This work focuses on the processing and deposit by suspension plasma spraying (SPS) of ZrO 2 -based ceramic materials for Thermal Barrier Coatings (TBC's) applications. The system of interest is ZrO 2 -16mol%Y 2 O 3 -16mol%Ta 2 O 5 (16YTZ). This ceramic has been reported to keep a non-transformable tetragonal phase (t'-phase), suitable to overcome the thermodynamic limits of the mostly used conventional 7-8wt.% yttria stabilized zirconia (YSZ). The research consists into evaluate the t'-phase stability and performance of the 16YTZ SPS coating. Synthesis of 16YTZ and, the evolution of the resulting microstructure in the dense ceramic and in the coating are a central part of the study. Sintering behavior in dense ceramics prepared from both precursor derived and milled powders is evaluated. Microstructural characterization by XRD, SEM and RAMAN spectroscopy of the as-deposited ceramic coating is presented and discussed.

In order to expand the fields of application and to improve the performance of graphite (Cg), it is necessary to reduce its permeability towards of oxygen and to limit its reactivity and especially its oxidation. It is, therefore, essential to protect it from the environment through the use of ceramic coatings. Adhesion between ceramic coatings and graphite is controlled by the mechanical stresses in the coatings and the thermodynamic work of adhesion. Different metal-graphite systems were examined which showed that the adhesion particularly depended on the thermal expansion coefficient mismatch between the two materials and on metal carbide stability. Thus, the role of the addition on the graphite surface of elements such as Cr, Mo, Al, Si, O on the adhesion of metals or ceramics to graphite has been identified.

Existing state of the art thermal spray processes (HVOF, D-Gun, Plasma Spraying) are limited to powder velocities of about 1 km/sec because they rely on the thermodynamic expansion of gases. A new thermal spray process using electromagnetic forces can accelerate powder particles to a final velocity of up to 2 km/sec. At this velocity powder particles have sufficient kinetic energy to melt their own mass and an equivalent substrate mass on impact. The process is based on railgun technology developed by the Department of Defense. A railgun is filled with argon gas and a high energy electrical pulse, provided by a capacitor bank, drives the gas down the railgun to a final velocity of up to 4 km/sec. This gas passes over a powder cloud and accelerates the powder through drag forces. The electrical and powder discharge frequency can be adjusted so that the deposition rate and thermal input to the substrate can be controlled.

Today, the efficiency of turbines is limited by different losses. Minimizing these losses is a main goal to reduce fuel consumption and produce more environmentally friendly machines. Observations on the scales of fast swimming sharks display a riblet structure. These riblets provide a significant reduction of drag losses, but are quite sensitive on pollution. Therefore, for a good performance, it is essential to combine these structures with self-cleaning properties. A lateral- and depth-selective distribution of particles with a negative thermal expansion coefficient (NTE) in a binder with positive thermal expansion coefficient can be used to deform the surfaces depending on the temperature. At high temperatures a riblet structure will be formed by local expansion or shrinkage and at cooling down the surface will be cleaned by the reversal of the deformation. Beside the production of a coating with a lateral- and depth-selective distribution of the NTE-ceramics within the binder the thermodynamical stability of the ceramics inside the binder is part of the investigations to provide a sufficient long-time stability of the coating.

Due to excellent mechanical properties and low density compared to super alloys (e.g. Ni-based alloys) Titanium Aluminide is often used as base material in the aerospace industry. But the thermodynamic conditions within turbines limit the capabilities of the material. At the moment γ-TiAl is used for parts, which have to withstand temperatures up to 700 °C. Above this temperature oxidation kinetics cause a thick oxide layer consisting of several oxides, which tend to fast chipping. Therefore the surface of the γ-TiAl is being destroyed and the material loses its excellent mechanical properties. To enable the use of this material at higher temperatures, the development of an oxidation protection coating is necessary. Several coating techniques e.g. EB-PVD were tried in the last years, but the oxidation behaviour of the γ-TiAl could not be significantly improved. Protective thermal spray coatings so far seem to be a promising technology in order to protect γ-TiAl components against oxidation. Therefore this technique was used within this work, which aims for the development of new oxidation protection coatings. A multilayer system was developed. The multilayer consists of a ceramic ZrO 2 -7Y 2 O 3 coating with a NiCoCrAlY top coat. In this case the ceramic coating avoids the diffusion of Ti or Al of the γ-TiAl into the MCrAlY coating or the other way around. The NiCoCrAlY coating improved the oxidation behaviour of the Titanium Aluminide by building a dense oxide layer on top of the multilayer. The paper will give an overview about the results of the oxidation tests with the new developed multilayer concept for protection of the γ-TiAl against oxidation.

