ASTM E1920-03(2008)
(Guide)Standard Guide for Metallographic Preparation of Thermal Sprayed Coatings
Standard Guide for Metallographic Preparation of Thermal Sprayed Coatings
SIGNIFICANCE AND USE
TSCs are used in a number of critical industrial components. TSCs can be expected to contain measurable levels of porosity and linear detachment. Accurate and consistent evaluation of specimens is essential to ensure the integrity of the coating and proper adherence to the substrate.
Example 1: By use of inappropriate metallographic methods, the apparent amount of porosity and linear detachment displayed by a given specimen can be increased, by excessive edge rounding, or decreased by smearing of material into voids. Therefore inaccurate levels of porosity and linear detachment will be reported even when the accuracy of the measurement technique is acceptable.
Example 2: Inconsistent metallographic preparation methods can cause the apparent amount of voids to vary excessively indicating a poorly controlled thermal spray process, while the use of consistent practice will regularly display the true microstructure and verify the consistency of the thermal spray process.
During the development of TSC procedures, metallographic information is necessary to validate the efficacy of a specific application.
Cross sections are usually taken perpendicular to the long axis of the specimen and prepared to reveal information concerning the following:
Variations in structure from surface to substrate,
The distribution of unmelted particles throughout the coating,
The distribution of linear detachment throughout the coating,
The distribution of porosity throughout the coating,
The presence of contamination within the coating,
The thickness of the coating (top coat and bond coat, where applicable),
The presence of interfacial contamination,
The integrity of the interface between the coating and substrate, and,
The integrity of the coating microstructure with respect to chemistry.
SCOPE
1.1 This guide covers recommendations for sectioning, cleaning, mounting, grinding, and polishing to reveal the microstructural features of thermal sprayed coatings (TSCs) and the substrates to which they are applied when examined microscopically. Because of the diversity of available equipment, the wide variety of coating and substrate combinations, and the sensitivity of these specimens to preparation technique, the existence of a series of recommended methods for metallographic preparation of thermal sprayed coating specimens is helpful. Adherence to this guide will provide practitioners with consistent and reproducible results. Additional information concerning standard practices for metallographic preparation can be found in Practice E 3.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E1920 − 03(Reapproved 2008)
Standard Guide for
Metallographic Preparation of Thermal Sprayed Coatings
This standard is issued under the fixed designation E1920; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2.3 taper mount, n—ametallographicspecimencreatedby
mounting a feature, typically an interface or thin coating, at a
1.1 This guide covers recommendations for sectioning,
small angle to the polishing plane, such that the visible width
cleaning, mounting, grinding, and polishing to reveal the
exhibited by the feature is expanded.
microstructural features of thermal sprayed coatings (TSCs)
3.2.4 TSC, n—thermal sprayed coating, including, but not
and the substrates to which they are applied when examined
limited to, those formed by plasma, flame, and high velocity
microscopically. Because of the diversity of available
oxyfuel.
equipment, the wide variety of coating and substrate
combinations, and the sensitivity of these specimens to prepa-
4. Significance and Use
ration technique, the existence of a series of recommended
methods for metallographic preparation of thermal sprayed
4.1 TSCs are used in a number of critical industrial compo-
coating specimens is helpful. Adherence to this guide will
nents. TSCs can be expected to contain measurable levels of
provide practitioners with consistent and reproducible results.
porosity and linear detachment.Accurate and consistent evalu-
Additional information concerning standard practices for met-
ation of specimens is essential to ensure the integrity of the
allographic preparation can be found in Practice E3.
coating and proper adherence to the substrate.
1.2 This standard does not purport to address all of the 4.1.1 Example 1: By use of inappropriate metallographic
safety concerns, if any, associated with its use. It is the methods, the apparent amount of porosity and linear detach-
responsibility of the user of this standard to establish appro- ment displayed by a given specimen can be increased, by
priate safety and health practices and determine the applica- excessive edge rounding, or decreased by smearing of material
bility of regulatory limitations prior to use. into voids. Therefore inaccurate levels of porosity and linear
detachment will be reported even when the accuracy of the
2. Referenced Documents
measurement technique is acceptable.
4.1.2 Example 2: Inconsistent metallographic preparation
2.1 ASTM Standards:
methods can cause the apparent amount of voids to vary
E3 Guide for Preparation of Metallographic Specimens
excessively indicating a poorly controlled thermal spray
E7 Terminology Relating to Metallography
process, while the use of consistent practice will regularly
3. Terminology
display the true microstructure and verify the consistency of
the thermal spray process.
3.1 Definitions—For definitions of terms used in this guide,
see Terminology E7.
4.2 During the development of TSC procedures, metallo-
graphic information is necessary to validate the efficacy of a
3.2 Definitions of Terms Specific to This Standard:
specific application.
