**4. Conclusions**

Several films of Cr-Ti-B-N with a pure Cr target at a constant radio frequency power of 200 W and a series of TiB2 compound targets were fabricated using the reactive magnetron sputtering system. The microstructure, mechanical, and tribological properties of the film was strongly influenced by the TiB2 target power. The cross-sectional morphology showed that the growth mechanism changed from the columnar crystal to isometric crystal with the addition of Titanium and Boron to CrN film. The Cr-Ti-B-N films exhibited a double face-centered cubic (fcc)-CrN and amorphous-BN structure. As the Ti and B elements were introduced, the hardness and elastic modulus of Cr-Ti-B-N films increased from 21 and 246.5 GPa to a maximum value of approximately 28 and 283.6 GPa for Cr38.7Ti3.7 B6.4 N51.2 film, and then decreased slightly with further increase of Ti and B contents. The hardness enhancement was attributed to the residual compressive stress. The friction and wear resistance of the film was improved obviously by the addition of Cr and B as compared with the CrN film under 200 ◦C. It can be conclude that the Cr38.7Ti3.7 B6.4 N51.2 film exhibited improved hardness and wear resistance over broad ranges of ambient conditions.

**Acknowledgments:** This work was supported by National Natural Science Foundation of China (Grants No. 51074080, 51374115 and 51574131).

**Author Contributions:** Lihua Yu conceived and designed the experiments; Lihua Yu and Huang Luo performed and analyzed the data the experiments; Jianguo Bian, Hongbo Ju and Junhua Xu contributed to data analysis. Lihua Yu wrote the paper; all authors participated and discussed this work and contributed to the submitted and published manuscript.

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