nanostructurated zro2+al2o3

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.Thin Solid Films 388 2001 208212

Interfacial phenomena in the plasma spraying Al O q 13 wt.%2 3TiO ceramic coating2

Yang YuanzhengU ,a, Liu Zhiguoa, Liu Zhengyib, Chuang Yuzhib

aFaculty of Materials and Energy, Guangdong Uniersity of Technology, Guangzhou, 510090, PR China

bDepartment of Mechanical and Electronic Engineering, South China Uniersity of Technology, Guangzhou, 510461, PR China

Received 15 January 2000; received in revised form 20 December 2000; accepted 19 January 2001

Abstract

The interface between the bond layer and ceramic layer as well as between the bond layer and the substrate in a duplex coatingsystem consisting of a NiCrAl bond coat layer and an Al O q13 wt.% TiO ceramic coat layer has been investigated by2 3 2scanning electron microscopy and transmission electron microscopy. The lamellar bond layer mainly consists of lamellae aligningapproximately parallel to the substrate surface, in which the individual lamellae consist of a thin outer amorphous andnanocrystalline film, an intermediate layer of fine columnar and equiaxed grains and an inner microcrystalline region of bigcolumnar grains. The morphology of the interfacial regions of the above two interfaces is shown to be similar and consists of athickness of 0.7 m, or far less, of a mixture of amorphous and nanocrystalline phases. However, besides a certain percentage ofarea of perfect contact, a small amount of area of non-contact has also been detected among the lamellae or at the interfaces. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Aluminium oxide; Coating; Interfaces; TEM

1. Introduction

Plasma spraying has been widely used to obtainceramic coatings to strengthen the surface of metal

w xsubstrates 1,2 . However, to a certain degree, the lowadhesion between ceramic coating and the substrateand the low cohesion within the coating seriously re-strict the applications of plasma sprayed ceramic coat-

w xing 2,3 . Therefore, it is necessary to understand fullythe nature of bonds within the coating and between thecoating and the substrate. However, studies of theinterfacial microstructure have been focused on the

w xZrO coating system 4,5 . Up to now, only a few2studies have been made into the interfacial phenomenain Al O ceramic coating using transmission electron2 3

U

Corresponding author. Tel.: q86-20-877-660-69; fax: q86-20-877-765-97.

.E-mail address: [email protected] Y. Yuanzheng .

. w xmicroscopy TEM 6,7 due to the difficulty in prepara-tion of the specimens for TEM observation. In thispaper, the microstructure of the bond layer and thecharacteristics of interface between the bond layer andthe substrate as well as between the bond layer andceramic layer in the Al O q 13 wt.% TiO coating2 3 2have been reported.

2. Experimental procedures

The starting materials used in this investigation werecommercial compound powders of the metal NiCrAland ceramic Al O q 13 wt.% TiO of sizes in the2 3 2range of 75143 m and 37105 m, respectively. Mildsteel plates, approximately 40 mm in diameter and10-mm thick, were used as substrates for spraying.After pre-cleaning and a fine grit blast, the specimenswere deposited with a duplex coating consisting of aNiCrAl bond coat layer approximately 50-m thick andan Al O ceramic coat layer approximately 200-m2 3

0040-6090r01r$ - see front matter 2001 Elsevier Science B.V. All rights reserved. .PII: S 0 0 4 0 - 6 0 9 0 0 1 0 0 8 0 6 - 9


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thick by atmospheric plasma spraying. On the samespraying condition, the single ceramic coating on the

.substrate without bond layer was also prepared. Theplasma spraying operating condition can be seen else-

w xwhere 8 . The pull test showed that the average bondstrength for the duplex coating was approximately 27.1MPa, appreciably higher than 16.8 MPa for the singlecoating. In view of engineering applications, high bondstrength is beneficial to improvement of the life of thecoating. So the present study is concentrated on theinterfacial microstructure of the duplex coating.

