ASTM D7187-20

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Designation: D7187 − 15 D7187 − 20
Standard Test Method for
Measuring Mechanistic Aspects of Scratch/Mar Behavior of
Paint Coatings by Nanoscratching
This standard is issued under the fixed designation D7187; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers thea nanoscratch method for determining the resistance of paint coatings on smooth flat surfaces
to scratch/mar.
1.2 PreviousOther methods used in scratch/mar evaluation first physically scratch or mar a sample’s surface with single or
multiple contact cutting, and then use visual inspection to assign a ranking. It has been recognized that loss of appearance is mainly
due to surface damages created. The philosophy of this method is to damage created. This method quantitatively and objectively
measuremeasures scratch/mar behavior by making the evaluation process two steps with emphasis on surface damages.damage.
Step one is to find the relationship between damage shape and size and external input (such as forces, contact geometry, and
deformation). Step two is to relate damage shape and size to visual loss of luster. The first step is covered by this method; in
addition, a survey in the appendix provides an example of an experiment to relate the damage to the change in luster.
1.3 There are three elementary deformation mechanisms: elastic deformation, plastic deformation and fracture; only the latter
two both contribute significantly to mar. This method evaluates scratch/mar based on the latter two damage mechanisms.
1.4 Although this standard was developed for paint coatings, it can also be applied to other types of similar polymer-based
coatings, for example, lacquers, varnishes, glazes and other decorative and protective layers deposited on hard substrates.
1.5 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.6 This standard does not purport to address all of the safety problems,concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and
determine the applicability of regulatory limitations prior to use.
1.7 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.
2. Referenced Documents
2.1 ASTM Standards:
D609 Practice for Preparation of Cold-Rolled Steel Panels for Testing Paint, Varnish, Conversion Coatings, and Related Coating
Products
D823 Practices for Producing Films of Uniform Thickness of Paint, Coatings and Related Products on Test Panels
D1005 Test Method for Measurement of Dry-Film Thickness of Organic Coatings Using Micrometers
D1044 Test Method for Resistance of Transparent Plastics to Surface Abrasion by the Taber Abraser
D3363 Test Method for Film Hardness by Pencil Test (Withdrawn 2020)
D3924 Specification for Standard Environment for Conditioning and Testing Paint, Varnish, Lacquer, and Related Materials
D5178 Test Method for Mar Resistance of Organic Coatings
D6037 Test Methods for Dry Abrasion Mar Resistance of High Gloss Coatings
D6279 Test Method for Rub Abrasion Mar Resistance of High Gloss Coatings
This test method is under the jurisdiction of ASTM Committee D01 on Paint and Related Coatings, Materials, and Applications and is the direct responsibility of
Subcommittee D01.23 on Physical Properties of Applied Paint Films.
Current edition approved Feb. 1, 2015June 1, 2020. Published April 2015June 2020. Originally approved in 2005. Last previous edition approved in 20102015 as D7187
– 10.– 15. DOI: 10.1520/D7187-15.10.1520/D7187-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For ASTM Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7187 − 20
D7027 Test Method for Evaluation of Scratch Resistance of Polymeric Coatings and Plastics Using an Instrumented Scratch
Machine
D7091 Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals
and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals
G171 Test Method for Scratch Hardness of Materials Using a Diamond Stylus
3. Summary of Test Method
3.1 This test method is based on representative samples of the paint film being scratched using a nanoscratch instrument. From
information receivedgenerated during a scratch test, values for plastic resistance (PR) and fracture resistance (FR) can be
determined. From these values, the mechanistic aspects of scratch/mar behavior of the coating can be compared.
3.2 From these values of plastic resistance and fracture resistance, the mechanistic aspects of scratch/mar behavior of the
coating can be subsequently compared.
4. Significance and Use
4.1 This test attempts to address two major drawbacks addresses two limitations in existing mar tests such as Test Methods
D1044, D3363, D5178, D6037, and D6279, namely:
4.1.1 Measured damage is caused by hundreds of contacts with differing contact geometries making it difficult or impossible
for mechanical quantities (force, displacement) at the contact points to be reliably determined.
