Industrial Tribology-Friction of Metals and Non-Metals

     

Friction of Metals and Non-Metals

   Friction of Metals

• The coefficient of friction of a particular materials depends on 3 factors:       1. Mating materials 2. Surface roughness and  3. Operating conditions

• When the metal surfaces are cleaned in high vacuum and placed in contact, strong adhesion is observed and consequently high friction is observed

• In vacuum typically 2 to 10 or even more
• With no interfacial contamination, the extent of junction growth is limited by ductility of material
• Gold is ductile but it does not forms oxide layers in the air, thus considerable amount of  junction growth in gold contact leads to high friction
• Most metals forms oxide layer in air and the layer will be in the range of 1 to 10nm These  films play important role in frictional characteristics

Variation of coefficient of friction with normal load for copper sliding on copper In air

• At low normal loads, the oxide films separate the two metals
• Coefficient of friction is low because the oxide has low shear strength
• At higher loads the surface films deforms and metallic contact occurs leading to high frictions                                                                                                                                             Note: for chromium very thin but strong oxide layer is formed and no metallic contact occurs for a wide range of normal loads leading to a low constant friction
• Friction of metals is affected by number of parameters like,

• 1. Sliding velocity

• 2. Contact Pressure

• 3. Temperature

• 4. Relative Humidity

• 5. Environmental conditions

Variation of coefficient of friction as a function of temperature for cobalt sliding on stainless steel

• Cobalt exhibits phase transformation from Hexagonal close packed structure to Cubic Packed structure at 4170c
• This is fully ductile
• This phase transformation leads to peak friction atm5000c
• The decrease in friction after 5500c is because of oxide film thickness and changes in oxides species from CoO to Co3O4

Friction of Non-Metallic Materials

• Ceramics combine low density with excellent mechanical properties (high strength,stiffness, hardness etc..) up to high temperatures

•  These are called engineering materials
•  These engineering materials include silicon nitride (SiN4), silicon carbide (SiC), Alumina (Al2O3), Zirconia (ZrO2) Ceramics are used in extreme conditions like high loads, high speeds, high temperatures and corrosive environments
• Because of different nature of bonds in ceramics compared with metals they show limited plastic flow at room temperatures
• Correspondingly much less ductility than metals
• Although adhesive forces are present the very low real area of contact makes them to have relatively low coefficient of friction
• In clean environment friction coefficient does not reach high values as in the case of metals show high friction in vacuum
• The reason for this less friction is the coefficient of friction decreases with an increase in fracture toughness

Friction as function of Fracture

Toughness

• This is for sharp diamond tip on (SiN4), (SiC), (Al2O3), (ZrO2) disks produced under various hot pressingconditions
•  Fracture is readily produced in concentrated contacts
• At low loads, friction is low and no fracture occurs with plastic grooving

Variation of coefficient of friction with normal load for 600

diamond cone sliding over the face of a silicon carbide

• With increase in load the friction increases and fracture occurs
• The role of normal load, sliding speed, temperature and test duration of friction of ceramics may be interpreted based on the tribo-chemical changes in the surface film and also extent of fracture in the contact zone
• Load and speed affects the temperature at the interface

Variation of coefficient of friction with sliding speed for

reaction bonded silicon carbide and hot pressed silicon nitride samples

• This is for self mated silicon carbide and silicon nitride
• This similar phenomenon is observed in alumina and zerconia
• With sliding speed interface temperature increases and this enhances the film formation in the sliding surface which decreases the friction

Variation of coefficient of friction as a function of

temperature for alumina and zerconia

• This is sliding of self mated pair in air
• The removal of adsorbed water results in initial rise of friction

Friction of Polymers

• Polymer includes elastomers and plastics
• The coefficient of friction for polymers ranges from 0.15-0.6 in general
• With the exception of PTFE (Polytetrafluoroethylene) which have very low coefficient of friction 0.05
• Thus in general polymers exhibit low coefficient of friction comparative to metals and ceramics
• Mostly used in the applications are self lubricating solids # PTFE, HDFE (High density polyethylene), polyphenylene sulphide (PPS), pluamide (Nylon), polyimide, acetal etc. are commonly used plastics # commonly used elastomers are natural and synthetic rubber, styrene butadiene rubber (SBR), silicon rubber etc.
• These self lubricated solids readily flow at moderate temperatures and pressures
• Since polymers lack in rigidity and strength, polymer composites are used to provide combination of mechanical strength with low friction and wear
• Carbon, graphite and glass are commonly used filler materials to make polymer composites
• When plastic slide against hard metal surfaces, transfer film of plastic is formed on the mating surface and this governs the friction and wear
• Sliding tend to occur at the interface of bulk polymer and transfer film leading to low wear rates
• The coefficient of friction for initial hard materials is 0.2-0.3
• As the sliding continues the coefficient of friction drops to much lower values