Deposition and characterization of WC-Co hard-metal coatings by high velocity oxy-fuel process combined with dry-ice blasting

Accepted Manuscript Deposition and characterization of WC-Co hard-metal coatings by high velocity oxy-fuel process combined with dry-ice blasting

Shujuan Dong, Hanlin Liao PII: DOI: Reference:

S0263-4368(16)30599-6 doi: 10.1016/j.ijrmhm.2016.12.007 RMHM 4384

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Revised date: Accepted date:

28 September 2016 1 December 2016 11 December 2016

Please cite this article as: Shujuan Dong, Hanlin Liao , Deposition and characterization of WC-Co hard-metal coatings by high velocity oxy-fuel process combined with dryice blasting. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Rmhm(2016), doi: 10.1016/j.ijrmhm.2016.12.007

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ACCEPTED MANUSCRIPT Deposition and characterization of WC-Co hard-metal coatings by high velocity oxy-fuel process combined with dry-ice blasting Shujuan Dong a,, Hanlin Liao b a State Key Laboratory of Silicate Materials for Architectures, Wuhan University of

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Technology, Wuhan 430070, China

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b University Bourgogne Franche-Comte, IRTES EA7274, F-90100 Belfort, France

Abstract: The residual stresses arising during High Velocity Oxy-Fuel (HVOF)

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process usually restrict the building up of thick coatings. The potential of in-situ

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dry-ice blasting treatment during HVOF process to deposit thick WC-Co coatings with high quality due to its efficient cooling and mechanical peening was investigated

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in the present work. Characterization of WC–Co coatings deposited by HVOF

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combined with dry-ice blasting was carried out and compared with that of the coatings prepared using conventional HVOF. Several techniques, including scanning

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electron microscopy, X-ray diffraction and energy dispersive spectroscopy, were used

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to characterize the microstructures and phase distribution of the powders and coatings. In addition, mechanical properties such as hardness and sliding wear resistance were evaluated. The results demonstrate that HVOF-sprayed WC-Co coating with dry-ice blasting is characterized by compact construction, grain refinement as well as high hardness. Different from the prevention phenomenon of decarburization observed for plasma spraying of austenitic steel, WC decarburization still occurs for

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E-mail addresses: [email protected] (S. Dong) Tel.: +86 27 87855097; Fax: +86 27 87855098. 1

ACCEPTED MANUSCRIPT HVOF-sprayed WC-Co coating with dry-ice blasting. The in-situ dry-ice blasting treatment results in minute improvement in wear resistance of WC-Co coatings. Keywords: High-Velocity Oxy-Fuel; Dry-ice blasting; WC-Co coating; Microstructure;

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Phase; Wear

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1. Introduction

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WC-Co coatings are well known for their use in abrasion/wear resistance applications due to their favorable combination of toughness and hardness [1-3]. The hard WC

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particles form the major wear resistant constituent, while the Co binder provides

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toughness. High velocity oxy-fuel (HVOF) has shown itself to be one of the better methods for depositing WC-Co powders because the higher velocities and lower

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temperatures experienced by the powder particles as compared to plasma based routes

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[4-6]. However, WC-Co powders tend to undergo detrimental decarburization and reaction between the WC and Co during spraying; resulting in the formation of

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undesirable brittle phases such as W2C, W, WO3 phases, etc. [5, 7, 8]. The

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decarburization of WC-Co powders during processing reduces the abrasion resistance of the material. The deposition of WC-Co coatings by high velocity oxygen fuel spray forming using nanostructured feedstock and by laser engineered net shaping (LENS) has been reported to obtain good sliding tribological performance [15]. In addition, residual stress limits the thickness of WC-Co coatings during or after thermal spraying, which may crack or spall from the substrate [9-12]. The combination process of HVOF and dry-ice blasting is suggested in this study as a

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ACCEPTED MANUSCRIPT new potential alternative method for spraying WC-Co powders due to the efficient cooling and shotpeening-like effects of dry-ice blasting. Application of dry-ice blasting during plasma spraying for the deposition of metal and ceramic coatings has been examined in the previous reports [13-15]. In the present work, it is expected that

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a thick WC-Co coating with high quality could be prepared by HVOF combined with

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dry-ice blasting. The microstructure, the phase composition and the wear resistance of

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WC-Co coatings were examined.