Tungsten carbide in nickel based self-fluxing alloy overlays has been dominating hardfacing applications due to its excellent properties, namely extremely high wear resistance. Nevertheless, there are still applications and limits which tungsten carbide has not conquered. This study focuses on (TiW)C 1-x which was deposited with several matrix materials and tested in wear, corrosion and impact resistance and benchmarked against tungsten carbide. Results for several other carbides such as (NbW)C 1-x , (VW)C 1-x , NbC 1-x and TiC 1-x overlays deposited by plasma transferred arc (PTA) and laser cladding (LC) will be presented and discussed. As a result of deposition trials and overlay testing, it was found that better thermodynamic stability of alloyed carbides allows them to be used in an iron based matrix and/or a matrix with a high chromium content, in applications requiring improved corrosion and oxidation resistance, better impact resistance and lower weight.

Most models used to simulate plasma torch operation consider only the gas domain with the electrodes included as boundary conditions, imposing a current density profile and temperature at the cathode tip. In order to achieve more realistic simulations, it is necessary to include the electrodes and arc column in the calculation domain. This paper discusses some of the issues, factors, and considerations involved in modeling arc-cathode and arc-anode interactions and their effect on the plasma jet. Descriptions of the arc column and plasma jet are based on the coupling of electromagnetic and fluid equations and require thermodynamic, transport, and radiative property values of the gas mixture. In order to capture the stochastic behavior of the arc, it is assumed that the model is 3D and time-dependent.

During air plasma spraying, molten metal particles flying in the plasma jet are oxidized. As a result, a part of the metal melt is converted into oxide melt. After the particle impact and solidification, oxidation continues as a gas – solid reaction. The present paper deals with oxidation of two binary Ni-based alloys. One of them, Ni-20%Cr, is frequently used in thermal spray applications. Another one was a Ni-Fe alloy with an approximate proportion of both components 1:1. The feedstock powders were plasma sprayed by a water-stabilized gun WSP. PAL 160. To analyze the reaction products of the in-flight oxidation stage, the flying particles were trapped and quenched in liquid nitrogen. Oxides resulting from both oxidation stages were studied in the as-sprayed deposits after their cooling down to room temperature. The oxide amounts in the samples were determined indirectly by oxygen level measurement using "extractive fusion" (LECO-method). Structure of the oxides, separated by dissolution of the metallic phase, was investigated by X-ray diffraction. Iron-containing oxides were also characterized by Mössbauer spectroscopy. From the point of view of reaction kinetics, both alloys behaved in a similar way. The particles quenched in liquid nitrogen contained less than 2% of oxygen, whereas in the deposits these values were higher, up to 4.5 %. Two oxide phases were found in all plasma sprayed Ni-Cr samples: a rhombohedral phase similar to (Ni,Cr) 2 O 3 and a tetragonally distorted spinel phase (Ni,Cr) 3 O 4 , both of them very rich in chromium. Another oxide, NiO, was present mainly in the deposits. In the oxidation products of Ni-Fe alloy, the dominant phase was similar to nonstoichiometric wüstite FeO. The results of thermodynamic calculations are in sound agreement with the experiments except for the presence of the tetragonal phase, the composition of which is near to Cr 3 O 4 , in oxidation products of Ni-Cr alloy.

This paper provides an overview of chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes and the advantages they offer physical vapor deposition for the application of friction and wear coatings for micromechanical assemblies and components. It explains how hard and solid lubricant phases can be applied by these non-line-of-sight deposition methods, achieving nanoscale conformality and coating uniformity on buried surfaces and interfaces. It also discusses inherent disadvantages and explains how plasma excitation can be incorporated in either process to overcome material limitations.

It has been well known that the coating quality of plasma spraying is strongly influenced by instability of jets in plasma spray due to the arc root fluctuation. A three dimensional (3D) unsteady modeling was employed in the research to analyze the arc root fluctuation in a DC non-transferred plasma torch. Numerical calculations on the distribution of gas temperature and velocity in plasma torch were carried out using argon as plasma gas. The electrical current density and potential were also discussed. The results indicate that the fluctuation of arc inside the plasma torch is mainly induced by the movement of the arc root on the anode surface. The arc root moves downstream with the flow of gas, and the arc will warp from the electromagnetic force simultaneous to the movement. While the arc warps close to the anode boundary, a new arc root is formed somewhere upstream of the original attachment. This article represents nature of fluctuation of arc root, also in this paper we will present that the voltage-drop calculated is larger than that measured experimentally based on the hypothesis of local thermodynamic equilibrium.

