**4. Conclusions**

A series of studies about the growth characteristics and properties of CVD TiN and TiO2 anti-coking coatings reveal that the effect of deposition parameters on the growth characteristics of TiN and TiO2 coatings is very different, and it is strongly dependent on deposition time, residence time, and partial pressure. The growth of TiN coating shows the characteristics of the island growth because the atoms (or molecules) of the deposit are more strongly bound to each other than to the substrate. Differently, the growth characteristics of TiO2 coating growth can be explained by the layer (Frank–van der) mode, which happens when the atoms (or molecules) of the deposit are more strongly bound to substrate than to each other. In general, the growth rate of the star-shaped TiN crystals is higher than that of crystals of other shapes. For the TiO2 coating, the growth characteristics of the layer mode indicate that the morphology of the TiO2 coating does not change significantly with the experimental conditions. The size of the TiO2 particles changed by the CO2 partial pressure is obviously larger than that obtained by changing the partial pressure of H2 and TiCl4. This indicates that the growth rate of the TiO2 grain is most sensitive to the water gas shift reaction.

A coking test was conducted to evaluate the properties of TiN and TiO2 coatings using cyclohexane steam pyrolysis at 770 ◦C for 1.5 h. Most interestingly, carbon deposits are very di fferent on the surface of TiN coatings with a di fferent morphology of irregular particles, needle-like structure, and fine dust-like particles. Flaky carbon deposits and irregular carbon particles of cokes on the surface of di fferent particle sizes of TiO2 coatings that are prepared by di fferent partial pressures are observed. The result suggested that the morphology of non-catalytic cokes is not only a ffected by temperature, pressure, and coking precursor, but is also closely related to the surface state of the coatings. In summary, from a comparison of the cokes on the surface of the TiN and TiO2 coatings, both TiN and TiO2 coatings can effectively prevent catalytic coking and eliminate filamentous cokes. In some cases, however, the N or O atoms in the TiN and TiO2 coatings may a ffect common carbon deposits formed by non-catalytic coking, such as formation of needle-like and flaky carbon deposits.

**Author Contributions:** S.T. and A.T. conceived and designed the experiments; T.L. and J.G. performed the experiments; S.T. and S.D. analyzed the data; A.T. contributed reagents/materials/ analysis tools; S.T. wrote the paper.

**Funding:** The research was funded by A Plan Project by Department of Science and Technology of Guizhou Province (Qiankehe LH Zi [2016] 7104), Natural science foundation of Guizhou Provincial Department of Education (Qianjiaohe KY Zi [2016] 014), and the Doctoral Scientific Research Foundation of Guizhou Institute of Technology.

**Acknowledgments:** We would like to sincerely thank Professor Zhu and Professor Wang of Sichuan University for their selfless help. In addition, we should also thank the provincial first-class platform of the Chemical Engineering Practice Teaching Center and the support of College Students'innovation and entrepreneurship.

**Conflicts of Interest:** The authors declare no conflict of interest.
