Role of Nanofillers on Mechanical and Dry sliding Wear Behavior of Basalt- Epoxy Nanocomposites

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ScienceDirect Materials Today: Proceedings 4 (2017) 8192–8199

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ICAAMM-2016

Role of Nanofillers on Mechanical and Dry sliding Wear Behavior of Basalt- Epoxy Nanocomposites Mahesha C R a,*, Shivarudraiahc, N Mohana, Rajesh Mb a

Department of Industrial Engineering and Management, Dr. Ambedkar Institute of Technology, Bangalure-560 056, India b Department of Mechanical Engineering , Dr.Ambedkar Institute of Technology, Bangalure-560 056, India. c Department of Mechanical Engineering, University Visvesvaraya College of Engineering, Bangalore -560001, India

Abstract Basalt fiber reinforced polymer composites are nowadays creating attention in both industrial and academic world. New developments are still under way to explore the application of these composites by tailoring their properties. The objectives of this research article is to evaluate the effect of the incorporation of Nano titanium dioxide (TiO2) alone and in combination with nano clay on the mechanical and tribological wear behaviour of Basalt fabric– Epoxy (BE) composites. A vacuum-assisted resin infusion technique (VARI) was employed to obtain a series of BE composites. The mechanical properties, including tensile strength, tensile modulus, elongation at break, and surface hardness, were investigated in accordance with ASTM standards. From the experimental investigation, it was found that the tensile strength and dimensional stability of the B–E composite increased with increasing fillers. The effect of different loads (10 to 30 N) and sliding distances from 2000 to 8000 m on the performance of the wear resistance of the composites were measured. Slight increase in coefficient of friction for nano Clay-BE composite, reduction of coefficient of friction was noticed for the nano TiO2 and TiO2/Clay filler filled composites. The wear loss of the composites decreased with addition of fillers and increased with increasing sliding distance. Failure mechanisms of the worn surfaces of the filled and unfilled composites were examined with SEM. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility ofthe Committee Members of International Conference on Advancements in Aeromechanical Materials for Manufacturing (ICAAMM-2016). Keywords:Sliding wear; Basalt, Epoxy; nanoclay; nano TiO2; VARI; Wear Mechanism.

* Corresponding author. Tel.: +91-988-681-4220; fax: +91-802-321-7789. E-mail address: [email protected] 2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility ofthe Committee Members of International Conference on Advancements in Aeromechanical Materials for Manufacturing (ICAAMM-2016).

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Nomenclature Ks ∆V L D v

specific wear rate volume loss applied normal load sliding distance sliding velocity

1. Introduction Natural fibers have recently attracted the attention of scientists and technologists because of the environmental issues and the advantages that these fibers provide over conventional reinforcement materials, and the development of natural fiber composites has been a subject of interest for the past few years. Flax and hemp fibers are good natural fiber reinforcement materials having acceptable mechanical properties and low cost, however, there is difficulty in maintaining their properties when exposed to high temperature. They are also sensitive to humidity, as well as experience adhesion problem with a polymer matrix [1]. A possible solution that takes into account the environmental issues is represented by the use of mineral fibres like basalt. It has good physical and mechanical properties, like high hardness, high strength, good wear resistance, excellent heat resistance, good resistance to chemical attack and low water absorption, which makes basalt, a suitable reinforcement for the tribological application of composites [2]. Basalt products have no toxic reaction with air or water, they are noncombustible and explosion-proof. When in contact with other chemicals they do not produce any chemical reaction that may damage health or the environment. It can be used in a wider temperature range of -2600C to about 8000C [3]. Despite this growing interest, only scarce attention has been devoted to the development of basalt epoxy composite. Functional fillers are added to the thermo set matrix for improving its physical, mechanical and tribological properties. The modification of the mechanical and tribological behavior of various polymers can be done by treating the surface by coupling agent, so that it provides compatibility between immiscible polymers, which are hydrophobic in nature with the hydrophilic nature of fibers, or by the incorporation of nano materials as reinforcing filler. Addition of filler material has shown a great promise, and hence it has been subject of considerable interest as reported by Naveed Anjum et.al [4]. Polymer based nanocomposites are commonly reinforced by nanofillers such as silica and oxides, carbides of different materials. Recent studies have shown that small amount of ceramic nanoclay in polymer composite improves mechanical and tribological properties [5]. Montmorillonite based clays (MMT) offer high interest from an industrial point of view since the use of small amounts of them is enough to improve the overall properties of a polymer matrix at a relatively low cost [6–8]. But the degree of dispersion of nanoclay in epoxy resin is reported to have a significant impact on the mechanical properties of nanocomposites [9]. Ha et al. [10] investigated the effect of the MMT concentration on the tribological behavior of MMT/epoxy based PNCs. Wear tests were performed on MMT/epoxy based PNCs with four different clay concentration levels to characterize the improvement in wear behavior due to the MMT concentration. The results showed that the friction coefficient of MMT/epoxy based PNCs with a low MMT concentration was larger than that of PNCs with a high MMT concentration. Lingaraju, et al. [11] developed nanoparticles reinforced plastics. The tensile strength, impact strength, flexural strength and hardness of PNCs were studied in accordance with ASTM standards. It was observed that the wear rate increased with applied load, time and sliding speeds. Recent investigation by Srinath and Gnanamurthy [12] and Dasari et al. [13] shows reduced friction and wear of nanocomposites. Thus by addition of some wt.% of exfoliated clays can lead to improvement in mechanical properties. The aim of this study was to evaluate the dry sliding wear behavior of nano TiO2 and nanoclay filled basalt epoxy hybrid composite for varying sliding distance and load. The effect of sliding distance and load on wear rate on this composite material has been discussed and the morphology of eroded surface is analyzed using micrographs.

