Atmospheric-pressure plasmas are used in a variety of materials processes. Traditional sources include transferred arcs, plasma torches, corona discharges, and dielectric barrier discharges. In arcs and torches, the electron and neutral temperatures exceed 3000/spl deg/C and the densities of charge species range from 10/sup 16/-10/sup 19/ cm/sup -3/. Due to the high gas temperature, these plasmas are used primarily in metallurgy. Corona and dielectric barrier discharges produce nonequilibrium plasmas with gas temperatures between 50-400/spl deg/C and densities of charged species typical of weakly ionized gases. However, since these discharges are nonuniform, their use in materials processing is limited. Recently, an atmospheric-pressure plasma jet has been developed, which exhibits many characteristics of a conventional, low-pressure glow discharge. In the jet, the gas temperature ranges from 25-200/spl deg/C, charged-particle densities are 10/sup 11/-10/sup 12/ cm/sup -3/, and reactive species are present in high concentrations, i.e., 10-100 ppm. Since this source may be scaled to treat large areas, it could be used in applications which have been restricted to vacuum. In this paper, the physics and chemistry of the plasma jet and other atmospheric-pressure sources are reviewed.

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The atmospheric-pressure plasma jet: a review and comparison to other plasma sources

This article intends to summarize our actual knowledge in plasma spraying with an emphasis on the points where work is still in progress. It presents successively: the plasma torches with the resulting plasma jets and their interactions with the surrounding environment; the powder injection with the heat, momentum and mass transfers between particles and first plasma jets and then plasma plume; the particles flattening and solidification, forming splats which then layer to form the coating; the latest developments related to the production of plasma sprayed finely structured coatings.

http://iopscience.iop.org/article/10.1088/0022-3727/37/9/R02/pdf

Understanding plasma spraying

There is increasing scientific and commercial interest in the development of nanostructured coatings, particularly those based on low-miscibility ‘ceramic–ceramic’ or ‘ceramic–metal’ crystalline/amorphous nanocomposite phase mixtures deposited by plasma-assisted PVD or CVD. In laboratory mechanical testing, extreme values of hardness (which may be in excess of 70 GPa) are often found for such films, similar to those claimed for intrinsically hard materials such as c-BN and diamond. High hardness is, however, often accompanied by an associated high elastic modulus, which although desirable in principle for cutting tool materials and/or coatings, may in practice limit coating durability, on low-strength, low-modulus substrates (e.g. low-alloy steels and the light alloys) and in many wear applications other than metal cutting. In this paper, we discuss the benefits of using the ratio of hardness to elastic modulus ( H / E ) as an indicator of coating durability since this parameter essentially describes the elastic strain to failure capability (and resilience ) of a candidate material. Furthermore, we consider the likely need for tribological coatings to accommodate some degree of substrate deformation; in this respect film toughness , i.e. ‘engineering toughness’ in the sense of an ability to absorb deformation energy (both elastic and plastic) needs to be considered. The concept of predominantly metallic films with a nanograined and/or glassy microstructure (containing little or no high-modulus ceramic constituents) is introduced, through which we point to the importance of retaining ‘sufficient’ coating hardness, whilst reducing coating elastic moduli to more closely match those of candidate substrate materials. With regard to the implications of H / E for practical tribological coating applications, we propose that closer matching of the coating/substrate interfacial elastic properties and thus an improved ability for the coating to accommodate substrate strain, where necessary, is often a more important factor in wear resistance than is extremely high hardness.

Design criteria for wear-resistant nanostructured and glassy-metal coatings

Calculated values of the viscosity, thermal conductivity and electrical conductivity of air and mixtures of air and argon, air and nitrogen, and air and oxygen at high temperatures are presented. In addition, combined ordinary, pressure, and thermal diffusion coefficients are given for the gas mixtures. The calculations, which assione local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30,000 K. The results for air plasmas are compared with those of published theoretical and experimental studies. Significant discrepancies are found with the other theoretical studies; these are attributed to differences in the collision integrals used in calculating the transport coefficients. A number of the collision integrals used here are significantly more accurate than values used previously, resulting in more reliable values of the transport coefficients.

