Mechanical properties of TiN/TiB2 multilayers deposited by plasma enhanced chemical vapor deposition

Nanometer scale TiN/TiB2 multilayer coatings with a thickness of approximately 2 μm could be deposited on steel substrates using PECVD. The coatings were prepared by alternate depositions of TiN and TiB2 using a gas mixture of TiCl4, H2, Ar, N2 and BCl3. The TiN grain size increased with increasing TiN layer thickness, while changes in the TiB2 layer thickness had no significant influence on the microstructure of the TiB2 layers. The plastic hardness of the coatings increased steadily with decreasing TiN layer thickness and with increasing TiB2 layer thickness. No significant change in hardness was observed with changes in the layer period. The hardness values of the multilayer coatings had a linear relationship with the TiB2 volume fraction. Therefore, the maximum hardness value of the multilayer coatings did not exceed that of the monolayer TiB2 coating. The wear resistance, however, increased by incorporating TiN layers. It was found that multilayer coatings with periods <24 nm showed better wear resistance than the monolayer coatings. It is believed that the wear resistance of the multilayer coatings was improved as crack propagation could be interrupted at the layer interface and the size of the debris detached from the coating could be reduced.