3.2.1 linear detachment, n—a region within a TSC in which
two successively deposited splats of coating material have not
4.3 Cross sections are usually taken perpendicular to the
metallurgically bonded.
long axis of the specimen and prepared to reveal information
concerning the following:
3.2.2 splat, n—an individual globule of thermal sprayed
4.3.1 Variations in structure from surface to substrate,
material that has been deposited on a substrate.
4.3.2 The distribution of unmelted particles throughout the
coating,
ThisguideisunderthejurisdictionofASTMCommitteeE04onMetallography
4.3.3 The distribution of linear detachment throughout the
and is the direct responsibility of Subcommittee E04.01 on Specimen Preparation.
coating,
Current edition approved Oct. 1, 2008. Published January 2009. Originally
4.3.4 The distribution of porosity throughout the coating,
approved in 1997. Last previous edition approved in 2003 as E1920–03. DOI:
10.1520/E1920-03R08.
4.3.5 The presence of contamination within the coating,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
4.3.6 The thickness of the coating (top coat and bond coat,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
where applicable),
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. 4.3.7 The presence of interfacial contamination,
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1920 − 03 (2008)
4.3.8 The integrity of the interface between the coating and 6.2.6 Faster blade speeds, 1675 m/min. (5500 surface ft/
substrate, and, min.) or greater, produce less coating damage. Slower blade
speedswillresultinmoredamagetothecutsurfaceandarenot
4.3.9 The integrity of the coating microstructure with re-
recommended.
spect to chemistry.
6.2.7 Generally, an abrasive cutoff blade selected to cut the
substrate effectively will be the best blade for the combination
5. Selection of Metallographic Specimens
of TSC and substrate.
5.1 Selection of specimens for metallographic examination
is critical if their interpretation is to be of value. Specimens
7. Cleaning
must be representative of the coating. Generally, the plane of
7.1 Cleaning of specimens prior to mounting is essential.
polish should be normal to the coating surface so as to display
All sectioning coolant shall be removed from the surface and
the entire coating thickness, the substrate, and all interfaces.
from any porosity connected to the surface. Use of an organic
solvent to aid in fluid removal and thorough drying is neces-
6. Sectioning
sary. Drying in an oven at low temperature (60 to 80°C or 140
6.1 Specimens to be mounted for metallographic prepara-
to 176°F) can accelerate this process. Any liquid residual may
tion are generally not larger than 12 mm by 25 mm (0.5 by 1.0
impedeimpregnationofporosity,aswellasretardthecuringof
in.). The height of the mounted specimen should be no greater
mounting compounds causing difficulty during grinding and
than necessary for convenient handling during polishing.
polishing.
6.2 In sectioning TSC specimens, care must be exercised to 7.2 Ultrasonic cleaning of TSC specimens is generally not
recommended,especiallyforfragileorbrittlecoatings,because
avoid affecting the soundness of the coating and the interface
coating particles may be lost during this energetic cleaning
between the coating and the substrate. Sectioning damage of
process. If ultrasonic cleaning is found to be necessary,
thecoatingandinterfacethatcannotberemovedbysubsequent
cleaning time should be kept to a minimum.
grinding and polishing must be avoided.
6.2.1 Friable, porous, or brittle coatings to be sectioned may
8. Mounting
be vacuum impregnated with epoxy mounting compound
before sectioning to protect the specimen.
8.1 General Information:
8.1.1 It is always necessary to mount TSC specimens to
6.2.2 Specimens should always be sectioned such that the
coating is compressed into the substrate. Sectioning techniques maintain the original structure of the specimen during grinding
and polishing. Both compression mounting and castable
which place the coating and interface in tension are strictly to
be avoided. Sectioning in tension may cause the coating to be mountingcompoundsarecommonlyusedwhenmountingTSC
specimens. However, only castable epoxy mounting com-
pulled away from the substrate or result in delamination of the
pounds should be used in the initial determination of the true
coating. During examination of the polished specimens, it is
characteristics of a coating before considering the use of any
likely that this type of damage will be mistakenly interpreted.
other mounting compound. For some TSC specimens castable
When sectioning some specimens, it may not be possible to
epoxy may provide the only acceptable mount. Refer to Table
avoid placing some areas of the TSC in tension. These areas
1 and Table 2 and Practice E3 for characteristics of various
should be noted and not included in the evaluation of the
mounting compounds.
specimen.
8.1.2 Byplacingpairsofspecimensinthesamemount,time
6.2.3 Sectioning with a hack saw will produce significant
and expense can be saved. When using this mounting method,
damage to the coating and interface and is not considered
acceptable.
6.2.4 Using an abrasive cutoff blade with a large particle
size abrasive produces a smoother surface than a hack saw, but
TABLE 1 Characteristics of Compression Mounting Compounds
still produces coating damage that may require considerable
A
Type of Compound Characteristics
grinding in subsequent preparation to remove. The choice of
Acrylic Cure time 10 to 15 min., optically clear, thermoplastic,
cutoff wheel, coolant, cutting conditions, and the type and
good impregnation, low hardness, degraded by hot
hardness of the coating and substrate will influence the quality etchants
of the cut surface. A poor choice of cutting conditions can
Diallyl phthalate Cure time 5 to 10 min., opaque, thermosetting, minimal
easily overheat some TSC specimens rendering the specimens
shrinkage, good resistance to etchants, high hardness
unusable for proper evaluation.
