Table 1 shows the chemical oompositions of specimen. Table 2 shows an example of mechanical and physical properties of WRS.
2.2 SURFACE MODIFICATION TREATMENT
Surface modification treatments, Ni-P alloy electroless plating (Ni-P plating) and electrolyzed and sulphurizing3 (sulphurizing) were examined.
In Ni-P plating treatment, after specimen was degreased, the specimen was dipped in reductive NaH2PO2 solution at room temperature, and a thin plating layer consisted of the 92wt%Ni-8% P composition was formed on the surface.
In sulphurizing treatment, after specimen was degreased, the specimen was dipped in rhodanic acid melted salt of about 463 ± 5 K, and the specimen was electrolyzed as an anode and the tank as a cathode. Sulphurizing layer of several μm was formed on the surface by the electrochemical reaction. This sulphufizing layer is poroum and soft, and it has the initial running-in condition with the plastic flow, and the scuffing initiation is suppreased by the increase in the true sliding plane area.
2.3 TEST ROLLER
Fig.1 shows shapes and dimensions of specimen. The dimensions of test roller were 40 mm in diameter and 10 mm in width. The rollers were ground after quenched and tempered. The faster roller was crowned. Two kinds of heat treatment were applied to the WRS. The surface hardness of the rollers were 370Hv (the following, WRS-370Hv) and 860Hv (WRS-860Hv); the former was the hardness of sliding materials in cylinders for diesel engine, and the latter for machinery. The surface hardness of the JIS-SUJ2 rollers was about 780Hv (SUJ2)
Ni-P plating and sulphufizing were treated to WRS-370Hv, and the thickness of layer was respectively about 10 /μm. Since the surface modification layer was very thin, the hardness of the layer was measured by a thin film hardness tester of the angle 115°trigonal pyramid indenter between edges (dynamic hardness DH115). The dynamic hardness measurement results of the Ni-P and the sulphurizing layer are respectively 300 DH115 and 70 DH115.
3. EXPERIMENTAL PROCEDURE
3.1 TWO ROLLERS SCUFFING TEST
The scuffing characteristics of specimen were evaluated using two rollers testing equipment, a spring load type as is shown in Fig. 2. Scuffing tests were carried out under the step-load method in which the load was in eased stepwise at a constant velocity ratio. The specific sliding was in the range of 50 to 90 % and the mean rolling speed was 16.7 Hz (1000rpm).
The normal load Pn was applied to the rollers by the loading spring, and the Hertzian pressure Pmax was calculated with the Pn and the dimensions of the rollers shown in Fig. 1. The initial Pmax was 0.8 GPa and the increments of pmax were 0.2 GPa at specific sliding σh=50〜60 % and 0.1GPa at σh≧70 %. Operating time at each load step was 300 seconds, which was enough for the outer surface temperature to reach an almost steady state.
The scuffing initiation was detected by a rapid incaease in both the frictional torque between two rollers and the outer surface temperature. The frictional torque was measured with a torque detector. The outer surface temperatures of rollers were measured by thermocouples, φ0.3 mm, which were set at a depth of 1 mm below the center of contact width of the outer surface.
Table 3 shows the properties of # 83 turbine oil used as a lubricant. The lubricant oil was pressure-fed into the engaging side of the roller pair at a rate of 5 ml/sec (:300 ml/min), and the oil temperature was adjusted to be 313 ± 1 K.
3.2 OBSEVATION of SCUFFING DAMAGE
After the scuffing test, the scuffing damage aspect of surface and cross section of the low speed roller specimen was investigated.
Scuffing damage aspect of the roller specimen surface was observed by the replica method. In order to copy the specimen surface the replica was affixed to it, and then the transfer replica was observed using the microscope. The scuffing damage micro aspects of circumferential cross section and axial cross section were observed using the optical microscope.
The products of the sliding plane of spedmen were examined using EPMA (EPMA-1600; Shimadzu Co., Lid), and XRD (MXP18; MAC Science Co., Ltd). The hardness of the cross sectiron in the sliding surface vicinity was measured by a thin film hardness tester (dynamic hardness DH115).
4. TEST RESULT and DISCUSSION
4.1 SCUFFING TEST RESULT
Fig.3 shows scuffing test result by the two rollers. The correlation in the scuffing initiation between normal load (Pn)s, maximum Hertzian stress (Pmax)s, outer surface bulk temperature (Tb)s, friction coefficient μs, and film thidmess parameter Λs, and specific sliding (σh) s was shown.
Now, the film thickness parameter Λs is defined in following equation.
Λs = hmin/Rrms ...(1)
Here, hmin: The smallest film thickness calculated by the equation of Hamrock-Dowson for bulk temperature (Tb)s.
Rrms: The synthesis surface roughness of high speed roller and low speed roller required from the surface roughness at test load before one step of the scuffing initiation;
Rrmsis defined in the following.
Rrms =1.13 x {(Ra1)2+(Ra2)2} 1/2 ...(2)
Here, Ra1 and Ra2 are respectively surface roughness of high speed and low speed roller.
Table 4 shows the list on specific sliding (σh)s and maximum Hertzian stress (pmax)s and friction coefficient (μ)s in the scuffing initiation.
