Porous thin films for thermal barrier coatings

A new approach is described in the deposition of thin films for thermal bar rier applications. Using controlled substrate motion, porous layers and cap ping layers were vacuum deposited in an alternating fashion, creating a new , multilayered film structure. Direct measurements of the thermal propertie s of these multilayers were made using the 3 omega and Mirage techniques. I n the 3 omega technique, heat is introduced into the coating by an AC curre nt flowing through an evaporated resistor with a frequency omega. A fit of resistor voltage as a function of frequency yields the thermal conductivity . In the Mirage technique, an oscillating temperature is induced immediatel y above the film using a pulsed laser. A second probe laser aligned paralle l to the surface is deflected by these temperature variations, and the ther mal diffusivity is then found by fitting amplitude and phase shift data to the solution of the three-dimensional diffusion equation. Typically, the 3w and Mirage techniques measure thermal constants in directions normal and p arallel to the substrate, respectively. Measurements using these methods le d to estimates of a reduction in thermal diffusivity of as little as 9% of that of films deposited entirely at normal incidence. Thermal simulations o f similar structures also predicted a substantial decrease in overall therm al conductivity. In a specific case, an improved conductivity of 18% of tha t of films deposited by standard techniques was estimated. (C) 2001 Elsevie r Science B.V. All rights reserved.