influence of aramid fibers on tribological behavior …

Proceedings of the 6th International Conference on Mechanics and Materials in Design,

Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26-30 July 2015

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PAPER REF: 5540

INFLUENCE OF ARAMID FIBERS ON TRIBOLOGICAL

BEHAVIOR OF PBT

Mihail Botan1, Constantin Georgescu

1, George Catalin Cristea

1, Lorena Deleanu

1(*)

1Faculty of Engineering, “Dunarea de Jos” University of Galati, Galati, Romania

(*)Email: [email protected]

ABSTRACT

This work presents the tribological behavior of PBT (polybuthylene terephthalate) + 10%

aramid fibbers and PBT, when sliding against steel in dry regime in order to point out the

influence of adding fibers. Tests were done on a block-on-ring tribotester, for normal loads of

5 ... 30 N and sliding speeds 0.25 ... 0.75 m/s, for a sliding distance of 5000 m. Based on the

experimental data, the authors recommend the composite for functioning in the tested ranges

for speed and force, as its wear and temperature in contact and friction coefficient are reduced

as compared to polymer.

The wear maps plotted based on the experimental results help to rank the tested materials and

to emphasis several advantages of the composite PBT and 10% wt short aramid fibers: it

allows for decreasing the temperature in contact (as measured at a lateral point of the contact)

with 5...10º C for the highest values of the testing parameters, but for low speed and load the

difference in temperature is almost insensible to the block made of neat polymer; for the

composite, wear is reduced almost twice for F = 15-30 N and all tested speed range; wear of

the composite is less dependent on speed, recommending it for applications with variable

regimes; the friction coefficient is only several percentages higher as compared to that

obtained for the block made of PBT, except for high load and speed (v = 0.75 m/s and F = 30

N), when the friction coefficient increases to 0.32-0.36). The SEM images pointed out a very

particular lumpy transfer, both for polymer and composite, meaning the leading factor is the

matrix material (PBT).

Keywords: PBT, short aramid fibers, block-on-ring test, wear map, friction coefficient.

INTRODUCTION

Researches on polymers and polymeric composites are focused on improving their properties

related to particular applications, including the tribological ones (Stachowiack, 2001),

(Bahadur, 2009), (Samyn, 2009), (Briscoe, 1981), (Thorpe, 1986), (Wardle, 2000), (Besnea,

2014) (Bang, 2004), sensors and aerospace materials (Meguid, 2005).

Polymeric composites with short aramid fibers have become of interest in the research field in

the last decade, both as friction and antifriction materials. For tribological applications,

length/diameter ratio characterizing the aramid fibers introduced in polymeric composites

varies from 20 to 550 µm (Dorigato, 2013). Composites with thermoplastic matrix are

especially used for their low friction coefficient and higher wear resistance as compared to the

neat polymer (Akbarian, 2008), (Arroyo, 2002), (Bijwe, 2006) (Zhu, 2010), (Tang, 2011).

Materials with rigid resins as matrix were tested for applications like brakes (Kim, 2001),

(Larsen, 2007), (Singh, 2015), (Xian, 2006). Shibulal added coated short aramid fibers in a

thermoplastic elastomer (Shibulal, 2011) and Kashani added aramid fibers in an elastomer for

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Tribology, Gears and Transmissions

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tires (Kashani, 2009). There are several restrictions in using aramid fibers as reinforcement in

tribological applications: a technological one, caused by the difficulty of molding / processing

mixture with relevant concentration of aramid fibers, the quality of the fiber dispersion, even

under laboratory conditions, and the aramid fiber sensitivity in fluids containing water.

Thermoplastics polymers used as matrix in composites with short aramid fibers included PA

(Botan, 2014), (Gordon, 2009), PP (Maity, 2008), (Arroyo, 2002), PTFE (Qiu, 2015), (Fung,

2005), PS (Mukherjee, 2006), PI (Li, 2009), (Zhao, 2013). Reports on tribology of PBT

composites are few (Kim, 2001), (Larsen, 2007) (Georgescu, 2012) and further investigation

on a mixture like PBT and fibers is necessary for better exploitation its unique set of

properties.

