ICMCTF2016 Session A1-1: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

The extreme operating conditions experienced by high temperature coatings include high temperature and heat flux, elevated stresses, and oxidative and corrosive settings. These conditions significantly influence their mechanical behavior, properties and eventual failure. Efforts to obtain key information on the response under realistic conditions have led to in-situ studies that reveal strain-dependent characteristics of ceramics under the effect of high-energy X-rays. Experiments have been conducted on ceramic coatings deposited via electron-beam physical vapor deposition on tubular samples. These include specimens pre-aged at 1000 °C, developing the thermally grown oxide (TGO) layers to thicknesses representative of different stages of a coating lifespan. High-energy x-rays were used to measure lattice strains in these multi-layer coatings with through-transmission synchrotron diffraction. Experiments were conducted under various combinations of applied thermal gradients and mechanical loads to determine the competing effects of these conditions on the mechanical behavior in the layers. The experimental results capturing this behavior are very valuable to improving life prediction models.

Thin coatings were synthesized by cathodic arc evaporation of powder metallurgical fabricated Al-Ni targets with the chemical compositions of Al₇₅Ni_25, Al₆₇Ni₃₃ and Al₅₂Ni₄₈ atomic percent. The coatings were produced in pure metallic vapour to avoid the influence of additional noble gases in the arc discharge on the layer condensation. Under these conditions, different Ni aluminide phases were formed. The coatings obtained from the Al rich target forms Al₃Ni while the formation of Al₃Ni₂ and AlNi phases has been found for the coatings synthesized from the targets with higher Ni content. In additional experiments, Al-Ni oxide coatings were synthesized from the same target compositions admixing oxygen to the cathodic arc discharge. All these coatings comprise the Al₃Ni, Al₃Ni₂ and AlNi phases in addition to the cubic-fcc Al₂NiO₃. All coatings have been annealed in ambient air up to 1200°C and were investigated by in-situ XRD acquisition in order to follow in detail their oxidation process. For all coatings, the formation of α-Al₂O₃ and Al₂NiO₃ could be observed after annealing forming a thin high temperature stable protective layer for the remaining AlNi intermetallic_. The coatings deposited in reactive atmosphere show the additional presence of NiO independently of their original composition. For the coating deposited without oxygen, NiO is found after annealing only in the coating deposited from the Ni rich target. A comparison with the oxide scale formation at bulk Ni aluminide material is given.

During Al die casting, it is important to prevent the soldering that frequently occurs between the Al alloy and the steel dies and core pins. The overall objective of this study is to develop coatings that are non-wetting with liquid Al with the long-term objective of circumventing the need to use liquid-based organic lubricants prior to each shot (lube free die casting). A simple and direct testing approach, the aluminum adhesion test (AAT), was developed that enables semi-quantitative measurements of the soldering behavior (adhesion behavior) between the die casting alloys and the coated dies. In this research, CrN and AlCrN coatings have been deposited on H13 tool steel by modulated pulsed power magnetron sputtering (MPPMS). The structural, mechanical, and tribological properties of these coatings have been characterized by X-ray diffraction, nanoindentation and pin-on-disk tests. The prepared CrN-based coatings were tested by the AAT. The alloy/coating interfaces are examined by using optical microscopy, scanning electron microscopy, focused ion beam and transmission electron microscopy. Several possible causes will be suggested to account for the different adhesive behaviors.

The analysis of microstructure evolution of coated single crystal Ni-base superalloy is a major issue to access guidelines for both design of parts and optimization of microstructure to increase life of these coated parts. Firstly, the influence of thermo-mechanical fatigue parameters on typical (Ni,Pt)Al coating microstructure evolution, namely, the respective influence of holding time and applied mechanical stress at high temperature has to be considered [1]. Moreover, some original results of thermal grow oxide (TGO) influence on rumpling will be presented by comparing thermal cycling in air and Ar atmosphere. These results enable to discuss some assumptions developed earlier in the literature [2] and point out the fact that without dense and highly adherent oxide layer, the rumpling could hardly occur.

Finally, the respective influence of surface roughness and phases constituting the coating on its brittleness will be discussed. To address this issue, typical coating microstructure inherited from thermal cycling and isothermal ageing conditions was analyzed and a series of room temperature mechanical testing was carried out up to crack detection. This methodology has enabled to determine a critical strain at which a crack could initiate [3]. This point will be discussed through experimental analysis of microstructure, in situ observation of both cracking and local strain measured by digital image correlation (DIC), and finally mechanical finite element analysis (FEA) of realistic morphology of the specimen surface. This study has clearly evidenced the criticality of roughness associated to rumpling, since an increase in local roughness will decrease the critical strain to failure. Nevertheless, long-term ageing is seen to be beneficial, increasing the ductility of the coating, by the increase in γ’ ductile phases content. The influence of roughness is seen to be limited both by the phase evolution of the coating, and geometrical effect. This latter point is again evidenced by FEA.

1. SALLOT P., MAUREL V., REMY L., N’GUYEN F., LONGUET A., Met. Mater. Trans. A, 46(10), 2015, pp 4589-4600

2. TOLPYGO V., CLARKE D., Surf. and Coat. Tech., 203(20-21), 2009, pp 3278–3285

3. ESIN V.A., MAUREL V., BRETON P., KÖSTER A., SELEZNEFF S., Acta Mat., 105, 2016, pp 505–518

In order to study the effect of titanium ion implantation on hydrogenation behavior of Zr-1Nb alloy, samples were implanted with titanium ions with different fluences using a filtered metal vapor vacuum arc source at the bias voltage of 1-1.5 kV. The crystalline structure, morphology, depth distributions of elements in the surface layer of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and glow discharge optical emission spectroscopy (GDOES) respectively. Hydrogen saturation were performed by Sievert method at 673 K temperature and 2 atm pressure. It was found that hydrogen absorption rate by Zr-1Nb alloy decreased with increasing the fluences of implanted titanium ions. Titinium implantation leads to formation of the modified layer which accumulates hydrogen and makes a hydrogen diffusion barrier. The mechanism of the hydrogenation of titanium-implanted zirconium was discussed.