Volume 6 Preprint 24
Influence of Organic Coating Macrostructure on its Resistance to Erosive Wear
Keywords: intensity of erosive wear, epoxy coating, glass microspheres
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Volume 6 Paper C051
Influence of Organic Coating Macrostructure
on its Resistance to Erosive Wear
Radom Technical University, Faculty of Mechanical Engineering,
Institute of Vehicles and Machines Maintenance, al. Chrobrego 45,
Radom 26-600, Poland, firstname.lastname@example.org
The paper describes an influence of polymeric (epoxy) coating
composition on coatings intensity of wear resulting from hard particles
of granulated alundum impacts. It was stated experimentally that
introduction of fillers to interlayer of epoxy coating influenced
advantageously the coating resistance to alundum particles impacts. It
was found too, that the angle of impact smaller the coating wear
intensity higher. Moreover, with extending of coating thickness
coating wear decreased. It should be explained by the fact that
impacting particle energy, released in coating – erosive particle
tribological system, is more effectively suppressed by the coating of
Keywords: intensity of erosive wear, epoxy coating, glass
Processes of erosive wear of machine elements with protective organic
coatings are different from wear processes characteristic for machine
elements without such kinds of coatings. From qualitative point of
view the wear process of machine elements – with or without coatings
– are similar because in both cases intensity of erosive wear depends
on inclination angle of element in relation to erosive particles flux. But
on quantitative score parameters of erosive wear (intensity of wear,
type of loss in element surface) are significantly different because of
different mechanism of these materials (organic and metallic) wear. In
case of steel the material destruction is first of all connected with
impact-abrasive nature of influence of hard particles impacting on
machine element surface [1,2]. However, destruction of polymeric
material is caused mainly by microcutting with hard abrasive particles
[3,4,5]. In this connection values of glancing angle of maximum wear
speed are different for steel and for polymers. For steel such values
are close to 45º [1,2] and for polymeric material of high elasticity are
equal 5-10º. Starting from 45º wear becomes stabilized and for
glancing angles of value 60º < α < 85º practically does not depend on
glancing angle of erosive particles . This is very important feature
that allows predicting which machine elements will be exposed to
intensive erosive wear and how to protect them with coatings.
Organic coatings are broadly applied as protective topcoats for
technical devices. During their service life they are frequently exposed
to corrosion as well as to mechanical influences. In many cases,
mechanical failure contributes to more intensive corrosive wear.
Erosive wear of an organic coating is a form of mechanical wear. This
type of damage is typical for organic coatings applied to agricultural
and building machinery, i.e. subjected to erosion caused by sand,
stones or lumps of soil.
The process of damage resulting from an impact of tiny solid or liquid
particles against the surface of a coating, combined with the loss of its
thickness (i.e. mass loss) is called erosive wear of an organic coating.
The most important factors influencing erosive wear of an organic
coating are: the friction coefficient between erosive particles and the
coating, the velocity of erosive particles and an angle a between the
direction of an impact of erosive particles and the surface of the
The type of reaction between an erosive particle and the surface of the
organic coating, i.e. reaction with slip or with microcutting, depends
on the value of the impact angle (α) and velocity of an erosive particle
(v) at the time of impact. Plastic strains of the coating are of minor
value if residual stresses generated by the collision do not exceed the
limit of elasticity of a high-molecular material. Exceeding the yield
point causes an increase in the plastic strain of the coating and results
in the forming of micro cracks. Propagation of micro cracks and their
junctions contributes to chipping of the coating's components (fillers,
pigments and high-molecular material) [8-16].
An improvement in resistance to erosive wear of an organic coating
can be achieved by modification of the composition of the coating with
various types of inorganic fillers (in the shape of carbon or glass
fibres, glass balls or microspheres) or with metallic fillers (e.g. flakes
of aluminium or zinc) [17-25]. The fillers mentioned above influence
changes in the mechanical properties of the coating, such as hardness,
tensile and tear strength, unit elongation and friction coefficient. It is
well known that erosive wear depends on those factors
[1,16,17,18,24]. The type of erosion predominating under set
conditions, i.e. deformational erosion or shearing erosion, depends on
the value of the impact angle α of the erosive particle. For an angle
α < 45° shearing erosion dominates because the force tangential to
the coating's surface, causing its shearing grows with reduction in the
α value. An improvement in resistance to shear erosion is achieved by
application of a high-hardness coating material [1,12].
