## 3 Results
### Tribological Results  
The analysis of wear tracks was conducted using Optical Microscopy (OM) and Scanning Electron Microscopy (SEM). As a representative example, Figure 5 presents OM (left) and SEM (right) micrographs of wear tracks on IPS e.max CAD samples prepared with different finishing techniques. The tribological tests were performed in an artificial saliva medium under a load of 1 N, a sliding velocity of 8 mm/s, and 10,000 fretting cycles. The results indicate that abrasive wear is the predominant wear mechanism.  
Within the wear tracks, deformation and damage of the smooth contact surface are evident in the form of tribo layers formation and the appearance of numerous micro cracks. As the sliding velocity increases due to the linear reciprocating motion, existing cracks gradually propagate over time. The delamination of tribo layers, caused by the constant sliding of the ball, leads to the fragmentation of wear debris, ultimately resulting in three-body abrasion. The process of material removal and the formation of new tribo layers is continuous and persists throughout the duration of the test.  
Wear tracks in the cases of 250 mN, 500 mN, and 750 mN were similar to the exhibited one for 1 N. However, with the increase in normal load (Fn), the wear track width visibly increased for all tested surfaces. Based on the aforementioned statements, the mathematically obtained wear rate (W) results for all finishing procedures demonstrated a trend of increasing wear rate with the rise in normal load and sliding velocity.  
Figure 6 presents a representative example of the coefficient of friction (COF) diagram for the polished contact surface of the commercial lithium disilicate material (IPS e.max CAD) under two selected testing conditions (F\({}_{1}\)V\({}_{1}\) = 250 mN, 4 mm/s, and F\({}_{4}\)V\({}_{3}\) = 1000 mN, 12 mm/s) over a duration of 10,000 cycles with the presence of artificial saliva in the contact zone.  
In the presented diagram (Figure 6), it is clearly observed that the COF decreases with increasing normal load. Generally, COF values declined with increasing force across all conducted tests of different surface techniques on the nanotribometer. The distribution and occurrence of peaks in the COF curve can serve as important indicators of various influences and changes in wear mechanisms during testing. The frictional behavior of the examined material surfaces varies depending on the different surface techniques.  
In general, the ground surface finish exhibited the best frictional properties under various testing conditions, which included three sliding velocities and four load levels. The COF values of the ground surface directly correlate with the wear rate for the same surface. This is due to the nature of contact formation, as the reciprocating linear motion of the Al\({}_{2}\)O\({}_{3}\) ball on the ground sample's contact surface occurs exclusively on the asperity peaks, which vary in height. As a result, the presence of wear debris in the contact zone is minimal, as the reciprocating linear motion facilitates its removal through numerous channels (grinding scars) formed by the valleys between asperities, aided by the artificial saliva flow. This phenomenon is clearly illustrated in the SEM image of the ground surface (Figure 5). Consequently, the influence of the \"third body\" effect on the COF values is minimized, unlike in polished and glazed surface finishes.  
Figure 5: OM **(left)** and SEM (**right**) micrographs of wear tracks on IPS e.max CAD samples (under different finishing techniques) in the presence of artificial saliva, \(\mathrm{Fn}=1\) N, v = 8 mm/s, and 10,000 fretting cycles. (MC—micro cracks, TL—tribo layers).