Characterization of plasma-sprayed layers of fully yttria-stabilized zirconia modified by laser sealing

Surface

and Coatings

Technology,

53

(1992) 75—86

75

Characterization of plasma-sprayed layers of fully yttria-stabilized zirconia modified by laser sealing K. Mohammed Jasim*, R. D. Rawlings and D. R. F. West Department

of Materials,

Imperial

College of Science,

Technology

and Medicine,

London

SW7 2BP (UK)

(Received November 27, 1991; accepted February II, 1992)

Abstract The effect of laser surface melting of plasma-sprayed layers of2Owt.% yttria-stabilized zirconia was investigated. The plasma-sprayed material consisted of approximately 90 mol% cubic (c) phase and approximately 10 mol% tetragonal (t) phase~on laser sealing at relatively high specific energies the proportion of the c phase increased. No t’ phase was observed either in plasma-sprayed or sealed layers. The microstructure of the sealed layers was of cellular or dendritic morphology depending on the processing parameters. Cracks formed during sealing penetrated through the sealed thickness and, in some cases, even through the unsealed region (plasmasprayed zone) down to the bond layer.

1. Introduction

2. Experimental procedures

Cracking and porosity are major problems associated with plasma-sprayed coatings. Significant cracking and porosity can lead to degradation at low temperature, together with destabilization, spalling and loss of thermal insulation and corrosion protection at higher temperatures. Fine microcracks and porosity allow penetration of the environment, including constituents of the fuel, through the coating and attack of the substrate. In recent years, there has been interest in sealing the porosity in plasma-sprayed ceramic coatings by laser surface melting; zirconia-based coatings have been the subject of several investigations [1—b]. Characteristic features of the sealed layers include reduced surface roughness compared with the plasma-sprayed material, fine-scale cellular or dendritic structures resulting from rapid solidification, cracking and the formation of a

A 2Owt.% yttria-stabilized zirconia (YSZ) powder with an average particle size of 100 ~tm was plasma sprayed to a thickness of approximately 350 ~tm.The substrate, with a bond layer (approximately 100 ~tm) of Ni23Cr6AlO.4Y (wt.%), used for the spraying process was a mild steel (discs: thickness, 6 mm; diameter, 25.4 mm). A few samples were processed without a bond layer. The plasma-sprayed samples were clamped with a suitable jig on a hydraulic powered x—y table which was moved relative to the stationary laser beam. The xdirection speed was in the range 3.2—370 mm s~. The y direction was traversed manually to produce single tracks on the sample. The laser was a 2 kW, fast axial continuous wave (CW) CO2 model (BOC 901 type). Argon gas was used

concavity and/or small surface depressions associated with gas evolution, These previous investigations were concerned with yttria-partially-stabilized zirconia (YPSZ) and calciastabilized zirconia (CaSZ). The effects of the main experimental parameters, including laser processing conditions, substrate preheating and plasma-sprayed thickness, on the structural features and dimensions of the sealed regions were reported. The present work is concerned with a fully-stabilized type of material containing 2Owt.% yttria.

to protect the lens from the plasma formed and also to protect the melt zone from the atmosphere; another high pressure argon supply was applied for effective shrouding of the sample. The laser processing conditions are listed in Table 1. TABLE I. Laser processing parameters studied _______________________________________________________

Power (P) Beam diameter (d) Traverse speed (V) Interaction time (t) 2) (PA) Power density Specific energy(4P/itd (P/dy) (S)

0.8—1.4 kW 5 mm 3.2—370 mm s 40.7—7 13.5—1000 1.3 W msmm2 0.43—50 J mm2

*On leave from the Scientific Research Council, Baghdad, Iraq.

0257—8972/92S5.00

~ 1992

—

Elsevier Sequoia. All rights reserved

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C/iarw terization of pla~nui—spravedfiiters of curia— stahili:ed :irconia

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tetragonal phase was mainly determined from scanning

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transmission electron microscopy (STEM). The thin foil discs (3 mm) of the sealed layers were prepared by grinding and polishing the plasma-sprayed layer after

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baser sealing to approximately 30 J.tm thick. The thin areas of the discs were produced using an ion beam.

