Post-deposition treatments of plasma-sprayed YBaCuO coatings deposited on nickel

Thin Solid Films. 193/194 (1990) 847 856

847

POST-DEPOSITION T R E A T M E N T S OF PLASMA-SPRAYED YBaCuO C O A T I N G S D E P O S I T E D ON N I C K E L D. I)UBI~, P. I.AMBERT, B. ARSENAULT AND B. CllAMPAGNE

National Research ('ouncil o f ('anada, 75 houle~,ard de Mortagne. Boucherville, Quebec, J4B6 Y4 ( Canada /

As-sprayed YBaCuO coatings do not exhibit superconductivity because of the non-equilibrium solidification conditions of molten particles on the substrate and to the deposit's loss of oxygen. Therefore post-deposition treatments are required to restore the superconductivity. In this study, post-deposition treatments were carried out on thick YBaCuO coatings (2001am) deposited on cold nickel substrates to modify their microstructure, to restore the oxygen content and to improve their superconducting properties. These treatments consist in heating the coatings at various temperatures above 950°C followed by controlled solidification cycles. The effect of these treatments on the microstructure of the coatings was assessed and the interaction between the coatings and the nickel substrate was also examined. Solidification cycles including a low cooling rate near the non-congruent melting temperature of YBa2Cu30~ and involving a temperature gradient were carried out to create a texture.

1. INTRODUCTION In plasma spraying of YBaCuO, the molten particles solidify in nonequilibrium conditions which result in the formation of non-superconducting phases and post-deposition treatments are thus required. It was shown that the superconductive YBa2CuOx phase may be recovered by heat treatment at about 930:'C followed by annealing in oxygen at about 500°C ~'2. Although such a treatment extensively modifies the microstructure with the recrystallization and the disappearance of splats and restores the superconductivity, it leads to a superconducting coating with a low critical current density because of weak coupling and low flux pinning 2-8. A promising quench-and-melt-growth thermal cycle was reported recently for creating a texture of crystallographically aligned grains in bulk materials 9 ~. It first consists in creating a fine dispersion of Y203 in a Ba-Cu-Orich matrix by quenching from high temperature. In a subsequent step, a homogeneous 211 dispersion is obtained for growing the 123 phase at a lower temperature. The use of a thermal gradient during the growth of the 123 phase leads to grain alignment which improves the critical current densities 9. Printed in The Netherlands

848

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In this work plasma-sprayed YBaCuO coatings deposited on nickel substrates were textured by a two-step process. The effect of the processing conditions on the microstructure is presented. 2. I..XPt~RIMENrAI. PR()('EI)URI!

A commercial powder described as 99.9",, pure. spray-dried and flame-sprayed Y B a ( ' u O spherical particles with good tlowing characteristics and Y : B a : ( ' u cation stoichiometry corresponding to 1:2:4 was plasma sprayed. An excess of copper ~as used to compensate for preferential copper losses that occur d u r m g the deposition of particles ol" size range between 10 and 74 I.tm. Plasma spraying was carried out in air using a Bay State torch operating at 76 V and 450 A. Nitrogen {70 I rnin ~) was the primary gas and oxygen (8.5 I min ~) was used as the carrier gas to generate a plasma jet with a substantial oxygen potential. The powder vs~as fed at a rate of 28 g min ~ into two external powder ports to improve the deposition efficiency. The substrate consisting of0.75 mm thick nickel strips of 99.5",, purity was cleaned and grit blasted with 24 mcsh alumina particles at a pressure o f 500 kPa. Coatings about 250 p.m thick were built by' depositing, at a stand-offdistance of 10cm. successive layers about 30 40 btm thick on non-preheated nickel substrates with about 1 rain between each intermediate pass. Overspray was reduced with nitrogen jets located on each side of the plasma gun. After spraying, the coatings were rapidly stored under vacuun3, to protect them from degradation b x. humidity and carbon dioxide. Indeed. as-sprayed coatings held in air are degraded h~ the formation of reaction products. Such an uncontrollable factor was thus eliminated. Two difl'erent procedures were used to study the microstructural transformations arising at the dilt'erent temperatures o f heal treatments. Ill the tirst, coatings were inserted into a preheated furnace at various temperatures m air betv, een 615 and 1095 (" l\~r 15rain and were then rapidl~ quenched in air. l h c second procedure consisted in rapidly preheating the coatings to 840 (" and maintaining them at that temperature t\~r 30rain in air. I\~llowed by heating up to different temperatures between 840 C and 1095 (" at a rate o f 2 C rain ~ and by air quenching. This second series o f treatments was performed to evaluate the effect o f a low heating rate on void formation. A two-step texturing process was also studied. As-sprayed Y B a ( ' u O Ni strips were inserted into the central heating zone o f a tubular furnace and homogenized in air for 5 rain at 1140 (" to transform the fine Y20.~ precipitates present in assprayed coatings in 211 sites by the peritectic reaction as suggested by' M u r a k a m i and coworkers '~ ~. These strips were then quickly transferred from this central zone to another where the thermal gradient was about 25 ( ' c m ~ and the temperature at one end o f the specimens was then rapidly decreased to 1()40 (" w h i l e t h e o t h e r e n d was kept at about 1140 C. The temperature at the hottest end o f t h e c o a t i n g was thereafter lowered at a rate o f 3 ( ' h ~ to 1040 ('. and then at 5 C h ~ t o 5 5 0 (7 for a 48 h period. The phases present in the coatings ~,ere analyzed b.~ scanning electron microscopy (SEM) using an X-ray dispersive spectrometer and by X-ra.~ difl'raclion ( X R D ) patterns of ground coatings extracted from nickel strips and mixed with

