Standard Guide for Metallographic Preparation of Thermal Sprayed Coatings
4.1 TSCs are used in a number of critical industrial components. TSCs can be expected to contain measurable levels of porosity and linear detachment. Accurate and consistent evaluation of specimens is essential to ensure the integrity of the coating and proper adherence to the substrate.
4.1.1 Example 1: By use of inappropriate metallographic methods, the apparent amount of porosity and linear detachment displayed by a given specimen can be increased, by excessive edge rounding, or decreased by smearing of material into voids. Therefore inaccurate levels of porosity and linear detachment will be reported even when the accuracy of the measurement technique is acceptable.
4.1.2 Example 2: Inconsistent metallographic preparation methods can cause the apparent amount of voids to vary excessively indicating a poorly controlled thermal spray process, while the use of consistent practice will regularly display the true microstructure and verify the consistency of the thermal spray process.
4.2 During the development of TSC procedures, metallographic information is necessary to validate the efficacy of a specific application.
4.3 Cross sections are usually taken perpendicular to the long axis of the specimen and prepared to reveal information concerning the following:
4.3.1 Variations in structure from surface to substrate,
4.3.2 The distribution of unmelted particles throughout the coating,
4.3.3 The distribution of linear detachment throughout the coating,
4.3.4 The distribution of porosity throughout the coating,
4.3.5 The presence of contamination within the coating,
4.3.6 The thickness of the coating (top coat and bond coat, where applicable),
4.3.7 The presence of interfacial contamination,
4.3.8 The integrity of the interface between the coating and substrate, and,
4.3.9 The integrity of the coating microstructure with respect to chemistry.
1.1 This guide covers recommendations for sectioning, cleaning, mounting, grinding, and polishing to reveal the microstructural features of thermal sprayed coatings (TSCs) and the substrates to which they are applied when examined microscopically. Because of the diversity of available equipment, the wide variety of coating and substrate combinations, and the sensitivity of these specimens to preparation technique, the existence of a series of recommended methods for metallographic preparation of thermal sprayed coating specimens is helpful. Adherence to this guide will provide practitioners with consistent and reproducible results. Additional information concerning standard practices for metallographic preparation can be found in Practice E3.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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