Papers

ICMCTF2004 Session A1-1: Coatings to Resist High Temperature Corrosion and Wear

Thursday, April 22, 2004 10:30 AM in Room Sunrise

Thursday Morning

Start	Invited?	Item
10:30 AM		A1-1-7 Architectural Structure Changes In Atmospheric Plasma Spraying WCCo Coatings For Improving Corrosion Response C. Godoy, M.M. Lima (Universidade Federal de Minas Gerais, Brazil); J.C. Avelar-Batista (Tecvac Ltd., Brazil); M.M.R. Castro (Colorado School of Mines) In this work, an attempt to improve the corrosion resistance of WCCo coatings was made by either modifying coating composition with the addition a NiCr alloy or by carrying out a melt post-deposition procedure, in order to disrupt the intrinsic lamellar microstructure.A duplex system comprising a WCCo top layer and a NiCrAl interlayer was also constructed. Five systems using an AISI 1020 steel substrate were studied: NiCrAl/steel; WCCo/steel; an as-sprayed and a post-melted 50%(WCCo)50%(NiCr); and a duplex system comprising a WCCo top layer and a NiCrAl interlayer. Immersion tests were performed in HCl1N media and potentiodynamic tests were performed in H₂SO₄ 1N. The immersion tests results showed more significant mass loss for NiCrAl monolayer system. Solution analyses obtained after these tests indicated that the introduction of an intermediary layer of NiCrAl between the steel and WCCo coating provided very effective protection of the substrate against corrosion in chloride acid solution. Although iron was presented in all solution analyzed, the amounts of Fe detected on duplex system were significant lower than on the others systems. High nickel dissolution was detected for 50%(WCCo)50%(NiCr),indicating the occurrence of preferential corrosion on this system for HCl environment.Melting after coating procedure decreased the alloy component dissolution, confirming the advantage of a non-lamellar structure with respect to corrosion resistance. Potentiodynamic curves for monolayer WCCo and duplex systems presented the same corrosion behavior. However chemical composition of solutions after the tests indicated a significant dissolution of Fe for WCCo composite compared to duplex conjugate. As-sprayed and post-melted 50%(WCCo)50%(NiCr) conjugates showed the same potentiodynamic-test curve shape, however the latest presented the lowest dissolution of all studied conjugates.Melting procedure ensured a better corrosion performance for this system.
10:50 AM		A1-1-8 Structural Analysis of AISI-SAE 4140 Steel Nitrided by Post-discharge Assisted Nitriding Process A. Medina-Flores (UMSNH Instituto de Investigaciones Metalurgicas, Mexico); P. Santiago (Instituto Nacional de Investigaciones Nucleares, Mexico); J. Ascencio (Instituto Mexicano del Petroleo, Mexico); J. Oseguera (ITESM-CEM, Mexico) AISI-SAE 4140 steel pieces were nitrided by a postdischarge assisted nitriding process. The nitrided pieces were characterized by, microhardness, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometry (EDS) and High Resolution Transmission Electron Microscopy (HRTEM). Molecular simulation techniques were applied to identify HRTEM and XRD images and patterns. Gamma prime to epsilon nitride transformation in the diffusion zone has been observed. The crystallographic orientation between these phases was measured.
11:10 AM		A1-1-9 Sulphidation Behaviour of Siliconised Mo and Nb by Pack Cementation at 850°C H.L. Du, Z.D. Xiang, J.S. Burnell-Gray, P.K. Datta (Northumbria University, United Kingdom) It is well known that refractory metals (particularly Mo and Nb) possess good high temperature (HT) sulphidation resistance. However their HT oxidation resistance is very poor in the environment of high oxygen potentials. Siliconising may provide protection in both oxidising and sulphidising environments at elevated temperatures. In this study, both Mo and Nb were siliconised using pack cementation. This process allowed the development of coatings of ~ 50µm thickness of MoSi₂ and NbSi₂ on both substrates. The siliconised materials were then exposed at 850°C in an atmosphere of an H₂/H₂S/H₂O, containing high sulphur (pS₂~6.8x10^-1 Pa) and low oxygen (pO₂~1.2x10^-15 Pa) potentials at 850°C. The sulphidation kinetics determined by a discontinuous gravimetric methods displayed parabolic kinetics. The coated substrates were characterised after exposure using scanning electron microscopy (SEM), energy-dispersive analysis by X-ray (EDX), wave-length dispersive spectroscopy (WDS) and X-ray diffraction (XRD). The kinetic results showed that siliconing significantly increased the sulphidation resistance of Mo and Nb. Siliconised Mo did not show evidence of the formation of SiO₂ while siliconised Nb developed a SiO₂ scale. This paper will discuss the mechanisms responsible for the processes of the scale formation, scale growth and scale breakdown.