The phase composition of the spray coatings wasdetermined using a Y-4 X-ray diffractometer. The mor-phology of these coatings was investigated with an

.S-505AMERY scanning electron microscope SEMand a JEM-200CX TEM or JEM-2000EX II HighResolution TEM. The cross-section specimens for SEMobservation were prepared by cutting, grinding andpolishing. To prevent charging of the insulating cer-

amic by the electron beam, all the specimens werecoated with a layer of carbon approximately 20-nmthick.

The preparation of cross-sectional foils for TEMexamination, which contains the interface between theceramic and bond coat layer as well as between thebond coat layer and the metal substrate, is relativelydifficult. After a series of steps of preparation with aspecial method, we got some thin disk specimens, ap-proximately 500-m thick and containing the desiredinterface. Next these disks were petrographicallythinned to a thickness of approximately 2030 m and

then attached by refractory conductive adhesive to aspecial brass ring, which is hollow in the center, with anoutside diameter of 3 mm and inside of 2.4 mm. Fi-nally, the thin disks were dimpled to 510 m and thenion thinned by an ion-beam milling technique untilperforated. The ion beam was produced from argonand the incidence angle was changed from 15 at thebeginning of thinning to 8 at the end of thinning inorder to increase the area of thinning. During thethinning the disc was rotated at 5 rev.rmin in order tochange the direction of bombardment.

3. Results

3.1. Morphology of the bond layer

The SEM morphology of the coating in cross-sectionis shown in Fig. 1. The ceramic layer and bond layermainly display a lamellar structure, in which the lamel-lae of both the ceramic layer and bond coat layer arealmost parallel to the substrate surface. The shapes oflamellae of the bond layer are rather regular strips .strictly speaking: are regular disks . However, for theceramic layer, in addition to the lamellae, spheres andshapes intermediate between them as well as some

Fig. 1. SEM morphology of Al O q 13 wt.% TiO coating in cross-2 3 2 . .section: a ceramic and bond layers; and b within bond layer.

unmelted particles can be observed. The morphology ofthe bond layer with a high magnification shows that,besides the area of perfect contact, at the corner oredge of some lamellae there exists a small amount of

area of non-contact, at which the gap may be up toapproximately 0.12-m thick.

The TEM morphology of the bond layer near to thesubstrate is presented in Fig. 2. The lamellar structureof the bond layer is much clearly illustrated. The thick-ness of the lamellae is non-uniform and can extend upto approximately 8 m as shown in Fig. 2a. The interfa-cial regions within the lamellae or between the lamel-lae and the substrate are clearly of a definite thicknessand non-uniform. From the bright and dark images,Fig. 2b,c, it is obviously demonstrated that the lamellaemainly consist of columnar crystallites, which are ap-

proximately 0.11 m in diameter and 34-m longand almost vertical to the substrate surface. In additionto the columnar grains, some approximately equiaxedgrains, up to 160 nm in diameter, are observed in theouter surface of the lamellae.

On close examination of the selecting area electrondiffraction for the area containing the interfacial re-gion between lamellae, one may observe that, besidesthe crystallite diffraction points, there exists a veryfaint spot ring caused by nano crystallites and a veryfaint diffuse ring by glassy matrix. This phenomenon isalso detected in the latter observation for interfacialzone between the bond coat layer and the substrate.

3.2. The interface between the bond coat layer and thesubstrate

As shown in Fig. 2a, the morphology of the interfa-cial regions between bond lamellae and betweenlamellarsubstrate is quite similar. The gap, approxi-mately 0.7-m thick as shown in Fig. 2ac, is preciselythe interfacial region between the bond layer and thesubstrate owing to the fact that the interfacial phasesare ready to be perforated during ion bombardment.Such a typical interface and the selecting area electron


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. . .Fig. 2. TEM morphology of the bond layer near to substrate: a bond layer near the substrate; b bright image of a bond lamella; and ccorresponding dark image.

diffraction pattern are shown in Fig. 3. The side ofcolumnar structure pertains to the bond layer and theother side to the steel substrate, in which there exists

almost no thin region owing to the difficulty in perfo-rating during ion bombardment. The selecting areadiffraction pattern shows that, besides crystallites of Ni .corresponding to diffraction points , some amount ofnano crystallites NiO and a certain amount of amor-

phous phases corresponding to the diffraction spot.ring and faint diffuse ring, respectively . Thus, it can be

concluded that the interfacial region of perfect contactbetween the bond metal layer and the substrate may be

totally up to 0.7 m or possibly far less, and consists ofamorphous phases and nanocrystalline grains layer,whereas for the region of imperfect contact at the

edges of the lamellae there may exist an extremely thingap.

