简介概要

Microstructure and performance of WC-Co-Cr coating with ultrafine/nanocrystalline structures

来源期刊：Rare Metals2018年第11期

论文作者：Xue-Zheng Wang Hai-Bin Wang Xue-Mei Liu Chao Hou Xiao-Yan Song

文章页码：968 - 975

摘要：The WC-lOCo-4Cr composite powder was synthesized firstly. Then the composite powder was agglomerated to prepare thermal spraying feedstock. The ultrafine/nanostructured WC-lOCo-4Cr coating was prepared by high velocity oxygen fuel thermal spraying. The phase constitution, elemental distribution and microstructure of the coating were characterized by X-ray diffraction and transmission electron microscopy, respectively. The wear resistance and corrosion resistance of the prepared composite coating were tested. The results show that the main phases of the coating include WC, binding phase with partial amorphous structure, with a little W₂C and Co（Cr）coexisting. The distributions of Co and Cr elements from the phase boundary to the eutectic area then to Co zone were analyzed quantitatively. The mechanisms for the formation of the microstructure and effects of Cr on the performance of the composite coating are proposed.

详情信息展示

稀有金属(英文版) 2018,37(11),968-975

Microstructure and performance of WC-Co-Cr coating with ultrafine/nanocrystalline structures

Xue-Zheng Wang Hai-Bin Wang Xue-Mei Liu Chao Hou Xiao-Yan Song

College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Education Ministry of China,Beijing University of Technology

作者简介：*Xiao-Yan Song,e-mail: xysong@bjut.edu.cn;

收稿日期：24 November 2016

基金：financially supported by the National Natural Science Foundation (No. 51601004);the Key Program of National Natural Science Foundation (No. 51631002);the National Science Fund for Distinguished Young Scholars (No. 51425101);

Microstructure and performance of WC-Co-Cr coating with ultrafine/nanocrystalline structures

Xue-Zheng Wang Hai-Bin Wang Xue-Mei Liu Chao Hou Xiao-Yan Song

College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Education Ministry of China,Beijing University of Technology

Abstract：

The WC-lOCo-4Cr composite powder was synthesized firstly. Then the composite powder was agglomerated to prepare thermal spraying feedstock. The ultrafine/nanostructured WC-lOCo-4Cr coating was prepared by high velocity oxygen fuel thermal spraying. The phase constitution, elemental distribution and microstructure of the coating were characterized by X-ray diffraction and transmission electron microscopy, respectively. The wear resistance and corrosion resistance of the prepared composite coating were tested. The results show that the main phases of the coating include WC, binding phase with partial amorphous structure, with a little W₂C and Co(Cr)coexisting. The distributions of Co and Cr elements from the phase boundary to the eutectic area then to Co zone were analyzed quantitatively. The mechanisms for the formation of the microstructure and effects of Cr on the performance of the composite coating are proposed.

Keyword：

In situ synthesis; High velocity oxygen fuel; Ultrafine/nanostructure; Composite coating; Phase constitution;

Received： 24 November 2016

1 Introduction

The thermal-sprayed WC-based cermet coatings have widespread use in a broad range of industries such as aerospace,automotive,petroleum,machinery manufacturing,electricity and printing due to their high hardness and wear resistance [ 1, 2, 3, 4, 5] .For the WC-Co coatings,the Co binder phase is easily corroded in the acidic,alkaline and humid environments.Thus,the WC particles may be pulled out in the wearing conditions.The failure of WC-Co coatings is eventually caused by the reciprocating effects of the above processes [ 6, 7, 8, 9, 10, 11] .Because of the weak corrosion resistance,the applications of WC-Co coating are restricted in certain fields.In recent years,many attempts have been made to improve the comprehensive performance of WC-based coatings.Typically,the methods include the decrease in grain size [ 12, 13, 14] ,modification of Co by addition of corrosion-resistant elements such as Cr and Ni [ 15, 16, 17, 18, 19, 20] ,optimization of thermal spray processes [ 21, 22, 23, 24, 25] ,sealing treatment of coating [ 26] ,etc.The previous studies show that,by adding a certain amount of Cr in the WC-Co coating,the coating not only had high hardness,high bonding strength and high wear resistance but also had superior corrosion resistance,which has become the development trend of wear and corrosion research on cermet coating.

