Microstructure Surface Properties..

8/13/2019 Microstructure Surface Properties..

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ByHead of DepartmentDr Rashid Hassan

Assistant ProfessorScience of Dental Materials Department

RAWAL INSTITUTE OF HEALTH SCIENCESRAWAL COLLEGE OF DENTISTRYISLAMABAD

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Atoms are bonded to each other by eitherprimary or secondary bondS By attraction ofpositive and negative charges of ions.

Thus they form a regularly spacedconfiguration , known a s space lattice orcrystal.

There are 14 possible lattice shapes and

forms.

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Triclinic simple.Monoclinic simple.Monoclinic body

centered.Orthorhombic

simple.Orthorhombic base

centered.Orthorhombic body

centered.

Orthorhombic facecentered.

Hexagonal.Rhombohedral.Tetragonal simple.Tetragonal body

centered.Cubic simple.

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SIMPLECUBICBODYCENTEREDCUBIC


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FACE CENTEREDCUBIC HEXAGONALCLOSE PACKEDORTHORHOMBIC


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Structures other than crystallineforms can also occur in solidstate.

Molecules are distributed in arandom pattern.

Dental waxes are an example ofamorphous structures.

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The increase in energy perunit area is called assurface energyor surfacetension. Interfacial tension that

exists between the twosurfaces due to unbalancedintermolecular forces. For adhesion to exist the

surfaces must be attracted

to one another at theirinterface.

The energy at the surface isgreater than that of itsinterior.

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At the surface the energy isgreater because theoutermost atoms are notequally attracted in alldirections. The surface atoms tend toform bond to other atomsin close proximity in orderto reduce the surfaceenergy. When primary bonding isinvolved, the adhesion iscalled HEMISORPTION

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Interfacial tension between a liquid and asolid resulting in a contact angle of less than90.Adhesion is negligible when the surface

molecules of the two materials are separatedby a distance greater than 0.7nm.

In case of true wetting of the surface,

adhesion failure should not occur.

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Cleanliness of the surface is very important tobetter adhesion.

Surface energy is very crucial.

Surface energy of waxes is very low that few ifany liquid can wet their surface.

Sometimes a synthetic resin is used whereadhesion is not desired.

e.g. Teflon Polytetraflouroethylene)

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The extent to which an adhesive can wet thesurface of adherend can be determined bymeasuring the contact angle between theadhesive and the adherend.

Contact angle is the angle formed at theinterface of the adhesive and the adherend.

If the molecules of adhesive are attracted

more to the molecules of the surface, theadhesive will spread completely over thesurface of the solid.

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Complete wetting occurs at 0 and no wettingoccurs at 180.

Entrapment of air bubbles during adhesiveapplication leads to voids and pores.

When a thermal energy is applied to suchadhesive, stress concentrations develop.

These voids may lead to crack propagation.

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Tooth is a composite material.The composition of both enamel and dentin

varies.

The materials that adhere to the organiccomponent of the tooth may not adhere tothe inorganic component.

Surface contamination (smear layer) reduces

the adhesion in cavity preparation. Inorganic component has strong affinity for

water.

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QUESTIONS???

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Questions

Microstructure Surface Properties..

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