稀有金属(英文版) 2018,37(02),79-94

Homologous layered InFeO₃(ZnO)_m:new promising abradable seal coating materials

Wei-Wei Qu Xiao-Xuan Zhang Bi-Fei Yuan Li-Dong Zhao

School of Materials Science and Engineering, Beihang University

China National Petroleum Corporation Greatwall Drilling Company

摘 要：

As promising abradable seal coating materials used in high temperature range, the homologous layered InFe03(Zn0)m(m = l, 2, 3...20) have attracted great attention. In this short review, we summary the research progress in InFe03(ZnO)m that were developed in our group.We first introduced a series of conventional abradable seal coating materials as a research motivation. Second, the phase composition and crystal structures of InFe〇3(ZnO)m system were presented. Then,their thermophysical properties,as the most important part,were introduced in detail. At last, the mechanical properties such as hardness, friction coefficient,erosion wear resistance of InFe03(ZnO)m system were also described. Our summary indicates that InFe03(ZnO)m sys?tems are promising abradable seal coating materials.

收稿日期：1 September 2017

基金：financially supported by the National Natural Science Foundation of China (Nos. 51671015, 51571007 and 51772012);the Beijing Municipal Science and Technology Commission (No. Z171100002017002);the Shenzhen Peacock Plan team (No. KQTD2016022619565911);

Homologous layered InFeO₃(ZnO)_m:new promising abradable seal coating materials

Wei-Wei Qu Xiao-Xuan Zhang Bi-Fei Yuan Li-Dong Zhao

School of Materials Science and Engineering, Beihang University

China National Petroleum Corporation Greatwall Drilling Company

Abstract：

As promising abradable seal coating materials used in high temperature range, the homologous layered InFeO₃(ZnO)_m(m = 1, 2, 3...20) have attracted great attention. In this short review, we summary the research progress in InFeO₃(ZnO)_m that were developed in our group.We first introduced a series of conventional abradable seal coating materials as a research motivation. Second, the phase composition and crystal structures of InFeO₃(ZnO)_m system were presented. Then,their thermophysical properties,as the most important part, were introduced in detail. At last, the mechanical properties such as hardness, friction coefficient,erosion wear resistance of InFeO₃(ZnO)_m system were also described. Our summary indicates that InFeO₃(ZnO)_m systems are promising abradable seal coating materials.

Keyword：

Abradable seal coating; InFeO₃(ZnO)_m; Thermal conductivity; Thermal expansion coefficient; Mechanical properties;

Author： Li-Dong Zhao is a Full Professor of Materials Science and Engineering at Beihang University, China. He received his B.E. and M.E. degrees in Materials Science from Liaoning Technical University and his Ph.D. degree in Materials Science from University of Science and Technology Beijing, China, in 2009. He was a postdoctoral research fellow in the ICMMO at University of Paris-Sud from 2009 to 2011 and continued as a postdoctoral research fellow in the Department of Chemistry at Northwestern University from 2011 to 2014. His research interests include the fabrication and characterizations of layered structural thermoelectrics, superconductors,and thermal barrier coatings.e-mail:zhaolidong@buaa.edu.cn;

Received： 1 September 2017

1 Introduction

Aviation engine is the power source and the core components of an aircraft,which must meet the requirements of both low fuel consumption and high efficiency.It is well known that the interspace between engine blade and casing increases the fuel consumption and then reduces the engine efficiency [ 1, 2] .However,an interface with size of about 2-3 mm is necessary during the engine designing to avoid the possible damages: [ 3] (1) the shaft deformation caused by different thermal expansion coefficients (TECs) of different components in the high-temperature working environment;(2) the deformation of the parts caused by constant vibration during working;(3) the elongation of engine blades in the radial direction which is generated during the rotor working at a high speed;(4) the error in the procedures of processing and assembling the components.Taken all above into account,there is a contradiction between the engine system design and the purpose for improving efficiency and reducing the fuel consumption.In order to solve the problems mentioned above,the seal coating materials which possess high thermal stability,low thermal conductivity,low friction factor,and excellent oxidation resistance were designed to coat on the aircraft engine.The seal coating materials can combine adjacent rotating and stationary parts more closely [ 4] ,minimize the clearance between the rotor blades and the turbine casing,improve engine efficiency,and increase the service life of the rotor blades [ 5] .

