# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 4 Effect of USRP on Mechanical and Tribological Properties of Engineering Materials
It can be noted that despite the refined microstructure, USRP-5 had the least likelihood of crack initiation, which was observed to occur at a higher depth compared to the rest of the specimens. Unlike the previously discussed study, the phenomenon was linked to the material's intrinsic properties, being closer to the crystal structure. With TD8 having a higher presence of the \(\alpha\) phase than the \(\beta\) phase, a smaller amount of slip systems can pin dislocations, thus having higher overall stability and resistance to crack propagation. In the case of USRP-15, the decrease in fatigue strength can be attributed to the formation of \(\alpha\) precipitates, which can increase the localized stress of continual fatigue loads, thus resulting in early crack initiation. [PERSON] et al. [74] conducted USRP on Ti6 Al4V and revealed enhancement in fatigue performance. The authors observed a GNS layer of 10 \(\upmu\)m which contains equiaxed grains. The average grain size at a depth of 10 \(\upmu\)m from the top surface is 91.5 nm. The USRP experiments not only enhanced fatigue performance in the LCF region but also in the HCF region. The fatigue limit of the base material corresponds to 10\({}^{7}\) cycles is 500 MPa and 610 MPa after USRP. This corresponds to a 22% increase in fatigue performance. The enhanced fatigue performance is attributed to the combination of high RCS, GNS layer, and work hardening due to SPD. Regarding the tribological performance of USRP'ed components, the combination of grain refinement, surface hardening, and compressive stresses largely improve the tribological properties similar to the improvement of general mechanical properties. Furthermore, understanding that a material's wear volume is inversely proportional to its hardness, it can be implied that USRP can profoundly impact the tribological performance of various materials, as per [PERSON]'s equation [75]. \[Q=K\frac{Wx}{H} \tag{1}\]  
where \(Q\) is the wear volume, \(K\) is the wear coefficient, \(W\) is the normal load, \(x\) is the sliding distance, and \(H\) is hardness. In the case of high-carbon high-chromium steel, [PERSON] et al. [76] recently demonstrated how the dissolution of carbides from USRP can improve its wear properties. In this work, the steel disc samples were subjected to an ultrasonic vibrational frequency of 27 kHz, ultrasonic vibrational amplitude of 10 \(\upmu\)m, static force of 700 N, and feeding speed of 1.4 mm min\({}^{-1}\). Prior to USRP, it was noted that there were several large primary (size > 5 \(\upmu\)m), large secondary (1 \(\upmu\)m < size \(\leq\) 5 \(\upmu\)m), and small secondary (0.1 \(\upmu\)m < size \(\leq\) 1 \(\upmu\)m) carbides impeded in the ferrite regions of the substrate. After applying USRP, the deformation layer of the specimen showed both increasingly refined and round carbides. When subjected to a reciprocating load of 5 N, the USRP sample showed fewer oxides to the surface and a smoother worn surface compared to the base specimen. Although each sample experienced two-body abrasive wear, there was a greater presence of deep holes along the surface of the untreated sample, which can be related to the removal of carbides, thus initiating cracking and fracturing along the wear track. Although the USRP sample demonstrated a slightly superior wear resistance to the base material, the dissolution of carbides lessened the probability of early crack propagation, which largely benefited the material's wear resistance. Similarly, [PERSON] et al. [64] also found an improvement in wear resistance of Inconel 690 alloy tubes from USRP. Although many of the previously stated works do not look at the influence of USRP on tube-like materials, this work provides great insight into how this surface treatment method can be applied to materials of various geometries.