## 2 The Materials and Methods
### Tribological Test  
Tribological tests in vitro were realized using a ball-on-flat configuration with the CSM nanotribometer, employing a linear reciprocating module (Figure 1a) in an artificial saliva medium. A commercial alumina ball (Al\({}_{2}\)O\({}_{3}\)) with a diameter of 1.5 mm was used as the stationary contact body (Figure 1b). Alumina is known for its ultra-high hardness (9 on the Mohs scale [21], just below diamond) and excellent wear resistance. All the tests were repeated three times, and the reproducibility of the tribological behavior was fairly confirmed. The characteristics of the conducted tribological tests are given in Table 1.  
The tribological tests were conducted in the presence of artificial saliva to simulate more realistic conditions of the oral environment. This setup is based on findings from previous studies [22], which demonstrated that artificial saliva could act as a coolant and lubricant during wear processes. Additionally, the presence of artificial saliva was shown to reduce the risk of damage to the contact surfaces of materials when compared to dry conditions. The lubrication mechanism in this case is facilitated by the complete separation of sliding surfaces by a thin layer of saliva [23].  
The tribological tests were aimed at tracking the wear and friction values as a function of the sliding distance. The coefficient of friction values was recorded in real time during the tests using TRIBOX 2.9.0 software. After each test, the samples were removed from the nanotribometer, and the wear tracks were documented using an Optical Microscope (MEJII Techno, Tokyo, Japan) and Scanning Electron Microscope (JEOL JSM-6610 LV, Tokyo, Japan).  
After each completed test, the wear volume of the samples was determined based on the measured width and length of the wear tracks, as measured through Optical Microscopy and the software Infinity Analyze, version number: 6.5.0, according to ASTM G133-05 [24; 25]. The wear volume (V) was calculated using the measured wear track length and its average cross-sectional area. The cross-sectional area was assumed to be a flat segment of a sphere corresponding to the geometry of the Al\({}_{2}\)O\({}_{3}\) ball. Measurements were taken at three equally spaced locations along the wear track to ensure uniformity, as required by the standard. Given that the wear of the alumina ball can be characterized as 'unmeasurable wear', only the wear of the flat sample was measured and  
\begin{table}
\begin{tabular}{c} \hline \hline
**Instrument Preferences** \\ \hline Linear reciprocating module (linear mode acquisition) \\ Half of amplitude: 0.5 mm \\ Frequency: 100 Hz \\ Ambient air temperature: 23 \(\pm\) 2 \({}^{\circ}\)C \\ \hline
**Tested samples** \\ \hline Commercial lithium disilicate—IPS e.max CAD (Ivoclar Vivadent, Schaan, Liechtenstein) \\ Finishing procedures of samples: polishing, glazing, and grinding \\ \hline
**Static body** \\ \hline Commercially supplied ball: alumina (Al\({}_{2}\)O\({}_{3}\)) \\ Ball diameter: 1.5 mm \\ \hline
**Normal load values** \\ \hline Fr: 250 mN; 500 mN; 750 mN; 1000 mN \\ \hline
**Maximum linear velocity values** \\ \hline v: 4 mm/s; 8 mm/s; 12 mm/s \\ \hline
**The test duration** \\ \hline
10,000 cycles (20 m) \\ \hline
**Environment contact zone** \\ \hline Artificial saliva medium \\ \hline \hline \end{tabular}
\end{table}
Table 1: Tribological input parameters.  
Figure 1: (**a**) Linear reciprocating module; (**b**) contact geometry [4].  
calculated. The wear rate (W), representing the material removal per unit sliding distance, was calculated using the following equation [26]:  
\[W=V/s,\,[\rm mm^{3}/m], \tag{1}\]  
where:  
\(V\)--is the wear volume of the material [mm\({}^{3}\)].  
\(s\)--is the sliding distance [m].  
The total sliding distance of 20 m was used for the calculations. The wear rate was calculated for each test to quantify the material's wear performance.