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2. Experimental procedure

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A commercial spray dried / sintered WC-17Co powder (Metco 73F-NS-1) was used as spray materials in this work. At a low magnification, it is observed that the size and

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shape of the powder vary significantly (Fig. 1a). It is observed from the particle

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morphology at a high magnification (Fig. 1b) that they have a significant amount of porosity. This powder has an essential lognormal distribution with a particle size of

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19.88 µm (d0.1), 34.56 µm (d0.5) and 58.51 µm (d0.9) (Fig. 1c).

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HVOF spraying was performed using a CDS-100 gun (Sulzer Plasma Technik, Wohlen, Switzerland), connected to an ABB robot arm with 6 degrees of freedom. Oxygen and methane were used as the combustion mixture at flow rates of 400 and 180 NL/min, respectively. Nitrogen was used as the powder carrier gas at a flow rate of 30 NL/min. The spray distance was 280 mm. Dry-ice blasting was carried out using a mobile blasting device (ic4000 system, HMRexpert, France), which comprises a similar-Laval nozzle with a rectangular

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ACCEPTED MANUSCRIPT outlet dimension of 9 × 40 mm, a mass flow controller with a pneumatic motor, a storage tank, and a compressed air supplier. For the present work, cylindrical dry-ice pellets (-78.50℃) with a diameter of 3 mm and a length of 3 to 10 mm are used for dry-ice blasting medium, mass flow rate of dry-ice pellets was 42 kg.h-1 under a gas

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pressure of 0.6~0.8 MPa. The distance between the axis-exit of the dry-ice blasting

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nozzle and substrate is about 25 mm.

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The HVOF and dry-ice blasting guns were mounted on the robot flange (Fig. 2) and moved in front of cylindrical holder (D=400 mm) which rotated with a speed of 80

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rev/min. By adjusting the rotation direction of the sample holder, it was assured that

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dry-ice blasting treats the samples just before the HVOF deposition of the coating. This route was referenced from that optimized and used during plasma spraying

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process [13].

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The deposited WC-Co hard-metal coatings were examined by an optical microscope (OM) on specimen cross-sections. Percentages of porosity were determined by image

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analysis using Image J (from National Institute of Health, NIH) software. The

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reported values are the average of 20 measurements made on cross-sections of each coating. The HVOF-sprayed coatings were also characterized for their average surface roughness (Ra) using an AltiSurf 500 roughness tester produced by Altimet. Crystal structures were determined by means of X-ray diffraction (XRD) with Co Kα (λ=1.78897 Å) radiation at 35 kV and 40 mA and a scanning rate of 10omin-1 in a scattering angular range (2θ) of 30-90o on a Siemens diffractometer. A scanning rate of 2omin-1 in a scattering angular range (2θ) of 35-47o was adopted in order to

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ACCEPTED MANUSCRIPT re-measure and calculate the relative content of different phases. Surface residual stresses were measured by X-ray diffraction only on the WC phase of the WC-Co coatings. For the conversion of strain to stress, the Elastic constants used for the {202} reflection of WC were S1=-3.247×10-7 MPa-1, 1/2S2=1.948×10-6 MPa-1, and

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Poison`s ratio of 0.20. Residual stress data were analyzed using special software of

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XRD measurement set-up. Measurements were taken with the samples oriented in

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four different azimuth angles, 15°, 24°, 35°, 45°, to determine the stress tensor. Microhardnesses were measured with a Vickers indentor with a load of 300 g applied

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during 15 s. The value given is the average of 10 measurements. A friction and wear

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test was carried out using a ball-on-disk CSEM tribometer (CSEM, Switzerland) in the air environment at room temperature. The counterparts were sintered WC-Co balls

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with a diameter of 6 mm. The applied load, sliding velocity and distance were 5 N, 15

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cm/s and 500 m, respectively. The friction force was measured with a linear variable differential transducer (LVDT) sensor and dynamically recorded into a computer at a

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frequency of 12 values per min. Before the friction and wear tests, the coatings were

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polished using SiC sandpaper and then polished using diamond slurries down to an average surface roughness of 1.6-2.2 µm. The friction force data were simply divided by the applied loads to give the friction coefficients. After the fraction tests, the worn surfaces and debris were observed by means of scanning electron microscopy (SEM, JSW-5800LV, JOEL) and were analyzed by the associated Energy Dispersive Spectrometer (EDS).