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2. Experimental details 2.1. Materials Basalt woven fabric 360 g/m2 was obtained from M/s. APS Austria. The basalt fabric of diameters 18 μm was used as reinforcement. Multifunctional epoxy-Bisphenol A-epichlorohydrin (MY 740) and cyclo aliphaticamine (HY 951) (room temperature cure system) were obtained from M/s. S& S POLYMERS, Bangalore, India. The resin is a clear liquid, its viscosity at 250C is 10000– 14500 mPa.s and density is 1.15-1.20 g/cc. The hardener is a liquid and its viscosity is 50–80 mPa.s and specific gravity is 1.59. The commercially available Nano clay and nano titanium dioxide powder was procured from M/s Sigma Aldrich, Bangalore, India. 2.2. Fabrication of Composite The composite fabrication consist of three steps: (a) mixing of the epoxy resin and filler using a mechanical stirrer, (b) mixing of the curing agent with the filled epoxy resin, and (c) fabrication of composites. In the first step, a known quantity of filler was mixed with hot epoxy resin using a high speed mechanical stirrer to ensure the proper dispersion of filler in the epoxy resin. In the second step, the hardener was mixed into the filler filled epoxy resin using a mechanical stirrer. The ratio of epoxy resin to hardener was 100:30 on a weight basis. To improve the consolidation and to reduce the voids and dry spots, the epoxy resin was manually smeared onto the basalt fabric before going to VARTM. The composites were cured at room temperature under a pressure of 14 psi for 24 h and it was post cured up to three hours at 1000 C. The unfilled basalt epoxy composites were designated as B-E, nano clay filled BE as CL-BE, TiO2 filled BE as T-BE and Hybrid composite as T+CL-BE respectively. The laminate of dimensions 300 mm × 300 mm × 2.8 ± 0.2 mm was fabricated and the specimens for the required dimensions of 5mm × 5mm were cut using a diamond tipped cutter for sliding test. 3. Testing Equations A dry sliding pin-on-disc (POD) setup was used for the sliding wear tests. The tests were conducted according to ASTM G99 standard. The fabricated specimen (5 mm × 5 mm × 2.6 mm) was glued to the pin of 10 mm diameter and 25 mm height, which came in contact with a hardened En32 steel disc of hardness 55 HRC. The steel disc was polished with water proof SiC emery paper (600 grade) in order to obtain a surface roughness of 0.65 lm. The test pin was mounted on the lever arm. The applied normal loads were 10 and 20 N, and the sliding velocity was 1.5 m/s. Sliding distances varied from 2,000 to 8,000 m in steps of 2,000 m. The surface of the specimen was cleaned with a soft tissue paper soaked in acetone and with compressed air before and after the test. The specimen weight is recorded using an electronic balance. The difference between initial and final weight of the specimen were measured as the sliding wear loss. The frictional force is measured by attaching a force transducer on pin-on-disc machine and it touches the loaded lever arm. The weight loss was converted into volume loss by using measured density of the specimen. A minimum of three trials was conducted to ensure repeatability of test data. The worn surface of the composite specimen was examined using a scanning electron microscope. The wear was measured by the loss in weight, which was then converted into wear volume using measured density data. The specific wear rate (Ks) was calculated from equation (1).

