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Tribological and Microstructural Characterization of Natural Diatomite Filled Epoxy Composites

Umut Savacı, Servet Turan

Anadolu University - Türkiye

Abstract Polymers and their composites are widely used and are in contact with abrasive particles in some applications such as bearing components in automobiles or protective coating of marine structures. In order to improve the wear properties of polymeric materials, different filler materials such as; nano-SiO2, nano-TiO2 and natural filler were used; however, there is very little information about the tribological performance of natural filler reinforced composite materials. Diatomite is one of the possible natural filler materials that is fossilized remains of diatoms, which is a type of algae that lives in oceans, are made from micron sized; tubular shaped silica cell wall that contains nano sized pores. In this study, change of the wear rate and friction coefficient of neat epoxy with addition of different amounts (2.5, 5, and 7.5 wt%) of diatomite natural filler were investigated by using ball-on-disc machine by running AISI 52100 steel ball with 6 mm diameter and morphologies of worn surfaces of epoxy composites were investigated by using scanning electron microscopy. According to the test results the friction coefficient and specific wear rate decreased up to 15% and 85%, respectively, with an addition of 5 wt% diatomite. 1. Introduction Polymers are becoming very important engineering material because of their low cost and ease of manufacturing. Because of this reason polymers are one of the mostly used material in wide range of applications like joint bearing surfaces, gears, car chassis where frictional forces become the most important challenge to overcome in order to prevent wear [1,2]. Wear is defined as loss of material and damage to surface of material due to the motion of contacting surfaces [3]. Different types of polymers (PEEK, PTFE, epoxy etc.) have different wear behavior, however mostly these properties do not satisfy the requirements of applications. Due to the fact that, polymers are rarely used as neat state, they are used as composite form that composed of polymer matrix reinforced with; (i) ceramic oxides like SiC, SiO2, TiO2 or (ii) carbon nano-tubes [1]. Epoxy matrix reinforced with SiO2, SiC or MWCNT is widely used for wear applications with improved wear

properties compared with neat epoxy. Wear properties like coefficient of friction and specific wear rates of neat epoxy is significantly improved with 2.2 vol% nano-SiO2 addition and SiO2 become an important additive for epoxy due to this reason [6]. Diatoms are single cell photosynthesizing algae that are found in all aquatic environments including fresh waters, oceans, soil, and almost anywhere moist. Each diatom cell has a cell wall known as frustule, which mainly made of amorphous silica. Diatoms are commonly between 20-200 microns in diameter or length and they can be in either pennate or centric geometry. Frustules have microstructure with pores around the cell walls. When diatom cells die their silica cell walls collect at the floor of the ocean, and become fossils. These fossils of the cell walls are called as diatomite and they are used in many applications like sound and heat insulation, filtration, etc., because of their characteristic properties like low density (2.33 g/cm3), high porosity and chemical inertness. Diatoms are used as reinforcement additive for epoxy in order to improve mechanical properties [7] however there are no information about the wear properties of this type of natural additive containing epoxy. 2. Experimental Procedure During the experiments epoxy resin used consisted of a bisphenol-A epoxy resin, which was mixed with a triethylenetetramine hardener in the ratio of 100:12 by mass. Epoxy resin first mixed with the diatom frustules (Alfa Aesar) with 2.5, 5 and 7.5 weight percentages, which equals to 1.2, 2.5 and 3.8 volume percentages, respectively. After initial mixing by hand, sonicator was used, in order to reduce the agglomeration and improve homogenization, for 2 minutes with 30 seconds intervals. During ultrasonic mixing, mixture was cooled with water and ice to room temperature, in order to prevent heat builds up which decrease the viscosity of the resin. Then, resin and diatom mixture was mixed with hardener gently by hand to prevent the formation of air bubbles. Mixture was casted into the silicon mold and applied vacuum was applied for 15 minutes to reduce the trapped air bubbles for 15 minutes. Epoxy then cured at room temperature for 15 hours. For the wear tests, test surfaces of the

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samples were polished down to 0.062 m surface roughness. Tribological properties of the polished samples were determined with ball on disk tribometer (CSM). During wear tests, 6 mm diameter 100Cr6 steel ball used with 3N normal force (FN) on sample surface with 0.1 m/s sliding speed for 55 mt sliding distance (L) with 3 mm route radius. The specific wear rate was determined by the following equation;

(1) where V is the volume of the removed material. In order to obtain volume of the removed material, first the cross section profile of the wear surface was measured with digital profilometer (Mitutoyo), after the calculation of the cross section area, volume of the removed material was calculated. In order to characterize the wear mechanism, microstructures were investigated by using scanning electron microscope (Zeiss Supra 50VP). Chemical analysis of diatom powder was done by X-ray fluorescence (XRF- Rigaku ZSX primus). Additionally, Vickers hardness values of the samples were measured with micro hardness test equipment by applying 2 N force for 6 seconds (Emcotest M1C). 3. Results and Discussion SEM analysis showed that diatom frustules have different morphologies from high aspect ratio tubes to low aspect ratio cylinders with different sizes as shown in Figure 1. Frustules have a broad range of size distribution (from 500nm to 100 m) as shown in Figure 2 and average particle size is about 18 micrometers.

