ICMCTF2014 Session A1-3: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

Al-Hf and Al-Hf-O coatings were synthesized by cathodic arc evaporation. The depositions were performed onto sapphire substrates utilizing composite targets with compositions of Al_0.75Hf_0.25, Al_0.70Hf_0.30 and Al_0.67Hf_0.33, respectively. The Al-Hf coatings were produced in non-reactive processes (without gas addition), while pure oxygen was utilized for Al-Hf-O coatings. The chemical composition was analyzed by EDX and RBS. For the Al-Hf coatings, it deviates distinctively from target composition. This was not the case for Al-Hf-O. XRD phase analysis proves the synthesis of Al₃Hf and Al₂ Hf intermetallic compounds for the Al-Hf coatings while the oxide coatings indicates the monoclinic HfO₂ phases in an amorphous matrix.

The coatings were annealed in ambient up to 1290°C to study the oxidation process. In-situ XRD analysis of the coatings was performed during annealing. Depending on composition, the intermetallic phases undergo phase transformations at temperatures between 690° and 850°. After a transition period charcterized by the evolution of characteristic phases, coatings stabilized at about 1100°C forming two oxide phases: alumina (corundum) and monoclinic HfO₂. Applying the same treatments to the Al-Hf-O coatings result in an improvement of crystallinity for the monoclinic HfO₂ phase starting at about 860°C. After a transition period to 1100°C, the corundum and monoclinic HfO₂ phases are formed.

The oxidation processes for the Al-Hf intermetallic phases and the Al-Hf-O oxides are compared placing particular emphasis on the transition region. The parameters of the deposition process and their influence on the stabilization of the oxides at already lower temperatures are discussed.

MoN and MoB(N) coatings demonstrate relatively high hardness and wear resistance, low friction coefficient and good adhesion to the steel substrates but their working temperatures are limited by 500-600⁰C. It’s well known that oxidation resistance and thermal stability of nitride and boride coatings can be enhanced by Si alloying. The goal of this work is a complex study of the Mo-Si-B-N coatings, including the investigation of high-temperature tribological characteristics, thermal stability, oxidation resistance, and diffusion barrier properties.

The multicomponent MoB_0.3Si_0.1 and MoB_0.2Si_0.4 composite cathodes produced by self-propagation high-temperature synthesis technology were subjected to magnetron sputtering either in a pure Ar or N₂ atmosphere, or in a gaseous mixture of Ar+N₂. Molybdenum, silicon, alumina, NiCrAlW and WC-Co alloys were used as substrate materials. To evaluate the high-temperature oxidation resistance, diffusion-barrier properties, and thermal stability, the coatings were annealed in air at various temperatures range from 500 to 1500⁰C. The structure of as-deposited and heat-treated coatings was studied by means of X-ray diffraction, scanning and transmission electron microscopy, glow discharge optical emission spectroscopy, X-ray photoelectron spectroscopy, Raman and FTIR spectroscopy. The mechanical properties of the coatings were measured using nanoindentation and scratch-testing. The tribological properties were evaluated in air using a high-temperature ball-on-disc tribometer. The properties of Mo-Si-B-N were compared with those of the MoN and Mo-B-(N) reference coatings.

The results obtained show that the Mo-Si-B-N coatings contained nanocrystalline nc-MoSi₂ and nc-MoB grains surrounded by amorphous a-SiN_x phase. The Mo-Si-B-N coatings with high concentration of Si and N demonstrated the following characteristics: high adhesion strength (critical load > 40 N), hardness up to 35 GPa, elastic recovery up to 65%, elastic modules less than 350 GPa, and wear rate less than 10^-5 mm³N^-1m^-1. The decreasing of friction coefficient from 0.8 (room temperature) to 0.3 (500⁰C) was observed and can be explained by the formation of MoO₃ phase acted as a solid lubricant. When the Si content was raised from 4 to 50 at.%, the oxidation resistance of the Mo-Si-B-N coatings was showed to increase from 600 to 1400⁰C and attributed to the formation of a SiO₂ top-layer, which protected the coatings from an intensive oxidation at high temperatures. The combination of relatively high hardness, wear, and high-temperature oxidation resistances makes the Mo-Si-B-N coatings promising candidates for high-temperature tribological applications.

Refractory alloys such as Nb-Si intermetallic composites exhibit low density and high strength above the operating temperature range of nickel-based superalloys. Due to these advantageous properties they have been investigated as replacement for nickel-base superalloys in structural components for high-temperature applications, such as aircraft or land-based turbines. However those alloys are not ready to withstand the higher-temperature hostile environments (high combustion gas flow rates, with presence of aggressive elements such as water vapor, oxygen and CMAS, component temperatures of more than 1150°C). Indeed they suffer from rapid degradation (pesting) in air and severe metal recession that hindered their industrial use.

Several strategies are developed to limit the oxidation rate of these compounds:

- Modification of the composition (Al, Si, Ti and Sn additions),

- Microstructure design using powder metallurgy processes,

- Manufacturing of a protective bond coat,

- Deposition of a bond coat plus an environmental/thermal barrier coating (E/TBC).

The research efforts made in these four directions allow obtaining Nb-Si intermetallic composites less susceptible to pesting at moderate temperature and with improved oxidation resistance at higher temperature. Moreover, protective coatings based on Ti₃X₃CrSi₆ (with X = Fe, Co and Ni) have been developed allowing their use up to 1200°C in air or air + steam. Finally, recent investigations evidenced the compatibility of these coating materials with TBC system like yttria partially stabilized zirconia (YSZ). From first results, an increase of the operating temperature for the Nb-Si based system can be expected.

