## 4 Results and Discussion
### The Friction Coefficient Curves with Different Load  
The friction coefficient curves of four kinds of friction disks under different loads are given in Figure 10. Figure 9(a) is the friction coefficient curve of test specimen T1. The maximum (average) friction coefficient and pressure value under 400 N, 500 N, and 600 N are given in Table 6.  
The maximum friction coefficient value under the load of 600 N is the largest. In the acceleration phase, the friction coefficient decreases first and then becomes stable. In the deceleration phase, the friction coefficient rises slowly first and then rapidly goes to locking friction. The friction coefficient value of 600 N in constant speed period is larger than that of 400 N and 500 N. The friction coefficient curves of 400 N and 500 N are close.  
Figure 10b is the friction coefficient curve of test specimen T2. When the speed is kept at 800 rpm, the friction coefficient values are basically the same. The fluctuations in the friction coefficient become larger than that of 45\({}^{\circ}\) cone angle. Figure 10c is the friction coefficient curve of test specimen T3. The fluctuation in the friction coefficient under 500 N is the largest. Figure 10d is the friction coefficient curve of test specimen T4. When the speed is kept at 800 rpm, the friction coefficient values of 400 N and 500 N are basically the same.  
Figure 10 shows the different COF behaviors of four kinds of friction disks with the same velocity. The cone height and angle of the friction disk greatly influence the strength and frequency of the stick-slip phenomenon. With the increase of cone angle, the frequency of stick-slip phenomenon increases. With the increase of cone height, the strength of stick-slip phenomenon decreases.  
The pressure of T2 is relatively low. The friction coefficient in the constant speed stage is relatively stable. The pressure of T3 is about 1 MPa, while the friction coefficient is almost equal to T1.  
The relative change rate of the friction coefficient is defined as the following Equation (4).  
\[\Delta=\left|\frac{\mu_{max}-\mu_{ave}}{\mu_{ave}}\right| \tag{4}\]  
The friction coefficient value of 600 N in the constant speed period is larger than that of 400 N and 500 N. At the same time, the friction coefficient under test conditions S4 (600 N, 800 rpm) is more stable and representative, so the data under S4 is used for comparative analysis in the next test.  
Where \(\Delta,\ \mu_{max}\), and \(\mu_{ave}\) are the relative change rates of friction coefficient, the maximum friction coefficient, and the average friction coefficient, respectively. The friction  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Test**} & \multicolumn{3}{c}{**Max COF [-]**} & \multicolumn{3}{c}{**Ave COF [-]**} & \multicolumn{3}{c}{**Pressure [MPa]**} \\ \cline{2-10}  & **400 N** & **500 N** & **600 N** & **400 N** & **500 N** & **600 N** & **400 N** & **500 N** & **600 N** \\ \hline T1 & 0.042 & 0.052 & 0.061 & 0.027 & 0.028 & 0.036 & 0.56 & 0.71 & 0.85 \\ \hline T2 & 0.046 & 0.065 & 0.075 & 0.026 & 0.027 & 0.027 & 0.41 & 0.51 & 0.61 \\ \hline T3 & 0.036 & 0.053 & 0.067 & 0.022 & 0.034 & 0.040 & 0.98 & 1.19 & 1.39 \\ \hline T4 & 0.044 & 0.064 & 0.087 & 0.031 & 0.032 & 0.052 & 0.56 & 0.71 & 0.84 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Friction coefficient and pressure value of friction disk (constant speed phase).  
Figure 10: Friction coefficient curve of friction disks.  
coefficients under 600 N of different friction specimens are listed in Table 7. The relative change rate of friction specimen T2 is the biggest.