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The oxidation of beta'-sialon

Citation for published version (APA):

van Dijen, F. K., & Metselaar, R. (1988). The oxidation of beta'-sialon. Science of Ceramics, 14, 327-332.

Document status and date: Published: 01/01/1988

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327

:i

THE OXIDATION OF BETAt-SIALON F.K. van Dijen and R. Metselaar

,__

qx...Lcfa:t~i..on

f.J:tud..Le.f.J have. be.e.n- pe.lt6oltme.d

_~n_~

l-f.J..La{on, S..L3A{30fNs,

--wi;:th

d.tt6e.lte.n:t f.J..Ln:te.It..Lng a..Ldf.J.

Re.f.Ju{:tf.J alte. g..Lve.n 601t ox...Lda:t..Lon a:t

"1000°C,

7-200°C and 1400°C -Ln a-Llt Olt ox.yge.n. A:t lZ00°C :the. palta-'

-13

Z

-4-Z

bo{-Le Ita:te. eonf.J:tan:t

-Lf.J

abou:t

S

x.

10

9 em

f.J

601t ox...Lda:t..Lon ..Ln

a..L1t 601t a{{ f.Jamp{e.f.J.

A:t 1400°C :the. lta:te.f.J valty be.:twe.e.n

k

~

10- 11 gZem- 4f.J-Z 601t

3

w:t% La

Z

0

3

, and k

~

10-9

gZem- 4 f.J-Z 601t

3 w:t% CaO dope.d f.J..La{on.

The. ox...Lde. f.Jea{e. eonf.J-Lf.J:tf.J 06 a m..Lx.:tulte. 06

~mu{{..L:te.,

eltif.J:toba{{-L:te. and o:the.1t phaf.J

e.f.J,

and

-Lf.J

e.nlt-Lehe.d ..Ln :the.

me.:ta{{..Le eompone.n:t 06 :the. f.J..Ln:te.It..Lng a..Ld.

In :the. adjaee.n:t f.J..La{on

~ult6aee.

{ayelt a dep{e.:t..Lon 06 :the. f.J..Ln:te.It-Lng

~-Ld -L~

obf.Je.ltve.d. The.

Ite~u{:t~

alte. eompalted w..L:th da:ta 6ltom {..L:telta:tulte.

INTRODUCTION

The applicability of struc~ural ceramics at high t~mperatures is of ten determined by their,oxidati6n resistan~e. ~ince most non-oxide materials are thermodynamically unstable against oxygen or air their lifetime is determined by the format ion of a protective oxide scale formed on the surface of the material. Actually the situation is quite comparable with that (or ~etals. Here too the properties of the scale are determined

tiy

th~ composition of the alloy, its microstructure and of course temperature and"partial oxygen pressure.

In the case of Si N

4 alloys

m~ny

studies have been performed 1

-7. We will

ment~on her~

a few results which ar.e most pertinent for " our 6~n study on B'-siàlon. Sy'far the6ighest Óxi4~tion rates~are observed in reacti~n-sintered materials because of oxidation in the por es until a continuous surface layer ?f Si0

2 is formed. The lowest rates are observed for Si N

4 layers grown 5y chemical vapour deposition. Such layers

a~e

dense, and chemically

pure~

The

low~oxi~atio~ rate !~ due to th~cfo~mation";of ~ cbh~§eijt and. ]0

protect1v~ Sl02 layer. It bas been shown for Sl, SiÇ, ' "MOS1

2 and Si

3N4 that the activat~on energy for oxidation o[ ~hese

cqm-pounds corresponds to that of diff~sion of molecular' oxygen __

through a,S~02-glass layer. Hot':'pressed Si~N4"materiáls'6èçi:lPY" an"", intermediate position. Widely different rates are founij-tn ~~- ,- , terials with different srntering aids d~eto the fbrmafi~n ~t

;

-different secondgry phases. In. general ~-sialons"seem tö sfiow ~" higher oxidàti?n'resistance5thancompositions close ~o th~"S13~4

corn~r of the phase diagram . ,

The most recent study of Bt-sialon, Si

6 AlO NB"'"',""with bigh

- -x x x -x

x-value, is due to Pomeroy and _ Hampshire.

These authors studiep a pressureless sintered sialon witb x

=

3, doped with 3.8 wt% yttria~ At 1380ó

c

in air ~short incubation

time is found, followed by a linear regime for 3.5 hr and then a parabolic oxidation occurs. Other authors observed a parabolic oxi-dation only. The mechanism of oxidation is also unclear. Both

(3)

EXPERIMENTAL

The present study is restricted to sial ons made by carbothermal convers ion of kaolinite, according to the reaction

MgO 3 .1 27

*

carbon black wt "/0

<

0.001

<

0.001

<

0.01 < 0.005 < 0.03

<

0.005

<

0.02

<

0.02 < 0.5 < O. 1 CaO 2.9 0.06 13.9 6.7 Kaolin+ wt "/0 45.6 38.6 0.34 4.37

TABLE 1. PROPERTIES OF THE RAW MATERIALS

+

*Monarch Elftex 120

TABLE 2. DENSITIES OF THE SINTERED SAMPLES SiO A1 2

6

3

F

72

0 3 T~02 CaO MgO K 20 Na 20 S ash loss on ignit~0~1 BET(m g ) .

