| Inconel 690, a nickel-based alloy with high chromium contents, is an important engineering material to substitute austenitic stainless steels and is widely used in aerospace, marine and nuclear industries. Despite its excellent corrosion resistance and high-temperature performances, this material suffers from wear damages under high mechanical loads. Plasma assisted nitriding (PAN) has been proved successful in improving the tribological properties without deteriorating the corrosion resistance of a varieties of alloys. In the case of austenitic stainless steels, PAN treatments produce a highly nitrogen-enriched YN layer that is responsible for the improved performances. In this paper, we investigate PAN of Inconel 690, both for engineering purposes and for enriching the knowledge on YN in alloys other than common austenite stainless steels. The PAN technique employed is pulsed d.c. plasma type in N2-H2 gas mixtures. As comparisons, we also carry out PAN of AISI 316L austenitic stainless steels as well as reactive magnetron sputtering coating of the YN layer on Inconel 690. We have characterized the microstructure, wear and corrosion resistances of the nitrided samples by a variety of techniques so as to have a comprehensive understanding on the nitriding behaviors of this material. In particular we have revealed and explained the nitriding thickness dependence on grain orientations. The main results are the following:The nitrided Inconel 690 samples show inhomogeneous nitrided layers at nitriding temperatures below 500℃, and the layer thickness is grain-orientation dependent, especially at temperatures around 425℃. The thickness is approximately a linear function of 9<100> which denotes the minimum angle between the nitriding direction and the <100>directions of the nitrided grains as measured by Electron Backscatter Diffraction (EBSD). The deepest penetration of nitrogen is associated with the <100>oriented grains. A model is established to explain this phenomenon: the anisotropic elastic modulus of this f.c.c. alloy results in the anisotropic strain and stress and hence in the anisotropic activation energy of the N diffusion.Similar to stainless steels, a nitriding processing of Inconel 690 at temperatures below 400 results in a two-layer structure: a top f.c.c. YN1 layer with relative high solid solution nitrogen (up to 30 at.%), and an f.c.c. YN2 sublayer with relative low solid solution nitrogen (<10 at.%). Nitriding at temperatures above 400℃ or at 400℃ for long durations (>4h) leads to the decomposition of the YN phase into the f.c.c. chromium nitride and austenite depleted in nitrogen and chromium.Electrochemical tests on the nitrided Inconel 690 samples show that the YN phase obtained by the 2h nitriding treatment at 350℃ exhibits a better corrosion resistance than theoriginal Inconel 690 substrate in a 0.5 mol/L H2SO4 aqua solution. Tribological tests reveal that the friction coefficients of the nitrided samples are greatly reduced, and the surface hardness is remarkably improved at the same time.Using the same pulsed d.c. plasma method, PAN of austenitic stainless steels at temperatures below 440癈 produces two distinct yN layers with well-defined interface: a top layer with a high solid solution nitrogen content, and a yN2 sublayer with a relative low solid solution nitrogen content and a small amount of enriched carbon. We prove that an in-situ ion bombardment cleaning in an Ar-H 2plasma before the nitriding is the necessary condition to produce such a dual structure in the nitrided layer.Reactive magnetron sputtering (RMS) of Inconel 690 in Ar-N2 gas mixtures is conducted to produce homogeneous coatings with well-defined composition of metastable f.c.c. N nitrogen solid solution. The saturated nitrogen content, as well as the lattice parameters of the rN phase prepared by RMS are nearly the same as those of the rN phase obtained in PAN. Real-time XRD observations on a YN coating heated in vacuum from room temperature to 525癈 reveal that the decomposition of the YN phase occurs nea...
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