V. M. Timchenko

Influence of the composition of the tetragonal phase in the surface layers of zirconia-based ceramics on their strength

The strength of partially stabilized zirconia-based ceramics is analyzed as a function of the porosity, the grain size, and the degree of tetragonality of the tetragonal phase. It is found that the strength of the studied ceramics, unlike conventional materials, is virtually independent of the porosity and the average grain size and is determined primarily by the content of the easily transformed tetragonal phase.

Influence of the size effect on the electrical conductivity of partially stabilized zirconia

The volume component of the electrical conductivity of bulk ceramics of the partially stabilized zirconia (ZrO2)0.94(Y2O3)0.05(Al2O3)0.01 is found to increase by a factor of 1.7 with the grain size decreasing from 600–800 to 200–300 nm. The observed effect is explained by the action of the pressure produced by surface tension forces, which shifts equilibrium toward the point of the polymorphic transition to the cubic phase.

Specific physical properties of nanocrystalline (La0.65Sr0.35)0.8Mn1.2O3 ± Δ samples obtained by cold isostatic pressing

Single-phase powders of manganites (La0.65Sr0.35)0.8Mn1.2O3 ± Δ with average crystallite sizes of 30, 50, and 500 nm were produced by co-precipitation. The samples studied were obtained by cold isostatic pressing of powders at a pressure of 1 GPa without subsequent sintering. It is shown that the size of particles has a significant effect on the electromagnetic properties of the manganite samples. As the crystallite size decreases, the electrical resistance and coercive force increase and the tunneling magnetoresistance of the samples and the Curie temperature decrease.

Wear of ceramics based on magnesia- and ceria-stabilized zirconia in dry friction against steel

Wear of two ceramic materials containing partially stabilized zirconia is studied in unlubricated friction against steel. The zirconia is stabilized by magnesia (Mg-PSZ ceramics) and ceria (Ce-TZP ceramics). The wear rate of the Mg-PSZ ceramics is found to be 1.5 times lower than that of the Ce-TZP ceramics. This difference is attributed to the fact that the wear of Ce-TZP ceramics occurs via tear of large grain blocks from the friction surface, whereas Mg-PSZ ceramics are worn by way of grain attrition.

Effect of Strain Rate and Preloading on the Fracture Toughness of ZrO2-Based Ceramics

We study the variation of the fracture toughness KIcofZrO2 - Y2 O3 ceramics (density ∼98%) as a function of the testing machine crosshead speed (0.005–50 mm/min) and preloading at KI < Kc. The fracture toughness is shown to be practically constant in the speed range from 0.05 to 5 mm/min. At a loading rate of 50 mm/min, the quantity KIc substantially decreases (by a factor of more than two), whereas at a rate of 0.005 mm/min it slightly increases. Preloading leads to a 1.5-fold increase in KIc. Variation of the fracture toughness is associated with structural transformations.

Stability of structural materials based on ZrO2

The thermal and mechanical stability of some high-strength ceramic materials from partially stabilized ZrO2 manufactured from various domestic and imported powders, including coprecipitated, sol-gel, and hydrothermal ones, with the use of CIP and sintering is considered. The thermal stability is tested under conditions close to the operating ones, i.e., under long-duration holds at 1000 and 1550°C and in water quenching. The mechanical stability is determined in impact-erosion wear and under combined loads of high pressure and multiple indentations by solid particles. It is shown that all the materials undergo degradation of various degrees but those most durable under normal conditions (hydrothermal and sol-gel materials, ceramics manufactured from imported press powders) are least stable. They have widely fluctuating properties under cyclic high-temperature loads, endure 900-1400°C, and withstand a pressure of at most 1.0-2.0 GPa in an abrasive, just like standard corundum ceramics; however, they are characterized by maximum wear resistance. At the same time, an original material from commercial coprecipitated PSZ powder has quite different features; its thermal stability allows it to withstand repeated quenchings from 1550°C in water, and the mechanical strength can attain 2.6-2.8 GPa, exceeding the strength of quenched tool steels in similar situations. Due to its refractoriness (2700°C) and chemical stability this material is the most versatile in operating under extreme conditions.

