Electron beam surface modification of a porous bronze-graphite composite

Elemental powders were mixed to obtain a 90 wt pct copper, 8 wt pct tin, and 2 wt pct graphite composite. The porosity level of the sintered specimens was reduced from 25 to 10 pct, which resulted in an increase in the macrohardness value from 17 Hv (90 MPa) to 67 Hv (355 MPa); the density of the sintered specimen was 7.80 g · cm⁻³. The synthesized material was then subjected to electron beam (EB) surface melting. The resultant surface was homogeneous and the microstructural features were refined. The segregation level and variation in the microhardness were drastically reduced. The morphology of the otherwise irregular pores changed to spherical, thereby reducing their interfacial energy. An intriguing modification in the EB melted layer had a density gradient with depth that is sensitive to the heating time of the material using EB. At a heating time of 250 ms, the upper region of the melted layer was dense and hard; the density and the hardness were 8.5 g · cm⁻³ and 103 ± 7 Hv, respectively, while the lower region had density of 6.7 g · cm⁻³ (porosity 22 pct). If the heating time was reduced to 17 ms, the distribution of pores was reversed; the density of upper and lower layers changed to 3.9 and 8.2 g · cm⁻³, respectively. In spite of the higher density of pores, the EB processed composite exhibited increased hardness, compressive strength, and tensile strength. The formation of pores in the lower EB melted region was explained using a qualitative fluid flow model. The combination of a dense substrate and porous surface was desirable, since the former improved the strength and the thermal conductivity of the composite and the latter could be impregnated with oil to achieve the required lubrication levels.