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6th Panhellenic Conference on Metallic Materials
Ioannina, 7-9 December 2016
Organised by the:
Laboratory of Applied Metallurgy, University of Ioannina
Co-organised by the: Hellenic Metallurgical Society
Editor: A.G. Lekatou
Tensile and bend behaviour of WC-Co-Cr HVOF coatings on steel
A. Koutsomichalis1,* K. Kalfountzos1, P. Makri1, M. Vardavoulias2, N. Vaxevanidis3
1 Hellenic Air-Force Academy, Department of Aeronautical Studies, Dekelia Air Force Base, TGA
1010, Attica, Greece, angelos.Koutsomichalis@hafa.haf.gr
2 Pyrogenesis S.A, Technological Park of Lavrio, GR 195 00, Lavrio, Greece,
mvardavoulias@pyrogenesis-sa.gr
3 School of Pedagogical & Technological Education, Dept. of Mechanical Engineering Educators, GR
141 21 Heraklion, Athens, Greece, vaxev@aspete.gr
Summary
This paper studies the effect of a cermet HVOF WC-Co-Cr coating on the mechanical behaviour of
steel substrate. The HVOF WC-Co-Cr coating was found to reduce the tensile strength of the steel.
The failure of the HVOF WC-Co-Cr coated steel specimens initiated by cracking on the coating
surface, transverse propagation of the cracks towards the coating - substrate interface and diversion
along the interface leading to local delamination and breakage. During the bend tests cracks developed
along the coating within the strain range and the main failure in the coating occurred due to the tensile-
shear deformation, particularly coating-substrate material interface. The coating thickness does not
appear to affect the bending strength. During bending the number of surface cracks per unit length
decreased with increasing coating thickness. Stereoscopic analysis showed that the thicker the coating
the deeper the surface cracks. When the critical stress for crack propagation reached defect sites at the
substrate-coating interface, the entire coating failed and peeled off from the substrate.
Keywords: HVOF coatings, WC-Co-Cr, tensile, bending
1. Introduction
Tungsten carbide coatings are attractive for various aeronautical applications such as landing gears,
propeller hubs, hydraulic actuators, green turbine engines, helicopters dynamic components, etc,
especially as an alternative to electrolytic hard chromium plating [1]. HVOF process has been
successfully employed for the replacement of electrolytic hard chromium coatings in such applications
by means of the deposition of cermet coatings such as WC–17Co and WC–10Co–4Cr. For these
applications, structural aluminum alloys and steels with high strength, toughness and fatigue properties
are commonly employed, whose resistance to wear can be increased by means of tungsten carbide
coatings. However, such an improvement in the tribological properties could give rise to a significant
decrease in the fatigue life of the coated components [2].
While the mechanisms of formation of such coatings have been investigated in depth, detailed
information on mechanical properties still remains difficult to obtain because of coatings
heterogeneity. The mechanical behaviour of the HVOF coatings is subject to inter-lamellar pores and
intralamellar cracks which reduce their elastic modulus. In addition to the surface area of pores and
cracks, the splat boundary area is also reported to contribute to the anisotropy and reduced elastic
modulus values in the perpendicular direction [3].
A large number of research studies have been devoted in the past few years to the analysis mostly of
the fatigue behaviour of different substrates, ferrous and non-ferrous, coated by HVOF thermal
spraying [4-6].
In the present study, HVOF coating of WC-10Co-4Cr agglomerated and sintered powder on steel is
examined. Mechanical properties of the coated system are examined through three-point bending and
tensile tests. Scanning electron microscopy was used to investigate the bend and tensile appearance of
the fractured coatings. Note also that the tensile and tribological behaviour of the same WC-10Co-4Cr
coating on aluminum alloy substrate; see Ref. [7].
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2. Experimental
A commercially available WC-Co-Cr agglomerated and sintered powder (WOKA 3653) was
deposited on the surface of a St 37 steel substrate. The powder characteristics were: Composition:
WC: 86 %, Co: 10 %, Cr: 4 %, Fe<0.3 %; particle size: -45+11 m; shape: spherical and
density: 14.7 g/cm3.
Two different types of specimens were prepared by the steel substrates: tensile dog bone shaped
specimens with dimensions 130x90x2.1 mm3 and three point bending specimens which were 23 mm
wide, 2 mm high and 131mm long. WC-Co-Cr coating was deposited on both sides of the tensile dog
bone shaped specimens in equal thicknesses in order to enable balanced loading during tensile testing.
