Scratch test forces

Figure 33 shows a typical scratch test curve of Sample 4. Both the penetration depth and the residual depth as well as the tangential force can be obtained from this curve. The critical load can be found from the transition stages plotted in the three curves. The critical load (L ) of Sample 4 is 86.63 mN. 

Let us consider one final example the application of atomic force microscopy (AFM) relating to nanoscale scratch and indentation tests on short carbon-fibre-reinforced PEEK/polytetrafluoroethylene (PTFE) composite blends (Han et al, 1999). In the scratch test, the tip was moved across the surface at constant velocity and fixed applied force to produce grooves with nanometre scale dimensions on the PEEK matrix surfaces. The grooves consisted of a central trough with pile-ups on each side. These grooves provide information about the deformation mechanisms and scratch resistance of the individual phases. In the nanoscale, indentation and... 

To model the complex physical processes occurring during the scratch test sev-eral approaches were taken. Benjamin and Weaver (1960) proposed two models based on the evaluation of the tangential force Fx (model 1) and the normal force (model 2). Model 1 can be described by Eq. (7.4a) as... 

The standard methods to determine the coating microhardness by indentation of a pyramid-shaped diamond indenter are described in ASTM E384-07 (2007). Depending on the shape of the indenter, Knoop- and Vickers-type diamonds as well as Berkovich indenters can be distinguished. The reported hardness number expressed in N mm-2 (MPa) is the force exerted on the specimen surface by the diamond indenter used to produce the impression. In principle, the technique is less affected by porosity than the scratch tests based on measuring the indenter travel caused by a specific increase in load. Microhardness tests are usually made... 

The Universal Nano-i-Micro Materials Tester (UNMT) has been developed to perform a variety of the common adhesion tests [2]. During any of them, it can simultaneously measure contact or surface electrical resistance displacement, deformation, or depth of penetration contact acoustic emission temperature forces in all three directions and digital video of the contact area. This report covers evaluation of the adhesion and delamination properties of coatings by the scratch test. 

In a scratch test, a diamond or WC, conical or pyramidal indentor is sheared across a specimen under a controlled normal load while monitoring the tangential force [117]. The adhesive bond and the tangential plowing force determine the total work performed by frictional forces. The coefficient of friction is determined from the ratio of the tangential to the normal load. 

EDX Energy-dispersive X-ray spectroscopy TEM Transmission Electron Microsoopry XRD X-ray Diffraction Nl Nanoindentations NIT Nano-Impact Test IT Impact Test ITMFS Impact Test with Modulated Force Signal BC Ball Cratering Test WLS White light scanning 3D measurements NT Inclined Impact Test S Scratch test NS Nano-scratch test RC Rockwell C OIF DiFfusion test OX Oxidation test TRM TRiboMeter... 

Force data measured during ductile scratching test on surface of GOE91 (see ig... 

Scratch testing is one method that has been developed for quantitatively measuring coating adhesion. For this test, a diamond-tipped stylus is used to scratch the surface at an increasing loading force until the coating is removed. Examination of... 

Hardness is defined as the resistance of a material to indentation. Since this resistance strongly depends on the shape of the indenter and the force level, there are a large number of different testing methods. These different methods can be classified in three groups, scratch tests, indentation tests, and rehound tests. In general, it is not possible to calculate a hardness value given a value measured with another method - however, conversion tables for common materials are available. 

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. 

SEM, field emission SEM and X-ray diffraction (XRD) are the most common techniques used to characterize surface morphology, microstmcture, and the phase composition of conversion coatings, while energy-dispersive X-ray analysis (EDS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are the most common techniques to characterize the chemical composition and oxidation state of conversion coatings. Other characterizations include transmission electron microscopy (TEM), atomic force microscopy (AFM), scratch test, and hardness and elastic modulus measurements. 

Experimental tools have been developed using a spherical indenter to produce blunt contact while sharp contact is produced by conical or pyramidal tips as reviewed below. Also, scratch testing allows production of a lateral movement during the contact and hence a frictional force. These testing modes allow to approach real contact conditions. 

The scratch test is a method to determine the adhesion strength of a coated layer. In the scratch test, as schematically shown in Fig. 22.10, an indenter is translated across a coated substrate at a constant velocity under a normal force applied. The stresses are introduced to the interface between coating and substrate and at the critical load, the delamination of the coating occurs. Benjamin and Weaver (1963) performed a simple analysis for the scratch test by using the following equations. 

A popular scratch test for plastics used in automotive applications is the Five Finger Scratch and Mar Tester. This apparatus includes a movable sledge and five fingers that include scratch pins, either 1.0 mm or 7.0 mm spherical balls. Each pin is loaded with different weights, such that they exert a standard force on the test surface ranging fi om 0.6N to 25N. The speed of the test is 100 mm/second. 

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. 

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. 

Khrushchev (1957) considers that the need to measure the force T has not been sufficiently well substantiated, nor has a sufficiently precise and easy in service hardness tester been developed yet for determinations of this type. However, he appreciates the usefulness of scratch hardness tests, especially at low loads, as a non-destructive technique. He recommends these methods as very useful for hardness determination of metallic layers or of materials exposed to abrasive wear under operating conditions (plastics, organic coatings, such as varnishes and paints, etc.). Scratch methods are especially important in tests of anisotropic materials where a change in scratch width is the measure of anisotropy. In static indentation methods, the indentations obtained in anisotropic materials are misformed, varying... 

Some investigators believe that the best course to follow in scratch hardness determination is to find the tangential force acting on the surface of material under test required to obtain a scratch of width b. In this case, hardness is expressed as the ratio of that force to the scratch cross-section or a magnitude proportional to it, or else as a quotient of the work necessary to produce the scratch and its volume. Proponents of this method (Yushkin, 1971) consider that scratch hardness tests in the present form play only a marginal role. Unlike Shreyner (1949), who contended that the results obtained under these methods are less accurate than those for mineral-bymineral scratch after Mohs, they accept that these tests have certain usefulness, but only of a complementary nature. The differences of opinion arise from the different approach to the question of accuracy of the method of determination itself, since as can readily be proved, a strict relationship exists between hardness defined as the ratio of load P to square of... 

Fig. 4.3.6. Force distribution in scratch hardness tests. (After Grigorovich, 1965)...

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