Mechanical Properties and Oxidation Behavior of Al2O3–AlN–TiN Composites

The study examines the effect which the composition of hot-pressed electroconductive ceramics has on their structure, mechanical properties, and oxidation behavior, for ceramics of the type AIN–Al₂O₃–42 wt% TiN, differing in the AIN/Al₂O₃ ratio. The results are physico-mechanical property data, including density, hardness, strength, fracture toughness, and wear resistance. A correlation was found between the wear resistance and fracture toughness. The analysis of oxidation products revealed the formation of α-Al₂O₃ and rutile in the temperature range from 600° to 1100°C and aluminum titanate above 1200°C. The spallation of the oxide layer caused low oxidation resistance of Al₂O₃-rich composites above 1250°C. The oxidation of composites was compared with the oxidation of pure TiN. The relationship is discussed between material properties, composition, phases, and processing parameters.     

Fracture toughness, strength, and other characteristics of yttria-stabilized zirconium ceramics

The mechanical behavior of zirconium ceramics with different porosities (456 and 1330 MPa) is studied with an accent on the fracture toughness (the study of other characteristics in a wide temperature range plays an auxiliary role) The tests are predominantly performed with the use of a Vickers pyramid (theIF andIS methods) and under flexure (theSEVNB andSENB methods) The results of the indentation and subsequent bending of the specimens are used to plot aR-curve that turns out to be plane It is shown that despite the substantial difference in the strength of the studied variants of ceramics their crack resistance differs inconsiderably The values of the critical coefficients of stress intensity determined by various methods for the same ceramics are shown to differ The results obtained are analyzed using the data of a micro-Raman analysis The tests for crack resistance by the method ofSEVNB (bending of a beam with a polishedV-notch) show that the specimens do not undergo a tetragonal-monoclinic phase transformation during the deposition of a stress concentrator Transleted from Ogneupory i Technicheskaya Keramika, No. 8, pp. 7 – 13, August, 2000.     

A micro-Raman study of phase transformations of zirconia crystals upon introduction of a vickers indentor

A micro-Raman study of zirconia crystals and ceramics performed with the use of argon, He - Ne, and semiconductor lasers is described. The results of the study are shown to depend on the wavelength of the radiation source used. It is shown that argon radiation determines only the monoclinic phase formed upon indentation of low-strength crystals of partially stabilized zirconia, which is not observed in the case of tests of high-strength crystals. The indentation behavior of crystals of partially stabilized zirconia is shown to depend on the tetragonal-monoclinic phase transformation and the rotation of typical ferroelastic domains. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 6, pp. 4–9, June, 2000.     

Fracture behaviour of Y-TZP ceramics: New outcomes

Modern approaches to fracture resistance estimation of Y-TZP ceramics are critically analyzed. It is shown that fracture toughness test methods do not produce data that are reliable enough. The present investigation makes use of the edge fracture (EF) test method based on flaking the rectangular specimen edge. Chip scars formed on specimen edges after the Rockwell indentation were examined. It is established that not the chip scar shape but its surface area is decisive for the fracture resistance of these ceramics. The influence of indenter sharpness on fracture resistance estimates was elucidated: Rockwell indenter, 400-μm tip radius conical indenter, and Vickers indenter were used. The EF method ensures simultaneous determining a fracture resistance characteristic and plotting an FR-line some equivalent to a conventional R-curve. Therefore, such economically feasible tests, easily realizable in a conventional mechanical laboratory, provide quite exhaustive information on the fracture behaviour of ceramics. The proposed method of evaluating the fracture resistance of Y-TZP ceramics may be useful in materials science practice.     

Study of thermal shock resistance of fired and unfired zirconia refractories

Conclusions Comparative studies were made of fired and unfired refractories based on zirconium dioxide. The fired refractories are more resistant to cracking. However, the unfired materials possess a higher resistance to the development of cracking which is noted during the registration of surface deformations in the process of destruction of the specimen; and this is confirmed by analytical criterial evaluation.The determination of the destructive drop in temperatures before the development of the first crack in specimens based on granular bodies should not be considered as a reliable evaluation of the thermal shock resistance of the refractories.A new method was developed for observing the nature of the failure in specimens during thermal loading.Translated from Ogneupory No. 1, pp. 52–56, January, 1973.     

Fracture barrier estimation by the edge fracture test method

Known methods for determining the fracture resistance of brittle materials by indentation flaking of the rectangular specimen edge are compared. The edge fracture (EF) test method was chosen as an optimum one. The investigations demonstrated that silicon carbide ceramics, submicrometre alumina, and glasses possessed fracture resistance (F_R) values that can be located above the baseline in the EF base diagram. These results emphasize that the above materials display a higher resistance to the onset of fracture (fracture barrier). This barrier can be estimated by comparing EF test data resulting from Rockwell and Vickers indentations.