Influence of silica rich HNT/MoS2 hybrid reinforcements on mechanical, wear and corrosion characteristics of magnesium AZ31 alloy

This article examines the effects of combining silica-rich Halloysite Nano Tube (HNT) and Molybdenum di Sulphide (MoS2) as hybrid reinforcements, dispersed at volumes of 2, 4, and 6%, on the surface of AZ31 alloy through the application of the friction stir process (FSP). The prepared composites were analysed to evaluate their microstructure, mechanical properties, wear resistance, and corrosion characteristics. Microstructural observations indicate the occurrence of rapid recrystallization, resulting in reduced grain size and uniform dispersion. The surface composite demonstrates an increasing trend in hardness with the addition of HNT, while hardness decreases with the inclusion of MoS2. The micro tensile test results exhibits that the composite exhibits an increasing trend in strength, while the micrograph of the fractured surface of the micro tensile specimens reveals reduced ductility. The composite displays enhanced wear behaviour with the increasing volume percentages of HNT and MoS2 particles. Solid lubricant nature of the secondary reinforcement and enhanced hardness due to HNT addition and FSP leads to higher wear resistance of the developed hybrid composite. Additionally, the corrosion rate decreases with the addition of HNT, whereas higher concentrations of MoS2 lead to increased corrosion.


Introduction
The global demand for advanced lightweight materials with exceptional properties is increasing.One promising solution is the use of lightweight metal matrix composites as alternatives to steel and cast-iron components [1].Among structural materials, magnesium alloys, known for their low weight, have gained significant interest in various industries such as automotive, aerospace, audio, and electronics [2].This appealing characteristic has attracted automobile builders to substitute heavier materials like steels and cast irons with Mg based materials [3,4].Researches proven that when compared to monolithic counterpart, particulate strengthened magnesium composites exhibit greater srength along with enhanced mechanical and tribological characteristics [5,6].
Recently, nanoparticles such as carbon nanotubes (CNTs) have gained considerable interest in composite production due to their superior mechanical properties.Numerous composites utilizing CNTs as reinforcement have achieved impressive characteristics.Similarly, halloysite nanotubes (HNTs), which possess similar mechanical and physical characteristics to CNTs but are more cost-effective, have emerged as a suitable alternative for reinforcement.In particular, the silica-rich HNT is highly recommended for replacing CNTs in Mg-based composite materials due to its reinforcing capabilities.Halloysite (H 4 AL 2 O 9 Si 2 .2H 2 O) is a clay mineral commonly used as a filler in polymer materials and as a sorbent for the deactivation of hazardous chemicals in the event of uncontrolled leaks [7,8].However, only limited studies are reported on silica-based HNTs added metal composites.
Extensive research studies have clearly shown that both coating techniques and the incorporation of solid lubricants like graphite and molybdenum sulphide have a significant positive impact on improving the wear resistance of various materials.Among these options, graphite is the most commonly utilized solid lubricant, especially in Metal Matrix Composites (MMCs) reinforced with materials such as SiC, Al 2 O 3 , and B 4 C.The combination of a hard ceramic material with a soft solid lubricant in the matrix results in hybrid composites that exhibit exceptional wear resistance [9] and a low friction coefficient [10].The advantage of hybrid composites is that they demonstrate improved properties compared to composites reinforced with a single material, as they can harness the beneficial characteristics of each constituent reinforcement [11,12].Previous research efforts have investigated the wear behavior of Mg and Al hybrid composites, as well as the tribological properties of Al hybrid composites incorporating solid lubricants [13,14] FSP initiated from the friction stir welding process, has gained significant recognition as an innovative solidstate processing technique.FSP offers localized microstructural refinement by subjecting materials to severe plastic deformation without melting, thereby facilitating dynamic recrystallization [15].Consequently, FSPtreated materials exhibit a uniform and refined microstructure in the stirred section.It is achieved by employing a rotating tool comprising a pin and shoulder, which moves in a definite path [16].While FSP has been widely explored for the development of composites, previous studies have primarily focused on incorporating materials such as carbon nanotubes (CNTs) into base metals like Mg and Al.These investigations have demonstrated remarkable enhancements in mechanical properties, including enhanced tensile strength and yield strength, attributed to grain refinement compared to the base material [17,18].Other researchers also examined the microstructure, microhardness, and wear behavior of FSP-processed composites reinforced with multi-walled carbon nanotubes (MWCNTs) in Mg alloy (AZ31) [19].Their findings indicated that the even dispersal of reinforcements and grain refinement achieved through FSP mutually contributed to the observed improvements in mechanical characteristics.The processed zone exhibited ultrafine equiaxed grains, leading to significant enhancements in mechanical properties [19].Additionally, Turan et al studied the effects of MWCNTs on dry and corrosive wear in Mg composites.Their observations revealed a dispersed distribution of CNTs in the matrix without any macro-defects.The incorporation of MWCNTs resulted in improved hardness, and a 0.5 wt% MWCNT content in the composites exhibited significant wear performance under both dry and corrosive conditions [20].
