Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, 73-79
doi:10.4236/jsemat.2011.12011 Published Online July 2011 (http://www.SciRP.org/journal/jsemat)
73
Effect of Nitriding on Wear Behavior of Graphite
Reinforced Aluminum Alloy Composites
Bhujang Mutt G iri sh, Bhujang Mutt Sati s h , Hanyalu Ramegowda Vitala
Research and Development Center, Department of Mechanical Engineering, East Point College of Engineering and Technology,
Bangalore, Karnataka, India.
Received April 22nd, 2011; revised May 20th, 2011; accepted May 29th, 2011.
ABSTRACT
The paper evaluates the effect of nitriding on the wear behavior of graphite reinforced aluminum 6061 alloy compo-
sites. The composites were prepared using the liquid metallurgy technique. The content of graphite in the composites
was va ried fro m 3 to 7% (by weight) in steps of 2%. The nitriding process was carried out at 500˚C for 24 hours. Thre e
categories of specimens, namely, the nitrided composites, non-nitrided composites, as well as th e alloy specime ns were
tested for their wear behavior. Pin-on disc equipment was used for wear testing. X-Ray Diffraction technique (XRD)
was used to confirm the implantation of nitrogen in the composites. It was observed that the nitrided composites have
better wear resistance than the non-nitrided composites.
Keywords: Composites, Nitriding , Surface Engineer i ng , Wea r
1. Introduction
Aluminum (Al) and its alloys posses numerous advan-
tages such as low specific weight, high strength-to-
weight ratio and low cost, and hence find wide applica-
tions i n t he au to moti ve, a viat io n and s pac e i ndus trie s [1] .
However, the main limitation of their wider application
for commercial purpose is the lack of surface hardness,
mechanical strength and wears resistance, together with
low thermal and chemical stability. Therefore, a surface
modification with the aim to improve the above proper-
ties is necessary for further industrial use. This can be
achieved by formation of aluminum nitrite (AlN) on Al
surfa ce, obt ained by ni tro gen insert io n into t he s urfac e of
bulk Al as AIN exhibits excellent tribological properties
combined with high thermal and chemical stability [2].
The use of aluminum for light weight machine parts
and car components has recently increased, although
non-metallic materials, such as resin, are also used for
this purpose. Al alloys have advantages over non-meta l-
lic materials such as light weight, corrosion-resistance,
and are workable and have good thermal conductivity.
However, the hardness, wear and seize resistance of Al
alloys is lower than that of the steel, and hence there is a
limit on the application as sliding parts. Thus, research
has been carried out on surface modification technology
to increase the applicability of Al alloys as sliding parts
[3].
MMCs (Metal Matrix Composites) have received in-
creasing attention in the recent decades as engineering
materials. MMCs are primary candidate materials for
industrial applications in the aerospace, automotive and
power utility industries. However, their properties such
as strength, toughness, and wear and corrosion properties
depend to a great extent on several factors of which ma-
trix properties are very important [4].
A more widespread use of light metal alloys in tribo-
logical applications like guide bars, bearing plates, seat
supports or bushings demands powerful functional sur-
face coatings to provide wear protection as well as com-
pressive strength. The direct contact of uncoated light
metal substrates with sliding or oscillatory counterparts
results in severe wear, seizing and high frictional co-
efficient, even under lubricated cond itions.
There are different surface treatment processes availa-
ble commercially for aluminum alloys to increase the
corrosion and wear resistance, such as nitriding, anodic
oxidation, elec trol ysis, nickel pla ting etc. N itridin g is o ne
surface modification technique that is widely used to
increase the fatigue, mechanical strength and wear resis-
tance of machine parts of carbon or alloys steels. It is a
process of introducing nitrogen in to the metals so as to

Effect of Nitriding on Wear Behavior of Graphite Reinforced Aluminum Alloy Composites
74
modify the surface properties, and increase the hardness,
wear resistance and corrosion resistance [5].
The present paper aims to report the evaluation of
wear behavior of nitrided aluminum MMCs reinforced
with graphite particles and the same is compared with
non- nitrided MMCs.