4.1.2 The damage is evaluated using subjective visual assessments, which provide only a qualitative sense of wear with little
information about mar mechanisms.
4.2 This test provides a quantitative assessment of a paint coating’s mechanistic aspects of scratch/mar behavior in various
conditions. The ability to control testing variables such as rate loading rate, speed, and temperature allow the study of the
scratch/mar behavior in a variety of environments.
4.3 This test method is particularly suitable for measurement of paint coatings on laboratory test panels.
4.4 The accuracy and precision of scratch/mar performance may be significantly influenced by coating surface
nonuniformitynon-uniformity and irregularities.
4.5 A correlation has been observed between good mar resistance in field studies and a combination of high Plastic Resistance
and high Fracture Resistance (terms are defined below). plastic resistance and high fracture resistance. When coatings have had
either high Plastic Resistanceplastic resistance and low Fracture Resistance,fracture resistance, or low Plastic Resistanceplastic
resistance and high Fracture Resistance,fracture resistance, there have been contradictory results in field studies.
4.6 Mar resistance characterizes the ability of the coating to resist light damage. The difference between mar and scratch
resistance is that mar is related to only the relatively fine surface scratches which spoil the appearance of the coating. The
mechanistic aspects of mar resistance depend on a complex interplay between visco-elastic and thermal recovery, yield or plastic
flow, and micro-fracture. Polymers are challenging because they exhibit a range of mechanical properties from near liquid through
rubber materials to brittle solids. The mechanical properties are rate and temperature dependent and visco-elastic recovery can
cause scratches to change with time. One such test for evaluating polymeric coatings and plastics is Test Method D7027.
4.7 Since this method measures mechanical qualities, such as forces and displacements (deformations) during the damage
making process, rate dependence, temperature dependence, and visco-elastic-plastic recovery can be further investigated and visual
impacts of damage can be related to deformation mechanisms.
5. Apparatus
5.1 Paint Application Equipment, as described in Practices D609 and D823.
5.2 Nanoscratch Instrument, consisting of an instrument with a well-defined indenter, which translates perpendicular to the
coating surface and has the capacity to produce an instrumented scratch of controlled and variable normal force and continuously
measured displacement during testing. The normal force mustshall be feedback controlled, in order to quickly respond to variations
in surface morphology. The force of the instrument should have a maximum normal force of at least 50 mN (mN should be read
as milli-Newtons) with a resolution of at least 0.1 mN. 50 mN 6 0.1 mN. The maximum tangential force, if measured, should be
at least 50 mN with a resolution of at least 6 0.5 mN. The range of the displacement sensors should be at least 50200 μm with
a resolution of at least 20 nm. Displacement and tangential force response of the coating should be measured with a high data
acquisition rate, such as a maximum of five5 μm between data points.
5.3 Suggested Range for Testing Parameters:
5.3.1 Indenter size should range from 11 μm to 100 microns 100 μm and should be spherical in geometry. Indenter material
should be diamond.
5.3.2 The scratch should be applied at a rate of 0.50.5 mm to 10 millimetres 10 mm per minute.
5.3.3 The loading rate of the normal force should be applied at 55 mN to 200 mN 200 mN per minute.
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5.3.4 The scanning preload should be conducted with an applied force of 0.10.1 mN to 1 mN.1 mN.
5.4 The following is an example of one particular application of the test ranges. This example is based on automotive clear coats
on a metal substrate.
5.4.1 Indenter size of 2 microns.2 μm.
5.4.2 Scratch ratespeed of 3 millimetres 3 mm per minute.
5.4.3 Loading rate of 40 mN per minute.
5.4.4 Scanning preload of 0.2 mN.
5.4.5 Data acquisition rate of 3 μm between data points.
NOTE 1—To optimize test parameters for a particular coating, it should be remembered thatexperiments may need to be conducted as different
combinations of applied load and indenter radius will cause differing damage in polymeric coatings. A smaller indenter radius (sharper tip) will tend to
cut the coating and apply a higher contact pressure, whereas a larger indenter radius (blunter tip) will tend to tear the coating and apply a lower contact
pressure.
