**2. Sample manufacturing and surface texturing**

The geometries of the sintered and laser textured samples had either curved surfaces (ball-shape) or plain surfaces (face of the disc). The geometrical dimension of samples followed standard dimensions imposed by the tribometer manufacturer, see Section 3. Balls are ø19.05 mm and discs ø46 6 mm.

Reference steel material (AISI 52100), defined as NP (Non-Porous), was used as the benchmark for comparing the results obtained from surface modifications (both from randomly distributed surface pores and deterministic laser textures). NP samples were purchased from the test rig manufacturer with controlled roughness (*Sq* = 20 nm) and dimensions; therefore, no further operations were needed. Samples with surface features (either pores or texture) were always tested against reference NP material (either ball or disc). Sintered against sintered (or textured against textured) condition was avoided to facilitate results interpretations and isolate the effect of surface features. The sintering process for powder metallurgy (PM) samples was followed by machining to obtain the final sample geometry, whereas laser texturing was directly applied on reference steel samples present in the final geometry for testing.

#### **2.1 Powder metallurgy, sintering techniques and porosity**

Powder metallurgy (PM) refers to the conjunction of processes and operations for obtaining components from powders mixtures through a sintering process. The drive for sintering is the reduction of free energy between particles and consequently diffusion during sintering [34, 35]. Details of conventional PM processes are out of the scope of this work and can be found in [22].

Besides them, Spark Plasma Sintering (SPS) [36] has attracted attention in the last decades [37–41] due to high densification achieved in reduced time and grain grow inhibition [42–44], as SPS is a pressure-assisted process. The powder mixtures are placed in an electrically conductive die (often graphite) that is mounted in a low vacuum chamber. Two graphite punches apply load on the die and permit a pulsed electric current passage that increases samples' temperature and enhances sintering. SPS also allows high heating rates (up to 100 °C/min), and is applied to reduce porosity and improve densification [45, 46]. SPS' monitoring and controlling system of temperature and pressure allow obtaining materials with a range of densifications and compositions. Hence, SPS makes it possible to tailor and prototype different materials [47].

Pores strongly influence mechanical and tribological properties of sintered samples [23, 26, 28, 41, 48]. Therefore, precise control and characterization of pores are required in PM. The quantity and characteristics of pores depend either on powder characteristics (shape, dimensions, distribution, particle mechanical properties) or on the sintering processes and parameters (pressure, sintering time and temperature) [36]. Particles with similar size distribution and irregular shape improve densification during the compacting phase and reduce porosity in the final

components [36]. Similarly, increasing the applied pressure reduces total porosity. Pore characteristics including area, size, perimeter, average distance between pores, distribution and shape can be controlled by sintering temperature and time. The increase of sintering temperature promotes diffusion and increases densification, reduces total porosity, pore dimensions and irregular shape [27, 49], and the occurrence of grain growth. Archimedes' method [50] can estimate the total porosity, and pore characteristics can be evaluated through image analysis. In this study, images were taken in five different areas on the sintered samples'surface and then processed using the freely available Image J software [51] to highlight and characterize the pores. Only pore size and circularity index were considered here as main factors [52]. The circularity index ranges between 0 (very irregular) and 1 (perfectly circular) [52].

Surface roughness is another crucial parameter that strongly influences the tribological performance of tribosystems. For PM materials, roughness evaluation is not trivial since pores represent geometrical depressions that significantly increase the root mean square roughness (*Sq*) of the surfaces. Alternatively, the *plateau* root mean square roughness (*Spq*) parameter generally used to characterize honed cylinder liner topographies [53–55] is adopted to quantify the roughness of PM samples. Besides, an *ad-hoc* measuring and postprocessing procedure, including the choice of the roughness filter and cut-off values), was developed to weakening pore contours' effects on roughness characterization. More details of this procedure can be found in previous publications [29, 56–58]. The surface roughness of the reference smooth material NP was evaluated using the *Sq* parameter.

For this study, a commercially available steel powder mixture (Astaloy85Mo) was used. The Fe-based mixture contains 0.85 wt.% Mo, 0.3 wt.% C. Ball-shape samples were manufactured via SPS, whereas disc-shaped samples via conventional sintering. As SPS only generates cylinders and conventional sintering produced some surface irregularities, ball and disc samples had to be machined to achieve the final geometry for testing. All sintered samples were water quenched at 850°C and polished.

