the influence of hybrid composites structure on their tribological

the influence of hybrid composites structure on their tribological

14: 1 (2014) 18-22
Andrzej Posmyk1, Jerzy Myalski2
1 Silesian
University of Technology, Faculty of Transport, ul. Krasińskiego 8, 40-019 Katowice, Poland
University of Technology, Faculty of Materials Engineering and Metallurgy, ul. Krasińskiego 8, 40-019 Katowice, Poland
* Corresponding author: E-mail: [email protected]
2 Silesian
Received (Otrzymano) 1.02.2014
THE INFLUENCE OF HYBRID COMPOSITES STRUCTURE
ON THEIR TRIBOLOGICAL PROPERTIES
The paper presents the influence of a hybrid composite structure containing two reinforcing phases i.e. porous, spherical
Al2O3 ceramic particles, coated with glassy carbon film which carries out the function of solid lubricant. This composite is
manufactured by a new method developed in 2013, which consists of three stages i.e. pre-form manufacturing from porous
foam by means of gelcasting, foam saturation with a glassy carbon precursor and its pyrolysis, followed by the infiltration of
the pre-form coated with glassy carbon by a liquid alloy. The new method helps to obtain composites with homogenous glassy
carbon distribution which improves their tribological properties. The presence of aluminum oxide increases the hardness
locally whereas the presence of glassy carbon mitigates the shear strength. Comparative tribological studies of a composite
with an AC-AlCu3Mg1 matrix alloy containing 10% Al2O3 and the hybrid composite in which aluminum oxide spheres have
been coated with glassy carbon, both sliding against GJL-300 cast iron in air, confirmed the positive influence of carbon on
both the friction coefficient and wear. The friction coefficient at a sliding speed of v = 2.5 m/s and unit pressure p = 2 MPa
after wearing-in of the sliding surfaces is 0.08÷0.12 for the composites with glassy carbon and 0.25÷0.32 for the composite
which contains only ceramic spheres.
Keywords: hybrid composite, structure, alumina, glassy carbon, tribological properties, solid lubricant
WPŁYW STRUKTURY KOMPOZYTÓW HYBRYDOWYCH
NA ICH WŁAŚCIWOŚCI TRIBOLOGICZNE
Przedstawiono wpływ struktury kompozytu hybrydowego zawierającego dwie fazy zbrojące, tj. porowate, sferoidalne,
puste wewnątrz cząstki Al2O3 jako umocnienie pokryte warstewką węgla szklistego, pełniącego funkcję smaru stałego.
Kompozyt ten jest wytwarzany według nowej metody opracowanej w 2013 roku. Metoda składa się z trzech etapów, tj.
wytwarzania preformy z porowatej pianki metodą Ŝelowania spienionej zawiesiny, nasączania tej pianki prekursorem węgla
szklistego i jego pirolizy oraz infiltracji ciekłym stopem osnowy pokrytej węglem szklistym preformy. Dzięki tej metodzie
uzyskuje się zupełnie inną strukturę kompozytu niŜ dotychczasową metodą mieszania. Podczas mieszania uzyskuje się
kompozyt o niezbyt równomiernym rozłoŜeniu węgla szklistego, przez co na powierzchni biorącej udział w tarciu moŜna
znaleźć obszary o mniejszej zawartości węgla, co sprzyja lokalnemu sczepianiu adhezyjnemu z współpracującym ślizgowo
Ŝeliwem. Według nowej metody, uzyskuje się kompozyt z bardziej równomiernym rozłoŜeniem węgla szklistego, co poprawia
właściwości tribologiczne. Obecność tlenku glinu zwiększa lokalnie twardość, a węgla szklistego zmniejsza wytrzymałość na
ścinanie. Doboru materiałów fazy zbrojącej dokonano zgodnie z hipotezą tarcia opracowaną przez Ernsta i Merchanta,
wg której w strefie tarcia powinny być dwa materiały, tj. jeden o duŜej twardości, a drugi o małej wytrzymałości na ścianie.