On the study of wear and corrosion mechanisms of WC-Co-Cr coating,Han et al. [ 27] suggested that a rich Cr was formed during high velocity oxygen fuel (HVOF) spraying process and the WC particle was completely coated with Cr-rich phase,so that the coating has higher corrosion resistance.Souza et al. [ 28] suggested that a dense Cr oxide layer with spinel structure could be preferentially formed on the top surface of the coating and hindered the etching process.So the WC-Co-Cr coating with a certain amount of Cr element had better corrosion resistance.In this paper,the nanostructured WC-Co-Cr thermal spray feedstock powder was firstly prepared using the in situ synthesized WC-Co nanocomposite powder [ 29, 30, 31, 32, 33, 34, 35] .Then,a series of sliding wear and corrosion tests were performed on the prepared WC-Co-Cr coating so as to investigate the wear and corrosion mechanisms of the coating.

2 Experimental

2.1 Materials preparation and thermal spraying process

Commercially available tungsten oxide,cobalt oxide and carbon black powders were used as the raw materials.The raw powders were mixed by ball milling and then were subjected to in situ reactions in a vacuum furnace.In this work,the WC-10.4 wt%Co composite powder was firstly synthesized using the above-mentioned method [ 8, 29, 30, 31, 32, 33, 34, 35] .Then a commercially available Cr powder was added into the WC-10.4 wt%Co composite powder by milling for20 h with a ball-to-powder ratio of 3:1.Then the WC-10 wt%Co-4 wt%Cr powder was obtained.The spray drying was carried out after the WC-Co-Cr powder was mixed with 2.0 wt%polyvinyl alcohol (PVA),1.0 wt%polyethylene glycol (PEG) and 30.0 wt%distilled water to form stable slurry.Then the spray-dried particles were heat-treated at 1215℃for 5 h in a vacuum tubular furnace with the pure argon as the protective gas.After physical grinding and air classification,the WC-Co-Cr feedstock powder with particle sizes of 10-45μm was obtained [ 12] .The coatings were deposited on the AISI 1045 steel substrate by HVOF-K2 spraying system.The fuel flow rate,oxygen flow rate,spraying distance,powder feeding rate and carrier gas (Ar) flow rate were 24 L·h^-1,900 L·min^-1,340 mm,98 g·min^-1,and 7.5 L·min^-1,respectively.Prior to the spraying process,the substrate samples(78 mm×58 mm×5 mm) were degreased in acetone and grit-blasted with 30 meshes Al₂O₃.

2.2 Characterizations

2.2.1 Phase and microstructure

The phases in as-sprayed coatings were examined by X-ray diffractometer (XRD,Rigaku D/max-3c) using Cu Kαradiation with a step of 0.02°.The morphologies and microstructures of the as-sprayed coatings were observed by scanning electron microscope (SEM,Nova NanoSEM)and transmission electron microscope (TEM,FEI Tecnai F30) equipped with an energy-dispersive spectroscopy.An image analyzer was done to measure porosity.The crosssectional SEM images were taken from different positions to measure the porosity of the coating.Microhardness measurements were taken on the transverse section of the coating under a load of 2.94 N for 15 s using a Vickers microhardness tester.At least 15 measurements were taken for each specimen to ensure the data repeatability.

