ISO 21456:2025

International
Standard
ISO 21456
First edition
Determination of the residual
2025-03
stress of TGO layer in thermal
barrier coating by photoexcitation
fluorescence piezoelectric
spectroscopy
Détermination de la contrainte résiduelle de la couche TGO
dans les revêtements barrières thermiques par spectroscopie de
photoexcitation fluorescente et piezoélectrique
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
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 . 5
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
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
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
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)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 107, Metallic and other inorganic coatings.
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
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 potential causes of 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 micrometres.
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
International Standard ISO 21456:2025(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.

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,
leading to premature failure. In order to improve the physical compatibility and alleviate the thermal expansion
mismatch between the ceramic coat and the substrate, a metal bond coat is often introduced between the substrate
and the ceramic coat. At the same time, the oxide film generated by the oxidation of the bonding layer can also improve
the high-temperature oxidation resistance and corrosion resistance of the substrate alloy. At present, MCrAlYX (M =
Ni and/or Co, X = Hf, Ta, Si, etc.) is widely used as a bond coat.
Key
1 substrate
2 bond coat
3 thermally grown oxide
4 ceramic insulation top coat
Figure 1 — Diagram of a section of the thermal barrier coating system
3.2
thermally grown oxide
TGO
oxide grown between top and bond coat when the coating system is heated
Note 1 to entry: In the preparation process and high-temperature service environment, the oxygen molecules in the
air and the oxygen atoms of the ceramic coat diffuse to the interface between the ceramic coat and the bond coat and
react with the metal elements diffused from the bond coat to form the thermally grown oxide layer (TGO). As Al has
the strongest diffusion activity, it is the first to react to form dense TGO with α-Al O composition.
...