Epoxy Cure time 5 to 10 min., opaque, thermosetting, minimal
6.2.5 Sectioning can be completed with minimal damage to
shrinkage, good resistance to etchants, high hardness,
the cut surface by selection of one of the two following good impregnation
abrasive cutoff blades:
Phenolic Cure time 5 to 10 min., opaque, thermosetting,
6.2.5.1 Use a diamond wafering blade with a maximum
shrinkage during cure can leave crevice at specimen
interface, degraded by hot etchants, moderate
thickness of 0.63 mm (0.025 in.).
hardness
6.2.5.2 Use an ultra–thin aluminum oxide abrasive blade
A
These compounds often contain filler materials, such as glass fibers or mineral
approximately 0.76 mm (0.030 in.) thick, which will break
particulate.
down during cutting to help reduce sectioning damage.
E1920 − 03 (2008)
TABLE 2 Characteristics of Castable Mounting Compounds
poured into the mold after the chamber has been evacuated.
Type of Compound Characteristics Vacuum pressure should be in the range of 630 to 760 mm (25
Acrylic Cure time 8 to 15 min, moderate shrinkage, low to 30 in.) of mercury. The vacuum should be maintained for
hardness, opaque
about two to ten min before allowing air into the chamber. Low
viscosity epoxy resins are recommended for best results during
Epoxy Cure time 1 to 12 h, negligible shrinkage, low level of
heat generated during polymerization, moderate
vacuum infiltration and subsequent preparation of TSC speci-
hardness, transparent, low viscosity formula is best for
mens.
vacuum impregnation
8.3.3 Addition of fluorescent dyes (1) or other agents, such
Polyester Cure time 30 to 60 min, high shrinkage, peak curing as Rhodamine B (2), that produce color response upon fluo-
temperature up to 120°C (248°F), moderate hardness,
rescent excitation or with crossed polarized illumination,
transparent
respectively, to the castable compound during mixing will aid
Polyester–Acrylic Cure time 8 to 15 min, negligible shrinkage, peak
in the identification of impregnated porosity during micro-
curing temperature up to 120°C (248°F), high
scopical examination.
hardness, opaque
8.3.4 Care should be used when handling castable mounting
compounds. Supplier’s instructions for the use and handling of
these materials should be followed.
the two coated surfaces should face each other. It may be
9. Grinding and Polishing
possible to place more than one pair of specimens in a single
9.1 Due to the many different types of TSCs and substrate
mount.
materials, grinding and polishing sequences will vary. Proper
8.1.3 Mounting expenses may be reduced by employing the
choice of polishing surface, lubricant, abrasive type and size,
sandwich mount technique. A sandwich mount is made by
time, force, polishing wheel speed and relative rotation will
using a small amount of a better, more expensive, mounting
produce accurate and consistent results. Some TSCs may
compoundasthecriticallayerincontactwiththespecimenand
require specialized preparation techniques. However, many
then topping off the mount with a less expensive compound.
coatings can be prepared on automatic and semi–automatic
Care must be taken to use only mounting materials that are
polishing equipment by use of one of the three procedures
compatible.
listed in Table 3, Table 4, and Table 5.
8.1.4 Taper mounting may be useful for examination of the
9.1.1 Useofautomaticandsemi–automaticpolishingequip-
interfaces between bond coating and top coating, as well as
ment has been found to reduce inconsistency in polished
between coating and substrate.
specimens and is therefore required in the metallographic
8.1.5 All components of mounting compounds including
preparation for TSC specimens. These machines provide re-
resins and catalysts, as well as any dyes or colorants, should be
producibility in wheel speed, specimen position on the wheel,
handled in accordance with the manufacturer’s instructions.
applied pressure, relative rotation, and polishing time. Manual
Material Safety Data Sheets are available from the manufac-
grinding and polishing is not recommended because of the
turer.
inherent inconsistency and the sensitivity of these specimens to
8.2 Compression Mounting Compounds:
preparation technique. (3,4,5,6)
8.2.1 Curing of compression mounting compounds is ac-
9.1.2 Most of the equipment for automatic and semi–auto-
complished by the use of a heated press designed for and
matic grinding and polishing move the specimen around a
dedicated to metallographic mounting. Compression mounting
rotating wheel covered with abrasive so that the specimen
compounds require the use of heat (140 to 150°C or 284 to
follows an epicycloidal path. The scratch pattern consists of
302°F) and pressure (up to 29 MPa or 4200 psi) to properly
random arcs, and upon inspection, should be uniform across
cure the mount. Therefore, mounting compounds of this type
the entire specimen surface before proceeding to the next step
should only be used to mount dense, non–friable coatings with
in the preparation sequence.
substrates a minimum of 1.5 mm (0.060 in.) thick.
