Research Proposal:
Study of Tribological Properties of Magnesium Composites
for Weight-Critical Applications
1 Introduction
As reported, 414.8 hundred million USD was lost in UK each year due to friction, corrosion, and
wear of the industrial products[1]. Consequently, a number of institutions for the research of
tribology were funded in many countries to study the anti-wear properties for the prolongation of
materials life time[2]. On the other hand, solving the problem of tribology can improve the efficiency
of transportation by reducing fuel power consumption, ultimately resulting in lower carbon footprint.
Weight-reduction is the most cost-efficient choice for decreasing the burdens on fossil fuel energy
and reducing greenhouse emissions[3]. With the fast growth of the vehicle industry, the demand
for lightweight construction materials has been augmented. Recent years, magnesium
composites have attracted researchers’ attention in the field of weight-critical applications
including aerospace and automobile, owing to their relatively light weight. However, their
engineering performance are limited to a large extent due to the poor wear resistance, which
decreases the lifetime of magnesium-based workpiece and making them undesirable for
manufacturing bearing, gears, and cylinders, etc. In this project, the self-lubricating properties of
nano-filler enhanced magnesium composites will be studied for the weight critical tribological
applications; the mechanism of improved mechanical properties and the structure of the resulting
metal matrix composites (MMCs) will be investigated.
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2 Review
2.1 Green Tribology
Tribology is clarified in the Oxford dictionary as ‘the branch of science and technology concerned
with interacting surfaces in relative motion and with associated matters’. Generally, it involves the
study of friction, wear, lubrication and the design of bearings[4].
In 2001, the term “green tribology” made its debut at a national symposium on tribology in China[5],
in which the concept and target of green tribology were presented. The objective of green tribology
is usually associated with interacting surfaces in relative motion that have great impacts on energy
or environmental sustainability. The technique emphasizes on the control of friction and wear,
which is essential for the fields relating to energy conservation and conversion[2], such as surfacemodification and tribological properties improvement for green applications and transportation
industry like wind-power turbines, tidal turbines or automobile engine blocks.
2.2 Materials for Weight-critical Applications
Steel and cast irons were widely applied in vehicle industry owing to their low cost and producibility.
New metal and composites are considered to substitute conventional materials and incorporate
into vehicles if they perform to be more beneficial with an affordable price. For instance, since
ferrous materials are relatively heavy, remarkable value can be obtained by exploiting light metals
and improve fuel power efficiency as well as drivability. In this light, there is a growing trend for
replacing cast irons and steel by Al and Mg to address the weight-critical issues, primarily due to
their low density[6].
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Aluminum alloy owns plenty of outstanding properties such as low density and thermal expansion
rate, and high chemical resistance, endowing them with great potential for automotive
applications; thus, a lot of casting techniques were proposed for their mass production. However,
as a soft metal, pure aluminum often displays poor resistance to wearing and friction, making
them vulnerable when sliding hard materials.
Magnesium is the lightest among all the engineering metals with a density of 1.74 g/cm3[7], which
is 33 percent and 75 percent lighter than aluminum and steel/cast-iron, respectively, while Mg
alloy also possess advantages of higher chemical resistance and better manufacturability over
aluminum die-cast alloys ferrous materials[6]. Therefore, vehicle manufacturing industries have
conducted the most intensive research on magnesium and its alloys. On the contrary, the
application of magnesium and its alloy in automotive such as pistons and cylinders for vehicles
are also limited as a result of its low thermal stability and wear resistance[8] compared to steel,
iron, and aluminum alloys. This gives rise to the need for light metal-based nanocomposite with
additives improving its tribological properties that will be present in section 2.3.
2.3 Self-lubricating MMCs
As wear is introduced by the plastic deformation of soft metals, to overcome their tribological
disadvantages, reinforcement fillers of as nanosized ceramics, e.g., silicon carbide and aluminum
oxide, which show higher mechanical properties are often added into the metal matrix. For
example, Mg-based MMCs reinforced with 1.11 vol.% of alumina nanoparticles exhibited
significantly improved wear resistance as a result of the increased strength of the MMC by pinning
the dislocations using the reinforcement particles[9].
