Tribology/Wear
Is it *ideal for our restorative materials to wear at the same rate as enamel*? If so, why?
YES, because if materials do not wear at the same rate as enamel then differences in the heights of the material and enamel become apparent. With some resin composites, the wear can progress to the point of exposing dentin. *As the wear progresses, this can also expose porosities.* This is bad because this can enhance the wear rate and also make the tooth more susceptible to caries. --> see IMAGE
Are there both static and kinetic (dynamic) coefficients of friction (μ)? What is the difference between the two?
YES, the kinetic (dynamic) coefficient of friction takes into account distance, represents work (work is dissipated as heat or deformation)
Can surface damage occur without the exchange of material, WITH the GAIN of material or with the LOSS of material?
Yes, these are all possibilities
Which is more abrasive, amalgam or denture base resin?
*AMALGAM is more abrasive* than denture base resin
What is the best hardness scale to use?
*Mohs hardness scale* - uses scratch resistance and a scale of 1-10 *Talc (baby powder) = 1* *Diamond = 10* - better than Knoop and Vickers scale
Which is more abrasive, pumice or gold alloy?
*PUMICE is more abrasive* (Mohs scale 6-7) than gold alloy meaning if you rubbed pumice on gold alloy you would note wear that occurred
Which is more abrasive, resin composite or denture base resin?
*RESIN is more abrasive* than denture base resin
Which is the *more effective mechanism of abrasive wear*, two-body or three-body?
*TWO-body* mechanism of abrasive wear is much more effective at causing wear they *typically produce 10-1000 times as much loss as a three-body system* --> see IMAGE
What is *surface damage that results in the LOSS of material*? ***IMPORTANT
*WEAR* ("the loss of material resulting from removal and relocation of materials through the contact of two or more materials")
Where does *wear* normally occur?
*at the tips of the highest asperities (aka the peaks of the roughness)* --> see IMAGE
What is the *relationship between a material and the abrasive hardness*? ***IMPORTANT
*if the abrasive is significantly HARDER than the material being abraded, then MORE abrasive wear occurs* "As the abrasive hardness increases relative to the material, that ratio goes down. as that ratio goes down we see around 1 to 1, below that we see dramatic changes in the amount of abrasive wear and that is experienced and then it starts to plateau if I want to abrade something, I want to use something that is much harder than the material that I am trying to abrade. it's important then to connect to hardness. It's the ability of one material to be plastically deformed by indenting or scratching with another material."
What is a *lubricant that the oral cavity uses during function*?
*saliva* if it is not present, as in xerostomic patients, we see that wear INCREASES
If *friction force is created via HIGH-speed cutting (50,000-300,000 rpm)* of a bur with air-water spray on, and the tooth is then restored with a temporary zinc-oxide eugenol (ZOE) restoration, what kind of response is generated?
*there is NO significant pulpal response* *NO acute inflammatory cells present* small amounts of chronic inflammatory cells that stimulate a little bit of reparative dentin, but not much TAKE HOME: *HIGH speed handpieces generate LESS friction force than low speed handpieces resulting in LESS of an impact on tooth structure* --> see IMAGE
Are there technological options that can be used to evaluate the wear a tooth/restoration experiences over time?
YES --> see IMAGE
When work results in deformation rather than heat, can the result be *surface damage*?
YES (topographical and/or microstructural changes) surface damage is generally created in consecutive small steps by a number of different mechanisms
If you *double the AREA of an object *that is experiencing friction, how does this affect the amount of friction? ***IMPORTANT
it will *NOT affect the amount of friction* because F (friction force) is independent of the area of contact and spreading the load has NO effect on F (friction force)
What is an example of a solid lubricant?
putting graphite into a key lock to make the key work better
Types of wear: What is *corrosive wear*?
related to the chemical activity of the wear surfaces
What is *hardness*?
the ability of one material to be plastically deformed by indenting or scratching with another material
What is *tribology*?
the science and technology of interacting surfaces in relative motion aka the study of *friction, wear and lubrication*
What is *friction*?
when two stationary bodies in contact resist the sliding of one on the other *the force required to overcome the resistance is measured in the plane of the interface*
What is the *two-body mechanism of abrasive wear*?
when we see one side that is abrasive ties to an opposite substrate that isn't abrasive Example: soflex discs, diamond cutting burs
What is the *three-body mechanism of abrasive wear*?
where we put an abrasive, harder particle between two surfaces that go into motion --> the result is surface impact and degradation Example: polishing pumice, polishing paste
What are the different *abrasive sizes (aka grit)*?
- coarse - medium coarse - medium - fine - superfine (mesh screens are used to separate these particle sizes when abrasive products are manufactured, *the number of holes per square inch in the screen determines the grit size of the particles*) (*generally, the more coarse, the LARGER the grit size and the more AGGRESSIVE the wear patterns.* however, this depends on the materials being used. *the closer the hardness values of two materials in abrasive contact are, the less the statement about the grit size hold up*. "zirconia is so hard that the difference is not great enough for these grit sizes to make a huge difference. In fact there are people advocating that you use smaller grit sizes to cut zirconia mostly because you can use them to get rid of debris easier and you're not seeing any great advantage of those grit sizes. We do through polishing systems and burs.")
What *factors impact friction*?
- interlocking of roughness - chemical bond formation - welding
What are some examples of static coefficient of frictions (μ) at a few different interfaces? ***IMPORTANT
--> see IMAGE lubrication = low friction dry = high friction
Discuss the *laws of friction*: ***IMPORTANT
--> see IMAGE for the definition of the *coefficient of friction (μ)* F is independent of the area of contact (aka *spreading the load has NO effect on F*) *F (friction force) is proportional to W (normal force)* (this allows for the definition of the *coefficient of friction (μ)*)
What are the two ways that *work is dissipated*?
1. as *heat* 2. results in *deformation*
What is *lubrication*?
any substance used to reduce friction and wear between moving surfaces may be a solid, liquid or gas
If *friction force is created via LOW-speed cutting (6000-20,000 rpm)* of a bur with air-water spray on, and the tooth is then restored with a temporary zinc-oxide eugenol (ZOE) restoration, what kind of response is generated?
energy has been created during this frictional interaction *generating acute inflammatory cells *for the first 10 days the patient experiences a moderate response of *post-operative sensitivity* after 15 days chronic inflammatory cells predominate, but they decrease over time as odontoblasts begin to produce reparative dentin after 20 days the reparative dentin ultimately creates a thicker amount of dentin between the restoration and the pulp to protect the tooth from the sensitivity that was caused by the initial LOW-speed cutting friction force --> see IMAGE
Types of wear: What is *adhesive wear*?
formation and disruption of micro-junctions
Types of wear: What is *abrasive wear*?
harder material plowing or cutting a softer material and surface loss occurs the softer surface is mostly affected two-body and three-body abrasive mechanisms of wear exist
Types of wear: What is *surface fatigue wear*?
high localized stresses producing surface or subsurface cracks
If you *double the WEIGHT of an object *that is experiencing friction, how does this affect the amount of friction? ***IMPORTANT
it will *DOUBLE the amount of friction*
