Plasmochemical modification of aluminum-zinc alloys using NH₃-Ar atmosphere with anti-wear coatings deposition

Highlights

• Surface properties of Al-Zn alloy after plasma processes are investigated.

• Modification in a RF reactor was the second stage of ageing.

• The N⁺ ion treatments of aluminum substrates was justified.

• SiCNH coatings obtained on Al alloys significantly improve mechanical parameters.

Abstract

This paper constitutes a continuation of studies on modification technologies for 7075 series aluminum alloys (Al-Zn) in plasmochemical conditions using the RF CVD (Radio-Frequency Chemical Vapor Deposition) method. This technique is simultaneously the second stage of alloy ageing. The presented results concern optimization of alloy surface modification using N⁺ ions (in NH₃ or NH₃/Ar atmosphere) before obtaining a DLC (Diamond-Like Carbon) layer doped with Si and N.

From the results it can be concluded that the most profitable mechanical properties (H, ca. 12 GPa and E, ca. 115 GPa) are obtained when the SiCNH coating process is preceded by Al-Zn alloy surface modification with nitrogen ions. These ions are provided by a flowing NH₃ and Ar gas mixture (1:1 ratio). In these process conditions, the lowest tribological wear of the surface is also observed. Furthermore, the obtained coating exhibits a fine-grained structure.

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.