This paper describes the procedure that has been developed for absolute porosity measurement using Image Analysis (IA). Because of the crumbly nature of the composite substrate, it was not possible to proceed with standard method. The IA conducted on Optical Microscopy did not show enough contrast between pores and other features to be automated. The IA conducted on Scanning Electron Microscopy (SEM) with back scattered electron imaging gives enough contrast for automatic threshold determination. The SEM magnification is a parameter to be considered because it filters the information. Three frames at 500X magnification are enough for measuring the porosity of homogeneous supersonic induction plasma sprayed 18 mm samples (thickness 50-100 µm). The established calibration almost shows a 1 to 1 ratio for the image analysis as measured porosity versus the Archimedean porosity. Application of this absolute porosity determination by IA can be found in the Functionally Graded Materials (FGM) which composition is not constant over the layer thickness.

This paper examines ways to control the porosity of thermally sprayed deposits. All spraying was done with a water-stabilized plasma system using different combinations of alumina, zircon, Ni, and Al powders. Sandwiched structures with alternating ceramic and metal layers were sprayed as were thick deposits consisting of metal and ceramic mixtures. Porosity was characterized by methods such as gas permeability, water immersion, MIP, SEM, and SANS. In addition, several post-processing methods were tested to determine their effect on porosity volume. For example, removing metallic phases by leaching or by annealing at temperatures above the melting point was found to effectively increase porosity, while the use of sealing materials proved effective at reducing porosity. Another method tested was calcination, which resulted in an increase or decrease of porosity depending on the deposit's chemistry and annealing conditions.

Thermal barrier coatings (TBCs) are used on heat engine parts to impart protection to components against failure under excessive heat loads, to increase inlet temperatures with consequent improvements in efficiency, and to reduce requirements for cooling. Control of thermal conductivity is addressed since low thermal conductivity depends not only on the nature of the yttria stabilized zirconia (YSZ) layer, but also on the morphology of pores and cracks, which are closely linked to process parameters. This paper will present the influence of feedstock characteristics (particle size distribution and powder morphology) and thermal cycling on porosity content and thermal conductivity of zirconia coatings. The results show increased porosity with particle size, due to an increase in the degree of particle fragmentation and unmelted particles, leading to lower thermal conductivity. Coatings sprayed with powders of different powder morphology yielded changes in porosity and interlamellar contact, thus, influenced thermal conductivity. Sintering effects during thermal cycling resulted in reduced porosity and increased thermal conductivity.

For utilization of free-standing ceramic parts produced by plasma spraying it is very important to know the temperature dependence of the linear thermal expansion coefficient and its relation to the porosity of the structure. Zircon ZrSiO4 and gray alumina (96 wt % AI2O3) were plasma sprayed by the water stabilized plasma gun WSP PAL 160. Samples of both materials were cut from thick coatings with respect to their orientation to the gun axis during the gun's horizontal spraying cycling with a constant speed. Thermal expansion coefficients and the differential thermal analysis were performed using SETARAM complex measuring system (up to 1750 °C), the density/porosity was measured by several techniques, such as Archimedean weighing, helium pycnometry, etc. It was found that both, the porosity and the thermal expansion coefficient, change for different locations in the thick deposit due to the varying trajectories of individual particles/droplets in the plasma stream. Measured data for deposits are then compared with data for bulk ceramics. The dependence of the thermal expansion coefficient on porosity in a given location was determined and its general applicability for free-standing plasma spraying is then discussed in the paper.

During plasma spraying in-flight particle characteristics are influenced by the many operating parameters associated with the deposition process. The distinctive temperature and velocity signals given by particles as they exit the plasma torch can be used to develop processing maps for defining the optimal operating envelope. Knowing the temperature and velocity history of the particles, the evolution of the microstructure, the amount of porosity and the phase composition can potentially be predicted. In this paper, the relationship of system parameters (stand-off distance, torch power, plasma gas composition and process gas flow) was correlated to in-flight particle characteristics of yttria stabilized zirconia and compared to the resulting coating features such as thickness, microstructure, porosity and phase composition. The appearance of the coating (i.e., color) was also compared after the deposition process. Yttria stabilized zirconia was deposited on grit blasted samples using an F4 (Sulzer Metco) plasma torch. Before depositing each sample on the substrate, the particle properties were measured at the desired stand off distance perpendicular to the particle jet covering an area of 18x18 mm 2 using the Tecnar DPV2000 inflight particle analyzer. The coatings were cross-sectioned for microstructure analysis, thickness measurements and deposition efficiency. Free standing films were used for mercury intrusion porosimetry. Grey levels of the coatings were obtained by optical microscopy and subsequent digital image recording. X-ray-diffraction analysis was also used to obtain the phase composition. Results showed that different particle temperature and velocity conditions lead to specific porosity and varying colors of the deposit. The color of the deposit was correlated directly to the amount of monoclinic phase in the as-deposit material.

