Investigation of Surface Properties and Mechanical and Tribological

8/18/2019 Investigation of Surface Properties and Mechanical and Tribological

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Investigation of surface properties and mechanical and tribological

behaviors of polyimide based composite coatings

Camelia Demian a,⁎, Hanlin Liao a, Remy Lachat b, Sophie Costil a

a Laboratoire IRTES-LERMPS, Université de Technologie de Belfort-Montbéliard, 90010 Belfort Cedex, Franceb Laboratoire IRTES-M3M, Université de Technologie de Belfort-Montbéliard, 90010 Belfort Cedex, France

a b s t r a c ta r t i c l e i n f o

Article history:

Received 17 June 2012Accepted in revised form 19 August 2013

Available online 29 August 2013

Keywords:

Polyimide based composite coatings

Total surface energy

Friction coef cient

Wear rate

Thermal stability

This article focuses on the development and characterization of thermoset polyimide (PI) based composite coat-

ings on aluminum substrates. In order to improve the tribological behavior, PTFE and SiC llers were added into

pure PI to develop composite coatings. A thermal study to validate the condition of the pure PI coating after the

elaboration process was performed using DSC analyses and the Tg evolution with the temperature of a pure

polyimide sample was investigated.

Then, the inuence of thellers (PTFE and SiC) on surface properties, and mechanical and tribological behaviors

of the PI composite coatings is considered. Results showed that, by adding PTFE particles into the pure PI, lower

surface energies and lower and stable friction coef cients can be obtained. Besides, the addition of SiC particles

improved the mechanical behavior such as hardnessand wearresistance of the composite PI–PTFE–SiC coatings.

Following the obtained results, correlations between wear rate depending on total surface energy and

microhardness were established. Thermogravimetric analysis (TGA) of the PI and PI composite coatings was

carried out. The results revealed that the addition of llers into PI pure matrix improved the thermal stability

of the composite coatings.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Advanced composites exhibit appropriate physical and chemical

properties that include light weight characteristics coupled with high

stiffness and strength, dimensional and thermal stability, chemical

resistance, exural performance and relatively easy processing. The

resin systems used to manufacture advanced composites concern

two basic types: thermosetting and thermoplastic polymers. Ther-

mosetting resinssuch as epoxy, polyurethane, phenolic and amino, and

bismaleimide (BMI, polyimide) are widely used in advanced composite

manufacturing. As a thermoset polymer, polyimide (PI) possesses

outstanding properties such as excellent mechanical and electrical

insulating properties, good thermal stability and chemical inertness,

high wear resistance and resistance to radiation which make it suitable

fora wide range of applications [1]. In spite of the good characteristics of

polyimide, studies have been carried out to improve their surface prop-

erties and tribological behavior by dispersing different ller materials

throughout the polymer matrix. Thus, the PI based composite becomes

a promising material with controlled mechanical or tribological proper-

ties. Besides, llers contribute to optimize the operational properties in

different applications, such as microelectronics or biomedical devices,

components for electrical, aerospace or automotive industries [2–4].

Polyimide is also known as a thermo-stable polymer due to its excel-

lent properties at elevated temperatures (250 and 350 °C). The thermal

properties of different synthesized polyimides were already studied by

many researchers. For example, high glass transition temperature of

polyimide up to 310–315 °C [5,6] or 340 °C [7] and no melting point

[5–8] have been observed on different DSC curves. The excellent ther-

mal stability of polyimide from ambient temperature up to 350 °C [9]

or 420 °C [6] has been also reported. Moreover, the polyimide has a

degradation temperature above 500 °C [5,6].

Polymer based composites reinforced at micro or nanoscale with dif-

ferent llers such as bers or solid lubricates have gained development

andbecome promisingbulk or coatingmaterials fortribological applica-

tions. The mechanical properties of the glass–epoxy composites can be

improved with the addition of SiC ller and this composite presents a

lower wear rate [10]. The addition of the SiC particles in the polymer

matrix as a secondaryller along withgraphite into glass–epoxy matrix

increases the wear resistance of the composite material [11]. Like many

kinds of polymer materials, the PI based composite coatings were also

studied. For example, the PI resins lled with solid lubricants such as

graphite, MoS2 and PTFE particles and reinforced with carbon bers

(CF) have shown better friction and anti-wear behavior under water-

lubrication ratherthandry sliding [1]. Theaddition of appropriate content

of Al2O3 nanoparticles into PI improved signicantly the tribological be-

havior [12] of the specimens prepared by compression molding method.