2.2.2 Sliding wear test

The sliding wear tests were conducted using a reciprocating sliding tribometer (CFT-I tribometer,Lanzhou ZKKH Science and Technology Development Co.,Ltd.).In the tests,the upper pin of the silicon nitride ceramic ball(Φ5 mm) was stationary,which was used as the counter material.The counter-face disk (20 mm×10 mm) slides reciprocatingly.Prior to the tests,all the coating samples were polished.The sliding wear tests were conducted at the load of 80 N,the sliding speed of 5m·min^-1 and the total time of 15 min.The wear volume was estimated by the section profiles at three different positions of the wear scar.Three tests were performed on the coating for each load [ 8] .The friction coefficient and sliding time were recorded automatically during the tests.The particle size of the powder and the grain size of the coating were measured by the linear intercept method on SEM and TEM micrographs,respectively.

2.2.3 Electrochemical corrosion test

The electrochemical behavior of the coating samples was examined at room temperature in the aerated and unstirred3.5 wt%NaCl solution,using the Ivium electrochemical measurement system (CompactStat.e 10800,Ivium Technologies) in a three-electrode test cell.The sample was taken as the working electrode,a platinum wire as a counter electrode and a saturated calomel electrode as a reference electrode.The specimens for electrochemical tests were closely encapsulated in a non-conducting epoxy resin with the rear side of the specimen soldered to a wire,leaving only an area of 1.0 cm² exposed to the electrolyte.The electrochemical tests included measurement of the open circuit potential (OCP),potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS)test to compare the corrosion resistance of HVOF-sprayed nanostructured WC-Co-Cr coatings.And the specimens were cold-mounted using epoxy resin,polished on the diamond resin polishing disks with the diamond particle sizes of 20 and 9 (μm,then were subjected to a cloth polishing using the diamond polishing paste of 1.5 (μm,then degreased in acetone in an ultrasonic bath and dried in warm air.The tests commenced after 100 min of free corrosion to allow full wetting of the surface of the coating and OCP stabilization.As the OCP became steady,potentiodynamic polarization curves were measured with a fixed sweep rate of 1 mV·s^-1.The corrosion current density (i_corr) and corrosion potential (E_corr) were obtained as the intersection point of linear fits to the anodic and cathodic polarization curves,according to the Tafel extrapolation technique.EIS test was conducted at the OCP by applying a sinusoidal potential excitation of 10 mV amplitude over frequency range of 100 kHz-10 mHz [ 21] .Each test was repeated at least thrice to make sure a good repeatability of the experiment result.

3 Results and discussion

3.1 Powder characterization

Figure 1 shows the morphology of the in situ synthesized WC-Co composite powder,which has a particle size of about 186 nm.Figure 2 shows the cross-sectional microstructure of a single feedstock particle (Fig.2a) and morphology of the spray powder (Fig.2b),respectively.It is clearly shown that the powder particles have nearly spherical shape without adherent satellite particles in the range of 15-45μm.As a reason,the feedstock powder has excellent flowability and could be well fed into the spray system.The apparent density and flowability of the spray powder are 3.92 g·cm^-3 and 0.3494 s·g^-1,respectively.The amounts of total carbon,Co and Cr of the feedstock powder are 5.23 wt%,9.47 wt%and 3.83 wt%,respectively,with the balance W,which is well consistent with the component design of the WC-10Co-4Cr spray powder.

3.2 Coating characterization

Figure 3 shows the microstructure of the as-sprayed coating on a polished cross section,which was fabricated by the feedstock made of the in situ synthesized composite powder.It is evident from Fig.3 a that the coating is dense and well bonded to the substrate with an average thickness of350μm.The average porosity of the coating is less than1%.From Fig.3b,it can be noted that tungsten carbides with high content are uniformly embedded into the coating.The binder matrix with different grayscales is observed,and regions with higher brightness are associated with significant dissolution of WC into the binder phase.Besides,some pores are observed from the micros true ture.The coating has an average hardness more than HV_0.31400.

