**4. Results of physical simulations of cavitation generators with WC/C protective coating**

Surface engineering is a field of materials engineering enjoying one of the highest growth rates, distinctive by the fact that a core (substrate) material of the engineering element is frequently subjected to such procedures as heat treatment, thermochemical treatment or others which thus have influence on a marked improvement of mechanical properties of the engineering material surface. Given the multiple techniques enabling to improve the strength of the surface of engineering materials used for elements in a machine working in difficult cavitation environment conditions, two coating deposition methods play an important role in industrial practice, namely: physical vapour deposition (PVD) or its variants such as xxPVD (e.g.: PAPVD, LAPVD) and chemical vapour deposition (CVD) or its variants such as xxCVD (e.g.: APCVD, LPCVD).

A modern thermochemical treatment technology such as PVD deposition onto a material surface is distinct for a coating deposition process using ionised plasma where vapours of the given material crystallise on the surface of the treated substrate. The material which is subject to PVD treatment is enhancing mechanical parameters considerably and its strength properties are also additionally improved, being the whole constructional element or just part of it. Such modern surface engineering technologies enable to dedicate a metallic material, which was previously not considered for use as a constructional element due to its reduced useful properties. In addition, by implementing such technology as PVD onto the material surface, the constructional materials being the substrate (core) can be used more widely, with such materials having until now medium or insufficient mechanical properties. Such technology of 'enhancement' with a composite coating or layer exceeds by far the strength parameters of the given constructional element's core. The methods of depositing composite coatings or layers onto metallic cores (substrate) were also applied to improve the strength and life of constructional elements used in a cavitation environment.

It is very important is to describe a correlation between technologies of deposition of PVD coatings consisting of composite layers and their strength and mechanical-fatigue properties as well as the way of their degradation caused by a cavitation environment. The strength properties of coatings deposited by PVD methods are highly appreciated in mechanical terms and are dedicated at the same time to such branches of industry as: power, heating, aviation, automotive, railway, chemical, petroleum, gas, river and maritime sector. For this reason, elements of machines and devices are becoming more and more popular, which are made of non-alloy and low-alloy steels enhanced through various types of surface treatment, whereas a given project's economy and budget are naturally the main argument for such design. The coatings deposited by PVD are distinct for their high hardness, resistance to oxidisation, a low friction coefficient and antiwear and anticorrosion properties. Because most of industrial machines and devices are exposed in their work to impact and fatigue loads working in a variable cycle, the knowledge of PVD coatings degradation mechanisms under a dynamic load is especially important. PVD technologies have been used successfully as coatings resistant to wear deposited onto the surface of constructional materials.

PVD coatings significantly improve the impact and fatigue strength and resistance to cavitation wear. A disadvantage of hard coatings is that they crack easily in the brittle and cracking mode during use in extreme environments. A mechanical behaviour of coatings depends on which degradation mechanism dominates during deformation in a given operational environment. A degradation mechanism is connected with a PVD coating's structure, namely: morphology, phase composition, number of components of individual layers, number of coatings, thickness of individual coatings and total thickness of coatings. Moreover, a coating degradation mechanism is dictated by mechanical properties such as: Young's modulus, hardness and adhesion, as well as by the substrate's (core's) properties and the frequency of external interactions linked directly to the working environment. Multilayer (composite) or monolayer (single) coatings are produced to achieve a coating with special properties [7].

**4. Results of physical simulations of cavitation generators with** 

stream and flow device (**Figure 2a**) for (a) magnification of 500× and (b) magnification of 2000×.

constructional elements used in a cavitation environment.