In 2001,the seal coating materials were successfully deposited on the gas turbine engine component by General Electric Company,which could reduce 0.2%-0.6%thermal loss and improve 0.3%-1.0%output power of the aviation engine [ 6] .In 2002,the Rolls-Royce used the seal coating materials on their own developed engine,which made its thrust fuel consumption reduce by 0.5%and greatly lengthened the service life of Trent 500 [ 7] .Padture et al. [ 8] modified the incoming system of plasma spraying equipment and prepared high-porosity ceramic coating by liquid materials.However,this process and investment are difficult to achieve because of its low deposition efficiency and low bonding strength.Sulzer Metco Company [ 9] and General Electric Company [ 10] used hollow zirconia ceramic powders as raw materials for plasma spraying in their patents to prepare the coating which has the characteristics of low hardness and large porosity of about 25%-30%.Lin et al. [ 11] used spherical amorphous yttria-stabilized zirconia (YSZ) as raw materials and prepared YSZ coating with special microstructure constituted by nanoparticle and cellular networks.This structure can solve the problem of low porosity.

With the continuous increase of aviation engine service temperature,the seal coating materials are faced with new requirements:not only to provide the mechanical protection,but also to provide thermal protection.Therefore,it is an essential research direction for seal coating materials to improve its abradable and thermal insulation performance at higher working temperature.The thermal insulation seal coating materials are a combination of the thermal barrier coating and the seal coating materials.Therefore,it has both the characteristics of the above two materials,and its application can effectively improve engine performance and service life [ 12] .To meet the requirement of harsh application environment,the work temperature of new generation high-performance aircraft engine has reached 1350℃ [ 13] .The conventional coating materials (YSZ,BN) can be used for high-temperature (>1000℃) abradable seal coating materials;however,they will be sintered after serving for a long time and have phase transformation over 1200℃,then the tips of the engine blade will be wore down and discarded under the increasing friction.Therefore,a new high-temperature abradable seal coating material,which possesses better thermal insulation and is easier to be abraded at high temperature,is in urgent need.

Recently,the thermal spraying seal coating materials and ceramic matrix high-temperature coating develop rapidly because of its obvious advantages.Thermal spraying seal coating materials can be pided into two kinds according to its applications:(1) The abradable seal coating materials with soft texture and high porosity are sprayed on the static band matching the rotating components,so it should meet the requirements of lubricity,erosion resistance,and thermal stability [ 14, 15, 16, 17, 18, 19] ;(2) the new wear-resistance high-temperature seal coating materials with high hardness can be treated as thermal barrier coating and wear-resistance seal coating materials [ 20] ,which meet the imperious demands of heat insulation and abrasion at high temperature [ 21] .Both of them are applied to control the interspace between the rotating and stationary parts.

There are several kinds of low and medium temperature seal coating materials:(1) Al/polymer coating can well cooperate with titanium alloys blade under 320℃for a longtime service [ 22] .However,the polymer will become high hardness,low wearability,and disabled in a longtime service;(2) Al/C coating extends the working temperature to450℃.It has better abradability than Al/polymer coating,but the erosion resistance is slightly worse [ 23] .In addition,if the coating was exposed in air for a long time,it can be particle loss and even invalid;(3) AlSi/BN coating can effectively improve the corrosion resistance,maintain the good abradability and the erosion resistance;(4) Ni/C coating is widely used in compressor parts working at below500℃.Based on this,the Ni/Si composite material was produced and its superior erosion resistance makes it suitable to be coated on the turbine parts under 750℃ [ 24] .

The high-temperature seal coating materials are the guarantee for aeroengines serving in the harsh environment and become a research hot point in recent years.There are several representative types of high-temperature seal coating materials:(1)β-NiAl/polyester coating shows good abradability under 850℃and good antioxidant capacity under 900℃ [ 25] .The metal phaseβ-NiAl with 30%Al possesses a high melting point,and the filling material of polyester can form tiny holes after ablation,which reduced the hardness of coating,improved the abradability and the thermal shock resistance;(2) CoNiCrAlY/BN coating has been put into application under 850℃ [ 26] .Cobalt-based superalloy CoNiCrAlY possesses superior resistance to oxidation and corrosion under 1000℃.The soft phase h-BN plays a role of reducing hardness,enhancing lubrication and abradability;(3) yttria-stabilized zirconia (YSZ)possesses high melting point,low thermal conductivity,excellent thermal stability,and high-temperature creep property.However,the density of directly sprayed YSZ ceramic coating is high;meanwhile,the thermal expansion coefficients of YSZ and metal substrate are not matched well,it will generate cracks and even cause the coating flack off.

