Investigation of the concentration rate and aggregation behaviour of nano-silver added colloidal suspensions on wear behaviour of metallic materials by using ANOVA method

Highlights

• Aggregation tendency of nanoparticles has been analysed for wear performance.

• The effect of nano-silver concentration rate was investigated.

• The penetration behavior of colloidal suspensions was compared.

• Wear resistance increased at concentrations above 4%.

• The ethylene glycol showed higher performance in terms of penetration ability.

Abstract

In this study, the tribological performance of nano-silver particles prepared at different concentrations was investigated by the parameters of temperature, friction coefficient, weight loss and surface roughness. Moreover, the effects of the aggregation behaviour of nanoparticles were analysed. According to the results, when the nano-silver concentration increased from 4% to 12%, weight loss decreased by 33.99%, surface quality increased by 21.13%, friction coefficient decreased by 18.16% and wear zone temperature decreased by 74.31%. Increased aggregation behaviour adversely affected the wear resistance. Additionally, it was determined that the ethylene glycol has a significant effect on the penetration ability of the nanoparticles.

Introduction

Using the fluids in industrial applications subjected to wear has significance in terms of sustainability. The concept of sustainability can be defined as the improvement of the lifecycle of mechanical parts by reducing the wear. The fluids are expected to have higher lubrication and cooling performance to reduce the wear in the applications where high forces and temperatures occur. Especially in the mechanical systems, the heat occurring on the contact area of the materials due to the friction causes deformation on the surface. Therefore, increasing the heat transfer becomes compulsory in the applications where both quality and efficiency are required. For increasing the cooling and lubricating properties of fluids, additives can be used as a performance agent. By adding solid particles (CuO, MoS₂, Al₂O₃, ZnO, ZrO₂, TiO₂) in the size of micron and millimetre to water- or oil-based fluids, an attempt was made to increase the capability of the cutting fluids [1]. However, the size of the solid particles limits the penetration performance and affects the tribological properties of the wear atmosphere negatively [2,3].

In recent years, for minimising the disadvantages of the traditional fluids, the use of nano-fluids (nano TiO₂, nano graphite, nano CuO, nano Al₂O₃, nano MoS₂, nano Cu etc.) has gained great importance in the areas where high heat transfer is required such as automotive applications, nuclear reactors and machining industry [[4], [5], [6], [7], [8], [9], [10]]. In the literature, the effect of nano-fluids on the working performance of mechanical systems and tribological properties of materials was studied by many researchers [11]. Asrul et al. [12] experimentally analysed the lubrication mechanism and tribological properties of the surface-modified copper oxide (CuO) nano-particle in the liquid paraffin. When the wear experiment was analysed for the nano-lubricants prepared with CuO nanoparticles with a non-modified surface, the lowest wear coefficient value (0.185) in a concentration of 0.2% and the highest wear coefficient value (0.247) in a concentration of 3% were obtained. As for the nano-lubricants prepared with surface-modified CuO nanoparticles, the lowest wear coefficient value (0.123) in a concentration of 3% and the highest wear coefficient value (0.247) in a concentration of 3% were obtained. Ghaednia et al. [13] investigated the tribological performance of polyethene glycol (PEG) nano-lubricant enriched with the silver nanoparticle. In their study, first, the nano-lubricant mixed with silver nano-particle was prepared, and then, tribological performance of it was analysed through the analyses of nano-lubricant rheology and Stribeck curve. For the rheology analysis, the measurement of viscometer was conducted, and it was found that the particles of Ag are very effective in reducing both the friction coefficient and the wear. Lee et al. [14] investigated the effect of graphite nano-lubricant on tribological properties through the wear experiments and surface analyses. In their study, as the base lubricant, the industrial gear oil having 220 cS kinematic viscosity in 40 °C was used. They determined that the nano-lubricant affects the tribological properties of the material positively because they both reduce the metal contact and increase the heat transfer.

In the literature, the tribological performance of nanoparticles was examined mainly in the wear and machining environments. The tribological performance of the nanoparticles depends on their chemical and physical characteristics [11]. The aggregation tendency is one of the most important parameters determining the characteristic behaviour in nanosize [[15], [16], [17], [18]]. In the literature, there is no mention of the aggregation tendency of nanoparticles and performance loss occurred due to this tendency. The aggregation tendency of the nanoparticles synthesised without any ligand is high due to the colloidal instability. Especially in long-term use, this trend will increase more. The nanoparticles starting to aggregate behave like abrader in the wear region due to their increasing dimensions. Thus, the colloidal behaviours of the nanoparticles should be analysed before and after the wear experiments to understand aggregation behaviour.

The originality of this paper is to determine the effect of different nano-silver concentrations on wear behaviour of metallic materials. For this aim, three different colloidal suspensions were prepared by adding the nano-silver in different concentrations (4%, 8%, 12%) into ethylene glycol. The performance of the colloidal suspensions was examined by comparisons with the conditions of the dry wear, the pure water and the ethylene glycol (EG) [[19], [20], [21]]. The results of friction coefficient, weight loss, temperature change and surface roughness obtained from the wear experiments were analysed by using the variance method. Development of the temperature measurement methodology and obtaining the temperatures in wear region for understanding the effect of nano-silver concentration has significant originality for literature. Additionally, another originality of this paper is that the colloidal suspension behaviours of the used nanoparticles (aggregation trends) were evaluated with the graphics of ultraviolet and visible light (UV–Vis) absorption spectroscopy. The exterior surface of the particles was coated with gelatine to prevent the aggregation behaviour of nano-silver particles [3]. The wear mechanisms on the surfaces were examined via scanning electron microscope (SEM) and elemental analysis (EDX). Moreover, for comparing the performances of the developed suspensions on surface quality, 3d topography images of worn surfaces were evaluated. By this means, the effect of different environmental conditions (dry, pure water, EG, EG+4%, EG+8%, EG+12%) on the surface morphology was investigated.