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The nucleation and growth of chimney pores during powder-bed electron-beam additive manufacturing is investigated using in situ infrared thermography and micro-computed tomography. The chimney pores are found to nucleate heterogeneously at dimples on the side surfaces of additively manufactured components, and to grow through a molten-film rupture process. Further, these nucleation and growth processes are found to be strongly influenced by the beam diameter. Several strategies for suppressing the formation of chimney pores are discussed in light of these results.

A new set of heat spreader coatings consisting of multilayers of diamond/AlN/diamond were deposited on high heat capacity substrates of molybdenum and silicon nitride. Bonding of the heat spreaders to the device wafers using gold-tin eutectic solder was carried out after metallization layers of titanium, gold and copper were deposited on diamond. Prior to bonding, backside of the silicon wafers was also metallized with titanium, gold and copper and the gallium arsenide wafers with titanium, copper-germanium alloy and gold. Characterization of the multilayer diamond films was carried out by Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The bonded wafers were tested for adhesion strength, resistance against peeling due to thermal cycling and failure under stress. Further, the bonded regions were characterized by scanning electron microscopy, energy dispersive spectroscopy and X-ray mapping of different elements. The heat spreader characteristics of the single layer diamond and the multilayer diamond substrates were tested by infrared imaging. The results illustrate that the multilayer diamond heat spreader coatings provide better heat dissipation and also possess better adhesion strength and resistance against peeling under thermal cycling. These novel multilayer diamond/AlN/diamond heat spreaders are expected to considerably improve the life of high frequency power devices.

Thermal diffusivity of automotive grade SAE G3000 (d) gray cast iron has been measured as a function of graphite flake morphology, chemical composition and temperature. Cast iron samples used for this investigation were cut from “step block” castings designed to produce iron with different graphite flake morphologies resulting from different cooling rates. Samples were also machined from prototype and commercial brake rotors, as well as from a series of cast iron slugs with slightly varying compositions. Thermal diffusivity was measured at room and elevated temperatures via the flash technique. Graphite flake morphology of the various cast iron samples was quantified stereologically with image analysis techniques. Several geometric features of the graphite flake morphology were quantified. It was found that the thermal diffusivity of these gray cast irons increases with carbon equivalent and has a strong linear correlation to graphite flake length. For gray iron with the same chemical composition, a four fold increase in the graphite flake size results in a 50% increase in thermal diffusivity. Amongst the commercial rotors, room temperature thermal diffusivity varied from 0.156 to 0.200 cm2/s.