ASTM F1854-15
(Test Method)Standard Test Method for Stereological Evaluation of Porous Coatings on Medical Implants (Withdrawn 2024)
Standard Test Method for Stereological Evaluation of Porous Coatings on Medical Implants (Withdrawn 2024)
SIGNIFICANCE AND USE
5.1 All these test methods are recommended for elementary quantification of the morphological properties of porous coatings bonded to solid substrates.
5.2 These test methods may be useful for comparative evaluations of different coatings or different lots of the same coating.
5.3 All the methods should be performed on the same set of images of fields.
5.4 A statistical estimate can be made of the distributions of the mean coating thickness and the volume percent void. No estimate can be made of the distribution of intercept lengths.
5.5 There are limits to the accurate characterization of porosity, depending on spacing between the lines in the line grid (or points in the point grid) and the individual and cumulative fields used for the measurements. Increasing the size of the fields, increasing the number of fields, or decreasing the grid spacing will increase the accuracy of the measurements obtained.
5.6 This method may be suitable for ceramic coatings if an accurate coating cross section can be produced. Producing an accurate ceramic coating cross section may require other techniques than standard metallographic techniques.
5.7 For coatings having a mean thickness less than 300 microns, it is not recommended to attempt to determine the volume percent void or the mean intercept length.
SCOPE
1.1 This test method covers stereological test methods for characterizing the coating thickness, void content, and mean intercept length of various porous coatings adhering to nonporous substrates.
1.2 A method to measure void content and intercept length at distinct levels (“Tissue Interface Gradients”) through the porous coating thickness is outlined in 9.4.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 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.
WITHDRAWN RATIONALE
This test method covered stereological test methods for characterizing the coating thickness, void content, and mean intercept length of various porous coatings adhering to nonporous substrates.
Formerly under the jurisdiction of Committee F04 on Medical and Surgical Materials and Devices, this test method was withdrawn in May 2024 in accordance with section 10.6.3 of the Regulations Governing ASTM Technical Committees, which requires that standards shall be updated by the end of the eighth year since the last approval date.
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This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
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Designation: F1854 − 15
Standard Test Method for
Stereological Evaluation of Porous Coatings on Medical
Implants
This standard is issued under the fixed designation F1854; 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 3.1.4 porous coating—coating on an implant deliberately
applied to contain void regions with the intent of enhancing the
1.1 This test method covers stereological test methods for
fixation of the implant.
characterizing the coating thickness, void content, and mean
3.1.5 substrate—the solid material to which the porous
intercept length of various porous coatings adhering to nonpo-
coating is attached.
rous substrates.
3.1.6 substrate interface—the region where the porous coat-
1.2 A method to measure void content and intercept length
ing is attached to the substrate.
at distinct levels (“Tissue Interface Gradients”) through the
porous coating thickness is outlined in 9.4.
3.1.7 tissue interface—the surface of the coating that shall
have first contact with biological tissue (i.e., the top of the
1.3 The values stated in SI units are to be regarded as
coating).
standard. No other units of measurement are included in this
standard. 3.1.8 working surface—the ground and polished face of the
metallographic mount where the images of the fields are
1.4 This standard does not purport to address all of the
captured.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Summary of Test Method
priate safety and health practices and determine the applica-
4.1 Mean Coating Thickness—Evenly spaced parallel grid
bility of regulatory limitations prior to use.
lines are oriented perpendicular to the coating-substrate inter-
face on a field. For each gridline, the distance from the
2. Referenced Documents
coating-substrate interface to the last contact with the porous
2.1 ASTM Standards:
coating material is measured as the coating thickness. The
E3 Guide for Preparation of Metallographic Specimens
average of all of the coating thickness measurements obtained
E883 Guide for Reflected–Light Photomicrography
from all the measured fields is the mean coating thickness for
that coating.
3. Terminology
4.2 Volume Percent Void—A regular grid of points is super-
3.1 Definitions of Terms Specific to This Standard:
imposed on a field from the working surface. The percentage of
3.1.1 field—the image of a portion of the working surface
points that are in contact with void areas in the coating is the
upon which measurements are performed.
volume percent of void present in that field.
3.1.2 intercept—the point on a measurement grid line pro-
4.3 Mean Void Intercept Length—Measurement grid lines
jected on a field where the line crosses from solid to void or
are oriented parallel to the substrate interface in a field. The
vice versa.
average length of the line segments overlaying the void space
3.1.3 measurement grid lines—an evenly spaced grid of
is the mean void intercept length for that field. This is a
parallel lines all of the same length.
representative measure of the scale, or size, of the pores in a
porous structure.
