Substrate Function
A nickel iron chromium superalloy base provides high mechanical strength and corrosion resistance for high temperature thin film sensor testing. When thin film gauges are evaluated, an Inconel 718 substrate mimics the structural environment of gas turbine engine components. This base alloy sustains high mechanical loads at temperatures up to seven hundred degrees Celsius without undergoing appreciable plastic deformation.
Material Property
High yield strength and excellent fatigue resistance characterize this superalloy, making it the industry standard for aerospace and power generation applications. The material has a thermal expansion coefficient that matches closely with many ceramic dielectric layers, reducing the thermal stress generated at the interface. This compatibility reduces the likelihood of coating delamination during rapid heating and cooling cycles.
Surface Preparation
Surface polishing and chemical etching are used to prepare the alloy surface for deposition of the dielectric layer. Polishing reduces surface roughness to a sub micron level, which is necessary to prevent electrical shorts in the subsequent thin films. This preparation also removes the natural oxide layer, ensuring that the deposited films bond directly to the clean metal surface.
Thermal Adaptation
Preoxidation treatment is often applied to the superalloy to grow a stable, protective chromia layer on the surface before thin film deposition. This grown oxide layer improves the adhesion of the dielectric barrier and limits further oxidation of the substrate during high temperature operation. The thickness of this grown oxide must be carefully controlled to prevent it from becoming a mechanical point of failure.
This preoxidation is executed under vacuum with controlled oxygen partial pressure to select for specific oxide phases.
