ASTM G171-03
(Test Method)Standard Test Method for Scratch Hardness of Materials Using a Diamond Stylus
Standard Test Method for Scratch Hardness of Materials Using a Diamond Stylus
SIGNIFICANCE AND USE
This test method is intended to measure the resistance of solid surfaces to permanent deformation under the action of a single point (stylus tip). It is a companion method to quasi-static hardness tests in which a stylus is pressed into a surface under a certain normal load and the resultant depth or impression size is used to compute a hardness number. Scratch hardness numbers, unlike quasi-static hardness numbers, involve a different combination of properties of the surface because the indenter, in this case a diamond stylus, moves tangentially along the surface. Therefore, the stress state under the scratching stylus differs from that produced under a quasi-static indenter. Scratch hardness numbers are in principle a more appropriate measure of the damage resistance of a material to surface damage processes like two-body abrasion than are quasi-static hardness numbers.
This test method is applicable to a wide range of materials. These include metals, alloys, and some polymers. The main criteria are that the scratching process produces a measurable scratch in the surface being tested without causing catastrophic fracture, spallation, or extensive delamination of surface material. Severe damage to the test surface, such that the scratch width is not clearly identifiable or that the edges of the scratch are chipped or distorted, invalidates the use of this test method to determine a scratch hardness number. Since the degree and type of surface damage in a material may vary with applied load, the applicability of this test to certain classes of materials may be limited by the maximum load at which valid scratch width measurements can be made.
The resistance of a material to abrasion by a single point may be affected by its sensitivity to the strain rate of the deformation process. Therefore, this test is conducted under low stylus traversing speeds. Use of a slow scratching speed also minimizes the possible effects of frictional heating.
This test uses measurem...
SCOPE
1.1 This standard describes laboratory procedures for determining the scratch hardness of the surfaces of solid materials. Within certain limitations, as described in this guide, this test method is applicable to metals, ceramics, polymers, and coated surfaces. The scratch hardness test, as described herein, is not intended to be used as a means to determine coating adhesion, nor is it intended for use with other than specific hemispherically-tipped, conical styli.
1.2 This standard may involve hazardous materials, operations, and equipment. 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: G 171 – 03
Standard Test Method for
Scratch Hardness of Materials Using a Diamond Stylus
This standard is issued under the fixed designation G 171; 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 (e) indicates an editorial change since the last revision or reapproval.
kP
1. Scope
HS 5
P
w
1.1 This standard describes laboratory procedures for deter-
mining the scratch hardness of the surfaces of solid materials.
where:
Within certain limitations, as described in this guide, this test
HS = scratch hardness number,
P
method is applicable to metals, ceramics, polymers, and coated
k = a geometrical constant,
surfaces. The scratch hardness test, as described herein, is not
P = applied normal force, and
intended to be used as a means to determine coating adhesion,
w = scratch width.
nor is it intended for use with other than specific
NOTE 1—The constant k may be chosen to include conversion factors
hemispherically-tipped, conical styli.
forexpressing HS inunitsofGPa.For HS inGPa, Pingrams-force,and
P P
1.2 This standard may involve hazardous materials, opera-
w in µm, k = 24.98.
tions, and equipment. This standard does not purport to
3.2.2 scratching force, n—the force that opposes relative
address all of the safety concerns, if any, associated with its
motion between a moving stylus and the surface that is being
use. It is the responsibility of the user of this standard to
scratched by that stylus, and which is perpendicular to the
establish appropriate safety and health practices and deter-
normal force exerted by the stylus.
mine the applicability of regulatory limitations prior to use.
3.2.3 stylus drag coeffıcient, n—in scratch testing, the
2. Referenced Documents dimensionless ratio of the scratching force to the normal force
applied to the stylus; namely,
2.1 ASTM Standards:
F
E 177 Practice for Use of the Terms Precision and Bias as
scr
D 5
2 sc
P
Applied to Measurement of a Property of a Material
E 178 Practice for Dealing with Outlying Observations
where:
G40 Terminology Relating to Wear and Erosion
D = stylus drag coefficient,
sc
G117 Guide for Calculating and Reporting Measures of
F = scratching force, and
scr
Precision Using Data from Interlaboratory Wear or Ero-
P = normal force.
sion Tests
4. Summary of Test Method
3. Terminology
4.1 This test involves producing a scratch in a solid surface
3.1 Definitions—For definitions of terms applicable to this
by moving a diamond stylus of specified geometry along a
standard see TerminologyG40.
specified path under a constant normal force and with a
3.2 Definitions of Terms Specific to This Standard:
constant speed. The average width of the scratch is measured,
3.2.1 scratch hardness number, n—a quantity, expressed in
and that value is used to compute the scratch hardness number
units of force per unit area, that characterizes the resistance of
in units of pressure.
a solid surface to penetration by a moving stylus of given tip
4.2 As an option, the scratching force may be measured
radius under a constant normal force and speed; namely,
during this test and used to compute a stylus drag coefficient,
which is a dimensionless measure of the resistance of the test
surface to deformation by a tangentially-moving stylus.
