M Henzel, J Kovalcik, J Dusza - Instrumented indentation of mosii based materials - страница 1

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ВІСНИК ЛЬВІВ. УН-ТУ VISNYK LVIV UNIV.

Серія фізична. 2006. Bun. 39. С. 253-260_Ser. Physic. 2006. N 39. P. 253-260

УДК 538.911

PACS number(s): 62.20.-x

INSTRUMENTED INDENTATION OF MoSii BASED MATERIALS

M. Henzel*, J. Kovalcik, J. Dusza, A. Juhasz*, J. Lendvai*

Institute of Materials Research, Slovak Academy of Sciences, Watsonova, 47, 043 53 Kosice, Slovak Republic tel.:+421 556 3381 15, fax: +421 556 3371 08

*Department of General Physics, ELTE, Budapest, Hungary e-mail: henzel@imrnov.saske.sk

As-recieved and as-deformed MoSi2 have been studied using an instrumented hardness testing device. Micro-nanoindentation tests at loads from 10 to 2 000 mN were performed on the as recieved and pre-strained (at 1300°C and 15 MPa, 24 hours) MoSi2 using depth-sensing method. The Martens, universal and classical hardness values were calculated at different indentation loads. The indentation load size effect was calculated directly from loading curves. According to the results the pre-strain reduces the micro-nano hardness values of the investigated material, probably due to the activated slip systems during the high-temperature deformation.

Key words: molybdenum disilicide, instrumented indentation, pre-strain, Martens hardness, universal hardness, depth-sensing curves.

Molybdenum disilicide (MoSi2) is a candidate material for high temperature structural applications as a furnace heating element and an electrical conductor in silicon intergrated circuit design or parts of engines [1]. MoSi2 is known for a high melting point of 2030°C, exhibits excellent high temperature oxidation resistance, and posseses many convenient properties such as high stiffness, high thermal conductivity, relatively low density, and high strength at elevated temperatures [2]. However, a major difficulty in the application of MoSi2 as a structural material has been a lack of ductility and fracture toughness at temperature range 900-1400°C. Toward this temperature range, with the onset of dislocation climb and diffusional creep processes, does MoSi2 show significant plasticity in compression, bending and tension in both single crystal and polycrystalline materials [3, 4, 5]. Many approaches have been taken to reduce brittle-ductile transition temperature of MoSi2 or to enhance the capability for plastic flow and obtain increasing of thougness at temperature range 900-1400°C. The crystal structure of MoSi2 is tetragonal ( C11b type), space group 14/mmm [6, 7]. The lattice parameters are a=0,3205 nm and c=0,7845 nm with c/a=2,45 (Fig. 1). MoSi2 is also reported to have the

hexagonal C40 structure above 1900°C [8].

© Henzel M., Kovalcik J., Dusza J. et al., 2006

Fig. 1. Tetragonal unit cell of MoSi2

In this material the dislocation-density limitation outgoes from a lack of overdue numbers of surface or internal dislocation sources. There is existing absence of knowlegde concerning the relative mobilities of edge and screw dislocations, and information on the different dislocation types <100>, <110>, 1/2<111> and 1/2<331>, their glide planes, and the operative slip systems as a function of temperature, strain rate and crystallographic orientation is only partially developed [9, 10].

Studies of the slip systems by means of hardness indentation for single crystal MoSi2 has found that primary and secondary slip systems were {100}<001> and {110}< 001> respectively [9, 10, 2]. Berkowitz et al. [11] reported that {110} is the slip plane in single crystal MoSi2 deformed between 625 and 1125°C under compressive load along three different directions. They concluded that the slip direction is <11-0>. Umakoshi et al. [12] reported that slip occurs in <3-31> directions on both {110} and {103} planes.

The materials used in this investigation were monolithic MoSi2 prepared by Cesiwid, Erlangen, Germany. Samples for microstructure analysis were prepared using standard procedure and investigated using optical microscopy, as well as scanning and transmission electron microscopy (SEM and TEM). Microstructure of MoSi2 is shown in Fig. 2 [13]. Prestrain (Fig. 3) was performed by compressive creep test at the applied load of 15 MPa at 1 400°C for 24 hours. This procedure may cause increasing of dislocation density in tested material.

