X. ZHOU et al.: EFFECT OF HEAT TREATMENT ON THE MICROSTRUCTURE AND PERFORMANCE ...
885–894
EFFECT OF HEAT TREATMENT ON THE MICROSTRUCTURE AND PERFORMANCE OF HIGH-VELOCITY AIR-FUEL SPRAYED
WC-10Co4Cr COATINGS
VPLIV TOPLOTNE OBDELAVE NA MIKROSTRUKTURO IN LASTNOSTI WC-10Co4Cr PREVLEK, IZDELANIH Z ZELO HITRIM NAPR[EVANJEM V
PLINSKI ATMOSFERI ME[ANICE PROPANA IN ZRAKA
Xuedong Zhou, Haijun Liu, Jishi Zhang, Jinsheng Ji, Wensheng Zhao, Zhimin Zhang, Qiang Wang, Yong Xue*
School of Materials Science and Engineering, North University of China, Taiyuan 030051, China Prejem rokopisa – received: 2021-08-23; sprejem za objavo – accepted for publication: 2021-11-02
doi:10.17222/mit.2021.245
In this study a high-velocity air-fuel (HVAF) flame-sprayed WC-10Co4Cr coating was heat-treated at (240; 300; 400) °C for 2 h in an air atmosphere. The effect of the heat treatment on the hardness, fracture toughness, wear resistance, corrosion resistance, phase composition and microstructure behaviour of the coatings was investigated. It could be concluded from the X-ray diffrac- tion (XRD) pattern that the phase of the coatings was mainly composed of tungsten carbide, an amorphous phase, a small amount of W2C and trace metal tungsten. However, the heat-treated coating had a small increase in W2C compared to the origi- nal coating, although the amount of amorphous phase did not decrease significantly. The results indicated that as the heat-treat- ment temperature increased, the hardness of the coating first increased and then decreased, while the fracture toughness in- creased. The polarization test confirmed that the heat-treated coating had higher corrosion resistance than the original coating.
In addition, the results of the reciprocating friction and wear test indicated that small amounts of W2C strengthening phases were formed in the WC-10Co4Cr coating after heat treatment at 400 °C. This process did not eliminate many of the tougher Co and WC phases. Therefore, this coating had the best wear resistance among all the comparative coatings.
Keywords: heat treatment, WC-10Co4Cr coatings, abrasion resistance, corrosion resistance.
Avtorji v tem ~lanku opisujejo {tudijo napr{evanja prevlek na osnovi WC-10Co4Cr s postopkom zelo hitrega plamenskega napr{evanja (HVAF; angl.: high-velocity air-fuel) v atmosferi zgorevanja plinske me{anice propan-zrak. Izdelane prevleke so nato dve uri na zraku toplotno popu{~ali pri temperaturah (240; 300; 400) °C. Sledilo je ugotavljanje vpliva toplotne obdelave na trdoto, lomno `ilavost, odpornost proti obrabi, korozijsko obstojnost, fazno sestavo in mikrostrukturo prevlek. Avtorji na osnovi rentgenske difrakcije (XRD) ugotavljajo, da so prevleke v glavnem sestavljene iz WC, amorfne (steklaste) faze, majhne vsebnosti W2C in sledov kovinskega volframa. Vendar pa toplotno obdelane prevleke vsebujejo nekoliko ve~jo vsebnost W2C, kot jo vsebujejo prevleke pred toplotno obdelavo, medtem ko se vsebnost amorfne faze ni veliko zmanj{ala. Rezultati preiskav so pokazali, da je s povi{ano temperaturo toplotne obdelave trdota prevlek najprej narasla nato pa padla, medtem ko se je lomna
`ilavost stalno pove~evala. Polarizacijski testi so potrdili izbolj{anje korozijske obstojnosti prevlek po njihovi toplotnoi obdelavi. Rezultati izmen~nega preizkusa drsnega trenja in obrabe so pokazali, da majhna vsebnost W2C utrjuje prevleko WC-10Co4Cr po toplotni obdelavi pri 400 °C. Ta proces ni pretirano zmanj{al vpliva bolj `ilavih faz na osnovi Co in WC. Zato ima ta prevleka v primerjavi z ostalimi najbolj{o odpornost proti obrabi.