11:30 AM		A1-1-10 The Influence of the Oxidation Behaviour on the Thermal Stability of Hard PVD Coatings. G. Nayal (Institute of Materials Research, United Kingdom) The oxidation behaviour in nano-structured and monolithic hard PVD coatings (TiAlN/VN, TiAlYN/VN, TiAlN/CrVN, TiAlN/CrN, TiAlYN, TiN, and Cr-C) was investigated within a temperature range of 25 to 900°C. Changes to the microstructure and properties of the coating materials with temperature were evaluated using a range of characterising techniques including TGA, SEM, TEM, and X-RD. Additionally, the thermo-tribological behaviour of these coatings was evaluated using a heated pin-on-disc tribometer and alumina as a counterpart material. In this investigation, two major processes, the oxidation and annealing, were significant to the thermal stability of coating materials. When the oxidation behaviour in a coating material was more kinetic and was associated with lower activation energy for oxidation, as in VN containing coatings, for example, the oxidation process was more dominant than the annealing process. This resulted in a momentous degradation in mechanical and sliding wear properties at a relatively modest temperature range below 500°C. Conversely, when the oxidation behaviour was less kinetic and associated with higher activation energy for oxidation, as in CrN containing coatings, the dominance of the oxidation process over the annealing process was reduced, and the decline in thermo-mechanical and sliding wear properties in CrN containing coatings was significantly delayed (700-800°C). The relative enhancement in the thermal stabilities of CrN containing coatings was mainly attributed to the formation of thermally more stable impervious and well-adhered Cr rich oxide, which acted as a restrictive barrier, thus limiting the progress of the oxidation process.
11:50 AM		A1-1-11 Relationship between Particle Erosion and Lamellar Microstructure for Plasma Sprayed Alumina Coatings C.J. Li, G.J. Yang, A. Ohmori (Xi'an Jiaotong University, P.R. China) The lamellar structure determines the mechanical properties of a thermal spray coating. An erosion model of thermally sprayed ceramic coatings resulting from the debonding of flattened ceramic particles is proposed based on the examination of the erosion mechanism of the coating. The theoretical relationship between erosion rate and microstructural parameters for an idealized structural model is established. The microstructural parameters involved are lamellar bonding ratio and thickness of the lamellae. The erosion rate at impact angle of 90° was measured under a fixed blast erosion test conditions. Based on the structural parameters estimated, the correlation of theoretical model with the observed structural parameters and erosion data of plasma-sprayed alumina coatings is examined. It is revealed that the theoretical relationship agrees well with the observed relation. It is clear shown that the erosion of plasma sprayed ceramic coatings is determined by the lamellar structure of thermal spray coating.
12:10 PM		A1-1-12 Study of Pit Initiation on Ar Implanted Stainless Steel L. Martinez (Universidad Complutense de Madrid, Spain); B. Malki, G. Berthomé, B. Baroux (Institute Nacional Polytechnique de Grenoble, France); F.J. Pérez (Universidad Complutense de Madrid, Spain) Ion Implantation has been widely used as a surface modification technique to improve surface properties. It is particularly interesting when the dimensions of the piece cannot be modified. Ions such as Cr, Mo, Ni, Ce or Si have been implanted to modify the corrosion resistance of stainless steels. However, a few works have been performed using argon as selected ion to implant. The surface bombardment with noble gases, mainly produce structural changes, modifying topography and morphology, but also introducing chemical changes. In this work, doses ranging from 1.10¹⁵ upto 1.10¹⁷ Ar⁺/cm² were implanted at energy of 80 KeV in two different stainless steels. The corrosion test were carried out by menas of electrochemical noise at open circuit potential. Quantitative analysis of samples were also carried out by X-ray photoelectron Spectroscopy (XPS).