ISO 21618:2019

INTERNATIONAL ISO
STANDARD 21618
First edition
2019-02
Fine ceramics (advanced ceramics,
advanced technical ceramics) —
Test method for fracture resistance
of monolithic ceramics at room
temperature by indentation fracture
(IF) method
Céramiques techniques (céramiques avancées, céramiques techniques
avancées) — Méthode d’essai pour déterminer la résistance à
la rupture par indentation (IF) des céramiques monolithiques à
température ambiante
Reference number
©
ISO 2019
© ISO 2019
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ii © ISO 2019 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Apparatus . 3
5.1 Testing machine . 3
5.2 Indenter . . 3
5.3 Verification by standard reference materials . 3
5.4 Metallurgical microscope or travelling microscope . 3
6 Test specimen . 3
6.1 Thickness . 3
6.2 Surface finish . 3
7 Procedure. 4
7.1 Specimen placement . 4
7.2 Specimen levelling . 4
7.3 Cleaning of the indenter . 4
7.4 Adjustment of stage movement . 4
7.5 Application of test force . 4
7.6 Acceptability of indentations . 4
7.7 Number of indentations . 5
7.8 Measurement of indentation size . 6
7.9 Measurement of crack size . 6
8 Calculation . 8
9 Test report . 9
Annex A (informative) General machining procedure for test piece preparation .10
Annex B (normative) Adjustment of the axis of travelling-stage and calibration of the
length measurement using the travelling stage .12
Annex C (informative) Interlaboratory evaluation of the indentation fracture (IF) test
procedure .14
Bibliography .16
Foreword
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iv © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 21618:2019(E)
Fine ceramics (advanced ceramics, advanced technical
ceramics) — Test method for fracture resistance of
monolithic ceramics at room temperature by indentation
fracture (IF) method
1 Scope
This document describes a test method that covers the determination of fracture resistance of
monolithic ceramics at room temperature by the indentation fracture (IF) method.
This document is intended for use with dense monolithic ceramics and whisker- or particulate-
reinforced ceramics which are regarded as macroscopically homogeneous. It does not include monolithic
silicon nitride ceramics for bearing balls and continuous-fibre-reinforced ceramics composites.
This document is for material development, material comparison, quality assurance, characterization
and reliability data generation.
Indentation fracture resistance, K , as defined in this document is not to be equated with fracture
I,IFR
toughness determined using other test methods such as K and K .
Isc Ipb
NOTE K is an estimate of a material’s resistance to cracking as introduced by an indenter and has
I,IFR
correlations with wear resistance and rolling contact fatigue performance as well as machining processes, since
[1]-[3]
these properties are governed by the resistance to crack extension in localized damage areas . By contrast,
fracture toughness properties K and K are intrinsic properties of a material and are relevant to macroscopic
Isc Ipb
and catastrophic fracture events with long cracks rather than those phenomena caused by microscopic and
successive damage accumulation associated with short cracks.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 4287, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Terms, definitions
and surface texture parameters
ISO 6507-2, Metallic materials — Vickers hardness test — Part 2: Verification and calibration of testing
machines
ISO 6507-3, Metallic materials — Vickers hardness test — Part 3: Calibration of reference blocks
ISO 14705, Fine ceramics (advanced ceramics, advanced technical ceramics) — Test method for hardness of
monolithic ceramics at room temperature
ISO 17561, Fine ceramics (advanced ceramics, advanced technical ceramics) — Test method for elastic
moduli of monolithic ceramics at room temperature by sonic resonance
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
fracture resistance
measure of resistance of extension of a crack introduced by an indenter
3.2
fracture resistance value
K
I,IFR
value of crack-extension resistance as measured by the IF method
Note 1 to entry: The indentation fracture resistance, K , as used here is not to be equated to fast fracture
I,IFR
toughness K . K is an estimate of a material’s resistance to cracking as introduced by an indenter. K is
Ic I,IFR Ic
considered to be an intrinsic property of a material and is independent of the test method.
4 Principle
This document is for material development, material comparison, quality assurance, characterization and
reliability data generation of dense monolithic ceramics. The method determines the indentation fracture
resistance value, K , from the elastic modulus and indentation force by forcing a Vickers indenter
I,IFR
(diamond pyramid) into the surface of a test piece and measuring the lengths of both the diagonals and
the associated cracks of the indentation that are left in the surface after removal of the indenter (see
[4]
Figure 1) . The method is applicable to the half-penny-crack type but not to the Palmqvist type. The
two types of crack profile can be estimated using the ratio of the crack length, 2c, to the diagonal length,
2a. If 2c/2a is more than 2,5, the crack is half-penny type. But the critical 2c/2a values can be smaller
than 2,5 for some ceramics. In such cases, direct observation of crack morphology by the decoration
technique or serial sectioning of the indented surface is also useful to differentiate the two crack profiles.