Węgiel szklisty cechuje się małą wytrzymałością na ścianie (30÷50 MPa) i duŜą mikrotwardością porównywalną do SiC
(230÷340 HV - SIGRADUR), co zmniejsza siły tarcia. Cechy stereologiczne sfer ceramicznych, takie jak udział
powierzchniowy (ilość cząstek na 1 mm2) i średnia średnica decydują o ilości i sposobie rozmieszczenia węgła osadzonego na
wewnętrznych i zewnętrznych ściankach sfer. Regulując intensywność i czas osadzania prekursora węglowego na podłoŜu
ceramicznym, moŜna dobrać ilość fazy węglowej tworzącej się na ściankach tych sfer ceramicznych. WydłuŜenie czasu
osadzania prekursora węglowego pozwala na uzyskanie warstewek o większej grubości. W badanym kompozycie
zaobserwowano, w zaleŜności od wielkości i połoŜenia sfer, powłoki węglowe o grubości od 1 do 5 µm. Cieńsze warstewki
wytworzono w miejscach, do których dostęp płynnego prekursora był ograniczony, np. przez zbyt małe lub częściowo
zamknięte pory w sferach Al2O3. Proces pirolizy prekursora wywiera równieŜ wpływ na właściwości tribologiczne
wytworzonego węgła szklistego. Temperatura i czas karbonizacji, z reguły, wpływają na zwiększenie wytrzymałości
i twardości węgla szklistego. Infiltracja ciekłym stopem osnowy pokrytych węglem szklistym preform ceramicznych wywiera
wpływ na właściwości wytrzymałościowe kompozytu. Za niskie ciśnienie i za mała płynność stopu mogą spowodować, Ŝe nie
wszystkie sfery zostaną wypełnione całkowicie, a powstałe pustki zmniejszą wytrzymałość. Porównawcze badania
tribologiczne kompozytu z osnową ze stopu AC-AlCu3Mg1, zawierającego 10% Al2O3 oraz kompozytu hybrydowego, ze
sferami Al2O3 pokrytymi węglem szklistym, we współpracy z Ŝeliwem GJL-300 w warunkach tarcia technicznie suchego
wykazały dobry wpływ węgla na współczynnik tarcia i zuŜycie. Współczynnik tarcia przy prędkości v = 2,5 m/s i nacisku
jednostkowym p = 2 MPa po dotarciu skojarzenia wynosił 0,08÷0,12 dla kompozytu z węglem szklistym i 0,25÷0,32 dla
kompozytu zawierającego tylko ceramiczne sfery.
Słowa kluczowe: kompozyty hybrydowe, struktura, tlenek glinu, węgiel szklisty, właściwości tribologiczne, smar stały
The influence of hybrid composites structure on their tribological properties
19
INTRODUCTION
The structure of engineering materials is one of the
most essential factors which modify their functional
properties. Their tribological properties such as friction
coefficient and resistance to sliding wear determine the
kind of material to be used for the design of sliding
parts in machines. Therefore, the proper selection of
materials for sliding pairs seems absolutely fundamental. This selection should be focused on the chemical
composition as well as the structure. From the theoretical point of view, the magnitude of friction forces
depends on the shear strength of one of the component
materials or on their structure, hardness and sliding
surface topography. To reduce friction forces, a material having a possibly low shear strength, high hardness
and low roughness should be selected. It is not easy
though to find solid materials which would meet these
requirements. Therefore, manufactured hybrid composite materials are composed of a matrix and two kinds of
discrete phases. One of them increases the hardness,
whereas the other one decreases the shear strength.
These principles constituted the basis for the investigation project [1], carried out at the Silesian University of Technology, on a hybrid composite containing
two reinforcing phases i.e. aluminum oxide which increases the strength properties (compressive strength
and hardness) and glassy carbon which mitigates the
friction.
The additional function of porous aluminum oxide
spheres and glassy carbon layers is to reduce the composite density. Glassy carbon has a density of 1420÷
÷1540 kg/m3. The manufacturing technology of ceramics, porous pre-forms used for the production of the
composite has been described in [2-4] while that of the
composite itself in [5, 6]. The present paper discusses
the effect of the structure of the hybrid composite containing ceramic spheres and thin films of glassy carbon
as the reinforcing phase on the tribological properties in
sliding contact with gray cast iron.