Despite these research efforts, investigations focusing on magnesium composites with nanotube reinforcement remain limited.Although some Mg-based composites have been addressed with different kinds of CNT by several researchers, there have been no reported studies on HNT based hybrid reinforcement system.Though HNT is cost effective, abundant, bio compatible and ecofriendly than CNTs, only minimal researches were ported with HNT as reinforcement.Therefore, this study aims to examine the significant effects of HNT and MoS 2 on Mg composites, with varying weight percentages (2, 4, and 6), developed through FSP.The examination will involve Scanning Electron Microscope (SEM) and Optical Microscope (OM) characterizations, mechanical property evaluations through tensile testing and microhardness measurements, as well as examinations of wear and corrosion behavior.The ultimate goal is to develop Mg composites suitable for applications in the automotive and aerospace industries.

Materials and methods
For the study, AZ31 alloy plates of 150 mm × 70 mm × 8 mm dimensions were employed as base material.AZ31 alloy is known for its superior mechanical properties widely applied in aircraft industries which has aluminium and zinc as major alloying elements of around 3 and 1% respectively.The choice of silica-rich HNT as the primary reinforcement was motivated by its high strength, green nature, corrosion resistance, wear resistance, and non-toxic properties [21,22].HNT with a structural formula of (H 4 AL 2 O 9 Si 2 .2H 2 O) with an average particle size of 40 nm is utilized.Additionally, micro-particles of amorphous MoS 2 , with a mean size of 20 μm, were used as secondary reinforcement.MoS 2 is a solid lubricant commonly added to metals to enhance wear characteristics.Figure 1 depicts the SEM micrograph of HNT and MoS 2 , along with x-ray Diffraction (XRD) and Energy-Dispersive Spectroscopy (EDS) analysis of HNT particles, confirming the presence of rich silica in HNT.
In view of adding reinforcements into the surface of AZ31 plate, 4 mm deep groove was precisely cut at the middle through wire cut electrical discharge machining.The width of the groove varied depending on the volume percentage of reinforcement particles, ranging from 0.3, 0.5, to 0.75 mm to fill 2, 4, and 6 vol.% reinforcements based on the calculations [23].This groove was then tightly filled with a mixture of silica-rich HNT and MoS 2 powder, following the plan outlined in table 1.After filling the reinforcements, a FSP tool without pin was passed over to prevent the particles from dispersing during the FSP.The FSP tool had a pin with a diameter and length of 5 mm and a shoulder diameter of 18 mm, tilted at a 1.5°angle made of HCHCr steel oil quenched to achieve a hardness of 60 HRC is utilized.The tool advancement speed remained constant at 30 mm min −1 , while the tool rotation was fixed at 900 rpm, applying an axial force of 4 kN. Figure 2 shows the base AZ31 plate, FSP process and the resultant surface composite.As a consequence of employing optimized FSP settings, the surface of the materials that underwent friction stir processing was discovered to be smooth and defect free.To investigate the impact of reinforcement phase and FSP on the stirred region of the base plate optical microscopy is utilized.SEM micrographs of the FSP region were utilized to analyze the distribution of HNT and MoS 2 on the magnesium surface, enabling the identification of the bonding between the matrix and reinforcement.To characterize the microstructure, the specimens were polished according to standard metallographic procedures.Test specimens were then cut from the samples, following the ASTM standard.The polished specimens were etched using a laboratory-prepared etchant consisting of 15 ml H 2 O 2 and 2.5 g Fe 3 Cl mixed with 100 ml distilled water for 8 seconds.The hardness tests were performed using a Matsuzawa MMT X7 hardness tester with a defined load of 500 g applied for 10 seconds.Micro tensile tests were carried out on the composite specimens with varying HNT and MoS 2 volume percentages using a Tinus Olsen H50KS universal testing machine.The results were compared with the values obtained for AZ31.Tensile test specimens were cut in the FSP direction from the middle of the stirred area, as depicted in figure 2(d).
