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Objective:
• To develop quantitative techniques for small scale measurements of surface mechanical and viscoelastic properties on polymeric materials as a function of time, maximum load, and loading rate,
• To develop relationships between material properties, complex stress states during scratch loading, and the resultant plastic deformation (damage), and
• To develop techniques to assess surface optical properties of virgin and damaged polymeric materials.
Problem:
The scratch and mar resistance of glassy polymeric materials is critical for industries whose products require appearance durability. For these products, scratch visibility, i.e. the consumer perception of scratch resistance, is the fundamental basis for assessing durability. There are currently ten standards for scratch and mar resistance available from both ASTM (five standards) and ISO (five standards). The standards provided by these organizations do not include specialized tests required in specific industries, such as automotive manufacturer requirements for OEM suppliers. There are three problems with the current scratch standards: 1) The proliferation of multiple standards increases the resources needed to validate one product using multiple tests, 2) None of the current tests focus on scratch visibility, and 3) The link between material properties and scratch resistance is poorly understood.
The major difficulty with scratch mechanical analysis is that the applied stress at small scales is extremely complex due to the interrelations between processing conditions, microstructure, and mechanical properties. Furthermore, the time-dependent nature of many polymer properties largely controls their deformation behavior and thus their performance. This deformation behavior is generally well-understood for uniform and/or controlled loading conditions in a linear elastic regime. However, more realistic or service-relevant loading involves more complex stress distributions. These complex fundamental deformation mechanisms are not understood and not easily reproduced in the laboratory. The correlations are particularly difficult for polymer coatings because of the temperature and rate-dependent nature of their material properties. In addition, the thickness of the coatings is on the order of a few mils, and deformation behaviors may be influenced by the underlying substrate, further complicating experimental and modeling efforts.
Indentation and micro-scratch tests are often used to measure differences in scratch and mar resistance of polymeric materials. Unfortunately, more research is required to demonstrate how these types of tests can show good correlation with the in-service performance of polymers. The methodology is beginning to gain acceptance. Most recently, ASTM has introduced a scratch standard based on commercially available nanoindenters, although this test does not focus on visibility.
Approach:
Ultimately, industrial products are marketed based on appearance durability. Current scratch and mar testing protocols focus on the onset of plastic deformation or fracture events. These protocols do not account for the visual appearance of the scratch or mar; consequently, no relationships exist between the size of a scratch or mar and the coating appearance and service life. This project seeks to bridge the gap between the appearance of the damaged coating and the mechanical and time-dependent properties of the polymeric coating. This eliminates the need for an understanding of the complete scratch process and allows the researchers to focus on only those areas that affect the coating optical properties.
The primary focus of this research is the development of needed test methods for measuring at small scales surface properties of polymers and evaluating the deformation mechanisms of polymeric materials under complex states of stress. New experimental procedures and techniques are designed and refined, particularly with regard to studying rate dependence of material properties and surface deformation at small scales. The experimental procedures and techniques are evaluated and modified based on experimental observations and sample complexity. An initial model for the scratch behavior of homogeneous model coatings has been developed, and virtual bulk tests are being simulated to link model parameters to material properties.
To correlate local surface properties to deformation behavior from the submicrometer scale to the visual scale, a state-of-the-art depth-sensing indentation device is used in conjunction with a state-of-the art optical scattering instrument to first inflict damage and evaluate the physical and optical properties of the damaged area. In the second phase of this research, the complexity of the polymeric coatings has been increased in order to probe different aspects of materials properties that are relevant to scratch resistance besides the glass transition temperature, Tg. The surface mechanical properties of these polymer coatings are still evaluated using indentation and scratch testing techniques established at NIST through the PIC consortium.
For the third phase studies, we have begun to add complexity to our materials: (1) a series of poly(methyl methacrylate) (PMMA) samples with different organic pigments will be used to understand the effect of color on scratch visibility; (2) a series of PMMA samples with different organic pigments will be studied to determine the effect of color on mechanical properties; (3) a series of epoxy materials with varying Tgs will be used to investigate the ability of nano-DMA to determine differences in polymer architecture; (4) the temperature dependant viscoelastic behavior of a PVDF/PMMA blend and polyurethane coating will be measured using indentation in an environmental chamber; and (5) FEA simulations will be conducted to determine our ability to simulate scratches in glassy polystyrene and PMMA materials.
Finally, evaluating the scratch or mar resistance of a polymeric material involves not only the characterization of the deformation but also the extent to which the deformation affects material’s optical properties. Through collaborations with NIST researchers working on optical properties and appearance issues, optical scattering and visual perception models will be used to understand the effect of the size of a scratch or mar on optical properties. New optical scattering and/or imaging methods are being developed for evaluating scratch and mar resistance in terms of interactions with light.
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