Klju~ne besede: toplotna obdelava, prevleke na osnovi WC-10Co4Cr, odpornost proti abraziji, korozijska obstojnost
1 INTRODUCTION
WC-Co coatings are widely applied as wear-resistant layers for a variety of applications due to their excellent wear resistance; such applications include the metal- lurgy, energy and other industries.1–7Higher particle ve- locities were achieved through the HVAF spray process at lower particle temperatures.8High particle velocities have interesting possible applications in various pro- cesses, because they have a positive effect on the coating density and increased lamellar cohesion, and high parti- cle temperatures are generally avoided due to increases in the carbon loss with increasing temperature and the formation of brittle structures in the coating.9–13
However, W2C phases and other amorphous phases could inevitably be exhibited in the sprayed coatings be- cause decarburization would be generated by WC during spraying.14The generation of these phases decreased the mechanical properties of the WC-Co coatings, limiting their applications.15 Some researchers have found that a heat treatment could significantly improve the hardness and wear performance of WC-Co coatings coupled with a transformation in the phase composition.16,17 Nerz et al.18reported that when WC-12Co coatings were treated at 875 °C for 1 h under argon gas, they recrystallized the matrix phase to form Co2W4C and Co6W6C carbides. The wear resistance of HVOF-sprayed WC-12Co coatings was increased after 5 min of heat treatment at 900 °C as a result of the formation of new carbide phases. Sohi et al.19also observed a similar phenomenon. Li et al.20 re-
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 55(6)885(2021)
*Corresponding author's e-mail:
YongXue395@163.com (Yong Xue)
ported that the complex carbide Co6W6C was trans- formed through crystallization of the amorphous phase at 600 °C for 6 h. In general, the composition and phase of the coatings led to a change in the mechanical properties, which mainly affected the wear resistance of the heat-treated coatings. When the heat treatment tempera- ture was over 450 °C, in a previous study of WC-Co coatings, the coating began to appear as a new oxide phase, CoWO4. A new phase formed in the coating after heat treatment at temperatures higher than 550 °C.21WC and Co were oxidized into new phases, such as CoWO4, C2CoO4, WO3, and C6WO6. As the temperature in- creased, the amount of WC, which gradually decreased in the coating, transformed into these new brittle struc- tures, and the brittle phase was harmful.
Nitesh Vashishtha et al.22used steel as a coating car- rier that could withstand a high heat-treatment tempera- ture. The temperature of a 2A12 aluminium alloy be- came too high at approximately 530 °C, and the heat-treatment range of the substrate was limited, so this alloy is only suitable for heat treatment at lower tempera- tures. There have been many studies on the heat-treat- ment temperature of WC-Co coatings higher than 500 °C and relatively few studies at temperatures lower than 500
°C. However, there have been fewer studies on the heat treatment of WC-Co-Cr coatings, and there are almost no data on the microstructure, mechanical properties, wear and corrosion behaviour of the coatings after low-temperature heat treatment. The properties of the WC-Co and WC-Co-Cr powders are relatively similar.
Therefore, the 2A12 aluminium alloy was selected as the substrate and WC-10Co4Cr as the coating to study the effect of heat treatment on the overall performance of the coating. The purpose of this paper is to study the effect of a low-temperature heat treatment on the micro- structure, mechanical properties, wear and corrosion be- haviour of WC-10Co4Cr coatings.
2 EXPERIMENTAL PART
2.1. Materials and Thermal spray
In the experiment, a commercial thermal spray pow- der (WC-10Co4Cr) was selected as the spray material.
The base material was 2A12 aluminium alloy, which was machined in a plate sample with a size of 100 mm × 80 mm × 10 mm. Before spraying the sample, the sprayed surface was sandblasted (coarsened) with 16#
brown corundum. After sandblasting, the surface of the substrate, which was blown with compressed air, was de- contaminated with acetone and ethanol. A Kermetico AK HVAF supersonic flame-spraying system was used for the test. Propane was used as fuel, air was used as the oxidant, and nitrogen was used as the powder feeding gas. The spraying parameters of the AK-07 spray gun are shown in Table 1. Compressed air was used simulta- neously to cool the back of the aluminium alloy sub- strate. After many tests and comparisons, it was proven that a spraying distance of between 180 mm and 300 mm
was a more appropriate spraying distance. In this paper, 200 mm was selected as the spray distance for subse- quent research on the performance of the coating.
