Tribology

Question 1

Tribology

Answer 1

The science and technology of interacting surfaces in relative motion
i.e: the study of friction, wear, and lubrication

Question 2

Friction

Answer 2

• Two stationary bodies in contact resist the sliding of one on the other

Question 3

In what plane is the force required to overcome friction resistance measured in?

Answer 3

• the plane of the interface

Question 4

The resistance to friction occurs through multiple things. Name 3

Answer 4

• Interlocking of (surface) roughness
  asperities (under a microscope) are the ruff surfaces and appear like the mountain tops of peaks and valleys and when a surface is rough, they interact w/each other and increase resistance
• Chemical bond formation
• Welding

Question 5

Laws of Friction (2)

Answer 5

1. F is indep of the area of contact (spreading the load over a greater area has no effect on F)
2. F is prop to W (the force is prop to a load or weight being moved: this rlnshp allows for the definition of the coefficient of friction μ)

μ = F (Friction force)/ W (normal force)
(represents static coefficient of friction)

Question 6

What is the diff btwn the coefficient of static (μs) vs coeff of kinetic (dynamic) friction?

Answer 6

They are similar but with coeff. of kinetic friction μk, distance must be taken into acct due to the motion involved

Question 7

Kinetic friction represents

Answer 7

• work that is being done

Question 8

Kinectic friction results in either 1 of these 2 things:

Answer 8

• deformation of or cutting of one surface (thus surface damage) or
• generation of heat (work dissipated at the surface)

Question 9

Surface damage

Answer 9

Topographical and/or microstructural changes in a surface layer

Question 10

Different levels of surface damage

Answer 10

• w/out exchange of material
• involving gain of material
• involv loss of material: wear

Question 11

Define surface damage by wear

Answer 11

The loss of material resulting from removal and relocation of materials through the contact of two or more materials

Question 12

Where does the wear occur and why?

Answer 12

• at the tips of the highest asperities; coz they are the places that experience the highest stress (hence will fracture and fail first)

Question 13

Types of wear (that contribute to material loss 6)

Answer 13

Abrasive wear
Solid particle erosion
Adhesive wear
Fatigue wear
Chemical or corrosive wear
Fretting wear

Question 14

Define abrasive wear

- which surface is mostly affected?

Answer 14

Occurs when a hard, solid particle or asperity comes in contact with a softer surface and surface loss occurs.
The softer surface is mostly affected.

Question 15

Ways in which abrasive wear happens (2)

Answer 15

1. 2-body abrasion
   
   • two surfaces with low loose particles that are coming in contact with each other ex. machining, & abrading tooth structure is two body abrasion
2. 3-body abrasion
   
   • instead of being attached, loose particles are moving around as two substrates are sliding relative to each other ex. pumice, prophy paste, food btwn teeth etc

Question 16

Compare 2-body systems vs 3-body systems

Answer 16

• 2-body systems typically produce 10-1000 times as much loss as 3-body systems
  (= 2 body syst. more aggressive)

Question 17

Which is the most efficient direction to prep a tooth?

Answer 17

• counterclockwise coz burs move in a clockwise direction.

If you prep in a clockwise direction, you force debris into the interface as you abrade the tooth which then creates a 3-body abrasion rather han a 2-body abrasion hnce interfering with contact and rate of abrasion.

Question 18

If a pt calls u in 2 days regarding post-op pain after placing a restoration, what should you think about if you had used a slow-speed cutting bur in a dry environment?

Answer 18

• an inflammatory response was created just from the generation of heat and with time reparative dentin forms. Chronic cells become predominant but reparative dentin forms as those cells drop off.

Question 19

Contrast high speed (50,000 - 300,000rpm) cutting with air water spray vs low speed (6000 - 20,000rpm) with air water spray

Answer 19

With high speed cutting w/air water spray,

• there is no significant acute pulp response
• the curves (graph) are much flatter
• there are no acute cells present

With low speed

• acute cells predominate for 10 days creating a moderate response
• after 15 days chronic cells predominate but decrease over time as odontoblasts begin to produce reparative dentin

Question 20

Two important factors that play a role in the amt of 2-body abrasion of restorative dental materials

Answer 20

(i) the type of composite
(ii) the type of filler in the composite
(iii) whether or not that filler is appropriately silanolated

Question 21

Of the 2 types of amalgam - Spherical and AgSn+AgCu, which demonstrates more abrasion (2-body)

Answer 21

Spherical (7.0) > AgSn + AgCu (5.6)

see slide 19

Question 22

why is it that glass filler has wear that is higher than what we see with quartz filler?

Answer 22

the harder the material that is doing the abrading relative to the material that is being abraded, the more the abrasion

Question 23

State the rlnshp between the amount of abrasive wear and the ratio of material hardness to abrasive hardness

Answer 23

As the ratio of material hardness to abrasive hardness goes down, there is an increased amt of abrasive wear

Question 24

Hardness

Answer 24

• The ability of one material to be plastically deformed by indenting or scratching with another material

Question 25

Mohs hardness

Answer 25

A scale of hardness of 1-10 based on scratch resistance (wear); 1- talc vs 10- diamond

• determined by observing whether its surface is scratched by a substance of known or defined hardness

Question 26

The hardness of dental materials is generally reported (measured) in

Answer 26

Knoop hardness

Question 27

Knoop hardness is calculated from

Answer 27

• the permanent surface deformation after removal of the load

Question 28

Difference btwn Knoop and Mohs hardness

Answer 28

Mohs test facilitates the identification of minerals in the field, but is not suitable for accurately gauging the hardness of industrial materials such as steel or ceramics; Knoop hardness does this

Question 29

Lubrication

Answer 29

Any substance - solid, liquid, or gas - used to reduce friction and wear between moving surfaces

Question 30

Abrasive size is identified by GRIT and plays a role in amt of abrasive wear seen. The particle sizes are separated into (5)

Answer 30

Superfine 
Fine
Medium
Medium coarse
Coarse

Question 31

What is Y-TZP?

Answer 31

Yttria, Tetragonal, Zirconia, Poly crystal

Almost all zirconia crowns today are Y-TZP

Question 32

Some pple would argue that ceramic crowns shlould be re-glazed not just polished after occlusal adjustment. However, with Zirconia crowns in particular, a polished surface is better than a glazed surface. Exlplain.

Answer 32

Putting a glaze on zirconia (i.e. putting feldspathic porcelain on top of it) causes more greater amt of wear than if left as is.

Question 33

Zirconia surface roughness

Answer 33

Glazed was smoothest followed by polished

As-machined and bur finish had higher surface ruffness

Question 34

Zirconia vertical height loss

Answer 34

Glazed - greatest wear
Polished - least wear
As-machined & bur finish - comparable wear