Lech Pawlowski

The relationship between structure and dielectric properties in plasma-sprayed alumina coatings

Abstract Nine commercially available powders, with different morphologies, chemical compositions and particle size ranges, were plasma sprayed. The microstructures of the coatings obtained were analysed using optical microscopy and X-ray diffraction methods. The densities and open porosities were determined using hydrostatic weighing; they varied from 3.25 g cm -3 to 3.45 g cm -3 and 5.5% to 7.9% respectively. The volume resistivity of “assprayed” coatings were within the range of 10 9 – 10 10 Ω cm. Baking the coatings at 120 °C results in an increase in resistivity to 10 13 – 10 14 Ω cm. The dielectric constant at 1 kHz is in the range 5.9 – 8.3 depending on the kind of powder used and on the coating pretreatment. The loss tangent measured at the same frequency varies from 0.012 to 0.051. The dielectric strength of the coatings depends on the coating porosity and is in the range 90 – 180 kV cm -1 .

Structure‐thermal properties—relationship in plasma sprayed zirconia coatings

Plasma sprayed coatings of zirconia stabilized with 7, 8, and 20 wt.% of yttria were investigated. The microstructure of the coatings was characterized by optical microscopy and x‐ray diffraction. The thermal properties of the coatings, i.e., thermal diffusivity, thermal dilatation, specific heat, and resulting thermal conductivity were determined for each zirconia coating and the results were correlated with the microstructure.

Sintering of Fine Particles in Suspension Plasma Sprayed Coatings

Suspension plasma spraying is a process that enables the production of finely grained nanometric or submicrometric coatings. The suspensions are formulated with the use of fine powder particles in water or alcohol with some additives. Subsequently, the suspension is injected into plasma jet and the liquid additives evaporate. The remaining fine solids are molten and subsequently agglomerate or remain solid, depending on their trajectory in the plasma jet. The coating’s microstructure results from these two groups of particles arriving on a substrate or previously deposited coating. Previous experimental studies carried out for plasma sprayed titanium oxide and hydroxyapatite coatings enabled us to observe either a finely grained microstructure or, when a different suspension injection mode was used, to distinguish two zones in the microstructure. These two zones correspond to the dense zone formed from well molten particles, and the agglomerated zone formed from fine solid particles that arrive on the substrate in a solid state. The present paper focuses on the experimental and theoretical analysis of the formation process of the agglomerated zone. The experimental section establishes the heat flux supplied to the coating during deposition. In order to achieve this, calorimetric measurements were made by applying experimental conditions simulating the real coatings’ growth. The heat flux was measured to be in the range from 0.08 to 0.5 MW/m2, depending on the experimental conditions. The theoretical section analyzes the sintering during the coating’s growth, which concerns the fine particles arriving on the substrate in the solid state. The models of volume, grain boundary and surface diffusion were analyzed and adapted to the size and chemistry of the grains, temperature and time scales corresponding to the suspension plasma spraying conditions. The model of surface diffusion was found to best describe the sintering during suspension plasma spraying. The formation of necks having the relative size equal to 10% of particle diameter was found to be possible during the thermal cycles occurring at the coatings’ deposition. Transmission electron microscopic observations of the agglomerated zone hydroxyapatite coating confirm the sintering of some of the fine grains.

A model of the temperature distribution in an alumina coating during plasma spraying

Abstract An attempt to develop a theory for the calculation of the temperature distribution in a plasma-sprayed coating is presented. This theory takes into account both the physical properties of sprayed (not the bulk) material and the effect of thermal resistance between the coating and the substrate. The calculation procedure employed (finite differences) permits the theoretical prediction of the temperature distribution in samples of limited dimensions. The theory was used to determine the temperature distribution in alumina coatings plasma sprayed onto copper or mild steel substrates. A comparison between the theoretical predictions and experimentally determined surface coating temperatures is also presented. The experiments were carried out using IR thermography. For these measurements it was necessary to determine the emissivity of the sprayed alumina at the IR wavelength of interest. The influence of the spraying distance, the substrate material and cooling of the sprayed samples on the coating surface temperature was investigated. The predicted temperatures are within 20% of the experimental values.

