Influence of Isothermal Heat Treatment on Porosity and Crystallite Size in Axial Suspension Plasma Sprayed Thermal Barrier Coatings for Gas Turbine Applications

Abstract

Axial suspension plasma spraying (ASPS) is an advanced thermal spraying technique, which enables the creation of specific microstructures in thermal barrier coatings (TBCs) used for gas turbine applications. However, the widely varying dimensional scale of pores, ranging from a few nanometers to a few tenths of micrometers, makes it difficult to experimentally measure and analyze porosity in SPS coatings and correlate it with thermal conductivity or other functional characteristics of the TBCs. In this work, an image analysis technique carried out at two distinct magnifications, i.e., low (500×) and high (10,000×), was adopted to analyze the wide range of porosity. Isothermal heat treatment of five different coatings was performed at 1150 • C for 200 h under a controlled atmosphere. Significant microstructural changes, such as inter-columnar spacing widening or coalescence of pores (pore coarsening), closure or densification of pores (sintering) and crystallite size growth, were noticed in all the coatings. The noted changes in thermal conductivity of the coatings following isothermal heat treatment are attributable to sintering, crystallite size growth and pore coarsening.

Failure Analysis of Multilayered Suspension Plasma-Sprayed Thermal Barrier Coatings for Gas Turbine Applications

“…4 in several regions as also indicated by the arrows. This phenomenon has also been observed in previous works (Ref 16,21) and is understood to occur due to the sintering of ceramic material because of exposure to high temperatures for long durations in TCF testing. Sintering of the fine particles in coating results in reduction in the intra-columnar porosity and thus shrinkage of the column (Ref 16, 21).…”

“…Since SPS coatings show porosity in a wide range from several micrometers to a few nanometers, image analysis was performed in two steps by taking SEM images at low and high magnifications similar to the approach undertaken by Ganvir et al (Ref 13,21). Images at low magnification (500 9) could capture large features in the coatings such as intercolumnar gaps, large cracks and micrometric pores, while the images at high magnification (5000 9) could capture the fine porosity mainly inside the columns.…”

Thermal Stability of YSZ Coatings Deposited by Plasma Spray–Physical Vapor Deposition

“…In recent years, to improve the service temperature of the hot section of an aircraft engine, besides cooling gas film, thermal barrier coatings (TBCs) consisting of an oxidation-resistant metallic bond coat and a thermally insulating ceramic topcoat of yttria-stabilized zirconia (YSZ) have been applied due to their low thermal conductivity and high thermal expansion coefficient [1][2][3][4]. At present, air plasma spraying (APS) and electron beam-physical vapor deposition (EB-PVD) are the two main technologies that are used for TBC deposition [5].…”

Crack Propagation and Effect of Mixed Oxides on TGO Growth in Thick La–Gd–YSZ Thermal Barrier Coating

“…Surface and cross-section of the prepared samples were characterized by using scanning electron microscope (SEM; JEOL JSM 6010 LA/LV, Kajang, Malaysia) equipped with energy dispersive spectrometer (EDS). The porosity analysis was conducted using ASTME 2109-01(201) standards [26] to determine the percentage area of porosity in the BC. The BC porosity was characterized using the percentage of pore surface content on the surface of the whole BC area (%).…”

“…The higher porosity is due to the open-air atmospheric production of APS. Meanwhile, HVOF exhibits low porosity due to the direct stream of hot gas and powder towards the surface of the substrate that is to be coated [26]. The powder is partially melted in the stream and is deposited upon the Ni-substrate.…”