4.3 This test is usually conducted under unlubricated con-
This test method is under the jurisdiction of ASTM Committee G02 on Wear
and Erosion and is the direct responsibility of Subcommittee G02.30 on Abrasive
ditions and at room temperature; however, it is possible to
Wear.
conduct scratch hardness tests under lubricated and elevated
Current edition approved Jan. 10, 2003. Published April 2003.
temperature conditions. The provisions of this standard allow
Annual Book of ASTM Standards, Vol 14.02.
Annual Book of ASTM Standards, Vol 03.02. testing under both conditions provided that requirements for
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
G171–03
valid scratch hardness testing are met and that the testing 6.1.1 Stylus—The stylus shall be conical of apex angle 120
conditions are fully reported. 65°,andtheconeshallterminateinahemisphericaltipof200
µm (6 10 µm) radius.The material of the tip shall be diamond.
4.4 Effects of moisture in the air and other ambient atmo-
spheric conditions may affect results depending on the sensi-
NOTE 2—The smaller the tip radius, the higher the contact stress under
tivity of the test material to the environment. If such effects are a given normal force. If a tip radius other than that indicated here is used,
results shall indicate that a modified version of the standard was used, and
either expected or observed during the course of testing,
the size of the tip radius shall be reported (see also 10.1.1).
precautions to control the surrounding atmosphere and to
document the relative humidity level should be taken and 6.1.2 Apparatus—A means to traverse the specimen under
reported. the stylus, or the stylus across the specimen, under constant
speed and normal force, shall be provided. Fixtures shall be
5. Significance and Use sufficientlyrigidtowithstandthenormal,lateral,andtangential
forces associated with the scratching process without undue
5.1 Thistestmethodisintendedtomeasuretheresistanceof
elastic or plastic deflection. The path of the stylus may be in a
solid surfaces to permanent deformation under the action of a
straight line or an arc, as produced using a rotating table-type
single point (stylus tip). It is a companion method to quasi-
device.
static hardness tests in which a stylus is pressed into a surface
6.1.3 Scratch Width Measurement System—A means for
under a certain normal load and the resultant depth or impres-
measuring the width of the scratch shall be provided. This can
sion size is used to compute a hardness number. Scratch
consistofanyimagingsystemthatiscapableofmagnifyingthe
hardness numbers, unlike quasi-static hardness numbers, in-
scratch such that its width can be accurately determined. The
volve a different combination of properties of the surface
measuring system shall be capable of measuring the width of
because the indenter, in this case a diamond stylus, moves
the scratch to a precision of at least 2 %. For example, the
tangentially along the surface. Therefore, the stress state under
required resolution for a measuring optical microscope needed
the scratching stylus differs from that produced under a
for an average 50 µm-wide scratch shall be (0.02 3 50 µm) =
quasi-static indenter. Scratch hardness numbers are in principle
1.0 µm or better. Reflecting-type, optical microscopes using
a more appropriate measure of the damage resistance of a
monochromatic illumination or interference-contrast and hav-
material to surface damage processes like two-body abrasion
ingameasuringeyepiecearesuitableforscratchmeasurement.
than are quasi-static hardness numbers.
Alternatively, photographic or video images may be used as
5.2 This test method is applicable to a wide range of
long as the magnifications are properly calibrated.
materials. These include metals, alloys, and some polymers.
6.1.4 Scratching Force (Optional)—A load cell or similar
The main criteria are that the scratching process produces a
force-sensing device can be used to measure the scratching
measurable scratch in the surface being tested without causing
forces generated during sliding. This standard does not specify
catastrophic fracture, spallation, or extensive delamination of
a method for measuring the scratching force, only that the
surface material. Severe damage to the test surface, such that
sensor shall be capable of being calibrated in the direction of
the scratch width is not clearly identifiable or that the edges of
the scratching force and in line with the contact point between
the scratch are chipped or distorted, invalidates the use of this
the stylus and surface.
test method to determine a scratch hardness number. Since the
7. Calibration
degree and type of surface damage in a material may vary with
applied load, the applicability of this test to certain classes of 7.1 Thepartsoftheapparatusthatrequirecalibrationare(1)
the normal force application system, (2) stylus traverse speed,
materials may be limited by the maximum load at which valid
scratch width measurements can be made. and optionally (3) the scratching force sensor.
