DETERMINATION OF THIN FILM'S MECHANICAL PROPERTIES

11
It has also been reported that the tribological properties depend on the
microstructure of the boride layer. The dual-phase FeB-Fe 2 B layers are not inferior to
those of monophase Fe 2 B layers, provided that the porous surface zone directly
beneath the surface is removed (W. Liluental et al, 1983). Alternatively, a thinner
layer is favored because of less development of brittle and porous surface zone
formation and flaking.
Typical properties of the FeB phase are:
• Microhardness of about 19 to 21 GPa.
• Modulus of elasticity of 590 GPa.
• Density of 6.75 g/cm 3 .
• Thermal expansion coefficient of 23 ppm/ o C
• Composition with 16.23 wt % boron.
• Orthorhombic crystal structure with 4 iron and 4 boron atoms per unit cell
• Lattice parameters: a= 4.053 A o , b= 5.495 A o and c= 2.946 A o
The formation of single phase Fe 2 B layers with a saw tooth morphology is
desirable in the boriding of ferrous materials (D.N. Tsipas et al, 1988). A single Fe 2 B
phase can be obtained from a double FeB-Fe 2 B phase by a subsequent vacuum or salt
bath treatment for several hours above 800 o C, which may be followed by oil
quenching to increase substrate properties (P.A. Dearnly et al, 1986).
Typical properties of Fe 2 B are:
• Microhardness of about 18 to 20 GPa
• Modulus of elasticity of 285 to 295 GPa
• Thermal expansion coefficient of 7.65 ppm/ o C
• Density 7.43 g/cm 3
• Composition with 8.83 wt% boron
• Body-centered tetragonal structure with 12 atoms per unit cell
• Lattice parameters: a=5.078 A o , c= 4.249 A o
The solubility of boron in ferrite and austenite is very small (< 0.008 % at 900
o C) according to the Fe-B phase diagram (T.B. Massalski, 1986). According to the
Brown and Nicholson (M.E. Nicholson, 1954), the phase diagram exhibits an / γ /