SUBCRITICAL CRACK GROWTH IN METAL-CERAMIC-INTERFACES

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SUBCRITICAL CRACK GROWTH IN METAL-CERAMIC-INTERFACES

M. Müller, K. Kromp, V. Gerold

To cite this version:

M. Müller, K. Kromp, V. Gerold. SUBCRITICAL CRACK GROWTH IN METAL- CERAMIC-INTERFACES. Journal de Physique Colloques, 1988, 49 (C5), pp.C5-653-C5-658.

�10.1051/jphyscol:1988584�. �jpa-00228081�

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Colloque C5, suppl6ment au nolO, Tome 49, octobre 1988

SUBCRITICAL CRACK GROWTH I N METAL-CERAMIC-INTERFACES

M. ~ L L E R ,

K.

KROMP and

V.

GEROLD

Max-Planck-Institut fiir Metallforschung, Seestrasse 9 2 , 0-7000 Stuttgart 1, F.R.G.

Nous presentons un m6thode d'estimation de la solidit6 des 11adh6sion de films m6- talliques minces sur des substrats d'alumine. L16tude porte sur la propagation d'une fissure d'interface lors d'un essai de flexion 3 points. Le matiere est compos6 d'une couche d'alliage AgPd de 10 i 50 pm d16paisseur et d'un substrat d'alumine dense

i

96%. Les Qprouvettes de flexion obtenues par collage de blocs d'alumine sur le substrat d'alumine et sur le film m6tallique sont entaillees tr6s pr6s du film m6tallique. La pr6fissuration

i

1' interface est r6alis6e par un chargement asymstri- que de 116prouvette. Par la suite, la propagation de fissure est observ6e l'aide d'un microscope optique situ6 mobile dans le plan de la fissure. La resistance

i

la propagation R est calcul6e

5

partir de 1 'enregistrement charge deplacement du point d'appui et de de la mesure optique directe de la longueur de la fissure.

Abstract

A method is presented to investigate the adhesion strength of thin metal films on alumina substrates. For this purpose the extension behavior of an interface crack in the composite was observed in a 3-point bending test. The composite consists of a 96-percent alumina and an AgPd-alloy with a thickness of 10-50 pm. A compound system is produced from the composite by glueing alumina blocks to the alumina substrate and to the metal film respectively. Bending specimens are cut out of this compound system and are notched near the interface layer. By asymmetrical preloading an interface crack is produced as a crack starter. During the following bending test the controlled crack-extension is observed with a travelling microscope. The load- displacement diagram and the optically measured crack length are used to calculate the crack resistance R, which characterizes the adhesion strength of the metal film.

1.

Introduction

The multitude of applications for metal-ceramic composites has resulted in a steady increase in their significance. Because of the very different chemical and physical properties of metals and ceramics, problems with the adhesion of the composites often arise during production. The usual testing methods for adhesion strength are pull-out, shearing- and peel-tests. The results of these tests depend strongly on the specimen preparation and thus they do not characterize the adhesion strength of composites very we1

1

.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988584

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Here a method i s presented which gives accurate experimental r e s u l t s t o describe t h e adhesion s t r e n g t h o f t h i n metal f i l m s on ceramic substrates. The crack extension o f an i n t e r f a c e crack i n t h e t e s t e d composite i s observed i n a c o n t r o l l e d 3-point-ben- d i n g t e s t . 'The crack-resistance, which describes t h e adhesion s t r e n g t h , can be c a l - c u l a t e d from t h e

load-displacment-diagramm

and t h e o p t i c a l l y measured crack l e n g t h . The aim o f t h i s paper i s t o describe t h e specimen p r e p a r a t i o n and t h e experimental Procedure t o o b t a i n a c o n t r o l l e d i n t e r f a c e crack.

2. P r e p a r a t i o n o f t h e samples

The composite used c o n s i s t s of an alumina s u b s t r a t e ceramic and a b u r n t - i n AgPd- a l l o y . Some p h y s i c a l p r o p e r t i e s and t h e composition o f t h e AgPd-alloy are l i s t e d i n Table 1.

Table 1 : p r o p e r t i e s and composition o f t h e b a s i c m a t e r i a l s

j - z x l y

s u b s t r a t e

qua1 i t y : 96% + 4% Si02

g r a i n s i z e : 3.56 0.8 ym

E [GPa] : 340

P [g/cm31: 3.75

thermal expansion

c o e f f i c i e n t [1/K] : 4.4-8.0.10-~

composition o f t h e b u r n i n g - i n a l l o y

15 % Pd 46 % Ag

1-5% B i ,Pt,Ni ,Zn

<1 % Mg,Al

+

metal oxide

+

SiO2

+

o r g a n i c paste

A t f i r s t , the AgPd-paste i s p u t on t h e alumina s u b s t r a t e by a s p e c i a l p r i n t i n g tech- nique ( F i g . 1 ) [I]. A f t e r i t has become d r y ( i n a5r a t room temperature), t h e AgPd- paste (about 50 ym t h i c k ) i s b u r n t i n according t o a s p e c i a l temperature p r o f i l e w i t h a maximum a t 840 ^{O C .} During t h i s procedure t h e o r g a n i c p a r t o f t h e paste burns away. The r e s u l t i s a m e t a l l i c f i l m w i t h a t h i c k n e s s o f about 10 t o 25 ym.