Besides, the tribological properties of polyimide based composites

demonstrated good friction behavior and improved wear resistance

Surface & Coatings Technology 235 (2013) 603–610

⁎ Corresponding author at:Laboratoire SPCTS, UMR CNRS 7315, Université de Limoges,

12 rue Atlantis, 87068 Limoges Cedex, France. Tel.: +33 0 5 87 50 24 03.

E-mail address: [email protected] (C. Demian).

0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.surfcoat.2013.08.032

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if they are lled with carbon nanotubes (CNT) and Al2O3 [12,13]. The

graphite additives can control the interfacial sliding conditions, either

acting abrasively or adhesively for sintered PI composites [14]. By the

addition of graphite as internal lubricant into pure polyimide, lower

friction coef cients can be obtained under high loads and high sliding

velocities [15]. Besides, increasing SiC whisker content in a SiC–AlN–

polyimide matrix composite increased the toughness and decreased

the exural modulus [16].

The use of polyimide based composites as surface protection can

improve the tribological behavior and surface properties of different

substrate materials. For example, aluminum is widely used in industrial

applications for diverse parts and devices, but it possesses poor frictionproperties. Moreover, polyimide based composite coatings also offer

high surface thermal stability when used in tribological applications,

even at elevated temperatures. It is already demonstrated that PTFE

signicantly improved the performances of other polymers such as

PEEK [17] andPI [18] because it controls thefrictionand wear behaviors.

Besides, SiC particles as a solid ller also contribute to enhance the

mechanical and tribological properties of PI based composites. Howev-

er, few information about the mechanical and tribological behaviors of

polyimide composite coatings lled with PTFE and SiC was reported in

the literature. The aim of this study is to investigate the mechanical

and tribological behaviors of PI–PTFE and PI−PTFE–SiC composite

coatings under dry condition. For this purpose different PTFE weight

concentrations as well as a x amount of SiCparticleswere incorporated

into a fully imidized polyimide resin matrix to fabricate composite

coatings. The DSC and TGA analyses were performed on the polyimide

solution and pure polyimide and its composite coatings in order to

determinethe structureafter the heat treatmentas well as thephenom-

ena which occur during the polymer heating such as glass transition,

desolventation and thermal characteristics. Microscopic observations

of the coating surface and cross-section were performed in order to

reveal the surface morphology. The results of microhardness and tribo-

logical tests were correlated with the total surface energy determined

by sessile drop method.

2. Coating elaboration and characterization methods

2.1. Composite coating elaboration

PTFE particles with an average diameter of 30 μ m wereadded in 20,

30 and 40 wt.% into a polyimide fully imidized P84 solution (provided

by Evonik Industries, Evonik Fibres GmbH, Austria) and well mixed.

Polyimide P84 solution is presented in the NEP (N-ethyl-pyrrolidone)

Fig. 1. DSC curve of the PI coatings.

G l a s s t r a n s i t i o n t e m p e r a t u r e

T g ( O C )

Isothermal holding (OC)

100

150

200

250

300

350

150 200 250 300 350 400 450

Fig. 2. Evolution of Tg with temperature. Fig. 3. Surface morphology of PI–

20% PTFE–

5% SiC composite coating before polishing.

604 C. Demian et al. / Surface & Coatings Technology 235 (2013) 603–610


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solvent, which represents 75% of the total mass. 5 wt.% SiC particles

having an average diameter of 1.7 μ m were added into the PI–PTFE

mixture as the second solid ller. Because of the high viscosity of the

PI–PTFE mixture, the chosen amount of SiC particles is almost the

limit in order to favor the application on the Al substrate. The resulting

composite mixtures were then applied on Al plates of 50 × 50 × 3 mm

to nally obtain a coating thickness between 60 and 120 μ m. Then,

the coatings were dried in a forced-air furnace at 80 °C for a few

hours to evaporate the solvent, and then heated up to 315 °C

with a heating rate of 1.5 °C/min to remove the residual solvent and

to stabilize the properties of the composite material.

