| The Cr3C2-NiCr coating prepared by high velocity oxy-fuel(HVOF)spraying technology shows good wear resistance and corrosion resistance,but under high temperature and salts corrosive environments,problems such as low hardness and high porosity make the wear resistance and corrosion resistance deteriorate.In recent years,CrN film deposited by physical vapor deposition(PVD)technology has also been used for surface modification of mechanical parts.Compared with Cr3C2-NiCr coating,it has higher hardness and denser structure,and exhibits excellent wear and corrosion resistance.However,under extreme conditions such as high temperature,high load,wear and corrosion,the crack in the film caused by substrate softening,hardness/modulus differences,and the mismatch of the coefficient of thermal expansion,which will seriously limit the practical application of the CrN film.In order to obtain more excellent high-temperature tribological properties and hot corrosion resistance,based on the idea of synergistic performance advantages of Cr3C2-NiCr coating and CrN film,a PVD/HVOF duplex system composed of a top CrN film as the surface functional layer prepared by PVD and a Cr3C2-NiCr interlayer as a hard support layer and transition layer prepared by HVOF is designed in this paper.Based on this,the microstructure,mechanical properties and thermal stability of CrN film,Cr3C2-NiCr coating and CrN/Cr3C2-NiCr duplex coating were investigated.Comparative analysis of tribological and hot corrosion behavior of duplex coating,and the related wear mechanisms and hot corrosion mechanisms were revealed in detailed.The main results and conclusions are as follows:The microstructure,mechanical properties and thermal stability of the three films/coatings were evaluated by scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS),atomic force microscopy(AFM),X-ray diffractometer(XRD),nanoindenter,scratch tester,high temperature vickers micro hardness tester.The results showed that the CrN film had a dense structure and high hardness(24.5 ± 3.8 GPa),but low adhesion strength,poor bearing capacity,fracture toughness and thermal stability.Cr3C2-NiCr coating had higher porosity and low hardness(826 ± 6 HV0.3),but higher bonding strength(73.9 MPa).The thicker coating resulted in higher residual stresses and poor thermal stability of the coating.CrN/Cr3C2-NiCr duplex coating had high surface roughness.The top CrN film played a role in sealing the Cr3C2-NiCr interlayer to a certain extent.The Cr3C2-NiCr interlayer obviously provided good support for top CrN film.The hardness and adhesion strength reached 33.3 ± 6.4 GPa and 37 N,respectively.The bearing capacity and fracture toughness were significantly improved compared with single CrN film.The tribological behavior of the three films/coatings was investigated using a reciprocating friction tester,SEM,EDS,AFM,and the 3D profilometer and Raman spectrometer(Raman)were used to research the wear mechanisms.The results indicated that the cross-sectional wear area of the wear track was only 12.64 μm2,and the wear resistance was relatively good.The friction coefficient of Cr3C2-NiCr coating fluctuated greatly.Due to the low hardness,the wear was severe.The cross-sectional wear area of the wear track reached 467.83 μm2.Due to the high hardness,the CrN/Cr3C2-NiCr duplex coating showed excellent wear resistance.The cross-sectional wear area of the wear track was only 1.27 μm2.The main wear mechanism was abrasive wear.With the increase of temperature,the friction coefficient of CrN film had a downward trend,but the wear resistance had deteriorated.The wear was the most severe at 350℃,and the film had experienced severe adhesive wear,accompanied by abrasive wear and oxidative wear.At 350℃,Cr3C2-NiCr coating was severely worn,and the cross-sectional wear area of the wear track reached 580.20 μm2.At 550℃,the wear resistance was good.The wear mechanisms were mainly oxidative wear,delamination wear and slight abrasive wear.The CrN/Cr3C2-NiCr duplex coating had poor wear resistance at high temperatures,and the top CrN film worn out.The wear mechanisms were mainly adhesive wear,oxidative wear and abrasive wear.SEM,EDS,XRD,X-ray photoelectron spectroscopy(XPS)were used to study the hot corrosion behavior of the three films/coatings at 550℃ in mixed salts(NaCl+KC1+Na2SO4),and the related hot corrosion mechanisms was discussed.The results illustrated that the CrN film was completely corroded and peeled during the hot corrosion,and the hot corrosion resistance was the worst.The weight of Cr3C2-NiCr coating increased severely during the hot corrosion process.High porosity was not conducive to the hot corrosion resistance of the coating,and corrosion pits had occurred in the pores on the coating surface.The thickness of corrosion oxide scale with voids and cracks was 13 μm,and the corrosive medium penetrated into the coating and reacted with the oxides to form volatile chlorides,resulting in the formation of more voids and cracks,which led to the peeling of the coating and showed poor hot corrosion resistance.The CrN/Cr3C2-NiCr duplex coating had a dense and uniform Cr2O3 oxide protective scale formed on the coating surface.The thickness of the oxide scale was about 1/10 of that of the Cr3C2-NiCr coating.Therefore,the duplex coating exhibited superior hot corrosion resistance.However,the Cr3C2-NiCr interlayer corrosion at the pores where the surface of the duplex coating was not completely covered by the CrN film was the main reason for the failure of the duplex coating. |
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