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3. Results and discussion

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3.1 . Murostructural features of plasma—sprayed layers Plan view observations showed porosity (approxi-

mately 15 vol.°o) and cracking (Fig. 1(a)). Transverse

0)

Plasma sprayed layer~~ ~

sections showed a higher amount of porosity near the bond layer (Fig. 1(b)). The surface roughness was approximately 6 ±0.5 ~tm. X-ray diffraction demonstrated the presence of cubic (c) (approximately 90 mol°o)and tetragonal (t) (approximately 10 mol%) phases (Table 2); the former were interpreted as corresponding to the regions of high yttria content as determined by EDS and STEM (Table 3). The yttria content of the t phase was calculated from the c a ratio [10] and found to be 5.7 and 5.8 wt.°o for the powder and the plasma-sprayed layer respectively (Table Hardness 3). measurements on the top surface and on polished transverse sections gave values in the range 850 920 Hv. These values are higher than those mea-

_____

jim

Bond layer (h)

1-1g. I. Scanning electron mlLlographs of as-sprayed 20 wt.°

0 YSZ: a) plan vicv, showing the ~cneral appearancc~)b) high transscrse section shooing the bonding hetoecn the bond la~cr and ceramic

sured previously in 8 and 8.5 wt.°o YPSZ plasmasprayed layers produced under approximately similar conditions (580 750 Hv) [7, 11].

layer.

TABLE 2. Data obtained by X-ras diffraction on the phases present in pooder. plasma-spra’,ed and sealed layers (mo!°) Material

c phase

Pooder

Plasma-sprayed layer Sealed layer (1 J mm

t phase

8!) 80

~90 2~

~90

2)

~97

8 -

~l0

m phase 0

0 I Trace

Trace

~ll)

2.8

~0.2

Scanning electron microscopy (SEM) of the surface and trans~ersesections was used to evaluate the struc-

tural features of the sealed tracks. The chemical composition was determined using energy dispersive spectroscopy (EDS). Microhardness and roughness measurements (centre line average) were made using a Leitz miniboad (500 g) and a Talysurf respectively. The phases present, including quantitative analysis, were determined using X-ray diffractometry with an X-ray step scanning programme. The composition of the

3.2. General j~’aturesafter laser sealing Layers sealed at specific energies of less than approximately 2 J mm 2 had an off-white colour, whereas with higher specific energies a shiny yellow coloured surface was produced. In a previous study on YPSZ sealed layers [5], it was reported that, although there was some variation with processing conditions, under all conditions the layers could be described as being basically a shade of yellow, and shiny. Observations of the width and depth of the sealed layers (Figs. 2 and 3) showed the expected trend of decreasing values with increasing traverse speed and decreasing power (i.e. with decreasing specific energy). The maximum sealed width was close to 3 mm, i.e. less than the beam diameter (5 mm), and the maximum depth was approximately 0.13 mm. With 0.8 kW power and speeds of more than 230 mm s only partial sealing was obtained (Fig. 4). but with I kW power complete sealing was obtained up to a traverse speed of 370 mm s (Fig. 5). Low magnification scanning electron micrographs of

K. Mohammed Jasim ci a) ./ Characteri:ation of plasma-sprayed lasers of viiria—siahili:ed :irconia

77

TABLE 3. Phase compositions (wt.%) in 20 wt.% YSZ determined by EDS and STEM Oxide

Powder (average)

Plasma (average)

Sealed (c phase)

Sealed (t phase)’

ZrO, Y,0 3

75—81

78

77

92

19—21 With zirconia 0.1 0.8-I 0.5-I 0.2-0.5 0.5—I 0.03-- 1.75

20 1.8 0.1 NA NA NA NA NA

21

6 NA NA NA NA NA NA NA

J-lfO,

A12O3 SO, TaO TIO, Other oxide Organic sol.

.8 0.1 NA NA NA NA NA

Y,O3 (wt.%) in t phase from c/a (powder) Y2O5 (wt.%) in t phase from c/a (plasma-sprayed layer) Y2O3 (wt.%) in I phase from c/a (sealed layer)

5.7 5.8 6(1

~Determined by STEM. NA. not analysed. 4000

o

0.8kW • 1.0 kW

3000

2000

~

1000 \ \

Partial scaling

0

.

0

100

°~

200 300 Traverse speed, mm/s

400

increasing power (Fig. ID). The percentage of depressions per unit sealed area showed a similar trend (Fig. II). The origin of these depressions is attributed to gas

Fig. 2. Relationship between sealed width and iraverse speed. 150

0

~-

evolution and has been discussed previously [7] for 8 wt.% YPSZ.