For a frictional system consisting of a high-molecular material and an
erosive particle an increase in the friction coefficient is observed when
plastic deformation of the element made from a high-molecular
surface increases . It affects the course of the characteristic of
erosive wear, i.e. its maximum moves into the direction of smaller α
As a result of the striking of erosive particles against the surface of the
polymer material, roughness of this surface increases, sometimes even
several times [5,14]. Various types of fillers improve the resistance of a
polymer material to erosive wear. For example, the wear of polyamide
PA6 modified with carbon fibres is four times lower than for the
unmodified one . Moreover, high resistance to erosive wear also
reveals also following materials: polyurethane elastomers, fluorine
plastics, high-molecular plastics modified with glass fibres, plastics
filled with quartz fibres [4,5].
2.1. The examination method of erosive wear of an epoxy coating
The examination method of erosive wear of an epoxy coating
employing the testing device (see Fig. 1), recommended by the Polish
Standard PN-76/C-81516, was used.
Fig. 1. a) Apparatus for testing erosive wear of organic coatings:
1. Container for erosive material, 2. Pipe transporting erosive
material, 3. Rotational holder for fixing metallic test
specimen with an organic coating, 4. Container collecting
erosive material after the test.
b) Particles of granulated alundum (magnification 50 times)
In order to learn the influence of an impact angle of the erosive
particle on the wear of an organic coating, the test specimen was
mounted in a specially designed specimen holder which allowed
precise setting of the angle of the specimen's surface, which was
subsequently subjected to testing. The tilt range was from 0 to 90°.
Particles of granulated alundum of grain size 0.60-0.71 mm
(according to the Polish Standard PN-76/M-59111) have been used as
the abrasive material. Aluminium trioxide (Al2O3) is the main
constituent (99% by weight) of the abrasive while SiO2, Fe2O3, Ca0 and
Na2O make up its residual part. The mass of the one charge of
alundum delivered to container 1 (see Fig. 1) was 3.5 kg, while at the
end of the test, i.e. when the substrate material was exposed, the
charge of alundum was reduced to 0.5 kg. In order to assess the
resistance of the coating to erosive wear I-criterion, calculated from
Eq. (1), was used.
where I is the intensity of erosive wear of organic coating (µm /kg), G
is the coating thickness (µm) and M is the mass of erosive particles
The above formula displays the ratio of the coating thickness to the
total mass of erosive particles producing the total wear of the coating
within the tested area, i.e. generating the exposure of the substrate
material in the elliptic shape of the minor diameter of d = 3.6 ± 0,1
2.2. Preparation of an epoxy coating
The first type of the coating examined consisted of three layers of the
epoxy. The second type of coating consisted of three layers of the
epoxy with the composite interlayer  containing 10 wt % of glass
microspheres (see Figs. 2 and 3) of diameter of φ < 30 µm . The
third type of coating consisted of three composite layers. All three
types of coating were produced on steel test plates.
The epoxy coatings subjected to the wear resistance tests were
prepared from red oxide epoxy paint which is resistant to chemicals.
This paint was blended with a polyamide curing agent at the mass
ratio 77:23, respectively. In the case of a modified coating, filler in the
form of glass microspheres was added into the mixture. Then, 30 min
mixing was performed and after a period of two 2 h the production of
coatings began. Three layers were deposited consecutively by means
of air-operated spraying. Each layer was subjected to a two-stage
hardening for 24 h at a temperature of 20 ± 2°C and then for 30 min
at a temperature of 120°C. Before the testing procedure was
performed all the specimens were subjected to 10 days acclimatisation
at a temperature of 20 ± 2°C and at a relative humidity of 65 ± 5%.
Fig. 2. Morphology of glass microspheres (magnification 2000 times)
Fig. 3. Cross-section of a glass microsphere (magnification 4000
times): 1. Inside of the glass microsphere, 2. Wall of the glass
microsphere, 3. Polymer finish
The thickness measurements of the coating were performed by means
of an electromagnetic thickness gauge (A-52) and the average
thickness of the three-layer coating was 120 µm (Fig. 4). During
examination of coating thickness influence on its erosive wear
intensity the thickness of coatings varied from 90 to 199 µm (Fig. 6).
