3.2. Micro-Structural Analysis of Basalt Samples
In Figure 2
, Scanning electron microscopy (SEM) micro-photos of both RB and GCB samples are shown as before the cavitation process. Raw basalt, as the base of the sample tested, Figure 2
a, was formed from a microcrystalline plagioclase with microlitic structure. Fenocrystals, olivines, rhombic pyroxene, and rarely basic plagioclases have been identified. The sample is limonitis, partially. Basalt stone structure is represented with olivine–pyroxene basalt. The basis of the tested GCB sample was cryptocrystalline with presence of small tiny crystalline crystals, Figure 2
b. Sample structure is a non-homogenous one and consists of different aggregates with a clear boundary in between. During the cryptocrystalline glass base of basalt, some crystals are thermally altered.
shows the structure of RB and GCB sample as recorded on pole-selecting microscope. Large olivine crystals are incorporated into the base mass of plagioclases, Figure 3
a. Phenocrystals of basic plagioclases are elongated, Figure 3
b. The structure of GCB sample is a cryptocrystalline glass base, Figure 3
c, in which the transformed spinels and pyroxenes including bubbles are partially filled with glass, Figure 3
The structure of RB and GCB samples contains bubbles of various sizes filled with air or glass, Figure 4
. Bubbles present on the sample surface cause surface roughness and appearance of pits. RB sample contains a large number of tiny bubbles, Figure 4
a, while the GCB samples contained bubbles of larger dimensions that are embedded in the cryptocrystalline–glass base, Figure 4
During cavitation test, changes of bubbles contained in the basalt base and the presence of pits on both RB and GCB samples surface were monitored.
3.4. Mass Change
Mass loss measurement applied for samples subject to cavitation during the test sequence is shown in Figure 5
a. Mass loss resulting from cavitation damage is displayed on the y-axis, and time intervals are displayed on x-axis. It has been shown that GCB samples have a significantly higher cavitation erosion resistance with an average cavitation rate of 0.03 mg/min when compared to RB samples with a cavitation rate of 0.74 mg/min.
Having analyzed erosion progression of GCB samples, it may be concluded that the mass loss is low; within the first 15 min, mass loss was 1.29 mg and then slightly increased to 3.53 mg during 120 min of exposure. In RB samples, it is evident that for the first 15 min, mass loss of RB sample was up to 15 mg; as the exposure time increases, cumulative mass loss of the sample gradually increases in an almost linear manner up to 88.5 mg during 120 min of exposure.
Higher erosion rate in RB samples in comparison to GCB samples can be explained by means of a rough structure of the olivine-pyroxene basalt of RB samples and through their comparison with compact structure of GCB samples which contributes to an increased resistance of GCB samples to the cavitation effect.
3.5. Image Analysis
Photos of both RB and GCB samples were taken before and during the cavitation erosion test, as shown in Figure 6
. It was noted that GCB sample presented lower scope of surface damage than RB samples. There were almost no dimensional changes in pits that were present on the sample surface prior to test. Profile lines of GCB sample are uniform with individual peaks present at the same locations on the surface of the sample. Presence of individual pits caused by presence of bubbles in the structure were identified prior to the test.
On the RB sample, initial pits on the surface and the roughness present (during t
= 0 min) were changing and increasing in size during cavitation exposure time, as it was reflected on RB sample profile lines producing major changes of the same during the test. These profile lines indicate that degradation was happening in the center of the sample surface, as the intensities of the profile lines edges were changing and increasing between a 15 min exposure and a 120 min one where significant surface area damage of the RB sample arises. The results shown in Figure 6
are in line with the results of surface damage of RB and GCB samples as determined with application of image analysis on photos of sample surfaces taken during the cavitation time. The same were then processed and analyzed using the Image Pro Plus software, as shown in Figure 5
At the end of 120 min test of GCB samples, minor changes were observed on the surface of the sample, with a significantly lower number of small pits in reference to RB samples where the surface was damaged to a higher extent with presence of a number of pits here and there joined to create larger and deeper pits. This is in line with the results of a gradual sample mass loss during the test, Figure 5
a. At the end of cavitation exposure, GCB sample surface damage amounts to 12%, while RB sample damage is over 35.9%, as shown in Figure 5
b. This is in line with the results obtained for the average surface of pit formation, Pav
shown in Figure 5
d shows the dynamics of pit formation on the RB and GCB samples surface during exposure to cavitation effect.
