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HVOF sprayed WC–Co coatings: Microstructure, mechanical properties and friction moment prediction
Materials and Design 31 (2010) 1431–1437
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HVOF sprayed WC–Co coatings: Microstructure, mechanical properties and friction moment prediction Tahar Sahraoui a,*, Sofiane Guessasma b, M. Ali Jeridane a, Mohamed Hadji a a b
Division of Surface Treatments and Materials (LTSM), University of Blida, BP 270, Blida 09000, Algeria INRA, rue de la géraudière, 44316 Nantes, France
a r t i c l e
i n f o
Article history: Received 25 July 2009 Accepted 25 August 2009 Available online 28 August 2009 Keywords: Cermets (A) Spraying (C) Microstructure (F)
a b s t r a c t This study aims at gaining a better understanding of the microstructural features that control the mechanical and the tribological performances of WC–12 wt.% Co coatings under High Velocity Oxygen Fuel (HVOF) spraying conditions. This paper looks at the influences of the HVOF process parameters for WC–12Co material on the microstructural and the tribological behaviours of the coatings. The correlation between the coating microstructure and the wear behaviour is investigated by observing and analysing the microstructure and by studying the friction moment using enhanced statistical tool based on neural computations. According to the experimental and the numerical results, it has been shown that the spray parameters affect the phase composition, hardness and porosity of HVOF sprayed WC–12Co coatings and the correlations with HVOF process parameters are fully predictable in the steady-state regime. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Electrolytic Hard Chromium (EHC) plating is a versatile technique offering a wide spectrum of applications where high wear resistance is needed. Due to the environmental restrictions against the use of EHC, HVOF spraying of cermets has become a promising alternative with equivalent production cost compared to chrome [1–7]. In addition to their related acceptable cost of production, these materials can handle severe wear mechanisms including erosion, adhesion, abrasion, among others [6]. High Velocity Oxygen Fuel (HVOF) is one of the leading thermal spray techniques [6,8]. It allows the fabrication of variety of coatings characterised by a low or intermediate melting point (mainly metals and polymers). The main advantage of HVOF compared to other thermal spray techniques is the ability to accelerate the melted powder particles of the feedstock material at a relatively large velocity. Large velocity handled using HVOF confers to the designed thick-formed components a fairly dense microstructure (see for example [2] and references quoted there). In addition, lower temperature regimes in HVOF compared to plasma spraying technique ensure less decomposition of WC. This statement does not mean that HVOF is the perfect processing solution. Indeed, compared to sintered WC–Co, HVOF coatings still suffer from decarburisation and decomposition as shown by several authors. These phenomena favour the formation of undesirable phases such as W2C, W and Co–W–C. * Corresponding author. Tel.: +213 (0) 21636609; fax: +213 (0) 21636604. E-mail address: [email protected] (T. Sahraoui). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.08.037
It has to be mentioned that the nature of the microstructure phases and their percentage in HVOF coatings depend mostly on the heat and mass transfer between the gas jet and the in-flight particles prior coating formation [9]. The nature and the stoichiometry of the fuel and the corresponding combustion gases are crucial operating conditions that tune the microstructure and thus the coating properties. Thermally sprayed chromium alloyed coating can be used as a single sprayed part or as a component in a duplex coating system [10,11]. Thanks to the development of HVOF technique, superior wear resistance can be obtained for varieties of complex coating structures based on WC material. Major properties of WC–Co coatings in duplex structures are large hardness, better adhesion and small difference in stiffness between substrate and the top layer part [10,12]. Indeed, WC–Co material provides a better distribution of the contact stress and thus avoids severe delamination problems. As a single part, the properties of WC-hard metal coatings using different additional alloying elements has been investigated [13,14] using conventional, fine [15] or nanostructured [16] powders. Compared to plasma sprayed coatings such as alumina–titania coatings, WC-based coatings exhibits the largest toughness because of the ductile matrix (principally due to metal binders such as Co or Ni) and the composite microstructure [14,17]. High wear resistance comes from the incorporation of hard WC particles, which increases the coating hardness. Regarding plasma sprayed of cermet-based coatings, improvement of wear resistance is achieved using laser remelting [18]. Because plasma-spraying results in large porosity content compared
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Table 1 Chemical composition of the WC-12 wt.%Co (WC–12%Co) powder. Commercial name
Production method
Element
Co
WC
C (free)
Fe
Amdry 1301
Agglomerated and sintered
wt.%
11.4
Bal.
<0.20
<2.0
to HVOF, laser processing lead to denser coating, harder and consequently wear resistant. The remelting technique is less efficient for HVOF coatings since no substantial gain in porosity reduction is observed [19]. Analysis of abrasive resistance under dry conditions shows that WC-coatings perform similarly to electrolytic hard chromium [17,20]. This statement cannot, however, be generalised to different tribological conditions [15]. Based on Pin-On-Disc experimental results, cermet-based coatings form a smooth and compact tribofilm by local plastic deformation, which confers to these materials similar performance to Cr2O3 and superior properties to Al2O3–TiO2 coatings. They key parameter for the best performance of cermet-based coatings is related to the stability of the tribofilm, which is usually brittle. If the coating process conditions are not well optimised, critical contact pressure would enhance tribofilm detachment, which in turn provokes severe wear. In such a way, the purpose of the present work is to study the effect of HVOF process on the wear properties of WC– 12%Co coatings from the optimisation point of view. Several HVOF operating conditions are used to control the microstructure of WC-based coating and their in-service properties. This control is mainly impacted by the presence of different cermets-based carbides that are the key factors to improve the coating mechanical performance. Among the interesting microstructural effects are the spatial distribution and size of carbides as well as the amount of free carbon in the matrix. Indeed, excellent wear resistance is obtained when the material contains a large ratio and a homogenised distribution of fine WC particles [13,21]. Generally speaking, severe decarburisation acts against wear performance at high design temperature, as this is the case of thermal spray techniques. In order to stick to the optimisation problem, a modelling technique is developed to predict optimal performance with regard to process conditions. In a more general context, Modelling activity has become an increasing route for studying mechanical performance and wear behaviour of WC-based coatings. Finite element method is used to assess fracture behaviour of HVOF sprayed WC–Co coatings under different load conditions [22]. The same methodology is also used for the prediction of residual stress in duplex structures [10,12]. Simulation of thermal processes using analytical approaches is undertaken to assess optimal conditions for the formation of WC-based coatings [3,23]. Modelling and Control of coating properties is also undertaken either using deterministic/ stochastic [24,25] or enhanced statistical approaches [20]. In particular, neural computation is extensively used to predict causal correlation between operating conditions and tribological behaviour of varieties of coatings [26–28]. Neural computation methodology is used here to investigate the effects of HVOF operating parameters on the microstructural, mechanical and tribological behaviours of thermal sprayed coatings [20,29].