The wear resistance property of the specimens was analysed using a pin-on-disc wear test rig (DUCOM).Pin samples, 30 mm in height, were taken from the stirred region for the wear test and the test was conducted under dry conditions, with the pin specimen sliding against a hardened steel disc under a load of 10, 20 and 30 N, while the disc rotated at a velocity of 1 m/s for a duration of 1500 m.The wear rate of the specimen was determined based on the weight loss.The corrosion characteristics of the developed surface were analysed using a VERSASTAT3-400 Electrochemical workstation under a 3.5% NaCl solution.

Microstructure of Mg/HNT/MoS 2 hybrid composites
The optical microscope analysis results of Mg surface composites produced using FSP with varying volume percentages of HNT and MoS 2 as secondary phases is illustrated in figure 3. A careful examination of figures 3(a)-(c) exposes that the composites exhibit a finer and more equiaxed grain structure.As the volume percentages of silica-based HNT and MoS 2 increase, the grain size becomes smaller.The grain size difference between the base material and stirred region can be easily identified from figure 3(c) that can be attributed to the favourable conditions for plastic deformation and elevated temperature resulting from friction during the FSP process, leading to recrystallization.The change in grain size during FSP can be categorized into two types.Firstly, an increase in grain size can be attributed to annealing processes caused by heat generation.Secondly, a reduction in grain size occurs due to the emergence of nucleation sites resulting from continuous mixing, as noted by Barmouz et al [24].The downsizing of grains can also be attributed to significant recrystallization resulting from intense mixing.The presence of hybrid reinforcements also contributes to a reduction in grain size compared to AZ31 material, as observed in figures 3(a)-(c).
The Heat-Affected Zone (HAZ) interface in figure 3(c) refers to the region of the material that has undergone some degree of thermal alteration due to the heat generated during the FSP.The HAZ is the zone adjacent to the stirred material that experiences elevated temperatures but does not undergo the same level of mechanical mixing and deformation as the main stirred region.The HAZ interface represents the boundary between the surface composite and the surrounding base material.This region is important to understand because it has distinct microstructural and mechanical properties (usually low hardness) compared to both the base material and the stirred composite.The HAZ interface exhibit changes in microstructure compared to the base material.The rapid heating and cooling in this zone result in grain growth and changes in grain morphology as depicted in figure 3(c).
The set of SEM micrographs presented in figure 4 clearly demonstrates the presence of reinforced HNT/MoS 2 particles within the magnesium matrix.Furthermore, as the amount of reinforcement increases, 4 the distance between the HNT/MoS 2 particles decreases.The distribution of HNT/MoS 2 within the magnesium matrix appears to be relatively uniform, which can be attributed to the use of optimal tool rotation and transverse speed during the fabrication process.No voids or reaction products are observed in the SEM micrographs, indicating a finer bonding between the reinforced HNT/MoS 2 particles and the base matrix.This enhanced bonding between the reinforced phase and matrix is expected to significantly improve the mechanical and tribological properties of the material, as anticipated.

