Plasma-sprayed ceramic coatings on the base of powders diffusion alloyed. Part I. Properties of powders
Eugene Feldshtein, Oleg Devojno, Olga Yatskevich
Quarterly No. 1, 2010 pages 81-85
DOI:
keywords: alumina powders diffusion alloyed, particles shape, phase composition
abstract The parts of contemporary machines frequently need the protection of their work surfaces against corrosion medium influence, mechanical wear and destruction. With that end in view plasma spraying is used widely. In latter years, powders with compound composition are used in increasing frequencies to raise strength properties of coatings and their cohesion with the base material. The process of diffusion alloying is the simple method to influence on phase composition and structure of a starting powder. It is known, alumina have the great hardness, wear resistance, does not enter interaction with metals when heating to high temperatures. Then, the purpose of this work is the study of properties of powders on the alumina base, which particles were subject to diffusion alloying. As alloyed elements, molybdenum and boron were used. Alloyed compositions Al2O3-Мо and Al2O3-В were received after heating into oven during 2 h. Mixtures of alumina powder with molybdenum powder (8% of general mass) or boron powder (10% of general mass) with addition of ammonium chloride (4% over the general mass) were used. To meet identical conditions, starting powder was heated with conditions similar to diffusion alloying. The powder shapes analysis fulfilled on the scanning microscope „Mira” allowed to find, that the base alumina powder consists of two type particles - rounded and in the form of conglomerate of microcrystals. Alloying powders have crystal shape, and boronizing forms more uniform faceted crystals. The X-ray structure analysis fulfilled on universal diffractometer ДРОН-3,0 using program package ARSANAL allowed to find phase composition of clean alumina powder and powders alloyed by boron and molybdenum. It is determined, the share of high temperature phase alpha-Al2O3 in powder increases due to conversion gamma → delta → alpha in consequence of gradual rebuilding of crystal grating. The temperature and time conditions of diffusion alloying are quite enough to realize phase transformation process into ceramic and to lock in it the hexagonal -modification of alumina. Because the presence of impurities influ-ences essentially on temperature and speed of polymorphous transformation in alumina, one can suppose that molybdenum and boron presence into alumina reduces the temperature of gamma → alpha transformation and increases the speed of alpha phase formation. Molybdenum presents into alloyed ceramic powder as clean chemical element both oxides. However, boron presents into alloyed ceramic powder as clean chemical element only, boron compounds into powder particles were not discovered.