.

oxide scale are reported to be rate determining mechanisms. From the work of Pomeroy and Hampshire 'it is clear that the scale for-mation occurs in different ste~s.

Since the oxidat10n process seems to be strongly dependent on both the x-value and the impurities or sintering aids, we have performed oxidation experiment~1on samples with different

additives and a constant x

=

3 .

3 (2 Si0

2.A1203·2 H20) + 5 N2 +

i5

C - 2 Si3A1303N5 + 15 CO + 6 H20 Details of the reaction have been published elsewhere11 and we

ment ion only the most important results here. As a starting mate-rial we used Monarch kaolin and carbon black. Table 1 gives data on the raw materials. As shown in this table Fe and Ti are the main impurities. The mixture of káolinite and carbon is pelletized and heated under N

2 at 1500oC. The ratio CJkaolinite has to be adjusted carefully to obtain the sialon phase. The resulting powder wa~ milled and sintering was performed,with powders of about 8 m Jg. Aftel' addition of sintering aids isostatically pressed pellets were sintered at 1675°C during 20 hours in a

powder bed consisting of sialon powder with 5 wt"/o BN. Densities of the sintered samples varied between 2.9 and3.3, depending on the sintering aid used (Table 2). The Ti and Fe impurities in the raw material lead to the formation of small TiN and FeSi

2 inclusions in the sintered samples. The grain size is relatively large due to the long sintering times (fig. 1). Only closed porosity is present.

Sintering aid_~3 wt"/o) Density (g cm )

(4)

329

Fig. 1. Optical micro-graph of an etched

Si1A130~Ns with 3 wt% Y' 0 , s!nte'red for 20 h at f7000C. The length of the bar is 10 ~m.

,Oxidation experiments were performed in air or oxygen at 1000 0 C,

1200aC and 1400°C. The oxidation experiments in air were carried out in a tube furnace inside an alumina tube. The samples were heated up to the oxidation temperature within 2 hours. Af ter oxi-dat ion they were pul led out of the hot furnace. The experiments in oxygen were performed in a thermobalance. (Setaram Microbalance MTB 10.-

8).

In this case the samples were heated in nitrogen at a rate of 200 0 Clper hour and next oxidized and cooled in oxygen. The cooling rate was again 200 0 C/h.

Af ter exposure the samples were sectioned and polished and sub-sequently examined by optical microscopy and electron probe micro-analysis (JEOL 733 Superprobe with both WDX and EDX analysis).

RESULTS

A number of measurements were performed in the thermobalance. Fig. 2 shows an example of the'weight change. Some oxidation occurs already during the heating'of the sample in nitrogen con-taining about 5 ppm of o~ygen. The linear-relation between the weight gain squared (~g) against time t ~hows

a

parabolic i~e.

diffusion limited oX~dation,~ehaviour. The ~arabolic ra te constant k is defined by (~gJ

=

kt A , where A-is tbe surface area of the sample. Table 3 shows the result of oxidation experiments in air at different temperatures for samples with different additives. The value of the sample-with3 wt%Y

20 ~ox~dized at 1400 0 C is of

the same order of magnitude as observeJ by Pomeroyand Hampshir!12

f~r ~4s~~ple~with 3~8 wt% Y

20 oxidi~ed at 1380 0 C (k

=

8.2 x 10 g cm s ). The rate constant 3 for the calcia doped sample is consi-derably larger than tor other dopants, at temperatures above

1200 0 C. Best results were obtained for La

20 doped samples. To in-vestigate the influence of the partial oxygJn pressure we have

TABLE 3. PARABOLIC RATE CONSTANTS FOR OXIDATION IN AIR AS A FUNCTION OF TEMPERATURE FOR SAMPbES" WITH DIFFEREN.T

SINTER ING ADDITI VES

C3

WT%) Sintering aid 1000 0 C 1200 0 C 1400°C Ce0 2 0.1-0.6 24 CaO 0.1-0.6 950 MgO 0.1-0.6 20

(5)

DISCUSSION

Fig. 2 •. Weight' gain o·f siAl':l'(D'~ÎN;s ,

;~i

tri 3

'~rt%.

Ge0

2, i n

oxyg~-à

at

T.,2d~o

' ;as Pa."Ju~ct~on of:time. Saniple\ arêa

2~~,6A;';'cm . ~",,­ ',.'"' '.