Microhardness of Mg/HNT/MoS 2 hybrid composites
The hardness results presented in figure 5 clearly indicate an enhancement in hardness within the FSP zone compared to unprocessed magnesium.Microhardness in the unprocessed section is approximately 36 Hv, which slightly decreases to around 34 Hv in the vicinity of the heat-affected zone (HAZ) indicating that distinct hardness zones can be obtained through FSP.Subsequently, it amplifies to approximately 53-62 Hv in the stir zone according to the quantity and type of reinforcement combination i.e., % of HNT and MoS 2 .This difference in hardness at different zones is due to different gain size as depicted in figure 3(c) owing to recrystallization during FSP.The stir zone exhibits higher hardness compared to the base material due to the intense plastic deformation and grain refinement.The hardness in the thermo-mechanically affected zone which is in between base metal and stir zone is higher than that of the base material but lower than the stir zone.This zone undergoes less severe plastic deformation than the stir zone, leading to a moderate change in microstructure and hardness.Maximum hardness of 62 Hv in the FSPed region strengthened with 4% silica-based HNT and 2% MoS 2 is visualized.This increase in hardness can be credited to grain boundary strengthening and grain refinement, resulting in improved hardness.Higher hardness values are observed where the concentration of silica-rich HNT is higher while least hardness is observed when MoS 2 % is higher in the Mg.It is clear from figure 3(a) and (b) that Mg composite reinforced with 6 vol% reinforcement possess finer grain structure than the Mg composite incorporated with 2 vol% hybrid reinforcement which shows the effect of reinforcement content on the microstructure refinement.This finer grain structure leads to higher hardness for that hybrid composite.A microhardness value of 60 Hv is recorded for magnesium with 3% HNT and 1% MoS 2 , while a value of 59 Hv is recorded for magnesium with 3% HNT and 3% MoS 2 which shows the negative impact of MoS 2 on hardness.This reduction in hardness is attributed to the soft solid lubricant nature of the reinforced MoS 2 .The consistent dispersal of HNT/MoS 2 , noteworthy dislocation density, and overall grain refinement contribute to the enhanced hardness of magnesium through FSP [25,26].

Tensile behaviour of Mg/HNT/MoS 2 hybrid composites
The tensile properties of base AZ31 and Mg/HNT/MoS 2 composite are shown in figure 6.The raw AZ31 exhibits the greatest value when compared to surface MMCs, while strength improves when the volume percentage of silica-rich HNT in the MMC rises.Additionally, it was found through studies that the ductile  quality of the material decreased as the vol% of HNT increased.Numerous metallurgical criteria, including the attachment of the matrix metal with the reinforced elements and grain fineness, often govern the tensile behaviour of any MMC [27].The interlinking sites of AZ31 with HNT and MoS 2 are susceptible to cracking, which makes their MMCs' tensile strength susceptible to decline.Contrary to the above assertion, it has been shown that for recently developed surface MMCs, the tensile strength tends to increase as the volume % of hybrid reinforcement increases.This phenomenon of higher tensile strength when secondary phase is raised is consistent with the observation that reinforcements have a tendency to resist grain developments secondary phase volume percentage increases.The improvement in MMC's hardness together with its grain refinement is credited with the increase in yield strength that results from the use of HNT/MoS 2 .Observed a reduction in % elongation when hybrid reinforcement particles were added to the AZ31 matrix.This striking fact may be explained by the decrease in dislocation caused by the presence of reinforcement particles, which results in a reduction in ductile characteristic.
The results presented in the figure 6 clearly indicate that the ductile nature of the developed AZ31 surface MMCs decreases as the quantity of HNT/MoS 2 addition increases.This observation is further supported by SEM micrographs of the fractured surface of the tensile specimens shown in figure 7, which reveal the presence of dimples and voids in the samples without reinforcements, indicating a characteristic of ductile behavior.However, with an increase in HNT/MoS 2 content, the extent of composite fracture increases, suggesting a transition towards a more brittle form of the base metal.These surfaces exhibit an expansion in size when HNT/MoS 2 is introduced into the AZ31 matrix, further indicating a reduction in the plastic deformation of the magnesium surface composites.

