Jie MAO, Min LIU, Ziqian DENG, Kui WEN, Changguang DENG,Kun YANG, Zhikun CHEN

The Key Lab of Guangdong for Modern Surface Engineering Technology, National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangdong Institute of New Materials, Guangzhou 510651, China

KEYWORDS Deposition mechanism;Flow field characteristic;Numerical simulation;Orientation difference;Plasma spray - physical vapor deposition;YSZ coating

Abstract The YSZ coatings are prepared by the plasma spray-physical vapor deposition(PS-PVD)technology based on a specific experimental design. The structure, thickness and growth angle of YSZ coatings on the entire circumferential surface of the cylindrical sample are studied.The results indicated that the structure, thickness and deflection growth angle of YSZ coatings are related to the orientation of deposition location.The numerical simulation of the multiphase mixed fluid near the substrate is carried out and the deposition regularity and mechanism of YSZ coatings prepared by PS-PVD is deduced. The growth rate is related to the local characteristics of the plasma flow field,and is directly proportional to the field pressure and inversely proportional to the field velocity. The growth angle of the coating is generally affected by the flow direction of the plasma jet.Especially, the normal component of velocity vector, Vnorm, mainly affects the speed at which the coating grows vertically upwards.The tangential component of velocity vector,Vtan,determines the degree that the coating growth direction deviates from the vertical direction.When Vtan ≠0,the coating forms a fine column with a certain deflection angle and finally develops into an oblique columnar structure.

1. Introduction

Thermal barrier coatings (TBCs) are extensively applied onto hot-components of turbine engines, which have a complex multi-layered structure. A metallic bondcoat of MCrAlY(M=Ni, Co or both) is used for oxidation/corrosion resistance and a ceramic topcoat of yttria stabilized zirconia(YSZ) for thermal protection.1Generally, electron beamphysical vapor deposition (EB-PVD) and air plasma spraying(APS) are the two main methods of depositing TBCs.2However, APS can quickly produce coatings with large thickness(300-3000 μm) and high porosity (10%-35%), which has good thermal insulation but poor thermal shock resistance.3,4The columnar structured TBCs of EB-PVD exhibit high-strain tolerance but inadequate thermal insulation. In addition, higher investment cost and lower deposition rate also need to be considered.5In some focused studies, APS was developed to prepare TBCs with vertical cracks, and suspension plasma spray (SPS) was used to prepare columnar crystal TBCs. However, some challenges need to be addressed such as low deposition rate, short spray distance and sensitive coating microstructure to the bondcoat roughness, spray angle and spray parameters.6-8

As a promising new technology combined with the advantages of APS and EB-PVD, plasma spray - physical vapor deposition (PS-PVD) has the ability to manufacture columnar structured coatings by vapor phase deposition using thermal spray technology.9,10Compared to low pressure plasma spraying (LPPS) or vacuum plasma spraying (VPS), plasma jet of PS-PVD is greatly expanded to more than 2 m in length and 200-400 mm in diameter due to low working pressure(ranging from 0.5-1.5 mbar) and high power input.11,12Moreover, high electrical input power (>100 kW) and highenthalpy plasma gas are used to achieve high temperature over 6000 K and possible powder evaporation for obtaining columnar structured coatings.10Besides, due to its high velocity and large dimension, PS-PVD plasma jet is capable of flowing along complex surface or around the barriers, and forced the incorporated vaporized material to deposit on the shadowed surface. It is the outstanding advantage called non-line-of-sight deposition for coating complex shaped parts, which cannot be achieved by PVD or other conventional plasma spray techniques.11,13-15

Recently,many investigations focused on the process development of PS-PVD, especially, on the process characteristics of non-line-of-sight deposition of TBCs based on the application of multiple guide vane and other complex hot-components of turbine engines.16-19In this paper,based on a specific experimental design,the representative YSZ coatings were prepared by the MultiCoatTMhybrid plasma-coating system at very low ambient pressure. The deposition regularity and microstructure of YSZ coatings were studied according to the experimental results. The relationship between the characteristics of PSPVD plasma jet and YSZ coatings was deduced, integrating the numerical simulation and experimental results, which are helpful to understand the unique advantage of non-line-ofsight deposition of PS-PVD technology.

