Nanoscratching

Since the early 1980s, the study of mechanical properties of materials on the nanometre scale has received much attention, as these properties are size dependent. The nanoindentation and nanoscratch are the important techniques for probing mechanical properties of materials in small volumes. Indentation load-displacement data contain a wealth of information. From the load-displacement data, many mechanical properties such as hardness and elastic modulus can be determined. The nanoindenter has also been used to measure the fracture toughness and fatigue properties of ul-... 

In order to study microscale friction and wear, scientists have developed the friction force microscope (FFM), nanoindentation and nanoscratch tester which serve as excellent tools in micro tribological research [1,6-9]. In this chapter, we first compare the differences between macro and micro friction and wear, and then introduce some results of our research group on microscale friction and wear of ordered films, thin solid films, and multilayers. 

Fig. 9. AFM image of post CMP surface showing nanoscratches produced by slurry particle...

Tests with Progressive Load In this context a method was developed in the DuPont Marshall Lab. (Philadelphia) that generates and evaluates a single scratch on a surface (nanoscratch method). This method is used for tests along with development work for clearcoats and it is constantly being improved. 

To measure the hardness and elastic modulus of thin films while avoiding the influence of the substrate, peak indentation depth cannot exceed about 30% of the film thickness.Because commercial nanoindenters can make a minimum penetration depth of 10-15 nm, hardness and elastic modulus of films thinner than 30 nm cannot be measured. Clearly, new techniques for fabricating sharper indenters and new nanoindentation theories are needed to extend this technique. For film thicknesses less than 30 nm, nanoscratch tests are widely accepted to evaluate the mechanical properties (discussed later). Alternatively, assuming the hardness and elastic modulus of a film do not change with thickness, thicker films can be used. 

Nanoscratch tests have been used to simulate the effect of third-body particulate wear debris on component surface scratching during use. The load at which the co-efficient of friction or friction force suddenly increases is identified as the critical load, and is used to evaluate scratch resistance and adhesion strength. The depth-sensing nanoindenter, usually equipped with a conical indenter, can elucidate the mode of failure, whether elastic/plastic deformation, cracking, or delamination. 

Atomic Force Microscopy, Scanning Nearfield Optical Microscopy and Nanoscratching... 

M.M. Shokrieh, M.R. Hosseinkhani, M.R. Naimi-Jamal, and H. Tourani, Nanoindentation and nanoscratch investigations on graphene-based nanocomposites, Polymer Testing, 32 (1), 45-51,2013. 

Figure 7.33 Nanoscratch depth profiles for PU and its nanocomposite thin films (a) 3 m s" and (b) 5 m s . Reproduced from Ref [43] with permission.

Figure 7.43 Nanoscratch depth profiles for the PU and the PU/4.4wt% GONP nanocomposite at scratch rates of 3 pm s" (a) and 5 pm s" (b), respectively. Reproduced from Ref. [53] with permission.

Pig. 6. Nanoscratching of a PEEK matrix sample. Reprinted from Ref. 49 (Fig. 3), Copyright (1999), with kind permission from Kluwer Academic Publishers. 

Microfriction in many cases is lower than macrofriction (65,74). This phenomenon is suggested to be due to a reduction in real area of contact and the degree of wear arises from increased hardness and modulus of elasticity with micro/nanoscratch. In addition, the small apparent area of contact reduces the number of particles trapped at the interface, and thus minimizes the ploughing contribution to the friction force (73,74). 

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). 

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