SELECTION OF STRUCTURE COMPONENTS
FOR SLIDING COMPOSITES
From the tribological point of view, the basic criterion for designing hybrid composites is the friction
hypothesis elaborated by Ernst and Merchant [7], which
helps to calculate the friction coefficient. According to
this hypothesis, the friction coefficient in sliding contact can be calculated from the following equation:
µ=
τ
H
+ tgα
(1)
where: τ - the mean shear strength of the adhesion junction in the contact zone [MPa], usually the shear
strength of the weaker material (shearing takes place
only inside glassy carbon layers, shear strength:
τ = 30 MPa); H - the hardness of the hardest material in
the pairing (ceramic spheres) MPa; α - the mean angle
of inclination of real contact areas to the direction of the
tangent force, equal to the angle of inclination of the
roughness peaks (the cast iron surface is worn-in, with
very low roughness).
The tribological properties of a hybrid composite are
conditioned by, inter alia, the number of phases and
stereological features of each phase. The amount of
glassy carbon, stereological features of the particles and
the method of its pyrolysis determine the shear strength,
and consequently the friction forces. The shear strength
depends upon the temperature and time of pyrolysis.
The amount of ceramic material, its stereological features and the distribution method in the matrix material
determine the strength properties such as hardness and
wear intensity of the tribological partner e.g. cast iron.
A larger amount of oxide in the form of irregular particles with sharp edges intensifies the wear. The optimum
amount of aluminum oxide ranges from 10 to 20%. The
least wear intensity of cast iron at sliding contact is
guaranteed by composites with aluminum oxide in the
form of fibers or spheres. The latest solution which
reduces the wear intensity of a sliding partner is a composite which contains ceramic foams made of porous
spheres e.g. aluminum oxide [2, 3, 8]. The lower hardness of porous spheres in comparison with fully dense
particles reduces the wear of cast iron but at the same
time gives rise to some failures in the composites like
wall crushing. Fragments of aluminum oxide might
locally intensify the wear, however, coating their surfaces with continuous glassy carbon film mitigates the
negative results of crushing.
INFLUENCE OF CERAMICS UPON SLIDING
PROPERTIES OF COMPOSITES
In composites containing irregular Al2O3 particles
manufactured up to the present, the oxide was introduced into the matrix after proper preparation. The
distribution of the Al2O3 particles was closely dependent on their preparation and matrix stirring at the introduction of the particles, as well as upon the casting
conditions. In the proposed technology, a pre-form with
porous Al2O3 microspheres is produced first. The preform is obtained by means of gelcasting of a suspension
of an α-Al2O3 powder (Alcoa CT 3000 SG) followed by
high temperature sintering. In the ceramic skeletons, the
microspheres merge (Fig. 2b) and interpenetrate in such
a way that continuity of the pre-form is ensured and
open porosity is generated. The spaces between the
spheres can be filled with Al2O3 (concentrated polycrystalline phase) or air. The first option significantly
increases the preform strength, whereas the other option
makes it weaker. The diameters of the microspheres
range between 200 and 1000 µm, with a mean value of
400 µm. In single spheres, holes of 40÷200 µm in
diameter can be observed [8]. Such a structure of
microspheres in the pre-form makes it possible to fill
Composites Theory and Practice 14: 1 (2014) All rights reserved
20
A. Posmyk, J. Myalski
them with liquid substances i.e. a glassy carbon precursor and composite matrix alloy. The dimensions of the
microspheres and holes in their walls can be altered
with the parameters of the production process. Spheres
of larger diameters facilitate the infiltration process
with metal. However, when the matrix is filled with
metal, areas with a surface close in size to that of an
inner cross-section of a sphere which contain nothing
but the matrix alloy are formed in the composite. This
might mean weaker sliding properties due to the possible occurrence of adhesive tacking with a sliding partner. Smaller diameters of microspheres cause an increase in their number, thickness reduction of their
walls and contraction of the area filled exclusively with
the matrix alloy. Uniform distribution of ceramics over
the friction surface will prevent the tendency to adhesion and reduce local friction forces inside the spheres
filled with the matrix metal. The pre-form can feature
diverse porosity. From the tribological point of view,
the Al2O3 content should range between 10 and 20% i.e.
the porosity of the ceramic pre-form should be 80÷90%.
INFLUENCE OF GLASSY CARBON ON SLIDING
PROPERTIES OF COMPOSITES
Composites manufactured in recent years have contained glassy carbon introduced into the matrix, after
a)
prior preparation, in the form of irregular particles [5].
