## 1 Introduction
They noted that every treatment led to a considerable increase of surface roughness, especially those including the fewest number of passes and the highest jet pressure, whereas a strategy including multiple passes with a gradually decreasing pressure was more beneficial. Consequently, this strategy also provided the highest hardening effect on the surface, being able to prolong the fatigue life as well. [PERSON] et al. [12] found that although the surface quality deteriorated considerably after WJ processing, the fatigue life was able to be slightly improved in some cases, whereas this type of treatment was definitely useful for obtaining highly compressive residual stresses. By using a carefully chosen jet path, the desired shape of the surface can be achieved without the use of a mask [13]. However, the efficiency of pure WJ and AWJ machining should be evaluated by multiple criteria, including not only surface roughness, texture, and integrity, but also geometrical accuracy [14]. Depending on the parameters chosen, a WJ can lead to significantly lower roughness, but in some cases, the difference between a WJ and AWJ is negligible [15]. Although some authors have investigated the appropriate conditions in order to minimize damage during AWJ processing of composites [16], a comparison regarding the use of an AWJ and a pure WJ for pocket milling of composite materials [17] pointed out that the jet energy and milling strategy were the most crucial parameters to be controlled in order to avoid fiber damage. Moreover, [PERSON] and [PERSON] [18] pointed out the influence of pressure fluctuations and vibrations on surface quality during AWJ use, concluding that pressure fluctuations only altered the depth of penetration, whereas vibrations were correlated with surface roughness. Surface quality is also important for medical implants, as a specific level of roughness and texture is required in order to promote osseointegration and adsorption of substances [19; 20]. More specifically, Sa, Ssk, Sal, and Sdr parameters were proven to be the most important for the characterization of biomedical surfaces, whereas jet pressure,standoff distance, and number of passes affected the nature of the surface, its height, and its waviness, respectively [21]. Apart from surface roughness, waviness should also be studied, as in the work of [PERSON] and [PERSON] [22] who conducted a comprehensive analysis of 2D and 3D roughness and waviness indicators during pocket milling of aluminum 6061-T6. [PERSON] et al. [23] used a PWJ under two passes in order to improve the surface of a laser-clad titanium workpiece, showing a clear improvement in comparison to the use of electron beam irradiation, although the combined used of a PWJ and EBM did not provide any further improvement both for average roughness and waviness. Moreover, the use of a plain WJ has proven to be beneficial in cases with a significant amount of embedded particles on the surface; thus, apart from surface quality and integrity, various authors attempted to address this major challenge in AWJ processes [24]. [PERSON] et al. [25] compared the effect of an AWJ, a WJ, and a WJ with suspension under two different levels of jet pressure, abrasive mass flow rate, and feed rate for two types of steel workpieces. It was shown that although the pocket depth was not considerably affected by the treatments, in some cases, a noticeable decrease in surface roughness was observed due to the release of embedded abrasive particles, chips, or damaged material from the workpiece. Furthermore, [PERSON], [PERSON], and [PERSON] [26] proposed a multi-stage processing of ceramic materials, including roughing with a hard abrasive and finishing with a much softer one. Their findings indicate that the use of the softer abrasive was beneficial as embedded particles were removed and the surface roughness was slightly improved. [PERSON] et al. [27] conducted a comprehensive work on the effect of a pure WJ treatment on the surface quality of specimens already machined by an AWJ.