Tensile testing was performed in accordance with ASTM E8 standard on an Instron 4482 testing
machine. Three point bending was accomplished by means of a computer controlled INSTRON 5544
instrument. The samples were tested in a configuration so as to place the coatings in tension (Fig. 1a).
(a) (b)
Fig. 1 (a) Schematic view of three-point bending test set-up; (b) A photograph of a workpiece after the
three-point bending test.
The tests were performed in air at room temperature at a constant cross-head speed of 50m/min. The
photograph of a workpiece after the bending test is shown in Fig. 1b. In situ observations were carried
out during the test with an optical microscope. The specimens were examined by scanning electron
microscopy (SEM) before and after the test.
The steel substrates were grit blasted with aluminum oxide mesh 90 and cleaned with ethyl alcohol
before spraying. The cermet powder was deposited by HVOF equipment. This step was accomplished
by means of a Diamond Jet 2700 Sulzer Metco gun, employing a mixture of propane as fuel and
oxygen. The spraying conditions were the same as in [7]. During spraying the substrate temperature
was measured using a hand held thermocouple temperature detector.
3. Results and discussion
Figure 2 shows the cross section of the coating which exhibits dense structure and is free of cracks.
The microstructure consists of dark and light areas; the dark areas was revealed to be mostly Co
particles with aggregate of WC, while the microstructure of the light areas was mostly consisted of
sintered aggregate WC particles. The coating microstructure is composed of equiaxed WC grains in a
Co matrix rich in Cr [8]. The coating is built up by deposits of lenticular splats, one over the other, in a
uniform manner, throughout the coating.
Coating-substrate interface shows no gaps or cracks indicating good adhesion between the coatings
and the substrate. WC–Co-Cr coating has good adhesion to substrate and shows W2C, W in addition to
WC and Co phases of powder. WC-Co-Cr particles are injected into the hot flame of up to 3500 °C
and high velocity of up to 1000 m/s. The particles impact on the surface forming gradually the coating.
Upon impact, bonds form and the surface and coating undergo quenching at a very high cooling rate,
in excess of 106 K/s, forming coatings with good adhesion to substrate as shown in Fig. 2 [9-11]. The
interface exhibits plastic deformation due to the impact of the alumina particles (grit blasting) and also
exhibits scattered residues of such embedded particles onto the substrate surface. Such defects are
expected to have a deleterious effect on the fatigue performance of the coated system, as reported
previously [12-14].
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Fig. 2 SEM migrographs and SEM/EDS of the WC-Co-Cr coating cross section.
The XRD pattern of the coating indicated the presence of WC and also the decarburized brittle
W2C phase formed during deposition. These brittle phases have been correlated with reduced
toughness and overall wear resistance [15] and are normally distributed in the peripheries of splats,
leading to a weak interface between the splats [7, 16].
Fig. 3 shows the load–displacement curves for both the as received steel and the WC-Co-Cr coated
steel. It can be seen that the plastic region for the coated specimens initiates at lower load levels for the
same flexural development which means that the uncoated steel test piece is more ductile compared
with the coated. Abnormal behaviour in some curves indicates that the gradual deformation of
interface of the substrate material probably cannot relieve the stress levels at the interface. The
coated steel specimens exhibit similar behaviour regardless of the coating thickness. The coating
thickness does not appear to affect the bending strength.
Fig. 3 Load and displacement characteristics of the coated systems after three-point bend tests.
For all he coatings the number of cracks initiated at the coating surface per unit length () was also
measured. Results obtained indicated, clearly, that during bending the number of surface cracks per
unit length decreases with increasing coating thickness. On the other hand it was also observed that the
thicker the coating the deeper the surface cracks (fig. 4). This phenomenon might be attributed to
internal stresses. The development of internal stress, due to tensile-shearing force, creates local stress
concentrations, particularly at defect sites in the region of coating-substrate interface. In this case, the
defect site has a significant effect on the failure mechanism. It should be noted that stress
concentrations at defect sites are, in general, higher than the mean internal stresses. Under the tensile-
shear loading delamination occurs above the plastically deformed region. The compressive stress
generated at the top surface of the workpiece does not generate failure, such as peeling due to elastic
strain energy stored in the coating.