As promising abradable seal coating materials used in high-temperature range,the homologous layered InFeO₃(ZnO)_m (m=1,2,3...20) have attracted great attention.In this article,we reviewed the thermal transport properties,the thermal stability,and the mechanical performance of InFeO₃(ZnO)_m.We firstly introduced the phase composition and crystal structures of InFeO₃(ZnO)_m system.Then,their thermophysical properties,as the most important part,were introduced in detail.At last,the mechanical properties such as hardness,friction coefficient,erosion wear resistance of InFeO₃(ZnO)_m system were described.Excellent thermal stability,low thermal conductivity,high thermal expansion coefficients,and good mechanical properties make InFeO₃(ZnO)_m be very promising abradable seal coating materials in high-temperature application.

2 Synthesis,phase composition,and crystal structure of InFeO3(ZnO)m

2.1 Synthesis

Kimizuka et al. [ 27] firstly synthesized polycrystalline InFeZnO₄ using In₂O₃ (99.9%),Fe₂O₃ (99.99%),and ZnO(99.99%) as raw materials through a solid-state reaction.According to this method,Ln₂O₃ (Ln=In,Yb,Gd),Fe₂O₃,and BO (B=Zn,Mg) were mixed with ethyl alcohol and were grinded by planetary ball mill (QM-3SP4) at400 r·min^-1 for 10 h [ 28] .Rare earth oxide (REO) powder can highly absorb water and carbon dioxide in the air;therefore,REO needs to be calcined at 1000℃for 6 h under air atmosphere to remove the physical adsorption and carbonate.Other oxides were calcined at 600℃for6 h to remove moisture before ball milling.The mixtures were dried in an oven at 110℃for 2 h.The obtained powders are cool-pressed and sintered according to the parameters in Table 1.

2.2 Phase composition

X-ray diffraction (XRD) patterns of InFeO₃(ZnO)_m(m=1-5) are shown in Fig.la [ 29] compared with the JCPDF card,all the samples can be indexed well,and there are no impurity peaks.As shown in Fig.lb-d [ 28, 30, 31] ,no impurities are observed after In substituted by Gd/Yb and Zn substituted by Mg.

Table 2 [ 32] and Table 3 [ 28, 30] show the chemical compositions and the deviation fraction of InFeO₃(ZnO)_m(m=1-5),Gd/Yb/Mg doped InFeZnO₄ using energy dispersive spectroscopy (EDS).The deviation fraction was calculated by Eq.(1):

where x_exp and x_nom are the experimental mole ratio and the nominal mole ratio,respectively.It can be found that there is a little deviation of mole ratio within the margin of error for the homologous compounds,InFeO₃(ZnO)_m (m=1-5)and Yb/Gd/Mg doped InFeZnO₄.Based on the results of XRD and EDS,the homologous compounds were successfully synthesized by the solid-state reaction.

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Table 1 Sintering parameters for InFeO₃(ZnO)_m

2.3 Crystal structure

As shown in Fig.2 [ 29] ,InFeO₃(ZnO)_m possess hexagonal structure,which is similar to YbFe₂O_4.Kimizuka et al. [ 27] and Beznosikov and Aleksandrov [ 27, 33] found that these compounds possess two space groups, (extinction law:-h+k+l≠3n) when m is odd number and P6₃/mmc(l≠2n for hkl) when m is even number.Table 4 [ 27, 33] lists the crystal data of InFeO₃(ZnO)_m (m=1-9),and the lengths of a-axis and c-axis have a close relationship with the wurtzite structure parameters of ZnO (a=0.3249 nm and c=0.5205 nm).Beznosikov and Aleksandrov [ 33] reported that the structure of InFeO₃(ZnO)_m can be considered as systems originating from the intergrowth of two structural types,InFeO₃ and ZnO (wurtzite),so the length of c-axis can be calculated:( +mC_znO)×(z/2),in which is 1.2202 nm and z is the molecular numbers in a cell.Kimizuka et al. [ 27] considered that the InFeO₃(ZnO)_m are stacked by InO_1.5 layers,(FeZn)O_2.5layers,and ZnO layers and the numbers of them are represented by u,w,x.Based on the construction rules mentioned above,the length of c-axis can be estimated from the following relationship:

where CInO_1.5+(Fezn)O_2.5 is the total thickness of InO_1.5 layer and (FeZn)O_2.5 layer,which is equal to 0.8698 nm.The contrast of C_obt, shows that the value of can well match with the obtained data in Table 4.It means that the hypotheses suggested by Kimizuka et al. [ 27] are reasonable.

