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..pdfStudies on friction and tramjer layer using inclined scratch |
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The EN8 steel flat was moved to a position where a minimum scratch length of 10 mm could be obtained. The tester was programmed such that pin would be lowered till it touches the flat, then the horizontal slide was moved at a speed of 2 mm/sec. The horizontal and vertical forces can be registered once the pin touches the surface, which increases as the pin slides on the EN8 steel flat. The normal and tangential forces are continuously acquired using a personal computer.
The coefficient of friction was calculated using formula given by equation (1).
T |
F Nsin8+ Frcos8 |
(1) |
p=-= |
F Ncos8- Frsin8 |
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where '6'is the angle of inclination of the EN8 steel flat, F r is the recorded traction force and FN is the recorded normal force at any instant.
The pins were slid at perpendicular and parallel to the uni-directional grinding marks generated by emery papers on EN8 steel flat. For the 8-ground and randomly polished surface, the direction of sliding was not important. The profiles and roughness parameters of the ENS steel flat were measured with a standard contact-type profilometer in the direction of the sliding on the bare surface away from the scratches. Later, the pins and ENS flats were observed using a scanning electron microscope (SEM) to study the surface morphology.
RESULTS
The average surface roughness (Ra) values of all ENS steel flats were measured for all four kinds of surfaces and are presented in table 1. It can be seen that surface roughness values for different textured surfaces are comparable with each other when they are ground against same grinding media.
Figure 3 (a) shows a typical variation of the normal load and tangential load obtained in the inclined scratch experiments. The recording is for an experiment carried out under dry conditions with an ENS flat with uni-directional grinding marks (Ra = 0.2359 11m) and the scratch direction being perpendicular to the uni-directional grinding marks. Figure 3(b) shows the variation of coefficient of friction with the sliding distance. It can be seen that the coefficient of friction does not vary much with the normal load, within the present load range in which the tests are conducted.
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Scratching of materials and applications |
Table 1: The surface rouglmess values for different textured surfaces.
Surface |
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Ra (microns) |
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0.4816 |
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0.2754 |
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600 |
0.2359 |
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800 |
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0.1823 |
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0.1171 |
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220 |
0.3965 |
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8-Ground |
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0.2539 |
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0.1681 |
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0.1055 |
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0.0911 |
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220 |
0.2028 |
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Uni-directional Parallel |
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0.1993 |
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0.1685 |
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800 |
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0.1167 |
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0.0845 |
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0.0237 |
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Random |
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0.1043 |
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0.2253 |
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0.1993 |
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100.--------------------------------- |
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120 |
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Sliding Distance (rum) |
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Fig. 3(a). |
Variation of forces with distance for AI-Mg alloy pin on ENS steel flat (Ra = 0.2359 |
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microns). |
Scratch direction is perpendicular to uni-directional grinding direction. |
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Figure 4 shows the variation of coefficient of friction for scratches conducted perpendicular to the uni-directional grinding marks with various roughnesses. Tests conducted under dry and lubricated experiments are shown on the same graph. It can be seen, for the dry experiments, that the coefficient of friction does not vary much with rouglmess (within the present range of
Studies on friction and transfer laver using inclined scratch |
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roughness) and the values crowd around a coefficient of fiiction value of 0. 7. For the lubricated experiments the coefficient of fiiction drops to a value of around 0.3. Here too, the fiiction values are independent of the roughness value.
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Sliding Distance (mm)
Fig. 3 (b). Variation of coefficient of fiiction with distance for AI-Mg alloy pin on EN8 steel flat. (R.= 0.2359 microns).
1.0 |
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R =0.4816 |
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R =0.2754 |
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0.9 |
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R = 0.2359 |
__.._.. R =0.1823 |
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0.7 |
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0.6 |
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0.5 |
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0.2 |
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0.1 |
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Lubricated |
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0.0 |
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Sliding Distance (mm)
Fig. 4. Variation of coefficient of fiiction with sliding distance for AI-Mg alloy pins on EN8 steel flat of different roughness (Ra in microns) under dry and lubricated conditions. Scratch direction is perpendicular to uni-directional grinding marks.
Figures 5, 6 and 7 show the variation of friction for the scratches carried out on the 8-ground EN8 flats, parallel to the uni-directional grinding marks and for the randomly ground surfaces. In all the above experiments it can be seen that, for a given kind of surface, the coefficient of
Studies on friction and transfer layer using inclined scratch |
271 |
- R =0.0237 |
- R =0.1043 |
a |
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- o - R = 0.2253 |
__._ R = 0.1993 |
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2
Sliding Distance (mm)
Fig. 7. Variation of coefficient of friction with sliding distance for AI-Mg alloy pins on randomly ground EN8 steel flat of different roughness (Rain microns) under dry and lubricated conditions.
1 . 0, -------------------- , 1.0 2.0
0.9 |
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Coefficient of Friction |
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0.9 |
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Fractal Dimension |
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Roughness |
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0.8 |
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·or- Dry condition |
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L--'-----'-----'-----'-=::......0 0 J1.0 |
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U-PD |
8-Ground U-PL |
Random |
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Nature of Surfaces
Fig. 8. Variation of coefficient of friction, surface roughness (Ra) and fractal dimension with nature of surfaces.
The range in which the coefficient of friction values fall for the various surfaces under both the dry and lubricated conditions are plotted in Fig. 8. The range of the roughness values and fractal dimension for each of the surfaces is also plotted in the figure. Many of the engineering surfaces are identified as fractal in nature and have a fractal dimension [36], a quantity that has been related to the friction and wear characteristics of surfaces. The fractal dimension is
Studies on friction and transfer layer using inclined scratch |
275 |
and reduces monotonically for the S ground, uni-directional parallel scratches and the randomly ground ENS surface. The figure also clearly shows that the adhesion component of friction, which is the difference between the coefficient of friction for the dry experiments and the lubricated experiments, is more or less constant but least for the randomly ground ENS flats.