Wear characterization
The enhancement of the wear characteristics of the AZ31 is the primary objective of the development of surface composites.Surface composites of AZ31 incorporated with HNT/MoS 2 demonstrate increased wear resistance than that of base matrix, strongly supporting the assertion.Figure 8 shows the pattern of wear resistance that tends to increase when HNT and MoS 2 dispersion levels rise.The specimen 10 provided in the figure 8 represents the unreinforced AZ31.This is primarily caused by the following factors: (i) evenly dispersed HNT/MoS 2 in the matrix; (ii) amplified hardness of the manufactured surface composites; and (iii) solid lubricating nature of MoS 2 reinforcement.
One key factor that attributed the enhancement in wear resistance of AZ31 when silica-based HNT and MoS 2 are combined is the increase in the hardness of the stirred material owing to reduction in grain size that occurs during stirring [28].When AZ31, a normally formable and ductile material, comes into contact with a hard steel counter disc, a significant amount of adhesion wear caused by plastic deformation occurs.However, the addition of silica based HNT in AZ31 reduces the agitated region's ductility, which in turn lessens plastic deformation.This significantly reduces wear based on adhesion in surface composites; yet, wear based on abrasion makes a significant contribution.Additionally, the wear resistance of the Mg improved further with MoS 2 addition.MoS 2 is a solid lubricant known for its excellent lubricating properties, even under high loads and temperatures.When MoS 2 particles are incorporated into the composite material, they act as lubricating agents, reducing friction between the sliding surfaces.This results in lower wear and decreased wear rate.The presence of MoS 2 particles on the surface of the composite creates a protective layer that reduces direct contact between the mating surfaces.This layer acts as a barrier, preventing metal-to-metal contact and reducing wear.MoS 2 reinforcement improves the load-bearing capacity of the composite material.The particles distribute the applied load more evenly, reducing localized stresses and preventing excessive wear in specific regions.This enhanced load-bearing capacity contributes to the reduction in wear rate.Further, MoS 2 has anti-adhesive properties, meaning it prevents the sticking or adhesion of materials on the surface.This reduces the likelihood of material transfer and abrasive wear between the sliding surfaces.But, the addition of solid lubricants in higher  amount may also leads to higher wear rate as a result of decreased hardness owing to soft nature of the reinforcement.Wear rate increases when the load applied increases.As a result of higher pressure between the sliding parts at higher load condition, the base and composite material's wear rate was observed to be rising with load increment [29].However, the rate of wear for 4% HNT + 2% MoS 2 reinforced magnesium is discovered to be lower compared to AZ31 and other composites developed, demonstrating the greater wear resistance of the Mg/HNT/MoS 2 material.Among the developed composites, the Mg hybrid composite with 4% HNT + 2% MoS 2 reinforcement exhibited finer microstructure (figure 3(b)).The grain refinement due to intense plastic deformation leads to increased hardness, which enhances wear resistance.

Corrosion studies
Influence of hybrid reinforcements on corrosion behaviour of the AZ31 material shown in figure 9.At earlier stage magnesium surface forms an oxide layer at cathodic reaction.The stability of the formed inhibition layer is affected by the applied increased potential.The chloride ion presents in the corrosive electrolyte get adsorbed over the passive layer that initiates the corrosion.This adsorption paves the way for dissolution of magnesium by breaking the formed passivation layer followed by initiation and propagation of pits.Metals go through cathodic and anodic reactions when they are corroded.During the anodic reaction, metal dissolves or changes its ion [30,31].The cathodic process also results in passive layer development or hydrogen evolution.Figure 9 shows that with the inclusion of silica-based HNT and MoS 2 vol.%, the corrosion potential changes towards the cathodic area.
