ASTM C849-88(2020)

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: C849 − 88 (Reapproved 2020)
Standard Test Method for
Knoop Indentation Hardness of Ceramic Whitewares
This standard is issued under the fixed designation C849; 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 pyramidal diamond indenter having specified face angles,
under a predetermined load, into the surface of the material
1.1 This test method covers the determination of the Knoop
under test and to measure the long diagonal of the resulting
indentation hardness of ceramic whitewares and the verifica-
impression after removal of the load.
tion of Knoop indentation hardness testing machines using
standard glasses.
NOTE 1—Ageneral description of the Knoop indentation hardness test
is given in Test Method E384. The present method differs from this
1.2 This standard does not purport to address all of the
description only in areas required by the special nature of ceramic
safety concerns, if any, associated with its use. It is the
whitewares.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter- 5. Significance and Use
mine the applicability of regulatory limitations prior to use.
5.1 The Knoop indentation hardness is one of many prop-
1.3 This international standard was developed in accor-
ertiesthatisusedtocharacterizeceramicwhitewares.Attempts
dance with internationally recognized principles on standard-
havebeenmadetorelateKnoopindentationhardnesstotensile
ization established in the Decision on Principles for the
strength, grinding speeds, and other hardness scales, but no
Development of International Standards, Guides and Recom-
generally accepted methods are available. Such conversions
mendations issued by the World Trade Organization Technical
arelimitedinscopeandshouldbeusedwithcaution,exceptfor
Barriers to Trade (TBT) Committee.
specialcaseswhereareliablebasisfortheconversionhasbeen
obtained by comparison tests.
2. Referenced Documents
2.1 ASTM Standards:
6. Apparatus
E4Practices for Force Verification of Testing Machines
6.1 Testing Machines:
E384Test Method for Microindentation Hardness of Mate-
6.1.1 There are two general types of machines available for
rials
making this test. One type is a self-contained unit built for this
3. Terminology
purpose,andtheothertypeisanaccessoryavailabletoexisting
microscopes. Usually, this second type is fitted on an inverted-
3.1 Definitions:
stage microscope. Good descriptions of the various machines
3.1.1 Knoop hardness number (KHN)—a number obtained
3,4
are available.
by dividing the applied load in kilograms-force by the pro-
6.1.2 Designofthemachineshouldbesuchthattheloading
jected area of the indentation in square millimetres, computed
rate, dwell time, and applied load can be standardized within
from the measured long diagonal of the indentation and the
the limits set forth in 3.1.1. It is an advantage to eliminate the
included edge angles of the diamond; it is assumed that the
human element whenever possible by appropriate machine
indentation is an imprint of the undeformed indenter.
design. The machine should be designed so that vibrations
4. Summary of Test Method
induced at the beginning of a test will be damped out by the
time the indenter touches the sample.
4.1 This test method describes an indentation hardness test
6.1.3 Thecalibrationofthebalancebeamshouldbechecked
using a calibrated machine to force a pointed, rhombic-base,
monthlyorasneeded.Indentationsinstandardglassesarealso
used to check calibration when needed.
ThistestmethodisunderthejurisdictionofASTMCommitteeC21onCeramic
Whitewares and Related Productsand is the direct responsibility of Subcommittee
6.2 Indenter:
C21.03 on Methods for Whitewares and Environmental Concerns.
Current edition approved Nov. 1, 2020. Published December 2020. Originally
approved in 1976. Last previous edition approved in 2016 as C849–88(2016).
DOI: 10.1520/C0849-88R20. SpecificationsforKnoopindenterscanbefoundinSmall,L., Hardness Theory
For referenced ASTM standards, visit the ASTM website, www.astm.org, or and Practice, Part 1, Service Diamond Tool Co., Ann Arbor, MI, 1960,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM pp.241–243.
Standards volume information, refer to the standard’s Document Summary page on Mott, B. W., Micro-Indentation Hardness Testing, Butterworth’s Scientific
the ASTM website. Publications, London, 1956.
Copyright ©ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA19428-2959. United States
C849 − 88 (2020)
6.2.1 The indenter shall meet the specifications for Knoop 7.1.2 Surface Finish—Aspointedoutabove,theaccuracyof
indenters. See Test Method E384. the test depends on the surface finish. However, if one is
6.2.2 Fig. 1 shows the indenter and its maximum usable
investigating a surface coating or treatment, he cannot grind
dimensions. The diagonals have an approximate ratio of 7:1, and polish the sample. Experience has shown that six inden-
and the depth of the indentation is about ⁄30 the length of the
tations on a ground and polished surface of glass will repro-
long diagonal. A perfect Knoop indenter has the following
duce within 61%. Six indentations on an “as-received”
angles:
surfacemaybeasbadas 610%.Groundandpolishedsurfaces
6.2.2.1 Included longitudinal angle 172° 30 min 00 s.
should be used. If this is not possible, the number of indenta-
6.2.2.2 Included transverse angle 130° 00 min 00 s.
tions should be increased.
6.2.3 Theconstant C foraperfectindenteris0.07028,and
p
7.1.3 Radius of Curvature—The KHN obtained will be
thespecificationsrequireavariationofnotmorethan1%from
affected even when the curvature is only in the direction of the
this value.
shortdiagonal.CareshouldbeusedwhenrelatingKHNvalues
6.3 Measuring Microscope—The measurement system shall obtained on curved surfaces to those obtained on polished flat
be so constructed that the length of the diagonals can be surfaces.
determined with errors not exceeding 60.0005 mm. The
apparent length of the diagonal is corrected in the calculations
8. Preparation of Apparatus
for the limit of resolution of the objective being used in the
8.1 Verification of Load—Most of the machines available
microscope.
for Knoop hardness testing use a loaded beam. This beam
should be tested for zero load. An indentation should not be
7. Test Specimens
visible with zero load, but the indenter should contact the
7.1 The Knoop indentation hardness test is adaptable to a
sample.Avisibleindentationshouldbeobtainedwithaloadof
wide variety of ceramic whiteware specimens. In general, the
0.1 gf. Other methods of verifying the load application are
accuracy of the test will depend on the smoothness of the
given in Practice E4.
surface and, whenever possible, ground and polished speci-
8.2 Verification by Standard Glasses—Table 1 gives the
mens should be used. The back of the specimen shall be fixed
so that the specimen cannot rock or shift during the test. Knoop hardness of several National Institute of Standards and
Technology (NIST) standard glasses. Knoop hardness mea-
7.1.1 Thickness—As long as the specimen is over ten times
as thick as the indentation depth, this will not affect the test. In surement on a piece of one of these glasses that has been
general, if specimens are at least 0.10 mm thick, the hardness ground and polished within the last 24 h should agree with the
will not be affected by variations in the thickness. value in the table 65%. Tests should be made using 100 gf.
FIG. 1 Knoop Indenter Showing Maximum Usable Dimension
C849 − 88 (2020)
A,B
TABLE 1 Knoop Hardness of NIST Standard Glasses
GE Fused GE Fused
NIST 710 NIST 711 NIST 71
...