Zhao et al. [ 29, 34] investigated the microstructure of InFeO₃(ZnO)_m and confirmed these homologous layered structures.As shown in Fig.3 [ 29] ,the crystal symmetries of allowing{0 0 3n} [ 35] reflections for m=1,5 and the symmetry of P6₃/mmc allowing{0 0 2n} [ 36] reflections for m=2,4 are corresponding well to the structural models.The layered structure of InFeO₃(ZnO)_m consists326+of InO₂^-layers interleaved with (FeZn)O²⁺layers along c-axis.

Zhao et al.observed the layer structure of InFeO₃(Z-nO)_m using electron diffraction [ 29] .Figure 4 shows the lattice image of InFeO₃(ZnO)_m (m=1-5) projected along the[110]direction.The layers of In-O are imaged as single darker lines while the layers of (Fe,Zn)^-O are imaged as brighter contrast.The difference in the crystal structure of the compounds when m is odd number and even number is also shown in Fig.4.When m=1,3,5,the numbers of(Fe,Zn)^-O layers are 4,7,10,as shown in Fig.4a,c,e.In other words,there are three (Fe,Zn)^-O layers different between In-O layers for the adjacent samples with odd m values.There is a similar rule between adjacent samples with even m values,but the number of differences of (Fe,Zn)^-O layers is only one.

Fig.1 XRD patterns of a InFeO₃(ZnO)_m [29] b In_1-xYb(Gd)_xFeZnO₄ [31] c In_1-xYb_xFeZnO₄ [30] d In_0.4Yb_0.6FeZn_1-xMg_xO₄ [28]

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Table 2 Chemical compositions of InFeO₃(ZnO)_m (m=1-5) [32]

Figure 5 shows the fracture micrographs appearance of InFeO₃(ZnO)_m (m=1-5) observed by scanning electron microscopy (SEM),which exhibits the feature of layered structure with the thickness of 1-3μm.Owing to the different directions of particles in nucleation stage,lamellas are disordered and grow up to different directions of layers.In addition,the same layered structure appeared in Gd/Yb/Mg doped InFeZnO₄,as shown in Figs.6 and 7.

3 Thermophysical properties

3.1 Phase stability

In our work,we determined the thermal stability using heat treatment and differential scanning calorimetry (DSC).DSC curves were measured by simultaneous thermal analyzer (STA 449C) with the heating rate of 15 K·min^-1 in argon atmosphere from 20 to 1350℃.DSC curves of InFeO₃(ZnO)_m (m=1-5) are shown in Fig.8a.For m=1-5,there is no any endothermic or exothermic peak on DSC curves,indicating that InFeO₃(ZnO)_m (m=1-5)have no phase transformation from room temperature to1400℃ [ 31, 32] .In addition,Fig.8b shows XRD patterns of InFeO₃(ZnO)_m (m=1-5) after heat treatment at1450℃for 48 h.Compared with JCPDF card,it can be well indexed.Zhang et al. [ 32] measured DSC curves and XRD patterns of In_1-xYb(Gd)_xFeZnO₄ (x=0,0.1,0.2),as shown in Fig.9.Similarly,XRD patterns of In_0.4Yb_0.6-FeZn_0.5Mg0.5O₄ were measured,as shown in Fig.10.Considering DSC curves and XRD patterns ofInFeO3(ZnO)m (m=1-5),we can conclude that InFeO3(ZnO)m (m=1-5) possess good phase stability and thermal stability at high temperature.

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Table 3 Chemical compositions of InFeZnO₄ doped by Gd/Yb/Mg [28,30]

Fig.2 Crystal structures of InFeO₃(ZnO)_m:a R3m space group with odd m numbers and b P6₃/mmc space group with even m numbers.In atoms at octahedral site,Fe and Zn atoms at trigonal bipyramidal sites and part of Zn atoms at tetrahedral sites [29]

3.2 Thermal conductivity and thermal expansion coefficient

3.2.1 InFeO3(ZnO)m (m=1-5)

The thermal conductivity (λ) can be calculated according toEq.(3):

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Table 4 Crystal data of InFe₃(ZnO)_m [27,33]

z,molecular numbers in a cell;u,number of InO1.5 layers;w,number of (FeZn)O7.5 layers;x,number of ZnO layers;a,length of a-axis;Cobt, obtained length of c-axis;

length of c-axis calculated by Kimizuka et al. [27] .

; length of c-axis calculated by Beznosikov et al. [33]

Fig.3 Electron diffraction patterns of InFeO₃(ZnO)_m (m=1,2,4,5) taken with incident beam along[110],[100]directions of hexagonal lattice:a₁,a₂ m=1,b₁,b₂ m=2,c₁,c₂ m=4,and d₁,d₂ m=5 [29]

whereαis thermal diffusivity,C_p is the specific heat capacity,and p is the density of the specimen.αcan be measured by a laser flash apparatus (Netzsch LFA 427) in argon gas atmosphere at 25-1000℃.The C_p can be calculated by the Neumann-Kopp law [ 37] using the heat capacity of the raw materials such as In₂O₃,Fe₂O₃,ZnO obtained from the thermodynamic database [ 38] .The p was tested by the Archimedes'method.

