ASTM C1326-99
(Test Method)Standard Test Method for Knoop Indentation Hardness of Advanced Ceramics
Standard Test Method for Knoop Indentation Hardness of Advanced Ceramics
SCOPE
1.1 This test method covers the determination of the Knoop indentation hardness of advanced ceramics.
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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Standards Content (Sample)
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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Designation: C 1326 – 99
Standard Test Method for
Knoop Indentation Hardness of Advanced Ceramics
This standard is issued under the fixed designation C 1326; 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.
1. Scope 3.1.1 Knoop hardness number (HK), n—an expression of
hardness obtained by dividing the force applied to the Knoop
1.1 This test method covers the determination of the Knoop
indenter by the projected area of the permanent impression
indentation hardness of advanced ceramics.
made by the indenter.
1.2 This standard does not purport to address all of the
3.1.2 Knoop indenter, n—a rhombic-based pyramidal-
safety concerns, if any, associated with its use. It is the
shaped diamond indenter with edge angles of 172° 308 and
responsibility of the user of this standard to establish appro-
130° 008.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
4. Summary of Test Method
2. Referenced Documents 4.1 This test method describes an indentation hardness test
using a calibrated machine to force a pointed, rhombic base,
2.1 ASTM Standards:
pyramidal diamond indenter having specified face angles,
C 730 Test Method for Knoop Indentation Hardness of
2 under a predetermined load, into the surface of the material
Glass
under test and measures the surface projection of the long
C 849 Test Method for Knoop Indentation Hardness of
diagonal of the resulting impression after removal of the load.
Ceramic Whitewares
E 4 Practices for Force Verification of Testing Machines
NOTE 1—A general description of the Knoop indentation hardness test
E 177 Practice for Use of the Terms Precision and Bias in
is given in Test Method E 384. The present test method differs from this
description only in areas required by the special nature of advanced
ASTM Test Methods
ceramics.
E 380 Practice for Use of the International System of Units
4 NOTE 2—This test method is similar to Test Methods C 730 and C 849,
(SI) (the Modernized Metric System)
but differs primarily in the choice of load and the rate of loading. In
E 384 Test Method for Microhardness of Materials
addition, the length correction factor for the resolution limits of optical
E 691 Practice for Conducting an Interlaboratory Study to
microscopes is not utilized.
Determine the Precision of a Test Method
5. Significance and Use
2.2 European Standard:
CEN ENV 843-4 Advanced Technical Ceramics, Mono-
5.1 For advanced ceramics, Knoop indenters are used to
lithic Ceramics, Mechanical Properties at Room Tempera-
create indentations. The surface projection of the long diagonal
ture, Part 4: Vickers, Knoop, and Rockwell Superficial
is measured with optical microscopes.
Hardness Tests
5.2 The Knoop indentation hardness is one of many prop-
2.3 ISO Standard:
erties that is used to characterize advanced ceramics. Attempts
ISO 9385 Glass and Glass Ceramics—Knoop Hardness
have been made to relate Knoop indentation hardness to other
Test
hardness scales, but no generally accepted methods are avail-
able. Such conversions are limited in scope and should be used
3. Terminology
with caution, except for special cases where a reliable basis for
3.1 Definition:
the conversion has been obtained by comparison tests.
5.3 For advanced ceramics, the Knoop indentation is often
preferred to the Vickers indentation since the Knoop long
diagonal length is 2.8 times longer than the Vickers diagonal
This test method is under the jurisdiction of ASTM Committee C-28 on
for the same load, and cracking is much less of a problem (1).
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on
Properties and Performance. On the other hand, the long slender tip of the Knoop indenta-
Current edition approved Jan. 10, 1999. Published April 1999. Originally
tion is more difficult to precisely discern, especially in mate-
published as C 1326 – 96. Last previous edition C 1326 – 96a.
rials with low contrast. The indentation loads chosen in this test
Annual Book of ASTM Standards, Vol 15.02.
Annual Book of ASTM Standards, Vol 03.01.
Annual Book of ASTM Standards, Vol 14.02.
5 7
Available from European Committee for Standardization, Brussels, Belgium. The boldface numbers in parentheses refer to the list of references at the end of
Available from International Standards Organization, Geneva, Switzerland. this test method.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
C 1326 – 99
method are designed to produce indentations as large as may be 7.1.3 The calibration of the balance beam should be checked
possible with conventional microhardness equipment, yet not monthly or as needed. Indentations in standard reference
so large as to cause cracking. materials may also be used to check calibration when needed.