Surface engineering is a field of materials engineering enjoying one of the highest growth rates, distinctive by the fact that a core (substrate) material of the engineering element is frequently subjected to such procedures as heat treatment, thermochemical treatment or others which thus have influence on a marked improvement of mechanical properties of the engineering material surface. Given the multiple techniques enabling to improve the strength of the surface of engineering materials used for elements in a machine working in difficult cavitation environment conditions, two coating deposition methods play an important role in industrial practice, namely: physical vapour deposition (PVD) or its variants such as xxPVD (e.g.: PAPVD, LAPVD) and chemical vapour deposition (CVD) or its variants such as xxCVD (e.g.: APCVD, LPCVD). A modern thermochemical treatment technology such as PVD deposition onto a material surface is distinct for a coating deposition process using ionised plasma where vapours of the given material crystallise on the surface of the treated substrate. The material which is subject to PVD treatment is enhancing mechanical parameters considerably and its strength properties are also additionally improved, being the whole constructional element or just part of it. Such modern surface engineering technologies enable to dedicate a metallic material, which was previously not considered for use as a constructional element due to its reduced useful properties. In addition, by implementing such technology as PVD onto the material surface, the constructional materials being the substrate (core) can be used more widely, with such materials having until now medium or insufficient mechanical properties. Such technology of 'enhancement' with a composite coating or layer exceeds by far the strength parameters of the given constructional element's core. The methods of depositing composite coatings or layers onto metallic cores (substrate) were also applied to improve the strength and life of

**Figure 10.** Result of cavitation wear of the surface of a constructional element made of P265GH steel after operation in a

**WC/C protective coating**

16 Cavitation - Selected Issues

A special low-friction tungsten carbide (WC/C) coating, applied by the PVD technique, was deposited to improve functional properties, tested in a cavitation environment, for 500 PMHs in an independently designed and fabricated author's stream and cavitation device, where cavitation generators made of P265GH and X2CrNi18-9 steel were implemented. Mass and surface roughness were measured and microscope examinations were carried out before and after use in a stream and flow device, operated continuously in a closed cycle (**Figure 2**), to identify the degree of wear of the cavitation generators with a WC/C coating applied. **Figure 11** shows a representative image of a cavitation generator with a special low-friction tungsten carbide (WC/C) coating deposited by PVD onto a P265GH steel substrate. It was found, with an analysis made in several points of the sample, that the average coating thickness is 1.55 μm.

Constructional elements such as cavitation generators with a WC/C coating deposited were operated for 500 PMHs in a stream and flow device generating a cavitation environment in continuous operation in a closed cycle. The constructional elements with a PVD coating were examined after prior cleaning in an ultrasonic cleaner, and then weighed on an analytical scale, AS/X by RADWAG. Surface roughness was determined with a Surtronic 25 contact profilometer by Taylor Hobson. At least four measurements, along the length of 16 mm, in different areas of the cavitation generator surface with a PVD coating, were made to determine surface roughness of each constructional element.

The results of examinations of mass and surface roughness measurements of cavitation generators with a PVD coating are shown, respectively, in **Figures 12** and **13**.

A cavitation generator with a ferritic-pearlitic substrate structure (P265GH) with a WC/C coating applied before use, weighed 58.3913 g before use and had a surface roughness factor

(304L) steel with a WC/C coating deposited by PVD after use in cavitation wear conditions in a continuous flow blast

Effects of Applying WC/C Protective Coating on Structural Elements Working in Cavitation…

of 0,161, thus falling to the 10th surface roughness class. A mass loss of approx. 0.03 g was

On the other hand, the other generator with an austenitic substrate structure (X2CrNi18-9) with a WC/C coating, operated in a stream and flow device, had the weight of 58.9554, losing only about 0.01 g during operation (**Figure 12**). The highest decrease of the roughness factor

from 0.112 to 0.065 (**Figure 13**) was seen for this generator, hence its roughness class was

In order to verify the obtained wear results, cavitation generators made of P265GH and X2CrNi18-9 (304L) steel with a WC/C coating deposited by PVD after operation in a stream and flow device in continuous operation in a closed cycle for 500 PMHs were examined in a modern contactless profilometer, Profilm3D, by Filmetrics. This innovative measuring device serves, among others, to examine a profile of the examined material's surface topography and roughness and to measure the coating thickness by comparing a layer of the uncoated

The aim of contactless examinations with a profilometer was to determine the topography profile for the part of the surface of the constructional element working in a cavitation wear environment. The part of the cavitation generator was examined as a first featuring a ferriticpearlitic substrate structure (P265GH) with a protective WC/C coating, which had the biggest material loss in the area of the generator's straight-through openings. A change of the profile shape at the measuring distance of 600 μm, directed towards the cavitation generator's straightthrough opening, was found by analysing the profile. It is seen that the profile was lowered by about 10 μm with the physical measuring distance of 400 μm towards the opening. The results

substrate with the height of the given coating with the accuracy of up to 1 nm [8].

fell from 0.161 to

of cavitation generators made of P265GH and X2CrNi18-9

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19

identified as a result of operation for 500 PMHs and a roughness factor R<sup>a</sup>

changed from 10th to 11th acc. to PN-EN ISO 1302:2004.

