that continuous removal of material by the tip is periodically interrupted by the breaking-off of 
larger amounts of material after a critical number of wear cycles N, (see figure 7). This can be ex- 
plained as low-cycle material fatigue: The periodic compression of the surface by the tip leads to an 
increasing densification and/or plastic deformation of material. Reaching the critical shear stress of 
the substance microcracks are formed, finally leading to a sudden removal of material. This sudden 
loss of material is strongly influenced by the micro- and nano-structure of the film. 
One of the structural specialities of Me-DLC is the fact 
0 that the coatings are grown in a low temperature vacuum 
w deposition process, resulting in a distinct columnar growth 
) structure with typical column diameters of 100-300nm. 
Figure 8 shows the lateral distribution of material break 
off along the wear trace and as function of time. It can be 
observed that always the top part of a complete growth 
’ column is breaking-off during wear process, while 
U neighbouring columns still persist for a certain time until 
= they also break. Consequently growth column interfaces fas 
can be identified as weakest part of the material structure he 
where material breaks first under cyclic loading. rer 
4.5 pm Fig. 9 shows the critical number of wear cycles N_ at loads respe 
Figure 7: on-line wear image of a W- of 1.9 mN for W-DLC as a function of its metal content. It 
DLC (7 at% W) at load of 1.9mN. Z- is found that material fatigue only occurs below 15 at% W, 
scale = 70nm. which corresponds to the percolation threshold of the 
tungsten carbide particles within W-DLC. Below this nn 
ee threshold, N, decreases with increasing metal concentra- 
e tion indicating that more and larger particles of tungsten fi 
bo carbide incorporated in the film worsen its wear behaviour. m 
mis This is probably due to the reduced volume fraction of on 
n highly cross-linked diamond-like carbon matrix. N“ 
10 Above the percolation threshold on the other hand, the 
| critical number of wear cycles is infinite, i.e. there occurs es 
. 50 no material fatigue at all and only the slower continuous a 
‘ Load = 1,9 mN wear is observed. Obviously the coalescence of carbidic $ ” 
c 0 RETRY particles inside the film causes an additional three- a 
W [at%] dimensional cross-linking which reinforces the structure of 8 
the coating. At higher loads of F > 3,2 mN material fatigue 
Figure 8: number of wear cycles until can be observed also above the percolation threshold, i.e. 
first material break-off as a function of higher pressures are able to break the polycrystalline parti- 
metal concentration in W-DLC films. cle network. 
It can be concluded that columnar growth structure and 
particle percolation are important factors influencing the mechanical and tribological properties of 
Me-DLC. A more detailed analysis presenting also semi-quantitative models for the description of 
load- and time-dependence of microwear can be found in (19). 
High Resolution Nanoindentation 
Recent developments of small electrostatic force transducers (20) working in conjunction with an 
AFM open new dimensions of highly resolved, quantitative mechanical characterisation of materi- 
als by nanoindentation. The force transducer replaces the normal AFM head and can be used to im- 
Q