**2. Experimental**

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: Al<sup>2</sup> O3 —TM-DAR produced by Taimicron Inc., Japan (the mean crystallite size of 130 nm), and yttria-stabilized ZrO<sup>2</sup> 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: **A**, **Z**, **AZ**, and **ZA** for alumina, zirconia, alumina/zirconia composite, and zirconia/alumina composite, respectively.

Silicon carbide (**SC**) samples were prepared utilizing commercial powder (SIKA FCP 15, Saint-Gobain). Compaction conditions were identical as for oxide materials. Sintering procedure was as follows: heating 10°C/min up to 1800°C, 5°C/min in the range of 1800–2150°C. Dwelling time at 2150°C was 1 h and the sintering atmosphere was argon.

Silicon nitride (**SN**) material was prepared on the base of Si3 N4 H.C. STARCK powder and oxide additives in 4 wt.% Y<sup>2</sup> O3 (POCh, Lublin, Poland) and 6 wt.% of Al<sup>2</sup> O3 TM-DAR. The final powder was prepared by rotation-vibration wet mixing of constituent powders for 1 h in the environment of isopropyl alcohol. Composite powders after separation from milling media (5 mm silicon nitride balls) were dried and granulated. Compaction conditions were identical as for previously described materials. Sintering process was carried on at 1800°C for 2 h in nitrogen atmosphere.

Densification (relative density *ρ*) of each material was calculated as a reference of apparent density measured by Archimedes method (at 21°C) to the theoretical values (dZrO2 = 6.10 g/cm<sup>3</sup> , dSiC = 3.21 g/cm<sup>3</sup> , dSi3N4 = 3.21 g/cm<sup>3</sup> , dAl2O3 = 3.99 g/cm<sup>3</sup> , dY2O3 = 5.01 g/cm<sup>3</sup> ). Relative density for silicon carbide samples was calculated considering the content of phases arising due to oxide addition. Densification of materials (as relative density values) was collected in **Table 1**.

Basic mechanical properties were determined as follows: hardness (*HV*) and fracture toughness (*K*Ic) were investigated by the Vickers indentation method. The values of *K*Ic parameter were calculated basing on the Niihara model [19]. Data for calculations were collected utilizing Nanotech MV-700 equipment. The load was 49.05 N for hardness and 98.1 N for *K*Ic measurements. The data for bending strength (σ) analysis were delivered by the four-point bending tests performed on 45 mm × 4 mm × 3 mm bars (Zwick Roell testing machine). The ultrasonic method was used for Young's moduli of sintered body determination.

Cavitation erosion process was examined utilizing jet-impact device, described in detail in [7]. 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 m<sup>3</sup> /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.


**Table 1.** Properties of investigated materials.

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 [1–3]. 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) [4–6]. 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 [9–18] 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 nonoxide (silicon carbide, silicon nitride) subjected to intensive, long-lasting (6000 min) jet-impact

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 fabri-

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: **A**, **Z**, **AZ**, and **ZA** for alumina, zirconia, alumina/zirconia composite, and zirconia/alumina composite, respectively. Silicon carbide (**SC**) samples were prepared utilizing commercial powder (SIKA FCP 15, Saint-Gobain). Compaction conditions were identical as for oxide materials. Sintering procedure was as follows: heating 10°C/min up to 1800°C, 5°C/min in the range of 1800–2150°C. Dwelling

Taimicron Inc., Japan (the mean crystallite size of 130 nm), and yttria-stabilized ZrO<sup>2</sup>

O3

—TM-DAR produced by

powder

cation of sintered bodies, commercial powders were utilized: Al<sup>2</sup>

time at 2150°C was 1 h and the sintering atmosphere was argon.

tests was investigated.

30 Cavitation - Selected Issues

**2. Experimental**