2. Experimental

All experiments were carried out on a MultiCoatTMhybrid plasma-coating system, as shown in Fig. 1. The 316L stainless steel cylinder with a dimension of ?16 mm×10 mm(diameter×height)was employed as substrate for YSZ coating deposition. Feedstock agglomerated 7.5 wt% yttria partially stabilized zirconia powder (1-30 μm, Metco 6700, Oerlikon Metco) was deposited on the substrate. Prior to spraying, the substrate pre-coated by NiCrAlY bondcoat was polished and degreased with acetone.The surface roughness of cylinder outside is less than 2 μm. The plasma power over 120 kW was used to deposit YSZ coatings.The O3CP plasma gun was fixed on a six-axis robot manipulator inside a 22 m3vacuum chamber with an automatic pumping system to assure that the chamber pressure was maintained at 150 Pa during spraying.The detailed spray parameters are listed in Table 1. The Helium was used as primary gas and Argon was employed as the secondary gas. The spray distance was set as 950 mm for all experiments.

Fig. 1 MultiCoatTM hybrid plasma-coating system.

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The representative YSZ coatings were prepared by a specific reciprocating experimental design. During the coating deposition, the cylindrical substrate was placed so that the axis of plasma jet is perpendicular to the axis of cylindrical substrate and the spray gun sweeps up and down, as illustrated in Fig. 2. The reversal spots are outside of the substrate to prevent overheating. Technically, the way the gun moves up and down, rather than being fixed in a particular location against the substrate, will avoid two problems. One problem is that the substrate temperature can be controlled and does not increase dramatically in a very short time, causing overheating and deformation. Another problem is that when the spray gun sprays to one position on the substrate instead of reciprocating movement (the substrate is not rotated in this experiment), can cause more serious turbulence near the space above the substrate. The shadow effect of coating growth is too prominent to truly represent the structure difference, not to mention to analyze the deposition regularity depended on orientation difference in PS-PVD plasma jet. In addition, the gun sprays along a certain trajectory rather than being fixed in a particular location against the substrate, in this way is closer to the actual situation of coating production.

The cross-sectional microstructures of YSZ coatings at different outer circumferential direction were observed by field emission-scanning electron microscope (FE-SEM, Nova-Nono430, FEI). The coating thickness was measured by the image analyzer (Leica DMIRM).

Fig. 2 Illustration of experiment design and coating spots with different orientation.

3. Results and discussion

3.1. Coating appearance and phase

Fig. 3 is the overall view of as-sprayed cylindrical sample.Fig. 3(a) shows the appearance of the coating facing the plasma jet. It can be seen from Fig. 3(a) that the YSZ coating is white and that the oxygen supplementation during the spraying process has a good function.It is worth noting that the surface of the coating at the front position is uneven and there are many small protrusions which are the tops of columnar crystals well-developed during the competition growing process.

The appearance of the coating on the back of cylindrical sample is shown in Fig.3(b),demonstrating that the YSZ coating in the back position of cylindrical sample (i.e., 180 ° position in Fig.2)is also white.But on both sides of the white area,the color becomes a little gray. The visible difference in color indicates that the thickness of the ceramics coating is very thin,revealing the color of metallic bondcoat underneath.

Fig.4 is the XRD patterns of the original powder and YSZ coatings at different circumferential positions (‘‘×” in Fig. 4 designates the NiCrAlY bondcoat). The original powder consists mainly of monoclinic phase(m-ZrO2).Under the action of plasma jet heating, m-ZrO2is converted to tetragonal phase(t-ZrO2) at ～950°C, and then converted to cubic phase(c-ZrO2) at ～2370°C. During rapid solidification on the substrate, c-ZrO2is first converted back to t-ZrO2and eventually replaced by metastable t’-ZrO2because t-ZrO2is more difficult to exist at room temperature.As shown in Fig.4,the coatings are composed of t’-ZrO2phase and have no m-ZrO2phase remained from original YSZ powder, indicating that most powder particles had been vaporized or molten well. The XRD spectrum of YSZ coating on the back of cylindrical sample (i.e., 180 ° position in Fig. 2) arise a weak peak of NiCrAlY,indicating that the coating thickness of YSZ ceramics is very thin and the metallic bondcoat of NiCrAlY was detected.

Fig. 4 X-ray diffraction patterns of four different YSZ coatings deposited at different circumferential positions by PS-PVD.