0.8kW

• i 0kW 100

50

“~,0~,,Partjal

0

S

0

100

plasma-sprayed layer and the sealed layer is shown in Fig. 6(c). Transverse section observations showed that, for the complete range of speeds studied, cracks penetrated through the whole depth of the seabed layer (Fig. 7) and sometimes into the residual plasma-sprayed material. The crack width (distance between faces of the crack) and spacing (distance between cracks) increased with increasing power and decreasing traverse speed (Figs. 8 and 9). The average size (diameter) of the surface depressions decreased with increasing speed, but increased with

~

200 300 Traverse speed, mm/s

400

Fig. 3. Relationship between sealed depth and traverse speed.

the surfaces of samples sealed with a laser power of more than 1 kW showed the presence of depressions and cracks .(Fig. 6(a)). A higher power (1.4 kW) produced mainly layers without depressions, but with severe cracks (Fig. 6(b)). The difference in roughness between the

As reported in previous studies [4, 6], cross-sections of the tracks perpendicular to the direction of traverse showed the tracks to be concave. The depth of concavity decreased with increasing traverse speed and power (Fig. 12). At a speed of approximately 230 mm si no concavity was observed. This effect may result from the temperature rise being below that required for significant evaporation, beading to a reduction in evaporation loss. Also, at 0.8 kW, only partial sealing was obtained at 230mm ~ The surface roughness was low at 0.8 kW and bow speeds, but increased slightly when the traverse speed was so fast that only partial sealing was achieved (Fig. 13). At 1.0 kW power the roughness decreased with the traverse speed. The bow roughness at the highest speed at 1.0 kW power is due to optimum sealing associated with the small depression size and the low ratio of depressions to sealed area; the high level of roughness at 1.0 k W and low speeds is because of severe cracking.

78

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.i,i.siin et il.

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ii

at on ‘1 plas,na—sjrai si fat

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Fig. 4. Plan views of the 20 sst.°o YSZ scaled lasers showing the effect of traverse speed laser power. 11-8 kW: beam diameter. 5 mm): ti) 53 mm (partial spalling revealing area of substrate); (hi 82 mm complete sealing): Ic) 23(1 mm s ) partial sealing): Id! 370 mm s no sealing).

than I J mm ~

______ _________

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--______ _____ --

500

p.m

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F ic. 5. Plan view of as-sealed 20 wt. -o YSZ las-er processed at a laser power of I kW, beam diameter of 5 mm and traverse speed of 230 mm showing complete scaling s-sub cracks and depressions ~s-ec I- ig. 41c)).

3.3 Microstructure and constitution

Higher magnification observations on plan views revealed two types of microstructure depending on the specific energy. At low values of specific energy (less

2)

a fine cell structure consisting mainly

of cubic solid solution and some t phase (as determined by X-ray diffraction) was observed (Fig. 14(a) and

Table 2). At higher values of specific energy. a dendritic type of structure was formed (Fig. 14(b)); the structure was determined to be mainly c phase on the basis of X-ray analysis, and this was consistent with EDS and transmission electron microscopy (TEM) chemical analysis and microstructural examination (hackscattered electron) which indicated that the t phase was present in only a small amount. Transverse sections (Fig. 7) showed the depth of sealing and the interface between the sealed and plasma-sprayed layers. The types of structure shown in Fig. 15 were observed in some regions of the seabed bayer (which also consisted of c phase and t phase). In these regions growth appeared to occur readily from a central nucleating crystal giving a eutectic-like appearance. These may form due to the relatively high concentrations of impurities present in the starting powder. such as SiO-, and Ab20s-. which may form a eutectic. EDS (area analysis) 1Vo) detected small amounts of alumina in these eutectic-like regions. (approximately I wt.

K. Mohamm&d .Ja.sim ci al.

( haracterization of plasma—spra tel ía vs rs of t iiria—srahili:cil

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)e) Fig. 6. Scanning electron micrographs (plan views) of as-sealed 2) showing the 20 wt.% YSZ: (a) low specific energy (0.7 J mm presence of depressions and network cracks; (b( high specific energy (17 J mm _2) showing the absence of depressions and presence of severe cracks; (c) the interface between as-sprayed and sealed layers (specific energy, 0.91 J mm2).

Fig.different 7. Transverse as-sealedlayer 20 wt.% YSZatlayers processed at traversesections speeds of showing damage low speed and deep cracks at high speed (laser power, I kW; beam diameter, 5 mm): (a) 22 mm s~ (lateral and vertical cracking); (b) 130 mm s —‘ (vertical cracking); (c) 230 mm s -- (vertical cracking).

