Spatial visualization of thermally sprayed microstructure ... · PDF fileSpatial visualization of thermally sprayed microstructure based ... expressed by spatial spherical contact

Spatial visualization of thermally sprayed microstructure based on light microscopy Pavel Ctibor1 and Radka Lechnerova2 1Institute of Plasma Physics ASCR, Prague, Czech Republic 2Private College of Economic Studies, Prague, Czech Republic.

A short survey is given of various techniques of spatial visualization of voids and other structural defects that are inherently present in thermally sprayed structures. It is proven that the only condition for visualization is possibility to set up a distinguishable threshold based on the gray scale of the microscopical image of the structure.

Segmented images are then further processed by commercial software for image analysis, animations and anaglyph files creation. Several softwares were applied and combined. The visualization technique was demonstrated on a ceramic plasma sprayed coating, where two kinds of voids – semi-globular pores and large interlamellar pores could be separated. Similarly, this visualization technique can be used to distinguish between pores and oxide particles in case of metallic coatings. The visualization is done in a Cartesian space without any transformation. The results can be useful mainly for: a) teaching to get a better insight into the microstructure of thermally sprayed coatings, b) research, where in combination with mathematical quantification the visualized structure can show important distinctions between coatings sprayed by different techniques and with variety of process parameters.

Keywords: image analysis, spatial visualization, reconstruction, plasma spraying, porosity, oxidation

Introduction

Plasma guns as a coating production tool

Plasma spraying belongs to the fast developing group of technologies known as thermal spraying. The vast majority of plasma spraying around the world is done by the gas stabilized plasma guns (GSP). However, a flow of a gas is not the only possible means of stabilization of an electric arc and production of a jet of thermal plasma. Evaporation of the liquid (water) wall is the principal mechanism that produces the plasma medium in water stabilized plasma gun (WSP®) used at the Institute of Plasma Physics ASCR. The goal of all thermal spray processes is to deliver a powder particle to the substrate in such a state, that after impacting the substrate the particle will flatten as one piece, not bouncing off, not spattering. To achieve that the particle (droplet) must be partly or completely molten. The right melting for a given chemistry results from a proper combination of the power, feedstock’s size and feed rate and placement of the feeding port and coating substrate versus the plasma gun exit nozzle. The formation of a coating depends on the interaction between a droplet and the substrate or the previously deposited layers, i.e., spreading of a droplet, the formation of a splat (lamella) and its solidification. The difference in the degree of a splat flattening results in the difference in porosity and its shape as well as distribution, and these factors could affect also the bonding between lamellae. Bonding between lamellae affects markedly the cohesion and elastic properties of the coating. The main spray parameters of the system – feeding distance (i.e., spray gun to feeding point distance) and spray distance (i.e., spray gun to substrate distance) are able to control the majority of the above-mentioned features. Taking into account the enormous amount of available heat in WSP® compare to GSP, larger powder sizes are needed to avoid evaporation or bouncing-off of the injected particles from the plasma jet. The

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principle of a coating formation is the same for WSP® and GSP, but the droplet size can affect some features of the coating, also the deep knowledge of the microstructure is desirable.

