Glass systems

Journal of Materials Processing Technology, 38 (1993) 465-482

465

Elsevier

The sinterability systems

and electrical

conductivity

of some

TiSi 2

/Glass

P H i n g "÷ and A Adotey+, "Division of Materials Engineering, School Of Applied Science, N a n y a n g Technological University, Singapore 2263. + Work was conducted at T h o r n EMI Central Research Lab., Dawley Road, H a y e s , Middlesex UB3 1HH, UK.

Abstract This p a p e r describes some preliminary studies on the d e v e l o p m e n t of a family of ceramic composites in the TiSi,]2MgO.2A1203.5SiO 2 system. The composite can be electrically non-conducting or electrically conducting, depending on the compositions and heating rates used during the sintering process. The conducting composite comprises an integral surface layer of electrically nonconducting glass ceramics surrounding an electrically conducting core, all formed during the sintering operation. Both types of composites are formed by conventional powder- processing routes a n d pressure-less sintering. These are also found to be oxidation r e s i s t a n t in air up to 1400°C. The effects of various processing p a r a m e t e r s on the microstructures, the electrical conductivity of the core and the thickness of the insulating surface are presented and discussed.

1. I N T R O D U C T I O N The development of thick film heaters on a substrate, as shown schematically in Fig.l, requires a protective glaze on the metallic conducting inks used as h e a t i n g elements. Since the metallic conducting inks are prone to oxidation, a s t u d y was initiated to look at the possibility of substituting metallic inks by metallic silicides. It is well known t h a t MoSi,~WSi 2 are capable of being u s e d as h i g h - t e m p e r a t u r e heating elements in air up to 1800°C. These silicides require t e m p e r a t u r e s >1500°C to densify, which m e a n s t h a t the glass-ceramic coated m e t a l s u b s t r a t e will not be able to w i t h s t a n d the sintering t e m p e r a t u r e s . One of the r e q u i r e m e n t s is the ability to sinter the conducting inks preferably below the g a m m a - a l p h a transition in the steel s u b s t r a t e to avoid distortion. This consequently places stringent r e q u i r e m e n t s on the m a t e r i a l s s y s t e m s t h a t can be incorporated in the thick film heater plates. This p a p e r presents the results of some p r e l i m i n a r y studies on the sinterability and electrical conductivity of TiSi,~ containing cordierite glasses. 0924-0136/93/S06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

466

2. EXPERIMENTAL T i t a n i u m disilieide of 98.5% purity from BDH with a mean size ,.)f aboui i() micron were &'y mixed with cordierite powders from Baikoski of m e a n siz< i.:i micron. To the blended powders, 33 parts by weights of de-ionised w a t e r m d 1 p a r t by weight of polyvinyl alcohol of molecular weight 125000 were ad&~d :_~s a solution, and the slurries wet milled for 16 hours using high-purity alumina balls and mill. The approximate particle size distribution of the resultant slurries is shown in Fig.2. The slurries were dried in a microwave over~ ~t~d sieved t h r o u g h a 250 micron aperture stainless steel sieve. The powders wer~ t h e n pressed into discs using a compacting pressure of 200MPa. The discs were sintered in air from 1000"C to 1400°C with heating rates of from 20!'C/minuu to 250°C/minute. Some samples were sintered in a nitrogen a t m o s p h e > , The a m o u n t of t i t a n i u m disilicide was varied from 20% to 80% by weighi,. ~i~h~ electrical resistance, hence the electrical resistivity and conductivity, ~)i {.i~( r e c t a n g u l a r samples was measured at room t e m p e r a t u r e using a fott~-p()i~i probe. The t inch (25.4mm) diameter sintered discs with the insulating i~y(~c were cut into r e c t a n g u l a r slabs using a diamond saw'. The probe was placed ~)~ the cut surface of the slab. Table I summarises the compositions studied. Fig.5 depicts the phase diagrams for titanium and silicon showing various disilicide~,: phases and Fig.4 shows the cordierite phase in the MgO.AI~Oa.SiO~ s y s t e m

