Mechanical and Dry Sliding Wear Behavior of Particulate Fillers CaCO3 and CaSO4 Filled Vinyl ester Composites

The improved performance of polymers and their composites in industries and many other applications by the addition of part iculate fillers has shown great advantages and so has lately been the subject of considerable interest. In this paper, mechanical and tribological behavior of particu late fillers CaCO3 and CaSO4 filled v inyl ester composites have been presented. Wear tests were carried out in dry sliding conditions on a pin-on-disc friction and wear test rig. (DUCOM) at room temperature under slid ing velocity (1.57, 2.62 and 3.67 m/sec.), normal load (20, 40 and 60 N), filler content (0, 10 and 20 wt.%) and sliding d istance (1000, 3000 and 5000 m). The plans of experiments is based on the Taguchi technique, was performed to acquire data in a controlled way. An orthogonal array and analysis of variance (ANOVA) were applied to investigate the influence of process parameters on the coefficient of friction and sliding wear behaviour of these composites. The coefficient of frict ion and specific wear rate were significantly influenced with increase in both the filler content. The results show that for pure vinyl ester the coefficient of frict ion and specific wear rate increases with the increase of normal load, sliding velocity and sliding d istance. The coefficient of friction and specific wear rate for CaCO3 filler decreases with the increase of filler content. But, for filler CaSO4 the coefficient of frict ion and specific wear rate decreases at 10 wt.% and then increases at 20 wt.%. It is believed that a thin film formed on stainless steel counterface was seems to be effective in improving the tribolog ical characteristics. The worn surfaces examined through SEM to elucidate the mechanis m of friction and wear behaviour.


Introduction
An informal nu mber of papers dealing with the tribological behaviors of polymer materials have been published. That is why the polymers are extending over a great area used in sliding co mponents like such as gears, cams, breaks, clutches, bearings, wheels and bushes. Adhesive wear includes galling, fretting, scuffing and surface fatigue. This refers to the damage produced when two mat ing surfaces move relative to each other under a normal load. Surface asperities interact and very high stresses, strain, and strain rates are generated in localized regions [1]. This type of wear occurs in bearings, piston rings, cylinders and in electrical contacts. In recent years attention has been focused on the sliding wear behavior o f poly mers and their co mposites due to their increasing use as bushings and seals in machinery.
The research by various authors [2][3][4] reported that the friction between poly mers can be attributed by the two ma in mech an is ms i. e, d e fo r mat io n an d ad h es io n . Th e deformation mechanism involves dissipation of energy in the contact area. The adhesion components of friction of polymer results from the breakage of bonds between the polymer and mating sliding surface [3]. Fran klin [5] studied the friction and wear behavior of POM poly mer and reported that the effect of slid ing speed on the wear of polymers does not always follow the generally accepted engineering rule of "h igher sliding speed, the higher wear rate".
Fro m the point of view of the coefficient of friction, Brentnall and Lancaster [6] reported that the friction coefficient of poly mers rubbing against metals decreases with the increase in load. So me researchers [7][8][9] reported that the coefficient off friction value increases with the increase in load. Finally, Byett and Allen [10] and Friedrich et al. [11] have reported that the coefficient off frict ion value increases with the increase in load. Many poly meric materials have an excellent strength-to-weight ratio, good corrosion resistance, wider choice of the materials, dimensional stability, high impact strength, light weight and ease to manufacturing. So me poly mers also possess excellent tribolog ical p roperties [12]. The poly mers can be considered to be one of the competit ive materials for tribological applications because of their low friction values against steel counterparts, good damping properties, and self lubricating abilities.
It has been observed that by assimilating filler part icles in the polymer based composites, synergistic effects may be achieved in the form of h igher modulus and reduction of the material cost [13][14][15]. The inclusion of such particles into polymers for co mmercial applications is focused at the cost reduction and stiffness improvement [16,17]. Various kinds of polymers and polymer-matrix co mposites with different types of fillers such as Al 2 O 3 , SiC, TiO 2 , fly ash etc. have been the subject of extensive research in recent years as found from the literature [18][19][20][21]. But the potential of particulate fillers CaCO 3 and CaSO 4 in vinyl ester matrix has not been reported so far. So, in this paper we are using these fillers (CaCO 3 and CaSO 4 ) with vinyl ester matrix and studying the effect of such fillers on mechanical and dry sliding wear behavior of the composites. Polymer composites containing different fillers and/or reinforcements that are frequently used for applicat ions like auto motive parts, gear assemblies, tub/ shower industries etc. in which friction and wear are critical issues. Calciu m carbonate (CaCO 3 ) and calciu m sulfate (CaSO 4 ) are the fillers, which are used in the automobile parts and tub/ shower industries respectively. Ho wever, study of the effect o f such filler addition is necessary to ensure that the mechanical properties of the co mposites are not affected adversely by the addition of such fillers.
The importance of mechanical and tribolog ical properties has convinced many researchers to study the friction and wear behaviour and to improve the wear resistance of polymer based composites. Fro m the above mentioned literature it is understood that there is tremendous potentials of these fillers used for research on vinyl ester composites and studied under dry slid ing conditions. Since, the purpose of this paper is to study the mechanical and dry slid ing wear behaviour of fillers CaCO 3 and CaSO 4 filled v inyl ester composites. Taguchi method is used to optimize the process parameters of sliding wear in order to reduce the nu mber of experiments without sacrificing the informat ion.

