ISO/TR 26946:2011
(Main)Standard method for porosity measurement of thermally sprayed coatings
Standard method for porosity measurement of thermally sprayed coatings
ISO/TR 26946:2011 describes a method for characterizing the porosity of thermally sprayed coatings by metallographical examination. This method is particularly applicable to oxide coatings, such as Al2O3, ZrO2 and TiO2, produced by plasma spray. It also considers the purposes to test the size, shape and density of pores for thermally sprayed coatings.
Méthode normalisée de mesure de la porosité des revêtements obtenus par projection thermique
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 26946
First edition
2011-11-15
Standard method for porosity
measurement of thermally sprayed
coatings
Méthode normalisée de mesure de la porosité des revêtements obtenus
par projection thermique
Reference number
©
ISO 2011
© ISO 2011
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Published in Switzerland
ii © ISO 2011 – All rights reserved
Contents Page
Foreword . iv
1 Scope . 1
2 Purpose . 1
3 Classification . 1
4 Principle . 1
5 Apparatus . 2
6 Metallographic preparation . 2
6.1 General . 2
6.2 Sectioning . 2
6.3 Cleaning . 2
6.4 Mounting . 3
6.5 Grinding and polishing . 3
7 Metallography procedure. 5
8 Presentation of porosity . 6
9 Test report . 6
Annex A (informative) List of t values with different n and values . 8
Annex B (informative) Report of an international round robin test on the determination of porosity
in plasma sprayed ceramic coatings by using image analysis of metallographically
prepared cross sections . 9
B.1 Introduction . 9
B.2 Scheme of the round robin test . 9
B.2.1 Participants . 9
B.2.2 Sample preparation . 9
B.2.3 Guideline of procedures for porosity measurement . 10
B.3 Results and discussion . 11
B.3.1 Initial results . 11
B.3.2 Second round test carried out in Japan . 12
B.4 Reassessment of the round robin tests by the grinding/polishing conditions . 15
B.5 SEM imaging mode . 19
B.6 Summary . 19
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 26946 was prepared by Technical Committee ISO/TC 107, Metallic and other inorganic coatings.
iv © ISO 2011 – All rights reserved
TECHNICAL REPORT ISO/TR 26946:2011(E)
Standard method for porosity measurement of thermally
sprayed coatings
1 Scope
This Technical Report describes a method for characterizing the porosity of thermally sprayed coatings by
metallographical examination.
This method is particularly applicable to oxide coatings, such as Al O , ZrO and TiO , produced by plasma
2 3 2 2
spray. It also considers the purposes to test the size, shape and density of pores for thermally sprayed
coatings.
2 Purpose
The main purpose of porosity measurement is to determine the quality of a thermally sprayed coating and its
freedom from porosity, particularly on those areas of the significant surface that demand a functional
requirement.
This Technical Report provides a standard process that is suitable for determining the porosity of thermally
sprayed coatings, as part of the total quality assurance programme.
This Technical Report is also intended to provide a standard way to present the porosity of thermally sprayed
coatings.
3 Classification
The microstructure of a thermally sprayed ceramic coating is characterized by the existence of various pores,
microcracks, splat boundaries and unmelted particles, because of the nature of the process. Although different
terms are used, both the pores and the microcracks are volumetric spaces, which are free from coating
material. The pores can be divided into closed pores, open pores and micropores. Closed pores appear as
isolated clustered voids in the coating and have no connection with the surface; open pores appear as the
same voids but have a connection with the atmosphere, either directly or from one pore to another;
micropores are either closed or open pores which show dimensions only detectable on a microscopic scale.
The difference between pores and microcracks lies mostly in their aspect ratios (ratio of the major axis over
the minor axis), so, they are collectively treated as pores. The fraction of volumetric space covered by the
pores in thermally sprayed coatings is defined as porosity.
4 Principle
The porosity of thermally sprayed coatings is determined by preparing an area of the inspected coating with a
cross-section of high microscopic surface quality, which can be viewed using a light microscope or a scanning
electron microscope (suggested). A quantitative assessment of the porosity of the inspected coatings is
carried out by using an image analysis technique on the microscope.
