**3. Results and discussion**

**Table 1** collects data concerning basic properties of investigated materials. The level of densification is described as relative density value. All investigated materials were dense, and the level of total porosity did not exceed 1.5% in any case. Basic mechanical properties, hardness, modulus of elasticity, bending strength, and fracture toughness were on the level which is typically reported for similar materials.

The basic results of the stream-impact test of oxide ceramics were collected in **Figure 1**. It presented the volumetric wear of the investigated samples. As it was predicted, ceramic phases were resistant to cavitation wear, yet the difference between alumina **A** and zirconia **Z** was distinct. The most interesting fact resulting from the wear investigations was that both composites **AZ** and **ZA** had much better cavitation resistance than zirconia.

Microstructural observations of eroded surfaces performed by means of SEM technique allowed to recognize differences in destruction mechanisms for investigated materials. Relatively high rate of erosion measured for alumina material could be explained by mechanism which could be distinctly recognized after eroded surface examination revealed in micrographs (**Figure 2**).

Destruction of alumina material happened by removing of whole grains. This process accelerated during the test duration and after 2400–3000 min was very intensive. Process of grain fragmentation was not observed. Transgranular cracking was detected in very rare number of cases (like a large grain in the center in **Figure 2** micrograph at the bottom). Seeing that, alumina grains were relatively large; degradation process after long exposition on cavitation was very significant.

Erosion process in zirconia materials runs in different ways. Microstructural documentation of this process was presented in **Figure 3**. Individual zirconia grains were removed from the surface, and this act consequently induced microcracks in this region [12]. Such situation made more probable possibility of removing the next grain in the nearest neighborhood created whole. This process runs not parallel to the sample surface but perpendicularly to it. This was the reason why erosion in zirconia developed in relatively limited surface area, and

**Figure 2.** SEM microstructures of alumina material (**A**) on different stages of destruction—starting degradation of polished surface (after 600 min) (at the top left side) and advanced level of degradation (after 2400 min) (at the top right

Cavitation Wear of Structural Ceramics http://dx.doi.org/10.5772/intechopen.79510 33

The way of degradation of composites depends on the major phase content. In **Figure 4**, selected areas of **AZ** composite microstructures were presented. When dispersion of constituent phases in composite was very good (on single micrometer level), surface was degraded uniformly. The mechanism of degradation was similar like in pure alumina material (whole grains removing), but in **AZ** composite, grains were much smaller than in pure alumina due to restraining influence of inert particles of minor phase (see Zener effect [20]). Additionally,

consequently the removed volume of the material is limited.

side and at the bottom).

**Figure 1.** Results of volumetric loss measurements during stream-impact cavitation test of investigated oxide materials.

**3. Results and discussion**

32 Cavitation - Selected Issues

typically reported for similar materials.

micrographs (**Figure 2**).

was very significant.

**Table 1** collects data concerning basic properties of investigated materials. The level of densification is described as relative density value. All investigated materials were dense, and the level of total porosity did not exceed 1.5% in any case. Basic mechanical properties, hardness, modulus of elasticity, bending strength, and fracture toughness were on the level which is

The basic results of the stream-impact test of oxide ceramics were collected in **Figure 1**. It presented the volumetric wear of the investigated samples. As it was predicted, ceramic phases were resistant to cavitation wear, yet the difference between alumina **A** and zirconia **Z** was distinct. The most interesting fact resulting from the wear investigations was that both com-

Microstructural observations of eroded surfaces performed by means of SEM technique allowed to recognize differences in destruction mechanisms for investigated materials. Relatively high rate of erosion measured for alumina material could be explained by mechanism which could be distinctly recognized after eroded surface examination revealed in

Destruction of alumina material happened by removing of whole grains. This process accelerated during the test duration and after 2400–3000 min was very intensive. Process of grain fragmentation was not observed. Transgranular cracking was detected in very rare number of cases (like a large grain in the center in **Figure 2** micrograph at the bottom). Seeing that, alumina grains were relatively large; degradation process after long exposition on cavitation

**Figure 1.** Results of volumetric loss measurements during stream-impact cavitation test of investigated oxide materials.

posites **AZ** and **ZA** had much better cavitation resistance than zirconia.

**Figure 2.** SEM microstructures of alumina material (**A**) on different stages of destruction—starting degradation of polished surface (after 600 min) (at the top left side) and advanced level of degradation (after 2400 min) (at the top right side and at the bottom).

