Dysprosia stabilized zirconia coatings with large globular pores have good potential as TBC topcoats. In previous work, such coatings have been produced by air plasma spraying with the aid of a polymer pore former. The aim of this work is to optimize the spraying parameters. A design of experiments approach was used to create a two-level full factorial test matrix based on spray distance, powder feed rate, and hydrogen flow. An agglomerated and sintered dysprosia stabilized zirconia (DySZ) powder mixed with polymer particles was sprayed on Hastelloy X substrates that had been prepared with NiCoCrAlY bond coats. The coatings obtained were evaluated based on thermal conductivity, thermocyclic fatigue life, and morphology, which are shown to correlate with spray parameters and in-flight particle properties.

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.

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.

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.

Porosity is a key feature of thermally-sprayed coating microstructure. Porosity is made of pores and cracks of various orientations. Both pores and cracks can be intralamellar or interlamellar due to coating build-up which leads to lamellae from impinging of droplets. Pores are interconnected with cracks, which results in a 3-dimensional porosity network. Direct observation of this network is intricate and remains somewhat limited. A 3-dimensional simulation of this network was therefore developed in this work based on the building-up of objects which simulated the lamellae in the sprayed microstructure. These objects were constructed from morphological measurements using confocal microscopy of actual lamellae, i.e. “splats”, obtained from “linescan”-typed plasma-sprayed experiments. This simulation, in the lamella building-up, involves randomly cracks and pores the characteristics of which (i.e. content, orientation, size, …) were determined from thorough quantitative image analysis of cross-section plasma-sprayed alumina microstructures. Using 3-dimensional images resulting from the simulation, finite element calculations were performed to study dielectric properties of plasma-sprayed alumina as a function of porosity. The influence of anisotropy is discussed in particular and calculated values compared to experimental values.

This paper describes an advanced three-dimensional surface finishing system for quantifying the surface finish of thermally applied cylinder bore coatings. The system can measure surface finish values over the total area of use with and without including the effect of porosity, as well as size distribution and percent area occupied by surface porosity. It can also quantify torn and folded material developed during the honing or grinding process, as well as hone angles in cylinder bores. The system provides a powerful tool for cylinder bore finish development and monitoring the effect of porosity on the finish of plasma coated cylinder surfaces. The paper also discusses the development of replicate material and the comparison of finish values developed through direct imaging and replication. Additionally, it explores the quantification of torn and folded material and crosshatch angles in the cylinder bore finish. Overall, the system improves the quality of finishing data and provides a time-saving solution for production or research and development environments.

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.

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.

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.

Three kind of commercial ceramics powders, Al 2 O 3 , ZrO 2 and TiO 2 were deposited on 304 stainless steel plates by plasma spraying technique. Their porosities were evaluated with digital image analysis method. As the accuracy of this method depends significantly on metallographic preparation and metallography procedure for coating sample, we studied the effects of surface roughness, magnification and number of fields of view on the porosity. The proper values for the three parameters are discussed.

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.

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.

New concept and method of testing for the interconnecting porosity of thermally sprayed nonconductive ceramic coatings is proposed. This process is useful to differentiate the open porosity from the closed porosity. Thermally sprayed ceramic coating with metallic substrate is plated, and the intergranular gaps in the coating are filled by deposited metal. Typically, 304 stainless steel substrate deposited with atmospheric plasma sprayed alumina coating is immersed in copper(II) sulphate bath, and electroplated. Inward of the alumina coating, plating solution penetrates toward the interface of coating/substrate via interconnected porosity, and attains the coating-substrate interface to deposit metallic copper. Deposit of copper is gradually grown up along the coating intergranular. The exposure of deposited copper on the coating surface can be visible. Because these phenomena occur only in the interconnected pore structure, it is easy to differentiate the interconnected porosity from the closed porosity. Also the proposed process suggests the unprecedented possibility of manufacturing method of three-dimensional structure of thermally sprayed ceramic coating.

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.

In this study, porous and dense layers of alloy 625 are deposited on nickel foam sheets using a modified twin wire arc spraying process. Sandwich panels with arc-sprayed alloy 625 skins on nickel foam cores were fabricated then subjected to four-point bend testing. The effects of skin porosity on flexural rigidity and overall mechanical behavior are investigated. The ductility of porous alloy 625 skins was improved after heat treatment at 1100 °C for 3 h.

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.

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.