ASTM D8075-16(2021)

Standard Guide for Categorization of Microstructural and Microtextural Features Observed in Optical Micrographs of Graphite

SIGNIFICANCE AND USE
4.1 The purpose of this guide is to provide a framework for consistent description of microstructural and microtextural features visible in optical micrographs of graphite. It also provides some guidance on sample preparation and image processing.
SCOPE
1.1 This guide covers the identification and the assignment of microstructural and microtextural features observed in optical micrographs of graphite. The objective of this guide is to establish a consistent approach to the categorization of such features to aid unambiguous discussion of optical micrographs in the scientific literature. It also provides guidance on specimen preparation and the compilation of micrographs.  
1.2 The values stated in SI units are to be regarded as the standard.  
1.3 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Mar-2021
• ICS: 77.040.01 - Testing of metals in general
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.F0 - Manufactured Carbon and Graphite Products

Relations

• Refers: ASTM D7219-19 - Standard Specification for Isotropic and Near-isotropic Nuclear Graphites
• Effective Date: 01-Nov-2019
• Refers: ASTM D7219-08(2014) - Standard Specification for Isotropic and Near-isotropic Nuclear Graphites
• Effective Date: 01-May-2014
• Refers: ASTM D7219-08 - Standard Specification for Isotropic and Near-isotropic Nuclear Graphites
• Effective Date: 01-May-2008
• Refers: ASTM D7219-05 - Standard Specification for Isotropic and Near-isotropic Nuclear Graphites
• Effective Date: 01-Nov-2005

Overview

ASTM D8075-16(2021): Standard Guide for Categorization of Microstructural and Microtextural Features Observed in Optical Micrographs of Graphite provides a unified framework for systematically describing and categorizing microstructural and microtextural features visible in optical micrographs of graphite. This standard supports the scientific and industrial community by promoting consistent terminology, improving specimen preparation techniques, and guiding the processing and analysis of graphite micrographs.

Through precise definitions and practical guidance, ASTM D8075 helps ensure clarity in the discussion, documentation, and comparison of optical micrographs in research, quality control, and manufacturing processes.

Key Topics

• Consistent Categorization: Details a standard approach for identifying and assigning microstructural (e.g., grain, porosity, accommodation cracks, filler, binder) and microtextural (e.g., domains, mosaics, supra mosaics, highly oriented particles) features in optical micrographs of graphite.
• Sample Preparation Guidance: Recommends specimen encapsulation using low-viscosity resins with optional fluorescent dye for clear visualization of pores and microfeatures.
• Image Calibration: Explains spatial calibration techniques for accurate measurement, such as use of graduated reticles or known spatial references within the microscopy field.
• Optical Microscopy Methods: Describes the use of bright field, fluorescence, and polarized light for distinguishing various graphite features. It highlights how changes in illumination techniques reveal different microstructural and microtextural details.
• Terminology and Hierarchy: Establishes precise vocabulary for features at macro, micro, and nano scales in graphite, facilitating unambiguous technical communication.

Applications

Standardized categorization of graphite micrographs, as outlined in ASTM D8075, is valuable in a variety of practical settings:

• Materials Research: Enables reproducible comparison of graphite’s microstructure and texture in scientific studies, especially related to graphite processing, aging, and irradiation behavior.
• Quality Assurance: Assists manufacturers of nuclear, electrochemical, or structural graphite in evaluating batch consistency and detecting production anomalies.
• Failure Analysis: Provides a structured approach for examining fractured, aged, or irradiated graphite components, supporting root-cause investigations.
• Graphite Manufacturing: Supports process optimization and product development by establishing objective criteria for evaluating graphite microstructures and the effects of raw material, binder, or process changes.
• Documentation and Reporting: Helps create clear, repeatable micrograph documentation for regulatory, contractual, or R&D purposes.

Related Standards

The following standards are commonly referenced in conjunction with ASTM D8075 to provide comprehensive coverage on graphite characterization:

• ASTM D7219: Specification for Isotropic and Near-isotropic Nuclear Graphites
• ASTM D121: Terminology of Coal and Coke
• IEEE/ASTM SI 10: Standard for Use of the International System of Units (SI)

Practical Value

By providing a common language and systematic methodology for characterizing microstructural and microtextural features in graphite using optical microscopy, ASTM D8075 plays a crucial role in:

• Ensuring repeatability and comparability of micrograph analysis worldwide
• Enhancing cross-industry and cross-research collaboration
• Facilitating improvements in graphite product quality, durability, and performance

Keywords: graphite microstructure, optical micrograph, microtexture, polarized light microscopy, sample preparation, ASTM D8075, consistency, materials analysis, quality control, graphite features