Erosion process in zirconia materials runs in different ways. Microstructural documentation of this process was presented in **Figure 3**. Individual zirconia grains were removed from the surface, and this act consequently induced microcracks in this region [12]. Such situation made more probable possibility of removing the next grain in the nearest neighborhood created whole. This process runs not parallel to the sample surface but perpendicularly to it. This was the reason why erosion in zirconia developed in relatively limited surface area, and consequently the removed volume of the material is limited.

The way of degradation of composites depends on the major phase content. In **Figure 4**, selected areas of **AZ** composite microstructures were presented. When dispersion of constituent phases in composite was very good (on single micrometer level), surface was degraded uniformly. The mechanism of degradation was similar like in pure alumina material (whole grains removing), but in **AZ** composite, grains were much smaller than in pure alumina due to restraining influence of inert particles of minor phase (see Zener effect [20]). Additionally,

**Figure 3.** SEM microstructures of zirconia material (**Z**) on medium advanced level of destruction (after 2400 min), smaller magnification showing "paths" of removed grains (left side) and bigger magnification showing local depth of mentioned "paths"(right side).

residual stress state caused by coefficients of thermal expansion mismatch (αAl2O3 = 9.2∙10−6°C−1; αZrO2 = 11.0∙10−6°C−1) [21] kept alumina matrix in average compressive stress state. As an effect of both mentioned factors acting, one can observe significantly limited cavitation erosion rate for **AZ** composite.

Detailed microstructural investigations showed that if some microstructural flaws were present in the composite (**Figure 4** right side) and homogeneity of its microstructure was not perfect (on the level of a few microns), large alumina agglomerates behave like pure alumina phase. Degradation of such agglomerates was faster than areas with well-dispersed zirconia grains, and the mechanism of degradation was identical than that observed for pure alumina (**A**).

Evidences of erosion in **ZA** composite presented in **Figure 5** proved that the main mechanism of material destruction was similar to that noticed for **Z** material. The **ZA** surfaces were covered by a net of erosion paths penetrating into material bulk. However, the surface density of mentioned paths is lower than that for **Z** material. Even if in some cases eroded areas reached diameters of a few microns (**Figure 5**, right side), a total erosion effect was smaller than that measured for pure zirconia phase.

the erosion rate was the small grain size of materials. The direction of residual stresses played a not so important role. It is also important to underline that degradation of all investigated

**Figure 4.** SEM microstructures of alumina/zirconia composite material (**AZ**): on medium advanced level of destruction, after 2400 min (at the top left side), and on strongly advanced level of destruction, after 5400 min (at the top right side), area with poor level of homogeneity (at the bottom). Light grains are zirconia ones; darker grains are alumina phase.

Cavitation Wear of Structural Ceramics http://dx.doi.org/10.5772/intechopen.79510 35

**Figure 6** presented volumetric losses of **A** and **Z** compared to non-oxide materials: silicon nitride (**SN**) and silicon carbide (**SC**). It is clearly visible that erosion rate for **SN** and **SC** was much smaller than that measured for oxide pure phases, but it is worth to notice that they

**Figure 7** illustrated the sequence of **SN** material degradation caused by cavitation. Microstructure of this material is composed of two elements—elongated silicon nitride grains (dark phase in micrographs) and oxynitride amorphous phase (light phase in micrographs) which was the liquid phase during sintering. The presence of liquid phase during sintering promoted very good densification of the material and helped to assure good mechanical properties. During

oxide materials went not linearly, but wear rate accelerated with the test duration.

were very close to values achieved for **AZ** and **ZA** composites.

It is worth to notice that the total level of volume loss for **AZ** and **ZA** materials was very similar. Such effect was not obvious because erosion rates for **A** and **Z** were distinctly different. Probably the strongest influence for such result has an effect of inhibition alumina matrix grain growth process in **AZ** material. Although the residual stress state in **AZ** and **ZA** materials was different (in **AZ** matrix was under compression, in **ZA** matrix was under tension), the total erosion rates were practically identical. In all investigated oxide materials, an elementary erosion act was the removing of the whole grain. The process of transgranular cracking was detected in very limited numbers of individual cases. It suggests that the decisive factor for

residual stress state caused by coefficients of thermal expansion mismatch (αAl2O3 = 9.2∙10−6°C−1; αZrO2 = 11.0∙10−6°C−1) [21] kept alumina matrix in average compressive stress state. As an effect of both mentioned factors acting, one can observe significantly limited cavitation erosion rate

**Figure 3.** SEM microstructures of zirconia material (**Z**) on medium advanced level of destruction (after 2400 min), smaller magnification showing "paths" of removed grains (left side) and bigger magnification showing local depth of

