Post on 18-Mar-2020
transcript
141
CHAPTER 5
STUDIES ON MECHANICAL, THERMAL, WEAR AND
MORPHOLOGICAL BEHAVIOURS OF MOLYBDENUM
DISULPHIDE FILLED NYLON 66/CARBON
BLACK/COMPOSITES
In this study, the effect of different weight fractions of MoS2 viz., 0, 0.5, 1.0,
2.0 and 3.0 wt. % content on the nylon 66/carbon black (CB) composites are studied.
With an objective to investigate the influence of molybdenum disulphides (MoS2) in
the presence of CB on wear characteristics of nylon 66. Nylon 66 was compounded
with MoS2 and CB in a co-rotating twin screw extruder. The fabricated composites
were evaluated for the surface friction, wear, laser etching resistance along with
physico-mechanical, thermal and morphological characteristic. The wear behaviour of
nylon 66/CB/MoS2 composites were investigated under dry sliding conditions at
different normal loads, sliding distances and sliding velocities at room temperature.
Additional techniques such as SEM and optical microscopy were performed to probe
the wear mechanism. It was found that the introduction of MoS2 in the presence of CB
has certainly reduced the friction, wear behaviour of nylon 66 with improvement in
laser etching resistance. MoS2 could increase the adhesion between the transfer film
and the counterface surface. The ability of the synergistic fillers in helping the
formation of thin, uniform and continuous transfer film would contribute to the
increase in wear resistance of nylon 66 composites.
5.1. Introduction
Polymer composites have been increasingly used for numerous tribological
purposes such as seals, gears and bearings, providing light weight alternatives to
metallic components. The feature that makes polymer composites so promising in
industrial applications is the possibility of tailoring their properties with special fillers,
low cost of materials and quantum of production. Due to the low coefficient of
friction and also the ability to maintain loads, some specific grades of polymer are
used in place of the traditional metal based materials in recent times [1,2].
Furthermore polymer gears and bearings can accommodate shock loading, shaft
142
misalignment and bending better than metal parts. Polyamides (PAs) and polyacetals
are widely used thermoplastic polymers for such applications. Polyamides such as
PA6 and PA66 are engineering thermoplastics which have found great attractions in
such applications due to their desirable mechanical properties, suitable tribological
characteristics and ease of processing.
It was reported that the friction and wear behaviour of nylons was fairly
satisfactory under dry sliding conditions and lubrication at higher speeds. However, in
order to keep pace with the modern technological innovations, ever increasing
demands are being placed on tribo-materials for enhanced performance for operating
under stringent conditions of loads, speeds, temperatures and hazardous
environments. In order to enhance the tribological characteristics of nylons efficiently,
solid lubricants may be added into the polymer matrix. A solid lubricant is defined as
a material that provides lubrication, under essentially dry conditions, to two surfaces
moving relatively to each other. The solid lubricants often lead to decrease of friction
coefficient and wear rate through the reduction in adhesion with the counterface or
creation a transfer film with a low shear strength at the interface [3,4]. A number of
scientists are reported on the improvement in wear resistance of various nylons such
as nylon 66, nylon 6, nylon 11, nylon 46, etc., with solid lubricants such as
polytetrafluoroethylene (PTFE), graphite (Gr) and MoS2 [5,6]. These are the pre-
dominant materials used as solid lubricants in thermoplastics.
Among these solid lubricants, MoS2 is a common dry lubricant in the form of
black crystalline powder and it has a layered structure. In its appearance and feel,
MoS2 is similar to graphite. It is a dark blue-grey or black solid, which feels slippery
or greasy to the touch. It has hexagonal layer lattice. It is widely used as a solid
lubricant because of its low friction properties, sometimes to relatively high
temperatures. MoS2 with particle sizes in the range of 1–100 µm is a common dry
lubricant. MoS2 is often a component of blends and composites where low friction is
sought. When added to plastics, MoS2 forms a composite with improved strength as
well as reduced friction. Polymers that have been filled with MoS2 include nylon.
MoS2 and graphite have a layered structure. The important effect is that the materials
can shear more easily parallel to the layers than across them. They can therefore
support relatively heavy loads at right angle to the layers while still being able to slide
easily parallel to the layers. This property is being effectively used for lubrication
process. The coefficient of friction is more or less equal to the shear stress parallel to
143
the layers divided by the yield stress or hardness perpendicular to the layers. Because
the low friction only occurs parallel to the layers, it follows that these solid lubricants
will only be effective when their layers are parallel to the direction of sliding. It is
also important that the solid lubricant should adhere strongly to the bearing surface;
otherwise it would be easily rubbed away and gives very short service life.
