ICMCTF2010 Session A1-3: Coatings to Resist High Temperature Oxidation

A high temperature indenter has been developed to determine the Ductile to Brittle Transition Temperature of diffusion aluminide coatings deposited by Chemical Vapor Deposition or pack cementation on martensitic-ferritic steels, austenitic steels and Ni-based alloys. An estimate of the transition temperature was based on hardness measurements as well as characterization of coating deformation and cracking after indentation at increasing temperatures. Creep tests were performed at temperatures below and above the DBTT to assess the impact of that parameter on the lifetime of coated specimens. The influence of the substrate and the coating process will be discussed.

The oxidation behaviour of hot-dipping aluminized steel was investigated at temperatures ranging from 700 to 800°C in air containing 100% water vapour (steam) at atmospheric pressure. The oxidation tests in 100% water vapour + N_(g) were also performed to justify effect of water vapour in the atmosphere without air. After oxidation test, the morphology, composition and structure of the scale were examined by means scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electron probe micro analysis (EPMA). The experimental results show that significant failure of protective coating occurred, resulting in substantial weight increase. The kinetics curve obeys linear law after long exposure times in the steam oxidation. The growth of Al₂O₃ scale and sporadic iron oxide nodules on the coating surface as well as at the interface are accelerated by the presence of water vapour in the atmosphere. In addition, when the temperature increases up to particularly 800^oC, the presence of water vapour greatly gives rise to internal oxidation of Al at the grain boundary in the substrate due to high atomic diffusion rate at this temperature.

Rapid formation of slow growing oxide scale on heat resistant alloys and coatings is one of the most important issues in order to improve reliability and lifetime, particularly of materials which have a low total aluminium content, such as coatings and thin foils. Al₂O₃ forming alloys and coatings usually form so-called “meta-stable” (γ, θ)-Al₂O₃ at an initial stage of oxidation, and later transforms to stable (α)-Al₂O₃. This meta-stable to stable phase transformation is known to require longer time when the alloys and coatings are exposed to temperatures below about 900ºC. Moreover, reactive element additions to alloys and coatings, in order to improve scale spallation performance, are known to be significantly detrimental by delaying this phase transformation. The growth rate of meta-stable Al₂O₃ is more than two orders of magnitude higher than stable Al₂O₃, therefore rapid formation of α-Al₂O₃ and suppression of metastable-Al₂O₃ scale formation are directly linked to improvement of the performance of heat resistant alloys and coatings.

The aim of this study was to investigate methods to accelerate the meta-stable to α-Al₂O₃ transformation. In this presentation we will introduce the direct formation of α-Al₂O₃ scale without the formation of meta-stable Al₂O₃ on Fe-50Al and Ni-50Al alloys with and without deposition of thin films, ~50nm, of various elements such as Fe, Cr, Ni, Ti, and Al at 900ºC in air. Additionally, the effect of coating elements on initial development of α-Al₂O₃ as well as the meta-stable to α-Al₂O₃ phase transformation will be discussed.

Acknowledgements

This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Exploratory Research, 20656115, (2008).

It is well known that the sputter coating deposition technique produces coating with metastable structures. Though the standard 304 and 310 stainless steels (SS) have a face-centered cubic (fcc), the microstructure of the as sputter deposited 304 and 310 SS coatings exhibits metastable bcc (αFe) and fcc (γFe) + bcc structures, respectively ^[1],[2]. However, the effect of thermal exposure on the stability of the metastable structure as a function of temperature is unknown and hence, a systematic study has been undertaken to identify the metastables phases present in the as sputter deposited 304 and 310 SS coatings and assess the stability of the phases as a function of aging time and temperature. The 304 and 310 SS coatings were deposited on 304 SS samples using a magnetron sputtering technique. The coated samples, along with a 310 SS target sample, were exposed to 500ºC, 650ºC, and 750ºC for up to 5000 hrs. For phase identification, x-ray diffraction (XRD) radiation was conducted on the as-deposited and exposed samples. The XRD results revealed that the microstructures of the as-deposited 304 and 310 SS coatings consisted of σ + αFe and γFe + αFe phases, and as expected, the target material exhibited fcc. Thermal exposure at the three temperatures investigated resulted in the reduction of σ phase and precipitation of γFe phase in the 304 SS coating. On the other hand, thermal exposure of the 310 SS coating led to precipitation of the σ phase and the amount of σ phase in the coating increased with the exposure time. The results will be discussed in terms of variation of composition and alloy segregation between the as-deposited 304 and 310 SS coatings.

In this study, high silicon ductile iron was coated by hot-dipping in pure Al and Al-10Si melt .their high temperature oxidation behaviour was tested at 750°C to 950°C for virous exposure time. After high temperature oxidation tests, the microstructure analysis of all samples were investigated by means of metallographical examination, scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometer (EDS) and X-ray diffractometry (XRD).The results showed that hot dipping in Al-10Si could reduce the thinkness of intermetallic layer. Hot dipping Al/Al-Si on high silicon ductile iron improved high temperature oxidation resistence by over 10 times compared with the uncoated high silicon ductile iron.