Different sintering parameters were used for varying porosity, see **Table 1**. Only one sintering condition is presented for the ball-shaped samples as this set of parameters (Ball-Sint) was the only one where open surface pores could be observed after machining [29, 56]. Three different compacting pressures were used with the conventional sintering technique for obtaining disc samples with different porosity. The main characteristics of sintered materials are summarized in **Table 2**.

#### **2.2 Laser texturing techniques**

As laser surface texturing is the technique most successfully used to apply deterministic textures on engineering surfaces [59], it was chosen to compare


#### **Table 1.**

*Sintering parameters used to manufacture the ball (SPS technique) and disc samples (conventional sintering).*


*Tribological Performance of Random Sinter Pores vs. Deterministic Laser Surface Textures… DOI: http://dx.doi.org/10.5772/intechopen.100245*

#### **Table 2.**

*Porosity characteristics, Vickers hardness and surface roughness values of sintered and NP samples.*

against sintered samples with random textures. Laser texturing of ball samples demonstrates the feasibility of processing curved surfaces, which is highly interesting for a wide range of applications (*i.e.* ball bearings). The texture design in terms of shape and dimensions is inspired by the most promising solutions from the literature [4, 11–14].

Two different laser systems were used to texture ball samples, which are based on different mechanisms of texture generation: an Nd:YAG laser (Quanta Ray Pro 290, Newport Spectra Physics) with a pulse duration of 10 ns and a passively modelocked ultrashort-pulsed Ti:Sapphire laser (Spitfire Pro XP, Newport Spectra Physics) with a pulse duration of 100 fs. Due to the different pulse durations of the laser systems, the ablation mechanism varies. In the case of the ns-system, the pulse duration is long enough to melt the material leading to a different texture morphology as it is the case for the fs-system, where the pulse duration is so short that the material is directly ablated, thereby preventing melt formation and keeping the heat-affected zone at a minimum [60]. The fs-Ti:Sapphire laser, having a wavelength of 800 nm and a repetition rate of 1 kHz, was used in a direct laser writing (DLW) process. In this process, the laser beam is focused by a collective lens with a focal length of 100 mm onto the sample surface. The sample is moved by a rotation stage, thereby scanning the laser beam over the surface and generating the desired texture shape. One single shot of the laser produces a circular dimple with a diameter of 50 μm and a depth of 0.1 μm. To generate dimples (Ball-Di) or grooves (Ball-LG), the rotation speed of the stage was adapted. For higher speeds, individual laser shots hit the surface (dimples), and for lower speeds, individual laser shots are overlapping by 50%, thus producing a homogenous line pattern (longitudinally oriented with respect to the sliding direction) [61]. The accumulated laser fluences were 3.87 and 5.57 J/cm<sup>2</sup> for the dimples and the grooves, respectively.

Grooves with an orientation perpendicular to the sliding direction were generated with the ns-Nd:YAG laser in a direct laser interference patterning (DLIP) process. The laser had a wavelength of 355 nm and a repetition rate of 10 Hz. In the DLIP process, the primary laser beam exiting the laser is split up into two subbeams by a beam splitter, which are then traveling through an optical setup guiding both beams to the sample surface. Upon overlapping of the two laser beams, a sinusoidal intensity distribution results from the interference of the beams. At the intensity maxima positions, the material is molten and removed, whereas the material stays virtually unchanged at the intensity minima positions, thus forming a groove-like texture. The laser fluence was kept constant at 1.29 J/cm<sup>2</sup> for all experiments. Due to the nature of the texture formation, textures of much smaller feature

**Figure 1.**

*Surface texture patterns on the ball and disc-shaped samples.*

sizes are generated, and a higher processing speed can be achieved. Further details related to DLIP and the optical setup can be found in [62, 63].

The flat surface of disc-shaped samples was textured using another fs-laser system. Radial lines of dimples were designed along the tested circumference of the discs, see **Figure 1**. The dimples have a diameter of 15 μm and a depth of 0.5 μm. The radial distance between dimples was maintained at 10 μm, and the circumferential distance between dimples is 300 μm. Additionally, two configurations of curved radial grooves were designed on discs. One configuration of grooves had a shallow depth (<0.5 μm) compared to the other one (depth > 1 μm). The groove width was around 20 μm for both cases.