5 Apparatus
The following equipment is necessary for the porosity measurement of thermally sprayed coatings.
5.1 Cut-off wheels (recommended) or diamond wire or high pressure water-jet cutting equipment,
(according to equipment in existence) for sectioning coating samples to a proper size with minimal damage.
5.2 Cleaning apparatus, with ultrasonic equipment.
5.3 Mounting equipment.
5.4 Grinding and polishing equipment, (semi-automated or automated grinding/polishing machines are
recommended for consistent reproducibility).
5.5 Scanning electron microscope (recommended) or light microscope, for viewing the inspected
sample on a cross-section and obtaining digital images.
5.6 Computer, with analysing software for porosity evaluation on digital images.
All equipment should undergo regular maintenance and calibration to assure reliability and repeatability of the
measurement. At the same time, all metallographic personnel should have the proper training to allow them to
perform the required functions and analyses.
6 Metallographic preparation
6.1 General
Metallographic preparation of thermally sprayed coatings is critical for the porosity results. The requirements
for detail and monitoring will vary from system to system, depending upon the degree of automation in the
preparation. The basic steps for the preparation are given in 6.2 to 6.5.
6.2 Sectioning
If sectioning is required, two commonly used methods are abrasive wheel cutting or diamond wire cutting. The
first one, which is comprised of a diamond or boron nitride saw (more effective in this purpose) that breaks
down readily exposed fresh cutting surfaces, is usually best for a wide range of coatings. Sectioning should be
done with the cutting force from coating to substrate and minimal clamping pressure on the sample. It will be
better to secure the specimen for sectioning with a soft cushion, such as wood, if possible. The sectioning
wheel should be as thin as possible to minimize damage, which must be removed in subsequent steps.
Minimum pressure should be applied on the wheel to minimize possible overheating, with cooling by water if
possible. The length of the test specimen should be greater than 1 cm. At least five test specimens should be
taken from each sample in different positions.
6.3 Cleaning
Cleaning is an important step for removing all contaminants from the surface of the specimen. Three methods
or any combination are recommended.
a) Washing samples with soap and water.
b) Brushing or soaking samples in solvent, such as acetone/alcohol, followed by application of heat
treatment to drive off any internal absorption.
c) Cleaning samples by performing an initial/extra vacuum step (if using vacuum impregnation in mounting)
to volatilize any entrapped materials.
2 © ISO 2011 – All rights reserved
6.4 Mounting
For the preparation of polished cross-sections, it is necessary to mount the selected region first so that a flat
polished area with minimal edge rounding is obtained. In this case, edge retention can be improved by coating
the outer surface of the sample with an additional layer during grinding and polishing. Electroless nickel
plating or sputtering with a metal layer are commonly used. The mounting procedure/material depends on the
following:
a) time available for mounting;
b) size of porosity and level of voids in the coating, and degree of interconnected porosity;
c) required viscosity of epoxy for impregnation of porosity is important (the viscosity of the cold-mount epoxy
should be medium, especially when porosity in the coating is small and difficult to impregnate);
d) hardness of coating vs. mounting material. (The mounting medium should be chosen to allow good edge
retention and be of comparable hardness to the coating, in order to minimize difficulties during grinding
and polishing.)
Cold mounting, which can be assisted by heat, with vacuum impregnation alone and/or pressure impregnation
is recommended.
6.5 Grinding and polishing
Generally, grinding and polishing parameters that must be considered/controlled in preparation are listed in
Table 1. Additional care must be taken to remove cut-off damage during initial grinding if the sectioning step
was used, and avoid over-polishing with colloidal silica in the final steps of preparation. During grinding,
examine the prepared area at each stage to ensure that all the damage from the previous stage has been
removed. In the case of polishing, the sample is polished with diamond paste down to 1 μm grade, then
alumina paste is used with 0,3 μm grade. Further polishing with colloidal silica may be required to obtain a
scratch-free surface. After polishing, clean the sample in suitable solven
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