Effect of short glass fiber and fillers on dry sliding wear behaviour of thermoplastic copolyester

Proceedings of the 2nd

International Conference on Current Trends in Engineering and Management ICCTEM -2014

17 – 19, July 2014, Mysore, Karnataka, India

71

EFFECT OF SHORT GLASS FIBER AND FILLERS ON DRY SLIDING

WEAR BEHAVIOUR OF THERMOPLASTIC COPOLYESTER

ELASTOMER COMPOSITES

R. Hemanth1, Prakash Sam Thomas

2, B. Suresha

3, M. Sekar

4

1, 2, 4

Karunya School of Mechanical Science, Karunya University, Coimbatore, India 3Department of Mechanical Engineering, National Institute of Engineering, Mysore, India

ABSTRACT

The dry sliding wear behaviour of thermoplastic copolyester elastomer (TCE) reinforced with fibers and fillers

were slid against a steel counterface of a pin-on-disc tribometer. The filler and fiber reinforcements used are

polytetrafluroethylene (PTFE), short glass fiber (SGF), short carbon fiber (SCF), silicon carbide (SiC), and alumina

(Al2O3). The parameters like filler content, sliding velocity and sliding distance on the specific wear rate have been

investaigated. In this study, a plan of experiments based on the techniques of Taguchi was performed to acquire data in a

controlled way. An orthogonal array L27 (313

) and analysis of variance (ANOVA) were applied to study the influence of

process parameters on the specific wear rate of TCE based composites. The experimental results reveal that the effect of

filler content was the major parameter on specific wear rate, followed by the sliding distance. The sliding velocity,

however, was found to have a neglecting effect. The worn surface topographies show asssorted features like tendency of

the matrix to adhere towards the fiber, network of microcracks, less debris formation, agglomeration of debris and

broken fibers on the sliding distance and velocity employed.

Keywords: TCE Based Composites, Taguchi’s Design of Experiments, Specific Wear Rate, Worn Surface Morphology.

1. INTRODUCTION

Polymers and their composites possess a unique combination of physical properties that are either unattainable

or difficult to reproduce in metal and ceramic materials. Polymer composites occupy a considerable market share

nowadays as one of the most common engineering materials. They provide a combination of various advantages, such as

ease in manufacturing, cost effectiveness and excellent performance, which cannot be attained by metals, ceramics, or

polymers alone [1]. In recent years, polymer composites have been extensively used to replace metallic materials in

engineering applications involving friction and wear. The advantages of polymers such as self lubricity, light weight,

corrosion resistant and ease of processing have allowed them to be the best choice [2]. Thermoplastic elastomers (TPE’s)

concern large industrial and commercial fields, as well as academic and applied research. They are novel constructional

polymers, which are physically cross-linked materials made up of a thermoplastic and an elastomer. Applications include

flexible couplings, ski boots, gears, high pressure hose lines, outer coverings for wire and optical fiber cables, seals, etc.

[3]. The choice of an appropriate matrix is of great importance in the design of wear resistant polymer composites. The

concept of adding particles of micro or nanoscale into polymers is one of the most intriguing subjects in the recent

decades. Fiber reinforcements like glass, carbon, and aramid fibers have frequently been applied in order to improve

mechanical properties. Solid lubricants (polytetrafluroethylene and graphite) are proved to be very helpful in developing

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a transfer film between the two counterparts and could drastically reduce the frictional coefficient of the composites [4].

Polytetrafluroethylene (PTFE) possesses low coefficient of friction, low stick–slip and high levels of anti-adhesive

properties, which make it a potential candidate for adhesive wear process. Unfortunately, PTFE has the following

drawbacks: low Young’s modulus, high visco-elasticity and poor wear resistance [5]. While, short fibre reinforcements,

such as carbon, glass and aramid fibers, could effectively improve the wear resistance of polymer composites by

undergoing most of the load during sliding processes. Short glass fiber (SGF) exhibited better adhesive wear

performance in severe operating conditions (applied load, sliding velocity, and sliding distance) [6]. Short carbon fiber

(SCF), is widely advocated as a decisive reinforcement component, show a remarkable capability to increase the wear

resistance [7]. Silicon carbide (SiC) particles have shown an increased wear resistance in dry sliding (adhesive) wear of

polymer composites [6]. The alumina (Al2O3) particles in polymer would strengthen the mechanical properties of

polymer composites [8].

