Mechanical and Tribological Behaviour of

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Mechanical and Tribological Behaviour of Epoxy Reinforced with

Nano-Al

2

O

3

particles

R.V. Kurahatti

a*

, A. O. Surendranathan

b

, A.V. Ramesh Kumar

c

,

V. Auradi

d

,

C. S. Wadageri

e

and S. A. Kori

a

R & D Centre, Dept. of Mech. Engg., BEC, Bagalkot - 587102, Karnataka, India

b

Dept. of Meta and Mater Engg, NITK, Surathkal, Karnataka, India

c

Naval Physical and Oceanographic Laboratory, Kochi - 682021, Kerala, India

d

R & D Centre, Dept. of Mech. Engg., SIT, Tumkur-572103, Karnataka, India

e

Dept. of Mech. Engg., MMEC, Belgaum-591113, Karnataka, India

*

rajukurahatti@gmail.com, aos_nathan@yahoo.com, avramesh.kumar@gmail.com, vsauradi@gmail.com, wadageri1969@gmail.com, kori.shivu@mailcity.com

Keywords:Epoxy, Nano-alumina, Mechanical properties, Friction and wear, SEM

Abstract: In the present work systematic study has been conducted to investigate the matrix properties by introducing nanosize Al2O3 (particle size 100 nm, 0.5–10 wt %) fillers into an epoxy resin. High shear mixing process was employed to disperse the particles into the resin. The experimental results indicated that frictional coefficient and wear rate of epoxy can be reduced at rather low concentration of nano-Al2O3. The lowest specific wear rate 0.7 x 10-4 mm3/Nm is observed for the composites with 1 wt.% which is decreased by 65% as compared to unfilled epoxy. The reinforcement of Al2O3 particles leads to improved mechanical properties of the epoxy composites. The results have been supplemented with scanning electron micrographs to help understand the possible wear mechanisms.

Introduction

Polymer composites are widely used as structural materials in aerospace, automotive and chemical industries, as they provide lower weight alternatives to metallic materials. Gears, cams, bearings and seals are tribological components, where the self-lubrication properties of polymers and polymer composites are of special advantage. The incorporation of well-dispersed nano- or micro-sized inorganic particles into a polymer matrix has been demonstrated to be quite effective to improve the tribological properties of the polymer composites [1-8]. It was verified experimentally by numerous researchers that nanoparticles of metallic or inorganic type, prove the ability to reinforce effectively thermoplastic and also thermosetting polymer matrices [10]. Specifically, the reinforcement covers improvements of the flexural modulus without loosing flexural strength. This effect is at the same time accompanied by improvements in fracture toughness and impact energy which, however, depend strongly on the filler volume content [11]. However, the unique nano-composite effects can only be effective, if the nanoparticles are well dispersed in the surrounding polymer matrix. It has been shown that, a considerable improvement of the mechanical and tribological properties can already be achieved at very low filler volume content, somewhere in the range of 1–5 vol% [12–14].

Epoxies exhibit superior qualities such as high glass transition temperature (Tg), high modulus, high creep resistance, low shrinkage at elevated temperature and good resistance to chemicals due to their densely cross-linked structure. Having ability to adhere to a variety of fillers, they are used for high-performance coatings for tanks and structures. There are special grades of epoxy for elevated-temperature service to about 176◦C. In the present work, nano-Al2O3 filled epoxy composites have been prepared by using high shear mixing process for particle dispersion. The filler content was varied in the range 0.5-10 wt%. The effect of filler content on mechanical and

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tribological properties of the nanocomposites has been studied. Epoxy nanocomposites containing 100 nm alumina particles have not been studied. Hence the present work is taken up.

Experimental Details:

Epoxy resin LY 556 (diglycidayl ether of bisphenol A) and hardener HY951 (triethylene tetramine) were supplied by Huntsman Advanced Materials Pvt. Ltd (Mumbai, India). Al2O3 nanoparticles were purchased from Sigma Aldrich (Bangalore, India). Al2O3 represents the ceramic nanocrystalline phase and consists of primary particles in the size of 100 nm. They have a specific surface area of 100m2/g. The epoxy resin of known weight (g) was taken in a beaker. Al2O3 nanoparticles were added into the resin. The mixture was stirred using a homogenizer (Miccra D-9, speed range of 11 000–39 000 rpm) for 2 h. The high shear mixing results in the separation of particle agglomerates. The hardener (10 wt% of resin) was then added into the mixture and stirred for 15 min. The mixture was then poured into the mould. Curing of the composite at 25°C for 24 h and post-curing at 70°C for 1 h were carried out. Composites with filler contents of 0.5, 1, 5 and 10 wt.% were prepared. Flexural tests of pure epoxy resin and the composites were carried out in a UNITEK 9450 PC universal testing machine in accordance with the ASTM D790 standard at a deformation rate of 1 mm/min, while un-notched impact tests were conducted in a FIE IT-30 tester. The hardness of the materials was measured using an FIE RASNB Rockwell hardness tester with ‘F’ scale (1/16” diameter ball indenter and 60 kg load). A loading time of 30 s was used.