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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Surface morphology of HVOF-sprayed WC-17Co coatings The surface morphologies of HVOF-sprayed WC-17Co coating are shown in Fig. 3. By observing the surface morphologies of the coating sprayed by conventional HVOF

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without dry-ice blasting (Figs. 3a and 3b), it seems that the coating has a lamination

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structure formed by fully or partially melted powder. At the same time, many obvious

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micro-pores were found at the sprayed surface. Contrary to the above, the coating sprayed by HVOF combined with dry-ice blasting exhibits a surface with less visible

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micro-pores at the sprayed surface (Figs. 3c and 3d). The micro-pores in the two type

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WC-17%Co coatings were associated with the incomplete filling and infiltration of the molten droplets, the partial melting and incomplete flattening of the powder

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particles, the air entrapment during the deposition process, the thermal stress induced

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in plasma spraying process, and so on. As for the surface roughness, it can be found that the WC-17%Co coating sprayed

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with dry-ice blasting has an equivalent value with the coating sprayed by conventional

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HVOF process (Fig. 4). For the coating sprayed by conventional HVOF, the Ra value reached 4.39±0.3 μm while the Ra value of the coating using dry-ice blasting was 4.28±0.2μm.

3.2 Cross-sectional microstructure of HVOF-deposited WC-17Co coatings Figs. 5 and 6 show the cross-sectional microstructures of WC-17Co coatings prepared by conventional HVOF and by the combination process of HVOF and dry-ice blasting,

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ACCEPTED MANUSCRIPT respectively. Fig. 5a illustrates a general view of the cross-sectional coating after metallographic preparation. The WC-17Co coating sprayed by conventional HVOF process appears to be quite dense. The coating thickness is 150±7.7 μm. However, at higher magnifications (Figs. 5b and 5c), the presence of pores and discretely

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distributed WC particles with different size and different shape embedded in the

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matrix is apparent. This indicates partial melting or decomposition of WC feedstock

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particles. The image analysis of this coating (Fig. 5d) yields an apparent porosity of 1.4±0.5%. The presence of the pores in the coating could be affected by the carbide

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size and Co content in the feedstock powders [16] and the HVOF spraying parameters,

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in particular the stand-off distance and fuel flow rates which determine the velocity of particles. Porosity level obtained using HVOF technique is usually smaller than those

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using plasma techniques. In the worst case, the porosity content of HVOF-sprayed

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WC-Co coatings does not exceed 6% [17]. Denser coatings are usually correlated to small stand-off distance and large fuel flow rates [16, 18, 19].

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By profiting the efficient cooling of dry-ice blasting, a very thick coating was

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prepared by HVOF process. In the case of the coating prepared by the combination process of HVOF and dry-ice blasting (Fig. 6), the coating possesses a very dense structure with small pores well distributed in the coating. The coating thickness is 957±11.6 μm. Compared with the coating deposited by conventional HVOF process, this coating has less and smaller pores. The image analysis of this coating yields a porosity value of 0.9±0.2% (Fig. 6 d). The decrease in the porosity can be attributed to the additional shotpeening brought by dry-ice blasting, apart from the shotpeening

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ACCEPTED MANUSCRIPT effect of HVOF process it-self. It is favorable to the bonding of the deposited particles with an additional peening of dry-ice blasting. Potentially, dry-ice blasting can remove off the poorly adhered particles or splashing splats in the former deposited layer and consequently contributes to less porosity in the coating, as observed in the

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plasma-sprayed coatings with dry-ice blasting [15]. Such peening effect can be

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indirectly confirmed by the actual profiles of the coating surfaces in Fig. 4 (see

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Section 3.1). With dry-ice blasting, the coating exhibits a visible convex topography which probably results from the mechanical effect of dry-ice blasting.

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Fig. 7 shows back scattered electron (BSE) micrograph at high magnification of

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WC-17Co coating prepared with dry-ice blasting. It can be observed that the microstructure mainly contains three different contrast regions indicated as regions 1,

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2 and 3. The elemental analysis determined by means of EDS allows the conclusion

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that the sharp and blunt particles and the dark regions in the binder are Co-rich regions, while the bright regions in the binder are known to be W-rich regions. These

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inhomogeneous compositions result from the dissolution of WC during HVOF

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spraying process and rapid cooling of the powders when they reach the substrate, as reported in the litterature [20].

