ISO 28079:2009

INTERNATIONAL ISO
STANDARD 28079
First edition
2009-07-15

Hardmetals — Palmqvist toughness test
Métaux durs — Méthode d'essai de dureté de Palmqvist




Reference number
ISO 28079:2009(E)
©
ISO 2009

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ISO 28079:2009(E)
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ii © ISO 2009 – All rights reserved

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ISO 28079:2009(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Symbols and units. 1
4 Test pieces and sample preparation. 2
4.1 Test piece size and sampling . 2
4.2 Surface preparation. 2
4.3 Surface condition. 2
5 Apparatus . 3
5.1 General. 3
5.2 Indentation. 3
5.3 Indentation and crack measurement . 3
6 Procedure and conditions of testing . 3
6.1 Indentations. 3
6.2 Indentation and crack length measurement . 3
6.3 Test validity . 5
7 Analysis . 6
7.1 Vickers hardness . 6
7.2 Toughness. 6
8 Measurement uncertainty . 6
9 Test report . 7
Annex A (informative) Report pro forma — Palmqvist toughness measurements on hardmetals . 8
Bibliography . 10

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ISO 28079:2009(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 28079 was prepared by Technical Committee ISO/TC 119, Powder metallurgy, Subcommittee SC 4,
Sampling and testing methods for hardmetals.

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ISO 28079:2009(E)
Introduction
Good test methods are those which enable a user or manufacturer to clearly discriminate between different
materials.
Fracture toughness values are required for three reasons:
a) for product design and performance assessment;
b) for selection of materials;
c) for quality control.
1)
A specific International Standard for the toughness of hardmetals has not been developed to date, primarily
because of the difficulty of introducing stable precracks into these tough but hard materials. However,
Palmqvist tests for toughness are widely used because of their perceived apparent simplicity. Cracks are
formed at the corners of Vickers hardness indentations and these can be used to calculate a nominal surface
toughness value. This value is sensitive to the method of measurement and to the method of surface
preparation of the sample. This International Standard outlines good practice to minimize uncertainties due to
these issues.
There are several possible methods for the measurement of the fracture toughness of hardmetals. The results
−3/2 −2
can be expressed either as a stress intensity factor, in MN·m , or as a fracture surface energy, in J·m .
−3/2 −3/2
The range of values for typical WC/Co hardmetals is from 7 MN·m to 25 MN·m . There is a general
inverse trend of hardness against fracture toughness (see [1] and [2] in the Bibliography).
When applied unqualified to hardmetals, “toughness” can have several meanings.
−3/2
a) Plane-strain fracture toughness, K , in MN·m , is a value obtained from tests on specimens with
Ic
appropriate geometries for plane-strain conditions and containing a well-defined geometry of crack. There
is no standard method for hardmetals and different organizations use different test methods for
introducing the precrack.
b) Strain-energy release rate (or work of fracture), G, is an alternative expression for toughness, often
2 2
obtained by converting plane-strain toughness, K, to G [i.e. G = K /E(1 − ν ), where E is Young's modulus
−2
and ν is Poisson's ratio]. G has units of J·m . Again, there is no standard method.
c) Palmqvist toughness, W, is a value obtained by measuring the total length of cracks emanating from the
four corners of a Vickers hardness indentation. For a given indentation load, the shorter the crack, the
tougher the hardmetal.
d) Finally, toughness is also widely used, in a loose sense, to describe the empirical relation between
perceived resistance to dynamic impacts. This is neither standardized nor quantified, but is clearly
important for many industrial applications of hard materials. Also, principally for hardmetals, it can be
more realistically assessed through either fatigue tests or high-rate strength tests, rather than a
conventional fracture toughness test.

1) Terminology — There is a range of terms used for this type of material, especially including cemented carbides and/or
cermets, as well as hardmetals. The word “hardmetals” has been used in this document. It includes all hard materials
based on carbides that are bonded with a metal. In ISO 3252 terminology, “hardmetal” is stated to be “a sintered material
characterized by high strength and wear resistance, comprising carbides of refractory metals as the main component
together with a metallic binder phase”. “Cemented carbide” is synonymous with “hardmetal”. A “cermet” is defined as “a
sintered material containing at least one metallic phase and at least one non-metallic phase, generally of a ceramic
nature”.
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ISO 28079:2009(E)
There is a considerable body of published information on Palmqvist toughness tests for hardmetals (see [5] to
[29] in the Bibliography). Palmqvist toughness, W, is a toughness value obtained by measuring the crack
lengths at the corners of a Vickers indentation. It can be evaluated by making indentations either at a single
load, usually 30 kgf, or from the inverse of the slope of a plot of crack length against load for a range of
applied loads. For hardmetals, the crack depth profile is normally of the Palmqvist type, i.e. independent
shallow arcs emanating from each indentation corner. The measurement of surface crack length is, however,
open to operator error. It is widely recognized that test surfaces are carefully prepared to remove the effects of
residual surface stresses (see [8] in the Bibliography). The test also has a poor fracture-mechanics pedigree
because of the uncertainties associated with residual stresses introduced by the indentation.
One advantage of the Palmqvist method is that parallel measurements are made of sample hardness, which
is required for quality-control purposes. The crack length, and thus toughness measurements, do not therefore
require much more effort and can yield equally useful material characterization data, provided the
measurements are obtained carefully in line with the methods proposed in this International Standard.
This International Standard is based on a “Good Practice Guide for the Measurement of Palmqvist
Toughness” published by the UK National Physical Laboratory in 1998. This International Standard
recommends good practice to minimize levels of uncertainty in the measurement process. The procedure has
2)
been validated through underpinning technical work within the VAMAS framework (see [29] in the
Bibliography). An interlaboratory exercise was conducted to generate underpinning technical information on
toughness tests for hardmetals. More than ten industrial organizations participated, either by correspondence,
supply of materials or by conducting tests. Eight organizations were able to complete Palmqvist tests. Good
statistics were obtained on the Palmqvist data that enabled a quantitative assessment of uncertainties to be
performed for this relatively simple test. Single-edge precracked beam data was thought to be closest to the
“true” value, and the mean values from the Palmqvist test data compared reasonably well with these results.
However, care was needed in test piece preparation to ensure a good correlation between data from the
Palmqvist tests and the single-edge precracked beam results.


