Effect of Adhesion, Film Thickness, and Substrate Hardness on the Scratch Behavior of Poly(carbonate) Films

ABSTRACT

The effect of adhesion, film thickness, and substrate hardness on the scratch behavior of poly(carbonate) (PC) films was investigated. Films of various thickness were prepared by spin-coating solutions of PC in chloroform onto glass, ferroplate, Al 1100, Al 6022, and Al 6111 substrates. Adhesion between the films and the substrates was controlled by pretreatment of the substrates and the thickness of the films was controlled by the concentration of the PC solutions. Adhesion of the films to the glass substrates was measured by a blister test. Scratch tests were performed using a custom-built, progressive-load scratch tester with interchangeable diamond indenters; the resulting scratches were observed by optical microscopy, atomic force microscopy (AFM), and environmental scanning electron microscopy (ESEM). The critical normal load (i.e., the smallest applied normal load for which delamination of the film from the substrate was observed) was used as a criterion to determine the scratch resistance of the films. It was found that better film/substrate adhesion resulted in a higher critical load for delamination. As film thickness increased, the critical load and, thus, scratch resistance also increased. Substrate hardness had a strong influence on the scratch behavior of the PC films. For a low-hardness substrate (i.e., Al 1100), the work from scratching was mainly consumed by deforming the substrate. In the case of substrates with intermediate hardness (i.e., Al 6022, Al 6111, and ferroplate), the substrates were more resistant to the stresses that were generated in the films; hence, the deformation of the substrates was less severe. A high-hardness substrate (i.e., glass) resisted the applied load and resulted in higher stress concentrations in the films and at the interface. Consequently, a rougher surface inside the scratch track was observed.

Keywords:

• Adhesion
• Blister test
• Scratch behavior
• Substrate hardness
• Plastic deformation
• Poly(carbonate)

ACKNOWLEDGMENT

The authors thank Giles Dillingham for discussions and suggestions. One of the authors (SW) acknowledges the Royal Thai Government for financial support.

Notes

Note: Different pretreatment was used to obtain different degrees of adhesion between the film and the glass substrate.

Note: A water flow rate of 0.2 ml/min was used for the blister tests.

Note: The film thickness was determined by weighing the substrate before and after spin-coating and assuming that the density of PC was 1.2 g/cm³. At least three samples for each concentration were measured.

Note: A load of 100 g was used for measuring the microhardness. At least five measurements were done per sample.

^a Values from www.matweb.com.

^b E₁ is the modulus of elasticity of the coating, which is PC in this case, and E₂ is the modulus of substrates.

^c Measured hardness values.

^d Value for steel [modulus of elasticity of Cr (recrystallized) is 248 GPa].

Note: The applied load was 0.90 N and the film thickness was 3.2 ± 0.4 µm. Scratch tests were done using a spherical indenter with tip radius of 89 µm, a full-scale load of 0.90 N, and a scratching speed of 0.033 mm/s.