Brief survey of the spray coating microstructure examining possibilities

Standard methods of examining the microstructure of coatings as well as other materials involve simplifying the spatial structure to a planar representation by a technique such as light or scanning electron microscopy, such as in our previous papers [1-8]. While planar representation of microstructures is widely used to give some idea of the microstructure morphology, it is not fully representative of the spatial structure of the material. Therefore, to visualize and fully understand the microstructure of a material, a technique should be used that can capture the three dimensional nature of that microstructure [9,10]. In the beginning technique of continuous polishing and successive imaging was used, adapted from earlier applications to bulk materials [11], also for hard coatings on a ductile substrate [5]. Later it has been applied for investigation of porosity in ceramic coating sprayed by Water Stabilized Plasma gun (WSP®) [2,3,7], where high capability of this approach to detect all large globular pores inside the coating was demonstrated. Further a special attention has been paid to interlamellar flat pores [6] and to microstructural features which cannot be easily tresholded out by image segmentation via image analysis tools [1]. The same approach was used to treat separately pores and oxides in metallic coatings [4, 8] for comparison of two thermal spray techniques: Arc Spraying (AS) and Atmospheric Plasma Spraying (APS) by WSP®. It is evident that transformation of experimentally measured data from real structure to useful computer data is not a simple process but it consist of several parts [8] and the method can be called computer-aided volumetric representation (CAVR). The main parts of CAVR are the following: Volumetric Reconstruction (VR), Volumetric Visualization (VV) and Volumetric Description (VD). This paper deals mainly with the VV part – the Volumetric Visualization. In theory other images (SEM or TEM) can be used for VV. Also x-ray computed microtomography (CMT) [13,14] or other non-destructive techniques [15,16] data arrays could be used as an input of VV, although in the case of CMT after complicated pre-processing [16]. There exist several various ways to utilize a reconstructed and ready-to-visualize volumetric data arrays for modeling purpose. One of them is to use the generated spatial data as an input for the finite element method (FEM) modeling [9], the other is construction of physical models by rapid prototyping [17] or processing the data in a virtual reality computer environment [17].

Figure 1: Scheme of the computer-aided volumetric representation (CAVR).

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Main microstructural features of coatings

It is important to keep in mind that any calculated data for the reconstructed space are in fact only the best estimates. The reason is that we are working with discrete information about the real structure obtained from a limited number of sections and we do not know the continuous changes of sizes and shapes of the structure in the real 3rd dimensions. Classical stereological methods are not needed because the stack of serial sections yields in fact spatial information directly. The discussion necessary for elucidation of edge effects and resolution limits associated with used approach was given elsewhere [1-8]. Spatial description of pores is easier than that of inclusions such as oxides. The reason is that pores can be identified more easily as individual objects than oxides. Parameters of sections through a roughly convex pore can be measured [8] (centroid coordinates, area, perimeter, angle and length of maximum diameter) and stored for each section. Then the 3D description is done with a computer program developed by the authors giving the following parameters: centroid coordinates, number of intersected sections, minimum and maximum diameter and volume. Input of this program contains results of the individual section measurements, using the knowledge about centroid coordinates, area and maximum diameter of pores in the sections. The non-convex pores must be detected manualy at each recorded section. The image analysis is then the same as for the convex pores. For the 3D description of non-convex individual objects other than pores was prepared a program giving the best estimate of the following parameters - centroid coordinates, number of intersected sections, volume of each 3D object. Interlamellar pores were represented as two flat parallel surfaces with zero gap between them [3]. Since these flat pores are mostly more or less perpendicular to the spraying direction, they are observed as approximately horizontal thin voids at the pictures of successive sections. These linear features were selected as combinations of segments whose projections in two-dimensional images of individual sections are lines. From these data the flat pores were reconstructed by triangulation between neighboring sectioning planes. The volume of pores was estimated with good precision by means of well-known Cavalieri method [2,3]. Besides description of individual pores, their spatial distribution was in our focus [2,3]. It can be expressed by spatial spherical contact distribution, nearest neighbor distribution, K-, L- and pair correlation functions of the point pattern of pore centroids – this pattern is shown on Fig. 2. Estimators of these characteristics were taken from the literature (see Refs. in 3). We restrict ourselves to the nearest neighbor distribution function in the model of complete spatial randomness (CSR) [3]. The clustering was observed in the graph for distribution function of nearest neighbor distances.

Figure 2: 3D view of point pattern of pore centroids of plasma (WSP®) sprayed MgTiO3-CaTiO3 ceramic coating. Visualization enabled by means of scatter plot function and combined with one ortho-slice and bounding box (program Voxler). For details see [1-3].


