Thermal barrier coatings (TBC‘s) being produced at present either by atmospheric plasma spraying (APS) or electron beam physical vapor deposition (EB PVD) are widely used in the hot-temperature sections of turbines to provide thermal and corrosion protections. An emerging technology of suspension plasma spraying has become interesting for the manufacturing of thermal barrier coatings thanks to the useful microstructure including columns similarly to the EB-PVD deposits associated with considerably lower price of production. A narrow window of optimal suspension plasma spraying (SPS) parameters remain an outstanding problem in creating the favorable microstructure. The recent studies demonstrated that the substrate roughness may play an important role in reaching columnar growth of the coatings. This study presents a follow up by showing how the substrate topography obtained by laser surface texturing may be controlled to create regular columnar structure thanks to. The laser generated peaks disposed regularly on the surface can promote columnar structure growth. The formulated suspensions were sprayed onto superalloy substrates coated with powder plasma sprayed bond coats. Optimized previously, plasma spray parameters were selected to generate columnar structures and to find out the influence of the suspension behavior on coating microstructures. The results indicate that columnar SPS coating microstructure can be controlled by optimizing the laser treatment parameters. The control of surface topography may be an important factor to improve the performances of TBC-SPS coatings.

During suspension plasma spraying, the evaporation of liquid from the solution precursor alters the composition of the working gases thereby changing their thermal transport properties. This aim of this work is to better understand how aqueous calcium-phosphate, used in the synthesis of hydroxyapatite, affects thermal transport in Ar-H 2 plasma gas mixtures. Transport properties of the working gases were determined before and after injection of the precursor solution using T&TWinner, a free computational tool for thermochemistry. The results show that a significant increase occurs in the thermal conductivity of the Ar-H 2 gas mixture after the injection of the calcium-phosphate solution, but there is little change in momentum transfer between the working gases and solution droplets based on viscosity calculations. Although the software predicts an increase in the heating ability of the Ar-H 2 plasma jet, the absence of fully melted splats in the coatings suggests that it is not enough to melt HA particles.

This paper examines the microstructure and morphology of zirconia coatings and demonstrates the calculation of elastic modulus and Martens hardness based on instrumented indentation test results. Coatings samples varying in microstructure, phase content, and chemical composition were deposited by suspension plasma spraying using different torches and different suspension formulations. Coatings produced from low-concentration suspensions with submicron-size powders had a columnar structure with long vertical pores between the columns and fine spherical pores within the columns. Coatings made from suspensions with high concentrations of solids and coarser, more irregular powders, on the other hand, were more uniform and their surfaces smoother. They are also shown to be harder and have higher elastic modulus based on indentation test results.

The paper discusses a possibility of metallization of polymers using low pressure cold spray (Dymet 413). The bonding mechanism of the coating is discussed as well as the influence of the number of spraying passes on coating microstructure. Two commercial powder were used (i) tin; and (ii) aluminum to obtain coatings on PA6 polymer substrate. The substrate topography was modified with sandblasting. The adhesion strength, residual stresses, electrical resistivity, and microstructure were determined and characterized. Finally the comparison with other metallization methods was made and the application of cold spray for producing local conductive paths was assessed.

In this study, 8YSZ and 24CeYSZ coatings were deposited on stainless steel by suspension plasma spraying. The suspensions were formulated using finely milled powder, water, and ethanol. Spraying parameters were modified by changing spray distance and torch scan speed and were the same for each material. Coating microstructure, phase composition, and porosity were assessed and thermal diffusivity was measured and used to calculate thermal conductivity.

The paper analyzes different working gases and their mixtures for possible application in cold gas spraying (CGS). The gases considered include Ar, CO 2 , and steam, alone and in combination. Typical pressures and temperatures were used, along with CFD simulation software, to calculate gas velocity profiles along the axis of a convergent-divergent nozzle. The profiles are compared with that of nitrogen, the standard gas for cold spray, flowing under the same conditions. Velocity and temperature profiles were also calculated for copper particles in the various gas flows as well as nitrogen. Differences in temperature and velocity are explained based on sonic velocities, viscosities, and thermal conductivities of the respective gases.

