Fabrication and characterization of novel Al-SiC-hBN self-lubricating hybrid composites

Highlights

• A novel Al-SiC-hBN self-lubricating composite fabricated by stir casting method.

• Al-5 wt.%hBN-4 wt.%SiC hybrid composite has higher wear resistance and hardness.

• Reinforcing materials are thermodynamically stable with the casting temperature.

• Uniform distribution of reinforcing phase within the matrix phase.

Abstract

The unabated thirst for fuel-saving, cost-efficient, attractive structural and lubricating properties of the materials have made the researchers focus on the new generation of aluminum matrix composites for automotive and aerospace application. Aluminum-based composites with hBN particle reinforcement could improve the tribological properties which reduce the wear rate of the components. The present study focused on the fabrication of a novel self-lubricating Al-SiC-hBN hybrid composites and its wear properties with unreinforced aluminium alloy. Structural and microstructural changes of the as-cast composites were examined by X-Ray diffraction and scanning electron microscope. Microstructural analysis reveals that uniform distribution of the reinforcing phase results in an increase in the density of the as-cast sample from 2.547 g/cm³ to 2.713 g/cm³. The pin-on-disc method is used to study the resistance to friction of the as-cast composite (Al-5 wt.% hBN-4 wt.% SiC) and found that, hBN particulates helps to minimize the wear loss at ambient temperature.

Introduction

In the present-day, modern engineering applications demand lighter, stronger, and in-expensive materials. Analyzing the present trend, researchers are showing significant interest in the development of materials with lightweight and high strength by variation of properties. In order to satisfy the recent demand of industries, advanced materials like aluminum-based composites gain the significant interest of the researchers due to its wide solidification range, appropriate structural and mechanical properties [[1], [2], [3], [4]]. However, the tribological behavior of aluminum-based composites is not superior which limits its application in various sectors [5]. To enhance the tribological properties of the composites, various methods had adopted by the researchers in the past. One of the methods among them was by applying liquid lubricant or solid lubricant coating on the surface of the materials to reduce the wear rate. But liquid lubricants are impractical in extreme environmental conditions and solid lubricant coating has limitations like oxidation, poor bonding, and low lifetime [6]. In order to overcome these issues, the incorporation of solid lubricating materials in the matrix phase is an alternative choice to reduce friction and wear without engaging liquid lubricants.

Graphite and molybdenum disulfide (MoS₂) is a common type of solid lubricating material which minimizes the friction and wear rate of the material in the absence of liquid lubricant [7]. These materials have a hexagonal closed packed layer structure, where a strong covalent bond exists in-between the atoms and a weak Van der Wall bond subsist between each layer of atoms. These solid lubricating materials are received the most attention because of its low coefficient of friction but in the presence of humidity the performance of MoS₂ decreases [8,9]. Hexagonal boron nitride (hBN) has unique lubricating properties with better thermal stability as compared to graphite and MoS₂. So hBN presents a unique opportunity to develop a new solid lubricating composite material for automotive industries [10].

Earlier researchers have been conducted various experimental and theoretical research work to fabricate and studied its microstructure and mechanical properties of aluminum alloy and its composites [[11], [12], [13], [14]]. Among them, the casting method is the utmost economical and scalable technique for the production of metal matrix composites by optimizing the processing parameters [[15], [16], [17], [18]]. Also, different kinds of ceramic particulates (carbides, nitrides, oxides, and borides) used as a reinforcing phase to make the composites using casting route [[19], [20], [21], [22]]. Radha and Vijayakumar [23] fabricated aluminium based composites using the stir casting method and used SiC and graphene as the reinforcing phase. They had studied the effect of SiC and graphene particulates on the properties of the composites and found that the elastic modulus, hardness, and strength of the composites increases with an increase in wt.% of the reinforcing particles. Moghadam et al. [24] fabricated Al-TiB₂-Al₂O₃ hybrid composite using the liquid metallurgy route and investigated the tribological performance of the as-cast composites with respect to process parameters. The effect of graphite particle reinforcement on the structural and mechanical properties of the AA6082 metal matrix composites fabricated by the conventional stir casting method was investigated by Sharma et al. [25] and they found that with an increase in graphite addition in the matrix phase, the density and hardness of the composites decreases in a significant way. Based on their investigation they suggested that graphite particles were not beneficial for the AA6082 metal matrix due to the non-uniform distribution of particles and decrease in hardness. Several authors also found that the addition of SiC in the matrix phase helps to increase the wear resistance, corrosion resistance, and lowering the fracture toughness as compared to the performance of matrix alloy alone [[26], [27], [28], [29]]. Majzoobi et al. [30] found that when the mass fraction of SiC was increased, the hardness and wear resistance of AA7075/SiC composite also enhanced. Harichandran and Selvakumar [31] fabricated aluminum metal matrix composites with three composites with varying B₄C content and keeping a fixed amount of hBN nanoparticles using ultrasonic cavitation assisted casting process. Their microstructural analysis reveals that, uniform distribution of reinforcing phase, grain refinement, and low porosity in the as-cast samples and found that the mechanical properties like tensile strength and hardness had significantly improved. A self-lubricating AA6061 matrix composite with different weight fractions of graphene, MoS₂, and hBN was fabricated by Wozniak et al. [32] through powder metallurgy process. The authors found that the addition of a solid lubricant phase decreases the density and mechanical properties of the composites. They concluded that with uniform distribution of the reinforcing phase, the physical and mechanical properties could be improved. Madhukar et al. [33] fabricated a novel Al-7150 nanocomposite by reinforcing different weight percentages of hBN by liquid metallurgy route. They had optimized the weight fraction based on wear rate using the Taguchi method. Singh et al. [34] investigated the tribological behavior of WS₂ dispersed Al-SiC based hybrid composite produced via powder metallurgy technique. The authors varied the percentage of WS₂ reinforcing phase and found that with an increase in the WS₂ phase, the density and hardness of the composites increased and also they optimized the processing condition for obtaining the desired properties with minimum wear loss. Biswal and Sahoo [35] fabricated WS₂ reinforced self-lubricating composite with an Al matrix using a powder metallurgy technique. The authors had optimized the sintering temperature and compaction pressure for obtaining a dense composite.

In the present investigation, Al-SiC-hBN hybrid composites were fabricated using the stir casting method by controlling the wt.% of SiC and hBN reinforcing phase. The effect of SiC and hBN reinforcing particles on the engineering properties of the self-lubricated hybrid composites were evaluated. Scanning electron microscopy was used to investigate the surface morphology of the composite sample before and after wear. Mechanical properties were evaluated through the microhardness test. Dry sliding wear test was performed on a pin-on-disk tribometer to find the wear rate and coefficient of friction to evaluate the self-lubricating property.