The objective of this research work is to evaluate the influence of fibers and fillers on wear rate and influence of

independent parameters such as sliding distance, filler content, and sliding velocity on wear performance of

thermoplastic copolyester elastomers (TCE) reinforced composites using design of experiments (DOE). The fibers and

fillers reinforcements used in TCE are SGF, SCF, PTFE, silicon carbide (SiC), and Al2O3. DOE was applied to study the

various parameters affecting the specific wear rate. The information generally comprises the relationship between

product and process parameters and the desired performance characteristic. In any process, the desired testing parameters

were either determined based on experience or by use of a handbook. It, however, does not provide optimal testing

parameters for a particular situation. Thus, numerous mathematical models based on statistical regression techniques

have been constructed to select the proper cutting or testing conditions. The Taguchi’s design can further simplify by

expending the application of the traditional experimental designs to the use of orthogonal array. This method is a simple,

efficient and systematic approach to optimize designs for performance, quality and cost [9].

Majority of research studied detailed experimental work i.e., the effect of one factor by keeping all other factors

fixed, this approach is not advisable because in an actual environment there will be combined effects of interacting

factors influencing the abrasive wear. Hence in this investigation an attempt is being made to study the interacting effects

of factors along with the main effect. In this paper, influence of filler content, sliding distance and sliding velocity on the

specific wear rate of TCE based composites was explored using Taguchi’s design of experiments. Dry slidng wear tests

were carried out on a pin-on-disc tribometer. Tribological tests were carried out at room temperature, adopting L27

ortogonal array. The experimental results were analyzed by using analysis of means and variance of the influence of

factors.

2. MATERIALS AND METHODS

2.1 Materials

In the present research work, the materials used for making polymer composites are TCE as matrix material,

SGF as fiber reinforcement, PTFE, SCF, SiC and Al2O3 as filler reinforcement. The sources of these materials are listed

in Table 1.

2.2 Fabrication of composites

The polymer granules, fibers and fillers were dried at 75º C for 10 h in an oven before compounding. Selected

compositions were mixed and extruded in Barbender co-rotating twin-screw extruder (Make: CMEI, Model: 16CME,

SPL, chamber size 70 cm3). The mixing speed of 100 rpm was maintained for all the compositions. The extrudates from

the die were quenched in cold water and then pelletized. In the melt blending, the temperature profile of the extrusion

were zone1 (200º C), zone 2 (210º C), zone 3 (220º C), zone 4 (240º C) and zone 5 (260º C) respectively. The extrudates

of the compositions were pelletized using pelletizing machine. The details of the composites fabricated for present

investigation are given in Table 2. The pellets of the extrudates were pre-dried at 100º C in vacuum oven for 24 h and

injection moulded in a reciprocating screw injection moulding machine (Windsor, 50 T), to produce test specimens. The

processing temperature at zone 1 (220º C) and zone 2 (250º C) were maintained respectively. The mould temperature

was maintained at 35º C.

Table 1: Supplier details of the materials procured

Polymer/Filler Source and supplier

TCE Gargi Enterprises, Bengaluru

PTFE Du Pont Co. Ltd.

SGF Fine organics, Mumbai

SCF Fine organics, Mumbai

SiC Carborundum India Ltd.

Al2O3 Triveni groups




73

2.3 Wear testing The dry sliding wear tests were performed on a pin on disc tribometer (Magnum Engineers, Bengaluru) setup as

per ASTM G 99-05 standard. The photograph of the test apparatus is shown in Fig. 1.

Table 2: Constituents of TCE based composites

Fig. 1: Photograph of the Pin on Disk apparatus showing 1) steel counterface, 2) sample holder and 3) load cell

Wear test samples of size 6 mm × 6 mm × 2.5 mm are glued to steel pin of 6 mm diameter and 30 mm length

and comes in contact with (EN31 grade, 62 HRC, 1.6 µ Ra) rotating disc. Prior to testing, the samples were polished

against medium grade sand paper (600 grit size) to ensure proper contact with counter face. Test parameters are given

below in Table 3. Normal load throughout the experiment is kept constant at 40 N; the pin along with the specimen was

then weighted in an electronic balance (0.1mg accuracy). Before and after wear testing, samples were cleaned with the

acetone to remove wear debris.

