Synthesis of composite coatings using rapid laser sintering of metallic powder mixtures

Presented here is a brief review of research of an authors’ collective dealing with laser treatment of materials, sintering metallic powders, and multiscale simulation. A theoretical analysis of the processes of structure formation upon the rapid laser synthesis of composite coatings has been performed. The experimentally obtained structural and phase characteristics of the sintered layers have been explained based on an analytical and numerical simulation of the dynamics of thermal fields in the zone of treatment, processes of melting, and subsequent solidification of porous materials. Upon rapid sintering and solidification, the effect of impurity trapping has been taken into account, which determines the chemical composition of the powders under nonequilibrium conditions of their formation. It has been shown that rapid laser treatment retains the composite structure of the powder layer due to the high rates of local heating/cooling and high rate of solidification comparable with the rate of diffusion of chemical components. The results obtained are applicable in the development of a wide class of functional-gradient composite materials.     

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.     

Production and service of high-alumina ceramic castables. 1. Ramming mixtures based on modified bauxite HCBS

The prospects of using bauxite-base HCBS in the production of high-quality ramming mixtures for lining blast furnace spouts are considered. A system of Chinese bauxite (filler) and bauxite HCBS (binder) is shown to give a ramming mixture with a porosity of 18 – 22% and σ_c of 120 MPa. The effect of some process factors on the properties of the new refractory material is considered. The life of the mixture in the main hearth spout is shown to be 70 – 90 thousand tons cast iron before the first repair at a consumption of 0.53 – 0.63 kg mixture per one ton cast iron. The processes that occur in the interaction between the material and blast furnace slags of various chemical compositions is considered. The characteristics of the developed mixture are compared with those of the standard ones. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 3, pp. 37–41, March, 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.     

Strength and crack resistance of zirconium dioxide crystals containing yttrium and terbium oxides

Conclusions We studied the mechanical behavior of the partially stabilized (with yttrium oxide) zirconium dioxide crystals in which a small quantity of terbium oxide was introduced as an additive. The obtained data indicate the suitability of this material for practical applications. The effect of heat treatment on the physical and mechanical properties of such materials was revealed. It was established that on cooling up to –150°C, their strength and thermal shock resistance are improved.The lack of correspondence between the values of the crack resistance determined according to the methods of bend testing and indentation was established. The existence of a significant barrier to fracture was observed. The two possible types of surfaces of the crystals under consideration were revealed and the relationship between the structure of these surfaces (and, also, the near-surface layers adjoining them) and the resistance to fracture was established.Translated from Ogneupory, No. 6, pp. 2–8, June, 1993.     

Critierional evaluation of thermal destruction of corundum concretes

Conclusions The resistance of corundum concretes to thermal destruction was studied using the experimentally plotted deformation diagrams and the standard thermal cycling method. The correlation between the values of thermal shock resistance obtained according to these methods was established for the corundum concretes.It was established that as compared to the concrete produced using a binder based on the Talyum cement, the concretes incorporating a hydraulically hardening (water-setting) binder prepared from the VTs-70 grade high-alumina cement exhibit better resistance to thermal stresses. With increasing strength and density of the concretes, their thermal destruction occurs catastrophically (abruptly) since the elements of a stronger structure (the concretes produced using the Talyum cement-based binder) are incapable of effectively hindering the growth of a potential crack.Modification of the structure of the concrete by introducing Cr₂O₃ additive leads not only to an increased thermal shock resistance in the high-temperature range due to an increased critical crack length, but also to an increased resistance ot crack displacement (opening).Translated from Ogneupory, No. 5, pp. 3–8, May, 1988.