Figure 4 shows XRD patterns of WC-Co-Cr spraying powder and resultant coatings,which were prepared by the in situ synthesized WC-Co composite powder.The WC-Co-Cr spraying powder consists of WC and Co phases mainly.In the coating,the dominant phase is WC,and minor W₂C phase is coexisting,due to slight decarburization.In addition,the Co peak disappears and a little amorphous peak appears in the coatings due to the rapid cooling rate of the fully molten sprayed particles,which impacts the substrate during the spraying process.

3.3 Friction coefficient and wear rate

For comparison,the commercially available WC-10Co-4Cr powder was also used to prepare the cermet coating.Both coatings made of the homemade WC-10mCr composite powder and the commercial powder were subjected to the reciprocated wear tests.

Figure 5 shows that the commercial coating is in the steady friction stage after wear for 6 min and has a friction coefficient in the range of 0.40-0.45.The coating prepared in the present work is in the steady friction stage after wear for 4 min and has a friction coefficient in the range of0.33-0.35.Table 1 compares the wear rates of the two coatings.The wear rate of the present coating is 48.6%lower compared with that of the coating made by commercial powder.

3.4 Electrochemical corrosion behavior

The potentiodynamic polarization curves of the as-sprayed coatings in 3.5 wt%NaCl solution are shown in Fig.6a.The corrosion potentials (E_corr) of the nanostructured coating prepared in the present work and the coating made by commercial powder are-0.30 and-0.51 V,respectively.The corrosion current density (i_corr)of the nanostructured coating is 1.07×10^-5 A·cm^-2,which is obviously lower than that of the coating made by commercial powder (1.32×10^-5 A·cm^-2).The superior corrosion resistance of the nanostructured coating is further verified by EIS results.Figure 6b shows EIS curves of the as-sprayed coating together with the coating made by commercial powder.

Fig.1 a SEM image of in situ synthesized WC-Co composite powder and b WC particle size distribution

Fig.2 SEM images of feedstock powder:a morphology of spray powder and b cross-sectional microstructure of a single feedstock particle

Fig.3 a SEM image of a transverse section of as-sprayed WC-10Co-4Cr coating and b enlarged image of coating

Fig.4 XRD patterns of WC-10Co-4Cr spraying powder and resul-tant coating

Fig.5 Sliding wear friction coefficients of coatings as a function of wear time

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Table 1 Wear rate of WC-10Co-4Cr coatings (mm³·N^-1·m^-1)

Fig.6 a Tafel curves of coatings and b EIS curves of coatings (Z_re and Z_im being real part and imaginary part of alternating current impedance,respectively

It is seen from EIS curves (Fig.6b) that both the impedance spectra reveal as capacitive semicircles in the testing range,and the diameter of the capacitive semicircle of the as-sprayed coating prepared in the present work is larger than that of the coating made by commercial powder.This indicates that the electrochemical mechanism is the same,but the corrosion rate is different.The diameter of the capacitive semicircle is related to the charge transfer resistance,i.e.,the corrosion resistance in the corrosion process;a larger diameter of the capacitive semicircle means a slower corrosion rate.Thus,it can be seen that the as-sprayed present coating has a slower corrosion rate,i.e.,better corrosion resistance than the coating made by commercial powder.

3.5 Microstructure of coating

Figure 7a shows a typical TEM image of the present coating,which contains WC and amorphous Co.Figure 7b shows the diffraction pattern of WC phase with hexagonal crystal structure (Fig.7b,corresponding to Region A).The Co binder becomes the amorphous phase of Co(Cr),as indicated by Fig.7c,d,corresponding to Region B.According to the parameters of crystallography (i.e.,the spacing and lattice parameters),the corresponding crystal planes are determined,as shown in Fig.7b.

Figure 8a shows another typical TEM image of the present coating.The marked grain has a round shape,which may be resulted from WC dissolution and precipitation process in the Co binder at the high-temperature spraying flame.During this process,carbon may react with oxygen,leading to the decarburization of WC grains.Figure 8b shows that the grain is W₂C with the hexagonal crystal structure (Fig.8a,corresponding to Region A).