3.3. The interface between the bond coat layer and ceramiccoat layer

The typical morphology of the interfacial regionbetween the ceramic layer and bond metal layer andthe selecting area electron diffraction are shown in

. .Fig. 3. a Local interface between the bond layer and the substrate and b the selecting area electron diffraction.


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.Fig. 4. A typical morphology of the interfacial region between the ceramic coating and bond metal layer at two magnification a, b and the .selecting area electron diffraction pattern c .

Fig. 4. The thickness of the interfacial region exhibitsalso non-uniform, possibly extending up to approxi-mately 0.7 m. The selecting area diffraction patternshows that micro crystallites of Al O and TiO , nano2 3 2crystallites of NiO, and amorphous phases exist. It isdemonstrated that the interfacial region is not onlynon-uniform but also contains amorphous phases, nano

and micro crystallites.

4. Discussion

As we know, the morphology of the characteristiclamellar microstructure is associated with the mecha-nism of the impact of a liquid droplet on the substrate.The ultra-rapid solidification of the surface of a flat-tening droplet caused by impact results in the forma-tion of amorphous and nanocrystalline constituents inthe contact layer due to homogeneous nucleation and

preventing grain growing. Since the amorphous filmacts as a thermal barrier, the rate of cooling decreaseswith the depth and hence leads to formation of anotherregion characterized by fine columnar and equiaxedgrains. For the inner major part of the flatted droplet,the cooling rate now reduces low enough to permit thegrowth of big columnar grains, which are aligned to thesurface of lamellae and oriented parallel to the direc-tion of the maximum heat transfer. Therefore, eachlamella of the bond layer may consist of an outermostthin amorphous and nanocrystalline film, an intermedi-ate region of fine columnar crystallites and approxi-

mately equiaxed grains, and inner main sub-layer oflarger columnar grains. Furthermore, within the experi-mental accuracy, the XRD results show that the NiCrAlcompound powder transfers into a supersaturated solid

. w xsolution Ni Cr,Al after plasma spraying 9 . Thus, dueto the oxidation of the outermost region of bond lamel-

w xlae with the entrapped gas in plasma jet 7,10 , NiOw . xmore accurately maybe Ni Cr,Al O rather than Al O2 3phase is to be expected at the interface. In short, the

interfacial region between the bond lamellae of perfectcontact consists of the outer amorphous films of neigh-boring lamellae.

In the interfaces within bond lamellae and betweenthe bond layer and substrate as well as between cer-amic and bond layer, the presence of amorphous filmmay mainly result from the same reasons. Besides, themorphology of the interfacial regions of these inter-faces is also quite similar. However, there exists somesubtle differences between them as follows. Generally,during spraying the higher current of spraying, the

higher heat of liquid droplets. In the present study, thecurrent used for spraying the ceramic and bond layersis 530 and 450 A, respectively. Thus, the heat of cer-amic droplets and the energy of impact may melt a partof the bond layer hence the interface between theceramicrbond layer will be widened and contain somecompositions of bond metal. However, the liquiddroplets of bond metal can melt almost no substrateowing to the lower heat of droplets and the more rapidheat conduction of the substrate, thus the bondrsub-strate interface contains no composition of the sub-strate or only a very small amount for the composition


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thermal diffusion. Therefore the interface between theceramicrbond layers might be divided into three sub-layers. The first one is adjacent to the metal bond layermainly consisting of metal composition; the secondregion is the main amorphous sub-layer containingceramic and metal composition; the third belongs to aceramic sub-layer of nano or micro crystallites mainlyof ceramic composition.

