Mathematical model of pressing double-phase powders

A phenomenological equation describing the compressibility of double-phase powders without a liquid that is binder based on continuity theory is suggested. It is established that in the case of a smooth experimental curve the parameters of a unified compressibility equation for all pressing stages can be calculated. The numerical parameters of the equation are determined for six powders with particles of various hardnesses. The greatest deviation of the calculated values of the relative density from the experimental values amounts to 11% in the first pressing stage and does not exceed 2.2 – 6.4% in the other regions of the compressibility curve.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 11, pp. 24–28, November, 1996.     

Isostatic pressing technology for refractory materials

Conclusions In the USSR and abroad numerous investigations have been carried out into the process of isostatic pressing of refractories and ceramics. The method is distinguished primarily by excellent physical properties, and density and strength which are uniform throughout the volume. The method is useful for making large, complex articles, including those which cannot be shaped by other methods.Isostatic pressing is widely used in various technologies. At present, Soviet and foreign industries have shown that it is advantageous to develop this method for the metallurgical industry, in particular for producing stopper-monoblocks and nozzles for the donkey ladles of continuous casting machines, crucibles, etc.Translated from Ogneupory, No. 8, pp. 6–13, August, 1982.     

Acoustic emission monitoring of uniaxial pressing of ceramic powders

Powder compaction is a critical step in a classical method for the production of zero-defect ceramic products. We have studied the development of this step to determine whether acoustic emission can be used to detect the presence of flaws in a compact. In uniaxial pressing at room temperature, the creation and extension of flaws occur principally during the ejection of the compacts. The type and extent of flaws depend on the mechanical properties of the powder used and on the pressing conditions. Acoustic emission was used to monitor the consolidation by uniaxial pressing of Al₂O₃ powders and a UO₂---PuO₂ powder. It was possible to detect flaws in the compacts, to determine their type for a given powder, and to identify the type of powder used by a direct examination of cumulative counts of acoustic emission events and by a statistical analysis of amplitude distributions. Hence, the introduction of this technique in a fabrication process offers the possibility of detecting the presence of defects in compacts as soon as they are created. This would decrease the difficulties involved in reuse in the process of the powder from defective products, and would limit further flaw development during sintering, which are two major problems in the nuclear industry.     

Determining pressing rates for refractory semidry bodies

Conclusions An inverse relationship was established for the permitted pressing rate and the ratio between the volume of the products and its double perimeter in plan. With an increase in the dimensions of the article it is necessary to reduce the pressing rate to exclude the formation of pressing cracks.Empirical equations are proposed which as a first approximation can be used for calculating the average rates and durations of pressing, and also for accumulating data connected with the question under discussion.Translated from Ogneupory, No. 5, pp. 17–21, May, 1971.     

Effects of pressing pressure on forsterite refractory density

Conclusions The density (porosity) in forsterite components has been examined as a function of pressure and represented as exponential and logarithmic equations.Berezhnoi's pressing equation and method have been used to calculate some pressing parameters for forsterite components.Translated from Ogneupory, No. 5, pp. 28–30, May, 1989.     

Making high-performance MoAlB ceramics by hot pressing combustion-synthesized powders

It is one of the most concerning issues to achieve high purity, low cost and excellent performance in MAB phases. Herein, the MoAlB powders were synthesized by an ultra-fast combustion synthesis (CS). The reaction mechanism was analyzed by changing components of raw materials. The high-purity MoAlB powders were successfully obtained with the mole ratio of Mo, Al and B of 1:1.3:1. After hot-pressing sintering, the powders were densified and formed compact MoAlB ceramics. Together with phase composition, microstructure, mechanical properties, toughening mechanisms, and oxidation resistance properties were collaboratively outlined. The flexural strength, fracture toughness and Vickers hardness of MoAlB ceramic hot-pressed at 1300 °C reach up to 640 ± 22.37 MPa, 6.82 ± 0.64 MPa m^1/2 and 13.82 ± 0.41 GPa, which far surpass these values of ternary layered compounds prepared by other methods. The oxidation resistance was investigated up to 1200 °C, where a passivating Al₂O₃ layer inhibited further oxidation even when treatment sample for 12 h. This work provides a guidance to industrially produce MAB phases with high performance.     

Visualizing isostatic pressing of ceramic powders using finite element analysis

Cold isostatic pressing, where a rubber bag is filled with ceramic powder, sealed and subjected to hydrostatic pressure, is a method of forming ceramic components with near-net shape. Cracking of the ceramic compact after pressing is one problem associated with the pressing of complex shapes. One mechanism responsible for the cracking of components is the interaction of the rubber bag with the component during the final stages of decompression where the elastomer can deform significantly and impose non-uniform loadings on the compact. Visualization of the detachment process and the stresses induced in the ceramic compact offer the opportunity for the design of press tooling which minimizes the potential for cracking of the components. In this paper, both 2D and 3D finite element models are developed to investigate this problem. The effect of different contact conditions between the compact and the rubber bag is discussed, and the distribution of stresses resulting from the interaction of the compact and the rubber tooling presented. These indicate methods for alleviating the stresses within the compact through suitable tool design.     

Densification of “active” zirconia powders during hot pressing

Conclusions We studied the influence of temperature and heat process time for zirconium nitrate on the phase composition, dispersion of grains and crystalline lattice defects in the resulting zirconia, and also on its capacity for subsequent compaction during hot pressing. This capacity is connected with the formation, during a short heating period up to 400°C, of metastable tetragonal and cubic modifications having a highly defective crystalline lattice.We also studied the densification during hot pressing of active zirconia obtained by decomposing zirconium nitrate. During decomposition of zirconium nitrate at 400°C with a short soak we noted the formation of zirconium dioxide which was actively compacted at 1100°C and a pressure of 200 kg/cm². We studied the influence of additions of CaO and the methods of adding it on the densification of active zirconia. An increase in the content of CaO leads to a reduction in the capacity of ZrO₂ for compaction during hot pressing. It is necessary to introduce the additive to the nitrate before it decomposes. During hot pressing of active zirconia containing additions of CaO and Y₂O₃ at low temperatures (1100–1300°C) cubic solid solutions are not formed owing to the short process time, and the zirconia in the specimens consists of monoclinic form. Subsequent heating of the pressed specimens forms cubic solid solutions.It was found that the introduction of 10–20% additive of zirconia obtained by heat processing the nitrate to industrial ultrapure zirconium dioxide yields dense specimens with a reduction in the optimum pressing temperature of 200°C.Translated from Ogneupory, No.7, pp. 39–45, July, 1967.     

Variants of hydraulic drives in semidry pressing of refractory parts

A solution is suggested for the problem of juxposing the compressibility and compression curves of a powder in a system of three coordinates and representing in three projections the deformation of the powder as a function of the pressing pressure and time and the pressure as a function of the time with preliminary calculation and plotting of the dependence of the deformation rate on the pressing pressure. The calculations and the plots in the three projections are presented for pump drives with a constant and variable capacity and for a pump-accumulator drive. Three calculation variants are given for each drive type, depending on the duration of the cycle and the pressing rate. It is recommended that drives with updated high-speed axial-piston pumps of variable capacity be used. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 7, pp. 18–21, July, 1997.