**Figure 13.** Variation of the surface roughness coefficient R<sup>a</sup>

Ra

Ra

0.142 (**Figures 12** and **13**).

machine in a closed cycle [6].

**Figure 11.** Cavitation generator fracture with visible WC/C coating deposited by PVD with coating thickness of 1.55 μm onto P265GH steel substrate.

**Figure 12.** Loss of mass of cavitation generator made of P265GH and X2CrNi18-9 (304L) steel with a WC/C coating deposited by PVD after use in cavitation wear conditions in a continuous flow blast machine in a closed cycle [6].

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profilometer by Taylor Hobson. At least four measurements, along the length of 16 mm, in different areas of the cavitation generator surface with a PVD coating, were made to determine

The results of examinations of mass and surface roughness measurements of cavitation gen-

**Figure 11.** Cavitation generator fracture with visible WC/C coating deposited by PVD with coating thickness of 1.55 μm

**Figure 12.** Loss of mass of cavitation generator made of P265GH and X2CrNi18-9 (304L) steel with a WC/C coating deposited by PVD after use in cavitation wear conditions in a continuous flow blast machine in a closed cycle [6].

erators with a PVD coating are shown, respectively, in **Figures 12** and **13**.

surface roughness of each constructional element.

18 Cavitation - Selected Issues

onto P265GH steel substrate.

**Figure 13.** Variation of the surface roughness coefficient R<sup>a</sup> of cavitation generators made of P265GH and X2CrNi18-9 (304L) steel with a WC/C coating deposited by PVD after use in cavitation wear conditions in a continuous flow blast machine in a closed cycle [6].

A cavitation generator with a ferritic-pearlitic substrate structure (P265GH) with a WC/C coating applied before use, weighed 58.3913 g before use and had a surface roughness factor Ra of 0,161, thus falling to the 10th surface roughness class. A mass loss of approx. 0.03 g was identified as a result of operation for 500 PMHs and a roughness factor R<sup>a</sup> fell from 0.161 to 0.142 (**Figures 12** and **13**).

On the other hand, the other generator with an austenitic substrate structure (X2CrNi18-9) with a WC/C coating, operated in a stream and flow device, had the weight of 58.9554, losing only about 0.01 g during operation (**Figure 12**). The highest decrease of the roughness factor Ra from 0.112 to 0.065 (**Figure 13**) was seen for this generator, hence its roughness class was changed from 10th to 11th acc. to PN-EN ISO 1302:2004.

In order to verify the obtained wear results, cavitation generators made of P265GH and X2CrNi18-9 (304L) steel with a WC/C coating deposited by PVD after operation in a stream and flow device in continuous operation in a closed cycle for 500 PMHs were examined in a modern contactless profilometer, Profilm3D, by Filmetrics. This innovative measuring device serves, among others, to examine a profile of the examined material's surface topography and roughness and to measure the coating thickness by comparing a layer of the uncoated substrate with the height of the given coating with the accuracy of up to 1 nm [8].

The aim of contactless examinations with a profilometer was to determine the topography profile for the part of the surface of the constructional element working in a cavitation wear environment. The part of the cavitation generator was examined as a first featuring a ferriticpearlitic substrate structure (P265GH) with a protective WC/C coating, which had the biggest material loss in the area of the generator's straight-through openings. A change of the profile shape at the measuring distance of 600 μm, directed towards the cavitation generator's straightthrough opening, was found by analysing the profile. It is seen that the profile was lowered by about 10 μm with the physical measuring distance of 400 μm towards the opening. The results were confirmed for several tested straight-through openings of the cavitation generator. A very steep cavitation wear profile was identified in some of the tested areas near the straightthrough openings, with visible parallel faults and grooves on its end being a destroyed edge of the constructional element's working opening, the result of which was a medium (water) flowing perpendicular to the direction of visible damages of the cavitation generator. A surface in the examined part of the generator had single and few craters and pits resulting from the cavitation processes the constructional element was subject to for 500 PMHs (**Figure 14**).

