J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 6 ( 1 9 8 7 ) 2 7 7 - 2 8 0

Effect of glass, rice-husk ash and wollastonite on transverse strength of porcelain

T. K. DAN, NAVIN CHAND, P. K. ROHATGI Regional Research Laboratory (CSIR), Hoshangabad Road, Near Habibganj Naka, Bhopal 462 026 (M.P.), India

In o rde r to reduce energy consumpt ion , a t t empts are now being made at add ing at t i t ives to ceramics. In the previous studies we have observed tha t add i t i on o f a small percentage o f addi t ivies such as glass, r ice-husk ash and wol las toni te to porce la in can reduce the firing t empera tu re of the t r iaxial body to a grea t extent [1, 2]. In this let ter the effect o f add i t ion o f glass, r ice-husk ash and wol las toni te on the s t rength and bond ing o f porce la in has been observed. A t t e m p t s have been made to app ly the Spr igg 's equa t ion for the s t rength o f such compos i tes and s t r e n g t h - d e n s i t y dependence is observed. The fracture surfaces o f these composi t ies have been observed by scanning e lec t ron microscopy .

China clay, flint, ball clay, waste glass powder and wol las toni te were ob t a ined f rom different sources and their chemical analysis was pe r fo rmed using the wet

chemical analysis m e t h o d (Table I). Rice husk ash was p repa red by firing washed rice husk.

R a w mate r ia l s were separa te ly g r o u n d to pass t h rough a 200 mesh B.S. Sieve. The conven t iona l porce la in mix (china clay 50%, fe ldspar 25% and flint 25%) was taken as the base mater ia l . Different po r t ions o f r ice-husk ash, glass and wol las ton i te were a d d e d to the porce la in (Table II) . The specimens were p repa red in the form o f rods by slip cast ing, using p las ter o f par is moulds . To ob t a in the p rope r consis tency o f the slip, a small quan t i t y o f e lectrolyte (1 :1 ra t io sod ium ca rbona t e and sod ium silicate mixture) was found essential in each case, bu t the a m o u n t var ied with the compos i t i on o f the body . The specimens were dr ied at 110 ° C for 48 h and were fixed at different t empera tu res between 1000 and 1200 ° C in

T A B L E I Chemical analysis and mineral composition of raw materials

Elements China clay Feldspar Quartz Rice-husk Glass powder Wollastonite (Kerala) (Tamil Nadu) (T.N.) ash (Kerala) (Rajasthan)

SiO 2 45.91 68.66 99.40 A1203 37.21 18.31 0.41 Fe203 0.90 0.24 TiO 2 0.67 Trace CaO Trace 0.55 0.08 MgO 0.27 0.36 0.04 K20 0.46 8.36 - Na20 0.19 3.22 Loss on ignition 14.37 1.20

Mineral composition Kaolinite 93.04 8.38 - Quartz 0.06 14.21 99.08 Feldspar 4.50 74.71 - Haematite 0.90 0.16 Rutile 0.67 - Calcite - - Magnesite 0.55 - Serpentine 2.44 - Carbonaceous matter 0.28

95.26 73.50 48.77 0.72 1.40 0.66 0.16 - 0.43 0.06 - Traces 0.78 9.80 48.02

0.60 0.06 0.67 - 0.02 0.46 14.70 0.11 1 .02 - 1 .68

T A B LE I I Composition of the mixes studied (wt %)

Batch no. China clay Feldspar Quartz Rice-husk Glass Wollastonite ash powder

I 50 25 25 II 45.00 22.50 22.50 lII 40.00 20.00 20.00 IV 35.00 17.50 17.50 V 45.00 22.50 22.50 VI 40.00 20.00 20.00 VII 35.00 17.50 17.50 VIII 47.50 23.75 23.75 IX 45.00 22.50 22.50 X 42.50 21.25 21.25

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Figure I Transverse strength as a function of bulk density of (a) glass-porcelain composite, (b) rice-husk ash-porcelain composite, (c) wollastonite porcelain composite. (e) Standard porcelain, and porcelain with: ( × ) 10% rice-husk ash; (zx) 20% rice-husk ask; (n) 30% rice-husk ash; (v) 10% glass powder; (®) 20% glass powder; (o) 30% glass powder; (50) 5% wollastonite; (v) 10% wollastonite; (A) 15% wollastonite.

an electric glober muffle furnace with a soaking period of 3 h at the maximum temperature. The physical properties in terms of bulk density, apparent porosity and transverse strength were determined according to ASTM no. C373-72.

