Mechanical properties of ion implanted glasses

253

Nuclear Instruments and Methods in Physics Research Bl (1984) 253-257 North-Holland, Amsterdam

MECHANICAL PROPERTIES OF ION IM.PLAN’IED GLASSES

G. BA’FTAGLIN ‘, R. DAL MASCHIO *, G. DELLA MEA“, G. DE MARCH1 l, V. GOTTARDI 2, M. GWGLIELMI ‘, P. MAZZOLDI 1 and A. PACCAGNELLA 3 I Unitd GNSM-CNR, Diprfimento di Fisica, Via Marzolo n.8, 35100 Padov~ Ita& 2 Istituto di Chimica Industriale, Facoltd di Ingegnerio, ’ Istituto di Fish, 41100 Modena, Ita&

Via Marzolo n.8, 35100 Paaba,

Ita&

It is well known that heavy ion implantation causes rn~~i~tions of the surface mechanical characteristics of glasses. At first a compaction is induced with increasing ion fluence, then a relief is observed after a maximum tension stress. In this paper we report the results of a study on the mechanical resistance of Ar implanted soda-lime glasses. Vickers microindentation tests have been performed by measuring both the percentage of cracks, which develop after application of a loaded square pyramidal indenter on the glass, with respect to the number of tests and the length of cracks. In order to clarify the mechanisms responsible for the ion induced mechanical modifications, the influence of the implantation parameters and the thermal stability of the modified surface layers have been investigated.

1. Introduction

Jensen et al. [l] studied the development of surface cracking produced by a Vickers diamond indenter on soda-lime glasses implanted with 480 keV H ions. The authors observed an induced compaction with increasing ion fluence (surface in tension) and a subsequent relief (surface compression) after a maximum tension stress attained at an implantation dose of 1015 +H/cm’. An improvement of the mechanical effects together with a change of the chemical durability has been observed by Chinellato et al. [2], after 50 keV-Ar+ implantation at doses lower than lOI ion/cm2, in soda-lime glasses. Arnold [3] used the shallow Knoop indenter, loaded with 15 g in order to measure hardness changes in the implanted layer of fused silica as a function of 250 keV A?+ dose. The results agree with those obtained by EerNisse [4] for 500 keV Ari implantation and indicate an induced compression (shorter cracks) for implantation doses higher than 1014 Ar/cm’ and a subsequent decrease as the ion dose increases above 1016 Ar/crr?. In this paper we extend the study of the surface mechanical m~ification due to Ar impl~tation in soda-lime glasses, including the effects of annealing processes [5].

2. Experimental Flat soda-lime glasses were implanted with Ar+ ions at 50 and 100 keV in the fluence range between XO13 and 4 X lO”j ions/cm2. The current density was maintained at a value of 1 PA/cm’. The glass chemical composition (in wt.%) was the following; SiO,, 72.0;

0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (Norm-Homed Physics Publis~ng Division)

Al,O,, 1.3; CaO, 8.2; MgO, 3.5; Na,O, 14.3; K,O, 0.3; other oxides 0.4. The sample sixes were 10 x 10 x 2 mm3. Vickers ~croinden~tion tests were. performed using a Durimet-Leitx Vickers tester with a load ranging from 15 to 500 g. The load application time was 15 s. Indenter impressions were observed by meOuls of a Zeiss optical transmission microscope 30 min after indentations. Some of the implanted samples were annealed for 30 min in a nitrogen atmosphere at low temperatures (50°C, lOO’C, 150%) and in a vacuum (typically 10m5 Torr) at high temperatures (250°C, 300°C, 350°C 4OOOC).