Utilizing big data to govern decisions is becoming increasingly valuable and the thermal spray process is no exception. The thermal spray process is unique as a material process in its capability to employ a wide range of advanced materials technologies: metals, ceramics, cermets, and oxides among others. Like any process, the thermal spray technology is most effective when utilizing material feedstock which is specifically designed for thermal spray. This paper will discuss how big data techniques can be employed to design disruptive materials technology. The thermal spray process presents unique challenges to modelling and simulation work due to the inherent complexity of the process. However, these challenges offer the opportunity to develop materials tailored for specific thermal spray processes to yield improved coating performance. Furthermore, big data material informatics can significantly accelerate the discovery of new alloy solutions. More than 100 years of experimental research underpins the science employed, but modern computational tools and materials informatics principles enable new decision strategies to be utilized. The big data approach relies on calculations which predict the microstructure of millions of alloy compositions and utilizing proprietary data mining algorithms to identify unique materials spaces which would never be discovered experimentally.

Plasma spray technology is widely employed by industry to apply coatings on different components to protect them from corrosion, wear and high temperature environments. The gases introduced into the DC plasma torch are heated by the arc and a plasma jet exits the torch. Powders are injected into the plasma jet where they are then accelerated, heated, and melted before impacting the substrate, which is placed at some distance from the outlet of plasma spray torch. Plasma arc exhibits strong voltage fluctuations which correspond to the movement of the anode arc root attachment. Understanding the arc movement within the torch and how it affects the flow and temperature fields of the plasma jet exiting the torch is of great importance. Understanding the flow, temperature and electromagnetic fields within the DC plasma torch is extremely challenging and there is a limited number of investigations in the literature. In order to provide unique sets of surface characteristics, e.g., thermal barriers, wear and corrosion resistance, a high quality coating with appropriate combination of powder and base materials must be produced. To produce a high quality coating, powder particles should be uniformly heated and accelerated, and then deposited onto the substrate. In this paper, an unsteady 3-dimensional model of the arc movement within the plasma torch is reported. The proposed model is employed to solve electric potential and magnetic vector potential equations in addition to continuity, momentum and energy equations. The k-ε turbulence model was used to model the turbulence of the flow field inside a non-transferred DC argon plasma torch. The geometry of the torch was that of SG-100 torch (Praxair). TO study the effect of the arc length on the voltage, first a steady-state model was considered for a range of arc lengths and arc-root radii. The results of this model provided the relation between arc length and arc voltage for a set of arc root radii and given argon flow rate. Then, given voltage fluctuation profile, the unsteady, arc root attachment movement was simulated from the estimation which found from steady models. Results show that the effects of velocity and temperature fluctuations at the outlet of the torch (where the particles are injected) are not negligible and such fluctuations exceed 15% of their average values. These will in turn affect the particle heating history and will negatively impact the microstructure of the coating.

The hypothesis of local thermal equilibrium (LTE) in thermal plasma has been widely accepted. Most of the simulation models for the arc plasma torch are based on the hypothesis of LTE and its results indicate a good validity to mimic the pattern of the plasma flow inside the plasma torch. However, due to the LTE hypothesis, the electrical conductivity near the electrodes is significantly low because of the lower gas temperature. Consequently, it is difficult for the flow of electrical current to pass between the anode and cathode. Therefore, a key subject for a model depending on the LTE assumption is to deal with the low electrical conductivity near the electrodes. In this study, two models, determining the electrical conductivity at the vicinity of the electrodes with two different assumptions, were employed to calculate the flow patterns inside a non-transferred DC arc plasma torch. A comparison of the gas temperature, velocity, voltage drop and the heat energy of the plasma arc between the two models were carried out. The results indicate that plasma arc inside the plasma torch fluctuates as simulated by both of the two models. It seems that the model can obtain comparable accuracy compared with the experimental results if the plasma gas electrical conductivity is determined by a nominal electron temperature.

A numerical model of a supersonic compressible plasma flow has been developed with the aid of CFD software to describe the thermodynamic and transport properties of a plasma jet in order to investigate the PS-PVD process and how to optimize it for thermal barrier coatings and, in particular, the formation of columnar microstructures. The required properties of the plasma gas mixtures were obtained as a function of temperature and pressure from thermodynamic calculations in chemical equilibrium with the effect of ionization. Two-dimensional Monte Carlo simulations were conducted to provide insight on the evolution of columnar microstructure, accounting for self-shadowing and vapor incidence angle but ignoring the effect of diffusion. Simulated structures and predicted values are presented and compared with actual images and measurements.

