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Key Engineering Materials
ISSN: 1662-9795, Vol. 735, pp 225-229
doi:10.4028/www.scientific.net/KEM.735.225
© 2017 Trans Tech Publications, Switzerland
Submitted: 2017-01-23
Accepted: 2017-01-25
Online: 2017-05-16
Nanoindentation Evaluation of Suspension Thermal Sprayed
Nanocomposite WC-Co Coatings
Omar Ali1,a , Rehan Ahmed1,2,b * , Nadimul H. Faisal2,3,c, Nayef M. Al-Anazi4,d,
Youssef O. Elakwah2,e and Matheus F. A. Goosen2,f
1
School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK
2
Office of Research & Graduate Studies, Alfaisal University, P.O. Box 50927, Riyadh 11533,
Saudi Arabia
3
School of Engineering, Robert Gordon University, Garthdee Road, Aberdeen, AB107GJ, UK
4
Materials Performance Unit, R D C, Saudi Aramco, Dhahran, 31311, Saudi Arabia
a
oa63@hw.ac.uk, b*R.Ahmed@hw.ac.uk, cN.H.Faisal@rgu.ac.uk, dnayef.anazi@aramco.com,
e
yelakwah@alfaisal.edu, fMGoosen@alfaisal.edu
Keywords: Thermal spray coating, Nanoindentation, Suspension spraying, WC-Co
Abstract. The aim of this paper is to evaluate the microstructural and nanohardness characteristics of
tungsten carbide-cobalt (WC-Co) cermet coatings deposited by liquid suspension spraying.
Commercially available WC-Co coating powder was milled and water based suspension was
produced as feedstock for the thermal spray coating process. Microstructural evaluations of WC-Co
cermet coatings included XRD (X-Ray Diffraction) and SEM (Scanning Electron Microscopy). Post
spraying nanomechanical evaluations were conducted using a Berkovich nanoindenter. Results
indicated relatively higher modulus but lower hardness of suspension coatings. The load
displacement curves during nanoindentation were characteristic of the complex coating
microstructure showing signs of microcracking and pile-up.
Introduction
Nanocomposite coatings have found numerous practical applications in industry ranging from
biomedical prostheses, to electronic data storage, to lightweight and high strength components in the
automotive and aerospace fields [1-3]. The increase in importance in nanocomposites is highlighted
by the number of publications and patent applications which have shown average annual growth rates
of 33% and 24%, respectively, as reported by Gan et al. [1]. Kawahara [2] and Srinivasulu and Sa [3]
reviewed recent trends in advanced coatings and applications as well as outlining new opportunities
arising from the shift towards solid oxide fuel cell (SOFC) applications.
Sliding wear evaluation of nanostructured tungsten carbide-cobalt (WC-Co) coatings deposited by
suspension high velocity oxyfuel (S-HVOF) and conventional HVOF (Jet Kote (HVOF-JK) and
JP5000 (HVOF-JP)) spraying were evaluated by Ahmed et al. [4, 5]. Additionally, Huang et al. [6]
and Picas et al. [7] employed heat treatment to improve the mechanical properties of sprayed
coatings. Nanoindentation of WC-12Co thermal spray coatings has been used to evaluate the elastic
modulus and hardness of coating on the polished surface of the coatings [8, 9]. While there has been
much progress overall, limited research has been reported on the deposition and evaluation of
WC-cermet coatings.
The focus of this paper was to understand the nanoindentation behavior of nanocomposite WC-Co
coatings and to compare them with conventional WC-Co coatings.
Experimental
Coating Deposition. Two different coating methods consisting of suspension and conventional
HVOF spraying were used to produce WC-12 wt.% Co layered samples on steel substrates. For the
case of suspension spraying, an agglomerated and sintered WC-12 wt.% Co spray feedstock powder
(China) was employed as the start powder. Due to coarse particle size of the start powder, the milling
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
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226
Advanced Materials Research VII
of the feedstock material was performed in a planetary ball mill. Two different variants of S-HVOF
spraying were used in this investigation. These are labeled as condition #1 and condition #2 in Table
1. The number of passes influences the coating thickness.
Table 1. Spray conditions, thermal spraying system used to produce them and microhardness
Coating
No. of
Microhardness
Spray Condition
Spray Process
thickness [µm]
passes
HV2.9N
Condition # 1
HVOF-TopGun
150
40
843±17
Condition # 2
HVOF-TopGun
265
80
796±19
Condition # 3
HVOF-Jet Kote
924 ± 127
In order to compare the performance of nanostructured coatings with conventional coatings,
HVOF coatings using agglomerated and sintered powders were also prepared. This is indicated as
condition #3 in Table 1. A liquid fuel HVOF (Jet Kote) system was employed to deposit these
WC-12% Co coatings. Spraying was carried out using Kerosene as the fuel gas and oxygen as the
powder carrier gas. For all cases of coating deposition considered in this investigation, the substrate
material was sand blasted prior to the coating process to improve the adhesive strength.
Microstructural Evaluations. The microstructure of the powders and both alloys was observed
via scanning electron microscope (SEM). The chemical compositions of different phases developed
in the powders and coatings were determined via XRD with Cu-Kα radiation (wavelength =
1.5406Å).
Nanomechanical Evaluations. Nanoindentation testing which included nanohardness and elastic
modulus measurements were performed using a calibrated nanoindentation system (NanoTest™ Micro Materials Limited, UK) equipped with a standard Berkovich nanoindenter. Measurements
were performed at room temperature (~23°C) in load control mode at a load of 50 mN for spraying
condition #1 to #3. The indentation procedures were programmed as three segments of trapezoidal
shape with loading, hold and unloading segments. A set of five equally spaced measurements were
performed on the sample cross-sections at various distances from the coating substrate interface.
