Key words: Thermal stability, high temperature, oxidation, corrosion, erosion.

One of the major challenges for TBC system is to determine the time to failure of ceramic layer during ageing. The aim of this paper is to propose new methodologies for both experimental assessment of adhesion and for analyzing the robustness of model of lifetime to spallation.

Recently, the use of LAser Shock Adhesion Test (LASAT) has shown its capability for both ranking different coating solutions and evaluating the evolution of a given coating as a function of aging [1-2]. The methodology developed in this study is based on the evaluation of interfacial crack propagation from a defect introduced by laser shock, an interfacial delaminated area, known in size and location. The evolution of interfacial delamination during thermal cycling was shown to be consistent with measured interfacial delamination measured by cross-sectioning. Thus, the influence of dwell time at high temperature has been clearly established, confirming that short dwell time was very detrimental as compared to long dwell at high temperature [3-4]. Moreover, this new method has shown a very small scatter as compared to other adhesion testing [2].

On the other hand, some lifetime to spallation models have been tested, focusing on two mains aspects. Firstly, the ability of existing models to reproduce the influence of loading parameters has been tested (e.g. the influence of dwell time at high temperature or the influence of maximum temperature) [3-5]. Secondly, the robustness of this kind of models has been analyzed through sensitivity analysis based on large numerical sampling. As a conclusion, the propagation of scatter from measurement to life modeling has been evaluated and guidelines for experimental focus points have been derived.

References

[1] Guipont, et al (2010). Journal of Biomedical Materials Research Part A, 95(4), 1096-1104.

[2] Guipont, et al. "Interfacial toughness evolution under thermal cycling by laser shock and mechanical testing of an EB-PVD coating system." (2018). http://dc.engconfintl.org/cgi/viewcontent.cgi?article=1033&context=tbcv

[3] Courcier, C et al (2011). Interfacial damage based life model for EB-PVD thermal barrier coating. Surface and Coatings Technology, 205(13-14), 3763-3773.

[4] Soulignac, Romain, et al. "Cohesive zone modelling of thermal barrier coatings interfacial properties based on three-dimensional observations and mechanical testing." Surface and Coatings Technology 237 (2013): 95-104.

[5] Chaboche, J. L., et al "Lifetime assessment tools for thermal barrier systems" in "Thermal Barrier Coatings IV",Eds, ECI Symposium Series, (2015). http://dc.engconfintl.org/thermal_barrier_iv/34

(Al MMCs) are more prominent due to the tool interaction with the particulates at high strain rate. Considering

the excellent mechanical and wear resistance properties of cathodic arc PVD coatings, they are expected to abate

the wear of tools during FSW of Al MMCs. Hence, it is the aim of this study to investigate the contribution of

cathodic arc PVD deposited AlCrN and TiAlN coatings in alleviating tool wear during FSW of Al MMCs. AlCrN

and TiAlN coated H13 FSW tools were used to butt weld 8 mm thick aluminum alloy 2124 reinforced with 17 vol

% SiC of 3 μm particle size. Interestingly, the coated tools exhibited significant improvement in wear resistance

of about 92 and 80%, respectively, over the bare H13 tool. Over 97% reduction in the wear of the FSW tool was

recorded during the plunging stage with the use of the coated tools. Additionally, AlCrN and TiAlN coated tools

considerably improved the surface finish of the welded Al MMC. The coated tools demonstrated superior wear

resistance due to the improved scratch crack resistance, high mechanical and oxidation resistance properties of

the coatings. Dominant wear mechanisms on AlCrN and TiAlN coated tools were abrasive erosion and chipping

off by sharp and hard SiC particles while severe striation and oxidation characterized the wear mechanism of the

bare tool. The use of these coatings did not deteriorate the weld properties, rather somewhat improvement in the

hardness of the nugget zone was observed due to the characteristic dynamic stirring and refinement of the Al

matrix and SiC particles.