Figure 9a-c shows the distribution of Co and Cr of the present coating by EDS analysis.From the WC grain to the eutectic Co area,the content of Co gradually increases,and the content of Cr gradually increases between WC grains and white-band region,reaches the highest value in the white-banded regions and then decreases between whiteband and Co region.The distribution of Cr in the coating is consistent with that in Ref. [ 36] .Because of the high temperature (about 2700℃) of HVOF spraying flame,most of the Co and Cr phases with melting points of 1495and 1857℃,respectively,could melt [ 27] .During the cooling stage,the Cr with higher melting point is preferentially precipitated from the liquid Co-Cr phase to form a Cr-enriched solid phase in the Co binder,as shown in Fig.9a.Moreover,due to the lower surface tension of Cr(σ=1628 mN·m^-1) compared with that of Co (σ=1870mN·m^-1) [ 27] ,Cr-rich zone is easy to form at the interface between WC and Cr particle.The content of Cr in the vicinity of Co region is obviously less than the average content of the spraying powder (Fig.9b).

Fig.7 a TEM image of the present coating,b diffraction patterns of Region A in a,c diffraction pattern and d EDS analysis of Region B in a

Fig.8 a TEM image of present coating and b diffraction pattern of Region A in a

Fig.9 a TEM image of present coating,b contents of Co and Cr at Regions A-D in a and c EDS analysis of coating

Figure 10 shows a TEM image of the coating made by commercial powder.As can be seen,the WC grain size of the coating is relatively larger and the average WC grain size is about 1μm.In the wear process,the Co binder phase was extruded firstly and worn away to expose the hard WC particles.Then the WC particles were involved in wear.Under the external load,the WC particles were broken,thinned and detached from the coating.Because of the lower bonding strength,the WC particles in the coating made by commercial powder are easy to be removed,resulting in the formation of large and deep holes compared with that in the present coating.So,the commercial coating has much higher wear rate.

Fig.10 TEM image of commercial coating

From Figs.7 and 8,it is estimated that the average WC grain size in the coating is about 200 nm.However,as shown in Fig.10,the average WC grain size in the commercial coating is about 1μm.With the decrease in WC grain size,the hardness and wear resistance of the coating increase.Moreover,the resistance against the crack propagation increases due to the increased amount of the WC grain boundaries and phase boundaries between WC and other phases.Previous study [ 31] reported that the areas of WC/WC grain boundary and WC/Co interface increased as the WC grain size decreased.As a reason,the resistance to intergranular fracture of the coating,as the dominant mechanism of crack formation during the wear process,would be improved.Therefore,the present coating prepared in this work has a much lower wear rate compared with the commercial coating.

The present coating has better corrosion resistance mainly because of the more homogeneous Cr distribution.The element Cr is easily oxidized to dense oxide layers,which endow the coating with good corrosion resistance.As can be seen in Figs.7,9 and 10,the elements Cr and Co in the coating exist in the form of Co-Cr composite in the vicinity of the WC grains.Because of the lager WC grain size,the distribution of Cr in the commercial coating is not uniform,resulting in relatively poor corrosion resistance compared with the ultrafine/nanostructured coating.

4 Conclusion

In the present work,the WC-Co-Cr composite powder was prepared,with the average WC particle size of about186 nm.The spray powder was then fabricated using the composite powder,and good spherical shape and excellent flowability of the spray powder are obtained.The prepared coating is dense and well bonded to the substrate with an average thickness of 350μm.The average porosity of the coating is less than 1%.Compared with the commercial coating,the present coating has much lower wear rate(decrease by 48.6%) and obvious higher corrosion resistance.

Acknowledgements This work was financially supported by the National Natural Science Foundation (No.51601004),the Key Program of National Natural Science Foundation (No.51631002) and the National Science Fund for Distinguished Young Scholars (No.51425101).