From t h i s composite a compound system was produced by g l u e i n g an alumina b l o c k t o t h e alumina s u b s t r a t e and a second one t o t h e metal f i l m . For t h i s purpose an ad- hesive based on an epoxide i s used, which has a t o t a l e l a s t i c behaviour. Now from t h e compound system bending specimens w i t h t h e dimensions 35x7x2.8 mm3 are c u t out.

To observe t h e crack, t h e specimens a r e p o l i s h e d on one s i d e . The Young's modulus o f each compact bending specimen i s measured i n a 3-point-bending t e s t by l o a d i n g them up t o 20 N. T h i s l o a d i n g a l s o serves t o t e s t t h e compound system i n r e s p e c t t o t h e adhesive. Now t h e specimens a r e notched t o ao/W

=

^{0.45 (ao}

⁼

depth o f t h e notch, W

=

h e i g h t o f t h e specimen). The n o t c h i s placed t o t a l l y w i t h i n t h e ceramic p a r t near t o t h e metal i n t e r f a c e ; i t has a w i d t h o f about 60 t o 80 pm.

metal paste, burning i n 60 pm t h i c k

n

e l a s t i c adhesive

c u t t i n g o u t g l u e i n g o f t h e p o l i s h e d and notched

o f t h e specimens a1 umina blocks bending specimen

Fig.1. p r e p a r a t i o n o f t h e bending specimen

3. Experimental Procedure

A l l experiments a r e performed w i t h a very s t i f f 3-point-bending-system i n a servo- h y d r a u l i c t e s t i n g machine [2]. The span i s 30 mm. The displacement i s d i r e c t l y measured a t t h e lower s i d e o f t h e specimen by a LVDT. Only i n a displacment-control- l e d experiment i t i s p o s s i b l e t o have c o n t r o l l e d crack extension. To perform such experiments i t i s a l s o necessary t o have a very low compliance o f t h e whole l o a d i n g system. A f t e r a good adjustment o f t h e specimens, t h e compliance o f t h e system was 0.021 f 0.003 pm/N.

During t h e experiment i t was p o s s i b l e t o observe and measure t h e crack l e n g t h by a t r a v e l l i n g microscope. I n a d d i t i o n , t h e crack l e n g t h was c a l c u l a t e d from t h e compliance i n t h e load-displacement diagram, obtained d u r i n g t h e experiment. T h i s e v a l u a t i o n i s o n l y p o s s i b l e i f t h e specimen shows 1 in e a r e l a s t i c a l behaviour 131.

The aim o f t h e experiment was t o i n v e s t i g a t e t h e crack propagation i n t h e metal- ceramic i n t e r f a c e . For t h i s purpose t h e crack f i r s t has t o be p o s i t i o n e d i n t o t h i s i n t e r f a c e , which c o u l d be performed by asymmetric pre-loading o f t h e specimen as shown i n Fig.2. A t t h e same t i m e t h e crack t i p was observed u s i n g t h e t r a v e l l i n g mi- croscope and t h e connected video system. Due t o t h e asymmetric loading, t h e crack i s d r i v e n towards t h e i n t e r f a c e . A f t e r i t has reached it, t h e experiment was stopped and t h e sample unloaded.

Then t h e sample was loaded again, t h i s t i m e symmetrically t o t h e i n t e r f a c e . When t h e upper l o a d bearing was a c c u r a t l y adjusted, t h e displacement c o n t r o l l e d experiment r e s u l t e d i n a crack extension d i r e c t l y i n t h e i n t e r f a c e . The adhesion s t r e n g t h should be comparable t o t h e s t r e n g t h o f t h e s i n g l e m a t e r i a l s r e s p e c t i v l y . During t h e experiment t h e load-displacement diagram and t h e crack l e n g t h were measured. The displacement r a t e had a value o f 2.4 pm/min f o r a l l experiments.

