Anisotropic microstructure of plasma-sprayed deposits

The microstructure of plasma sprayed deposits (PSD) is dominated by two void systems-interlamellar pores and intralamellar cracks-each with a different anisotropy, Anisotropy of these void systems and varying crack-to-pore ratios within PSDs are responsible for the anisotropic properties observed within deposits. While it is difficult to apply standard porosity measurement techniques to the assessment of anisotropic microstructures, novel techniques utilizing different approaches have recently emerged. Image analysis (IA) of impregnated PSD samples is the most direct technique. The structure is stabilized by impregnation and then polished and imaged. The limitations of IA lie in the impregnation process and in the subsequent polishing. Also, the images produced from anisotropic materials can be difficult to interpret quantitatively. The technique of small-angle neutron scattering (SANS) has recently been applied to the study of PSDs, The major advantages of SANS are that it does not require sample preparation and that quantitative information can be obtained concerning the separate crack and pore systems, including their distinctive anisotropies. However, the relationship between the SANS results and the underlying structure is more complex and less intuitive than for IA, and the availability of the SANS technique is limited by the need to have access to a powerful neutron source, such as a reactor. Also, the two techniques present different views of the microstructure because of the different sensitivities in different parts of the size range. This article compares results from IA and SANS for a set of thick plasma-sprayed ceramic deposits possessing a range of crack/pore microstructures and discusses how the two techniques might complement one another.