Ks =

∆

m3 / N-m ------ (1)

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4. Results and Discussion 4.1 Mechanical Properties The Mechanical properties of unfilled and nano TiO2 filled BE composites are illustrated in Table 1. Lancaster [14] and Ratner et al., [15] have done correlations of wear volume loss with selected mechanical properties such as (Se) factor (where, S is the ultimate tensile strength and e is the ultimate elongation), hardness (H) have been reported for single-pass studies of polymers without filler filled composites. Table 1: Mechanical properties of unfilled and nano TiO2 filled BE composites Properties

Composite specimen BE

3T-BE

3CL-BE

T+CL-BE

Tensile Strength (S) (MPa)

380±10.96

388.2±9.44

3911±9.22

394.17±20.32

Elongation at fracture(e) (mm)

3.2

3.26

3.29

3.32

Product(Se)

1216

1265.5

1286.4

1308.6

1/(Se) ProductX10-4

8.223

7.90

7.773

7.642

Hardness

81

82

83

85

Ratner et al., [15] proposed a model which states that the rate of material removal is inversely proportional to the product of stress and strain at rupture. The phenomenon is similar to what is reported in reference [14-15] in spite of the fact that polymeric materials, testing configurations and conditions are different. 4.2 Sliding Wear Loss and Specific Wear Rate Graphical plots of wear volume as a function of sliding distance for unfilled, 3wt.% nano clay filled, 3wt.% nano TiO2 ,1.5wt.% TiO2+ 1.5wt.% clay filled hybrid composite at two different loads with constant velocity of 1.884m/s for varying sliding distances (2000m-8000m) are shown in Fig.1 and Fig.2 The hybrid composite exhibits lower wear loss compared to unfilled, TiO2 filled and clay filled basalt epoxy composite. Wear volume loss showed an upward trend in the gradient, as the sliding distance increased, and the same is noted for all the composites tested at all the loads. This result may be due to an increase in temperature which occurred during the wear process. The order of wear resistance behavior of epoxy nano composites is as follows: T+CL-BE> T-BE>CL-BE>BE. The specific wear rate (Ks) of composite was observed to be linear up to 20N load and increased nonlinearly up to 30N. .

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4

BE 3T-BE 3CL-BE T+CL-BE

3 2.5

Wear volume loss X(10-3 gm)

Wear volume loss X(10-3gm)

3.5

(a)

2 1.5 1 0.5 0 2000

4000

6000

8000

14 13 12 11 10 9 8 7 6 5 4 2000

BE 3T-BE 3CL-BE T+CL-BE

(b)

4000

6000

8000

Sliding distance (m)

Sliding distance (m) Fig.1. Wear volume loss as a function of sliding Distance at (a) 10N and (b) 30N applied load

The specific wear rate decreases linearly with increase in sliding distance. The TiO2 and clay filled hybrid composite shows the smallest specific wear rate. It may be noted that the resistance offered to wear is larger in Hybrid (TiO2 and nano clay filled) basalt-epoxy composite compared to unfilled, clay and TiO2 filled basalt epoxy composite at different loads of 10, 20 and 30 N. Due to surface treatment of TiO2 and Nano clay being much harder than the epoxy matrix, the lesser wear in such composite is on the expected lines. This can be attributed to improved interfacial bonding between the matrix and nano particles. The scientific and technological attention is required to develop an understanding of the action of Nano fillers in reducing the wear. (a)

BE 3CL-BE

4

Specific wear rate X10-14 (m3/N-m)

Specific wear rate X10-14(m3/N-m)

5

3T-BE T+CL-BE

3 2 1 0 2000

4000

6000

8000

(b)

9 8 7 6 5 4 3 2 1 0

BE 3CL-BE 3T-BE T+CL-BE

2000

Sliding Distance (m)

4000

6000

8000

Sliding Distance (m)

Fig.2. Specific wear rate as a function of sliding distance at (a) 10N and (b) 30N applied load

In the case of lower sliding distance, polymer matrix rubs with the metallic counterparts. The wear debris consists of shear deformed polymer matrix containing broken pulverized matrix particles and wear powder of the metallic counter surface. The particles can either act as debris or form a transfer layer. In such cases, their component can cushion the counter surface asperities of the composite and reduce the effective toughness, but the pulverized