2) VAMAS, Versailles Project on Advanced Materials and Standards, supports trade in high technology products through
international collaborative projects aimed at providing the technical basis for drafting codes of practice and specifications
for advanced materials. The scope of the collaboration embraces all agreed aspects of enabling science and technology,
i.e. databases, test methods, design methods, and materials technology, which are required as a precursor to the drafting
of standards for advanced materials. VAMAS activity emphasises collaboration on pre-standards measurement research,
intercomparison of test results, and consolidation of existing views on priorities for standardization action. Through this
activity, VAMAS fosters the development of internationally acceptable standards for advanced materials by the various
existing standards agencies.
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INTERNATIONAL STANDARD ISO 28079:2009(E)

Hardmetals — Palmqvist toughness test
1 Scope
This International Standard specifies a method for measuring the Palmqvist toughness of hardmetals and
cermets at room temperature by an indentation method. This International Standard applies to a measurement
of toughness, called Palmqvist toughness, calculated from the total length of cracks emanating from the
corners of a Vickers hardness indentation, and it is intended for use with metal-bonded carbides and
carbonitrides (normally called hardmetals, cermets or cemented carbides). The test procedures proposed in
this International Standard are intended for use at ambient temperatures, but can be extended to higher or
lower temperatures by agreement. The test procedures proposed in this International Standard are also
intended for use in a normal laboratory-air environment. They are not intended for use in corrosive
environments, such as strong acids or seawater.
2 Normative references
The following referenced documents are indispensable for the application 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 3878, Hardmetals — Vickers hardness test
3 Symbols and units
For the purposes of this document, the following symbols and units apply.
Symbol Designation Unit
A Constant of value 0,002 8 —
d Indentation-diagonal mean value mm
d , d Indentation-diagonal individual values mm
1 2

−2
E Young's modulus N·mm
F Indentation load (toughness calculations) N
−2
G Strain-energy release rate J·m

−2
H Hardness kgf·mm
−2
HV(P) Vickers hardness at load P (kgf) kgf·mm
−3/2
K Plane-strain fracture toughness MN·m
lc
A Crack length at indent corner mm
n
P Indentation load (Vickers hardness method) kgf
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ISO 28079:2009(E)
Symbol Designation Unit
T Total crack length mm
t Tip-to-tip crack length mm
n
−1 −2
W Palmqvist toughness N·mm or J·m ;
G
−1 −2
1 N·mm = 1 000 J·m
−3/2
W Palmqvist fracture toughness MN·m
K
ν Poisson's ratio —
4 Test pieces and sample preparation
4.1 Test piece size and sampling
Any test piece shape can be used, provided that it can be prepared with a flat surface and a flat opposing face
for making the indentation. Hot mounting in a press gives flat and parallel faces. Cold mounting does not give
flat and parallel faces.
Diamond slicing or electrospark discharge machines are convenient to use for this purpose. However, the
surfaces shall then be polished. It is recommended that 0,2 mm of material be removed before the final polish
to ensure that material typical of the bulk is tested. For example, the ISO Vickers hardness test for hardmetals
(see ISO 3878) specifies the removal of 0,2 mm.
NOTE It has also been suggested, in a dissertation by M. Heinonen (UMIST) [19], that the test piece should be at
least as thick as ten times the crack length. Thinner test pieces might not give representative results because the stress
state will be dependent on the amount of material supporting the indentation and its associated cracks. It can be
convenient to mount the test pieces in hot-setting resins to directly provide flat and parallel faces. However, if the test
pieces are to be subsequently annealed to remove surface residual stresses then this can be a disadvantage, since the
test piece has to be removed from the mount to put it in the annealing furnace (typically 800 °C for 1 h in a vacuum).
4.2 Surface preparation
It is essential to pr
...