9.1.3 Each of the methods outlined in Table 3, Table 4, and
8.2.2 Compression mounts should be cooled to below 40°C
Table 5 requires the follow
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:E1920–97 Designation:E1920–03 (Reapproved 2008)
Standard Guide for
Metallographic Preparation of Thermal Sprayed Coatings
This standard is issued under the fixed designation E 1920; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 Thisguidecoversrecommendationsforsectioning,cleaning,mounting,grinding,andpolishingtorevealthemicrostructural
features of thermal sprayed coatings (TSCs) and the substrates to which they are applied when examined microscopically. Because
of the diversity of available equipment, the wide variety of coating and substrate combinations, and the sensitivity of these
specimens to preparation technique, the existence of a series of recommended methods for metallographic preparation of thermal
sprayed coating specimens is helpful. Adherence to this guide will provide practitioners with consistent and reproducible results.
Additional information concerning standard practices for metallographic preparation can be found in Practice E 3.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E3 Practice of Preparation of Metallographic Specimens Guide for Preparation of Metallographic Specimens
E7 Terminology Relating to Metallography
3. Terminology
3.1 Definitions—For definitions of terms used in this guide, see Terminology E 7.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 linear detachment, n—a region within a TSC in which two successively deposited splats of coating material have not
metallurgically bonded.
3.2.2 splat, n—an individual globule of thermal sprayed material that has been deposited on a substrate.
3.2.3 taper mount, n—ametallographicspecimencreatedbymountingafeature,typicallyaninterfaceorthincoating,atasmall
angle to the polishing plane, such that the visible width exhibited by the feature is expanded.
3.2.4 TSC, n—thermal sprayed coating, including, but not limited to, those formed by plasma, flame, and high velocity oxyfuel.
4. Significance and Use
4.1 TSCs are used in a number of critical industrial components.TSCs can be expected to contain measurable levels of porosity
andlineardetachment.Accurateandconsistentevaluationofspecimensisessentialtoensuretheintegrityofthecoatingandproper
adherence to the substrate.
4.1.1 Example 1: By use of inappropriate metallographic methods, the apparent amount of porosity and linear detachment
displayed by a given specimen can be increased, by excessive edge rounding, or decreased by smearing of material into voids.
Therefore inaccurate levels of porosity and linear detachment will be reported even when the accuracy of the measurement
technique is acceptable.
4.1.2 Example 2: Inconsistent metallographic preparation methods can cause the apparent amount of voids to vary excessively
indicating a poorly controlled thermal spray process, while the use of consistent practice will regularly display the true
microstructure and verify the consistency of the thermal spray process.
4.2 During the development of TSC procedures, metallographic information is necessary to validate the efficacy of a specific
application.
4.3 Cross sections are usually taken perpendicular to the long axis of the specimen and prepared to reveal information
concerning the following:
This guide is under the jurisdiction of ASTM Committee E–4 on Metallography and is the direct responsibility of Subcommittee E04.01 on Sampling, Specimen
Preparation, and Photography.
Current edition approved Dec. 10, 1997. Published February 1998.
This guide is under the jurisdiction of ASTM Committee E04 on Metallography and is the direct responsibility of Subcommittee E04.01 on Specimen Preparation.
Current edition approved Oct. 1, 2008. Published January 2009. Originally approved in 1997. Last previous edition approved in 2003 as E 1920–03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1920–03 (2008)
4.3.1 Variations in structure from surface to substrate,
4.3.2 The distribution of unmelted particles throughout the coating,
4.3.3 The distribution of linear detachment throughout the coating,
4.3.4 The distribution of porosity throughout the coating,
4.3.5 The presence of contamination within the coating,
4.3.6 The thickness of the coating (top coat and bond coat, where applicable),
4.3.7 The presence of interfacial contamination,
4.3.8 The integrity of the interface between the coating and substrate, and,
4.3.9 The integrity of the coating microstructure with respect to chemistry.
5. Selection of Metallographic Specimens
5.1 Selection of specimens for metallographic examination is critical if their interpretation is to be of value. Specimens must
be representative of the coating. Generally, the plane of polish should be normal to the coating surface so as to display the entire
coating thickness, the substrate, and all interfaces.
6. Sectioning
6.1 Specimens to be mounted for metallographic preparation are generally not larger than 12 mm by 25 mm (0.5 by 1.0 in.).
The height of the mounted specimen should be no greater than necessary for convenient handling during polishing.
6.2 In sectioning TSC specimens, care must be exercised to avoid affecting the soundness of the coating and the interface
between the coating and the substrate. Sectioning damage of the coating and interface that cannot be removed by subsequent
grinding and polishing must be avoided.
6.2.1 Friable, porous, or brittle coatings to be sectioned may be vacuum impregnated with epoxy mounting compound before
sectioning to protect the specimen.