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matrix particles and wear powder of the metallic counter surface can act as a third body abrasive, leading to enhanced roughening of the counter surface. Thus, specific wear rate of the composite depends on the various particles in the wear debris. During wear process, no transfer film was formed on the counter surface leading to higher specific wear rate for lower sliding distance. As the sliding distance increases the hard faced fibers enter the contact zone and give more resistance to wear so that rate of wear volume removal is minimal. Therefore, a decline in the specific wear rate is noticed. During the sliding process, nano particles were embedded in the soft matrix transfer films on the counter surface and grooved the sample surface. This widens the gap between the counter surface and the samples and the particles act as spacers. This in turn can cause a reduction in the adhesion between the contacting surfaces. Thus specific wear rate of filler filled composite reduces. The obtained results are similar to what is reported in reference [16], in spite of the fact that the testing configurations and conditions were different. When the polymer rubbing or sliding against metallic counter face, a thin layer of polymeric material forms on the surface of the metal element, known as tribo-film and it reduces the roughness of the steel surface and also frequently reduces the friction and other wear processes. Wielba et al. [17] experimentally studied the formation of tribo-film and have concluded that the friction develops only between two polymer layers and not between polymer and metal surface. Tribo-chemical reactions occur between the broken chains of the polymer located at the surface of the mating metal. Generally, some important factors influence the tribo-chemical reactions such as chemical composition, molecular structure of the polymer, friction conditions such as load / pressure, sliding velocity, type of contact, environment and interface temperatures. The rate of tribo-film formation depends on the type of polymer structure, durability, lubrication properties and forces of adhesion between the metal counter surface and polymeric materials. The transfer of adhesion from polymeric material to the counter face is strictly by molecular and electrostatic forces, physical and chemical interactions. Generation of heat due to friction rises and temperature at the mating surface of the polymer increases, which in turn leads to the loosening of bonds between intermolecular polymer chains. The polymer chains at the surface are subject to mechanical forces like compression, shearing and tension, which leads to molecular chain cracking and creation of molecules of different radicals. As the nanoparticles were free to move and tend to be dispersed uniformly over the transfer films during the wear process, this would result in a more uniform contact pressure between the contact surfaces and in turn minimizes the stress concentration. This ensured that the lower specific wear rate of filler filled basalt-epoxy at higher sliding distances. 4.3 Co-Efficient of Friction The nano particles usually play a major role in the interface quality of the nano composite material, the capability to transfer stresses and elastic deformation of the matrix materials[18]. If the filler matrix interaction is poor, the nanoparticles are unable to carry any part of the applied load. In that case, good wear resistance cannot be expected. If the bonding between the polymer matrix and fillers is instead strong enough; the yield strength of the filled composite can be higher than that of the unfilled composite [19]. Similar way, the difference in reducing the friction coefficient and improving the wear resistance of unfilled BE composite and filler filled composites is mainly from the adhesion strength of the filler-matrix. Major load is supported by the fillers, resulting in increased wear resistance of the composite. Nano filler improves the adhesion of the transfer film to the counter surface and thereby reduce the wear process. Addition of nano TiO2 and clay in BE composite system reduces the coefficient of friction and also nano particles have polishing effect on the hard steel, reducing the abrasive effect of the Nano composite. 5. Worn Surface Morphology The SEM feature of worn surface of unfilled, 3wt.% clay filled, 3wt. % TiO2 and TiO2+Clay filled hybrid BE composite at a load of 30N and 8000m sliding distance at constant velocity of 1.884 m/s shown in Figures 3 and 4. Several mechanism have been proposed to explain how the composite material is removed from the surface during the sliding wear process, which involve matrix facture, peeling of matrix, fiber-matrix de-bonding, fiber breaking, shear deformation of the fibers, fiber pull out and wear track edge deformation.[20]. With the help of SEM

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micrograph, it is possible to identify qualitatively the dominant wear mechanisms under sliding. In case of higher load and higher sliding distance, most of the matrix material was taken out and loosening of the fibers results in the exposure of the fibrous region to sliding contact. The area under the sliding contact, often transmitted the tribo-film, the fiber ends undergo severe thinning along the their length.

(a)

(b) Fig.3. SEM images of (a) unfilled BE and (b) C-BE composite at 30N, 8000m sliding distance

(a)

(b) Fig.4 SEM images of (a) T-BE and (b) T+CL-BE composite at 30N ,8000m sliding distance

The synergistic effect of nano fillers obstructs the wear of hybrid composite surface layer. Thus, smooth worn surface and hence lower wear volume loss was observed. The better wear resistance can be exhibited by the nano filler filled hybrid basalt epoxy composite system. It depends on the factors such as increasing bonding strength, less voids and formation of tribo-film by the filler materials.

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Conclusion From sliding wear studies of BE, CL-BE, T-BE and T+CL-BE composites, the following conclusions could be drawn: • Sliding wear loss increases with the increase in abrading distance/applied load for all the composites. However, the T+CL-BE filled BE composite showed better sliding wear resistance. • SEM studies of worn out surfaces support the damage to the matrix, exposure of fibres, crushed and fragmented fibres and de-bonding of matrix and filler in composite System • Severe sliding wear of unfilled Basalt epoxy has been changed into mild wear with the addition of nano-clay and nano-TiO2 particles. • The NanoTiO2 and Nano Clay filled BE composite system finds a place in between unfilled and TiO2-Clay filled BE system with regard to wear volume as well as specific wear rate. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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