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Advanced Mechanical Surface Testing

Mar Resistance of Paint Coatings by Nanoscratch Testing

Introduction

Mar is described as a physical damage that usually occurs within a few micrometers of the surface of the topcoats; these damages are usually the cause of the changes in appearance observed on the paint coatings [1].

Mar is therefore one of the major concerns of the automotive coating industry.

Some causes of mar include keys, fingernails, branches, blowing sand. The car wash is however believed to be the main cause [1] as it is one of the few periodic activities for many car owners.

The purpose of this application report is to demonstrate a mar behavior analysis methodology using the Anton Paar Nanoscratch Tester. This methodology, which allows an objective quantitative measurement of the mar behavior on automotive paint coating, will be detailed in this application report along with a typical example. An example of progressive load scratch on automotive clearcoat is shown in figure 1. The figure also shows a Panorama image, which is synchronized with load and depth signals recorded during the scratch procedure.

This test methodology is based on the International Standard D 7187-05.

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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).

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Experimental setup

The international standard D 7187-05 gives specific guidelines concerning the parameters to be employed for a mar behavior analysis.

Indenter size 1-100 mm

Scratch speed 0.5-100 mm/min

Loading rate 5-200 mN/min

Scanning load 0.1-1 mN

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

Anton Paar Nanoscratch Tester (NST3) is a suitable instrument for this type of application thanks to its double cantilever system that is used for precise application of the load. The double cantilever is combined with a piezoelectric actuator for fast feedback of the applied load and its corrections during scratching (caused by cracks or other failures). The Nanoscratch Tester also offers suitable force range as requested by the D7187-05 Standard. The tests detailed in this report have therefore been performed using this instrument.

The following experimental conditions were applied for all tests performed on two examined samples:

Indenter type Sphero-conical

Indenter radius 2 mm

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

Table 2 - Experimental testing conditions.

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

The substrate was, as per recommendations of the international standard (and what is usually the case for car and many other paints), a smooth plane and rigid surface.

Results and discussion

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

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

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

• Pre-scan – It is the first scan of the undamaged surface that allows measuring the topography of the coating. Usually a very low constant load is applied. In our case we used a scanning load of 0.05 mN.

• Theactualscratch- The indenter is drawn at a constant speed with progressively increasing or constant loading across the coating system to be tested. The penetration depth (Pd) is measured during this scratch.

• Post-scan – It is the scan of the scratch track. This pass allows the measurement of the topography of the damaged coating by measuring the residual depth (Rd). Usually the same load as during the pre-scan is used (0.05 mN).

Thanks to these three passes, the real penetration depth (Pd) and the elastic recovery ((Pd-Rd)/Pd) of the tested material can be investigated. By applying this method of depth calculation, any non-uniformity in the flatness of the sample are taken into consideration and the surface profile therefore does not influence the measurement of penetration and residual depth.

The following graph illustrates the typical penetration depth (Pd) and residual depth (Rd) data recorded during one of the performed tests. The residual depth represents the permanent damage created 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 visible on the residual depth curve. These fluctuations indicate changes on the surface of the coating which are associated to fractures of the coating. The optical observations performed post-scratch confirm the hypothesis of fracture of the coating.

The image bellow is an illustration of the first fracture observed on one of the two samples analyzed for this application.

Figure 3 - Failure corresponding to Lc1.

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

The tests on the two samples show differences between the two samples in terms of first fracture (Lc1). Sample A, with the additional top coating with 1 µm thickness, exhibits 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 determined 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 type of coating failure. The plastic resistance value is defined 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 lower residual depth (Rd) will show higher value of PR, i.e. better plastic resistance (resistance to permanent scratching) than material that will exhibit a higher value of Rd. In other words, materials with high value of PR will resist better permanent scratching than materials with a low value of PR.

The first fracture occurred, in both cases, under normal load above 2.5 mN. The normal load threshold for both normal load and permanent plastic deformation requested for the plastic resistance (PR) calculations was therefore set to 2.0 mN. In principle, 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 recommended to select a value reasonably high to avoid errors when evaluating residual depths at very low loads.

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

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

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

The results indicate slight differences between the two samples. These differences were expected due to different layer composition. The additional 1 mm layer on the top surface (Sample A) did not lead to improvement of the scratch resistance; on the contrary, the scratch resistance of sample A slightly decreased – although the Lc1 value was higher (see Figure 4). This example shows that the mar resistance gives a more complex picture of the coating compared to simple critical load values which do not necessarily catch all differences in scratch resistance of polymeric paints.

Conclusion

Following the guidelines detailed in the international standard D 7187-05 and employing the Anton Paar Nanoscratch Tester, it is possible to characterize the mar behavior of paint coatings.

• Thanks to the methodology given in the D 7187-05 International Standard which was followed and summarized in this report, it is possible to objectively and quantitatively characterize the mar behavior.

• The automotive coating industry can use this methodology to determine, compare and improve the quality and performance of the paint coatings.

For further details about this application report, do not hesitate to contact your local sales representative or Anton Paar directly.

References

[1] L. Lin, G.S. Blackman, R.R. Matheson, A new approach to characterize scratch and mar resistance of automotive coat-ings, 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).

AuthorMihaela Dubuisson, Anton Paar TriTec SA

Contact Anton PaarTel: +41 32 552 1600 Fax: +41 32 552 1610info.tritec@anton-paar.com www.anton-paar.com

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