Scanning electron microscopy (SEM: JEOL JSM. 5800LV), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (Cu Ka radia0 tion; i.e., 1.54 Å A wavelength) are used for powder and coatings observation and analysis. CDS Sultzer-Mecto gun (barrel size = 3 in.) is used to process WC–12%Co powder in an HVOF facility at a feed rate of 52 g min1. Coatings are designed using different HVOF operating conditions and deposited on ordinary steel (AFNOR 25CD4) substrates. Disc samples (dimensions: ø 25 mm h 20 mm) are mounted on a rotated (angular velocity = 160 rpm) cylindrical holder. The standoff distance (distance from the gun-substrate) is varied from 200 mm to 300 mm. A mixture of methane and oxygen is used to melt the feedstock powder. The fuel flow rate is varied from 145 N l min1 to 190 N l min1. The oxygen flow rate is fixed to 420 SLPM. Nitrogen is used as a carrier gas for the feedstock powder at a flow rate of about 20 SLPM. The horizontal flame jet is moved vertically while powder injection, thanks to an automatic manipulator, at a transverse speed of 53 mm s1. More than 30 passes are required to produce a fairly thick coating of about 250 lm in thickness. Samples are cooled with compressed air jets during and after spraying to obtain optimum spraying conditions. Four combinations of process parameters are used (see Table 2) for which the corresponding coating nomenclature ranges from A to D. Metallographic preparation consisted in: sample cutting using an abrasive saw, sample mounting in epoxy rings and pre-polishing and polishing using diamond slurries on an automatic polishing system to enhance reproducibility. Microstructures of studied coatings are determined using optical microscopy (Carl Zeiss2) at different magnifications. Illumination of the samples is kept constant for all conditions. Image acquisition is handled using CCD camera (SONY DSC-80) coupled to an optical microscope. Image analysis is undertaken using NIH image software from the public domain. Wear performance testing is based on an Amsler A135 tribometer. Fig. 1, illustrates the experimental configuration. The coated discs are sliding against brass discs (counter-samples). The force with which the samples are pressed together during the tests is 490 N. The radial velocities are 0.52 m s1 for the coated samples (down disc) and 0.47 m s1 for the brass discs (upper disc), inducing a sliding between the discs of 10%. The tests are carried out without any lubrication. Despite that friction coefficient is often used to quantify the wear of sliding parts [30,31], friction moment can also be used as a physical parameter to assess in varieties of sliding contact systems [32]. In a typical adhesive wear testing using Amsler machine, both friction moment and friction coefficient are related using the following relationship [33].
c¼ 2. Experimental layout The considered feedstock is WC–12%Co (Amdry1 1301). Table 1 presents the chemical composition and the manufacturing method of the powder. 1
Amdry : Sulzer-Metco, Rigackerstrasse 16, 5160 Wohlen, Switzerland.
M FR
ð1Þ
where F is the applied force. R is the radius of the counterpart disc. M is the friction moment. c is the friction coefficient. In our case, the friction moment is used to assess the influence of the processing parameters on wear behaviour of the studied materials. 2
Carl Zeiss : Light Microscopy, P.O.B. 4041 D-37030 Goettingen, GERMANY.
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T. Sahraoui et al. / Materials and Design 31 (2010) 1431–1437 Table 2 Operating conditions used to process WC–12%Co coatings using HVOF technique. #
Stand-off distance (mm)
Fuel flow rate (N l min1)
Powder composition
A B C D
300 300 300 200
150 190 180 145
WC, WC, WC, WC,
WC2, WC2, WC2, WC2,
Co3W3C, Co3W3C, Co3W3C, Co3W3C,
Coating composition
Co Co Co Co
WC WC, W6C2.54, W3C, Co3C, CoCx WC, W6C2.54, W3C, Co3C, CoCx WC, W2C
Larger W2C and elementary tungsten fractions in the deposited material are associated to fine powders [15]. This is probably due to small momentum inertia of small particles during their flight [25]. Conventional powders contain higher percent of undesirable brittle phases (W2C, CoxWyCz, etc.) compared to nanostructured ones [16]. Small WC size in the starting powder decreases the amount of WC phase decomposition.
Fig. 1. Amsler 135 tribometer.
3. Neural computation analysis Wear analysis is based on an enhanced statistical approach described in previous studies [20,28,29]. In the following, we stick to concise formulation of neural computation to highlight those aspects related to the present problem. A small-size feed forward neural network is built. Three inputs (fuel flow rate, stand-off distance and sliding distance) are related to the unique output of the wear analysis, namely friction moment. Table 3 gives the parameters ranges considered in this work. The experimental database required to feed the neural network includes about 600 samples. Each sample represents a sliding point (friction moment vs. sliding distance). Neural network optimisation is handled using a quick propagation training paradigm coupled to a testing procedure. After 1000 iterations, the residual error is less than 2.3%. The optimal neural net structure comprises only four neurons between input and output layers. 4. Results and discussion A proper choice of feedstock powder is known to highly influence phase composition in the final microstructure. Some of the main influences of powder characteristics can be summarized into several points.
Analysis of powder particle morphology of WC–12%Co material reveals a highly homogeneous powder with particle size between 11 and 45 lm (Fig. 2). Two populations are clearly discriminated: dark grey particles with smooth surface and clear grey particles having a rough surface. The last category is characterised by a higher ratio of the link phase (Co) and a small amount of free carbon. X-ray diffraction analysis of the starting powder reveals a main WC peak followed by lower intensities of WC2 and Co3W3C (Fig. 3). Small quantities of cubic elemental Co are also detected. The properties and performance of tungsten carbide cobalt coatings are attributed to several factors among them: carbide size, shape and distribution, matrix hardness and toughness, concentration of free carbon in the cobalt matrix. The manufactured coatings contain a high concentration of tungsten monocarbide crystals (WC), as distinguished by the higher proportion of dark grey phase (Fig. 4). Numerous authors have shown that several secondary phases appear in the as-sprayed coating despite the absence of any of these phases in the starting material [9,34]. The decarburisation of the WC–Co is driven by the amount of oxygen available in the supersonic jet because firstly of air entrainment and secondly of residual O2 in the spray gun [14]. The decomposition of WC to other species such as W2C is depicted in Table 1. It is also clearly established in a number of related studies [9,14]. In the work of Stewart et al. [9], it has been established that the formation of W2C upon splat quenching is caused by dissolution of WC in Co matrix whereas the formation of elementary W depended on the composition of the starting powder. Yang et al. [21] showed that larger degree of WC decomposition is correlated to a smaller carbide grain size in the starting powder (see Table 4). X-ray patterns of fabricated coatings depict broader peaks compared to the powder pattern. Peak broadening can be related to both microstrain development in the microstructure and small crystalline size. Microstrain development has to deal with plastic deformation of unmolten particles whereas small crystalline size
Table 3 Results of the coating characterisation.
a
#
Coatings
Surface
Microstructure
Parameters
Thickness (lm)
Average roughness Ra (lm)
A B C D
336 ± 40 300 ± 16 286 ± 24 248 ± 35
4.24 ± 0.84 3.80 ± 0.68 3.43 ± 0.40 <<–>>
Weight loss of the brass counterpart.
