Low Temperature Direct Liquid Injection Mocvd Of Amorphous Crcx Coatings In Large-Scale Reactors: An Original Route To Nanostructured Multilayer Coatings

The effect of low temperature deposition on the quality and the microstructure of hard a-CrCx coatings grown by Direct Liquid Injection Metal-Organic Chemical Vapor Deposition (DLI-MOCVD) was investigated. At deposition temperatures higher than 525 degrees C, the coatings are bi-phased and polycrystalline, essentially composed of Cr7C3 with Cr3C2 as a minor phase. By decreasing the temperature in the 525-350 degrees C range, they are amorphous and exhibit a monolithic glassy-like microstructure without grain boundary. For temperatures lower than 350 degrees C, the uniform amorphous microstructure changes for a lamellar multilayer structure, while the composition of the gas phase was kept constant at the inlet of the reactor for all these CVD runs. The origin of this surprising lamellar structure has been investigated. After having discarded the idea of a self-organized multilayer growth, sometimes encountered in CVD processes, evidence was found for rapid changes in the deposition mechanism. They are induced by oscillations of the surface temperature around a critical value due to the regulation of the furnace used for such large-scale reactors. These almost periodic temperature changes lead to modulations in the surface composition of nutrient species, and subsequently, to the growth of multilayers. The individual composition of lamellae changes regularly, while the overall composition of the coatings remains identical to that of uniform monolithic coatings. For growth conditions leading to local temperature close to the critical value, a regular and controlled nanostructuring can be obtained with a period close to a hundred nanometers. Similar to monolithic a-CrCx coatings, these a-CrCx multilayer coatings exhibit a good barrier behavior against high temperature oxidation, according to the test performed. As future prospects, the main interest for these lamellar coatings could be their mechanical properties thanks to the numerous interfaces capable of deflecting and dissipating the propagation of cracks.