**3.1 Effect of ball burnishing on the surface topography**

The values surface topography parameters of the height of the five burnished samples, in comparison to the turned one, are regrouped in **Table 2**.

As a consequence of the ball burnishing process, the root means square height of the surface Sq was decreased in all samples as compared to the untreated one. During ball burnishing, the first two passes significantly reduced the Sq parameter from 2.212 μm to 0.715 and 0.729 μm respectively. After three passes (i = 3), the Sq still decreased achieving 0.583 μm. However, further augmentation in the number of passes provoked an increase in the Sq parameter which can attain a value higher than the initial one after five passes (i = 5). The turned surface is characterized by a more negative skewness Ssk and higher kurtosis Sku than all burnished samples. The highest skewness and the lowest kurtosis are registered in the surface burnished with three passes (i = 3/Ssk = −0.784 Sku = 2.77). The other amplitude parameters (Sp, Sv, Sz, and Sa) of all samples decreased after ball burnishing. These parameters follow

*Surface Integrity of Ball Burnished 316L Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.101782*


**Table 2.**

*Comparison of height surface topography parameters of ground and ball-burnished samples.*

the same tendency with respect to the number of passes as the Sq parameter. Hence, it can be noted that three passes (i = 3) are the most appropriate if surface topography is aimed to be improved. Further augmentation in this parameter can lead to the deterioration of surface quality, which is indicated by the increase in the amplitude parameters. The main objective of ball burnishing is the reduction of the heights of surface irregularities. Effectively, this objective was reached because the results show that the height parameters of the surface structure were reduced by more than threefold, which is indicated in the results listed in **Table 2**.

The sample burnished with three passes (i = 3) shows the best height surface topography parameters. As a result, this surface is selected to be studied in terms of the other surface topography parameters. **Table 3** regroups the measured parameters of this surface as compared to the turned sample. Comparing these results, significant differences in the measurements can be highlighted between the turned and the burnished surface. After ball burnishing, the areal material ration Smr was significantly increased while the Smc and Sxp indicators were reduced. The Smr parameter of the turned sample was very low indicating a high peaky topography. After the application of ball burnishing, the value of Smr was sharply increased which impacts positively on the wear properties of the material. Indeed, a good bearing ratio indicates a good bearing capacity which improves the tribological behavior of the workpiece [10]. The Sxp parameter was reduced indicating reducing in surface roughness [24].

The spatial parameter Str of the burnished sample was almost similar to that of the turned sample indicating the micro-anisotropic texture of both surfaces. The micro-anisotropy is a natural result of the machining process [10]. In turning, and similarly burnishing, the single point cutting tool will generate a high degree of anisotropy to the machined surface. The std. parameter is used to indicate the marked direction of the surface texture for the y-axis, which means indicating the lay direction of the surface [25]. This parameter is applicable only for surfaces which does not have a uniform texture, i.e., when the Str > 0.5. It can be observed from **Table 3** that both turned and burnished surfaces have a lower value of Str < 0.5, which means that both surfaces have a pronounced lay pattern. The Std parameter gives the direction angle of the texture, which in the present results has increased from 72.01**°** for the turned sample to 106**°** after burnishing.


### **Table 3.**

*Comparison of surface texture parameters of turned and ball-burnished surfaces.*

All functional (volume) parameters were significantly reduced as a result of the ball burnishing process. The decrease in the material volume Vm indicates that an important part of surface irregularities was eliminated while the decrease in the void volume Vv refers to the elimination of valleys. This is also evident from the diminish of the other functional parameters Vmp and Vvc. The Vvv parameter characterizes the volume of fluid retention in the deepest valleys of the surface. Although this indicator was reduced for the burnished surface, this is not significant as this parameter is not affected by wear processes [26]. The wear resistance of components is directly related to the functional (volume) parameters and the enhancement resulting after BB impacts positively on reducing the quantity of material exposed to wear during the functioning of the workpiece.

For the functional parameters (stratified surfaces), all the parameters were reduced in the case of the burnished surface. The only exception is for the parameter Smr2. The lower value of Sk is desired for better sliding contact between contact surfaces while the decrease in the Spk parameter means that the volume of the material which is likely to be removed during the running in of the component was considerably restricted [26].

Based on the previously cited results, the effect of ball burnishing on surface topography can be remarked. As a consequence of the ball burnishing process, the functionality surface topography of 316L was efficiently improved which is characterized by the advantageous micro-geometric changes, namely: surface smoothness, elimination peaks and valleys and reduced peak heights and trough depths. The effect of a number of burnishing passes was also highlighted. It can be concluded that when the ball passes repeatedly over the surface of 316L, it deforms more asperities and produce smoother surface. However, this repetition should be limited 3 times to have the most improved surface, otherwise, surface flaking occurs due to excessive plastic deformation on the same surface layers [4].

**Figure 2** represents isometric views of the selected burnished sample with three passes (i = 3) which showed the best-enhanced surface topography parameters. According to the 3D images of the untreated surface (**Figure 3**(1)), we can notice that it is characterized by higher peaks and deeper valleys compared to the burnished surface (**Figure 3**(2)). Hence it can be confirmed that the burnishing treatment by applying three passes produced a smoother surface. The visible scratches on the turned surface are due to the machining process which generates significant roughness (Ra = 134 nm μm and Rq = 172 nm). After burnishing with three passes over the surface of the 316L, the scratches as well as the peaks have almost disappeared, which reduces the roughness Ra to 14.1 nm and Rq to 18.3 nm, i.e., a decrease by 89.4% and 68.3% respectively.