Many researchers have studied the friction and wear behaviour of MoS2 filled
polymers. Yinping Ye et al [7] have reported that high load and frequency promote
the formation of a compact transfer films. The compact transfer films are believed to
be the predominant mechanism giving rise to high load-carrying capacity, and
excellent wear-resistance performances of the bonded MoS2 solid film lubricants
[7,8]. Liu et al [9] pointed out that MoS2 was not very effective for reducing friction
and caused an increase in wear of nylon 6, while Steinbuch [10] reported that MoS2
filled nylon could reduce the wear rate but not its friction coefficient. Bijwe et al [11]
proposed that the addition of MoS2 alone did not impart a good wear resistance to
PTFE, especially during severe conditions of sliding. However, addition of lubricating
MoS2 in PTFE matrix and alone with reinforcing phase (glass fibers) had the potential
to reduce the abrasive wear by maintaining a low friction transfer film. This film
would have been disrupted by the glass fiber if only PTFE were present [12].
The objective of the present work was to look at the effects of the transfer film
and the tribo-chemical reactions between MoS2 filler and carbon black on the
tribological properties of nylon, in order to find out the important factors for reducing
wear and friction of nylon 66. Nylon 66 was compounded with MoS2 and CB. Carbon
black is a form of amorphous carbon that has a high surface area to volume ratio. It is
used as pigment and filler in rubber products, especially tires. Compounding of CB
with rubber improves the tensile strength and wear resistance, but it is not much
explored in the tribological performances of thermoplastics like nylon. Under these
circumstances, nylon 66 was compounded with varying amounts of, i.e., 0.5, 1.0, 2.0
and 3.0 wt%, MoS2 with one weight percentage of CB. Thus the fabricated nylon
66/CB/MoS2 composites have been evaluated for friction, wear, scratches and
morphological characteristics along with thermal and physico-mechanical properties.
The wear characteristics of the composites were evaluated using pin-on-disc
equipment.
144
5.2 Compounding and specimen preparation
CB powder (1 wt %) was premixed with varying amounts viz., 0.5, 1, 2 and 3
wt % of MoS2 powder and then mixed with nylon 66 in tumbling mixer for 15 min,
after pre-drying in hot air oven at 80°C for 8 h duration, and then melt blended using
a co-rotating intermesh twin screw extruder at a screw speed of 175 rpm with barrel
temperature ranging from 200 to 280 ºC. The extruder consists of nine nozzles and the
temperature zones maintained at each of the nozzles are different and lay in the range
200-280 o
C. The extrudate strand was pelletized and stored in sealed packs containing
desiccant. The test specimens for tensile behaviours, impact strength, and water
absorption were prepared using an R.H. Windsor India, SD-75 automatic injection
molding machine with 70 ton clamping pressure at 260-285 oC and an injection
pressure of 80 bars. After molding, the test specimens were conditioned at 23 ± 2 oC
and 50 ± 5% RH for 40 h according to ASTM D 618 prior to testing.
5.3 Results and Discussion
5.3.1 Physico - mechanical properties
The prepared nylon 66/CB/MoS2 composites were characterized for physico –
mechanical properties according to ASTM methods. The measured physico-
mechanical properties such as water uptake, density, void content and surface
hardness for nylon 66/CB/MoS2 are given in Table 5.1.
5.3.1.1 Water uptake behaviour
The results shown in Table 6.1 indicates that the incorporation of MoS2 has
reduced the water uptake from 2.7 (0.5% MoS2) to 2.1% (3.0% MoS2). When working
with nylons, it is important to examine the water absorption, because it severely
affects its mechanical and thermal properties.
5.3.1.2 Density
The density measurements were performed on all composites of nylon
66/CB/MoS2. The density values of nylon 66/CB/MoS2 composites falls in the range
1.136– 1.148g/cc. Table 5.1 shows that the density of composites increased linearly
with increase in MoS2 content. MoS2 being a higher dense material, the composite
material density increases with increase in MoS2 content.
145
Table 5.1. Physical properties of nylon 66/CB/ MoS2composites
MoS2 (%) Water
uptake (%)
Density (g/cc) Void content
(%)
Surface
hardness
(Shore D) Expt. Theo.
0 2.7 1.136 1.144 0.72 79
0.5 2.6 1.140 1.149 0.75 73
1.0 2.4 1.142 1.153 0.96 69
2.0 2.3 1.144 1.162 1.55 64
3.0 2.1 1.148 1.171 1.98 62
5.3.1.3 Surface hardness
Surface hardness is a measure of resistance to indentation. Surface hardness
indicates the degree of compatibility to certain extent. The surface hardness values of
nylon 66/CB/MoS2 composites falls in the range 79 - 62 shore D (Table 5.1). From the
table it is noticed that a significant decrease in surface hardness values with increase
in MoS2 content.
Table 5.2. Mechanical properties of nylon 66/CB/ MoS2composites
MoS2
content(%)
Tensile
strength (σ)
(MPa)
Tensile
modulus
(MPa)
Elongation
at break,
(e) (%)
Product
Parameter
(σ x e)
Impact
strength
(J/m)
0 65 2670 10.3 669 37.2
0.5 68 2712 13.2 897 38.4
1.0 73 2814 16.7 1219 41.4
2.0 76 2944 20.8 1581 42.5
3.0 78 2975 22.5 1755 43.2
5.3.1.4 Tensile behaviour
From the Table 5.2, it is noticed that tensile strength and percentage
elongation at break increases with increasing in MoS2 content. The tensile strength of
nylon 66/CB/MoS2 lays in the range 65 – 78 MPa. Similarly, percentage elongation at
break lies in the range 10.3 - 22.5 % for MoS2 filled nylon 66/CB composites. The
tensile modulus of the composites increased from 2670 to 2975 MPa. These results
146
indicate that the tensile properties increased significantly with increase in MoS2
content in nylon 66/CB matrix.