Speci men Preparati on
The matrix used in this work is vinyl ester resin (density 1.28 g m/cc) was supplied by Northern Poly mer Pvt. Ltd. New Delhi. Methyl ethyl ketone pero xide (M EKP-1.5%), Cobalt Naphthenate (1.5%) was used as catalyst and accelerator respectively. Three d ifferent types of composites are prepared for the study. The Calciu m Sulfate (CaSO 4 ) and Calciu m Carbonate (CaCO 3 ) having particle size 1.813 µm and 1.620 µm respectively were used as a filler material collected fro m the Pioneer Chemical Corporation, Delh i.
The composites are made homogeneously. Firstly they are properly sterilized in a jar and then simply pour into the 10mm d iameter test tubes. There are three different types of composites are made for the current study with 0, 10 and 20 wt.% of the filler content. The accelerator Cobalt Naphthenate 1.5% is mixed thoroughly in vinyl ester resin and then catalyst 1.5% Methyl ethyl ketone peroxide (MEKP) was mixed in the resins prior to reinforcement. Befo re pouring the composite solution in the test tube, the test tube is sprayed with a release agent (Silicon spray) to ensure that the part will not adhere to the test tube after the curing of the composites samples. The cast of each composite is cured for 24 hour at roo m temperature before it is removed fro m the test tube. The other composite samples with fillers CaCO 3 and CaSO 4 of fixed weights (10 wt.% and 20 wt.%) percentage were fab ricated by the same technique. The fillers CaSO 4 and CaCO 3 were mixed thoroughly in the v inyl ester resin mechanically before pouring into the test tubes. The composites prepared for this study are designated as WCGV 1 , WCGV 2 , W CGV 3 , WCGV 4 , and WCGV 5 respectively. The composition and designation of the co mposites prepared for this study are listed in Table 1.

Fricti on and Wear Test Apparatus
Dry sliding wear tests were conducted on a pin-on-disc friction and wear monitoring test rig (DUCOM ) as per ASTM G 99. The cylindrical pin specimens of 10mm diameter and 30mm length were tested against a disc made of hardened ground steel (EN-32, hardness 72HRC, surface roughness R a = 0.07 µm). The specimen was held stationary and the disc was rotated while a normal force was applied through a lever mechanis m. The schematic diagram of the pin-on-disc apparatus is shown in the Figure 1. Du ring the test, frictional fo rce was measured by the transducer mounted on the loading arm. The frictional force readings were taken as the average of 100 readings of every 40 seconds for the required time period. For th is purpose a microprocessor controlled data acquisition system was used. The average mass loss was used to calculate to the specific wear rate (K s ). The tests were conducted with sliding velocity (1.57, 2.62, 3.67 m/sec.), normal load (20,40, 60 N), filler content (0, 10, 20 %) for the sliding distance of 1000, 3000 and 5000 m. Sliding wear data reported here is the average of two runs. The initial weight before run and final weight after run is measured using a precision electronic balance with an accuracy of ± 0.01 mg. The specific wear rate (mm 3 /Nm) is then exp ressed on 'volume loss' basis. K s = ∆m L ρ F n Where, K s is the specific wear rate (mm 3 / Nm), is the mass loss in the test duration in g m, is the density of the composite (gm/cm 3 ), F n is the applied normal load (N), L is the slid ing distance (m). The parameters setting and levels for various control factors for wear test are shown in the Table 2.

