ADVANCED MATERIALS & PROCESSES | APRIL 2025 38 iTSSe TSS iTSSe TSS 7 Thermal spray coatings play a critical role in enhancing the performance and durability of hot section components in turbines, particularly those operating in aero-engine and industrial gas turbine (IGT) environments. These components are exposed to extreme temperatures, oxidation, and corrosion, necessitating the use of thermal barrier coatings (TBCs)[1–4]. TBCs are multilayered coatings that protect underlying substrates by reducing heat transfer and providing thermal insulation. This not only extends the service life of components but also improves their efficiency by allowing higher operating temperatures. CHALLENGES WITH CONVENTIONAL TORCHES Among the various methods for applying TBCs, atmospheric plasma spray (APS) technology is the most widely used due to its versatility and ability to produce coatings with desirable microstructural properties[5,6]. However, conventional APS torches are limited when it comes to coating internal diameters (IDs). The inability to effectively spray confined geometries presents a significant challenge in protecting components with complex shapes, such as small annular combustors, combustion cans, afterburners and transition ducts of gas turbine engines. IMPROVED TORCH FEATURES To overcome the challenges of coating confined geometries, Portech’s APS 3MB-PT-ID torch was specifically engineered with a compact operating length of 7.0 cm, enabling it to coat IDs as small as 150 mm (6-in.) while maintaining a safety gap of 5 mm (Fig. 1). It operates in any controller adapted for the legacy 3MB torch, and it is machined mounted in robots, being able to operate at maximum bore depths up to 1.4 m (56-in.). Equipped with an advanced heat protection system, it reliably operates in high-temperature, restricted environments. The torch features a high-power output of 40 kW with a 100% duty cycle, being able to operate with Ar/H2 or N2/H2 plasmas. Its lightweight design, weighing only 4.70 kg, ensures ease of operation, maintenance, and installation. These features make the torch a highly effective and innovative solution for producing high-quality thermal barrier coatings in complex industrial applications. CASE STUDY In this study, the torch was utilized to evaluate its effectiveness in producing high-quality TBCs. The coatings were applied using a NiCoCrAlY+HfSi (Amdry 386-4) bond coat (~175 microns thickness, ~12% porosity) and 8 wt% YSZ (Saint-Gobain #204) topcoat (~380 microns thickness, ~9% porosity). Microstructural analysis revealed a uniform distribution of porosity within both the bond coat and top coat layers, which aligns with the optimal parameters required for industrial applications. While the porosity levels in the bond coat were slightly higher than ideal, they were consistent with results achievable using other 3MBbased torches. The porosity levels of the 8 wt% YSZ topcoat are within the range of those reported for low-porous TBCs employed by aero-engines (8-15%). The cross-sectional microstructure demonstrated excellent layer uniformity, strong bonding of the bond coat to the substrate and seamless adhesion at the bond coat- topcoat interface (Fig. 2). The roughness of the bond coat, measured at Ra ~11 microns, met the minimum recommended values for enhancing the adhesion of the topcoat[7]. The bond strength of the coatings, measured via ASTM C633, averaged 15.4 ± 0.7 MPa across five samples, positioning this CHARACTERIZATION OF THERMAL BARRIER COATINGS PRODUCED USING A NOVEL ATMOSPHERIC PLASMA SPRAY TORCH A case study shows that new torch technology can overcome the common challenges of conventional atmospheric plasma spray torches. Hossein Shahbazi and Ehsan Alirezaei, Portech, Ottawa, Canada Fig. 1 — Portech’s APS 3MB-PT-ID torch. FEATURE
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