(a) (b)

(c) (d)

Figure 1 (a-d). SEM secondary electron images of the frustules.

Figure 2. Particle size distribution of diatome frustules obtained by using particle size analyzer. XRF analysis results (as shown in Table 1.) showed that frustules mainly composed of SiO2 and also there are some other oxides like Al2O3 and Fe2O3 in the composition. XRD results are compatible with XRF results and frustules composed of opal and quartz crystalline phases as well as with some amorphous phase due to the typical amorphous hump in the results given in Figure 3. Table 1. XRF results of the frustules.

Chemical Composition (mass%) SiO2 Al2O3 Fe2O3 K2O3 MgO 91.58 3.76 1.59 0.79 0.62 Na2O CaO TiO2 P2O5 Na2O 0.52 0.48 0.25 0.23 0.52

Figure 3. XRD pattern of frustules. Hardness of the composite is also an important parameter because hardness of the material controls the surface area of contact, which is inversely proportional to the hardness. Surface area of contact is also proportional to the friction between the moving parts [8,9]. Additionally change in hardness with the diatom addition was measured and results showed that with diatom addition hardness of the composite increases as shown in Table 2.

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Table 2. Change in hardness of the composites with diatom addition.

Hardness (Hv) Neat Epoxy 2.5 wt% 5 wt% 7.5 wt%

18.5 19.7 19.1 21.3 According to the ball on disc wear test results (Figure 5); addition of frustules into the epoxy matrix yielded with the decrease of friction coefficient and specific wear rate compared with the neat epoxy. With an increased amount of diatom up to 5 wt% both specific wear rate and friction coefficient decrease, however, above 5 wt% addition both parameters increase but still lower than neat epoxy. According to the SEM results (Figure 4) wear mechanism of neat epoxy is characterized by fatigue wear (as similar with other researchers results [6]) due to the material waves along the sliding direction and wear grooves are in continuous mode. With an addition of frustules decrease of these parameters can be a result of the breakdown of diatom frustules into the smaller particles (down to 120 nm according to the SEM results) and act as a lubricant layer between the composite and ball during wear tests even though wear mechanism is still fatigue wear according to the SEM results shown in Figure 4. Increase of the wear rate with 7.5 wt% frustule added composites can be the agglomeration of particles due to the high amount of filler loading even though the width of the wear mark is about the same (160 m) with 5 wt% diatom added composite that is lower than both neat epoxy (300 m) and 2.5 wt% composite (250 m).

(a)

(b)

(c)

(d)

Figure 4. SEM results of (a) neat epoxy, (b) 2.5 wt% additive, (c) 5 wt% additive, (d) 7.5 wt% additive (arrows indicate the sliding direction).

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Figure 5. Change in specific wear rate and coefficient of friction with diatom frustule/epoxy composites. 4. Conclusion According to the results obtained in this study, the diatom additive can be used to improve wear and mechanical properties of epoxy matrix. In this study lowest friction coefficient is obtained with 2.5 vol% (5 wt%) natural micron sized diatom frustules additive as similar with epoxy/nano-SiO2 composites in literature and the friction coefficient and specific wear rate decreased up to 15% and 85%, respectively, with an addition of 5 wt% diatomite. In order to understand wear mechanism to a great extent, extensive wear tests needs to be studied. Acknowledgement Authors would like to acknowledge Prof. Dr. Mustafa GÜDEN for diatom powders and Abdullah SERT for his help on the wear tests. References [1] B.J. Briscoe, S.K. Sinha, Tribology and Interface Engineering Series, 55 (2008) 1-14 [2] F. Klaus, Z. Zhang, A.K. Schlarb, Composites Science and Technology, 65 (2005) 2329-2343 [3] B.R. Raju, B. Suresh, R.P. Swamy, ARPN Journal of Engineering and Applied Sciences, 7,4 (2012) 485-491 [4] N.K. Myshkin, M.I. Petrokovets, A.V. Kovalev, Tribology International, 38(2005) 910-921 [5] H. Zhu, R.S.C. Woo, C.K.Y. Leung, J. Kim, Key Engineering Materials, 01(2007)605-608 [6] M.Q. Zhang, M.Z. Rong, S.L. Yu, B. Wetzel, K. Friedrich, Wear, 253 (2002) 1086-1093 [7] E. Gültürk, The Effects Of Diatom Frustule Filling On The Quasi-Static And High Strain Rate Mechanical Behavior Of Polymer Matrices, Ph.D Thesis, Izmir Institute of Technology, 2010, Izmir, Turkey

[8] B.J. Briscoe, S.K. Sinha Proc. Instn. Mech. Engrs, 216 (2002) 401-413 [9] J. K. Lancaster, Wear, 14 (1969) 223-239