Zhao et al. [ 34] firstly studied the thermal conductivity of InFeO₃(ZnO)_m,m=1,from room temperature to1200℃,as shown in Fig.11.At 800℃,InFeZnO₄ possesses lower thermal conductivity than La₂Zr₂O₇ and Gd₂Zr₂O₇.The thermal conductivity of InFeZnO₄ at1200℃is～1.33 W·m^-1·K^-1 and can be compared with～1.1 W·m^-1·K^-1 for Ba₂ErAlO₅ at 1000℃and～1.2 for Ba₂DyAlO₅ at 1000℃ [ 39] .

Zhang et al. [ 32] systematically tested the thermal conductivity of InFeO₃(ZnO)_m (m=1-5),and the results are shown in Fig.12a.At 1000℃,the thermal conductivity of InFeZnO₄ is 1.38 W·m^-1·K^-1 and is lower than2.1-2.2 W·m^-1-K^-1 for 8 YSZ [ 40] The low thermal conductivity may be caused by the lattice vibration,which is described by the phonon scattering [ 41] Fig.12a shows that InFeZnO₄ possesses the lowest thermal conductivity,one can see that the thermal conductivity with odd m number is lower than those with even m number.Namely,the thermal conductivity of InFeO₃(ZnO)3 is lower than that of InFeO₃(ZnO)₂ and the thermal conductivity of InFeO₃(ZnO)₅ is lower than that of InFeO₃(ZnO)₄.The reason is that there are different crystal structures for InFeO₃(ZnO)_m,trigonal system for odd m number and hexagonal system for even m number [ 27, 33] The complicated crystal structure,trigonal system,can lead to an anabatic anharmonicity of the lattice vibration and a strong phonon scattering,as a result,the mean free path and the thermal conductivity are lower [ 41, 42] .

Fig.4 Lattice images of InFeO₃(ZnO)_m (m=1-5) projected along[110]direction:a m=1,b m=2,c m=3,d m=4,and e m=5 [29]

Fig.5 SEM fracture images of InFeO₃(ZnO)_m (m=1-5):a m=1,b m=2,c m=3,d m=4,and e m=5 [29]

Fig.6 SEM fracture images of In_1-xYb(Gd)_xFeZnO₄ (x=0.1,0.2):a In_0.9Yb_0.1FeZnO₄,b In_0.8Yb_0.2FeZnO₄,c In_0.9Gd_0.1ZnO₄,and d In_0.8Gd_0.2ZnO₄

The thermal expansion coefficient (TEC) is a linear expansion coefficient,and the measurements uncertainty is within 3%.Figure 12b shows the thermal expansion coefficients of InFeO₃(ZnO)m (m=1-5) from 20 to 900℃.The TEC is related to the volume of unit cell,lattice sites,and average distance between atoms,which increases with temperature increasing [ 41, 43, 44, 45, 46] Thus,the TECs of InFeO₃(ZnO)_m (m=1-5) increase with temperature increasing.For InFeO₃(ZnO)_m system,the TEC of InFeZnO₄ is the largest,which may be associated with the decrease of layer spacing for In_1.5,(FeZn)O_2.5 and ZnO.The smaller the layers spacing is,the larger the thermal expansion is when the temperature rises.The TECs of InFeO₃(ZnO)_m for the samples with m=1-5 at 900℃are11.58×10^-6,10.97×10^-6,11.12×10^-6,11.28×10^-6,and 11.46×10^-6,respectively,which are better than the TEC of 8YSZ [ 32] .

3.2.2 In1-xYb(Gd)xFeZnO4 (x=0,0.1,0.2)

Considering that the thermal conductivity of InFeZnO₄ is not low enough from the room temperature to 800℃,Guo et al. [ 31] replaced In by Yb/Gd to reduce the thermal conductivity.Figure 13 shows the thermal conductivities,thermal diffusivity,and thermal expansion coefficient of In_1-xYb(Gd)_xFeZnO₄ (x=0,0.1,0.2) from 20 to 1000℃.The thermal conductivities obviously decrease at all temperature ranges with x increasing,and Gd-doped InFeZnO₄ possesses lower thermal conductivity than Ybdoped InFeZnO₄.In_0.8Gd_0.2FeZnO₄ possesses the lowest thermal conductivity,2.32-1.21 W·m^-1·K^-1 from 20 to1000℃.According to the theory of phonon thermal conduction [ 47] ,the lattice thermal conductivity is in proportion to the phonon mean free path .When In³⁺are partially replaced by Yb³⁺/Gd³⁺,the lattice will be distorted and the phonon scattering can be enhanced,causing a reduction in phonon mean free path and then low thermal conductivity.