6.2.2 Specimens should always be sectioned such that the coating is compressed into the substrate. Sectioning techniques which
place the coating and interface in tension are strictly to be avoided. Sectioning in tension may cause the coating to be pulled away
from the substrate or result in delamination of the coating. During examination of the polished specimens, it is likely that this type
of damage will be mistakenly interpreted. When sectioning some specimens, it may not be possible to avoid placing some areas
of the TSC in tension. These areas should be noted and not included in the evaluation of the specimen.
6.2.3 Sectioning with a hack saw will produce significant damage to the coating and interface and is not considered acceptable.
6.2.4 Using an abrasive cutoff blade with a large particle size abrasive produces a smoother surface than a hack saw, but still
produces coating damage that may require considerable grinding in subsequent preparation to remove. The choice of cutoff wheel,
coolant, cutting conditions, and the type and hardness of the coating and substrate will influence the quality of the cut surface. A
poorchoiceofcuttingconditionscaneasilyoverheatsomeTSCspecimensrenderingthespecimensunusableforproperevaluation.
6.2.5 Sectioning can be completed with minimal damage to the cut surface by selection of one of the two following abrasive
cutoff blades:
6.2.5.1 Use a diamond wafering blade with a maximum thickness of 0.63 mm (0.025 in.).
6.2.5.2 Use an ultra–thin aluminum oxide abrasive blade approximately 0.76 mm (0.030 in.) thick, which will break down
during cutting to help reduce sectioning damage.
6.2.6 Faster blade speeds, 1675 m/min. (5500 surface ft/min.) or greater, produce less coating damage. Slower blade speeds will
result in more damage to the cut surface and are not recommended.
6.2.7 Generally, an abrasive cutoff blade selected to cut the substrate effectively will be the best blade for the combination of
TSC and substrate.
7. Cleaning
7.1 Cleaning of specimens prior to mounting is essential. All sectioning coolant shall be removed from the surface and from
any porosity connected to the surface. Use of an organic solvent to aid in fluid removal and thorough drying is necessary. Drying
in an oven at low temperature (60 to 80°C or 140 to 176°F) can accelerate this process. Any liquid residual may impede
impregnation of porosity, as well as retard the curing of mounting compounds causing difficulty during grinding and polishing.
7.2 Ultrasonic cleaning of TSC specimens is generally not recommended, especially for fragile or brittle coatings, because
coating particles may be lost during this energetic cleaning process. If ultrasonic cleaning is found to be necessary, cleaning time
should be kept to a minimum.
8. Mounting
8.1 General Information:
8.1.1 It is always necessary to mount TSC specimens to maintain the original structure of the specimen during grinding and
polishing. Both compression mounting and castable mounting compounds are commonly used when mounting TSC specimens.
However, only castable epoxy mounting compounds should be used in the initial determination of the true characteristics of a
coating before considering the use of any other mounting compound. For some TSC specimens castable epoxy may provide the
only acceptable mount. Refer to Table 1 and Table 2 and Practice E 3 for characteristics of various mounting compounds.
E1920–03 (2008)
TABLE 1 Characteristics of Compression Mounting Compounds
A
Type of Compound Characteristics
Acrylic Cure time 10 to 15 min., optically clear, thermoplastic,
good impregnation, low hardness, degraded by hot
etchants
Diallyl phthalate Cure time 5 to 10 min., opaque, thermosetting, minimal
shrinkage, good resistance to etchants, high hardness
Epoxy Cure time 5 to 10 min., opaque, thermosetting, minimal
shrinkage, good resistance to etchants, high hardness,
good impregnation
Phenolic Cure time 5 to 10 min., opaque, thermosetting,
shrinkage during cure can leave crevice at specimen
interface, degraded by hot etchants, moderate
hardness
A
These compounds often contain filler materials, such as glass fibers or mineral
particulate.
TABLE 2 Characteristics of Castable Mounting Compounds
Type of Compound Characteristics
Acrylic Cure time 8 to 15 min, moderate shrinkage, low
hardness, opaque
Epoxy Cure time 1 to 12 h, negligible shrinkage, low level of
heat generated during polymerization, moderate
hardness, transparent, low viscosity formula is best for
vacuum impregnation
Polyester Cure time 30 to 60 min, high shrinkage, peak curing
temperature up to 120°C (248°F), moderate hardness,
transparent
Polyester–Acrylic Cure time 8 to 15 min, negligible shrinkage, peak
curing temperature up to 120°C (248°F), high
hardness, opaque
8.1.2 By placing pairs of specimens in the same mount, time and expense can be saved. When using this mounting method, the
two coated surfaces should face each other. It may be possible to place more than one pair of specimens in a single mount.
8.1.3 Mounting expenses may be reduced by employing the sandwich mount technique. A sandwich mount is made by using
a small amount of a better, more expensive, mounting compound as the critical layer in contact with the specimen and then topping
off the mount with a less expensive compound. Care must be taken to use only mounting materials that are compatible.
8.1.4 Taper mounting may be useful for examination of the interfaces between bond coating and top coating, as well as between
coating and substrate.
