ae gp the left-side edges of islands as well as the reduced lateral forces at the right-side edges of islands
Deg a are topographic artefacts. They result from incomplete cancelling of topography in friction-loop
8 op : N computation, due to steep island edges. Additionally figure 5 is an example for strong tip artefacts.
ME Surface N From the topographic image itis obviously that the tip consist of two apexes 48 nm apart and 36 nm
Te ihe | y difference in height, leading to a doubling of all surface features within the image. oo
ve ime IP In addition to qualitative mapping of friction force
nom bhp W-C:H distributions, FFM is also able to measure local fric-
A been str M [3 a tion forces quantitatively, if the instrument has been
before iy N7 S calibrated carefully (15). As an example figure 6
ling 3 ds J of 41 at% VW shows frictional forces as a function of load for two
of alg | different tungsten-DLC coatings, containing 9 and 41
ng . 0 po’ at% W respectively. A strong influence of the metal
omer of N ~ 4 content on the friction is evident. Additionally, a non-
Fi Sig : 3 N ; linear behaviour of friction versus load can be ob-
od Jat%e W served which results from spherically shaped single
Hat eo in : asperity tip and can be described by nn
| does not corre- 0% Fr ~ H*(R,-Fı) .
ances z the tun- 0 10 20 30 40 where Fy, F, are the friction and load respectively and
an hie load F, [NN] u* = m(3/4)**.S-E**’ = effective microscopic friction
and sample and coefficient, which depends on the shear stress S of the
leading in most Figure 5: Quantitative evaluation of FFM sample and the reduced Young’s modulus E* of tip
I images at different loads for W-DLC coa- and sample (16,17). Apart from these material de-
tings containing 9 and 41 at% W. pendent factors friction also depends on the tip radius
of curvature R,, wherefore only measurements taken
with the same tip are comparable. As a matter of course microscopic friction also depends on envi-
ronmental humidity, surface contamination or oxidation, etc. as in the macroscopic case.
cov hie
ne Microwear and Material Structure
ee Microwear experiments can be performed in AFM simply by increasing the load applied by the
into a quadrant cantilever while scanning the tip, leading to line-like or rectangular wear grooves. For hard materi-
coeficient, but als this can be realised using a diamond tip glued at the and of a stainless steel cantilever (force
scanned FEM constant 300-350 N/m) allowing application of forces from the micro- to the milli-newton range.
+ he alla N Imaging of the wear groove can be carried
Co [A] out directly by the same tip at minimum load
oer Jar -580 oe to measure three-dimensional wear volumes.
fees ml | brez An experimental procedure proposed by
Sl ne a Loubet et. al. (18) proved to be especially
are 10 100 au useful: It consists of taking an AFM image
during a linear oscillating wear experiments,
s AFM topogi?- 1500 giving the depth profile of the wear trace
| force image 05 along sliding direction as a function of time,
dislands o 00 so i.e. Z(X,t) is actually recorded in a cycle-by-
wc of N wear cycles cycle manner. In the following this kind of
wer frict! . , .
oe no Figure 6: time evolution of wear depth of a tungsten- wear map 18 called on-line wear mage.
oi Ww | DLC film (12 at% W) in a linear oscillating wear ex- Applying on-line wear imaging to Me-DLC
aphite bi periment (load = 1.9mN, trace lenght = 5 pm). nanocomposite materials it can be observed.
aryl forces dl
7
