A Comparative Study on Dry Sliding Wear Characteristics of Al 2 O 3 and Bone Powder Filled Hybrid Composites

The aim of the research was to develop E-glass/jute fiber reinforced epoxy composites with an addition of Al2O3 and bone powder by using hand layup technique and to compare tribological properties of these composites under similar test conditions. The wear experiments were designed according to Taguchi’s (L27) orthogonal array with three control variables such as sliding velocity, filler content and normal load. The results indicated that the normal load for Al2O3 and filler content for bone powder emerged as the significant factors affecting specific wear rate of hybrid composites. An addition of 10 wt% of bone powder or Al2O3 into E-glass/jute fiber reinforced epoxy composites increased the wear resistance considerably, and natural waste bone powder can be used instead of ceramic filler Al2O3 in hybrid composites. After the analysis of control factors, an optimal factor setting has been suggested for specific wear rate and coefficient of friction. Further, the scanning electron microscopy (SEM) images for worn surfaces of hybrid composites were studied. Finally, a confirmation test was carried out to validate the results.


Introduction
Composite materials can be described as engineering or naturally occurring materials made from two or more constituent materials with essentially different physical or chemical properties which remain separate and dis-tinct at the macroscopic or microscopic scale within the finished structure [1].As polymers can be filled with organic fillers and inorganic fillers, inorganic-filled composites have become attractive in polymer field due to its various advantages such as easiness in processing, cost effectiveness and excellent performance over the metals.Polymer composites have a special property of self lubrication and this made the composites suitable in tribological applications such as cams, seals, brakes, bearings etc. [2].
Many researchers showed interest in developing the new natural fiber composites because of their low cost, combustibility, lightweight, low density, high specific strength, renewability, non-abrasivity, non-toxicity and biodegradability [3]- [5].Natural fibers are renewable and biodegradable materials and are largely available in worldwide nature [6]- [8].Pine, apple leaf, oil palm fiber, hemp, sisal, jute, kapok, rice husk, bamboo and wood are the fibers most commonly used as reinforcing natural fibers in polymer matrix [9]- [11].Nowadays, in the field of composites, waste material is used as a filler like eggshell, uncarbonized bone and carbonized bone power.Eggshell contains about 95% calcium carbonate in the form of calcite and 5% organic materials such as type X collagen, sulfated polysaccharides and other proteins [12].Filler materials are used to reduce the material costs, to improve mechanical properties to some extent and in some cases to improve process ability.It also increases properties like abrasion resistance, hardness and reduce shrinkage.Properties of composites are strongly dependent on the properties of their constituent materials, their distribution and the interaction among them.The geometry of the reinforcement (shape, size and size distribution) influences the properties of the composite to a great extent.Natural fibers and glass fibers, reinforced thermoplastic composite, have successfully proven their high qualities in various fields of technical application.Natural fibers used as replacements for aramid and glass fibers due to their low density, good thermal insulation and mechanical properties, reduced tool wear, unlimited availability, low price, and problem free disposal.In view of the above, an attempt was made to study the effect of Al 2 O 3 and bone powders as a filler material in E-glass/jute epoxy composite.The wear characteristics of these hybrid composites were investigated and analyzed.

Materials
In this work, 7-mil E-glass fibers, woven jute fibers, epoxy L-12 (3202) and K-6 hardener were used for fabricating the composite materials [13] [14].Aluminium oxide particles size was 70 -230 mesh and uniformly distributed throughout the matrix.The average density of aluminium oxide was 3.89 g/cc.Goat bones were washed and cleaned.Goat bone powder was used as filler material since eggshell and bone belong to the same calcium group [12].Thin top surface layer of the bone was removed by using grinding machine and powder was prepared.An approximate density value of bone powder was 1.95 g/cc.