Several studies make comparison among composites with same matrix but different fibers,

including mixture of fibers. Zhao et al. [Zhao, 2013] evaluated friction and wear of

polyimides reinforced with carbon, glass and aramid fibers, under dry sliding against

sandpaper and steel, under three-body abrasive conditions. The best performance was

exhibited by inorganic fiber composites due to the load sharing. Brittle glass fibers have not

contributed to improve wear resistance under abrasive conditions, due to insufficient support

from polyimide matrix.

Short aramid fibers in a polymeric matrix could improve wear of polymeric composites but

tests are required in order to evaluate the particular mix of wear mechanisms and the contact

durability under conditions of the future application. Dorigato and Fambri (Dorigato, 2013)

used regenerated aramid fibers in PA12, pointing out the effect of fiber concentration in the

range of 10...30%, after a synthetic review of mechanical and tribological bahavior of

composites with short aramid fibers, the conclusion being that wear could be improved when

using these materials and their application could include either antifriction or friction

tribosystems, depending on the matrix characteristics. Results with composites with PBT

matrix are promising as concern wear and temperature in contact as compared, for instance, to

PA (Botan, 2014), (Maftei, 2011), (Yu, 1994) (Tang, 2011), (Ioffe, 2011). Other fillers, like

carbon fibers, clay (Hwang, 2010) were added in PBT for improving wear behavior.

Polybuthylene terephthalate is a polymer only recently used as matrix because of

technological reasons in molding parts made of this material, including properties’ control

with the help of thermal treatment. Thus, researches on composites with PBT matrix are few

and the future studies will help to introduce it (as neat polymer or composites) in tribological

and mechanical applications.

MATERIALS AND TESTING METHODOLOGY

The block-on-ring tribotester is often used for characterizing the tribological behavior of a

pair of materials, especially when dealing with polymeric materials sliding against steel

because of block simplicity and high accuracy of the rolling bearing external ring,

characterized by a high quality of surface texture and steel structure. The variables could be

the sliding distance (L), the block material, the specific load (W), the speed (v) and, if the

tribometer has a controlled enclosure, temperature and composition of the environment. Also,

it is easy to compare dry and lubricated regimes.

This study deals with a composite with matrix made of PBT (grade Crastin 6130 NC010 –

DuPont) and 10% wt short aramid fibers (Twaron D1088, 250 µm average length, supplied by

Teijin). The material code is PBX in all figures. For technological reason, the composite

contains 1% wt polyamide 6 (grade Relamid, produced by ICEFS) and 1% wt black carbon.



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Samples were obtained by molding at Research Institute for Synthetic Fibers Savinesti

(ICEFS) Romania and they were heat treated.

Table 5 - Studies on block-on-ring tribotester

Authors, year Block material,

Matrix/ fillers

Test parameters (dry regime)

L [m] W [N/mm] v [m/s]

(Li, 2007) PTFE and PI 7.14 ...40 0.424

(Georgescu, 2012) PBT/ glass bead, PTFE 5000, 7500, 1000 1.25 ...7.50 0.25 ... 0.75

(Wannasri, 2009) UHMWPE/ carbon nanotubes 3840 22.85 0.32

(Qiao, 2007) PEEK/ Al2O3 3000 28 0.42

(Jia, 2005) PI, PEEK /carbon fibers 3000 28.5 0.43

(Cong, 2008) PA 3000 7 0.42

(Zheng, 2007) PA6/ braided carbon fiber 1500 8.3...41.6 0.42

Experimental results are difficult to be compared, but qualitative evaluation was taken into

consideration for this work (Table 1). The authors, based on (Maftei, 2011),

(Georgescu, 2012), considered that 5000 m was reliable for pointing out a stable friction and

wear rate, for the tested materials and the test parameter ranges (v = 0.25 m/s, 0.5 m/s and

0.75 m/s, W = 1.25 N/mm, 2.5 N/mm, 3.75 N/mm and 7.5 N/mm).