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fi ^adhesive

^{( =}

5 0 pm thick)

AgPd-burning in alloy (10-50 pm thick)

n u A1203 - substrate

Fig.2. introduction o f the crack into the interface by asymmetric loading

4. Experimental Results

The base f o r all further analyses are the load-displacment diagrams together with

the optical measured crack length. A characteristic example is shown in Fig.3. It is

notable that the optically measured crack length in most cases is a little smaller

than the crack length calculated from the compliance. This could be due t o non

linear elastic or elastic-plastic behaviour o f the interface at the crack front. The

calculation of the crack length from the compliance is done assuming linear elastic

behaviour (i.e. it is assumed that each point o f t h e load-displacement diagram can

be connected t o the zero point t o calculate the crack length from alteration o f t h e

slope. A plastic deformation would result in a zero upset during unloading, the

slope would be a little higher and thus the calculated crack length a little smaller

(see Fig.

3,

optically measured crack length).

L o a d

C N 3

5 8

Temp.

-

^{2 1 OC v}

-

^{-3 um/min}

C r a c k

Cmm3

6 . 5

0

5

10

1 5 2 8 2 5

D i s p 1 a c e m e n t C u m 1

Fig.3. load-displacement curve i n c l u d i n g t h e crack-length (crack l e n g t h : ooo = o p t i c a l l y measured,

- - -

c a l c u l a t e d from t h e compliance )

To c a l c u l a t e t h e c r a c k - r e s i t a n c e (R-value) t h e f o l l o w i n g equation i s used /3/:

( B = specimen thickness; a = crack l e n g t h ; U = energy f o r crack extension; Wtot = t o t a l energy investment; W e l = e l a s t i c energy s t o r e d i n t h e specimen)

I n Fig.4 t h e R-curves o f t h e load-displacment-diagram o f Fig. 3 a r e shown. There i s a p l a t e a u corresponding t o t h e R-value o f t h e i n t e r f a c e . The i n i t i a l increase o f t h e R-curve may be caused by t h e b u i l d i n g - u p o f a process zone whereas t h e f i n a l i n c r e a - se o f t h e curve f o r l a r g e a/W values i s caused by t h e i n c r e a s i n g i n f l u e n c e o f t h e s t r e s s f i e l d c l o s e t o t h e upper l o a d i n g ram [4]. The R-value o f t h e i n t e r f a c e (R-value i n t h e r e g i o n o f t h e p l a t e a u ) i s a l i t t l e h i g h e r than t h e R-value o f t h e ceramic s u b s t r a t e . R-val ues o f o t h e r metal -cerami c i n t e r f a c e s

[5]

a r e comparable t o those o f the t h i n f i l m compounds i n v e s t i g a t e d here.

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13

- 6 5

. 7

. 7 5

. 8

. 8 5

a / W Fig.4. crack-resistance curves:

ooo = from t h e o p t i c a l l y measured crack l e n g t h

- - -

from t h e c a l c u l a t e d crack l e n g t h SP.Nr. J047C

.

^Temp.

-

^{2 1 ' C v = -3} u m / m i n ⁰

-

0 0

0

-

^OooOO 0

-

0 /

-

O 0 "9,"

O o o O O O

-

-

I I I I

5. Conclusions

A method i s described t o g e t c o n t r o l l e d crack extension i n t h e i n t e r f a c e o f a t h i n f i l m metal-ceramic compound. I t was p o s s i b l e t o c a l c u l a t e an R-value ( c r a c k r e - s i s t a n c e ) from t h e d i r e c t observation o f t h e crack extension i n t h e i n t e r f a c e and t h e measurement o f t h e dependence o f t h e l o a d on t h e displacement. A simultaneous c a l c u l a t i o n o f t h e crack extension from t h e compliance (assuming l i n e a r e l a s t i c be- h a v i o u r ) i n d i c a t e s e l a s t i c - p l a s t i c behaviour i n f r o n t o f t h e crack.

References

[I] M. M u l l e r , W.D. Vogel, K. Kromp: U n t e r k r i t i s c h e Rissausbreitung i n M e t a l l - Keramik-Grenzschichten; F o r t s c h r i t t s b e r i c h t e der Deutschen Keramischen Gesell- s c h a f t 1'387, i n p r i n t

[2] A.Bornhauser: PhD-Thesis, U n i v e r s i t y o f S t u t t g a r t , FRG, 1983

[3] A. Bornhauser, K. Kromp, R.F. Pabst: R-Curve Evaluation w i t h Ceramic M a t e r i a l s a t Elevated Temperatures by an Energy Approach using D i r e k t Observation and Com- p l i a n c e C a l c u l a t i o n o f t h e Crack Length; J. Mater. Sci., 20 (1985) 2586-2596.

[4] K. Kromp : High Temperature Creep Crack Growth i n SiSiC High Performance Ceramic Materia1:j;Seminar on t h e Development and Use o f Powder M e t a l l u r g y i n Engineering I n d u s t r i e s , Minsk, March 1985; ed. U n i t e d Nations Economic Comission f o r Europe, Geneva.

[5] W. G r o l l : PhD-Thesis, U n i v e r s i t y o f Erlangen, FRG, 1984