Influence of the type of ceramic reinforcement on the glass-wettability of composite chromium layers produced by plasma transfer arc method
Władysław Włosiński, Tomasz Chmielewski Politechnika Warszawska, Instytut Technologii Materiałowych, ul. Narbutta 85, 02-542 Warszawa
Annals 2 No. 5, 2002 pages 348-353
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abstract The experiments were concerned with composite chromium layers produced by the plasma transfer arc method (PTA), intended for use as protective coatings formed on tools for precision forming of glass. Thanks to the use of chromium as the coating matrix, the coatings are highly resistant to oxidation at high temperature, show an increased hardness (greater than that of the 3H13 steel substrates) and are relatively slightly wetted by glass. The composite material was reinforced with ceramic particles (Al2O3 or Cr3C2) with the aim to increase its hardness, frictional wear resistance and heat resistance. The reinforcement also appears to reduce essentially the wettability of the coating by glass. The paper discusses the results of examinations of the glass-wettability of the Cr+Al2O3 or Cr+Cr3C2 composite coatings. Glass industry knows several methods of fabricating protective coatings to cover the working surfaces of glass forming tools. The choice of an appropriate method depends on the requirements set for a given coating. In the process of hot forming (600÷700ºC) of glass, it is required that the tools not only shape the product to the specified geometry but also that they ensure its appropriate cooling to below the glass softening temperature during a specified period of time so as to avoid generating thermal stresses and, thus, cracking, deformation and shape changes. The tools intended for shaping glass products should not only be strongly resistant to oxidation at high temperature but also have a good thermal conductivity and reduced wettability by liquid glass. The ‘fidelity’ of copying the forming tool shape by the glass batch being formed and the heat transfer to the tool both strongly depend on how the working surface of the tool is wetted by the glass. The wettability also determines the highest permissible temperature of the forming process. In order to prevent the glass from adhering to the tool surface, the wettability of the tool coating should be reduced. The reduced glass wetting of the coatings described in the present paper was achieved thanks to the use of chromium as the coating matrix material. This is so since a thin, tightly adhering oxide film forms on the chromium surface. According to the literature [6], this film shows a low wettability since most often it forms an equilibrium system with the wetting liquid phase and, thus, it does not form solutions or other new phases with it. The results shown in Figure 8 suggest that the wettability of the chromium coating by kinescope-type glass can greatly be reduced by introducing a ceramic phase into it. It can also be seen from this figure that the wettability of the coating can further be reduced by increasing the amount of the reinforcing CrxCy carbide phase (coatings No 1 and 2), and that the use of another reinforcing constituent, namely Al2O3, in small amounts comparable with those added to coating No 1, decreases the wettability even more. Hence we can see that Al2O3 decreases the wetting angle more effectively than chromium carbides. The immediate advantages of the reduced glass wettability of the protective coatings formed on glass forming tools are: - reduction of the glass adherence to the tool surface during the glass forming process, and - possibility of reducing the glass forming temperature so as to increase its flowability.