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Properties of dental materials- Physical .pptx
- 1. Physical Properties of dentalmaterials 2nd Year theory
- 2. Thermal properties Coefficient ofthermal expansion Thermal diffusivity Thermal conductivity Denture bases in contact with mucosal sufaces Prevent thermal shock and trauma Deep cavities
- 3. Thermal conductivity Physical propertythat governs heat transfer through a material by conductive flow Second law of thermodynamics – higher temperature to lower temperature • Conductors • Insulators
- 4. Thermal diffusivity Measure ofspeed with which a temperature change will spread through an object when one surface is heated Enamel and dentin – effective thermal insulators
- 5. Coefficient of thermalexpansion Change in length per unit of the original length of a material when the temperature is increased by 1 degree Marginal microleakage or debond
- 6. Coefficient of thermalexpansion Inlay wax Denture teeth Metal ceramic restorations Porcelain veneer
- 7. Electrochemical properties Tarnish and corrosion Surface discoloration on ametal Slight loss or alteration of the suface finish line or luster Oxides, sulfides or chlorides
- 8. Corrosion Deterioration of metal reaction withits environment Severe and catastrophic disintegration Mechanical failure of a structure Phosphoric acid, acetic acid lactic acid
- 9. Fundamental basis ofcorrosion Chemical Electrochemical process First step – loss of an electron
- 10. Chemical corrosion Direct combination of metallic and nonmetallic elements Formationof silver sulfide Dental gold alloys containing silver – dry corrosion Dental amalgam
- 11. Electrochemical mechanism of corrosion Oxidationpotential Electrochemical cell – anode, cathode, electrolyte Anode – amalgam Cathode – gold alloy Saliva – electrolyte
- 13. Anode – positive ions areformed Cathode – metal ions are deposited Electomotive force
- 14. Electro motive or galvanicseries More positive potential – lower tendency to dissolve
- 15. Dissimilar metals Combinations of dissimilar metals– direct physical contact Flow of galvanic currents – continuous or intermittent Eg: Dental amalgam opposing gold alloy Sharp pain – galvanic shock
- 16. Heterogenous surface composition Nominally puremetals corrode slow Impurities in alloys Solder joints Homogenized solid solution Commercial dental alloys
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- 18. Stress corrosion Imposition of stresses increasethe internal energy Fatigue or cyclic loading Cold working of an alloy
- 19. Concentration cell corrosion Variationsin electrolytes Polishing – prevent pitting corrosion Crevice corrosion
- 20. Protection against corrosion Coatingsto provide corrosion protection Chromium passivating metal Electroplated with nickel followed by chromium Titanium – titanium oxide film
- 21. Clinical significance ofgalvanic currents Varnish coating on the surface of metallic restoration Discomfort to the patient Electro galvanism
- 22. Mechanical properties ofdental materials Defined by the laws of mechanics – physical science dealing with forces that act on bodies and the resultant motion, deformation or stresses
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- 24. Mechanical properties Hardness Impact strength Malleability Percentageelongation Proportional limit Shear strenght Tensile strength Yield strength
- 25. Stresses and strains Stressdistribution or stress gradient Stress – force per unit area Tensile stress Shear stress Compressive stress
- 26. Newtons third lawof motion External force acts on a solid Equal in magnitude but opposite in direction
- 27. External force or pressure Deformationor strain occurs
- 28. Strain Change in lengthper unit length Relative deformation of an object subjected to a stress • Elastic • Plastic • Elastic and plastic • Viscoelastic
- 29. Viscoelastic materials –viscous and elastic charecteristics Plastic strain – permanent deformation Elastic strain – reversible
- 30. Stress Magnitude and the typeof deformation Simple stresses Tensile Compressive Shear
- 31. Tensile stress Tensile strain– stretch or elongate a body • Deformation of a bridge • Bending force
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- 33. Shear stress Resist thesliding or twisting of one portion of a body over another Twisting or torsional action • Rough curved surfaces • Chamfer or bevel • Tensile failure is more likely
- 34. Flexural stress- bendingstress Three unit FPD Two unit cantilever FPD • Three point loading • Load along the unsupported section
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- 36. Elastic modulus –Young’smodulus or modulus of elasticity Stiffness or rigidity of a material Stiffness Increased by increasing the thickness –elastic modulus does not change
- 37. Elastic modulus Constant valuethat describes a materials relative stiffness as determined from a stress strain graph, which compensates for differences in cross sectional area and length by plotting force per unit area by the relative change in dimension, usually length, relative to its initial value.
