The focus of the 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. The mechanism that enables the development of a large Al flux at low temperature must be resolved to provide the fundamental understanding that will allow for a robust process control. 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 will be conducted to confirm the role of the Al5Fe2 phase in enhancing low temperature synthesis. An examination of the stability of the Al5Fe2 phase will allow for an understanding of the defect structure and diffusion kinetics. While the rapid kinetics is an advantage in initial coating synthesis, it is a concern during service since it results in depletion of Al and the loss of oxidation protection. In order to address the lifetime extension the multicomponent diffusion pathways will be assessed so that a novel kinetic biasing strategy can be employed to develop in-situ diffusion barriers and compliant layers as part of an integrated coating design. 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. An important component of the project effort will be the education of a graduate student. Further, lecture demonstrations on coating synthesis will be incorporated into outreach programs for high school and incoming undergraduate students.
Intellectual Merit Aluminide coatings as applied by pack cementation have a long history of valuable service in providing protection against oxidation under dry and moist high temperature environments. Similarly, as a result of a sustained effort the capability of the Cr-Mo P92 steels to provide adequate high temperature strength and creep-resistance has advanced to enable a cost effective approach to enhancing energy conversion efficiency. However, in order to realize the enhanced capabilities of high temperature alloy steels, they must be protected against environmental attack. An aluminide coating can provide the required protection, but the traditional synthesis approach involves high application temperatures that result in significant degradation of the high temperature mechanical properties. Recent advances in Al pack cementation have revealed an innovative low temperature synthesis pathway that was overlooked in past work and has the potential to enable the effective application of aluminide coatings without degradation of alloy steel properties. In order to realize the advantages of this advance in processing several key issues must be resolved to provide the fundamental understanding of the synthesis mechanism that will allow for a robust process control. One central aspect is the mechanism that enables the development of a large Al flux at low temperature. The synthesis of the Al5Fe2 phase under non-equilibrium conditions appears to be an important component of the reaction pathway. The Al5Fe2 phase is characterized by a low lattice site occupancy that yields a high vacancy content that can promote rapid kinetics even at low temperature. A quantitative analysis of the coating growth kinetics has been conducted to confirm the role of the Al5Fe2 phase in enhancing low temperature synthesis. At the same time an examination of the stability of the Al5Fe2 phase has allowed for an understanding of the defect structure and the influence of alloying on the stability and diffusion kinetics. Following the identification of effective coating designs in the initial part of the research a quantitative evaluation of the performance of the integrated aluminide coatings has been conducted in oxidation environments at service temperature and during thermal cycling treatments. The results of this evaluation have provided an essential fundamental foundation and database to establish a comprehensive kinetic model to provide lifetime predictions of the integrated aluminide coatings. While the rapid kinetics is an advantage in initial coating synthesis, it is a concern during coating service since it results in depletion of Al and the loss of oxidation protection. In the future in order to address the lifetime extension the multicomponent diffusion pathways should assessed so that in-situ diffusion barriers and compliant layers can be included as part of an integrated coating design. Broader Impact The advancement in the understanding and control of low temperature aluminide coating synthesis reactions resulting from the proposed research has yielded a deeper fundamental knowledge of the governing kinetic mechanisms, the role of the defect structure of the product phase on the enhanced kinetics and a kinetic lifetime model. The fundamental understanding provides the essential background in order to implement low temperature synthesis as an effective means to protect alloy steels in energy conversion applications. An important component of the project effort has been the education of graduate students and the research experience for undergraduate and high school students. Further, lecture demonstrations on coating synthesis were incorporated into campus outreach programs for high school and incoming undergraduate students.