In-service properties
Porosity ratio (%)
Mechanical properties Hardness (HV0.3Kg f)
Friction moment (N M)
Wear work (–)
Weight lossa (g)
0.23 0.69 0.50 0.38
977 ± 57 846 ± 34 993 ± 155 1253 ± 176
1.45 1.8 2.3 1.5
71 91 98 46
1.623 1.955 1.608 2.800
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Fig. 2. (a) SEM micrograph of WC–12%Co powder. (b) A zoomed view of a typical WC–12%Co particle detailing the particle morphology and surface roughness.
Table 4 Parameter window considered in neural computation.
WC-12%Co. Powder
Intensity
WC
30
WC W2C
WC W2C
35
W2C
Co3W3C
40
45 2 theta
Category
Parameter
Minimum
Maximum
Input 2 3 4
Stand-off distance (mm) Fuel flowrate (SLPM) Sliding distance (mm) Friction moment (N M)
200 140 0 0
300 200 2000 Not restricted
W2C
50
55
60
Fig. 3. X-ray diffraction spectrum of the WC–12Co powder.
is the consequence of rapid solidification of droplets upon substrate impingement [35]. Because of the reduced size range of inflight particles, it is believed that change in crystalline size is the parameter explaining peak broadening. Indeed, the absence of large size particles would normally lead to higher HVOF efficiency and a complete melting of in-flight particles. The presence of relatively large humps for 40° < h < 50° indicates the presence of nanocrystalline or amorphous Co phases as reported by several authors [17,36]. Microhardness values evolve between 810 and 1400. In the average, the observed hardness score range is comparable to what is commonly found for WC-based coatings [3,4,13,14,19]. The larg-
est hardness score is that of coating D. It corresponds to the smallest stand-off distance. In the work of Zhao et al. [4,7], the authors predicted an inverse correlation between WC-based coating hardness and spray distance. Such a correlation is mainly explained by large in-flight particle velocity and temperature when the standoff distance is small. In the same work [4,7], harder coatings are correlated to a large fuel flow rate, which is not the case of coatings A to C. Despite the large stand-off distance condition for coating A, B and C, the hardness of coating A seems to be unpredictable based on the above discussion. All these processing effects are related to complex physical phenomena depending on the microstructure architecture and composition. It is, for example, commonly admitted that: Hardness increases with the decrease of porosity level and improvement of splat cohesion [4,21]. These last factors are favoured by the increase of velocity and temperature of the inflight particles [3,7,21].
Fig. 4. (a) Top view of the surface topology of as-sprayed WC–12Co coating (condition C). (b) Phase contrast in WC–12Co coating (condition D).
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Hardness is sensitive to the amount of W2C in the coating since it is harder than WC phase [14,21]. The higher degree of WC decomposition, the better is the hardness. Conversely, retention of WC and Co phases lead to low hardness levels [21,37].
0.25
Prediction Linear fit (Num=0.98 x Exp , R²=0.79 ) 0.20
Num (-)
0.15
0.10
0.05
0.00 0.00
0.05
0.10
0.15
0.20
0.25
Exp (-) Fig. 5. Comparison between normalized experimental and numerical friction moment values for all studied conditions.
Fuel flow rate (SLPM) Numerical Experimental 150 Coating # A 160 170 180 Coating # C 190 Coating # B 200
0.40 0.35
Friction moment (-)
b
0.50 0.45
0.30 0.25
Porosity level obtained using HVOF technique is usually smaller than plasma techniques. In our study, the porosity content is less 0.7%, similar roughly to the work of Zhao et al. [4,7]. Larger porosity ratios (>0.9%) can be obtained using the same spraying process [13]. In the worst case, porosity content does not exceed 6%. Denser coatings are usually correlated to small stand-off distance and large fuel flow rates [4,24,25]. In our case, coating D seems to follow this trend whereas the result obtained for coating A is not fully converging towards this explanation. Large porosity ratio is generally obtained when the control of process conditions is not optimised. Simulation results show for example that optimal conditions for a fairly dense coating require a large total flow rate, a minimum entrainment of in-flight particles by the gas carrier and optimal ratio of fuel/oxygen [25]. All these conditions increase the
0.15
Friction moment (-)
a
Hardness of coatings depends on the carbide size. The general trend is that large hardness is correlated to a small carbide size [21,38]. In some cases, especially when using indentation techniques, an inverse correlation can be observed [21]. This is attributed to the fact that indentation footprints performed under large loads measure the effective properties of the coating including porosity and other structural features [39]. A large substrate temperature can be correlated to higher hardness [4] because of the improvement of splat flattening during spraying and consequently the improvement of coating cohesion. A small stand-off distance would result in such effect [40].
0.20 0.15
0.10
Stand-off distance (mm) Numerical Experimental 200 Coating # D 225 250 275 300
0.05
0.10 0.05
0.00
0.00 0
500
1000
1500
Sliding distance (m)
2000
0
500
1000
1500
2000
Sliding distance (m)
c
Fig. 6. (a) Predictive analysis of the influence of fuel flow rate. (b) Predictive analysis of the influence of stand-off distance. (c) Predictive analysis of the influence of both parameters on the friction moment.
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melting degree and velocity of in-flight particles resulting in optimal particle flattening. Generally speaking, HVOF coatings have superior surface finishing status compared to plasma sprayed coatings but comparable to electroplating process. In our case, average roughness of studied coatings is smaller than 5.1 lm. Bolelli et al. [17] report comparable values (less than 5 lm). Fig. 5 compares the experimental and predicted friction moments for all studied conditions. Each point represents here a couple of predicted and experimental sliding points. Knowing the fact that a perfect match between experimental and numerical trends would mean a slope of unity, the linear fit approximation gives a smaller slope with an acceptable correlation factor (R2 = 0.79). The major reasons explaining this slight deviation is, first, the misunderstanding of the largest friction moments (corresponding here to the transition regime) and second, the jagged character of the experimental friction moments at any sliding distance. Fig. 6a depicts the evolution of the friction moment as function of sliding distance for different fuel flow rates. Also are represented those combinations corresponding to coating A, B and C. Here, the stand-off distance is fixed to 300 mm. It is found that the friction moment evolves nonlinearly with the increase of fuel flow rate. In particular, for any sliding distance beyond the transition regime, the friction moment increases first with the increase of the fuel flow rate up to 170 SLPM before it decreases significantly. The parabolic evolution between the friction moment and the fuel flow rate is clearly