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Examples of visualization by VV and their application

Several softwares were applied and combined, such as Lucia (Laboratory Imaging, Czech Republic), Voxblast (VayTek, IA, USA), Amira (Mercury Computer Systems, MA, USA) and Voxler (Golden Software, CO, USA) for processing of segmented images of conventional materialographic sections. Images of ceramic coating (MgTiO3-CaTiO3) as well as metallic coating from steel AISI 316L were used for detailed analysis, animation and anaglyph files creation. Creation of a seamless ”assembly” of images covering all fields in the region of the plane was an important part of our work [1-8] In the case of ceramics the area was 450x350 µm, (i.e. 3460x2690 pixels) and 360x280 µm in the case of AISI 316L coating. Spacing between planes equal to 3 µm was used. In the case of MgTiO3-CaTiO3 sample 80 aligned planes, each consisting of 25 high-resolution ”assembled” images, was documented and all 2000 images stored in the computer to undergo the image analysis. For metallic samples there were 40 planes, each with 16 “assembled” images and thus 640 images for further work. For alignment of the images in subsequent planes fiducial markers by Vickers indents were used under an assumption that every new polished plane was parallel to the previous one. The validity of this assumption was demonstrated by the lack of any divergence in the diagonal dimensions of several widely-spaced microhardness indents after successive removal of the surface layer by polishing. The nature of microstructures reflects the character of as-sprayed coating; we were working only with voids large enough to be detected by optical microscopy and of course our selected resolution is a compromise between detection of as small voids as possible and covering representatively large area by images. Our approach gives in principle unlimited possibilities to enlarge a studied area, but the effort on materialography and image treatment is increasing also rapidly.

Figure 3: Single image of plasma (WSP®) sprayed stainless steel, light microscopy.


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Figure 4: Large Area Section of plasma (WSP®) sprayed stainless steel with scheme of the grid between single images, the single image from Fig. 3 represents here the left top corner. Most visualization techniques convert volume data to surfaces; however, software like Amira or Voxler use an alternative technique called direct volume rendering to render voxels directly. Render means to convert graphic information from a file format to a visual representation. A direct volume rendering is a three-dimensional display of data that simulates the transmission and absorption of light through the points in the volume. A voxel is a three-dimensional version of a pixel - derived from volume pixel - and is the smallest distinguishable box-shaped part of a three-dimensional image. Light rays are cast through the volume, where particles within the volume simultaneously emit and absorb light. The color of an individual pixel on the screen is computed by composing the contributions from each particle intersecting the ray. This allows visualization of non-homogeneity inside objects with appropriate adjustment of the opacity . Simulating the transmission of light through the volume makes it possible to display the data set from any view direction without constructing intermediate polygonal models. By exploiting modern graphics hardware, software is able to perform direct volume rendering almost in real time, even for data volumes over 50 megabytes. Moreover, multiple data sets can be volume rendered simultaneously. The somewhat irregular edges seen on the individual 3D objects were an artifact of reconstruction caused by slight rotational misalignments in the serial sections. The relatively small errors induced by rotation do not reduce the overall accurate representation of the pores and oxides morphology, see Fig. 5.


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Figure 5: Spatial microstructure of wire-arc sprayed steel AISI 316L coating. Visualization enabled by means of volume rendering of segmented images (program Voxblast). Details about shown volume of interest are given elsewhere [4,8].

The ‘3D Texture’ method renders slices parallel to the image plane with a blending function to composite the pixels between slices. The color data is applied via three-dimensional texture mapping hardware on the video card. This method does not exhibit the popping effect and uses less memory than the ‘2D Texture’ method; however, it requires three-dimensional texture support from the video hardware. The above-mentioned features of the visualization software enables us to observe on the screen the computer-designed ‘virtual materialographic sections’ called “slices”. A slice is a two-dimensional cut through a three-dimensional image. An ortho-slice is aligned with two axes of the volume. The ‘clip plane’ module clips input geometry according to a user-defined clipping plane. All geometry on one side of the plane is clipped (not drawn). Multiple clip planes can be connected to the same output geometry in order to get effects like quarter cutouts, etc. Combination of a slice and a clip plane is also possible, see Fig. 6. In the Figures 2 and 5 the z-dimension of the visualized volume is enlarged for a better clarity. The precise proportions of the volume of interest are maintained in the Figure 6.

