Combination of cold spray and micro-arc oxidation methods to fabricate a wear resistant multilayer coating on AZ31 alloy

Abstract

Magnesium alloys are widely used in automotive and aerospace industries. Owing to their high specific strength, weight reductions in order to decrease CO2 emmissions are possible. Applications range from engine covers to wheels and structural members in the automotive industry and gearboxes and electronic housings in aerospace can be given as examples. However, main problems inhibiting the adoption of magnesium alloys further is their low mechanical and surface properties. Due to these, wear and oxidation is prevalent in magnesium alloys. In order to increase these properties, surface modification techniques are widely studied. Among the various surface treatment methods, cold spray process allows the deposition of metallic or metal matrix composite coatings on the substrate without altering the microstructural properties of the substrate material and offers a cheap and functional solution to surface modification. There are various studies in the literature employing hard particles within a ductile matrix in order to improve wear resistance of the substrate. Micro arc oxidation (MAO) method offers an environmentally friendly solution to hard and wear resistant coatings by forming an oxide layer on the substrate. Studies on MAO for wear related applications are increasingly becoming popular. Among these studies, applications of MAO on a cold spray deposited surface is emerging as a new trend. Subsequent applications of cold spray and MAO enables a hard coating with an underlayer with increased load carrying capability. In this study, subsequent applications of cold spray and MAO has been conducted in order to increase the surface properties of magnesium alloys. The chosen composite coating system for cold spray process consisted of Al as a matrix and Al2Zr intermetallic compound as the reinforcing particle. This system presents strategic advantages over conventional ceramic reinforced coatings such as SiC, TiB2 or Al2O3. Low wetting characteristics of these particles leading to interfacial bonding problems and structural discontinuities in the Al matrix whereas Al2Zr particle posess a metallic character that enables excellent chemical and crystallographic coherency with Al matrix. This coherency eliminates any secondary surface activation step that is mandatory for ceramic reinforcements and makes the process more cost efficient, simpler and available. The intermetallic phases which impact the surface with high velocity during cold spraying, homogenously distribute within the Al matrix and form a functional layer on the surface without altering the substrate microstructure. The choice of Al2Zr for the MAO process also plays a determining role over the coating's mechanical characteristics. Al2O3 based ceramics formed by MAO naturally posess high hardness and low fracture toughness. This makes the Al2O3 susceptible to crack propagation. The structure to be achieved through the oxidation of Al2Zr intermetallic compound, especially ZrO2, is expected to increase the toughness within the ceramic matrix and improve wear resistance. This multiple coating synergy can be a promising technical solution to improve surface hardness and wear resistance of magnesium alloys. To the best knowledge of the author, no studies have been encountered in the literature focusing on the investigation of Al2Zr intermetallic compound's cold spraying and MAO behaviour. In this study, Al2Zr has been incorporated into the cold spray Al matrix as a hard reinforcing phase with 0 vol. %, 20 vol. % and 40 vol. % ratios and subsequently MAO process has been successfully applied with the aim of increasing wear resistance of the AZ31 magnesium alloy and fabricating zirconia toughened alumina (ZTA) regions with the MAO process. Commercial Al and laboratory synthesized Al2Zr powders via vacuum arc melting method were used with cold spray deposition while the MAO process is applied in an aluminate based electrolyte. In order to compare the structural and mechanical behaviours of cold spray coating and MAO modification, AZ31 alloy was also subjected to the MAO process in same condition. Structural characterization of the fabricated samples was done by optical microscope, X-ray diffractometer and an energy dispersive spectroscopy equipped scanning electron microscope (SEM EDS). Mechanical characterization steps involved Vickers micro hardness measurements on cross sectional cold sprayed samples and dry sliding wear tests, which were conducted on a linear reciprocating ball-on-flat tribometer against alumina ball counterface. The tests were conducted at room temperature under testing load of 3 N. After wear tests, wear tracks of the samples were examined with an SEM and a 2-D surface profilometer in order to determine the active wear mechanism and specific wear rates, respectively. Structural characterization of the cold sprayed samples revealed that the single phased Al and dual phased Al/Al2Zr composite coating was successfully deposited without impurities and contamination. Hard Al2Zr particles enabled the reduction of porosity within the cold spray coating. Through cross sectional examinations, Al2Zr particles has been found to fracture and crumble upon impact due to their brittle nature. Average hardness values of the composite cold sprayed coatings was measured as 96 HV0.3 for the 40 vol.% reinforced one and 69 HV0.3 for the 20 vol.% reinforced one while the single phase Al cold spray coating has a hardness of 59 HV0.3. Wear behaviour of the cold spray coatings showed that the 40 vol. % reinforced sample increased the wear resistance by 4-times through reducing the plastic deformation on the surface. On the other hand, unreinforced and 20 vol. % reinforced cold spray samples have been completely worn and failed to increase the wear resistance of AZ31 alloy. Considering the material transfer towards the counterface and the worn surface appearance; dominant wear mechanism was determined as adhesive wear for unreinforced and Al2Zr reinforced cold spray samples. After the MAO process, the resultant oxide layer on AZ31 alloy consisted of MgO and MgAl2O4 spinel oxides, oxide layer of the unreinforced cold spray sample consisted of alumina (-Al2O3, -Al2O3) phases and oxide layer of the particle reinforced cold spray samples consisted of Al and Zr based oxide structures (-Al2O3, -Al2O3, t-ZrO2 and m-ZrO2). Surface and cross sectional EDS studies revealed that the Al oxide is grown from the Al matrix and the Zr oxides evolved from the Al2Zr particles. With the increasing contribution of Zr into the MAO oxide, the resulting surface roughness and porosity exhibited a decreasing trend. On the other hand, surface micro-cracking of the MAO layer increased with increasing content of Zr. The wear test results revealed that while the MAO coating formed on the unreinforced cold spray layer was worn, Zr reinforced MAO samples and the AZ31 MAO sample retained their integrities under sliding contact. MAO samples which retained their integrity during wear tests increased the wear resistance of AZ31 alloy more than 1000-times when compared with cold sprayed samples. When the contact surfaces of the counterfaces slid against MAO coated samples were investigated, no material transfer has been found while scratches indicating abrasive wear have been identifed. MAO coated samples with sound integrity showed micro-cracking and local superficial spallation in the wear track. In this respect, the dominant wear mechanism of MAO coated samples was determined as fatigue wear (micro-cracking). MAO coating formed on AZ31 alloy exhibited more cracking in the wear track when compared with Zr containing MAO coatings. In accordance with this observation, MAO coatings formed on cold spray processed Al/Al2Zr composite layers showed more than 30% increase in wear resistance when compared with Mg based MAO coating.