3.3 Decarburization and crystalline size Fig. 8 presents diffraction patterns of the starting WC-17Co powder and the two coating samples. The XRD pattern of the powder clearly shows that only present phases are WC and α-Co. Whereas, the diffraction pattern of the coating prepared by

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ACCEPTED MANUSCRIPT conventional HVOF process illustrates the presence of Co6W6C phase. It is believed that such phase forms as the result of the decomposition of WC and reaction between the WC and Co [4, 5, 7]. In addition, it is known that the decarburization of the WC-Co is driven by the amount of oxygen available in the supersonic jet because

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firstly of air entrainment and secondly of residual O2 in the spray gun [21, 22]. Thus,

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it is reasonable to desire that the application of dry-ice blasting could potentially

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suppress the WC-Co decarburization since the sublimated CO2 gas could reduce the partial pressure of oxygen around the supersonic jet. However, in the case of the

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coating prepared by the combination process of HVOF and dry-ice blasting, the

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coating is also mainly composed of WC and Co6W6C phases. Co6W6C phase of each coating is quantified by taking the relative intensity of the most intense peaks for each

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phase. The phase content ratio of Co6W6C phase of the coating sprayed by

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conventional HVOF is 23.54% while 22.75% of the coating by the combination process of HVOF and dry-ice blasting. These results confirm that WC-Co powders

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suffer the typical decarburization in the two spraying processes. Moreover, no much

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difference can be distinguished between the Co6W6C phase contents of the two coatings. The application of dry-ice blasting during HVOF process caused no obvious change in carbide phases present in the coating. Such result is different from that observed during plasma spraying of austenite steel where the sublimated CO2 gas arising from dry-ice blasting prevents the decarburization and preserves the original austenite phase (CFe15.1) of feedstock [23]. Using dry-ice blasting, the XRD pattern of the sprayed coating depicts broader peaks and simultaneously the peak intensity

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ACCEPTED MANUSCRIPT decreased compared with the diffraction pattern of conventional HVOF-sprayed coating. Peak broadening can be related to both microstrain development in the microstructure and small crystalline size [22]. Microstrain development has to deal with plastic deformation of unmolten particles whereas small crystalline size is the

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consequence of rapid solidification of droplets upon substrate impingement [24]. The

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infrared thermometer has detected that the sample (WC-Co coating deposited on

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stainless steel substrate) temperature decreased from 140oC to 90oC after using dry-ice blasting. The mean size of the crystallites (here after referred to as D) can be

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inferred from the broadening of the X ray diffraction lines detected at 2θ angles

D  0.89

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between 40.5° and 43.5° (Fig. 8b) using the Scherrer formula:

B cos 

(Eq. 1)

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Where, B is the FWHM (full width half maximum) of the considered crystalline peak,

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λ (0.1798897 nm) is the wavelength of the X-rays and θ is the scattering angle. According to the Scherrer formula, the calculated crystalline size D of WC phase is

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46 nm for the conventional HVOF-sprayed coating, while it decreased into 24 nm

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after using dry-ice blasting due to its efficient cooling effect. The presence of relatively large humps for 40.5o <θ < 43.5o indicates the presence of nanocrystalline or amorphous Co phases, which is similar with the results reported by several authors [22, 25, 26].

3.4 Mechanical properties of HVOF deposited coatings 3.4.1 Microhardness

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ACCEPTED MANUSCRIPT The microhardness measurement of the sprayed coatings under different conditions was carried out. The Vickers microhardness of the coating increases from 1185±31HV0.3 to 1243±40HV0.3 using dry-ice blasting. Generally, hardness increases with the decrease of porosity level, the improvement of splat cohesion and

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the decrease in the carbide size [4, 5, 18, 27, 28]. Hardness is also sensitive to the

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phase composition in the coating. The higher degree of WC decomposition, the better

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is the hardness. Conversely, retention of WC and Co phases lead to low hardness levels [5, 27, 29]. In this work, the application of dry-ice blasting still corresponds to

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the WC decarburization in the HVOF-sprayed coating, thus it is believed that the

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decrease in crystalline size and the porosity is responsible for the microhardness

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3.4.2 Surface residual stress

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increase.