Fig. l a to c shows the transverse strength plotted against bulk density of porcelain with glass, rice-husk ash and wollastonite, respectively. It is common in all cases that the density increases linearly with increase in transverse strength of the composite. It is also observed that at a particular density, increase in volume fraction of rice-husk ash, glass and wollaston- ite increases the transverse strength. This indicates

Figure 2 Transverse strength as a function of porosity of (a) rice- husk asMporcelain composite. (O) Standard porcelain, and por- celain with: (v) 10%, (n) 20% and (zx) 30% rice-husk ash; (b) glass-porcelain composite (D) 10%, (zx) 20% and (v) 30% glass powder and porcelain; (c) wollastonite porcelain composite (o) 5%, (zx) 10% and (v) 15% wollastonite and porcelain. The solid curve is Equation 1 for pure porcelain indicating the fit of Spriggs equation.

that densification takes place in the following way: (a) the higher the softening point of the composite, the higher the temperature needed for a certain degree of densification. By addition of additives, the softening point of the base triaxial porcelain body had been altered; (b) if the amount of liquid phase is greater at a particular temperature, densification takes place faster. On the other hand, if an excessive amount of liquid phase formation takes place, it has adverse effects on the properties. The amount of liquid phase should be such that it just acts as a bond between the grains. This amount of liquid phase varied with the nature of additives and in its amount, which actually governed the degree of diversifcation; (c) by addition of additivies, it is possible to change the crystalline nature, its amount and size in the finished products; (d) the larger the particle size of the additives, the higher is the temperature needed for a certain densification.

Fig. 2 shows the transverse strength plotted against porosity for pure porcelain, porcelain-rice-husk ash (a), porcelain-glass (b) and porcelain-wollastonite (c) composites. These plots were made to understand the dependence of strength (a) on porosity (p) to fit Spriggs empirical equation [3].

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Figure 3 Hashin Hasselman plot for the composites. (a) (O) 10% and (A) 20% glass powder and porcelain, (v) 10% rice-husk ash and porcelain. (b) (o) Standard porcelain, and porcelain with (x) 5%, (Lx) 10% and (rq) 15% wollastonite.

zero porosities, respectively, and a is a constant . It is observed that the equat ion does not fit well except in the case o f pure porcelain. For porcelain composi te another empirical equat ion has been tried by rearranging Hash in -Hasse lman ' s equat ion [4] as follows:

E = Eo 1 + 1 - (A + 1)p (2)

where E is Young ' s moudlus, E0 is zero porosi ty modulus, and p is porosi ty and A is a constant . The above equat ion can be written for strength as follows:

Figure 4 Scanning electron micrographs of the fracture surface of the composites. (a, b) Pure porcelain, (c) 30% rice-husk ash- porcelain composite, (d) 30% glass~orcelain composite, (e) 15% wollastonite porcelain composite.

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Ap ) (3) a = a0 1 + I _ ( A + 1)p

where a and tr 0 are the transverse strength, and transverse strenth at zero porosity, respectively, and A is a constant and p is porosity. On rearranging the above equation can be written:

1 1 Kp - + - - ( 4 )

tr tr 0 1 - p

where K is a new constant = - (A)/tr o. Fig. 3a and b shows the fit of above equation for rice-husk ash- porcelain, glass-porcelain and pure porcelain, and wollastonite-porcelain composites.

Fig. 4a to e shows the fractographs of porcelain, porcelain-rice-husk ash, porcelain-glass and porcelain- wollastonite composites. From the pictures it is revealed that by addition of additives it is possible to change the crystallographic nature which is the main factor influencing strength of the final products. Because sintering of a ceramic is generally governed

by the diffusion route, it is possible to change the crystalline nature in the final products by changing the amount and composition of the liquid. On addition of rice-husk ash and wollastonite, crystallinity increases. This is probably not due to nucleation of the additive. Crystal growth takes place from the glassy phase on the finely divided particles by a diffusion mechanism, whereas on addition of excess glass there is not such a remarkable change in crystallinity.

References 1. T. K. DAN and K. J A Y A C H A N D R A N , Indian Ceram. 26

(9) (1983) 163. 2. Idem, Res. Ind. to be published. 3. R. SPRIGGS, J. Am. Ceram. Soc. 44 (1961) 628. 4. Z. HASHIN, J. Appl. Mech. 29 (1962) 143.

Received 31 July and accepted 19 August 1986

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