Extensive work has been undertaken to develop structural material able to work at higher temperature than nickel based superalloys, currently operating at 90 % of their melting point [1]. Among potential candidates, refractory alloys based on the Nb-Si system would allow a jump of 200°C of the operating temperature and a decrease by 25% of density in comparison to temperature capabilities and densities offered by current nickel based superalloys. However, their oxidation resistance is too low for a use as structural material without any protective coating, just like current nickel superalloys. Diffusion coatings based on ternary [2] of quaternary [3] complex silicides have been developed, but both systems presented limitation regarding their thermal stability at high temperature. As a consequence, the coating has to be made more refractory. Titanium silicides diffusion coatings could be good candidates for the protection of Nb-Si alloys as (i) they form a slow growing SiO₂ layer serving as a barrier against oxygen penetration (ii) they are perfectly and uniformly adherent with the substrate [4].

In the present work, halide activated pack cementation technique (HAPC) was applied to coat the Nb-Si alloys. Chlorides (CrCl₃) were used as activator and four compositions of masteralloy were employed : Ti₅Si₃ + Ti₅Si₄, TiSi₂ + Ti₅Si₄, TiSi₂ + TiSi, Si + TiSi₂. Deposits were performed for 9 h at 1200°C in vacuum (10^-6 mbar). Each subsequent coating was characterized by X-ray diffraction and scanning electron microscopy. Results allow the determination of the nature of the successive layers formed during the coating process at the surface of the substrate. Phase successions were considered regarding the available thermodynamic data of the system.

Then, oxidation tests were performed in air to assess the coating performances in a large range of temperature (isothermal oxidation at 1300°C and 1400°C, thermal cycling at 815°C and 1100°C). Regarding results, the coating formed using the pack with the higher silicon activity exhibited the best oxidation resistance both at 815°C and 1300-1400°C. However, limitations appear at 1100°C in cyclic conditions.

[ [1] ] DIMIDUK D. M., PEREPEZKO J. H., MRS Bull. 2003, 28, 639 .

[2] KNITTEL S., MATHIEU S., VILASI M. Surface and Coatings (2013) in press

[3] KNITTEL S. MATHIEU S., PORTEBOIS L., DRAWIN. S, VILASI M. Surface and Coatings (2013) in press

[ 4 ] BV COCKERAM, RA RAPP, Metallurgical and Materials Transactions A, 1995 - Springer

The dies used for moulding and forming require excellent mechanical properties in order to withstand the high levels of stress applied to them during the processing. Another requirement is low release forces to allow for the separation of the moulded/formed product from the surface of the die without fouling of the die surface or damaging the processed components.

Closed Field UnBalanced Magnetron Sputter Ion Plating (CFUBMSIP) is a well-known process capable of producing hard, wear resistant coatings such as CrN. Whilst CrN has been successfully used in a number of forming and moulding applications to enhance the hardness and wear resistance of dies, the release properties are not always sufficient. The addition of Ni in the coating can enhance the release properties against a number of polymers such as low-density polyethylene and epoxy resins.

This presentation reports on NiCrN coatings deposited using reactive CFUBMSIP from elemental Cr and NiCr alloy targets using nitrogen as the reactive gas. The variation of process parameters, such as Ni and N content has been carried out in order to obtain the desired mechanical and non-stick properties. Coatings have shown excellent results in practical moulding applications by reducing the release force and fouling of the die.

Lanthanum borosilicate glasses, B₂O₃-SiO₂-La₂O₃ based, are widely used as optical glass materials for lens utility in glass molding technology. To evaluate the feasibility of protective coatings on glass molding dies, Ta –Si–N coatings were sputtered on cemented tungsten carbide substrates. Lanthanum borosilicate glasses were placed on the surface of coatings and heat-treated in a glass molding atmosphere of 15 ppm O₂ – N₂ in a thermal cycling annealing at 270 and 600 ^oC. Up to 2,000 thermal cyclic annealing tests were performed and the holding time at the molding temperature was set at 1 min for each cycle. The variations in crystalline structure, nanohardness, surface roughness and chemical composition profiles in depth after various annealing durations were investigated.

The thermal durability of TBCs is closely related to their microstructure, determining thermo-mechanical properties and failure mechanism. Therefore, the control of microstructure in the top and bond coats is proposed as a new strategy for advanced TBCs. The TBC system consists of heat-resistant ceramic top coat, intermetallic bond coat, and nickel-based substrate. The nickel-based superalloys used as the substrate are well known for their excellent mechanical properties in high temperature applications, especially for gas turbines. However, the substrate should be protected by MCrAlY (where M= Co, Ni or Co/Ni) bond coat, due to their lack of the oxidation and corrosion resistances. The bond coat plays an important role in ensuring structural effectiveness and affording extra adhesion of the top coat to the substrate, which also prevents and/or delays oxidation of the substrate. Many techniques have been developed and employed to prepare the bond coat, such as low-pressure plasma spray, air plasma spray (APS), and high velocity oxy-fuel (HVOF) spray. Among these techniques, HVOF process is a promising technique for forming the bond coat in TBCs, owing to its good microstructure, better adhesive strength, and low operating costs compared with APS process. In this study, various deposition conditions, such as gun distance, powder feeding rate, and ratio of fuel/oxygen/air, were controlled to optimize the microstructure of bond coat in HVOF process. In order to understand the effects of microstructural design on mechanical properties in the bond coat, the mechanical properties with feedstock powder and coating parameter were investigated using various techniques and the optimized microstructure for the bond coat prepared by HVOF process was proposed.