8.1.5 All components of mounting compounds including resins and catalysts, as well as any dyes or colorants, should be
handled in accordance with the manufacturer’s instructions. Material Safety Data Sheets are available from the manufacturer.
8.2 Compression Mounting Compounds :
8.2.1 Curing of compression mounting compounds is accomplished by the use of a heated press designed for and dedicated to
metallographic mounting. Compression mounting compounds require the use of heat (140 to 150°C or 284 to 302°F) and pressure
(up to 29 MPa or 4200 psi) to properly cure the mount.Therefore, mounting compounds of this type should only be used to mount
dense, non–friable coatings with substrates a minimum of 1.5 mm (0.060 in.) thick.
8.2.2 Compression mounts should be cooled to below 40°C (104°F) while under pressure, preferably by use of a water cooled
mounting press, to prevent formation of a crevice between the mounting compound and the specimen as they contract unevenly
during cooling.
8.3 Castable Mounting Compounds:
8.3.1 CastablemountingcompoundsmaybeusedforallTSCspecimens,butareparticularlyusefulforporousspecimens.These
mounting compounds are usually prepared by mixing two components just prior to use. The specimen is placed in a mold, usually
a cup or ringform, into which the compound is poured.
8.3.2 Vacuum impregnation of porous specimens with an epoxy resin is required.The specimen and mold are put into a vacuum
chamber which will allow the castable compoundepoxy to be poured into the mold after the chamber has been evacuated. Vacuum
pressure should be in the range of 630 to 760 mm (25 to 30 in.) of mercury. The vacuum should be maintained for about two to
ten min before allowing air into the chamber. Low viscosity epoxy resins are recommended for best results during vacuum
infiltration and subsequent preparation of TSC specimens.
8.3.3 Addition of fluorescent dyes (1) or other agents, such as Rhodamine B (2), that produce color response upon fluorescent
E1920–03 (2008)
excitationorwithcrossedpolarizedillumination,respectively,tothecastablecompoundduringmixingwillaidintheidentification
of impregnated porosity during microscopical examination.
8.3.4 Care should be used when handling castable mounting compounds. Supplier’s instructions for the use and handling of
these materials should be followed.
9. Grinding and Polishing
9.1 Due to the many different types of TSCs and substrate materials, grinding and polishing sequences will vary. Proper choice
ofpolishingsurface,lubricant,abrasivetypeandsize,time,force,polishingwheelspeedandrelativerotationwillproduceaccurate
and consistent results. Some TSCs may require specialized preparation techniques. However, many coatings can be prepared on
automatic and semi–automatic polishing equipment by
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:E1920–03 Designation:E1920–03 (Reapproved 2008)
Standard Guide for
Metallographic Preparation of Thermal Sprayed Coatings
This standard is issued under the fixed designation E 1920; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 Thisguidecoversrecommendationsforsectioning,cleaning,mounting,grinding,andpolishingtorevealthemicrostructural
features of thermal sprayed coatings (TSCs) and the substrates to which they are applied when examined microscopically. Because
of the diversity of available equipment, the wide variety of coating and substrate combinations, and the sensitivity of these
specimens to preparation technique, the existence of a series of recommended methods for metallographic preparation of thermal
sprayed coating specimens is helpful. Adherence to this guide will provide practitioners with consistent and reproducible results.
Additional information concerning standard practices for metallographic preparation can be found in Practice E 3.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E3 Guide for Preparation of Metallographic Specimens
E7 Terminology Relating to Metallography
3. Terminology
3.1 Definitions—For definitions of terms used in this guide, see Terminology E 7.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 linear detachment, n—a region within a TSC in which two successively deposited splats of coating material have not
metallurgically bonded.
3.2.2 splat, n—an individual globule of thermal sprayed material that has been deposited on a substrate.
3.2.3 taper mount, n—ametallographicspecimencreatedbymountingafeature,typicallyaninterfaceorthincoating,atasmall
angle to the polishing plane, such that the visible width exhibited by the feature is expanded.
3.2.4 TSC, n—thermal sprayed coating, including, but not limited to, those formed by plasma, flame, and high velocity oxyfuel.
4. Significance and Use
4.1 TSCs are used in a number of critical industrial components.TSCs can be expected to contain measurable levels of porosity
andlineardetachment.Accurateandconsistentevaluationofspecimensisessentialtoensuretheintegrityofthecoatingandproper
adherence to the substrate.
4.1.1 Example 1: By use of inappropriate metallographic methods, the apparent amount of porosity and linear detachment
displayed by a given specimen can be increased, by excessive edge rounding, or decreased by smearing of material into voids.
Therefore inaccurate levels of porosity and linear detachment will be reported even when the accuracy of the measurement
technique is acceptable.
4.1.2 Example 2: Inconsistent metallographic preparation methods can cause the apparent amount of voids to vary excessively
indicating a poorly controlled thermal spray process, while the use of consistent practice will regularly display the true
microstructure and verify the consistency of the thermal spray process.