5.3.1.5 Impact strength
The impact strength of the composites depends upon many factors like
toughness properties of the polymers, the degree of miscibility and phase
morphology. The nature of the interface region is of extreme importance in
determining the toughness of the composites. Table 5.2 reveals increase in izod
impact strength from 37.2 to 43.2 J/m.
5.3.2 Thermal behaviours
5.3.2.1 Differential scanning calorimetry
The thermal properties of the nylon 66/CB/MoS2 composites were
investigated by DSC technique to analyze the effect of MoS2 content on the
crystallization melting temperature, enthalpy of melting and percentage of
crystallisation and shown in Figure 6.1.
Figure 5.1. DSC thermograms for nylon 66/ CB/MoS2 composites
147
The data obtained from DSC analysis are given in Table 6.3. Table reveals
that the onset of Crystallization temperature (To), peak melting temperature of
crystallization (Tm) and completion of crystallization temperature (Tc) are decreased
linearly with increase in MoS2 content. Similarly the enthalpy of crystallization (∆H)
and percentage crystallinity (χc) also decreased from 75.1 to 58.6 J/g and from 38.3 to
29.9% respectively with increase in MoS2 content. The above results clearly show that
the incorporation of MoS2 particles gone in-between the molecular layers of nylon 66
and reduced the crystalline behaviour of nylon 66. The reduction in ∆H and χc values
is due to the plasticization effect of MoS2.
Table 5.3. Thermal data (glass transition temperatures) obtained from DSC
thermograms for nylon 66/ CB/MoS2 composites
Composition
of MoS2 in
nylon 66/CB/
MoS2
composites
(%, wt/wt)
To
(ºC)
Tm
(ºC)
Tc
(ºC)
H (J/g) Crystallinity* χc
(%)
Exp. Cal. Dev.
(%) Exp. Cal.
Dev.
(%)
0 243 265 278 75.1 - - 38.3 - -
0.5 242 264 277 73.0 74.7 -2.4 37.2 38.1 -2.4
1.0 241 263 272 70.6 74.3 -5.3 36.0 37.9 -5.3
2.0 240 263 271 65.4 73.6 -12.5 33.4 37.6 -12.5
3.0 239 262 270 58.6 72.8 -24.3 29.9 37.2 -24.3
*The heat of fusion value of 100% crystalline nylon 66 is 196 J/g.
5.3.2.2 Dynamic mechanical analysis
Dynamic mechanical analysis (DMA) is one of the most appropriate methods
to study viscoelastic behaviour and relaxations in polymeric materials. The Tg is a key
process in most polymers and influences use and processability of the material,
possibly more than any other factor. This technique provides very revealing
information about these relaxations through the tan δ vs. temperature data. Tan δ is an
important parameter characterizing material’s viscoelastic behaviour [36]. The same
experiment also yields the stiffness (modulus) of the material versus temperature. The
DMA properties such as storage modulus (E'), loss modulus (E'') and loss tangent
148
(tanδ) are recorded as a function of temperature from 25 to 200°C are shown in
Figures 5.2, 5.3 and 5.4 respectively.
Figure 5.2. Plots of storage modulus versus temperature for nylon 66/CB/MoS2
composites
From the thermograms, it can be noticed that, the incorporation of MoS2 into
nylon, causes remarkable increase of E’ and E'' values as compared to pure nylon 66.
This indicates that the increase in MoS2 content in nylon 66 increases the energy of
dissipation. The obtained tan δ values (both predicted and experimental) from DMA
thermograms for nylon 66/CB/MoS2 composites along with Tg which was represented
by the peak temperature of the tan δ curve is addressed in Table 5.4.
Table 5.4. Data obtained from DMA analysis for nylon 66/CB/MoS2 composites
MoS2
wt. %
Tan δ Tg
Storage modulus (MPa)
Exp. Cal. Glassy region Rubbery region
0 0.088 - 64.8 903 361
0.5 0.087 0.087 65.1 954 387
1.0 0.085 0.086 65.6 1011 405
2.0 0.082 0.085 63.7 1095 406
3.0 0.080 0.084 63.0 1127 441
The storage modulus of nylon 66 measured at 40 °C was 903 MPa and it
increases after incorporating MoS2 into nylon matrix. The maximum storage modulus
value was noticed to be 1127 MPa for 3 % MoS2 loaded nylon 66/CB composites,
149
which is about 25 % higher than that of nylon 66. The results obtained in this study
are comparable to the literature data [37]. Table 5.4 also lists the average values of Tg
for different wt. % of MoS2. The measured Tg values of nylon 66/CB/MoS2
composites are in the range 65-63°C.