Figure 6: Spatial microstructure of plasma (WSP®) sprayed steel AISI 316L coating. Visualization enabled by means of clip plane and one ortho-slice (program Voxler). Details about shown volume of interest are given elsewhere [4,8].

The visualization software uses as input either two-dimensional images (like majority of here presented images) or tree-dimensional point data array (in our case pore centroid coordinates, Fig. 2), which must be organized into data-set definition files. Point data are collections of XYZ points in space,


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optionally with associated data values. Occasionally, this is called "XYZC data" where XYZ represent the three-dimensional position and C represents one or more data values at that position. For displaying of tree-dimensional point data following functions are suitable. The ‘Scatter Plot’ module displays a set of symbols at each point of a point set. The positions of the symbols, however, are maintained in three dimensions. For clarity of volume borders there is advantageous to display scatter plot together with ‘bounding box’, Fig. 2. A ‘bounding box’ is the smallest rectangle, complete with vertical and horizontal sides, that completely surrounds an object. All portions of the object lie within the bounding box. A special plug-in enables the deconvolution and visualization of microscopic z-stacks using maximum likehood deconvolution algorithm. This plug-in has integrated elementary volume viewer and slice viewer and it can work in both the bright field and the confocal regimes. The computed height profile (which is a product of 3D focusing) is used. The calculated height map allows to generate the spatial view of the object. Stereovision images (anaglyphs) can be generated from the 3D focused images as well. These images can be viewed via special glasses with color filters (e.g. red and green) that provide tree-dimensional impression. An example of anaglyph crated from eight-level stack (8 polished planes) made by segmentation of micrographs of plasma sprayed steel coating is given on Figure 7. Only oxides are highlighted, all other features (pores and metal matrix) were thresholded-out by image analysis. Anaglyph is made only from eight levels because of more instructive view inside the structure. This type of imaging is very useful for checking what has changed between few adjacent polishing planes but less practical for observation of large number of planes at once. For viewing the anaglyph depth the reader needs glasses with left eye red and right eye green and some 20 to 30 seconds of quiet concentration looking on the microstructure.

Figure 7: Microstructure of oxides in plasma (WSP®) sprayed steel AISI 316L coating. Anaglyph of 8 planes (program Lucia). For details see [4,8].


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Figure 8: Microstructure of plasma (WSP®) sprayed steel AISI 316L coating. Visualization enabled by means of three perpendicular ortho-slices (program Voxler). Metal is white, oxides grey and pores black.

Figure 9: Scheme belonging to the Figure 8. Visualization enabled by means of three perpendicular ortho-slices (program Voxler). A and B indicate unmelted particles and No. 1 a pore. The visualization enables us to study spatial features that have previously been studied only with two-dimensional techniques. For example Figures 8 and 9 show three perpendicular ortho-slices of the plasma sprayed steel coating. Near the crossing point of the slices two unmelted particles and large pore above them can be seen. The diameters of the unmelted particles (A and B in Fig. 9) are roughly 15 µm, which is several times less than the used feedstock size. The shape of the adjacent large pore (No. 1 in Fig. 9) can be described - by moving the ortho-slices positions, as stretched out. We can conclude that the particles A and B as well as the pore are most probably products of droplet splashing and fragmentation at the impact. Such findings can hardly be acquired from randomly selected 2D section.