Finally, the sprayed sample sheet was cut into a square with a size of 10 mm × 10 mm × 10 mm for subsequent tests.
Table 1:Thermal spraying parameters employed during deposition of the WC-10Co4-Cr coatings
Parameter Value
Fuels Propane
Nozzle AK-07
Spray distance (mm) 200
Powder feed rate (min–1) 4 Nitrogen carrier gas (kPa) 172.3
Air pressure(kPa) 627.4
Fuels pressure (kPa) 565.3 Porosity of the coatings 0.95 %
2.2 Heat treatment
The square sample was subjected to heat-treatment tests at different temperatures, and the heating equipment was a Nabertherm P300 box-type resistance furnace. The resistance furnace was equipped with a temperature-reg- ulating device, and the temperature-error range was no more than 3 °C. The samples were placed in a resistance furnace with test temperatures of (240; 300; 400) °C for 2 h and then air-cooled to room temperature.
2.3 Material characterization
Scanning electron microscopy (SEM: SU5000, Ja- pan) was used to characterize the powder morphology, the top surface of the coating, the cross-section and the wear trajectory. The samples were polished first with P400, P800 and P2000 silicon carbide sandpaper and then with diamond suspensions of 2.5 μm and 1 μm. The microhardness of the cross-section of the coating was measured using a Vickers microhardness tester (VMH- 002VM, Germany). The average value of 10 indentation tests was obtained on the cross-section of the coating polishing, in which the load was 2.94 N and the pressure holding time was 15 s. The indentation method was used in combination with the Evans & Wilshaw formula to calculate the fracture toughness of the coating. The equipment used was a VMH-002VM Vickers microhard- ness tester. The indenter was a diamond Vickers indenter, and the load was 19.61 N. The average value of 10 in- dentation cracks for each coating was measured to calcu- late the fracture toughness.23 To calculate the fracture toughness, the following equation was used (Equation (1)).
K P
a
a
c = ⎛ c
⎝⎜ ⎞
⎠⎟ ⎛
⎝⎜ ⎞
⎠⎟
0 079 4 5
. 3 2 log .
/ (1)
where P is the applied indentation load (mN), a is the indentation half diagonal (μm), andcis the crack length from the centre of the indent (μm).
In the WC-Co-Cr coating material, the crack pre- pared by the indentation method satisfied 0.6 <c/a< 4.5 (cwas the length from the centre of the indentation to the end of the crack, and 2a was the length of the diagonal length of the indentation). The load was maintained at 19.61 N for 15 s. The phase composition of the sprayed powder and coating was analysed by XRD (Rigaku, Ja- pan), a copper cathode was used (ka wavelength l= 0.15406 nm), the scanning angle was 10° < 2q< 90°, and the step size was 0.01°. Finally, JADE software was used to analyse the phase of the diffraction peak.
An MFT-4000 multifunctional material surface per- formance tester was used to test the wear on the polished coating surface. The indenter was made of SiC, the fric- tion length was 5 mm, the friction speed was 50 mm/min, the load was 50 N and 70 N, and the abrasion of the coating was tested by scratch friction with a load of 70 N.
All the measurements were carried out at (25 ± 1) °C in a water bath using a CHI660e electrochemical work- station. The electrochemical test involved a three-elec- trode system. The samples were used as the working electrode, a saturated calomel electrode (SCE) was used as the reference electrode, and a platinum counter elec- trode was also used. Before the test, the samples were polished and cleaned. The surface of the samples was washed with deionized water and then cleaned with an acetone alcohol solution. The noncoated surface was coated with epoxy resin adhesive to prevent the corro- sion solution from infiltrating, exposing the working sur- face (coated surface), which had an area of 100 mm2. The samples were immersed in a 3.5 w/% NaCl acetic acid solution with pH = 3 for 60 min until the open-cir- cuit potentials (OCPs) became almost stable. Potentio- dynamic polarization tests were conducted at a scan rate of 10 mVs–1from –500 mV to 2 V vs. OCPs. Electro- chemical impedance spectroscopy (EIS) tests were per- formed at the OPCs in the frequency range from 100 kHz to 10 mHz. The data were analysed with ZSimpWin software.
