ICMCTF2018 Session AP: Symposium A Poster Session

Atom probe tomography provides the opportunity to investigate the chemical composition of (coating) materials on the nm-scale. During the multimedia poster session differences (and similarities) between the integral coating composition determined by elastic recoil detection analysis and/or energy dispersive X-ray spectroscopy and the local composition determined by atom probe tomography will be presented. The effect of Si additions to Cr₂AlC which are known to affect the oxidation kinetics are presented as well as Mn additions to Cr₂AlC. Furthermore, differences (and similarities) in local and global chemical composition identified for the coating systems Mo₂BC as well as transition metal nitrides and oxynitrides are discussed.

The need to increase the efficiency in energy production in a sustainable way as well as reducing the environmental impact, has become evident. This leads to increasingly extreme conditions imposing more demanding performances on materials such as higher temperatures and pressures in more corrosive environments. Therefore, the main goal of "Coating Generation and Study for Materials Protection used in Extreme Atmosphere: Sustainability and Energy Efficiency” project is developing and optimizing protective coatings for structural materials of power plants generation to protect from aggressive environments present in power plants working under oxy-fuel atmospheres and employing biomass as fuel.

The technical goal aiming at studying the feasibility of using coatings as an alternative in a biomass oxy-combustion thermal plant working under supercritical conditions. Until now, it had been unexplored, so this project has assessed the corrosion behavior of bare and coated coupons of ferritic steels (T22 and P92) and a high-alloy multi-purpose austenitic stainless steel (Sanicro 28) which have been exposed to biomass oxy-combustion conditions on one side, and to supercritical steam on the other, both in laboratory and in pilot plants. The gravimetric evolution was analyzed at fixed intervals to study their oxidation kinetics. In addition, common characterization techniques have been used, such as x-ray diffraction (XRD), to explored the phases formed during the oxidation process, and scanning electron microscopy with energy dispersive detector (SEM-EDX), to evaluate the morphology and semi-quantitative composition.

On one hand, the samples were tested in pure steam at 650ºC and 0.1 and 30 MPa in order to evaluate the role of the pressure. After more than 1000 h, the coated materials exhibited an improved behavior compared with base materials. However, this parameter significantly affected the ferritic steels.

On the other hand, the oxy-combustion tests were firstly made in a laboratory with and without additions of K₂SO₄ and KCl and/or SO₂ in order to simulate the composition of thistle biomass, which has been used in experiments carried out after at the pilot plant. In all cases, ferritic steels have been severely affected, forming non protective oxides. Nevertheless, Sanicro 28 steel has been able to resist so it does not need to be covered. Regarding the coatings, they have successfully altered the catastrophic behavior of the ferritic steels. So it could contribute to reach the production of sustainable, clean and efficient electricity using existing infrastructures.

Characterization of as-cast microstructure and oxidation resistance of new type of cobalt based superalloys with γ/γʹ microstructure were presented in this investigations. As a referential material the Co-20Ni-7Al-7W (at.%) was used as the modified version of basic (Co-20Ni-7Al-7W). All investigated alloys were modified by addition of yttrium, hafnium and zirconium on the level of 0.5, 0.2 and 0.05 at.% respectively due to improvement of its oxidation resistance. All alloys were made in Institute of Materials Engineering of Silesian University of Technology in Poland.

The oxidation performance of all alloys were made during thermogravimetric investigations at temperature range from 25 to 1200°C. The main analyzed parameter was mass gain detected continuously during the test. After the test the overall and cross-sectional analysis of specimens was made and included analysis of oxide layer morphology on the surface of specimens with characterization of phase constituent of oxide layer. The detailed analysis of oxidized layer was made by scanning electron microscopy method when distribution of alloying elements was made with special attentions on yttrium localization after the test in oxide zone.

Obtained data showed that both addition of Y, Hf and Zr has a strong influence on oxidation performance of new Co based superalloys. Those influence is demonstrated mainly by different morphology of final oxide zone with strong segregations of yttrium containing oxidation products.

This work was supported by Institute of Materials Science of Silesian University of Technology, as a part of Statutory Research no BK-225/RM0/2017.

