It was confirmed that on WRS-370Hv, (pmax)s was higher and μs was lower than those on WRS-860Hv, and that therefore, the scuffing characterbrits depended on the surface hardness. Though (Tb)s of WRS-370Hv and WRS-860Hv were almost same for (σh)s ≦60 %, on WRS-370Hv of which the hardness was lower than WRS-860Hv, however, (Tb)s was high for (σh)s>70%, and the operation of it was possible to the higher temperature. For either (σh)s, Λs of WRS-370Hv was almost fixed, Λs=0.4, and Λs of WRS-860Hv was 0.4 and 0.1 each for (σh)s≦60 % and (σh)s≧ 70 %.
In the meantime, as there was no self-lubricating on SUJ2, (pmax)s was low and μs became high for either (σh)s, and it was confirmed that the anti-scuffing characteristics were very much inferior on SUJ2 and it was easy to cause the scuffing. In addition, on SUJ2, (Tb)s was lower than WRS, and Λs became higher.
In continuing, the surface modification treatments are examined. For (σh)s = 60 %, (pmax)s and (Tb)s of the Ni-P plating specimen were higher than those of SUJ2, but were lower than WRS without treatment. For (σh)s≧70 %, (pmax)s and (Tb)s of the Ni-P plating specimen rose, compared with WRS without treatment. For (σh)s=60 %, μsof the Ni-P plating was μs = 0.08, for (σh)s=70〜90%, μs=0.08, which was almost fixed, and for (σh)s≧80 %,μs lowered from that of WRS without treatment.
On the other hand, there was no large difference between the sulphurizing specimen and WRS without treatment, and so (pmax)s, (Tb)s, μs and Λs of the sulphurizing were almost equal to those of WRS without treatment. That is to say, the sulphurizing specimen seemed to show characteristics of the base material, WRS-370Hv, since the wear of the sulphurizing specimen relatively easily progressed, and sulphurizing layer on the sliding surface disappeared at the initial load step of the scuffing test.
4.2 OBSERVATION RESULT of SCUFFING DAMAGE
After scuffing test, the scuffing damage aspects of surface and cross section of specimen were investigated. For (σh)s=80 %, the results of 4 kinds of specimen, WRS-370Hv, WRS-860Hv, SUJ2 and the Ni-P plating (Base material: WRS-370Hv) are examined. It was omitted, because the sulphurizing specimen of which layer on the sliding surface disappeared at the initial load step of the scuffing test, was the damage aspects equal to WRS-370Hv.
Fig. 4 shows scuffing damage aspect of the surface of specimen. Fig. 4 (a)〜(d) are results of WRS-370Hv, WRS-860Hv, SUJ2 and Ni-P plating. The average damage widths were respectively 4.0 mm, 3.8 mm, 1.4 mm and 6.0 mm, and they were proportional for (pmax)s. Though damage width of the Ni-P plating was about 6.0 mm, the damage of the Ni-P plating layer was heterogeneous, of which the part remained for about 3.0 mm width, and the remainder had disappeared.
Fig. 5 and Fig. 6 show scuffing damage micro aspects of circumferential and axial cross section of specimen. Fig.5 (a), (b) are results of WRS-370Hv and SUJ2, and Fig.6 (a)〜(d) are results of WRS-370Hv, WRS -860Hv, SUJ2 and Ni-P plating.
(pmax)s of WRS-370Hv, WRS-860Hv and SUJ2 were respectively 2.1 GPa, 1.5 GPa and 1.0 GPa, as was shown in Fig. 3, and the anti-scuffing characteristics of WRS was very good. On WRS, the precipitations with the self-lubricating artificially were made to disperse in ordter to intend to improve the frictional characteristics. According to the image analysis result, the area ratio of the precipitations of WRS was about 2.0 %. The white layer existed at 30〜50 μm thickness on sliding surface of each spedmen. It was dearly observed in WRS-370, WRS-860 and Ni-P plating specimen in which (pmax)s was high, but it was indistinct a little in SUJ2. The cracks were initiated in white layer along the rotation direction of the roller specimen, and the peelings were produced in the part of the white layer.
Fig. 7 shows scuffing damage micro aspect of the axial cross section of the Ni-P plating at (σh)s = 60 %. According to scuffing test results shown in Fig. 3, the (pmax)s in the scuffing initiation had lowered, and the anti-scuffing characteristics lowered. The thin Ni-P plating was intermittently recognized, but white layer could not be recognized on sliding surface. The scuffing might be occuzurred before the white layer production. Though the testing condition of (σh)s = 60 % was not severe condition in ccomparism with (σh)s = 80 %, the cracks existed in surface vicinity of base material WRS-370Hv. The crack was not almost recognized on the Ni-P plating surface at (σh)s = 80%, the minute cracks were generated in great numbers and the surface roughness increases at (σh)s = 60%, and it is correspondent to the rise in friction coefficient μs of the Ni-P plating. As comparatively large Hertzian stress affected on the specimen, μs rose a little and the number of rotation increased at (σh)s = 60 %, the Ni-P plating might cause the rolling fatigue.
Qualitative analysis and semi-quantitative analysis by EPMA were carried out on sliding surface and inside (the position which left over 0.5 mm from the surface) of the Ni-P plating specimen at (σh)s = 60 % and 80 % in order to investigate composition of the scuftting white layer. Qualitative analysis and semi-quantitative analysis results were similar on both specimen, and the semi-quantitative analysis results on carbon of sliding surface and inside were 0.3〜0.9 wt %. In addition, according to XRD result of scuffing damage surface of the Ni-P plating specimen at (σh)s= 60 % and 80 %, the main products were Fe, Fe3C and FeO, and the products which corresponded for other specifically did not come. Namely, the white layer is the composition equal to the base material, and it is possble to form the similar layer by laser quenching treatment etc.
Table 5 shows the hardness (dynamic hardness DH115) of sliding surface and inside after the scuffing test at (σh)s = 80 %. Specimens are WRS-370Hv, WRS-860Hv, SUJ2 and Ni-P plating. The hardness of the white layer were almost 510〜570 DH115, (Hv ≧700), and it seems to be the quenched condition. And the hardness of the inside was respectively almost 340 DH115, 420 DH115, 540 DH115, and 340 DH115. Judging from heat treatment condition and compasition and mechanical properties of the specimen, it is estimated that the positon of 30〜50 μm of the sliding surface became very much a high temperature over the transformation temperature (about 1073 K).
