This paper reviews some of the tools used to characterize the pore microstructure of thermally sprayed thermal barrier coatings. It describes the capabilities of optical and SEM analysis, intrusion porosimetry, high-flux X-ray and neutron imaging, small-angle scattering, and X-ray microtomography. It explains why it is often necessary to combine multiple techniques and provides examples showing where it has been done and what it can achieve. It also reviews commonly used porosity descriptors.

Coating porosity is an important parameter to optimize for plasma-sprayed ceramics which are intended for service in molten metal environments. Too much porosity and the coatings may be infiltrated by the molten metal causing corrosive attack of the substrate or destruction of the coating upon solidification of the metal. Too little porosity and the coating may fail due to its inability to absorb thermal strains. This study describes the testing and analysis of tungsten rods coated with aluminum oxide, yttria-stabilized zirconia, yttrium oxide, and erbium oxide deposited by atmospheric plasma spraying. The samples were immersed in molten aluminum and analyzed after immersion. One of the ceramic materials used, yttrium oxide, was heat treated at 1000°C and 2000°C and analyzed by X-ray diffractography and mercury intrusion porosimetry. Slight changes in crystal structure and significant changes in porosity were observed after heat treatments.

The formation of the microstructure is influenced by a number of parameters. The spray angle is considered to be one of the most important and difficult to control parameters, especially for substrates with complex shapes. In this paper, the influence of the spray angle on the microstructure of gray aluminum oxide and yttrium oxide stabilized (8% by weight) zirconium oxide applications is investigated. Plots are made from each material at four spray angles (90 degree, 70 degree, 55 degree, and 45 degree). Their microstructure is characterized by means of intrusion porosimetry and SEM. The results show clear differences between the two materials. The aluminum oxide microstructure is significantly influenced by the spray angle, while the YSZ microstructure is almost independent of the spray angle. Paper includes a German-language abstract.

Yttrium oxide-stabilized zirconium dioxide is mainly used in thermal barrier coatings. However, the desired higher gas inlet temperatures have initiated the search for novel materials. Low thermal conductivity and high melting points are important criteria for the selection of these materials. This paper investigates a zirconate material with a pyrochlore structure and a high melting point. In addition, it investigates the sintering behavior of the coatings at elevated temperatures. Dilatometer tests are carried out at 1200 deg C for at least 70 hours. For samples which had been annealed for 24 hours at 1250 deg C, the change in the porosity distribution is determined by means of mercury porosimetry. The paper presents the first results of thermal cycling tests on a plasma-sprayed coating. These first results are very promising. Paper includes a German-language abstract.

Reducing the pore size and pore volume can lead to improved mechanical properties and enhanced corrosion resistance of plasma sprayed thermal barrier coatings. In this work, plasma sprayed 8 wt.% yttria stabilized zirconia coatings were removed from the substrate and machined to obtain 25x5x1 mm test specimens. These specimens were vacuum impregnated with alumina sol, calcined at 873 K for an hour and then heat treated at 1273 K for an hour to produce ceramic impregnated specimens. As-sprayed and impregnated specimens were investigated using optical microscopy, XRD, SEM, mercury intrusion porosimetry and electron microprobe analysis. This technique can impregnate the entire thickness of the specimens. Pores in the as-sprayed specimens were impregnated with α alumina grains, resulting in microstructural variations and reduction of the size and volume of the specimen pores.

Properties and characteristics of a material depend mainly upon its morphological structure. In the case of plasma sprayed deposits, the properties (i.e., mechanical, thermal, electrical, etc.) are especially related to the deposit void content. Voids are coming from pores, cracks and delaminations. Implementing image analysis and stereological protocols, the apparent void content (AVC) of a vacuum plasma sprayed deposit was studied, in terms of the pore size distribution and spatial distribution. Results were compared with data obtained implementing water immersion porosimetry. This study showed on the one hand that the pore size distribution is described by a bimodale function, and, on the other hand, that pores are spatially distributed evenly within the deposit (isotropic distribution). In such a condition, the stereological protocol can be implemented on a single random plane section of the deposit.