Fabrication and Specimen Preparation of Hybrid Composites
The hybrid fiber reinforced epoxy composites were prepared by keeping constant 40 wt% of E-glass fiber and 10 wt% of jute fiber.The composites filled with varying concentrations (0 wt%, 10 wt% and 15 wt%) of Al 2 O 3 and bone powder were fabricated by using hand layup technique [15].Jute plies were placed in between E-glass plies and also E-glass plies were placed at extreme ends of the composites.The epoxy, hardener and fillers were mixed in a basin and the mixture was subsequently stirred constantly till the filler was uniformly distributed throughout the mixture [16].The plies was placed one above the other and the epoxy mixture was brushed uniformly over the woven E-glass and the woven jute plies.Care was taken that the plies were properly coated with epoxy mixture.Entrapped air was removed manually with squeezers or rollers to complete the composite structure.The composites were put under load for about 24 hours for proper curing at room temperature.The fabricated hybrid composites were taken out from the mould and then specimens were prepared according to ASTM G99 standards.The composites were cut by using Zigzag board cutter machine.Three identical test specimens were prepared for each test.

Density Test
The theoretical density (ρ t ) of hybrid composite material in terms of weight fractions of different constituents can be obtained from the following Equation (1) [17].
where, W and ρ represent the weight fraction and density respectively.The suffixes g, j, m and f stand for the Eglass fiber, jute fiber, matrix (epoxy) and filler respectively.The actual density (ρ e ) of the composites can be determined experimentally by simple water immersion technique.The volume fraction of voids (V v ) in the composites was calculated using the following Equation (2) [17].

Brinell Hardness Test
Brinell hardness test was conducted on the specimen using a standard Brinell hardness tester [18].A load of 2500 N was applied on the specimen for 30 sec using 5 mm diameter hard metal ball indenter and the indentation diameter was measured using a microscope.The hardness was measured at three different locations of the specimen and the average value was calculated as shown in Table 2 [19].

Dry Sliding Wear Test Apparatus
DUCOM pin on disc test apparatus was used to investigate the dry sliding wear characteristics of hybrid composites.The disc made of hardened EN-31 hardened ground steel to 62 HRC, surface roughness 1.6 μm Ra, 120 mm track diameter and 8 mm thick.Specimen with size of 10 mm × 10 mm × 30 mm was subjected vertically to the counter face where the contact area was constant.Wear test was carried out for constant sliding distance of 2000 m, sliding velocities of 1.4 m/sec, 2 m/sec and 2.6 m/sec against the normal loads of 20 N, 40 N, and 60 N and wear rate data reported was the average of two runs.Care was to see that the specimen was continuously cleaned with woolen cloth to avoid the entrapment of wear debris and to achieve uniform experimental procedure.After each test the disc was cleaned using acetone solution.The initial weight before run and final weight after run was measured using a precision electronic balance with an accuracy of ±0.01 mg.
The specific wear rate (mm 3 /Nm) was then expressed on volume loss basis as shown below Equation (3) [20].

Taguchi Experimental Design
Taguchi technique was used for analyzing the influence of control factors on performance output [21] [22].The most important stage in the design of experiment lies in the selection of control factors.In the present work, the impact of three parameters such as sliding velocity, normal load and filler content were studied using L 27 (3 13 ) Taguchi orthogonal design [23].The experimental results were transformed into a signal-to-noise (S/N) ratio.The S/N ratio for minimum wear rate coming under "smaller is better" characteristic, and it was calculated as logarithmic transformation of the loss function as shown below Equation ( 4) [1].
( ) where n indicates the repeated number of trial conditions; 1 2 , , , n y y y indicates the response of the dry sliding wear characteristics respectively.For ''Lower is better" (LB) characteristic of the above S/N ratio transformation, it was suitable for minimizations of coefficient of friction and specific wear rate.The standard linear graph was used to assign the factors and interactions to various columns of the orthogonal array.
The plan of the experiments: The experiments were conducted as per the standard orthogonal array L 27 (3 13 ).The selection of the orthogonal array was based on the condition that the degree of freedom for the orthogonal array should be greater than or equal to the sum of those wear parameters.
In the present investigation L 27 Taguchi orthogonal array was chosen and this has 27 rows and 13 columns.The wear parameters chosen were: 1) sliding velocity; 2) normal load and 3) filler content, and their levels are indicated in Table 1.The experiments consisted of 27 tests (each row in the L 27 orthogonal array) and were assigned with parameters.The first column in the table was assigned to sliding velocity (S); the second column was assigned to normal load (L); the fifth column was assigned to filler content (FC) and the remaining columns were assigned to their interactions.The dry sliding wear tests result were subjected to the analysis of variance.