Scanning Electron Microscopy
A FEI quanta FEG450 was used to analyze the worn surfaces of the polymer co mposites. The composite samples are mounted on stubs with gold plating. To enhance the conductivity of the samples, thin films of p latinu m are vacuum evaporated onto them before the photomicrographs were taken.

Experi mental Design
Taguchi design of experiment is a powerful analysis tool which is adopted for optimizing design parameters. Taguchi method provides the designer with a systematic and efficient approach for experimentation to determine near optimu m settings of design parameters for performance, quality and cost [22][23][24][25]. The most impo rtant stage in the design of experiment lies in the selection o f the control factors. In the present work, the impact of the four such factors are studied using L 27 (3 13 ) orthogonal array wh ich has 27 rows corresponding to the number of tests (20 degree of freedo m) with 13 colu mns at three levels. The operating conditions under which sliding wear tests carried out are given in the Table 2.
In conventional full factorial experimental design, it would require 3 4 = 81 runs to study four factors each at three levels whereas, Taguchi's factorial experiment approach reduces it to only 27 runs offering a great advantage in terms of experimental time and cost. The experimental observations are transformed into a signal-to-noise (S/N) ratio. There are three S/ N ratios available depending upon the type of characteristics (smaller-the-better, larger-the-better, nominal-the better). The S/N ratio for minimu m (friction and wear rate) coming under smaller is better characteristic, which can be calculated as logarithmic transformation of the loss function as shown below [26] S N = −10 log� 1 n (y 1 2 + y 2 2 + ⋯ y n 2 � Where 'n' is the repeated number trial conditions and y 1 , y 2 ………y n are the response of the frict ion and slid ing wear characteristics. "Lower is better" (LB) characteristic, with the above S/N ratio transformation is suitable for minimizat ions of coefficient of friction and specific wear rate. The standard linear graph is used to assign the factors and interactions to various columns of the orthogonal array (OA).
The plan of experiments is as follows: the first column is assigned to the velocity (A), the second column to normal load (B), the fifth column to filler content (C) and the ninth column to sliding distance (D) the third and fourth colu mn are assigned to (A×B) 1 and (A×B) 2 respectively to estimate interaction between the velocity (A) and the normal load (B), the sixth and seventh column are to (B× C) 1 and (B×C) 2 respectively to estimate the interaction between the normal load (B) and the filler content (C), the eight and eleventh column are assigned to (A × C) 1 and (A × C) 2 respectively to estimate the interaction between the velocity (A) and the filler content (C) and the remain ing colu mns are used to estimate the experimental error. The linear graph for L 27 array is shown in the Figure 2.

Results and Discussion
The characterization of the co mposites reveals that inclusion of any particu late filler has very strong influence not only on the mechanical properties of co mposites but also on their sliding wear behavior. By incorporating these particulate fillers into the vinyl ester matrix, synergistic effects, as expected were achieved in the form of mod ified mechanical properties and improved slid ing wear resistance. A comparative study of modified behavior of the composites against the two different types of fillers is presented.

Density
The composite under this investigation consists of three components such as matrix, fiber and part iculate filler. Hence, the density of the co mposite can be calcu lated using rule-of-mixture as shown in the fo llo wing expression Agarwal and Brout man [27].
Where, W and represents the weight fraction and density respectively. The suffix m, f, and p stand for the matrix, fiber and particulate filler respectively.
The actual or experimental density of the composite, however, can be determined by simple water immersion technique (Archimedes princip le). The vo lu me fraction of voids (V v ) in the composites is calculated using the following equation: It can be noticed fro m Table 3 that composite density values calculated fro m weight fractions using Eq. (2) are not in agreement with the experimentally determined values. The difference between the co mposite or theoretical density and the actual or experimental density is a measure of voids and pores present in the composites. It is clear fro m the Table 3 that volume fraction of voids is small in WCGV 1 due to absence of particulate fillers in it. As the filler content of CaSO 4 and CaCO 3 co mposites (WCGV 2 -WCGV 5 ) increases from 10 wt.% to 20 wt.% the volume fract ions of voids increases as shown in the Table 3. The voids are more in the CaSO 4 filler as co mparison to the CaCO 3 filler. Th is may due to the particle size variat ions, because the particle size o f the CaSO 4 (1.813 µm) and CaCO 3 (1.620 µm) is higher respectively. The voids significantly affect some of the mechanical properties and even the performance of composites. Higher void contents usually mean lower fatigue strength, greater susceptibility to water penetration and weathering [27]. The knowledge of the void content is usually important for the estimation of the quality of the composites.