The geometry of the tribotester is given in Fig. 1. The blocks (16.5 mm × 10 mm × 4 mm)

were cut with the help of a water jet machine, from the mold plates. Figure 2 presents the way

of cutting the blocks from a mold plate; the molding nozzle is positioned in the centre of the

plate. Selected zone have a good dispertion of fibers and central and peripheral zone are

excluded because of high probablity of having a non-uniform dispersion of aramid fibers.

The blocks were dried in an oven immediately after being cut. The other triboelement was the

external ring of the tapered rolling bearing KBS 30202, made of steel grade DIN 100Cr6 (see

Table 2), with 60-62 HRC and Ra = 0.8 µm on the exterior surface.

Fig. 1 - Geometry of the block-on-ring tester Fig. 2 - Cutting of the block from a mold plate

Taking into account that an average stress on the contact is not relevant for evaluating such a

contact between two materials with very different mechanical properties, the maximum

pressure depending on the elasto-plastic behavior of the polymeric block. In order to compare

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some results from literature for block

linear load, this type of contact better characterizing by the linear load:

b

FW = [N/mm]

where F is the normal load, in N,

Table 6 - Chemical composition of steel grade 100Cr6 (DIN 17230) (wt%)

C Si Mn

0.90-1.05 0.15-0.35 0.25-0.45

RESULTS AND CONCLUSIONS

The values for the friction coefficient are good f

4), with lower values for the polymer as compared to the composite

N, but at F=30 N, this parameter is in a close range for both materials.

Fig. 3 - Evolution of friction coefficient in time, for different loads and sliding speeds


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some results from literature for block-on-ring tests (see Table 1), the authors calculated a

linear load, this type of contact better characterizing by the linear load:

[N/mm]

is the normal load, in N, b – the theoretical length of the contact, in [mm]

Chemical composition of steel grade 100Cr6 (DIN 17230) (wt%)

Mn P S Cr

0.45 ≤0.030 ≤0.025 1.35-1.65 ≤0.30

RESULTS AND CONCLUSIONS

The values for the friction coefficient are good for both tested materials (see Fig. 3 and Fig.

), with lower values for the polymer as compared to the composite, for normal load

ameter is in a close range for both materials.

Evolution of friction coefficient in time, for different loads and sliding speeds

, the authors calculated a

[N/mm] (1)

the theoretical length of the contact, in [mm].

Chemical composition of steel grade 100Cr6 (DIN 17230) (wt%)

Ni Cu

≤0.30 ≤0.30

or both tested materials (see Fig. 3 and Fig.

normal loads of 5–15

Evolution of friction coefficient in time, for different loads and sliding speeds


Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26

The temperature on the lateral side was monitored with the help of a thermographic camera

and the maximum recorded values

interested in having materials with low temperature in functioning and low influence of load

and speed on this temperature.

Fig. 4 - The influence of normal load and sliding

PROCESSES IN THE SUPERFICIAL LAYERS

Transfer processes are very important in selecting the pair of materials

tribological applications, especially when dealing with polymers

(Stachowiak, 2001), (Czicosh, 2009). Their generation and evolution influence friction and

wear. Transfer processes have to be studied on both triboelements. PBT and its composites

(Georgescu, 2012) have a very particular transfer pro

tribological behavior, as compared to other polymeric composites on steel. SEM images

reveal several aspects due to the combination

and f), abrasive wear (Fig. 5), transfer film

8f), wear debris generation and evolution (Fig. 8b and d).

SEM investigations proved that

polymer or composite: for instance

morphology: belt-like and spiral rod

against steel were adhesive wear and melting flow. The melting flow was not visible on PBT

block due to its better thermal character

a) v = 0.25 m/s

Fig. 5



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aximum recorded values are points on the maps in Fig. 9. An engineer will be


.

