Surface Mechanical Properties Flashcards
Surface hardness
It is the resistance of materials’ surfaces to be scratched, indented or even penetrated by hard objects
Importance of hardness
Indicative of the ease of finishing and polishing. high surface hardness hard to finishing and polishing.
✔low surface hardness –scratches
surface hardness of restoration equals to surface hardness of enamel- حشو
Scratches
Act as stress raisers
Compromise fatigue strength
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Rough surface
Premature failure
Staining and bacterial plaque adhesion
microcracks
Fracture-
Hardness test
Depends on penetration of a small symmetrically shaped object (indenter) into the surface of the material being tested under a standardized load.
✔Hardness is determined from the resultant indentation (area or depth).
Dimensions of the indentation vary inversely with hardness of the material being tested.
Methods for measuring the surface hardness
Brinell testing ductile materials as metals.
Rockwell-testing plastic materials as polymers.
✓ Vicker’s-universal test.
wide range of materials
micro hardness test
*✓ Knoop →universal test.
✓ Shore A→ testing rubber like materials (elastomers).
✔Barcol → measure the depth of cure of resin composite,
Nano-indentation → to measure the properties of the micro-phases(as in micro-filled composite) one should use indentations of nano-size.
Friction
It is the resistance to motion of one material body over another
Importance of friction
4 friction motion
Roughing the surface of dental implant to reduce motion between the implant and the adjacent tissue.
✔Friction (orthodontic wire & brackets due to pressing elastics reduce motion of teeth (not desired)-Increasing time of treatment
Wear opposite of hardness
It is the mechanical loss of material surface as a result of contact two or more materials.
✔Wear is always recognized at the surface of softer materials
Clinical significance of wear
Wear is highly desirable during finishing & polishing procedure.
✓ Wear is not desired during function, as:
Considered type failure
o It can produce biologically active particles, which can excite an inflammatory response
o Produce shape changes that can affect function
Examples of wear
Normal mastication cun make attrition of tooth structure and restorative materials physiological condition. Loss of mamelons.
Bruxism pathologic form of wear in which opposing surfaces slide
against each other.
Improper tooth brushing with a dentifrice abrasive wear
Factors affecting wear
Nature of wearing surfaces (non-homogeneity, phases & inclusion particles).
hardness strength 1 wear!
Surface hardness (soft materials are liable to wear than hard materials).
✔Microscopic contact between surfaces. point stress Concentration wear!
✔Presence of lubricant as saliva reduces the contact between the surfaces reduces the wear rate.
↑ pressure at the contact areas, the speed & total time of Movement → ⬆️wear rate.
Types of wear
Abrasive wear
Corrosive wear
Adhesive wear
Surface fatigue
Abrasive wear
A harder material cutting or blowing into a softer material.
Abrasive wear can be divided into two categories:
o Two-body wear: caused by direct contact of dentition surfaces.
o Three-body wear: abrasion by particles in the food or toothpaste.
Corrosive wear
Result from interaction of the environment (atmospheremoisture accumulation, bacteria, acids, chemicals) with sliding surfaces.
Sliding action accelerates corrosion, or wear may be accelerated by
corrosion of the sliding surfaces.
mechanical action activates chemical action (wear)
و العكس
- Adhesive wear
The action of one material sliding over another with surface interaction
Adhesive wear
The action of one material sliding over another with surface interaction and welding (adhesion) at localized contact areas. point not area. ↑ stress
✓Characterized by formation and disruption of micro junctions
Microregions are pulled from one object & transfer to the other
Surface fatigue
Particles with small area of contact contribute to high localized stresses and produce surface & subsurface cracks.
These particles break off under cyclic loading & sliding.