Transport coefficients of air, argon-air, nitrogen-air, and oxygen-air plasmas

Major industrial plasma processes operating close to atmospheric pressure are discussed. Applications of thermal plasmas include electric arc furnaces and plasma torches for generation of powders, for spraying refractory materials, for cutting and welding and for destruction of hazardous waste. Other applications include miniature circuit breakers and electrical discharge machining. Non-equilibrium cold plasmas at atmospheric pressure are obtained in corona discharges used in electrostatic precipitators and in dielectric-barrier discharges used for generation of ozone, for pollution control and for surface treatment. More recent applications include UV excimer lamps, mercury-free fluorescent lamps and flat plasma displays.

https://iopscience.iop.org/article/10.1088/0741-3335/46/12B/006/pdf

Atmospheric-pressure plasma technology

In this article, amorphous and nanocomposite thermally deposited steel coatings have been formed by using both plasma and high-velocity oxy-fuel (HVOF) spraying techniques. This was accomplished by developing a specialized iron-based composition with a low critical cooling rate (≈104 K/s) for metallic glass formation, processing the alloy by inert gas atomization to form micron-sized amorphous spherical powders, and then spraying the classified powder to form coatings. A primarily amorphous structure was formed in the as-sprayed coatings, independent of coating thickness. After a heat treatment above the crystallization temperature (568 °C), the structure of the coatings self-assembled (i.e., devitrified) into a multiphase nanocomposite microstructure with 75 to 125 nm grains containing a distribution of 20 nm second-phase grain-boundary precipitates. Vickers microhardness testing revealed that the amorphous coatings were very hard (10.2 to 10.7 GPa), with further increases in hardness after devitrification (11.4 to 12.8 GPa). The wear characteristics of the amorphous and nanocomposite coatings were determined using both two-body pin-on-disk and three-body rubber wheel wet-slurry sand tests. The results indicate that the amorphous and nanocomposite steel coatings are candidates for a wide variety of wear-resistant applications.

Wear-resistant amorphous and nanocomposite steel coatings

This paper describes a re-examination of a known process for the direct plasma thermal conversion of methane to acetylene. Conversion efficiencies (% methane converted) approached 100% and acetylene yields in the 90–95% range with 2–4% solid carbon production were demonstrated. Specificity for acetylene was higher than in prior work. Improvements in conversion efficiency, yield, and specificity were due primarily to improved injector design and reactant mixing, and minimization of temperature gradients and cold boundary layers. At the 60-kilowatt scale cooling by wall heat transfer appears to be sufficient to quench the product stream and prevent further reaction of acetylene resulting in the formation of heavier hydrocarbon products or solid carbon. Significantly increasing the quenching rate by aerodynamic expansion of the products through a converging–diverging nozzle led to a reduction in the yield of ethylene but had little effect on the yield of other hydrocarbon products. While greater product selectivity for acetylene has been demonstrated, the specific energy consumption per unit mass of acetylene produced was not improved upon. A kinetic model that includes the reaction mechanisms resulting in the formation of acetylene and heavier hydrocarbons, through benzene, is described.

Plasma Thermal Conversion of Methane to Acetylene

Particle temperature is a fundamental parameter in the thermal spray process. The measurement of particle temperature usually relies on the measurement of the radiance of the hot, incandescent, particles in two or more wavelength or color bands. Measurement techniques can be categorized as single particle and ensemble methods. Single particle methods use high-speed pyrometry to estimate the temperature of individual particles. The mean and standard deviation of the particle temperature distribution can then be obtained from observations of sufficient numbers of individual particles. Ensemble methods observe large numbers of particles simultaneously and yield an estimated mean temperature directly, but cannot provide information on the shape or width of the particle temperature distribution. Both techniques have inherent errors, strengths, and limitations that are examined and discussed.

Particle temperature measurement in the thermal spray process

The plasma-spray process features complex plasma-particle interactions that can result in process variations that limit process repeatability and coating performance. This paper reports our work on the development of real-time diagnostics and control for the plasma spray process. The strategy is to directly monitor and control those degrees of freedom of the process that are observable, controllable and affect resulting coating properties. This includes monitoring of particle velocity and temperature as well as the shape and trajectory of the spray pattern. Diagnostics that have been developed specifically for this purpose are described along with the demonstration of a closed loop process controller based on these measurements.

Diagnostics and control in the thermal spray process

An improved automatic feedback control scheme enhances plasma spraying of powdered material through reduction of process variability and providing better ability to engineer coating structure. The present inventors discovered that controlling centroid position of the spatial distribution along with other output parameters, such as particle temperature, particle velocity, and molten mass flux rate, vastly increases control over the sprayed coating structure, including vertical and horizontal cracks, voids, and porosity. It also allows improved control over graded layers or compositionally varying layers of material, reduces variations, including variation in coating thickness, and allows increasing deposition rate. Various measurement and system control schemes are provided.

W.D. Swank

Papers

Wear-resistant amorphous and nanocomposite steel coatings

Plasma Thermal Conversion of Methane to Acetylene

Particle temperature measurement in the thermal spray process

Diagnostics and control in the thermal spray process

Feedback enhanced plasma spray tool