7YSZ coating prepared by PS-PVD based on heterogeneous nucleation

Ziqian DENG,Xiaofeng ZHANG,Kesong ZHOU,Min LIU,Chunming DENG,Jie MAO,Zhikun CHEN

aSchool of Materials and Energy,Guangdong University of Technology,Guangzhou 510006,China

bNational Engineering Laboratory for Modern Materials Surface Engineering Technology&the Key Lab of Guangdong for Modern Surface Engineering Technology,Guangdong Institute of New Materials,Guangzhou 510650,China

cSchool of Materials Science and Engineering,South China University of Technology,Guangzhou 510640,China

1.Introduction

The plasma spray-physical vapor deposition(PS-PVD)process has been developed with the aim of depositing different structured functional coatings(such as thin,gas tight,columnar coatings,etc.)with large area coverage by plasma spray.1–3The PS-PVD is developed based on low-pressure plasma spray(LPPS),where electrical current up to 3000 A,plasma gas flow up to 200 L/min,and an input power level of 180 kW could be achieved.With operation pressure decreasing,the plasma plume expands to a length of more than 2200 mm and a diameter of 400 mm.Using appropriate parameters,it is possible to evaporate powder feedstock materials,resulting in variant micro structures and non-line-of-sight deposition.4,5Li et al.6reported that using an advanced PS-PVD process,the nanohardness and micro-hardness of prepared dense coatings were markedly higher than those of conventional YSZ coatings(i.e.,coatings fabricated by atmospheric plasma spray or electron beam-physical vapor deposition),even comparable with those of sintered YSZ polycrystal.Goral et al.7presented that columnar YSZ coatings were deposited from evaporating powders during the process of PS-PVD.The microstructures of coatings were affected by the feed rate,chamber pressure,sample rotation rate,and plasma gas ratio(Ar,He,and N2).Hospach et al.8produced columnar-structured YSZ coatings with a diameter between 20 and 750 μm through PS-PVD.Thinner and thicker coatings seem to be possible.The geometry and arrangement of a sample and the sample holder have a big influence on the coating quality.

Despite much investigation about PS-PVD have been done in the past few years,however,the basic process technology,such as heating of powder particles to spray-deposited molten,semi-molten droplets or vapor gas phase onto substrate surface,has remain edessentially the same.9,10There are still a lot of areas which have not been investigated thoroughly.These areas consist of particle-plasma interactions in the rarefied plasma,particle vaporization and its affect on plasma properties,and deposition mechanisms associated with different micro structures.Now,any further under standing in each of these areas will enable the spray community to more easily apply the PS-PVD technology to meet emerging coating challenges.11,12

To exploit the potential such as gas-phase deposition by PSPVD,the deposition mechanisms and their dependency on process conditions must be better understood.The PS-PVD process can be summarized as three steps9:(A)feedstock processing in plasma torch;(B)plasma jet formation and materials transport;and(C)deposition and coating growth.The third step mainly includes heterogeneous and homogeneous nucleation depending on the spray distance.13When the spray distance is set at the middle area in the axial direction of the plasma f l ame,coating deposition primarily relies on heterogeneous nucleation on the substrate surface.In this work,different structured coatings based on heterogeneous nucleation have been obtained,and these principles are summarized in this investigation.

2.Experimental procedure

The experimental set-up is based on an Oerlikon-Metco MulticoatTMPS-PVD system together with an O3CP plasma torch mounted on a robot manipulator of ABB insidea?2.5 m×4.5 m vacuum chamber.The PS-PVD system is obtained through a comprehensive reconstruction of an existing conventional LPPS system.In particular,the system is equipped with an additional vacuum pumping unit,a large vacuum blower to provide sufficient pumping capacity at low pressures and enhanced cooling capacity,additional power sources,a new torch transfer arc system,and new operational control units.In terms of the powder feeding system,two powder injectors are located in the cylindrical section of the O3CP nozzle(diameter=12.5 mm)close to the divergent part.

Feedstock agglomerated 7YSZ powders(Metco 6700,Oerlikon-Metco)were used and their grain sizes ranged from 5 to 22 μm.7YSZ coatings were deposited on graphite,sintered zirconia,and nickel-based superalloy K417 substrates(size ?25.4 mm × 5 mm and surface roughness ＜2 μm)at a spray distance of 950 mm,where the Ar-He hybrid plasma was operated at a 67 kW net power of O3CP under an operation pressure of 150 Pa.Meanwhile,the substrate pre-heating temperatures were controlled at 850°C and room temperature prior to deposition of 7YSZ coating,respectively.During the pre-heating or deposition process,the substrate remained still,while the plasma gun moved at a speed of 1000 mm/s.The detailed spray parameters are shown in Table 1.

The microstructures of 7YSZ coatings were characterized by field emission-scanning electron microscopy(FE-SEM,Nova-Nono430,FEI)and transmission electron microscopy(TEM,JEM2100F,JEOL).Additionally,before TEM characterization,test samples were prepared by focused ion beam(FIB,450S,FEI)milling.