This test method is under the jurisdiction of ASTM Committee F04 on Medical
4.4 Tissue Interface Gradients—The Volume Percent Void
and Surgical Materials and Devices and is the direct responsibility of Subcommittee
and the Mean Void Intercept Length are characterized in three
F04.15 on Material Test Methods.
200-µm-thick zones below the Tissue Interface in each field.
Current edition approved March 15, 2015. Published May 2015. Originally
approved in 1998. Last previous edition approved in 2009 as F1854 – 09. DOI:
5. Significance and Use
10.1520/F1854-15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
5.1 All these test methods are recommended for elementary
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
quantification of the morphological properties of porous coat-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. ings bonded to solid substrates.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1854 − 15
5.2 These test methods may be useful for comparative 8.1.1.2 If the angle between the tangent to the coating-
evaluations of different coatings or different lots of the same substrate interface at one edge of a field and the tangent to the
coating. substrate interface at the opposite edge of the field is greater
than 2º, the substrate curvature is too large.
5.3 All the methods should be performed on the same set of
8.1.1.3 There is a practical limit to the magnification that
images of fields.
can be used for measurement of the void content and mean
5.4 A statistical estimate can be made of the distributions of
intercept length. As magnification is increased, the number of
the mean coating thickness and the volume percent void. No
fields should be increased to obtain a representative sample. If
estimate can be made of the distribution of intercept lengths.
there are too few intercepts in the individual fields, the
5.5 There are limits to the accurate characterization of accuracy of the measurement could decrease.
porosity, depending on spacing between the lines in the line
8.2 Field Parameters:
grid (or points in the point grid) and the individual and
8.2.1 Resolution:
cumulative fields used for the measurements. Increasing the
8.2.1.1 The magnification used for the field should be high
size of the fields, increasing the number of fields, or decreasing
enough to resolve all the features that need to be measured.
the grid spacing will increase the accuracy of the measure-
8.2.1.2 For most porous coatings, the magnification should
ments obtained.
be high enough that features as small as 5 µm can be easily
5.6 This method may be suitable for ceramic coatings if an
distinguished. If digital imaging is used, the pixel size should
accurate coating cross section can be produced. Producing an
be less than or equal to 5 µm.
accurate ceramic coating cross section may require other
8.2.1.3 For digital images the pixel size in µm and the image
techniques than standard metallographic techniques.
field size in pixels shall be included with the images.
5.7 For coatings having a mean thickness less than 300
8.2.2 Field Dimensions:
microns, it is not recommended to attempt to determine the
8.2.2.1 The field height of the images shall be large enough
volume percent void or the mean intercept length.
to include all the features of any portion of the porous coating
for Mean Coating Thickness determination (section 9.1).
6. Apparatus
8.2.2.2 A good rule of thumb for an accurate measurement
of Mean Void Intercept Length is that the minimum field width
6.1 The procedures outlined in this test method can be
should be greater than or equal to 5 times the resulting Mean
performed manually or using digital image analysis techniques.
Void Intercept Length. For example, a Mean Void Intercept
6.2 Microscope, or other suitable device with a viewing
Length value of 200 µm should have a measurement field width
screen, photomicrographic capability, or digital image capture
of at least 1000 µm.
capability should be used to image the sample fields of interest
8.2.2.3 It is possible to measure the Mean Void Intercept
for these test methods.
Length in a field using a series of shorter non-overlapping grid
6.3 For manual measurement, a transparent sheet, with
lines. This does not change the requirement for the coating field
measurement grid lines or points is superimposed on the
length required for the calculation. Care should be exercised
viewing screen or photomicrograph for the measurements. The
using multiple short lines in a single field, because it is possible
line grid (or point grid) should consist of at least five uniformly
to make the grid lines so short that the accuracy of the result is
spaced, parallel lines (or rows).
affected.
8.2.2.4 If the magnification used produces an image with a
7. Metallography
height or width smaller than that which is required, multiple
7.1 The procedures outlined in this test method for charac- images may be carefully stitched together to produce a field of
terizing porous coatings require the preparation of metallo- sufficient height and width.
graphic sections. Good metallographic preparation techniques,
8.2.2.5 All four types of measurements shall be performed
in accordance with Guides E3 and E883 shall be used to
over a minimum coating area of 15 mm with no area being
prevent deformation of the surface of the section or creation of
measured more than once. For thinner coatings that require
any other artifacts that will alter the morphology of the
higher magnifications to allow reasonable measurements the
metallographic section. An example of an unacceptable artifact
minimum total measured field length shall be 20 mm with no
would be the absence of a portion of the porous coating, caused
part of that length being measured more than once.
by its removal, thereby creating an artificial void area.
