*5.3. Microtribological Investigations*

The coefficient of friction was calculated based on the performed scratch test for the cladding material. The microtribological investigations showed a coefficient of friction (CoF) of 0.4 for the considered hybrid bearing washer. This was in a good agreemen<sup>t</sup> to the CoF of industrial axial bearing made of AISI 52100, which was investigated in [14].

Figure 11 show the plastic behavior calculated by the nano-scratch tests. Without an additional forming and hardening of the axial bearing washer, a mean difference of 10.6 nm (standard deviation *SD* = 2.46) was achieved. The mean difference of the indentation depth with additional forming and hardening was about 9.5 nm (*SD* = 2.55), which corresponds to a decrease of about 11% between bearing washer with an additional forming step and without. However, the results of the industrial bearing washer made of AISI 52100 showed an even lower tendency to plastic deformation (8.6 nm with *SD* = 3.13) than the bearing washer made by means of the tailored forming process chain after the forming stage and a target heat treatment.

**Figure 11.** Evaluation of the scratch test of the cladding at different stages of the process chain.

These results indicate that the additional forming and heat treatment steps reduced the tendency to plastic deformation of the cladding material of the bearing washer made by the tailored forming process chain. However, industrially produced bearing washers have a lower tendency to plastic deformation than the hybrid bearing washer, so that individual process steps in the tailored forming process chain should be further optimized.

#### *5.4. Optical and Acoustic Microscopy*

Before fatigue testing, the washers were optically inspected. For the hybrid washers, the arithmetic mean value of the surface roughness Ra was measured as 95 ± 7 nm. The industrial standard is 80 nm. In applications with high speeds and high loads, bearing manufacturers usually recommend a mean roughness value of Ra < 0.2 μm for running surfaces in direct bearing arrangements, which was exceeded.

Via acoustic sectional images in axial direction (C-scan), various welding defects could be detected, as shown in Figure 12. At a depth between 150 and 300 μm from the surface, smaller pores are visible as white dots in the C-scan (circled in red in Figure 12a left). They have a typical size of 10–20 μm in diameter. The pores appeared randomly distributed. For the parameters investigated here with an axial load of 60 kN, the maximum of the Tresca equivalent stress is at a depth of approx. 125 μm. Since these pores occur in the zone of highest stress, they are rated as potentially critical. SAM images with higher magnification at this depth are shown in Figure 13 (bottom). The material transition zone between AISI 52100 and AISI 1022M lies in a depth of around 2.1 mm, which is shown in Figure 12a right. Here, larger cavities beyond 1 mm can be seen. These are arranged along the helical welding tracks (red arrows), which are about 10 mm apart. Concentric interference patterns are partially visible at the cavities, as there are repetitive echoes of the ultrasound signal. In order to support these findings, the sample was ground down layer by layer in the axial direction and compared to SAM imaging, see Figure 12b. The region of interest, marked in Figure 12a) on the right, is a 45◦ cutout. The cavities (circled in red in Figure 12b) open and close again in the course of 1.2 to 1.5 mm below the surface. Accordingly, the axial elongation is > 0.3 mm. By means of SAM (Figure 12a left), a greater degree of detail of the cavities is visible, as information was partially lost during the grinding process.

**Figure 12.** Scanning acoustic microscope images before testing: (**a**) Smaller pores near the surface (left) and larger cavities in the material transition zone (right); (**b**) comparison of scanning acoustic microscopy (SAM) (left) and optical microscopy after grinding (right) for di fferent depths.

**Figure 13.** Optical (top) and acoustic (bottom) microscope images after: (**a**) machining; (**b**) 400 h runtime at 250 min−<sup>1</sup> with surface damage; (**c**) test termination with 1100 h runtime at 250 min−1.

#### *5.5. Rolling Contact Fatigue Performance*