The phenomena occurring after injection of water-ethanol suspension of fine hydroxyapatite powder are simulated numerically. The mathematical modeling starts with the calculation of the map of velocity and temperature of working plasma gases. The map is calculated by taking into account the evaporation of the liquids included in the suspension. The suspension is injected through a mechanical injector into the anode-nozzle of the SG-100 torch. The plasma was generated with the use of working gases composed of 45 slpm of Ar and 5 slpm of H 2 and with the electric power input of 30 kW. The initial droplets of suspension were supposed to be spherical with a diameter equal to that of the injector, i.e., 500 µm. The trajectory of suspension was calculated until the evaporation of liquids. Then, the simulation of the movement and heating of solid hydroxyapatite (HA) started. The HA powder was home synthesized and exhibited a bimodal size distribution with two maxima around 3 and 10 µm. The equations describing the momentum and heat transfer from hot gas to the solids took into account the small size of solid particles. In particular, the thermophoresis force, as well as, the drag coefficient modified for non-continuum effect were used in the calculation of the trajectory of small particles. Similarly, the non-continuum effect was considered in the calculation of heat transfer. The obtained trajectories were tentatively correlated with the microstructure of the suspension plasma sprayed HA coatings.

Plasma generated by an SG-100 torch was applied to a spray suspension formulated with the use of ZrO 2 +8 wt% Y 2 O 3 (8YSZ) solid phase. The solids had a mean size of about 4.5 μm and were obtained by milling of commercial Metco 204 NS powder. The suspension was formulated with 20 wt% solid phase, 40 wt% water and 40 wt% ethanol. The plasma spray parameters were optimized with the electric power equal to 40 kW, working gases composition Ar 45 slpm and H 2 5 slpm, spray distance varying from 40 to 60 mm, and torch scan linear speed varying from 300 to 500 mm/s. Coatings with thicknesses ranging from 51 to 106 μm were sprayed onto stainless steel substrates. The porosity of the samples was found from the image analysis of metallographically prepared cross-sections of the samples to be in the range of 8 to 12%. Thermal diffusivity was measured with the use of the commercial NanoFlash system in the temperature range from room temperature to 523 K. The measurements were made with the use of the coatings sprayed on the substrate, and a 2-layer numerical model was developed to determine thermal diffusivity of the coatings. The diffusivity was in the range from 0.196 × 10 -6 to 0.352 × 10 -6 m 2 /s in room temperature depending on the spray parameters. The obtained data were then associated with the literature data of density and specific heat and experimental porosity to find thermal conductivity, which was in the range of 0.47 to 0.86 W/(mK) at room temperature, depending on the spray run.

Plasma generated by an SG-100 torch was applied to spray suspension formulated with the use of ZrO 2 +8 wt% Y 2 O 3 (8YSZ) and ZrO 2 +24 wt% CeO 2 +2.5 wt% Y 2 O 3 (24CeYSZ) as solid phases. The solids have the mean size of about 4.5 µm for 8YSZ and 3.9 µm 24CeYSZ and were obtained by milling of commercial powders Metco 204 NS and Metco 205NS, respectively. The suspensions were formulated with the use of 20 wt% solid phase, 40 wt% water, and 40 wt.% ethanol. The plasma spray parameters were optimized by keeping constant: (i) the electric power at 40 kW (ii) the working gases composition 45 slpm of Ar and 5 slpm of H 2 . On the other hand, the spray distance was varied from 40 to 60 mm and torch linear speed was varied from 300 to 500 mm/s. The coatings were sprayed onto stainless steel substrates to reach the thickness ranging from 70 to 110 µm (8YSZ) and about 70 µm (24CeYSZ). The coating microstructures were analyzed with the use of a scanning electron microscope. Mechanical properties were tested with the use of indentation and scratch tests. The indentation test was carried out with various loads ranging from 100 to 10,000 mN to determine elastic modulus and Martens microhardness. Young’s modulus of the coatings was in the range 71 to 107 GPa for 8YSZ and 68 to 130 GPa for 24CeYSZ. Scratch tests were conducted to determine the scratch macrohardness.