3.Results and discussion

3.1.Variation of coating microstructure

3.1.1.Effect of substrate materials

Taking graphite as a substrate without pre-heating by plasma flame(namely,the substrate temperature is controlled at room temperature),7YSZ coatings prepared by PS-PVD show typical columnar microstructure,as seen in Fig.1(a).The interface between the coating and the substrate has good bonding without crack.Among columns,there exist different sizes of gaps.Between columns,many small particles were formed,which were resulted from condensation of the vapor phase14,15,as shown in the magnified Fig.1(b).When the graphite substrate was replaced by sintered zirconia,similar columnar 7YSZ coating was generated with the same parameters at room temperature,as presented in Fig.2(a)and(b).According to the result of comparison,it can be known that the horizontal width of a single column is larger than that of a column deposited on graphite.Moreover,the deposition rate of 7YSZ on graphite is higher than that on sintered zirconia due to different thermal conductivities between graphite(129 W/(m·K))and zirconia(2.2 W/(m·K)).During the deposition process,the temperature gradient on graphite is larger than that on zirconia,which results in a higher growth driving force on graphite.Thus,graphite has a higher deposition rate.Due to similar properties,the interface between 7YSZ coating and sintered zirconia has a better bonding.Between columns,there is no apparent vertical gap,and no small particle as well appears in the gaps.Besides,the columns are made of f i ne grains and denser than those deposited on graphite.

Table 1 Parameters of 7YSZ coating by PS-PVD.

Fig.1 7YSZ coating deposited on graphite at room temperature.

Fig.2 7YSZ coating deposited on sintered zirconia at room temperature.

3.1.2.Effect of substrate temperature

With a net power of 60 kW,the inner O3CP gun provided high plasma energy density,and the electron excitation temperature is more than 10,000°C,so that most of the 7YSZ powders can be evaporated in the inner plasma torch and a short-distance flame appears ahead of the nozzle before jet expansion.5A columnar 7YSZ coating could be obtained by vapor phase deposition with a spray distance of 950 mm,as seen in Fig.3.It shows a typical well-arranged columnar 7YSZ coating deposited on K417 at a substrate pre-heating temperature of 850°C.Fig.3(a)illustrates that the growth directions of all columnar grains are perpendicular to the substrate.The gaps between the columnar grains are larger than the internal grains.The columnar coating was analyzed by TEM.Fig.3(b)shows the bright- field cross-sectional image.The internal structure of a column shows a feather-like structure and is separated by nano-gaps.In the internal feather-like columnar coating,nano-sized secondary columns(～20 nm length in the perpendicular direction to the substrate)called feather arms are clearly observed,as seen in the high-resolution image of Fig.3(c).

As the substrate temperature decreased to room temperature,at which no pre-heating was conducted by plasma plume to the substrate prior to 7YSZ coating deposition,a dense coating was obtained(seen in Fig.4(a))exhibiting a different microstructure from that of the coating deposited at 850°C as shown in Fig.3(a).The dense coating is thinner than the columnar coating after the same spray times.The magnified image of Fig.4(b)shows that the dense coating has a mixed structure consisting of fine grains and columnar grains.No large-size gaps exist in the mixed coating,where the columnar grains are surrounded by a large amount of fine grains.The fine-grain field is made of lots of nano-sized grains.Mostly,the shape of fine grains is spherical without an oriented direction,which is different from secondary columns in the columnar field through an observation by the bright- field image of TEM(Fig.4(c)).

3.2.Heterogeneous nucleation

According to the reported literature13,when the spray distance is set at 950 mm,heterogeneous nucleation of absorbed gas particles will occur on the substrate surface.When vapor particles in plasma plume impinge on the substrate,heterogeneous nucleation occurs on the substrate surface that lowers the critical free energy required to form a stable nucleus of mean dimensionr.Fig.5 shows a spherical cap-shaped solid nucleus on the substrate with a contact angle θ.16The critically-sized nucleus(r*)is related to the amount of under cooling by the following relation16,17:

Fig.3 7YSZ coating deposited on K417 at a substrate temperature of 850°C.

Fig.4 7YSZ coating deposited on K417 at room substrate temperature.

whereTmis the equilibrium freezing temperature of the feedstock powders, ΔHfis the latent heat of fusion,and ΔTis the amount of under cooling at which the nucleus is formed,and γvnis the specific surface free energy between the vapor and the nucleus.Thus,the free-energy change of heterogeneous nucleation can be given by

Fig.5 Schematic of heterogeneous nucleation on the substrate surface during vapor deposition.16

where ΔGvis the volume free energy of a nucleus,andnis the number of formed nucleus.