6. Test Specimen
6.1 The substrate for the paint coating should be a smooth, plane, rigid surface, such as those specified in Practices D609 and
D823.
6.2 The thickness of the coating being tested, determined in accordance with either Test Methods D1005, or D7091, should be
uniform within 500 nm. In order to minimize the effect of the substrate for maximum accuracy, the penetration depth should not
exceed one-half the 5 % of coating thickness.
6.3 At least three scratches shouldshall be performed on each test specimen.
6.4 The surface of the specimens shouldshall be free of any dirt and oils. Care should be taken when cleaning samples: solvents
should not be used as they may modify the surface properties of polymer-based coatings. For removing dust, it is recommended
to blow off particulates with compressed air from a clean source (without oil contamination).
6.5 The specimen size should be sufficient to be adequately secured to the nanoscratch instrument, but not so small as to
interfere with the movement of the indenter tip or its supporting cantilever.
NOTE 2—It is recommended that substrates with similar compliances be used when comparing different coatings.
FIG. 1 Example - Typical Data from a Nanoscratch Experiment C1 – Vertical Displacement of the Indenter During the Pre-scan C2 – Ver-
tical Displacement During Post-scan C3 – Vertical Displacement During Scratch C4 – Tangential Force C5 – Applied Normal ForceTest
(1)
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7. Conditioning
7.1 Cure the coated test specimens under conditions agreed upon between the purchaser and seller that reflect the conditions
of curing of the paint in actual service.
7.2 Condition and test the test specimens at 2323 °C 6 2ºC (73.52 °C (73.5 °F 6 3.5ºF)3.5 °F) and a relative humidity of
5050 % 6 5 % for at least 24 h, unless the purchaser and seller agree on more suitable test characteristics, as specified in the
Standard Atmosphere of Specification D3924.
8. Procedure
8.1 Secure the specimen to the moveable stage on the instrument with the surface to be measured located perpendicularly to
the indenter tip. Ensure the panel is held rigidly to the stage and cannot be moved by the action of the subsequent scratch test.
8.2 Carefully move this area under the indenter and bring the indenter tip close to the sample surface.
8.3 The complete scratch test consists of three distinct steps. In all three steps, the indenter follows the exact same path across
the sample surface.
NOTE 3—A set of sample test parameters can be found in 5.4.
8.3.1 Perform a prescan to measure the topography of the undamaged coating. Apply the lowest load that the instrument can
apply but that makes no permanent damage. The prescan, scratch, and postscan should all must be performed on the same line.
8.3.2 Instruct the instrument to begin making a scratch to produce damage to the coating. Allow the instrument to ramp to the
desired normal force at a controlled rate. At the end of the scratch, return the indenter tip to its starting position at the beginning
of the scratch.
8.3.3 Perform Without delay perform a postscan, where the indenter tip is scanned along the scratch, measuring the residual
topography of the damaged area. This should be done with the lowest load the instrument can apply.
NOTE 4—Prescan and postscan should only be used if the instrument has force feedback control, otherwise significant error may be incurred.
NOTE 5—The prescan and postscan need to be conducted consistently (with the same scanning parameters) before and after the scratch load is applied.
This is done to accurately measure recovery aspects since these aspects will vary with time.
8.4 The complete scratch test shouldshall be repeated 2two more times at different locations so that there are a total of 3three
scratches per test panel.
8.5 Typical results of Results of typical data from a nanoscratch test are presented in
Fig. 1. . The graph consists of five curves labeled 1C through 5.C . If needed,required, correct the data by curve fitting so that
1 5
zero indenter penetration and residual depth (RD) corresponds to zero applied normal force.
8.5.1 Curve 1
...