2.2. Characterization methods

Several investigation methods were used to observe the inuence

of ller particles on the surface characteristics and mechanical and

tribological behaviors of the composite coatings. Thermal investigations

of the PI coating, PI solution and PI based composite coatings were car-

ried out using differential scanning calorimetry (DSC) and thermal

gravimetric (TGA) measurements. These analyses were conducted in

air atmosphere using Q10 and Q50 TA Instrument apparatus, respec-

tively. Thedata were recorded using dedicated Universal AnalysisV4 ac-

quisition software. The sample was embedded in an aluminum pan in

the case of DSC analysis and then heated until 500 °C at a heating rate

of 10 °C/min. Following the same experimental protocol, the PI basedcompositecoatingswere also analyzed by a seriesof TGA tests. Thesam-

ples were heated until 800 °C at a heating rate of 10 °C/min.

Coating morphology was investigated using a JSM 5800 LV, JEOL,

Japan scanning electron microscope. The microscopic observations in

cross-section view have been performed using a Leica DMRM optical

microscope. The grinding and polishing processes of metallographic

samples, which were embedded in epoxy resin under vacuum, were

performed according to an adapted procedure using a SiO2 solution

for polishing.

The surface properties were evaluated using sessile drop method to

determine the wettability and surface energies according to Owen–

Wendt theory [19].

The sample surfaces used to evaluate the tribological properties of

the coatings and surface energy were polished to reach an average sur-

face roughness between 1 and 3 μ m.

The friction coef cients of the composite coatings were determined

by tribological tests using the pin-on-disk method at ambient tem-

perature. Friction tests were accomplished using a ball-on-disk CSM

tribometer (CSM Instruments, Swiss). The counterpart consisted of a

6 mm diameter 100C6 steel ball with a mirror nished surface (Ra =

0.02 μ m) and a hardness of 62 HRC. The friction force was measured

with a linear variable differential transformer sensor and dynamically

recorded into a computer. The friction coef cients were obtained when

the measured forces were divided by the applied load.

To determine the wear rate, prolometry traces were taken across

the wear tracks generated on the coating surface after tribological test,

using a 3D AltiSurf 500 prolometer (Altimet, France). The volume

loss of the coating was determined by multiplying the cross-section

area by the perimeterof thewear track. Thewear rate wascalculated as:

w ¼ V

d F ¼

A l

d F

mm3

N m

! ð1Þ

where,

V the volume loss (mm3);

D the sliding distance (m);

F the applied load (N);

A the cross-section area of the wear track (mm2);

L the perimeter of the circular track (mm): l = 2 × 3.142 × r

(mm), where r (mm) represents theradius of thewear track.

Finally, mechanical properties in terms of hardness were deter-

mined. At least 8 indentations using a load of 10 g were performed on

the metallographic samples in cross-section using a Leica VMHT 30A

Vickers tester. Then, to have a clear interpretation of the measures the

average and the standard deviation of the results were calculated.

3. Results and discussion

3.1. Thermal study of the polyimide matrix

ThePI matrixis a fully imidized solution containing 25%of polyimide

and 75% of NEP solvent, provided by Evonik Industries GmbH, Austria.

The thermal study of the pure PI coating aims to validate the condition

of the coating after the elaboration process. From DSC tests conducted

Fig. 4. PI based composite coating microstructures: A) PI–20% PTFE; B) PI–20% PTFE–5%

SiC.

Table 1

Coating thicknesses and surface roughness measurements.

Coating Thickness (μ m) ± SD Roughness Ra (μ m) ± SD

PI 78 ± 3 0.03 ± 0.01

PI–20% PTFE 69 ± 7 5.2 ± 0.3

PI–20% PTFE–5% SiC 63 ± 2 4.8 ± 0.6

PI–30% PTFE 109 ± 8 9.1 ± 0.3

PI–30% PTFE–5% SiC 95 ± 3 7.5 ± 0.9

PI–40% PTFE 120 ± 7 13.2 ± 1.1

PI–40% PTFE–5% SiC 81 ± 7 11.6 ± 1.0

605C. Demian et al. / Surface & Coatings Technology 235 (2013) 603–610


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on a PI solution sample, the Tg evolution with the temperature is ob-

served. It serves as an indicator of mastery of the coating.