Much smaller roundings of edges of working openings, at the same time with a steep profile of surface wear according to the water (medium) flow direction, were found for a cavitation generator made of austenitic X2CrNi18-9 steel with a WC/C coating, where the profile height difference was approx. 4 μm over a measuring distance of 850 μm for a physical measuring distance of ~560 μm. Moreover, numerous local pits and craters with a small volume were noticed, especially in the nearest surrounding of the straight-through openings, created as a result of use for 500 PMHs in a stream and flow device generating a cavitation environment in continuous operation in a closed cycle (**Figure 15**).

An imaging technique with a confocal microscope was applied to confirm the results (**Figures 14** and **15**) obtained with an optical profilometer, Profilm3D, by Filmetrics. Confocal microscopy is used, in particular, for examination of: materials surface topography, to identify microstructure and material surface defects and for precise measurements with a higher quality of imaging. Confocal microscopy is a modification of light microscopy featuring higher contrast, higher depth of sharpness and resolution capacity, where a narrow source of light is used in the form of a laser radiation beam, owing to which an image is achieved with large power concentrated in a given test point. An advantage of this measuring technique is that the tested samples are visualised, 3D and 4D images are reconstructed, and a series of optical sections are recorded at the different depth of the preparation, as well as high image resolution [9].

were examined with a Laser Scanning Microscope, LSM 5 Exciter, by Zeiss. The examinations were carried out with a diode laser with a wavelength of 450 nm. A cavitation generator made of P265GH steel, with a WC/C coating deposited, was characterised by a profile height difference of the examined surface of 5.7 μm along the measuring distance of 130 μm, with a physical measuring distance of 86 μm, whereas a very steep and brittle character of cavitation wear of the working opening edge was noticed, the effect which was an intensively flowing medium (water). The uneven wear of the opening edge was also observed, as a gentle parabolic shape of the edge or as

**Figure 15.** Topography profile of cavitation wear of part of the surface and working opening of the X2CrNi18-9 cavitation

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However, a cavitation generator made of X2CrNi18-9 steel, with a WC/C coating deposited, was characterised by a profile height difference of the examined surface of 12.7 μm along the measuring distance of 130 μm, with a physical measuring distance of 82 μm. An extensive strip of material loss was found for the examined sample area, based on which it can be concluded that the deposited coating was detached from the substrate material in a brittle way, whereas the process of rounding and a non-homogeneous profile of the material edge results from the intensively flowing medium (water) towards the inside of the constructional

Over the next stage of structural examinations, cavitation generators made of P265GH and X2CrNi18-9 steels with a monolayer protective WC/C coating deposited by the PVD technique, subjected to operation in a stream and flow device in continuous operation in a closed cycle for 500 PMHs, underwent a macroscopic analysis using a scanning electron microscope, SUPRA 35, with secondary electrons (SE) detection. The results of the examinations are presented in **Figures 18**–**21**. Numerous fatigue and flow processes were found for cavitation generators made of P265GH steel with a protective WC/C coating, subjected to operation in a stream and flow device for 500 PMHs, where the flowing water (medium) was destroying the coating surface in the form of incised bands (**Figures 18a** and **19a**) towards the inside of the working opening (orange arrow). The coming cavitating water caused considerable destructions as a result of which the coating cracked and collapsed (red arrow), developing oblong craters with a different height on the analysed part of the constructional element's

a sharp crack, detachment of part of the constructional element's material (**Figure 16**).

element's working opening (**Figure 17**).

generator with protective WC/C coating after operation for 500 PMHs.

Cavitation generators made of P265GH and X2CrNi18-9 (304L) steel with a (WC/C) tungsten carbide coating deposited, subjected to operation for 500 PMHs in a cavitation wear environment,

**Figure 14.** Topography profile of cavitation wear of part of the surface and working opening of the P265GH cavitation generator with protective WC/C coating after operation for 500 PMHs.

