Overview
ISO/TR 26946:2011 defines a standard, metallography-based method for measuring porosity in thermally sprayed coatings. Published as an ISO Technical Report, it is primarily aimed at oxide ceramic coatings produced by plasma spray - notably Al2O3, ZrO2 and TiO2. The document describes how to prepare cross-sections, acquire microscopic images and use image analysis to quantify pore size, shape and volumetric porosity for quality assurance and comparative studies.
Key topics and requirements
- Scope & purpose: Establishes a reproducible procedure for porosity characterization as part of coating quality control and for reporting porosity results consistently.
- Porosity classification: Defines closed pores, open pores and micropores; treats microcracks and pores collectively based on aspect ratio.
- Principle: Porosity is measured from high-quality cross-sectional images (light microscope or preferably SEM) and quantified by image analysis.
- Apparatus:
- Sectioning tools (cut-off wheels, diamond wire, or water-jet)
- Ultrasonic cleaning equipment
- Mounting and impregnation materials (cold-mount epoxies, vacuum/pressure impregnation)
- Grinding/polishing equipment (semi-automatic/automatic recommended)
- SEM (recommended) or optical microscope; computer with image-analysis software
- Metallographic preparation:
- Sectioning guidance (minimize damage; sample length > 1 cm; at least five specimens from different positions)
- Cleaning (soap/water, solvents, ultrasonic; vacuum steps for impregnation)
- Mounting: electroless nickel plating or sputtering, choice of epoxy viscosity and hardness to improve edge retention
- Grinding/polishing: staged abrasive sequence (diamond down to 1 μm, alumina to 0.3 μm and optional colloidal silica) and control of pressure, speed, lubricant and time to avoid pullouts and artifacts
- Pre-coating with a thin conductive film (10–20 nm carbon or gold) for SEM imaging
- Imaging & analysis: SEM is strongly recommended; backscattered electron imaging (BEI) is useful when pullouts are absent. Number of fields and magnification should match pore size distribution and desired accuracy.
- Reporting: Standardized presentation of porosity values and test conditions; annex includes round-robin results and guidance.
Applications and users
ISO/TR 26946:2011 is used by:
- Coating engineers and metallographers performing porosity characterization of thermal-spray coatings
- Quality assurance and failure-analysis laboratories in aerospace, power generation, automotive and industrial manufacturing
- Material scientists and researchers conducting comparative studies or method validation (round-robin testing guidance included)
Practical uses include process control for plasma spray parameters, acceptance testing of functional coating areas, and evaluation of porosity-related performance (insulation, wear resistance, corrosion protection).
Related standards
Consult other ISO standards on thermal spraying, metallographic preparation and image analysis, and documents from ISO/TC 107 for complementary guidance on thermal-sprayed coatings and surface characterization.