Figure 5.3. Plots of loss modulus versus temperature for nylon 66/CB/MoS2
composites
Figure 5.4. Plots of loss tangent versus temperature for nylon 66/CB/MoS2
composites
5.3.2.3 Thermo gravimetric analysis
TG analysis was carried out in order to understand the influence of MoS2
addition on the thermal stability of the nylon 66/CB/MoS2 composites. The TGA
thermograms of MoS2 loaded nylon 66/CB composites are shown in Figures 5.5 along
with inset thermograms. The temperature range of thermal degradation was analyzed
150
from the TGA thermograms and is given in Tables 5.5. TGA thermograms of the
composites indicate only single stage thermal degradation process. The decomposition
temperature of composites was started at 334°C and takes place upto 710°C, which
corresponds to the weight loss ranging from 95.9 to 98.2 %. From TGA curves it can
be clearly observed that the thermal stability markedly improved with increase in
MoS2 content.
Figure 5.5. TGA thermograms for nylon 66/ CB/MoS2 composites
Some characteristics TGA data related to the temperature corresponding to
weight loss such as T0 (temperature of onset decomposition), T10 (temperature for 10
% weight loss), T20 (temperature for 20% weight loss), T50 (temperature for 50 %
weight loss) and Tmax (temperature for maximum weight loss) are the main criteria to
indicate their thermal stability of the composites (Table 5.5). Higher the values of T10,
T20, T50 and Tmax higher will be the thermal stability of the composites [13].
Figure 5.5 and Table 5.5 data reveals that the initial stage thermal degradation
process pattern is almost same for all nylon 66/CB/MoS2 composites. Higher the
values of oxidation index (OI), higher will be the thermal stability [13-16]. From the
table it was observed that the OI values increases with increase in MoS2 content and it
lies in the range 0.178 - 0.476 (Table 5.5). This data indicates that the nylon
66/CB/MoS2 composites are more thermally stable.
151
Table 5.5. Thermal data obtained from TGA thermograms of nylon 66/
CB/ MoS2 composites
MOS2 content in nylon
66/CB composites
(%, wt.)
Temperature at different weight loss (± 2ºC) Oxidation
Index
(OI) T0 T10 T20 T50 Tmax
0 334 407 423 440 641 0.178
0.5 335 408 424 443 692 0.239
1.0 336 409 430 450 694 0.288
2.0 339 410 432 452 701 0.389
3.0 349 413 436 457 710 0.476
5.3.3 Wear studies
In order to improve the friction and wear behaviour of polymeric materials,
one typical concept is to reduce their adhesion to the counterpart material and to
enhance their hardness, stiffness and compressive strength. This can be achieved quite
successfully by using special fillers. To reduce the adhesion, internal lubricants are
frequently incorporated. One of the mechanisms of the corresponding reduction in the
coefficient of friction is the formation of transfer film on the surface of the
counterpart [17]. The wear resistance is increased when fillers decompose and
generate reaction products which enhance the bonding between the transfer film and
the counterface [18], MoS2 is one such filler and lubricant that will decompose and
produce MoO3, FeS, FeSO4 and Fe2(SO4)3 during sliding. These compounds could
increase the adhesion between the transfer film and the counterface surface.
Normally the matrix should possess a high temperature resistance to withstand the
high heat generated during the frictional dry sliding. Additional fillers that enhance
the thermal conductivity are often of great advantage, especially if effects of
temperature enhancement in the contact area must be avoided in order to prevent an
increase in the specific wear rate. Carbon black is one such conductive filler which
forms conductive pathways in the polymer matrix to dissipate the heat due to
frictional sliding. Here we have utilized both MoS2 and CB to have synergistic effect
152
in formation of thin, uniform and continuous transfer film would contribute to the
increase in wear resistance of nylon composites.
5.3.3.1 Wear loss
The plots of wear loss as a function of MoS2 content (0, 0.5, 1, 2, and 3 wt %)
at different applied loads (50, 100 and 150N) and sliding distances (1000, 1500 and
2000m) for all nylon 66/CB/MoS2 composites are shown in Figures 5.6(a)-(c). The
plots of wear loss as a function of loads, sliding distances and sliding velocities are
shown in Figures 5.7-5.9 respectively. All the plots indicate that wear loss decreases
with increase in MoS2 content. That means pristine nylon 66 has more wear loss than
blends containing MoS2 content for all loads investigated. It is clearly evident from all
the plots that the MoS2 content has significant influence on the wear behaviour of the
composites.