9. Procedure
7.2 Care shall be taken to ensure that the working surface is
perpendicular to the substrate interface.
9.1 Mean Coating Thickness:
9.1.1 An array of equally spaced parallel gridlines should be
8. Sample Working Surfaces and Fields
superimposed on the field perpendicular to the substrate
interface, as shown in Fig. 1. The gridlines should be spaced no
8.1 Sample Orientation:
more than 100 µm apart. Appendix X2 includes two typical
8.1.1 Normal Section Orientation:
sets of gridlines each with ten equally spaced parallel lines.
8.1.1.1 For accurate coating thickness measurements, the
orientation of sample working surfaces should be approxi- 9.1.2 At each gridline, the distance from the substrate
mately perpendicular to the plane of the substrate. interface to the last contact with a solid coating feature is
F1854 − 15
NOTE 1—The solid line is the measured distance.
FIG. 1 Illustration of Coating Thickness Measurement
measured. A measurement is only valid if the gridline is Interface during the metallographic mounting process and shall
oriented 90° 6 2° to the substrate interface. have an angle away from the substrate of less than 1º.
9.1.2.1 The surface of the substrate is usually rough due to
NOTE 1—Spring-loaded binder clips have been used to secure the
the processing involved in applying the coating. If the surface
attached flat metal plate to the tissues interface.
is too rough, a line indicating a subjective average substrate
9.1.5.2 The second method is to use the average of the
interface shall be made on the each field. Thickness measure-
longest 5% of the thickness measurements from the Mean
ments shall then be made from this line.
Coating Thickness procedure in section 9.1. Use that average
9.1.2.2 If a subjective average interface line is used, the
to draw a line, in any field examined, that distance from the
same line should be used for the positioning of the other
substrate, parallel within 1º to the substrate, to define the Tissue
measurements in this standard.
Interface.
9.1.3 Coating thickness measurements should be obtained
9.1.6 For rough surface coatings, such as plasma spray
over a linear distance of at least 20 mm of porous surface with
coatings intended to create a roughened surface if the spacing
no overlap between measurement sites.
of the thickness lines is too large it can be affect the
9.1.4 The average of all the individual coating thickness
repeatability of the thickness measurement. If the mean of the
measurements from all the fields that were measured is the
thickness plus the standard deviation of the thickness is more
mean coating thickness. The standard deviation estimator and
than one half of the standard deviation of the thickness below
the 95 % confidence interval should be calculated for all the
the value of the tissue interface, then the line spacing should be
fields. The equations for calculating these values are as
decreased and the thickness re-measured. For such coatings, it
follows:
may be better to start with the distance between the thickness
n
¯
measurement lines at 33 µm or less.
T 5 t (1)
i
(
M 3n
i51
9.2 Volume Percent Void:
where:
9.2.1 For this measurement, the field should be entirely
t = the individual magnified thickness line length,
i
contained between the Tissue Interface (section 9.4.1) and the
n = the number of thickness measurements,
substrate interface.
M = the magnification, and
9.2.2 An array containing at least 100 regularly spaced
¯
T = the mean coating thickness.
points should be superimposed on the field, as shown in Fig. 2.
n 2
The points should be spaced no more than 50 µm apart. If the
1 t
i
ˆ ¯
S 5Π3 2 T (2)
F G
(
void areas form a regular or periodic pattern, the use of a grid
n 2 1 M
i51
having a similar pattern should be avoided. The height of the
where:
array should include the coating from 10% to 90% of the tissue
Ŝ = the standard deviation estimator, and
interface value. The same grid positioned at 10% of the tissue
CI = the confidence interval.
interface away from the substrate should be used for all fields.
Appendix X2 includes two typical arrays each with at least 100
ˆ
S
CI 5 2 3 (3)
regularly spaced points.
=
n
9.2.3 The number of points overlying void areas (P )onthe
α
fields shall be counted and recorded. When using the manual
9.1.5 Define the Tissue Interface of the porous coating.
method, any points falling on a boundary between a void area
9.1.5.1 The first method to define the location of the Tissue
and solid features should be counted as one half. Any ques-
Interface is a physical one. Securely attach a flat metallic
tionable points should be counted as one half.
surface on the porous interface of the metallographic sample
prior to embedding the sample. The attached flat metal surface 9.2.4 The number of contact points in void areas (P ),
α
must show that it has not moved away from the Tissue divided by the total number of points on the grid (P ) times 100
T
F1854 − 15
FIG. 2 Illustration of Volume Percent Void Measurement
gives the percentage of grid points on the void for that field. 9.2.8 The volume percent void estimate is given by the
This should be calculated for each grid app
...
