Keywords: Polymer nanocomposites are being increasingly used in a variety of tribological applications owing to their structural features. In this study, high density polyethylene (HDPE) composites with three different nanoparticles such as alumina (Al₂O₃), multi-walled carbon nanotubes (MWCNT) and graphene were fabricated using a newly designed friction stir processing tool and fixture. Mechanical test results showed that the fabricated polymer nanocomposites possess enhanced mechanical properties at higher tool rotational speed and traverse feed. The frictional coefficient and wear properties of the fabricated polymer nanocomposites were evaluated under dry sliding conditions using a pin-on-disc tribometer. Further the surfaces of the fabricated samples before and after wear studies were evaluated using scanning electron microscope. The wear test results showed that the HDPE/MWCNT has a very low mass loss as compared with HDPE/graphene, HDPE/Al₂O₃ composites and HDPE parent material. Moreover MWCNT particles act as a lubricant during wear test causing the worn surface of HDPE/MWCNT nanocomposites to be smoother as compared to other fabricated nanocomposites. The higher reflection peak between 23.71° to 23.95° obtained using X-ray diffraction (XRD) for the HDPE nanocomposites fabricated with the nanoparticles MWCNT, Al₂O₃ and graphene reveal a uniform mixture of the polymer matrix with the nanoparticles. The XRD results also show that the addition of nanoparticles does not significantly alter the crystal structure of the HDPE matrix.

Among the many available polymers high density polyethylene (HDPE) is the third largest thermoplastic used worldwide with numerous applications. They are highly durable, chemically non reactive, inexpensive and easy to process.¹ However its lower tensile strength makes it unsuitable for heavy duty applications.² To increase the strength of the polymers they are normally reinforced with nano and micro particles. The reinforced polymer nanocomposites are increasingly being used in various assemblies and equipments including gear, bearings, joints, seals, food conveyors, semiconductors, automobile elements owing to their light weight, flexibility, excellent corrosion resistance, good processability and low cost.^3-11 Several techniques such as vacuum arc deposition, melt mixing and injection molding are being used to fabricate the polymer matrix composites (PMC) by mixing polymer matrix and nanoparticles under high temperature.^12-18
Chee et al.¹⁹ used ‘melt mixing’ technique to fabricate low density polyethylene composites with nano Al₂O₃ reinforcements at 125 ℃. Silane was used as a coupling agent to enhance the interfacial interactions between alumina and low density polyethylene. The results showed that among the various weight percentage proportions used, 1wt% nano-alumina successfully enhanced the mechanical properties of low density polyethylene material.
Sengupta et al.²⁰ observed an enhancement in mechanical and electrical properties of graphene reinforced HDPE nanocomposites. Bhattacharyya et al.²¹ used prereduction and insitu method to disperse graphene particles into ultra high molecular weight polyethylene for the fabrication of nanocomposite films. The nanocomposite film produced by prereduction method was found to possess higher crystalline characteristics, better modulus, strength, network hardening and creep resistance as compared to the ‘in situ’ method. Lin et al.²² used ‘melt blending’ technique to fabricate polyethylene/graphene oxide nanocomposites. The experimental results showed an increase of 20 and 13% in Young’s modulus and tensile strength respectively. Achaby and Quaiss²³ prepared HDPE/graphene nano sheets and HDPE/MWCNT nanocomposites with equal weight percentage using melt mixing technique. The result showed that HDPE/graphene nanocomposites with higher specific surface area exhibit better mechanical properties as compared to HDPE/MWCNTs nanocomposites. Behrouz et al.²⁴ developed a coarse grained model to capture the interactions between polymer chains and nanotubes. It was found that the mechanical properties of the nanocomposites can be estimated with higher accuracy and lower computational cost using coarse grained model simulation.
Recently, friction stir processing (FSP) a novel solid state processing technique has gained wide acceptance in fabricating polymer matrix composites. Composites fabricated by FSP were found to possess an inner matrix with better ductility and toughness and an outer surface with improved wear resistance and strength.1 During friction stir processing the tool pin serves two primary functions: (a) heating of work piece and (b) movement of material. The friction between the tool and the work piece develops heat which softens the material around the rotating pin. The rotation also causes material movement from the advancing to the retreating side of the pin. The tool shoulder consolidates the material that flows around the pin.²⁵ Puviyarasan and Senthil Kumar^26,27 fabricated aluminum AA6061/B₄C and AA6061/SiC_p metal matrix composites through friction stir processing and found that higher tensile strength and microhardness was achieved in the composites fabricated using optimum process parameters. Hosseini et al.²⁸ have observed a microstructural modification, grain size reduction and homogeneous distribution of reinforcements in aluminum Al5083 grade using friction stir processing method.
Prakash et al.²⁹ used FSP technique to develop alumina/0.5 graphene reinforced AA6061 hybrid composite sheets with higher hardness and minimum wear rate. Yet fabricating PMCs through conventional FSP tool is highly difficult owing to the differences in the material structure, morphology of the matrix, low melting temperature, low thermal conductivity, microhardness and short solidification time.^30-32 In order to avoid these defects Rezgui et al.³³ developed a new FSP tooling system that utilizes a stationary wooden shoulder to lap weld HDPE sheets. However the poor thermal conductivity of wood caused the formation of a small sized HAZ and discontinuities in the weld. Barmouz et al.1 fabricated HDPE/clay nanocomposites using FSP and studied the effect of process parameters on its morphology and mechanical properties.
Azarsa and Mostafapour³⁴ fabricated HDPE/copper powder polymer metal matrix composites using a FSP tool with stationary shoulder and a heater. The results showed that the ultimate tensile strength and elastic modulus of the fabricated composites were remarkably enhanced. Yet studies conducted on the fabrication of PMCs through FSP using a stationary shoulder are negligible.
In this work an attempt has been made to develop and apply a novel FSP tooling system using a stationary shoulder without any external heat source to fabricate HDPE/Al₂O₃, HDPE/MWCNTs and HDPE/graphene nanocomposites. The tensile strength and tribological behavior of the fabricated composites were evaluated. Microstructural analysis using scanning electron microscopy (SEM) and XRD was carried out to confirm the dispersion of nanoparticles in the HDPE without any prominent defects.