3. RESULTS AND DISCUSSIONS 3.1. S i n t e r i n g in nitrogen a t m o s p h e r e Some preliminary sintering studies were conducted in nitrogen containiiig about 1000ppm oxygen, the oxygen content was being m e a s u r e d usi~g ~J commercially available oxygen m e t e r The sample volume increased by 133% w h e n sintered at 1000°C in nitrogen without, any noticeable change or distort ioJ~ in the geometrical form. When the sample was sintered at. 1300°C for 2 h~a~s in nitrogen, the volume increased by 93%. In both cases, a gain in weight of i~'>,~ was recorded. The materials sintered at, ]300"C appeared to have: ::~m.~ mechanical strength judging from the difficulty in breaking it manually. Fia~ sample sintered at 1300"C started to show some sign of electrical eonductiv~n. All the sintered samples appeared dark grey. The results indicate t h a t a low density composite more akin to a timmed ceramic has been formed. It is possible that: the titanium disilieide has r e a c | e d with the nitrogen gas to form some new phases. The dilation could have com~ from the incomplete pyrolysis of the PVA in nitrogen. The work was ~oi p u r s u e d further, because of difficulties in achieving the required dimensional stability and electrical conductivity for the intended applications, n a m e l y the possibility of replacing the metallic conducting inks.

467

Metal (BS 430 Steel)

Glass-Ceramic

•

.

°+o+++r+

/ Connectors

Conductive Tracks

Glass Overglaze

Fig. [i] Thick- film heater plate

T a b l e [i] V a r i a t i o n of r e s i s t i v i t y a n d a p p e a r a n c e w i t h the a m o u n t of TiSi 2 in the TiSi 2 c o r d i e r i t e c o m p o s i t e Composition reference

Wt % TiSi,

Log p ( Ohm.cm )

Appearance

C5

80

-1A1

Light grey insulating layer, low resisdvity core, very small volume change, strong.

C6

60

-0.96

Light grey insulating layer, low resistivity core, very small volume change, strong.

C7

40

6.12

Light grey layer that was slightly detached from the sample at the comers, quite high resistivity core, very small volume change, strong.

C8

20

6.11

Lighl grey outer layer that was slightly detached from the sample at the comers, high resistivity core, very small volume clumge, strong.

T a b l e [2] V a r i a t i o n of layer t h i c k n e s s w i t h h e a t i n g rate for s a m p l e s s i n t e r e d

and resistivity at 1300°C

Sample

Heating rate ( "C/minute )

Log resistivity ( Ohm.cm )

Layer dlickness ( g m )!

1

250

- 1.74

30

2

75

- 1.03

25 25

3

30

-1.35

4

20

- 1.05

25

5

10

7.5

No layer

468

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01

1.

Microns Fig, [2] P a r t i c l e size d i s t r i b u t i o n s of t i t a n i u m d i s i l i c i d e mixture after 16 hours m i l l i n g ~t %

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of

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0

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4

-,.4 -,4

2

o Sintering

temperature

(°C)

470

3.2. S i n t e r i n g in air 3.2.1 Effect of s i n t e r i n g t e m p e r a t u r e s The s a m p l e s containing 80% by weight of TiSi z and 20% by weighi~ o~ cordierite, sintered at 1000°C in air for 2 hours, were found to be very e~>~k. T h e r e was some 8% increase in volume of the sintered s a m p l e a n d the m a t e r i a l s were also electrically non-conducting. Samples sintered at 1100"C using a heating r a t e of 250°C/minute for 2 hours, however, showed an incre~Jse in strength, judging from the difficulties of breaking the discs m a n u a l l y , q'h¢ sintered m a t e r i a l s were still non-conducting, indicating lack of silicide ~-rain connectivity. At 1200°C, the resistivity of the sample was lowered by 2 to :-~ oede~'~ of m a g n i t u d e , as shown in Fig. 6. Between 1200°C and 1300°C, the resistiviiy ~,f the s a m p l e s dropped rapidly; a drop of almost 9 orders of m a g n i t u d e m resistivity was recorded between 1100"C and 1300"C or above. T h e thickness of the insulating layers increased with sintering t e m p e r : ~ m e for a fixed composition, an example being shown in Fig. 6. The insul~tting layers h a d a light grey a p p e a r a n c e whilst the core of the composite was bl~_mk. It was noted t h a t the insulating layer was whiter and t h a t the eiectcicalty conducting core structure was m a i n t a i n e d even w h e n the s a m p l e s were- ~ak,m p a s t t h e i r melting points (>- 1500°C). 3.2.2. Effect of heating rates It was found t h a t for all compositions investigated the heating r a t e s h~d w~'5 m a r k e d influence on the electrical properties of the ceramic composit,:~, A critical h e a t i n g rate of g r e a t e r t h a n 25°C per m i n u t e was required to prod~w( composites with insulating layers and conducting (:ore. All the as-pressed s a m p l e s h e a t e d at the r a t e of less t h a n 25°C were completely non-con(luctin~. In fact, the samples showed no formation of an insulating layer and the bMk of the m a t e r i a l h a d a grey yellowish colour. In m o s t cases the s a m p l e s sinte~ed below 25°C per m i n u t e showed an increase in volume a n d some incre~s(~ i~, weight, without an appreciable alteration in their geometry. The effects of the h e a t i n g r a t e on the thickness of the insulating layer are s u m m a r i s e d in Tab](~ 2, whilst the effects of the heating r a t e on the electrical resistivities and ~he thickness of the insulating layers are illustrated in Figs. 7 and 8. It can be seen t h a t the heating r a t e has an over-riding effect on controlling w h e t h e r t i t a n i u m disilicideocordierite composites are electrically non-conductin~ or electrically conducting for a particular loading above the threshold values ~.ff t i t a n i u m disilicide. From a preliminary e x a m i n a t i o n (,f the n a t u r e ~f th~ crystalline p h a s e s formed, it a p p e a r s t h a t at lower heating rates, the solid-st,~te reactions b e t w e e n the t i t a n i u m disilicide and the cordierite p h a s e s are favourL~d, A n o t h e r possibility is t h a t the t i t a n i u m disilicide reacts with nitrogen in the air, which could account for the noticeable dilation in the size of the sintered discs. If the insulating layer surrounding the electrically conducting phas(~ is a s s u m e d to be due to the oxidation of the t i t a n i u m disilicide phase, t h e n ~ higher heating r a t e would result in a t h i n n e r layer as the s a m p l e is being exposed to shorter time at a particular t e m p e r a t u r e . Since the insulating layer