The influence of normal load and sliding speed on the average and the spead range of

PROCESSES IN THE SUPERFICIAL LAYERS

Transfer processes are very important in selecting the pair of materials

tribological applications, especially when dealing with polymers and their composites

(Czicosh, 2009). Their generation and evolution influence friction and


have a very particular transfer process, which differenciate their


aspects due to the combination PBT – aramid fibers: adhesion wear (Fig. 8a, c

and f), abrasive wear (Fig. 5), transfer film (Fig. 8a, b, c and e), fiber degradation (Fig. 7, Fig.

8f), wear debris generation and evolution (Fig. 8b and d).

that (Cong, 2008) wear debris shapes are strongly dependent

instance, PA46 formed two kinds of wear debris with different

like and spiral rod-like wear debris. The main wear mechanisms of PA46

wear and melting flow. The melting flow was not visible on PBT

block due to its better thermal characteristics (Figures 5 and 7).

b) v = 0.5 m/s c) v

- SEM images for blocks made of PBX, F = 15 N


. An engineer will be


average and the spead range of friction coefficient

Transfer processes are very important in selecting the pair of materials involved in a

and their composites

(Czicosh, 2009). Their generation and evolution influence friction and


cess, which differenciate their


aramid fibers: adhesion wear (Fig. 8a, c

(Fig. 8a, b, c and e), fiber degradation (Fig. 7, Fig.

ear debris shapes are strongly dependent on

kinds of wear debris with different

The main wear mechanisms of PA46

wear and melting flow. The melting flow was not visible on PBT

c) v = 0.75 m/s

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a) v = 0.5 m/s

Fig. 6

a) v = 0.25 m/s

Fig. 7 - Aramid fibers within the PBT matrix,

Because of their visco-elastic nature, the fibers almost perpendicular to the friction surface do

not remain up the surface, being defo

(Fig. 6b and c), even if the test condition differ from slow speed (v = 0.25 m/s) to high one (v

= 0.75 m/s) (Fig. 7). And the surface of aramid fiber is deformed, printed, but not scrached,

the wear debris have few and small fiber fragments (see Fig. 8c).

Comparing these SEM images to those presented by Chang and Friedrich (Chang, 2013), due

to the elasticity of fibers and to a good adherence of their adherence to the matrix, the fibers

do not remain outside the surface of the matrix and the load distribution would be better. The

fiber detaching from the matrix is taking place especially for fibers that are positioned along

or inclined to the surface (Fig. 6a and Fig. 8d). The surface of the fibers underg

seems to be less influenced by the sliding speed increase (Fig. 7), the fibers being deformed

and not fragmented or scratch by the asperities of the metalic surface. Even if the load

increases from 15 N to 30 N, this does not evidently change

matrix seems to be soften, but it has a good adherence to the fibers, even after testing at F =

30 N.

Few polymers exhibit a contin

(Myshkin, 2005) and high den

polyethylene (UHMWPE). Their similar behavior

profile’ or the absence of side groups in the

have a continuous transfer film on a steel pin (

difficult to investigate this aspect. The majority of polymers and polymer composites exhibit a

‘lumpy transfer’ when sliding against a hard counterface (Wieleba, 2007)

not too “smooth” and this could explain

sliding against. The size of these lumps depends on friction pair of materials, test conditions

and could vary in a large dimensional range (from microns to


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b) v = 0.5 m/s c) v

Fig. 6 - SEM images for blocks made of PBX, F = 15 N

b) v = 0.5 m/s c) v

Aramid fibers within the PBT matrix, F = 30 N, block made of PBX

elastic nature, the fibers almost perpendicular to the friction surface do

not remain up the surface, being deformed when the matrix is torn off from the tribolayer



ris have few and small fiber fragments (see Fig. 8c).