3.2. Cross-sectional microstructure

Fig. 5 shows the cross-sectional microstructures of YSZ coatings at different circumferential positions fabricated by PSPVD.The red spot in each subfigure represents the deposition orientation. The coatings are mainly composed of feather-like columnar structure with many branches, which is formed by the rapid deposition of vapor phase under shadow effect.19-21Fig. 5(a) is the cross-sectional morphology of the coating facing the plasma jet (i.e. the 0° position in Fig. 2).It can be seen from Fig. 5(a) that the YSZ coating exhibits a typical columnar crystal structure with a lot of branches, and looks like a feather pattern. The growth direction of the coating is basically perpendicular to the substrate surface. The coating deposited at this position has a large thickness,indicating that the growth rate is very high,as shown in Fig.6.Fig.6 is the coating thickness of YSZ coatings at different circumferential positions. The coating thickness at the 0° position is more than 750 μm. The cross-sectional morphology of the coating shows that the coating has a rugged, undulating surface, indicating that the surface roughness is large. There are some prominent columnar crystals looks taller and bigger in the coating, because of a rapid competitive growth of these individual columnar crystals and a corresponding suppression on the surrounding columnar crystals during the vapor deposition process.

Fig. 3 Overall view of as-sprayed cylindrical sample.

Fig. 5 Cross-sectional morphologies of YSZ coatings at different circumferential positions.

Fig.6 Thickness and deflection growth angle of YSZ coatings at different circumferential positions.

Fig.5(b)and(h)are the cross-sectional morphologies of the coatings near the position facing the plasma jet (i.e. 45° and 315° position in Fig. 2). In Fig. 5(b) and (h), the columnar crystal structure with many branches can also be seen in the YSZ coatings. The coatings are deposited about 450-510 μm in thickness. As shown in Fig. 6, the coating thickness decreases sharply as the deposition region goes away from the 0°position.The difference in coating thickness can be seen clearly in the field of view(about 1.5 mm wide on substrate)in Fig. 5(b) and (h). The closer to the center of plasma jet, the thicker the coating. The surface tends to be smooth and there is no particularly high columnar crystal in the coating. It is worth noting that the growth direction of the coating is no longer perpendicular to the substrate surface.There is a certain angle between the growth of the coating and the normal direction of the substrate surface,as shown in Fig.6.The deflection growth angle of YSZ coating at 45°or 315°position in Fig.2 is about 16°-19°.

Fig.5(c)and(g)are the cross-sectional morphologies of the outermost sides of the cylindrical sample (i.e. 90° and 270°position in Fig. 2). As shown in Fig. 5(c) and (g), the YSZ coating exhibits a distinct structure of columnar crystal and the gap between the columnar crystals becomes larger. With the increase of the distance from the 0° position, the coating on substrate surface drastically decreases to about 140-170 μm in thickness. In Fig. 5(c) and (g), the oblique growth phenomena of YSZ coatings can be seen very clearly, and the deflection growth angle of YSZ coating at 90°or 270°position in Fig. 2 is about 25°-30°, as shown in Fig. 6.

Fig. 5(d) and (f) are the cross-sectional morphologies of YSZ coatings near the back position facing the plasma jet(i.e. 135° and 225° position in Fig. 2). In Fig. 5(d) and (f), it can be seen that the YSZ coating exhibits an obvious columnar structure, but the columnar crystal grows thicker and the gap between them becomes larger. The coating thickness at these positions reaches a minimum of about 70-80 μm. As shown in Fig. 5(c) and (g), the coating grows up with a very slight oblique angle, and the angle has a range of 6°-9°.

Fig.5(e)shows the cross-sectional morphology of the coating at the back of the sample (i.e. 180° position in Fig. 2). It can be seen from Fig. 5(e) that the YSZ coating still exhibits an obvious columnar structure,but the columnar crystal is relatively thicker and the gap between them becomes larger. The coating thickness at this position has slightly increased to about 95-105 μm.In Fig.5(e),the coating does not grow obliquely, but perpendicularly to the substrate surface.

3.3. Flow field characteristic

The numerical simulations of the pressure field distribution(a),velocity field distribution (b) and velocity vectors (c) for the entire computational domain are exhibited in Fig. 7, respectively.Fig.8 is the local enlargements of the pressure field distribution(a),velocity field distribution and velocity vectors(c)near the substrate.

