IMPROVEMENT OF THE WEIBULL MODULUS OF CERAMICS PRESSED WITH A VIBRATORY ASSISTANCE

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IMPROVEMENT OF THE WEIBULL MODULUS OF CERAMICS PRESSED WITH A VIBRATORY

ASSISTANCE

F. Baylet, B. Rogeaux, P. Boch

To cite this version:

F. Baylet, B. Rogeaux, P. Boch. IMPROVEMENT OF THE WEIBULL MODULUS OF CERAMICS

PRESSED WITH A VIBRATORY ASSISTANCE. Journal de Physique Colloques, 1986, 47 (C1),

pp.C1-85-C1-89. �10.1051/jphyscol:1986113�. �jpa-00225527�

JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment au n02, Tome 47, fkvrier 1986 page cl-85

IMPROVEMENT OF THE WEIBULL MODULUS OF CERAMICS PRESSED WITH A VIBRATORY ASSISTANCE

F. BAYLET, B. ROGEAUX and P. BOCH

E.N.S.C.I., U A C.N.R.S. 320, F-87065 Lirnoges, France

R6sum6 - Les d6fauts de compaction introduits lors du pressage des poudres interviennent fr6quement c o m e microfissures initiant la rupture fragile des c6ramiques. Une assistance vibratoire 2 la compaction r6duit la dispersion de taille et limite la s6v6rit6 des d6fauts, ce qui entrahe une augmentation du module de Weibull. Lf6tude du pressage de differentes poudres ceramiques mon- tre que c'est sur le &arrangement initial des agglomgrats que se manifeste l'effet des vibrations.

Abstract - The defects of compaction induced during the pressing of powders frequently act as microcracks which initiate the brittle fracture of ceramics.

A vibratory assistance to the compaction reduces the scattering of size and limits the severity of flaws, which leads to an increase of the Weibull modu- lus. The study of the compaction of different ceramic powders shows that the vibrations mainly influence the initial rearrangement of agglomerates.

I

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The basic equation of linear fracture mechanics relates the strength of a piece (af) to the toughness of the material (Kc) and the "equivalent" size of the critical

"microcrack" (ac) :

(1) Kc

Of % Y V

(where Y is a dimensionless constant which depends on the geometry of the loading and on the microcrack configuration).

Many efforts have been made to increase the toughness of ceramics, in developing ne:v materials : silicon nitride, silicon carbide, partially stabilized zirconia etc. However, such efforts would be useless if they were not accompanied by a cor- relative improvement of the processes to limit, or even to decrease, the "ac" value.

For ceramics processed in the usual way (powders -t forming +thermal treatments and sintering) there is a general agreement on the importance of the forming stages.on the occurence of microcracks. For instance gross voids can be introduced by large powder agglomerates, and circumferential cracks can be induced by differential sin- tering in inhomogeneously compacted bodies /1/2/. A bad control of "ac" does not allow one to control the scattering of strengths in a batch of identical pieces, i.e. it leads to a low Weibull modulus : this is the main problem for the development of engineering ceramics, because most potential users are more afraid of such a scattering than of the value of the mean strength in itself. This study is thus de- voted to a technique - the vibratory assistance to the pressing

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986113

J O U R N A L DE PHYSIQUE

I1 - EXPERIMENTAL

Fig. 1 shows the schematic of the experimental equipment. The pressing of powders is performed at R.T. (20°C) in a floating cylindrical die, where the inferior punch is mechanically coupled with one or another of two devices, able to generate acous- tic waves in the ultrasonic (20 kHz)z/3/ or in the sonic (50 Hz) range /4/. The amplitudes of the vibrations of the punch are = 50 at low frequencies, and can vary from 10 to 40 um at high frequencies.

The samples are disc-shaped, 30 mm in diameter and = 4 mm in thickness in the as- pressed state, = 25 mm in diameter and = 3 mm in thickness (depending on shrinkage)

in the sintered state. Various ceramic powders are studied (alumina, silicon carbide, SiAlONS, tungsten carbide) ; however, all are in the micron or the submicron range

(e.g. A 16 SG Alcoa or RC 172 DBM Reynolds alumina, with a mean particle size of 0.6 jlm). Various organic binders are used, and all the powders are agglomerated into free-flowing granules (e.g. by spray-drying, or by screening through sieves) before pressing.

The sintering can be performed in a laboratory furnace (electric heating, low ther- mal inertia) or in industrial furnaces. The latter case is chosen for SiAlONS.

I1 - RESULTS AND DISCUSSION

Neither the high frequency nor The low frequency vibrations increase the green den- sities of compacts by more than = 0.5 to 1 % (green densities depending on the natu- re of the powders and on the maximum pressing pressure, but always remaining between 56 and 60 %). This shows the vibratory assistance does not noticeably improve the pressing efficiency. However, it can improve the mechanical properties of sintered samples /3/, by reducing the microcrack severity.

The samples are broken by using a biaxial flexure technique /5/, and the strength data are analyzed in accordance with the usual two-parameter Weibull probability function :

'f m (2) F = 1

-

1

' 0

where "m" is the Weibull modulus. The LnLn (1/F) \is Ln of diagrams are drawn by using at least 30 samples, to obtain sufficient accuracy (6).

Fig. 2 and 3 show the results for alumina and SiAlON materials, pressed without and with ultrasonic assistance (US). The US is clearly beneficial, because it leads to an increase of "m" from 6.5 and 6 (without US) to 15 and 14 (with US). The highest strength values are not sensibly increased, contrary to the lowest ones, which agrees with the previous statement that the US assistance results in a decrease of the "equivalent" size of the largest flaws.

