Coating 15
Coating 15 : Microstructure: Microstructure of coatings is routinely inspected and analyzed by metallographic preparation of cross sections. Although examination of metallic overlay and conversion coatings is relatively straightforward, graded coatings, which can include porosity and relatively loosely bonded material in the case of plasma-sprayed material, presents more difficulty. Special techniques (e. g. , epoxy infiltration) are required to preserve the coating microstructure. Procedures for metallographic preparation and microstructural analysis have been developed by coating producers and users but have not been codified as standard methods. Porosity, manifested as coating density, has significance for coating thermomechanical behavior, corrodent penetration, heat transfer, and centrifugal loading of turbine disks. Standards for measurement of connected porosity by use of the BET gas absorption technique are routinely used (e. g. , ASTM C-577-92-refractory permeability). Unconnected porosity is more difficult to measure. Sophisticated techniques (e. g. , small-angle neutron scattering) have been used in research to determine pore size and distribution. This technique is not suitable for routine laboratory or production use but may provide a means to synthesize standard materials for calibration of metallographic or other techniques
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