The measurement of the crack length and the observation of the crack tips are performed separately in
order to improve the accuracy. The crack length should not be measured in a single optical image because
doing so inevitably limits the magnification that can be used. Travelling microscopy is a solution that
allows both reading of the crack length and detection of the crack tips to be performed at high resolution,
albeit separately. Both an objective lens of 40 × or higher and a calibrated optical stage shall be employed
to ensure reliability. Both international and domestic interlaboratory comparison study (round robin)
projects on the advanced IF method are described in Annex C (see References [5] to [8]).
Key
2a diagonal length of the indent
2c diagonal length of the crack
Figure 1 — Crack lengths and diagonal sizes of the Vickers indentation
2 © ISO 2019 – All rights reserved

5 Apparatus
5.1 Testing machine
The testing machine shall be in accordance with ISO 6507-2. A suitable testing machine capable of
applying an indentation force of 49,03 N up to 196,1 N shall be used.
5.2 Indenter
The indenter shall meet the specification for Vickers indenters. See test method ISO 6507-2. The
diamond should be examined before the test, and if it is loose in the mounting material, chipped or
cracked it shall be replaced.
5.3 Verification by standard reference materials
The reference materials which are in accordance with ISO 6507-3 shall be used to verify the testing
machine and their Vickers hardness shall not vary from the hardness of the material to be measured by
more than 20 %.
5.4 Metallurgical microscope or travelling microscope
The metallurgical microscope equipped with calibrated stage movement or the travelling microscope
shall be employed for both detection of the crack tips and measurement of the crack length. The
magnification of the objective lens shall be 40 × or 50 × and the total magnification shall be 400 × or
more. The travelling stage shall move both vertically and horizontally and the readout resolution of the
table position shall be 1 μm or less. The coaxial vertical illumination with enough light intensity shall
be used for the observation of the crack tips.
6 Test specimen
6.1 Thickness
The thickness of the specimen shall be large enough so that the crack lengths are not affected by
variations in the thickness. As long as the thickness of the specimen is more than five times the crack
length (2c), the test will not be affected. In general, a specimen thickness of more than 3 mm is suitable.
6.2 Surface finish
Specimens shall have a ground and polished surface so that the crack lengths can be measured
accurately. The surface roughness, Ra, as defined in ISO 4287, shall be not more than 0,1 μm. Any
grinding-induced damage layer at the surface shall be removed completely by polishing so that the
crack lengths are not affected by any residual stress at the surface layer. The area fraction of tiny pits
due to dropout of a grain or fragmentation shall be as small as possible to enable the precious crack-
length measurement.
NOTE Annex A provides further information about a typical machining procedure.
The IF method is only applicable to dense ceramics. But if it is necessary to evaluate slightly porous
ceramics through the IF method, the porosity shall be described in the test report to show the precision
of measurements, because ceramics with some small porosity can experience local densification
beneath the indenter, which reduces the driving force for crack formation and results in overestimation
of K .
I,IFR
7 Procedure
7.1 Specimen placement
Place the specimen on the stage of the machine so that the specimen will not rock or shift during the
measurement. The specimen surface shall be clean and free from any grease or film.
7.2 Specimen levelling
The surface of the specimen being tested shall lie in a plane normal to the axis of the indenter.
7.3 Cleaning of the indenter
The indenter shall be cleaned prior to and during a test series. A cotton swab with ethanol, methanol or
isopropanol may be used. Indenting into soft copper can also help remove debris.
7.4 Adjustment of stage movement
Ensure that the horizontal movement of the stage of optics is coincident with the horizontal direction
in the field of optics. The magnitude of the stage movement shall be calibrated with an objective
micrometer. The procedures for the adjustment of the stage axis and for the length calibration shall be
carried out as specified in Annex B.
7.5 Application of test force
Indentations shall be made using a Vickers indenter under the following conditions:
Force: 196,1 N
Dwell time: 15 s
If indentations made at the test force of 196,1 N lead to no acceptable indentations (see Figure 2), use a
lower test force of 98,07 N or 49,03 N. The test force employed shall be described in the test report to
show the precision of measurements.
NOTE K measured at a force of 98,07 N or 49,03 N can be slightly smaller than that obtained at 196,1 N,
I,IFR
especially for those silicon nitrides with self-reinforced microstructures which produce rising R-curve behaviour.
Also, the accuracy of the measurement of crack length can become worse when the indentation size gets smaller
at the test force of 98,07 N or 49,03 N.
7.6 Acceptability of indentations
Only indentations whose four primary cracks emanate straight and radially from each corner shall
be accepted. Indentations with spalled edges, badly asymmetrical, split or forked cracks or gross
chipping shall be rejected, see Figure 2. If the difference between the horizontal crack length and the
vertical length is more than 10 % of the mean value of the horizontal and vertical lengths, the result
shall be rejected. If 2c/2a is less than 2,5, the results shall be rejected since the crack could
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