Properties of investigated materials.
The usage of advanced ceramic materials in the applications endangered by intensive cavitation could limit erosion phenomena distinctly. In the presented work, cavitation erosion resistance of ceramics the most commonly used in structural applications was investigated. These materials were oxide ones: α-alumina, yttria-stabilized tetragonal zirconia, and two composites selected from alumina/zirconia system. Otherwise, the most promising non-oxide materials were examined: silicon carbide and silicon nitride. Results showed significant difference in cavitation wear mechanisms of all investigated materials. Degradation of alumina proceeded from the beginning on the relatively large surfaces, and the dominant mechanism of destruction was removing of the whole grains. Degradation of zirconia also consisted on removing of the whole grains, but this process proceeded locally, along ribbon-like paths. Cavitation wear of composites was strongly influenced by the residual stresses caused by the thermal expansion coefficient mismatch. Cavitation erosion of silicon nitride proceeded by selective degradation of glassy phase present on grain boundaries. On the contrary, silicon carbide degradation proceeded by large grain fragmentation process.
- cavitation wear
- silicon carbide
- silicon nitride
Phenomenon of cavitation could be described as reproducible process of nucleation, growth, and violent collapse of clouds of bubbles within the liquid. As a consequence of implosion of cavitation bubbles, microstreams of liquid are produced, and pressure waves assisting bubble disappearing process become the main reason of material damage. This damage consists in a material loss called cavitation erosion. Mentioned process starts on material surface and depending on material properties develops locally on bigger surface areas or proceeds into material bulk. The nature of loading caused by the interaction between pressure waves, microstream blows, and intensive hydrodynamics parameters is presented by many researchers as fatigue process [1, 2, 3]. As a result of such approach, improvement of cavitation resistance of materials should be reached by the material hardness and micro-hardness increase, the mean grain size decrease, and introduction of internal compressive stresses (in the case of multiphase materials) [4, 5, 6]. Progress in cavitation resistance in metallic materials was reached by using intermetallic phases [7, 8]. Modern demands for reliability of fluid-flow machinery components forced application of ceramic phases as possible more resistant for cavitation damage than any metallic phase. Investigations of cavitation erosion of ceramics are not very often. Sparse reports [9, 10, 11, 12, 13, 14, 15, 16, 17, 18] concern such materials like monophase oxides (α-alumina, tetragonal zirconia), silicon nitride, or some types of glassy phases. The mentioned works gave, as a result, some experimental data which put in order cavitation wear resistance of ceramic phases, suggesting explanations how the microstructure of sintered bodies could influence their susceptibility to cavitation wear. The presented work summarizing results of investigations of cavitation erosion resistance of commonly used, in structural applications, oxide (α-alumina and tetragonal zirconia, composites in alumina/zirconia system) and non-oxide (silicon carbide, silicon nitride) subjected to intensive, long-lasting (6000 min) jet-impact tests was investigated.
The process of cavitation wear was investigated for six ceramic materials. Four of them were the widely used oxide materials: α-alumina, tetragonal zirconia, and two composites in alumina/zirconia system. The first one was an alumina-based material containing 10 vol.% of zirconia additive and the second one was zirconia based with 10 vol.% of alumina particles. For fabrication of sintered bodies, commercial powders were utilized: Al2O3—TM-DAR produced by Taimicron Inc., Japan (the mean crystallite size of 130 nm), and yttria-stabilized ZrO2 powder named 3Y-TZ manufactured by Tosoh, Japan (the mean crystallite size of 20 nm). Composite powders were manufactured by rotation-vibration mixing of constituent powders. The mixing procedure was conducted in ethyl alcohol suspension for 1 h. After separation from milling media (5 mm zirconia balls), composited powders were dried and granulated. Preliminary compaction of powders was performed uniaxially in ceramic die under pressure of 50 MPa. After that, samples were isostatically repressed under 300 MPa. Pressureless sintering process was conducted at 1500 (for alumina) or 1550°C (for the rest of oxide materials). The dwelling time of 2 h was the same for all the mentioned samples. Mentioned procedure allowed to achieve samples which have cylindrical shape of 20 mm in diameter and 6 ± 0.5 mm high. Description of oxide materials investigated in this work was as follows:
Silicon carbide (
Silicon nitride (
Densification (relative density
||Bending strength, σ, MPa|
||99.28||17.0 ± 1.2||379 ± 6||4.3 ± 0.2||600 ± 120|
||99.96||14.0 ± 0.5||209 ± 5||6.1 ± 0.3||1150 ± 55|
||98.50||17.0 ± 0.4||361 ± 5||5.1 ± 0.5||800 ± 120|
||99.12||15.0 ± 0.6||216 ± 4||6.0 ± 0.4||1050 ± 55|
||98.50||27.2 ± 0.8||392 ± 6||6.3 ± 2.0||550 ± 100|
||98.66||16.5 ± 0.9||301 ± 8||5.3 ± 1.1||720 ± 150|
Basic mechanical properties were determined as follows: hardness (
Cavitation erosion process was examined utilizing jet-impact device, described in detail in . The sample surface roughness, measured before the test (PGM-1 C profilometer), was less than 0.03 μm for all samples. Cavitation wear test consists in fast rotation of samples which stroke against the water stream. The samples were mounted vertically in rotor arms, parallel to the axis of water stream. Water was pumped continuously at 0.06 MPa through a nozzle with a 10 mm diameter, 1.6 mm away from the sample edge. Water flow intensity was constant and amounted to 1.55 m3/h. The wear rate was determined by sample weighing up to the total time of 6000 min. The wear rate was determined after each 600 min of the test as the weight loss of each sample. The samples were dried before weighing in a laboratory dryer at 120°C for 60 min. The volumetric wear rates were calculated using apparent density of each sample type and their weight loss. Surfaces of the worn materials were examined by means of the SEM technique using FEI Nova Nano 200 device.