The cell size (Fig. 16) decreased slightly with decreasing power, in general agreement with the E°4 and ~A°4 relationships previously found to apply to laserprocessed 8 wt.% YPSZ, where E and ~A are the energy

and power density respectively [9]. The cell size was also a function of the traverse speed and decreased by about 45% on changing the speed from 5.3 to 230 mm s~. Typical micrographs illustrating the range of cell

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200 300 Traverse speed, mm/s

I- ig. 8. Relationship bets-seen mad

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o idi It and

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speed -

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200 300 Traverse speed, mm/s

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I ig. I I - Relations-hip bets-seen percentage of depressions per unit sealed tire:, and traverse speed.

100

600

F

0

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0.8kW

0

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100 0

,

100

20

\~~ia~seaiing .

200

300

)0

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Traverse speed, mm/s

200

300

400

Traverse speed, mm/s

I ig. 9. Relationship bets-seen average crack spacing and travcrs~speed.

2. Relationship between maximum depth of concavity and tra-

Fig.

serse speed.

120

1

0 0.8kW

io~

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_0

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1.0kW

100

200

300

•

400

0

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Traverse speed, mm/s Fig. I I). Relationship between average depression size and tras-erse

200

o.siw 1.0 kW

300

400

Traverse speed, mm/s —

-

-.

.

I- ig. I 3. Relationship hctoeen roughness of the scaled layer 1- lA) and

speed.

traverse speed.

sizes are presented in Fig. 17. This cell size dependence on power and traverse speed is consistent with the decrease in cell size with an increase in the cooling rate predicted by solidification theory. The X-ray diffraction data from the starting powder

showed that the (Ill) peak for the c t phase dominated in the range 20=27.5—32 (Fig. 18) although them phase (111) and (111) peaks were also detected. The higher range of the step-scanning X-ray results (20 72 75.5 showed the presence of the (004) t phase peak which =

K. Mohammed Jasim ci al.

Characterization of plasma-spra~ed laver,s of t ,iria-,siahili:ed zirconia

81

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~

(

1p.m

20p.m

(at

h)

Fig. 14. High magnification of the 20 wt.% YSZ sealed layers: (a) plan view at low energy showing the presence of cells (light areas are e phase and dark areas are t phase); (b) plan view at high energy showing the presence of a dendritic structure (the phase is mainly c).

____________________________

4— —

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100p.m

.

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1’ I

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(h) Fig. IS. Anomalous eutectic-like structure observed in some regions of as-sealed 20 wt.% YSZ: (a) low magnification; (b) higher magnification. 3.0

2.5

~

0.8kw

parameter of thee phase is 0.5 14 nm. Quantitative results

• 1.0kw

from X-ray diffraction, EDS and STEM are shown in ~ 1~yi~13 for the powder, plasma-sprayed and

-

2.0

1.5 -

0

100

200

400

Traverse speed, mm/s Fig. 16. Relationship between average cell size and traverse speed.

was overlapped by a strong c peak (Fig. 18). Further evidence for the m phase, and also very weak peaks corresponding to a small amount of yttria, were observed in other regions of the X-ray spectrum. The lattice

The data showed an increase in the proportion of the c phase and a decrease in the t phase in the material as a result of plasma spraying (Fig. 19). Iwamoto et al. [12] also reported that the plasma-sprayed condition of a similar material (20 wt.% YSZ) was predominantly c phase; their starting powder was different, consisting entirely of c phase, whereas the plasma-sprayed material contained 73, 22 and 5 mol% c, t and m phases respectively. The laser-sealed layers in the present work consisted of c phase, with some t phase and a small amount of m phase (Table 2). The t phase regions in the sealed layers were small and spurious counts from the surrounding phase are probably included in the EDS analysis using SEM. Consistent with this suggestion are the results of a few analyses on thin foils by STEM; these yielded

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Ic) I- ig. 17. Plan vies-s-s of as-sealed 2t) wi. YSZ layers showing the cells processed at different tras-erse speeds and poss-ers beam diameter. 5 mm): (a) Ic) laser power, (t.8 kW: Id). Ic) laser power. I kW: a) 22 mm s : hI 175 mm s 1: Ic) 23(1 mm s- I: Id) 22 mm : Ic) 370 mm s -

yttria contents for the t phase of around 6 wt.°o,much lower than that obtained using EDS. It should be noted that the yttria content determined from the c a ratio (Table 3), using the experimental formula reported previously [10], gave a value similar to that from STEM analysis. The presence of t and c phases in the lasersealed regions is attributed to segregation during solidification and or to precipitation of the t phase in the solid state during cooling,

The phases present in laser-sealed 20 wt.°o YSZ are dependent on the process parameters and can he divided generally into two groups. (I) At specific energy values of less than I J mm 2 the phases present are c and t with a trace of m phase. The proportions of the phases are similar to those in the plasma-sprayed layer (i.e. 10 mol% t phase and 90 mol°o c phase). The average yttria content of the c phase is 22 wt.°/0and of the t phase is about 6 wt.°o.