Figure 5.6. Weight loss as a function of MoS2 content for nylon 66/CB
composites at varying sliding distances and at varying loads; (a) 50 N, (b) 100 N
and (c) 150 N
Figure 5.7. Weight loss as a function of load for MoS2 filled nylon 66/CB
composites at sliding distances of 1000 m, 1500 m and 2000 m
153
Figure 5.8. Weight loss as a function of sliding distance for nylon 66/CB/ MoS2
composites at varying loads; (a) 50 N, (b) 100 N and (c) 150 N
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0 0.5 1 2 3
Wei
ght
loss
(g)
MoS2 (%)
50 N 5 m/s
7
9
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0 0.5 1 2 3
Wei
ght
loss
(g)
MoS2 (%)
100 N 5 m/s
7
9
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0 0.5 1 2 3W
eigh
t lo
ss (g
)
MoS2 (%)
150 N 5 m/s
7
9
Figure 5.9. Weight loss as a function of MoS2 content for nylon 66/CB
composites at varying sliding velocities and at varying loads; (a) 50 N, (b) 100 N
and (c) 150 N
The improvement in wear resistance is due to the presence of solid lubricant
(MoS2) dispersed in the polymer matrix. These MoS2 dispersed in the polymer matrix
acts as barrier and also prevent large scale fragmentation of nylon 66/CB matrix. This
behaviour is clearly observed from SEM pictures. The MoS2 also acts as reinforcing
element, bears the load and reduces the wear rate. The wear resistance is maximum
for the inorganic MoS2 content of 3 wt. %. The wear loss of MoS2 filled nylon 66/CB
composites exhibiting an inverse relationship with all the parameters as the inorganic
filler content MoS2 goes on increases in the composites. It was reported that addition
of MoS2 with the base polymers lowers the wear rate when tested under unlubricated
dry conditions [7-8, 19]. When polymers slide against metal counterfaces, transfer
films are formed and the wear behaviour of a polymer in dry sliding condition is
strongly influenced by its ability to form a transfer film on the counterface [20-21].
The transfer films provide a shielding of the soft polymer surface from the hard metal
154
asperities [21-26]. This study indicates that anti-wear properties of nylon 66/CB
composites are improved markedly after incorporation of MoS2.
5.3.3.2 Specific wear rate
Specific wear rate as a function of MoS2 content (0, 0.5, 1, 2, and 3 wt %) at
different applied loads (50, 100 and 150N) and sliding distances (1000, 1500 and
2000m) for all nylon 66/CB/MoS2 composites are shown in Figures 5.10(a)-(c).
Specific wear rate as a function of loads, sliding distances and sliding velocities are
given in 5.11-5.12(a)-(c) respectively. In all these plots, specific wear rate is inversely
proportional to MoS2 content, load and sliding distance. It is evident that the increase
of MoS2 content from 0 to 3 wt % led to a remarkable decrease of specific wear rate.
This kind of variation has been reported for glass fabric reinforced epoxy composites
[27]. It is evident that the increase of MoS2 content from 0.5 to 3 wt % in nylon led to
a remarkable reduction in specific wear rate of the composites. Similar kind of
variation has been reported elsewhere for glass fabric reinforced epoxy composites
[28].
Figure 5.10. Specific wear rate as a function of MoS2 content for nylon 66/CB
composites at varying loads; (a) 50 N, (b) 100 N and (c) 150 N
Figure 5.11. Specific wear rate as a function of sliding distance for nylon 66/CB/
MoS2 composites at varying loads; (a) 50 N, (b) 100 N and (c) 150 N
155
Figure 5.12. Specific wear rate as a function of load for nylon 66/CB/
MoS2composites at sliding distances of 1000 m, 1500 m and 2000 m
The order of wear resistance behaviour of composites is as follows; 3 > 2 > 1
> 0.5 % by weight of MoS2. Also, it was noticed that 3 wt % MoS2 filled composites
exhibited the higher wear resistance under all sliding distances/loads. The reduction in
specific wear rate with increase in MoS2 content in nylon 66 matrix is due to the
transfer film formed on the counterface, which act as effective barrier to prevent
large-scale fragmentation of polymer. It is well known that the wear behaviour of a
polymer sliding against a metal is strongly influenced by its ability to form a transfer
film on the counterface [6].
5.3.3.3 Co-efficient of friction
The 'coefficient of friction' (COF), µ, is a dimensionless scalar value which
describes the ratio of the force of friction between two bodies and the force pressing
them together. The coefficient of friction depends on the materials used. A common
way to reduce friction is by using a lubricant, the science of friction and lubrication is
called tribology. One of the mechanisms of the corresponding reduction in the
coefficient of friction is the formation of a transfer film on the surface of the
counterpart [17]. It is also important that the solid lubricant should adhere strongly to
the bearing surface; otherwise it would be easily rubbed away and gives very short
service life. Yinping Ye et al [7] have reported that high load and frequency promote
the formation of a compact transfer films. The compact transfer films are believed to
be the predominant mechanism giving rise to high load-carrying capacity, and
excellent wear-resistance performances of the bonded MoS2 solid film lubricants
[7,8]. The variation in coefficient of friction as a function of MoS2 at 5 m/s sliding
velocity for MoS2 filled nylon 66/CB composites is tabulated in Table 5.6. The co-
156
efficient of friction decreases with increase in MoS2 content. This behaviour is
obviously relevant to the lubricating effect of MoS2 which can reduce the adhesion
between the composite with the metallic counterpart. MoS2 is composed of sheets and
layers. The layers themselves are strong but the bonding between the layers is weak.