5.4 The Knoop indentation is shallower than Vickers inden- 7.2 Indenter:
tations made at the same load. Knoop indents may be useful in 7.2.1 The indenter shall meet the specifications for Knoop
evaluating coating hardnesses. indenters. See Test Method E 384.
7.2.2 Fig. 1 shows the indenter and its maximum usable
5.5 Knoop hardness is calculated from the ratio of the
applied load divided by the projected indentation area on the dimensions. The diagonals have an approximate ratio of 7:1,
and the depth of the indentation is approximately 1/30 the
specimen surface. It is assumed that the elastic springback of
the narrow diagonal is negligible. (Vickers indenters are also length of the long diagonal. A perfect Knoop indenter has the
used to measure hardness, but Vickers hardness is calculated following angles:
from the ratio of applied load to the area of contact of the four 7.2.2.1 Included longitudinal angle 172° 30 min 00 s.
faces of the undeformed indenter.) 7.2.2.2 Included transverse angle 130° 00 min 00 s.
5.6 A full hardness characterization includes measurements 7.2.3 The constant C (defined in 12.2) for a perfect indenter
p
is 0.07028. The specifications require a variation of not more
over a broad range of indentation loads. A comprehensive
than 1 % from this value.
characterization of this type is recommended but is beyond the
7.2.4 The offset at the indenter tip shall not exceed 1.0 μm.
scope of this test method which measures hardness at a single,
See Test Method E 384.
designated load.
7.2.5 The four faces of the indenter shall meet at sharp
edges.
6. Interferences
7.2.6 The diamond should be examined periodically, and if
6.1 Cracking from the indentation tips can interfere with
it is loose in the mounting material, chipped, or cracked, it shall
interpretation of the exact tip location. The loads chosen for
be replaced.
this test method are sufficiently low that tip cracking, if it
occurs, will cause tiny, rather tight cracks at the indentation tips NOTE 3—This requirement is from Test Method E 384 and is especially
pertinent to diamond indenters that are used to measure hardness of
in advanced ceramics. Such cracks will have a negligible
ceramics. In addition, these indenters sometimes are used to precrack
interference on measurements of the long diagonal length (2)
advanced ceramic specimens at loads higher than customarily used for
(unlike Vickers indentations in ceramics).
hardness testing. Such usage can lead to indenter damage. The diamond
6.2 Cracking or spalling from the sides of the Knoop
indenter can be examined with a scanning electron microscope, or indents
impression may also occur, possibly in a time-dependent
can be made into soft copper to help determine if a chip or crack is present.
manner (minutes or hours) after the impression is made. Small
Indenters may also be inspected with an optical microscope with at least
amounts of such lateral cracking have little or no influence 500X power, but care should be taken to avoid damaging the microscope
lens.
upon measured hardness, provided that the tip impressions are
still readable and the tips are not dislodged (2).
7.3 Measuring Microscope:
6.3 Porosity (either on or just below the surface) may
7.3.1 The measurement system shall be constructed so that
interfere with measuring Knoop hardness, especially if the
the length of the diagonals can be determined with errors not
indentation falls directly onto a large pore or if the indentation
exceeding6 0.0005 mm.
tip falls in a pore.
6.4 At higher magnifications in the optical microscope, it
may be difficult to obtain a sharp contrast between the
indentation tip and the polished surface of some advanced
ceramics. This may be overcome by careful adjustment of the
lighting as discussed in Test Method E 384 and Refs (2, 3).
7. Apparatus
7.1 Testing Machines:
7.1.1 There are two general types of machines available for
making this test. One type is a self-contained unit built for this
purpose, and the other type is an accessory available for
existing microscopes. Usually, this second type is fitted on an
inverted-stage microscope. Descriptions of the various ma-
chines are available (4–6).
7.1.2 Design of the machine should be such that the loading
rate, dwell time, and applied load can be set within the limits
set forth in 10.5. It is an advantage to eliminate the human
element whenever possible by appropriate machine design.