Density
In the present research work, the theoretical and experimental densities of hybrid composites along with the corresponding volume fraction of voids are presented in Table 2.It was found that the composite density values calculated theoretically from weight fractions were not equal to the experimentally measured values.It was evident from Table 2 that the density values for hybrid composites increase with the addition of filler content.It was further observed that with the incorporation of fillers, the void fractions in these composites also increase.As similar observation has been reported earlier for hybrid composite filled with different filler content [24].

Wear Behavior of Bone Powder Filled Hybrid Composites
The experimental data for specific wear rate and coefficient of friction of bone powder filled hybrid composites reported in Table 3.The overall mean for the S/N ratio of the specific wear rate and the coefficient of friction were found to be 91.995db and 7.105 db respectively.

ANOVA and Effects of Factors on Specific Wear Rate
It was observed that the filler content (C) factor (P = 44.25%),normal load (B) factor (P = 16.67%) and the interaction between sliding velocity x filler content (P = 10.51%) have great influence on the specific wear rate and hence theses were physically and statistically highly significant from the ANOVA Table 4. However interaction (A × B) between sliding velocity × normal load (P = 9.09%), interaction between normal load x filler content (P = 7.57%) and sliding velocity (A) factor (P = 1.10%) have lesser effect on specific wear rate as error value (P = 10.87) was higher side hence less significant.From the analysis of ANOVA and response Table 5    the S/N ratio for specific wear rate as shown in Figure 1(a), it was observed that the filler content has major impact on specific wear rate followed by normal load and sliding velocity.The influence of each control factor (sliding velocity, normal load and filler content) on the specific wear rate was analyzed with a main effect plot as shown in Figure 1(a) and an interaction plot as shown in Figure 2(b).From the analysis of ANOVA and response Table 5 of the S/N ratio for specific wear rate, it was observed that the control factor filler content (C) has major impact on the specific wear rate followed by the normal load (B) and sliding velocity (A).It means that for bone powder filled hybrid composites with the increase of the filler content, sliding velocity and normal load the specific wear rate decreases i.e., good wear resistance.It was also observed that the filler content plays adverse effect, when the filler content was increased from 0 wt% to 10 wt% the specific wear rate was decreased due to good bonding strength between the fibers, filler and matrix.When the filler content was increased from 10 wt% to 15 wt% specific wear rate was increased due to presence of voids, weak bonding strength between fiber and matrix.The experimental results show that further increase in filler content increases specific wear rate [12].Analysis of these results leads to the conclusion that optimal values of the parameters for minimizing the wear rate when the sliding velocity and normal load were at level 3 and the filler content was at level 2.

ANOVA and Effects of Factors on Coefficient of Friction
ANOVA Table 6 for the coefficient of friction that the filler content factor (P = 69.33%),interaction between sliding velocity × normal load (P = 12.62%) and the interactions between sliding velocity × filler content (P =  6.56%) have significant influence on the coefficient of friction.However interaction of normal load x filler content (P = 3.94%), sliding velocity (P = 2.07%) and normal load factor (P = 0.05%) do not have a significant effect (both physically and statistical) on coefficient of friction as their values are quite smaller than error (P = 5.43%).So they were neglected.From the analysis of ANOVA and response Table 7 of the S/N ratio of coefficient of friction, it was observed that the control parameter filler content (C) has a major impact on coefficient of friction followed by normal load and sliding velocity.The influence of each control factor on the coefficient of friction was analyzed with a main effect plot as shown in Figure 2(a) and an interaction plot as shown in Figure 2(b).From the analysis of ANOVA and response Table 7 of the S/N ratio for coefficient of friction, it was observed that the control factor filler content (C) has major impact on the coefficient of friction.It means that for bone powder filled hybrid composites, with the increases of the filler content from 0 wt% to 10 wt%, coefficient of friction increases and from 10 wt% to 15 wt% coefficient of friction decreases.The same trend was observed for normal load.Analysis of these results leads to the conclusion that optimal values of the parameters for minimizing the coefficient of friction when the sliding velocity and normal load were at level 1 and the filler content was at level 3.