POST-DEPOSITIONTREATMENTSOF YBaCuO ON Ni

849

silicon powder as an internal standard. Differential thermal analysis (DTA) was also carried out with 20.-30 mg of ground coatings extracted from nickel strips. D T A thermal cycles consisted in heating the powder in alumina crucibles at a rate of 1 0 " C m i n t from 500°C to 1150~C in flowing air. Only the heating cycle was considered since an unavoidable contamination of the charge occurred because of its reaction with the crucible. 3.

RESULTSAND DISCUSSION

The optical micrograph of the starting powder shows that particles are made up of darkish and angular precipitates dispersed in a whitish matrix (Fig. I(a)). A microanalysis of the particles revealed that the precipitates are yttrium rich whereas the matrix is essentially constituted of barium and copper in a 1:2 atomic ratio. X R D analysis confirmed that the precipitates are Y203 and that the matrix is

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Fig. I. (a) Optical micrograph of YBaCuO particles as received: (b) SEM overviewof the surface: (c) SEM micrograph of the plasma-sprayedYBaCuOcoatings; (d) details of(c) with identification ofY203 and BaCu202 phases.

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BaCu2() 2 ( Fig. 2). Minor XRI) peaks corresponding to the 123 phase, Ba('uO2 and 211 phases were visible. A small contamination by barium carbonate (BaCO.O was also detected. These results thus indicate that the starting powder, prepared by llame melting of spray-dried agglomerates, brought about the formation of nonequilibrium phases. The physical aspect of the coatings {Figs. l(b) and I(c)) indicates that the spraying parameters led to almost complete melting of the particles. The thickness of the lamellae which ranges between 2 and 10 lain reveals a high velocity on impact and high solidification rates. However, an incomplete bonding between successive splats occurred and elongated porosities were t\)rmed. A conlined segmentation due to shrinkage during cooling is also present in the l\wm of transverse cracks (Fig. I(c)). Individual platelets (Fig. I(d)) tire constituted of a matrix containing an yttrium-rich second phase. The X R I) pattern of as-sprayed coatings reveals that the matrix is constituted of BaCu202 whereas the precipitates are Y20.~ ( Fig. 2). Particles underwent an additional melting during plasma spraying and assprayed coatings essentially contain the same phases as those found in the asreccived powder, i.e. Y20.~ and BaCu20 e ( Fig. 2). Contrary to the present study and as we reported extensively elsewhere' 2, previous investigations on plasma-sprayed YBaCuO coatings did not identify Ba('u2Oe but referred to an unknown "'cubic phase of lattice parameter 0.701 rim'" ~t.~ ,~ ~s. No significant traces of BaCO~ were detectcd in plasma-sprayed coatings ([:ig. 2) which could be explained by its decomposition during plasma spraying. BaCulO Y203 123

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POST-DEPOSITION TREATMENTS OF

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Even though microstructural transformation in the coatings were only noticeable at temperatures above 890 ° C, XRD showed that considerable structural modifications occurred at temperatures as low as 615 "C for which the appearance of the 123 phase and BaCuO2 was significant (Fig. 3). The 123 phase rapidly formed above 820 ':C and disappeared above 1000"C where the 211 phase, BaCu20~ and BaCuO 2 became dominant as indicated by the XRD patterns of quenched coatings. The metastable BaCu:O2 phase initially observed in the coatings was largely transformed above 615'~C, especially between 7 2 0 ' C and 990 ~C where it was BaCu202 Y203 123 211

