SULPHURIZING OF METAL SURFACES BY ELECTROSPARK-DISCHARGE ALLOYING. Pt. 1: Structural-Phase State of Sulphur-Containing Coatings on Constructional Steels

The methods of surface sulphur saturation of metal surfaces to provide them with special tribotechnical properties are reviewed and analysed. The main attention is focused on technologies based on the method of electrospark alloying (ESA). As shown, the process of sulphur saturation can be realised by using a special sulphur-containing saturating technical substance (STS). The methods of forming sulphided, sulphocarburized, sulphoaluminized, Al-C-S, and sulphomolybdenum coatings on steels using STS by ESA are considered. The results of sulphur distribution in the surface layer during ESA sulphurizing with a metal electrode using STS are presented. As shown, the sulphur concentration on the surface is of about 0.53-0.60% that gradually decreases deeper into the substrate. The topography of the treated surface and its structure after sulphocarburized of steel surfaces with a graphite electrode using STS containing sulphur are investigated. As found, the coating consists of several layers: a 'soft' layer saturated with sulphur, a hardened layer saturated with carbon, and the substrate metal. The thickness, microhardness, and continuity of the coating increase with the discharge energy. The qualitative parameters of sulphoaluminized coatings obtained by the ESA method with an aluminium electrode using STS are analysed. The microstructures reveal three zones: a near-surface, non-continuous loose layer with sulphur enrichment, 10-100-um thick, and microhardness of 1368-2073 MPa; a 'white' hardened layer containing aluminium, 20-40 um-thick, and microhardness of 4094-5157 MPa; a diffusion zone; and a substrate material. The sulphoaluminized-coatings' phase composition depends on the ESA energy parameters. Intermetallics FeAl and FeAl2 are formed in the surface layer. The structural-phase state and properties of sulphomolybdenum coatings obtained by the ESA method with a molybdenum electrode using STS are discussed. The near-surface loose layer saturated with sulphur contains up to 8% of molybdenum disulphide formed due to ESA. Beneath this layer is a hardened layer saturated with molybdenum and having a microhardness of 10596-10731 MPa. It is proposed to use sulphurizing methods based on ESA using STS as cheap and effective methods of surface modification of friction surfaces to reduce seizure and friction coefficient.