materials
Article
Tribological Performance of Graphite Nanoplatelets Reinforced
Al and Al/Al
2
O
3
Self-Lubricating Composites
Emad Omrani
1,2
, Afsaneh Dorri Moghadam
1
, Ashish K. Kasar
2
, Pradeep Rohatgi
1,
*
and Pradeep L. Menezes
1,
*
Citation: Omrani, E.; Moghadam,
A.D.; Kasar, A.K.; Rohatgi, P.;
Menezes, P.L. Tribological
Performance of Graphite
Nanoplatelets Reinforced Al and
Al/Al
2
O
3
Self-Lubricating
Composites. Materials 2021, 14, 1183.
https://doi.org/10.3390/ma14051183
Academic Editor: Maria
Cristina Ramirez
Received: 8 December 2020
Accepted: 25 February 2021
Published: 3 March 2021
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4.0/).
1
Department of Materials Science and Engineering, University of Wisconsin-Milwaukee,
Milwaukee, WI 53201, USA; eomrani@uwm.edu (E.O.); afdorri@gmail.com (A.D.M.)
2
Department of Mechanical Engineering, University of Nevada, Reno, NV 89557, USA;
akasar@nevada.unr.edu
* Correspondence: prohatgi@uwm.edu (P.R.); pmenezes@unr.edu (P.L.M.)
Abstract: In the present work, the effect of graphite nanoplatelets (GNPs) on tribological proper-
ties of the aluminum (Al), and Al/alumina (Al
2
O
3
) composite are studied. GNPs are multilayer
graphene sheets which were used as a solid lubricant material. Two sets of composites, Al/GNPs and
Al/GNPs/Al
2
O
3
with varying amounts of reinforcements, were synthesized by powder metallurgy
that involves cold compaction followed by hot compaction. The hardness of the composites increased
with the addition of GNPs and Al
2
O
3
. The Al/GNPs composite with 1 wt.% of GNPs (Al/1GNPs)
showed a 20% increase in hardness whereas Al/GNPs/ Al
2
O
3
composite with 1 wt.% GNPs and
2 wt.% Al
2
O
3
(Al/1GNPs/2Al
2
O
3
) showed 27% increases in hardness compared to the pure Al. The
coefficient of friction measured at 20 N was observed to be 22% and 53% lesser for Al/1GNPs and
Al/1GNPs/2Al
2
O
3
, respectively, compared to corresponding alloys without graphene Al. The X-ray
diffraction and scanning electron microscopy analysis revealed the presence of GNPs at the worn
surface after the tribology tests. The wear rate was also reduced significantly. In comparison with
pure Al, the Al/1GNPs and Al/1GNPs/2Al
2
O
3
composites resulted in 5- and 20-times lesser wear
rate, respectively. The addition of Al
2
O
3
caused reduction in wear rate due to higher hardness and
load carrying ability, whereas composites with more than 1 wt.% GNPs showed higher wear rate
due to lower hardness and higher porosity. The Al/1GNPs/2Al
2
O
3
composite exhibited the least
coefficient of friction (0.2–0.25) and wear rate (1 × 10
−6
–4 × 10
−6
mm
3
/N.m) compared to other
GNPs and Al
2
O
3
reinforced Al composites. The worn surfaces were further analyzed to understand
the wear mechanism by Raman spectroscopy, transmission electron microscopy, and x-ray diffraction
to detect the Al
4
C
3
phase formation, chemical bonding, and defect formation in graphene.
Keywords: graphite nanoplatelets; wear rate; self-lubricating composite
1. Introduction
The current demand to fulfil the requirement of high-performance materials is chal-
lenging with existing metals and alloys as they have limited properties. When metals
are reinforced with suitable reinforcements, the manufactured metal matrix composites
(MMCs) contain improved properties that can be optimized to have a wide range of
properties such as being lightweight, high specific strength, high hardness, and superior
tribological properties. Generally, it has been observed that MMC’s properties improve
with a decrease in the size of the reinforcement [1,2]. The current technologies allow pro-
duction of particles below 100 nm that can be used as a reinforcement to improve desired
properties. Accordingly, a metal matrix composites reinforced with nanosized particles
are termed as a metal matrix nanocomposite (MMNCs). The addition of nanoparticles can
lead to overcome of the shortcomingsof composites such as poor ductility, machinability,
and lower fracture toughness. Generally, the size of reinforcement influences mechanical
Materials 2021, 14, 1183. https://doi.org/10.3390/ma14051183 https://www.mdpi.com/journal/materials