## 2 Preparation  
The polyamic acid (PAA), as the precursor of PI, was purchased from the Pome Sci tech Co., Ltd., Beijing, China. This PAA product (ZKPI-305) was of PMDA-ODA type. The chemical structure and the preparation process of the PMDA-ODA type PAA/PI have been extensively studied in previous literature [22; 29; 30]. The N-methyl pyrrolidone (NMP), in which the nanoparticles were pre-dispersed, was obtained from Xuntu Chemical Co., Ltd., Shanghai, China. The DNs were obtained from the Sino-Crystal Micro-Diamond Co., Ltd., Zhengzhou, China, with an average particle size of 50 nm. The SCWs were produced by Sinet Advanced Materials Co. Ltd., Changsha, China, with an average diameter of 300 nm and lengths ranging from 10 to 50 \(\upmu\)m.  
Ball-milling and mechanical stirring experiments were performed to mix the DNs, SCWs and PI coating resin. The result showed that the reinforcement materials were not well dispersed in the matrix when directly mixed together, which might be due to the high viscosity of the PI resin. Figure 2a shows the dispersion result of a direct mixture process (8 h ball-milling followed by 4 h mechanical stirring). It can be seen that some SCWs were tangled up to form whisker clusters. Furthermore, the SCWs were prone to break off during the ball-milling process, leading to degradation in the enhancement effect. Thus, for the preparation of the composite film, it was a challenge to disperse these two different reinforcement materials in the matrix with uniform distribution. Figure 2: The diamond nanoparticle (DN)/ silicon carbide whisker (SCW)/ polyimide (PI) composite films prepared by (**a**) direct mixture process and (**b**) the pre-dispersion method. After a large number of experiments, a mixing method was developed with a good dispersion result. Whiskers and particles are randomly distributed in the matrix as shown in Figure 2b. In this method, the reinforcement materials were pre-dispersed respectively before mixing with PI. The DNs were pre-dispersed in the NMP by ball-milling before mixing with PI. Since the SCWs were not suitable for ball-milling, they were pre-dispersed by a mechanical stirring process in ethanol. The ethanol was chosen as the pre-dispersing matrix for its low viscosity and high volatility. The low viscosity was beneficial for the pre-dispersion of SCWs, and the high volatility was necessary for the subsequent mixing process with PI. Since the ethanol was easy to volatilize during the mixing process, the mixture becomes viscous again to suppress the sedimentation and stratification of the fillers. The preparation process of the composite film is illustrated in Figure 3. The details are described as follows. 1. The DNs were mixed with the NMP. Then the mixture was ball-milled for 2 h with a rotation rate of 500 r/min;
2. The PI precursor was added into the mixture and then was ball-milled for another 8 h to fully disperse the nanoparticles in the PI matrix;
3. The SCWs were added into the ethanol at a mass ratio of 1:20 and then mechanically stirred for 1 h with a rotation rate of 900 r/min. The vessel was sealed during the stirring process to avoid the volatilization of the ethanol;
4. The ball-milled DN/PI mixture was added into the SiC/ethanol mixture. Then it was treated with ultrasonication and mechanical stirring alternately. During this process, the vessel was open and the ethanol continued to volatilize. This process took about several hours until most of the ethanol was volatilized and the mixture became viscous;
5. The DN/SCW/PI composite was placed in vacuum for 20 min to remove the air from the mixture;
6. The composite was spin-coated on the silicon wafer;
7. The wafer was placed in an oven with nitrogen atmosphere and the DN/SCW/PI precursor was cured through a stepped heating process (the same process as curing the PI matrix). The temperature curve is shown in Figure 4.  
Figure 3: The preparation process of the composite film. Scanning electron microscopy (SEM) was used to observe the morphology of the composite film.