Uniform distribution of the carbon particles depended
upon the kind of applied preparation and the conditions of their introduction into the liquid metal by
stirring. The conditions of composite casting in a mould
were also very important. In spite of the fact that
a homogeneous composite suspension is obtained, the
transfer of particles may occur during crystallization
caused either by a crystallization front or sedimentation
due to the different specific gravities of the carbon particles and matrix alloy (Fig 1a). In consequence,
a greater concentration of particles is observed upon the
outer surface or inside the cast. In practice, it is not
possible to obtain a specific composite product with
uniformly distributed carbon particles using casting
methods.
In the case of the composite under investigation,
a liquid carbon precursor is introduced into a ceramic
foam and not into the matrix. Then pyrolysis of the
precursor is performed [1]. The method enables uniform distribution of glassy carbon over the walls of the
ceramic spheres (Figs. 1b, 2b and 3) and eliminates
stirring or any hazards of sedimentation or conglomeration present in composite casting with glassy carbon
particles. As a result, the composite features better tribological properties since places deprived of glassy
carbon functioning as a solid lubricant are reduced to
a minimum.
b)
Fig. 1. Structure micrograph of composite produced using stirring (a) and infiltration of ceramic pre-form with glassy carbon precursor (b): 1- Al2O3,
2 - matrix alloy, 3 - phase precipitations, 4 - glassy carbon/particles layer
Rys. 1. Mikrofotografia struktury kompozytu z węglem szklistym wytworzonego metodą mieszania (a) i przez nasączanie ceramicznej preformy
prekursorem węgla szklistego (b): 1 - Al2O3, 2 - stop osnowy, 3 - wydzielenia fazowe, 4 - cząstki/warstewka węgla szklistego
a)
b)
Fig. 2. Structure micrograph of composite after infiltration with matrix alloy (a) and joint of three ceramic spheres (1 in Fig. b) coated with layer of
glassy carbon and filled with matrix alloy
Rys. 2. Mikrofotografia struktury kompozytu po infiltracji stopem osnowy (a) i połączenia trzech sfer ceramicznych (1 na rys. b) pokrytych warstewką
węgla szklistego (4) i wypełnionych stopem osnowy (2)
Composites Theory and Practice 14: 1 (2014) All rights reserved
The influence of hybrid composites structure on their tribological properties
a)
21
b)
Fig. 3. Micrographs of border zone between glassy carbon coated ceramic sphere wall and matrix alloy (a) and sphere filled with matrix alloy,
partially coated with glassy carbon (b)
Rys. 3. Mikrofotografie granicy pomiędzy pokrytą węglem szklistym ścianką ceramicznej sfery i wypełnieniem stopem osnowy (a) oraz sfera
zapełniona stopem osnowy (b) pokryta częściowo węglem szklistym
The wear conditions are mitigated and local adhesion of the sliding surfaces is prevented. Moreover, the
production costs of machine parts are lower. Stirring is
not required thus casting tools and stirrers are not worn
down constantly by a stream of matrix alloy with sharp
edged ceramic particles.
The thermal properties of glassy carbon such
as thermal conductivity λ ranges from 1.59 to
6.3 W/(K·m) and the thermal expansion coefficient α =
= 2.2x10–6 in 20÷100°C and 3.2 x10–6 at temperatures
of 100÷1000°C improve the dimensional stability of the
composite which allows the application of this composite in the production of heat engines e.g. combustion
engines. Low shear strength (30÷50 MPa) and high
hardness 230÷340 HV reduce the friction forces.
A compressive strength of 480 MPa is sufficient for
sliding pairings to operate under high compressions.
Comparative tribological studies on a composite with
an AC-AlCu3Mg1 alloy matrix containing 10% Al2O3
and the hybrid composite in which aluminum oxide
spheres have been coated with glassy carbon sliding
against GJL-300 cast iron in air, demonstrate the positive influence of glassy carbon upon the friction coefficient and wear of both the cast iron and the composite.
The friction coefficient at the relative speed of 2.5 m/s
and unit pressure of 2 MPa after contact wearing-in was
0.08÷0.12 for the composite with glassy carbon and
0.25÷0.32 for the composite with ceramic spheres
exclusively.