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D8075 − 16 (Reapproved 2021)
Standard Guide for
Categorization of Microstructural and Microtextural Features
Observed in Optical Micrographs of Graphite
This standard is issued under the fixed designation D8075; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope exactly match those adopted in general scientific usage but
should not be at variance. General terms have not been
1.1 This guide covers the identification and the assignment
redefined with graphite-specific meanings or optical
of microstructural and microtextural features observed in
microscopy-specific meanings. As with the identification of
optical micrographs of graphite. The objective of this guide is
featuresinmicrographs,somedefinitionshavebecomeunclear
to establish a consistent approach to the categorization of such
to differences in usage and this guide provides the basis for a
features to aid unambiguous discussion of optical micrographs
more consistent approach.
in the scientific literature. It also provides guidance on speci-
men preparation and the compilation of micrographs. 3.2 Definitions:
3.2.1 accommodation cracks, n—(also referred to as
1.2 The values stated in SI units are to be regarded as the
Mrozowski-like cracks) cracks and voids formed between
standard.
basal planes and at domain interfaces throughout the graphite
1.3 This standard does not purport to address all of the
microstructure from thermal contraction of the graphite during
safety concerns, if any, associated with its use. It is the
carbonization/graphitization (sometimes referred to as calcina-
responsibility of the user of this standard to establish appro-
tioncracks),fromchemicaldecompositionoftheliquidcrystal
priate safety, health, and environmental practices and deter-
hydrocarbon precursor in graphite manufacture (also referred
mine the applicability of regulatory limitations prior to use.
to as calcination cracks) and following cooling after graphiti-
1.4 This international standard was developed in accor-
zation(manufacture).Inirradiatedgraphite,theyalsocomprise
dance with internationally recognized principles on standard-
cracks arising from anisotropic responses to irradiation.
ization established in the Decision on Principles for the
3.2.2 agglomerate, n—in manufactured carbon and graph-
Development of International Standards, Guides and Recom-
ite product technology,compositeparticlecontaininganumber
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee. of grains.
3.2.3 binder, n—substance such as coal tar pitch or petro-
2. Referenced Documents
leum pitch, used to bond the coke or other filler material prior
2.1 ASTM Standards:
to baking.
D7219 Specification for Isotropic and Near-isotropic
3.2.4 crystallite, n—in manufactured carbon and graphite
Nuclear Graphites
product technology, a region of regular crystalline structure
having parallel basal planes.
3. Terminology
3.2.5 filler, n—in manufactured carbon and graphite prod-
3.1 The definitions listed below cover terms used in this
uct technology,particlesthatcomprisethebaseaggregateinan
guide and apply specifically to the optical microscopy of
unbaked green-mix formulation (also referred to as coke
graphite. Properties and features not apparent under the optical
particles, grist particles, or filler grains).
microscope are avoided where possible. Definitions may not
3.2.6 filler-binder phase, n—in manufactured carbon and
graphite product technology, mix of finely ground filler (flour)
This guide is under the jurisdiction of ASTM Committee D02 on Petroleum
and binder comprising the matrix in which the filler is bound.
Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcom-
mittee D02.F0 on Manufactured Carbon and Graphite Products.
3.2.7 grain, n—in manufactured carbon and graphite, par-
Current edition approved April 1, 2021. Published May 2021. Originally
ticle of filler material (usually coke or graphite) in the starting
approved in 2016. Last previous edition approved in 2016 as D8075–16. DOI:
10.1520/D8075-16R21.
mix formulation. Also referred to as granular material, filler
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
particle, or aggregate material. The term is also used to
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
describe the general texture of a carbon or graphite body, as in
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. the descriptions listed below:
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D8075 − 16 (2021)
3.2.7.1 coarse grained, adj—containing grains in the start- 3.2.16 porosity, n—fractionofthetotalvolumeofamaterial
ing mix that are substantially greater than 4 mm in size. occupied by both open and closed pores and cracks.
3.2.17 void, n—unfilledspaceenclosedwithinanapparently
3.2.7.2 medium coarse grained, adj—containing grains in
solid carbon or graphite body.
the starting mix that are generally less than 4 mm in size.
3.2.7.3 medium grained, adj—containing grains in the start-
4. Significance and Use
ing mix that are generally less than 2 mm in size.
4.1 The purpose of this guide is to provide a framework for
3.2.7.4 medium fine grained, adj—containing grains in the
consistent description of microstructural and microtextural
starting mix that are generally less than 1 mm in size.
features visible in optical micrographs of graphite. It also
provides some guidance on sample preparation and image
