Overview
ISO 21456:2025 specifies a standardized, non‑destructive test method to determine the residual stress in the thermally grown oxide (TGO) layer of thermal barrier coatings (TBCs) using photoexcitation fluorescence piezoelectric spectroscopy (PFPS). The method requires a Cr-containing bond coat (Cr must be present in the bond coat so Cr3+ can act as the probe) and is applicable regardless of the TBC preparation route - with electron beam‑physical vapour deposition (EB‑PVD) systems noted as particularly favorable. ISO 21456:2025 also provides guidance on converting fluorescence spectral data into reliable residual stress estimates and on estimating measurement uncertainty.
Key topics and requirements
- Principle: PFPS is a Raman‑type fluorescence technique that monitors Cr3+ fluorescence lines (R1, R2) in TGO. Peak positions (notably near ~14 300–14 500 cm−1 / wavelengths ~694.3 nm and 692.9 nm) shift with stress and are used to quantify residual stress.
- Prerequisite: Bond coat must contain chromium (Cr) so Cr3+ enters the TGO layer and produces detectable fluorescence.
- Scope of applicability: Not limited by substrate size/shape or TBC manufacture method; EB‑PVD and PS‑PVD architectures are well suited. APS coatings may increase scattering and require optimized excitation settings.
- Test procedure elements (documented): specimen preparation, PFPS device calibration, detection condition settings, sample focusing, Raman/fluorescence peak detection, data acquisition, stress calculation, and uncertainty estimation.
- Penetration and resolution: PFPS probes only tens of micrometres into the TGO (shallow depth), offering high spatial resolution compared with bulk X‑ray methods.
- Reliability and reporting: The standard instructs on evaluating measurement reliability and specifies required content of the test report; includes an informative example (Annex A).
Applications and users
ISO 21456:2025 is intended for:
- Materials and coatings engineers evaluating TBC life and failure mechanisms
- R&D laboratories studying TGO growth, stress evolution, and coating architectures
- Nondestructive testing (NDT) and quality control teams in aerospace, power generation, and turbine manufacturing
- Failure analysis groups and service centers performing lifetime assessments and maintenance planning
Practical uses include periodic inspection of TBCs, comparing coating processes (EB‑PVD vs APS), validating finite‑element stress models, and providing input data for service‑life prediction.
Related standards
- ISO 14188 - test methods for thermal cycle/thermal shock resistance of TBCs
- ISO 19477 - measurement of Young’s modulus of TBCs by beam bending
Keywords: ISO 21456:2025, TGO residual stress, thermal barrier coating, PFPS, photoexcitation fluorescence piezoelectric spectroscopy, EB‑PVD, TBC inspection, nondestructive testing.
Open preview (PDF)Determination of the residual stress of TGO layer in thermal barrier coating by photoexcitation fluorescence piezoelectric spectroscopy
