Experimental investigations of electrodeposited Zn–Ni, Zn–Co, and Ni–Cr–Co–based novel coatings on AA7075 substrate to ameliorate the mechanical, abrasion, morphological, and corrosion properties for automotive applications
Authors: Sundaramali Govindaswamy , Aiyasamy Jeeva P. , Karthikeyan Sambantham , Kandavel Thanjavur K. , Arulmurugan Balasubramanian , Rajkumar Sivanraju , Sharma Shubham , Li Changhe , Dwivedi Shashi Prakash , Kumar Abhinav , Singh Rajesh , Eldin Sayed M. 
Published: July 2023

Abstract
The aluminum (Al) alloy AA7075 is widely used in various industries due to its high strength-to-weight ratio, which is comparable and replaceable to steel in many applications. However, it has poor resistance to wear and corrosion compared to other Al alloys. The conventional pressure die coating with Cr and cadmium has led to premature failure while the load is applied. It is indeed to develop a novel coating method to improve the mechanical, wear, and corrosion properties of AA7075 Al alloy. In the present investigation, the binary and ternary metals such as zinc–nickel (Zn–Ni), zinc–cobalt (Zn–Co), and nickel–chromium–cobalt (Ni–Cr–Co) are electroplated on the substrate material (AA7075). In order to ensure optimal coating adhesion, the surface of the substrate material was pre-treated with laser surface treatment (LST). The mechanical and corrosion studies have been carried out on the uncoated and coated materials. It is observed from the findings that the ternary coating has higher wear resistance than the binary-coated material. The ternary coating has 64% higher resistance in the non-heat-treated status and 67% higher resistance in the heat-treated condition compared to the uncoated specimens. The tensile strength (MPa) of Ni–Cr–Co on AA7075 pressure die casting (PDC) is higher than the other deposits (582.24 of Ni–Cr–Co > 566.07 of Zn–Co > 560.05 of Zn–Ni > 553.64 of uncoated condition). The presence of a crystalline structure with the high alignment of Co and Ni atoms could significantly improve the corrosion resistance of Ni–Cr–Co coatings on AA 7075 PDC substrates when compared to binary coatings. The scanning electron microscopy (SEM) images, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy findings on the coated materials have been corroborated with the analyses on mechanical and corrosion properties. The XRD analysis of the Zn–Ni binary coating has reported that the diffraction peaks of γ-NiZn3 (831), γ -Ni2Zn11 (330), and 631 with 2θ values 38, 43, and 73° are confirming the presence of Zn–Ni binary deposit on AA7075 PDC substrate. The XRD pattern of Zn–Co-coated material has revealed that the presence of three strong peaks such as Zn (110), Co (111), and CoZn (211) and two feeble peaks such as ε-CoZn3 (220) and ε-CoZn3 (301) are clearly visible. The XRD pattern of Ni–Cr–Co ternary coating has exhibited that the Ni–Cr–Co ternary deposit is a solid solution with a body-centered cubic structure due to the formation peaks at lattice plane such as (110), (220), and (210) with a crystal lattice constant of 2.88 A°. The SEM image for both the binary- and ternary-coated materials has exhibited that the deposited surface has displayed many shallow pits due to hitting by progressive particles. The SEM image has illustrated the presence of Zn–Ni atoms with smaller globular structure. The surface morphology of binary Zn–Co coating on the PDC AA7075 substrate has unveiled the evenly distributed dot-like structure and submerged Co particles in the galaxy of Zn atoms. To understand the effectiveness of bonding by laser texturing, cross-section SEM has been carried out which furthermore revealed the effective adhesion of Ni–Cr–Co on AA7075 PDC; this could also be the reason for the enhancement of microhardness, wear, and corrosion resistance of the said coating.