The experimental model permitting the detection of coating non-homogeneity is suggested for standard depth sensing indentation tests.It is assumed that the shear modulus varies arbitrarily with depth in a nanostructured coating. It is defined a notion of the stiffness as a function of radius of a contact zone for nanostructured coating coupled with homogeneous half-space, which makes it possible to classify the non-homogeneity of a coating.Using numerical examples, the influence of different laws of shear modulus variation in a coating on the stiffness is studied.
Powdered aluminomagnesia spinel, MgA1204, is extensively used as the raw material for refractories and design ceramics [1], and also as a material for hot spraying multifunctlonal protective coatings [2, 3].MISiS in collaboration with the Central Energy and Nonferrous Metals Production Association has developed a technology for obtaining fine spherical powders of MgAI204 based on the treatment of magnesia-alumina batch in a plasma discharge of a high-frequency induction plasmatron, whose design is described in [4].Batch was prepared with magnesium oxide (TU 6-093023-79) and GK commercial alumina (GOST 6912-87), in a molar ratio of 1:1; this was subjected to combined grinding in a rubber-lined ballmill, using alumdum griding bodies for 0.3-0.5 h. The addition of up to 0.3%* surface-active agents from the amine group of compounds provided a high level of batch homogeneity, reduced its hygroscopicity, and prevented particle conglomeration.The resulting batch transferred from the hoppers to the feeders, was sprayed with a carrier gas into the plasma discharge zone through water-cooled probes fitted into the transfer unit under the section of the discharge chamber. Under these conditions, the currents of gas-powder mixture were disposed by the probes so that the batch particles moved towards the running stream of plasma-forming gas, and then, losing velocity -together with it. The batch feed of this type increased the dwell-time of the particles of material being processed in the high-temperature zone.Entering the discharge zone, the batch particles were partly or completely volatilized. The reaction between the MgO and A1203 occurred both in the liquid and the solid phases. From the reaction zone the biphase mixture of reagents moved with the current of plasma-forming gas into the "cold" reactor, in which the final product was chilled, owing to the sharp tall in temperature.Air was used as the plasma-forming and gas-carrying medium: The consumption of batch delivered was in the range 8.5-23.0 kg/h, and the specific energy outlay for processing it was 13.7-16.5 kW.h/kg. Using an oxygen-air mixture, or oxygen alone, as the working gases in combination with preheated batch, due to the utilization of waste-gas heat, reduced the energy outlay on average by 28-35 %.In most cases the yield of final product was 93-95%. The material in the main (up to 60%) was deposited in the reactor; somewhat less so in the cyclone (up to 40%), and even less (3-10%) in the sleeve filter.Powders from the reactor and cyclones consisted of spherical powders, diameter 50-160 and 5-50/zm respectively (Fig. la, b). Optical microscope examination of the powders in transmitted tight did not reveal porosity or cracking of the particles. However, when the consumption of plasma-forming and transporting gases exceeded the above limits, we noted particles with spherical, gaseous inclusions arranged in the central part of the granule (Fig. 2). This apparently is due to their entrapment in the sprayed particle, which is deformed under the action of the ...
In recent years, fused periclase (MgO) and the products based on it have been finding ever increasing applications in high-temperature technology as efficient refractories and electrical insulating materials. However, the specifications concerning the composition, the properties, and the structure of periclase and the demand for high-quality peric!ase products are not completely met by modern production technology because of the differences in the nature of the raw materials used, the specific features of the melting practice, and the distribution of the products in the block [i].In particular, the specifications concerning periclase used in tubular electrical heaters (TEH) as a particulate electrical-insulating material are extremely stringent. The reliability and the stability of the insulation characteristics of TEH depend on the quality of the insulating particulate charge. It must have a high electrical resistivity (1.2"i0v-5.5"I0 ~ ~-cm), a high dielectric strength (1.1-1.3 kV/mm), and satisfactory flowability [2]. The electrophysical properties of periclase depend on the impurities and their distribution in its lattice, its electrical resistivity is decreased by impurities such as Fe=Os, MnO, and AI203 contained in the raw materials and introduced during the crushing operation on the production cycle. The electrophysical properties (electrical resistivity and stability of the electrical parameters) of periclase depend to a significant extent on its hydration resistance, which can be improved by creating hydrophobic particle surfaces in the powder [2].It is desirable that the periclase grains havenot only a specific size but also spherical shape in order to ensure high flowability of the powder and, consequently, a high packing density of TEH.We studied the production technology of spheroidized periclase in the discharge of a high-frequency induction (HFI) plasmotron (Fig. I).Grade-3 electrical-engineering periclase (according to COST 13236-83) was used as a raw material.A particulate charge having a size dispersion of 100-2500 ~m was fed from the bunkers of the feeders into the zone of plasma discharge through the water-cooled probes fitted in the transition assembly located below the discharge chamber. The probes used for feeding the charge were shifted so that the particles of the powder-gas mixture move towards the inflowing plasma-forming gas, lose their speed, and move along with the gas. Such a scheme of charging made it possible to increase the duration of residence of the particles of the experimental material in the high-temperature zone. Furthermore, it did not disturb the stability of the burning process of the plasma discharge. The following parameters were maintained: voltage across the anode U a = 34-36 kV; current I a = 5.3-5.5 A; consumption of the plasma-forming gas Gp~ = 10-12 m3/h; and consumption of the transporting gas Gtr = 0.4-0.5 m3/h.After the introduction of the electrical-engineering periclase into the high-temperature zone of the plasma discharge, heating up of the particles and...
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