Table 3: Test conditions for the present study

Weight loss of the test samples gives the measure of sliding wear loss. Volume loss was calculated from

measured weight loss using density data of the test specimen.

Sl. No.

Matrix material (wt %) Composition (wt %)

TCE Particulate fillers

1 80 20 PTFE

2 68 12 PTFE + 20 SGF

3 60 10 PTFE + 17.5 SGF + 2.5 SCF + 5 SiC + 5 Al2O3

Parameters Units Tested values

Sliding velocity m/s 0.5, 1.0 and 1.5

Sliding distance m 2000, 4000 and 6000

Normal load N 40 constant




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The specific wear rate (Ws) was calculated using equation (1).

DL

VWs

×

= (1)

where, V the volume loss in mm3, L the load in Newton, D the sliding distance in m.

2.4. Experimental Design Design of experiments (DOE) is the powerful analysis tool for modelling and analyzing the influence of the

control factors on the performance output. The most important stage in the design of experiment lies in the selection of

the control factors [9]. Taguchi uses a special design of orthogonal arrays to study the entire process parameter space

with only a small number of experiments [10]. The Taguchi design of experiment approach eliminates the need for

repeated experiments and thus saves time, material and cost. The most important stage in the design of experiment lies in

the selection of the control factors. The Taguchi approach to experimentation provides an orderly way to collect, analyze,

and interpret data to satisfy the objectives of the study. In the design of experiments, one can obtain the maximum

amount of information for the amount of experimentation. Taguchi parameter design can optimize the performance

characteristics through the setting of design. Three parameters namely filler content (A), sliding velocity (B), and sliding

distance (C). The Table 4 given below indicates the factors and level. The experiments were conducted as per the

orthogonal array with level of parameters given in each array row.

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.

Table 4: Control factors and levels used in the experiment

For lower is the better performance characteristic S/N ratio is calculated as per the given formula: -

n

y

n

s ∑−=

2

log10 (2)

Where “n” is the number of observations and “y” is the observed data. “Smaller is the better” characteristic, with

the above S/N ratio transformation, is suitable for minimization of wear rate. The sliding wear test results were subject to

the analysis of variance (ANOVA). The purpose of ANOVA is to investigate parameters which significantly affect the

performance characteristic. With DOE and ANOVA analysis; the optimal combination of wear parameters is predicted to

acceptable level of accuracy. The optimal process parameters obtained from the parameter design [11]. The use of

ANOVA is to analyze the influence of wear parameters like (A) filler content, (B) sliding velocity, and (c) sliding

distance.

3. RESULTS AND DISCUSSION

3.1 Analysis of control factor Analysis of the influence of each control factor (A, B, and C) on the specific wear rate was performed with a so-

called S/N response table, using a Minitab 16 computer package.

Table 5 shows the experimental plan and their results with calculated S/N ratios for the specific wear rate of

TCE based composites. The right side of the table included the results of the specific wear rate and the calculated S/N

ratio. Overall mean for S/N ratio was found to be 224.1818 dB. The response table of the specific wear rate is presented

Control factors

Level

I II III Units

A : Filler content 15 32 40 %

B : Sliding velocity 0.5 1 1.5 m/s

C : Sliding distance 2000 4000 6000 m




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in Table 6. It indicates the S/N ratio at each level of control factor and how it was changed when settings of each control

factor were changed from level 1 to level 2 and level 2 to level 3. The control factor with the strongest influence was

determined by differences values. The higher the difference, the more influential was the control factor. It can be seen in

Table 6 that the strongest influence was exerted by filler content (factor A), followed by sliding velocity (factor B) and

sliding distance (factor C), respectively.

Table 5: Test conditions and output results using orthogonal array

Sl.

No.