Glass microspheres used as filler are spherical particles of aluminosilicate filled with carbon dioxide CO2 and nitrogen N2. The main
constituents of their walk are silicon dioxide SiO2 (49-61%) and
aluminium trioxide Al2O3 (26-30%). Production of glass microspheres
is based on fly-ashes which are by-products of burning bard coal in
Well developed surfaces of the glass microspheres as well as coating
their surfaces with a specially invented polymer finish, composed of
methyl methacrylate and methacrylic acid , ensure strong and tight
binding of the filler and the epoxy plastic. Differentiated diameters of
the glass microspheres as well as their irregular shape enable effective
filling of internal voids in the structure of the epoxy plastic, which
yields lower porosity of the coating. This improves both the
mechanical properties and thermal resistance of the coating. It also
reduces the ability of the plastic to absorb water and aggressive
3. Results and discussion
3.1. Influence of morphology of epoxy coating on resistance to erosive
The organic coating consisting of three layers of composite revealed
the lowest resistance to erosive wear (see Fig. 4).
It is worth noting that the low resistance of the walls of the glass
microspheres to shear stresses is responsible for this effect. The
epoxy coating with the composite interlayer was the most resistant to
erosive wear. The reason for this effect is the dissipation of energy,
released during the collision of alundum particles with the coating by
the composite interlayer. The reduction in intensity of mass loss of the
organic coating with the composite interlayer may be due to erosive
wear products accumulation on the insides of the spherical caps of the
Intensity of erosive wear
Impact angle α, (o)
Fig. 4. Influence of epoxy coating composition on intensity of wear
resulting from alundum particles impacts:
1 - three-layer epoxy coating modified with glass microspheres
2 - three-layer epoxy coating,
3 - three-layer epoxy coating with composite interlayer
3.2. Influence of an impact angle of alundum particles on erosive wear
of an epoxy coating
The process of erosive wear of an organic coating was investigated for
the values of alundum particles impact angle α ranging from 5 to 85°.
Close examination of these figures shows that the lower α - angle, the
higher the intensity of the erosive wear of a coating. The most
intensive erosive wear resulting from an impact of alundum particles
was recorded for the impact angles α < 45°, while the lowest was for
the impact angle α > 60°.
Morphology of the surface of the epoxy coating modified with glass
microspheres which was subjected to erosive wear tests at an impact
angle α = 45° is presented in Fig. 5.
Fig. 5. Erosive wear of the surface of the epoxy coating modified with
glass microspheres (magnification 1500 times)
At the initial stage of the erosive wear of the composite coating, the
walls of the glass microspheres situated at the surface of the coating
are subjected to shearing, while the surface layer of the coating
around the filler particles undergoes strong plastic deformation
(Fig. 5). During the subsequent stages, microcutting of the epoxy
material and shearing of the walls of the glass microspheres proceeds,
until the steel base material appears (Fig. 6).
On the base of carried out examination it can be stated that resistance
of epoxy coating to erosive wear increases as its thickness increases
too. It means, of course, that intensity of the coating wear decreases.
It should be explained by more effective dissipation of thermal energy
(releasing during impacts of hard particles) by the coating of bigger
thickness (volume). But increase of coating thickness over critical value
leads to increase of internal stresses in the coating. It can be a reason
of micro cracks generation in the coating resulting in intensity of
coating erosive wear increase.
Intensity of erosive wear
µm / kg
Thickness of coatings
Fig. 6. Influence of coating thickness on intensity of erosive wear:
1 - three-layer epoxy coating modified with glass
microspheres (composite coating),
2 - three-layer epoxy coating,
3 - three-layer epoxy coating with composite interlayer
On the basis of the examination results the following can be
The filler in the shape of glass microspheres included in the interlayer
of the epoxy coating remarkably improves resistance of the coating to
erosive wear generated by the action of alundum particles. It can be
accounted for by the damping of energy released during the collisions
between the impacting particles and the organic coating by glass
microspheres. The three-layer epoxy coating, modified with glass
microspheres, revealed the highest level of erosive wear, compared
with the other coatings tested, because of the relatively high
roughness of the surface combined with the low shear resistance of
the walls of the glass microspheres.
Kinetics of wear of the coatings tested depends heavily on the impact
angle α of the erosive particle. For an impact angle α < 45° the erosion
process is dominated by micro-shearing. This contributes to higher
intensity of wear than in the case of an impact angle bigger than 60°.
Thickness of epoxy coating has an essential influence on the coating
resistance to hard particles action. The thicker the coating the bigger
the resistance of the coating to erosive wear because the coating of
bigger thickness more effectively suppresses energy of impacting
erosive particles what results in lower intensity of the coating wear.
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