Each measurement set contained seven measurements and averaged values of each measurement set
are reported in the results section. The force-displacement (P-h) profiles were analyzed using
standard methods with the area function for the Berkovich indenter which was determined by
indentations into fused silica with elastic modulus of 69.9 GPa. The raw data (P-h profile) were
utilized to evaluate hardness and reduced elastic modulus (Er) using Oliver and Pharr method [10].
Results and Discussion
Microstructural Characterization. Fig. 1 shows the SEM observations of the as deposited coatings
(Table 1). Fig. 1(a) displays a typical SEM of the S-HVOF coating (condition #2), which was similar
to that deposited using condition #1. Fig. 1(b) illustrates the SEM of a conventional HVOF layer set
down under coating condition #3. The SEM in Fig. 1(a) demonstrates that the nano-structured
features are well distributed within the coating. The lamella structure is also consistent with good
wettability and flattening of powder particles on impact. Comparison of S-HVOF coatings observed
in Fig. 1(a) with conventional HVOF coatings shown in Fig. 1(b) indicates that the carbide size which
is typically around 3 to 5 microns in Fig. 2(b) is well preserved and distributed in the deposited layer.
The coating porosity is lower than that of the S-HVOF layer.
The XRD pattern of the conventional HVOF coating (Fig. 2(b)) showed sharp WC peaks which
are well retained in the deposition process. This indicated optimization of the HVOF coating
parameters with only a small amount of carburization around 2-theta of 39 degrees. Contrary to this,
the XRD patterns of S-HVOF coatings indicated not only retained WC peaks but also broadening of
peaks between 2-theta values of 35 to 45 degrees. This is consistent with the decarburization of
WC-Co coatings during coating deposition. Analysis of Fig. 2(a) shows that the STS deposition
process led to thermal decomposition of WC, since the X-ray spectra of as-sprayed coating point to
Key Engineering Materials Vol. 735
227
the occurrence of higher amounts of secondary phase tungsten carbide (W2C) than in the spray
powder, and also some eta phases (Co3W3C) which are expected in the broadened peak between
2-theta values of 35 to 45 degrees. This is consistent with published literature [4, 5].
a)
b)
Fig. 1. SEM observations of deposited coatings, a) cross-section observation of coating deposited
under spray condition #2, b) cross-section observation of coating deposited under spray condition #3.
Fig. 2. XRD pattern of WC-12wt.%Co (a) spray condition #2, (b) spray condition #3.
Fig. 3. Reduced indentation modulus (Er- left axis) and indentation hardness (H – right axis) of
WC-12 wt. % Co coatings and substrate. Zero at x-axis represents the location of coating-substrate
interface and the distances represented are measured from this interface.
Nanohardness. Fig. 3 demonstrates the reduced modulus and indentation hardness values in the
coating and substrate. The difference in the coating microstructure and phase composition influenced
the nanohardness of both layers. The average elastic modulus values indicate relatively uniformity
through thickness values for the S-HVOF coatings, which are higher than the conventional HVOF
coating (Condition #3). The standard deviation of conventional HVOF coating was also higher. The
average values are comparable to previously observed by values reported by Stewart et al. [11, 12] in
post-treated WC-Co coatings. The average hardness values of S-HVOF coatings point to a trend
which is opposite to that of indentation modulus, where spray condition #3 shows the higher
hardness. This is consistent with the microhardness values shown in Table 1.
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Advanced Materials Research VII
The load displacement (P-h) curves along with the SEM images of indents for S-HVOF coating in
Fig. 4 illustrate evidence of sink-in and pile-up of material around the indent contact residual
impression during the nanoindentation process. There is some indication of microcracking during
indentation as well. These features are consistent with the complex microstructure of these coatings
where secondary and carbide phases and porosity leads to microcracking and densification of
microstructure, respectively. For the indents (#1, 2, 3) mark in the coatings presented in Fig. 4, the
displacements are lower bound but very similar compared to the higher bound displacement in indent
(#4) mark in the substrate, representing significant difference in hardness. The Fig. 4 (i.e. zoomed
view in P-h loading/unloading curves for the indents (#1, 2, 3) mark) also displayd features of
‘pop-in’ during early stage of indenter loading cycle. Further investigation is required to confirm if
this is due to cracking or phase transformation.
Fig. 4. Indentation on coating produced under Condition #1 with respective hysteresis plots of
loading and unloading.
Conclusions
1) The microstructure of S-HVOF coatings indicate nano-composite features inherited from the
milled powder and decarburization during the coating process.
2) A comparison of S-HVOF and conventional HVOF coatings points toward phase transformations
occurring in the suspension spraying which led to nanocrystalline or amorphous phases. This
phase transformation in the conventional HVOF coatings was minimal.
3) The elastic modulus of S-HVOF coatings was on average higher than the conventional HVOF
coating, whereas a reverse trend was observed for the hardness values.
4) The load displacement curves show features which are consistent with the complex coating
microstructure with evidence of micro-cracking and pile-up.
Acknowledgements
Financial support of Saudi Aramco (Contract 6000074259) is acknowledged for the project
“Nano-Composite Carbide Coatings for Wear Resistance Applications”. Support of Fraunhofer IWS
(Germany) in preparation of STS coatings is also recognized.
Key Engineering Materials Vol. 735
229
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