demonstrated by both the numerical and the experimental trends. We should mention that this non-linear effect of the fuel flow rate could be inferred to the flame energy available to melt feedstock powder, which in turn has a direct consequence on the microstructure characteristics. When varying the stand-off distance, it is predicted that the friction moment increases with the increase of the stand-off distance whatever is the fuel flow rate (Fig. 6b). This increase becomes moderate when the stand-off distance is beyond 225 mm. Further increase would probably affects the friction moment because the process efficiency decreases (small in-flight velocity and temperature). However, this effect is not highlighted enough through the process parameter window considered in this study. Starting from several combinations of fuel flow rates and standoff distances, the optimised neural network is able to predict, in the studied parameter window, the corresponding friction moment. Fig. 6c illustrates such a result for a fixed sliding distance of 2000 m. Both effects of the process parameters described earlier are depicted in the friction moment map. The largest friction moments correspond to a large central region for which the product of the fuel flow rate and stand-off distance increases. This region is bounded by the following combinations: (200 mm, 150 SLPM) and (290 mm, 180 SLPM). Regarding the weight loss correlation with hardness score, if we exclude coating D, weight loss is negatively correlated to hardness. Such correlation has to be handled with care since only a single measurement is available for each condition. In a more general context, weight loss–hardness relationship is also found for abrasion resistance in other materials [41]. In the work of Zhao et al. [7], the authors found that wear rate loss are similar. The authors claim that almost coatings had similar weight loss using wheel abrasion test. However, a rough tendency can be deduced. It can be related to a genuine effect of the total gas flow rate, which improves in-flight particles characteristics. In the counter part, Pin-On-Disc experiments revealed in the same work no clear correlation between hardness and weight loss. Following the work of Yang et al. [21], the sliding wear rate of WC–12%Co against alumina is more related to the carbide grain size rather than to hardness and fracture toughness of the studied coating. Wank et al. [15] showed that wear of HVOF WC-based coatings
with different Co content decreases with increasing coating hardness. 5. Summary The design of HVOF WC–12Co coatings results in different phase compositions depending on the spray conditions. For all studied conditions, the porosity content is less than 1%, which is a typical result obtained using HVOF technique. Substantial improvement of coating hardness can be obtained using the smallest stand-off distance and fuel flow rate but this does not mean best tribological performance. Indeed, the largest friction moment is obtained in the opposite case. Neural computation is used complementary to coating characterisation to study the tribological performance of the designed coatings. The use of neural computation technique cannot be justified for the analysis of hardness, porosity results because of the small umber of available data. However, it offers a major advantage in the prediction of friction moment – sliding distance correlation for all studied conditions. Indeed, the statistical analysis of the wear properties of WC– 12Co coatings reveals that possible correlations with HVOF process parameters are fully predictable in the steady-state regime. In particular, friction moment is found to increase linearly with the increase of the fuel flow rate. In addition, friction moment has a parabolic correlation with the stand-off distance. References [1] Legg KO, Graham M, Chang P, Rastagar F, Gonzales A, Sartwell B. The replacement of electroplating. Surf Coat Technol 1996;81:99–105. [2] Rastegar F, Richardson DE. Alternative to chrome: HVOF cermet coatings for high horse power diesel engines. Surf Coat Technol 1997;90:156–63. [3] Sobolev V, Guilemany JM, Miguel JR, Calero JA. Influence of thermal processes on coating formation during high velocity oxy-fuel (HVOF) spraying of WC–Ni powder particles. Surf Coat Technol 1996;82:121–9. [4] Zhao L, Maurer M, Fischer F, Lugscheider E. Study of HVOF spraying of WC– CoCr using on-line particle monitoring. Surf Coat Technol 2004;185:160–5. [5] Nakajima A, Mawatari T, Yoshida M, Tani K, Nakahira A. Effects of coating thickness and slip ratio on durability of thermally sprayed WC cermet coating in rolling/sliding contact. Wear 2000;241:166–73. [6] Stokes J, Looney L. HVOF system definition to maximise the thickness of formed components. Surf Coat Technol 2001;148:18–24. [7] Zhao L, Maurer M, Fischer F, Dicks R, Lugscheider E. Influence of spray parameters on the particle in-flight properties and the properties of HVOF coating of WC–CoCr. Wear 2004;257:41–6. [8] Ak NF, Tekmen C, Ozdemir I, Soykan HS, Celik E. NiCr coatings on stainless steel by HVOF technique. Surf Coat Technol 2003;174–175:1070–3. [9] Stewart DA, Shipway PH, McCartney DG. Microstructural evolution in thermally sprayed WC–Co coatings: comparison between nanocomposite and conventional starting powders. Acta Mater 2000;48:1593–604. [10] Bemporad E, Sebastiani M, Casadei F, Carassiti F. Modelling, production and characterisation of duplex coatings (HVOF and PVD) on Ti–6Al–4V substrate for specific mechanical applications. Surf. Coat. Technol. 2007;201:7652–62. [11] Bolelli G, Lusvarghi L, Montecchi M, Mantini PF, Pitacco F, Volz H, et al. HVOF-sprayed WC–Co as hard interlayer for DLC films. Surf Coat Technol 2008;5–7:699–703. [12] Bemporad E, Sebastiani M, De Felicis D, Carassiti F, Valle R, Casadei F. Production and characterization of duplex coatings (HVOF and PVD) on Ti–6Al–4V substrate. Thin Solid Films 2006;515:186–94. [13] Berger L-M, Saaro S, Naumann T, Wiener M, Weihnacht V, Thiele S, et al. Microstructure and properties of HVOF-sprayed chromium alloyed WC–Co and WC–Ni coatings. Surf Coat Technol 2008;202:4417–21. [14] Celik E, Culha O, Uyulgan B, Ak Azem NF, Ozdemir NF, Turk A. Assessment of microstructural and mechanical properties of HVOF sprayed WC-based cermet coatings for a roller cylinder. Surf Coat Technol 2006;200:4320–8. [15] Wank A, Wielage B, Pokhmurska H, Friesen E, Reisel G. Comparison of hardmetal and hard chromium coatings under different tribological conditions. Surf Coat Technol 2006;201:1975–80. [16] He J, Schoenung JM. A review on nanostructured WC–Co coatings. Surf Coat Technol 2002;157:72–9. [17] Bolelli G, Cannillo V, Lusvarghi L, Manfredini T. Wear behaviour of thermally sprayed ceramic oxide coatings. Wear 2006;261:1298–315. [18] Mateos J, Cuetos JM, Fernández E, Vijande R. Tribological behaviour of plasmasprayed WC coatings with and without laser remelting. Wear 2000;239: 274–81. [19] Chen H, Xu C, Zhou Q, Hutchings IM, Shipway PH, Liu J. Micro-scale abrasive wear behaviour of HVOF sprayed and laser-remelted conventional and nanostructured WC–Co coatings. Wear 2005;258:333–8.
