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High Temperature Material Processes: An International Quarterly of High-Technology Plasma Processes
RECENT DEVELOPMENTS IN THERMAL SPRAYING FOR IMPROVED COATING CHARACTERISTICS AND NEW APPLICATIONS/ PROCESS CONTROLS AND SPRAY PROCESSES
LERMPS laboratory, University of Technology of Belfortâ€MontbÃ©liard
Thermal spraying has experienced very significant developments over the past five years, in terms of process controls and of developments of new processes. Controlling thermal spray processes requires firstly the capability to diagnose them. In-flight particle characteristics; i.e., their velocity and temperature, but also their diameter and their flux at a given location, constitute, with the accurate determination of the coating temperature during spraying, the most important parameters to record. Several systems, based on fast infrared pyrometry or on fluxmetry, were developed during these past few years. Controlling thermal spray processes requires secondly a robust strategy to correlate the diagnosed characteristics to the operating parameters. Very significant efforts were made as well to develop these strategies to reach, in a short to middle term, an on-line control of the processes. The efforts were also directed towards the development of new processes. Suspension plasma spraying (SPS), using d.c. or R.F. plasma spray torches, consists in injecting the powder particles into the plasma flow using a liquid as carrier media, instead of the classical carrier gas. This new approach offers unique possibilities to carry nano-sized particles to manufacture finely grained coatings. Very Low Pressure Plasma Spraying (VLPPS) is another way to manufacture thin and homogenous coatings with high deposition rate. Hybrid thermal spraying consists in coupling to a classical thermal spray gun a high power (i.e., a few MW.m−2) laser beam. Depending on the characteristics of the beam, several physical mechanisms can be induced. From surface ablation prior to particle spreading to surface heating and in situ complete or partial remelting, hybrid thermal spraying permits to modify locally the coating structures and hence their properties. Cold spraying differs from the other spray processes in the sense that the particles impact the surface to be covered in the solid state: the flattening results exclusively from the plastic deformation by dissipation of the kinetic energy. The system is based on the implementation of a De Laval nozzle through which a warm gas is released to produce a high velocity gas stream into which particles are injected. This process offers the unique possibility to process particles without melting them: oxidation does not take place and metastable materials can be considered as candidates to produce coatings.
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