Mar Resistance of Paint Coatings by Nanoscratch Testing

Table of Contents

Introduction
Experimental Setup
Results and Discussion
Conclusion

Introduction

Mar can be defined as a physical damage that typically occurs within a few micrometers of the surface of the topcoats; these damages are commonly the cause of the changes in appearance seen on the paint coatings [1].

Mar is thus one of the big concerns of the automotive coating sector.

Some reasons for mar include keys, branches, blowing sand and fingernails. The car wash is however considered as the main cause [1] as it is one of the few repeated activities for most car owners.

The purpose of this article is to show a mar behavior analysis methodology with the aid of the Anton Paar Nanoscratch Tester. This methodology, which allows an objective quantitative measurement of the mar behavior on automotive paint coating, will be comprehensive in this article together with a typical example. An example of progressive load scratch on automotive clearcoat is illustrated in Figure 1. The figure also illustrates a Panorama image, which is synchronized with depth signals and load recorded during the scratch procedure.

This test methodology is according to the International Standard D 7187-05.

Figure 1. Panorama image showing typical scratch track on an automotive clearcoat together with recorded signals of penetration depth (Pd) and residual depth (Rd). Vertical line indicates critical load (Lc1).

Experimental Setup

The international standard D 7187-05 offers specific guidelines regarding the parameters to be used for a mar behavior analysis.

Table 1. Testing conditions - standard D 7187-05.

. .
Indenter size 1-100 µm
Scratch speed 0.5-100 mm/min
Loading rate 5-200 mN/min
Scanning load 0.1-1 mN

Anton Paar Nanoscratch Tester (NST3) is an ideal instrument for this type of application, especially because of its double cantilever system that is used for correct application of the load. The double cantilever is combined with a piezoelectric actuator for rapid feedback of the applied load and its corrections during scratching (triggered by cracks or other failures). The Nanoscratch Tester also offers appropriate force range as stipulated by the D7187-05 Standard. The tests explained in this article have thus been performed using this instrument.

The experimental conditions mentioned below were applied for all tests performed on two inspected samples:

Table 2. Experimental testing conditions.

. .
Indenter type Sphero-conical
Indenter radius 2 µm
Type of Scratch Progressive
Begin load 0.5 mN
End load 20 mN
Loading rate 39 mN/min
Scratch lenght 1 mm
Scratch speed 2 mm/min

Two similar samples (A and B) were investigated, both displaying ~70 µm thick car body paint on a steel substrate. Sample A had an extra 1 µm layer of top coating.

The substrate was, according to the regulations of the international standard (and what is commonly the case for car and many other paints), a smooth plane and rigid surface.

Results and Discussion

The coating appearance and performance is directly connected to the two permanent deformation mechanisms, namely fracture and plastic deformation [2].

These two deformation mechanisms can be quantified using the depth recorded data during a scratch test together with the optical observations post scratch.

Each scratch test by the Anton Paar NST3 includes three passes of the indenter over the same, pre-established test track:

  • Pre-scan – It is the first scan of the intact surface that allows measuring the topography of the coating. Typically a very low constant load is applied. Anton Paar used a scanning load of 0.05 mN.
  • The actual scratch - The indenter is drawn at a constant speed with gradually increasing or constant loading across the coating system to be analyzed. The penetration depth (Pd) is measured during this scratch.
  • Post-scan – It is the scan of the scratch track. This pass permits the measurement of the topography of the damaged coating by calculating the residual depth (Rd). Typically the same load as during the pre-scan is used (0.05 mN).

Due to these three passes, the real penetration depth (Pd) and the elastic recovery ((Pd-Rd)/Pd) of the tested material can be examined. By applying this technique of depth calculation, any non-uniformity in the flatness of the sample is considered and the surface profile thus does not impact the measurement of residual depth and penetration.

The graph bellow shows the common penetration depth (Pd) and residual depth (Rd) data observed during one of the performed tests. The residual depth indicates the permanent damage caused on the surface of the coating by scratching with the diamond stylus.

Figure 2. Penetration depth, Residual depth and Applied load as a function of Scratch length.

Fluctuations are seen on the residual depth curve. These fluctuations specify alterations on the surface of the coating which are linked to fractures of the coating. The optical observations performed post-scratch verify the hypothesis of fracture of the coating.

The figure bellow shows the first fracture observed on one of the two samples tested for this application.

Figure 3. Failure corresponding to Lc1.

The normal load where the primary fracture is optically observable on the test track is defined as the critical load (Lc1). According to the critical load results, the resistance of the material to scratching failure can be assessed and compared.

The tests on the two samples indicate differences between the two samples in terms of first fracture (Lc1). Sample A, with the extra top coating with 1 µm thickness, shows better fracture resistance.

Figure 4. Comparison of Lc1 critical load on both tested samples.

The plastic resistance (PR) value, characterizing the resistance of the scratched material to permanent deformation, is established by dividing the normal force (Fn) by the residual depth (Rd) value. This property is to be calculated on the data recorded before the occurrence of the first fracture or other similar types of coating failure. The plastic resistance value is established as follows:

where Fn is the normal force in milinewtons, and Rd is the residual depth (also called permanent plastic deformation) in micrometers. If the plastic resistance is measured at the same load, the material that has lesser residual depth (Rd) will exhibit higher value of PR, i.e. better plastic resistance (resistance to permanent scratching) than material that will show a higher value of Rd. In other words, materials with a high value of PR will resist better permanent scratching than materials compared to a low value of PR.

The first fracture happened, in both cases, under regular load above 2.5 mN. The regular load threshold for both normal load and permanent plastic deformation requested for the plastic resistance (PR) calculations was thus set to 2.0 mN. In theory, any value of normal load below the first critical load of 2.5 mN can be used for the calculations of the plastic resistance. However, it is suggested to choose a value practically high to avoid errors when assessing residual depths at very low loads.

Due to the high sensitivity of the Nanoscratch Tester and the depth data recorded during the tests, the penetration depth and residual depth corresponding to the permanent damage caused by the 2.0 mN applied load could be retrieved.

The following graph compares the calculated plastic resistance (PR) of the two tested samples.

Figure 5. Comparison of plastic resistance for both tested samples.

The results reveal small differences between the two samples. These differences were expected due to different layer composition. The extra 1 mm layer on the top surface (Sample A) did not result in improvement of the scratch resistance; in contrast, the scratch resistance of sample A somewhat decreased – although the Lc1 value was higher, as seen in Figure 4. This example illustrates that the mar resistance offers a more intricate picture of the coating compared to simple critical load values which do not automatically catch all differences in scratch resistance of polymeric paints.

Conclusion

It is possible to characterize the mar behavior of paint coatings by following the recommendations detailed in the international standard D 7187-05 and using the Anton Paar Nanoscratch Tester.

  • Due to the methodology stipulated in the D 7187-05 International Standard which was followed and summarized in this article, it is possible to quantitatively and objectively characterize the mar behavior
  • The automotive coating sector can use this methodology to define, compare, and enhance the quality and performance of the paint coatings

References

[1] L. Lin, G.S. Blackman, R.R. Matheson, A new approach to characterize scratch and mar resistance of automotive coatings, Progress in Organic Coatings 40 (2000).

[2] Test method for measuring mechanistic aspects of scratch/Mar behavior of paint coatings by nanoscratching (D 7187-05).

This information has been sourced, reviewed and adapted from materials provided by Anton Paar TriTec SA.

For more information on this source, please visit Anton Paar TriTec SA.

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