Unlubricated pin-on-disc sliding wear tests were carried out in order to determine the tribological properties of the nanocomposites. Sliding was performed in air with the ambient temperature of around 25°C over a period of 60 min. at a sliding velocity of 0.84 m/s and a normal pressure of 0.8 MPa. The test samples were accurately weighed before and after the wear run using electronic balance (0.1 mg accuracy) to determine the wear loss. The friction coefficient was calculated by taking into account the normal load and the friction force. The wear performance is described by specific wear rate (K_s):

where ∆ m is the mass loss in grams (g), ρ is the measured density of the sample in (g /mm3), FN is the normal load in (N) and L is the sliding distance in meters (m). The average of the three readings was adapted in our results. Scanning electron microscopy examinations on a Jeol-5400 was used to study the morphology of fracture surfaces from flexural testing and the morphology of worn surfaces.

Results and Discussions

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Tribological behaviour: In general, the friction and wear properties do always describe the whole tribological system rather than a material property alone. Fig. 1a shows the specific wear rate (K_s) of the nanocomposites as a function of content of Al2O3 nanoparticles. It was found that the nano-Al2O3/epoxy composites exhibited decreased Ks in comparison to the neat epoxy. The Ks sharply decreased when nano particle content was below 1 wt%. 1 wt% nano-Al2O3/epoxy exhibited the lowest K_s_._{Although, the}K_{s increased with increasing Al}₂_O₃_{content above 1 wt. %, it was still}

Table1. Shows mechanical properties of epoxy composites containing different concentrations of nano-Al2O3 examined the influence of microstructure on the tribological performance of nanocomposites by different compounding methods. They confirmed that the dispersion state of the nanoparticles and micro-structural homogeneity of the fillers improve the wear resistance significantly. The phenomenon reported in the present work is similar to the one reported by Rong et al [9]. Besides improving the wear resistance, the nanoparticles also reduce the coefficient of friction as shown in Fig. 1b. Evidently, the nano-composite with 1 wt% Al2O3 has the lowest friction coefficient. The coefficient of friction decreases from 0.57 for unfilled epoxy to 0.44 at 1 wt%. The detached nanoparticles might also act as solid lubricant. These account for the lower specific wear rates and friction coefficients of the nanocomposites.

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Fig. 1. (a) Specific wear rate of unfilled epoxy and nano-Al2O3/epoxy composites (Normal pressure, 0.8MPa; Sliding velocity, 0.84m/s; sliding distance, 3014m) (b) Friction coefficient of unfilled epoxy and nano-Al2O3/epoxy composites.

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deep grooves in the sliding direction (Fig. 2a). In the case of filled nanocomposites, the appearances are completely different and become rather smooth. Although the ploughing grooves are still visible on the sample surface, the groove depths are shallow on 1 wt% filled epoxy (Fig. 2b) and at some region, the grooves are simply invisible. It seems the low filler loading is sufficient and can bring about significant improvement in wear resistance. During sliding, a rolling effect of nanoparticles could reduce the shear stress, the friction coefficient and the contact temperature. The matrix damages in the interfacial region were reduced by this rolling effect. However, the wear resistance suffers if the filler loading exceeds 1 wt% and the large amount of nanoparticles cannot provide any wear reducing effect. Abrasive wear is accompanied by delamination and fatigue cracking of the matrix (Fig. 2c).

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Fig. 2. Micro-photographs showing worn surface features of (a) unfilled epoxy, (b) 1wt% Al2O3 /epoxy and (c) 5 wt% Al2O3/epoxy nanocomposites.

Conclusions

This study focused on the development of nanocomposites with properties superior to the neat matrix. The following conclusions can be drawn:

1. The reinforcement of Al2O3 leads to improved mechanical properties. Mechanical properties do not exert remarkable influence on the wear behaviour of the materials.

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3. Nano-Al2O3 filled epoxy composites show signs of mild abrasive wear due to the hard ceramic particles. The main wear mechanism of the composite changed from the severe abrasive wear (for neat epoxy matrix) to mild abrasive wear.

Acknowledgements

The authors thank Director Dr K.U. Bhaskar Rao, DMSRDE Kanpur for permitting to conduct the present research work.

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Mechanical and Tribological Behaviour of Epoxy Reinforced with Nano Al2O3 Particles