Consequently MoS2 is strong in compression but weak in shear. This is advantageous
for producing low friction [29-34]. As the real area of contact and shear strength of
polymer substrate changes during sliding, the coefficient of friction increases with
increase in sliding load. Similar trends were observed at other sliding distances and
velocities investigated during the current studies.
Table 5.6. Coefficient of friction for nylon 66/ CB / MoS2 composites at 5 m/s
velocity
Load
(N)
Sliding
length
(m)
Co-efficient of friction for nylon 66/ CB / MoS2 composites
with varying MoS2 (%)
0 0.5 1.0 2.0 3.0
50
1000 0.169 0.157 0.154 0.144 0.132
1500 0.200 0.190 0.185 0.179 0.170
2000 0.219 0.207 0.199 0.188 0.181
100
1000 0.185 0.179 0.168 0.155 0.145
1500 0.231 0. 220 0.211 0.199 0.189
2000 0.245 0.238 0.229 0.216 0.199
150
1000 0.211 0.194 0.181 0.174 0.165
1500 0.248 0.237 0.220 0.215 0.210
2000 0.264 0.250 0.242 0.229 0.221
5.3.3.4 Surface morphology of worn surfaces
Scanning electron microscopic images are used for correlating the wear data.
Figures 5.13.(a)-(e) indicate the positive effect of MoS2 content on the worn surface
of the nylon 66/CB composites. The worn surface of 0 wt. % MoS2 filled nylon 6/CB
composites is relatively rough with more matrix damage and wear tracks compared to
other loadings (1.0, 2.0 and 3.0 wt MoS2). It can be seen from Figure 5.13(a) that the
ploughed marks appeared on the rubbing surface of composite. Figure 5.14 shows the
SEM images of MoS2 filled nylon 66/CB composites at 1000 m sliding distance, 50
N load at (a) 5 m/s and (b) 9 m/s.
157
Figure 5.13. SEM images of nylon 66/CB composites at 5 m/s sliding velocity for
1000 m sliding distance and with varying amounts of MoS2 content;
(a) 0 %, (b) 0.5 %, (c) 1.0 %, (d) 2 % and (e) 3 %.
A close examination of SEM images reveals that even though both are
looking similar, but more surface damage and wear track is oberserved for higher
speed (9 m/s) than that is observed for lower speed (5 m/s). Similarly more sliding
distance (2000 m) shows more damage to the wear surface and breaking of
delaminated layers than that is observed for lower sliding distance as seen in Figure
5.15 (a) -(b). Similar results are observed at higher loads (150N) (Figure 5.16(a)–(b)).
From above four SEM images it can be conclueded the addition of MoS2 protects the
wear damage to the polymeric surface by forming a barrier film, however, both
a b
d c
e
158
increase in load and sliding distances does not prevent the surface damage or surface
delamination
Figure 5.14. SEM images for MoS2 filled nylon 66/CB composites at 1000 m
sliding distance, 50 N load at (a) 5 m/s and (b) 9 m/s.
Figure 5.15. SEM images for MoS2 filled nylon 66/CB composites at 5 m/s
sliding velocity, 100 N load (a) 1000 m and (b) 2000 m.
Figure 5.16. SEM images of nylon 66/CB/MoS2 composites at 5 m/s sliding
velocity, 150 N load (a)1000 m and (b) 2000 m.
b
a b
a
159
5.3.4 Regression analysis
The wear model for the tested materials was developed based on the applied
load, sliding velocity and sliding distance. The process parameters for the purpose of
analysis are shown in Table 5.7 Furthermore regression analysis and analysis of
variance (ANOVA) are employed to investigate the characteristics of the materials.
The dry sliding wear of composites depend on several parameters such as size, shape,
contents, environment and test conditions such as load, speed and temperature A
mathematical model will be developed by using analysis techniques such as ANOVA
and regression analysis whereby the mathematical model (Table 5.8) shows the
relationship between the input parameters and the input responses.
5.3.4.1 Process parameters
Table 5.7 Process parameters employed for analysis of variance
Levels Load (N) Sliding velocity (m/s) Sliding distance (m)
1 50 5 1000
2 100 7 1500
3 150 9 2000
Table 5.8. Regression equations for nylon 66/CB/MoS2 composites
%MoS2 Regression Equation
0 0.000897 + 0.000014 L + 0.000292 Sl.Vel +0.0036D
0.5 0.000064 + 0.000010 L + 0.000158 Sl.Vel +0.00028D
1.0 0.000117 + 0.000007 L + 0.000150 Sl.Vel +0.00030D
2.0 0.000019 + 0.000006 L + 0.000125 Sl.Vel +0.00025D
3.0 0.000139 + 0.000005 L + 0.000117 Sl.Vel +0.00024D
where, L = Load, Sl. Vel = Sliding velocity and D = Sliding distance
The analysis of variance for nylon 66/PTFE composites is shown in Table 5.9.