For electrical or thermal insulation, the porosity of an air plasma sprayed (APS) coating is an important property to control. Moreover in aggressive environment the interconnected porosity is responsible for the substrate corrosion. To solve, at least partially, this problem, deposition by mutitechniques (APS and a PECVD) was used to close interconnected and opened porosities. In this study, titanium alloy (TA6V) substrates were coated by alumina using either one or both deposition processes. Electrochemical characterization technique was used to evaluate the open porosity in alumina coatings. It consists of evaluating the polarization resistance of the reference sample surface (uncoated substrate) and to compare it to coated ones. After different tests for selecting the electrolyte solution, the influences of different parameters (thickness and relative position) of each deposition process on coating porosity were examined.

Microstructure of plasma-sprayed thermal barrier coating has an influence on the lifetime under service conditions in aero engine. This paper focuses on problems of preparing and comparing ceramic samples and characterising microstructure features such as porosity and cracks within coating cross sections by image analysis. The results can be used for developing standards and best practices for characterization plasma-sprayed coatings. The methodology was evolved to relate coating microstructure to spraying parameters and properties of coating. It would help to develop new ceramics coatings or optimise them.

The surface finish characteristics of thermally applied coatings are an important component in establishing the resultant wear and friction values of the tribological systems. An ongoing issue, in terms of specifying and measuring the surface finish of the thermally applied coatings, is the inability to separate the inherent surface finish from the effect of porosity. The paper provides data developed utilizing a recently developed three dimensional surface finish measuring system for quantifying finish of thermally applied cylinder bore coatings. The advanced Three Dimensional Surface Finish System quantifies: • Surface finish values over the total area of use with a 95% confidence value with and without including the effect of porosity • Size distribution and percent area occupied by the surface porosity • Torn and folded material developed as a result of the honing or grinding process in applications such as engine cylinder bores • Hone angles in cylinder bore or grinding lead angles on seal surfaces.

Ceramic coatings are applied in mechanical components subject to high temperature conditions, normally are deposited by plasma thermal spraying process. In this work, the porosity of YSZ ceramic coatings, deposited with different parameters conditions were analyzed by optical microscopy, scanning electron microscopy using back-scatter electron (SEM-BSE) detector and ultrasonic technique. It was verified that porosity measurement by optical and scanning electron microscopy is very sensitive with respect to metallographic preparation, mainly cutting process, and gray level adjustment. SEM-BSE technique showed less scatter results with easier porosity visualization, compared with optical microscopy. The porosity of the coatings was also measured by ultrasonic technique. It was observed that the ultrasonic velocity increase with porosity reduction. Ultrasound technique showed a good correlation with OM and SEM porosity measurement.

The purpose of this work is a general assessment of the evolution of the porosity of a thermal barrier coating (PSZ : Partially Stabilized Zirconia with 7% wt yttria) used as an SOFC electrolyte sprayed by a High Power Plasma system Plazjet using commercial powders. The properties of these coatings are investigated using a microstructural and a physical approach. The different steps concern : - the optimisation of the spraying parameters of the Plazjet gun, in order to obtain either very dense, or porous top coat microstructure. We focused on parameters such as the gun (external, or step) or the morphology of the powder. The use of a strict methodology to analyze the results. Characteristics obtained by various measures, qualitative and quantitative, are compared and discussed.

A new method was introduced to fabricate porous coating and to control its porosity by changing spray parameters. Two models were constructed to simulate the deformation of molten particle and the deposition process of splats on substrate. Considering the gap-pore, step-pore and shield-pore in the presented models, the influence of spray angle on the porosity of coating was mainly investigated. Besides, other spray parameters which influence porosity were analyzed, including the velocity of spray gun, particle velocity and particle size. Moreover, a new forming method of graded porous coating is discussed.

This study evaluates the through-porosity of HVOF-sprayed Hastelloy C coatings on carbon steel with respect to coating thickness and combustion pressure. The amount of through-porosity in the NiCrMo alloy layer is determined by chemical analysis, using ICP emission spectroscopy, and compared with electrochemical measurements. Paper includes a German-language abstract.