Mechanical Properties
The experimental values of the properties of the particulate filled CaCO 3 and CaSO 4 co mposites under this investigation are presented in Table 4.  The tensile test is performed on the universal testing mach ine (UTM) Hounsfield H25KS as per ASTM standard D 3039-76 [28]. It is seen that in all the samples irrespective of the filler material the tensile strength of the composite decreases with increase in filler content. The pure vinyl ester has strength of 39.04 MPa in tension and it may be seen from Table 4  Among the two fillers taken in th is study, the inclusion of CaSO 4 filler causes maximu m reduction in the co mposite strength. It may occurs due to the interface bonding between the vinyl ester matrix and CaSO 4 filler is not good to transfer the tensile stress as that in CaCO 3 filler content. The one more reason is that the corner points of the irregular shaped particulates result in stress concentration in the vinyl ester matrix. Now, it is interesting to note that the properties for tensile modulus is increasing with the addition of the both the filler CaSO 4 and CaCO 3 at 10 wt.% and 20 wt.% respectively as shown in Figure 4.  The flexu ral test is conducted on the same UTM as per ASTM standard D 2344-84 [29]. Figure 5 shows the comparison of flexu ral strengths of the co mposites obtained experimentally fro m the 3-point bend tests for composites (WCGV 1 -WCGV 5 ). The flexural strength for the filler CaCO 3 is increasing with the addition of the 10 wt.% to 20 wt.% filler content. But, for the filler CaSO 4 it decreases with the addition of the filler content. Now, fro m the results it may now be suggested that CaCO 3 is potential candidate to be used as filler in making high flexural strength composites with the increase of the reinforcement of the filler in comparison to CaSO 4 filler. The flexu ral properties are of great importance for any structural element. Co mposite materials used in structures are prone to fail in bending and therefore the development of new composites with imp roved flexu ral characteristics is essential. Fro m the results it may now be suggested that CaCO 3 is potential candidate to be used as filler in making high flexural strength composites with the increase of the reinforcement of the filler in comparison to CaSO 4 filler. CaSO 4 holds good flexural strength at 10 wt.% filler content also more than WCGV 1 and WCGV 4 co mposites as shown in Figure 5. There may be one reason for this that the voids in WCGV 2 (1.9381) is mo re in comparison to WCGV 4 (0.8362).

Anal ysis of Experi mental Results
The experimental data for coefficient o f friction and specific wear rate (K s ) for CaCO 3 and CaSO 4 fillers is reported in the Table 5 and 8 respectively. The data reported here is the average of two runs. From Table 5 the overall mean for the S/N ratio of the coefficient of friction and the specific wear rate for CaCO 3 are found to be 4.1395 db and 92.5339 db respectively.
On the other hand, from Table 8 the overall mean for the S/N ratio of the coefficient of friction and the specific wear rate for CaSO 4 are found to be 3.3670 db and 89.8949 db respectively. Here, we saw that the overall mean fo r the S/N ratio of the coefficient of frict ion and specific wear rate is more in CaCO 3 in co mparison to CaSO 4 wh ich means that the coefficient of friction and specific wear rate is less in CaCO 3 . The analysis of the experimental data is carried using the software MINITA B 16 specially used for the design of experiment applications. Before analy zing the experimental data using this software for predict ing the measure of performance, the possible interaction between control factors are considered. Thus factorial design incorporates a simple means of testing for the presence of the interaction effects.  Figure 8 and 9 for CaCO 3 shows graphically the effect of four control factors on coefficient of friction and specific wear rate of the composite specimens WCGV 1 , WCGV 4 and WCGV 5 . The analysis of the results gives the combination factors resulting in minimu m coefficient of frict ion and specific wear rate o f the co mposites. Analysis of these results leads to the conclusion that factors combination A 1 , B 3 , C 3 and D 2 gives min imu m coefficient of friction as shown in the Figure 8. It is observed that the interaction B× C shows significant effect on the coefficient of friction. Similarly the combination factors A 3 , B 3, C 3 and D 2 gives minimu m specific wear rate as shown in the Figure 9. It is observed that interaction A×C has significant effect on the specific wear rate. For CaSO 4 Figure 10 and 11 shows graphically the effect of four control factors on coefficient of friction and specific wear rate of the composite specimens WCGV 1 , WCGV 2 and WCGV 3 . Analysis of these results leads to the conclusion that factors combination A 1 , B 3, C 2 and D 1 gives minimu m coefficient of friction as shown in the Figure 10. It is observed that the interaction B×C again shows significant effect on the coefficient of friction as in case of CaCO 3 filler. The combination factors A 3 , B 3 , C 2 and D 1 gives minimu m specific wear rate as shown in the Figure 11. It is observed that interaction A×C has significant effect on the specific wear rate.