471

4O Zh V

/

U .U 14

~0

/ Fig. [6] V a r i a t i o n of the t h i c k n e s s of the i n s u l a t i n g layer w i t h the s i n t e r i n g temperature.

Im r-I

H

o 1.150

1.200

1250

Sintering

1.300

I

I

L3&O

1.400

i.4SO

temperature(°C)

35

i"

v

25

Fig. [7] V a r i a t i o n of the t h i c k n e s s of the i n s u l a t i n g layer with the h e a t i n g rate for c o m p o s i t i o n C6

16

10

0 lO

--

I 20

I 30

I

75

Heating ,'ate (°C/minute)

25O

472

""

8

o

0

Fig, [8] V a r i a t i o n of the r e s i s t i v i t y w i t h the h e a t i n g rate for c o m p o s i t i o n c6 (the samples were sintered at 1300°C for 2 h o u r s in air)

4

43

"~

2

-,4 43 o

(2)

0

I

10

U

_ _ _ _ f

. . . . . . .

~.

. . . . . . . . . . . .

20 30 75 H e a t i n g rate ( e C / m i n u t e )

] 1

~'%¢

8

0

6

-r-4

4

Fig, [9] V a r i a t i o n of r e s i s t i v i t y with wt% TiSi2(the composite was s i n t e r e d at 1300°C; the h e a t i n g rate was 250°C/minute; and the r e s i s t i v i t y was measured p e r p e n d i c u l a r to the d i r e c t i o n of presssing) ~

0 I

1

,I

,

, t

Wt% Tisi2(in TiSi2/Cordierite composite)

