To reveal the flame-retardant mechanism of the hybrid SiR-GO coating, the residual char of PU@SiR-GO 1.00% composite was investigated. In particular, the outstanding flame retardancy of this sample during the combustion process was considered. The components inside and outside the skeleton surface were characterized by FTIR spectra, as shown in Fig. 7(a). The combusted PU@SiR-GO composites maintain their structural integrity (see Fig. 7(b)). After carefully cutting the PU@SiR-GO composites with a blade, the inside zone exhibits a good condition, though appearing dark, which is likely due to the presence of GO nano-sheets. Although both the outside and inside zones keep the integrity and dark appearance, it is worthy to note that white powders are found outside the skeleton surface but not inside. To clarify the composition of this white powder, the white powder was carefully collected and characterized using FTIR and XRD. The red curve in Fig. 7(a) is the FTIR spectrum of the white powder. Two distinct peaks were observed: one peak at 1080 cm
-1 corresponds to the stretch vibration of the Si
-O
-Si bond, and the other peak at 794 cm
-1 corresponds to the stretch vibration of the Si
-C bond. Meanwhile, the stretching vibration of the C
-H band at 2980 and 2880 cm
-1 is not detected, indicating the SiR has completely decomposed under thermal attack. The XRD curve obtained for the white powder is plotted in Fig. S3 (cf. ESM); a diffraction pattern peak at approximately 2
q = 23°
-24° was detected, which can be attributed to the diffraction peak of amorphous SiO
2 [
56]. These results further confirmed that the white powder layer is mainly composed of nano-silica, which is likely formed during combustion. The green curve in Fig. 7(a) reflects the inner zone of PU@SiR-GO 1.00% after combustion. The bonds at 3368, 2968, 1011, and 1269 cm
-1 are assigned to the stretch vibration of N
-H, C
-H, Si
-O
-Si, and C=C bonds, respectively. The peak at 1431 cm
-1 is the vibration of the aromatic ring, and 1269 cm
-1 conforms to the symmetric deformation vibration of Si
-CH
3. The bands at 763 cm
-1 and 696 cm
-1 are in accordance with C
-H planar swing vibration and Si
-C stretch vibration, respectively. The presence of N
-H and Si
-CH
3 indicates that the inner zone does not show an evident thermal degradation of SiR and PU, where the structural integrity and composition are preserved well.