**4. Conclusions**

AZ91D coatings and SiC-reinforced AZ91D composite coating were fabricated using cold spraying. Compressed air was used as the main gas, and the effects of gas temperature were studied. It was found that the gas temperature should be higher than 450 ◦C for cold spraying of spherical AZ91D powder particles. The deposition efficiency increases greatly with the gas temperature and it is lower than 10% for all coating specimens. The coating average porosity remained between 3.6% and 3.9% when the gas temperature is in the range of 500~600 ◦C. Coatings are denser when the gas temperature is higher. XRD results showed that no phase transformation or oxidization occurred during the cold spraying process of AZ91D. The microhardness of the coatings (approximately 100 HV) was much higher than that of as-casted bulks due to the work hardening effect. Gas temperature does not show an obvious influence on microhardness. Partial impact melting was found between AZ91D particles in the coatings fabricated at 550 ◦C and 600 ◦C.

15 vol % SiC powder particles were added to feedstock powder as reinforcement to improve the coating performance. Results showed that the SiC content of coatings is 10.7 ± 3.2 vol %. Reduced SiC content means that the deposition efficiency of the SiC powder particles is lower than the deposition efficiency of the AZ91D particles. The addition of SiC particles reduces the porosity and increases the microhardness of cold-sprayed AZ91D coatings. Both the AZ91D coating and SiC-reinforced AZ91D composite coating serve as the cathode for the magnesium substrate due to the relatively higher corrosion potential. The addition of SiC particles increases the open circuit potential of the AZ91D coating, meanwhile increasing the galvanic potential and decreasing the negative galvanic current of the coating-substrate couple.

**Acknowledgments:** This investigation is financially supported by the National Natural Science Foundation of China under grant no. 51601158 and the Qihang Science Research Foundation of Southwest Petroleum University, no. 2015QHZ013.

**Author Contributions:** X.S., M.P., and H.L. conceived and designed the experiments; Y.W. and X.S. performed the experiments; X.S., Y.W., and J.T. analyzed the data; B.N., M.P., and H.L. contributed materials; J.T. contributed analysis tools; and Y.W., X.S., and B.N. wrote the paper.

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