473 is thicker, this indicates t h a t the mechanism(s) of formation of the insulating l a y e r is somehow more involved t h a n just oxidation of the t i t a n i u m disilicide. A plausible explanation as to how the higher heating r a t e could r e s u l t in the f o r m a t i o n of the insulating layer m a y be due to the surface of the s a m p l e being at a n appreciably higher t e m p e r a t u r e s t h a n the bulk, t h u s causing the r a p i d oxidation of the t i t a n i u m disilicide. It is therefore conceivable t h a t at the s a m e t i m e the surface oxidised products of the t i t a n i u m disilicide r e a c t with the cordierite to form an impervious glass ceramic insulating layer, since at higher t e m p e r a t u r e s the viscosity of the glass ceramics will be reduced sufficiently to cause the surface layers to flow. This explanation is consistent with the increase in the thickness of the layer with increasing h e a t i n g r a t e s and also with increasing sintering t e m p e r a t u r e s . 3.2.3. E f f e c t o f c o m p o s i t i o n C e r a m i c composites containing up to 800/0 by weight of t i t a n i u m disilicide were studied. It was found t h a t the resistivity s t a r t e d to show a m a r k e d decrease at a b o u t 40% by weight. When the a m o u n t of t i t a n i u m disilicide was increased to about 60% the resistivity values had dropped by 7 orders of m a g n i t u d e . F u r t h e r increase in t i t a n i u m disilicide showed very little decrease in the resistivity. The s h a r p drop in the resistivity indicates p r e s u m a b l y t h a t a percolation b e h a v i o u r is occurring and t h a t the degree of connectivity of t i t a n i u m disilicide phases has increased markedly, resulting m o s t likely in point-to-point contact. The detail microstructures at and around the threshold values of electrical conduction need f u r t h e r study. It was found also t h a t the insulating layer thickness surrounding the conducting core increased with an increase in the cordierite content for all samples sintered with a h e a t i n g r a t e of 250°C per minute. It is of interest to note t h a t the slope of the g r a p h of log resistivity versus composition, as shown in Fig. 9, is less steep t h a n the slope of log resistivity versus sintering t e m p e r a t u r e s (Fig. 5). This suggests t h a t it would be m u c h easier to control the resistivity of the sample by varying the composition t h a n b y trying to obtain a particular resistivity value by altering the sintering t e m p e r a t u r e s b e t w e e n 1000°C - 1300°C. 3.3. L o n g - t e r m t h e r m a l s t a b i l i t y Table 3 s u m m a r i s e s the result of the variation of layer-thickness and weightgain after firing samples at 1300°C for approximately 500 hours in air using a heating r a t e of 10oc per minute. It can be seen t h a t the thickness of the insulating layer increases with the cordierite content. However, it m u s t be stressed t h a t the samples containing less t h a n 400/o of t i t a n i u m disilicide, as in the case of composition n u m b e r C8, were entirely non conducting. The weightg a i n after 500 h o u r s exposure was almost negligible, a m o u n t i n g to about 5x10 -9 to 20x10 9 g/s at m o s t at 1300°C. These results show t h a t the composites obtained h a v e good oxidation resistance at 1300°C, the long-term oxidation b e h a v i o u r also a p p e a r i n g promising. However, f u r t h e r work will be required to establish w h e t h e r or not the long t e r m oxidation resistance can be increased by several orders of magnitude.

474

It was observed also t h a t a composite with an insulating layer s u r r o u n d i n g the conducting core h a d a self-heating property. For instance, w h e n a s a m p l e t h a t was originally sliced with a diamond saw was later r e h e a t e d in air at 1300°C with a heating r a t e of 10°C per m i n u t e and held for some 500 hours, an i n s u l a t i n g a r e a reformed on t h a t particular cut surface. It is also of int~rest to note t h a t in this case the surface layer thickness was only a b o u t 45 micron. This f u r t h e r indicates t h a t when the sample is sufficiently dense, su.~!h ,, t h i c k n e s s of the insulating layer is adequate to provide oxidation resistanc~ tbr the conducting core for quite prolonged periods. In the case of the fairly dense sample, it a p p e a r s t h a t oxidation occurred quickly on the surface to form a~ impervious layer. However, t h a t w h e n a porous compact disc was h e a t e d with a r a t e of less t h a n 25°C, the resulting sintered m a t e r i a l s was completely no~conducting. Moreover, there was an increase in volume of about 16.3% without a n y noticeable distortion or change in the original geometry. The tow densit,y of the composites together with its good oxidation resistance and a p p a r e n t high s t r e n g t h are an interesting combination of properties with potential commerci,~l applications. The theoretical density of the composites e s t i m a t e d using the m~te of mixture, a s s u m i n g there is no interaction between the p h a s e s and !he s u r r o u n d i n g e n v i r o n m e n t , ranges from 2.99 g/co for an 800/0 cordierite content composite to 3.89 g/cc for an 80% t i t a n i u m disilicide composite. For an incr~s~: in volume of 16.3% recorded in a 60% t i t a n i u m disilicide composite, the density will be a b o u t 87.5% theoretical (~ 87.5% of the effective density of 3.4 ~/cc) a s s u m i n g no p h a s e change or interactions. However, the density of the completely oxidised sample with this composition was a b o u t 2.5 g/c~:. This m e a n s t h a t either the composite is highly porous (only 73.5% dense a s s u m i n g no p h a s e interactions) or composites with very different p h a s e s h a d been formed. Since the X-ray diffraction p a t t e r n indicated the f o r m a t i o n of highly crystalline p h a s e s of as yet unidentified nature, f u r t h e r studies on the syst~-m to identify a n d quantify the phases present in these light-weight composites would be useful.