Detailed microstructural investigations showed that if some microstructural flaws were present in the composite (**Figure 4** right side) and homogeneity of its microstructure was not perfect (on the level of a few microns), large alumina agglomerates behave like pure alumina phase. Degradation of such agglomerates was faster than areas with well-dispersed zirconia grains, and the mechanism of degradation was identical than that observed for pure

Evidences of erosion in **ZA** composite presented in **Figure 5** proved that the main mechanism of material destruction was similar to that noticed for **Z** material. The **ZA** surfaces were covered by a net of erosion paths penetrating into material bulk. However, the surface density of mentioned paths is lower than that for **Z** material. Even if in some cases eroded areas reached diameters of a few microns (**Figure 5**, right side), a total erosion effect was smaller than that

It is worth to notice that the total level of volume loss for **AZ** and **ZA** materials was very similar. Such effect was not obvious because erosion rates for **A** and **Z** were distinctly different. Probably the strongest influence for such result has an effect of inhibition alumina matrix grain growth process in **AZ** material. Although the residual stress state in **AZ** and **ZA** materials was different (in **AZ** matrix was under compression, in **ZA** matrix was under tension), the total erosion rates were practically identical. In all investigated oxide materials, an elementary erosion act was the removing of the whole grain. The process of transgranular cracking was detected in very limited numbers of individual cases. It suggests that the decisive factor for

for **AZ** composite.

mentioned "paths"(right side).

34 Cavitation - Selected Issues

alumina (**A**).

measured for pure zirconia phase.

**Figure 4.** SEM microstructures of alumina/zirconia composite material (**AZ**): on medium advanced level of destruction, after 2400 min (at the top left side), and on strongly advanced level of destruction, after 5400 min (at the top right side), area with poor level of homogeneity (at the bottom). Light grains are zirconia ones; darker grains are alumina phase.

the erosion rate was the small grain size of materials. The direction of residual stresses played a not so important role. It is also important to underline that degradation of all investigated oxide materials went not linearly, but wear rate accelerated with the test duration.

**Figure 6** presented volumetric losses of **A** and **Z** compared to non-oxide materials: silicon nitride (**SN**) and silicon carbide (**SC**). It is clearly visible that erosion rate for **SN** and **SC** was much smaller than that measured for oxide pure phases, but it is worth to notice that they were very close to values achieved for **AZ** and **ZA** composites.

**Figure 7** illustrated the sequence of **SN** material degradation caused by cavitation. Microstructure of this material is composed of two elements—elongated silicon nitride grains (dark phase in micrographs) and oxynitride amorphous phase (light phase in micrographs) which was the liquid phase during sintering. The presence of liquid phase during sintering promoted very good densification of the material and helped to assure good mechanical properties. During

**Figure 5.** SEM microstructures of zirconia/alumina composite material (**ZA**) on relatively advanced level of destruction area with very good constituent phase homogeneity (left side) and area with poor level of homogeneity (right side).

The most resistant for jet-impact cavitation test was silicon carbide material (**SC**). In **Figure 8** different stages of its degradation were presented. In this case mechanism of erosion was different from described previously. Volume of material loss proceeded in **SC** case not by the whole grains removing but by cracking of material (**Figure 8** left side) and removing of small parts of it (**Figure 8** right side). **Figure 9** illustrated development of mentioned process shoving a large part of eroded surface after different time of exposition for cavitation (3600

**Figure 7.** SEM microstructures of silicon nitride (**SN**) material at the first stages of destruction, 1800 min (at the top left side) and on more advanced levels of destruction, 3600 min (at the top right side) and 6000 min (at the bottom).

Cavitation Wear of Structural Ceramics http://dx.doi.org/10.5772/intechopen.79510 37

During jet-impact test procedure of data collecting consisted in measure of weight loss after every 600 min of test. In was not very dense net of experimental points due to rather high resistance of investigated materials for cavitation wear. Anyway, even not very frequent collection allowed to detect an important difference between oxide and non-oxide materials at the first stages of erosion. Measurable effect of material loss in oxide materials was detected from the beginning of the test. Measurements after 600 min showed distinct wear rate. For non-oxide

and 5400 min).

**Figure 6.** Results of volumetric losses during stream-impact cavitation test of alumina, zirconia, silicon carbide, and silicon nitride materials.

cavitation test, this phase seemed to be the weakest element of **SN** material microstructure. Erosion of **SN** started in oxynitride phase volume, and it proceeded through this phase. When this process was advanced enough, the whole silicon nitride grains could be removed. Probably, elongated shape of silicon carbide grains was profitable for erosion rate decrease. These elongated grains were trapped in bulk material, and they have not been removed so easily as it was observed for isometric oxide grains in previously mentioned materials (**A**, **Z**).