Load cells

Fig. 3. Pre-strain of MoSi2

For hardness tests mirror polished samples have been used.The depth sensing tests were performed with Shimadzu DUH device with Vickers sharp indenter for all investigations [14]. Nominal peak loads of 10 to 2 000 mN were used and the dwell time at maximum load was 10 seconds in these experiments.

Fig. 4. Instrumented testing device

Total penetration depth h {including elastic deformation} and relations for the Vickers geometry gives an apparent hardness called universal or Martens hardness. Universal hardness can be calculated from the equation as follows:

Hu = F / 26,43 h2,

where F is applied load.

With a known Youngs modulus of the tested material, an analytic solution separates the contribution of elastic deformation, converting Hu into the conventional hardness Hv, which is related to the plastic indent size [15]. Hv is calculated from the equation as follows:

* Hv = 4.Hu / {1 + V( 1 - 12.Hu / E*)}2,

where E* is the effective contact stiffness, which can be determinate from the following equation:

E* = {( 1 - Vs2) / Es + ( 1 - Vi2) / Ei}-1, where Ei is Youngs modulus of indenter Vi is Poissons ratio of indenter, Es is Youngs modulus of the tested material, Vs is Poissons ratio of the tested material.

The next figures 5, 6 present the different F-h curves obtained for the MoSi2 intermetallics under study. Values of universal hardness and conventional hardness are recorded in the same figure. Both universal and conventional hardness present obvious load size effect. The approximate value of hardness can be taken from steady state curve of universal and conventional hardness. Shape of the depth-sensing curve shows elasto-plastic behaviour of MoSi2. Values of conventional and universal hardness taken from

steady state curves are recorded in table 1. Both state as-recieved and as-deformed are presented.

As received state-max.indentation load 50 mN

- load /7

As received state-max.indentation load 50 mN

0,1 0,2 0,3 0,4

depth (|jm)

load /

7/

0,1 0,2 0,3 0,4

depth (jjm)

50

50

40

40

CL

20

20

10

10

0,0

0,5

0,0

0,5

Fig. 5. Depth-sensing curve of as-received Fig. 6. Depth-sensing curve of as- deformed

state with max. load 50 mN state with max. Load 500 mN

Figures 7, 8 show the shapes of both states used in the study. Both shapes of the depth-sensing curve present elasto-plastic behaviour of MoSi2. There is variety in shape of both curves. As-deformed state shows larger area of plastic deformation, material stiffness is decreasing. This effect is caused by high-temperature deformation in pre-strain material. The total indentation work also acquires higher values in pre-strained material. Looking at the table 1 it can be seen, that values of both universal and conventional hardness are decreasing in as-deformed state.

Table 1

Hardness calculation from P-h curves

Max. load [mN]

Hardness [GPa]

 

As-recieved state

As-deformated state

 

Hu

Hv

Hu

Hv

500

7,5-8

10-11

5-5,9

5,2-7,5

50

10-11

13,5-15

8-10

10-12,5

Comparison-state as received and deformed-max.load 500 mN Comparison-state as received and deformed-max.load 50 mN

I......................... 0-1-,-,-,-,-,-,-,-,-,-,

0.0   0.2   0.4   0.6   0.8   1.0   1.2   1.4   1.6   1.8   2.0   2.2   2.4 0,0 0,1 0,2 0,3 0,4 0,5

depth (^m) depth (^m)

Fig. 7. Comparison of as-recieved state and as-deformed    Fig. 8. Comparison of as-recieved state and as-with max. load 500 mN deformed with max. load 50 Mn

Shapes of the depth-sensing curve obtained after instrumented indentation present elasto-plastic behaviour of MoSi2.

Shapes of the load-depth curves are different for the as-received and as-deformed materials with larger area of plastic deformation (lower hardness) in the case of as-deformed material. This effect is probably caused by introduction of slip systems during high-temperature deformation in pre-strained material.

The total indentation work also acquires higher values in the pre-strained material. Values of both universal and conventional hardness are lower in the as-deformed state compared to the as-received material.