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gradually replaced by BaCuO2. However. Ba('u20 2 reappearcd in coatings air quenched from 1000 (" and above. Therefore, BaCu202 may also form under moderate solidification rates for this specilic composition in the YBa('uO system. Various degrees ot" porosity and cracks were observed after heat treatment above 840' C (Fig. 4(a)). The porosity in the coatings increased with temperature (Figs. 4(b) and 4(c)). The evolution of the microstructure with temperature revealed that the volume proportion of the voids suddenly' increased at specitic temperatures: 930 C and 1000 ('. The appearance of large voids seems to be closel'~ related to melting events as confirmed b,, the presence of endolhermic peaks on the DFA curve at these temperatures (Fig. 5). Since Y203 B a ( ' u , O , coatings rapidly transtormed in the 123 phase dt, ring heating above 615 ('. the small endothcrmic peak (AJ observed at 932 (" is probably related to the reacti\.c melting ot" the 123 phase with CuO as reported in a prexious investigation of the liquidus relations in the YBaCu() system in air'". The second and strongest endothermic peak (B) at 1010 C corresponds to the incongruent melting of the 123 phase ~''. This interpretation is supported by the X R l) data of coatings quenched l'rom IO00 C (Fig. 3).

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to those observed at highcr temperature (920 9 3 0 ' C) and this suggests that the void formation is also a time-dependent process. In the first step of the texturing process, consisting in a rapid heating up to 1140 '~C for 5 min and quelching to 1040 ~ C, a severe reaction between the coating and the nickel substrate occurred. The extent of this reaction is represented in Fig. 6 which shows the average nickel concentration in the coatings as a function of the heat treating temperature for a 15 min period. A drastic increase in the contamination occurs above 1000 °C and reaches 13 at.~o at 1095 ~'C(Fig. 6). The duration of the first texturing step was thus reduced to lower the nickel contamination. However, a significant nickel contamination still occurred and led to nickel substitution in the 211 phase, BaCuO2 and BaCuzO 2 which were also the major phases detected by XRD. This significant nickel substitution probably occurs on the copper sites and therefore the phases should be denoted in this study as Y2Ba(Cu,Ni)O5, Ba(Cu,Ni)O 2 and Ba(Cu,Ni)20 2 respectively. An accurate nickel analysis of these phases was difficult owing to their very small size. Y2Ba(Cu,Ni)O5 precipitates were about 1-5 ttm in size and were quite similar to those recently reported in a previous study on bulk materials 9. Energy-dispersive X-ray analysis indicated that the nickel content of Ba(Cu,Ni)O 2 ranges between 6 and 8 a t . ~ . After the second texturing step consisting in a slow crystallization from 1040 ~ C, only the end of the coating located in the colder zone underwent an effective 123 grain texturing (Fig. 7(a)-7(c). For the colder end, the texturing process took place in a discontinuous way and led to the formation of large domains of aligned 123 grains and elongated voids (Fig. 7(b). The existence of such domains was also reported in other studies and was correlated to low temperature gradients. In the present study, the temperature gradient was about 25~'Ccm - ' , which is

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POST-DEPOSITIONTREATMENTSOF YBaCuO ON Ni

855

relatively low and did not favor a continuous solidification front ~~. Even though an excellent directionality was obtained within domains, the microstructural defects and discontinuities such as voids largely affected the normal orientation of the crystallization front with respect to the temperature gradient (Fig. 7(c)). The nickel concentration in the YBaE(Cu,Ni)307_ a phase after the texturing treatment was between 4 and 7 at.','/o. Even though textured coatings obtained during the present study were annealed in oxygen at 500 ° C for 48 h, the high nickel concentration in the 123 phase largely decreased the onset of the superconducting transition temperature to about 40 K (Fig. 8). Indeed, it is known 18' ~9 that nickel substitution for copper in the 123 phase largely decreases the onset of To. For the hottest end of the coating, the large contamination of the coating by nickel produced spheroidal 123 grains not connected together. No transition temperature was observed there.

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After texturing, typical reaction layers were between 20 and 30 lam thick and were constituted of two different zones: a nickel oxide underlayer and a top layer of a copper-rich nickel oxide corresponding to Nio.sCu0.2Ox. The interaction region between the nickel substrate and the coating was about 5 -10 pm thick after the first texturing step. 4. CONCLUSIONS As-sprayed YBaCuO coatings mostly consist o f Y 2 0 3 and BaCu202 and were not superconducting. The restoration of the 123 phase begins at temperatures as low

I). I)t;lll'! c! d/.

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as 615 C hut rapidly occurs above 820 ('. Other microstructural moditications such as the generation of porosities also occur for heat treatments above solidus at the onset of inching events. A two-step process has been used to texture plasmasprayed YBaCuO coatings deposited onto nickel even in thernaal gradients as l o ~ as 25 C c m t. An effective orientation consisting of large domains of aligned nickeldoped 123 grains was obtained. [ h e reaction between the nickel and the coating leads to the contamination of the superconducting phase which reduces the transition temperature. The process is promising but must be optimized to reduce the [brmation of voids and the contamination during texturing, and also to increase the orientation of the 123 phase in larger thermal gradients. A('KN()~%'I.EI)(;MIIN IS The authors

wish

to thank

It. H a r v e ) ,

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arid M. lhibodeau

For

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