2.1 Experimental procedures

For the present investigation iron based (using atomized iron powders, produced by Heigazús AB, Sweden) and Fe-1.5Mo based materials (using the prealloyed AstaloyMo powder, produced by Höganäs AB, Sweden) were produced. Graphite was added to the Fe and Fe-1.5Mo powders to produce the materials to be tested in the as-sintered condition. In Table 2.1 the materials produced are shown, along with the codes used for their designation.

Material	Heat treatment	Code
Fe-0.6%C	as sintered	Fe06C
Fe	carbonitrided	Fe-CN
AstaloyMo+0.5%C	as sintered	AMo05C
AstaloyMo	carbonitrided	AMo-CN

Table 2.1 Materials investigated

All materials were produced in the form of disks for the rolling-sliding tests (diameter: 40 mm; thickness: 10 mm). The compacting pressures were selected in order to reach a final density of about 7.0 g/cm3 for all the materials (the total porosity was about 10%). Sintering was carried out at 1120°C for 20 minutes in an atmosphere of N₂/5% H₂. The carbonitriding treatment comprised a soaking period at 860°C for 60 minutes, quenching in oil at 60°C and tempering at 180°C for 60 minutes.

The two as sintered materials are characterized by an hardness of 110 HV 30 (Fe06C) and 160 HV 30 (AMoO5C). Carbonitriding produced different hardened layers, as outlined by the microhardness measurements shown in Table 2 (a Vickers indenter was used with a load of 50 g). It can be noted that the surface peak hardness is similar in both materials, whereas hardness decreases more sharply in Fe-CN than in AMo-CN as the depth is increased.

Material	surface	100 μm	250 μm	500 μm	750 μm	1000 μm
Fe-CN	900	860	750	500	200	190
AMo-CN	880	850	780	660	560	500

Table 2.2 Microhardness values of the surface treated materials at different depths

The lubricated rolling-sliding tests were carried out in a disc-on-disc configuration using an Amsler testing rig, described in detail in ref. [7]. The specimen disk rotated at an angular speed of 400 rev./minute, whereas the counterface disk rotated at 360 rev./minute, thus producing a contact sliding velocity of 0.084 m/s. In every test both the specimen and the counterface disks were made of the same material.

As a lubricant, SAE 15W-40 oil was used (with a dynamic viscosity of 0.153 Pa s at 30°C and at a pressure of 1 bar). During each test the oil temperature did not exeed 80°C. The contact load was set at 2000 N for every test and produced a maximum contact pressure of 700 MPa.

The microscopic subsurface damage was investigated by examination, by optical microscopy, of cross-sections taken parallel to the sliding direction.