Analysis of Adhesion Strength of Automotive Bumper Paints Using a Micro Scratch Tester
Anton Paar TriTec (formerly known as CSM Instruments) has developed a new method for measuring the adhesion strength of many different coatings on various layers. The Micro Scratch Tester (MST) is based on the scratch resistance test widely accepted by the industry for the qualitative measurement of thin films. Measurements are quick and simple, allowing direct comparison of similar coatings.
Adhesion Analysis of Automotive Bumper Coatings Using Micro Scratch Tester
In the scratch test on the sample, a diamond tip creates controlled stress and strain on the surface coating. The mechanical response can be measured thanks to simultaneous recording of changes in surface morphology, acoustic emission, and transient friction.
Raman imaging of the scratched area provides information about the molecular composition of all tribochemical deposits formed.
Additionally, it provides information about local stress and strain distribution.
This article presents the effectiveness of MST in distinguishing coated bumpers supplied by a major automotive manufacturer; it examines a comparison of scratch resistance properties of three different bumper samples with the same paint coating.
Experimental Work
A standard Rockwell C diamond tip with a 200 μm radius was found to lack sufficient sharpness to obtain good results. Based on this finding, a hardened metal tip with a 10 μm radius was used for the scratch test of three different samples due to the soft nature of the paint coating.
Subsequently, Raman imaging was taken to examine stress and strain distribution throughout each scratch from beginning to end.
Figure 1. Two consecutive scratch images obtained by applying a maximum load of 10 N on the paint coating
Results
Partial layer delamination at critical Lc 1 load and total layer delamination at critical Lc 2 load were monitored in three sample coatings showing both main fracture modes. The magnitude of both critical loads provided insight into the scratch resistance of the coating/substrate system.
For example, for a total load of 10 N, a poor bumper material had an Lc 1 load of 1 N and an Lc 2 load of 2.7 N, while a good bumper material had an Lc 1 load of 3.4 N and an Lc 2 load of 6.8 N. Two typical scratches are shown in full in Figure 1, and the detail of the material's response to partial and total layer delamination is shown in Figure 2.
Figure 2. Detail of the two main fracture modes observed in all three samples; (a) partial, and (b) total layer delamination
A Raman spectrum of a good bumper material that examines in detail stress and strain distribution is deterministic before and after the scratch test in the same area.
Conversely, a poor bumper material shows significant stress and strain variation after deformation (for example, stress develops in the coating/substrate system). This condition helps explain the mechanism occurring at the interface.
Good bumper plastic is elastic and susceptible to extension from loading. Therefore, it prevents fracture in the paint coating. Poor bumper material does not have this property and increases strain, leading to early deterioration in the coating.
Summary
This study has demonstrated that combining two different analytical techniques for characterization of the coating/substrate system, which has major industrial importance, provides substantial benefits.
As MST can be used in small-scale research work for developing better materials and processes, as well as in routine quality control of production line coatings and surface improvement processes, the test equipment is poised to open new frontiers in the industry.
Esra Ertani - Sales Engineer - Anton Paar Ölçüm Aletleri Ticaret Ltd. Şti.
Related Publications
C. Julia-Schmutz and H. E. Hintermann; Microscratch testing
to characterise the adhesion of thin layers, Surf. &
Coat. Tech., 48 (1991) 1-6.
D. Vaughn, B. G. Frushour and W. C. Dale; Scratch indentation,
a simple adhesion test method for thin films on
polymeric supports, J. Adhesion Sci. Tech., 8 (1994) 635-
650.
P. J. Burnett and D. S. Rickerby; The scratch adhesion test:
an elasticplastic indentation analysis, Thin Solid Films,
157 (1988) 233-254.
K. Ajito, J. P. Sukamto et al.; Strain imaging analysis of Si
using Raman microscopy, J. Vac. Sci. Tech. A, 13 (1995)
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