Tafel polarization values of the developed composites are given in table 2. Typically, in polarization curve, the anodic slope represents the rate of anodic reaction concerning to potential, and the cathodic slope represents the rate of cathodic reaction.Higher value of Tafel slope implies the major reaction occurs during corrosion condition.From table 2 it can be observed that the values of cathodic Tafel is higher than anodic Tafel which represents major occurrence of cathodic reaction.Corrosion potential E corr , represent the equilibrium circuit potential of a developed composite in saline environment.Shifting the E corr to a more noble value can help to mitigate corrosion by making the metal less prone to oxidation reactions.Among the developed composite, 4%  HNT/2% MoS 2 reinforced Mg matrix showcased higher noble value that implies 4% HNT with 2% MoS 2 help in decreasing the oxidation of Mg.Similarly lower corrosion density I corr is observed for the same combination which delivers high corrosion resistance.Since corrosion current density is a measure of electron flow between the anode and the cathode during the corrosion process, a higher corrosion current density indicates a more rapid flow of electrons, corresponding to a faster rate of corrosion.When the corrosion current density is low, it means that the developed composite is corroding at a slower pace.The combined effect of 4% HNT with 2% MoS 2 might contribute to reducing the corrosion current density by hindering the electrochemical reactions that drive corrosion reaction.These evenly distributed HNT particles reduce the active spots and block the porousness of Cl-ions, which slows the rate of corrosion.It may be because the reinforced HNT has the impact of increasing the grain refinement of produced surface composites and hence increase in silica rich HNT % results in a decrease in the corrosion current density.These factors enhance the generated oxide layer's capacity to resist peeling, and rise in HNT amplifies the layer thickness.That serve as an inhibitory block amongst electrolyte and the composite.This process lessens the flow of ions or the conversion of metal ions, which lessens the corrosion current density.Better water hydrophobic and infiltration-resistant qualities are possessed by HNT particles [32].Therefore, a surge in HNT creates a solid barrier passive coating that prevents saline electrolyte from penetrating the metal surface and electrolyte diffusion, thereby reducing corrosion current density [33].Similarly, incorporation of MoS 2 in magnesium matrix, decreases the chloride ions adsorption over magnesium surface thus delays the corrosion initiation in earlier stages.Another aspect is, magnesium surface undergoes chloride ions attack when immersed in 3.5 wt% solution.The surface of Magnesium layer forms a passive inhibiting layer that are destabilized by chloride ions that initiates corrosion.In case of MoS 2 incorporated surface composite, the direct attack of chloride ions decrease since these chloride ions will not be attached to MoS 2 distributed area.It can be observed that addition of MoS 2 improve the corrosion resistance of composite up to a limit however further increment increases the corrosion rate.Addition of MoS 2 upto 2 wt% showcase lower corrosion current density which might be owing to the higher diffusion barrier and its chemically inert basal.

Conclusion
Silica-rich HNT and MoS 2 were effectively incorporated into the AZ31 surface using FSP, resulting in the development of a surface composite.The key findings of the study are outlined below: • The microstructure analysis revealed that the addition of HNT and MoS 2 led to finer and more equiaxed grain structures in the composite.The grain size reduced due to recrystallization during FSP, resulting in improved properties.
• The microhardness of the composite materials increased compared to the base AZ31 alloy.The hardness increased with the addition of HNT, but it decreased with MoS 2 inclusion due to its solid lubricant nature.A maximum hardness enhancement of 35% was observed for the composite with 4% HNT and 2% MoS 2 .
• The tensile strength improved with increasing HNT/MoS 2 content, indicating strengthening effects.However, the ductility of the composites decreased as reinforcement content increased, leading to reduced elongation values.
• The developed composites exhibited enhanced wear resistance as the volume percentages of HNT and MoS 2 increased.The combination of improved hardness, solid lubrication from MoS 2 , and even reinforcement dispersion contributed to higher wear resistance.
• The corrosion behavior improved with the addition of HNT but deteriorated with higher MoS 2 content.The HNT particles contributed to the formation of a protective oxide layer, reducing corrosion rates.MoS 2 , acting as a solid lubricant, hindered chloride ions' attack and initial corrosion.