Optimization of Dry Sliding Wear Behaviour of Zirconium Filled Bismaleimide Nanocomposites

Optimization of Dry Sliding Wear Behaviour of Zirconium Filled Bismaleimide Nanocomposites
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  Proceedings of the 2 nd  International Conference on Current Trends in Engineering and Management ICCTEM -2014 17 – 19, July 2014, Mysore, Karnataka, India   62 OPTIMIZATION OF DRY SLIDING WEAR BEHAVIOUR OF ZIRCONIUM FILLED BISMALEIMIDE NANOCOMPOSITES S.M. Darshan 1 , B. Suresha 2 , B.N. Ravi Kumar 3 1, 3 Department of Mechanical Engineering, Bangalore Institute of Technology, Bangalore, Karnataka, 560004, India 2 Department of Mechanical Engineering, The National Institute of Engineering, Mysore, Karnataka, 570008, India ABSTRACT The mechanical properties and dry sliding wear behavior of bismaleimide BMI) nanocomposites with varying weight percentage of zirconium dioxide (ZrO 2 ) filler have been studied in the present work. The influence of independent parameters such as sliding velocity (A), normal load (B), filler content (C), and sliding distance (D) on dry sliding wear behaviour has been considered using Taguchi's L 27  orthogonal array. The results showed that the coefficient of friction of ZrO 2 filled BMI nanocomposites decreased with the ZrO 2 content increased. The specific wear rate of the BMI and ZrO 2 filled BMI nanocomposites decreased as the sliding distance increased and a clear transition in friction and wear behavior was observed in neat BMI and their nanocomposites. The high wear resistance of the nanocomposites is discussed in terms of impact and dynamic mechanical strengths. Improvements in mechanical and tribological properties of these nanocomposites make them promising candidates for bearing applications. It was found that the silane modification of the ZrO 2 particles improved the wear resistance, impact strength, and hardness BMI nanocomposites as well. Taguchi's results indicate that the normal load played a significant factor affecting the wear rate followed by sliding velocity, sliding distance and ZrO 2  loading. Keywords: ZrO 2 Filled BMI Nanocomposites, Wear, Specific Wear Rate, Wear Mechanisms. 1.   INTRODUCTION Present day industries are experiencing an escalating trend in the applications of particulate and fiber reinforced polymer matrix composites. Polymers and their composites are extensively used in tribological application because of light weight, excellent strength to weight ratios, resistance to corrosion, non-toxicity, easy of fabrication, design flexibility, self-lubricity, better coefficient of friction, and wear resistance. Some of these applications related to mechanical engineering experience surface interactions with the surroundings as well as with the pairing element. Such applications call for better understanding of the tribological behavior of the material under study. A few examples of components used in tribological applications are gears, cams, wheels, brakes, bearing liners, rollers, seals, clutches, bushings, transmission belts, pump handling industrial fluids and pipes carrying contaminated water [1]. An improvement in the tribological behavior of polymers is usually expected as a result of introducing fillers into the matrix. Studies of introducing additional phases, e.g., inorganic fillers, have been successfully developed and thus extended its applications in the past decades [2, 3]. Effects of fillers on the mechanical performances of composites strongly depend on their properties, shape, size and aggregate degree, surface characteristics and loading. Studies have shown that mechanical properties of polymer composites filled with smaller particles (fine particles) are superior to those with larger ones at micron level. In recent years, great efforts devoted to the development of nanoparticle-filled polymer composites have made it possible to investigate the effect on the mechanical properties [4].   INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 5, Issue 9, September (2014), pp. 62-70 © IAEME: Journal Impact Factor (2014): 7.5377 (Calculated by GISI)   IJMET   © I A E M E    Proceedings of the 2 nd  International Conference on Current Trends in Engineering and Management ICCTEM -2014 17 – 19, July 2014, Mysore, Karnataka, India   63 In recent years, nanoparticles have been used as fillers in polymeric composites for improving the tribological performance of the materials (nano-Al 2 O 3  /polyphenylene sulfide (PPS), nano-TiO 2  /epoxy, nano-SiO 2  /polyarylate, nano-ZrO 2  /polyetheretherketone (PEEK), nano-Si 3 N 4  / bismaleimide). When nanoparticles are incorporated into the matrix, both microstructure and properties of the composites can be improved. Zhang et al. [5, 6] systematically studied the wear resistance of epoxy filled with short carbon fiber, graphite, polytetrafuroethylene (PTFE) and nano-TiO 2  under different sliding conditions. The spherical nano-TiO 2  was able to apparently reduce the friction coefficient during sliding and consequently reduce the shear stress, contact temperature and wear rate of fiber reinforced epoxy composites [6]. In order to improve the friction and wear behavior of polymeric materials, one typical concept is to reduce their adhesion to the counterpart material and to enhance their mechanical properties. This can be achieved quite successfully by using inorganic fillers. Ng et al. [7] verified that nanoparticles can remarkably reduce the wear rate, while micron sized particles cannot. Rong et al. [8] conformed that the dispersion state of the nanoparticles and micro-structural homogeneity of the fillers improved the wear resistance significantly. The way of nanoparticle incorporation must be considered as a very important key point to receive the desired material properties. The addition of different fillers favorably stiffens the material and may also increase the strength under certain load conditions. Bismaleimide (BMI) Resins are relatively young class of thermosetting polymers that are gaining acceptance by industry because they combine a number of unique features including excellent physical property retention at elevated temperatures and in wet environments, almost constant electrical properties over a wide range of temperatures, and nonflammability properties. Their excellent processibility and balance of thermal, mechanical, and electrical properties have made them popular in advanced composites. The application of BMI composite materials is being expanded, especially for military aircraft structures. BMI was synthesized from 4, 4’- bismaleimidodiphenylmethane and maleic acid anhydride, with the synthesis being followed by cyclodehydration. BMI was synthesized according to the method for synthesis of allyl ether novlak described in [9]. Many modified BMI resin systems have been developed. Among them a two-component high performance resin system based on BDM and O,O’-diallyl bisphenol A (DBA), coded as BDM/DBA has been proved to have outstanding toughness, good humidity resistance, excellent thermal and mechanical properties [10]. Hence, the BDM/DBA resin was chosen as the base resin in the present work. Zirconium dioxide, which is also referred to as zirconium oxide or zirconia, is an inorganic metal oxide that is mainly used in ceramic materials. Zirconium dioxide succeeds zirconium as a compound of the element zirconium that most frequently occurs in nature [11]. Its great hardness, low reactivity, and high melting point have made it the oldest mineral that can be found on the earth. Majority of research studied detailed experimental work with effect of one factor by keeping all other factors fixed, this approach is not advisable because in actual environment there will be combined effects of interacting factors influencing the abrasive wear. Hence in this study an attempt is being made to study the interacting effects of factors along with the main effect. To achieve this, design of experiments based on Taguchi method is adopted. This method is advocated by Taguchi and Konishi [12]. Taguchi’s technique uses special design of orthogonal arrays to study the entire parameter space with only a small number of experiments. Taguchi’s technique also helps in optimizing the critical parameters [13]. In this study, we have developed a new type of ZrO 2  nanoparticles filled BMI composites. A high shear mixing procedure was used to uniformly disperse the ZrO 2 nanoparticles into the BMI resin system. Effect of incorporation of ZrO 2 nanoparticles on impact strength, dynamic mechanical strength and dry sliding wear behaviour have been investigated. The influence of independent parameters such as sliding velocity (A), normal load (B), filler content (C), and sliding distance (D) on dry sliding wear behaviour has been considered using Taguchi's L 27  orthogonal array. 2.   