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[31] Wong KC, Lub X, Cotter J, Eadie DT, Wong PC, Mitchell KAR. Surface and friction characterization of MoS2 and WS2 third body thin films under simulated wheel/rail rolling–sliding contact. Wear 2008;264:526–34. [32] Bishop NE, Waldow F, Morlock MM. Friction moments of large metal-on-metal hip joint bearings and other modern designs. Med Eng Phys 2008;30:1057–64. [33] Qiao H-B, Guo Q, Tian A-G, Pan G-L, Xu L-B. A study on friction and wear characteristics of nanometer Al2O3 /PEEK composites under the dry sliding condition. Tribol Int 2007;40: 105–10. [34] Celika E, Culha O, Uyulgan B, Ak Azem NF, Ozdemir NF, Turk A. Assessment of microstructural and mechanical properties of HVOF sprayed WC-based cermet coatings for a roller cylinder. Surf Coat Technol 2006;200:4320–8. [35] Bolelli G, Lusvarghi L, Giovanardi R. A comparison between the corrosion resistances of some HVOF-sprayed metal alloy coatings. Surf Coat Technol 2008;202:4793–809. [36] Bolelli G, Giovanardi R, Lusvarghi L, Manfredini T. Corrosion resistance of HVOF-sprayed coatings for hard chrome replacement. Corros Sci 2006;48:3375–97. [37] Chivavibul P, Watanabe M, Kuroda S, Shinoda K. Effects of carbide size and Co content on the microstructure and mechanical properties of HVOF-sprayed WC–Co coatings. Surf Coat Technol 2007;202:509–21. [38] Usmani S, Sampath S, Houck DL DL. Effect of carbide grain size on the sliding and abrasive wear behavior of thermally sprayed WC–Co coatings. Tribol Trans 1997;40:470–8. [39] Alcalá J, Gaudette F, Suresh S, Sampath S. Instrumented spherical microindentation of plasma-sprayed coatings. Mater Sci Eng A 2001;316:1–10. [40] Zhao L, Lugscheider E. Influence of the spraying processes on the properties of 316 L stainless steel coatings. Surf Coat Technol 2003;162:6–10. [41] Bolelli G, Cannillo V, Lusvarghi L, Ricco S. Mechanical and tribological properties of electrolytic hard chrome and HVOF-sprayed coatings. Surf Coat Technol 2006;200:2995–3009.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
HVOF sprayed WC–Co coatings: Microstructure, mechanical properties and friction moment prediction Tahar Sahraoui a,*, Sofiane Guessasma b, M. Ali Jeridane a, Mohamed Hadji a a b
Division of Surface Treatments and Materials (LTSM), University of Blida, BP 270, Blida 09000, Algeria INRA, rue de la géraudière, 44316 Nantes, France
a r t i c l e
i n f o
Article history: Received 25 July 2009 Accepted 25 August 2009 Available online 28 August 2009 Keywords: Cermets (A) Spraying (C) Microstructure (F)
a b s t r a c t This study aims at gaining a better understanding of the microstructural features that control the mechanical and the tribological performances of WC–12 wt.% Co coatings under High Velocity Oxygen Fuel (HVOF) spraying conditions. This paper looks at the influences of the HVOF process parameters for WC–12Co material on the microstructural and the tribological behaviours of the coatings. The correlation between the coating microstructure and the wear behaviour is investigated by observing and analysing the microstructure and by studying the friction moment using enhanced statistical tool based on neural computations. According to the experimental and the numerical results, it has been shown that the spray parameters affect the phase composition, hardness and porosity of HVOF sprayed WC–12Co coatings and the correlations with HVOF process parameters are fully predictable in the steady-state regime. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Electrolytic Hard Chromium (EHC) plating is a versatile technique offering a wide spectrum of applications where high wear resistance is needed. Due to the environmental restrictions against the use of EHC, HVOF spraying of cermets has become a promising alternative with equivalent production cost compared to chrome [1–7]. In addition to their related acceptable cost of production, these materials can handle severe wear mechanisms including erosion, adhesion, abrasion, among others [6]. High Velocity Oxygen Fuel (HVOF) is one of the leading thermal spray techniques [6,8]. It allows the fabrication of variety of coatings characterised by a low or intermediate melting point (mainly metals and polymers). The main advantage of HVOF compared to other thermal spray techniques is the ability to accelerate the melted powder particles of the feedstock material at a relatively large velocity. Large velocity handled using HVOF confers to the designed thick-formed components a fairly dense microstructure (see for example [2] and references quoted there). In addition, lower temperature regimes in HVOF compared to plasma spraying technique ensure less decomposition of WC. This statement does not mean that HVOF is the perfect processing solution. Indeed, compared to sintered WC–Co, HVOF coatings still suffer from decarburisation and decomposition as shown by several authors. These phenomena favour the formation of undesirable phases such as W2C, W and Co–W–C. * Corresponding author. Tel.: +213 (0) 21636609; fax: +213 (0) 21636604. E-mail address: [email protected] (T. Sahraoui). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.08.037
It has to be mentioned that the nature of the microstructure phases and their percentage in HVOF coatings depend mostly on the heat and mass transfer between the gas jet and the in-flight particles prior coating formation [9]. The nature and the stoichiometry of the fuel and the corresponding combustion gases are crucial operating conditions that tune the microstructure and thus the coating properties. Thermally sprayed chromium alloyed coating can be used as a single sprayed part or as a component in a duplex coating system [10,11]. Thanks to the development of HVOF technique, superior wear resistance can be obtained for varieties of complex coating structures based on WC material. Major properties of WC–Co coatings in duplex structures are large hardness, better adhesion and small difference in stiffness between substrate and the top layer part [10,12]. Indeed, WC–Co material provides a better distribution of the contact stress and thus avoids severe delamination problems. As a single part, the properties of WC-hard metal coatings using different additional alloying elements has been investigated [13,14] using conventional, fine [15] or nanostructured [16] powders. Compared to plasma sprayed coatings such as alumina–titania coatings, WC-based coatings exhibits the largest toughness because of the ductile matrix (principally due to metal binders such as Co or Ni) and the composite microstructure [14,17]. High wear resistance comes from the incorporation of hard WC particles, which increases the coating hardness. Regarding plasma sprayed of cermet-based coatings, improvement of wear resistance is achieved using laser remelting [18]. Because plasma-spraying results in large porosity content compared
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Table 1 Chemical composition of the WC-12 wt.%Co (WC–12%Co) powder. Commercial name
Production method
Element
Co
WC
C (free)
Fe
Amdry 1301
Agglomerated and sintered
wt.%
11.4
Bal.
<0.20
<2.0
to HVOF, laser processing lead to denser coating, harder and consequently wear resistant. The remelting technique is less efficient for HVOF coatings since no substantial gain in porosity reduction is observed [19]. Analysis of abrasive resistance under dry conditions shows that WC-coatings perform similarly to electrolytic hard chromium [17,20]. This statement cannot, however, be generalised to different tribological conditions [15]. Based on Pin-On-Disc experimental results, cermet-based coatings form a smooth and compact tribofilm by local plastic deformation, which confers to these materials similar performance to Cr2O3 and superior properties to Al2O3–TiO2 coatings. They key parameter for the best performance of cermet-based coatings is related to the stability of the tribofilm, which is usually brittle. If the coating process conditions are not well optimised, critical contact pressure would enhance tribofilm detachment, which in turn provokes severe wear. In such a way, the purpose of the present work is to study the effect of HVOF process on the wear properties of WC– 12%Co coatings from the optimisation point of view. Several HVOF operating conditions are used to control the microstructure of WC-based coating and their in-service properties. This control is mainly impacted by the presence of different cermets-based carbides that are the key factors to improve the coating mechanical performance. Among the interesting microstructural effects are the spatial distribution and size of carbides as well as the amount of free carbon in the matrix. Indeed, excellent wear resistance is obtained when the material contains a large ratio and a homogenised distribution of fine WC particles [13,21]. Generally speaking, severe decarburisation acts against wear performance at high design temperature, as this is the case of thermal spray techniques. In order to stick to the optimisation problem, a modelling technique is developed to predict optimal performance with regard to process conditions. In a more general context, Modelling activity has become an increasing route for studying mechanical performance and wear behaviour of WC-based coatings. Finite element method is used to assess fracture behaviour of HVOF sprayed WC–Co coatings under different load conditions [22]. The same methodology is also used for the prediction of residual stress in duplex structures [10,12]. Simulation of thermal processes using analytical approaches is undertaken to assess optimal conditions for the formation of WC-based coatings [3,23]. Modelling and Control of coating properties is also undertaken either using deterministic/ stochastic [24,25] or enhanced statistical approaches [20]. In particular, neural computation is extensively used to predict causal correlation between operating conditions and tribological behaviour of varieties of coatings [26–28]. Neural computation methodology is used here to investigate the effects of HVOF operating parameters on the microstructural, mechanical and tribological behaviours of thermal sprayed coatings [20,29].