3.4. X - r a y d i f f r a c t i o n s S u d i e s Some p r e l i m i n a r y studies were carried out on the composite witil the i n s u l a t i n g surface layer surrounding the electrically conducting core. "Fh~÷ insulating layer was analyzed by scraping off small pieces from the surface, the m a t e r i a l s t h e n being h a n d ground. Since the deposit was hard, it was difficult. to s e p a r a t e it from the substrate, so some of the bulk m a t e r i a l was also included in the analysis. The sample was analyzed using a single- crystal silicon s u b s t r a t e in a Philips multi-purpose XRD S y s t e m MAD 1880 at Philips Application Laboratory, Cambridge, UK. The following conditions were used: 40 Kv/50 ma; copper K a radiation with slit divergence of I°C; receiving slit; set at 0.02 ram; and scattering slit at I°C. No filter was used a n d the scan r a n g e was f r o m 4-60°C for 20. The total scan time was 3 hours. T h e XRD r e s u l t s are s h o w n in Figs. 10(a), (b) and (c), The following p h a s e s were identified : TiSi2, TisSia, SiO2, low cristobalite, Mg2A14SiO,s (tndialite), t o g e t h e r with a small a m o u n t of rutile and anatase. T h e r e are also some unidentified peaks, these being m a r k e d with a dot. Indialite a p p e a r s to be a p o l y m o r p h of cordierite.

475

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15.0

20.0

300

250

Fig. [10a] XRD s h o w i n g low c r i s t o b a l i t e .

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the p r e s e n c e

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I

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30.0

35.0

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Fig. [10b] XRD s h o w i n g small a m o u n t of f u t i l e and anastase.

20

476

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1.00

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15.0

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40.0

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15 0

20.0

25.0

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350

400

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Fig. [10c] XRD s h o w i n g the p r e s e n c e of TiSi 2, Tissi 3 and i n d i a l i t e (a polym o r p h of c o r d i e r i t e 2MgO.2AI203.5SIO2) 1.2 P

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Fig. [lla] Thermal expansions of Tisi 2 (1), cordierite (2), and 80% Tisi 2- 20% c o r d i e r i t e (2MgO.2AI20~.5SiO~)(3)

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Fig. [llb] T h e r m a l e x p a n s i o n s of s i n t e r e d a l u m i n a ( 1 ) , 80% titanium disilicide 2 0 % c o r d i e r i t e ( 2 ) , a n d 430 f e r r i t i c steel (3) . i.z I |

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expansions of 80% titanium cordierite(1), and 20% titanium cordierite composite(2),

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478

Fig. [12a] A p p e a r a n c e of i n s u l a t i n g surface layer (bright area at the top) in 80% t i t a n i u m d i s i l i c i d e - 20% c o r d i e r i t e composite.

lO0

Fig. [12b] SEM p r e s e n t i n g an e x p a n d e d v i e w s h o w i n g the interface b e t w e e n the i n s u l a t i n g layer (top) and the e l e c t r i c a l l y c o n d u c t i n g t i t a n i u m d i s i l i c i d e core (bottom).

479

Fig. [13] S E M of the c o n d u c t i n g core s h o w i n g a h i g h d e g r e e of c o n t a c t b e t w e e n the d i s i l i c i d e grains. V o i d s are v i s i b l e also.

480 F r o m the above investigation it a p p e a r s t h a t m o s t of the non-conducting layer' contains low cristobalite and a small trace of futile a n d a n a t a s e , a n d the bulk consists of TiSi,, and TisSi 3. Indialite (most likely a p o l y m o r p h of cordierite: 2MgO.2A120a.5SiO2) is expected to be present both on t h e surface a n d in the bulk, since the cordierite p h a s e is incorporated uniformly in the c e r a m k ' composite t h r o u g h the prolonged wet milling process. The p h a s e s identified suggest t h a t the t i t a n i u m disilicide first oxidises to various p o l y m o r p h of titania a n d low cristobalite. These phases, eogether with the cordierite pha~se incorporated in the composite, p r e s u m a b l y react to form a protective, insulating a n d oxidation-resistant layer. The insulating layer, moreover, a p p e a r s to be impervious to f u r t h e r diffusion of oxygen from the s u r r o u n d i n g a t m o s p h e r e . e v e n at elevated t e m p e r a t u r e s . Some more detailed studies, however, will be n e e d e d to confirm the n a t u r e of the reactions and p h a s e transformat;Jons occurring on the surface.

