Microstructure and Corrosion and Tribo-Corrosion Behaviors of Si-Based and Ti-Based Aerospace Coatings Produced by PECVD

Microstructure and corrosion and tribo-corrosion behaviors of Ti-based and Si-based coatings have been systematically investigated. A series of Ti-based and Si-based coatings with different silicon and/or carbon contents were prepared by plasma enhanced chemical vapor deposition (PECVD). Various experimental techniques were employed for the microstructural characterization of the coatings, e.g., X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The mechanical and tribological properties were assessed using nano-indentation, micro-scratch test and pin-on-disk wear test. Corrosion resistance was measured using potentiodynamic polarization test and analyzed using electrochemical impedance spectroscopy (EIS), while the tribo-corrosion behavior was characterized using reciprocating wear test in corrosion environment in tribo-corrosion apparatus.;Interface structure of the TiN coating system was designed in such a way that the hardness increased gradually with the distance from the substrate to the coating surface. This was done by applying a Cr interlayer in order to enhance the adhesion and simultaneously improve the load bearing capacity. In addition, the Cr interlayer further enhanced the corrosion resistance of the TiN-based nano-composite coatings.;The Ti-Si-C coatings mainly consisted of nanocrystalline TiC particles embedded in the a-SiCx:H and a-C:H matrix. The refinement of the TiC grains and the increase of the amorphous fraction simultaneously took place as more Si and/or C incorporate in the Ti-Si-C coatings. This gave rise to improvement of the electrochemical properties to the Ti-Si-C coatings, which could be attributed to the superior corrosion resistance and homogeneous dense feature of the a-SiCx:H and a-C:H matrix surrounding the TiC nanoparticles.;In the case of the PECVD Si-based coatings, such as a-SiCx:H, a-SiNx:H and a-SiCxNy:H coatings that possessed amorphous structure, the corrosion tests in 1 wt.% NaCl electrolyte revealed that the coated substrates exhibited superior corrosion resistance compared to the stainless steel 301 substrate. The results from tribo-corrosion and dry wear tests showed that the coatings had very different performance in the dry and wet environments. This was especially evident for the a-SiC xNy:H coating, demonstrating clearly that the corrosive environment played a significant role during the tribo-corrosion process. The a-SiC x:H coating had the best corrosion and tribo-corrosion resistance among the tested coatings, which made it a good candidate for the application in the wear and corrosive environments.;The grain size refinement took place as the Si and C incorporated into the TiN coating. At the same time, the microstructure of the coatings changed and a transition from TiN columnar structure into densely packed homogeneous nano-composite structure was observed for nc-TiN/a-SiNx and nc-TiCN/a-SiCN coatings. This gave rise to a further improvement of the corrosion resistance by a factor of ~20 compared to TiN.;Following the investigation of the Si-based single-layered coatings, we attempted to develop a more sophisticated multilayer coating system -- Ti-6Al-4V/SiN/SiC/a-C. The excellent tribo-corrosion performance of this coating was attributed to the combined advantageous properties of respective layers in the multilayer coating system. This coating had Ti-6Al-4V substrate with superior corrosion resistance and light weight compared to stainless steels, the a-SiN:H interlayer with good adhesion to both the substrate and the a-SiC:H interlayer and the a-SiC:H interlayer with excellent corrosion and tribo-corrosion resistance; in addition, the a-C:H top layer had very good tribological properties. Such a multilayer coating could be prepared in a single PECVD system, and it exhibited a great potential to be used in various working environments.