The residual stresses of the two coatings were determined by X-ray diffraction

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technique concern only the WC phase, since WC could represent the dominant phase

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of the WC-Co composite. Similar work have been reported by many researchers [30-32]. Such surface residual stress was to be compared to investigate the effects of dry-ice blasting on the coatings. The coating sprayed by conventional HVOF process displays a tensile stress of 188 MPa while the coating sprayed with dry-ice blasting has a tensile stress value of 293 MPa. By the same measurement technique, tensile stresses have also been measured by other researchers [30, 31]. Pina et al. [30] have evaluated the residual stresses of

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ACCEPTED MANUSCRIPT HVOF-sprayed WC-12Co coating by means of X-ray diffraction technique, employing the “sin2ψ” method, and have reported tensile stresses of ~165 MPa. Wang et al. [31] have also reported a similar value of ~112 MPa. As reported by Wenzelburger et al. [33], the impulse transfer of the impinging

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particles to the coated surface as well as the differences in thermal expansion of the

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coating material and the substrate could contribute to the differences in residual

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stresses of the coating systems. In this work, compared with the value of the coating sprayed by HVOF, the higher tensile stress of the coating sprayed by the combination

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process of HVOF and dry-ice blasting could reflect dominances of thermal quenching

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over peening mechanism of dry-ice blasting. The thermal effect is related with the

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3.4.3 Wear resistance

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rapid quenching of dry-ice blasting on the molten particles with high temperature.

The Ball-on-disk test results obtained for the two type coatings are shown in Fig. 9,

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where it can be seen that the coating sprayed by conventional HVOF process has a

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higher friction coefficient during the whole sliding period than the sample sprayed by the combination process of HVOF and dry-ice blasting. Fig. 10 shows the worn surface morphologies of the coating prepared by the combination process. It seems that the worn track is shallow. The mean track width is 446±5.5 μm. Moreover, the enlarged image at the bottom of the worn track exhibits a smooth surface with some shallow grooves (Fig. 10a), which indicates that the abrasive cuts through the larger WC particles as well as through the binder. The

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ACCEPTED MANUSCRIPT binder material around the WC grains probably has been partially removed. At one edge of the worn track after a sliding distance of 500 m for 5 kg load, fine wear debris were observed (Fig. 10c). The average size of the debris particles is less than 1 μm. These wear debris correspond to the oxide, as evidenced in Figs. 10d-f. When the

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counterpart material (sintered WC-Co ball) slides over the sample, a tribo-chemical

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process occurred. During such process, the enhanced oxidation of the surface

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components occurred to form multi-oxide films of CoO or WO3. As for WC-Co coating prepared by conventional HVOF process in Fig. 11, the mean track width is

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453±4.8 μm, which is a little larger than that of the coating by HVOF with dry-ice

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blasting. Generally under the same test condition the wider the worn track of the coating, the higher the wear resistance. This indirectly indicates that WC-Co coating

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prepared by HVOF with dry-ice blasting has a higher wear-resistance than that by

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conventional HVOF process. Similarly, a smooth surface with some shallow grooves as well as fine wear debris and multi-oxide films could be observed at the bottom of

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the worn track of WC-Co coating prepared by HVOF with dry-ice blasting.

4. Conclusions

It is possible to apply dry-ice blasting into the HVOF prepation process for the deposition of thick WC-Co coatings. The deposited coating exhibits a compact microstructure as well as few micro-pores and low roughness at the surface. Small crystalline size was obtained with the efficient cooling of dry-ice blasting, although the WC decarburization occurred in the coating which was different from the

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ACCEPTED MANUSCRIPT prevention phenomenon of decarburization observed for plasma spraying of austenitic steel. Corresponding to the small crystalline size and low porosity, a higher microhardness and a lower fraction coefficient were also obtained compared with that of the conventional one. Shallow groove and fine wear oxide debris were observed on

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the worn surface.

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Acknowledgments

The authors wish to gratefully acknowledge the financial support of the National

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Natural Science Foundation of China (Grant No. 51501137). The authors express their

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gratitude to Mr. Bernard Hansz for the technical support under the Oseo IC5000 program (Grant No. LS 10003 CR-01). This research is also financially supported by

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the project (Grant No. P2016-04) of State Key Laboratory of Materials Processing

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and Die & Mould Technology at Huazhong University of Science and Technology. Dr. Shujuan Dong also wishes to thank the Wuhan University of Technology for the

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financial support for this research, to the Fundamental Research Funds for the Central

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Universities (WUT: 2015IVA055). The authors would like to express their deepest gratitude to European Marie-Curie programme (IPACTS PIRSES-GA-2010-268696) and Oseo IC5000 program (LS 10003 CR-01) for financial support.