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(a) (b)
Fig. 4 Plan view of the bent surface for a coating 220 m (a) and 460 m thick (b).
Since the coating is placed in the bottom (opposite) site where the three-point bend indented is
applied, the coating section of the workpiece is subjected to tensile-shear force only. Consequently,
the main failure in the coating occurs because of the tensile-shear deformation, particularly coating-
substrate material interface.
Fig. 5 shows SEM micrographs of the tensile surface as well as the cross-section of the workpiece
after the bending tests. Multi-crack deformation in the coating is evident, which shows that coating
does not conform to the plastic deformation in the substrate material. In some regions, crack spacing is
small indicating that the sliding and splitting deformation occurs in the coating. However, in general,
crack spacing is large and in some regions total elimination of coating is resulted (peeling off from the
substrate surface) due to shear deformation at coating substrate interface [14]. In addition, the coating
fracture was brittle and no sign of plastification was observed. The crack in the coating is formed due
to the tensile load and initiated at the free surface of the coating as shown in Fig. 5b and Fig. 4.
When the critical stress for crack propagation is reached in defect sites at the interface, the entire
coating fails and peels off from the substrate material (fig. 5d). If the crack propagation is limited to
local region, the fracture of the coating is resulted. In this case, internal stress in the coating is relaxed
around the crack sites. If the energy used to propagate the crack is dissipated, the crack cannot extend
beyond the substrate material, i.e., it terminates at the free surface of the substrate material.
From Fig 5a the multi-cracking deformation of the coating is apparent which illustrates that the
cermet coating is unable to follow the plastic deformation of the substrate. In some areas, the distance
between cracks is small indicating fracture of the coating due to slip and traversal. However, in
general, the spacing between cracks is large and in some areas the exfoliation of the coating due to
shear stresses in the substrate-coating interface is limited. Rupture of the coating is brittle in nature
and where no peeling off has occurred cohesive failure within the coating is observed (fig. 5c).
Fig. 5 Coated specimen with thickness 395m (a) cross section after bending (b) plan view after
bending (c) cohesive failure area (d) coating exfoliation area.
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The engineering stress–strain curves for the WC-Co-Cr coated steel substrate with various coating
thickness were very similar to the ones reported in [7] for aluminium substrate. The presence of the
WC-Co-Cr coating results in a lower tensile strength of the coated laminated composite in comparison
with the tensile strength of the uncoated steel. The reduction of the tensile properties can be attributed
to the brittle characteristics of the coating, while fracture analysis revealed that brittle cracks, formed
in the coating, are the primary cause of the tensile failure of the coated specimens.
During tensile tests the coating exhibited cracking in the transverse direction of the tensile load. The
initiation of these cracks was at the free surface of the coating, before the specimens enter the plastic
region, something that has been also mentioned by other researchers [15, 16]. Cracks in the coating
surface were observed prior to the transition in the plastic region and are attributed to its brittle nature.
As the applied tensile strain increased the number of transverse cracks, also, increased (fig. 6a) until
they reached saturation. After the saturation of the multiple cracks, the cracks stopped on the interface
between the top coat and substrate (fig. 6b). The initiated cracks moved towards the coating-substrate
interface in a tree like manner (fig. 6c) and propagated between the laminations of the coating causing
delamination (fig. 6d). Some of them passed through all the thickness of the coating and were evolved
at the substrate-coating interface causing extensive peeling of the latter. The different Young’s
modulus of the two different constituents was the main reason for that and the subsequent creation of
shear stress which leaded to the observed peeling. The interfacial delamination cracks have been
studied in coated systems, they are attributed to the linkage of previously formed cracks and are both
relative to the residual stresses and associated with the fracture toughness of the coating [17-19].
The coating-substrate adhesion influences the tensile performance of the HVOF WC-CoCr coatings.
The coating-substrate adhesion is related to the substrate surface roughness which enables mechanical
interlocking of the WC-Co-Cr splats with the substrate surface. Substrate surface roughness is
accomplished by grit blasting. Embedment of grit into the substrate or the presence of grit remnants
may act as stress raisers and influence interfacial crack propagation and delamination of the coating
upon stress induction [20].
Fig. 6 (a) Plan view of the fractured WC-Co-Cr coating, (b) transverse cracks in the cermet coating
passing through the interface, (c) tree like cracks (d) enlargement of crack tip region in (b).
208
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