Scanning electron microscopy (SEM: JEOL JSM. 5800LV), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (Cu Ka radia0 tion; i.e., 1.54 Å A wavelength) are used for powder and coatings observation and analysis. CDS Sultzer-Mecto gun (barrel size = 3 in.) is used to process WC–12%Co powder in an HVOF facility at a feed rate of 52 g min1. Coatings are designed using different HVOF operating conditions and deposited on ordinary steel (AFNOR 25CD4) substrates. Disc samples (dimensions: ø 25 mm h 20 mm) are mounted on a rotated (angular velocity = 160 rpm) cylindrical holder. The standoff distance (distance from the gun-substrate) is varied from 200 mm to 300 mm. A mixture of methane and oxygen is used to melt the feedstock powder. The fuel flow rate is varied from 145 N l min1 to 190 N l min1. The oxygen flow rate is fixed to 420 SLPM. Nitrogen is used as a carrier gas for the feedstock powder at a flow rate of about 20 SLPM. The horizontal flame jet is moved vertically while powder injection, thanks to an automatic manipulator, at a transverse speed of 53 mm s1. More than 30 passes are required to produce a fairly thick coating of about 250 lm in thickness. Samples are cooled with compressed air jets during and after spraying to obtain optimum spraying conditions. Four combinations of process parameters are used (see Table 2) for which the corresponding coating nomenclature ranges from A to D. Metallographic preparation consisted in: sample cutting using an abrasive saw, sample mounting in epoxy rings and pre-polishing and polishing using diamond slurries on an automatic polishing system to enhance reproducibility. Microstructures of studied coatings are determined using optical microscopy (Carl Zeiss2) at different magnifications. Illumination of the samples is kept constant for all conditions. Image acquisition is handled using CCD camera (SONY DSC-80) coupled to an optical microscope. Image analysis is undertaken using NIH image software from the public domain. Wear performance testing is based on an Amsler A135 tribometer. Fig. 1, illustrates the experimental configuration. The coated discs are sliding against brass discs (counter-samples). The force with which the samples are pressed together during the tests is 490 N. The radial velocities are 0.52 m s1 for the coated samples (down disc) and 0.47 m s1 for the brass discs (upper disc), inducing a sliding between the discs of 10%. The tests are carried out without any lubrication. Despite that friction coefficient is often used to quantify the wear of sliding parts [30,31], friction moment can also be used as a physical parameter to assess in varieties of sliding contact systems [32]. In a typical adhesive wear testing using Amsler machine, both friction moment and friction coefficient are related using the following relationship [33].
c¼ 2. Experimental layout The considered feedstock is WC–12%Co (Amdry1 1301). Table 1 presents the chemical composition and the manufacturing method of the powder. 1
Amdry : Sulzer-Metco, Rigackerstrasse 16, 5160 Wohlen, Switzerland.
M FR
ð1Þ
where F is the applied force. R is the radius of the counterpart disc. M is the friction moment. c is the friction coefficient. In our case, the friction moment is used to assess the influence of the processing parameters on wear behaviour of the studied materials. 2
Carl Zeiss : Light Microscopy, P.O.B. 4041 D-37030 Goettingen, GERMANY.
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T. Sahraoui et al. / Materials and Design 31 (2010) 1431–1437 Table 2 Operating conditions used to process WC–12%Co coatings using HVOF technique. #
Stand-off distance (mm)
Fuel flow rate (N l min1)
Powder composition
A B C D
300 300 300 200
150 190 180 145
WC, WC, WC, WC,
WC2, WC2, WC2, WC2,
Co3W3C, Co3W3C, Co3W3C, Co3W3C,
Coating composition
Co Co Co Co
WC WC, W6C2.54, W3C, Co3C, CoCx WC, W6C2.54, W3C, Co3C, CoCx WC, W2C
Larger W2C and elementary tungsten fractions in the deposited material are associated to fine powders [15]. This is probably due to small momentum inertia of small particles during their flight [25]. Conventional powders contain higher percent of undesirable brittle phases (W2C, CoxWyCz, etc.) compared to nanostructured ones [16]. Small WC size in the starting powder decreases the amount of WC phase decomposition.
Fig. 1. Amsler 135 tribometer.
3. Neural computation analysis Wear analysis is based on an enhanced statistical approach described in previous studies [20,28,29]. In the following, we stick to concise formulation of neural computation to highlight those aspects related to the present problem. A small-size feed forward neural network is built. Three inputs (fuel flow rate, stand-off distance and sliding distance) are related to the unique output of the wear analysis, namely friction moment. Table 3 gives the parameters ranges considered in this work. The experimental database required to feed the neural network includes about 600 samples. Each sample represents a sliding point (friction moment vs. sliding distance). Neural network optimisation is handled using a quick propagation training paradigm coupled to a testing procedure. After 1000 iterations, the residual error is less than 2.3%. The optimal neural net structure comprises only four neurons between input and output layers. 4. Results and discussion A proper choice of feedstock powder is known to highly influence phase composition in the final microstructure. Some of the main influences of powder characteristics can be summarized into several points.
Analysis of powder particle morphology of WC–12%Co material reveals a highly homogeneous powder with particle size between 11 and 45 lm (Fig. 2). Two populations are clearly discriminated: dark grey particles with smooth surface and clear grey particles having a rough surface. The last category is characterised by a higher ratio of the link phase (Co) and a small amount of free carbon. X-ray diffraction analysis of the starting powder reveals a main WC peak followed by lower intensities of WC2 and Co3W3C (Fig. 3). Small quantities of cubic elemental Co are also detected. The properties and performance of tungsten carbide cobalt coatings are attributed to several factors among them: carbide size, shape and distribution, matrix hardness and toughness, concentration of free carbon in the cobalt matrix. The manufactured coatings contain a high concentration of tungsten monocarbide crystals (WC), as distinguished by the higher proportion of dark grey phase (Fig. 4). Numerous authors have shown that several secondary phases appear in the as-sprayed coating despite the absence of any of these phases in the starting material [9,34]. The decarburisation of the WC–Co is driven by the amount of oxygen available in the supersonic jet because firstly of air entrainment and secondly of residual O2 in the spray gun [14]. The decomposition of WC to other species such as W2C is depicted in Table 1. It is also clearly established in a number of related studies [9,14]. In the work of Stewart et al. [9], it has been established that the formation of W2C upon splat quenching is caused by dissolution of WC in Co matrix whereas the formation of elementary W depended on the composition of the starting powder. Yang et al. [21] showed that larger degree of WC decomposition is correlated to a smaller carbide grain size in the starting powder (see Table 4). X-ray patterns of fabricated coatings depict broader peaks compared to the powder pattern. Peak broadening can be related to both microstrain development in the microstructure and small crystalline size. Microstrain development has to deal with plastic deformation of unmolten particles whereas small crystalline size
Table 3 Results of the coating characterisation.
a
#
Coatings
Surface
Microstructure
Parameters
Thickness (lm)
Average roughness Ra (lm)
A B C D
336 ± 40 300 ± 16 286 ± 24 248 ± 35
4.24 ± 0.84 3.80 ± 0.68 3.43 ± 0.40 <<–>>
Weight loss of the brass counterpart.