ANOVA and Effects of Factors
In order to understand the impact of various control factors like velocity (A), normal load (B), filler content (C) and sliding distance (D) and interaction on the response of experimental data it is desirab le to develop the analysis of variance (A NOVA) to find the significant factors as well as interactions. ANOVA allows analyzing the influence of each variable on the total variance of the results. For CaCO 3 , Table 6a shows the results of ANOVA for the specific wear rate and Table 7a shows the results of ANOVA for coefficient of frict ion and for CaSO 4 , Table 9a shows the   Table 10a shows the results of ANOVA for coefficient of frict ion. The analyses are performed with a level of significance 5% means at 95% level of confidence. In ANOVA table, the column shows the percentage contribution (P) of each variable in the total variation indicating the influence of specific wear rate and coefficient of frict ion. For filler CaCO 3 it can be observed fro m the ANOVA Table 6a for specific wear rate that the filler content (P=57.981%), velocity (P=17.981%), normal load (P=5.523%) and the interactions A×C (P=10.218%), B×C (P=3.283) and A×B (P=3.097%) has significant influence on the specific wear rate. However, the control factor sliding distance (P=0.303%) does not have a significant effect (both physically and statistically) on specific wear rate as their values are quit smaller than error (P=1.613%) so they are neglected. From the analysis of ANOVA and response Table  6b of the S/N ratio of specific wear rate, it is observed that the control factor filler content (C) has major impact on the specific wear rate fo llo wed by velocity (A), normal load (B) and sliding distance (D). It means that with increasing the filler content, velocity and normal load the specific wear rate decreases i.e., increase the wear resistance as observed from the Figure 9. In the same way fro m the ANOVA Tab le 7a for coefficient of friction the filler content (P=64.607%), normal load (P=12.540%), velocity (P=10.265%), sliding distance (P=1.743%) and the interactions B×C (P=7.279%), A × C (P=1.889%) has significant effect on the coefficient of friction. But, interaction A×B (P=0.755%) does not have a significant effect (both physically and statistically) on coefficient of friction as its value is quit smaller than error (P=0.922%) so it may neglected. So, fro m the analysis of ANOVA and response Table 7b of the S/N ratio of coefficient of frict ion, it is observed that the filler content (C) has majo r influence followed by normal load (B), velocity (A) and sliding distance (D) as for the coefficient of friction.