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: F1854 − 09 F1854 − 15
Standard Test Method for
Stereological Evaluation of Porous Coatings on Medical
Implants
This standard is issued under the fixed designation F1854; 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 This test method covers stereological test methods for characterizing the coating thickness, void content, and mean intercept
length of various porous coatings adhering to nonporous substrates.
1.2 A method to measure void content and intercept length at distinct levels (“tissue interface gradients”)(“Tissue Interface
Gradients”) through the porous coating thickness is outlined in 9.4.
1.3 The alternate sample orientation method in 8.2 is not suitable for the tissue interface gradients method in 9.4.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 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
E883 Guide for Reflected–Light Photomicrography
3. Terminology
3.1 Definitions:Definitions of Terms Specific to This Standard:
3.1.1 field—the portion of image of a partportion of the working surface upon which measurements are performed.
3.1.2 intercept—the point on a measurement grid line projected on a field where the line crosses from solid to void or vice versa.
3.1.3 measurement grid lines—aan evenly spaced grid of parallel lines all of the same length.
3.1.4 porous coating—coating on an implant deliberately applied to contain void regions with the intent of enhancing the
fixation of the implant.
3.1.5 substrate—the solid material to which the porous coating is attached.
3.1.6 substrate interface—the region where the porous coating is attached to the substrate.
3.1.7 working surface—the ground and polished face of the metallographic mount where the measurements are made.
3.1.7 tissue interface—the surface of the coating that shall have first contact with biological tissue (that is, (i.e., the top of the
coating).
3.1.8 working surface—the ground and polished face of the metallographic mount where the images of the fields are captured.
4. Summary of Test Method
4.1 Mean Coating Thickness—Evenly spaced parallel grid lines are oriented perpendicular to the coating-substrate interface.
interface on a field. For each gridline, the distance from the coating-substrate interface to the last contact with the porous coating
This test method is under the jurisdiction of ASTM Committee F04 on Medical and Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.15 on Material Test Methods.
Current edition approved June 15, 2009March 15, 2015. Published August 2009May 2015. Originally approved in 1998. Last previous edition approved in 20012009 as
F1854 – 01.F1854 – 09. DOI: 10.1520/F1854-09.10.1520/F1854-15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1854 − 15
material is measured as the coating thickness. The average of all of the coating thickness measurements obtained on a working
surface from all the measured fields is the mean coating thickness for that working surface. coating.
4.2 Volume Percent Void—A regular grid of points is superimposed on a field from the working surface. The percentage of points
that are in contact with void areas in the coating correlates with is the volume percent of void present.present in that field.
4.3 Mean Void Intercept Length—Measurement grid lines are oriented parallel to the substrate interface. interface in a field. The
average length of the line segments overlaying the void space is the mean void intercept length. length for that field. This is a
representative measure of the scale, or size, of the pores in a porous structure.
4.4 Tissue Interface Gradients—The volume percent voidVolume Percent Void and the mean void intercept lengthMean Void
Intercept Length are characterized in three 200-μm-thick zones below the tissue interface.Tissue Interface in each field.
5. Significance and Use
5.1 All of these test methods are recommended for elementary quantification of the morphological properties of porous coatings
bonded to solid substrates.
5.2 These test methods may be useful for comparative evaluations of different coatings or different lots of the same coating.
5.3 With the exception of using the alternate mounting method, all the All the methods should be performed on the same
working surfaces. The alternate mounting method can only be used for set of images of fields.9.2 and 9.3.
5.4 A statistical estimate can be made of the distributions of the mean coating thickness and the volume percent void. No
estimate can be made of the distribution of intercept lengths.
5.5 There are limits to the accurate characterization of porosity, depending on spacing between the lines in the line grid (or
points in the point grid) and the individual and cumulative fields used for the measurements. Increasing the size of the fields,
increasing the number of fields, or decreasing the grid spacing will increase the accuracy of the measurements obtained.
5.6 This method is notmay be suitable for ceramic coatings for whichif an accurate coating cross sections cannot be produced
using section can be produced. Producing an accurate ceramic coating cross section may require other techniques than standard
metallographic techniques.
5.7 This test method does not address characterization of For coatings having a mean thickness of less than 300 μm.microns,
it is not recommended to attempt to determine the volume percent void or the mean intercept length.
6. Apparatus
6.1 The procedures outlined in this test method can be performed manually or using digital image analysis techniques.
6.2 Microscope, or other suitable device with a viewing screen, photomicrographic capability, or digital image capture
capability should be used to image the sample fields of interest for these test methods.