Suspension plasma spraying is a process which enables production of finely grained nanometric or submicrometric coatings. The suspensions were formulated with the use of fine powder of ceramic particles of yttria stabilized zirconia in water with alcohol. The present paper focuses on the theoretical analysis of the formation process of sintering of fine solids impacting the growing coating’s surface. The heat flux input to the coatings was estimated and their surface temperature at spraying was measured. The theoretical analysis of sintering during the coating’s growth was carried out. The different models of sintering were analyzed and adapted to the suspension plasma spraying conditions. The model of surface diffusion was found to be the most appropriate to describe the sintering during suspension plasma spraying. The formation of the necks having the relative size equal to 10 % of the particle diameter was found to be possible during the coatings deposition.

Titanium oxide coatings were suspension plasma sprayed onto different substrates. The suspension was formulated using fine rutile pigment in the mixture of water with ethanol. The zeta potential of the suspension was determined. The spray process parameters were designed using a full factorial plan using spray distance and torch scan velocity as the variables. The temperature at spray process was monitored using a pyrometer. The X-ray diffraction (XRD) analysis enabled to find out the crystalline phases in sprayed deposits and, in particular, the anatase content. Scanning electron microscope (SEM) enabled to characterize the coatings’ microstructure. The coatings included well molten lamellas and zones of loosely agglomerated and sintered grains. The scratch test of the coatings enabled to determine their mechanical properties such as critical load and scratch hardness.

In this investigation, titanium dioxide and hydroxyapatite (HA) suspensions are plasma sprayed onto stainless steel, titanium, and aluminum substrates and the structure and properties of the resulting layers are correlated with spraying conditions. The suspensions were formulated with fine TiO 2 pigment and HA milled from spray-dried powder or synthesized from calcium nitrate and ammonium phosphate. In some experiments, an atomizer was used to inject the suspensions into the plasma jet, and in others, the suspensions were fed into the jet using continuous stream injection. The deposits are characterized on the basis of morphology, chemical and phase composition, scratch hardness, and dielectric strength.

The synthetic hydroxyapatite (HA, Ca 10 (PO 4 ) 6 (OH) 2 ) is a very useful biomaterial for numerous applications in medicine, especially in view of using as fine powder for suspension plasma spraying. The powder was synthesized using aqueous solution of ammonium phosphate (H 2 (PO 4 )NH 4 ) and calcium nitrate (Ca(NO 3 ).4H 2 O) in carefully controlled experiments. The synthesized fine powder was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The powder was formulated into water and alcohol based suspension and used to carry out the initial tests of plasma spraying onto titanium substrate. The phase analysis of sprayed coating was made with the used of XRD.

Spray process of hydroxyapatite was optimized by an advanced statistical planning of experiments. Full factorial design of 24 experiments was used to find effects of four principal plasma spray parameters, i.e. electric power, plasma forming gas composition, carrier gas flow rate and distance of spraying onto microstructure of hydroxyapatite (HA) coatings and powders. The Nemrod software has been applied to obtain the mathematical model of influence of these parameters onto experimental response. The chosen response was the volume fraction of HA crystal phase with regard to its decomposition phases. Two most important factors influencing this response are electric power supplied to torch and art of powder injection. The crystal phase content of powders and coatings was determined using X–ray diffraction (XRD) quantitative analysis. The morphologies of coatings surfaces, cross sections were characterized using scanning electron microscope (SEM).

The paper aims at the development of coating having a gradient of crystal grain size. Thick, inner layer was plasma sprayed using coarse TiO 2 powder. This layer has the thickness ranging from 30 to 50 µm. Thin, outer layer of thickness smaller than 10 µm, was plasma sprayed using different aqueous suspensions of fine powders of TiO 2 . The morphology of coarse and fine powders was characterized using scanning electron microscope (SEM) X-ray diffraction (XRD). Electronic emission was tested using home made setup. X-ray diffraction enabled to find out an interesting result which is formation of a mixture of rutile and anatase in suspension sprayed coatings. This was also confirmed by Raman spectroscopy investigations. The technology of suspensions plasma spraying was optimized to obtain homogeneous and dense deposits. The sizes were in the range of tenth to one hundred nanometers in initial powders and get clearly smaller in the coatings sprayed using coarse powder but remained quite similar in suspension sprayed films. X-ray photoelectric spectroscopy (XPS) was used to analyze quantitatively TiO 2 powders and coatings. Electronic emission was correlated with phase composition of the coating and their grain size.