A multiscale model is being built in order to better understand and predict high-temperature corrosion and erosion properties of thermal spray coatings and materials in general. The approach uses molecular dynamics to predict diffusion kinetics, constrained free energy to determine reactions, and FEA to simulate structure. To obtain oxidation behavior data for validation, surface polished bulk materials and thermal spray coatings were exposed to various temperatures and exposure times. Oxidation depth and diffusion were assessed by optical emission spectroscopy and cross-sectional SEM examination and surface oxidation in grain and lamellae boundaries was characterized by 3D profilometry and SEM-EDS. Rough validation of the model was achieved using indentation test data, and a more complete validation will be done when high-temperature erosion test results are available.

Anode erosion is a common concern in dc plasma spray torches. It depends largely on the heat flux brought by the arc and the dimensions, residence time, and mode of the arc attachment to a given location on the anode wall. This paper compares anode arc attachment modes predicted by LTE (local thermodynamic equilibrium) and 2-T (two-temperature) arc models that include the electrodes in the computational domain. The analysis is based on a commercial cascaded-anode plasma torch operated at high current (500 A) and low gas flow rate (60 NLPM of argon). It shows that the LTE model predicted a constricted anode arc attachment that moves on the anode ring while the 2-T model predicted a diffuse and steady arc attachment. The comparison between the predicted and measured arc voltage indicated that the 2-T prediction is closer to the actual voltage. A post-mortem observation of a new anode ring on a plasma torch operated under the same conditions confirmed the diffuse arc attachment predicted by the 2-T arc model.

In this study, high-emissivity NiCr oxide coatings, one with TiO 2 and one with TiB 2 compounds, were deposited on stainless steel substrates by spray granulation and atmospheric plasma spraying. The main phases in both coatings are NiTiO 3 , spinel (NiCr 2 O 4 ), and residual Cr 2 O 3 . The emissivity of the layer with TiO 2 was found to be higher in the 2.5-8 μm range than that of the layer with TiB 2 , which is attributed to higher surface roughness and larger particle size. These results indicate that the NiCr oxide coatings with TiO 2 compounds are more suitable for high-temperature IR applications.

This study investigates the effect of gas composition on the flow characteristics of a three-cathode air plasma torch. A numerical model that couples fluid dynamic, electromagnetic, and thermal relationships is used to simulate temperature and velocity fields at the outlet of the torch. Different gases, including argon, nitrogen, and hydrogen, and gas mixtures are examined in the context of the study. The results show that the use of N 2 or H 2 as a secondary gas improves the output power and efficiency of the torch.

Pratt & Whitney's upper stage rocket engine development program, designated "RL60", has incorporated cold-sprayed copper to improve the design and function of this new engine. Combustion chamber designs contain two stainless steel manifolds connected by a series of copper tubes. The manifold where the hydrogen fuel exits is located near the injector face. The combustion gases from the injector would cause over-heating of this manifold. Thick copper application was needed to actively cool this manifold by conducting the cold temperatures from the hydrogen fuel inside the copper tubes. Plating copper greater than 0.050-inch thick resulted in poor adhesion following a subsequent braze cycle and required 2 weeks to plate. Cold sprayed copper was attempted which has surpassed plated copper in its ability to adhere through this braze cycle and can be applied in a few hours. In addition, the hazardous chemicals associated with copper plating have now been eliminated.

Throughout the years, parametric and computational approaches have been used extensively for the design of High Entropy Alloys (HEA) and Multicomponent Element Alloys (MEA). Machine learning (ML) approaches have been extensively used to circumvent the fundamental issues that challenge the proposed theories, models, and methods for conventional alloys. Highly accurate ML models rely heavily on the quality of data and the design features that are used as inputs to the model. The applicability of these methods for phase formation predictions is questionable when it comes to the design of thermally sprayed HEA coatings using gas or water atomized powders as feedstock material. Phase formation from liquid state depends on the cooling rate during solidification which is several orders of magnitude higher when compared to arc melted as-cast HEAs. In addition, during plasma spray the powder melts in the flame and re-solidifies under different cooling rates during deposition. To our knowledge, almost all ML algorithms are based on available datasets constructed from relatively low cooling rate processes such as arc melting and suction casting. A new approach is needed to broaden the applicability of ML algorithms to rapid solidification manufacturing processes similar to gas and water atomization by making use of existing data and theoretical models. In this study the authors introduce a cooling rate dependent design feature that can lead to accurate predictions of the HEA powder phase formation and the subsequent phases found in the spray coated materials. The model is validated by comparing the predictions with existing coating related data in the literature.