SIFAT MEKANIK KERAMIK

keramik

Pada temperatur kamar, baik keramik yang kristalin maupunnon-kristalin selalu patah sebelum terjadi plastic deformationakibat beban yang diberikan

Proses brittle fracture pada keramik terdiri dari pembentukan(formation) dan perambatan (propagation) melalui penampangbahan dalam arah tegak lurus terhadap beban yang diberikan

Retak (crack) pada keramik kristalin muncul melewati butir dansepanjang bidang tertentu (misalnya bidang dengan densitasatom yang tinggi

Kc = the plane strain fracture toughnessY = dimensionless parameter, tergantung pada geometrispesimen dan retakan = beban yang diberikana = panjang retakan di permukaan

Rambatan retakan tidak akan terjadi jika

MECHANICAL PROPERTIES OF CERAMICS

Flexural Strength

The stress at fracture using this flexure test is known as the flexural strength.

Flexure test :which a rod specimen having either a circular or rectangular cross section is bent until fracture using a three- or four-point loading technique

Callister, W., D., (2007), Materials Science And Engineering, 7th Edition,8

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9 3-point bend test to measure room-T flexural strength.

Adapted from Fig. 12.32, Callister & Rethwisch 8e.

Flexural Tests Measurement of Flexural Strength

FL/2 L/2

d = midpoint deflection

cross section

R

b

d

rect. circ.

location of max tension

Flexural strength: Typical values:

Data from Table 12.5, Callister & Rethwisch 8e.

Si nitrideSi carbideAl oxideglass (soda-lime)

250-1000100-820275-700

69

30434539369

Material fs (MPa) E(GPa)

223bd

LFffs (rect. cross section)

(circ. cross section)3RLFf

fs

10

Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used.

-- tensile tests are difficult for brittle materials.

Adapted from Fig. 12.32, Callister & Rethwisch 8e.

Flexural Tests Measurement of Elastic Modulus

FL/2 L/2

d = midpoint deflection

cross section

R

b

d

rect. circ.

Determine elastic modulus according to:F

x

linear-elastic behaviord

F

dslope =

3

3

4bdLFE

d (rect. cross section)

4

3

12 RLFEd

(circ. cross section)

Callister, W., D., (2007), Materials Science And Engineering, 7th Edition,11

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ELASTIC BEHAVIOR

Elastic modulusThe elastic modulus E [GPa] of almostall oxide and non-oxide ceramics isconsistently higher than that of steel.

This results in an elastic deformation ofonly about 50 to 70 % of what is foundin steel components.

The high stiffness implies, however, thatforces experienced by bondedceramic/metal constructions mustprimarily be taken up by the ceramicmaterial.

http://www.keramverband.de/brevier_engl/5/3/4/5_3_4.htm13

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HARDNESS

Technical ceramic components are thereforecharacterised by their stiffness and dimensional stability.

Hardness is affected from : porosity in the surface, the grain size of the microstructure and the effects of grain boundary phases.

http://www.dynacer.com/hardness.htmhttp://www.keramvaerband.de/brevier_eng/5/3/%_3_5.htm

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Hardness.htm14

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Porosity

Technical ceramic materials have no open porosity.

Porosity can be generated through the appropriate selection of raw materials, the manufacturing process, and in some cases through the use of additives.

This allows closed and open pores to be created with sizes from a few nm up to a few m.

http://www.ucl.ac.uk/cmr/webpages/spotlight/articles/colombo.htm

Change in elastic modulus with the amount of porosity in SiOC ceramic foams obtained from a preceramic polymer

http://www.keramverband.de/brevier_engl/5/3/5_3_2.htm15

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Besarnya modulus elastisitas (E) berkurang terhadap fraksi volumeporositas (P) dengan persamaan :

Eo = modulus elastisitas material tidak berpori

Porosity

http://www.keramverband.de/brevier_engl/5/3/5_3_2.htm16

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porositas yang merusak terhadap kekuatan lentur karena :

pori-pori mengurangi luas penampang dimana bebandiberikan

bertindak sebagai konsentrator stres

Hubungan porositas dengan Flexural Strength

Test procedures for determining the hardness according to Vickers, Knoop and Rockwell.

Some typical hardness values for ceramic materials are provided below:

The high hardness of technical ceramics results in favourable wear resistance. Ceramics are thus good for tribological applications.

http://www.dynacer.com/hardness.htm18

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Density

The density, (g/cm) of technical ceramics lies between 20 and 70% of the density of steel.

The relative density, d [%], has a significant effect on the properties of the ceramic.

http://www.keramverband.de/brevier_engl/5/3/4/5_3.htm19

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A comparison of typical mechanical characteristics of some ceramics with grey cast-iron and construction steel

http://www.keramverband.de/brevier_engl/5/5_2.htm20

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Strength

The figure for the strength of ceramic materials, [MPa] is statistically distributed depending on

the material compositionthe grain size of the initial material and the additivesthe production conditions the manufacturing process

Strength distribution within batches

http://www.keramverband.de/brevier_engl/5/3/3/5_3_3.htm21

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Toughness

Ability of material to resist fracture

affected from,

temperaturestrain raterelationship between the strenghtand ductility of the material and presence of stress concentration (notch) on the specimen surface

http://www.subtech.com/dokuwiki/doku.php?id=fracture_toughness22

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MECHANICAL PROPERTIES OF CERAMICSMaterial KIc (MPa-m1 / 2)

Metals

Aluminum alloy (7075) 24

Steel alloy (4340) 50Titanium alloy 44-66Aluminum 14-28CeramicsAluminum oxide 3-5Silicon carbide 3-5Soda-lime-glass 0.7-0.8Concrete 0.2-1.4PolymersPolystyrene 0.7-1.1CompositesMullite fiber reinforced-mullite composite

1.8-3.3

Some typical values of fracture toughness for various materials

http://en.wikipedia.org/wiki/Fracture_toughness23

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