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Tensile and bend behaviour of WC-Co-Cr HVOF coatings on steel


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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.
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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,
2 Pyrogenesis S.A, Technological Park of Lavrio, GR 195 00, Lavrio, Greece,
3 School of Pedagogical & Technological Education, Dept. of Mechanical Engineering Educators, GR
141 21 Heraklion, Athens, Greece,
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].
    
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.
    
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).
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    
... They showed that cracking of the coating started at the surface and propagated in the deposit towards the substrate. Then, the crack changed direction and spread to a limited extent in the coating-substrate interface and in the coating itself just above the interface [36,37]. In the (Cr 3 C 2 -25(Ni20Cr))-(Ni25C) deposit deposited on the Al alloy substrate, an intense crack was observed in the interface zone, contrary to those sprayed on steel ones. ...
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In this work, the effect of the substrate, Al 7075 alloy and 1H18NT9 stainless steel, on the microstructure and tribological properties of cold sprayed (Cr3C2-25(Ni20Cr))-(Ni-graphite) coatings was investigated. Both coatings were dense and did not reveal any discontinuities at the interfaces. They had similar Cr3C2 and graphite contents. Their microstructures showed a variety of grain sizes of the matrix phase between the inner part of the splat, showing large ones, and their boundaries, where elongated and nanostructured grains were formed during the deposition process. The coating deposited on the steel substrate revealed a slightly higher hardness and lower abrasive wear with the Al2O3 loose abrasive particles. The force required to destroy the durability of the coating–steel substrate system in the three-point bending test was higher than those of the other ones. The cermet deposit cold sprayed on steel and examined at 25 °C under 10 N revealed the best wear resistance and the lowest friction coefficient.
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This research focuses on studying the powder reactions and coating microstructures produced by thermal spraying various particle sized (micro and nano-sized) tungsten carbide cobalt powder. High Velocity Oxy Fuel (HVOF) and Atmospheric Plasma thermal spray methods were used to perform a multi-layer coating applied onto carbon steel specimens (typical of those used in oil/gas industry). The thermally sprayed powders/coatings were analyzed using X-ray powder diffraction (XRD), Environmental scanning electron microscope (ESEM), and Energy Dispersive Spectrometry (EDS) to predict the metallic powder reactions. In addition, surface roughness measurement of the coating, hardness assessment, and evaluation of coating porosity and adhesion were conducted to determine the coating characterization. The comparative metallographic results of the various test coatings are discussed. Results show that higher phase transformation occurred in the plasma thermal spray and more cobalt evaporation occurred during the deposition of the nano-sized powder. The plasma spray of nano-sized powders yielded lower coating roughness (from 5.5 μm to 4.6 μm), whereas it increased the roughness when sprayed with the HVOF system (from 3.1 μm to 4.3 μm). Nano-sized powders improved porosity but not significantly (to 1.2%). However, the higher percentage of nano-sized particles in the starting powder increased the hardness in the coatings to 1367 HV. Among the four produced coatings, the results concluded that the coating produced by HVOF using nanostructured WC–12Co powder was found to have the best coating microstructure due to its low porosity, high density, good adhesion, and fracture resistance.
The present investigation has been carried out in order to study the fatigue behavior of a SAE 1045 steel substrate coated with a WC–10Co–4Cr cermet, of approximately 200 μm thick, deposited by HVOF thermal spraying. Particular emphasis has been paid to the influence of the substrate roughness prior to HVOF deposition, as a mean of improving the mechanical bonding of the coating, on the fatigue life of the coated system. Fatigue tests were conducted under rotating bending conditions (R = −1), employing samples with different surface roughness, including as-polished, fine-grinding with abrasive paper grit 400 and coated and grit blasted with alumina particles of two different sizes (<1 and <3 mm, respectively) and coated. For comparative purposes, tests were also carried out on as-grit blasted specimens employing alumina particles of 3 mm in size. Fatigue tests were carried out at maximum alternating stresses in the range of 358–588 MPa, depending on the condition of the material, in order that the number of cycles to fracture varied in the range of 105 to 106. Selected samples tested at different applied stresses were analyzed after fracture by SEM techniques, which allowed the determination of the crack nucleation and propagation sequence. The results indicate that the presence of the cermet coating gives rise to a delay in the initiation of fatigue cracks at deep notches and alumina particles embedded at the steel substrate after grit blasting, leading to an insignificant fatigue strength debit of the coated specimens, in comparison with the as-polished ones. Fine-grinding, on the other hand, impairs the mechanical bonding of the coating, giving rise to its delamination from the substrate at elevated maximum alternating stresses.
The objective of this program was to generate a life prediction model for electron-beam-physical vapor deposited (EB-PVD) zirconia thermal barrier coating (TBC) on gas turbine engine components. Specific activities involved in development of the EB-PVD life prediction model included measurement of EB-PVD ceramic physical and mechanical properties and adherence strength, measurement of the thermally grown oxide (TGO) growth kinetics, generation of quantitative cyclic thermal spallation life data, and development of a spallation life prediction model. Life data useful for model development was obtained by exposing instrumented, EB-PVD ceramic coated cylindrical specimens in a jet fueled burner rig. Monotonic compression and tensile mechanical tests and physical property tests were conducted to obtain the EB-PVD ceramic behavior required for burner rig specimen analysis. As part of that effort, a nonlinear constitutive model was developed for the EB-PVD ceramic. Spallation failure of the EB-PVD TBC system consistently occurred at the TGO-metal interface. Calculated out-of-plane stresses were a small fraction of that required to statically fail the TGO. Thus, EB-PVD spallation was attributed to the interfacial cracking caused by in-plane TGO strains. Since TGO mechanical properties were not measured in this program, calculation of the burner rig specimen TGO in-plane strains was performed by using alumina properties. A life model based on maximum in-plane TGO tensile mechanical strain and TGO thickness correlated the burner rig specimen EB-PVD ceramic spallation lives within a factor of about plus or minus 2X.
High velocity oxy-fuel (HVOF) spray coating of micron (m) and nano (n) WC–Co powders has been studied for the improvement of durability of sliding machine components (SMC). In this work, optimal coating process (OCP) is obtained from the best surface properties of coating prepared by the Taguchi program. Hardness of coating is strongly dependent on powder size and spray parameters (SP) because of their strong influence on in-flight parameters. Hardness of n WC–Co is lower than that of m WC–Co since the degree of hard WC decomposition to less hard W2C, W and graphite is larger due to the larger specific surface area. Coating is porous since the decomposed graphite forms carbon oxide gasses by reaction with excess oxygen, and the gas evolution from coating makes porous coating. Porosity of n WC–Co coating is larger than that of m WC–Co because of larger evolution of carbon oxide gasses through n WC–Co coating. Friction coefficient (FC) is strongly dependent on the coating process (CP) since hardness and porosity of coating are dependent on the CP. FC of n WC–Co is lower than that of m WC–Co both at 25 °C and 500 °C, because of the more decomposition of n WC–Co. FC increases with increasing coating temperature (CT) from 25 °C to 500 °C both at m and n WC–Co because of the increase of adhesion by increasing surface temperature. WC–Co coating is very protective for the machine component since hardness of the coating is 2–3 times higher than those of machine component materials. Stick friction on WC–Co coating surface occurs easily at higher temperature due to the higher FC at the higher temperature.