Our Research

Aluminide Coatings for High Efficiency Power Generation

Efforts to increase efficiency during power generation often rely upon materials that will perform at higher temperatures. Development of material solutions must also maintain cost effectiveness in order for them to be accepted widely (e.g. a solution for jet engines is not practical for power plants). Current steel technology is well established, but the temperature and aggressive environments are pushing the limits of the current protection systems. Therefore, the focus of this research is to develop an understanding of an innovative low temperature synthesis pathway for oxidation resistant aluminide coatings without degradation of alloy steel properties that occurs with high temperature processing. During aluminization, the Al5Fe2 phase appears to be an important component of the reaction pathway and is characterized by a high lattice defect content. A quantitative coating growth kinetics analysis is being conducted to confirm the role of the Al5Fe2 phase in enhancing low temperature synthesis.

The advancement in the understanding and control of low temperature aluminide coating synthesis reactions resulting from the research will yield a deeper fundamental knowledge of the governing kinetic mechanisms. The introduction of a novel kinetic biasing strategy in order to develop an in-situ diffusion barrier will provide a key missing component that is required in order to implement low temperature synthesis as an effective means to protect alloy steels in energy conversion applications to allow for increased power generation and efficiency. The coating design strategies developed in this research have a general application as a cost effective means to enhancing the resistance of structural materials to degradation under aggressive environments at high temperature.

High Temperature Aerospace Materials

The advent of hypersonic flight has led to the investigation of alternative materials for the aerospace and aeronautical industries. The environment for sustained hypersonic flight introduces not only higher levels of performance in the usual requirements of temperature capability, specific strength, and materials compatibility, but also new demands such as environmental resistance at high temperature in an ionized atmosphere (i.e. plasma) that were not faced previously. Available thermal protection materials will not survive the extreme
temperatures (>2000°C) for extended service.

Research has recently focused on ultra-high temperature ceramics (UHTCs), notably ZrB2 for its high melting temperatures (>3000°C), lower density (compared to other diborides such as HfB2) and resistance to chemical attack. It has recently been shown that additions of SiC to ZrB2 help to improve the high-temperature oxidation resistance of the material, forming a SiO2 layer that has a lower volatility than the B2O3 layer seen in the oxidation of ZrB2. To date the focus has been on SiC additions of 10-30 vol% but it is obvious that there are benefits to higher (~80 vol%) additions of SiC.

SiC/C composites composed of a two-dimensional mesh of carbon fiber within a protective SiC matrix are also materials of interest to the aerospace and aeronautical industries for their low density and good mechanical properties at high temperatures. The thermal expansion mismatch between SiC and C fiber forms cracks in the protective SiC coating allowing for the inward diffusion of oxygen. The oxygen rapidly oxidizes the C fiber leaving a SiC skeleton. More recent interest has shifted to SiC/SiC composites (SiC fiber within a protective SiC matrix) that have better oxidation resistance than SiC/C composites.

A coating of Mo-Si-B can be applied to SiC/C composites to provide protection from oxygen ingress. The Mo-Si-B coating has been shown to be robust in oxidative environments due to the formation of an aluminosilicate protective layer. Additionally the Mo-Si-B coating has been shown to have excellent compatibility with a variety of materials, including SiC. While SiC displays excellent high temperature oxidation resistance, there are problems with oxidation protection of SiC/C at lower temperatures (~800°C) and in environments containing water vapor. The Mo-Si-B coating has been shown to protect from oxidation in a large temperature regime (up to 1500°C) as well as in the presence of water vapor. Current research is aimed toward application of Mo-Si-B to SiC-based composites and ceramics.

Glass Stability and Nanocrystallization

The major research activities of the current project center about nucleation and growth phenomena in alloys far from equilibrium. Aluminum based glasses are in a highly undercooled state and the extremely high product number density achieved during annealing treatments is not well understood from a kinetic viewpoint. Within this scope, we are working toward a nucleation model informed by medium range order information obtained through new FEM (Fluctuation Electron Microscopy) techniques. An initial goal of this work is to develop predictive capabilities within the Al-Y-Fe-TM system as well as a generic framework for other MRO mediated precipitation reactions. Nucleation from a heterogeneous amorphous matrix is a relatively unexplored subject within the scientific community; thus, another goal of this work is to educate the community about the new insight that is made available from this study.

The development of rapidly quenched glasses that undergo primary crystallization to nano-crystalline dispersions also raises a number of intriguing fundamental issues with regard to thermal stability, nucleation mechanism and atomic arrangements in the amorphous phase. In the current program these issues are being addressed through the systematic calorimetric and micro-structural study of kinetics, the use of fluctuation electron microscopy (FEM) to probe medium range order and modeling of the reaction process. The approach involves the exploration of new techniques (i.e. FEM) to probe atomic arrangements in amorphous phases and the development of a non-conventional kinetics model that is unproven but offers the possibility for a new paradigm for the analysis of nanoscale microstructure synthesis.