7.2 Loading System—The normal force applied to the stylus
5.3 The resistance of a material to abrasion by a single point
while it is traversing the surface shall be calibrated in such a
may be affected by its sensitivity to the strain rate of the
way that the normal force is known to within 1 %. For
deformation process. Therefore, this test is conducted under
example, a normal force of 1 N shall be applied to within an
low stylus traversing speeds. Use of a slow scratching speed
accuracy of 6 0.01 N.The means to calibrate the scratch tester
also minimizes the possible effects of frictional heating.
shall be determined by its individual design; however, the
5.4 This test uses measurements of the residual scratch
method of normal force calibration shall be stated in the report.
width after the stylus has been removed to compute the scratch
hardness number.Therefore, it reflects the permanent deforma- NOTE 3—One method to calibrate the normal force on the stylus is to
use a quasi-static system such as a button-type load cell placed under the
tionresultingfromscratchingandnottheinstantaneousstateof
stylus tip in the position where the test specimen is located.
combined elastic and plastic deformation of the surface.
7.3 Stylus Traverse Speed—The speed of the stylus across
6. Apparatus the surface s may be calibrated in any suitable manner such as
timing the period t required to produce a scratch of length L.
6.1 General Description—The apparatus consists of (1) the
Thus:
rigidstylusmountandspecimenholdingfixture,(2)ameansto
L
apply a normal force while traversing the stylus along the
s 5 (1)
t
surfaceatconstantspeed,and(3)ameanstomeasurethewidth
of the scratch. Optionally, the apparatus can be equipped with 7.4 Scratching Force Sensor (Optional)—The scratching
a sensor to detect the magnitude of the scratching force. force sensor shall be calibrated periodically in the direction of
G171–03
the scratching force, and as closely as possible in line with the scratching. Lower the stylus to apply the load on the specimen
point of contact between the stylus and specimen. The interval surface gently to avoid impact damage.Activate the traversing
between calibrations shall be determined by the user to ensure drive to produce the scratch of desired length. Raise the stylus
accurate readings of scratching force and compensate for any off of the surface. Select another location at least 5 scratch
electronic signal drift. widths away from the previous scratch and produce another
scratch parallel to the first. Repeat as necessary, but with a
minimum of three (3) scratches per value of the normal force.
8. Procedure
Measure the scratch width as described in 8.8.
8.1 Specimen Preparation—The test specimen shall be
8.8 Scratch Width Measurement—Using a measuring mi-
prepared in such a way as to represent the application of
croscope or other calibrated magnifying or surface profiling
interest or polished to facilitate observation and measurement
system, measure the width of each scratch at three locations
of scratch width. A surface may be unsuitable for scratch
spaced approximately equally along the length of the scratch.
testing if its roughness or porosity is such that the edges of the
The width of the scratch shall be determined optically, as
scratch are indistinct or jagged, or if the stylus cannot traverse
shown by the examples in Fig. 1. Owing to acceleration and
thesurfacewithoutskippingalongitorcatchinginapocket.In
deceleration effects, scratch widths should not be measured
a polished condition, the surface should be as free as possible
near the ends of the scratch.
from preparation artifacts such as grinding-induced cracks,
gross grinding marks, and grain pull-out. Surface roughnesses
NOTE 5—Other methods, such as surface profiling, may produce values
of0.02to0.05µmR (arithmeticaverageroughness)aretypical different from optical measurements. Therefore, to improve consistency,
a
widths should be measured on enlarged images.
of polished surfaces. Surfaces may be scratch tested in the
as-fabricated condition as long as the characteristics of the
8.8.1 Special Considerations in Optical Scratch
scratch do not display the types of artifacts described in this
Measurement—Thecharacteristicsofthesurfacesbeingtested,
paragraph.
such as their roughness, color, degree of light diffusion, extent
8.2 Specimen Cleaning—Since many different kinds of
of plastic deformation, and reflectivity, will all affect the ease
materials can be scratch tested, one specific cleaning treatment
or difficulty in precisely locating scratch edges. In general,
cannot be given. Specimens shall be cleaned in such a way that
finer scratches present more difficulties in width measurement
the surface is free from grit, grease, fingerprints, or other
than wider scratches (see also 11.2). It may be necessary to use
contaminants. Metals and alloys may be cleaned in non-polar
special lighting methods, such as oblique illumination, polar-
solvents. Plastics may require alternative cleaning with eye-
ized light, or differential inference contrast microscopy to
glass cleaner or similar. If contact with solvents or cleaners
provide sufficient contrast to measure the scratch widths
could result in changes to their properties, surfaces may be
optically. Report the use of special lighting methods, when
tested as-received. The method of cleaning, if any, shall be
applicable.
described in the report.
9. Calculations
8.3 Inspection of the Stylus—Inspect the stylus tip with a
microscope or other topographic inspection method to ensure
9.1 Scratch Hardness Number—The scratch hardness num-
that there are no defects (cracks, chips), wear or adhering ber is calculated by dividing the applied normal force on the
material left from manufacturing or resulting from a previous
stylus by the projected area of scratching contact, assuming
test.Wiping the stylus with a soft cloth moistened with acetone thatthehemispherically-tippedstylusproducesagroovewhose
or other cleaning solvent is usually suitable.
leading surface has a radius of curvature r, the tip radius of the
stylus. The projected area of the contact surface is therefore a
NOTE 4—
...
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