This study assesses the influence of atmospheric plasma spraying parameters on splat stacking and porosity formation in bioglass coatings prepared from commercial powders. Coating samples were deposited on stainless steel substrates using spraying parameters established through numerical simulations. Different Ar-H 2 mixtures were used as the forming gas, and plasma current and spraying distance were varied. Coating microstructure and phase composition were determined by SEM and XRD analysis. Although numerical simulations for each parameter set predicted a suitable Sommerfeld number for proper splat stacking, Na 2 O and P 2 O 5 volatilization occurred during spraying, promoting the formation of porosity in the coatings. Denser coatings were obtained, however, by adjusting the gas mixture ratio, plasma current, and spraying distance such that enthalpy of the plasma jet is sufficient to overcome the glass transition temperature of the powder and at the same time avoid the evaporation of volatile oxides.

The paper describes the major features of a recently developed high voltage – low current air plasma spray (APS) process and torch that is based on combined wall and gas stabilizations of plasma (C+Plasma). It is shown that the C+Plasma process is capable of efficiently generating stable plasmas without drifting or pulsing. Plasma gas selection includes N 2 , N 2 -H 2 , N 2 -Ar-H 2 , Ar-H 2 (He), etc. Availability of stable N 2 -H 2 plasmas having enthalpies within 30-70 kJ/sl range offers a new level of APS efficiency and coating quality. The paper includes illustrations of the C+Plasma durability, stability and expanded operating window. The process capability is also illustrated by properties of the advanced MCrAlY bond coatings and dense segmented TBCs (thermal barrier coating).

A recently developed air plasma spray (APS) process and a torch based on the combined wall and gas stabilizations of plasma (C+Plasma) has demonstrated extremely wide operating window and capability to efficiently generate both argon and nitrogen based plasmas including high enthalpy N 2 - H 2 ones. The present paper aims to describe new areas of the extended C+Plasma operating window, in which a significant increase in the APS sprayed coating quality and process efficiency should be expected. The paper includes a brief analysis of properties of different plasmas as well as specifics of coating formation. Analysis showed that with the high enthalpy N 2 -H 2 plasmas a significantly higher heat transfer potential and higher flexibility in controlling the particle acceleration can be achieved than with the argon based plasmas. Consequently, C+Plasma can enable better treatment of sprayed particles, which results in better deposit efficiency and improved coating properties.

In this study, a wide range of suspension plasma spraying conditions are used to produce YSZ coatings for intended use in liquid gas engines. To meet specifications, the coatings must exhibit a homogeneous microstructure with no vertical cracks or columns, low surface roughness, and low thermal conductivity. The properties of the plasma jet (velocity, enthalpy, stability), droplets (trajectory, number, size), and particles (velocity) were measured during spray trials and are correlated with coating microstructure. Suspension plasma spraying conditions necessary for depositing disk-shaped splats and achieving finely structured coatings with no stacking defects are described along with substrate cooling requirements.

Yttrium oxide (Y 2 O 3 ) can be used in different applications such as corrosion resistance, high temperature applications and semi-conductor production equipment due to its very high thermal and chemical stability. In the current research, yttria coatings were processed using a new type of DC plasma gun consisted of molecular gases CO 2 +CH 4 . Physical and structural properties were compared with the coating made by SG-100 plasma torch. Gas mixture of CO 2 +CH 4 improves the torch efficiency due to its high thermal enthalpy and conductivity which leads to increased particle temperature and complete fusion of the sprayed particles during the process of coating. SEM study of the structure revealed that the coating has higher density and lower porosity compared to the coating produced by SG-100 torch. No unmelted particles can be observed in the coating. XRD analysis of the coating showed that the coating contains no amount of harmful metastable monoclinic phases. This all proves the better quality of the coatings deposited by CO 2 +CH 4 gas mixture in comparison to the conventional coatings.

Very low pressure plasma spraying has been intensively studied in recent years especially the properties of plasma jet. These properties are affected by plasma generating and working conditions. These operating parameters such as arc power, plasma gas flow rate and chamber pressure have influences on specific enthalpy and temperature of plasma jet. In this work, the measurements under very low pressure were performed using enthalpy probe which was previously modified (increase of the internal diameter and depositing TBC coating (Ni/Al and ZrO 2 + Y 2 O 3 ) on the head). Different parameters, for instance, current intensity, hydrogen gas flow rate and detecting distance were changed in order to point out their effect on the characteristics of plasma jet. The specific enthalpy, temperature and quantity of heat in this situation were obtained.

Arc instability in conventional plasmas causes variations in particle properties that can lead to poor coating quality and low deposit efficiencies. In this paper, the authors explain how the use of molecular gases with higher enthalpy mitigates the effects of arc instability, resulting in higher spray rates, better coating quality, and lower cost per kilogram of material deposited. They also describe the design and operation of a commercial high-enthalpy plasma spraying system and present and analyze coatings of different materials thereby produced. Paper includes a German-language abstract.

Some applications of thermally sprayed coatings need a metallurgical bonding of substrate and coating. This can be reached by laser remelting of a thermally sprayed coating, which causes, on the other hand, a certain dilution of the substrate elements into the coating. This article discusses the influence of reaction enthalpies on the microstructure formation in the alloying systems Ni-Al and Ti-Al. Experimental work and simulation were done to examine the time constants of solidification influenced by laser dwell time and reaction enthalpy. It was observed that, for short dwell times, the reaction heat dominates the solidification process and the microstructure formation.

Plasma spraying of metals and metallic alloys performed in controlled atmosphere or soft vacuum results in coatings with a low oxidation level and excellent thermomechanical properties. Unfortunately, the spraying cost is drastically increased by one or two orders of magnitude compared to air plasma spraying (APS). Thus the minimisation of oxidation during APS is a key issue for the development of such coatings. Oxygen concentrations sucked into plasma jets have been measured by an enthalpy probe linked to a mass spectrometer. This technique allows to determine simultaneously plasma composition, temperature and velocity distributions within the plasma plume. Results have been compared to those obtained with a two-dimensional turbulent flow model. The obtained results have shown that surrounding air entrainment is reduced when using adequate Ar/H2/He mixtures which viscosity is higher than that of Ar/H, mixtures, limiting the turbulence in the jet fringes and pumping of the surrounding atmosphere.

Experiments have shown that the mechanical properties of plasma-sprayed coatings depend to a large extent on the details of the spraying process, in particular, they are strongly dependent on the details of the solidification and deformation history of the individual droplets which are in turn highly affected by the substrate conditions such as its temperature, material, and surface thermal contact resistance. In this study, droplet-substrate interactions are investigated through a complete numerical solution of droplet impact and solidification for a typical thermal spray process. The energy equation is numerically solved for both droplet and substrate regions; the solution is based on the Enthalpy Method for the liquid and solidified parts of the droplet, and the conduction heat transfer in the substrate. The numerical solution for the complete Navier-Stokes equations is based on the modified SOLA-VOF method using rectangular mesh in axisymmetric geometry. The developed model is suited for investigating droplet impact and simultaneous solidification permitting any desired condition at the substrate. The splat shape, the solidification front, and the temperature profile in the entire droplet and substrate regions are obtained at any desired time elapsed after the impact. Through these results, the nucleation and growth of solidification and droplet-substrate interactions are extensively studied.