3.2.7.5 fine grained, adj—containing grains in the starting
processing.
mix that are less than 100 µm in size.
5. Optical Microscopy Methods
3.2.7.6 superfine grained, adj—containing grains in the
starting mix that are less than 50 µm in size.
5.1 Three different methods of illumination are generally
employed in optical microscopy: optical or bright field (BF),
3.2.7.7 ultrafine grained, adj—containinggrainsinthestart-
ing mix that are less than 10 µm in size. fluorescence under UV light, and polarized light. While bright
fieldandpolarizedlightmethodscanbeundertakendirectlyon
3.2.7.8 microfine grained, adj—containing grains in the
apreparedgraphitesurface,fluorescencerequiresthesampleto
starting mix that are less than 2 µm in size.
be impregnated with a resin incorporating a fluorescent dye
3.2.7.9 Discussion—All of the above descriptions relate to priortopreparationofthegraphitesurface.Itiscommonforall
the generally accepted practice of measuring the sizing frac- threemethodsofilluminationtobeusedinthecharacterization
tions with a criterion that 90% of the grains will pass through of graphite microstructure and texture so that resin impregna-
the stated sieve screen size in a standard particle sizing test. tion is a standard procedure in sample preparation. It should
also be noted that resin impregnation stabilizes the graphite
3.2.8 highly oriented region, n—an area of uniform color
matrix and protects porosity from dust intrusion during polish-
under polarized light associated with a relatively crystalline
ing of the surface being prepared for examination.
unidirectional (at the observed magnification) orientation.
5.2 If the sample requires impregnation, a low-viscosity
3.2.9 isotropic nuclear graphite, n—graphite in which the
resin is used to impregnate and encapsulate the sample. The
isotropy ratio based on the coefficient of thermal expansion
resin can have a small amount of fluorescent dye added for
(25°C to 500°C) is 1.00 to 1.10.
observation under ultraviolet (UV) light. Once impregnated
3.2.10 mesophase, n—fluid phase (discotic nematic liquid
with resin and cured, the encapsulated sample is ready for
crystal phase) converted to graphite during pyrolysis.
preparation of an examination face.
3.2.11 mosaics, n—term used to describe texture consisting
5.3 The selected face of the sample is prepared for micro-
of a grouping of isochromatic domains, often subdivided by
scopic examination by grinding it using progressively finer
grain size.The following terms may be encountered relating to
silicon carbide (SiC) papers to 2500 grit (8.4µm 6 0.5µm).
these microtextural features:
The face is then further polished with a diamond suspension to
3.2.11.1 mosaic cluster, n—an identifiable grouping of
a 1µm finish. The same procedure is employed for both
similar-sized mosaic texture.
untreated and impregnated graphite samples.At this stage, the
prepared face of the sample is ready for optical examination.
3.2.11.2 mosaic ribbon, n—an identifiable ribbon-shaped or
strand grouping of mosaic texture.
5.4 With BF illumination, the sample is observed using
white light at normal incidence. Within the constraints of the
3.2.11.3 supra mosaic, n—aligned region of coarse mosaics
optical resolution, this method of illumination allows micro-
exhibiting a largely acicular shape.
structural features in the sample to be seen.
3.2.12 Mrozowski cracks, n—a subset of accommodation
5.5 With fluorescence microscopy, incident UVlight causes
cracks formed between basal planes within coke particle
the dye in the resin to fluoresce, thus showing the extent of
crystallites and the filler-binder phase from mismatches in
resin penetration into the sample and an indication of areas of
thermal contraction of the graphite following cooling after
open porosity. This method requires full impregnation of the
graphitization (manufacture). These may also occur between
accessibleporositybytheresin,whichcanbeinfluencedbythe
crystallites if crystallite binding energies allow.
viscosity of the resin and extent of evacuation. The method is
3.2.13 optical domain, n—the smallest region of local pre-
less revealing in terms of characterizing microstructure in
ferred orientation with relatively small misorientation angles
fine-grained material because of incomplete penetration of the
appearing isochromatic under polarized light with a sensitive
porosity by the resin.
tint plate.
5.6 Illumination with polarized light merits a more detailed
3.2.14 optical texture, n—fine structure in an optic array
explanation. The random variations in a light beam are in
giving rise to color variations under polarized light, attributed
directions normal to the direction of propagating light. If the
to variations in the optic axis of domains.
light beam is passed through an optically active crystalline
3.2.15 pore, n—see void. material (a plane polarizer), some directions of vibrations will
D8075 − 16 (2021)
be suppressed and others rotated. The net result is that specific Determination of optical texture can be influenced by the
directions of vibrations are favored on passing through the magnification employed, and the user should be aware that
polarizer. If the transmitted plane-polarized light is examined magnification requirements may differ depending upon the
after passing through a second optically active material, and nature of the graphite under investigation.
this second optically active material is at right angles to the