Figs. 7(a) and (a) show the pressure field distribution around the substrate. From Figs. 7(a) and (a), it can be seen that after the high-velocity plasma jet impacts the front of the substrate, the plasma bounces against the substrate and forms a turbulent zone above the surface,resulting in an obvious increase in pressure. The smaller the deposition angle (in the range of θ=0°-90°),the larger the turbulence area formed by the plasma rebound,and the greater the pressure on the surface.As the spray angle increases,the pressure on both sides of the cylindrical sample gradually decreases. Due to the obstructing effect of the cylindrical sample, the jet pressure on the back of the cylinder surface is much lower than the pressure on the front and periphery surface. However, it is worth noting that due to the large pressure difference between the back and the periphery of the cylinder, a ‘‘swirling” of the plasma will be formed, eventually resulting in a slight increase in pressure at 180° position. Between the positions of 90° and 180°,most of the area is located between the turbulent swirling zone and the high-velocity flowing zone, eventually forming a negative pressure zone,such as 135°position.The difference in jet pressure reflects the difference in the concentration of the gas phase material in plasma jet. In general, the higher the pressure,the higher the concentration of gas phase.Therefore,the concentration of the gas phase is highest on the surface in front of the cylindrical sample,resulting in the thickest coating deposited in this area.With the deposition angle increases,the coating deposition efficiency gradually decreased,reaching the lowest value at 135° position. Then, with a slight increase in pressure, the deposition rate at 180° position is similar to at 90°. Due to the symmetry of the designed spray experiment,the same trend of pressure change and deposition distribution appears between 180° and 360°.

Fig. 7 Fluid field simulations for the entire computational domain.

Figs. 7(b) and 8 (b) show the velocity field distribution around the substrate.From Figs.7(b)and 8(b),the jet velocity approaches zero in front of the substrate (θ=0°). It is due to the high-velocity jet that bounces against the substrate,causing turbulence and decreasing jet velocity. The smaller the turbulence, the greater the jet velocity on the substrate surface.Therefore, with the spray angle increases from θ=0° to 90°,the jet velocity gradually increased on both sides of the cylindrical sample. Due to the obstructing effect of the cylindrical sample, the jet velocity on the back of the cylinder surface is much lower than the velocity on the front and periphery.The high-speed jet on both sides of the cylinder will drive the surrounding plasma to move together. The closer to the high-speed jet, the more obvious the driving effect. Therefore,on the back of the cylindrical sample (θ=90° to 180°), the plasma velocity on the substrate surface sharply decreases with increasing spraying angle due to the distance from the highvelocity jet.Therefore,in the range of 0°to 180°,the jet velocity around the substrate shows a tendency that increases first and then decreases with the deposition angle increasing, and the inflection point is at 90 degree. Due to the symmetry of the designed spray experiment, the same trend of velocity change appears between 180° and 360°.

Figs. 7(c) and 8(c) show the velocity vector distribution around the substrate. As shown in Figs. 7(c) and 8(c), the velocity vector is perpendicular to the normal direction of substrate surface at two positions(the θ=0°and 180°position in Fig. 2). That is to say, the velocity vector has no effect on the vertical growth of the coating on the substrate surface. But in other positions, there is a certain angle between the velocity vector and the normal direction of the substrate surface,which affects the upward growth of the coating and causes the columnar structure to grow in the similar direction as the jet flow.

3.4. Deposition mechanism analysis

According to the above results, the characteristics and deposition mechanism of YSZ coatings prepared by PS-PVD is depended on the deposition orientation in plasma jet,and generally is affected by three factors, as the pressure, the velocity and the flow direction of plasma jet at the coating growth position.

Fig. 9(a) shows the position of point A on a cylindrical sample with an angle of θ between the normal direction and the axial direction of the plasma jet. Moreover, the plasma pressure and velocity at the position A is PAand VA, respectively. In the case where only vapor deposition is considered,in general,the coating deposition at the position A is subjected to the following growth process.