6.3 For manual measurement, a transparent sheet, with measurement grid lines or points is superimposed on the viewing screen
or photomicrograph for the measurements. The line grid (or point grid) and should consist of at least five uniformly spaced, parallel
lines (or rows).
7. Metallography
7.1 The procedures outlined in this test method for characterizing porous coatings require the preparation of metallographic
sections. Good metallographic preparation techniques, in accordance with PracticeGuides E3 and Guide E883, shall be used to
prevent deformation of the surface of the section or creation of any other artifacts that will alter the morphology of the
metallographic section. An example of an unacceptable artifact would be the absence of a portion of the porous coating, caused
by its removal, thereby creating an artificial void area.
7.2 Care mustshall be taken to ensure that the working surface is perpendicular to the substrate interface. When using the
alternative mounting method shown in 8.1.2, extreme care must be taken to keep the substrate interface parallel to the final working
surface.
8. Sample Working Surfaces and Fields
8.1 Sample Orientation:
8.1.1 Normal Section Orientation:
8.1.1.1 For accurate coating thickness measurements, the orientation of sample working surfaces should be approximately
perpendicular to the plane of the substrate.
8.1.1.2 If the angle between the tangent to the coating-substrate interface at one edge of a field and the tangent to the substrate
interface at the opposite edge of the field is greater than 2º, the substrate curvature is too large.
8.1.1.3 There is a practical limit to the magnification that can be used for measurement of the void content and mean intercept
length. As magnification is increased, the number of fields should be increased to obtain a representative sample. If there are too
few intercepts in the individual fields, the accuracy of the measurement could decrease.
F1854 − 15
8.1.2 Alternative Orientation Method:
8.1.2.1 An alternate orientation may be used for the volume percent void and mean intercept length measurements. The section
should be prepared such that the working surface is parallel to the substrate interface and the measurements should be taken at a
fixed distance from the substrate interface. It is recommended that the measurements be made at about 50 % of the mean coating
thickness.
8.1.2.2 At least one additional section immediately adjacent to the fields used on the working surface shall also be prepared
perpendicular to the working surface. This shall confirm that the substrate interface is parallel to the working surface and allow
measurement of the distance from the working surface to the substrate interface.
8.1.2.3 This test method is not suitable for substrate interfaces with a radius of curvature less than 25 mm.
8.1.2.4 Since this test method also requires more aggressive porous surface removal to reach 50 % of the mean coating
thickness, it may be more susceptible to creation of metallographic artifacts. Care should be exercised to ensure that the
metallographic sections that are used are free of artifacts.
8.2 Field Parameters:
8.2.1 Resolution:
8.2.1.1 The magnification used for the field should be high enough to resolve all the features that need to be measured.
8.2.1.2 For most porous coatings, the magnification should be high enough that features as small as 5 μm can be easily
distinguished. If digital imaging is used, the pixel size should be less than or equal to 5 μm.
8.2.1.3 For digital images the pixel size in μm and the image field size in pixels shall be included with the images.
8.2.2 Field Dimensions:
8.2.2.1 The field height must include the full thickness of of the images shall be large enough to include all the features of any
portion of the porous coating for mean coating thickness (Mean Coating Thickness determination (section 9.1).
8.2.2.2 A good rule of thumb for an accurate measurement of mean void intercept lengthMean Void Intercept Length is that the
minimum field width should be greater than or equal to 5× 5 times the resulting mean void intercept length.Mean Void Intercept
Length. For example, a mean void intercept lengthMean Void Intercept Length value of 200 μm should have a measurement field
width of at least 1000 μm.
8.2.2.3 It is possible to measure the mean void intercept lengthMean Void Intercept Length in a field using a series of shorter
non-overlapping grid lines. This does not change the requirement for the number of fieldscoating field length required for the
calculation. Care should be exercised using multiple short lines in a single field, because it may be is possible to make the grid
lines so short that the accuracy of the result is affected.
8.2.2.4 If the magnification used produces an image with a height or width smaller than that which is required, multiple images
may be carefully stitched together to produce a field of sufficient height and width.
8.2.2.5 All four types of measurements shall be performed over a minimum coating area of 15 mm with no area being measured
more than once. For thinner coatings that require higher magnifications to allow reasonable measurements the minimum total
measured field length shall be 20 mm with no part of that length being measured more than once.
9. Procedure
9.1 Mean Coating Thickness:
9.1.1 An array of equally spaced parallel gridlines should be superimposed on the field perpendicular to the substrate interface,
as shown in Fig. 1. The gridlines should be spaced no more than 100 μm apart. Appendix X2 includes two typical sets of gridlines
each with ten equally spaced parallel lines.