- 38. Proportional limit, elastic modulus, ultimate compressive strengthof enamel are greater than dentin Unsupported enamel is susceptible to fracture Dentin – flexible and tougher
- 39. Elastic modulus Elastic modulusconstant • Unaffected by elastic or plastic stresses • Independent of the ductility • Not a measure of plasticity or strength • High elastic modulus – low or high strength
- 40. Polyether material – greater stiffness Lower the strain– greater the modulus Ratio of elastic stress to strain
- 41. Dynamic Young’s modulus Dynamic& static Velocity of sound wave and density of material – elastic modulus and Poisson’s ratio
- 42. Flexibility High value ofelastic limit High modulus of elasticity is desirable – inlay or impression material Maximum flexibility – flexural strain that occurs Material is stressed to its proptional limit
- 43. Resilience Interatomic space increases – internal energy increses Stress isnot greater than the proportional limit – resilience
- 44. Resilience Amount of energyabsorbed within a unit volume of a structure when it is stressed to its proportional limit
- 45. Poisson’s ratio Tensile force –longer and thinner Compressive force- shorter and thicker Ratio is 0.5 – 0.25 and 0.30
- 46. Strength properties Strength isequal to the degree of stress necessary to cause either fracture or a specified amount of plastic deformation • Proportional limit • Elastic limit • Yield strength • Ultimate tensile strength, shear strength, compressive strength
- 47. Proportional limit It representsthe maximum stress above which stress is no longer proportional to strain
- 48. Elastic limit Defined asthe greatest stress to which the material can be subjected such that it returns to it original dimension when the force is released
- 49. Yield strength Proportional limitcannot be determined Represents stress value at which small amount of plastic strain has occurred
- 50. Permanent(Plastic)deformation The object doesnot return to its original dimension when the force is removed • Bent • Stretched • Compressed • Plastically deformed
- 51. Cold working(Strain hardeningor work hardening) Repeated plastic deformation – embrittlement of the deformed area of the wire Deform in small increments
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- 53. Flexural strength Transverse strengthand modulus of rupture – strength test of a bar supported at each end
- 54. Biaxial flexural strength Determinedby subjecting a material to a cyclic stress of a maximum known value and determining the no of cycles required to cause fracture Fatigue failure – microscopic flaws
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- 56. Impact strength Energy requiredto fracture a material under an impact force Reaction of a stationary object in collision with a moving object
- 57. Toughness Amount of elasticand plastic deformation energy required to fracture a material Measure of energy required to propagate critical flaws
- 58. Brittleness Relative inability ofthe material to sustain plastic deformation before fracture of material
- 59. Ductility Ability of amaterial to sustain a large permanent deformation under a tensile load upto the point of fracture Metal – long thin wire
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- 61. Gold– most ductileand malleable pure metal Silver – second Platinum – third in ductility Copper – third in malleability
- 62. Tests used forductility Percentage elongation after fracture The reduction in area of tensile test specimens The maximum no of bends performed in a cold bend test
- 63. Hardness Resistance to indentation Relatedproperties- Compressive strength, proportional limit and ductility
- 64. Hardness Ability of thesurface of the material to resist penetration by a diamond point or steel ball under a specific load • Barcol • Brinell • Rockwell • Shore • Vickers • Knoop
- 65. Brinell hardness test Usedextensively Proportional limit and ultimate tensile strength of dental gold alloys Oldest tests
- 66. Rockwell hardness test similarto Brinell test • Steel ball Vickers hardness test – same principle used in brinell • Square based pyramid
- 67. Knoop hardness test Diamondtipped tool Impression rhombic Knoop and vickers – microhardness test
- 68. Shapes of hardnessindenter points