Microstructural design of functionally graded coatings composed of suspension plasma sprayed hydroxyapatite and bioactive glass

Various bioactive glass/hydroxyapatite (HA) functional coatings were designed by the suspension plasma spraying (SPS) technique. Their microstructure, scratch resistance, and apatite-forming ability in a simulated body fluid (SBF) were compared. The functional coatings design included: (i) composite coating, that is, randomly distributed constituent phases; (ii) duplex coating with glass top layer onto HA layer; and (iii) graded coating with a gradual changing composition starting from pure HA at the interface with the metal substrate up to pure glass on the surface. The SPS was a suitable coating technique to produce all the coating designs. The SBF tests revealed that the presence of a pure glass layer on the working surface significantly improved the reactivity of the duplex and graded coatings, but the duplex coating suffered a relatively low scratch resistance because of residual stresses. The graded coating therefore provided the best compromise between mechanical reliability and apatite-forming ability in SBF.

Microstructural study of plasma-sprayed alumina and nickel chromium coatings☆

Abstract The microstructure of arc-plasma-sprayed alumina and nickel chromium is analysed by use of different methods. X-ray analysis provides the phase composition of the coatings. A quantitative metallurgical analysis carried out with the aid of an automatized image analyser is used to determine porosity and pore distributions as well as the content of non-melted grains within the sprayed alumina coatings. This method permits the estimation of the influence of the kind of powder used for spraying and the distance between the plasma torch and the substrate on the coating microstructure. Some details of metallurgical analysis are also discussed and the coating characteristics are complemented by density measurements.

The properties of plasma sprayed aluminium-aluminium oxide cermets☆

Abstract The plasma spraying process has been applied to deposit aluminium-aluminium oxide cermets. These cermets, containing 5 to 95 vol.% of aluminium, were sprayed with the use of fine (particle sizes: -45+20 μm) and coarse (particle sizes: -90+45 μm) aluminium powders. Optical microscopy made it possible to characterize the microstructure of the sprayed coatings. The density and the open porosity of the coatings as well as their thermal expansion coefficients in the temperature range 300–400 K have been determined.

Injection of Hot Particles in the Plasma Flame

This study deals with experimental and theoretical investigations of preheating of the particles injected into a flame during the process of atmospheric plasma spraying (APS). Prior to the injection in the flame, the particles, delivered from the powder feeder, were heated in a carrier gas. The experimental devices to preheat the carrier gas, based on a principle oflinear, superficial, andvolume heating are discussed. The particle velocity and temperature in the pipeline connected to a preheating device were estimated theoretically using a simple analytical model. Two types of preheating devices were submitted to preliminary tests. The temperature of the carrier gas was measured using a thermocouple for avolumetric device working with a gliding arc discharge. The microstructure and properties of chromium oxide coatings plasma sprayed with the aid of a superficial preheating device are also presented and discussed.

Editorial: the globalization of JTST: a forum for a world wide network

“The Journal of Thermal Spray Technology publishes contributions on all aspects — fundamental and practical — of thermal spray science including processes, feedstock manufacture, testing, and characterization. As the primary vehicle for thermal spray information transfer, its mission is to synergize the rapidly advancing thermal spray industry and related industries by presenting research and development efforts leading to advancements in implementable engineering applications of the technology”.

A comparison of microstructure and of thermal transport properties of yttria and ceria stabilized zirconia coatings deposited by suspension plasma spraying

The single cathode plasma torch SG-100 was used to spray suspension prepared with the use of two kinds of commercial powders ZrO2+8 wt. % Y2O3 (8YSZ) and ZrO2 + 24 wt.% CeO2 + 2.5 wt.% Y2O3 (24CeYSZ). The suspensions were formulated using finely milled solid phase, water and ethanol. The operational spray parameters being the same for each powder were modified by changing: (i) spray distance; and (ii) torch scan linear speed. The coatings microstructures were analyzed with the use of optical and scanning electron microscopes as well as by X-ray diffraction. Their porosities were found with help of the image analysis of metallographically prepared cross-sections. Thermal diffusivity was measured with the use of commercial system LFA 447 NanoFlash® working at the temperatures limited to 300°C. The measurements were made with the use of the coatings sprayed on the steel substrate and a 2-layers numerical model was applied to determine thermal diffusity of the coatings. The obtained data were used to calculate thermal conductivity of stabilized zirconia coatings.