K. Mohammed Jasim ci al. / Characterization of plasma-sprayed layers of vttria-siahilized zirconia

83

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r-~,

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75

200 Fig. 18. X-ray step scanning of 20 wt.% YSZ powder showing the presence of m, c and t phases.

Fig. 20. X-ray step scanning ofas-sealed 20 wt.% YSZ plasma-sprayed layer processed at high specific energy.

—

TABLE 4. I~(lII), 13400) and i3400)/1~011) for the sealed layers compared with the starting powder, plasma-sprayed layer and standard c zirconia

ci

Condition

Peak hkl

l,~

!3400)/131 II)

Standard zireonia

111 400 III 400 III 400 III 400 III 400

100 5 100 5 100 10 100 10 20 40

0.05

Powder Plasma-sprayed layer Sealed (low specific energy) Sealed (high specific energy)

28

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0.05

the proportion of c phase is approximately 97 mol%, with a small amount of t phase (Fig. 20). The amount -

I I

of yttria in the c phase is slightly less (approximately 19 wt.%) than after processing at lower specific energies. The low than range0.2 angle of step showsis the presence of less mol% m scanning phase, which marginally

Fig. 19. X-ray step scanning of 20 wt.% YSZ plasma-sprayed layer

higher than in the sealed layers processed at lower

showing absence of m phase and presence of c and t phases.

specific energy (only trace).

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Microhardness

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Fig, 2 I - Setunning elect ron microgra phs ,mf 20 wi” ,, ‘i’SZ sealed lay-cr showing the cracks propagated trans-cell ularly (due to niechanical stressd Li rung cutting) and intercellularly (due to therintul stresses): (a) los-v magnifIcation shos-sing the presence of both t\’pes of cr:ick 5: I hI higher mtugni fication of (a I showing the intercellular crack ) primary’): )c) higher magnification of (a I showing the intercel I ular crack I secondary-): (dl higher magnification of ti) showing the trtunscellular crtuck.

An interesting result observed is that the 13400).13111) ratio from the baser-scaled material at high specific energy is significantly higher than that from the starting powder, from the plasma-sprayed layer material and according to the standard data file [131 (Table 4). This is attributed to a texture in the laser-sealed material processed at high specific energies. This texture does not appear to be so marked at lower specific energies.

3.4 .Mic rohardness and fracture toughness

The microhardness values of the 20 wt.°/oYSZ layers are effectively independent of the laser conditions (Table 5). However, the fracture toughness (as calculated from optical microscopy of the cracking associated with microhardness indentations, see refs. 14 and IS) is slightly higher in the sealed layers processed at low specific energies. which is attributed to a higher volume fractton

K ~!ohaniniedJasini ci al.

( haractcri:ai tim

of

plasma—spraveil lois rs of

stabilized :ir onia

)~(fl~~

85

ET~ •

~

100 p.m

100 p.m )h)

