What is tribology?

Tribology is the science and engineering of interacting surfaces with relative motion.

What are the three areas of study in tribology?

Friction

Lubrication

Wear

What are the 4 things tribology aims to address?

- The effect of surface properties (e.g. structures and textures) on the processing of materials.
- The impact of wear, lubrication and friction in manufacturing processes.
- Surface treatment for the enhancement of appearance and performance of manufacturing products.
- Destructive and non-destructive inspection methods for manufactured parts.

What is surface engineering?

A sub-discipline of material science that deals with the surface of solid matter.

What are solids composed of?

A bulk material (bulk phase) covered by a surface (surface phase).

What is the surface an interface between?

Between the bulk material and the surrounding environment.

What can degrade the surface?

interaction between the surface and the environment.

What does surface engineering involve?

Altering the properties of the surface in order to reduce the degradation.

What kind of properties does the bulk material provide?

The overall mechanical properties (e.g. strength, ductility, etc).

What kind of properties does the surface provide?

Essential properties (e.g. friction coefficient, wear rate, etc).

What are 3 examples where the surface provides the primary property/function of the product? (name the primary and secondary properties in each case).

- Break pads: primary properties: high coefficient of friction, low wear rate over a range of temps. Secondary properties: ductile and form-able to shape.
- Machine tools: primary properties: the ability to perform cutting at a wide range of temps which requires high wear resistance and oxidation resistance. Secondary properties: the need to make the correct shape.
- Mirrors: primary properties: need to provide reflective surfaces. Secondary properties: supportive strength.

What is surface topography?

the geometric configuration of the surface.

What are surfaces like?

Irregular, rough and comprise of hollows and peaks (asperities).

What are the three classifications for tribology to characterize the surface topography?

Tribology - macro-scale (10-3)

Micro -tribology - micro-scale (10-6)

Nano-tribology - nano-scale (10-9)

What are the three techniques to characterize surface topography?

- Optical microscope
- Electron scanning microscope
- profilometers (2D and 3D).

What is talysurf?

obtain a quantitative measure to confirm that the proposed surface finishing has been met with high accuracy (reach up to nm scale).

What does talysurf give?

A single statistical measure of the roughness of a surface.

What is average roughness (eqn and description)?

Ra = 1/n (sum from i=1 to n) of hi. The CLA value of vertical deviations.

What is mean square roughness (eqn and description)?

Rq = (1/n (sum from i =1 to n) of hi^2)^1/2

RMS value of vertical deviations.

How is surface roughness represented in engineering drawings?

-By a symbol and a value.

What does the symbol refer to in surface roughness representation?

The manufacturing process that’s required/allowed to achieve the proposed roughness.

What does the value refer to in surface roughness representation?

The roughness index in microns or micro-inches.

What does a basic symbol look like and when is it used?

A basic symbol looks like: ….

and it is used when the process is provided.

What does adding a bar to the basic symbol look like and mean?

Means the removal of material is required by machining (draw pic).

What does adding a circle mean and look like?

Draw pic.. means the removal of material is not permitted.

What does adding a line to the long arm look like and mean?

Draw pic.. means a special surface characteristic is required.

On page 11 there is a drawing, what do all the letters stand for?

Ra = center line average (CLA) h = waviness height w = waviness width S = roughness width L = length of measurement surface

What does it mean to say that Ra and Rq are not unique surface characteristic features?

It means that two or more different surfaces could have the same Ra or Rq values.

What do alternative measures for surface roughness describe?

Used to describe surface topography based on probability distribution of the surface height ordinates.

What is the probability density of height function?

P(z) = [1/(Rq * 2pi^1/2)] * e^(-z^2/2Rq^2).

- i think this gives the true area of contact (asperities).

What is a bearing ratio/ bearing area curve used for?

Used primarily for the analysis of load carrying surfaces. It gives the percentage of material to air in the surface profile at any level.

What are hertzian stresses?

Stresses which arise from contact between solids.

Where does the peak shear stress in the softer material occur for the single contact condition between a hard rider and a soft surface?

0.5-0.7a below the surface.

what is the equation for equivalent youngs modulus?

1/E = 1-v1^2/E1 + 1-v2^2/E2.

What does a stand for?

The impression radius.

When does the material yield?

When contact stress (sigma) >= 3.1 yield stress in shear (k).

What are the tresca and von mises criteria for yield?

Sy = k for single uniaxial stress state Sy = 2k for plane stress.

Why is this yield criteria as such?

Because of the hydrostatic component of stress that acts all round the point of maximum stress.

What happens when contact stress is greater than 3.1 k?

Plastic zone will form

What happens when contact stress >= 6k?

Materials flow will occur.

What will a surface coating of t=a do?

Virtually embrace the stress field so that in hertzian contacts it behaves as though it were the bulk material.

What is the eqn for indentation hardness?

H = 6k = 3 Sy

What is the brinell hardness number eqn and its units?

BHN, HB (kg/mm^2) = 2F/ [pi*D*(D- sqrt(D^2 - d^2)]

F = applied force (kgf)

D = diameter of indenter (mm)

d = diameter of indentation (mm).

What is the mean contact radius for contact between two curved surfaces?

1/ R = 1/R1 + 1/R2

where

R = mean contact radius

R1, R2 = the principal radii of curvature of the 2 bodies.

What is the plasticity index?

A measure of whether the stress levels at the point of contacts (asperities) between 2 surfaces will cause elastic deformation (index less than 1), plastic deformation (index greater than 3) or a combination of the two.

What is the eqn for plasticity index?

Pi = E/H * [sigma/beta]^1/2

where

E= equivalent modulus of elasticity

H = hardness of softer component

Beta = mean radius of curvature of the asperity tips surface

Sigma = rms of asperity peak height or Rq.

What happens if Pi

most of the asperities are deformed elastically; plastic deformation occurs only under high contact pressures.

What happens if 3>= Pi >= 1?

Intermediate area (mix of both elastic and plastic deformation)

What happens if Pi > 3?

Most of the asperities will be plastically deformed already under low contact pressures.

Example material with a high E/H ratio?

Steel

What does it mean if a material has a high E/H ratio?

It means that the material has a high plasticity index.

With a material with a high plasticity index (E/H ratio high) what do we have to do to be in the elastic area?

We have to lower surface roughness or contact pressure (at significantly higher surface roughness this is not sufficient). Only finely polished surfaces will remain in the elastic area.

Example of material with low E/H ratio?

Ceramic

For a material with a low E/H ratio what state will the contacts be in?

Majority of contacts will be elastic even at slightly higher surface roughness.