However, these protruding hard-ceramic fillers can, on the other hand, elevate the coefficient of
friction (COF) and increase the abrade between the counter surface material at a certain loading
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level[10]. To mitigate this effect, lubricants have been commonly employed between contacting
areas to decrease friction and wear. Whereas, the major issue of lubricants between materials is
the lack of continuous supply of the solid lubricant without liquid phase to produce the shear force
between sliding surfaces, while liquid lubricants are often toxic and undegradable thus not
environmental-friendly. One of the approaches that can avoid the external usage of lubricants is
to disperse MMCs with additive lubricant particles and made them self-lubricating[11]. These
lubricants, such as graphite[12], MoS2 or WS2[13] usually comprised of a lamellar crystal structure
that atoms are strongly bonded via covalence bond within the sheet and weakly interacted by
Van der Waals force between layers. Since these lamellar structured materials can be sheared
more easily parallel to the layers than the vertical direction, they can bear heavier loads at an
angle that sliding can easily take place parallel to the layers. The mechanism of these selflubricating MMCs is believed as the transfer and affinity of the embedded lubricant additions
towards the metal surface and thus the formation of a film of lubricant which prevents the
immediate contact of the surfaces. This process eliminated the demand for external lubricants.
2.4 Challenges and Project Plan
Based on the discussion above, magnesium alloys have been intensively studied in recent years
due to their superior advantage of lightweight. However, their poor mechanical properties
compared with extensively used construction materials like steel, iron and Al alloys, hindered the
further application of Mg alloys in weight-critical engineering field. Ceramic reinforcement fillers
are often added to increase their strength and wear resistance, which, on the negative side
increases the COF of MMCs. The self-lubricating additives seem to be an solution for improving
their tribological properties.
There are two main challenges lying in the manufacturing of self-lubricating MMCs. The first one
is the distribution of lubricants in the matrix. There is a trend for the agglomeration of the
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nanofillers occur in the metal matrix at a relatively high loading level of the additive, making it
difficult to form a homogenous distribution of the reinforcement, and finally resulting in poor
mechanical properties of thus produced MMCs. For example, Meysam and his co-workers[14]
reported the importance of the amount of graphene in Al matrix. When less than 1 wt.% graphene
was added, a well-distributed graphene phase in the matrix could be achieved and the MMC
shows good coefficient of friction, while more than 5 wt.% graphene in the matrix caused a nonuniformed distribution and decreased the mechanical properties of MMC.
Another factor is the formation of bonding between fillers and matrix. The anti-wear properties
can be significantly elevated with the existence of chemical interaction between the substrate and
fillers. This is mainly because the interaction can generate chemical tribofilms on the wear track
with favourable mechanical properties which can control wear rate and protect the substrate from
abrasion[15]. Sulfide, such as MoS2 and WS2, often shows good anti-wear properties due to their
“affinity” to cover on the metal surface. WS2 is considered to be an excellent additive since it can
be used in either dry and wet environment and also possesses high-temperature stability[16]. As
one type of WS2, inorganic fullerene-like (IF)-WS2 can reduce the COF of MMC when it was added
as the lubricant, but their anti-wear properties are not that superior as a result of lacking chemical
bonding with the substrate metal[17], while another structure flat sheets (2H)-WS2 can serve as
both friction modifier and anti-wear fillers because of its reactivity with metal wear track.
Up to date, some reports have exhibited the increases in mechanical properties of magnesiumbased MMCs by adding WS2 nanostructures[18,
19].
However, barely have there been
investigations on the effects of WS2 on the tribological properties of these magnesium matrix
composites. In this project, self-lubricating Mg/2H-WS2 composites will be prepared via spark
plasma sintering or hot press for efficient and greener tribological parts. Systematic microstructure
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characterisation of the prepared nanocomposites will be carried out, to establish the processstructure-property relationship.
3 Methodology
The project will be conducted in Advanced Materials Research Group (AMRG), Faculty of
Engineering, The University of Nottingham individually for three years. The main work comprises
of design, fabrication, and characterisation of magnesium nanocomposites. Tribology
performance of the prepared Magnesium nanocomposites will be evaluated at different conditions
(dry, oil lubricated and at elevated temperature), as well as the mechanical and thermomechanical properties. The following experimental instrumentations and characterization
techniques will be required: spark plasma sintering (SPS) or hot compaction machine, ball milling
machine, scanning electron microscope (SEM), dynamic ultra-micro hardness tester, Vickers
hardness tester, ball-on-plate test, Auger electron spectrometer (AES), X-ray photoelectron
spectroscopy (XPS).
3.1 Materials and Processing methods
Magnesium and 2H-WS2 will be purchased from commercial sources. The plate or ball specimens
studied in this research will be the pure magnesium or Mg MMCs reinforced with 20 wt.% 2HWS2. Composite will be mixed via ball milling machine for 6 hours, following by the SPS or hot
press process at a temperature ranging from 400-600 °C.