Moving from a 2-dimensional to a 3-dimensional approach to microstructure and properties has been expected eagerly for a long while to result in a dramatic increase in the knowledge of thermally-sprayed processes and coatings. To meet these expectations, in the present work, microtomography and electrochemical impedance spectroscopy (EIS) were carried out to simulate the microstructure of plasma-sprayed alumina. As-sprayed and excimer laser-processed deposits were studied. Some unexpected but relevant results, e.g. regarding pore orientation in the coatings, could be obtained. EIS simulation led to the establishing of an electrical circuit equivalent to the microstructure which simulated the insulating properties as a function of interfaces and pore interconnection. The latter was studied by microtomography. From this 3-dimensional simulation, a finite element analysis of mechanical properties was developed and compared to experimental measurements. Using this approach to microstructure and properties, excimer laser surface processing was shown to be an innovative process to modify insulating characteristics of plasma-sprayed alumina.

The demand of industry for metallic thermal sprayed coatings with controlled porosity until now is fulfilled by the spraying of metallic powders mixed with additives (organic element in many cases) which play the role of pores. The new technology of cold spray can lead to the formation of innovate coatings of controlled porosity by using pure metallic (or alloy) powder without any further addition. A fine Al-12Si powder (<45 µm) was sprayed with a cold spray system (CGT Kinetic 3000-M) on stainless steel substrate under different spraying conditions. In the present study, the new polymeric nozzle PBI-33 of CGT was used for the formation of al-based coatings. The microstructure, the porosity, the Vickers microhardness and the superficial Rockwell hardness (R15Y) of the produced coatings are examined.

Cold gas dynamic spraying (CGDS) can be used to deposit oxygen sensitive materials, such as titanium, without significant chemical degradation of the powder and with minimal heating of the substrate. The process is thus believed to have potential for the deposition of corrosion resistant barrier coatings. However, to be effective a barrier coating must not allow ingress of a corrosive liquid and hence must have minimal interconnected porosity. Thus the aim of the present study was to investigate the effects of processing, including a post-spray annealing treatment, on the deposit meso- and microstructures and corrosion behavior. Commercially pure titanium powder was deposited using pre-heated nitrogen as main and powder carrier gas using a CGT Kinetiks 4000 system to produce coatings on stainless steel. Selected coatings were debonded from the substrate, and the resultant free standing deposits heat treated at 1050° C in vacuum for 60 minutes. Changes in microhardness were measured and correlated with microstructural changes. Optical microscopy, scanning electron microscopy, X-ray diffraction (XRD), helium pycnometry and mercury porosimetry were all employed to examine the microstructural characteristics of coatings and free standing deposits, before and after heat treatment. Their corrosion performance was also investigated using potentiodynamic polarization tests in 3.5 wt% NaCl. The influences of heat treatment and corrosion behavior will be analyzed and discussed in terms of pores structure evolution and microstructural changes.

Thermally sprayed ceramic coatings such as plasma-sprayed alumina exhibit a composite microstructure due to the presence of defects such as pores, interlamellar and intra-lamellar cracks. These second phase typed features influence the mechanical behaviour of the coating dramatically. In this study, an excimer laser surface treatment of plasma-sprayed alumina surface was developed for the optimization of component properties of a wireline tool used in the oil industry. In contrast to liquid phase treatment realized with CO 2 or YAG laser, an excimer laser processing presents short wavelength which means that for ceramic materials, the energy is absorbed in a region of the surface. This condition leads to surface treatment free of cracks. Effect of laser operating parameters, i.e. wavelength, pulse number and power density, on microstructure and the sealing quality of the coating are discussed. First, surfaces and cross sections of the microstructures were studied using image analysis of scanning electron microscope (SEM). Surface roughness and coating ablation were characterized according to laser treatment. Then, three dimensional (3D) microstructures were obtained using X-ray microtomography to evaluate the 3D porosity after laser treatment. Finally, nanoindentation and Electrochemical Impedance Spectroscopy (EIS) were carried out to characterize respectively the mechanical and electrical properties of the modified coating microstructure. The excimer laser surface processing was shown to be an innovative process to control the insulating characteristics of plasma-sprayed alumina.