3 RESULTS AND DISCUSSION 3.1 Structure of the powders and coatings
WC-10Co4Cr is an agglomerated and sintered pow- der with a common micron structure. Scanning electron microscopy (SEM) images of the sprayed powder are shown inFigure 1. The morphology of the sprayed pow- der is regular spherical particles with a size of approxi- mately 5–45 μm. It can be seen that the powder is loose and porous. This structure enables it to fully melt during the spraying process. The spray particles were not blown away due to their small size.
Figure 2 shows that the phase structures of the mi- cron- and submicron-coatings were prepared by the WC-10Co4Cr thermal spray powder in this experiment.
It can be seen from the XRD spectra that the powder and coatings have similar characteristic peaks, specifically those of WC, W2C, Co, and Cr, but the intensities of the peaks are different. The characteristic Co and Cr peaks of the coatings are broad to different degrees than the powder. During the spraying process, the metallic phase can first dissolve a certain amount of W and C. After be- ing sprayed on the surface, the metallic phase cools quickly and becomes amorphous.
The heat treatment of WC-12Co4Cr coatings was conducted in an air atmosphere. The powder and coating composition were tested by XRD (Figure 2). A diffrac- tion peak was observed at 40° to 45°, where W2C, W and amorphous phases were all present. Coating A was the unheated coating, coating B was heat treated at 240 °C, coating C was heat treated at 300 °C, and coating D was heat treated at 400 °C. There was no significant change in the XRD results of coatings A and C, except that a small amount of the W2C phase increased after heat treatment at 240 °C and 400 °C.
SEM images of the WC-10Co4Cr coating heat treat- ment are shown in Figure 3. These SEM images show
Figure 2:XRD pattern of feedstock powder and WC-10Co4Cr coat- Figure 1:SEM images of feedstock powder WC-10Co4Cr ing
the uniform distribution of WC within the ductile Co and Cr matrix. The SEM images also show that WC particles have block-like and angular shapes; these shapes are shown inFigure 3b (not specified inFigure 3b, 3fand 3h). Because the three heat-treatment temperatures were relatively low, there was no significant change in the size of the WC. Figure 3i shows the WC-10Co4Cr coating after heat treatment at 400 °C, indicating that the coating did not oxidize during the heat treatment. The energy spectra of the other coatings were similar, so they are not shown in thisFigure 3i.
Figure 3shows that the microstructure of the coating is homogeneous and dense, with a few pores, but no cracks are found. Table 1shows the results of porosity tests on the coating. The pores in five cross-sectional pic-
tures of the coating were counted and calculated with re- lated software. The porosity of the coating is the ratio of the black pore area to the entire area. The calculated re- sults show that the porosity of the coating was approxi- mately 0.95 %. HVAF technology has a low flame tem- perature, high particle-flight speed, and short residence time in the flame (oxidizing atmosphere). The porosity of the coating was reduced.
The cross-sectional microstructure of the coating in Figure 3shows that the white block WC was distributed in the grey binder, and the local binder dissolved into more W and C, resulting in a brighter colour, as shown in the box in Figure 3d, while those inFigure 3b, 3f and 3h are not shown. The coatings keep the original WC particle size of the powder. Some WC particles in the coating hit the substrate and were embedded in the sur- rounding binder phase. Some of the micropores in the coating were distributed around the larger WC particles with a polygonal structure, as shown in the circle inFig- ure 3f, while others were distributed around the WC par- ticles that were broken by the impact.24,25It is considered that the fragmentation of WC particles during spraying may have been caused by the low melting point of the binder phase and the weakness of the particles.
3.2 Effect of heat treatment on coatings’ hardness and fracture toughness
Figure 4a shows the microhardness of the coating.