3.2 Characterization
The density of the samples will be calculated by Archimedes’s principle using electronic balancer;
the dispersion and morphology of nanosized 2H-WS2 will be examined by SEM; Young’s modulus
will be monitored using a dynamic ultra-micro hardness tester, and the hardness will be tested by
a Vickers hardness tester; the determination of wear and friction properties will be conducted on
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a ball-on-plate type test rig; AES will be employed to study the elemental distribution across the
wear track, and XPS will be performed to characterize the interaction between WS2 and
magnesium matrix and thus confirm the chemical composition of generated tribofilms.
4 Project Motivation
This project will be focused on generating major impacts in both academic and industrial
communities. 1) To apply nanoparticles in practical engineering has been a challenge for a long
time due to the dispersion, property gain over cost and other issues associated with metallic
matrix; 2) The potential application will lead to significant weight reduction and thus less fuels and
lower carbon footprint, leading to significant environmental and energy benefits, in addition to
immense economic gains.
University of Nottingham is an excellent university ranked #8 overall in the UK by the 2018 QS
Graduate Employability Rankings, and the Advanced Materials Research Group (AMRP) in the
University of Nottingham specialises in innovative research focussed on providing solutions and
expertise in the field of functional nanomaterials. All these factors convinced me this is an ideal
place to conduct the project.
5 Reference
[1]
Zhang, S. Current industrial activities of tribology in China Plenary Lecture to the China Int.
in Symp. on Tribology (CIST 2008)(Beijing,). 2008.
[2]
Nosonovsky, M. and B. Bhushan, Green tribology: principles, research areas and
challenges. 2010, The Royal Society.
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[3]
Dieringa, H. and K. Kainer, Magnesium–der Zukunftswerkstoff für die Automobilindustrie?
Materialwissenschaft
und
Werkstofftechnik:
Entwicklung,
Fertigung,
Prüfung,
Eigenschaften und Anwendungen technischer Werkstoffe, 2007. 38(2): p. 91-96.
[4]
Bhushan, B., Introduction to tribology. 2013: John Wiley & Sons.
[5]
Zhang, S., Investigation concerning the developing directions of tribology in China.
Tribology21, 2001: p. 321-323.
[6]
Cole, G. and A. Sherman, Light weight materials for automotive applications. Materials
characterization, 1995. 35(1): p. 3-9.
[7]
Davies, G., Materials for automobile bodies. 2012: Butterworth-Heinemann.
[8]
Mordike, B. and T. Ebert, Magnesium: Properties—applications—potential. Materials
Science and Engineering: A, 2001. 302(1): p. 37-45.
[9]
Lim, C., et al., Wear of magnesium composites reinforced with nano-sized alumina
particulates. Wear, 2005. 259(1-6): p. 620-625.
[10]
Dasgupta, R., Sliding wear resistance of Al-alloy particulate composites: An assessment
on its efficacy. Tribology International, 2010. 43(5-6): p. 951-958.
[11]
Menezes, P.L., P.K. Rohatgi, and M.R. Lovell, Self-lubricating behavior of graphite
reinforced metal matrix composites, in Green Tribology. 2012, Springer. p. 445-480.
[12]
Menezes, P.L., P.K. Rohatgi, and M.R. Lovell, Self-Lubricating Behavior of Graphite
Reinforced Metal Matrix Composites, in Green Tribology: Biomimetics, Energy
Conservation and Sustainability, M. Nosonovsky and B. Bhushan, Editors. 2012, Springer
Berlin Heidelberg: Berlin, Heidelberg. p. 445-480.
[13]
Huang, S., et al., Electrical sliding friction and wear properties of Cu–MoS2–graphite–WS2
nanotubes composites in air and vacuum conditions. Materials Science and Engineering:
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[14]
Tabandeh-Khorshid, M., et al., Tribological performance of self-lubricating aluminum matrix
nanocomposites: role of graphene nanoplatelets. Engineering Science and Technology, an
International Journal, 2016. 19(1): p. 463-469.
[15]
Stachowiak, G. and A.W. Batchelor, Engineering tribology. 2013: Butterworth-Heinemann.
[16]
Shi, X., et al., Tribological behavior of Ni3Al matrix self-lubricating composites containing
WS2, Ag and hBN tested from room temperature to 800 C. Materials & Design, 2014. 55:
p. 75-84.
[17]
Niste, V.B. and M. Ratoi, Tungsten dichalcogenide lubricant nanoadditives for demanding
applications. Materials Today Communications, 2016. 8: p. 1-11.
[18]
Huang, S.-J., et al., Mechanical behavior enhancement of AZ31/WS2 and AZ61/WS2
magnesium metal matrix nanocomposites. Advances in Mechanical Engineering, 2018.
10(2): p. 1687814017753442.
[19]
Huang, S.-J., et al., Advanced AZ31 Mg alloy composites reinforced by WS2 nanotubes.
Journal of Alloys and Compounds, 2016. 654: p. 15-22.
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