Primary Plasma Gas Research Articles

Characteristics of Thermally Sprayed Alumina-Titania Ceramic Coatings obtained from Conventional and Nanostructured Powders - A Review

ABSTRACT Alumina-titania thick coatings with finely layered microstructure are usually deposited using Air Plasma Spraying (APS) process. In the APS process, nitrogen or argon is used as the primary plasma gas. Microstructural characteristics of alumina-titania coatings are typically affected by the percentage of Titania (TiO2) present in the coatings. On the other hand, coatings obtained from nanostructured agglomerated alumina-titania powders have shown superior wear resistance over the corresponding conventional coatings. The properties of conventional Alumina-Titania coatings can also be modified by reinforcing them with some fillers like graphene, Carbon Nanotubes (CNTs), etc. From the past decade, significant research work has been in progress both industrial and academia side and still there is a lot of scope for future research on this process. This detailed review gives an overall view of Alumina-Titania coatings including their significance, microstructural, mechanical, wear, corrosion behavior and the effect of various reinforcements on their characteristics.

Effect of Critical Plasma Spray Parameter on Characteristics of Nanostructured Alumina-Titania Coatings

Micro and nanostructured alumina-13wt% titania coatings were sprayed via air plasma spraying (APS) technique using nitrogen as primary plasma gas. Microstructural characteristics and mechanical properties of nanostructured coatings are regularly influenced by percentage of unmelted/partially melted regions. Hence, the coatings were obtained as a function of critical plasma spray parameter (CPSP) defined as the ratio of arc power to the primary plasma gas flow rate. It was watched that for nanostructured coatings properties like porosity, micro hardness and percentage of partially melted/ unmelted regions were seen to be monotonic functions of CPSP. However, influence of CPSP on volume fraction of partially melted regions apparently is much smaller in nanostructured Al 2 O 3 -13wt%TiO 2 coatings deposited using nitrogen as compared to argon as the primary plasma gas

Influence of Critical Plasma Spray Parameter on Microstructural and Tribological Characteristics of Nanostructured Tungsten Carbide-Cobalt Coatings

Nanostructured tungsten carbide-cobalt coatings were deposited by air plasma spraying (APS) technique, using nitrogen as primary plasma gas. The characteristics of nanostructured coatings are typically affected by the percentage of unmelted/partially-melted regions present in the coatings. Hence, the coatings were obtained as a function of critical plasma spray parameter (CPSP), which is defined as the ratio of arc power to the primary plasma gas flow rate. It was observed that tungsten (W) rather than tungsten carbide (WC) was the dominant phase in nanostructured coatings due to decarburization. CPSP was found to have a significant effect on wear performance of APS nano-structured coatings. Wear resistance was highest for coatings deposited at lowest CPSP. Flaking was more prevalent in nanostructured coatings deposited at the highest CPSP.

Investigation of some process parameters using microwave plasma technology for the treatment of radioactive waste

ABSTRACTThe production of nuclear energy and the application of other nuclear technologies produce large volumes of low- and intermediate-level radioactive wastes. To investigate a novel means of treating such wastes, plasma is investigated for its efficacy. Plasma treatment promises to simultaneously treat all waste types without any previous sorting or pre-treatment. Microwave-driven plasma torches have the advantage of high-energy efficiency and low-electrode wear. In small-scale experiments, several design variations of an open plasma oven were assembled in order to investigate constraints caused by the materials and oven geometry. The experimental set-up was modified several times in order to test the design characteristics and the variation of plasma-specific proprieties related to the radioactive waste treatment and in order to find a suitable solution with the minimum complexity that allows a representative reproducibility of the results obtained. A plasma torch controlled by a 2.45 GHz microwave signal of up to 200 W was used, employing air as the primary plasma gas with a flow rate of ∼2 L/min. Different organic and inorganic materials in different shapes and sizes were treated besides a standardized mixture resembling mixed wastes from nuclear plants. The results prove that the chosen microwave plasma torch is suitable for a combined combustion and melting of organic and in-organic materials. Investigation of the specimen size to be treated is influential in this process: the power is still too low to melt larger samples, but the temperature is sufficient to treat all kinds of material. When glass particles are added, materials melt together to form an amorphous substance, proving the possibility to vitrify material with this plasma torch. By optimization of the oven configuration, the time needed to combust 25 g of standard sample was reduced by ∼50%. Typical energy efficiencies were found in the range of 8–20% for melting of metal chipping, and ∼90% for melting of zinc powder.

Characterization of air plasma-sprayed yttria-stabilized zirconia coatings deposited with nitrogen

Air plasma-sprayed (APS) yttria-stabilized zirconia (YSZ) conventional and nanostructured coatings were deposited with nitrogen as the primary plasma gas. Nanostructured coatings were deposited as a function of critical plasma spray parameter (CPSP). Microstructural characteristics, e.g., phase distribution, porosity, microhardness, percentage of partially melted (PM) regions, surface roughness, and wear performance, were evaluated for deposited coatings. Effect of CPSP on wear performance of coatings was most prominent at highest normal load considered. Conventional and nanostructured coatings deposited with nitrogen showed different trends for specific wear rate as a function of normal load. While specific wear rate for conventional coatings was nearly constant, it increased with increasing load for nanostructured coatings. Thus, even though nanostructured coatings provided better wear resistance at lower normal loads, conventional YSZ coatings become more wear resistant at higher loads. For example, at highest normal load considered, specific wear rate of conventional coatings was around 12 % smaller than that for corresponding nanostructured coatings.