When the heat-treatment temperature was increased to 240 °C, the hardness increased. The hardness decreased at 300 °C. It starts to increase again at 400 °C. However, the hardness of all the coatings changed slightly, and the range of the change was within 10 %. The hardness of the intermediate coating was higher for the same coating, and the hardness of the coating at the interface with the substrate was slightly higher than that of the surface coating. It was higher because the porosity of the inter- face and intermediate coatings was low, and the degree of crushed WC particles in the intermediate coating was low. The diffraction peak appeared at 2q = 38° for coat- ings B and D but not for coatings A and C. Coatings B and D were slightly harder than coatings A and C be- cause a small amount of the W2C phase is beneficial for increasing the hardness. Many researchers have also re- ported that the microhardness of WC coatings can reach above 1000 HV.26–28The fracture toughness of the coat- ing is shown in Figure 4b. Compared with coating A, coatings B, C and D show greater fracture toughness.
The fracture toughness of the WC-10Co4Cr coatings was similar after heat treatment at 240 °C and 300 °C, and the maximum fracture toughness was at 400 °C.
The fracture toughness of the coatings decreased be- cause more brittle carbide phases formed in the coating after high-temperature heat treatment. The fracture toughness in this paper was measured using coatings that were heat treated at low temperature. The trends in change were slightly different from those observed by
Figure 3:Section morphologies SEM micrographs and EDS pattern of: a and b) coating A, c and d) coating B, e and f) coating C, g, h and i) coating D
earlier researchers. The change in fracture toughness is related to the change in residual stress caused by heat treatment and the mismatch of the thermal expansion co- efficient between the coating and the substrate. During heat treatment at (240; 300; 400) °C, the heat-treatment temperature increased and the residual stress decreased.
The thermal expansion coefficients of the coating and the substrate were better matched, and the WC phases and the adhesive further merged, possibly due to the increase in the fracture toughness.
3.3 Coefficient of friction and abrasive loss of the coat- ings
Figure 5ashows the relationship between the coeffi- cient of friction (COF) and the heat treatment tempera- ture of the coating. When the load was 50 N, the COF of coating A was 0.142. The COF of coatings B, C and D were 0.128, 0.133, and 0.125, respectively. The results show that the wear rate of the coating after heat treat- ment was lower than that of the original coating. The coating material had the best wear resistance after heat
treatment at 400 °C. The COF showed a downwards trend, and the value dropped by 6–12 % as the heat treat- ment temperature was increased to 400 °C, but it fluctu- ated slightly at 300 °C. The COF of all the coatings in- creased with increasing load when the load was 70 N.
However, the overall trend was the same, and the COF showed a downwards trend as the heat-treatment temper- ature was increased. When the heat treatment tempera- ture was 300 °C, the COF of the coating differed by ap- proximately 35 %. These differences were mainly related to the hardness of the coating and the additional load.
The adhesion due to surface roughness and ploughing play an important role in the change in the COF. With re- ciprocating movement, the friction increased with in- creasing load, which caused some particles in the coating to peel off, leading to a higher friction coefficient. Fig- ure 5b shows that the wear of the coatings varied with the heat treatment temperature at an initial load of 70 N.
The wear of the coating is similar to the trend in the COF. The overall trend is downwards, and there is a slight fluctuation at 300 °C. After heat treatment, the
Figure 4:a) Microhardness, b) indentation fracture toughness of as-sprayed and heat-treated WC-10Co4Cr coatings
Figure 5:a) Coefficient-of-friction response at loads of 50 N and 70 N, b) coefficient-of-abrasion loss at a load of 70 N
wear of the coating was approximately 8–15 % lower than that of the coating without heat treatment.
The wear mechanisms were investigated using the SEM images, as shown inFigure 6ato6d. The worn-out surfaces of the A, B, C and D coatings at a load of 70 N are shown. The brighter areas in the image of coating A are the WC-rich areas, while the darker areas are the co- balt-rich bonding phases that are preferred for wear. Dur- ing the wear process, the reciprocating friction of the in- denter gradually exposes the carbide particles in the coating until they become particles that are completely separated from the adhesive.
