Thermal barrier coatings for diesel engines

Sammanfattning: The upward trend in internal combustion engine efficiency is driven by the demands for emission reduction and fossil fuel depletion. No present alternative fuel can produce energy comparable to that produced by conventional oil; this necessitates the reasonable, efficient usage of oil. For several decades, thermal barrier coatings (TBCs) have been studied as an answer for increasing the thermal efficiency of gas turbine engines. However, TBCs have not been extensively evaluated for application to internal combustion engines owing to their emissions, costs, and demanding working conditions. Much effort is needed to simultaneously address this problem and expand the applicability of thermal barrier coatings.The objective of this study is to investigate the relationships between the spraying parameters, microstructural variations, thermal properties, and performance of TBCs applied to diesel engines. The further objective is to harness this knowledgeto fabricate coatings that result in high automotive engine efficiencies. Different feedstock materials were used with various spraying methods and classified into separate TBC types. The first TBC type had a lamellar bond coat deposited by atmospheric plasma spray (APS) and lamellar yttria-stabilized zirconia (YSZ) APS top coat. The second TBC type was derived from the first TBC type and had a lamellar APS bond coat, and the top coat was deposited by APS using a feedstock along with a porosity former, resulting in high-porosity top coats. The third TBC type had a dense bond coat deposited with high velocity air fuel (HVAF) and a columnar top coat deposited by suspension plasma spray (SPS) using a feedstock of YSZ or gadolinium zirconate (GZO). The SPS technique can generate a variety of microstructures, and the TBCs containing these microstructures were tested in an internal combustion engine for the first time. The fourth TBC type had a dense bond coat deposited HVAF, a columnar top coat produced with SPS and an additional top layer, which functioned as the sealing layer.For the thermophysical property investigation of all coating types, experimental and modeling techniques—laser flash analysis (LFA) and object-oriented finite element (OOF) analysis, respectively—were employed. To evaluate the optical properties of the coatings, two methods were adopted, namely, spectral normal hemispherical reflectivity at room temperature (SNHRRT) and spectral normal emissivity at high temperature (SNEHT). The functional performance of the coatings was evaluated on the basis of the TBC behavior under cyclic thermal loads; thermal cyclic furnace test, flame rig test, thermal swing test, and a single cylinder engine experiment were conducted. The coatings were characterized by scanning electron microscopy (SEM) before and after the functional performance test. The coating performance was correlated to the microstructural, thermophysical, and optical properties of the coatings.The results of this study infer that the TBC type significantly influences the thermal properties and thermal cyclic performance, which can be correlated with the porosity levels and the pore types. The complex substrate geometry of the piston resulted in inherent variations in spray angle and spray distance, leading to different coating microstructures and porosities owing to the changes in the particle trajectory and in-flight characteristics. Further, the single-cylinder engine evaluation demonstrated that the high-emissivity second TBC type or the third TBC type with a porous microstructure and a low thermal effusivity resulted in a high engine efficiency.