Several thermal events were observed on the DSC curve of the PI

coating presented in Fig. 1. The rst part of the curve corresponds to

thePI heat capacity. Then, twoexothermic eventsoccur at temperatures

of 128 °C and 410 °C, respectively. The second exothermal event which

could represent the expulsion of the residual solvent enclosed in the

polymer molecules is overlapped on the beginning of the degradation

process of the polymer. This process is possible due to the relaxation

of the molecular chainsduringthe glass transition phenomenon. Subse-

quently, an acceleration of heat ow can be observed.A similarthermal

event was observed on the DSC curve of PI ber [6].

In order to detect the position of the Tg on the DSC curve of the PI

coating, a complementary DSC analysis was performed on a PI solution

sample. Fig. 2 shows the Tg evolution with the temperature. During the

heating several isothermal holdings were performed as follows:

150 min at 80 °C, 80 min at 250 °C and 300 °C, and 20 min at 350 °C

and 400 °C. It can be observed that the Tg points follow a logarithmic

evolution with the temperature increase, but a logarithmic trendline

cannot be drawn due to point displacements. The increase of Tg with

temperature is due to the solvent expulsion which, at high tempera-

tures, acts as a plasticizer.

Therefore, associating these results with the DSC curve presented in

Fig. 1, the position of the Tg point of the pure PI coating can be dened

around the temperature of 263 °C. Besides, using a fully imidized PI

solution no melting point was observed on the DSC curve as shown inFig. 1.

3.2. Composite coating morphology

The composite coatings with different ller concentrations presented

similar morphology. For example, Fig. 3 presents the surface morphology

of PI–20% PTFE–5% SiC coating. PTFE and SiC particles appear quite

homogeneously dispersed into the PI matrix with respect to the

different particle diameters: about 30 and 1.7 μ m (particle dimensions

between 0.4 and 6.5 μ m) for the PTFE and SiC particles, respectively.

The cross-section microstructuresof the PI basedcomposite coatings

are shown in Fig. 4. A relative homogenous distribution of llers into PI

matrix can be distinguished. PTFE content does not affectthe composite

coating morphology (Fig. 4A). When SiC particles are added, a homoge-

neous distribution of the reinforcing agents can be observed (Fig. 4B).

The composite coating thickness and roughness are listed in Table 1.

Both thickness and roughness increase with the ller content. The PI

coating has a very smooth surface with an average roughness Ra of

0.03 μ m compared with the PI based composites. Thesurface roughness

considerably increases with the increase of the PTFE particle content in

PI matrix. Theaddition of SiCparticleshas a positive effecton thesurface

roughness for all composite coatings. When a mass amount of PTFE

above 20% is added, the composite coating thickness increases from

69 to 109 and 120 μ m corresponding to PI–30% PTFE and PI–40% PTFE,

respectively. Moreover, the roughness also increases from 5 μ m corre-

sponding to the PI–20% PTFE to 9 and 13 μ m corresponding to PI–30%

PTFE and PI–40%PTFE, respectively. The addition of SiC particles slightly

reduces the thickness and roughness values of the composite coatings

lled with 20, 30 and 40 wt.% PTFE.

3.3. Evolution of the surface properties of coatings

The measured contact angles and surface energies of the PI and PI

based composite coatings are listed in Table 2. The wetting occurs if

the contact angle θ between the water droplet and the surface is lessthan 90°. Consequently the surface is consideredhydrophilic. Converse-

ly, non-wetting occurs if the contact angle θ is higher than 90° and the

surface is considered hydrophobic. For the pure PI coating the water

contact angle has a value of 73°. As a result,the surface is considered ei-

ther wetting or hydrophilic. Adding PTFE and SiC reinforcing agents the

angle increases up to 98° and the surface becomes hydrophobic for PI–

40% PTFE and PI–40%PTFE–5% SiCcoatings.Besides, thetotal surface en-

ergy is directly related to the wettability and represents an important

parameter which can strongly affect the surface properties. The total

Table 2

Contact angles and total surface energies of PI and PI composite coatings.

