Tribology lecture 2 Flashcards

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1
Q

What is friction?

A

Friction is the resistance to relative sliding between two surfaces/bodies in contact under a normal load.

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2
Q

What is the simple explanation for the cause of friction forces?

A

The normal load between two contacted surfaces forms junctions, when surfaces move relatively, these junctions are sheared resulting in friction forces.

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3
Q

What are the friction laws for dry friction?

A
  1. The friction forces is proportional to the normal contact force. The coefficient of proportionality is known as the coefficient of friction. Often, 2 values are quoted: the coefficient of static friction (which applies to the onset of sliding and the coefficient of kinetic friction (which applies during sliding motion).
  2. The coefficient of friction is independent of the apparent area of contact
  3. The static coefficient is larger than the kinetic coefficient
  4. The coefficient of kinetic friction is independent of the sliding velocity (except in special cases).
  5. When tangential motion occurs, the friction force acts in the same direction a the relative velocity but in the opposite sense (except in special cases).
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4
Q

What does bowden and tabor theory say about the area of contact between two surfaces?

A

Two clean dry surfaces (unlubricated) contact each other only at a fraction of their apparent area of contact known as the asperity contacts or junctions.

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5
Q

What is true contact area eqn?

A

Draw diagram.. At = a1 + a2 +… + an

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6
Q

What happens under light loads?

A

The normal stress is low, elastic behaviour

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7
Q

What happens as load/stress increase?

A

Plastic deformation occurs, this can form new junctions/ asperities. Adhesive bonds (micro-welds) can develop at the junctions.

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8
Q

What is the eqn for max load before yield?

A

W = At * Sy

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9
Q

In bowden and tabor theory how are frictional forces determined?

A

By a combination of shearing and ploughing components.

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10
Q

What is the shear component for contact between a hemispherical rider and a soft surface?

A
S = At * Ssy
Ssy = shear strength of soft material
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11
Q

What is the eqn for the additional force required to displace a wall of materials ahead of the sphere?

A

P = A’ * Sy
Where projected area AOBC, A’ = d^3/12r.
(see diagram.. doesnt make sense because no O in diagram.. ask lecturer)

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12
Q

When does sliding between the two bodies occur?

A

When a tangential force, F, is applied under the normal load, W.

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13
Q

What is the total friction force for this case of contact between a hemispherical rider and soft material?

A

F = S + P

= At*Ssy + d^3/12r * Sy

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14
Q

For a small normal load what is the ploughing component, P?

A

Negligible, so

F = At * Ssy

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15
Q

What is the eqn for coefficient of friction?

A

m = F/Q = At. tao/At. normal stress = Ssy/Sy
where,
tao = shear stress (strength)
Sy is the yield stress.

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16
Q

What is the coefficient for friction for most metals?

A

0.2 (if environmental and surface condition remain constant)

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17
Q

How can friction coefficient be lowered?

A

By depositing a thing layer of soft metal onto a hard substance.

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18
Q

When is the coefficient of friction ratio (Ssy/Sy) no longer valid?

A

if the interface shear stress reaches yield shear stress, the workpiece will start to deform by shearing and the ratio is no longer valid.

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19
Q

For perfect lubrication what is friction coefficient?

A

0

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20
Q

For sticking what is friction coefficient?

A

1

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21
Q

What is wear?

A

The progressive removal of material from surface.

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22
Q

What is adhesive wear?

A

Shearing of the junctions at the original interface of the two bodies under a load and tangential applied force. When the adhesive bond is stronger than the base material (soft), fracture at the asperity of the soft material can cause fragments to be attached to the harder material and become loose wear particles. (draw images of this)

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23
Q

What does archards wear law give?

A

The probability of adhesive wear.

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24
Q

What is wear rate?

A

The volume of material (of softer component) removed per unit sliding distance.

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25
Q

What is archards wear equation?

A
Vi = Ki.F.s
where
V = volume of wear debris produced
K = specific wear rate coefficient
F = normal load
s = sliding distance
26
Q

How to get this into wear distance, h?

A

hi = V/A (A= area subject to wear)
since P = F/A (P= contact pressure)
then
hi = Ki.P.s

27
Q

What is K and it’s units?

A

K = the specific wear rate coefficient, given in m^3/Nm or m^2/N.

28
Q

What does K depend on and how is it found?

A

K is like m, it depends on many parameters. It is therefore found through experiments: pin and disc wear test is used.

29
Q

Can you draw a diagram of a pin and disc wear test?

A

yes or no

30
Q

What is K a measure of and what are typical values?

A

K is a measure of severity of wear. Typically for mild wear, K=10^-8 and for severe wear K=10^-2.

31
Q

What is abrasive wear?

A

Hard foreign metal debris or dust from the work environments abrade the rubbing surfaces of both materials unintentionally. (draw diagram).

32
Q

For a good design to avoid abrasive wear?

A
  1. Avoid using similar materials to prevent solubility
  2. Use of hard materials
  3. Good lubrication
33
Q

What is fatigue wear?

A

Cyclic stresses set up by repeated loading and unloading between surfaces eventually form surface and sub-surface cracks.

34
Q

For a good design to avoid fatigue wear?

A
  1. Low contact load
  2. Apply surface finish
  3. Avoid thermal fatigue
35
Q

What is erosive wear?