The machine should be designed so that vibrations induced at
the beginning of a test will be damped out by the time the
indenter touches the sample. FIG. 1 Knoop Indenter Showing Maximum Usable Dimensions
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
C 1326 – 99
NOTE 4—Stage micrometers with uncertainties less than this shall be
Technology can be used to verify that an apparatus produces a
used to establish calibration constants for the microscope. See Test
Knoop hardness within6 5 % of the certified value.
Method E 384. Ordinary stage micrometers which are used for determin-
ing the approximate magnification of photographs may be ruled too coarse
10. Procedure
or may not have the required accuracy and precision.
10.1 Specimen Placement—Place the specimen on the stage
7.3.2 The numerical aperture (NA) of the objective lens
of the machine so the specimen will not rock or shift during the
shall be between 0.65 and 0.90.
measurement. The specimen shall be clean and free of any
grease or film.
NOTE 5—The apparent length of a Knoop indentation will increase as
the resolving power and NA of a lens increases. The range of NA specified
10.2 Specimen Leveling:
by this test method corresponds to 40 to 1003 objective lenses. The higher
10.2.1 The surface of the specimen being tested shall lie in
power lenses may have higher resolution, but the contrast between the
a plane normal to the axis of the indenter.
indentation tips and the polished surface may be less.
10.2.2 If one leg (one half) of the long diagonal is more than
10 % longer than the other, or if the ends of the diagonal are not
7.3.3 A filter may be used to provide monochromatic
both in the field of focus, the surface of the specimen may not
illumination. Green filters have proved to be useful.
be normal to the axis of the indenter. Align the specimen
surface properly, and make another indentation.
8. Test Specimens
8.1 The Knoop indentation hardness test is adaptable to a NOTE 7—This tolerance is more stringent than the 20 % that is specified
in Test Method E 384, but is less stringent than requirements in Test
wide variety of advanced ceramic specimens. In general, the
Methods C 730 and C 849.
accuracy of the test will depend on the smoothness of the
surface and, whenever possible, ground and polished speci-
10.2.3 Leveling the specimen to meet these specifications is
mens should be used. The back of the specimen shall be fixed
facilitated if one has a leveling device.
so that the specimen cannot rock or shift during the test.
10.3 Magnitude of Test Load—A test load of either 9.81 N
8.1.1 Thickness—As long as the specimen is over ten times
(1 kgf) or 19.61 N (2 kgf) is specified. 19.61 N is preferred.
as thick as the indentation depth, the test will not be affected.
9.81 N may be used if the higher load is not available on the
In general, if specimens are at least 0.50 mm thick, the
apparatus, if cracking is a problem, or if preferred for a specific
hardness will not be affected by variations in the thickness.
requirement. If cracking is a problem at 9.81 N, lower loads
8.1.2 Surface Finish—Specimens should have a ground and
may be used and the reporting procedure of 12.6 shall be used.
polished surface. The roughness should be less than 0.1 μm
(The precision might be less and the bias greater with the lower
rms. However, if one is investigating a surface coating or loads.)
treatment, one cannot grind and polish the specimen.
10.4 Clean the Indenter—The indenter shall be cleaned
prior to and during a test series. A cotton swab with ethanol,
NOTE 6—This requirement is necessary to ensure that the surface is flat
methanol, or isopropanol may be used. Indenting into soft
and that the indentation is sharp. Residual stresses from polishing are of
copper also may help remove debris.
less concern for most advanced ceramics than for glasses or metals.
References (2) and (7) report that Knoop hardness was insensitive to
NOTE 8—Ceramic powders or fragments from the ceramic test piece
surface finish for surfaces prepared with 1 μm or finer diamond abrasive.
can adhere to the diamond indenter.
Hardness was only affected when the surface finish had an optically
resolvable amount of abrasive damage (7). (Extra caution may be 10.5 Application of Test Load:
appropriate during polishing of transformation toughening ceramics, such
10.5.1 Start the machine smoothly. The rate of indenter
as some zirconias, since the effect upon hardness is not known.)
motion prior to contact with the specimen shall be 0.015 to
0.070 mm/s. If the machine is loaded by an electrical system or
8.1.3 Radius of Curvature—Care should be used when
a dash-pot lever system, it should be mounted on shock
relating hardness values obtained on curved surfaces to those
absorbers which damp out all vibrations by the time the
obtained on polished, flat surfaces. The hardness values ob-
indenter touches the specimen.
tained will be a
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