9.1.2 At each gridline, the distance from the substrate interface to the last contact with a solid coating feature is measured. A
measurement is only valid if the gridline is oriented 9090° 6 2° to the substrate interface.
NOTE 1—The solid line is the measured distance.
FIG. 1 Illustration of Coating Thickness Measurement
F1854 − 15
9.1.2.1 The surface of the substrate is usually rough due to the processing involved in applying the coating. If the surface is too
rough, a line indicating a subjective average substrate interface shall be made on the each field. Thickness measurements shall then
be made from this line.
9.1.2.2 If a subjective average interface line is used, the same line should be used for the positioning of the other measurements
in this standard.
9.1.3 Coating thickness measurements should be obtained over a continuous linear distance of at least 1020 mm of porous
surface with no overlap between measurement sites.
9.1.4 The average of all the measurements individual coating thickness measurements from all the fields that were measured
is the mean coating thickness for that working surface. thickness. The standard deviation estimator and the 95 % 95 % confidence
interval should be calculated for each working surface.all the fields. The equations for calculating these values are as follows:
n
¯
T 5 t (1)
( i
M 3n
i51
where:
t = the individual magnified thickness line length,
i
n = the number of thickness measurements,
M = the magnification, and
T¯ = the mean coating thickness.
n
1 t
i
ˆ ¯
S 5 3 2 T (2)
Œ
F G
(
n 2 1 M
i51
n 2
1 t
i
ˆ ¯
S 5Π3 2 T (2)
F G
(
n 2 1 M
i51
where:
Ŝ = the standard deviation estimator, and
CI = the confidence interval.
ˆ
S
CI 5 23 (3)
=n
9.1.5 Define the Tissue Interface of the porous coating.
9.1.5.1 The first method to define the location of the Tissue Interface is a physical one. Securely attach a flat metallic surface
on the porous interface of the metallographic sample prior to embedding the sample. The attached flat metal surface must show
that
...
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: F1854 − 15
Standard Test Method for
Stereological Evaluation of Porous Coatings on Medical
Implants
This standard is issued under the fixed designation F1854; 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 3.1.4 porous coating—coating on an implant deliberately
applied to contain void regions with the intent of enhancing the
1.1 This test method covers stereological test methods for
fixation of the implant.
characterizing the coating thickness, void content, and mean
3.1.5 substrate—the solid material to which the porous
intercept length of various porous coatings adhering to nonpo-
coating is attached.
rous substrates.
3.1.6 substrate interface—the region where the porous coat-
1.2 A method to measure void content and intercept length
ing is attached to the substrate.
at distinct levels (“Tissue Interface Gradients”) through the
porous coating thickness is outlined in 9.4.
3.1.7 tissue interface—the surface of the coating that shall
have first contact with biological tissue (i.e., the top of the
1.3 The values stated in SI units are to be regarded as
coating).
standard. No other units of measurement are included in this
standard. 3.1.8 working surface—the ground and polished face of the
metallographic mount where the images of the fields are
1.4 This standard does not purport to address all of the
captured.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Summary of Test Method
priate safety and health practices and determine the applica-
4.1 Mean Coating Thickness—Evenly spaced parallel grid
bility of regulatory limitations prior to use.
lines are oriented perpendicular to the coating-substrate inter-
face on a field. For each gridline, the distance from the
2. Referenced Documents
coating-substrate interface to the last contact with the porous
2.1 ASTM Standards:
coating material is measured as the coating thickness. The
E3 Guide for Preparation of Metallographic Specimens
average of all of the coating thickness measurements obtained
E883 Guide for Reflected–Light Photomicrography
from all the measured fields is the mean coating thickness for
that coating.
3. Terminology
4.2 Volume Percent Void—A regular grid of points is super-
3.1 Definitions of Terms Specific to This Standard:
imposed on a field from the working surface. The percentage of
3.1.1 field—the image of a portion of the working surface
points that are in contact with void areas in the coating is the
upon which measurements are performed.
volume percent of void present in that field.
3.1.2 intercept—the point on a measurement grid line pro-
4.3 Mean Void Intercept Length—Measurement grid lines
jected on a field where the line crosses from solid to void or
are oriented parallel to the substrate interface in a field. The
vice versa.
average length of the line segments overlaying the void space
3.1.3 measurement grid lines—an evenly spaced grid of
is the mean void intercept length for that field. This is a
parallel lines all of the same length.
representative measure of the scale, or size, of the pores in a
porous structure.
This test method is under the jurisdiction of ASTM Committee F04 on Medical
4.4 Tissue Interface Gradients—The Volume Percent Void
and Surgical Materials and Devices and is the direct responsibility of Subcommittee
and the Mean Void Intercept Length are characterized in three
F04.15 on Material Test Methods.