Material c oa tings C onta ct angle, θ (°) Total surface energy (mN/m)

H2O (water) C2H6O2 (ethylene-glycol) C10H7Br (α-bromonaphthalene) γsP γs

D γsT

PI 73.1 39.7 8.8 5.1 41.2 46.4

PI–20% PTFE 80.5 57.7 42.1 4.6 31.0 35.6

PI–20% PTFE–5% SiC 79.1 60.3 41.0 5.0 30.4 35.4

PI–30% PTFE 83.9 62.9 44.7 3.8 29.5 33.2

PI–30% PTFE–5% SiC 87.5 69.4 54.6 3.5 25.1 28.6

PI–40% PTFE 97.9 76.9 54.9 0.9 25.5 26.5

PI–40% PTFE–5% SiC 98.1 72.4 49.0 0.6 29.1 29.7

0

5

10

15

20

25

30

H V 0 . 0 1

Fig. 5. Vickers microhardness of PI and PI based composite coatings.

F r i c t i o n c o e f f i c i e n

t µ

Sliding distance d (m)

F = 5 N; v = 0.6 m/s

PI PI-20%PTFE-5%SiC PI-30%PTFE-5%SiC

PI-40%PTFE-5%SiC PI-20%PTFE PI-30%PTFE

PI-40%PTFE

PI coating

PI based composite coatings

0

0.05

0.1

0.15

0.2

0.250.3

0.35

0.4

0 200 400 600 800 1000

Fig. 6. Friction coef cient of PI and PI based composite coatings depending on sliding

distance.



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surface energy decreases with thePTFE content. TheSiC content hasless

inuence on the total surface energy of the composite coatings. A minor

reduction of the total surface energy can be observed in the case of PI–

20% PTFE–5% SiC andPI–30% PTFE–5% SiC, respectively. An exception

can be noticed in the case of PI–40% PTFE, where adding the SiC

particles slightly increases the total surface energy.

3.4. Mechanical behavior

The mechanical behavior of the PI and PI based composite coatings

was determined by microhardness measurements. The HV 0.01 experi-

mental results are listed in Fig. 5.

PI coating microhardness reached a value of about 27 HV 0.01 which is

higher than that of the PI based composite coatings (Fig. 5). If differentwt.% of PTFE particles are added into the polyimide matrix a signicant

decrease in microhardness of the composite coatings was observed.

However, adding SiC particles into the PI–PTFE matrix the microhardness

of the PI–PTFE–SiC composite coatings slightly increases.

3.5. Tribological behavior

The tribological tests were conducted at room temperature and lab

air environment. For an applied load of 5 N the sliding speeds of 0.4,

0.6 and 0.8 m/s were selected to investigate their inuences on thefric-

tioncoef cients and wear rates.For every test, the ball slides 1000 m on

the coating surface. Fig. 6 illustrates the friction coef cient curves of the

PI and PI based composite coatings versus sliding distance. Generally,

the friction coef cient of composite coatings has a relatively stablevalue from the start of the sliding track.

Table 3 illustrates the data input for the friction tests as well as the

average friction coef cients obtained for all composite coatings and

the calculated wear rates.

PI coating has a friction coef cient of 0.34 which is approximately

two times higher than that of the PI based composite coatings. For

each PI composite coating, the friction coef cient decreases slightly

along with sliding velocity increase but its value varies over a small

range. Forexample, in thecase of PI–20% PTFE–5% SiC, the friction coef-

cients have the values of 0.18 and 0.17 for the sliding velocities of 0.4

and 0.6 and 0.8 m/s, respectively. Besides, depending on the ller con-

tent, the composite coatings have different friction behaviors. From

the results listed in Table 3, it can be noticed that the addition of PTFE

into the PI matrix leads to a friction coef cient decrease in comparison

with the pure PI coating. For example, adding 20 wt.% of PTFE in the PI

matrix the friction coef cient decreases from 0.34 to 0.16 for a sliding

velocity of 0.6 m/s. Along with the increase of PTFE content into the PI

matrix, the friction coef cients vary in a small range for PI–30% PTFE

and PI–40% PTFE. By theaddition of SiC particles into the PI–PTFE matrix

a slight increase of the friction coef cient can be observed. Similar

friction coef cient of 0.15 was reported for PI–15% PTFE composite

coating, but under harder conditions (sliding velocity of 1.4 m/s and

an applied load of 200 N) [18].The wear dependence of PI and PI based composite coatings on slid-