In-service properties
Porosity ratio (%)
Mechanical properties Hardness (HV0.3Kg f)
Friction moment (N M)
Wear work (–)
Weight lossa (g)
0.23 0.69 0.50 0.38
977 ± 57 846 ± 34 993 ± 155 1253 ± 176
1.45 1.8 2.3 1.5
71 91 98 46
1.623 1.955 1.608 2.800
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Fig. 2. (a) SEM micrograph of WC–12%Co powder. (b) A zoomed view of a typical WC–12%Co particle detailing the particle morphology and surface roughness.
Table 4 Parameter window considered in neural computation.
WC-12%Co. Powder
Intensity
WC
30
WC W2C
WC W2C
35
W2C
Co3W3C
40
45 2 theta
Category
Parameter
Minimum
Maximum
Input 2 3 4
Stand-off distance (mm) Fuel flowrate (SLPM) Sliding distance (mm) Friction moment (N M)
200 140 0 0
300 200 2000 Not restricted
W2C
50
55
60
Fig. 3. X-ray diffraction spectrum of the WC–12Co powder.
is the consequence of rapid solidification of droplets upon substrate impingement [35]. Because of the reduced size range of inflight particles, it is believed that change in crystalline size is the parameter explaining peak broadening. Indeed, the absence of large size particles would normally lead to higher HVOF efficiency and a complete melting of in-flight particles. The presence of relatively large humps for 40° < h < 50° indicates the presence of nanocrystalline or amorphous Co phases as reported by several authors [17,36]. Microhardness values evolve between 810 and 1400. In the average, the observed hardness score range is comparable to what is commonly found for WC-based coatings [3,4,13,14,19]. The larg-
est hardness score is that of coating D. It corresponds to the smallest stand-off distance. In the work of Zhao et al. [4,7], the authors predicted an inverse correlation between WC-based coating hardness and spray distance. Such a correlation is mainly explained by large in-flight particle velocity and temperature when the standoff distance is small. In the same work [4,7], harder coatings are correlated to a large fuel flow rate, which is not the case of coatings A to C. Despite the large stand-off distance condition for coating A, B and C, the hardness of coating A seems to be unpredictable based on the above discussion. All these processing effects are related to complex physical phenomena depending on the microstructure architecture and composition. It is, for example, commonly admitted that: Hardness increases with the decrease of porosity level and improvement of splat cohesion [4,21]. These last factors are favoured by the increase of velocity and temperature of the inflight particles [3,7,21].
Fig. 4. (a) Top view of the surface topology of as-sprayed WC–12Co coating (condition C). (b) Phase contrast in WC–12Co coating (condition D).
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Hardness is sensitive to the amount of W2C in the coating since it is harder than WC phase [14,21]. The higher degree of WC decomposition, the better is the hardness. Conversely, retention of WC and Co phases lead to low hardness levels [21,37].
0.25
Prediction Linear fit (Num=0.98 x Exp , R²=0.79 ) 0.20
Num (-)
0.15
0.10
0.05
0.00 0.00
0.05
0.10
0.15
0.20
0.25
Exp (-) Fig. 5. Comparison between normalized experimental and numerical friction moment values for all studied conditions.
Fuel flow rate (SLPM) Numerical Experimental 150 Coating # A 160 170 180 Coating # C 190 Coating # B 200
0.40 0.35
Friction moment (-)
b
0.50 0.45
0.30 0.25
Porosity level obtained using HVOF technique is usually smaller than plasma techniques. In our study, the porosity content is less 0.7%, similar roughly to the work of Zhao et al. [4,7]. Larger porosity ratios (>0.9%) can be obtained using the same spraying process [13]. In the worst case, porosity content does not exceed 6%. Denser coatings are usually correlated to small stand-off distance and large fuel flow rates [4,24,25]. In our case, coating D seems to follow this trend whereas the result obtained for coating A is not fully converging towards this explanation. Large porosity ratio is generally obtained when the control of process conditions is not optimised. Simulation results show for example that optimal conditions for a fairly dense coating require a large total flow rate, a minimum entrainment of in-flight particles by the gas carrier and optimal ratio of fuel/oxygen [25]. All these conditions increase the
0.15
Friction moment (-)
a
Hardness of coatings depends on the carbide size. The general trend is that large hardness is correlated to a small carbide size [21,38]. In some cases, especially when using indentation techniques, an inverse correlation can be observed [21]. This is attributed to the fact that indentation footprints performed under large loads measure the effective properties of the coating including porosity and other structural features [39]. A large substrate temperature can be correlated to higher hardness [4] because of the improvement of splat flattening during spraying and consequently the improvement of coating cohesion. A small stand-off distance would result in such effect [40].
0.20 0.15
0.10
Stand-off distance (mm) Numerical Experimental 200 Coating # D 225 250 275 300
0.05
0.10 0.05
0.00
0.00 0
500
1000
1500
Sliding distance (m)
2000
0
500
1000
1500
2000
Sliding distance (m)
c
Fig. 6. (a) Predictive analysis of the influence of fuel flow rate. (b) Predictive analysis of the influence of stand-off distance. (c) Predictive analysis of the influence of both parameters on the friction moment.
