Microstructure control in precious metal coatings for high temperature applications, obtained by metallorganic chemical vapour deposition

Abstract

In most high temperature applications, efficiency increases with service temperature of the component. Especially in combustion chambers, higher efficiency is translated into less propellant consumption and therefore a lower weight and a reduction of operational cost. In these components, the use of high performance coatings may simultaneously increase the service temperature and decrease its price. From the many available coating processes, Metal organic Chemical Vapour Deposition (MO-CVD) is one of the few capable of producing coatings of most precious metals in internal surfaces with intricate geometry. Nevertheless the microstructure control of the films is extremely complicated and characterised by the formation of dendrite structures. This morphology eliminates the coating efficiency, as it is not compact. EADS has had the chance to use the research infrastructure of the AMTT in Seibersdorf to characterise the coatings and determine how the transformation from the compact to the non-compact microstructure takes place. The understanding of this phenomenon should allow us to develop a process able to produce 100 % dense coatings independently of the coating thickness in an industrial scale.

Overview of Coating Growth Mechanisms in MOCVD Processes as Observed in Pt Group Metals

“…Depositions: Three MOCVD reactors were used to deposit platinum group metals [2][3][4][5]29]. The gas (argon, hydrogen and oxygen) streams rates were controlled and monitored using mass flow meters (range: 1-24 L h -1 ).…”

“…It can be affirmed that the carbon content is above the solubility limit (2.5 at.-%). [2,4] Observations suggest that precursor molecules permit a residual amount of carbon content in the deposited coatings. From the early stages of deposition, this residual carbon concentrates itself in a superficial layer on the top of the metallic compact layer.…”

“…Deposition conditions, as well as substrate material, have a strong influence on coating microstructure and composition. [2][3][4][5]14,22,26,27] In order to understand the mechanism of coating growth and the effects of deposition temperature, the initial growth stages of depositions were observed in the case of iridium.…”

“…In the case of Pt and Ir, an unexpected structure change from compact to columnar was observed under various conditions [2,4,5] at the same thickness on different substrates (glass and metal). In order to understand why this happens, investigations were made into the initial stages of growth.…”