ing velocity is illustrated in Fig. 7. The wear rate of PI coating is lower

than that of PI–PTFE and PI–PTFE–SiC composite coatings. Besides, for

all composite coatings the wear rate decreases with the sliding velocity

increase. An exception can be noticed in the case of PI coating, whose

wear rate increases with the sliding velocity increase, e.g. from a wear

rate of 0.42 to 0.71 mm3/N·m for sliding velocities of 0.4 and 0.6 m/s,

respectively. For the PI based composite coatings, the wear rate in-

creases along with the PTFE content. For example, at a sliding velocity

of 0.6 m/s, the wear rate of PI–20% PTFE coating has a value of

0.58 mm3/N·m which increases at 2.99 and 4.19 mm3/N·m for the

PI–30% PTFEand PI–40% PTFE, respectively. The PI–40% PTFE composite

coating reaches a maximum wear rate of 14 mm3/N·m for a sliding ve-

locity of 0.4 m/s. In general, for all composite coatings, the addition of

SiC ller leads to a wear rate decrease. For example, in the case of PI–

20% PTFE matrix, the addition of SiC particles led to a slight decrease

of the wear rate from 0.56 to 0.52 mm3/N·m. This difference is

highlighted with the increase of PTFE content.

When developing wear protection coatings for tribological applica-

tions, the total surface energy also plays an important role. Fig. 8a and

b illustrates the linear regression of wear rate depending on total sur-

face energyas well as wear rate dependingon microhardness. Thelinear

regression curve of wear rate depending on total surface energy is illus-

trated in Fig. 8a. The addition of PTFE llers results in a wear rate

increase and a surface total energy decrease. Besides, with the addition

of SiC particles, both wear rate and total surface energy decrease. The

linear regression presented in Fig. 8b shows that except the PI coating,

the composite coatings with a relatively high hardness exhibit less

wear, especially for the composite coatings that contain SiC particles.

3.6. Thermogravimetric analysis of the pure PI and PI based composite

coatings

Thermal stability of PI and PI based composite coatings were evalu-

ated by TGA (Fig. 9). The pure PI coating did not lose any signicant

mass, other than due to the loss of moisture or the residual solvent

expulsion, until about 400 °C. For example, an amount of 1.4%moisture

evaporation occurred at a temperature of 93 °C and then, a weight loss

of 4% at a temperature of 378 °C.Above 400 °C, the degradation process

started slowly, and an offset point can be observed at a temperature of

569 °C. After that, the masslosswas muchfasterand by700 °C thecoat-

ing was completely burnt.

The TGA curves show that the composite coatings are more stablethan the pure PI coating. This may be due to the PTFE ller melting

point (about 330 °C) which occurs in the same temperature range as

the expulsion of solvent. On the other side, PTFE has a lower degrada-

tion temperature than the PI, resulting in the degradation of PI based

composite coatings. SiC ller was found as a residue but it does not

alter the degradation process of the composite coatings.

The real coating compositions were determined by TGA tests. Ac-

cordingly with TGA curves shown in Fig. 9, the proportion of different

components in the solidied coatings should be noted which are

displayed in Table 4.

Theproportions found in the coatingresidues contain a SiCller. Be-

sides, it is not possible to distinguish the proportion of PI and PTFE

which are two organic polymers that degrade in the same temperature

range. The only distinction that is possible to realize is that due to the

Table 3

Friction coef cients at different sliding velocities and an applied load of 5 N as well as a

calculated wear rate.

