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Erosion of solids by liquid droplets is a phenomenon which is a compromise between mechanical properties of the material and droplet hydrodynamic parameters. While a number of studies deal with the deformation of drops, the deformation evolution inside the material has not yet been revealed, mainly from the point of view of the time action of the impinging drops The mechanical response of AISI 316L was investigated under gradually increasing numbers of impingements of liquid droplets, with a droplet volume of Vd ≅ 0.9 mm3, generated by an ultrasonic pulsating water jet with the frequency f = 40 kHz from 1 to 20 s. The surface roughness and the wear rates were determined using a laser profilometer. The cross-section of the selected samples was subjected to microhardness measurement with a load of 0.150 N in a 2D grid, which included the entire perimeter of the deformed area. The minimal microhardness measurement grid under the groove had dimensions of 15 × 15 indents, equal to an area of approximately 450 × 600 µm. A maximum hardness increase was observed at the lowest measured depth of 30 µm. An increase in hardness was observed at 300 µm below the surface. The hardening in the deeper subsurface area was most likely caused by shear stress. This shows the high degree of similitude between the solid and liquid droplet impingements. The results indicate that the currently accepted theory on the development of erosion over time has shortcomings, as demonstrated in this work by the ratio between the utilised droplet diameter and the grain size of the material.

The erosion of 25CrMo4 (EA4T) steel was studied to determine the surface and subsurface damage due to the high-frequency impingement of water droplets using an ultrasonic droplet generator. The material under investigation is railway axle steel, where the pulsating water jet can be used as a surface treatment method to increase the fatigue resistance. The surface processing is related mainly to standoff-distance which was changed to explore the effect of pulsating water jet while the other parameters were kept constant. Surface and subsurface conditions of the material were analyzed by means of light microscopy, scanning electron microscopy and transmission electron microscopy. By altering the standoff distance, it is possible to investigate droplet impingement from the point of view of the pulsating water jet process for different purposes, such as for maximal erosion or for an increase of fatigue resistance by introduction of severe plastic deformation to the surface. The maximum erosion state was reached for a standoff distance of 35 mm when a crater depth of 750 μm and a width of 1500 μm was observed. The mild conditions, when only plastic deformation is reached occurred at a standoff distance of 21 mm. The occurrence of subsurface cavities in materials with higher hardness indicates a significant penetration ability of the periodic high-density action of water droplets at subsonic speeds. • Surface and subsurface erosion manifestations on EA4T steel were analyzed. • Microchannels due to penetration ability under the visible groove were observed. • The subsurface hardening on the sides of the groove is stochastic. [ABSTRACT FROM AUTHOR]

Surfaces exposed to natural forces in the form of water droplets are structurally deformed over time through changes in their surface morphology. Plastic deformation in thin subsurface layers where compressive stress prevails is typical for this stage of erosion. The stress accumulation does not exceed the fatigue limit, so the structural integrity is not broken. Information about this stage has been obtained by post-experimental assessments using various observation techniques. This article considers the changes to a surface using techniques to assess a specific site before and after exposure to the erosive action of water droplets. An electron backscatter diffraction analysis was conducted pre-exposure of water droplets for detection on an electrochemically polished surface of stainless steel AISI 316 L. Specific areas that were exposed to the effects of water droplets at subsonic speed were marked with indents. The droplets were generated by an ultrasonic pulsating water jet (PWJ) with a nominal frequency of 40 kHz and supply pressure of p = 50 MPa. To assess the development in the very early stages of erosion, individual runs were performed with a time range of 1–3 s. The erosion development was compared with control runs, where a continuous water jet (CWJ) with a frequency of 1 Hz was used with a time range of 3–6 s. A post-exposure electron backscatter diffraction analysis showed a real change in the grain orientation using kernel average misorientation. It was found that multiple droplet impingement changes the grain geometry and results in an increase in misorientation inside the grains. The misorientation distribution in the zone treated by the water jet was not homogeneous over the entire cross-section of the sample; the CWJ required double or even triple the time to achieve a similar level of plastic deformation when compared to the PWJ. • An EBSD-based procedure for observation of the incubation stages was applied. • The misorientation inside the grains was numerically evaluated. • The misorientations around 1° form walls resulting in subgrain-like structures. • Grain tilting was established as the main surface deformation mechanism. [ABSTRACT FROM AUTHOR]

The objective of the study is to investigate the erosion effects of periodic water clusters interacting with flat AISI 304 surface to evaluate erosion regimes, surface morphology and variation in the subsurface microhardness below the disintegrated grooves. Stairs trajectory within the standoff distance 5–101 mm with step height 2 mm and length 20 mm was used. The pulsating water jet was changed by varying the supply pressure of the liquid from 40 MPa–100 MPa, at a traverse speed of 5 mm/s which is equivalent to constant impingement distribution of 4000 impingements per mm. Morphology of the PWJ defines different erosion regimes based on specific value of supply pressure and standoff distance. Increasing of supply pressure shifts prevalence of both impact pressure and stagnation pressure at higher standoff distances leading to the delay in formation of incubation, acceleration, culmination, depletion, and termination regime. Impinged surface features such as micro-pits, craters, upheaved surface and isolated voids near the periphery of the trace were observed along with PWJ footprint. The values of the micro-hardness showed strengthening of material subsurface at selected standoff distances. At certain standoff distance effect of supply pressure is inverse to micro-hardness values. Determination of studied technological parameters can be used for effective surface peening operation at v > 5 mm and surface activation of the metals. Results opens a new avenue for determination of erosion resistance of material. • Erosion effects of pulsating water jet as function of standoff distance and flow rate was investigated. • Linear offset of erosion stages and erosion intervals due to interaction of water flow rate and standoff distance was found. • Different erosion stages were observed due to non-uniform plastic deformation with the variation of standoff distance. [ABSTRACT FROM AUTHOR]