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melting degree and velocity of in-flight particles resulting in optimal particle flattening. Generally speaking, HVOF coatings have superior surface finishing status compared to plasma sprayed coatings but comparable to electroplating process. In our case, average roughness of studied coatings is smaller than 5.1 lm. Bolelli et al. [17] report comparable values (less than 5 lm). Fig. 5 compares the experimental and predicted friction moments for all studied conditions. Each point represents here a couple of predicted and experimental sliding points. Knowing the fact that a perfect match between experimental and numerical trends would mean a slope of unity, the linear fit approximation gives a smaller slope with an acceptable correlation factor (R2 = 0.79). The major reasons explaining this slight deviation is, first, the misunderstanding of the largest friction moments (corresponding here to the transition regime) and second, the jagged character of the experimental friction moments at any sliding distance. Fig. 6a depicts the evolution of the friction moment as function of sliding distance for different fuel flow rates. Also are represented those combinations corresponding to coating A, B and C. Here, the stand-off distance is fixed to 300 mm. It is found that the friction moment evolves nonlinearly with the increase of fuel flow rate. In particular, for any sliding distance beyond the transition regime, the friction moment increases first with the increase of the fuel flow rate up to 170 SLPM before it decreases significantly. The parabolic evolution between the friction moment and the fuel flow rate is clearly demonstrated by both the numerical and the experimental trends. We should mention that this non-linear effect of the fuel flow rate could be inferred to the flame energy available to melt feedstock powder, which in turn has a direct consequence on the microstructure characteristics. When varying the stand-off distance, it is predicted that the friction moment increases with the increase of the stand-off distance whatever is the fuel flow rate (Fig. 6b). This increase becomes moderate when the stand-off distance is beyond 225 mm. Further increase would probably affects the friction moment because the process efficiency decreases (small in-flight velocity and temperature). However, this effect is not highlighted enough through the process parameter window considered in this study. Starting from several combinations of fuel flow rates and standoff distances, the optimised neural network is able to predict, in the studied parameter window, the corresponding friction moment. Fig. 6c illustrates such a result for a fixed sliding distance of 2000 m. Both effects of the process parameters described earlier are depicted in the friction moment map. The largest friction moments correspond to a large central region for which the product of the fuel flow rate and stand-off distance increases. This region is bounded by the following combinations: (200 mm, 150 SLPM) and (290 mm, 180 SLPM). Regarding the weight loss correlation with hardness score, if we exclude coating D, weight loss is negatively correlated to hardness. Such correlation has to be handled with care since only a single measurement is available for each condition. In a more general context, weight loss–hardness relationship is also found for abrasion resistance in other materials [41]. In the work of Zhao et al. [7], the authors found that wear rate loss are similar. The authors claim that almost coatings had similar weight loss using wheel abrasion test. However, a rough tendency can be deduced. It can be related to a genuine effect of the total gas flow rate, which improves in-flight particles characteristics. In the counter part, Pin-On-Disc experiments revealed in the same work no clear correlation between hardness and weight loss. Following the work of Yang et al. [21], the sliding wear rate of WC–12%Co against alumina is more related to the carbide grain size rather than to hardness and fracture toughness of the studied coating. Wank et al. [15] showed that wear of HVOF WC-based coatings
with different Co content decreases with increasing coating hardness. 5. Summary The design of HVOF WC–12Co coatings results in different phase compositions depending on the spray conditions. For all studied conditions, the porosity content is less than 1%, which is a typical result obtained using HVOF technique. Substantial improvement of coating hardness can be obtained using the smallest stand-off distance and fuel flow rate but this does not mean best tribological performance. Indeed, the largest friction moment is obtained in the opposite case. Neural computation is used complementary to coating characterisation to study the tribological performance of the designed coatings. The use of neural computation technique cannot be justified for the analysis of hardness, porosity results because of the small umber of available data. However, it offers a major advantage in the prediction of friction moment – sliding distance correlation for all studied conditions. Indeed, the statistical analysis of the wear properties of WC– 12Co coatings reveals that possible correlations with HVOF process parameters are fully predictable in the steady-state regime. In particular, friction moment is found to increase linearly with the increase of the fuel flow rate. In addition, friction moment has a parabolic correlation with the stand-off distance. References [1] Legg KO, Graham M, Chang P, Rastagar F, Gonzales A, Sartwell B. The replacement of electroplating. Surf Coat Technol 1996;81:99–105. [2] Rastegar F, Richardson DE. Alternative to chrome: HVOF cermet coatings for high horse power diesel engines. Surf Coat Technol 1997;90:156–63. [3] Sobolev V, Guilemany JM, Miguel JR, Calero JA. Influence of thermal processes on coating formation during high velocity oxy-fuel (HVOF) spraying of WC–Ni powder particles. Surf Coat Technol 1996;82:121–9. [4] Zhao L, Maurer M, Fischer F, Lugscheider E. Study of HVOF spraying of WC– CoCr using on-line particle monitoring. Surf Coat Technol 2004;185:160–5. [5] Nakajima A, Mawatari T, Yoshida M, Tani K, Nakahira A. Effects of coating thickness and slip ratio on durability of thermally sprayed WC cermet coating in rolling/sliding contact. Wear 2000;241:166–73. [6] Stokes J, Looney L. HVOF system definition to maximise the thickness of formed components. Surf Coat Technol 2001;148:18–24. [7] Zhao L, Maurer M, Fischer F, Dicks R, Lugscheider E. Influence of spray parameters on the particle in-flight properties and the properties of HVOF coating of WC–CoCr. Wear 2004;257:41–6. [8] Ak NF, Tekmen C, Ozdemir I, Soykan HS, Celik E. NiCr coatings on stainless steel by HVOF technique. Surf Coat Technol 2003;174–175:1070–3. [9] Stewart DA, Shipway PH, McCartney DG. Microstructural evolution in thermally sprayed WC–Co coatings: comparison between nanocomposite and conventional starting powders. Acta Mater 2000;48:1593–604. [10] Bemporad E, Sebastiani M, Casadei F, Carassiti F. Modelling, production and characterisation of duplex coatings (HVOF and PVD) on Ti–6Al–4V substrate for specific mechanical applications. Surf. Coat. Technol. 2007;201:7652–62. [11] Bolelli G, Lusvarghi L, Montecchi M, Mantini PF, Pitacco F, Volz H, et al. HVOF-sprayed WC–Co as hard interlayer for DLC films. Surf Coat Technol 2008;5–7:699–703. [12] Bemporad E, Sebastiani M, De Felicis D, Carassiti F, Valle R, Casadei F. Production and characterization of duplex coatings (HVOF and PVD) on Ti–6Al–4V substrate. Thin Solid Films 2006;515:186–94. [13] Berger L-M, Saaro S, Naumann T, Wiener M, Weihnacht V, Thiele S, et al. Microstructure and properties of HVOF-sprayed chromium alloyed WC–Co and WC–Ni coatings. Surf Coat Technol 2008;202:4417–21. [14] Celik E, Culha O, Uyulgan B, Ak Azem NF, Ozdemir NF, Turk A. Assessment of microstructural and mechanical properties of HVOF sprayed WC-based cermet coatings for a roller cylinder. Surf Coat Technol 2006;200:4320–8. [15] Wank A, Wielage B, Pokhmurska H, Friesen E, Reisel G. Comparison of hardmetal and hard chromium coatings under different tribological conditions. Surf Coat Technol 2006;201:1975–80. [16] He J, Schoenung JM. A review on nanostructured WC–Co coatings. Surf Coat Technol 2002;157:72–9. [17] Bolelli G, Cannillo V, Lusvarghi L, Manfredini T. Wear behaviour of thermally sprayed ceramic oxide coatings. Wear 2006;261:1298–315. [18] Mateos J, Cuetos JM, Fernández E, Vijande R. Tribological behaviour of plasmasprayed WC coatings with and without laser remelting. Wear 2000;239: 274–81. [19] Chen H, Xu C, Zhou Q, Hutchings IM, Shipway PH, Liu J. Micro-scale abrasive wear behaviour of HVOF sprayed and laser-remelted conventional and nanostructured WC–Co coatings. Wear 2005;258:333–8.
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