Sputter-grown hierarchical nitride (TiN & h-BN) coatings on BN nanoplates reinforced Al7079 alloy with improved corrosion resistance

Highlights

• Development of BN nanoplates reinforced aluminum alloy (Al-7079) metal matrix by mechanical stir casting.

• The MM nano-composite shows better wetting and anti-corrosion properties as compare to base Al7079 alloy.

• TiNNCs and h-BNNPs were employed on the BNNPs-Al7079 composite surface via a single-step PVD technique.

• Better hydrophobicity and high corrosion resistance shown by BNNPs coated sample compared to TiNNCs coated sample.

Abstract

A boron nitride nanoplates (BNNPs) reinforced aluminium alloy (Al7079) based ex-situ metal matrix (MM) nano-composite was successfully developed via mechanical stir casting process. This MM nano-composite shows better wetting and anti-corrosion properties in comparision of bare Al7079 alloy. To further enhance the aforementioned properties, two dissimilar hard coatings (TiN & h-BN) with hierarchal rough surfaces were employed on the BNNPs functionalized MM composite surface via a single-step PVD technique, which provide a tough barrier with non-wetting surface to prevent the base material from corrosion attack. The addition of these layers led to a significant change in physical, chemical, mechanical and anti-corrosion properties. The results also revealed the hydrophobicity of developed coatings with optical contact angle meter of TiNNCs and h-BNNPs surfaces ~102.1° and 141.6°, respectively. The collective results obtained from electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests revealed the one order higher corrosion resistance of h-BN coated sample than TiN coated sample under saline media (3.5 wt%). This coating may be a promising candidate in various applications, where the protection of materials against corrosion is necessary.

Introduction

Due to the emergence of new materials for various industrial sectors, researcher needs to know the various properties of these new materials and their control methods. There are so many new materials developed with unique properties to fulfil the criteria of corresponding applications. Metal matrix composites (MMCs) [1] material have given us a new era of materials for the use of various application purposes. It is very much required to know the properties of the constituent's material and the properties desired in the final composite material. It is also essential to understand how the composite structure may respond to various sets of conditions. Advanced materials like aluminium matrix composites have attracted considerable attraction for the researchers to exploring their potential applications in aerospace defense sectors, automotive, sports, military, and shipbuilding, owing to their advanced mechanical properties, large specific strength, lightweight, and corrosion resistance [2], [3]. Thus viewed as an ideal candidate in the new generation of high-strength and lightweight materials, under excellent elevated temperature resistance, high specific strength, good isotropic properties, stiffness, modulus as well as high wear resistance with low fabrication costs. Moreover, the better resistance to corrosion of Al-based composites as compared to many other metals makes this material as a predominant choice in materials selection for various applications in shipment industries, where greatest anti-corrosion property is required [4], [5].

Anti-corrosion property is a gradual vicious phenomenon that comes from the environment via strong electrochemical reaction with a corrosive environment. It can cause severe loss to various fields of industries such as airplanes, automobiles, home appliances, industrial plants highway bridges, [2], [5], [6]. Metallic corrosion because of the contact of the metal as well as its atmospheric reasons a great economic loss every year [7], [8]. Although corrosion can be stopped, it cannot be stopped. This is the reason why the focus of recent research is on how to slow down its kinetics. Thus, the recent research is focused on slowing the kinetics of corrosion by modifying it's mechanism [9]. To the date, several approaches have been used to enhance the corrosion resistance of metals, i.e., cathodic safety, synthesis of corrosion resistant coatings, mainly usage of inhibitors of corrosion, or arrangements of the procedures as revealed earlier [10], [11], [12] In addition to these, a mixture of the coatings such as nano-paints, where nanomaterials get incorporated in the polymer matrix is being used. Apart from this, multiphase nanoparticle coatings that can bear high temperatures have also been tried [13], [14]. Despite these advantages, material hardness is the major issue with these coatings [15].