Section 3.1.1 has demonstrated that the width of a columnar grain deposited on zirconia(a2)is larger than that of a column on graphite(a1).The corresponding sketch can be seen in Fig.6.For occurrence of heterogeneous nucleation,the substrate must be wetted by the vapor phase.Fig.5 indicates that a nucleus is formed on the substrate surface,creating a contact angle θ between the nucleus and the substrate.Importantly,the contact angle θ depends solely on the surface properties of the involved materials,and will affectr*and Δ.The contact angle θ between a nucleus and the zirconia substrate is larger than that between a nucleus and graphite due to a higher surface tension of zirconia at room temperature.Thus,r*and Δwill increase with a decrease of the contact angle θ based on Eqs.(1)and(2),resulting in a larger width of columns deposited on the zirconia substrate.Meanwhile,the total amount of columns will reduce.

Fig.6 Schematics of different structures of columnar coatings deposited on different substrate materials.

3.3.Coating growth

It can be inferred from Eqs.(1)and(2)that with a decrease of the substrate temperature,r*and Δwill decrease correspondingly.Using superalloy K417 as a substrate,the coating structure will change from oriented columns to fine grains when the substrate pre-heating temperature varies from 850°C to room temperature,as described in Section 3.1.2.Due to a lower surface energy than those of inorganic materials such as graphite and zirconia,the coating tends to be a dense structure instead of porous columns.

Most of the feedstock 7YSZ powders when charged with a rate of 18 g/min will be vaporized into vapor particles before impinging on the substrate surface.13,17,18Some of the vapor particles will be absorbed,but most of them will be rebounded by the rigid substrate and vortex flow.As a consequence,a boundary layer will be formed ahead of the substrate surface,where the number of vapor particles in a unit volumeCVis variable and given by19,20

whereC0is the number of vapor particles without a rebound effect,K0is the equilibrium partition coefficient,Rxis the absorbed rate of vapor particles by the substrate that is proportional to the distancexahead of the substrate surface,andDis a constant for a certain vapor material.Thus,the critical temperature of heterogeneous nucleation can be calculated by the following equation,which is influenced byCV:

whereTAis the melting temperature of the feedstock material,andmis a constant.TVis proportional to the distancexfrom the substrate surface as plotted in Fig.7.Whenx=0,the interface temperatureTibetween the substrate and the coating is given by the following relation:

The practical temperatureTDof the vapor phase ahead of the substrate depends on the gradient temperatureG（dT/dx）,and it can be described by the following equation:

Eq.(6)indicates thatTDhas a linear relationship with the distancexfrom the substrate surface.The coating structure deposited on the substrate mainly depends on the amount of undercooling whenTD＜TV.When the substrate pre-heating temperature was set at 850°C,the amount of undercooling ΔT(1)and the practical temperatureTD(1)are shown in Fig.7(a).With the substrate pre-heating temperature decreasing,ΔT(2)andTD(2)decrease as well,which are shown in Fig.7(b).Moreover,when the substrate pre-heating temperature reduces to room temperature,Fig.7(c)indicates ΔT(3)andTD(3).Fig.7 indicates that undercooling of the interface between the substrate and the vapor phase has the relationship of ΔT(1)＜ ΔT(2)＜ ΔT(3),and the slope ofTDincreases along with a decrease of the substrate pre-heating temperature(TD(1)＜TD(2)＜TD(3)).In Fig.7(a)and(b),with an increase of the distancex,ΔTincreases first and then decreases whenTD＜TV.As opposite to Fig.7(c),ΔTdecreases continuously.If ΔTis low,a columnar coating can be obtained,which has been demonstrated in Fig.3,and when ΔTdecreases,a mixed structure composed of columnar coating and f i ne grains can be obtained as illustrated in Fig.4.Thus,if ΔTreduces continuously,a dense coating completely made of f i ne grains can be fabricated.

Fig.7 Different amounts of undercooling related to variant coating structures.

4.Conclusions

Different columnar structured 7YSZ coatings were prepared by plasma spray-physical vapor deposition on graphite and zirconia substrates.The effects of the substrate pre-heating temperature on the coating structure deposited on metallic materials have been investigated.The following gas-deposition principles based on heterogeneous nucleation can be drawn:

(a)Without pre-heating,the coatings deposited on the graphite and zirconia substrates are of a columnar structure.Due to the higher surface energy of zirconia,the horizontal width of columns is larger than that deposited on graphite.

(b)Undercooling of the interface between substrate and vapor phase plays an important role in the coating structure.With the substrate pre-heating temperature decreasing,a typical columnar structure will transform into a mixed structure made of columns and f i ne grains.

Acknowledgements

We would like to acknowledge f i nancial supports from National Key Research Program(2017YFB0306100),Guangdong Academy of Sciences(No.2017GDASCX-0843),GuangdongTechnicalResearchProgram (Nos.201707010385,2014B070706026,2013B061800053),Guangdong Natural Science Foundation(No.2016A030312015),National Natural Science Foundation of China(No.51501044),and Guangzhou Technical Research Program(No.201707010385).

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