3. Results Indentations result in the formation of surface cracks. We determined the number of cracks, for fixed applied loads, as a function of the implantation fluence and energy and after the annealing process. Ten indentations at the same load were performed on each sample. Fig. 1 shows some typical Vickers indenter marks obtained on implanted samples. Four long (radial) cracks were clearly visible near the comers of the mark. Sometimes some shorter cracks developed near the longer ones (at the right and left-hand comers in fig. lb) but their appearance is not systematically connected to the experimental parameters. For this reason we neglected them in the analysis of the experimental results. The evaluation of crack number induced by indentation is performed only considering the long (radial) ones. II. RADIATION MODIFICATION

254

G. Battaglin et al. / Mechanical properties of ion implanted glasses

Fig. 1. Indenter impressions (a) after 385 g applied load on 10’ 3 Art -/cm* at 100 keV, (b) after 500 g on 4 X lOI Ar+/cm* at 50 keV energy, implanted glasses.

From fig. 2 we observe that the crack development is the same in unimplanted and low dose (< 1014 Ar+/cm*) implanted samples. A considerable reduction in crack number is evident in the dose range between 1014 and 4 X 1016 Ar’/cm2 with a minimum for doses of the order of 1015 Ar’/cm’. This minimum is more pronounced for lower applied loads. For 100 keV-implanted glasses (see fig. 3) a reduction of crack development is observed for all implantation fluences and this reduction is even more apparent than in the case of 50

As four is the maximum number, which may occur in each indentation, we normalized the measured crack number for each sample to 40. Besides the number we also measured the crack length, c (see inset in fig. 4) with respect to the centre of the mark. In fig. 2 we report for three indenter loads the crack percentage for a 50 keV-Ar+ implanted glass as a function of the implantation dose. In the case of an unimplanted glass, the percentage reaches 100% for applied loads higher than 100 g.

I

I

I

I

I

50 k&i-Ar+

I

0

700 -

I

0

A

2

“80s ‘3360z 3 9403 0

D

zo-

or

I

5

I

I

ld”

5

I

KY DOSE

I

1

5

10”

I

5

(ions/cm’)

Fig. 2. Percentage of cracks versus implantation dose for three applied loads. The implant energy was 50 keV and the Ar+ current density 1 PA/cm’.

255

G. Battaglin et al. / Mechanical propertzes of ion implanted glasses

I

I

I

I

I

I

r

I

100 O\

\

A

ii!

(;

100 keV - Ar’ \

0 ! -

\

.l.\my$;,,>\\,J

!

\ 5

1013

1014

5

IO’”

5

?O16

5

DOSE (ions/ctn2) Fig. 3. Percentage density 1 pA/A2.

of cracks

versus implantation

I

dose for three applied

I

I

loads. The implant

I

energy was 100 keV and the Ar+ current

I

50 keV- At-’ l2=L ‘r; q

+

O-f

-

-

____-_---__-q-l___

+

s :

-I-

2

-2_

t

T

1

E g-34

5009 A385g l

z-4_ 2 0

93009 -5-

k---2c --I I 5

I 5

I 7014

I 7o15

I 5

1 1o16

I 5

DOSE ( ions/cm2) Fig. 4. Change

of radial

crack

length

as a function

of implantation

dose for three applied

loads. The implant II. RADIATION

energy

was 50 keV.

MODIFICATION

G. Battaglin et al. / Mechanical properties 4 ion .implanted glasses

I

a-,-

1 I 100 keV-A?

I

I

P

8

4

t-

z6 szl

-3-

P]

4-

P

2 Q--5-

P

I

‘-,

-

0 500 g A 385 g

8 -6-

I

0 300 g

1

1013 Fig. 5. Change

5

I

I

1014

I

1

,/

5 1d5 DOSE (ion.s/cm2)

or radial crack length as function

I

,

of implantation

1

I

100

I

I

1

1

1

200 LOAD

10’”

dose for three applied

700 keV

1

5

*;

300

5

loads. The implant

I

I

5 x ld5Ar+/cm2

I 400

I

energy was 100 keV.

I

1

500

(gc)

Fig. 6. Percentage of cracks versus applied load for 100 keV Ar+ implanted The implantation dose was 5 X 10” ions/c&.

glasses after 30 min annealing

at different

temperatures.

