Velocity, scratch behavior

Individual scratch is often done with a constant loading rate (which includes constant load) and constant scratch rate. The scale of damage of interest varies from macroscopic features to those of nanometer scale. This renders comparisons between studies difficult. The recent concerted industry-university effort (6,7,44) has led to the development of a scratch test method and universal scratch testing equipment to facilitate good scratch test practices and fimdamental study of scratch behavior of polymers. Figure 4 presents the scratch tester that was developed. It aims to provide reasonable capability to vary experimental conditions for capturing the essential scratch characteristic of the materials with reliable and reproducible results. One unique feature of the tester is its ability to alter scratch load and velocity at a linear rate. This feature is foimd to be useful in understanding scratch behavior of polymers, to be presented in later sections. 

Effect of Scratch Velocity and Temperature. Another frequently investigated experimental parameter on the effect of scratch behavior of materials is the scratching rate or the scanning velocity. Since polymers are viscoelastic-viscoplastic in nature, the importance of scratching rate lies in its ability to change the strain and strain rate at the interface of the sample and the indenter and thus alters the deformation mode. The relationship between velocity and coefficient of fiction will depend on the relaxation state of the surface concerned, especially for nanoscratch (21,56,57,63-65). It was shown that rate and temperature interplay to affect nanotribological behavior of polymer films (57). 

With these complications in mind, research in this area has blossomed rapidly. Two main foci of research in this area are on (1) how external conditions (such as levels of loadings, the use of different indenters, and scratch rate) and (2) intrinsic materials properties (such as modulus and crystallinity) affect the tribological behaviors of the polymers. Apart from examining the scratch resistance of polymers, a closely related quantity which is of interest would be changes in coefficient of friction. Studies relating mechanical properties (3-5,9,36,71,75,76), deformation patterns (18,33,63,71,77-81), fabrication process (3,5,35,72,77,82-86) with respect to experimental parameters, snch as temperature (18), loading effect (24,71,72,87-96), indenter geometry (21,33,75,82,95,97), and scratch velocity (21,56,57,59,64,65,96,98) have been carried ont. In addition, scratch maps for different polymers have been produced (32,33), and various scratch resistance properties estimated (33,37,56,58,59,99). 

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