This work was realized with the financial support of the Slovak Grant Agency, under the contract No. 2/1166/21 and by NANOSMART, Centre of Excellence, SAS.

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2. Alcala J. Instrumented Micro-Indentation of Zirconia Ceramics, J.Am.Ceram. Soc., 83. 1977-84. 2000.

3. Berkowitz-Mattuck J., Rossetti M. Basic Factors Controlling Pest in High-Temperature Systems, NASW-1887, Cambridge, MA, 1971.

4. Dusza J., Hvizdos P., Steinkellner W., Kromp K. Bending creep behaviour of pressureless sintered MoSi2, Scripta Materialia. Vol. 37, № 4. 1997. Elsevier Science.

5. Inou S., Toyokura N. et al. Properties of Molybdenum Silicide Film Deposit by CVD, J.Electro. Soc. 130 [7], 1983. P. 1603-1607.

6. Gibala R., Ghosh A.K., Van Aken D.C. et al. Mat. Sci. Eng., A 155. 1992. Р. 147­158.

7. Gibala R., Chang H., Czarnik C.M., Campbell J.P. Plasticity enhancement mechanisms in MoSi2, Mat.Science and Eng., A 261. 1999. Vol. 122. Р. 122-130.

8. Yamaguchi M. Mechanical Properties of BCC Metals, AIME, Warrendale, PA 1982. P. 31-40.

9. Krell A., Schadlich S.  Nanoindentation hardness of submicrometer alumina

ceramics, Mat. Sci. Eng., A 307. 2001. P. 172-181.

10. Mitchell T.E., Castro R.G., Petrovic J.J. et al., Dislocations, twins, grain boundaries and precipitates in MoSi2, Mat.Science and Eng., A 155. 1992. Vol. 241.P. 241-249.

11. Noebe R.D., Gibala R. Structure and Deformation of Boundaries, The Metall.Soc., AIME, Warrendale, PA. 1986. P. 89-108.

12. Umakoshi Y., Sakagami T. et al. Planar Faults in MoSi2 single crustals deformed at high temperatures, Phil. Mag. Lett., 59 (4), 1989. 159-64.

13. Unal O., Petrovic J.J., Carter D.H., Mitchell T.E. Dislocations and Plastic deformation in Molybdenum disilicide, J.Am.Ceram.Soc., 73 (6) 1752-57. 1990.

1752.

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15. Vahldiek F.W., Mersol S.A. Phase Relations and Substructure in Single-crystal MoSi2, J.Less-Common Met 15, 1968. 165-71.

АПАРАТНЕ ВИЗНАЧЕННЯ МІКРОТВЕРДОСТІ СПОЛУК НА ОСНОВІ MoSi2

М. Гензель*, Й. Ковальчік, Й. Душа, А. Юхаш*, Й. Лендвай*

Інститут дослідження матеріалів, Словацька академія наук вул. Ватсонова, 47, 043 53 Косіце, Республіка Словаччина тел.:+421 556 3381 15, факс: +421 556 3371 08 * Відділ загальної фізики, ЕЛТЕ, Будапешт, Угорщина e-mail: henzel@imrnov.saske.sk

Сполуку MoSi2 одразу після отримання та деформації вивчали за допомогою пристрою для вимірювання мікротвердості. Тести на мікровм'ятини при навантаженнях від 10 до 200 мН були проведені лише на щойно отриманих зразках (витриманих при температурі 1 3000C і тиску 15 МПа протягом 24 годин). Універсальну та класичну твердість Мартенса розраховано при різних навантаженнях. З кривих навантаження оцінено розмірні ефекти. З отриманих результатів робимо висновок, що витримка зразків при заданій температурі і тиску зменшує значення мікро/нано твердості досліджуваного матеріалу. Це відбувається, ймовірно, унаслідок процесів, що мають місце при високотемпера­турних деформаціях.

Ключові слова: дисилікат молібдену, апаратне визначення мікротвердості, деформація, твердість Мартенса, універсальна твердість, криві навантаження.

Стаття надійшла до редколегії 23.05.2003 Прийнята до друку 21.11.2005

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M Henzel, J Kovalcik, J Dusza - Instrumented indentation of mosii based materials