**4. Conclusions**

242 Atomic Force Microscopy – Imaging, Measuring and Manipulating Surfaces at the Atomic Scale

In the case of fiber A, in addition to surface roughness, chemical heterogeneity due to two regions with different friction coefficients (0.04 and 0.12) was observed by LFM. The great standard deviation of "µs" as well as its bimodal distribution, during fiber-fiber friction are

The low friction coefficient values obtained by LFM may be explained by the fact that sliding of AFM tip on a fiber sample surface is easier (µ < 1), than that of a fiber on another fiber surface (µs~1-10). Moreover in LFM, the friction force analyzed is a dynamic one, while in this case, it is the static friction coefficient which is being evaluated. The friction coefficient values disparities can be related to scale difference of measurements and to the nature and surface of contact which are different in both cases. In AFM, a Si3N4 tip having a radius of curvature of about 30 nm, is in contact with the fiber surface while during fiber-

The theoretical contact area during friction, can be calculated by using the DMT and JKR theories of contact mechanics. But according to Pashley, as the glass fiber is rigid and the fiber radius as well as its surface energy small, the DMT theory should be applied (Pashley, 1984). For orthogonally placed fibers, if contact occurs at one point, the contact radius derived by the DMT is 0.118 µm . However, contact between the two fibers occurs on a length of a circular arc of 2r (/360) (where the wrap angle =2.3° and the fiber radius: r=5.5 . 10-6 m (see Fig. 3). Thus the area of contact 'A' between the two fibers is approximately an ellipse, and it is evaluated to be A=0,12 µm² (details of this result is

The AFM/LFM measurements of the fiber A revealed two regions of distinct friction coefficient (µ~0.04 and 0.12 respectively), each of them having a surface area comprised between approximately 0.1 and 1µm², that is nearly the same contact area as that during fiber-fiber friction. Thus, the main types of contacts that can take place would have the

Fig. 10. Modelling the different contacts possible during fiber-fiber friction of the sized fiber

thus due to surface roughness and chemical heterogeneity.

published in another paper ( N. Behary et al., 2000).

A (the friction coefficients are those evaluated by LFM)

configurations illustrated in Fig. 10.

fiber friction measurements sized fibers of diameter 11 µm are in contact.

**3.4 Determining the theoretical contact area during the fiber-fiber friction** 

**3.5 Types of contact possible during the fiber-fiber friction of the fiber A** 

In light of the above results and discussions, both techniques of measuring friction forces, by Lateral force microscopy and an electronic microbalance, seem to be invaluable methods for characterizing frictional properties of sized glass fibers. The AFM/LFM successfully determines the topography and chemical nature of sized and desized glass fibers. Sized fibers have both physical and/or chemical heterogeneities while the desized bare glass fiber is completely plain and smooth.

AFM/LFM results also help to better understand friction force results obtained at a larger macro scale, particularly the widespread values of friction coefficients during fiber sliding. Nanosccale friction values by AFM/LFM are smaller than 'micro' friction values during fiber-fiber friction because the nature and the area of contact are different in both cases.

## **Part II: Air-atmospheric plasma treatment of PET (Polyethylene Terephtalate) woven fabrics studied by atomic force microscopy**
