Effect of Spray Power on the Microstructure and Thermal Shock Resistance of 8YSZ / NiCrAlY Protective Coatings

The use of ceramic materials with low thermal conductivities as thermal barrier coatings (TBCs) on critical components of internal combustion engines, such as piston end faces, has recently become a popular research topic. The objective is to enhance the insulation and thermal efficiency of internal combustion engines. These engines frequently undergo start-stop cycles, subjecting the heated components to rapid thermal fluctuations. Consequently, TBCs on the heated surfaces of internal combustion engines must exhibit excellent resistance to thermal shock. During the preparation of yttria-stabilized zirconia (YSZ) TBCs, the axial velocity and melting state of the sprayed powder are critical factors affecting coating quality. While other spray process parameters (such as carrier gas flow rate, powder feed rate, and spray distance) remain fixed, the voltage and current (i.e., power) play a significant role in determining the coating quality. Although researchers have extensively studied the impact of various parameters on the microstructural properties of YSZ coatings, research on their thermal shock resistance, especially at extreme conditions of 800 degrees C with up to 800 thermal cycles (which closely resemble the severe start-stop process of internal combustion engines), remains relatively scarce. In this study, NiCrAlY / YSZ bilayer (bond layer + ceramic top layer) thermal barrier coatings were prepared on GH4169 and Inconel718 high-temperature alloy substrates (Phi 24mmx8mm) using an atmospheric plasma spraying system (9MC, Sulzer Metco, USA). Before spraying, the surfaces of GH4169 and Inconel718 Ni-base superalloy substrates were polished with 80-mesh coarse sandpaper to remove oil, then sandblasted (corundum sand of 80 similar to 120 mesh, pressure of 0.4 similar to 0.6MPa), and finally subjected to ultrasonic cleaning with acetone solution applied twice to keep the fresh surface clean. The YSZ coating thickness was approximately 600 mu m, with a transition bond coat thickness of approximately 100 mu m. The prepared YSZ coating samples were labeled as C1 to C4 based on increasing spray power from 35.75 to 42.0 kW, respectively. The microhardness of the coating surfaces was measured using an MH-5 Vickers hardness tester. Additionally, we evaluated the coating-substrate bond strength according to ASTM C633 using a SANS WDW-200 microcomputer-controlled electronic universal material testing machine. X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) were used to characterize the phase composition and microstructure of YSZ powder and coatings. Furthermore, we assessed the thermal fatigue resistance of YSZ coatings through a heating-water quenching process at 800 degrees C (although a real start-stop thermal cycling in internal combustion engines is less severe, this accelerated fatigue test serves as a preliminary evaluation). We found that as the spray power increased from 35.75 to 42 kW, the YSZ coating phase consisted predominantly of tetragonal zirconia. The unmelted zone in the coating decreased or disappeared, resulting in distinct columnar and equiaxed grain regions. At low spray power, the flat particles were not completely stacked and there were more pores and microcracks in the coating, resulting in a lower coating density and cohesive strength. As the spraying power increased, the number of pores and microcracks decreased, leading to a denser coating. The microhardness initially increased and then decreased, whereas the bond strength gradually improved. Among the prepared coatings, C4 (42 kW) exhibited favorable microhardness (817.15 +/- 58.65 HV300g) and bond strength (66.37 +/- 4.90 MPa). After 800 thermal cycles at 800 degrees C, the C4 coating surface showed no significant signs of spallation, cracking, or spalling, demonstrating the best thermal shock resistance. However, microstructural damage to the coating was severe, approaching the thermal fatigue limit of the coating. The C4 (42 kW) spray power condition is suitable for preparing YSZ thermal barrier coatings on the heated surfaces of critical components of internal combustion engines because it exhibits good microstructural properties and performance. All in all, in this study we provide theoretical support and data for the application and further development of thermal barrier coatings in internal combustion engines.