Plasma Sprayed WC-12%Co-Coatings for TBC Applications on Diesel Engine Piston

Plasma Sprayed WC-12%Co-Coatings for TBC Applications on Diesel Engine Piston

Debasish Das, Rajeev Verma, Vipul Pathak
DOI: 10.4018/IJSEIMS.2019010103
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In the present study, plasma sprayed WC-12%Co coatings with 100µm NiCrAlY bond coat on a substrate of A336 cast aluminum alloy have been investigated for a thermal barrier coating (TBC) application. The coatings deposited with varying topcoat thickness up to 500µm were deposited on the piston top surface of an Indian hatchback diesel car to act as a thermal barrier and enhance the thermal efficiency of the engine. Although all the specimens with distinct coating overlays survived 350 thermal cycles, the one with 200µm thickness exhibited the best thermal shock behavior as they exuded the most cycles to surface cracks initiation. Moreover, SEM analysis also suggested 200 µm thick coating to be optimal for thermal shock behavior in diesel engine components. The coating phase analysis by XRD and the lattice strain analysis performed by a Williamson-Hall (W-H) analysis did not reveal any structural changes after the thermal shock experiment.
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The plea of energy conservation is increasing every day and the present energy scenario necessitates researchers to strive for effective use of energy conversion systems. These systems used in different engineering applications like power sector, aerospace, and the automotive industry have to deal with sufficiently high temperatures. Increased combustion temperatures in heat engines such as gas turbine and diesel engine can improve their thermodynamic efficiency and are also desirable for environmental reasons like reduction in pollutant emissions especially, NOx. Due to the metallurgical constraints, the increased heat supplied may however, deteriorate the engine parts thereby requiring an extra cooling arrangement which may further affect the efficiency of the system. Reportedly, almost 22% of the heat is rejected to the cooling fluid in a diesel engine which affect the thermal efficiency (Sharma & Kumar, 2015).

Alternatively, curtailing the heat rejected through the system may be realized by applying the ceramic coating on the exposed parts to reduce the heat loss through the cylinder head, valve, and piston. A low heat rejection (LHR) engine having insulated combustion chamber walls by a ceramic coating is ought to have an enhanced thermal efficiency. Apparently, by lowering the substrate’s thermal conductivity by half, the surface temperature had been reduced by about 55°C (Maloney, 2001).

The refractory ceramic coatings act as thermal barrier coatings (TBCs) and is a point of interest for many explorations especially diminishing the in-cylinder heat rejection of adiabatic engine (Costa et al., 2018; Garud et al., 2017). Keeping this in view, the present research focuses on the effective use of thermal energy in a high-temperature engineering component of an internal combustion (IC) engine. A typical TBC configuration would comprise a ceramic topcoat, and an intermediate bond coat layer (Lima, Cinca, & Guilemany, 2012). Ceramic topcoat manifests the requisite high-temperature properties like creep resistance owing to high specific heat capacity, low thermal diffusivity, and coefficient of thermal expansion, besides good wear and corrosion resistance (Bakan & Vaßen, 2017). Whereas, the bond coat imparts the requisite adhesion among the metallic substrate and ceramic coating, and extensively is MCrAlY where M refers to a metal like Fe, Ni and/or Co depending on the type of super-alloy (Jiang et al., 2018).

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