Thermal and Cold Spray: Recent Developments

Abstract

Thermal spraying consists in a technology aiming at producing coatings whose thicknesses range from 10 μm to a few millimeters onto mechanical components to confer them specific and unique functional properties, such as wear and corrosion resistances, friction coefficient adaptation, thermal and electrical insulation, biocompatibility, repair, etc., among the principals. Thermal spraying consists in injecting in a viscous enthalpic jet (animated by a momentum) powder with particles which average size ranges from 0.01 to 100 μm. These particles are melted and simultaneously accelerated towards the surface of the part to be covered. They form, after impact, spreading and solidification, near-circular lamellae the stacking of which form the coating. Due to the versatility of the available processes exhibiting a wide range of enthalpic and momentum contents, virtually any kind of material exhibiting congruent melting behavior can be processed, from alloys and ceramics to polymers, ever since its melting temperature differs from its vaporization or decomposition temperature by at least 300 K and that it can be processed previously under the form of powder particles or wires. Thermal spray techniques offer the unique capability to manufacture a large variety of coatings on components of a large variety and geometry. However, thermal spraying constitutes a special process for which the coating service properties derive mostly from the structure and indirectly from the selection of the operating parameters. Very significant improvements over the past years permitted to diagnose the in-flight particle characteristics, mostly in terms of velocity and temperature. Recently, these new capabilities have made possible the development of on-line process controls. This should participate to a drastic increase in coating reliability. In convetntional thermal spraying processes, a pulverulent feedstock (i.e., powder particles) is injected within the plasma jet via a carrier gas. This approach does not permit to process small diameter particles; i.e., nano-sized particles, which could permit to form finely grained coatings. Replacing gas by liquid to carry particles offer the unique possibility to process nano-sized particles. Cold gas spraying may appear as an alternative process to reach the same goal. Indeed, thermal spray processes experienced very significant developments over the past years, opening new doors to manufacture coatings with a high reliability and superior properties. This papepr indend at presenting some of those developments.

Development of YSZ Thermal Barrier Coatings Using Axial Suspension Plasma Spraying

“…Porous bands existing between successive passes of the spraying gun provide an easy pathway for the propagation of branching cracks under thermal stress. The formation of porous bands is attributed to poorly-heated un-molten particles or resolidifized particles travelling in the jet fringes [36]. The extent of porous bands is largely reduced by axial feeding compared with radial feeding.…”

Corrosion Resistance of Boronized, Aluminized, and Chromized Thermal Diffusion-Coated Steels in Simulated High-Temperature Recovery Boiler Conditions

“…Some studies have even explored the use of nanostructured yttria-stabilized zirconia as a top coat for a multi-layered environmental barrier coating system for the boiler tubes [26]. However, due to the high operating temperatures (3,500-10,000 • C) of the thermal spray processes [29][30][31], and possible formation of detrimental cracks and delamination of the final coating, these coatings may not be suitable for all substrate materials and practical conditions. In addition, there is a real challenge to apply thermal spraying techniques for protection of the inner surface of long tubing, as well as the components with complex shapes.…”

Hladna metalizacija

“…Za razliku od HMVP, ako se prah ubrizgava u struju gasa u mlaznici, u pitanju je hladna metalizacija niskim pritiskom (HMNP), Slika 3b, pri čemu je pritisak gasa 6-8 bar a temperatura gasa 20-550 °C [4,9,[11][12][13]. Usled delovanja struje gasa velike brzine, čestice praha mogu dostići brzinu do 1200 m/s [14]. Čestice dodatnog materijala udaraju u pripremljenu površinu, usled čega dolazi do deformacije površine podloge i čestica praha, i na taj način formira se veza čestica/podloga.…”