200-µm-thick zones below the Tissue Interface in each field.
Current edition approved March 15, 2015. Published May 2015. Originally
approved in 1998. Last previous edition approved in 2009 as F1854 – 09. DOI:
5. Significance and Use
10.1520/F1854-15.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
5.1 All these test methods are recommended for elementary
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
quantification of the morphological properties of porous coat-
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. ings bonded to solid substrates.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F1854 − 15
5.2 These test methods may be useful for comparative 8.1.1.2 If the angle between the tangent to the coating-
evaluations of different coatings or different lots of the same substrate interface at one edge of a field and the tangent to the
coating. substrate interface at the opposite edge of the field is greater
than 2º, the substrate curvature is too large.
5.3 All the methods should be performed on the same set of
8.1.1.3 There is a practical limit to the magnification that
images of fields.
can be used for measurement of the void content and mean
5.4 A statistical estimate can be made of the distributions of
intercept length. As magnification is increased, the number of
the mean coating thickness and the volume percent void. No
fields should be increased to obtain a representative sample. If
estimate can be made of the distribution of intercept lengths.
there are too few intercepts in the individual fields, the
5.5 There are limits to the accurate characterization of accuracy of the measurement could decrease.
porosity, depending on spacing between the lines in the line
8.2 Field Parameters:
grid (or points in the point grid) and the individual and
8.2.1 Resolution:
cumulative fields used for the measurements. Increasing the
8.2.1.1 The magnification used for the field should be high
size of the fields, increasing the number of fields, or decreasing
enough to resolve all the features that need to be measured.
the grid spacing will increase the accuracy of the measure-
8.2.1.2 For most porous coatings, the magnification should
ments obtained.
be high enough that features as small as 5 µm can be easily
5.6 This method may be suitable for ceramic coatings if an
distinguished. If digital imaging is used, the pixel size should
accurate coating cross section can be produced. Producing an
be less than or equal to 5 µm.
accurate ceramic coating cross section may require other
8.2.1.3 For digital images the pixel size in µm and the image
techniques than standard metallographic techniques.
field size in pixels shall be included with the images.
5.7 For coatings having a mean thickness less than 300
8.2.2 Field Dimensions:
microns, it is not recommended to attempt to determine the
8.2.2.1 The field height of the images shall be large enough
volume percent void or the mean intercept length.
to include all the features of any portion of the porous coating
for Mean Coating Thickness determination (section 9.1).
6. Apparatus
8.2.2.2 A good rule of thumb for an accurate measurement
of Mean Void Intercept Length is that the minimum field width
6.1 The procedures outlined in this test method can be
should be greater than or equal to 5 times the resulting Mean
performed manually or using digital image analysis techniques.
Void Intercept Length. For example, a Mean Void Intercept
6.2 Microscope, or other suitable device with a viewing
Length value of 200 µm should have a measurement field width
screen, photomicrographic capability, or digital image capture
of at least 1000 µm.
capability should be used to image the sample fields of interest
8.2.2.3 It is possible to measure the Mean Void Intercept
for these test methods.
Length in a field using a series of shorter non-overlapping grid
6.3 For manual measurement, a transparent sheet, with
lines. This does not change the requirement for the coating field
measurement grid lines or points is superimposed on the
length required for the calculation. Care should be exercised
viewing screen or photomicrograph for the measurements. The
using multiple short lines in a single field, because it is possible
line grid (or point grid) should consist of at least five uniformly
to make the grid lines so short that the accuracy of the result is
spaced, parallel lines (or rows).
affected.
8.2.2.4 If the magnification used produces an image with a
7. Metallography
height or width smaller than that which is required, multiple
7.1 The procedures outlined in this test method for charac- images may be carefully stitched together to produce a field of
terizing porous coatings require the preparation of metallo- sufficient height and width.
graphic sections. Good metallographic preparation techniques,
8.2.2.5 All four types of measurements shall be performed
in accordance with Guides E3 and E883 shall be used to
over a minimum coating area of 15 mm with no area being
prevent deformation of the surface of the section or creation of
measured more than once. For thinner coatings that require
any other artifacts that will alter the morphology of the
higher magnifications to allow reasonable measurements the
metallographic section. An example of an unacceptable artifact
minimum total measured field length shall be 20 mm with no
would be the absence of a portion of the porous coating, caused
part of that length being measured more than once.
by its removal, thereby creating an artificial void area.
9. Procedure
7.2 Care shall be taken to ensure that the working surface is
perpendicular to the substrate interface.