3.5. T h e r m a l e x p a n s i o n T h e t h e r m a l expansions of the t i t a n i u m disiticide/cordierite composites were e v a l u a t e d on a Netzsch high t e m p e r a t u r e dilatometer. The t h e r m a l expansion of the composites decreased, as expected, with increasing cordierite compos]tJ¢>n, since the l a t t e r is a low-expansion phase. All the t i t a n i u m disilicide/cordierite composites have expansion values lower t h a n those of steel a n d alumina. The composites are therefore likely to have good t h e r m a l shock resistance, The t h e r m a l expansions are shown in Figs. ll(a), (b) and (c), 3.6. M i c r o s t r u c t u r e s The m i c r o s t r u c t u r e s of the t i t a n i u m disilicide/cordierite composites sini~ced using a h e a t i n g r a t e above 25°C per m i n u t e are c h a r a c t e r i s e d by the f o r m a t i o n of a n insulating layer a n d an electrically conducting core, as s h o w n in Figs° 12(a) a n d 12(b). Generally the structure of the non- conducting layer a p p e a r s coarser t h a n the bulk; some porosity can be seen also in the insulating l:ayer, A typical microstructure of conducting core of t i t a n i u m disilicide/eordierite composite is shown in Fig. 13. It can be seen t h a t the m i c r o s t r u c t u r e is c h a r a c t e r i s e d by a high degree of connectivity b e t w e e n the various t i t a n i u m disilicide grains. Some large voids can be seen also+

4.CONCLUSION F r o m the results, it is concluded t h a t : (1) A family of m o d e r a t e l y strong and electrically- conducting ceramic composite w i t h a n integral insulating layer can be formed in the system TISiJ2MgO.2AlzO3.5SiO 2 between 1100°C and 1300°C in air using a heating r a t e in excess of 25°C per minute, the preferred heating r a t e being in excess of 100°C per m i n u t e to effect the rapid formation of the insulating surface layer. A plausible explanation for the formation of the insulating surface layer with h e a t i n g r a t e s h a s been advanced.

481 (2) The insulating layer is found to provide good oxidation resistance to the materials at 1300°C for over 500 hours. (3) The moderately dense sintered composite with the insulating layer has a self-healing property, namely, an inherent capability of the system to reform an oxygen-impervious, oxidation-resistant layer on a cut or fracture surface when exposed in an oxidising environment. (4) A family of moderately strong, dense and light weight non-conducting ceramic composites is obtained when the heating rate is below 20°C per minute. These composites also have good oxidation resistance at 1300°C for over 500 hours, with the potential of being developed into high specific-strength oxidation resistant composites. (5) The phases on the surface of the insulating layer consist mostly of low cristobalite, a trace of rutile and anatase, with indialite and some unidentified phase(s). (6) The phases in the bulk are TiSi,~ and Ti3Sis, indialite and possibly some u n k n o w n phase(s). (7) The inability to sinter to high density at the required t e m p e r a t u r e s is a t t r i b u t e d to the r a t h e r coarse particle size distributions of the resulting mixtures, despite intensive milling. (8) F u r t h e r work is needed to enhance the densification and reduce the sintering t e m p e r a t u r e s of titanium disilicide ceramic composites.

REFERENCES o

2. 3. .

S P Muraka, Silicides for VLSI - Academic, New York, 1983. Handbook on Superkanthal, Bullen-Kanthal, 1987. I E Campbell and E Sherwood, High T e m p e r a t u r e Materials and Technology, Wiley, 1967 E.M. Levin, C.R. Robins and H.F. McMurdie, Phase Diagrams for Ceramicists, p.246. Fig.712. Edited and published by American Ceramic Society, Inc., 1964.

ACKNOWLEDGEMENTS

The authors would like to t h a n k the Director of T h o r n Emi Central Research Laboratories, Hayes, Middlesex, UB3 1HH, UK for permission to publish the paper. We would like to express our t h a n k s to Philips Laboratories in Camdridge, UK for providing the facilities for the XRD work. P. Hing would like to t h a n k Prof. Fong Hock Sun, Head of the Materials Division, School of Applied Science, Nanyang Technological University, Singapore, for the encouragement to take part in the First Asia Pacific Conference on Materials Processing and Dr. Zhu Dewei for useful comments on the paper.