G. Battaglin et al. / Mechamcal properties of ion implanted glasses

keV implantation. For indentation at low loads there is a wide fluence range where the crack formation is absent. In figs. 4 and 5 we report the difference, AC, of the average crack length, c, between the implanted and the unimplanted glasses, at the same indenter load, as a function of the ion dose. The crack lengths for the unimplanted glass were: (40 f 0.5) pm at an indenter load of 300 g; (49 f 0.5) pm at 385 g and (58.5 k 0.5) pm at 500 g. For both the implantation energies we observed a reduction in the crack length, the maximum of which occurred for implantation doses of the order of 5 x 1Or5 ions/cm2 and was more pronounced at the higher energy. Clearly we have not reported in the figures the points corresponding to ion doses and indenter loads at which no or few cracks developed. We can observe a qualitative correlation between the number of cracks which develop at fixed ion doses and their length. A reduction of crack formation is accompanied by a shorter length. While the number of cracks at a fixed ion dose and energy depends on the indenter load, the variation in the crack length AC, seems to be nearly practically independent of these parameters. We studied the thermal stability of the surface mechanical resistance of the implanted glasses by performing indentation tests on samples annealed at different temperatures and for a constant time. The annealing process was carried out on 100 keV implanted samples at a dose of 5 X 1015 ions/cm2. This dose corresponds to a low observed probability for crack formation (see fig. 2). In fig. 6 the crack number percentages are reported as a function of indenter load for the different annealing temperatures. For comparison the results obtained on unimplanted and 5 X lOI Ar+/cm2 implanted samples are reported.

100 keV

5- 10” A&n2

251

We observe an increase of the crack formation probability as a function of the annealing temperature. We tried to analyze the annealing results by studying the temperature dependence of the P* load, at which the crack percentage reaches the value 50%. In fig. 7 we show the parameter P*, using a logarithm scale, as a function of l/T, where T is the annealing temperature. An activation energy of 0.07 eV can be obtained for the annealing process. 4. Discussion The characteristic dimension of the outward-extending “radial crack”, c, is related to the fracture surface energy (resistance to crack extension) [l]. In the presence of a surface layer containing residual-stress, one expects the radial cracks either to contract, in the case of compression, or to expand, in the case of tension. The collision cascade process, induced by ion implantation, determines the generation of defects and in particular the formation of interstitial clusters [16]. The defect distribution leads to an expansion of the bombarded zone in the subsurface layer. Since this is constrained by the substrate, intense lateral compressive stresses may be generated. We did not observe a change in the characteristic dimension of the deformation impression which is connected to the resistance to irreversible processes such as plastic flow or structural densification. Such a model is justified by the annealing process results which give an activation energy value for the defects’ recovery, characteristic of point defect annihilation. For high dose irradiation a higher vacancy concentration near the surface facilitates the vacancy agglomeration which causes voids and blistering [7]. The crack length increase for high implantation doses may be explained in terms of the above mentioned subsurface modifications. Work is in progress to clarify the nature of defects, their relation to the structural arrangement of introduced Ar ions and influence on mechanical and chemical glass surface properties. This work has been partially supported by Minister0 Publica Istruzione. We are grateful to Dr M. Prosperi for English language revision. References [l] T. Jensen, B.R. Lawn, R.L. Dalglish and J.C. Kelly, Rad. Eff. 28 (1976) [2] V. Chinellato,

[3] [4] [5] [6]

Fig. 7. Applied load at which the percentage of cracks reaches a value of 50% versus the annealing temperature. The implantation energy was 100 keV and the dose 5 x 10” Ar+/cm*.

[7]

245.

V. Gottardi, S. Lo Russo, P. Mazzoldi, F. Nicoletti and P. Polato, Rad. Eff. 65 (1982) 31. G.W. Arnold, Rad. Eff. 65 (1982) 17. E.P. EerNisse, J. Appl. Phys. 45 (1974) 167. P. Mazzoldi, Nucl. Instr. and Meth. 209/210 (1983) 1089. C. Wang, Y. Tao and S. Wang, J. Non-Crystalline Solids 52 (1982) 589. B. Rauschenbach and W. Him, Silikattechnik 27 (1976) 406.