When a soft material (Al-Mg alloy) is pressed and slid on a harder surface (ENS steel) the softer material will deform plastically to follow the contour of the harder material. The higher the normal pressure the closer would the softer material follow the contour of the surface of the harder material. For the sliding experiments carried out perpendicular to the uni-directionally ground ENS flats, which has a wave like surface, the softer aluminium alloy material will have to climb over the asperities. But, for the randomly ground surfaces, which has a hill and valley surface the softer aluminium alloy material could flow around the asperities. Only the material that comes directly in the central path of the asperity flows over the asperity. A schematic of the possible flow pattern is shown in Fig. 12.
More Plane Strain Condition |
More Plane Stress Condition |
Fig. 12. Schematic of flow pattern of a soft material over a hard cylinder and a sphere.
Flow over the uni-directionally ground ENS flats is shown as flow over cylindrical asperities and flow over the randomly ground ENS flat is shown as flow over spherical asperities. The flow pattern for sliding experiments carried out perpendicular to the uni-directionally ground EN8 flats induces a flow pattern that is more constrained and a state of stress that is more towards plane strain conditions. For the randomly ground ENS flats the flow pattern is more unconstrained and has a state of stress that is more towards plane stress conditions. The flow pattern for the sliding experiments carried out on the S-ground EN8 flats and parallel to the unidirectionally ground EN8 flats is, then, expected to fall in between these two extremes. This is because the constraint for flow induced by the S-ground surface would be lower than the unidirectional perpendicular experiments and would further reduce for the uni-directional parallel experiments. The results shown in Fig. S confirm this trend.
For a given Ra value, it can be expected that the cylindrical asperity height would be the same as that of the spherical asperity height. In such a situation the shear stresses ahead of the asperities
276 |
Scratching of' materials and applications |
while flowing over a cylindrical asperity (uni-directionally ground EN8 flats) would be much higher than the shear stresses generated ahead of the asperities while flowing around a spherical asperity (randomly ground EN8 flat). This again means that one could expect a higher plowing component of coefficient of fiiction for the uni-directionally ground EN8 flat, of similar Ra, when compared to the randomly ground EN8 flat. This is in agreement with the results of the present set of experiments. Further, the shear stresses generated for the dry experiments would be higher ahead of the asperities, which could lead to shearing of the material ahead of the asperities. This would lead to collection of transfer layer ahead of the asperities. From Fig. 9a, which shows the transfer layer formed on the uni-directionally ground EN8 flats, it can be clearly seen that the transfer layer forms ahead of the asperities. Here too, the results for the sliding experiments carried out on the 8-ground EN8 flats and parallel to the uni-directionally ground EN8 flats would fall between these two extremes.
Adhesion component of fiiction was characterized by the difference between coefficient of fiiction for the experiments carried out under dry and lubricated conditions, respectively. It is seen that the adhesion component of fiiction is almost same for all the surfaces. Real area of contact is high when adhesion component is high. This would increase the damage of the pin. If it is the adhesion component that is causing the damage, a drop of lubricant on the surface should reduce the damage. This is because the adhesion component of fiiction would be minimized. This is in fact what is observed when comparing the damage on the pin (compare Figs. 9b, 9d and lOb, lOd). This is evident from the fact the amount of transfer layer on the EN8 flat (Fig. 10 (a)) and damage on the pin (Fig. IO(b)) is high for the dry experiments. If, it is the adhesion component of friction that causes the large amount of transfer layer and damage of the pin one would then expect the transfer layer on the EN8 flat and the damage on the pin to be low under lubricated conditions. In fact, the amount of transfer layer is almost non-existent for the lubricated EN8 flat (Fig. IO(c)) and the damage of the pin reduces drastically (Fig. IO(d)). Thus one can conclude that the plowing component of fiiction is controlled by the constraint to flow induced by the surface. The higher the constraint to flow the higher would be the plowing component of fiiction. Finally, though the roughness, as given by R., and fractal dimension does not have a relation to the coefficient of friction other hybrid parameters that define a surface might have a relation. This has to be checked and work is progressing in this direction.
In the simulations done for sheet metal forming and other forming operations [24-28] the coefficient of fiiction is taken to be a constant or can be set at different values at various locations. The criterion for choosing different values of coefficient of friction at different locations is arbitrary or based on intuition. The present results give a better basis for choosing the coefficient of friction at various locations. In fact, the results can be used to choose a particular finish at different locations of the die so that the coefficient of fiiction could be varied according to the requirement. In locations where the coefficient of friction need to be high a uni-directionally ground surface with the flow perpendicular to the grinding marks can be machined and in locations where the coefficient of fiiction need to be low a randomly ground surface can be machined. Previous work carried out on dies with various surface roughness [9- 13] did not differentiate between the surfaces that offer various levels of constraint to flow. The work conducted by Lakshmipathy and Sagar [14] clearly showed that the fiiction factor, as estimated by ring tests, was lower for the die with a criss-cross surface when compared to a die with uni-directional surface. They, however, did not differentiate the role of constraint to flow which results in the reduction in fiiction factor. The control on the coefficient of friction under lubricated conditions could be based on this criterion but more work needs to be done to understand the exact reasons that control the adhesion component of fiiction. The role of adhesion has been summarized by Urbakh et al. [35] at much lower loads and speeds