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were confirmed for several tested straight-through openings of the cavitation generator. A very steep cavitation wear profile was identified in some of the tested areas near the straightthrough openings, with visible parallel faults and grooves on its end being a destroyed edge of the constructional element's working opening, the result of which was a medium (water) flowing perpendicular to the direction of visible damages of the cavitation generator. A surface in the examined part of the generator had single and few craters and pits resulting from the cavitation processes the constructional element was subject to for 500 PMHs (**Figure 14**). Much smaller roundings of edges of working openings, at the same time with a steep profile of surface wear according to the water (medium) flow direction, were found for a cavitation generator made of austenitic X2CrNi18-9 steel with a WC/C coating, where the profile height difference was approx. 4 μm over a measuring distance of 850 μm for a physical measuring distance of ~560 μm. Moreover, numerous local pits and craters with a small volume were noticed, especially in the nearest surrounding of the straight-through openings, created as a result of use for 500 PMHs in a stream and flow device generating a cavitation environment

An imaging technique with a confocal microscope was applied to confirm the results (**Figures 14** and **15**) obtained with an optical profilometer, Profilm3D, by Filmetrics. Confocal microscopy is used, in particular, for examination of: materials surface topography, to identify microstructure and material surface defects and for precise measurements with a higher quality of imaging. Confocal microscopy is a modification of light microscopy featuring higher contrast, higher depth of sharpness and resolution capacity, where a narrow source of light is used in the form of a laser radiation beam, owing to which an image is achieved with large power concentrated in a given test point. An advantage of this measuring technique is that the tested samples are visualised, 3D and 4D images are reconstructed, and a series of optical sections are recorded at the different depth of the preparation, as well as high image resolution [9]. Cavitation generators made of P265GH and X2CrNi18-9 (304L) steel with a (WC/C) tungsten carbide coating deposited, subjected to operation for 500 PMHs in a cavitation wear environment,

**Figure 14.** Topography profile of cavitation wear of part of the surface and working opening of the P265GH cavitation

generator with protective WC/C coating after operation for 500 PMHs.

in continuous operation in a closed cycle (**Figure 15**).

20 Cavitation - Selected Issues

**Figure 15.** Topography profile of cavitation wear of part of the surface and working opening of the X2CrNi18-9 cavitation generator with protective WC/C coating after operation for 500 PMHs.

were examined with a Laser Scanning Microscope, LSM 5 Exciter, by Zeiss. The examinations were carried out with a diode laser with a wavelength of 450 nm. A cavitation generator made of P265GH steel, with a WC/C coating deposited, was characterised by a profile height difference of the examined surface of 5.7 μm along the measuring distance of 130 μm, with a physical measuring distance of 86 μm, whereas a very steep and brittle character of cavitation wear of the working opening edge was noticed, the effect which was an intensively flowing medium (water). The uneven wear of the opening edge was also observed, as a gentle parabolic shape of the edge or as a sharp crack, detachment of part of the constructional element's material (**Figure 16**).

However, a cavitation generator made of X2CrNi18-9 steel, with a WC/C coating deposited, was characterised by a profile height difference of the examined surface of 12.7 μm along the measuring distance of 130 μm, with a physical measuring distance of 82 μm. An extensive strip of material loss was found for the examined sample area, based on which it can be concluded that the deposited coating was detached from the substrate material in a brittle way, whereas the process of rounding and a non-homogeneous profile of the material edge results from the intensively flowing medium (water) towards the inside of the constructional element's working opening (**Figure 17**).

