ISO 21456

FINAL DRAFT
International
Standard
ISO/FDIS 21456
ISO/TC 107
Determination of the residual
Secretariat: KATS
stress of TGO layer in thermal
Voting begins on:
barrier coating by photoexcitation
2024-12-13
fluorescence piezoelectric
Voting terminates on:
spectroscopy
2025-02-07
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MADE IN NATIONAL REGULATIONS.
Reference number
ISO/FDIS 21456:2024(en) © ISO 2024

FINAL DRAFT
ISO/FDIS 21456:2024(en)
International
Standard
ISO/FDIS 21456
ISO/TC 107
Determination of the residual
Secretariat: KATS
stress of TGO layer in thermal
Voting begins on:
barrier coating by photoexcitation
fluorescence piezoelectric
Voting terminates on:
spectroscopy
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2024
IN ADDITION TO THEIR EVALUATION AS
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BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
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INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
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MADE IN NATIONAL REGULATIONS.
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Published in Switzerland Reference number
ISO/FDIS 21456:2024(en) © ISO 2024

ii
ISO/FDIS 21456:2024(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 3
4.1 General .3
4.2 Principle of measuring residual stress by photo-excited fluorescence piezoelectric
spectroscopy .3
5 Test methods . 3
5.1 General .3
5.2 Test specimen .3
5.3 PFPS device calibration .4
5.4 Setting of detection conditions .4
5.5 Sample focusing .4
5.6 Detection of Raman peaks .4
5.7 Data acquisition .4
6 Calculation of stress . 4
7 Reliability . 7
8 Test report . 7
Annex A (informative) Example of the determination of the residual stress of the TGO layer in
TBC by photoexcited fluorescence piezoelectric spectrum . 9
Bibliography .11

iii
ISO/FDIS 21456:2024(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
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in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
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Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
ISO/FDIS 21456:2024(en)
Introduction
The thermally grown oxide (TGO) layer of a thermal barrier coating (TBC) is the fundamental cause of
interface crack and eventual spalling failure of the ceramic layer. Therefore, the TGO layer and its interfaces
with each layer are risky areas for TBC failure and peeling. The residual stress in the TGO of a TBC can
be determined using the photoexcitation fluorescence piezoelectric spectroscopy (PFPS) method. This
provides an important basis for the lifetime evaluation of TBC and to understand the failure mechanism of
the TBC.
This method to test the residual stress in the TGO layer is a non-destructive testing method, unlike the
curvature and drilling methods, which cause damage to the sample. Unlike x-ray diffraction, the penetration
depth is only tens of micrometers.
3+
The inclusion of Cr in the TGO of a TBC is a prerequisite for testing the residual stress of the TGO layer of
TBC by photoexcited fluorescence piezoelectric spectroscopy. No matter what method is used to prepare the
TBC system, the bond coat contains a Cr element.
The size, shape and composition of the substrate material are not specified and differentiated. In addition,
the preparation method of the TBC is not specified and differentiated.
The residual stress of the TGO layer is one of the main factors causing the failure of the TBC. However, no
standard document is available for the test method process and the result of the photoexcited fluorescence
piezoelectric spectroscopy test of residual stress in the TGO layer of the TBC. Therefore, it is necessary to
develop a standardized and unified test method process that is conducive to the formation, simulation and
testing of residual stress in the TGO layer of the TBC and even the prediction of the service life of the TBC.

v
FINAL DRAFT International Standard ISO/FDIS 21456:2024(en)
Determination of the residual stress of TGO layer in thermal
barrier coating by photoexcitation fluorescence piezoelectric
spectroscopy
1 Scope
This document specifies a test method for the determination of the residual stress of the TGO layer in
thermal barrier coating (TBC) by photoexcitation fluorescence piezoelectric spectroscopy.
This test method specifies that there is a Cr element in the bond coat of the TBC.
This test method to determine the residual stress in the TGO layer of the TBC system is not limited by the
preparation method of the TBCs. Particularly, the TBC system prepared by electron beam-physical vapour
deposition (EB-PVD) has a better effect.
This method provides guidance on determining reliable estimates of residual stresses from fluorescence
spectral data and estimating uncertainties in the results.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 14188, Metallic and other inorganic coatings — Test methods for measuring thermal cycle resistance and
thermal shock resistance for thermal barrier coatings
ISO 19477, Metallic and other inorganic coatings — Measurement of Young's modulus of thermal barrier coatings
by beam bending
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 14188 and ISO 19477 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
thermal barrier coating
TBC
two-layer coating consisting of a metallic bond coat and a ceramic top coat in order to reduce heat transfer
from outside of the top coat through the coating to the substrate
Note 1 to entry: Thermal barrier coating is a thermal protection technology that combines ceramic materials, known
for high temperature resistance and low thermal conductivity, with substrate alloy in the form of coating to reduce the
surface temperature of hot-end components, enhance resistance to high-temperature oxidation corrosion in substrate
materials and ultimately improve the engine's thrust-to-weight ratio, thermal efficiency and the service life of hot-end
components under high temperature and stress.

ISO/FDIS 21456:2024(en)
Note 2 to entry: Thermal barrier coating systems usually consist of a metal bond coat and an insulating ceramic coat
(see Figure 1). In the thermal barrier coating system, due to the large difference in thermal expansion coefficient
between the ceramic coat and the substrate material, it is easy to produce large thermal stress in the service process,
lea
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