiations of the plasma jet and of theparticles as well as of the spray spot are recorded over thevisible spectrum (Fig. 15).

PFI-S has been developed for industrial application. The totalplasma spray process (plasma jet, particle flux and spray spot) isoptically recorded and characterized by a few parametersresulting from approximations with ellipses. Physical changesof the hot carrier medium as well as of the particle flux or thespray spot are sensitively detected. By means of smallexpenditure of technology, an on-line monitoring of the overallprocess is realized allowing an efficient quality assurance ofplasma spraying processes.

5. Summary

In this paper, the three innovative diagnostic techniques,plasma computer tomography (PCT), particle shape imaging(PSI), and particle flux imaging (PFI), are presented: PCT hasbeen developed for investigation of the plasma jet by

recording the plasma jet radiation under different directions.For data evaluation, a numerical algorithm has been created,which is carried out by a PC. A new version of PCT actuallybeing under development will allow an application also onnonstationary plasma jets as they are generated by conven-tional plasma gun types. With PSI diagnostic, a set of dynamicand geometric particle parameters as velocity, size and alsoshape can be determined in the plasma spray jet. Theimproved experimental set up PSI 2 allows additionally anunambiguous determination of particle positions and a betterdata evaluation procedure. PFI diagnostic looks at the sprayingprocess as a whole, because it images simultaneously theplasma jet as well as the flow of the luminous particles. In aspecial version also the spray spot on the surface to be coatedis monitored. The PFI technique provides not detailedquantitative physical parameters but by monitoring the processit is a very useful tool for quality control.

References

[1] G.T. Herman, Image Reconstructions from Projections, The Fundamentalsof Computerized Tomography, Academic Press, New York, 1980.

[2] K.D. Landes, G. Forster, J. Zierhut, M. Dulko, D. Hawley, Computertomography of plasma jets – applied on a TRIPLEX II torch, InternationalThermal Spraying Conference 2004, Osaka, 2004.

[3] S. Zimmermann, K. Landes, Particle diagnostics in plasma spray jets,Second International Conference on Spray Deposition and Melt Atomiza-tion, SDMA, 2003.

[4] S. Zimmermann, K. Landes, Particle shape imaging – a modern innovativeplasma spraying diagnostic method, 48th Ilmenau Scientific Conference, Mate-rials of the Electrical Engineering and Electronics (Plasma Processing), 2003.

[5] J. Zierhut, Entwicklung von Diagnostikverfahren zur Optimierung vonPlasmaspritzsystemen, Universität der Bundeswehr München, Dissertation,2000 (May).

diagnostics in plasma spraying techniques

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