Under a load of 50 N, the frictional force of coating A, beginning at the initial stage of wear, shows a down- wards trend with time. The friction force is reduced by approximately 25 % due to the instability of the transi- tion layer (20 μm away from the coating surface). When the critical sliding distance is reached, the friction stabi- lizes. Because Co serves as a good binder between the WC particles, the coating stability is further maintained after the critical sliding distance is reached (a certain friction time). The wear mechanisms of coatings B, C
Figure 6:SEM images of the worn-out surfaces at a load of 70 N of the: a) coating A, b) coating B, c) coating C, d) coating D
Figure 7:Force friction response at loads of 50 N and 70 N under different heat treatments: a) room temperature, b) 240 °C, c) 300 °C, d) 400 °C
and D are similar to that of coating A. Under a load of 70 N, the friction first increases, then decreases and fi- nally becomes stable. While the frictional force of coat- ing A increases initially, later it shows a decreasing trend and subsequently stabilizes after sliding some distance.
After heat treatment, the critical sliding distance of the coating is relatively short. As shown inFigure 7, during the heat treatments at 240 °C and 400 °C, the increasing load of the coatings has little effect on the variation trend of friction force. However, the friction of coatings A and C has obvious fluctuations when the load is increased.
Under a load of 70 N, coating A has more wear than the other coatings because the binder of the unheated coating is relatively easy to break, which causes carbide particles to fall off the surface of the coating. The hard carbide particles that fall off the surface of coating A serve as ex- cess abrasive media, which further wears down the coat- ing. Another cause of high wear may be the peeling of the unstable transition layer of the coating. As the sliding distance increases, the reason for the final stability is that the high temperature due to sliding forms a friction oxide layer, which prevents further wear. Compared with the 50-N load, the critical sliding distance is increased due to the higher load. The wear of coatings A and C is higher than that of coatings B and D. Because coatings B and D have relatively few carbide particles peeled off, the wear of these coatings is also relatively lower. After the sur- face materials of the coating are removed, the oxides formed by WC, Co, and Cr after abrasion and the harder part of the intermediate coating make the later wear of the coating stable.
3.4 Corrosion properties of the coatings
The potentiodynamic polarization curves for the coat- ings in 3.5 w/% NaCl acetic acid solution with pH = 3 are presented inFigure 8. The polarization curve of the coatings shows exponential growth with increasing po-
tential and finally flattens and remains basically un- changed. According to Faraday’s law, when one gram equivalent of metal is dissolved at the anode, the amount of electricity passed is 1 Faraday (1 F = 96500 Cou- lomb). If the atomic weight of the metal isA, the metal valence is n, and the corrosion current density is icorr, then Equation (2) can be used to calculate the corrosion rateVcorrof the coating. To calculateVcorr, the following Equation (2) was used
v i A
corr nF
= corr⋅
(2) Using the Tafel extrapolation method, the corrosion potential and corrosion current density of the four coat- ings were calculated, as shown inTable 2. The self-cor- rosion potentialsEcorrof the four coatings in solution are sorted from high to low as D > B > C >A; the corrosion rate is A > C > B > D. It can be concluded from the ex- perimental results that coating D has the best corrosion resistance because the WC particles in the coating are more uniformly distributed after heat treatment at 400 °C (Figure 3h), as shown in the red circle. In addition, an oxide passivation film is formed on the coating surface in the solution,29which further reduces the chance of a cor- rosive liquid entering the coating/substrate interface, and at the same time, the oxide passivation film slows down the corrosion of the coating surface.
Table 2:Results of electrochemical corrosion tests
Designation Corrosion potential (mV)
Corrosion current density (A/cm2)
A –472 7.8×10–7
B –322 4.9×10–7
C –351 4.7×10–7
D –265 2.2×10–7
The Cl–in the solution easily exposes the coating sur- face to pitting corrosion. Once pitting corrosion occurs, the lamellar coating cracks and peels off, which eventu- ally leads to a gradual increase in weight loss of the coat- ing. Some passivation films soon form on the coating surface in a corrosive environment, and the coating en- ters the passivation area. The self-corrosion current den- sity is also significantly reduced or no longer greatly in- creased, and the passivation film formed is also in a dynamic equilibrium in continuous dissolution destruc- tion and regeneration. For coating D, when the potential reaches –265 mV, the polarization curve of the coating begins to enter the anode dissolution zone, and the cur- rent density increases with increasing corrosion poten- tial. When the potential rises to 95 mV, the polarization curve of coating D enters passivation. The potential con- tinues to rise to 462 mV, and the current density begins to increase sharply with increasing corrosion potential.
