Adhesion Analysis of Automotive Bumper Paints Using a Micro Scratch Test Device

Anton Paar TriTec (formerly known as CSM Instruments) has developed a unique method to test the adhesion properties of many different types of coatings on various substrates. The Micro Scratch Test instrument (MST) is based on the scratch adhesion test, which is widely accepted by many industries for qualitative measurement of thin film adhesion.

This method is simple and fast, and also allows direct comparison between similar coatings. In the scratch test on the sample, controlled stress and strain are created on the coating surface by means of a diamond tip. Measured and simultaneously recorded friction waves, acoustic emission and changes in surface morphology provide mechanical response. Subsequently, Raman mapping of the scratched area provides additional information about the molecular composition of tribochemical residues obtained together with local stress and strain. In this study, the effectiveness of such an instrument was tested on coated car bumpers supplied by a major automotive manufacturer. The focus was on comparing the adhesion properties of three different bumper types with similar paint coatings.

Experimental Section

The standard Rockwell C diamond tip was found to be insufficiently sharp to obtain good results. For this reason, a hardened metal tip was used for the scratch test of three different samples with soft paint coating structure. Raman mapping was performed to observe stress and strain distribution throughout each scratch. Results All three samples exhibited two main failure modes: partial removal of the paint coating (at critical load Lc 1) and subsequent complete removal (at critical load Lc 2). The magnitude of these critical loads provides an indication of coating/substrate resistance to failure. For example, a poor bumper material had Lc 1 load of 1.0 N and Lc 2 load of 2.7 N, while a good bumper material had Lc 1 load of 3.4 N and Lc 2 load of 6.8 N. With a good bumper material, Raman spectra confirm that stress and strain distribution are identical for the same area before and after the scratch test. Conversely, a poor bumper material exhibits significant stress and strain variation after deformation (such as stress accumulation in the coating/substrate system). This helps explain the mechanisms occurring at the interface level. Good bumper plastic is elastic and therefore can relax after loading. Thus, failure of the paint coating is prevented. Poor bumper material lacks such ability and accumulates strain leading to premature failure of the paint coating.

Assessment

This study has demonstrated the benefit of combining two different analytical techniques that provide mutual advantages for complete characterization of coating/substrate systems for major important industries. The Micro Scratch Test instrument provides routine quality control of production line coatings and surface refinements, and at the same time opens the door to a new generation of specialist testing tools used in smaller-scale research environments to develop better materials and processes. Esra Ertani Sales Engineer Anton Paar Ölçüm Aletleri Ticaret Ltd. Şti.

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