Conclusive remarks

A short survey was given of various techniques of spatial visualization of voids and other structural defects that are inherently present in thermally spray structures. It is proven that the only condition for visualization is possibility to set up a distinguishable threshold based on the gray scale of the microscopical image of the structure. Segmented images were processed by commercial software for image analysis, animations and anaglyph files creation. All tested softwares are suitable for microstructure visualization and the resolution in the third – computed – dimension is similar despite certain differences in the used algorithms. The softwares differ mostly in the ability to use the


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reconstructed volume for further modeling, data processing and transfer to other programs or platforms, but this part of their performance was not an object of our current interest. The usefulness of the visualization technique was demonstrated on ceramic plasma sprayed coating, and also on metallic coatings produced by two different spray processes. A good ability of the used technique to distinguish between pores and oxide particles in case of metallic coatings has been shown. The visualization simple to control because it is done in a Cartesian space without any transformation. The results can be useful mainly for distinctions between coatings sprayed by different techniques and with variety of process parameters. Software performance is advancing ahead rapidly; also we hope that various ways of visualization of microstructure will become a common part of the thermal spray research and development soon.

Acknowledgment. The authors from IPP ASCR acknowledge support of the Academy of Science of the Czech Republic under project CV 1QS 200430560. Work at Faculty of Mathematics and Physics, Charles University in Prague (statistical treatment of the data) was supported by grant GACR 201/06/0302.

References

[1]. P. Ctibor, O. Roussel and A. Tricoire: Unmelted particles in plasma sprayed coatings, Journal of the European Ceramic Society, 23 (2003), pp. 2993-2999.

[2]. Juzkova R., Ctibor P. and Benes V.: Analysis of porous structure in ceramic plasma sprayed coating, Image Analysis and Stereology, 23 (2004), pp. 45–52.

[3] P. Ctibor, R. Lechnerova and V. Benes: Quantitative analysis of pores of two types in a plasma-sprayed coating, Materials Characterization 56 (2006), pp.297-304.

[4] P. Ctibor, S. Nourouzi, M. Inizan, N. Bonnet and R. Enzl: Structural analysis of plasma- and wire arc- sprayed stainless steel coatings, Acta Technica CSAV, 49, (2004), pp. 411-422.

[5] P. Ctibor, P. Chraska, O. Roussel, A. Tricoire: 3 D reconstruction of structure of plasma spray deposits, Proceedings from the 1st International Surface Engineering Congress and the 13th IFHTSE Congress, Columbus, Ohio (2002) pp. 357-360.

[6]. Juzkova R., Ctibor P. and Benes V.: Interlamellar flat pores analysis in plasma-sprayed coating, Proceedings from the 9th European Congress on Stereology and Image Analysis and 7th STERMAT International Conference on Stereology and Image Analysis in Material Science, Zakopane, Poland (2005), Vol. 1, pp. 65-72.

[7]. R. Juzkova, P. Ctibor and V. Benes: Analysis of the microstructure in plasma-sprayed coating, Final programme and abstracts the 25th European Meeting of Statisticians, Oslo, Norway (2005), p. 515.

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[14]. Margadant N, Siegmann S, Keller T, Wagner W, Kulkarni A: Insight to spraying conditions, microstructure and properties and their statistical correlation for different thermal spraying processes using complementary characterization methods, Proceedings of Int. Thermal Spray Conf., Marple B & Moreau C - editors, (2003), pp.1053-1061


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[15]. Kulkarni AA, Goland A, Herman H, Allen AJ, Ilavsky J, Long GG, Johnson CA, Ruud JA: Microstructure - Property Correlations in Industrial Thermal Barrier Coatings, Journal of the American Ceramic Society, 87, 7(2004), pp.1294-1300.

[16]. Kulkarni AA, Goland A, Herman H, Allen AJ, Ilavsky J, Long GG, De Carlo F (2005): Advanced Microstructural Characterization of Plasma-Sprayed Zirconia Coatings Over Extended Length Scales, Journal of Thermal Spray Technology, 14 (2), pp. 239-250.

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Spatial visualization of thermally sprayed microstructure ... · PDF fileSpatial visualization of thermally sprayed microstructure based ... expressed by spatial spherical contact