Surface hardening, a process that includes a wide variety of techniques (Carburizing, Nitriding, Nitrocarburizing, Boriding, and Thermal diffusion process), is used to improve the wear resistance of parts without affecting the more soft, tough interior of the part. In particular, resistant layers of borides are produced in ferrous and no-ferrous materials though the well-developed process of boriding. In the present work, the AISI 9840 steel was pack-borided in the temperature range 1123-1273 K for 2- 8 h to form a compact layer of Fe₂B at the material surface. A recent kinetic approach, based on the integral method, was proposed to estimate the boron diffusion coefficients in the Fe₂B layers formed on AISI 9840 steel in the temperature range 1123-1273 K. In the present model, the boron profile concentration in the Fe₂B layer is described by a polynomial form based on the Goodman’s method. As a main result, the value of activation energy for boron diffusion in AISI 9840 steel was estimated as 193.08 kJmol^-1 by the integral method and compared with the values available in the literature. Three extra boriding conditions were used to extend the validity of the kinetic model based on the integral method as well as other diffusion models. An experimental validation was made by comparing the values of Fe₂B layers’ thicknesses with those predicted by different diffusion models. Finally, an iso-thickness diagram was proposed for describing the evolution of Fe₂B layer thickness as a function of boriding parameters.

The paper presents results of pre-oxidation experiments of ɣ-TiAl alloy with Si-modified aluminide coatings intended for application as bondcoatings for Thermal Barrier Coatings. The goal of this work was to provide insight into the effect of different oxidation atmospheres and temperatures on the initial stages of oxide scale formation on bare and coated ɣ-TiAl. It is assumed that the formation of a thin (several hundreds of nanometers) and slow growing TGO consisting of α-alumina will enhance the adherence and lifetime of the TBCs on ɣ-TiAl.

The coatings were produced by pack cementation method applying a mixture consisting of Si and Al-containing powders and NH₄Cl activator. The additional surface modification by pre-oxidation under various atmospheres was performed to foster the formation of oxide scales characterized by various thicknesses as well as phase and chemical compositions. The pre-oxidation experiments were performed in various gases including a mixture of Ar and O₂ (from 0.01 to 10 %) as well as pure O₂ at 900 °C for 0.5, 2 and 4 hours. After the pre-oxidation experiments systematic studies were performed using Scanning Transmission Electron Microscopy (STEM) method in order to characterize the phenomena at the metal-scale interface and study the phase and chemical compositions as well as the microstructures of the formed oxide scales.

Effective oxidation and corrosion protection of turbine blades in aircraft engines used at high temperature in the atmosphere of exhaust gases is still a major technological challenge and is the subject of intensive research. In this work, the microstructure of coatings deposited on Inconel 738 alloy used for turbine blades was investigated. In the first stage of the research work MCrAlYHfSi (M=Ni) coatings made of Amperit 405.001 material with granulation +22-45 µm were deposited using HVOF method. The coatings were applied with variable process parameters by HVOF method using the Thermico system and the CJS K5.2-N burner. Then thermal treatment of the sprayed coatings was performed at 1050°C for 4 hours in argon atmosphere. After the heat treatment vapor phase aluminizing using the “out-of-pack” method was carried out at 1050°C for 5 hours. After each treatment thickness of the coatings was measured, the microstructures were investigated using SEM and chemical composition analysis (EDS) and their surface topography and roughness were determined using 3D profilometry.

In the present study, the mechanical properties of a free-standing MCrAlY coating and their evolution were scrutinized when it is aged in air for a large range of time and temperature. Free-standing micro-tensile specimens were prepared [1] and tested after different time/temperature ageing [4]. Based on digital image correlation to derive the strain evolution during loading, the stress-strain behavior has been identified. Thus a crucial effect of intrusive oxidation from deposition process defects and surface oxidation has been evidenced to drive both the ductility and the fracture mechanism of the tested material. For coarse intrusive oxidation, a loss of ductility has been identified, whereas metallic phase transformation (from b to g) increases the ductility of the material from 0.5 up to 3%. This composite effect will be analyzed through systematic microstructure and homogenization analyses. This methodology could be straightforward to determine the coupling between coating cracking and substrate failure under thermo-mechanical fatigue conditions [5].

References

[1] D. Texier, D. Monceau, J. C. Salabura, R. Mainguy, E. Andrieu, Materials at High Temperatures, 33 (2016) 325–337.

[2] D. Texier, D. Monceau, Z. Hervier, E. Andrieu, Surface and Coatings Technology, 307 (2017) 81–90

[3] Alam, M.Z., Satyanarayana, D.V.V., Chatterjee, D., Sarkar, R., Das, D. K., Materials Science and Engineering: A, 641 (2015) 84-95.

[4] Straub, T., Baumert, E.K., Eberl, C., Pierron, O.N., Thin Solid Films, 526 (2012) 176–182.

[5] Esin, V.A., Maurel, V., Breton, P., Köster, A., Selezneff, S., Acta Materialia, 105 (2016) 505–518.