Abstract
Samples of AISI 4340 steel have been borided in a plasma initiated in a gas mixture of 2.7% diborane in argon. The plasma was sustained by inductive coupling to a 27.12 MHz generator. The effect of net r.f. power, pressure, position in the reactor and pre-sputtering of the steel surface on the microstructure of the surface layer and its hardening have been investigated. The plasma-borided surfaces have been investigated by scanning electron spectroscopy, X-ray diffractometry, Auger electron spectroscopy and microhardness measurements. As a result of the dissociation of the diborane in the plasma, boron is deposited onto the surface of the steel substrate and diffuses into the substrate. According to the plasma conditions, orthorhombic FeB, tetragonal Fe 2B or cubic Fe 4.5Ni 18.5B 6 is formed. Because of the formation of the boride phases, the surface microhardness increased from the original value of 2650 MPa (270 kgf mm −2) to a maximum value of 9815 MPa (1000 kgf mm −2). The largest increase in the surface microhardness was obtained when crystalline Fe 2B was formed.
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