In the RB sample structure there are numerous minor bubbles (Figure 4
a) forming multitude of small pits on the sample surface thus causing an increased surface roughness (Figure 6
, RB sample for t
= 0 min). With the activity of cavitation process, number of newly formed pits is gradually increased in up to 60 min of exposure; then, after 120 min, number of pits is slightly lowered to indicate that the pits get connected in some places, Figure 5
d. Increment of the number and size of pits—which in some places get connected to form larger and deeper pits—causes an increased surface damage of the samples, Figure 5
c. Larger and deeper pits on the damaged RB sample surface allow for the focus of energy of the shock waves caused by implantation of cavitation bubbles intensifying the cavitation effect. An analysis of the RB samples image showed that generation of a larger number of smaller pits on the sample surface and their merging into larger and deeper pits contributed to the extent of damage of the sample surface. After 120 min of cavitation activity, damage of surface amounted to 35.9%, Figure 5
In the GCB sample structure, there are individual larger bubbles (Figure 4
b), as well as individual larger or smaller pits before the beginning of cavitation activity, (Figure 6
, GCB, for time t
= 0 min). On the surface of the GCB sample cavitation changes are noticed after 30 or 60 min of cavitation activity. From the beginning of cavitation activity up to 30 min, a very small number of tiny new pits are formed; up to 60 min the number of pits is rapidly decreasing, most likely due to their interconnection; from 60 to 120 min of exposure the number of formed pits is slightly increased, Figure 5
d. The middle surface of formed pits is gradually widened, Figure 5
c. Analysis of GCB samples showed that initial pits on the sample surface were most likely due to presence of bubbles in the structure; further, these did not change during exposure, as can be seen on photos of GCB samples during the test; a smaller number of pits caused less damage to the GCB sample surface. Therefore, surface damage is below 12% after 120 min of cavitation activity, Figure 5
The mechanism of pit formation and growth on the RB samples surface could be monitored by having the image analyzed, starting from the early phase of the cavitation process then after 15 min of exposure to the end of the test in duration of 120 min. In the GCB samples, tiny pits formed on sample surface after 30 min of cavitation activity changed very little in shape and size by the end of the test. Changes in morphology of both RB and GCB sample surfaces over the test time were monitored by the scanning electronic microscopy method, Figure 7
and Figure 8
shows a change in the surface area of the RB samples during exposure to the cavitation process. By performing a 15 min cavitation process, the existing roughness of the surface increases and the shallow pits form (Figure 7
a). The pit in the olivine-pyroxene basalt structure is sharp edged, most likely due to erosion of edges of the crystal around the existing bubble (Figure 7
b). A 30 min exposure influenced increment of the sizes of the pits formed (Figure 7
c). Presence of pits with surfaces eroded due to removal of the grain and mass loss caused by cavitation effect is shown in Figure 7
d. Cavitation activity lasting for 60 min influenced further degradation of sample surface. A number of new pits were formed while the existing pits increased having eroded their surface to lead to a further mass loss of the samples concerned (Figure 7
e,f). At the end of the test—with cavitation effect for 120 min—the RB sample surface was highly deformed (Figure 7
g). The crystals of olivine and pyroxene minerals, respectively present, eroded in different ways (Figure 7
On the Figure 8
, at the beginning of the test (in 15 min), there was no change in the sample surface. The pits existing on the surface were caused by bubbles in the crystalline base of basalt (Figure 8
a,b). With the cavitation effect, during 30 min, there was no change in the sample surface. The bubbles existing in the cryptocrystalline structure of GCB samples were filled with glass mass (Figure 8
c,d). Cavitation activity during 60 min shows erosion surface with pits occurred near the bubble, Figure 8
e and surface deformation of the pits filled with glass mass, Figure 8
f. At the end of the 120-min test, erosion of pits occurred near the bubble in the base of basalt, Figure 8