Overview
ISO 18555:2016 specifies a laboratory method to determine the thermal conductivity of thermal barrier coatings (TBCs) made of a metallic bond coat (BC) and a ceramic top coat (TC). Measurements are taken normal to the coating surface at room temperature. The standard uses pulse‑heating (“flash”) techniques and multi‑layer analytical models to derive thermal conductivity from measured thermal diffusivity, specific heat capacity and bulk density.
Keywords: ISO 18555:2016, thermal barrier coatings, thermal conductivity, thermal diffusivity, flash method, bond coat, top coat, TBC testing
Key topics and technical requirements
- Measurement principle: Thermal conductivities are calculated from thermal diffusivity data combined with separately measured specific heat capacity and bulk density.
- Specimens: Three specimen types are required - bare substrate, substrate with bond coat (BC), and substrate with full TBC (BC + TC). Specimens are flat disks or square plates with lateral dimensions typically 10–15 mm and substrate thickness typically 1–2 mm (values given in the standard).
- Apparatus: Uses a pulse heating light source, infrared radiometer, thermocouple, specimen holder and chamber as per the flash method apparatus described in ISO 18755:2005.
- Data acquisition: Measure temperature‑rise curves on the specimen rear surface after pulse heating (normalized temperature‑rise vs time). Key derived quantities include apparent thermal diffusivity, half‑rise time and areal heat diffusion time.
- Analytical models: The thermal diffusivities of BC and TC are obtained by applying a multi‑layer analytical model - either the areal heat diffusion time method or the theoretical temperature‑rise curve method (chosen by agreement between parties).
- Specimen preparation: Coating surfaces are smoothed and both faces are coated with a thin opaque (preferably black) layer for radiative uniformity, following ISO 18755:2005. Coating thicknesses of BC and TC are measured from cross‑section images (ISO 1463).
- Reporting: The standard defines required reporting items including test conditions, specimen geometry, thermal diffusivities, specific heat, bulk density and calculated thermal conductivities.
Practical applications and users
ISO 18555:2016 is used by:
- Materials and coatings laboratories for quality control and R&D of TBC systems.
- Turbine and aero‑engine manufacturers evaluating thermal protection performance of bond/top coat systems.
- Failure analysis and lifetime assessment where accurate through‑thickness heat transfer data are needed.
- Certification and procurement teams specifying thermal properties for hot‑section components in power generation and aerospace.
Practical uses include material selection, process validation, comparative testing of coating systems, and inputs for thermal‑mechanical modeling of coated components.
Related standards
- ISO 18755:2005 - Determination of thermal diffusivity of monolithic ceramics (laser‑flash method)
- ISO 1463 - Measurement of coating thickness - Microscopical method
- EN 821‑3 / ASTM E1269 - Methods for specific heat capacity measurements
For implementing ISO 18555:2016, laboratories should follow the referenced standards for apparatus calibration, specimen preparation and complementary measurements.
Open preview (PDF)Metallic and other inorganic coatings — Determination of thermal conductivity of thermal barrier coatings
