PULSED PLASMA SURFACE MODIFICATION OF GREY CAST IRON

The work is devoted to the investigation of the effect of high-energy pulsed plasma treatment on the surface modification and tribological characteristics of grey cast iron with the initial microhardness of about 220 HV. The modification was performed in the air atmosphere using an electro-thermal axial plasma accelerator with an arc discharge voltage of 3.0 kV and 4.0 kV to produce a plasma flux with a power density of 1.0 . 109 W/m(2) and 1.75 . 109 W/m(2) respectively. The study was carried out using numerical modelling, optical (Eclipse M200 Nikon) and electron scanning (JSM-6510 JEOL) microscopy, EDX (JED-2300 JEOL), X-ray diffraction (X'Pert PRO PANalytical diffractometer with CuK alpha source), microhardness measurement, and wear testing according to "Three-body abrasion" (Al2O3 sand) and dry-sliding "Ball-on-Disk" schemes. The results showed that a single plasma impulse with a power density of 1.0 center dot 10(9) W/m(2) led to a surface modification of up to 12 mu m depth without melting to form ultrafine-grained martensite with the microhardness of 359-652 HV. An increase in power density to 1.75 center dot 10(9) W/m(2) resulted in a modification of up to 28 mu m depth while the surface was melted to 10-12 mu m depth. Under the melting the graphite dissolved causing a carbon-enrichment of the melt with a consequential formation of ultra-fine "Ledeburite" eutectic cells and high-carbon martensite having microhardness of 697-817 HV. Plasma-induced surface dopping by cathode elements (W, Cu) through the melt resulted in the crystallization of globular carbide WC and in over-enrichment of ferrite by copper (of about 3 wt. %). The solute trapping of copper in the ferrite lattice occurred due to the ultra-high cooling rate (2 center dot 10(6) K/s) after the plasma-induced heating. The pulsed-plasma modification of grey cast iron led to a two-fold increase in its abrasive wear resistance while the dry-sliding friction coefficient was slightly increased too due to the worsening of self-lubrication caused by the graphite dissolution.