2. Experi m ental Method
2.1. Preparation of Composites
Aluminum 6061 alloy with the chemical composition
given in Table 1 was used as the matrix material. The
optical emission spectrometer was used determine the
chemical composition of the alloy. The reinforcement
material used was graphite which is a solid lubr icant with
adequate resistance to wear. The composites were fabri-
cated by liquid metallurg y techniq ue, the details o f whic h
are available elsewhere [6]. Graphite particles (0%, 3%,
5% and 7% by weight) were heated separately to a no-
minal temperature to remove moisture content if any, and
introduced into the vortex of the effectively degassed Al
6061 molten alloy. The molten alloy was stirred rigo-
rously using a stirrer having a ceramic coated steel im-
peller.
2.2. Nitriding of Specimens
Nitriding is a promising method for surface treatment to
improve hardness, corrosion, wear and fatigue resistance
of materials. Nitriding process can improve the tribolog-
ical and mechanical properties by enriching the near sur-
face r egio n with ni troge n. Nitrides produced by the com-
bination of nitrogen with alloying elements posses a
higher hardness as compared with the iron nitrides. It is
due to this fact that the nitriding pr ocess is primarily used
to increase the hardness and wear resistance of the sur-
face of several metals including steel.
The specimens prepared for wear testing were washed
in distilled water, and then with acetone and later sub-
jected to the nitriding process. Nitriding was accom-
plished in a muffle furnace where ammonia gas was in-
troduced into the air-tight furnace chamber heated to a
temperature of 500˚C. T he spec imens were exposed for a
duration of 24 hours and the ammonia feed rate was
20CFH (Cubic feed/hr).
Ammonia gas decomposes giving the nitrogen in nas-
cent fo rm or mona tomic nitro gen, which is the only fo rm
capable of entering the material and diffusing in it.
2.3. Wear Test Equ ipm e nt
The wear tests were conducted in accordance with
ASTM G99 standards using a pin-on-disc sliding wear
testing machine, which is similar to the one, used by
Poonawala et al., [7]. EN24 steel disc of diameter 200
mm and chemical composition: C-0.45, Si-0.35,
Mn-0.70, Cr-1.40, Mo-0.35, Ni-1.80, S-0.05, P-0.05 in
weight %, was used as the counter face on which t he test
specimens slide. Hardness of the steel disc was HRc 57
achieved by oil quenching at 850˚C and tempering at
550˚C for 2 hours. Arrangements were made to hold a
specimen and also for application of the load on t he spe-
cimen. The test specimen was clamped in a vertical sam-
ple holder and held against the rotating steel disc. In the
pre sent invest igation, loads of 20-160 N in steps of 20 N
were used. The rotational speeds employed were 200,
250, and 300 rpm, which at an average distance of 80
mm from the center gave corresponding linear speeds of
1.25, 1.56, and 1.87 m/s respectively.
2.4. Testing of Specimens
The ‘weight loss’ method was adopted in the present
study in which the pins of the material under investiga-
tion were 6 mm in di ameter and 15 mm in length. A fr e s h
disc was used each time and before each test, the disc
was cleaned with acetone to remove any possible traces
of grease and othe r sur face c onta minant s. The specimens
were cleaned with ethanol and weighed before and after
the tests using a balance accurate to ± 0.001g. The wear
results were computed from weight loss measurements.
The duration of each test was exactly 60 minutes. The
wear volume was calculated from the ratio of weight loss
to density and wear rate was calculated using sliding dis-
tance and wear volume. Usage of such relations to calcu-
late the wear parameters is common and has been used
by Rohatgi et al., [8]. The data for the wear tests was
taken from the average of three measurements. The
standard deviation was about 5%. The surfaces of the
worn specimens were cleaned thoroughly to remove the
loose wear debris and then observed using a scanning
electron microscope (SEM). Along with the test speci-
mens, even the surface of the counterpace steel disc was
subjected to the SEM analysis in order to draw as much
vital information as possible about the wear behavior of
the composite specimens.