A schematic view of texture design on ball and disc samples is presented in **Figure 1**. The main geometrical characteristics of textured and sintered features are summarized and schematically represented in **Table 3** to facilitate the input into the ML algorithm (Section 5). Note that the coverage area (*Ca*) represents the percentage of texture area to the tested surface. The parameter P was introduced to evaluate the shape and orientation of the texture, and it was arbitrary assigned the values of 1 for perpendicular features, 0 for dimples and 1 for longitudinal features.

### **3. Test rigs**

#### **3.1 Coefficient of friction measurements**

The tribological tests were carried out in a ball-on-disc tribometer (MTM2 from PCS Instruments) under varying slide-to-roll ratio (SRR) conditions. In this tribometer, the ball and the disc are driven by two independent motors allowing continuous variations of SRR (see **Figure 2**). The coefficient of friction (COF) is calculated through the vertical load on the disc and the transversal force on the ball arm measured by load cells. The disc is mounted in a temperature-controlled bath, where a precise amount of lubricant is inserted. Another important feature of the test rig is the electric contact resistance (ECR) measurement. Although the ECR is


*Tribological Performance of Random Sinter Pores vs. Deterministic Laser Surface Textures… DOI: http://dx.doi.org/10.5772/intechopen.100245*

#### **Table 3.**

*Schematic configuration and geometric characteristics of sintered and textured samples. W is the feature width, D feature depth, Ca is the coverage area, C circularity, P perpendicularity, and Sq is the surface roughness.*

not a direct film thickness measurement, it enables identifying the lift-off speed, *i.e.* the minimum speed from which the contact surfaces are entirely separated by a thin film of lubricant [64].

Stribeck-like curves (COF *vs.* entrainment speed *Ue* at fixed SRR) were obtained at different SRRs for the sintered and textured materials. All tests were performed with synthetic base oil PAO6 at the constant temperature of 40 °C and constant maximum Hertzian pressure of 0.6 GPa. Each test was repeated three times to

#### **Figure 2.**

*Schematics of tribometers used for the coefficient of friction (MTM2) and film thickness measurements (EHD).*


#### **Table 4.**

*Test parameters adopted for friction (MTM2 rig) and film thickness (EHD rig) measurements.*

assess repeatability. The detailed test parameters for the MTM2 rig are summarized in **Table 4**.

#### **3.2 Film thickness measurements**

The optical interferometry technique based on the spacer layer imaging method (SLIM) [65–67] was used for film thickness measurements. A steel ball is loaded against a glass disc coated with a Cr layer and a SiO2 spacer layer (see **Figure 2**). Light passing through the disc is reflected by the ball surface and recombined by the light reflected by the Cr layer. Differences in light wavelength are used to calculate the central EHD lubricant film thickness of non-conformal contacts.

The used EHD test rig (PCS Instruments) with the SLIM setup had to be slightly modified for measuring film thickness on rough surfaces, according to Guegan [68– 70]. Basically, this is achieved by two LED light sources to obtain brighter images, and the measurement is triggered to a specific ball position to always measure the film in the same position. Besides measuring central film thickness, the contact area and the minimum and maximum film thickness of a specific area can be calculated [68–70].

Only pure rolling (SRR 0%) was tested with this setup (see **Table 4**) since the increase of sliding may cause premature damage of the spacer layer, and it was proved that film thickness depends mainly on the entrainment speeds and only marginally on the SRR [69–71].

## **4. Experimental results and discussion**

The authors have already published most of the experimental work for sintered [29, 37, 41, 57, 58] and textured materials [33, 56, 72, 73] presented in the following *Tribological Performance of Random Sinter Pores vs. Deterministic Laser Surface Textures… DOI: http://dx.doi.org/10.5772/intechopen.100245*

sections, and the main goal of this work is to provide a critical comparison between the different solutions. Hence, only the main results are reported and selected so that the tribological effects of surface pores and laser textures can be thoroughly evaluated and contrasted.

#### **4.1 Sintered samples**

**Figure 3a** shows the Stribeck-like curves of the sintered discs for SRR 100%. The results for distinct SRRs presented a similar trend. As can be observed, the decrease of total porosity and pore dimensions (**Table 2**) decreased the COF of the PM discs. The reduction of porosity goes conform with the decrement of surface roughness. As explained in Section 2.1, the alteration of surface pores is directly connected with surface morphology and, consequently, surface roughness. These parameters (surface roughness and porosity) should always be evaluated together when interpreting the friction results of PM materials.

