**3. Results of physical simulations of cavitation generators without protective coating**

Cavitation generators, made of two P265GH and X2CrNi18-9 (304L) steels selected for this aim, were then tested in the conditions of cavitation wear continuously for 500 PMHs, with a specially designed and constructed author's stream and flow device (**Figure 2a**) generating a cavitation environment. The impact of surface roughness was investigated before the operation of the generators in the conditions of cavitation wear on the roughness and a mass loss after the above experiment. For this reason, prior to installation of a cavitation generator in a stream and flow device generating a cavitation environment (**Figure 2**), as well as after 500 PMHs of continuous work in such device, the cavitation generator was cleaned 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 4 measurements, along the length of 16 mm in different areas of the constructional element, were performed to determine surface roughness of each generator. Detailed macroscopic examinations with a scanning electron microscope, SUPRA 35, at the accelerating voltage of 5–20 kV using secondary electrons (SE) detection, with the magnification of 100–2000×, were undertaken for preliminary identification of the cavitation wear results [6].

The results of mass loss and surface roughness measurement examinations before and after use are shown, respectively, in **Figures 5** and **6**.

The results of macroscopic examinations of the applied P265GH and X2CrNi18-9 (304L) steels in conditions of cavitation wear made with a SUPRA 35 electron scanning microscope using secondary electrons (SE) detection are shown in **Figures 7**–**10**.

A constructional element such as a cavitation generator made of ferritic-pearlitic steel, designated as 200-P265GH, wet sanded with sandpaper with the grain size of 200, weighed 57.6271 g before use and featured a surface roughness factor R<sup>a</sup> of 0.627, thus falling to the 8th surface roughness class according to PN-EN ISO 1302:2004. A negligible mass loss of approx. 0.03 g was also found as a result of generator operation and a roughness factor R<sup>a</sup> fell from 0.627 to 0.41, which is further classified as the 8th surface roughness class. A constructional element made of ferritic-pearlitic steel designated as 1000-P265GH, i.e. wet sanded with sandpaper with the grain size of 1000, had the weight of 57.1835 g and a surface roughness factor Ra of 0.15, thus falling to the 10th surface roughness. The highest mass loss, of as much as 0.1752 g in relation to all the operated generators, and the growth of the roughness factor R<sup>a</sup> from 0.15 to about 0.5, was found after operating a generator marked as 1000-P265GH, which

was selected characterised by a relative clearance Pp of 11.1 [%], which has reached an inlet pressure of the medium of 244,000 [Pa], for the number of cavitations (content of steam) of 0.98 [%], as shown in **Figures 3** and **4**. The areas most susceptible to cavitation wear were also defined by analysing numerical results, with such areas being most of all the areas of straight-through openings, and a central opening with the diameter of 4 mm was found to be most susceptible, in particular.

**Figure 4.** Implosion distribution in form of content of steam on the one-twelfth (30°) section of the field area of model of

Cavitation generators, made of two P265GH and X2CrNi18-9 (304L) steels selected for this aim, were then tested in the conditions of cavitation wear continuously for 500 PMHs, with a specially designed and constructed author's stream and flow device (**Figure 2a**) generating a cavitation environment. The impact of surface roughness was investigated before the operation of the generators in the conditions of cavitation wear on the roughness and a mass loss after the above experiment. For this reason, prior to installation of a cavitation generator in a stream and flow device generating a cavitation environment (**Figure 2**), as well as after

**3. Results of physical simulations of cavitation generators without** 

**protective coating**

constructional element [5].

12 Cavitation - Selected Issues

**Figure 5.** Loss of mass, i.e. the mass of cavitation generator made of P265GH and X2CrNi18-9 (304L) steel after use in cavitation wear conditions in a continuous flow blast machine in a closed cycle [6].

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

**Figure 7.** Result of cavitation wear of the surface of a constructional element made of P265GH steel after operation in a stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of 100×; (b) magnification of 150×.

It was also confirmed on the basis of macroscopic examinations undertaken using a scanning electron microscope for a constructional element such as a cavitation generator made of P265GH steel, designated as 2500-P265GH, that numerous places exist on the edges of straight-through openings, especially on the edges with the biggest opening area in the central part of the cavitation generator, which were rounded by the flowing water (**Figure 7**). Cavitation craters and pits were formed in the first stage, then such craters were piling up and successive centres of cavitation wear were being formed, leading to either complete damage and breaking of the material part from the edge or to the material flowing towards the opening interior according to the medium flow direction (**Figures 8** and **9**). It was noticed for magnifications of 500–2000× that the surface of the cavitation generator made of P265GH steel bears traces of intensive wear in the form of irregularly spaced pits and craters and clusters of surface degradation of the constructional elements created in operation lasting 500 PMHs in a cavitation environment (**Figure 10**) [6].