Table 1. Composition of Al-6061 alloy.
Elements Cu Mg Si Fe Mn Ni Zn Pb Sn Ti Cr Al
% by Wt 0.36 0.99 0.80 0.12 0.02 0.01 0.01 0.07 0.05 0.01 0.12 Bal

Effect of Nitriding on Wear Behavior of Graphite Reinforced Aluminum Alloy Composites
75
3. Results and Discussion
Wear of composite materials although appears to be sim-
ple, the actual process of material removal is quite com-
plex. This can be attributed to the fact that a large num-
ber of factors influence wear such as metallurgical fac-
tors (weight percentage, chemical composition, and size
of the reinforcement) or service (i.e., speed, pressure) or
other contacting factors (lubrication, corrosion, etc.) The
factors that have been considered in the present study to
explain the wear behavior of the composites are the ap-
plied load, the applied speed, and the weight percentage
of the reinforcement.
Scanning electron microscopy were carried out on
samples which were ground to 600 grit SiC and polished
using alumina powders up to 1 µ, and etched using Kel-
ler’s reagent. The microphotograph of the 7% graphite
composite showing uniform distribution of the rein-
forcement is shown in Figure 1. The Figure 2(a) is the
EDX of a nitrided Al6061 alloy specimen. The nitrogen
peak is clearly visible indicating the presence of nitrogen.
The Figure 2(b) shows the thickness of the coating that
was obtained and it was found to be 1.10 mm.
3.1. Effect of Load and Speed on the Wear Rate
The specimens tested fall into three categories, namely,
the unreinforced and non-nitrided specimen, the non-
nitrided co mposite, and the nitrided composite. Three p in
specimens were tested from each category at each speci-
fied load and speed.
The results were averaged to obtain the final wear rate,
which are presented graphically in Figures 3-5. In the
grap hs sho wn i n Figure 3-5, ‘UN’ means unnitrided spe-
cimens, while ‘nit’ means nitrided specimens. The wear
rate of the unreinforced alloy is also plotted in order to
enable comparison with the other category specimens. It
is found from the graphs that the wear rate of the speci-
mens belon ging to all the thre e categories increased with
the applied load. The wear rate of the non-nitrided com-
posite as well as the nitrided composite specimen re-
duced with the increase in graphite content. It is clearly
evident from the graphs that there exists a certain load,
i.e., a transition phenomena at which there is a sudden
increase in the wear rate of the specimens belonging to
all the three categories. However, the transition loads for
the nitrided composites were much higher than that ob-
served for the non-nitrided composites, and that of the
non-nitrided composites was much higher than the un-
reinforced alloy and also the transition load increased
with the increase in graphite particle content.
The Figure 3 presents the behavior of the specimens
tested at a speed of 1.25 m/s and load ranging from 20 to
200 N. It was observed that the unreinforced matrix alloy
Figure 1. SEM showing uniform distribution of graphite
part icles in the alloy matrix.
(a)
(b)
Figure 2. (a) EDX of a nitrided composite specimen; (b)
SEM showing the thicknes s of the coating on the specimen.
showed a transition from mild to severe wear at a load of
60 N, while the 3 and 5% graphite reinforced composites
showed a transition at 140 and 160 N respectively. The

Effect of Nitriding on Wear Behavior of Graphite Reinforced Aluminum Alloy Composites
76
0
1
2
3
4
5
6
20406080100 120 140 160 180 200
Load (N)
Wear rate (Cu.mm/km)
0% Gr
3% G r (UN)
5% G r (UN)
7% G r (UN)
3% G r (Ni t)
5% G r (Ni t)
7% G r (Ni t)
Figure 3. Gr aph of wear rate v/s load at a speed of 1.25 m/s.