Surface Morphology of Bone Powder Filled Hybrid Composites
When compared with sliding velocity and normal load the effect of filler content on specific wear rate was more as seen from the main effect plot as shown in Figure 1(a).For analysis, varying concentration of filler content composite samples were selected and at 20N normal load condition.The micrograph in Figure 3

Wear Behavior of Aluminium Oxide (Al 2 O 3 ) Filled Hybrid Composites
The experimental data for specific wear rate and coefficient of friction of aluminium oxide (Al 2 O 3 ) filled hybrid composite reported in Table 3.The overall mean for the S/N ratio of the specific wear rate and the coefficient of friction were found to be 88.789 db and 7.905 db respectively.

ANOVA and Effects of Factors on Specific Wear Rate
It was observed that from the ANOVA Table 8 the normal load (B) factor (P = 37.64%) and filler content (C) factor (P = 17.63%) have great influence on the specific wear rate and hence these were physically and statisti-   cally highly significant.However sliding velocity (A) factor (P = 14.19%), (B × C) interaction between normal load × filler content (P = 8.18%), interaction (A × B) between sliding velocity × normal load (P = 5.99%) and interaction (A × C) between sliding velocity × filler content (P = 2.11%) have lesser effect on specific wear rate as error value (P = 14.25%) was higher side hence less significant.From the analysis of ANOVA and response Table 9 of the S/N ratio for specific wear rate, it was observed that the normal load has major impact on specific wear rate followed by filler content and sliding velocity.The influence of each control factor on the specific wear rate was analyzed with a the main effect plot as shown in Figure 4(a) and an interaction plot as shown in Figure 4(b).From the analysis it was observed that the control factor normal load (B) has major impact on the specific wear rate followed by the filler content (C) and sliding velocity (A).It means that for Al 2 O 3 filled hybrid composites, with the increases of the sliding velocity and normal load the specific wear rate decreases.It was also observed that as the filler content (Al 2 O 3 ) increases up to 10 wt% the specific wear rate decreases and further increase in filler content increases the specific wear rate.Analysis of these results leads to the conclusion that optimal values of the parameters for minimizing the wear rate when the sliding velocity was at level 3, normal load was at level 3 and the filler content was at level 2.

ANOVA and Effects of Factors on Coefficient of Friction
From the ANOVA Table 10 for the coefficient of friction that the filler content (C) factor (P = 51.08%) and interaction of normal load (B × C) filler content (P = 16.47%) have significant influence on the coefficient of friction.However the interaction (A × C) between sliding velocity × filler content (P = 9.92%), interaction between (A × B) sliding velocity x normal load (P = 5.55%), normal load (B) factor (P = 2.23%) and sliding velocity (P = 1.39%) do not have a significant effect both physically and statistical on coefficient of friction as their values were quite smaller than error (P = 13.33%).So they were neglected.From the analysis of ANOVA and response Table 11 of the S/N ratio of coefficient of friction, it was observed that the control parameter filler content (C) has a major impact on coefficient of friction followed by normal load and sliding velocity.
The influence of each control factor on the coefficient of friction was analyzed with a main effect plot as shown in Figure 4(a) and an interaction plot as shown in Figure 4(b).From the analysis of ANOVA and response Table 11 of the S/N ratio for coefficient of friction, it was observed that the control factor Al 2 O 3 as a filler content (C) has major impact on the coefficient of friction.It means that for aluminum oxide filled hybrid composites, with the increases of the filler content from 0 wt% to 10 wt% coefficient of friction decreases and from 10 wt% to 15 wt% coefficient of friction increases.The same trend was observed for sliding velocity but the variation was very less, so the influence of sliding velocity on coefficient of friction was neglected.With the increase of normal load coefficient of friction increases due to thin transfer layer between the sliding materials formed because of plastic deformation of the epoxy and broken E-glass/jute fibers along with the Al 2 O 3 fillers results in the easy shear during sliding [25].Analysis of these results leads to the conclusion that optimal values of the parameters for minimizing the coefficient of friction when the sliding velocity and filler content were at level 2 and the normal load was at level 1.

