Pencil scratch tests

Figure 9.8 SEM images of the coatings treated by tetraethyl orthosUicate chemical vapor deposition (TEOS CVD) for 3 h after 2EI (a and b), 3EI (c and d), 4EI (e and f) and 5EI (g and h) pencil scratching tests. Reproduced from [52] with permission of The Royal Society of Chemistry.

A commonly used quick characterization of relative hardness is provided by the pencil scratch test [25] in which a series of standardized, sharp pencils are drawn across the coating at a specific angle to find the softest one that causes a scratch. Thus, coating hardness is reported as that of the scratching pencil (e.g. 4B-4H). The test is obviously subject to a certain amount of subjectivity, since it depends on the force applied to the pencil, the pencil consistency (i.e. different manufacturers of pencils make pencils of slightly different hardness), pencil sharpness and inclination. For this reason, pencil hardness should only be used as a comparative measure of coating hardness, when the same person performs the test on a variety of coatings with the same pencils. 

As mentioned earlier, one of the main purposes of reinforcing thin sol-gel silica films is to improve their wear and scratch resistance, and for this reason scratch tests are usually performed to measure the scratch resistance and the fi iction coefficient of the coating [33,35,40,41]. These tests are usually conducted by using nanoindentation equipment or pencil scratch tests, but traditional wear tests using a tribometer can also be found in the literature [5,42]. Impact and erosion resistance [43] are also mechanical properties of sol-gel coatings that can be improved by the addition of fillers. 

Reference books list the Mohs hardness for many known minerals and a simple scratch test with a hardness pencil can be used with any unknown materials (providing there is a large enough surface available for scratching). It is conceivable to reverse the situation in that the scratching is done with a particle (or particles) of the material under test, on a surface made of the reference material but no standard procedure is known to the author. 

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. 

Although hardness is a somewhat nebulous term, it can be defined in terms of the tensile modulus of elasticity. From a more practical side, it is usually characterized by a combination of three measurable parameters (1) scratch resistance (2) abrasion or mar resistance and (3) indentation under load. To measure scratch resistance or hardness, an approach is where a specimen is moved laterally under a loaded diamond point. The hardness value is expressed as the load divided by the width of the scratch. In other tests, especially in the paint industry, the surface is scratched with lead pencils of different hardnesses. The hardness of the surface is defined by the pencil hardness that first causes a visible scratch. Other tests include a sand-blast spray evaluation. 

Table 14.3 contains comparative hardness values for five hardness scales including the classical Mohs scale, which ranges from the force necessary to indent talc given a value of 1 to that needed to scratch diamond given a Mohs value of 10. In the field, a number of relative tests have been developed to measure relative hardness. The easiest test for scratch hardness is to simply see how hard you have to push your fingernail into a material to indent it. A more reliable approach involves scratching the material with pencils of specified hardness (ASTM-D-3363) and noting the pencil hardness necessary to indent the material. 

Resistance to abrasion is measured by determining the weight loss at the abraded surface. Scratch resistance may be measured by using materials of known hardness to scratch the polymer surface. Pencils with known degrees of hardness may be used for this test. 

For Section I of the test (the multiple-choice questions), you will be provided a test booklet and an answer sheet. The answer sheet is a bubble sheet, and the test booklet contains all of the multiple-choice questions. You are not allowed to use scratch paper on the test, and you will have to put your calculator away. Any extra writing you need to do (e.g., calculations) can be done in the margins of the test booklet—not on the answer sheet As far as the bubble sheet goes, first make sure you re using a 2 pencil—that way, the marks you make will be dark enough for the scoring machine to read. In addition, you must make sure that your answers are neatly bubbled into the ovals. 

In the Mohs hardness test, the resistance of the sample to scratching is tested. The Mohs hardness scale is divided into ten degrees of hardness. These are fixed arbitrarily (e.g., talc = 1, Iceland spar = 3, quartz = 7, diamond = 10). A similar hardness scale is based on the scratching power of pencils of different hardness. 

Kohinoor test n. A test for scratch hardness employing a series of pencils of different hardnesses. 

Pencil hardness n. A measure of coating hardness based on the scratching of the film with pencil leads of known hardness. The result is reported as the hardest lead, which will not scratch or cut through the film to the substrate. Paint and coating testing manual (Gardner-Sward Handbook) MNL 17, 14th edn. ASTM, Consho-hocken, PA, 1995. 

Surface hardness of coatings and molded objects is also measured by scratch and abrasion resistance. In one simple test for painted surfaces, the hardness can be quantified in terms of the pencil hardness (HB, 4H, etc.) that leaves an indentation. As a rule, polymer systems cannot approach the surface hardness of silica glass, which is itself a highly cross-linked structure. Some highly cross-linked laminates do have scratch resistance nearly like that of glass. 

A common apparatus used in the paint industry to measure scratch resistance on smooth surfaces and coatings is the Wolff-Wilbom PencU Hardness Test. Using constant pressure and pencils of varying hardness (usually 6B to 6H), the pencil is held at a fixed angle of 45° to the specimen surface and pushed about 6.5 mm away from the operator under a fixed pressure of 7.5N. This test is repeated using softer lead until one is found that will not cut through the coating or indent the surface. 

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