On the other hand, according to the self-cleaning lotus leaf effect, hydrophobic surfaces have been considered as a promising method to increase metals corrosion resistant performance, as surface contact can be inhibited with atmospheric humidity [16], [17], [18]. In brief, better adhesion and greater surface area of nanomaterials to the substrates offer effectual resistance to corrosion and can be employed for protective coatings on several surfaces [19]. Herein context, h-BN (boron nitride) may be a promising material for preventing the underlying alloys from galvanic corrosion because of it's insulating character. Additionally, BN has also various significant inherent properties, including im-permeability, high mechanical properties, and thermal conductivity in evaluation to graphene due to its large bandgap [20], [21], [22]. In this prospect, hexagonal boron nitride (h-BN) coating is chosen to protect the base material from corrosion attack [10], [23]. It's stable and soft character from all other polymorphic structures and existence in various morphologies including nanosheets, nanorods, nanotubes, nanofibers, and spherical nanoparticles make it suitable for this application [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]. The monolayers, multilayers, and a combination with other compounds of h-BN have been developed using various techniques. The most commonly used technique (in research laboratories along with in industries) for this purpose is PVD (sputtering and PLD) technique [37]. The main reason to choose the PVD (sputtering technique) is that it simply control the physical properties of the coatings on large surface area [15], [38], [39].

In order to prevent the base material from corrosion, a large number of coating material are available in the reported literatures. Among them, h-BN and TiN materials are chosen due to their extreme hardness, high friction index, excellent thermal stability and hydrophobicity [10], [40], [41]. These coatings provide a tough barrier to the corrosion, resulting longer life times. Although a large number of reported literature are available on the studies of corrosion resistance for these coatings [41], [42] but, we are interested here to exploring the impact of surface modification and surface functionalization on corrosion resistance and wetting property of the coatings, which is still lacking field in this context. Many studies in the literature suggest that hydrophobicity is also add-on a significant contribution to preventing low-energy surfaces such as coated aluminium from corrosion [43], [44], [45]. Consequently, the study of coating wettability plays a vital part in analyzing the chemical processes of corrosion [46] and opens new field in different area for instance, engineering, medicine, and science [40], [47], [48]. Be contingent on the nature of the system, the wettability is a function of the adhesive and cohesive forces [49]. This study is performed mainly by determining the contact angle amongst the sample's surface and a tangent along the drop surface [50], [51], [52]. The combined function of chemical potential and surface tension of a system is termed as wettability. Depending on the angle formed by the liquid-gas and solid-gas surface tension, all the surfaces show either hydrophobic or hydrophilic behaviour. The wetting is said to be perfect when the contact angle approaches zero [51].

Herein, boron nitride nanoplates (BNNPs) reinforced aluminium-7079 (Al7079) based ex-situ metal matrix (MM) nano-composite via mechanical stir casting process has been developed. This MM nano-composite is found to be more advantageous than the bare Al-7079 alloy in terms of better wetting and anti-corrosion properties. Thereafter, two dissimilar hard coatings (TiN & h-BN) with hierarchal rough surfaces (TiN-nano-clusters & h-BN-nano-plates) were employed on the BNNPs functionalized MM composite surface via a single-step PVD technique. The influence of adding these layer was investigated in terms of it's physical, chemical, mechanical and anti-corrosion properties. The results indicated that the addition of these layers led to a significant change in surface micro-structure, root mean square surface roughness, wetting characteristics and to greatly enhanced anti-corrosion as well as corrosion durability of MM composite, while keeping their physical and mechanical properties. The coatings with the surface bearing micro/nanorough structure and a greater contact angle demonstrate appropriate corrosion properties in the saline medium (3.5 wt% aqueous solutions of NaCl). The durability of the fabricated samples was verified in comparision with fresh samples. Therefore, anti-corrosion performance of the nitride coated and functionalized Al7079 surfaces was studied using techniques like EIS (electrochemical impedance spectroscopy) also Tafel polarization. The non-wetting performance of the samples (coated and uncoated) was investigated via contact angle measurement (Sessile drop method). The illustrated graphical abstract of the present work is presented in Fig. 1.