The influence of chemical groups on the mechanical properties of SiCNH coatings deposited on 7075 aluminum alloy

This paper presents the results of research into the influence of surface modification of 7075 aluminum alloy with nitrogen ions on the type and quantity of chemical bonds and on the mechanical properties of Si:C:N:H coatings deposited with use of the Micro Wave Chemical Vapor Deposition method. Fourier Transform Infrared Spectroscopy results show that during coating deposition the following chemical bonds are mainly formed: CH2 in SiCH2Si, CC, NH, NH2, CH3, and CH2. Their relative proportions are directly linked to the hardness and modulus distribution of the coatings. The modification of Al alloy substrate with N+ ions below 670 K promotes an increase (by a factor of 2) in the formation of NH and NH2 groups relative to CC double bonds and the CH2 group in the SiCH2Si chain as compared to unmodified samples. However, above 670 K the mechanism changes whereby more CC and SiCH2Si groups are formed as compared to CH2, CH3, NH, and NH2 groups. The change of mechanism determines the hardness of the coatings, which is approximately 8 GPa in the case of modification above 670 K, but does not exceed 3 GPa if modification was conducted below 670 K. Still, coatings deposited on substrates modified above 670 K delaminate in scratch tests by propagation of cracks within the scratch track. This behavior is not observed for coatings deposited below 670 K (on nitrogen-modified substrates). In this case, only plastic deformation in the sample/indenter contact region is present.

► We proved impact of the N+ modification of Al on type of bond of the SiCNH coating.
► We proved influence types of bonds of the SiCNH coating on mechanical properties.
► We model formation of different types of bonds during SiCNH coating deposition.