Filler

content

(wt. %)

Sliding

Velocity

(m/s)

Sliding

distance

(m)

Specific wear

rate x e-12

(mm3/Nm)

S/N

ratio

(db)

1 15 0.5 2000 22.6250 212.98

2 15 0.5 4000 20.3625 213.82

3 15 0.5 6000 15.8375 216.00

4 15 1.0 2000 21.4930 213.35

5 15 1.0 4000 15.8375 216.00

6 15 1.0 6000 9.0500 220.86

7 15 1.5 2000 12.0660 218.36

8 15 1.5 4000 6.7875 223.36

9 15 1.5 6000 3.3937 229.38

10 32 0.5 2000 9.0000 220.91

11 32 0.5 4000 6.9000 223.22

12 32 0.5 6000 4.6000 226.74

13 32 1.0 2000 7.9850 221.95

14 32 1.0 4000 5.3300 225.46

15 32 1.0 6000 3.0160 230.41

16 32 1.5 2000 7.0000 223.09

17 32 1.5 4000 4.0000 227.95

18 32 1.5 6000 1.3300 237.52

19 40 0.5 2000 6.2100 224.14

20 40 0.5 4000 6.0370 224.38

21 40 0.5 6000 2.9000 230.75

22 40 1.0 2000 8.6250 221.28

23 40 1.0 4000 3.5800 228.92

24 40 1.0 6000 2.8750 230.82

25 40 1.5 2000 6.9000 223.22

26 40 1.5 4000 3.4500 229.24

27 40 1.5 6000 1.1500 238.78

Fig. 2 shows the main effect plots for S/N ratio. Highest S/N ratio will give the minimum specific wear rate.

From the plots it is clear that factor combination of A3B3C3, gives minimum specific wear rate. Thus minimum specific

wear rate is obtained when filler content, sliding velocity and sliding distance is at maximum level. It is evident that

increasing the filler content of the matrix material the specific wear rate is decreasing. It is observed that increase in the

level of both filler content and sliding distance causes specific wear rate to decrease with the effect of filler content

dominating the effect of sliding distance on wear rate [11]. In the case of specimen TCE filled with SGF, SCF and

ceramic fillers addition of ceramic materials i.e. SiC will help to detach the material from the specimen. The material

detached from specimen forms wear debris. The ceramic material forms wear path on the disc. The debris engages

between the specimen and the counter face helps to reduce the wear rate. It is understandable that increase in sliding




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distance causes the transfer film formation on the wear disc and this contributes to reduce the wear rate under longer

duration of sliding. Addition of PTFE to the polymer will cause the film formation this will decrease the wear rate [12].

403215

230.0

227.5

225.0

222.5

220.0

1.51.00.5

600040002000

230.0

227.5

225.0

222.5

220.0

filler content

Mean

of

SN

rati

os

sliding velocity

sliding distance

Main Effects Plot for SN ratiosData Means

Signal-to-noise: Smaller is better

Fig. 2: Main effect plot for S/N ratio

Table 6: Response table for S/N ratio - smaller is better

Level Filler

content

Sliding

velocity

Sliding

distance

1 218.2 223 221.5

2 226.4 223.2 223.6

3 229.5 227.9 229.0

delta 11.3 4.9 7.5

rank 1 3 2

Fig. 3 illustrates the interaction effects of control parameters. It is well known that interactions do not occur when

the lines on the interaction plots are parallel and strong interactions occur between parameters when the lines cross. In

order to find out the correlation of the interaction factor and wear rate ANOVA analysis carried out.

Fig 3: Interaction plot for S/N ratio




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3.2 ANOVA The analysis of variance (ANOVA) was used to investigate which design parameters significantly affect the

quality characteristic. It was accomplished by separating the total variability of the S/N ratios, which is measured by sum

of the squared deviations from the total mean S/N ratio, into contributions by each of the design parameters and the

errors.

The percentage contribution of each control factor is employed to measure the effect over Ws. Table 5 shows the

results of the ANOVA with the specific wear rate. This analysis was undertaken for a level of significance of 5% that is,

for level of confidence 95%. Examination of the calculated values of Fishers (F) for all control factors also showed a

very high influence of factor A and low influence of factor B on specific wear rate of TCE based composites (Table 7).

The interaction between A x B (7.50%) have got the significant influence, A x C and B x C have got least significance in

dry sliding wear behaviour of TCE composites. The present analysis indicates that dry sliding wear test parameters and

their interactions have both statistical and physical significance in TCE composites.

3.3 Worn surface morphology

The worn surface morphology was observed by scanning electron microscopy (SEM) to study the effect of

increase in filler content, sliding distance and sliding velocity.