EXPERIMENTAL PROCEDURE 2.1 Materials 4, 4’ –Bismaleimidodiphenylmethane (BDM) and O,O’ –diallylbisphenol A(BA) were supplied by ABR Organics Limited, Hyderabad (India), Zirconia(ZrO 2 ) nanoparticles were purchased from sigma Aldrich, Bangalore(India). The zirconia represents the ceramic nanocrystalline phase with size range of 60-100 nm (Fig 1). Along with a spherical shape, their large number is characterized by a very high specific surface area of 100m 2  /g. This powder contains particle agglomerates with sizes in the micrometer range (Fig 1), which consists of ZrO 2  primary nanoparticles sticking strongly together. Primary nanoparticles attract each other due to adhesive inter particle ‘van der Waals’ forces, which srcinate from the materials surface energy.  Proceedings of the 2 nd  International Conference on Current Trends in Engineering and Management ICCTEM -2014 17 – 19, July 2014, Mysore, Karnataka, India   64 Fig. 1: Microstructure of zirconium nanoparticles 2.2 Preparation of ZrO 2  filled BMI nanocomposites Appropriate content of zirconia nanoparticles were added into BA at room temperature (25 ◦ C) with vigorous mechanical agitating. The mixture was agitated for 2 h followed by ultrasonic stirring for another 2 h to obtain a homogeneous suspension. The detailed procedure of fabrication of zirconia filled BMI nanocomposites were given in Elsewhere [3]. 2.3 Dynamic Mechanical Analysis DMA is a technique, which is used to study the stress, temperature and frequency of the material when it is subjected to a small deformation by sinusoidal load. DMA is used to measure the stiffness and damping in terms of storage   modulus, loss modulus and tan δ . The approach is often used to determine glass transition temperature, as well. The dimensions of the specimens were 3.2mm×12.5mm×63.5mm. The tests were conducted at a heating rate 5.0°C/minute from 0 o C to 280 o C at a frequency of 1.0Hz for the neat BMI and nano ZrO 2  filled BMI. 2.4 Dry sliding wear measurements Unlubricated pin-on-disc sliding wear tests were carried out in order to determine the tribological properties of the nanocomposites. The disc material is made up of En-32 steel (diameter 160mm and 8 mm thickness) having hardness value of HRc 65. The surface roughness of the disc varies from 0.02 to 0.06 µm. A constant 114 mm track diameter was used throughout the experimental work. Sliding was performed in air with the ambient temperature of around 25 ◦ C, over different sliding distance at a sliding velocity of 0.5 m/s and a normal load of 40 N. Prior to wear testing, all specimens were cleaned, that is, the sample was abraded with water-abrasive paper (600 grit) and a super-fine water-abrasive paper successively. Then both the steel ring and the specimen were cleaned with acetone and distilled water. The wear process takes some material away from the sample. This mass loss can accurately be measured by determining the weight of the specimen before and after the experiment. A characteristic value, which describes the wear performance under the chosen conditions for a tribo-system is the specific wear rate (K s ):      ⋅⋅∆= m N mm LF mK   N s 3  ρ    (1)   Where ∆ m is the mass loss, ρ  is the measured density of the composite, F N is the normal load applied and L is the sliding distance. In order to take repeatability into account, results from the friction and wear tests were obtained from three readings and the average value was adapted in our results. 2.5 Experimental design Design of experiments is the powerful analysis tool and analyzing the influence of the control factors on the performance output. The most important stage is the design of experiments lies in the selection of the control factors. Four parameters, namely, sliding velocity(A), normal load(B), filler content(C), and sliding distance(D) each at three levels, are considered in this study in accordance with L 27 (3 13 ) orthogonal array design. Control parameters and their levels are indicated in table. Four parameters each at three levels would require 3 4 = 81runs in a full-factorial experiment,  Proceedings of the 2 nd  International Conference on Current Trends in Engineering and Management ICCTEM -2014 17 – 19, July 2014, Mysore, Karnataka, India   65 whereas Taguchi’s factorial experiment approach reduces it to only 27 runs offering a great advantage. The plan of the experiment is as follows: the first column of the Taguchi orthogonal array is assigned to the sliding velocity (A), the