Sample Sliding velocity v

(m/s)

Average friction

coef cient μ ± SD

Wear rate w

(mm3/N·m)

PI 0.4 0.38 ± 0.08 0.42

0.6 0.34 ± 0.03 0.71

PI–20% PTFE 0.4 0.16 ± 0.01 0.67

0.6 0.16 ± 0.01 0.58

0.8 0.16 ± 0.01 0.56PI–20% PTFE–5% SiC 0.4 0.18 ± 0.01 0.62

0.6 0.17 ± 0.01 0.57

0.8 0.17 ± 0.01 0.52

PI–30% PTFE 0.4 0.16 ± 0.01 13.03

0.6 0.15 ± 0.01 2.99

0.8 0.15 ± 0.01 1.71

PI–30% PTFE–5% SiC 0.4 0.18 ± 0.01 0.74

0.6 0.17 ± 0.01 0.61

0.8 0.17 ± 0.01 0.60

PI–40% PTFE 0.4 0.17 ± 0.02 14.02

0.6 0.16 ± 0.01 4.19

0.8 0.16 ± 0.01 2.34

PI–40% PTFE–5% SiC 0.4 0.18 ± 0.01 1.95

0.6 0.17 ± 0.01 1.47

0.8 0.17 ± 0.01 1.34



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high concentrationof PTFE in thecomposite coatings thethermal events

associated to PI aremasked by those associated to PTFE. This justiesthe

main studies concerning the thermal behavior of the pure PI coating.

3.7. Discussion

DSC analyses performed on the pure PI coating revealed a high Tg

point which along with the TGA results, indicate that the PI matrix is

thermally stable above 400 °C. Moreover, the analysis performed on a

PI solution sample demonstrated that the Tg increaseswith the temper-

ature increase and results in a thermallystable PI at an elevated temper-

ature. Still, a mass loss of PI coating was detected on the TGA curve

beginning with the temperature of 310 °C (Fig. 9) which could corre-

spond to the residual solvent evaporation. A complete desolventation

of the polyimide is hard to accomplish, because it is directly related to

the temperature andtime andwith a temperature increase,the polymer

degrades. If the heat treatment of the PI coating is performed at a tem-

perature above 350 °C the polymer could be damaged, but it contains

a less solvent amount, which could inuence the mechanical properties

of the composite coating matrix.

Concerning the stability of pure PI matrix in the absence of solvent,

an incorrectheat treatment which ended up with a lower Tg could affect

the properties of the nal coatings, e.g. a decrease of the mechanicalproperties of the nal composite coatings. The effect of degassing pro-

cess results in an effective process control for the drying procedure of

the PI–PTFE–SiC composite coatings.

Analyzing the results presented in Tables 2 and 3, it can be noticed

that low total surface energies lead to low friction coef cients. This is

dueto the additionof thePTFE ller whichis known for itslowtotal sur-

face energy. Thetotal surface energy decreases along with the PTFE con-

tent increase from 46 to 26 mN/m2 for PI and PI–40% PTFE coatings,

respectively. Moreover, along with total surface energy decrease the

friction coef cient decreases from 0.34 to 0.16 which corresponds to a

sliding speed of 0.6 m/s for PI and PI–40% PTFE coatings. However

when increasing the amount of PTFE ller into the PI matrix, the wear

rate increases quickly (PTFE presents a low hardness). For example, in

the case of a sliding velocity of 0.4 m/s, the wear rate of PI is0.42 mm3/N·m against 14.02 mm3/N·m for PI–40% PTFE. The addition

of SiC ller reduced thewearrate caused by theaction of strong cyclical

solicitationsand kept in thesame time thelow friction coef cients. Con-

sequently, the addition of 5 wt.% SiC particles into the fully imidized PI

resin demonstrated an improved mechanical behavior by a slight in-

crease in hardness. Moreover, the wear rate signicantly decreases in

the case of PI–30% PTFE–5% SiC with about 94% in comparison with

PI–30% PTFE or with 86% for PI–40% PTFE–5% SiC in comparison with

PI–40% PTFE. For PI–20% PTFE–5% SiC coating, the wear rate has a low

reduction of 8% in comparison with PI–20% PTFE. The PI coating demon-

strated a low wear rate because of its high hardness. From the point of

view of both friction and wear behaviors, it can be observed that the

addition of 20% PTFE and 20% PTFE + 5% SiC into PI matrix has the

best behavior in terms of a moderate friction coef

cient, a low wear

0

2

4

6

8

10

12

14

W e a r w ( m m 3 / N 3 m )

Sliding velocity v (m)

0.4 0.6 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8

Fig. 7. Wear rate of PI and PI composite coatings depending on sliding velocity.