Comparative study of air plasma sprayed and high velocity oxy-fuel sprayed nanostructured WC-17wt%Co coatings

Nanostructured WC-17wt%Co coatings were deposited using air plasma spraying (APS) and high velocity oxy-fuel (HVOF) spraying techniques. While air plasma sprayed coatings were deposited as a function of critical plasma spray parameter (CPSP) with nitrogen as the primary plasma gas, HVOF sprayed coatings were obtained with propane as the fuel gas for different oxygen to fuel ratios. Microstructural characteristics and wear performance were evaluated for the deposited coatings with the aim to correlate microstructural and tribological characteristics of coatings. Effect of CPSP on wear performance of nanostructured coatings deposited using APS was seen to be much more significant as compared to the effect of oxygen to fuel ratio for corresponding HVOF sprayed coatings. The effect of normal load on weight loss in wear tests for nanostructured coatings deposited using APS was seen to be more prominent than that for corresponding HVOF sprayed coatings. Tungsten was found to be the dominant phase in nanostructured coatings deposited using APS, while WC is the dominant phase in corresponding HVOF sprayed coatings. Further, wear resistance of nanostructured WC-Co coatings deposited using APS was found to much smaller than that for corresponding HVOF coatings, particularly at higher contact pressures.

MICROSTRUCTURES, MICROHARDNESS, AND CRYSTALLIZATION BEHAVIORS OF AMORPHOUS Al2O3-YSZ COATINGS PREPARED BY AIR PLASMA SPRAYING

Three types of amorphous Al 2 O 3-yttria-stabilized zirconia (YSZ) coatings were prepared using air plasma spraying (APS). The effects of the APS parameters on the microstructure, porosity, amorphous-phase content, and microhardness of the coatings were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The results indicated that the microstructure of the coatings was dependent on the APS parameters used. The porosity of the coatings and the number of unmelted particles deposited increased with the flow rate of the primary plasma gas increasing. The amorphous-phase contents and microhardness values of the coatings exhibited a trend opposite to that of their porosity and the number of unmelted particles present in them. The crystallization behaviors and microhardness values of Al 2 O 3-YSZ coatings annealed at different temperatures were also investigated. The results indicated that the coatings with higher amorphous-phase contents possessed homogeneous nanocrystalline structure and exhibited higher microhardness values after the heat treatments.

Conductive Transparent TiNx/TiO2 Hybrid Films Deposited on Plastics in Air Using Atmospheric Plasma Processing

The successful deposition of conductive transparent TiNx/TiO2 hybrid films on both polycarbonate and silicon substrates from a titanium ethoxide precursor is demonstrated in air using atmospheric plasma processing equipped with a high‐temperature precursor delivery system. The hybrid film chemical composition, deposition rates, optical and electrical properties along with the adhesion energy to the polycarbonate substrate are investigated as a function of plasma power and plasma gas composition. The film is a hybrid of amorphous and crystalline rutile titanium oxide phases and amorphous titanium nitride that depend on the processing conditions. The visible transmittance increases from 71% to 83% with decreasing plasma power and increasing nitrogen content of the plasma gas. The film resistivity is in the range of ∼8.5 × 101 to 2.4 × 105 ohm cm. The adhesion energy to the polycarbonate substrate varies from ∼1.2 to 8.5 J/m2 with increasing plasma power and decreasing plasma gas nitrogen content. Finally, annealing the film or introducing hydrogen to the primary plasma gas significantly affects the composition and decreases thin‐film resistivity.

Microstructural characterization of plasma sprayed conventional and nanostructured coatings with nitrogen as primary plasma gas

Air plasma sprayed conventional and nanostructured Al2O3–13wt.%TiO2, WC–12–17wt.%Co, and ZrO2–7–8wt.%Y2O3 coatings were deposited using nitrogen as the primary plasma gas. In nanostructured coatings, as critical plasma spray parameter (CPSP) – which is defined as the ratio of gun or arc power to the primary gas flow rate – controls the volume fraction of unmelted/partially melted regions, which in turn control the properties of coatings, coatings for all systems considered in this work were obtained as a function of CPSP. Coating properties, e.g. porosity, microhardness and percentage of partially melted/unmelted regions, for nanostructured coatings, were seen to be monotonic functions of CPSP. However, the effect of CPSP on percentage of partially melted/unmelted regions was significantly smaller for nitrogen as compared to argon as the primary plasma gas, particularly for nanostructured Al2O3–13wt.%TiO2 coatings.

