22UCH101 - Engineering Chemistry
(Semester I)
2022 - 2023
, Unit- IV
Engineering Materials
UNIT IV ENGINEERING MATERIALS 6
Refractories – classification – acidic, basic and neutral refractories – properties
(refractoriness, refractoriness under load, dimensional stability, porosity, thermal spalling)
– manufacture of alumina, magnesite and zirconia bricks.
Lubricants – mechanism of lubrication, liquid lubricants, - properties – viscosity index,
flash and fire points, cloud and pour points, oiliness) – solid lubricants – graphite and
molybdenum sulphide.
4.2. Lubricants:
4.2.1. Introduction
A lubricant is a substance introduced between two moving surfaces to reduce
friction. This property of reducing friction is termed lubricity or slipperiness and the
process of reducing friction between two moving surfaces by the introduction of lubricants
is known as lubrication. Before going into what lubrication is, what a lubricant is and how
it functions, we need to know what friction is. Friction is the force that resists the relative
motion between two bodies in contact. The surfaces of all material, no matter how
smooth they appear, are not truly smooth on an atomic scale. They are rough, with sharp
and rugged projections called asperities (Fig. 4.2.1(a)). When two surfaces come in
contact with each other, initially, they only touch at a few of these asperity points
(Fig. 4.2.1(b)). When a small load is applied on a metal surface, local high pressure
develops at these points (asperities) of contact, which then tend to interlock, thus
developing a resistance to motion. When a metal slides, the peaks get broken resulting
in wear and tear of the surfaces. Further, if the pressure is high, the asperities deform
1
, plastically increasing the contact area between the two surfaces. A sufficiently high
pressure leads to the formation of a welded junction between asperities. Additional force
is then required to break these junctions. This further adds to the frictional resistance.
Figure 4.2.1. (a) Magnified view of a real surface.
(b) Contact between two surfaces on application of load.
Thus, frictional resistance harms the system in three ways.
• It causes energy loss. Extra work has to be done to overcome the force of
friction.
• Friction causes generation of heat in areas in contact. These heated parts may
melt and get deformed, leading to seizure (welding of two surfaces due to heat)
of the system.
• Breaking of asperities brings about abrasion or surface wear and tear.
4.2.2. Functions of Lubricants
• A lubricant reduces frictional resistance by keeping the moving surfaces of a
system apart and prevents their direct contact so that the asperities do not
interlock.
• It reduces surface deformation, and wear and tear by keeping the moving surfaces
apart.
• The lubricating oil absorbs the generated heat and acts as a coolant.
• Lubricants increase the efficiency of the machine.
2
(Semester I)
2022 - 2023
, Unit- IV
Engineering Materials
UNIT IV ENGINEERING MATERIALS 6
Refractories – classification – acidic, basic and neutral refractories – properties
(refractoriness, refractoriness under load, dimensional stability, porosity, thermal spalling)
– manufacture of alumina, magnesite and zirconia bricks.
Lubricants – mechanism of lubrication, liquid lubricants, - properties – viscosity index,
flash and fire points, cloud and pour points, oiliness) – solid lubricants – graphite and
molybdenum sulphide.
4.2. Lubricants:
4.2.1. Introduction
A lubricant is a substance introduced between two moving surfaces to reduce
friction. This property of reducing friction is termed lubricity or slipperiness and the
process of reducing friction between two moving surfaces by the introduction of lubricants
is known as lubrication. Before going into what lubrication is, what a lubricant is and how
it functions, we need to know what friction is. Friction is the force that resists the relative
motion between two bodies in contact. The surfaces of all material, no matter how
smooth they appear, are not truly smooth on an atomic scale. They are rough, with sharp
and rugged projections called asperities (Fig. 4.2.1(a)). When two surfaces come in
contact with each other, initially, they only touch at a few of these asperity points
(Fig. 4.2.1(b)). When a small load is applied on a metal surface, local high pressure
develops at these points (asperities) of contact, which then tend to interlock, thus
developing a resistance to motion. When a metal slides, the peaks get broken resulting
in wear and tear of the surfaces. Further, if the pressure is high, the asperities deform
1
, plastically increasing the contact area between the two surfaces. A sufficiently high
pressure leads to the formation of a welded junction between asperities. Additional force
is then required to break these junctions. This further adds to the frictional resistance.
Figure 4.2.1. (a) Magnified view of a real surface.
(b) Contact between two surfaces on application of load.
Thus, frictional resistance harms the system in three ways.
• It causes energy loss. Extra work has to be done to overcome the force of
friction.
• Friction causes generation of heat in areas in contact. These heated parts may
melt and get deformed, leading to seizure (welding of two surfaces due to heat)
of the system.
• Breaking of asperities brings about abrasion or surface wear and tear.
4.2.2. Functions of Lubricants
• A lubricant reduces frictional resistance by keeping the moving surfaces of a
system apart and prevents their direct contact so that the asperities do not
interlock.
• It reduces surface deformation, and wear and tear by keeping the moving surfaces
apart.
• The lubricating oil absorbs the generated heat and acts as a coolant.
• Lubricants increase the efficiency of the machine.
2