Over the next stage of structural examinations, cavitation generators made of P265GH and X2CrNi18-9 steels with a monolayer protective WC/C coating deposited by the PVD technique, subjected to operation in a stream and flow device in continuous operation in a closed cycle for 500 PMHs, underwent a macroscopic analysis using a scanning electron microscope, SUPRA 35, with secondary electrons (SE) detection. The results of the examinations are presented in **Figures 18**–**21**. Numerous fatigue and flow processes were found for cavitation generators made of P265GH steel with a protective WC/C coating, subjected to operation in a stream and flow device for 500 PMHs, where the flowing water (medium) was destroying the coating surface in the form of incised bands (**Figures 18a** and **19a**) towards the inside of the working opening (orange arrow). The coming cavitating water caused considerable destructions as a result of which the coating cracked and collapsed (red arrow), developing oblong craters with a different height on the analysed part of the constructional element's

**Figure 16.** Topography profile of cavitation wear of part of the surface and working opening of the P265GH cavitation generator with protective WC/C coating after operation for 500 PMHs.

Long, axial coating detachment and delamination towards the working opening edge (green arrow) was identified in case of cavitation generators made of X2CrNi18-9 steel with an austenitic substrate structure with a WC/C coating deposited by PVD and operated in the conditions of cavitation wear. An area of cavitation wear with a polygonal shape with a different height from the plane of the constructional element (green arrow) and void places (cavities) after the removed tungsten carbides on the examined piece of the constructional element's area (yellow arrow) were also observed (**Figure 20a** and **b**). Cavitation wear effects were also found near straight-through openings in the form of brittle cracks and delamination as a mesh of the WC/C coating implemented in several places of the tested sample on a large area. The cracked coating was moving during operation towards the working opening, which can be signified by even gaps between particular plates of the WC/C coating, until the coating is completely detached from the substrate material and its larger parts falls apart due to activity of the medium under high pressure (violet arrow) (**Figure 21a**). Another degradation mechanism of part of the cavitation generator's surface in the operation process over the time of

**Figure 19.** Result of cavitation wear of the surface of a constructional element made of P265GH steel with a WC/C coating deposited by PVD after operation in a stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of

**Figure 18.** Result of cavitation wear of the surface of a constructional element made of P265GH steel with a WC/C coating deposited by PVD after operation in a stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of

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2500× and (b) magnification of 4500×.

7500× and (b) magnification of 3000×.

**Figure 17.** Topography profile of cavitation wear of part of the surface and working opening of the X2CrNi18-9 cavitation generator with protective WC/C coating after operation for 500 PMHs.

part. Numerous void places (cavities) were observed in **Figures 18** and **19** (yellow arrow) due to removing the droplets of the deposited carbides as a result of operation in a cavitation environment for 500 PMHs. Additionally, a brittle mechanism of WC/C coating cracking was noticed, characterised by being situated in parallel to the edge of the constructional element's working opening, where destruction was initiated near the edges of the infused or removed tungsten carbides (blue arrow) (**Figure 19b**).

**Figure 18.** Result of cavitation wear of the surface of a constructional element made of P265GH steel with a WC/C coating deposited by PVD after operation in a stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of 2500× and (b) magnification of 4500×.

**Figure 19.** Result of cavitation wear of the surface of a constructional element made of P265GH steel with a WC/C coating deposited by PVD after operation in a stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of 7500× and (b) magnification of 3000×.

Long, axial coating detachment and delamination towards the working opening edge (green arrow) was identified in case of cavitation generators made of X2CrNi18-9 steel with an austenitic substrate structure with a WC/C coating deposited by PVD and operated in the conditions of cavitation wear. An area of cavitation wear with a polygonal shape with a different height from the plane of the constructional element (green arrow) and void places (cavities) after the removed tungsten carbides on the examined piece of the constructional element's area (yellow arrow) were also observed (**Figure 20a** and **b**). Cavitation wear effects were also found near straight-through openings in the form of brittle cracks and delamination as a mesh of the WC/C coating implemented in several places of the tested sample on a large area. The cracked coating was moving during operation towards the working opening, which can be signified by even gaps between particular plates of the WC/C coating, until the coating is completely detached from the substrate material and its larger parts falls apart due to activity of the medium under high pressure (violet arrow) (**Figure 21a**). Another degradation mechanism of part of the cavitation generator's surface in the operation process over the time of

part. Numerous void places (cavities) were observed in **Figures 18** and **19** (yellow arrow) due to removing the droplets of the deposited carbides as a result of operation in a cavitation environment for 500 PMHs. Additionally, a brittle mechanism of WC/C coating cracking was noticed, characterised by being situated in parallel to the edge of the constructional element's working opening, where destruction was initiated near the edges of the infused or removed

**Figure 17.** Topography profile of cavitation wear of part of the surface and working opening of the X2CrNi18-9 cavitation

**Figure 16.** Topography profile of cavitation wear of part of the surface and working opening of the P265GH cavitation

generator with protective WC/C coating after operation for 500 PMHs.