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
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 . 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
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 (
Evidences of erosion in
It is worth to notice that the total level of volume loss for
Figure 6 presented volumetric losses of
Figure 7 illustrated the sequence of
The most resistant for jet-impact cavitation test was silicon carbide material (
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 materials (
After the first period of stability, during the rest of performed cavitation test, the wear rates of
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 degradation of materials subjected to cavitation and differences in measured wear rates.
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 manner, independently on test duration.
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 interact with stresses caused by cavitation.
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 during SEM observations and Dr. Robert Jasionowski from Maritime Academy Szczecin for its involvement in cavitation tests.
Brennen CE. Cavitation and Bubble Dynamics. New York: Oxford University Press; 1995
Briggs LJ. The limiting negative pressure of water. Journal of Applied Physics. 1970; 21:721-722
Trevena DH. Cavitation and Tension in Liquids. Bristol: IOP Publishing Ltd; 1987
Plesset MS, Chapman RB. Collapse of an initially spherical vapor cavity in the neighborhood of a solid boundary. Journal of Fluid Mechanics. 1971; 47(2):283-290
Hickling R, Plesset MS. Collapse and rebound of a spherical bubble in water. Physics of Fluids. 1964; 7(1):7-14
Naude CF, Ellis AT. On the mechanism of cavitation damage by non-hemispherical cavities collapsing in contact with a solid boundary. Journal of Basic Engineering. 1961; 83:648-656
Jasionowski R, Przetakiewicz W, Zasada D. The effect of structure on the cavitational wear of FeAl intermetallic phase-based alloys with cubic lattice. Archives of Foundry Engineering. 2011; 11(2):97-102
Schneibel JH, George EP, Anderson IM. Tensile ductility, slow crack growth and fracture mode of ternary B2 iron aluminides at room temperature. Intermetallics. 1997; 5:185-193
Tomlinson WJ, Matthews SJ. Cavitation erosion of structural ceramics. Ceramics International. 1994; 20(3):201-209
Tomlinson WJ, Kalitsounakis N, Vekinis G. Cavitation erosion of aluminas. Ceramics International. 1999; 25(4):331-338
Niebuhr D. Cavitation erosion behavior of ceramics in aqueous solutions. Wear. 2007; 263(1-6):295-300
Garcia-Atance Fatjo G, Hadfield M, Tabeshfar K. Pseudoplastic deformation pits on polished ceramics due to cavitation erosion. Ceramics International. 2011; 37:1919-1927
Lua J, Zum Gahr K-H, Schneider J. Microstructural effects on the resistance to cavitation erosion of ZrO2 ceramics in water. Wear. 2008; 265:1680-1686
Pedzich Z. The abrasive wear of alumina matrix particulate composites at different environments of work. In: Zhang D, Pickering K, Gabbitas B, Cao P, Langdon A, Torrens R, et al., editors. Advanced Materials and Processing IV. Vol. 29-30. Switzerland: Trans Tech Publications; 2007. pp. 283-286
Pedzich Z. Fracture of oxide matrix composites with different phase arrangement. In: Dusza J, Danzer R, Morrell R, Quinn GD, editors. Fractography of Advanced Ceramics III: Key Engineering Materials. Vol. 409. Switzerland: Trans Tech Publications; 2009. pp. 244-251
Pędzich Z, Jasionowski R, Ziąbka M. Cavitation wear of ceramics—Part I. Mechanisms of cavitation wear of alumina and tetragonal zirconia sintered polycrystals. Composites Theory and Practice. 2013; 13(4):288-292
Pędzich Z, Jasionowski R, Ziąbka M. Cavitation wear of ceramics—Part II. Mechanisms of cavitation wear of composites with oxide matrices. Composites Theory and Practice. 2014; 14(3):139-144
Pędzich Z, Jasionowski R, Ziąbka M. Cavitation wear of structural oxide ceramics and selected composite materials. Journal of the European Ceramic Society. 2014; 34(14):3351-3356. DOI: 10.1016/j.jeurceramsoc.2014.04.022
Niihara K. A fracture mechanics analysis of indentation. Journal of Materials Science Letters. 1983; 2:221-223
Kang S-JL. Sintering: Densification, Grain Growth and Microstructure. Amsterdam: Elsevier; 2005
Grabowski G, Pedzich Z. Residual stresses in particulate composites with alumina and zirconia matrices. Journal of the European Ceramic Society. 2007; 27(2, 3):1287-1292