The application of aluminum or magnesium alloys
as matrix materials lowers the composite density but
worsens the tribological properties due to the tendency
of the metals to form adhesive tacking with the majority of engineering materials. Therefore the best sliding
properties could be achieved when precipitation hardened alloys (e.g. alloys for plastic processing) are used
as a matrix. The precipitations of phases distributed
inside the aluminum oxide spheres reduce the tacking
tendency which in turn reduces the friction forces and
wear. Figures 1b and 2a show the matrix alloy with
precipitations inside the ceramic spheres before sliding
and Figure 4 after sliding against cast iron. The matrix
material should facilitate the formation of sliding film
from the wear products of glassy carbon. As a result,
sliding contact, e.g. cast iron-matrix alloy, would be
replaced by cast iron-glassy carbon sliding contact.
a)
b)
INFLUENCE OF MATRIX MATERIAL UPON
SLIDING PROPERTIES OF COMPOSITES
Pre-forms made of ceramic foam coated with glassy
carbon are subjected to pressure infiltration with the
selected matrix alloy. Aluminum or magnesium alloys
can be used to produce low density composites. If the
use of matrix material with an elevated melting point
appears necessary, e.g. copper or silver, the process of
infiltration will have to be carried out in a vacuum or
under protective gas to prevent glassy carbon oxidation
and deterioration of the sliding properties.
Fig. 4. Micrograph of inside of ceramic sphere filled with hardened
matrix alloy precipitations (a) and chemical analysis of precipitations (b)
Rys. 4. Mikrofotografia wnętrza sfery ceramicznej wypełnionej stopem
umacnianym wydzieleniowo (a) i analiza składu chemicznego
wydzieleń (b)
Composites Theory and Practice 14: 1 (2014) All rights reserved
22
A. Posmyk, J. Myalski
The change of the composite colour after sliding is
the evidence of good adhesion between the wear debris
of the glassy carbon and the matrix material. Dark areas
upon the surface after friction account for the deposition of a sliding film. Figure 4 shows micrographs
of the composite surface after sliding contact with
GJL-300 cast iron in air.
a)
b)
Fig. 5. Surface view of composite after sliding against cast-iron GJL-350
in air (a) and border zone between sphere and matrix alloy with
initial exposure of a sphere inside (b)
Rys. 5. Widok powierzchni kompozytu po współpracy ślizgowej z Ŝeliwem GJL-350 w warunkach tarcia technicznie suchego (a) i granicy sfery i stopu osnowy z początkiem odsłonięcia wnętrza
sfery (b)
The exposure of aluminum oxide spheres coated
with glassy carbon film (darker circular areas) are visible upon the composite surface (Fig. 5a) together with
a sliding film of glassy carbon and graphite from cast
iron formed during friction. The lighter areas represent
the places where the sliding film has been removed and
the brightest areas represent the fragments of exposed
ceramics. Figure 5b presents the junction of ceramic
spheres filled with the matrix alloy with the deposited
sliding film. The wear debris of glassy carbon and
graphite from cast iron have been deposited at cross-sections of the ceramic walls as well. Exposure of the
inside of the ceramic sphere took place as a result of
wear followed by slight wear of the carbon film which
coated the inner part of the sphere.
CONCLUSION
The structure of a hybrid composite containing
porous ceramic spheres as the reinforcing phase and
Composites Theory and Practice 14: 1 (2014) All rights reserved
layers of glassy carbon functioning as a solid lubricant
can be shaped in a three-stage manufacturing process
consisting of the production of a foamy preform by the
gelcasting method, saturation with the carbon precursor
followed by precursor pyrolysis and infiltration with
the matrix alloy. The first stage enables one to obtain
the required porosity of a pre-form which determines
the amount of ceramics in the composite and stereological features of the spheres such as dimensions of the
pores or wall thickness. The second stage opens the
possibility to control the shear strength and hardness of
glassy carbon with the amount of carbon precursor
introduced and the process parameters. By selecting
a matrix alloy with hard precipitations and parameters
of infiltration, it is feasible to control the tendency of
the matrix material to adhesive tacking. The tribological
properties in the final composite constitute the operational superposition on all the manufacturing stages.
The structure and tribological properties of the presented composite have been obtained under the established conditions based on theoretical analysis and
literature available as well as the authors’ professional
experience. However, further multifactor optimization
studies which will determine the best composite structure for tribological purposes are required.
Acknowledgements
Financial support of Structural Funds in the Operational Programme - Innovative Economy (IE OP)
financed from the European Regional Development
Fund - Project No POIG.0101.02-00-015/08 is gratefully acknowledged.
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