ISO 21222:2020

INTERNATIONAL ISO
STANDARD 21222
First edition
2020-01
Surface chemical analysis — Scanning
probe microscopy — Procedure for
the determination of elastic moduli
for compliant materials using atomic
force microscope and the two-point
JKR method
Analyse chimique des surfaces — Microscopie à sonde locale —
Lignes directrices pour la détermination des modules d’élasticité des
matériaux souples en utilisant un microscope à force atomique
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols (and abbreviated terms) . 4
5 Review of contact mechanics . 4
5.1 Introduction . 4
5.2 Hertzian model . 5
5.3 Derjaguin-Muller-Toporov (DMT) Model . 5
5.4 Johnson-Kendall-Roberts (JKR) model . 6
5.5 JKR–DMT transition . 6
6 Procedure of determination of elastic modulus . 6
6.1 Introduction and limitations . 6
6.2 Measurement of deflection sensitivity and spring constant . 7
6.3 Measurement of tip radius . 7
6.4 Measurement of force-distance curve . 7
6.5 Force-distance curve conversion . 8
6.6 JKR two-point method . 9
6.7 Uncertainties . 9
6.8 Reporting results . 9
Annex A (informative) Example measurements .11
Annex B (informative) Result of Inter-laboratory Comparison .15
Bibliography .17
Foreword
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This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 9, Scanning probe microscopy.
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iv © ISO 2020 – All rights reserved

Introduction
Atomic force microscope (AFM) is a member of the scanning probe microscope (SPM) family and is
used to image surfaces by mechanically scanning a probe over the surface. In AFM, a surface force is
monitored as the deflection of a compliant cantilever, which has a probe tip at its free end in order to
interact with surfaces. AFM can provide amongst other data: topographic, mechanical and chemical
information about a surface depending on the mode of operation and the property of the probe tip.
Accurate force measurements and sample deformation measurements are needed for a wide variety
of applications, especially to determine the elastic moduli of compliant materials such as organics
and polymers at surfaces. For quantitative force measurements, it is necessary to select an adequate
contact mechanic model used to calculate the elastic modulus, and also use the appropriate calculation
procedure.
This document describes a procedure for the determination of the elastic moduli for compliant materials
using AFM. Force-distance curves are obtained on the surfaces of compliant materials and are used for
the calculation of elastic modulus based on Johnson-Kendall-Roberts (JKR) two-point method.
INTERNATIONAL STANDARD ISO 21222:2020(E)
Surface chemical analysis — Scanning probe microscopy
— Procedure for the determination of elastic moduli for
compliant materials using atomic force microscope and the
two-point JKR method
1 Scope
This document describes a procedure for the determination of elastic modulus for compliant materials
using atomic force microscope (AFM). Force-distance curves on the surface of compliant materials are
measured and the analysis uses a two-point method based on Johnson-Kendall-Roberts (JKR) theory.
This document is applicable to compliant materials with elastic moduli ranging from 100 kPa to 1 GPa.
The spatial resolution is dependent on the contact radius between the AFM probe and the surface and is
typically approximately10-20 nm.
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 18115-2, Surface chemical analysis — Vocabulary — Part 2: Terms used in scanning-probe microscopy
ISO 11775, Surface chemical analysis — Scanning-probe microscopy — Determination of cantilever normal
spring constants
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 18115-2 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
force-distance curve
force-displacement curve
pairs of force and distance values resulting from a mode of operation in which the probe is set
at a fixed (x, y) position and the probe tip is moved towards or away from the surface as the force is
measured
Note 1 to entry: The force is usually monitored using the cantilever deflection.
[SOURCE: ISO 18115-2:2013, 5.56]
3.2
normal spring constant
spring constant
force constant
k
z
quotient of the applied normal force at the probe tip by the deflection of the cantilever in that
direction at the probe tip position
Note 1 to entry: The normal spring constant is usually referred to as the spring constant. The full term is used
when it is necessary to distinguish it from the lateral spring constant.
Note 2 to entry: The force is applied normal to the plane of the cantilever to compute or measure the normal force
constant, k . In application, the cantilever in AFM can be tilted at an angle, θ, to the plane of the sample surface
z
and the plane normal to the direction of approach of the tip to the sample. This angle is important in applying the
normal spring constant in AFM studies.
[SOURCE: ISO 18115-2:2013, 5.92, modified — Note 1 to entry has been deleted and the following notes
renumbered.]
3.3
pull-in force
pull-on force
force exerted by the surface on the probe tip at snap-in
[SOURCE: ISO 18115-2:2013, 5.123]
3.4
pull-off force
force required to pull the probe free from the surface
Note 1 to entry: This force is generally measured from the force-distance curve as the value between the force
minimum and the zero of force as the probe moves away from the surface.
[SOURCE: ISO 18115-2:2013, 5.124]
3.5
tip radius
radius describing the surface curvature in a region at the apex of
a stylus or probe tip
Note 1 to entry: It might be necessary to describe the tip by radii in different azimuths.
Note 2 to entry: In practice, tips can only approximate a sphere for a very small region at the tip.
[SOURCE: ISO 18115-2:2013, 5.161]
3.6
tip-sample contact radius
maximum radius of the contact area between the tip and the sample at the maximum indentation depth
[SOURCE: ISO 18115-2:2013, 5.163]
3.7
work of adhesion
energy required when two condensed phases, forming an interface of unit area, are separated reversibly
to form unit areas of the free surfaces of those two phases
Note 1 to entry: This term is sometimes also known as the work of separation or the Dupré work of adhesion.
[SOURCE: ISO 18115-2:2013, 5.175]
2 © ISO 2020 – All rights reserved

3.8
Hertzian model
model of tip and surface contact between elastic solids that ignores any surface forces and adhesion
hysteresis
Note 1 to entry: This approach, derived by Hertz and described in Reference [1] describes the contact between
elastic solids. It ignores any surface forces and adhesion hysteresis and applies at high loads where there are no
surface forces present.
[SOURCE: ISO 18115-2:2013, 4.4]
3.9
DMT model
Derjaguin-Müller-Toporov model
model of tip and surface contact in which adhesion forces are taken into account but the tip-sample
[2]
geometry is constrained to be Hertzian
Note 1 to entry: This approach applies to rigid systems with low adhesion and small radii of curvature. The
adhesion forces are taken in
...