Surface Morphology of Al 2 O 3 Filled Hybrid Composites
When compared with sliding velocity and filler content the effect of normal load on specific wear rate was more

Confirmation Experiments
The confirmation experiment was conducted to validate the inference drawn during the analysis phase and considered as a final test in design of experiment process.The new set of factor setting A 2 B 3 C 2 was considered to predict the coefficient of friction for confirmation experiment.The estimated S/N ratio for coefficient of friction can be calculated with the help of following predictive Equation (5) [24].
Similarly a prediction Equation ( 7) was developed for estimating S/N ratio of specific wear rate For each of performance measures an experiment was conducted for different combination of factors and results were compared with those obtained from the predictive equation for specific wear rate (Table 12) and for coefficient of friction (Table 13) [24].The above Equations ( 6) and (8) were capable of predicting the coefficient of friction and specific wear rate to the acceptable level of accuracy.An error for the S/N ratio of the coefficient of friction and for the S/N ratio of the specific wear rate was observed and shown in Table 12 and Table 13.However if number of observations of performance characteristics were increased further these errors can be reduced.

Conclusions
From the results and discussion the following conclusions can be made: • Fabrication of hybrid composites consisting of E-glass/jute fiber reinforcement in epoxy with Al 2 O 3 and bone powder filler was done successfully.The incorporation of Al 2 O 3 and bone powder as secondary phase in the epoxy matrix improved the tribological characteristics; • Design of experiments approach by Taguchi method enabled to successfully analyze the friction and wear behavior of the composites with sliding velocity, normal load and filler content as test variable; • The wear resistance was increased for the addition of 10 wt% of Al 2 O 3 and bone powder filled composites.
Normal load for Al 2 O 3 and filler content for bone powder were the main factors that had the highest physical and statistical significance in influencing the specific wear rate; • The coefficient of friction decreased for the addition of 10 wt% of Al 2 O 3 but it increased for bone powder filled hybrid composites due to weak bonding strength of composite material, and layers were transformed between the sliding materials.Filler content was the main factor that had the highest physical and statistical significance in influencing the coefficient of friction for Al 2 O 3 and bone powder filled composites; • The predictive equations based on Taguchi approach were successfully used for the prediction of effect of various factors and predicted results were consistent with experimental observations.SEM studies of worn surfaces supported the involved mechanisms and indicated micro-cracking, debris, wear track, exposure of fibers, fiber breakage and wedge formation.

Figure 1 .
Figure 1.Effect of (a) control factors; (b) Interaction of factors on specific wear rate of bone powder filled hybrid composites.

Figure 2 .
Figure 2. Effect of (a) control factors; (b) Interaction on COF of bone powder filled hybrid composites.
(a) unfilled hybrid composite show the wedge formation, fiber exposure and plastic deformation of matrix which results maximum wear.The SEM image as shown in Figure 3(b) 10 wt% of bone powder filled hybrid composite illustrate only a few fibers exposure when compared with Figure 3(a) and less deformation of the matrix which indicates less wear.

Figure 3 (
c) 15 wt% of bone powder filled hybrid composite show more plastic deformation of the matrix which slightly increases the wear rate when compared with Figure 3(b).Similarly, coefficient of friction was affected by filler content as seen from the main effect plot Figure 2(a).So SEM images were selected for varying concentration of filler content of hybrid composites.The worn surface of Figure 3(d) unfilled hybrid composite and Figure 3(f) 15 wt% of bone powder filled hybrid composite show the wear track, minor crack and only few fibers exposed from the matrix which has less coefficient of friction.The SEM image for Figure 3(e) 10 wt% of bone powder filled hybrid composite sample show the wear track on the surface, breakage of fibers and more number of fibers exposed which increased the coefficient of friction due to the rubbing action of the fibers with the disc surface.

