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SCRATCH TEST METHODOLOGY FOR POLYMERS

A new scratch test methodology is developed here. Different test conditions are used in conducting the scratch tests. The results is compared and assessed to determine the best method. Results from a concurrent study using finite element analysis (FEA) will also be presented.

Model Material System and Test Procedures：

In this study, four PP-based material systems are selected and their compositions are shown in Table 4.3. For these material systems, the PP resin and a dark gray coloring pigment was provided and blended by Solvay Engineered Polymers. Talc additive was provided by Luzenac. Injection molding of the plaques, having dimensions of 340 mm × 180 mm × 3 mm, was performed by Advanced Composites, Inc. For testing, the plaques were cut and machined into dimensions of 140 mm × 1 mm × 3 mm. All test specimens were prepared according to ASTM D 618-00 Procedure A [64].

Three sets of scratch tests (Tests A – C) were conducted. In Test A, aconstant stylus rate of 100 mm/s with a linear increasing normal load of 0 to 50N was performed. While in Test B, a 30 N dead load was utilized with a constant stylus rate of 100 mm/s, which is consistent with the Ford five-finger test. Finally for Test C, a dead weight of 30 N was used with a linearly accelerated stylus rate of 0 to 140 mm/s. The scratch lengths of all tests were set to be 100 mm and tests were conducted at room temperature. Stainless steel ball with a diameter of 1 mm was used as the scratch stylus tip.

Evaluation of Scratch Damage:

Transmission optical microscopy (TOM) observation, using an Olympus? BX60 microscope, of thin sections of PP systems was performed to study the scratch damage of selected cross-sections along and across the scratch groove. The thin sections were prepared by cutting the polymer strips into 2-cm long rectangular blocks, and mounted in an epoxy resin. The mounted polymer block was glued onto a microslide and further cut down to a 2-mm thick section by an ISOMET? 1000 diamond saw. The thick sections were then polished to a thickness of 100–150 ?m, using polishing papers stepwise with roughness from grit 800 to grit 4000 (grain size 5 ?m) to achieve the final polish.

Scanning electron microscopy (SEM) was also performed to study the microscale surface damage features using a JEOL JSM-6400 system. A flatbed scanner with a resolution of 1,200 dpi was used to scan the test specimens and generate digital images for the quantification of scratch damage.

To quantify the scratch damage, measurements were taken from the TOM, SEM and scanned images using the definitions of scratch widths and depths by Kotaki et al. [65], as shown in Figure 4.2. SW1 represents the inner width of the scratch groove. SW2 represents the outer width of the scratch groove, i.e., the distance between the points where the slopes of the hills meet the unscratched plane. SD1 represents the depth of the scratch groove calculated from the unscratched plane. SD2 is the height of the peak to the trough of the scratch groove. For spherical indenters, the scratch grooves generally show a symmetric cross-sectional profile. In cases where asymmetry occurs, i.e., one side of the pile-up is higher than the other; the higher point was taken to obtain scratch depths.

Finite Element Analysis:

In the concurrent work by Goy Teck Lim [52], in the mechanics of scratch, the finite element method [66] is used as the numerical tool to help elucidate the phenomena observed in the experiments. A well-established commercial package ABAQUS/Explicit? [67] has been adopted to perform the finite element analysis (FEA) of the concerned problem.

The modeling work is primarily set out to model the scratch problem as closely and realistically as possible to the actual testing conditions. A computational model of 50 mm × 10 mm × 3 mm was first considered. By exploiting the plane of symmetry, the computational model was reduced to the dimensions of 50 mm × 5 mm × 3 mm, as illustrated in Figure 4.3. Not only will it save computational resources, the results of the reduced computational model can be extended to those of the original model. For a more detailed discussion of the boundary and loading conditions of the computational model and various considerations of the FEA, one can refer to the literature.

Results and Discussion:

Experimental Results The scratch damage cross-sectional profile is reported based on an average of five specimens for each test condition. For Test A, the cross-section was taken at a location equivalent to 30 N load. While for Test C, the cross section was taken at a location equivalent to 100 mm/s speed. In this way, the three tests could be compared under the same loads and speeds of 30 N and 100 mm/s.

Following the definition specified in Figure 2, the trend suggests that the scratch width is the greatest for Test C, followed by Test B and Test A (Figure 4.3a). This trend has also been observed in FEA modeling (Figure 4.3b), which will be discussed in the next section. For Test C, the accelerating scratch tip will induce both horizontal (in the direction of scratch) and vertical inertias (acting downwards). The vertical inertia induced is due to the frictional effect.

Both of the inertia will increase the normal and tangential forces acting on the substrate, thereby increasing the scratch width and depth. While the increasing load imposed in Test A also induced additional vertical inertia, the magnitude of the induced inertia is much smaller than that for Test C. With the presence of induced inertia, it is however contrary to the engineering intuition that the scratch width for Test A is smaller than that for Test B, where there should not be any additional inertia induced. One possible reason for such an anomaly is because of the pre-existing high penetration depth due to the high initial dead load for Test B, which leads to a much higher resistance against horizontal sliding. This, in turn, induces a higher ‘scratching force’ required to drive the scratch tip to maintain a constant speed of 100 mm/s, when compared to Test A.

Comparing the scanned images of the scratch morphology of a talcfilled PP copolymer under the three test conditions, the scratch width remains constant along the scratch path for Test B and C conditions; while there is a gradual increase in scratch width along the scratch path for Test A (Figure 4.4).

The damage induced in the scratch groove undergoes a transition as the scratch progresses in Test A. Minimal surface features are observed in the beginning while severe damage with prominent ripple marks is present toward the end of the scratch. It is found that the ripple marks are actually curved fracture lines that appear periodically. The same phenomena are also observed in other model PP systems. It should be noted that the existing initial scratch width of 0.33 ~ 0.45 mm found in specimens is caused by the pre-existing small mass of the scratch tip and the load control unit, which measures about 5 N. Future improvement to the test device will be made to minimize such a pre-existing dead load prior to testing.

It is apparent that the linear load increase test, i.e., Test A, is a more sensible test method in characterizing scratch damage resistance in polymers.

Subsequent tests done on different material systems will demonstrate the usefulness and effectiveness of this test. The test has shown that copolymer systems suffer more damage than homopolymer systems (Figure 4.5). This is to be expected as the Young’s modulus and yield strength of copolymer PP are lower than those of the homopolymer PP [70]. Interestingly, the addition of talc does not cause significant changes in the size of scratch damage as quantified by the scratch depths and scratch widths. The test also found that all scratch depths and scratch widths show the same general trend between the copolymer and homopolymer PP, and between neat and talc-filled PP systems.

Figures 4.6 and 4.7 illustrate a typical complex surface feature and its sub-surface damage profile after a scratch is performed on a polymer. It is evident that complex surface damage mechanisms, such as plastic ironing,brittle fracture, fibril drawing, filler debonding, stick-slip, etc., can evolve,causing the scratch depths to vary within the same scratch pass. Thus, it is recommended that scratch widths, as opposed to scratch depths, be considered as a more reliable and consistent measure to quantify scratch damage.

Adopting the scratch widths as a measure of severity of surface damage can be quite practical since flatbed scanners can be used for the measurement.

However, it should be highlighted that the scanned images generally have a relatively lower resolution than TOM or SEM images. Hence, one cannot easily distinguish between SW1 and SW2 from the scratch widths measured from scanned images. Nevertheless, scanned images allow one to have a quick assessment of the scratch damage. More sophisticated imaging tools can always be used for a more detailed study, if needed.

2019-04-10 16:18

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