The Durbin- Watson statistic is used to establish the correlation amongst the
variables. If the Durbin–Watson statistic is substantially less than 2, there is evidence
of positive serial correlation. As a rough rule of thumb, if Durbin–Watson is less than
1.0, there may be cause for alarm. Small values of d indicate successive error terms
are, on average, close in value to one another, or positively correlated. If d > 2,
160
successive error terms are much different in value with one another, i.e., negatively
correlated. In the table DW values for all the compositions are less than 2 indicating
the presence of positive serial correlation which can be further implied that the
material’s resistance to wear.
Table 5.9. Analysis of variance (ANOVA) for nylon 66/CB/MoS2 composites
Source % MoS2 in nylon 66/CB
0% 0.5% 1% 2% 3%
DOF 2 2 2 2 2
SS 5.12E-06 2.20E-06 1.35E-06 8.57E-07 6.53E-07
MS 2.56E-06 1.10E-06 6.73E-07 4.28E-07 3.27E-07
F 32.55 126.57 121.2 136.06 220.5
P 0.105 0.698 0.387 0.843 0.075
S 0.00028 0.00009 0.00007 0.00006 0.00004
R-SQ(%) 91.6 97.7 97.6 97.8 98.7
Coefficient 0.000897 0.000064 0.000117 0.000019 0.000139
SE Coeff 0.000471 0.000157 0.000125 0.000094 0.000065
T 1.90 0.41 0.93 0.21 2.15
R. Error 4.72E-07 5.22E-08 3.33E-08 1.89E-08 8.89E-09
DW Statistic 1.93 2.34 1.87 1.88 2.13
DOF = Degree of freedom; SS = Sum of variance; MS = Mean square; P = %
contribution; S = Standard deviation; D-W Statistics = Durbin-Watson statistics;
R. Error = Residual error, T = No. of observations.
5.3.5 Laser assisted etching behaviour
The effect of MoS2 content and power of laser on the etching of the surface of
the nylon composites has been studied. The laser etched surface has been
characterized by surface roughness in z-direction. The surface roughness values (Ra)
of laser etched specimens at different laser parameters are tabulated in Table 5.10.
The Ra values showed decreasing trend with increase of MoS2 content in the nylon
66/CB composites. That shows surface roughness decreases with increasing MoS2
content. However, between 50% and 100% power, 100% power has more roughness
than 50% power. This result indicates that MoS2 content control the etching behaviour
of the composite. Also the increase in power of etching increases the surface
roughness of the specimens.
161
Table 5.10. Surface roughness results for laser etched nylon 66/CB/MoS2/
composites
MoS2 content in
nylon 66/CB (wt. %)
Surface Roughness (Ra)
500 mm/s, 5 kHz
50 % power 100 % power
0 4.9 5.6
0.5 4.2 5.0
1.0 3.9 4.2
2.0 3.7 3.8
3.0 3.2 3.5
5.3.5.1 Surface morphology of laser etched surfaces
Figure 5.17(a)-(e) shows the optical photomicrographs of laser etched surfaces
of nylon 66/CB/MoS2 composites for, (a) 0.5 %, (b) 1.0 %, (c) 2.0 % and (d) 3.0%
MoS2 at 50 % power, 5 kHz frequency for 500 mm/s velocity and this figure reveals
that the surface roughness decreases and co-hesive strength increases with increase in
MoS2 content.
Figure 5.17. Photomicrographs of laser etched nylon 66/ CB/ MoS2 composites
with (a) 0%, (b) 0.5%, (c) 1.0%, (d) 2.0% and (e) 3.0 % MoS2 content at 5 kHz
frequency, 50% power and 500 mm/s velocity
a
b c
d e
162
Figure 5.18(a)-(e) shows the optical photomicrographs of laser etched surfaces of
nylon 66/CB/MoS2 composites for (a) 0.5%, (b) 1.0%, (c) 2.0% and (d) 3.0% MoS2 at
100 % power, 5 kHz frequency for 500 mm/s velocity. At the outlook, these pictures
look similar in roughness; however a close examination reveals that smoothness
increases with increase in MoS2 content.
