ASTM D7027-20 Standard Test Method for Evaluating Scratch Resistance of Polymeric Coatings and Plastics Using an Instrumented Scratch Machine

The standard “ASTM D7027-20 Standard test method for evaluating scratch resistance of polymeric coatings and plastics using an instrumented scratch machine” published by the American Society for Testing and Materials (ASTM) describes a laboratory test method that uses an instrumented scratch machine to produce and measure surface damage under controlled conditions. This test method can characterize the scratch resistance of polymers by measuring many important material parameters. The scratch triggering and data collection process is automated to avoid user-induced influences that could affect the results.

For the purposes of this standard, in addition to the terms and definitions given in ASTM D883 and ASTM E456, the following definitions apply:

ASV (automatic scratch visualization) software is a computer program that automates the identification of the failure point in load tests by comparing failure criteria. ASV software determines failure when there is a continuous change in contrast between the scratch groove and the undamaged material surface. The continuity criterion is defined as a region equal to 2 diameters of the scratch pencil, with 90 percent or more of the region exceeding the contrast criterion. The lowest load point on the scratch where there is a continuous collection of contrast is considered the failure point. This program is useful for visual analysis of testing and can also be used for other applications such as pass-fail criteria for scratch visibility.
Critical normal load refers to the normal load at which the material first fails in the scratch groove.
Normal load refers to the load applied to the scriber in a vertical downward direction, perpendicular to the surface of the sample.
The point of failure is the point connecting the loads to the scratch path at which surface damage is initially considered unacceptable.
The coefficient of friction expresses the ratio of tangential force to normal load.
Scratch depth refers to the vertical distance measured from the groove of the scratch groove to the intact sample surface or the tops of the scratch path.
Scratch resistance is the ability of a material to withstand damage that comes with the large deformation typically associated with shear indentation of protrusions, which may involve compression, plowing, and shearing. Quantification of scratch resistance can be accomplished by measuring critical normal loads, scratch depth, scratch width, and other geometric or visual characteristics of the scratch.
Scratch width is the horizontal distance between two peaks on either side of the scratch groove.
Scratching is the process of surface deformation (displacement or mechanical removal of material, or both) caused by the sliding of one or more asperities or protrusions, or both, along the surface.
Tangential force is the force acting at the interface between the scratch tip and the sample opposite to the direction of motion of the scratch tip.
Whitening is a phenomenon that occurs as a result of light scattering due to surface deformation as a result of the scratch process, causing the scratch path to become brighter or whiter than the intact background surface.

This test method uses an automated scratch machine to perform controlled scratch tests on polymeric samples. Two basic test modes (test modes A and B) are presented.

Test mode A: A scratch is applied to the specimen surface at a constant scratch rate of 2 m/s plus/minus 50 m/s at a distance of 0,1 m plus/minus 0,0001 m under increasing normal load from 0,1 to 0,0005 N. This test mode is intended to determine the critical normal load for failure for a material system. Test A is considered valid if the failure point occurs in the second or third quadrant of the test length. For materials that do not fail in this range, the load range can be modified to ensure that the failure point occurs in the middle of the scratch path.
Test mode B: A scratch is applied to the specimen surface at a distance of 0,1 m plus/minus 0,0001 m, at a constant scratch speed of 0,1 m/s plus/minus 0,0005 m/s, under a constant normal load of 30 N plus/minus 0,1 N. This test mode is designed to evaluate the load-dependent homogeneous response of the material and to determine the scratch friction coefficient. If 30 N is not sufficient to cause damage to the specimen, the constant load value can be increased.

The scratched surface can be visually inspected or assessment tools can be used to examine surface damage. For test mode A, the critical normal load is determined by the failure criterion point determined for that test. Measurement of scratch width or depth, or both, can also be taken to aid in quantification of scratch resistance. ASV software can be used to automate measurement of the failure point for scratch visibility.

The scratch friction coefficient can be calculated for material characterization using tangential force and normal load data recorded during the tests.

Scratch tests are performed on samples for the following purposes:

To evaluate the scratch resistance of a particular material
Ranking the relative scratch resistance of different materials
Determining the scratch friction coefficient of materials

Since polymers exhibit mechanical properties that are highly dependent on temperature, the test standard provided herein is designed to provide reproducible results when users perform tests on samples of the same material and surface texture under similar test conditions and subjected to the same conditioning procedures.

Some polymers are self-healing (recoverable) when subjected to scratches and other physical deformations due to their viscoelastic and relaxation properties. It is important to note the difference between instantaneous (if easily measurable) and permanent scratch damage and compare results accordingly to ensure repeatability. It is recommended to allow 24 hours for viscoelastic recovery when considering permanent scratch depth.

Scratched surface whitening is a significant damage mechanism that is of great concern in automotive and other applications where surface aesthetics are important. Such damage is undesirable because it is visible to the human eye. The critical normal load at which this phenomenon occurs serves as a criterion for ranking material performance, especially from an aesthetic perspective.

Our organization, which has been trying to support businesses from every sector with its testing, measurement, analysis and evaluation studies carried out in a wide range for years, has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. In this context, testing services are also provided to businesses in accordance with the standard "ASTM D7027-20 Standard test method for the evaluation of scratch resistance of polymeric coatings and plastics using an instrumented scratch machine".