ISO/TR 26946:2011

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

• Status: Published
• Publication Date: 14-Nov-2011
• ICS: 25.220.20 - Surface treatment
• Technical Committee: ISO/TC 107 - Metallic and other inorganic coatings
• Drafting Committee: ISO/TC 107/WG 1 - Thermal spraying
• Current Stage: 6060 - International Standard published
• Completion Date: 15-Nov-2011

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/TR 26946:2011(E)
©
ISO 2011

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ISO/TR 26946:2011(E)

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©  ISO 2011
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ISO/TR 26946:2011(E)
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

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ISO/TR 26946:2011(E)
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.
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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.
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ISO/TR 26946:2011(E)
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.
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ISO/TR 26946:2011(E)
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 solvents in an ultrasonic bath to remove all
polishing debris. It should be noted that porosity evaluation is relatively a complicated process and grinding
and polishing parameters should be chosen properly for reproducible porosity results. Typical procedures
involving both grinding paper and disc formats are shown and suggested in Tables 2 and 3. These procedures
will require modification for different coating types and equipment available in the specific laboratories. Semi-
automatic/automatic machines in conjunction with written procedures that monitor/control critical parameters
are recommended, which will result in consistent and reproducible results.
The kind and amount of consumables used in the metallographic process are obviously very critical to the final
result. It is important to know the changes in consumable suppliers and these should be considered carefully.
The specific trial samples should be run to assure similar performance and results, if changes have to be
made to an already established procedure with new consumables.
Research should always be conducted to judge the preparation by SEM micrographs to confirm that no
coarse feature occurs during metallographic preparation which is significant of the presence of pullouts, which
inevitably result in deviation. Surface roughness is suggested as a crucial parameter to evaluate the quality of
the preparation, which is connected with porosity range in the inspected coating and should be as low as
possible.
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ISO/TR 26946:2011(E)
Table 1 — Grinding and polishing parameters considered/controlled in preparation
Parameter Description
Pressure Load/mount area
Speed Both table and specimen holder
Rotation direction Relative rotation of head with respect to table
Grinding disc vs. grinding papers
Format
Polishing: no-nap vs. high-nap clothes
Diamond, SiC, colloidal silica, Al O
Abrasive
2 3
Orientation How samples are placed in holder with respect to wheel rotation
Frequency How often is lubricant/abrasive applied
Kind of lubricant Oil, water, alcohol
Quantity of lubricant ml/min
Time Processing duration for each step

Table 2 — Typical procedure with the grinding paper format
Surface Grit size Pressure Speed Time Abrasive Lubricant Rotation
10 min.
Grinding Usually
(enough papers to flatten
180 40 kPa 300 rpm SiC Complementary
papers water
specimen and remove
damage/edge effects)
Grinding 400,600 and Usually
40 kPa 300 rpm 20 min. SiC Complementary
papers 800 water
30 min.
Grinding 1000,1200 Usually
40 kPa 300 rpm SiC Complementary
(usually 2 papers
papers and 2000 water
per grit size)
Can be in the Poly- or
Usually
No- range of Can be in the range mono-
40 kPa 300 rpm water or Complementary
nap cloth 1 to 6 μm of 2 to 4 min. crystalline
alcohol
diamond diamond
Usually in the Usually
Higher- Colloidal
range of 0,3 40 kPa 300 rpm Usually 4 to 6 min. water or Complementary
nap cloth silica, Al O
2 3
to 0.5 μm alcohol

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ISO/TR 26946:2011(E)
Table 3 — Typical procedure with the disc format
Surface Grit size Pressure Speed Time Abrasive Lubricant Rotation
Fixed
Poly- or mono-
diamond or Usually
40 to 60 μm 40 kPa 300 rpm 15 min. crystalline Complementary
composite water
diamond
disc
Fixed
Poly- or mono-
diamond or Usually
6 to 9 μm 40 kPa 300 rpm 30 min. crystalline Complementary
composite water
diamond
disc
Can be in the
Poly- or mono- Usually
No- range of Can be in the range
40 kPa 300 rpm crystalline water or Complementary
nap cloth 1 to 6 μm of 2 to 4 min.
diamond alcohol
diamond
Usually in the Usually
Higher- Colloidal silica,
range of 0.3 40 kPa 300 rpm Usually 4 to 6 min. water or Complementary
nap cloth Al O
2 3
to 0,5 μm alcohol
7 Metallography procedure
It should be noted that metallographic examination is only meaningful for the well-prepared sample, as well as
appropriate visual and numerical standards based on significant statistical analyses.
Pre-coating is necessary with a thin (10 to 20 nm) conducting film of carbon or gold (recommended) to avoid a
change in the surface which would result in poor quality images.
Scanning electron microscopy (SEM) is strongly recommended as porosity result from optical microscopy is
generally unreliable for the poor field depth, especially with higher magnification. Both SEI and BEI images
can be employed depending on the metallographic preparation mentioned above. BEI image is recommended
when no coarse feature such as pullout appears, which will be misunderstood as pores.
The magnification and the number of fields of view depend on both sample characters, such as porosity range
and pore size distribution, which also depend on the deposition method and materials deposited, as well as
measurement accuracy. It is contradictory to balance accuracy and the area of field of view. To achieve this
goal, at least 15 fields of view at 1 000  magnification, chosen randomly across the whole sample to ensure
unbiased results, is recommended. Several more images may be necessary when porosity or its size
distribution is high.
Focus the microscope on the area to be examined and optimize contrast conditions to distinguish the size and
area of the pores clearly and suppress background variations in the image. The micrographs should be
corrected prior to carrying out the analysis; features touching the image edges should be discarded for this
purpose during the analysis.
It is very important to follow the manufacturer's instructions when implementing the software to determine the
pore-area fraction. Generally, the pore edge in an image should be defined by a suitable threshold level. It is
strongly recommended that the threshold level be adjusted by comparing the processed images with the
original ones, in order to ensure that they are a reliable representation. It should be mentioned that this
process should be carried out on each image unless they are obtained from the same SEM equipment and
with the same parameters.
To increase the confidence in the measurements, statistical parameters, such as the mean diameter and
standard deviation for a group of measurements, can be calculated.
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ISO/TR 26946:2011(E)
8 Presentation of porosity
The porosity of a coating is an averaged value. To make sure that the averaged features of the entire coating
can be represented by several small domains, the report of porosity from the statistical view is necessary.
Many statistical methods for evaluation of experimental data exist, such as probabilistic estimations, mean
values, standard deviation (error). In practice a statistical treatment of experimental data is recommended for
the presentation of porosity. In the case of a thermally sprayed coating, the real porosity should be predicted
and presented from the finite measurement values as a mean value with standard deviation. The mean value
comes from the porosity of each image, while the standard devia
...