The agglomerated YSZ powder, after being fed into the plasma jet,rapidly breaks up and disperses in the plasma inside the spray gun,absorbs heat,and is converted into vapor-phase material in the form of atoms and clusters.19,22Once the PSPVD plasma jet is out of the nozzle exit, the scale will expand dramatically. This expanded high-speed plasma jet will carry the vapor-phase materials in the form of atoms and clusters to the surface of the substrate, as shown in (1) of Fig. 9(b).When adsorbed by relatively cold substrate surface, the vapor-phase atoms and clusters will undergo mutual transformation, forming a non-equilibrium dynamic system, but the atoms are mainly condensed into clusters. Under the action of energy fluctuation, atomic cluster condenses and forms the initial crystal nucleus, which changes from dynamic nonequilibrium state to stable state ((2) of Fig. 9(b)).23-25Subsequently, the initial crystal nucleus can further capture the vapor-phase atoms in the surrounding plasma or the migrated vapor-phase atoms of the nearby substrate,and the initial crystal nucleus gradually develops on the substrate surface, forming a small ‘‘island” structure ((3) of Fig. 9(b)). The small‘‘island” structure grows in size and develops into a large ‘‘island”structure on the substrate surface by incorporating smaller surrounding ones and merging with each other((4)of Fig.9(b)).As the spraying continues,some large‘‘island”structures can grow upward along the direction perpendicular to substrate surface,while new small‘‘island”structures may appear and supplement in the gaps between the large islands ((5) of Fig. 9(b)).26-28Therefore, in the initial stage of PS-PVD vapor-phase deposition, a dense coating is formed on the substrate surface, consisting of well-arranged fine columns, as shown in (6) of Fig. 9(b).29In the initial stage of PS-PVD vapor-phase deposition, the shadow effect has little effect on the deposition due to the easier migration of atoms.The structure of the coating is mainly determined by the size and quantity of crystal nucleus formed in this stage. Generally, the size and number of crystal nucleus are affected by the concentration of vapor-phase material, the substrate temperature and the velocity of plasma jet.As the spraying continues,these fine columns will continue to grow, both vertically in height and horizontally in size. At this growth stage of the coating, the shadow effect appears obviously,which becomes the main factor affecting the deposition of the vapor-phase material, and its influence is increasingly intensified.These fine column structures accelerate in the upward growth rate, and squeeze one another in the horizontal direction. And the fine column,which grows faster upward and larger horizontally, becomes the dominant columnar structure,becoming higher and larger,forming the shadow effect.In other words,it has a better position, easier access to more vapor-phase atoms and further growth.At the same time,the neighboring columnar structure is under shadow effect,and the vapor-phase materials are getting fewer and fewer, and the growth is increasingly inhibited or even stopped, which is divided among the larger welldeveloped columnar structures.In the final stage,the structure of PS-PVD coating is basically formed, presenting an obvious columnar feature and surface morphology of cauliflower top,as shown in (8) of Fig. 9(b).

Fig. 9 Schematic diagram of growth model and typical structure of YSZ coating.

It is worth noting that the plasma jet and the vaporized spray materials carried in the jet form a high-speed moving fluid. When the multiphase mixed fluid encounters the substrate, the multiphase mixed fluid will rebound, deflect, and form a boundary layer and turbulence.If the substrate surface is complex,such as a convex or concave surface of the cylinder or sphere, the change of multiphase mixed fluid is more dramatic.

●The concentration of the vapor-phase spray materials carried in the plasma jet can be considered to be constant near the local area of the substrate. The multiphase mixed fluid will undergo pressure changes due to the influence of the substrate profile.In some positions,the jet is compressed and the fluid pressure is increased accordingly. In some other positions,the jet is expanded and the fluid pressure is decreased accordingly.This means that at the position where the jet compresses and the fluid pressure increases, the concentration of the vapor-phase spray materials is high. And at the position where the jet expands and the fluid pressure decreases,the concentration of the vapor-phase spray materials is low. The concentration of the vapor-phase spray materials determines the rate of nucleation and coating growth. Therefore, it can be concluded that the coating growth rate at the position A is related to the pressure of the multiphase mixed plasma jet near the local area of the substrate in a certain proportional relationship.

where RA,growthis the coating growth rate at the position A on the substrate, nA,maiterialsis the concentration of the vapor-phase spray materials at the position A, and PAis the fluid pressure at the position A.