A

Occurs when particles (solid or liquid) carried by a gas or liquid strike a surface with velocity. Low impact angles remove the metal by plastic flow, this is governed by:

  1. Yield strength and youngs modulus of the surface material
  2. Angle of attack of fluid/solid in relation to ductility of material (draw diagram/graph)
36
Q

What is corrosive wear?

A

When 2 surfaces are in contact under a corrosive environment, oxidation or other electrochemical reactions can occur which form reaction products subsequently removed via sliding or abrasion between 2 surfaces.

37
Q

For a good design to avoid corrosion wear?

A
  1. Select suitable/compatible materials
  2. Apply low loads/ reduce stresses
  3. use surface coating
38
Q

What is lubrication?

A

When 2 surfaces slide against each other under high pressure, without protective layers at the interface, friction/wear will be high. Lubrication can reduce the wear coefficient significantly.

39
Q

For lubricated conditions, how may the wear coefficient be effected?

A

The power value of typical wear coefficient may be doubled (i.e. from 10^-5 to 10^-10).

40
Q

What is reynolds steady state equation?

A
F directly prop. to v.n/D
F= frictional (drag force)
v = sliding velocity
n = bulk fluid viscosity
D = film lubricant thickness
41
Q

What type of lubrication can you get?

A

Full, mixed or boundary as determined by the extent of the load.

42
Q

How can boundary lubrication/film reduce adhesive wear?

A

By reducing the coefficient of friction between the sliding surfaces.

43
Q

What does proper lubrication depend on?

A
  1. the right oil
  2. in the right place
  3. at the right time
  4. in the right amount
44
Q

What are surface engineering processes?

A

The processes and treatments which modify the properties and characteristics of surfaces and sub-surfaces. Several mechanical, thermal, electrical and chemical methods can be used to modify surfaces of components/products to improve frictional performance, resistance to wear, corrosion and surface appearance.

45
Q

What are category 1 surface engineering methods?

A

Modifies surfaces without altering the chemistry, such as transformation hardening (depth up to 750 micrometres)

46
Q

What are category 2 surface engineering methods?

A

Altering the chemistry of the surface regions such as anodising (depth up to 100 micrometres)

47
Q

What are category 3 surface engineering methods?

A

Adding a layer of material to the surface, such as vacuum (chemical and physical) vapour deposition (depth up to 5 micrometres).

48
Q

What is transformation hardening?

A

Strengthens certain alloys by introducing a phase change to a harder and stronger phase. Two-phase aluminium bronzes (copper alloys) and some manganese bronzes can be quenched and tempered to increase their strength without compromising ductility. These alloys are hardened by cooling rapidly from a high temperature (e.g. by a laser beam) to produce a martensitic type structure. They are then tempered (slow cooling) at a lower temperature (oven 600deg for 2 hours) to stabilize the structure and partly restore ductility and toughness. This process also applies to carbon steel.

49
Q

What machining techniques does transformation hardening require?

A

Peening and cold deformation.

50
Q

What is shot peening?

A

A cold work process, in which the metal part is struck by a stream of small hard spheres (shot) creating numerous overlapped dimples on the part surface. Results in a formation of compression stressed skin of about 250 microm thickness.

51
Q

What materials are used as shot media?

A

Glass, steel or ceramic balls.

52
Q

What properties can be improved by shot peening?

A
  1. Fatigue strength
  2. Stress corrosion cracking
  3. Corrosion fatigue
  4. Intergrannular corrosion resistance
  5. Galling resistance
  6. Wear resistance
53
Q

What is anodising (include diagram)?

A

Anodising is an oxidation process in which the surfaces of the substrate are converted to a hard and porous oxide layer that provides corrosion and wear resistance and a decorative finish. The acid solutions used for the electrochemical process may include organic dyes of various colours. Anodised surfaces are also required for good adhesion for paint or adhesive joints. Aluminium and titanium are widely used for anodising.

54
Q

What does the anodising process include?

A

Thermochemical diffusion treatment, plasma nitridation, pre-oxidation and ion implantation.

55
Q

What is laser nitriding?

A

A material is places in the reactive gas environment and directly irradiated with a later light and nitrogen is fed through a nozzle into the melt pool. On a time scale of hundreds of nanoseconds and at a high intensity pulse-laser irradiation in ambient nitrogen atmosphere, it is capable to generate 1-1.5microm thickness nitride layer. draw diagram.

56
Q

What is vacuum vapour deposition?

A

In this process, the substrate is subjected to a chemical reaction by gases that contain chemical compounds of the materials to be deposited in very low vacuum conditions. Normally this process is limited to a very thin coating thickness. The deposited materials may include metals, oxides or ceramics. The substrates may include metals, plastics, ceramics or paper. Typical applications include tools and dies, automotive ceramic components, and decorative/textured surfaces.

57
Q

If the vapor source is a liquid or solid then what is the process called?

A

Physical vapor deposition (PVD)

58
Q

If the vapor source is a chemical precursor then what is the process called?

A

Chemical vapor deposition

59
Q

What are some vapor deposition processes?

A

Weld overlay thermal/plasma spray, electroplating/electro-less plating, polymer coating, hot dipping and laser alloying.

60
Q

What is physical vapor deposition?

A

PVD: a substrate or the product to be coated is placed in a vacuum chamber. The metal coating is evaporated through a thermal process and the vapor is deposited onto the substrate in a low temp environment. when the evaporated metal is removed from the vacuum chamber and returns to its solid state, the result is a hard, uniform, thin metallic coating on the substrate. This process presents several advantages in surgical and medical devices.