ISO 24449:2021

Overview

ISO 24449:2021 specifies a standardized method to determine the thermal conductivity of the ceramic top coat (TC) of thermal barrier coatings (TBCs) in the direction normal to the coating surface, from room temperature up to 1 000 °C. The method uses flash-type thermal diffusivity measurements on heat-treated specimens and calculates TC thermal conductivity from measured thermal diffusivity, specific heat capacity and bulk density using a multi‑layer analytical model.

Key topics and technical requirements

• Scope: Measurement of thermal conductivity for ceramic top coats (TC) in TBC systems (substrate + bond coat + top coat) at elevated temperatures up to 1 000 °C.
• Specimen types and preparation: Uses two specimen types - substrate alone and TBC (substrate+bond coat+TC). Specimens are flat disks or square plates with lateral dimensions in the range given by the standard (typical diameters/side lengths ≈ 10–15 mm). The standard defines thickness ranges (substrate and total TBC) and thickness tolerances and requires controlled polishing and surface treatment.
• Heat treatment: TBC specimens are heat-treated at the maximum specified test temperature for 2–3 hours prior to measurement to stabilize microstructure.
• Measurement principle: Thermal diffusivity is measured by the flash (pulse) method (see ISO 18755), recording temperature‑rise curves on the rear/coating surface with an infrared radiometer following a pulse heating light source. Both substrate and TBC temperature‑rise curves are measured at each test temperature.
• Data and calculations: Thermal diffusivity of the TC is derived using a multi‑layer analytical model (areal heat diffusion time method or theoretical temperature‑rise curve method) combined with separately measured specific heat capacity and bulk density to compute thermal conductivity.
• Specimen surface treatment: Both faces are coated with a thin opaque black layer (e.g., colloidal graphite or sputtered metal + graphite) to ensure high absorption and emissivity. Polishing with abrasive paper P800 or finer is specified for TC surface finish.

Applications and users

• Who uses it: Coating manufacturers, materials testing laboratories, R&D groups, turbine OEMs, quality assurance teams and failure-analysis engineers.
• Practical uses: Characterizing high‑temperature insulating performance of TBCs for gas turbine blades, vanes, combustors and aero‑engine hot‑section components; validating coating process changes; comparing TBC materials and architectures; supporting lifetime and thermal modeling studies.

Related standards

• ISO 18755 - thermal diffusivity by laser/flash method
• ISO 18555 - thermal conductivity of TBCs at room temperature
• ISO 1463 - coating thickness (microscopical)
• ISO 14188 - TBC thermal cycle and shock tests
• EN 821‑3 - specific heat capacity for ceramics

Keywords: ISO 24449:2021, thermal barrier coatings, thermal conductivity measurement, thermal diffusivity, flash method, high‑temperature testing, ceramic top coat, TBC testing.