Main Effects Plot for SN ratios
For filler CaSO 4, it can be observed from the ANOVA Table 9a for specific wear rate that filler content (P=44.375%), velocity (P=27.788%) and the interactions A×C (P=20.423%) and B× C (P=2.474%) has significant effect on specific wear rate. Ho wever, the control factors normal load (P=1.326%), slid ing distance (P=0.815%) and interaction A×B (P=0.859%) do not have a significant effect (both physically and statistically) on specific wear rate as their values are quit smaller than residual error (P=1.941%) so they are neglected. Fro m the analysis of A NOVA and response Table 9b of the S/N rat io for specific wear rate, it is observed that the control factor filler content (C) has major impact on the specific wear rate fo llo wed by velocity (A), the normal load (B) and slid ing distance (D). It means that for filler CaSO 4 , with the increases of the filler content, velocity and normal load the specific wear rate decreases i.e., the wear resistance is good as observed fro m the Figure 11. But fro m the Figure 11 we also observed that for CaSO 4 the filler content plays adverse effect when filler content is increases fro m 10 wt.% to 20 wt.%. At 10 wt.% for CaSO 4 the specific wear rate is decreased and for 20 wt.%, it further increases. In the same way fro m the A NOVA Table 10a for coefficient of frict ion the filler content (P=69.460%), velocity (P=6.655%), normal load (P=6.199%), sliding distance (P=4.085%) and the interactions B×C (P=5.866%) and A × B (P=4.174%) has significant effect on the coefficient of friction. However, the interaction A × C (P=0.472%) do not have significant effect on the coefficient of friction as their values are quite smaller than the residual error (P=3.088%), so it may neglected. Fro m the analysis of the ANOVA and the response Table 10b for coefficient of friction it is observed that the filler content (C) has major influence follo wed by the velocity (A) normal load (B) and the sliding distance (D). Fro m the Fig. 10 for CaSO 4, it is observed that the coefficient of frict ion is also increase with the increase of the filler content. It means that in 10 wt.% the coefficient of friction is less as comparison to 20 wt.% i.e., the reinforcement of the CaSO 4 filler at 10 wt.% is mo re wear resistance. The micrograph in Figure 12a shows the resinous and matrix region. The filler CaCO 3 covered the matrix region which results in less wear. Figure 12b shows the debris and wedge format ion regions due to long slid ing distance. Vinyl ester debris was adhered into the filler reg ion and micro cracks were identified which increases the wear rate. Figure  12c showing the thin layer fo rmation and debris which results the nominal wear.

Surface Morpholog y
Similarly   Figure 13a shows the resinous and filler region as in case of CaCO 3 wh ich results in the lesser wear rate because the filler covers the maximu m matrix region. Figure 13b found the maximu m wear rate due to wedge formation and matrix debris adhered into the filler region as in the case of CaCO 3 which increases the wear rate. Figure 13c shows the nominal wear rate due to thin transfer layer fo rmation.

