Spray Tips: Relationship between Particle Melting and Coating Structure

November 3, 2018
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In addition to the effects of particles in the periphery of the spray stream, resulting from poorly classified materials or nonoptimized process parameters, consideration should be given to particles in the spray stream and how variations in the degree of melting and velocity affect coating structure.

Particles in the center of the spray pattern may include one or all of the following states on impact:

• Fully molten, or just above the material melting temperature
• Superheated, well above the melting temperature and perhaps close to the vaporization point
• Semimolten, with the outside liquid but the core still solid
• Molten then resolidified while in flight before impact
• Under- and over-accelerated particles

Fully melted particles at or just above the material melting point arrive at the substrate, impact, flow, and flatten, as shown in the diagram. Particulate material spreads and cools rapidly as heat is conducted into the substrate.

The classic lamellar splat morphology is thus achieved, as shown in (b). Lamellar particle thickness depends on particle velocity, particle temperature, substrate temperature, and other minor factors. Superheated particles may splash on impact, sending out fine droplets radially that can end up in the coating as debris, as shown in (c). Debris formed from splatter differs from debris produced in the jet, in that splatter lodges in the coating, building up at the first ridge that stops its radial travel. Air blasting does not remove splatter debris. Superheating is a condition that generally should be avoided, in that it produces fumes, oxidation, and lowered deposition efficiency.

Particles that resolidify in flight after having been melted may either not deposit or they may appear as embedded unmelted particles, usually with clearly discernible oxide layers on the particle surfaces, as shown in (a).

Volume 5A, Thermal Spray Technology -> Introduction to Coating Design and Processing -> Coating Deposition

Carbide-based thermal spray coating cuts corrosion and biofouling to keep ships at sea

Source:

Swinburne University of Technology, Australia, announces that its researchers, in collaboration with many other scientists, have developed a new corrosion-resistant flame-spray coating that has reduced the build-up of algae and barnacles on ship hydraulic components by half. The coating consists of a single layer of carbide-based material that is applied by supersonic combustion flame spray.

Swinburne University researchers are collaborating with experts from the Defence Materials Technology Centre, MacTaggart Scott Australia, United Surface Technologies, and the Defence Science and Technology Group to advance the new technology. The scientists and industry experts came together to tackle the issue of corrosion and biofouling, which combine to impose a massive expense on the shipping industry worldwide.

“We have developed new materials and used a supersonic combustion flame jet to coat hydraulic machinery parts, and found these new protective coatings reduce biofouling by roughly 50 per cent compared to current standard coatings,” says Dr. Andrew Ang of Swinburne, one of the lead scientists on the team.

The team tested the protective coating on more than 100 samples, immersing them in seawater at three field sites around Australia from 2015 to 2017. The treatment is likely to be too expensive to be used on entire ship hulls, but it could make a big difference for critically important machinery that helps provide propulsion or heavy lifting capabilities.

http://www.swinburne.edu.au/news/latest-news/2018/10/swinburne-researcher-develops-coating-to-prevent-corrosion-on-ships.php

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