A novel 2.45 GHz/200 W Microwave Plasma Jet for High Temperature Applications above 3600 K

State of the art atmosphere plasma sources are operated with frequencies in kHz/MHz regions and all high power plasma jets make use of tungsten electrodes. A microwave plasma torch has been developed at FH Aachen for the application in various fields. The advantages over other plasma jet technologies are the high efficiency combined with a maintenance-free compact design and non-tungsten electrodes. In this paper the development of a 200 W torch is described. Argon is used as the primary plasma gas and a second gas can be applied for additional purposes. For the plasma generation a microwave at 2.45 GHz is sent through the torch. The special internal topology causes a high electric field that ignites the plasma at the tip and leads to the ionization of the passing Argon atoms which are emitted as a jet. By designing the copper electrode as a cannula it is possible to gain plasma temperatures higher than the electrode's melting point. The electric field simulations are made with Ansoft HFSS. Experiments were carried out to verify the simulations. The upcoming steps in the development will be the scale-up to higher power levels of several kW with a magnetron as power source.

Yttria-Stabilized Zirconia Thermal Barriers Sprayed Using N2-H2 and Ar-H2 Plasmas: Influence of Processing and Heat Treatment on Coating Properties

Thermal barrier coatings were produced using both Ar and N2 as the primary plasma gas. Various aspects of the process and the coatings were investigated. It was found that higher in-flight particle temperatures could be produced using N2, but particle velocities were lower. Deposition efficiencies could be increased by a factor of two by using N2 as compared to Ar. Coatings having similar values of porosity, hardness, Young’s modulus, and thermal diffusivity could be produced using the two primary gases. The coatings exhibited similar changes (increased hardness, stiffness, and thermal diffusivity) when heat-treated at 1400 °C. However, the N2-processed coatings tended to have lower values of Young’s modulus and thermal diffusivity following such treatment. The results point to the potential advantage, in terms of reduced powder consumption and increased production rate, of using N2 as compared to Ar as the primary plasma gas for TBC deposition.

Probing instabilities in arc plasma devices using binary gas mixtures

This paper presents an experimental approach to identify the sources of instabilities in arc plasma devices. The phenomena of demixing in arcs have been utilized to explore the characteristics of different instabilities. Problems in explaining the observed behavior with our current understanding of the phenomena are discussed. Hydrogen is used as a secondary gas with argon as the primary plasma gas for this study. Results indicate that the observed behavior such as steady, takeover, and restrike modes of instabilities in arcs may essentially originate from the thin boundary layer over the anode wall primarily at the location of the anodic arc root. The bulk core flow apparently does not play any significant role in such instabilities. Arc currents rather than flow rates control the behavior of the instabilities in frequency space. Bifurcation of the system behavior and evidence for the existence of quadratic zones in flow space of binary gas mixtures separating steady and unsteady behavior are discussed.

Velocity and temperature distributions of alumina–titania in-flight particles in the atmospheric plasma spray process

Control of the thermal spray process generally considers the monitoring of the mean in-flight particle characteristics (i.e., mean velocity and temperature) during the coating process. The correlation with process parameters is often established without considering the modifications of characteristic distributions. The aim of this work was to demonstrate the need for these distributions. Three processing parameters (arc current, primary and secondary plasma gas flow rates) were correlated to the characteristic distributions in the case of alumina–titania in-flight particles. A high speed two color pyrometer was used to measure, at the centre of the particle flow, individual in-flight particle characteristics, which were used to build velocity and temperature distributions. Results showed a change on characteristic distributions and demonstrated a significant effect of the energetic parameters in the control of the in-flight particle characteristics.

Fabrication of amorphous silicon materials and solar cells at high temperatures using ECR deposition techniques

We report on the preparation and properties of a-Si:H films and p-i-n devices produced using a remote-electron-cyclotron-resonance deposition at high temperature are electronic quality, with significantly improved stability compared to standard glow-discharge-deposited films. The bandgap and H content of the films depend critically upon which gas is used as the primary plasma gas. We have also made devices in these new materials, deposited on tin-oxide substrates, and they are also of good quality.

Effect of plasma spray processing variations on particle melting and splat spreading of hydroxylapatite and alumina

Splats of hydroxylapatite (HA) and alumina were obtained via plasma spraying using systematically varied combinations of plasma velocity and temperature, which were achieved by altering the primary plasma gas flow rate and plasma gas composition. Particle size was also varied in the case of alumina. Splat spreading was quantified via computer- aided image analysis as a function of processing variations. A comparison of the predicted splat dimensions from a model developed by Madejski with experimental observations of HA and alumina splats was performed. The model tended to underestimate the HA splat sizes, suggesting that evaporation of smaller particles occurred under the chosen experimental conditions, and to overestimate the observed alumina splat dimensions. Based on this latter result and on the surface appearance of the substrates, incomplete melting appeared to take place in all but the smaller alumina particles. Analysis of the spreading data as a function of the processing variations indicated that the particle size as well as the plasma temperature and velocity influenced the extent of particle melting. Based on these data and other considerations, a physical model was developed that described the degree of particle melting in terms of material and processing parameters. The physical model correctly predicted the relative splat spreading behavior of HA and alumina, assuming that spreading was directly linked to the extent of particle melting.