**Figure 9.** Result of cavitation wear of the surface of a constructional element made of P265GH steel after operation in a

stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of 500× and (b) magnification of 1000×.

**Figure 8.** Result of cavitation wear of the surface of a constructional element made of P265GH steel after operation in a

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stream and flow device (**Figure 2a**) for 500 PMHs: (a and b) magnification of 500× [6].

decreases the roughness class from 10th to 8th. A small mass loss from 56.5545 to 56.5182 g and roughness factor growth from 0.09 to 0.285, i.e. fall from 10th to 9th roughness class, was seen for the last cavitation generator made of ferritic-pearlitic steel, designated as 2500-P265GH, operated for 500 PMHs.

Far better results were achieved for a constructional element such as a cavitation generator made of austenitic X2CrNi18-9 (304L) steel, wet sanded with sandpaper with the grain size of 2500, with the weight of 59.4399 g and with the surface roughness factor R<sup>a</sup> of 0.1, which is grouped in the 10th roughness class. The weight dropped to only 0.0002 g after operation, i.e. it was within the measurement error range, and the roughness factor rose to 0.455, which ranks it in the 8th roughness class [6].

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**Figure 8.** Result of cavitation wear of the surface of a constructional element made of P265GH steel after operation in a stream and flow device (**Figure 2a**) for 500 PMHs: (a and b) magnification of 500× [6].

**Figure 9.** Result of cavitation wear of the surface of a constructional element made of P265GH steel after operation in a stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of 500× and (b) magnification of 1000×.

**Figure 7.** Result of cavitation wear of the surface of a constructional element made of P265GH steel after operation in a

decreases the roughness class from 10th to 8th. A small mass loss from 56.5545 to 56.5182 g and roughness factor growth from 0.09 to 0.285, i.e. fall from 10th to 9th roughness class, was seen for the last cavitation generator made of ferritic-pearlitic steel, designated as 2500-P265GH,

Far better results were achieved for a constructional element such as a cavitation generator made of austenitic X2CrNi18-9 (304L) steel, wet sanded with sandpaper with the grain size

is grouped in the 10th roughness class. The weight dropped to only 0.0002 g after operation, i.e. it was within the measurement error range, and the roughness factor rose to 0.455, which

of 0.1, which

of cavitation generators made of P265GH and X2CrNi18-9

stream and flow device (**Figure 2a**) for 500 PMHs: (a) magnification of 100×; (b) magnification of 150×.

(304L) steel after use in cavitation wear conditions in a continuous flow blast machine in a closed cycle [6].

of 2500, with the weight of 59.4399 g and with the surface roughness factor R<sup>a</sup>

operated for 500 PMHs.

ranks it in the 8th roughness class [6].

**Figure 6.** Variation of the surface roughness coefficient R<sup>a</sup>

14 Cavitation - Selected Issues

It was also confirmed on the basis of macroscopic examinations undertaken using a scanning electron microscope for a constructional element such as a cavitation generator made of P265GH steel, designated as 2500-P265GH, that numerous places exist on the edges of straight-through openings, especially on the edges with the biggest opening area in the central part of the cavitation generator, which were rounded by the flowing water (**Figure 7**). Cavitation craters and pits were formed in the first stage, then such craters were piling up and successive centres of cavitation wear were being formed, leading to either complete damage and breaking of the material part from the edge or to the material flowing towards the opening interior according to the medium flow direction (**Figures 8** and **9**). It was noticed for magnifications of 500–2000× that the surface of the cavitation generator made of P265GH steel bears traces of intensive wear in the form of irregularly spaced pits and craters and clusters of surface degradation of the constructional elements created in operation lasting 500 PMHs in a cavitation environment (**Figure 10**) [6].

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

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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].

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 thick-

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

wear deposited onto the surface of constructional materials.

ness is 1.55 μm.

**Figure 10.** Result of cavitation wear of the surface of a constructional element made of P265GH steel after operation in a stream and flow device (**Figure 2a**) for (a) magnification of 500× and (b) magnification of 2000×.