**Figure 7.** SEM microstructures of silicon nitride (**SN**) material at the first stages of destruction, 1800 min (at the top left side) and on more advanced levels of destruction, 3600 min (at the top right side) and 6000 min (at the bottom).

The most resistant for jet-impact cavitation test was silicon carbide material (**SC**). In **Figure 8** different stages of its degradation were presented. In this case mechanism of erosion was different from described previously. Volume of material loss proceeded in **SC** case not by the whole grains removing but by cracking of material (**Figure 8** left side) and removing of small parts of it (**Figure 8** right side). **Figure 9** illustrated development of mentioned process shoving a large part of eroded surface after different time of exposition for cavitation (3600 and 5400 min).

During jet-impact test procedure of data collecting consisted in measure of weight loss after every 600 min of test. In was not very dense net of experimental points due to rather high resistance of investigated materials for cavitation wear. Anyway, even not very frequent collection allowed to detect an important difference between oxide and non-oxide materials at the first stages of erosion. Measurable effect of material loss in oxide materials was detected from the beginning of the test. Measurements after 600 min showed distinct wear rate. For non-oxide

cavitation test, this phase seemed to be the weakest element of **SN** material microstructure. Erosion of **SN** started in oxynitride phase volume, and it proceeded through this phase. When this process was advanced enough, the whole silicon nitride grains could be removed. Probably, elongated shape of silicon carbide grains was profitable for erosion rate decrease. These elongated grains were trapped in bulk material, and they have not been removed so easily as it was

**Figure 6.** Results of volumetric losses during stream-impact cavitation test of alumina, zirconia, silicon carbide, and

**Figure 5.** SEM microstructures of zirconia/alumina composite material (**ZA**) on relatively advanced level of destruction area with very good constituent phase homogeneity (left side) and area with poor level of homogeneity (right side).

observed for isometric oxide grains in previously mentioned materials (**A**, **Z**).

silicon nitride materials.

36 Cavitation - Selected Issues

**Figure 8.** SEM microstructures of silicon carbide material (**SC**) at the first step of degradation, 1800 min (left side), and on relatively advanced level of destruction, 3600 min (right side).

**4. Conclusions**

ner, independently on test duration.

nitride materials at the first stages of destruction.

**Acknowledgements**

interact with stresses caused by cavitation.

Performed jet-impact cavitation test of a group of ceramic materials confirmed their relatively high resistance for cavitation erosion. Test revealed differences between mechanisms of deg-

**Figure 10.** Results of volumetric losses during jet-impact cavitation test of alumina, zirconia, silicon carbide, and silicon

Cavitation Wear of Structural Ceramics http://dx.doi.org/10.5772/intechopen.79510 39

Oxide materials degradation consisted in the whole grains removing from the bulk. Silicon nitride material eroded by faster degradation of amorphous phase which was the remnant of sintering process. Silicon carbide destruction is run by grain cracking and fragmentation. Degradation of all oxide materials started relatively fast and proceeded in accelerated manner during the whole test. Contrary to that, non-oxide materials had a period of stability when any measurable mass losses were detected. After this period materials eroded in a stable man-

Composites in alumina/zirconia system have much better resistance for cavitation wear than alumina or zirconia monophase materials. This improvement could be described to profitable microstructural changes (finer grain size) and the presence of residual stresses which locally

Author would like to thank Dr. Magdalena Ziąbka from the Department of Ceramics and Refractory Materials of AGH University Krakow for very patient and competent assistance

radation of materials subjected to cavitation and differences in measured wear rates.

**Figure 9.** SEM microstructures of silicon carbide material (**SC**) after 3600 min (left side) and 5400 min (right side) duration of the jet-impact test.

materials (**SN** and **SC**), the first measurable effect of erosion was detected after 1800 min of test (**Figure 10**). This fact does not directly confirm that cavitation-caused erosion could be treated as an effect of a specific type of fatigue test. It confirms that different materials have a different threshold for degradation to start.

After the first period of stability, during the rest of performed cavitation test, the wear rates of **SC** and **SN** materials were practically stable contrary to systematical increment of wear rates for oxide materials (**Figure 6**).

**Figure 10.** Results of volumetric losses during jet-impact cavitation test of alumina, zirconia, silicon carbide, and silicon nitride materials at the first stages of destruction.