The oxidation behavior of surface aluminizing T91 boiler-used steel (T91A) was investigated over the temperature range of 600 ~ 900 ˚C in dry air. The surface-aluminizing parameters for T91A were to heat the T91 samples at 800 ^oC for 4 hr in 7% AlF₃ at a 200 cc/min flow rate of Ar. The results showed that the oxidation kinetics of T91A followed a single-stage parabolic-rate law at 600 ~ 750 ˚C, while a two-stage parabolic-rate law was observed at 800~900 ˚C. The steady-state oxidation rates at 850 ~ 900 ^oC were significantly slowly than those at lower temperatures. The oxidation rates of the aluminizing alloy were significantly lower than those of the T91 substrate by 6.2 orders of magnitude at 850 ˚C. Both α- and θ-Al₂O₃ formed on top of T91A surface after the oxidation. The amount of θ-Al₂O₃ gradually reduced and the amount of α-Al₂O₃ gradually increased with increasing temperature. It was found that the growth of α-Al₂O₃ is strongly dependant on the oxidation temperature and duration of time. The formation of α-Al₂O₃ is responsible for the lower oxidation rates of the T91A alloy at 850 ~ 900 ˚C.

Applications such as high-speed machining and dry cutting of metals demand the development of superior materials to withstand severe operating conditions. High loads and temperatures exceeding 1000°C in contact between the tool and the work piece require protection of the tool surface by advanced hard coatings. Under such conditions, thermal stability and oxidation resistance of the coated tool is of outmost importance. A solid understanding of the diffusional mechanisms resulting in the phase transformation of metastable phases and oxidation of the coating with subsequent deterioration of the mechanical properties is thus the basis for establishing strategies for improved high-temperature behaviour of the protective coatings. The coating performance may also be dramatically affected by structural changes driven by elements diffusing from the tool material into the coating.

In this work, AlCrSiN coatings with a Si content up to 10 at. % were deposited on cemented carbide and high-speed steel substrates by cathodic arc evaporation. To study the oxidation behaviour, the samples were annealed in ambient atmosphere at temperatures up to 1200 °C for 1 h. Powders made out of the coatings were investigated via differential scanning calorimetry and thermogravimetry. Additionally, an AlCrSi₁₀N coating on cemented carbide annealed at 1400 °C for 1 h was characterized by cross-sectional, position-resolved synchrotron X-ray nanodiffraction analysis to gain information about structure, texture and residual stresses across the coating thickness including the oxide scale formed on its surface. To investigate the influence of diffusion of substrate elements into the coating, samples were annealed in vacuum in the same temperature range and analysed by means of scanning electron microscopy and energy dispersive X-ray spectroscopy. Additionally, one selected sample was characterized by cross-sectional, position resolved synchrotron X-ray nano-diffraction analysis to study the microstructural evolution, phase transformations and development of residual stresses near the substrate-coating interface affected by pronounced diffusion of substrate elements.

Based on the results of this study, we will suggest strategies to suppress diffusion of Co from the cemented carbide and Fe from the steel substrates into the coating and enhance the oxidation resistance of AlCrN coatings to ensure high-temperature stability of the coating and thus enhanced operation performance of the coated tool.

Carbon-carbon(C/C) composites , an important high-temperature structural material, face the severe problem of oxidation only above 500℃. Coating technology with ultra-high temperature ceramics (UHTCs) have been proved to be highly effective to improve the oxidation resistance of C/C composites^[1]. Currently, the main coating techniques for C/C composites include pack cementation, plasma spraying, polymer derived ceramic coatings and so on. Among them, polymer derived ceramic coatings have gained more and more attention due to its easy for implementation, no need for special equipment and low sintering temperature. However, the technique of polymer derived ceramic coating usually needs multiple casting-sintering cycles to densify the prepared coating, which requires ultra long operation time thus affect its real application. In this paper, we present a novel strategy to fabricate gradient SiBCN composite coating with just one casting-sintering cycle by adopting polyborosilazane ceramic precursor^[2] as the main raw material, well controlling the viscosity of the polyborosilazane-filler slurry and a low temperature pre-oxidation treatment.

The microstructure morphology, elements composition, thermal performance, and anti-oxidation property of the ceramic coating have been investigated. The coating surface macro appears uniform, no cracks, while microscopic loose and porous. The coated C/C composite exhibits excellent anti-oxidation property with weight loss of only 0.06% after oxidation at 1500 C for 30min. Due to the gradient structure, the coating shows no crack and no peeling off after 8 cycles of thermal shock from 1500^oC to room temperature. The XRD analysis showed that the surface of the coating was oxidized to form a zirconium silicate material, thereby preventing oxygen from further penetrating and achieving the oxidation resistance. All above of the results indicate that SiBCN ceramic coating has high potentials for anti-oxidation protection of C/C composites.

Reference

[1] Davies I J, Rawlings R D.(1994) ,Microstructural investigation of low-density carbon-carbon composites, J. Mater. Sci., 29, 338-344.

[2] Zongbo Zhang, Caihong Xu.(2011), Synthesis and characterization of a new liquid polymer precursor for Si–B–C–N ceramics, J Mater Sci., 46,5940–5947.