Abstract
Abstract The bonding of AlN ceramic substrates with tungsten can only be achieved by incorporating CaO into the metallic powder. This oxide improves the sintering process of W and leads to calcium aluminates at the AlN/W interface, whose enrichment in CaO through the metal gives compositions CA, C 12 A 7 and C 3 A with decreasing eutectic temperatures and viscosity. This allows a liquid phase to diffuse at firing temperatures. A good adhesion needs: (i) a CaO content threshold to form substantial amounts of liquid phase, which depends on the amount of alumina available from the nitride surface; and (ii) temperatures above 1490°C, a eutectic temperature in the system CaO/WO 3 , because the solid Ca tungstate film simultaneously formed near the W/AlN interface may prevent Ca aluminate from diffusing when not displaced towards the metal core by means of a dissolution-precipitation mechanism.
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