Plasmochemical modification of aluminum-zinc alloys using NH3-Ar atmosphere with anti-wear coatings deposition
Graphical abstract
Introduction
Aluminum and its alloys are becoming increasingly more popular as construction materials for use in the automotive and aircraft industries. The chemical compositions of these materials are modified with various chemical elements (including Si, Zn, Mg, etc.) in order to obtain new alloys with increased mechanical resistance [1], [2]. Technologies of Al alloy manufacturing include the processes of oversaturation and ageing, which lead to alloy component precipitates. These precipitates are formed during specific heat treatments and subsequently hinder dislocation movement, resulting in increased material strength. With respect to 7xxx series alloys, these processes are related to manufacturing of, inter alia, MgZn2 and MgAlCu phases from an oversaturated solution of solid Al-Zn-Mg-Cu alloy [3], [4], [5]. Due to the attractiveness of the previously mentioned aluminum alloys compared to other construction materials (e.g. steel) it is possible to manufacture more light and corrosion resistant elements. These alloys exhibit a wide range of interesting physicochemical properties, including low density, ductility, toughness, resistance to fatigue and high strength [6], [7], [8]. Constant development in areas using construction materials, especially those demanding light, resistant to fatigue materials, resulted in a continuous search for technologies capable of improving aluminum alloy utility properties. This search was mainly necessary due to the insufficient resistance of the alloys against tribological wear [9], [10]. Mechanical and heat processing of Al alloys often lead to unsatisfactory results in improving wear-resistant properties. Therefore, studies have been performed to modify their surface layer. These modifications were conducted via coating deposition of, inter alia, DLC (Diamond-Like Carbon) [11], WC/C [12], TiN [5], [13], or surface hardening by means of nitridation and oxidation [14], [15]. The goal of using the aforementioned processes was to increase alloy surface resistance against wear. Various surface plasma technologies have been proposed and investigated in order to improve the friction and wear properties of Al alloys. This includes magnetron sputtering [5], [13], plasma-enhanced CVD method [14], [16], plasma electrolytic oxidation [17] and plasma immersion ion implantation [18]. The plasma-enhanced chemical vapor deposition (PE CVD) technique attempts to combine some of the advantages of both the CVD and PVD methods. In the case of Al-Zn alloy modifications for application in the motor and aviation industries, especially for use as construction elements with complex shapes (e.g. gear boxes), the most perspective methods are based on plasma-chemical modifications.
This paper constitutes a continuation of the research [19], [20], [21] on Al-Zn alloy surface modification in reduced pressure conditions carried out in order to improve their utility properties, including hardness and tribological features. It specifically presents the results of the studies related to chemical composition optimization of the gas mixture (NH3-Ar) used in the surface modification process performed on the alloys with N+ ions prior to the low friction SiCNH coating deposition process. The RF CVD (Radio Frequency Chemical Vapor Deposition) method was used for this purpose. This technique enables performing the processes in plasma conditions on substrates with complex micro-geometry. Techniques, typical for material engineering, were applied in the conducted study. Microstructure and topography of the modified alloy surface was determined from the research results obtained via scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. X-ray diffraction (XRD) was used for phase analysis, while information on the structure of the obtained coating in terms of short-range order was received by means of FTIR spectra analysis. These studies were completed by analyzing the chemical composition using the EDS method. The nanoindentation method was used to determine mechanical and tribological properties (hardness, Young modulus). The wear of the examined surfaces was determined on the basis of a scratch test.
Section snippets
Materials and methods
The process of Al-Zn alloy (series 7075) modification was conducted in reduced pressure conditions using a RF CVD apparatus (Elettrorava S.p.A., Italy), presented in Scheme 1.
The average chemical composition of the alloys was as follows: 5.3 Zn % wt., 3.1 Mg % wt., 1.7 Cu % wt., ≤0.4 Fe % wt., ≤0.3 Mn % wt., ≤0.3 Si % wt., ≤0.2 Cr % wt., and the remaining percentage consisted of Al.
Optimization of the gas mixture composition (NH3-Ar), supplied to the reactor in the alloy surface modification
Structural, surface topography and composition
Al alloy microstructures, after plasmochemical processes with SiCNH coating deposition (samples A, C and D), are characterized by a compact structure, in which grains sizes do not exceed 0.4 μm. Additionally, the analysis of surface topography revealed a considerable decrease in the roughness parameter Ra from 9.5 nm to 6.4 nm (sample D) compared to the reference sample of Al-Zn alloy without modification. Fig. 1 and Fig. 2 present SEM and AFM images of the surface of selected Al-Zn alloys.
In
Conclusion
From all the conducted experiments, it can be concluded that the combination of heat processing (constituting the second stage of the Al-Zn alloys ageing process) with N+ ion modification increases coating adhesion to the modified surface. Therefore, this is an essential stage in the elaborated technology. In the case of the surface subjected only to the process of SiCNH coating deposition, peeling and loosening of the coating from the substrate was observed.
SiCN:H coatings deposited on alloy
Acknowledgements
This work has been supported by the Polish State Committee for Scientific Research under project no. NN 507 269540.
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