Effect of the pressure of cold isostatic pressing and the sintering temperature on the properties of ceramics made of partially stabilized ZrO2

The physicomechanical properties of a ceramics composed of ZrO2+3 mol.% Y2O3 and fabricated by cold isostatic pressing (CIP) at up to 0.8 GPa with sintering at 1440–1620°C are described. It is shown that the material with a low content of admixtures sinters only at 1620°C at a pressure of 0.6–0.8 GPa and possesses a density of 6.0 g/cm3, an ultimate bending strength of 900 MPa, and a fracture toughness of 8 MPa·m1/2. For the material with a high content of admixtures sintered at 1550°C and pressed at a pressure of 0.6 GPa the same parameters are 5.95 g/cm3, 700 MPa, and 12 MPa·m1/2. It is established that CIP promotes sintering of Y-PSZ ceramics. CIP has the best effect on the mechanical properties of high-purity materials.

Cold isostatic pressing as a method for fabricating high-strength ceramic materials based on ZrO2

High-strength ceramic materials fabricated with the use of cold isostatic pressing (CIP) at ≤0.8 GPa and sintering by different regimes from yttria-stabilized zirconia produced by leading foreign and Ukrainian firms are described. All the materials exhibit two peaks of mechanical properties, one at a low CIP pressure (0.1–0.3 GPa) attributable to destabilization of the press-powder and characterizing the stability of the material as a whole and the other at a high CIP pressure (above 0.6 GPa) attributable to attainment of close packing of the particles. In accordance with the set of standard properties the materials can be classified into two types, namely, intermediate-strength ones with an ultimate bending strength of 700–900 MPa,K1c ranging from 7 to 12 MPa·m1/2, a density of about 6 g/cm2, and an optimum CIP pressure of 0.2–0.3 GPa (the first peak) and high-strength materials with an ultimate bending strength of about 1200 MPa,KIc ranging from 6 to 9 MPa·m1/2, a density of about 6.1 g/cm3, and an optimum CIP pressure of 0.1 GPa. The intermediate type comprises the majority of domestic materials and some foreign ones and is characterized by higher stability and crack resistance. The high-strength type comprises mainly foreign materials with high strength and lower stability.

Effect of Quasi-Hydrostatic Compression on the Mechanical Properties of Ceramics in the ZrO2 + 3 MOL.% Y2O3 System

The effect of quasi-hydrostatic compression on the strength of ZrO2 + 3 mol.% Y2O3 ceramic specimens of two series was studied. The series 1 ceramic was a powder commercially available from TOSOH Co. (Japan), with a density of 6.1 g/cm3, and the series 2 ceramic was a powder with a density of 5.9 g/cm3 prepared under laboratory conditions at the IPM Research Institute (National Academy of Sciences, Ukraine). The pressure range was up to 1.2 GPa, and the pressure-transmitting medium was a coarse-grained corundum powder. In the series 1 specimens, the strength increases with pressure over the entire pressure range (from 670 MPa to 1098 MPa at 1.2 GPa); in the series 2 specimens, the strength increases only to a pressure of 0.8 GPa (from 695 MPa to 828 MPa) and then, with further increase in pressure drops sharply to nearly zero (30 MPa at 1.2 GPa). It was proposed that the observed effect might be associated with a martensite transformation in the zone of structural imperfections (discontinuities). On reaching a critical value determined by the strength of the matrix, the martensite transformation becomes a cause of failure of the material.

The effect of prestressing on the fracture toughness of ZrO2-(3, 4) mol % Y2O3 ceramics

It is shown that the fracture toughness of prestressed ZrO2-(3, 4) mol % Y2O3 ceramics monotonically increases, the growth being as high as ∼50% of the initial value. It is believed that prestressing causes slow isothermal martensite transformation of some grains in the material. During mechanical tests, the degree of transformation rises, which shows up as increased fracture roughness.