2. The UNSM Technology Ultrasonic Nanocrystal Surface Modification (UNSM) stands out for ability to generate specifically nanoskin on the surface of material. The main concept and mechanism of UNSM shown in Figure 1 are as following [7, 8, 9]. A tungsten carbide ball attached to an ultrasonic device strikes the surface of a work piece 1.2 ~2.4 x 106 times per minute with 1,000 to 100,000 shots per mm2 and under contact pressure range from 3 to 100 GPa. These strikes bring severe plastic-elastic deformation to surface and subsurface. Thus uniformed micro dimple pattern on the surface and nanocrystalline structure and compressive residual stress in subsurface layer are induced. This change has potential to simultaneously improve static and fatigue strength, as well as surface hardness and surface roughness of the work piece. Figure 1. UNSM Mechanism and UNSM Device with CNC Lathe for Hub Bearings 2.1 Top surface properties: roughness and micro dimples texture Figure 2. Micrograph images and 3-D topology of AISI52100 The uniform micro dimple pattern of the surface is the signature mark of the UNSM treatment. Figure 2 shows modification of the surface pattern of SAE52100 bearing steel ground specimen treated with UNSM. Ra roughness is improved from 0.19 μm to 0.11 μm and uniform micro dimple structure improved entrapped oil volume [9, 10, 11]. Table 1 shows that dimensions of micro-dimples measured by AFM are rather in nano order. Table 1. Micro dimple size for AISI52100 treated with UNSM Conditions Feedrate, mm/rev Amplitude, µm Lathe Spindle Speed, rpm Static Load, N Tip diameter, mm Micro dimple diameter/depth, μm UNSM I 50 0.8/0.07 UNSM II 100 0.56/0.05 UNSM III 0.07 30 150 60 2.38 0.42/0.04
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