This work is mainly focused upon the analytical examination of the physical and mechanical response of plastics undergoing an induced scratch deformation caused by a semi-spherical scratch tip under a linearly increasing normal load. Evaluation of the scratch deformations in this study was based upon visual and optical observations and upon observations of failure and fracture mechanisms as well as Electron Microscopy examinations. In the first section of this study an effort was made to correlate the scratch resistance observed in Polypropylene (PP) thin sheets with material properties, such as molecular weight and surface crystallinity. In the second section of this work the scratch behavior of epoxy nanocomposites was examined and a conclusion was made based upon the effects of the addition of nano-additives with various natures into the epoxy matrix. Furthermore, a region of the scratch path prior to the onset of scratch visibility known as the mar region, which was an obscure area of deformation on a microscopic scale, was thoroughly investigated for the epoxy systems and various conclusions were made based upon those results. Finally, based on these findings and previous studies, it was shown that failure and fracture mechanisms of polymeric materials under scratch deformations are dependent on the type and physical nature of the material, whereas brittle and ductile materials show various behaviors under the specified conditions. Based on the failure mechanism which the material exhibits subsequent to the scratch deformation process and the physical and mechanical characteristics of the material, several factors were shown to effect the materials ability to scratch resistance.

Scratch recovery is a desirable property of many polymer systems. The reason why some materials have demonstrated excellent scratch recovery while others do not has been a mystery. Explaining the scratch resistance based upon the hardness of a material or its crosslink density is incorrect. In this thesis, novel polymers were tested in an attempt to discover materials that show excellent scratch recovery - one of the most important parameters in determining the wear of a material. Several hypotheses were developed in an attempt to give an accurate picture of how the chemical structure of a polymer affects its scratch recovery. The results show that high scratch recovery is a complex phenomenon not solely dependent upon the presence of electronegative atoms such as fluorine.

A new scratch test methodology is proposed. The new test methodology is developed based on the principles of materials science and solid mechanics, which include the consideration of material parameters, use of microscopy for image analysis and the finite element method (FEM). The consistency and reproducibility of test results are shown using a new scratch test device on two sets of neat and talcfilled polypropylene (PP) systems. Three different test conditions, i.e., linear load increase under constant rate, constant load under constant rate, and linear rate increase under constant load, have been conducted to determine the most effective, informative test conditions for evaluation of scratch resistance of polymers. Experimental observations and FEM results show a good qualitative correlation. The unique advantages of the new scratch test method for evaluating scratch resistance of polymers are discussed. A systematic study of surface damage effected by a progressive scratching load is performed on model polypropylene (PP) systems. Mar-scratch and stress -whitening transitions can be readily observed, and the corresponding critical loads determined. Distinctive scratch hardnesses and surface damage features are found for different material systems. Visibility of scratched surface is quantified using gray level analysis via a flatbed scanner and a commercial image analysis tool. It is found that the onset of scratch visibility can be determined accurately and reproducibly using the custom -built scratcher under progressive loading condition. Talc particles are found to be responsible for the increased light scattering, leading to greatly increased visibility. The observed scratch visibility is also found to be related to the measured frictional force profiles. Approaches for producing scratch resistant PP are discussed.

This dissertation work is focused on the analytical and numerical examination of the mechanical response of polypropylene (PP) under scratch deformation by a semispherical indenter. The finite element (FE) method is employed as the analysis technique and ABAQUS??, a commercial FE package is adopted to perform the analysis. Important physical and computational considerations on the implementation of FE analyses for the scratch problem are reviewed. It is shown through the discussion of the generated results that a good understanding can be gained on how different scratch conditions can affect scratch behavior of PP. A phenomenological deduction of the scratch damage process and mechanisms is also established. Considering the two main damage modes of polymers, shear yielding and crazing, it is shown that the two damage modes not only exist in the scratch deformation, and moreover, that they may compete against each other for dominance. A parametric study is also performed to assess the influence of material and surface properties on scratch response of material. A secondary research effort is also made to investigate the material constitutive modeling of polymers. Focusing on elastomeric or rubbery materials, a new mixed network model between the Gaussian and eight-chain non-Gaussian models is proposed. This mixed model inherently preserves the good predictive power of these two models and yields better predictions over a wider range of deformation than that of the rubber model adopted by ABAQUS??.

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Polymeric packaging films see widespread use in the food packaging industry, and their mechanical integrity is paramount to maintaining product appearance, freshness, and overall food safety. Current testing methods, such as tensile or puncture tests, do not necessarily correlate well with field damages that are observed to be scratch-like. The standardized linearly increasing load scratch test is investigated as a new means of evaluating the mechanical integrity of packaging films. Mechanical clamp and vacuum fixtures were considered for securing the films to a set of backing materials and tested under various testing rates and film orientation conditions. Film performance was evaluated according to their puncture load. Based on the above study, the vacuum fixture offers the most consistent and meaningful results by providing a more intimate contact between film and backing and minimizing uncontrolled buckling of the film during testing. Additional testing was also carried out on a commercial film to confirm similarity between damage observed in the scratched films and that from the field. The scratch test gives good correlation between field performance and scratch test results on a set of commercial films. The usefulness of the scratch test methodology for packaging film mechanical integrity evaluation is discussed. Scratch-induced damages on multi-layer commercial packaging films are investigated using cross- and longitudinal-sectioning. Scratch test results show clear distinction between the two tested systems on both the inside and outside surfaces. Microscopy was performed to investigate the feasibility of utilizing this methodology as a tool for packaging film structure evaluation by determining the effect each layer has on the resistance of scratch damages. It is shown that the film showing superior scratch test results also shows significantly better stress distribution through its layers during the scratch test, as well as better layer adhesion during severe deformation. The scratch test shows good ability to provide more in-depth film mechanical integrity testing by allowing for layer-by-layer analysis of damages and layer adhesion after testing.

