African Fusion July 2020

Thermal spray coating processes

Figure 5: Microstructure of a plasma sprayed alumina coating with 50 µm average thickness after sealingwithamethylmethacrylate based sealer on concrete, torch: GTV Penta, powder feed rate: 300 g/min, surface velocity: 1.13m/s, passes: 1, coverage rate: 1.36m2/min, deposition efficiency: 78%.

Figure 3: Alumina coating deposited on concrete in a single pass using plasma spray torch GTV Penta.

Figure 6: Alumina coating on a concrete sample after partial (right side) sealing with a polymer sealer. on visual inspection were evaluated with respect to coating bond strength and their capacity to avoid water absorption both in as- sprayed and sealed states. Sealing with water glass is clearly less effective compared to the use of the methyl methacrylate based sealer. Water glass forms a thin layer on top of the plasma sprayed alumina coating, while the polymer sealer penetrates the defect network of these coatings, right down to the concrete substrate. After sealing with the polymer sealer, a segmentation crack network in the plasma sprayed alumina coatings becomes clearly visible (Figure 6). Also, polymer sealed surfaces show strong hy‑ drophobicity (Figure 7). Due to the superior penetration behaviour, only polymer sealed alumina coatings were evaluated in tightness tests. Tightness tests were carried out by filling Karsten tubes with 4.0mℓ of tapwater. Sample surfaces were exposed to tapwater for 72 hours. In this time, evaporation of 0.15mℓ water was measured on the reference tile and the Karsten tubes on all polymer sealed alumina coatings showed exactly the same loss of water, which means that the surfaces are perfectly watertight. On the contrary, concrete samples covered with unsealed alumina coatings lost from 3.3 mℓ up to the full volume of 4.0 mℓ, which indicated that the plasma spray coatings alone could not act as an effective barrier between tapwater and concrete substrates. Bond strength tests showed an excellent bond strength of

Figure 4: Local damage of an alumina coating on concrete because of excessive heat transfer. layer (Figure 3). For the production of coatings with an average thickness of 50 µm, 70 to 80% deposition efficiency and a cover‑ age rate of 1.36 m 2 /min was achieved. In extensive investigations on the influence of spray distance, surface speed, track offset, number of passes and pre-heating tem‑ perature, it was found that the formation of defects such as cracks, local coating delamination or concrete spallation (Figure 4) clearly increases with the heat transferred to the concrete substrate and with increasing coating thickness. So high surface velocities and long spray distances are beneficial, while large numbers of passes and pre-heating are disadvantageous. Relatively smooth surfaces of concrete samples formed on formworkspermiteddepositionof consistent aluminacoatings that showed a comparable microstructure to the coatings deposited on metallic substrates (Figure 5), with low porosity and a network of fine microcracks. On the other hand, large surface pores and roughness peaks prevented complete coverage of the concrete sample surfaces. Heat affected zones were not observed in basalt mortar substrates. Coated concrete samples that did not show obvious defects

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July 2020

AFRICAN FUSION