Similar phenomena are also observed in coatings A, B and C. Compared with coating A, the corrosion resis- tance of coatings B, C, and D is improved to different de- grees. Because the heat treatment may make the coating
Figure 8:Potentiodynamic polarization curves of four kinds of coat- ings in 3.5w/% NaCl solution with pH = 3
structure more uniform, the Co and Cr in the bonding phase are more likely to generate oxide films, which pre- vents further corrosion of the coating by the solution. Af- ter the coating is heat treated at three different tempera- tures, the 400 °C-treated coating has a relatively high self-corrosion potential and low corrosion current den- sity, so it has better corrosion resistance and can better protect the substrate.30Figure 9shows the Bode plots of the four coatings in 3.5w/% NaCl acetic acid solution at pH = 3. The bode magnitude plots (Figure 9a) present the decreased impedance at low frequency with the in- creased heat-treatment temperature. However, the imped- ance at a low frequency after heat treatment at 240 °C is slightly higher than that after heat treatment at 300 °C. In theory, the impedance at low frequency reflects the pro- tectiveness of the coating.30,31 The impedance of coating A is < 1200W·cm2, indicating poor corrosion resistance.
Bode phase-angle plots (Figure 9b) display the reduced value of the phase angle with decreased heat-treatment temperature, which also signifies a decreased corrosion resistance. For EIS data simulation, a general equivalent circuit model in Figure 10is proposed to describe the Bode plots based on related research. The specific mean- ings of these parameters in the equivalent circuit are as follows:Rsis the resistance of the solution;R1andR2are the charge-transfer resistances,R1is the resistance of the coatings, and R2 is the reaction resistance at the metal/coating interface.C1is the capacitance of the coat- ing, andC2is the capacitance of the double layer. The re-
sults of the electrochemical impedance spectroscopy show that theR1andR2values of coating D are the larg- est, followed by those of coatings B, C and A. Based on the results of electrochemical impedance spectroscopy, it can be inferred that the corrosion resistance of the four coatings is D > B > C > A.
4 CONCLUSIONS
The coatings were heat treated at three different tem- peratures. The effects of the performance were evaluated through microstructure and corrosion-behaviour analy- ses. Some conclusions can be drawn.
1) No new substances formed in the WC-10Co4Cr coating after heat treatment at (240; 300; 400) °C for 2 h.
However, the content of W2C in the coatings increased slightly after heat treatment at 240 °C and 400 °C, and a small amount of W2C was beneficial for increasing the hardness of the coating.
2) The maximum hardness of the coating after heat treatment at 400 °C was 1143 HV; however, the hardness of all the coatings changed slightly, and the range of change was within 10 %. The coating had the best frac- ture toughness after heat treatment at 400 °C, and its value was 8.08 MPa·m1/2. The mechanical properties of the coating after heat treatment at 240 °C and 300 °C were similar, and both were better than those of the un- heated coating.
3) The wear resistance of the coating after heat treat- ment at 400 °C was preferably slightly higher than that of the coating after heat treatment at 240 °C, and its value was 0.02237 mm2. The friction force of coating A and the unheated coating after heat treatment at 300 °C noticeably fluctuated when the load was increased.
4) Electrochemical tests showed that the corrosion rate of the heated coatings was reduced, and it exhibited higher corrosion resistance than the unheated coatings.
Based on the electrochemical tests, in the 3.5w/% NaCl
Figure 9:Electrochemical impedance spectroscopy of the different coatings in 3.5w/% NaCl acetic acid solution with pH = 3
Figure 10:Equivalent circuit models used for fitting the EIS experi- mental data
acetic acid solution with pH = 3, the coating after 400 °C treatment had a relatively high self-corrosion potential and low corrosion current density, so it had better corro- sion resistance.
Acknowledgment
The present research was supported by the National Natural Science Foundation of China (Grant No.
51675492).
Author Contributions
Conceptualization, Writing Review and Editing, X.
Z.; Date curation, Methodology, H. L., J. Z., J. J.; Writ- ing Original Draft, W. Z.; Validation, Z. Z., Q. W.; Inves- tigation, Y. X. All authors have read and agreed to the published version of the manuscript.
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