Figure 13c shows that Yb-doped InFeZnO₄ possesses higher thermal expansion coefficient than Gd₂O_3-doped In_0.8Yb_0.2FeZnO₄.The higher thermal expansion benefits from a lower bond energy of Yb-O (397.9 kJ·mol^-1) than Gd-O (716 kJ·mol^-1),resulting in lower binding energy between the atoms and greater atomic thermal vibration amplitude.

3.2.3 In1-xYbxFeZnO4 (x=0,0.1,0.2…0.9,1.0)

The above investigations of In_1-xYb(Gd)_xFeZnO₄ (x=0,0.1,0.2) show that Yb₂O₃ and Gd₂O₃ doping can effectively reduce the thermal conductivity of InFeZnO₄.Meanwhile,Yb₂O₃ doping can maintain the thermal expansion coefficient.Based on this,Zhang et al. [ 30] measured the thermal conductivities of In_1-xYb_xFeZnO₄(x=0,0.1,0.2…0.9,1.0) from 20 to 1000℃.As shown in Fig.14a,the minimum lattice thermal conductivity of1.19 W·m^-1·K^-1 was obtained at 1000°C for In_0.4Yb_0.6FeZnO₄.

With Yb3+fractions increasing,the lattice distortion was enlarged,which leaded to a low thermal conductivity.Theoretically,when x=0.5,the lattice distortions of InFeZnO₄ and YbFeZnO₄ are maximum thus in lower thermal conductivity.Figure 14b shows that the optimized Yb fractions can lead to a minimum lattice thermal conductivity with x=0.6,which is due to lower thermal conductivity of YbFeZnO₄.

3.2.4 In0.4Yb0.6FeZn1-xMgxO4 (x=0,0.1,0.2...0.9,1.0)

Figure 15 shows the thermal conductivities of In_0.4Yb_0.6FeZn_1-xMg_xO₄ (x=0,0.1,0.2...0.9,1.0) as function of temperature and Mg content [ 28] .It can be found that Mg doping on Zn site can efficiently decrease the thermal conductivity of In_0.4Yb_0.6FeZn_1-xMg_xO₄.With the increase of x value,the thermal conductivity goes down and then starts to increase at x=0.5,which may be due to the maximum lattice distortion when x=0.5.The lowest thermal conductivity of In_0.4Yb_0.6FeZn_1-xMg_xO₄ is about1.13 W·m^-1·K^-1 at 1000℃,and about 5%lower than1.19 W·m^-1·K^-1 of In_0.4Yb_0.6FeZnO₄ and about 21.5%lower than 1.44 W·m^-1·K^-1 of InFeZnO₄.

Fig.7 SEM fracture images of In_0.4Yb_0.6FeZn_1-xMg_xO₄:a₁,a₂ x=0,b₁,b₂x=0.2,c₁,c₂ x=0.4,d₁,d₂ x=0.6,e₁,e₂ x=0.8,and f₁,f₂x=1.0 [28]

The thermal expansion coefficients of In_0.4Yb_0.6-FeZn_0.5Mg0.5O₄ were measured,and the results are shown in Fig.16.Compared with the TECs of Sm₂Zr₂O₇ [ 48] which is a representative rare earth-doped zirconium oxide coating material with broad development prospects,In_0.4Yb_0·6FeZn_0.5Mg_0.5O₄ shows a higher thermal expansion coefficient from the room temperature to 1200℃.

3.3 Calculated thermal conductivity of InFeO3(ZnO)m(m=1-5)

Zhang et al. [ 29] calculated theoretical thermal conductivity of InFeO₃(ZnO)_m (m=1-5) using Debye model.Considering the contributions of Umklapp phonon-phonon scattering,stacking faults and optical branch,the calculated thermal conductivities were in good agreement with the experimental data.As shown in Fig.17a,when only considering the effect of Umklapp phonon-phonon scattering,there is a great deviation between calculations and experimental data,especially in low temperature range.The reason is that the stacking faults play an important role in reducing the thermal conductivity of this system.Figure 17b shows the comparison between calculations and experimental data after considering the effect of stacking faults.In all temperature ranges,the above two are in good agreement.Optical branch can also make a big difference in this system.As shown in Fig.17b,the calculated thermal conductivity of InFeO₃(ZnO)_m matches well with the experimental data when joining the effect of optical branch.

