Scratch tester

A nano scratch tester (CSEM) was employed to carry out the scratch test. A Rockwell diamond tip with a radius of 2 fim was used to draw at a constant speed 3 mm/min across the coating/substrate system under progressive loading of 130 mN maximum at a fixed rate 130 mN/min. The total length of the scratch scar is 3 mm. The critical load (L ) here is defined as the smallest load at which a recognizable failure occurs. The failure can be observed both by the built-in sensors and by the optical microscope. 

The same nano scratch tester was used to carry out the friction tests. The Rockwell diamond tip (radius 2 /u.m) was used to draw at a constant speed 3 mm/min across the sample surface under a constant load of 20 mN for which no scratches occurred for all the samples. Feedback circuitry in the tester ensures the applied load is kept constant over the sample surface. The sliding distance is 3 mm. The friction coefficient is defined normally as the ratio of the friction force and the applied load. 

A commercial instrument by the Swiss company CSEM (now CSM) was tested in the course of a project at the Research Institute for Pigments and Paints (Forschungs-institut fur Pigmente und Lacke e.V. (FPL)). This instrument is based on the method developed at the DuPont Marshall Lab. Objective of the project - where manufacturers of paints, raw materials and automobiles worked together - was the evaluation of the CSEM nano-scratch-tester regarding reproducibility, accuracy and applicability under the aspect of a realistic determination of scratch resistance. 

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. 

Mg. 4. Custom-made scratch tester in PTC, Texas A M University. 

Prasad and Kosel have described a fixed-depth scratch tester Figure 2.20 is a diagram of such an apparatus taken from their paper. In such an apparatus the specimen is moved by hand horizontally relative to the stylus and care is taken in constructing the specimen mounting block to ensure that it maintains the specimen surface parallel to the direction of movement and at an angle of 90° to the stylus tip. Constant-depth conditions are achieved by using the two outer load arms to carry most of the applied load. Thus if the stylus encounters a hard second-phase particle and tries to rise over it, the outer frame is lifted which causes the normal load on the stylus to increase sharply. 

The crack initiation test was performed for silica, Tempax, and soda-lime glasses in water and in dehydrated heptane by using the scratch tester. Figure 5 shows an... 

Figure 2.24 Photo of Taber Shear/Scratch Tester.

A scratch was created on the coated specimen using a rod with a diamond tip on a scratch tester (IMC-1552, Imoto Machinery Co. Ltd.). The load for the scratch was 30 g. The scratched specimen was put in a holder and connected to a conductive wire, followed by immersion in a 0.0005 M NaCl corrosive solution, maintained at 40°C. The electrochemical impedance spectroseopy of the scratched specimen was measured at intervals of 4 h, or more, for a total of 24 h. Measurement data were analysed to calculate the polarisation resistance after the test. Scratched specimens were also immersed in a 0.0005 M NaCl corrosive solution at 40°C for 7 days. The surface appearance of the specimens was observed after the corrosion test to confirm the self-healing properties of the coatings. The seratched part of the specimen was analysed after the corrosion test by Electron Probe Micro-Analysis (EPMA). 

Pencil hardness measurements were performed as per the protocol specified in the ASTM D3363 standard. A pencil scratch hardness tester from PPH-I was used to test samples (color chips) of different resins under a load of Ikgf at a temperature of 23°C. Mitsubishi-UNI pencils specified by the JIS S6006 standard were used in the evaluation. The hardness ratings, going from softer to harder, range from 6B to 7H and take the form 6B.3B, 2B, B, HB, F, H, 2H, 3H...7H (B=Black, F=Firm and H=Hard). Scratch resistance was also evaluated using a different method compliant with the ISO 1518 standard. An Erichson Scratch Tester (Type 413) was used to measure the resistance of the material to penetration when scratched with a spherical needle 1mm in diameter under a load of 6N. 

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