http://www.solidfilm.cn/up/pdf/2009051127548345.pdf

Preparation and thermophysical properties of CeO2 doped La2Zr2O7 ceramic for thermal barrier coatings

Thermal barrier coatings (TBCs), which protect metallic components from high-temperature environments, have been widely applied to the fields of high-temperature and corrosion-resistant structural parts such as gas turbine engines, diesel engines, and power generation systems. Yttria-stabilized zirconia (YSZ) is one of the most widely used materials for TBCs owing to its excellent shock resistance, low-thermal conductivity, and relatively high coefficient of thermal expansion. In this paper the properties of YSZ and the recent developments of YSZ-TBCs are reviewed. The failure mechanism of YSZ-TBCs and corresponding methods for lengthening the lifetime of YSZ-TBCs are discussed. The advantages of graded thermal barrier coatings and the problems in processing are elucidated.

Yttria-stabilized zirconia thermal barrier coatings — a review

The effects of heat treatment and gas atmosphere on thermal conductivity of atmospheric plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) partially Y 2 O 3 stabilized ZrO 2 (PYSZ) thermal barrier coatings (TBCs) were investigated. Two-layer samples that had an EB-PVD coating deposited on bond coated nickel-base superalloy IN625 substrates, free-standing APS and EB-PVD coatings as well as a quasi-free-standing EB-PVD PYSZ coating (coating on semitransparent sapphire) were included in the study. Thermal diffusivity measurements for determining thermal conductivity were made from room temperature up to 1150 °C in vacuum and under argon gas using the laser flash technique. To investigate the effect of heat treatment on thermal conductivity, coatings were annealed at 1100 °C in air. For both the APS and EB-PVD PYSZ coatings the first 100 h heat treatment caused a significant increase in thermal conductivity that can be attributed to microstructural changes caused by sintering processes. Compared to the measurements in vacuum, the thermal conductivity of APS coatings increased by about 10% under argon gas at atmospheric pressure, whereas for the EB-PVD coatings, the influence of gas on thermal conductivity was relatively small. The effect of gas on the thermal conductivity of APS and EB-PVD PYSZ coatings can be attributed to amount, shape, and spatial arrangement of pores in the coating material.

The effects of heat treatment and gas atmosphere on the thermal conductivity of APS and EB-PVD PYSZ thermal barrier coatings

Synthesis, structure, microstructure and mechanical characteristics of MOCVD deposited zirconia films

The interfacial fracture of thermal barrier coatings (TBCs) at the ceramic-bond coat interface occurs during the cooling period following exposure to high temperatures. Previous studies showed that, the presence of multiple vertical surface precracks that span over the entire coating thickness, was beneficial in reducing the propagation of interface cracks. In this paper, the effect of different surface precrack morphologies, such as precrack length and precrack density, on an interface crack during a thermal shock caused by a high heat flux is presented. The results show that shorter surface precracks and a larger precrack density can delay the interfacial fracture. In particular, this behavior is shown to be more prominent in thicker TBCs.

Effect of pre-existing surface crack morphology on the interfacial thermal fracture of thermal barrier coatings: a numerical study

Thermal barrier coated high pressure turbine blades were characterized before and after the service by microstructural analysis and Cr3+ photostimulated luminescence piezo-spectroscopy. Thermal barrier coatings, in this study, consisted of electron beam physical vapor deposited yttria partially stabilized zirconia (YSZ; ZrO2–8 wt.% Y2O3), vapor-deposited aluminide bond coat and Ni-base superalloy. Compressive residual stress in thermally grown oxide, measured by Cr3+ photostimulated luminescence piezo-spectroscopy, was observed to be in the order of 2.5∼3.0 GPa and varied slightly as a function of substrate geometry. X-Ray diffraction and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy were utilized to investigate the microstructural development of thermal barrier coatings. The as-deposited non-equilibrium tetragonal (t′) phase in the YSZ coatings was observed to decompose after the service, but the monoclinic (m) phase was only found in the YSZ coatings with concave substrate curvature on the pressure side of the HPT blade. Also, a significant sintering of ZrO2–8 wt.% Y2O3 coating after the service was observed in the microstructure. Localized spallation of YSZ occurred within the thermally grown oxide (mostly α-Al2O3) and within the ZrO2–8 wt.% Y2O3 coating for pressure and suction sides of the serviced high pressure turbine blade near the tip, respectively.

E.Y. Lee

Papers

Microstructural characterization of thermal barrier coatings on high pressure turbine blades