Fig.8 a DSC curves and b XRD patterns of InFeO₃(ZnO)_m (m=1-5) after heat treatment [32]

Fig.9 XRD patterns and DSC curves of In_1-xYb(Gd)_xFeZnO₄ (x=0,0.1,0.2):a XRD patterns of as-synthesized samples,b XRD patterns of samples after heat treatment,and c DSC curves [31]

Fig.10 a DSC curve and b XRD patterns of In_0.4Yb_0.6FeZn_0.5Mg0.5O₄ before and after heat treatment [28]

Fig.11 Thermal conductivities of InFeZnO₄,La₂Zr₂O₇,Gd₂Zr₂O₇,Ba₂ErAlO₅,and Ba₂DyAlO₅ [39]

It can be seen from the calculation results that stacking faults play a decisive role in reducing thermal conductivity,especially in low temperature ranges.Meanwhile,the effect of optical branch on the thermal conductivity cannot be ignored.

4 Mechanical properties

4.1 Hardness

The hardness was measured by the electric surface Rockwell hardness tester (HSRD-45) with the staff gauge of HR_45Y.Rockwell hardness is calculated as: [ 49]

Fig.12 a Thermal conductivities and b thermal expansion coefficient (TECs) of InFeO₃(ZnO)_m (m=1-5) [32]

Fig.13 a Thermal conductivity,b thermal diffusivity,and c thermal expansion coefficients (TECs) of In_1-xYb(Gd)_xFeZnO₄ (x=0,0.1,0.2) [31]

Fig.14 Thermal conductivities of In_1-xYb_xFeZnO₄ samples as function of a temperature and b Yb composition [30]

Fig.15 Thermal conductivities of In_0.4Yb_0.6FeZn_1-xMg_xO₄ samples as function of a temperature and b Mg content [28]

Fig.16 Thermal expansion coefficient (TEC) as function of temper-ature for In_0.4Yb_0.6FeZn_0.5Mg0.5O₄ and Sm₂Zr₂O₇ [28]

where N is a dimensionless constant (100),h is the residual depth of indentation,S is a constant (0.001 mm) representing one unit surface Rockwell hardness.According to Eq.(4),the smaller the residual depth is,the harder the coating material is;when h>0.1 mm,the value of hardness is negative and the other hardness scale should be adopted.

Taken five points expressed in x₁,x₂,x₃,x₄,x₅ on the InFeZnO₄,In_0.4Yb_0.6FeZn_0.5Mg0.5O₄,and 8YSZ samples for the hardness test and the calculated average x are listed in Table 5 [ 28] .Owing to the layered structure,the Rockwell hardness of InFeZnO₄ (HR_45Y 76) and In_0.4Yb_0.6FeZn_0.5Mg0.5O₄ (HR_45Y 73) is much smaller than that of 8YSZ (HR_45Y 87).Both of InFeZnO₄ and In_0.4Yb_0.6FeZn_0.5Mg0.5O₄ with low hardness show good application in the seal coating materials.

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Table 5 Rockwell hardness of InFeZnO₄ and 8YSZ (HR_45Y) [28]

4.2 Friction coefficient

The friction coefficients of InFeZnO₄ were measured by HT-1000 high-temperature friction and wear test machine for 20-40 min at room temperature,200,400,and 800℃,respectively.In addition,Zhao et al. [ 28] measured the friction coefficient of In_0.4Yb_0.6FeZn_0.5Mg0.5O₄ and 8YSZ using SRV-4 high-temperature friction and wear test machine for 20 min at room temperature,400,and 800℃,respectively.

As shown in Fig.18a,b,compared with YSZ and 8YSZ,InFeZnO₄ and In_0.4Yb_0.6FeZn_0.5Mg0.5O₄ possess lower friction coefficient.It is due to the weak van der Waals force between the layers,which can be easily damaged by the friction.Then,the slice atomic layer can adjust the direction to parallel to the relative sliding direction under the friction,making it easy to continue sliding.This property endorses InFeZnO₄ and In_0.4Yb_0.6FeZn_0.5Mg0.5O₄to be the potential abradable seal coating materials.