●At different positions on the substrate, the flow velocity of the multiphase mixed fluid composed of the plasma jet and the vapor-phase spray materials is different.Even on a planar substrate perpendicular to the axial direction of the jet,there is a difference in velocity in the radial direction.When the cylindrical substrate is encountered, the multiphase mixed fluid will bounce, deflect and form a boundary layer and turbulence. When the surface of the substrate is complex, the change in the flowing velocity of the multiphase mixed fluid becomes more dramatic.As far as the position A on the substrate is concerned, the greater the fluid flow velocity above it, the less likely the atoms or clusters of the spray materials in the vapor-phase are adsorbed at the position A. As a result, the time for the nucleus becomes shorter, also as the period that the coating can continue to capture the vapor-phase atoms or clusters of the spray materials and maintain stable growth. The coating growth rate is also slower. On the contrary, the smaller the flow velocity above the position A, the easier the atoms or clusters of the sprayed materials in the vapor-phase are adsorbed.As a result,the time for the nucleus is prolonged,also as the period that the coating can continue to capture the vapor-phase atoms or clusters of the spray materials and maintain stable growth.The coating growth rate is relatively faster. Therefore, it can be concluded that the coating growth rate at the position A on the substrate is related to the velocity of the multiphase mixed fluid nearby the position A, and has a certain inverse relationship.

where vA,maiterialsis the flow velocity of the vapor-phase spray materials at the position A,and VAis the flow rate of the fluid at the position A.Combine Eqs.(1)and(2),that is

●When the multiphase mixed fluid composed of the plasma jet and the vapor-phase spray materials encounters the substrate, the multiphase mixed fluid will rebound and deflect,forming a change in flow direction. That is, a certain angle is formed between the normal direction of the deposition position of the coating and the direction of the fluid velocity. As far as the position A on the substrate is concerned,the fluid velocity vector VAat that position can be decomposed into the component Vnormin the normal direction of the position and the component Vtanin the tangential direction.The velocity component Vnormin the normal direction mainly affects the speed at which the coating grows vertically upwards, that is, affects the deposition thickness (deposition efficiency) in normal direction.The velocity component Vtanin the tangential direction determines the extent to which the coating growth direction deviates from the vertical direction. When Vtan=0, the upward growth of the coating is unaffected,forming a vertical fine column,such as(6)of Fig.9(b),which subsequently develops into a vertical columnar structure, such as (8) of Fig. 9(b). When Vtan≠0, the upward growth of the coating is affected by the fluid flow in the tangential direction, and the deflection grows toward the tangential direction to form a fine column with a certain inclination angle to the vertical direction,such as (7) of Fig. 9(b). It then develops into an oblique columnar structure, such as (9) of Fig. 9(b).The larger the tangential component Vtanof the fluid velocity vector VAat the deposition position of the coating, the more easily the vapor-phase sprayed material moves to the side of the fluid movement under fluid-carrying conditions. At the same time, under the influence of the fluid the vaporphase atoms or clusters adsorbed at the position A are more susceptible to adsorption and diffusion to the coating on the side of the fluid movement.Near the deposition position of the coating, it is actually formed a dynamic equilibrium environment for nucleation and growth, and the coating can deposit toward the side of the fluid movement. This is an important reason to ensure that the coating with a certain deflection growth angle can be obtained.

4. Conclusions

Based on a specific experimental design, the representative YSZ coatings were prepared by the PS-PVD technology.Three important conclusions are listed as follows:

(1) The structure,thickness and growth angle of the coating exhibit a regular distribution over the entire circumferential section of the cylindrical sample. The structure,thickness and deflection growth angle of the coating are related to the orientation of the deposition location on the circumferential section of the cylindrical sample.

(2) Integrating the numerical simulations of the multiphase mixed fluid near the substrate, the deposition regularity and mechanism of YSZ coatings prepared by PS-PVD is deduced. The structure, thickness and growth angle of the coating is generally affected by three factors, as the pressure, the velocity and the flow direction of plasma jet at the coating growth position.

(3) The velocity component Vnormin the normal direction mainly affects the speed at which the coating grows vertically upwards.The velocity component Vtanin the tangential direction determines the extent to which the coating growth direction deviates from the vertical direction.When Vtan=0,the coating forms vertical fine column, and subsequently develops into vertical columnar structure.When Vtan≠0,the coating forms fine column with a certain inclination angle and then develops into an oblique columnar structure.

Acknowledgments

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (51771059),R&D Program in Key Fields of Guangdong Province of China(2019B010936001), National Science and Technology Major Project of China (2017-VI-0010-0081), Science and Technology Project of Guangdong Province of China(2017A070701027, 2014B070705007) and Sciences Project of Guangdong Academy of China (2019GDASYL-0104022).