Fig. 4 shows the compaction response of alumina powders without and with US.lVithout m e p/po vs Ln P diagram exhibits its usual features : two linear domains, with a slope variation which defines the yield point. The low pressure domain is usually attributed to the particle sliding and the agglomerate rearrangement, whereas the yield point is the beginning of the particle deformation and the agglomerate fractu- re, such a yield point being a qualitative index of the fracture stress of agglome- rates /7/8/. With US, the density under low pressures (< 0.1 MPa) is strongly increa- sed (by = 20 %), which means the US assistance is "equivalent" to an increase of pressure of about one order of magnitude. Besides, the yield point is not so well defined as it is without US. However, in the high pressure field (> 20 ma), the density is only slightly improved by the US (0.5 to 1 %, as previously noticed).

Hence, the ultrasonic assistance appears to improve the particle sliding and/or the agglomerate rearrangement, and to lead to a more homogeneous state with a better packing, which is shown by the higher density. Beyond the yield point the increasing pressure results in a progressive elastic deformation of particles and a breaking of the softest agglomerates, but it does not seem to modify the main features of the

x French Patent no 82.07701 (Legrand, Limoges)

packing. That means even a high pressure cannot heal some "vault effects" around loosely packed hard aggregates. Thus the l a r g e s t microcracks appear t o be induced by the i n i t i a l stage of the pressing. This i s confirmed by t e s t s where the US is applied i n d i f f e r e n t ranges of pressure. For a low pressure US (point A i n Fig. 4, and curveA i n Fig. 5) t h e Weibull modulus is = 16, whereas f o r a high pressure US (point B i n Fig. 4 and curve B i n Fig. 5) it is = 10 only. A l l powders lead t o similar r e s u l t s , i . e . the US has a beneficial influence on the "m" value (with a typical increase from

= 8-10 t o = 12-16) only i f applied a t pressures below the y i e l d point. The highest values of "m" here found (= 16) may seem rather low i n comparison with c e r t a i n values given i n t h e l i t e r a t u r e . However, we indeed found m < 16 i n most of t h e t e s t s we per- formed on some commercially available ceramics, advertised t o have a high Weibull modulus. Finally, it must be underlined t h a t t h e specimens were broken i n t h e i r as- sintered s t a t e , without any surface machining, and t h a t a polishing should lead t o higher values f o r both "af'' and "m".

A s f a r a s the amplitudes of vibrations a r e concerned, the lowest amplitude (= 10

m)

always leads t o b e t t e r r e s u l t s than t h e highest one (= 40 pm). Working on t h e low frequency (50 Hz) device, a t a v i b r a t i o n a l amplitude of = 50 pn, leads t o similar r e s u l t s a s the 40 um/20 kHz equipment, which does not make it possible t o judge a possible influence of t h e frequency.

I11 - CONCLUSION

This study points out t h e importance of t h e low pressure stage i n the pressing of ceramic powders and shows t h a t a bad i n i t i a l rearrangement of agglomerates leads t o flaws - probably gross voids as t h e one shown i n Fig. 6, which corresponds t o the f r a c t u r e surface of an alumina sample pressed without the US - which cannot be e l i - minated by t h e subsequent high pressure stage. I t a l s o demonstrates t h a t a noticeable improvement can be obtained with t h e help of u l t r a s o n i c assistance.

IV

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The authors g r a t e f u l l y acknowledge D r . W. MUSTEL, from CERAVER, f o r t h e supplying of SiAlON materials.

V

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/1/ Dynys, F.W. and Halloran, J . W . , J . Am. Ceram. Soc.

66

67

(1984) 596.

/ 2 / Kellet, B. and Lange, F.F., J . Am. Ceram. Soc.

67

/3/ Rogeaux, B . and Boch, P., i n press i n J. Mat. Sc. Lett (1985).

/4/ Rogeaux, B., Thesis, Limoges (1985).

/5/ Shetty, D.K. e t a l . , Am. Ceram. Soc. Bull.

59

/6/ Glandus, J . C . and Boch, P., J. M a t . Sc. L e t t .

3

/ 7 / Frey, R.G. and Halloran, J . W . , J. Am. Ceram. Soc. -- 67 (1984) 199.

/8/ Dimilia, R.A. and Reed, J.S., i n "Forming i n Ceramics", Mangels and Messing E d i t , Am. Ceram. Soc

.

/9/ International Commission on Glass, 'Tech. Conunitte 6 (1984).

J O U R N A L DE PHYSIQUE

? 4

// ////// microns

Fig. 1 Schematic of the devices for vibration assistance

Fig. 2 Iieibull's diagram for A16SG Fig. 3 Weibull's diagram for Ceravel Alcoa alumina, pressed (1) without US, SiAlON, pressed (1) without US,

( 2 ) with US (2) with US

0.0 1 0 1 1 10 100

PRESSURE MPa

I.'ig. 4 Ilffcct of IIS on compaction of IIC1721)13M Ileynolds al~nn i r x t powders : (1) without. 1 6 , ( 2 ) with \IS

Fig. 5 Weibull's diagram f o r RC Fig. 6 Fracture i n i t i a t i n g void i n 1 7 2 DBM pressed with US : ( 1 ) US alumina pressed without US.

a p p l i e d below t h e y i e l d p o i n t (A, i n Fig. 4 ) ; ( 2 ) US a p p l i e d beyond t h e y i e l d p o i n t ( B , i n Fig. 4 ) .