0
1
2
3
4
5
6
7
20406080100 120140 160180 200
Load (N)
Wear rate (Cu.mm / km)
0% Gr
3% Gr (UN)
5% Gr (UN)
7% Gr (UN)
3% Gr (Ni t)
5% Gr (Ni t)
7% Gr (Ni t)
Figure 4. Gr aph of wear rate v/s load at speed of 1.56 m/s.
same observation was made at a load of 140 N in case of
7% graphite reinforced composites. This observation
0
1
2
3
4
5
6
20406080100 120 140 160180 200
Load (N)
Wear rate (Cu.mm/km)
0% Gr
3% Gr (UN)
5% Gr (UN)
7% Gr (UN)
3% Gr (Nit)
5% Gr (Nit)
7% Gr (Nit)
Figure 5. Graph of wear rat e v/s load at a speed of 1.87 m/s.
which is evident from Figure 3 indicates that the pres-
ence of graphite reinforcement delays the transition from
mild to severe wear, and increases the transition load of
the 7% reinforced composite by almost 2.5 times with
respect to the unreinforced alloy. Interestingly, similar
transition in the case of nitrided composites was ob-
served at 200 N in those having a reinforcement content
of 3% graphite, while the 5 and 7% reinforcement com-
pos ites show no t ransition at all even at 200 N.
It follows from the results ob tained that comparatively
low wear rates exist at lower loads, thereby indicating th e
regime of mild wear. In this regime of mild wear, the
composites demonstrate significant wear resistance than
the alloy cou nterpart. At high er loads, the materials e xhi-
bit rapid increase in wear rate. At loads greater than the
transition loads, severe wear occurs leading to seizure of
the materials. The severe wear manifests itself by a rapid
rate of material removal in the form of generation of
coarse metallic debris, and also by massive surface de-
formation and material transfer to the counter face.
The composites behave very differently from the un-
reinforced alloy. The alloy shows a transition at 60 N
when tested at 1.25 and 1.56 m/s, while the same is ob-
served at 40 N in case of 1.87 m/s test. Similarly the
composite with 3 % reinforcement shows a transition at
140 N when tested at 1.25 m/s, at 120 N when tested at
speed of 1.56 m/s and 100 N at 1.86 m/s. Similar obser-
vations for 5 and 7% composites as well as the nitrided

Effect of Nitriding on Wear Behavior of Graphite Reinforced Aluminum Alloy Composites
77
composites are clearly evident form the graphs shown in
Figures 4 and 5.
The composite with 5% graphite shows a transition at
160 N when tested at 1.25 m/s, at 140 N at speed of 1.56
m/s, and the same is observed at 120 N in case of
1.87m/s test. The above observations clearly indicate that
the sliding speeds employed have a significant effect on
the wear rate tra nsition on t he mater ials. The tr ans ition in
wear rate decreases with the increase in speed in all the
materials. The results obtained are on par with the one
obtained by Lee, et al., [9] who have reported that the
wear mechanisms are strongly dependent on the sliding
speeds.
The mild wear of the alloy is oxidation dominated
wear at low sliding speeds and loads. In the case of
composites, due to the existence of graphite particles, the
oxide film of the metal is not continuous and tenacious. It
is removed by friction forces in the following sliding
friction resulting in oxidation assisted mild abrasion
wear. Hence it can be considered that the dominating
wear mechanism is the removal and reproduction of the
oxide film. This kind of wear is maintained until higher
loads are employed, under which condition the wear
mechanism transforms from mild to severe wear. The
morphologies of wear surfaces of 7% graphite reinforced
composites are shown in Figure 6(a) -( c). Fig ure 6(a)
shows the wear tracks for unreinforced alloy, while Fig-
ure 6(b) and (c) represent the wear tracks for composite
and nitrided composite respectively. The wear tracks in
Figure 6(b) and (c) show typical abrasive wear for the
composites tested at low loads of 20 and 60 N respec-
tively. Hence it can be concluded that the dominating
wear mechanism is abrasive wear at low loads.
It was obser ved that at higher loads a transition occurs
from mild to severe wear, and the wear rate quickly in-
creases by tremendous rate. Due to the high loads em-
ployed, the friction and wear increased obviously. I n thi s
condition, the removal and formation of oxide films are
faster than that of mild oxidation, thereby resulting in
relatively higher wear rates.
