77 Friction of metals Introduction - www.lmu.edu

77

Friction of metals
• Introduction
o Values of the coefficient of static friction





Table 7. Typical values of the coefficient of static friction for combinations of metals in
air and without lubrication (Hutchings, 1992).

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• Clean metals in high vacuum
o

o



o

o
o

Figure 88. The effect of oxygen on the friction of pure iron (Hutchings, 1992).

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• Self-mated metals in air

o

o

o




o



•
•



•
•
•

Figure 89. The variation of the coefficient of friction with applied normal load for
unlubricated copper sliding against copper in air (Hutchings, 1992).

o

80

• Dissimilar metals and alloys
o
o
o

Figure 90. The variation of coefficient of friction with normal load for unlubricated plain
carbon steels sliding against themselves in air (Hutchings, 1992).

For the 0.4% carbon (Figure 90):
•

•
•

Surface segregation of minor constituents can influence friction
•
•
•

o Some alloys have a low coefficient of friction (Table 7)



•

•

•


•

•

• Effect of temperature 81
o As temperature increases: Cubic close-packed




o

Body-centered cubic

Hexagonal close-packed

Figure 91. The variation of coefficient of friction with temperature for various pure
metals sliding against themselves in ultra-high vacuum: (a) c.c.p. metals; (b) b.c.c.

metals; (c) h.c.p. metals (Hutchings, 1992).

82

o Similar trends for the influence of friction on hardness


Figure 92. Influence of surface hardness on the coefficient of friction as a function of
applied load (Wang, 2002).

o For metals heated in air:



Figure 93. The dependence of coefficient of friction on temperature for an austenitic
stainless-steel sliding against pure nickel in air (Hutchings, 1992).

83
o Influence of sliding

When surface are sliding at high speeds and becomes molten,
friction is decreased (decrease in interfacial shear strength)

Figure 94. Effect of surface melting on the friction of a sliding steel interface (Wang,
2002).

Figure 95. The variation of coefficient of friction with sliding speed for pure bismuth and
copper sliding against themselves (Hutchings, 1992).

o Phase transformations can occur

84

o Solid state phase transformation
Cobalt
•

•
•
•

Figure 96. The variation of coefficient of friction with temperature for cobalt sliding
against itself in vacuum (Hutchings, 1992).

Tin
•
•
•
•

Figure 97. The variation of coefficient of friction with temperature for tin sliding against
itself (solid curve). The addition of a small amount of bismuth suppresses the allotropic

transformation to grey tin (broken curve) (Hutchings, 1992).

85

o Order-disorder transformation







Figure 98. The variation of coefficient of friction with temperature (in vacuum) for the
alloy Cu3Au which exhibits an order-disorder transformation at 390°C (Hutchings, 1992).

Friction of ceramic materials
• Introduction
o

o

o
o
o
• Chemical structures
o

o

o

o

o

o

86

• Surface chemical reactions
o

o

Figure 99. Illustration of the effect of environment on the friction of hot-pressed silicon
nitride. The values of μ are derived from pin-on-disc tests at a sliding speed of 150
mm/s, carried out in the environments indicated (Hutchings, 1992).

o For TiN:




o For diamond:

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• Fracture on the sliding surface
o

o

Figure 100. Variation of coefficient of friction with normal load for a 60° diamond cone
sliding over the (0001) face of a silicon carbide single crystal. The increase for loads
above ~4 N is associated with fracture along the sliding path (Hutchings, 1992).
• Effects of service conditions
o
o

Figure 101. Variation of coefficient of friction with temperature for magnesia-partially
stabilized zirconia and alumina sliding against themselves in air (Hutchings, 1992).

88

Figure 102. Variation of coefficient of friction with sliding speed for reaction-bonded
silicon carbide and hot-pressed silicon nitride samples sliding in self-mated couples in air

(Hutchings, 1992).
References
Books

• K.L. Johnson, Contact Mechanics, Cambridge University Press, 1985.
• I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward

Arnold, London, 1992.
Other

• Q. Wang, Introduction to Tribology, Lecture Notes, Northwestern University,
2002.