This thesis presents a surface micromachining process to fabricate scratch drive actuator (SDA). Besides, various parameters (plate length, plate width, plate shapes, support beam width, spring type, one-plate SDA/four-plate SDA, dimple number, bushing length and etching hole¡Ketc.) have been designed to find appropriate design parameters of the SDA and to reduce the driving-voltage. According to the results, we can demonstrate three points below: (i) Adding etching holes at the end of plate can reduce residual electric charge and increase life time of SDA. (ii) Changing normal wafer for low resistance wafer can reduce the driving-voltage of SDA about 35%. (iii) The life time of triangle SDA is longer than those of other plate shapes. Finally, discussions and suggestions for the design of SDA are presented in this thesis.

The lack of a widely-accepted quantitative methodology for evaluating the scratch behavior of polymeric materials has resulted in the development and establishment of a new methodology recently standardized as ASTM D7027-05. Using a custom-built instrumented scratch machine, it is possible to produce controlled, repeatable scratches on polymer surfaces under constant or linearly increasing loading conditions at constant or increasing scratch rates. Software-aided digital image analysis along with material science tools (SEM, OM, FTIR, etc.) allows polymer scratch behavior to be analyzed without the ambiguity inherent in the past. The current work will serve to describe the motivation for the development of this methodology as well as illustrate the effectiveness of the increasing load/constant rate test mode in three case studies. First, it will be shown that an acrylic coating on a steel system exhibits three zones of scratch damage: adhesive delamination, transverse cracking and finally buckling failure. It will be discussed how increases in ductility and thickness serve to improve the scratch resistantance of this coating/substrate system. Improvements in the scratch behavior of thermoplastic olefins (TPOs) through the use of surface-treated talc fillers and the slip agent erucamide will be shown in the second case. It was found that the surface-treatment of the talc likely allows for enhanced migration of the erucamide to the TPO surface, thus lowering the surface friction and greatly increasing scratch resistance. Finally, the effects of processing conditions, namely injection molding, on the scratch behavior of neat i-polypropylene will be represented by the results of scratch tests conducted where the scratch direction was oriented both along and transverse to the polymer melt flow direction. Based on the findings of the study, there appears to be a high degree of surface anisotropy introduced to injection-molded polymers due to complex fluid flow regimes as well as non-uniform cooling properties.

Doctor of Philosophy / The drastic improvements in stiffness and strength even with the addition of small percentage of clay to a polymer are commonly traded-off with significant reductions in fracture toughness. It is believed that the presence of a stiff nano-filler will restrict the mobility of the surrounding matrix chains, and thus limit its ability to undergo plastic deformation, thereby decreasing their fracture toughness. To understand the role of rigid nano-fillers, like clay and their constraint effect on the surrounding polymer matrix, the effects of preferentially organized polyamide 6 lamellae in the vicinity of organoclay layers on the toughening processes are studied and compared with polyamide 6 filled with an elastomeric additive (POE-g-MA). It is suggested that to impart high toughness to polymer/organoclay nanocomposites, full debonding at the polymer-organoclay interface is necessary so that shear yielding of large volumes of matrix material can be enhanced. However, due to the strong tethering junctions between the individual organoclay layers and the matrix, full-scale debonding at the polymer-organoclay interface is rarely observed under stress conditions indicating that the constraint on the polymer adjacent to the clay is not relieved. Therefore, this has led to the development of ternary nanocomposites by adding a soft elastomeric dispersed phase to polymer/clay systems to obtain well-balanced mechanical properties. Polyamide 66/SEBS-g-MA/organoclay nanocomposites are prepared with four different blending protocols to understand the effect of blending protocol on the microstructure, mechanical properties and fracture mechanisms of the ternary nanocomposites so as to obtain new insights for producing better toughened polymer nanocomposites. In general, it is found that the level of enhancement of fracture toughness of ternary nanocomposites depends on: (i) the location and extent of dispersion of organoclay and (ii) the internal cavitation of rubber particles leading to effective relief of crack-tip tri-axial constraint and thus activating the matrix plastic deformation. Based on the wear/scratch damage studies on different polymer nanocomposite systems, it is suggested that elastic modulus and toughness of polymer nanocomposites are not the predominant factors controlling the material removal or friction coefficient and cannot be the sole indicators to compare and rank candidate materials. It is also found that nano-fillers by themselves, even if uniformly dispersed with good interfacial interaction with the matrix, do not irrevocably improve the wear (and friction) properties. Although it is important to consider these factors, it is necessary to thoroughly understand all microstructural parameters and their response to wear/scratch damage. Other important factors that should be considered are the formation of a uniform and stable transfer film on the counterface slider and the role of excessive organic surfactants or other modifiers added to disperse nanoparticles in a polymer matrix. It is also emphasized that the mechanisms of removal of materials during the wearing/scratching process should be studied meticulously with the use of high resolution microscopic and other analytical tools as this knowledge is critical to understand the surface integrity of polymer nanocomposites.