A new technique for coating intermetallic powders by ceramic powders has been developed in laboratory It has been drawn from mechanical alloying. Its principle consists to applied on the powders a strong compression forces, by hammers against inner side of the chamber's wall, attrition and shear forces. The forces engender heat which is therefore responsible for welding of the two types of powders. This technique has been perfected to avoid grinding and the contamination of powders by apparatus components. The main part of this apparatus is constituted by a cylindrical chamber 47 mm in height and 100 mm in diameter, inside of which there is a static circular piece. The piece is composed of two hammers and two scrapers of 44 in height, placed symmetrically to each others (fig. 1). The assembly is placed on a rotating plateau. The intermetallic NiAl (diameter < 45 mu m) (fig. 2) and ceramic powders ZrO2 (diameter < 0,6 mu m) are introduced, in appropriate proportion, inside the cylindrical chamber. The system is subsequently set in motion. As a result of centrifugal forces, the powders are compressed whilst passing through the remaining space between the inner side of the chamber's wall and the hammers. The friction forces originate heat which is therefore responsible for the adhesion of the types of powders forming cermet. The present work has focused on the investigation of two parameters. The first parameter consists of varying the time of the powders stays inside the rotating cylindrical chamber. The space between the chamber's wall and the hammers, on one hand, and scrapers on the other hand, were adjusted to one millimeter. The chamber was first fumed on during two hours, in order to make the new surfaces, in their nascent state, are susceptible to forces of adhesion. The surfaces so achieved allow improved adherence among particles of a different nature. Afterwards, the ceramics powders (12 % wt) were added gradually to avoid any agglomeration. Later on, the time of rotation was extended up to 55 h. The results of Scanning Electron Microscopy, after 8 h of rotation, have revealed the existence of small NiAl particles deriving from a partial reduction of initial size. The micrographs of cross sections show that those having a large diameter are covered partially by a fine layer of ZrO2 (fig. 3) whereas the small ones (diameter less than 10 mu m) are completely coated by ZrO2 (fig. 4a). The particles of different natures are joined to each other because of strong compression. It has been noted that after 15 h of rotation, the micrographies depict a considerable reduction on NiAl particles sizes down to 5 mu m. Such a reduction is likely due to strong compression. The powder compression generates strong friction forces which lead the temperature rising up to 200 degrees C inside the chamber. The cross section of small particles (diameter < 10 mu m), exhibit a second type of the particles; a major constituent of the core are Zr and Al, however, the surface composition is mainly Ni (fig. 4b). After 55 h, the majority of the particles are spherical and become completely covered by other microparticles, all of which seem to be welded on the surface (fig. 5). The analysis shows that these microparticles are compounded by Al, Ni and Zr. The second part of this work deals with the effect of the initial powders compositions on the quality of coating. In this case we have increased the remaining space between the hammers and the inner side of the chamber up to 2 mm in order to avoid the reduction of NiAl particles sizes; as it has already been noted in the first part. The variation of the compositions, from 12 to 40 in weight percent of ZrO2, was associated with an increase in the quality of the coating. The thickness of ZrO2 layer varies from a few hundred of manometer for 12 % wt to 7 mu m for 40 % wt (fig. 6). It has been concluded that the new process is well adapted in coating the intermetallic powder by ceramic ones. It makes to obtain an even ceramic layer of several micrometers in thickness during an experiment less than 8 hour's and with a 2 mm between hammers and the chamber wall. The coating powders were then annealed or used in plasma spraying experiments. The use of coated particles ensure better homogeneity of deposits than projection performed with uncoated powders.