5.8 To correctly determine the size of optical objects or to
polarizer, then the light will be cut off completely. When the
measure distances on optical microscope images, or both, a
two optically active materials are in this position they are said
spatial calibration must be performed. There are two basic
to be crossed. The second optically active material is termed
ways to perform spatial calibrations: either by using known
the analyzer. Polarization will occur on reflection from most
spatialreferencesintheimage,orthroughestimationsbasedon
crystalline materials, even when they are isotropic. Examina-
camera and lens optical characteristics.
tion with crossed polarizers allows the polarization caused by
5.8.1 Calibrations based on known spatial references are
interactionwiththespecimensurfacetobestudied.Thedegree
more accurate, and should be used whenever trustful spatial
of polarization will depend on the angle between the incident
references are available and can be imaged in identical optical
light and specific crystal planes in the material. Also, qualita-
conditions as the area of interest on the graphite specimen.
tive analysis of the specimen’s surface relative crystallography
When possible, a commercially available graduated reticle
and degree of crystallinity can be made.
should be used as spatial reference. Using the tools available
5.6.1 If a sensitive tint plate is placed between the polarizer
with modern microscopy image acquisition software, calibra-
and analyzer, orientations of isotropic materials can be distin-
tion should be done by repeatedly measuring linear segments
guished. A 1 λ plate is most commonly used but ⁄2 λ plates
drawn between known reference points on the reticle and
may also be employed.Asensitive tint plate consists of a slice
saving the results along with the spatial distance values in
of some birefringent (birefringence is the difference between
appropriateunits.Ifagraduatedscaleisnotavailable,thenany
the highest and lowest refractive indices for anisotropic crys-
other object whose size can be accurately measured can serve
tals) material that is cut parallel to the optic axis of the crystal.
as a spatial reference.
If plane-polarized light is transmitted through the sensitive tint
plate, then the emergent ordinary and extraordinary rays will 5.8.2 Fig. 1 shows an example of a graduated reticle. On
have a path difference of exactly one wavelength for light of top, a low magnification image shows a segment about 7mm
one particular wavelength. In this case, the wavelength of long of the graduated scale, with marks at every 1mm (left),
greenlightisused,suchthatthetransmittedlightiswhitelight 0.1mm (center) and 0.01mm (right). The image is composed
minus the green wavelength, which is magenta in color. of 21 by 3 individual images stitched together. This image is
5.6.2 A feature that appears dark with crossed polarizers useful for calibration of a series of equally large areas of
will appear magenta with a sensitive tint plate in its 45º interest on graphite specimens, acquired in exactly the same
position. Other features with differing orientations and differ- optical conditions as illustrated with the lower image in Fig. 1.
ingpolarizationcharacteristicswillsuppressotherwavelengths
5.8.3 In Fig. 2, portions of the high density marks on the
and appear as a characteristic color (white light minus the
same reticle from Fig. 1 are shown, acquired with three
suppressed wavelength). In this way, different orientations will
differentobjectives(20×,40×,and100×magnificationpower).
produce different colors in cross-polarization optical micros-
The shortest distance between the mark centers is 0.01mm in
copy. The strength of the colors observed will indicate the
all figures. These images can be used for spatial calibration of
degree of long-range order and crystallinity within any one
high magnification images acquired in identical conditions
feature.
with each of the respective objective lenses.
5.7 The specification for optical microscopy equipment will 5.8.4 Estimating the size of objects imaged by optical
be determined by the resolution required to observe micro- microscopy is also possible if the magnifying power of the
structural features of interest. Typically, low magnification objective lens and camera are known. For example, the image
images are taken with a 5× objective lens. The total magnifi- in Fig. 1 was collected using an objective lens with 40×
cation of the image will depend upon the strength of the ocular magnificationandacamerawith10×magnificationpower.The
lens and the image capture arrangement. For more detailed image recorded by the camera has a combined magnification
images,objectivelensesmayrangeupto100×,althoughimage equaltotheproductofmagnificationpowersofthecameraand
quality can be challenging at this level of magnification. objective lens (400× in the example of Fig. 1).
Upper image shows a 1 mm (left), 0.1 mm (center), and 0.01 mm scale; lower image shows an example optical micrograph.
FIG. 1 Use of a Graduated Reticle to Show Scale on Optical Micrographs
D8075 − 16 (2021)
not accessible to the resin from the outside surface of the
sample.Thepolarizedlightimage,similarincontrasttotheBF
image,showssomevariationsincolorduetodifferentlamellar
orientations.
5.11 Principal microstructural features that can be observed
i
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