Ying Zhang

Bio: Ying Zhang is an academic researcher from Tennessee Technological University. The author has contributed to research in topics: Coating & Aluminide. The author has an hindex of 24, co-authored 45 publications receiving 1300 citations.

Martensitic transformation in CVD NiAl and (Ni,Pt)Al bond coatings

Abstract: The martensitic phase transformation in single-phase β-NiAl and (Ni,Pt)Al coatings was investigated. After isothermal exposure to 1150 °C for 100 h, the β phase in both types of coatings was transformed to a martensite phase during cooling to room temperature. Martensitic transformation was also observed in the (Ni,Pt)Al bond coat with and without a YSZ top layer after thermal cycling at 1150 °C (700 1-h cycles). The transformation took place due to Al depletion in the coating from the formation of the Al2O3 scale and interdiffusion between the coating and superalloy substrate. The effects of the martensitic transformation on coating surface stability (‘rumpling’) via volume changes during the phase transformation are discussed with regard to TBC failure.

Effects of Pt incorporation on the isothermal oxidation behavior of chemical vapor deposition aluminide coatings

Abstract: The effects of Pt incorporation on the isothermal oxidation and diffusion behavior of low-sulfur aluminide bond coatings were investigated. Aluminide (NiAl) coatings and Pt-modified aluminide (Ni,Pt)Al coatings were synthesized on a low-sulfur, yttrium-free single-crystal Ni-based superalloy by a high-purity, low-activity chemical vapor deposition (CVD) aluminizing procedure. The isothermal oxidation kinetics and scale adhesion behavior of CVD NiAl and (Ni,Pt)Al coatings before and after isothermal oxidation were determined by electron microprobe analysis. Platinum did not reduce oxide-scale growth kinetics. No significant differences in bulk refractory metal (W, Ta, Re, and Mo) distributions were observed as a result of Pt incorporation. Spallation of the alumina scale and the formation of large voids along the oxide-metal interface were commonly observed over the NiAl coating grain boundaries after 100 hours at 1150 °C. In contrast, no spallation of Al2O3 scales occurred on (Ni,Pt)Al coating surfaces or grain boundaries, although the sulfur content in the CVD (Ni,Pt)Al coatings was higher than that of the CVD NiAl coatings. Most significantly, no voids were observed at the oxide-metal interface on (Ni,Pt)Al coating surfaces or cross sections after 200 hours at 1150 °C. It was concluded that a major beneficial effect of Pt incorporation on an aluminide coatings oxidation resistance is the elimination of void growth at the oxide-metal interface, likely by mitigation of detrimental sulfur effects.

Comparison of the cyclic oxidation behavior of β-NiAl, β-NiPtAl and γ–γ′ NiPtAl coatings on various superalloys

Abstract: The cyclic oxidation behavior of simple aluminide and platinum aluminide coatings on various superalloy substrates was investigated at three temperatures. Coatings of β-NiAl, β-NiPtAl and Pt-modified γ–γ′ were tested and compared in dry oxygen at 1050, 1100 and 1150 °C. There was a clear influence of substrate sulfur content on the oxidation behavior of β-NiAl coatings at all temperatures. The impact of substrate impurities, especially S content, was significantly reduced for β-NiPtAl coatings at all temperatures. Simple Pt-modified γ–γ′ coatings showed excellent oxidation behavior up to 1100 °C on some alloys, but were more sensitive to substrate composition, especially Hf content, than were β-NiPtAl coatings.

LaCrO3-based coatings on ferritic stainless steel for solid oxide fuel cell interconnect applications

Abstract: A thin layer of doped lanthanum chromite on ferritic steel may act as a protective coating to mitigate the Cr volatility problems and facilitate the use of metallic interconnect in solid oxide fuel cells operated at intermediate temperatures. In this paper, the LaCrO3 thin film was successfully synthesized on a ferritic stainless steel substrate by two approaches, i.e. reactive formation and sol–gel processing. The coating structures and surface morphologies were analyzed using X-ray diffraction and scanning electron microscopy. After isothermal oxidation at 850 °C for 100 h in air, the electrical resistance of the sol–gel coated samples remained very low, as compared to that of the uncoated sample after similar thermal exposure. The sol–gel coating also provided effective protection for the interconnect steel during oxidation of twelve 100-h cycles at 800 °C in air, whereas significant spallation and weight loss were observed for the uncoated steel. The two coating processes (i.e. reactive formation and sol–gel processing) were compared and their advantages and drawbacks were outlined.

Effect of cycle length on the oxidation performance of iron aluminide coatings

Abstract: One of the lifetime issues of particular concern for application of iron aluminide coatings is the possible compatibility problems between Fe–Al coatings and substrates which can have substantially different coefficients of thermal expansion (CTEs). This difference could cause deformation or cracking and reduce coating lifetime. The present study has focused on the effect of cycle length (1 h vs. 100 h) on the cyclic oxidation behavior of aluminide coatings on representative commercial ferritic (Fe–9Cr–1Mo) and austenitic (type 304L stainless steel) alloys at 700°C in air with 10 vol.% H 2 O. The ferritic and austenitic steel specimens were aluminized in a laboratory-scale chemical vapor deposition (CVD) reactor. Testing of the coating specimens indicated that high frequency thermal cycling (1 h cycle time) could significantly degrade the coating performance. Comparison of these results with those from similar specimens with a longer cycle time (100 h) or after isothermal exposure showed that this degradation was not due to Al loss from the coating into the substrate by interdiffusion, but most likely was caused by the thermal expansion mismatch between the coating and substrate.

The Superalloys: Fundamentals and Applications

Abstract: 1. Introduction 2. The physical metallurgy of nickel and its alloys 3. Single crystal superalloys for blade applications 4. Superalloys for turbine disc applications 5. Environmental degradation: the role of coatings 6. Summary and future trends.

Advances in Coating Design for High-Performance Gas Turbines

Abstract: Surface engineering is now a key materials technology in the design of future advanced gas-turbine engines. This article focuses on coating systems for hot-gas-path components, which can vary from low-cost aluminide diffusion coatings to the more exotic, and therefore expensive, thermal-barrier coatings. Available coating systems and their relative benefits are reviewed in terms of performance against manufacturing complexity and cost. Future trends in the design of environmental- and thermal-protection coatings are discussed, including the addition of multiple reactive elements, modified aluminide coatings, diffusion-barrier concepts, the design of “smart” corrosion-resistant coatings, and the development of structurally modified, low-thermal-conductivity thermal-barrier coatings.