22 Cavitation - Selected Issues

tungsten carbides (blue arrow) (**Figure 19b**).

generator with protective WC/C coating after operation for 500 PMHs.

biggest cavitation wear effects were also noticed, confirmed with photographs from an

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**2.** The smallest mass loss with its value at the level of a measurement error was recorded for a cavitation generator made of austenitic X2CrNi18-9 (304L) steel, sanded with sandpaper with the grain size of 2500, however, significant cavitation wear was recorded on its surface in form of axial brittle cracks inside the material, initiated towards the edges of the

**3.** The roughness factor Ra was greatly reduced for a constructional element made of P265GH steel with a ferritic-pearlitic structure with a high surface roughness factor Ra in the initial condition after a process in a cavitation environment for 500 PMHs. It can be concluded that such a process (surface smoothing) may concern a majority of engineering materials

**4.** The cavitation generators featuring a low surface roughness factor Ra in the initial state have increased - as a result of the impact of the cavitation environment - the roughness factor Ra regardless the steel structure, either ferritic-pearlitic P265GH or austenitic

**5.** The deposition of a monolayer protective WC/C coating onto constructional elements which were subjected to wear in a cavitation environment for 500 PMHs did not prevent the mass loss of cavitation generators with a ferritic-pearlitic P265GH structure and austenitic X2CrNi18-9 structure, however, it significantly slowed down this process (by refer-

**6.** The surface roughness factor Ra of cavitation generators, onto which a WC/C coating was deposited, subjected to operation in a cavitation environment for 500 PMHs, fell indepen-

**7.** Topography examinations of the surface of constructional elements onto which a WC/C coating was deposited, using a modern contactless profilometer and a confocal microscope with the CLSM technique, have revealed extensive cavitation wear of the surface, especially near the edge of the cavitation generator's working opening regardless the substrate

**8.** A monolayer WC/C coating deposited on P265GH steel was wearing in a cavitation environment in a distinctive manner by collapsing parallel to the direction of the flowing water

**9.** A surface of a constructional element in the form of tungsten carbide, deposited on X2CrNi18-9 steel, was wearing during operation in a cavitation environment for 500 PMHs with coating flakes detaching with a brittle cracking mechanism and with plastic wear of the substrate in the form of substrate waving due to a fatigue-cyclic character of the work-

**10.**The tests results obtained allow to conclude that the application of special low-friction protective coatings allows to reduce costs associated with selection of engineering materials for a substrate of constructional elements working in a cavitation wear environment. P265GH steel is 4 times cheaper than austenitic chromium-nickel X2CrNi18-9 steel, and if a WC/C coating is deposited in this case, this considerably extends the working time of such

which would be subjected to operation in a cavitation environment.

ring to cavitation generators without a coating at least four times).

dently from the substrate applied, onto which a coating was deposited.

and by brittle cracking of the coating on the edge of the working opening.

electron scanning microscope.

working opening.

X2CrNi18-9 steel.

material.

ing environment.

elements in a cavitation environment.

**Figure 20.** Result of cavitation wear of the surface of a constructional element made of X2CrNi18-9 steel with a WC/C coating deposited by PVD after operation in a stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of 250× and (b) magnification of 2500×.

**Figure 21.** Result of cavitation wear of the surface of a constructional element made of X2CrNi18-9 steel with a WC/C coating deposited by PVD after operation in a stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of 2500× and (b) magnification of 7500×.

500 PMHs were fatigue processes caused by long-term interaction of the cavitation environment, the result of which was the plastic waving of the substrate material (white arrow) made of X2CrNi18-9 steel, which also led to its significant destruction, cracking of the WC/C coating and consequently to its detachment from the substrate (**Figure 21b**).