4.2 During the development of TSC procedures, metallographic information is necessary to validate the efficacy of a specific
application.
4.3 Cross sections are usually taken perpendicular to the long axis of the specimen and prepared to reveal information
concerning the following:
This guide is under the jurisdiction of ASTM Committee E04 on Metallography and is the direct responsibility of Subcommittee E04.01 on Sampling, Specimen
Preparation, and Photography. Preparation.
Current edition approved May 10, 2003.Oct. 1, 2008. Published July 2003.January 2009. Originally approved in 1997. Last previous edition approved in 19972003 as E
1920–97. E 1920–03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1920–03 (2008)
4.3.1 Variations in structure from surface to substrate,
4.3.2 The distribution of unmelted particles throughout the coating,
4.3.3 The distribution of linear detachment throughout the coating,
4.3.4 The distribution of porosity throughout the coating,
4.3.5 The presence of contamination within the coating,
4.3.6 The thickness of the coating (top coat and bond coat, where applicable),
4.3.7 The presence of interfacial contamination,
4.3.8 The integrity of the interface between the coating and substrate, and,
4.3.9 The integrity of the coating microstructure with respect to chemistry.
5. Selection of Metallographic Specimens
5.1 Selection of specimens for metallographic examination is critical if their interpretation is to be of value. Specimens must
be representative of the coating. Generally, the plane of polish should be normal to the coating surface so as to display the entire
coating thickness, the substrate, and all interfaces.
6. Sectioning
6.1 Specimens to be mounted for metallographic preparation are generally not larger than 12 mm by 25 mm (0.5 by 1.0 in.).
The height of the mounted specimen should be no greater than necessary for convenient handling during polishing.
6.2 In sectioning TSC specimens, care must be exercised to avoid affecting the soundness of the coating and the interface
between the coating and the substrate. Sectioning damage of the coating and interface that cannot be removed by subsequent
grinding and polishing must be avoided.
6.2.1 Friable, porous, or brittle coatings to be sectioned may be vacuum impregnated with epoxy mounting compound before
sectioning to protect the specimen.
6.2.2 Specimens should always be sectioned such that the coating is compressed into the substrate. Sectioning techniques which
place the coating and interface in tension are strictly to be avoided. Sectioning in tension may cause the coating to be pulled away
from the substrate or result in delamination of the coating. During examination of the polished specimens, it is likely that this type
of damage will be mistakenly interpreted. When sectioning some specimens, it may not be possible to avoid placing some areas
of the TSC in tension. These areas should be noted and not included in the evaluation of the specimen.
6.2.3 Sectioning with a hack saw will produce significant damage to the coating and interface and is not considered acceptable.
6.2.4 Using an abrasive cutoff blade with a large particle size abrasive produces a smoother surface than a hack saw, but still
produces coating damage that may require considerable grinding in subsequent preparation to remove. The choice of cutoff wheel,
coolant, cutting conditions, and the type and hardness of the coating and substrate will influence the quality of the cut surface. A
poorchoiceofcuttingconditionscaneasilyoverheatsomeTSCspecimensrenderingthespecimensunusableforproperevaluation.
6.2.5 Sectioning can be completed with minimal damage to the cut surface by selection of one of the two following abrasive
cutoff blades:
6.2.5.1 Use a diamond wafering blade with a maximum thickness of 0.63 mm (0.025 in.).
6.2.5.2 Use an ultra–thin aluminum oxide abrasive blade approximately 0.76 mm (0.030 in.) thick, which will break down
during cutting to help reduce sectioning damage.
6.2.6 Faster blade speeds, 1675 m/min. (5500 surface ft/min.) or greater, produce less coating damage. Slower blade speeds will
result in more damage to the cut surface and are not recommended.
6.2.7 Generally, an abrasive cutoff blade selected to cut the substrate effectively will be the best blade for the combination of
TSC and substrate.
7. Cleaning
7.1 Cleaning of specimens prior to mounting is essential. All sectioning coolant shall be removed from the surface and from
any porosity connected to the surface. Use of an organic solvent to aid in fluid removal and thorough drying is necessary. Drying
in an oven at low temperature (60 to 80°C or 140 to 176°F) can accelerate this process. Any liquid residual may impede
impregnation of porosity, as well as retard the curing of mounting compounds causing difficulty during grinding and polishing.
7.2 Ultrasonic cleaning of TSC specimens is generally not recommended, especially for fragile or brittle coatings, because
coating particles may be lost during this energetic cleaning process. If ultrasonic cleaning is found to be necessary, cleaning time
should be kept to a minimum.
8. Mounting
8.1 General Information:
8.1.1 It is always necessary to mount TSC specimens to maintain the original structure of the specimen during grinding and
polishing. Both compression mounting and castable mounting compounds are commonly used when mounting TSC specimens.