Smooth reference samples NP showed significant lower COF when compared to the discs with pores. Again, this could be justified by the significant difference in surface roughness. For a better comparison between porous and NP samples, results can be evaluated in terms of the specific film thickness (*Λ*), see **Figure 3b**.

The sintered ball samples presented higher COF than NP for all SRRs and entrainment speeds, similarly to sintered discs. These outcomes are not explicitly

**Figure 3.**

*a) Stribeck-like curves in terms of entrainment speed (COF vs. Ue) of sintered discs for SRR 100% and b) in terms of measured specific film thickness (COF vs. Λ) of NP and sintered balls for SRR 100%.*

reported here and the reader is referred to the original reference for a more in-depth result examination [29]. As for the sintered disc, the improved performance of NP compared to sintered balls could be attributed to the significant difference in surface roughness of almost one order of magnitude between the two sets of samples (see **Table 2**).

Film thickness was only evaluated for the sintered (Ball-Sint) and NP balls. The optical interferometry technique requires glass discs, which excludes the use of sintered discs. The central region of every interferogram was selected, and the mean film thickness was analysed. The COF results of NP and sintered balls were compared in **Figure 3b** as a function of *Λ* for SRR 100%. As can be seen, the COF of sintered balls was lower than the NP samples for all *Λ* values, indicating beneficial friction behavior promoted by the surface porosity. Although only results for SRR 100% are shown here, this trend was also confirmed for other SRRs. These results show how surface pores can potentially have a similar effect as classical surface texturing. It should be noted that NP was tested at lower speeds than Ball-Sint for obtaining the COF in the same ranges; therefore, the increase of porosity and roughness in porous samples increased the lift-off speed compared to smooth NP surfaces.

Previous work by Li *et al.* [74] proved that the tribological behavior of PM materials is not only dependent on the total surface porosity but also on the pore characteristics (morphology, shape, contour, area). When using sintered balls (Ball-Sint) with small and regular-shaped pores, the friction performance was similar to that of conventional dimple textured surfaces [75, 76]. Pores with dimensions smaller than the contact area and shallow depth (generally <0.5 μm) can also improve the hydrodynamic load-carrying capacity in non-conformal contacts due to the action of the so-called "micro-hydrodynamic bearing effect". The passage of the counter-body over the convergent region of the pores or textures can promote a local increment of the lubricant viscosity and pressure, thus increasing the hydrodynamic load capacity [3]. In addition, lubricant can be "sucked" into the texture or pore when the inlet pressure in the diverging region is lower than the ambient pressure [8, 9].

#### **4.2 Laser textured samples**

The Stribeck-like curves of the laser textured samples for SRR 100% are shown in **Figure 4**. Similar trends were also obtained for distinct SRRs. Since film thickness could not be measured for textured discs, COF results were plotted together with ECR values to highlight different lubrication regimes and lift-off speed.

Dimple-textured discs (Disc-Di) promoted friction reduction compared to the reference disc NP, especially at low speeds (<100 mm/s). Shallow radial grooves (Disc-G1) also slightly reduced friction compared to NP, whereas deeper textures (Disc-G2) produced significant higher COF. As previously demonstrated in [6, 75, 77] and observed in the present results, shallow features can boost the hydrodynamic load-carrying capacity in non-conformal EHD contacts due to the local increase in fluid pressure. On the contrary, deeper textures (Disc-G2) can disturb the lubricant film locally and, therefore, induce asperities contact and COF rise [77]. Considering the results of ECR and COF in **Figure 4**, this is following the behavior observed here. Boundary lubrication characterized by an ECR of 0% started roughly at the same speed for all samples (line "a" in **Figure 4**), whereas full-film EHD lubrication regime (ECR 100%) was achieved at different speeds (lines "b" and "c" in **Figure 4**). Dimples (Disc-Di) presented lower lift-off speed, having the steepest increase in ECR value and therefore film thickness and reaching hydrodynamic lubrication at the lowest speed. Dimples were followed by shallow radial grooves (Disc-G1), NP, and

*Tribological Performance of Random Sinter Pores vs. Deterministic Laser Surface Textures… DOI: http://dx.doi.org/10.5772/intechopen.100245*

**Figure 4.** *Friction and contact resistance results of textured disc samples tested at SRR 100%.*

**Figure 5.**

*a) Relative differences in friction plotted as a function of Ue using SRR 100% and b) measured minimum film thickness for NP and textured balls at low-speed (<800 mm/s) and lift-off speed values (vertical dashed lines).*

finally deep radial grooves (Disc-G2). This trend proved that shallow configurations (Disc-Di and Disc-G1) had a micro-bearing effect under mixed lubrication, promoting full-film lubrication at lower speed than NP and Disc-G2, thus leading to reduced friction. This trend is also reflected in the COF *vs. Ue* dependence.