Figure 3 .
Figure 3. SEM images of composites for bone powder filled hybrid composites.
-noise: Smaller is better (b)

Figure 4 .
Figure 4. Effect of (a) control factors; (b) Interaction of factors on specific wear rate of Al 2 O 3 filled hybrid composites.

Figure 4 (
a), So SEM images were selected for different normal load conditions.Figures6(a)-(c) show the SEM images for normal load 20 N, 40 N and 60 N conditions respectively for 10 wt% of Al 2 O 3 filled epoxy composites.The micrograph in Figure6(a)shows the wedge debris where more amount of epoxy matrix has been worn out which results in maximum wear.Few fibers were exposed as shown in Figure6(b) due to temperature rise between the surfaces and surface matrix layer has been worn out.When compared with both the samples (a & b) the fibers were not exposed and matrix worn out from the surface of sample (c) where less which results in less wear as shown in Figure6(c).Similarly, when compared with sliding velocity and normal load the effect of filler content (Al 2 O 3 ) on coefficient of friction was more as seen from main effect plot Figure5(a), So SEM images were selected for different filler content of hybrid composites.The worn surface of Figure 6(d) unfilled hybrid composite and Figure 6(f) 15 wt% of Al 2 O 3 filled hybrid composite show the plastic deformation of matrix, fibers were exposed and breakage of fibers which increases coefficient of friction due to rubbing of fiber and matrix with the disc surface [25].The SEM for Figure 6(e) 10 wt% of Al 2 O 3 filled hybrid composite sample show breakage of fiber and matrix due to weak bonding strength results in less coefficient of friction [16].
average; T -overall experimental average and 2 3 A B and 2 C -mean response for factors and interactions at designed levels.By combining all the terms the above Equation (5

2 C
-mean response for fac- tors and interactions at designated levels.By combining all the terms the above Equation (7

Figure 5 .
Figure 5.Effect of (a) control factors; (b) Interaction on COF of Al 2 O 3 filled hybrid composites.

Figure 6 .
Figure 6.SEM images of composites for Al 2 O 3 filled hybrid composites.

Table 1 .
of Levels of variables used in the experiments.

Table 2 .
Theoretical and experimental densities of hybrid composites.

Table 3 .
Experimental design using L 27 array for Al 2 O 3 and bone powder filled hybrid composites.

Table 4 .
ANOVA table for specific wear rate of bone powder filled hybrid composites.
DF-degree of freedom; Seq SS-sequential sum of squares; Adj SS-extra sum of squares; Seq MS-sequential mean squares; F-F-test and P-percent contribution.

Table 5 .
Response table for specific wear rate of bone powder filled hybrid composites.

Table 6 .
ANOVA table for COF of bone powder filled hybrid composites.
DF-degree of freedom; Seq SS-sequential sum of squares; Adj SS-extra sum of squares; Seq MS-sequential mean squares; F-F-test and P-percent contribution.

Table 7 .
Response table for COF of bone powder filled hybrid composites.

Table 8 .
ANOVA table for specific wear rate of Al 2 O 3 filled hybrid composites.
DF-degree of freedom; Seq SS-sequential sum of squares; Adj SS-extra sum of squares; Seq MS-sequential mean squares; F-F-test and P-percent contribution.

Table 9 .
Response table for specific wear rate of Al 2 O 3 filled hybrid composites.

Table 10 .
ANOVA table for COF of Al 2 O 3 filled hybrid composites.

Table 11 .
Response table for COF of Al 2 O 3 filled hybrid composites.

Table 12 .
Results of the confirmation experiments for the Specific wear rate.

Table 13 .
Results of the confirmation experiments for the Coefficient of friction.