side the surface of the matrix and the load distribution would be better. The


or inclined to the surface (Fig. 6a and Fig. 8d). The surface of the fibers underg



increases from 15 N to 30 N, this does not evidently change the fibbers behavior. Only the


Few polymers exhibit a continuous film transfer, similar to that of PTFE (Stachowiack, 2001),

and high density polyethylene (HDPE) and ultra-high

. Their similar behavior would be caused by a ‘smooth molecular

profile’ or the absence of side groups in the molecular chain. Yet, polyamide was proved to

sfer film on a steel pin (Maftei, 2010), but in actual tribosystems

this aspect. The majority of polymers and polymer composites exhibit a

ing against a hard counterface (Wieleba, 2007). PBT molecule

not too “smooth” and this could explain its tendency not to creep when a metallic surface is

The size of these lumps depends on friction pair of materials, test conditions

and could vary in a large dimensional range (from microns to millimeters) (Fig.

c) v = 0.75 m/s

c) v = 0.75 m/s

PBX

elastic nature, the fibers almost perpendicular to the friction surface do

rmed when the matrix is torn off from the tribolayer





side the surface of the matrix and the load distribution would be better. The


or inclined to the surface (Fig. 6a and Fig. 8d). The surface of the fibers undergoing the load



the fibbers behavior. Only the


(Stachowiack, 2001),

high-molecular weight

be caused by a ‘smooth molecular

Yet, polyamide was proved to

, but in actual tribosystems it is

this aspect. The majority of polymers and polymer composites exhibit a

. PBT molecules are

not to creep when a metallic surface is

The size of these lumps depends on friction pair of materials, test conditions

millimeters) (Fig. 8a). Some


Editors: J.F. Silva Gomes & S.A. Meguid, P.Delgada/Azores, 26

polymer materials have better lubricating properties than others (i.e. PTFE), however,

exhibit low mechanical properties and

polymer matrix, like PEEK (Greco, 2011)

a) lump transfer on the steel ring,

having a variable thickness (block

made of PBX)

d) F = 15 N, v = 0.25 m/s

(block made of PBT)

Due to wear debris generation and fragmentation of the already transferred islands, the dry

regime of a friction couple polymeric material

tribosystem (Myshkin, 2005).

In this study, on both PBT materials, fatigues crac

cracks appear on transferred lumps (see Fig. 8c) and possible detaching of a wear particle will

worsen the surface quality of the ring. Load and speed variation does not appear to give a

strong correlation between film transfer covering area on the steel ring and its thickness, the

islands of polymer adhesion on steel being similar for both polymer and composite (compare

Fig. 8b and Fig. 7a), this conclusion being also pointed out by Laux and Schwartz (Laux,

2013), for different grades of PEEK. A strong correlation does not appear to exist between

film thickness and volumetric wear.

Aramid fibers are not crushed and fragmented as the glass fibers and glass beads do under the

load (Georgescu, 2012), (Maftei, 2010). SEM

aramid fibers from the tribolayer and even the way they are fixed on the steel texture imply a

“drag” of these worn fibers (but not fragmented) along the contact (Fig. 8d and

the friction coefficient to oscillate (see Fig. 3, for load F

individual lands meet, merge and detach, forming a discontinuous transfer film. Analyzing



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polymer materials have better lubricating properties than others (i.e. PTFE), however,

mechanical properties and, therefore, they are used as fillers to other more robust

, like PEEK (Greco, 2011), (Li, 2009), (Ye, 2014).

b) wear debris, laminated and

rolled (block made of PBX)

c) detail from a

e) transfer of neat PBT on the steel

ring, F = 15 N, v = 0.25 m/s

f) Aramid fibber, torn off from the

matrix, bent because of the uneven

fixing on the metallic texture,

made PBX,

Fig. 8 - Wear mechanisms

ear debris generation and fragmentation of the already transferred islands, the dry

regime of a friction couple polymeric material – steel should be analyzed as a ‘third body’

