A new breed of nano-structured coatings are being developed around the suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) processes. SPS and SPPS require more energy per deposited mass than conventional powder spraying because a liquid carrier must be vaporized before the plasma can interact with the coating material.
The 100HE, currently used in multiple SPS and SPPS production systems, is well equipped to meet this increased energy requirement through the use of high enthalpy gas mixtures (Ar-N2-H2 or Ar-N2-He) and operating powers up to 105 kW.
The small particle/droplet sizes within SPS and SPPS increase the sensitivity of the feedstock materials, and thus the microstructure, to changes in the plasma flow. The ternary gas mixtures and high arc voltages (>200 V) of the 100HE generate an extremely stable plasma, allowing for consistent feedstock injection and heating.
Progressive has also developed a closed-loop feed system, called the LiquifeederHE, which provides accurate and consistent flows for SPS and SPPS materials in the 10-100 ml/minute range.
It is the nature of suspensions to settle out and produce clogs within the feed system. As a result, the LiquifeederHE includes several design features to counteract these problems during operation. An impeller within the reservoir ensures the powder particles remain suspended. Settling between the reservoir and gun is prevented by the correct choice and routing of feed lines. Inline filters are used to trap larger particle agglomerates or foreign bodies, stopping these objects from clogging an injection orifice.
Suspensions or solution precursors can also produce clogs if these materials remain stagnate within the feed system between uses. To solve this issue, the LiquifeederHE includes the ability to constantly circulate the feedstock during delays between spraying, which also enables quick transitions between torch preheating and coating application. When not in use, proper cleaning of the feed system is essential in preventing clogs within a suspension/solution feeder. The LiquifeederHE is designed to facilitate cleaning through integrated flush and waste capture features.
The performance characteristics of the 100HE and LiquifeederHE have proven to yield a very effective and versatile SPS/SPPS platform that is capable of spraying coatings from 25 wt% solid suspensions and solutions (dissolved solid content) at flow rates of up to 90 ml/min with deposition efficiencies ranging from 50 – 80%.
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