0

1

2

3

4

5

0 10 20 30

W e a r r a t e w ( m m 3 / N · m )

HV0.01

0

1

2

3

4

5

20 30 40 50

W e a r r a t e w ( m m 3 / N · m )

Total surface energy sT (mN/m2)

a

b

Fig. 8. Linear regression curves of PI and PI based composite coatings: a) wear rate

depending o n total surface energy; b) wear rate depending on microhardness.



7/8

rate and an acceptable mechanical behavior. Under harder dry sliding

conditions, the addition of 15 wt.% PTFE and 10 wt.% CNT into a PI

matrix highly decreased the friction coef cient to 0.12 [18].

A correlationbetween the mechanical and tribological behaviors can

be noticed. The PI–PTFE–SiC composite coatings which demonstrated a

good hardness exhibit less wear. Another trend suggested by this study

is the possibility to use the PI–PTFE–SiC composite coatings for high

temperature applications, due to the thermal stability of the matrix.

Moreover, this is also underlined by the thermal stability of SiC and

PTFE particles like llers and their positive inuence on hardness, total

surface energy and friction coef cient of the composite coatings.

4. Conclusions

This study proposed to investigate the surface properties as well as

the tribological and mechanical behaviors of the PI–PTFE–SiC composite

coatings, especially the effect of PTFE and SiC reinforcing agents. In ad-

dition, DSC and TGA investigations of the pure polyimide solution sam-

ples and PI and PI based composite coatings were performed. Based on

the results obtained withrespect to PI composite coating investigations,

several conclusions can be made. PTFE addition has a positive inuence

on the surface properties of the PI composite coatings but negative with

respect to the mechanical and tribological behaviors in terms of hard-

ness and wear rate of the PI composite coatings. The addition of the

SiC particles has no inuence on the surface properties of the PI based

composite coatings but signicantly improves the mechanical and tri-

bological behaviors of the PI–PTFE–SiC composite coatings.

DSC analysis revealed a high Tg point of thepure PI coating. Besides,

the Tg of the pure polyimide solution increases with the temperature

increase.

Sessile drop methodinvestigations revealed that increasingthe PTFE

amount up to 30 wt.% the PI composite coatings have a hydrophilic

character then a hydrophobic character is noticed for the PI–40% PTFE

and PI–40% PTFE–5% SiC. The calculated total surface energy decreases

with the increase of the PTFE wt.%. Thus, PI–20% PTFE–5% SiC composite

coating demonstrated a lower total surface energythan thepure PI coat-

ing. Increasing the amount of PTFE to 30 and 40 wt.% and adding SiC

particles the total surface energy still decreases.

Moreover, the PTFE ller considerably decreases the friction coef -cient of the PI–PTFE–SiC composite coatings. Thereby, the addition of

PTFE signicantly contributes to the total surface energy decrease and

consequently to the reduction of the friction coef cient of the PI based

composite coatings. The hardness as well as the wear rate increases

with the increase of PTFE amount into the PI–PTFE composite coatings.

Adding 20 wt.% of PTFE ller into the pure PI matrix is suf cient to

have interesting results with respect to the tribological and mechan-

ical behaviors of the PI based composite coatings. Thus the PI–20%

PTFE composite coating revealed a low friction coef cient. By the ad-

dition of 5 wt.% SiC particles, the PI–20% PTFE–5% SiC composite

coating still presents a low friction coef cient, a slightly decreased

wear rate and a satisfactory hardness in comparison with the other

PI–PTFE–SiC composite coatings.

The TGA results showed that the PI coating is thermally stable at an

elevated temperature (above 400 °C). Moreover, the addition of PTFE

and SiC llers into pure PI matrix thermally stabilizes the PI–PTFE–SiC

composite coatings.

Acknowledgments

The authors are grateful to the company Evonik Industries-Evonik

Fibres GmbH, Austria for the polyimide P84 solution supply.

References

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1578.

Fig. 9. TGA curves of PI and PI composite coatings.

Table 4

Composition of the PI composite coatings, according to TGA results.

Initial composition of the

composite coatings

(Before the furnace drying)

Final composition of the

composite coatings

(After the furnace drying)

PI–20% PTFE PI–50% PTFE

PI–20% PTFE–5% SiC PI–44% PTFE–12% SiC







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Investigation of Surface Properties and Mechanical and Tribological

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