•0~

_ mo

10 p.m

II

p.m

(dl

~ el

~~~100p.m

.

10p.m

((‘1

Fig. 22. Scanning eleciron micrographs showing the effect of contact with Na 2HPO4~12H20 at l2)Ut C or 42 h on the quality of as-sprayed and .is--se.iled 20 wt.% YSZ layers: (a) plasma-sprayed layer (low’ magnification, SE image); (b( BSE image showing the penetration of species: (c) higher magnification of (a), SE image; (d) as-scaled layer (low- magnification. SE image(; (e( BSE image; (f) higher magnification showing the penetration of species in the depressions of the sealed zone.

of the t phase. The toughness values of the laser-sealed layers produced in the studyYPSZ are inferior to those previously obtained onpresent a 8.5 wt.% material [10]. This is attributed to the greater proportion of t’ and/or phase in the laser-sealed 8.5 wt.% YPSZ. It is interesting

to note that the lowest indentation toughness value for 32 (similar to the the 8.5 wt.% YPSZ 2.3 MN m present values) and was this corresponded to the structure with the greatest proportion (48 mol%) of the c phase. SEM examination of the plan views showed that the

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uif,

Chuirau’tcri:uit ion of plasma—s pro veil lat’c’r.s of iu(riuu—.siahili:cul ziru-oniuu

cracks propagated extensively, which reflects the low fracture toughness of the c phase. As in YPSZ sealed layers both transcellular and intercellular cracking were observed. There was a tendency for the cracking associated with solidification and cooling to be intercellular, whereas cracking due to mechanical stresses produced during cutting was mainly transcellular (Fig. 21). 3.5. High temperature corrosion test To explore the effect of high temperature corrosion on the sealed layer, a sample with single-track sealed layers was covered with a coating of Na2HPO4 l2H2O and heated in a furnace in air at 1200 “C for 42 h. Scanning electron micrographs show the severe attack on the plasma-sprayed layer (Fig. 22). In contrast, the sealed layer generally showed good resistance to this attack, although the depressions behaved as pockets for species concentration leading to some preferential corrosion.

Acknowledgments We wish to thank Professor D. W. Pashley (FRS) for providing the facilities of the laser laboratory and Mr. R. Sweeney, Department of Materials, Imperial College for assistance and cooperation during the X-ray analysis. K. Mohammed Jasim wishes to thank the Ministry of Education and the Scientific Research Council, Baghdad, Iraq and the Committee of Vice-Chancellors and Principals of the Universities of the United Kingdom for a scholarship award.

References I I. Z. Zaplatynsky, Thin Solid turns, 95 (1982) 275. 2 R. A. Miller and C. ~‘. Berndt. Thin Solid Films, 119 (1984) 195. 3 F. S. Galasso and K. Veltri. J. Am. Cc’rarn. Soc. Bull. 2(1983) 253.

4 K. Mohammed Jasim, D. R. F. West and W. M. Steen. .1. Mater. St-i. Leit.. 7(198811307. 5 K. Mohammed Jasim. D. R. F. West, W. M. Steen and R. D.

4.

Conclusions

(I) Two types of microstructure were observed on the upper surface of laser-seabed layers of 20 wt.% YSZ: cellular at specific energies of less than I J mm~2 and dendritic at energies higher than I J mm 2, (2) The sealed layers consist of more than 90 mol% c phase, the proportion being2.higher at specific The balance is t energy phase levelsa of greater I J No mmhigh yttria content t’ phase with trace of mthan phase. was observed in any of the processed layers. (3) In contrast with the shallow cracking formed in YPSZ sealed layers, perpendicular cracks penetrated through the sealed layer and sometimes into the plasmasprayed layer of the present material even at low specific energy values, thus posing a severe problem. (4) The hardness levels of the sealed layers are relatively high (similar to that of YPSZ, approximately 1500 Hv). However, the toughness is inferior to that of YPSZ sealed layers due to the predominance of the c phase in the layers produced in the present study.

Rawlings, in G. J. Bruck (cdl, Laser Materiuil.s Processing (‘JCALEO 8(~’). Springer-Verlag IFS (Publication( Ltd.. UK (in association with Laser Institute of America(, 1989, p. 17. 6 K. Mohammed Jasim. D R. F. West and R D Rawlings, in A N Lan and S. K. Ghosh (cds.(, Laser itt Industry (Laser-5). IITT.

1989. p. 90. 7 K. Mohammed Jasim, R. D. Rawlings and D. R. F. West, J. Mutter. Sc’i., 26 (19911 909. S K. Mohammed Jasim, R. D. Raw-lings and D. R. F. West, J. Mutter, 27(1992) 1937 9 Sd.. K. Mohammed Jasim, R. D. Rtuwlings and D. R. F. West. J. Mu,tur, Sci., 992, to he published. 10 K. Mohammed Jasim. R. D. Rawlings and D. R. F. West. Mater. Scm. Technol. 8 (1992 83. II K. Mohammed Jasim, R. D. Rawlings and D. R. F. West. unpublishecl work, 1991 12 N. Iwamoto, N. Umesaki and F. Endo, Tins, Solid Films, 127

(19851 129. 13 Powder D(tfracuion File (JCPDS, International Centre for Diffraction Data. Swarthmore, USA, 1989. 14 C. Ponton and R. D. Rawlings. Mater. Sci. Technol., 5 119891 865. 15 C. Ponton and R. D. Rawlings, Mcuter. Sc-i. Technol., 5 (1989)

961.