J.F. Dinhut

Growth stresses in α-Cr2O3 thermal oxide films determined by in situ high temperature Raman spectroscopy

Growth stresses have been investigated in relation with the microstructure in the case of α-Cr2O3 growing oxide films on NiCr30 alloy. The equibiaxial growth stresses have been measured thanks to a technique coupling Raman spectroscopy and in situ high temperature oxidation of the NiCr30 alloy in the temperature range (700−900 °C). It is established that the compressive growth stress in such oxide films can reach more than 2 GPa, before additional thermal stress arises on cooling. Moreover, the growth stress kinetics—subsequent establishment and relaxation—are highly microstructure sensitive: in particular, as the oxidation temperature rises, the chromia mean grain size also increases, and it consequently retards the occurrence of the creep relaxation phenomena which needs an additional stress level to start.

Phosphating of bulk α-iron and its oxidation resistance at 400 °C

Fe bulk specimen have been treated with phosphoric acid in order to obtain a conversion surface layer and the oxidation resistance has been studied by means of in situ thermogravimetric experiments in artificial air (20% O2; 80% N2). For oxidation time of 48 h at 400 8C it shows an important decrease of the oxidation kinetic in comparison witha-Fe. Moreover, the kinetic can be described by two successive parabolic stages. This behaviour is respectively related to an X-ray photoelectron spectroscopy (XPS) analysis of the surfaces and to previous results obtained on the oxidation of a-iron. # 2002 Elsevier Science B.V. All rights reserved.

Low-energy high-flux nitriding of Ni and Ni20Cr substrates

Abstract The influence of a low-energy high-flux nitrogen implantation on the structure and hardness of Ni and NiCr 20 is examined. These implantation–diffusion experiments carried out at 475 and 450 °C, respectively, show that the nitriding process is very difficult for the pure Ni metal, whereas in the NiCr 20 substrate the formation of an expanded austenite γ N phase is identified by X-ray diffraction. Vickers microhardness indentation tests indicate an important hardening effect of the NiCr 20 substrate surface. The absence of any detectable nitridation effect in pure nickel substrates can be correlated with the low solubility of nitrogen. An increase in surface roughness and the presence of porosity is nevertheless observed by scanning electron microscopy and atomic force microscopy on both metallic substrates surfaces after the implantation process. This behaviour can be correlated with the sputtering effect produced by the very high implantation dose.

Structural characterisation of phosphated α-iron oxidised at 400 °C

α-Iron specimens were treated with phosphoric acid to obtain a conversion surface layer and the oxidation resistance of samples was studied at 400 °C in air. For oxidation time of 48 h, an important decrease in the oxidation kinetics was obtained in comparison with untreated α-Fe. The samples were characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses of the surfaces and these results are related to previous data on the oxidation of α-iron.

Effects of an ion nitriding treatment on the oxidation behaviour of an ODS FeAl alloy

A B2 oxide-dispersion-strengthened (ODS) iron aluminide has been treated at 400 °C with a high flux and low energy nitrogen ion beam. The oxidation resistance has been investigated by in situ thermogravimetric experiments and related to structural changes studied by X-ray diffraction, scanning electron microscopy and chemical analysis of the surface. After the nitriding process the hexagonal AlN phase is detected, and during the oxidation tests between 800 and 950 °C a hematite layer is formed.

Substrate grain boundary effect on residual stress distribution at micrometer scale in chromia oxide thin films

Residual stresses in chromia oxide films formed at 700°C on NiCr30 have been investigated with confocal Raman spectroscopy. The high lateral resolution allows us to determine stress distribution in the oxide growing above both substrate grains and grain boundaries. It is demonstrated that the magnitude of the equibiaxial compressive stress is related to local oxide microstructure evolution: during the initial oxidation, stress is smaller in the chromia located above substrate grain boundaries as compared to that above substrate grains. Subsequently, this gap in stress magnitude progressively vanishes due to the occurrence of oxide grains forming bridges across depressions in the oxide scale above substrate grain boundaries.

On the discrepancy between theoretical and experimental thermal stress in chromia oxide films

Growth and residual stresses have been determined, thanks to Raman spectroscopy in chromia oxide films thermally grown in air from a Ni33at %Cr alloy between 700 and 900 °C. Due to the good sensibility of the in situ experimental device, the growth stress kinetics even at short oxidation times has been recorded and has been shown to be microstructure sensitive. At different oxidation times, the corresponding residual stress at room temperature has also been investigated and the experimental thermal stress has been deduced. The comparison between the respective magnitudes of stress variations induced by creep relaxation during isothermal oxidation, and by the additional thermal stress occurring upon cooling, has led to the definition of a stress threshold finally proposed to explain the residual stress values and the difference between theoretical and experimental thermal stress components.

Chronological study of the oxidation of phosphated α-iron

In the present work, the morphological evolution of the iron-oxide layers growing at 400 °C on α-Fe and phosphated α-Fe in artificial air at 1 atm is investigated. The oxidation kinetics of α-Fe and phosphated α-Fe are firstly presented for various oxidation times. Then a detailed chronological study is undertaken of the microstructural states in the different stages of the complex oxidation kinetics. Correlations are established between the morphology of the oxides layers and the successive parabolic oxidation stages. It is shown that the phosphate conversion layers seem to modify the growth mechanisms of the oxides.

Graded Nitrogen Ingress in FCC Metallic Structures and the Related Microstructures and High Temperature Oxidation Behaviour

Nitriding by low energy high flux processing has been carried out at about 400°C in fcc metal substrates (pure Ni, Ni-20Cr model alloy and a conventional AISI 304L stainless steel). The gradual ingress of this element into the structures will be shown to depend markedly on the chemical composition of the substrate. The associated expansion of the fcc lattices and surface roughness will be discussed in this work with the support of X-ray diffraction, atomic force, scanning and transmission electron microscopy techniques. In light of the resulting composition, microstructures and thickness of nitrided layers, some preliminary results of their behaviour under isothermal oxidation conditions at high temperatures will be discussed. The high temperatures will provoke decomposition of the expanded austenite into a conventional gamma phase and some chromium nitrides. Trapping of chromium therefore shall explain a reduction of the high temperature oxidation resistance against the untreated substrates.

High Resolution MicroRaman Study of Blisters in Cr2O3 Thermal Oxide Films

Using Raman microprobe spectroscopy made it possible to study the buckling phenomenon in chromia films grown at 900°C in air from a Ni30at%Cr alloy. Blisters have been optically observed to be circular and, from the top view, the mean radius has been measured with an accuracy of about 1%m. An autofocus device allows the characterisation of the profile of each blister and the shift of the A1g Raman peak of chromia gave the local stress far from the blister and all along the buckled zone. From these observations, the induced spalling has been related to the blister morphology.