ASTM-D7187 › Standard Test Method for Measuring Mechanistic Aspects of Scratch/Mar Behavior of Paint Coatings by Nanoscratching
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1.1 This test method covers the nanoscratch method for determining the resistance of paint coatings on smooth flat surfaces to scratch/mar.
1.2 Previous methods used in scratch/mar evaluation first physically scratch or mar a sample’s surface with 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 quantitatively and objectively measure scratch/mar behavior by making the evaluation process two steps with emphasis on surface damages. 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, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
Significance and Use
4.1 This test attempts to address two major drawbacks in existing mar tests such as Test Methods , , , , and , 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 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 surface nonuniformity 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). When coatings have had either high Plastic Resistance and low Fracture Resistance, or low Plastic Resistance and high 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.
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.
coatings; gloss; mar or organic coating mar resistance; scratch resistance;; ICS Number Code 87.040 (Paints and varnishes)
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April 15, 2015