Excellent biocompatibility of hydroxyapatite (HA) is the main reason of application plasma-sprayed coatings onto orthopedic prostheses. A careful optimization of spray parameters is necessary to avoid thermal decomposition of HA onto less biocompatible products such as e.g. tricalcium phosphate, tetracalcium phosphate, calcium oxide and amorphous calcium phosphates. The spray parameters influence considerably the decomposition and the present study is devoted to understand this influence using on an experimental way. The design of experiments (DOE) was made using two-level 2N plan of experiments (N=5). In total, 32 experiments of spraying were carried out by varying following operational parameters: (i) composition of plasma working gases; (ii) electric power input; (iii) art of spraying (into water or onto substrate); (iv) carrier gas flow rate and; (v) art of injection (external and internal). Plasma-sprayed coatings and powders were analyzed by Fourier Transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The X-ray diagrams enabled to find the content of crystal phases. The content was a first response function described by a polynomial regression equation. The morphology of obtained deposits was also characterized using Scanning Electron Microscope (SEM). Their porosity was estimated using image analysis of coatings cross section images.

The powders of Bi 2 O 3 and Bi 2 WO 6 were prepared by calcination, sintering and crushing. Their sizes were screened out to be in the range from 50 to 150 µm. The initial spray parameters were optimised using numerical codes simulating the behaviour of powders in plasma jet. Spray experiments onto metallic substrates were carried out using commercial plasma torch SG-100, equipped in an external powder injector. The spraying of coatings was carried out by varying electric power input to plasma, distance of spraying and the geometry of injector. The morphology of sprayed coatings was investigated using scanning electron microscope (SEM). X-ray Diffraction (XRD) of Bi 2 O 3 coatings revealed the α-Bi 2 O 3 phase, present in the initial powder, and also β-Bi 2 O 3 in some of the samples. Reduction of oxide into metallic Bi at spraying process was not discovered using XRD and Differential Thermal Analysis (DTA). Bi 2 WO 6 powder was crystallized in an orthorhombic phase that transformed into tetragonal one at spraying. Finally, the X-ray Photoelectric Spectroscopy (XPS) investigations revealed a possible presence of Bi(OH)3 phase in the coatings sprayed using Bi 2 O 3 powder.

Optical selective surfaces, i.e., surfaces with optical properties varying according to the frequency of the impinging radiation, have been exploited in several technical fields. These surfaces consist generally of doped semiconducting films, such as mixed oxides of Indium and Tin as well as Aluminium and Zinc. Thay are currently obtained by physical vapour deposition or sol-gel techniques. The present work aimed at demonstrating that coatings retaining optical selectivity can be obtained also by plasma spraying. Powders of Indium Tin Oxide (ITO) were prepared by an agglomeration technique and sprayed with a plasma torch under air and inert gas atmospheres. Both powders and coatings were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Optical reflection coefficients of the coatings were determined in the wavelength range 0.3-20 µm, i.e., in the visible and in the infrared regions of the spectrum. The experimental results indicated that it was possible to deposit, by plasma spraying, coatings possessing optically selective properties.

The powders of alloys Al 2 O 3 +13wt.%TiO 2 and Al 2 O 3 +40wt.% TiO 2 were plasma sprayed onto aluminum rolls coated initially with Ni20Cr bond deposits. The coatings were subsequently ground, polished and submitted to laser engraving. The engraving, similar to that applied for anilox rolls manufacturing, enabled production of three different patterns with the line densities of 60, 100 and 200 1/cm and depths of about 80, 50 and 10 µm correspondingly. The patterns were engraved under angle of 60° that corresponds to a honeycomb geometry of cells. Emission current, for the electric fields up to 150 V/µm, was in the range of 10-7-10-4A for coatings containing 13 wt.% of titania and, in the range 10-11–10-9 A for that of 40 wt.% of titania. The SEM observations of engraved coating morphology was made in order to find the electron emission spots.

The aim of this paper is to develop a model that describes the heating of porous aluminum oxide particles in the plasma jet of the APS process. The model is then used to analyze sintering effects and thermal gradients in particles of varying porosity in-flight. Paper includes a German-language abstract.