Figure 5.18. Photomicrographs of laser etched nylon 66/ CB/ MoS2 composites
with (a) 0%, (b) 0.5%, (c) 1.0%, (d) 2.0% and (e) 3.0 % MoS2 content at 5 kHz
frequency, 100% power and 500 mm/s velocity
5.4. Conclusions
With the objective to improve the physico-mechanical and tribological
performance of the nylon 66 matrix, the MoS2 was incorporated as filler along with
CB. The MoS2 content was varied selectively from 0.5 to 3.0 wt % keeping CB
content constant at 1 wt %. Thus, the wear resistance of resultant nylon 66/CB/MoS2
composite increased with increase in MoS2 content, but decreased with increase in
load or sliding distances. Similarly the specific wear rate and friction of nylon
66/CB/MoS2 decreased with increase in MoS2 content, load and sliding distances. The
a
b c
d e
163
impact strength of the nylon 66 matrix increased from 37.2 to 43.2 J/m for the
incorporation of MoS2 content. The tensile properties- tensile strength, tensile
elongation and tensile modulus, increased linearly with increase in MoS2 content. The
addition of MoS2 renders this material somewhat stiffer and dimensionally more
stable than nylon 66. MoS2-filled nylon provides a degree of self-lubrication suited to
applications where external lubrication is impractical, contaminating, or difficult to
maintain leading to an improvement in wear. This, combined with lower water
absorption extends the range of applications that MoS2-filled nylon. All of the above
research findings suggest, that incorporation of two or more filler materials each
having a distinct functionality (one act as a lubricant and another act as heat
dissipater), can result in a composite with the potential of enhancing tribological
performance.
164
5.5 References
1. I.M. Hutchings, Tribology, CRC Press, London, (1992) pp. 156–162.
2. P.K. Mallick, Fiber Reinforced Composites, Marcel Dekker, Inc., (1993) New
York.
3. J. K.Lancaster, Tribology, 12(1973) 219.
4. B.B.Jia, T.S.Li, X.J.Liu and P.J.Cong, Wear, 262(2007) 1353.
5. S.K.Sinha, Wear Failure of Plastics, in ASM Handbook, Vol. 11, Failure
Analysis and Prevention. W.T. Becker and R.J.Shipley (eds), ASM Intl., OH,
2002; pp. 1020-1027.
6. J.K.Lancaster, Wear, 22 (1972) 412.
7. Ye Yinping, Jianmin Chen and Huidi Zhou, Wear, 266 (2009) 859.
8. M.H.Cho, S.Bahadur and A.K.Pogosian, Wear, 258 (2005) 1825.
9. W.M. Liu, C.X. Huang and L. Ling, Wear, 151 (1991) 111–118.
10. R.T. Steinbuch, Wear, 5 (1962) 458–466.
11. J. Bijwe, C.M. Logani and U.S. Tewari, Wear, 138 (1990) 77–92.
12. J.Khedkar, I. Negulescu and E.I. Meletis, Wear, 252 (2002) 361–369.
13. J. Chambers J.Jircny and C. Reese, Fire and Mater., 5(1981) 133.
14. M.Herrera, G.Matuschek and A.Kettrup, Polym. Degrad. & Stability, 78
(2003) 323.
15. L.Gabriela, E.Avram, G.Paduraru, M.Irimia, N.Hurduc and N.Aelenei, Polym.
Degd. Stab., 82 (2003) 73.
16. J.Ziegler and R.H. Schuster, KGK Kautschuk Gummi Kunststoffe, 56 (2003)
159.
17. A.M. Ha¨ger, M. Davies, in: K. Friedrich (Ed), Advances in Composite
Tribology, 1993, pp-107.
18. B.J. Briscoe, The tribology of composite materials: a preface. In: Friedrich K,
editor. Advances in composite tribology. Pipes RB, editors. Composite
materials series, vol. 8. Amsterdam, The Netherlands: Elsevier; 1993. p. 3–15.
19. K.G. Mclaren and D.Tabor, Nature, 197(1963) 856–858.
20. N.Chang, A.Bellare, R.E. Cohen and M.Spector, Wear, 241(2000) 109-117.
21. H. Unal and F.Findik, Tribology, 60(2008) 195 - 200B.Suresha,
Siddaramaiah, Kishore, S.Seetharamu and P.S.Kumaran, Wear, 267 (2009)
1405
165
22. E.Basavaraj and Siddaramaiah, J. Macromol. Sci.-Pure & Appl. Chem., A47
(2010) 558.
23. G.Shi, M.Q.Zhang, M.Z.Rong, B.Wetzel and K. Friedrich, Wear, 254(2003)
784.
24. W.Chen, F.Li, G.Han, J.Xia, L.Wang, J.Tu and Z.Xu, Tribology Letter, 15
(2003) 275-278.
25. M.Palabiyik and S.Bahadur, Wear, 246(2000) 149–158.
26. G.Srinath and R. Gnanamoorthy, J. Matl. Sci., 40(2005) 2897 – 2901.
27. B.Suresha, G.Chandramohan, N.M.Renukappa and Siddaramaiah, J. Appl.
Polym. Sci., 103 (2007) 2472.
28. X. Du-XinLi, J.Deng, J.Wang, K.Yang and X. Li, Wear, 269(2010) 262-268.
29. W.O. Winer, Wear, 10(1967) 422–52.
30. R. Holinski and J.Gansheimer, Wear, 19 (1972) 329–42.
31. A.I. Brudnyi and A.F. Karmadonov, Wear, 33 (1975) 243–9.
32. J.P.G.Farr, Wear, 35 (1975) 1–22.
33. N. Takahashi and K. Okada Wear, 33(1975)153–67.
34. N.Takahashi, Wear, 124 (1988) 279–89.