second column to the normal load (B), the fifth column to the fiber content (C), the ninth column to sliding distance (D), and remaining columns are assigned to their interactions and experimental errors. Table 1: control factors and levels used in the experiment  LEVELS Control factor I II III Units A: Sliding velocity 0.5 1 1.5 m/s B: Normal load 20 40 60 N C: Filler content 0 5 10 % D: Sliding distance 1000 2000 3000 M The experimental observations are transformed into signal-to-noise (S/N) ratio. There are several S/N ratios available depending on the type of characteristic, which can be calculated as logarithmic transformation of the loss function. For lower is the better performance characteristic S/N ratio is calculated as per : S/N = -10 log [1/n ( Σ y 2 )] (2) Where “n” is the number of observations and “y” is the observed data. “Lower is the better” (LB) characteristic, with the above S/N ratio transformation, is suitable for minimization of wear rate. A statistical analysis of variance (ANOVA) is performed to identify the control parameters that are statistically significant. With the S/N ratio and ANOVA analyses, the optimal combination of wear parameters is predicted to acceptable level of accuracy. Finally conformation of experiments is conducted to verify the optimal process parameters obtained from the parameters design. 3. RESULTS AND DISCUSSION 3.1 Effects of concentrations of ZrO 2 on the impact strength The impact tests were performed using a Charpy impact tester according to the ASTM-D256. The impact strength of the neat BMI is 1.4 kJ/m 2  and the impact strength of nanocomposites increases to 2.2kJ/m 2  for 5 wt% ZrO 2  filled BMI composite. It is obvious that all the nanocomposites have improved impact strength. All ZrO 2  filled BMI nanocomposites show better performance, because they have more contact area with BMI resin at the same particle loading and also because more interaction forces can occur, such as hydrogen bonding and Van der Waals interaction. According to the craze mechanism, the addition of nanoparticles can lead to formation of more crazes, and more impact energy can be absorbed compared with micron particles [14]. Of all the ZrO 2  filled BMI composites, the 10 wt% ZrO 2  filled BMI composite showed the highest impact strength (2.8 kJ/m 2 ). This is about two times that of neat BMI. This can be explained by the fact that ZrO 2 nanopartilces have the special two-dimensional nanostructure and fewer agglomerates compared with other nanostructures. It is well known that nanopartilces have a high surface area, which results in excellent interfacial combinations of BMI resin with silanated ZrO 2 nanoparticles. In addition, the ZrO 2 nanoparticles embedded in the BMI matrix work like load bearing material in reinforced polymer, forming crack pinning and immobilizing the polymer, thus leading impact energy to be conceded quickly. 3.2 Effects of concentrations of ZrO 2 on the dynamic mechanical strength The storage modulus versus temperature curve provides valuable information about the stiffness of a material as a function of temperature, and it is sensitive to structural changes. DMA results for the nanocomposite systems show a consistent trend of decreased storage modulus over the pure BMI (Fig.2). It seems reasonable to assume that a better impregnation of the 5% wt ZrO 2  nano-filler in to BMI will amplify the effect of stress transfer under loaded condition due to increased filler-matrix bonding and degree of crosslinking action. On the other hand, large microsized clusters formed during the mechanical agitation leads to marginal changes in storage and loss moduli from 5% to 10% of filler loading. Loss modulus is the capacity of a material to dissipate energy when placed when stressed. The addition of ZrO 2  to the BMI matrix should increase the loss modulus. This is due to the fact that polymer segments bond to the surface, the loops and chains that extend toward the bulk matrix are expected to support a mechanical interlocking with the bulk chains. Glass transition temperature reported here is the temperature corresponding to the peak of tan δ  curves. Referring to Fig 2, increase the glass transition temperatures from 86 0 C to 102 0 C was observed with increase in filler content from 5% to 10% of filler loading. This increase in, T g , is probably due to the molecular weight effect, more
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