3.2. Effect of Graphite Particles on Wear Rate
It follows from the observations that the graphite par-
ticles play a very strong role in enhancing the wear resis-
tance of the composites. It was found that the transition
load increases with the increase in graphite content and
also the wear rate of the composites was lower than that
of the base alloy without graphite. This is obviously due
to the release of graphite particles by the composite spe-
cimens on to the mating surface during sliding which
provides resistance to wear. The release of graphite onto
the sliding interface ca uses for mation of a thin film such
that the relative movement of the mating surfaces pro-
(a)
(b)
(c)
Figure 6. (a) SEM show ing wear trac ks in the unreinforced
alloy; (b) SEM showing wear tracks in the 7% graphite
reinforced non-nitrided composite; (c) SEM showing wear
tracks in the 7% graphite reinforced and nitrided compo-
site.

Effect of Nitriding on Wear Behavior of Graphite Reinforced Aluminum Alloy Composites
78
motes easy shear bet ween the la mellar plane s of graphite.
The buildup of the fi lm i s a significant feature of graphite
tribology. Gra phite has a hexa gona l la yere d str uct ure a nd
the bonds between the parallel layers are relatively weak
(van der Walls type). The key to graphite’s value as a
self-lubricating solid lies in its layered lattice structure
and its ability to form strong chemical bonds with gases
such as water vapour. The adsorption of water vapour
and other gases from the environment ont o the cr ys t alline
edges weakens the interlayer bonding forces, resulting in
easy shear and transfer of the crystalline platelets on to
the mating surfaces. Graphite also performs well under
boundary lubrication conditions because of its affinity fro
hydrocarbon lubricants [10].
Hence it can be concluded that the ability of the
sheared reinforcement layers to adhere to the sliding sur-
face decides the effectiveness of the graphite particles in
reducing the wear rate of the composite materials.
3.3. Effect of Nitriding on Wear Rate
Nitriding is a surface treatment process where nitroge n i s
supplied into the chamber of specialized equipment at
relatively high temperature. A nitrided thin layer on the
surface will be formed containing aluminum alloy based
phases. The addition of graphite to the base alloy signif-
icantly improves the wear resistance. This is further im-
proved by several folds in the case of nirtided compo-
sites. The hardness of the nitrided composites increases
quite significantly as shown in the Ta ble 2. T he primary
cause for the increase in hardness and hence wear resis-
tance is the presence of AlN and nitrogen enriched alu-
minum at the surface. The presence of the phases is
sho wn by the XRD a na l ysi s whic h is p r esented i n Figure
2(a).
3.4. SEM Analysis
In view of brevity and convenience, the SEM micro-
graphs of only 7% composites at speed of 1.87 m/s have
been presented. However, the explanation holds good
even for the composites with 3 and 5% reinforcement as
well. The SEM micrographs of a typical worn surface of
the 7% graphite reinforced composites are presented in
Figure 6(a), (b) and (c) which shows the wear track
morphology of the specimens tested at various loads.
It can be seen that a lot of parallel, continuous and
Table 2. Vickers micro hardness number for nitrided and
non-nitrided specimens.
Specimen Type VHN
Non-Nitrided Specimens 68
Nitrided Specimens 82
deeply ploughed grooves exist on the wear surface of the
composites and there is an abrasion phenomenon ob-
served at low loads. The parallel grooves suggest abra-
sive wear as characterized by the penetration of the gra-
phite particle s into a so fter sur face, which is an i mportant
contributor to the wear behavior of composites. The worn
surfaces in some places reveal patches of material re-
moved from the surface of the material during the c ours e
of wear and smeared on to the sliding surface.
4. Conclusions
The addition of graphite particles to the aluminum alloy
improves the wear resistance of the composite in spite of
the s ignifi cant i mpro vement to wear resi stance. Nitridin g
whi ch is a s urface treatment process further improves the
wear resistance by several folds. The primary cause for
the increase in hardness and hence wear resistance is the
presence of AlN and nitrogen enriched aluminum at the
surface.
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