However, only castable epoxy mounting compounds should be used in the initial determination of the true characteristics of a
coating before considering the use of any other mounting compound. For some TSC specimens castable epoxy may provide the
only acceptable mount. Refer to Table 1 and Table 2 and Practice E 3 for characteristics of various mounting compounds.
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TABLE 1 Characteristics of Compression Mounting Compounds
A
Type of Compound Characteristics
Acrylic Cure time 10 to 15 min., optically clear, thermoplastic,
good impregnation, low hardness, degraded by hot
etchants
Diallyl phthalate Cure time 5 to 10 min., opaque, thermosetting, minimal
shrinkage, good resistance to etchants, high hardness
Epoxy Cure time 5 to 10 min., opaque, thermosetting, minimal
shrinkage, good resistance to etchants, high hardness,
good impregnation
Phenolic Cure time 5 to 10 min., opaque, thermosetting,
shrinkage during cure can leave crevice at specimen
interface, degraded by hot etchants, moderate
hardness
A
These compounds often contain filler materials, such as glass fibers or mineral
particulate.
TABLE 2 Characteristics of Castable Mounting Compounds
Type of Compound Characteristics
Acrylic Cure time 8 to 15 min, moderate shrinkage, low
hardness, opaque
Epoxy Cure time 1 to 12 h, negligible shrinkage, low level of
heat generated during polymerization, moderate
hardness, transparent, low viscosity formula is best for
vacuum impregnation
Polyester Cure time 30 to 60 min, high shrinkage, peak curing
temperature up to 120°C (248°F), moderate hardness,
transparent
Polyester–Acrylic Cure time 8 to 15 min, negligible shrinkage, peak
curing temperature up to 120°C (248°F), high
hardness, opaque
8.1.2 By placing pairs of specimens in the same mount, time and expense can be saved. When using this mounting method, the
two coated surfaces should face each other. It may be possible to place more than one pair of specimens in a single mount.
8.1.3 Mounting expenses may be reduced by employing the sandwich mount technique. A sandwich mount is made by using
a small amount of a better, more expensive, mounting compound as the critical layer in contact with the specimen and then topping
off the mount with a less expensive compound. Care must be taken to use only mounting materials that are compatible.
8.1.4 Taper mounting may be useful for examination of the interfaces between bond coating and top coating, as well as between
coating and substrate.
8.1.5 All components of mounting compounds including resins and catalysts, as well as any dyes or colorants, should be
handled in accordance with the manufacturer’s instructions. Material Safety Data Sheets are available from the manufacturer.
8.2 Compression Mounting Compounds :
8.2.1 Curing of compression mounting compounds is accomplished by the use of a heated press designed for and dedicated to
metallographic mounting. Compression mounting compounds require the use of heat (140 to 150°C or 284 to 302°F) and pressure
(up to 29 MPa or 4200 psi) to properly cure the mount.Therefore, mounting compounds of this type should only be used to mount
dense, non–friable coatings with substrates a minimum of 1.5 mm (0.060 in.) thick.
8.2.2 Compression mounts should be cooled to below 40°C (104°F) while under pressure, preferably by use of a water cooled
mounting press, to prevent formation of a crevice between the mounting compound and the specimen as they contract unevenly
during cooling.
8.3 Castable Mounting Compounds:
8.3.1 CastablemountingcompoundsmaybeusedforallTSCspecimens,butareparticularlyusefulforporousspecimens.These
mounting compounds are usually prepared by mixing two components just prior to use. The specimen is placed in a mold, usually
a cup or ringform, into which the compound is poured.
8.3.2 Vacuum impregnation of porous specimens with an epoxy resin is required.The specimen and mold are put into a vacuum
chamber which will allow the castable epoxy to be poured into the mold after the chamber has been evacuated. Vacuum pressure
should be in the range of 630 to 760 mm (25 to 30 in.) of mercury. The vacuum should be maintained for about two to ten min
before allowing air into the chamber. Low viscosity epoxy resins are recommended for best results during vacuum infiltration and
subsequent preparation of TSC specimens.
8.3.3 Addition of fluorescent dyes (1) or other agents, such as Rhodamine B (2), that produce color response upon fluorescent
E1920–03 (2008)
excitationorwithcrossedpolarizedillumination,respectively,tothecastablecompoundduringmixingwillaidintheidentification
of impregnated porosity during microscopical examination.
8.3.4 Care should be used when handling castable mounting compounds. Supplier’s instructions for the use and handling of
these materials should be followed.
9. Grinding and Polishing
9.1 Due to the many different types of TSCs and substrate materials, grinding and polishing sequences will vary. Proper choice
ofpolishingsurface,lubricant,abrasivetypeandsize,time,force,polishingwheelspeedandrelativerotationwillproduceaccurate
and consistent results. Some TSCs may require specialized preparation techniques. However, many coatings can be prepared on
automatic and semi–automatic polishing equipment by use of one of the three procedures listed in Table 3, Table 4, and Table 5.
9.1.1 Use of automatic and semi–automatic polishing equipment has been found to reduce inconsistency in polished specimens
and is therefore required in the
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