In this study, on both PBT materials, fatigues cracks were not noticed, but fatigue micro



m transfer covering area on the steel ring and its thickness, the



different grades of PEEK. A strong correlation does not appear to exist between

film thickness and volumetric wear.


load (Georgescu, 2012), (Maftei, 2010). SEM images reveal the detaching mechanisms of


“drag” of these worn fibers (but not fragmented) along the contact (Fig. 8d and

oscillate (see Fig. 3, for load F = 5 N and 30 N). During sliding,


polymer materials have better lubricating properties than others (i.e. PTFE), however, they

are used as fillers to other more robust

c) detail from a) (block made of

PBX)

Aramid fibber, torn off from the

bent because of the uneven

fixing on the metallic texture, block

F = 15 N, v = 0.75 m/s

ear debris generation and fragmentation of the already transferred islands, the dry

steel should be analyzed as a ‘third body’

ks were not noticed, but fatigue micro-



m transfer covering area on the steel ring and its thickness, the



different grades of PEEK. A strong correlation does not appear to exist between


images reveal the detaching mechanisms of


“drag” of these worn fibers (but not fragmented) along the contact (Fig. 8d and 8f), making

5 N and 30 N). During sliding,


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Fig. 8a, 8b and 8c, one may notice a high dynamics of generating, adhering and detaching,

partially or entirely, of the wear debris; big debris are rare but make the friction coefficient to

oscillate.

Wear maps are very useful in comparing tendencies of wear evolution for different materials

sliding on the same surface (here, hardened steel). Figure 9 evidences a lower temperature for

the friction couple with the block made of composite with aramid fibers, for higher speed (0.5

m/s...0.75 m/s) and higher loads (15 N... 30 N). For low values of speed and load, it seems

that the neat polymer also keeps the contact temperature close to that of the composite. Other

parameters will make the selection between the two materials.

The wear maps were plotted using MATLAB R2009b, the wear parameter (mass loss) being

represented for each material as a function of sliding speed and normal force, with the help of

a cubic interpolation.

Fig. 9 - Temperature maps (maximum values on the lateral side of the contact) at the end of the test

Fig. 10 - Wear maps generated with the help of experimental data (sliding distance of 5000 m)

Mass loss of the block is lower for the composite, especially at low speed v = 0.25 m/s (Fig.

10). This could assume that abrasive wear under F = 30 N is more intense for the polymer and

thus, it may be assumed that fibers avoid the polymer to be plough and tear off from the

tribolayers. The composite is sensitive to low load (F = 5 N) and have higher values for the

mass loss because the fibber are easier to be detached from the matrix when normal load is

low.



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CONCLUSIONS

Based on the experimental data, the authors recommend the composite for functioning in the

tested ranges for speed and force, as its wear (see Fig. 10) and temperature in contact (see Fig.

9) are reduced and the friction coefficient is higher only with less 20% as compared to that

exhibits by neat polymer, under the same test conditions.

The wear maps plotted based on the experimental results help to rank the tested materials and

to point out several advantages of a block made of PBX:

• it allows for decreasing the temperature in contact (as measured at the lateral point of the

contact) with 5...10º C for the highest values of the testing parameters (F = 20...30 N and

v = 0.5...0.75 m/s); but, for low speed and load, the difference in temperature is almost

insensible to the block material;

• wear is reduced almost twice for F = 15...30 N and all tested speed range;

• wear is less dependent on speed, recommending the composite for applications with

variable parameter regimes;

• the friction coefficient is only several percentage higher as compared to that obtained for

the block made of PBT, except the higher speed (v = 0.75 m/s).

The SEM images pointed out a very particular lumpy transfer, both for polymer and

composite, meaning the leading factor is the matrix material (PBT).

ACKNOWLEDGMENTS

The work of MIHAIL BOTAN, CONSTANTIN GEORGESCU and LORENA DELEANU

was supported by Project SOP HRD /159/1.5/S/138963 - PERFORM.

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influence of aramid fibers on tribological behavior …

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