It is surprising that the thickness of the interfacesbetween ceramicrbond and between bondrsubstrate isaccidentally almost equal. Generally, the cooling rateof ceramic droplets is lower than that of bond metaldroplets due to the higher rate of heat conduction ofthe substrate than that of bond lamellae with an amor-phous film. As a result, the outermost amorphous filmof ceramic lamellae neighboring the bond layer is thin-ner than that of bond metal lamellae adjacent to thesubstrate. On the other hand, ceramic droplets canmelt a part of the bond layer then widen the interfacial

amorphous film. Therefore, the thickness of interfacialamorphous film between ceramicrbond including theouter amorphous films of neighboring ceramic andbond lamellae might be accidentally equal to that ofthe amorphous film between bondrsubstrate.

As mentioned in Section 2, the average bond strengthfor the duplex coating is approximately 27.1 MPa, ap-preciably higher than 16.8 MPa for the single coating.It suggests that the NiCrAl bond layer is beneficial tothe adhesion strength of the ceramic coating. From theabove discussion, in the duplex coating there exists fourkinds of interfaces, namely, ceramicrceramic, cer-

amicrbond, bondrbond and bondrsubstrate, while inthe single coating system, only two types of interfaces,ceramicrceramic and ceramicrsubstrate. Since the in-terfaces within bond lamellae and between bondrsub-strate have greater chemical, i.e. metallurgical bondingresulting from interatomic forces and the generation ofinterdiffusion zones, such metalrmetal interfaces willown relatively high adhesion strength. Additionally, theinterfaces within ceramic lamellae in the single andduplex coating show the same. Thus the adhesionstrength of the coatings is governed by the interfacesbetween the ceramicrbond layer in the duplex coatingand between the ceramicrsubstrate in the single coat-

ing system, respectively. By comparison with the sub-strate, the bond layer has a far less smooth surface oreven contains some micro-cavities and pores, then re-sults in an increase in the bonding area and the role ofriveting and interlocking between the protrusions ofthe ceramic and bond layer. Thus the ceramicrbondinterface would have better mechanical bonding oreven physical bonding than the ceramicrsubstrate in-terface. Therefore, the better mechanical andror phys-ical bonding, possibly together with the amorphousinterfacial film, are believed to be responsible for thehigh adhesive strength of the ceramicrbond interface.

5. Conclusion

The microstructure within bond layer and the inter-facial phenomena within the bond layer and betweenceramicrbond layer as well as between bondrsubstratein a duplex coating system consisting of a NiCrAl bondcoat layer and an Al O q 13 wt.% TiO ceramic coat

2 3 2layer have been studied. The plasma spraying bondlayer is of a characteristic overall lamellar nature,which is associated with the mechanism of the impactof a liquid droplet. Each droplet forms a single lamella.The individual platelet consists of an outer amorphousand nanocrystalline film, an intermediate layer madeup of fine columnar and equiaxed grains, and a maininner part of big columnar crystallites extendingthrough their thickness. The NiCrAl bond layer isbeneficial to the adhesion strength of the ceramic coat-ing, which is discussed with respect to the improvementof mechanical andror physical bonding at the cer-

amicrmetal interface.The morphology of the interfacial regions of the two

interfaces between ceramicrbond and betweenbondrsubstrate looks similar and consists of a certainthickness of a mixture of amorphous and nanocrys-talline phases. These interfacial phases are formed dueto the ultra-rapid solidification of liquid droplets duringplasma spraying. Furthermore, within the bond lamel-lae and at the interface between bondrceramic lamel-lae as well as between bondrsubstrate, besides a cer-tain percentage of area of perfect contact, a smallamount of area of non-contact is also detected at the

edges or corner of the lamellae.

Acknowledgements

This work was supported by Doctor Foundation ofMinistry of Education of China and by the NaturalSciences Foundation of Guangdong Province, China.

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