# Wear of 17-4 PH Stainless Steel Patterned Surfaces Fabricated Using Selective Laser Melting
## 1 Introduction  
Friction and wear reduction are among the most critical pursuits in engineering since almost one-fifth of all energy produced worldwide is annually used to overcome friction [1]. Recent studies have focused on functional surfaces featuring bio-inspired superficial patterns or textures obtained using various technologies, including laser micro-machining or coining [2, 3, 4]. In particular, [PERSON] et al. [2] explored the possibility of introducing a regular surface microporosity in a tribo-pair to reduce refetting fatigue. It was found that the micropores can trap wear debris, removing it from the contact zone, thereby leading to an +88% increase in fatigue life with respect to the baseline value. It was observed that the depth of micropores may be unimportant once it exceeds a characteristic value related to wear debris size. Later on, [PERSON] et al. [3] studied the influence of dimples in lubricated conditions. The dimples were introduced to trap wear debris and lubricant and increase the load-carrying capacity. An influence of the running-in phase on the steadywear phase was observed, as well as a proportionality between the coefficient of friction and wear, under the conditions tested. It was found that, within a given range of dimple area density (i.e., the ratio between total dimple area and total surface area), the linear wear of the tested assembly could be reduced by about 27%, or else the effect of dimples was negative because of an increase in unit pressure. [PERSON] [4] provided analytical proof, supported through experiments, about the existence of a minimum coefficient of friction (_COF_) related to the real area of contact under dry sliding. These studies are mainly focused on the tribological behaviour of negative surface textures (dimples); however, comprehensive investigations have been also performed on positive (protruding) surface textures [5]. As reported by [PERSON] et al. [6], many body armours found in nature (e.g., those belonging to fish, snakes and armadillos) are created through the combination of finite size protruding elements whose arrangement allows significant enhancement of strength and toughness, often accompanied by reduced friction and wear. Several manufacturing technologies are currently used for such complex surface structures, including micro-casting and 3D printing [7; 8]. Additive manufacturing (AM) has been shown to be very promising and is currently being largely used to build customized products in a variety of industries, including automotive, aerospace and biomedical, implying a cost drop in the case of high-complexity parts [9]. One of the most popular AM technology for plastics and metals is represented by laser beam powder bed fusion (LB- PBF). Such technology allows us to build components with complex shapes by using a bottom-up approach, whereby mechanical parts are fabricated by melting and fusing either plastic or metallic powder layer by layer using a high-powered laser source. Among the large set of AM materials that are currently available, the precipitation-hardening steel 17-4 PH (low carbon martensitic/austenitic steel) is making its way in a variety of applications because of its interesting mechanical properties, including high fracture toughness, fatigue strength and corrosion resistance [10; 11; 12]. Despite the growing interest, there is still a paucity of contributions focused on the tribological behaviour of LB- PBF 17-4 PH steel [13], of which wear resistance in conventionally manufactured conditions has been assessed in the past [14]. The present work aims to fill this gap and investigate the tribological behaviour of 17-4 PH steel disks featuring either a baseline as-produced surface or an array of protruding hexagonal prisms arranged in a honeycomb pattern. The investigated process is selective laser melting (SLM), which belongs to the powder bed fusion technologies.