Table 7: ANOVA response table for Ws

Source D S JS JM P%

Filler content (A) 2 5.88801 5.88801 2.94400 55.10

Sliding velocity (B) 2 1.47654 1.47654 0.73827 13.80

Sliding distance (C) 2 2.01698 2.01698 1.00849 18.90

(A x B) 4 0.80196 0.80196 0.20049 7.50

(A x C) 4 0.32160 0.32160 0.08040 3.00

(B x C) 4 0.08519 0.08519 0.02130 0.80

Error 8 0.09630 0.09630 0.01204 0.90

Total 26 10.68658 100

S = 0.109715, R-Sq = 99.10% and R-Sq (adj) = 97.07%.

D: DOF, degrees of freedom; S: Seq SS, sequential sum of squares; JS: Adj SS, adjusted sum of squares; JM: Adj MS,

adjusted mean squares; P%: percentage of contribution.

The SEM micrographs are shown in Fig. 4. The arrows show the direction of the adhesive wear. Fig. 4a and 4b

shows the worn surface micrograph of TCE + PTFE filled composite, and Fig. 4c, 4d and 4e shows the worn surface

micrograph of TCE + fibers + ceramic fillers reinforced composite with different magnifications. The circles show the

wear debris. P stands for debonding of PTFE particulate and SGF stands for short glass fiber. From the micrographic

pictures we can infer that the detachment of the particles from the specimen cause wear debris. This wear debris

entrapped between the specimen and the counter face. This will cause the reduction of the wear rate. Ray and

Gnanamoorthy [13] explained three mechanisms namely (i) matrix material loss, (ii) filler wear, and (iii) debonding at

the interface are operative in filled polymer composites and dominance of one factor over the other control the wear

behaviour of composites.

In the case of specimen with TCE and PTFE wear debris is more. In the case of specimen with TCE, PTFE and

SGF i.e. short glass fiber is easily detached from the surface, since the glass fiber is harder than the steel material. It will

cause wear to the disc. So wear rate decreases. In the case of specimen with TCE, PTFE, SGF, SCF, SiC, and Al2O3

ceramic material cause wear on the counter face. The wear debris trapped inside the wear path and cause decrease in the

wear rate.




78

Fig. 4a: SEM micrographs of TCE + PTFE composite slid at 6000 m, 0.5 m/s

Fig. 4b: SEM micrographs of TCE + PTFE composite slid at 6000 m, 1 m/s

Fig. 4c: SEM micrographs of TCE + fibers + ceramic fillers filled composite slid at 6000 m, 0.5 m/s

Cracks

P

P

Deep fragmentation

Severe damage

P

Micro cracks

Void

Micro cracks

Fragmentation

Furrows and ridges

Void

Micro crack




79

Fig. 4d: SEM micrographs of TCE + fibers + ceramic fillers filled composite slid at 6000 m, 1 m/s

Fig. 4e: SEM micrographs of TCE + fibers + ceramic fillers filled composite slid at 6000 m, 1.5 m/s

4. CONCLUSIONS

Experimental investigation carried out on TCE composites to study the effect of filler content, sliding distance

and sliding velocity on specific wear rate. The following conclusions could be drawn from the results of the dry sliding

wear behaviour of fiber and filler filled TCE composites.

�� Specific wear rate is least in TCE composite reinforced with fiber and ceramic fillers, in comparison to that of

TCE + PTFE and TCE + PTFE + SGF reinforced composites. A synergistic effect of the fibers and fillers on

improving the wear resistance was reported.

�� Specific wear rate decreases with increase in sliding distance. As the filler content of the specimen increases the

specific wear rate also decreases. From Taguchi analysis it is concluded that control factors cause significant effect

on specific wear rate and they are ranked as filler content > sliding distance > siding velocity.

�� Ananlysis of variance provides the effect of interaction of control factors on specific wear rate. Filler content has

more significant effect on the specific wear rate of TCE composites followed by sliding distance and sliding

velocity. Interaction between sliding distance and sliding velocity possess more significant effect followed by

filler content and sliding velocity on the specific wear rate of TCE composites.

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[2] H. L. Stein, Ultra high molecular weight polyethylene (UHMWPE), Engineered Materials Handbook 1999,

167–171.

Furrows and ridges




80

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