Results of textured balls were normalized using NP as a reference in **Figure 5a**, the negative values representing friction reduction. Differences between the samples are evident at *Ue* lower than 600 mm/s. Similarly to textured discs, the configuration of selected dimples (Ball-Di) improved frictional response. Perpendicular grooves (Ball-PG) generally behaved quite similarly to NP, whereas longitudinal grooves (Ball-LG) brought drawbacks and significantly increased COF. This behavior was observed for all the SRRs studied. To understand the reasons, film thickness results are presented in **Figure 5b**.

Particularly interesting are the values of minimum film thickness since the liftoff speed can be estimated from them. Comparing the different surface textures, dimples (Ball-Di) featured full-film conditions at a lower speed. Perpendicular grooves (Ball-PG) and NP showed similar lift-off speeds, whereas a complete separation between the rubbing surfaces was significantly delayed to higher speeds for longitudinal grooves (Ball-LG). COF and lift-off speed results are in good correlation, confirming the micro-bearing effect of dimples again. Perpendicular grooves did not significantly improve frictional performance, probably as a result of a great extent of the grooves in transversal direction (1 mm) relative to the contact dimensions (170 μm), leading to a reduction of pressure build-up as the lubricant flows transversally in the grooves upon contact with the counter body. Finally, longitudinal grooves (Ball-LG) did not improve the hydrodynamic load-carrying capacity, which can be traced back to a more effortless lubricant flow inside the grooves when the entrainment speed is in the direction of the textures. Upon contact, lubricant is thus squeezed out of the tribological contact, and the textures contribute to higher friction instead of local increment of pressure/ viscosity and film thickness [2, 12].

Sintered and textured samples were compared both from a manufacturing, topographical, and tribological point of view. The manufacturing of sintered materials inherently entails surface features that can potentially improve tribological performance. However, pores are randomly distributed, and the geometrical characteristics of the single features can only be influenced but not precisely tailored by the sintering parameters (**Table 2**). Furthermore, surface pores in sintered material increased surface roughness, making the direct geometrical and tribological comparison with smooth samples challenging (see **Figure 3**). On the other hand, the manufacturing of laser texturing requires extra effort, but it ensures a very precise tuning of the feature's geometry and distribution.

The topographical comparison between sintered and textured materials is quite demanding since the geometrical and spatial distributions of the features are significantly different. A series of parameters was selected to statistically compare the two classes of features, *i.e.* width *W* and depth *D* for dimension, area coverage *Ca* for spatial distribution, perpendicularity *P* and circularity *C* for shape, see **Table 3**. Only main representative parameters were selected here to reduce the complexity since they were also used as inputs for the ML model in the next section.

The tribological improvements of textured materials were observed in **Figures 4** and **5**. Particularly, dimple-configurations (Ball-Di and Disc-Di) reduced both COF and lift-off speed, increasing the load-carrying capacity compared to NP. Surface pores in sintered balls decreased friction compared to NP only at the same specific film thicknesses, see **Figure 3b**. The large difference in COF at equal entrainment speed between sintered and NP (see **Figure 3a**) is due to the extensive difference in surface roughness, see **Table 2**. Therefore, it should be noted that the presented

*Tribological Performance of Random Sinter Pores vs. Deterministic Laser Surface Textures… DOI: http://dx.doi.org/10.5772/intechopen.100245*

pore configurations increased the lift-off speed since the increase of roughness requires higher speeds for reaching hydrodynamic conditions, see **Figure 3**. The tribological performances of dimples and pores in sintered balls can be comparable due to the geometrical similarities of surface features (see **Table 3**) and the small extent of cavities compared to the contact area.

A further comparison between sintered and textured materials is made in the following ML section. The geometrical parameters were used as inputs to characterize very different surfaces, and the COF was used as the output of the predicting model.