4

8 12 16 20 24

(HOURS>

-(mg)2

O. 5

~,,-~"-'-""T'"A""

0.4

0.3

0.2

0.1

heated'a Ce0

2 doped sample in pur~ exygen instead of air. At -'

1000~C a catastrophic oxidation occurre~ and the sample cra~ked -However, at h~gher temperatures parabolic oxidation was_~~served~

F~om_5h!1s10peof fig-. 2 we find !\212~OO~4k_f 0.6·x 10 g e m s j a t 1 4 0 0 o C k = 2 6 0 x 1 0 gcm s

To g~t some insight in the oxidation mechanism we performed

electron microprobe analyses on samples oxidized in air at 12000C

and 1400oC. Fig. 3a shows an example of an electron micrograph

with superposed yttrium concentration profile. Independent of the nature of the additive always four different zones are observed. This is indicated schematically below the picture~ In zone 1 the bulk concentration of the dopant is present. In zone 2 we are s t i l l in the matrix but a depletion of the sintering aid has occurred. The oxide layer consists of two parts, an inner layer

(zone 3) and an outer layer (zone 4). The oxide Iayers are inhomo-geneous and contain pores, mullite, crÎstoballite and other phasesj therefore a concentration measurement is rather difficult. It wàs tried to carry out a line scan along a suitable part of the layer. The oxide layer clearly contains yttrium. A similar behaviour is observed for other dopants; e.g. fig: 3b shows results for a Mg doped sample. The impurity ions Ti and Fe of the kaolinite are present as TiN and FeSi

2 inclusions in the sialon and are not affected by the oxidation. The SEM picture also shows that large pores (10 - 20 pm) arê present in the oxidation layer. As men-tioned in ref. 7 the surface of the scale shows signs of nitrogen eruption. Many questions, especially concerning the oxidation mechanism, are s t i l l unanswered. For instanèe, in HPSN 5 wt% MgO doped samples'show an oxidation rate about two orders1~f mag-nitude higher than a sample with 6 % 3i0

2 + 12 % Y20~ . This has been a

7

tributed to the difference'in viscosity of Eh~ liquid phase formed . However, the results shown in table 3 do not confirm this. Y2G':l and MgO doped samples show similar k-values, while CaO has a k-va:lue about 36 x t-imes larger. A matter which also deser-ves more attention is the influence of the partial oxygen and nitrogen pressures. For instance, for an undoped hot p~essed B'-sialon with z = 2.85 Desmaison et al. 13 find a paralinear oxi-dation kinetics at 1450o~1~n 1 atm oxyge~ (P02 = 1 atm) or 1 atm carbon dioxide (p02 ~ 10 atm). In most cases we observe para-bol ic oxidation, however, in the ce02 doped sample heated in 1 atm oxygen instead of air, catastroph~c 5xidation occurred at 1000oC.

So far we can only say that both diffusion of cations and anions play a role.

(6)

331

Fig. 3a) Line scan

ana-lysis and backscattered

electron image of a

sample with 3 ~t% Y2D~,

aft er oxidation in air

at 1400°C during

64 hours.

ZONE

SCHEMATIC Y

CONCENTRATION

b) Similar picture for

a sample with 3 wt% MgD.

ZONE

SCHEMATIC Mg

CONCENTRATION

(7)

RKFERENCES

-

,

- 1. SINGHAL, S. C. in Ni t!;cQgen Ceramics! ed.' RILEY, F. L., London: Noordhoff Publ. Comp~,'p. 601- 626' (19T7) •

2. SCHLICHTING, J. in N~trog,en'Ceramies, ed. RILEY, F. L., London: Noordhoff Publ. Comp:' p.

62/'r

-,634 (J977).

3. PORZ, F. in 'Progress' in Ni ttogen Ceramics " ed. RILEY; F. L. , Th e Ha g u e : Mar t i nu s

Ni,jfl.ciT,t.

Pub 1 . ,

p.

5

3

9 - 5 4 6 (1 9 8

3 ) .

4. THUMMLER, F. & GRATWOHJ1i G~~-' in Progress in Ni trogen Ceramics, ed. RILEY, F.L., The:'H~gue: Martinus' Nijhoff ,Publ., p. 547 - 555

( 198 3 ) • -::.F.~

5. S CHL I CHTIN G, J. & GAUCK,f#lFi~', L. J . , P0 W

ct

er Met all • Int.

2. ,

36 - 39 (1977). --~

6. PORZ, F. & THUMMLER, F.,,::~. Mat'er. Sci. 19, 1283 - 1295- {1984). 7. POMF:ROY, M.J. & HAMPS~IRE, S:, Mater. Chem. Phys •

.!l,

437 - 448 (1985).

8. MOTZFELD, K., Acta Chem~ Seand.,

..l.!!.

1596 - 1606 ( 1964). 9. FITZER, E. & EBI, R. ''c,D,echema Frankfurt

U

972). .

1O. FIT ZER, E. & REI NMUTH.-;",~ ..X., i n Ho c h tem per a t u r We rk s tof f e, ed. BENESOVSKY, Wien: Sp@inger Verlag {1969).

11. VAN DIJEN, F.K., Thesr~, Eindhoven University of Technology. (1986).

12. BABINI, G.N.

&

VINCE~ZINI, P~, in Progress in Nitrogen Cera-mies, ed. RILEY, F.L., The Hague, Martinus Nijhoff Publ. Comp.,

p. 4 2 7 (1 9 8 3 ).

13. DESMAISON, J:, BROSSA~D~ M~, DESMAISON-BRUT, M.

&

GOURSAT, P.,

~in Progr~ss in Nitrogen Ceramies, ed. RILEY, F.L., The Hague:

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