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[31] Wang J, Li K, Shu D, He X, Sun B, Guo Q, Nishio M, Ogawa H. Effects of structure and processing technique on the properties of thermal spray WC-Co and NiCrAl/WC-Co coatings. Mat. Sci. Eng. A 2004;371:187-92. [32] Murthy JKN, Rao DS, Venkataraman B. Effect of grinding on the erosion behaviour of a WC-Co-Cr coating deposited by HVOF and detonation gun spray processes. Wear 2001;249:592-600. [33] Wenzelburger M, Lopez D, Gadow R. Methods and application of residual stress

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2006;201:1995-2001.

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ACCEPTED MANUSCRIPT Figure captions: Fig. 1 Scanning electron microstructures of WC-17%Co powder (a) at low magnification, (b) at high magnification and (c) its size distribution. Fig. 2 (a) Installation photo and (b) schematic diagram of the spray systems.

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Fig. 3 Top view of the surface topology of HVOF-sprayed WC-17Co coatings: (a, b)

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without dry-ice blasting; (c, d) with dry-ice blasting.

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Fig. 4 Roughness of the HVOF-sprayed WC-17Co coatings: (a, b) without dry-ice blasting; (c, d) with dry-ice blasting.

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Fig. 5 Cross-sectional microstructures of the WC-17Co coating prepared by

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conventional HVOF process: (a) general OM view, (b) OM microstructure and (c) SEM microstructure at higher magnifications, (d) binary image corresponding to Fig.

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5b (Figs. 5b and 5c represent a magnification of the rectangular frame in Fig. 5a).

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Fig. 6 Cross-sectional microstructures of the WC-17Co coating prepared by HVOF combined with dry-ice blasting process: (a) general OM view, (b) OM microstructure

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and (c) SEM microstructure at higher magnifications, (d) binary image corresponding

6a).

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to Fig. 6b (Figs. 6b and 6c represent a magnification of the rectangular frame in Fig.

Fig. 7 (a) Back scattered electron (BSE) micrograph of WC-17Co coating prepared with dry-ice blasting and (b) EDS analysis results corresponding to regions 1, 2 and 3 in Fig. 7a. Fig. 8 (a) XRD spectra of the starting WC-17Co powder and two coatings prepared by conventional HVOF and by HVOF combined with dry-ice blasting process, (b)

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ACCEPTED MANUSCRIPT enlargements of the two coatings at 2θ angles between 40.5° and 43.5°. Fig. 9 Evolution of the friction coefficients of the two coatings as a function of sliding distance. Fig. 10 SEM micrographs of the worn surface of the coating prepared by the

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combination process with dry-ice blasting: (a) at low magnification, high

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magnifications of (b) zone A and (c) zone B, (d-f) cartographic analysis corresponding

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Fig. 11 SEM micrographs of the worn surface of WC-Co coating prepared by

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conventional HVOF: (a) at low magnification, high magnifications of (b) zone A’ and

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(c) zone B’, (d-f) cartographic analysis corresponding to Fig. 11a.

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(b)

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Fig. 1

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(c)

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(a) (b)

Dry-ice blasting gun

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HVOF gun

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Fig. 2

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(b)

(a)

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micropores

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(c)

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Fig. 3

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micropores

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WC-Co coating sprayed without dry-ice blasting

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WC-Co coating sprayed with dry-ice blasting

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Fig. 4

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(a)

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(a)

Resin

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Coating

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Substrate (d)

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(c)

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pores

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Resin

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(d)

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small pores

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Fig. 7

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Fig. 9

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(b)

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zone A

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Sliding direction

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zone B

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Fig. 10

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zone A’

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Sliding direction 446 μm

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Fig. 11

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Highlights

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(1) Dry-ice blasting was in situ applied into HVOF process.

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(2) Thick WC-Co coatings were successfully prepared.

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(3) Compact construction, grain refinement and high hardness were observed. (4) Different from plasma spraying of austenitic steel WC decarburization still

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occurred.

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