Confirmati on Experi ments
The confirmation experiment is the final step in the design of experiments process. It predicts and verifies the improvements in the observed values through the use of optimal co mbination level o f control factors. For filler CaCO 3, the confirmat ion experiment was performed by taking an arbitrary set of factor co mbination A 2 B 2 C 3 D 1 to predict the coefficient of friction and for specific wear rate factor setting is A 2 B 2 C 1 D 1 . Now, the estimated S/ N rat io for coefficient of frict ion can be calculated with the help of the following predict ive equation.
η CaCO 3 = T � + ( A � 2 − T �) + ( B � 2 − T �) + ( C � 3 − T �) Where, η CaCO 3 is the predicted average of CaCO 3 for coefficient of frict ion, � is the overall experimental average A � 2 C � 3 , B � 2 C � 3 and D � 1 is the mean response for factors and interactions at designed levels. By co mb ining all the terms Eq. (4) reduces to η CaCO 3 = A � 2 C � 3 + B � 2 C � 3 + D � 1 − C � 3 − T � A new co mb ination of factor levels A 2 B 2 C 3 D 1 are used to predict the S/N ratio of coefficient of frict ion through predictive Eq. (5) and is found to be η CaCO 3 = 5.6781 db. Fo r each of performance measures an experiment is conducted for different comb ination of factors and results are compared with those obtained from the predictive equation as shown in the Table 11. Similarly a predict ive equation is developed for estimating S/N rat io of specific wear rate as shown in Eq. (6).
Where, η CaCO 3 is the predictive average of CaCO 3 for specific wear rate, � is the overall experimental average A � 2 C � 1 , B � 2 C � 1 and D � 1 is the mean response for factors and interactions at designed levels. By co mb ining all the terms Eq. (6) reduces to η CaCO 3 = A � 2 C � 1 + B � 2 C � 1 + D � 1 − C � 1 − T � A new co mb ination of factor levels A 2 B 2 C 1 D 1 are used to predict the S/N ratio of specific wear rate through predictive Eq. (7) and is found to be η CaCO 3 = 81.1478 db. For each of performance measures an experiment is conducted for the prediction equation as shown in Table 11. The resulting equations seem to be capable of predict ing the coefficient of friction and specific wear rate. An error of 6.49% for the S/N ratio of the coefficient of frict ion and 5.41% for the S/N rat io of the specific wear rate is observed. For filler CaSO 4, the confirmation experiment was performed by taking an arb itrary set of factor co mbination A 2 B 2 C 3 D 1 to predict the coefficient of friction and for specific wear rate factor setting is A 2 B 2 C 1 D 1 as same for filler CaCO 3 . No w, the estimated S/N ratio for coefficient of friction can be calculated with the help of the following predictive Eq. (8).
Where, η CaSO 4 is the predicted average of CaSO 4 for coefficient of frict ion, � is the overall experimental average A � 2 B � 2 , B � 2 C � 3 and D � 1 is the mean response for factors and interactions at designed levels. By co mb ining all the terms Eq. (8) reduces to η CaSO 4 = A � 2 B � 2 + B � 2 C � 3 + D � 1 − B � 2 − T � (9) A new co mb ination of factor levels A 2 B 2 C 3 D 1 are used to predict the S/N ratio of coefficient of frict ion through predictive Eq. (9) and is found to be η CaSO 4 = 3.2593 db. For each of performance measures an experiment is conducted for different comb ination of factors and results are compared with those obtained from the predictive equation as shown in the Table 12. Similarly a predict ive equation is developed for estimating S/N ratio of specific wear rate as shown in Eq. (10).
Where, η CaSO 4 is the predictive average of CaSO 4 for specific wear rate, � is the overall experimental average A � 2 C � 1 , B � 2 C � 1 and D � 1 is the mean response for factors and interactions at designed levels. By co mb ining all the terms Eq. (10) reduces to η CaSO 4 = A � 2 C � 1 + B � 2 C � 1 + D � 1 − C � 1 − T � A new co mb ination of factor levels A 2 B 2 C 1 D 1 are used to predict the S/N ratio of specific wear rate through predictive Eq. (11) and is found to be η CaSO 4 = 82.2855 db. For each of performance measures an experiment is conducted for the prediction equation as shown in Table 12. The resulting equations seem to be capable of predict ing the coefficient of friction and specific wear rate. An error of 9.18% for the S/N ratio of the coefficient of frict ion and 8.04% for the S/N rat io of the specific wear rate is observed.

Conclusions
An experimental study has been carried out for frict ion and dry sliding wear of v inyl ester matrix with fillers CaCO 3 and CaSO 4 slid ing against smooth stainless steel counterface using Taguchi experimental design. Taguchi's design of experiment method can be used to analyze the coefficient of friction and the dry sliding wear of poly mer matrix composites as presented in this research paper. The following conclusions can be drawn fro m the present study:-(1) The tensile strength for both the fillers CaCO 3 and CaSO 4 decreases with the increase of the filler content. While the tensile modulus for both the fillers increases with the increase of filler content.
(2) For filler CaCO 3 , flexural and co mpressive strength increases with the increase of filler content. While, hardness is increases at 10 wt.% and then decreases at 20 wt.%.
(3) For filler CaSO 4 , flexu ral and co mpressive strength decreases with the increase of filler content. While, the hardness is increases at 10 wt.% and then decreases at 20 wt.% as same in case of CaCO 3 filler content.
(4) For pure v inyl ester the coefficient of friction and specific wear rate increases with the increase of normal load, sliding velocity and slid ing distance.
(5) The coefficient of frict ion and specific wear rate for CaCO 3 filler decreases with the increase of filler content. But, for filler CaSO 4 the coefficient of friction and specific wear rate decreases at 10 wt.% and then increases at 20 wt.%.
(6) For both the fillers CaCO 3 and CaSO 4 it is observed that the control factor filler content (C) has major impact on the specific wear rate followed by velocity (A), normal load (B) and slid ing distance (D). (7) The p redictive equations based on Taguchi approach is successfully used for the prediction of effect of four control factors and predicted results are consistent with the experimental observations.
(8) It is demonstrated that Taguchi approach based on ANOVA well reflect the effects of various factors on the friction and sliding wear loss.
The significance of this current research is to create the interest between the various young researchers towards such fillers wh ich are good not only in the mechanical behaviour but also good in the friction and dry sliding wear behaviour.