4.3 Erosion wear resistance

The erosion wear resistance of a material can be characterized by its volumetric erosion rate,which is the volume loss caused by the unit erosion particle eroding.The larger the volumetric erosion rate is,the worse the erosion wear resistance is,and the volumetric erosion rate (ω) can be expressed by Eq.(5):

Fig.17 Comparison between calculations and experimental data of thermal conductivities (κ):a only considering effect of Umklapp phonon-phonon scattering,b considering effect of Umklapp phonon-phonon scattering and stacking faults,and c on previous basis,adding effect of optical branch [29]

Fig.18 Comparisons of friction coefficient:a YSZ and InFeZnO₄ and b In_0.4Yb_0.6FeZn_0.5Mg0.5O₄ and 8YSZ [28]

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Table 6 Volume erosion rate of In_0.4Yb_0.6FeZn_0.5Mg_0.5O₄ at differ-ent erosion angles at room temperature [28]

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Table 7 Volume erosion rate of In_0.4Yb_0.6Fezn_0.5Mg0.5O₄ at differ-ent erosion temperatures with erosion angle of 60°

where M₁ is the weight of sample before erosion,M₂ is the weight of sample after erosion,M₃ is the weight of erosion particles,and p is the density of the material.

The erosion wear resistance of In_0.4Yb_0.6-FeZn_0.5Mg0.5O₄ was measured by sandblasting-type airflow erosion test machine with control variable method,fixing erosion temperature (20℃) at different angles (30°,60°,90°),and fixing erosion angle (60°) at different temperatures (20,400,800℃).The erosion speed was set to30 m·s^-1,and 100 g corundum sand was chosen as the erosion particle.Table 6 [ 28] shows the volume erosion rate of In_0.4Yb_0.6FeZn_0.5Mg0.5O₄ at different erosion angles at room temperature.It can be found that theωincreases with the erosion angle (α) increasing.The relationship betweenωandαcan be expressed by Eq.(6): [ 50]

where A and B are the constants.If the material is completely toughness,A=0;If the material is completely brittleness,B=0.Most materials are not completely ductile or brittle,and their brittleness can be varied with the external environment.In general,when the erosion angle is20°-30°,most of the ductile materials can be destroyed;when the erosion angle is 90°,most of the brittle materials can be destroyed.Based on this,it is easily judged that In_0.4Yb_0.6FeZn_0.5Mg0.5O₄ is a kind of brittle material.

The volume erosion rates of In_0.4Yb_0.6FeZn_0.5Mg0.5O₄at different erosion temperatures (20,400,800℃) with erosion angle of 60°are shown in Table 7. [ 28] It can be seen that rising temperature increased the erosion rate.In summary,the erosion resistance of In_0.4Yb_0.6-FeZn_0.5Mg0.5O₄ is not good and requires improving.However,the material abrasion resistance and its erosion resistance are antagonism,so we need to keep balance for two sides to choose the best solution.

5 Summary and conclusion

The homologous layered InFeO₃(ZnO)_m can be prepared by the solid-state reaction.These homologous compounds reveal good thermal stability from room temperature to1400℃.Thermal conductivity of InFeO₃(ZnO)_m with odd m number is lower than that with even m number.InFeZnO₄ (m=1) has the lowest thermal conductivity of1.38 W·m^-1·K^-1 at 1000℃,which is about 30%lower than that of YSZ (2.1-2.2 W·m^-1·K^-1 at 1000℃).Yb/Gd doping can effectively lower thermal conductivities of InFeZnO₄,and the thermal conductivities show a decreasing trend with Yb/Gd contents rising.The thermal expansion coefficients (TECs) of InFeO₃(ZnO)_m are in the range of 10×10^-6-12×10^-6·K^-1 at 900℃,which are similar to those of YSZ.Compared with YSZ bulks,InFeO₃(ZnO)_m exhibit relatively low Rockwell hardness and friction coefficient,showing excellent abradability.Unfortunately,the erosion wear resistance of this compound is too poor to use as a thermal barrier coating material;besides,there is still room for thermal conductivity to decrease.In the future research,suitable doping,compositing,and second phase introducing may enhance the erosion wear resistance and reduce thermal conductivity of this compound.In summary,the properties that we investigated,excellent thermal stability at high temperature,low thermal conductivities,high thermal expansion coefficients,good mechanical properties,make homologous layered compounds,InFeO₃(ZnO)_m,show very promising application as abradable seal coating materials.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos.51671015,51571007 and 51772012),the Beijing Municipal Science and Technology Commission (No.Z171100002017002),and the Shenzhen Peacock Plan team (No.KQTD2016022619565911).The authors are grateful to Professors N.Dragoe,D.Berardan,J.Q.He,S.K.Gong,H.B.Guo,Y.-L.Pei,and Y.Liu for their plentiful discussions and fruitful collaborations.

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