Polymer cold spray has yielded lower deposition efficiency (DE) and quality deposits compared to metal cold spray. The disparity stems from metals being studied far longer than polymers in cold spray; in addition, polymers exhibit richer thermo-mechanical behavior. An experimental study was conducted to examine the effects of polymer feedstock degree of crystallinity (D) on cold sprayed deposits of polyetherketoneketone (PEKK), a thermoplastic used in aerospace and other high-performance applications. As deposition relies on the plastic deformation of the impacting particle, polymers with high D may inhibit deposition, reducing deposit quality and efficiency. This study evaluates three PEKK grades produced using different ratios of terephthalic (T) to isophthalic (I) monomer moieties (T/I = 60/40, 70/30, 80/20). The ratios control D, with higher proportions of T monomers corresponding to higher crystallization rates and degrees of crystallinity. A parametric study was completed to evaluate functional process set points of system carrier gas temperature and powder mass flow rate. Using operational parameters common among the PEKK grades, spray cycles were completed for each material and quantitative responses to variation in crystallinity were evaluated through a suite of analyses. DE of the materials was assessed gravimetrically, deposit porosity was evaluated by scanning electron microscopy, and thermophysical changes to the feedstock during the spray cycle were determined by differential scanning calorimetry. Overall, we found that cold spray processing of powders of lower D formed less porous deposits with a higher DE than more crystalline powders sprayed at the same process conditions. PEKK grades with lower T/I ratios achieved DEs in the range of 60-75%, whereas the most T enriched grade only reached ~10% DE.

Cold spray is a high-speed solid state deposition technique which allows for fabricating coatings and free standings structures by careful tuning of process parameters. However due to the complex dependencies of the process parameters a machine learning approach is utilized in this work to predict the coating properties. A machine learning (ML)-based data driven platform for determining the deposition porosity for cold sprayed deposition with the goal of reducing product cost and time has been developed. In this work, five ML models - Linear Regression, Decision Tree Regression and Random Forest Regression, XGBRegressor and LGBMRegressor are compared for the prediction of coating porosity from spray and material parameters namely Heat Capacity Ratio of carrier gas, Processing Gas Temperature, Processing gas pressure, Standoff Distance, Average Powder Diameter, Powder Material Density, and the Substrate density. A total number of 227 data sets were extracted from an extensive literature survey on cold spray deposition of metal/alloy powders which were used to train the ML models. The data analysis showed strong and weak correlations of several processing parameters with the coating porosity. The processing gas temperature and pressure have a negative and average powder diameter has a positive correlation with the deposition porosity. The coating porosities of 10 unknown sets (which were not included in the training or the validation data sets) of processing parameters predicted by the trained algorithms were compared with each other. Decision Tree regression algorithm showed the most appropriate predictions with R2 Fit of 0.75 and MAE of 2.93, while the Linear Regression model did the worst predictions with R2 Fit of 0.27 and MAE of 5.064.

Fabrication of Thermal Barrier Coatings (TBCs) with higher lifetime and relatively cheaper processes is of particular interest for gas turbine applications. Suspension Plasma Spray (SPS) is capable of producing coatings with porous columnar structure, and it is also a much cheaper process compared to the conventionally used Electron Beam Physical Vapor Deposition (EB-PVD). Although TBCs fabricated using SPS have lower thermal conductivity as compared to other commonly used processes, they are still not commercialized due to their poor lifetime expectancy. Lifetime of TBCs is highly influenced by the top coat microstructure. The objective of this work was to study the TBCs produced using axial SPS with different process parameters. The bond coat was deposited using High Velocity Air Fuel (HVAF) spray. Influence of the microstructure on lifetime of the coatings was of particular interest and it was determined by thermal cyclic fatigue testing. Thermal conductivity of the coatings was determined by laser flash analysis. The results show that axial SPS could be a promising method of producing TBCs for high temperature gas turbine applications.

Providing for the DZ400 arc spray system, the new design high-velocity arc spray gun with three different nozzles is developed to produce the coatings with the structure of superfine and low porosity. This system can be used to spray the three normal wires of 4Crl3, FeCrAl and 7Crl3 (flux cored wires). With the SEM to analysis shape and particles size sprayed by the nozzles by different parameters, as well as with the S-3500N SEM (made by HITACHI) and the INCA ESA (made by OXFORD) identify the content of the oxides, porosity and thickness of the coatings. The result shows that the porosity in the coatings of solid wire is less than 2%, of the flux-cored wires is less than 4%, and the coating sprayed by the nozzle with the secondary supplementary airflow is typically shown to be distributed in the form of high-density lamellar splat structure and the average lamellar thickness is around 5 μm.