9.1 Mean Coating Thickness:
9.1.1 An array of equally spaced parallel gridlines should be
8. Sample Working Surfaces and Fields
superimposed on the field perpendicular to the substrate
interface, as shown in Fig. 1. The gridlines should be spaced no
8.1 Sample Orientation:
more than 100 µm apart. Appendix X2 includes two typical
8.1.1 Normal Section Orientation:
sets of gridlines each with ten equally spaced parallel lines.
8.1.1.1 For accurate coating thickness measurements, the
orientation of sample working surfaces should be approxi- 9.1.2 At each gridline, the distance from the substrate
mately perpendicular to the plane of the substrate. interface to the last contact with a solid coating feature is
F1854 − 15
NOTE 1—The solid line is the measured distance.
FIG. 1 Illustration of Coating Thickness Measurement
measured. A measurement is only valid if the gridline is Interface during the metallographic mounting process and shall
oriented 90° 6 2° to the substrate interface. have an angle away from the substrate of less than 1º.
9.1.2.1 The surface of the substrate is usually rough due to
NOTE 1—Spring-loaded binder clips have been used to secure the
the processing involved in applying the coating. If the surface
attached flat metal plate to the tissues interface.
is too rough, a line indicating a subjective average substrate
9.1.5.2 The second method is to use the average of the
interface shall be made on the each field. Thickness measure-
longest 5% of the thickness measurements from the Mean
ments shall then be made from this line.
Coating Thickness procedure in section 9.1. Use that average
9.1.2.2 If a subjective average interface line is used, the
to draw a line, in any field examined, that distance from the
same line should be used for the positioning of the other
substrate, parallel within 1º to the substrate, to define the Tissue
measurements in this standard.
Interface.
9.1.3 Coating thickness measurements should be obtained
9.1.6 For rough surface coatings, such as plasma spray
over a linear distance of at least 20 mm of porous surface with
coatings intended to create a roughened surface if the spacing
no overlap between measurement sites.
of the thickness lines is too large it can be affect the
9.1.4 The average of all the individual coating thickness
repeatability of the thickness measurement. If the mean of the
measurements from all the fields that were measured is the
thickness plus the standard deviation of the thickness is more
mean coating thickness. The standard deviation estimator and
than one half of the standard deviation of the thickness below
the 95 % confidence interval should be calculated for all the
the value of the tissue interface, then the line spacing should be
fields. The equations for calculating these values are as
decreased and the thickness re-measured. For such coatings, it
follows:
may be better to start with the distance between the thickness
n
¯
measurement lines at 33 µm or less.
T 5 t (1)
( i
M 3n
i51
9.2 Volume Percent Void:
where:
9.2.1 For this measurement, the field should be entirely
t = the individual magnified thickness line length,
i contained between the Tissue Interface (section 9.4.1) and the
n = the number of thickness measurements,
substrate interface.
M = the magnification, and
9.2.2 An array containing at least 100 regularly spaced
¯
T = the mean coating thickness.
points should be superimposed on the field, as shown in Fig. 2.
n
The points should be spaced no more than 50 µm apart. If the
1 t
i
ˆ ¯
S 5Π3 2 T (2)
F G
( void areas form a regular or periodic pattern, the use of a grid
n 2 1 M
i51
having a similar pattern should be avoided. The height of the
where:
array should include the coating from 10% to 90% of the tissue
Ŝ = the standard deviation estimator, and
interface value. The same grid positioned at 10% of the tissue
CI = the confidence interval.
interface away from the substrate should be used for all fields.
Appendix X2 includes two typical arrays each with at least 100
ˆ
S
CI5 2 3 (3) regularly spaced points.
=n
9.2.3 The number of points overlying void areas (P ) on the
α
fields shall be counted and recorded. When using the manual
9.1.5 Define the Tissue Interface of the porous coating.
method, any points falling on a boundary between a void area
9.1.5.1 The first method to define the location of the Tissue
and solid features should be counted as one half. Any ques-
Interface is a physical one. Securely attach a flat metallic
tionable points should be counted as one half.
surface on the porous interface of the metallographic sample
prior to embedding the sample. The attached flat metal surface 9.2.4 The number of contact points in void areas (P ),
α
must show that it has not moved away from the Tissue divided by the total number of points on the grid (P ) times 100
T
F1854 − 15
FIG. 2 Illustration of Volume Percent Void Measurement
gives the percentage of grid points on the void for that field. 9.2.8 The volume percent void estimate is given by the
This should be calculated for each grid application. following relationship:
P V 5 P (8)
α v v
P 5 3 100 (4)
v
P
T
9.3 Mean Void Inter
...
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