## 4 Discussion  
Experimental results for each condition are shown in Figures 4-10. In Graphs 3 and 4, which had relatively low pressure and low speeds, the occurrence of ultrafine particles was not confirmed. The emission of relatively large particles between 1 micrometer and 10 \(\upmu\)m can be found. In Graphs 5 and 6, the emission of ultrafine particles of less than 10 nanometers increased with respect to rpm increase. In Figure 8, the pressure increased to 0.66 MPa, and the overall pattern of the graph was similar to 0.25 MPa. Figures 8-10 show that the amounts of ultrafine particles produced increased several times to several tens of times as rpm increased. Comprehensive data are shown in Figure 11. Wear particles larger than 0.75 \(\upmu\)m in diameter appeared to increase as the friction condition became harsh; this was found by comparing visibly seen red dots and black dots. However, some conditions showed similar results, and in order to quantitatively evaluate them, the brake power equation was substituted [28]. Accordingly, the conditions changed from the pressure and speed values to the deceleration of the vehicle. If listed sequentially through the calculation formula, values such as those in Table 5 could be obtained. Figure 10: Number concentration graph with particle diameter of condition 0.66 MPa 240 rpm. Figure 9: Number concentration graph with particle diameter of condition 0.66 MPa 180 rpm. Assuming the number concentration as the amount of wear, the wear factor could be obtained for each particle diameter. Comparing the data between the deceleration and the amount of wear in Table 5, the form can be shown in a graph like that of Figure 12. The graph was linear overall, and the slope varied slightly from diameter to diameter. Between 1.2 and 3 \(\upmu\)m, the average diameter showed the highest number, and the number of moments when it falls below 0.75 and above 7.3 \(\upmu\)m decreased dramatically (black, brown dots). As the above graph shows, it is reasonable to insist that the fine particles from the micrometer region were caused by mechanical abrasive wear. The abrasion of coarse particles dominated the friction process, and the worn particles rolled against the bulk materials, which caused multiple body abrasions and led to increasingly complicated friction. We assume that the main reason for this result was the difference in the contact area of the brake pad specimen and the complexity of the pad material. The surface of the pad was not flat, and friction occurred in the actual contact area but not in the apparent area. Consequently, the temperature of the actual pad surface was distributed irregularly, and the pressure distribution changed continuously for the same reason [29]. This imbalance affected the sheer force of the junction, resulting in a change in the amount of wear debris. Figure 11: Comprehensive graph of number concentration of each friction condition cases. Black dot for 0.25 MPa, red dot for 0.66 MPa. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline Pressure (MPa) & \multicolumn{3}{c}{0.25} & \multicolumn{3}{c}{0.66} \\ \hline RPM & 120 & 180 & 240 & 300 & 120 & 180 & 240 \\ \hline Deceleration (m/s\({}^{2}\)) & 4.91 & 7.36 & 9.81 & 12.3 & 8.83 & 13.24 & 17.66 \\ \hline \end{tabular}
\end{table} 
Table 5: Deceleration values corresponding to friction conditions. By sorting the fine particles in region B in the same way, a graph, such as that shown in Figure 13, could be derived. In the case of fine particles, in the region under 1 micrometer, the focus was on chemical action rather than mechanical wear in the junction. As the deceleration increased, the number of ultrafine particles generated tended to increase linearly at the log scale of number concentration. Overall, a large number of abrasion particles with a diameter of 0.1 micrometer occurred. It can be seen that, when the generated particles were replaced with the amount of wear, it increased exponentially as the deceleration increased.