Derecelendirilmiş mullit esaslı çevresel bariyer kaplamaların üretimi ve termal davranışının incelenmesi = Graded mullite based environmental barrier coatings production and thermal behavior investigation

Abstract

Gaz türbinlerinde yaygın olarak kullanılan termal bariyer kaplamalar, gaz türbinlerinin metalik bileşenlerini yüksek sıcaklıktan korumakla birlikte yanma sıcaklığının artırılarak daha verimli gaz türbinleri üretilmesine olanak vermektedir. Daha yüksek yanma sıcaklığı gereksinimi ile yüksek sıcaklıklara (>1300°C) dayanıklı malzemelere ihtiyaç ortaya çıkmaktadır. Süper alaşım esaslı metalik malzemelerin kullanımı yerine gaz türbinlerinde seramik matrisli kompozitlerin (CMC) kullanımı ortaya çıkmıştır. CMC yüksek sıcaklıklarda üstün mekanik özellikler sağlasa dahi yanma atık gazlarında bulunan su buharı ile korozyona uğramaktadırlar. Bu nedenle çevresel bariyer kaplama olarak adlandırılan ve seramik matrisli kompozitleri su buharından koruyacak kaplamalara ihtiyaç duyulmaya başlanmıştır. Yanma odalarında kullanılan CMC genellikle SiC/C kompozitlerinden üretilmekle birlikte oksit malzemelere göre nispeten düşük termal genleşme özelliğine sahiptirler. Bu nedenle kaplama malzemesi olarak kullanılabilinecek Çevresel Bariyer Kaplamalar (ÇBK) termal genleşme katsayısını sağlayabilecek malzemeler genellikle silikat esaslı malzemelerden seçilmektedir ve silikat esaslı malzemelerde de su buharı korozyonu problemi ortaya çıkmaktadır. Çalışmanın amacı, termal genleşme katsayısı yüksek olsa dahi su buharı korozyonuna daha dayanıklı olan oksit kaplamaların termal genleşme katsayısı CMC ile uyumlu olan silikat esaslı oksitler ile altlıktan en dışa doğru kompozisyonu değiştirilerek en dışta oksit esaslı kaplama katmanı elde edilmesi ve altlık kaplama arayüzünde ise silikat esaslı malzeme uygulanarak TGK uyumsuzluğunun giderilmesidir. Bu amaç doğrultusunda dört farklı kaplama tozu, müllit (3Al2O3.2SiO2), zirkon (ZrSiO4), aluminyum oksit (Al2O3), yitriya (Y2O3) tercih edilmiştir. Mullit- Yttria, Zirkon-Alumina ve Mullit-Alumina olacak şekilde 3 farklı kaplama üretimi gerçekleştirilmiştir. Üretimi gerçekleştirilen farklı kaplamalarda her katman sonrası kompozisyon oksit diğer oksit bileşenin kompozisyonu %25 artırılarak kaplamalar elde edilmiştir. Kaplama yöntemi olarak plazma sprey püskürtme (APS) yöntemi kullanılmıştır. Kaplamaların optik mikroskop, SEM, XRD, termal yayınım ve termal şok analizleri yapılmıştır.Yapılan deneysel sonuçlar neticesinde termal yayınım değerlerinin artan sıcaklık ile azaldığı görülmüştür fakat düşük termal yayınım her zaman düşük termal iletim katsayısına denk gelmeyeceği göz önüne alınmalıdır. Ancak yapılan çalışmanın özü itibari ile kaplamada beklenen termal yayınım değerinin sıcaklığa bağlı değişimi literatür ile uyumlu olduğu görülmektedir.

High-temperature industrial applications, particularly in the energy and aerospace sectors, place extreme demands on materials used in critical components. These components must endure severe thermal, chemical, and mechanical stresses, often in highly oxidative and corrosive environments. As a result, the need for advanced materials and surface modification techniques that can withstand these harsh conditions is crucial. Among the various approaches, thermal spray technologies have become one of the most important methods for enhancing material properties, offering solutions for wear resistance, thermal insulation, and corrosion protection. Thermal spray coatings involve the deposition of molten or semi-molten particles onto a substrate, which solidify to form a protective layer. This process is particularly beneficial in applications where direct exposure to extreme conditions would lead to rapid degradation of the structural components. By forming a strong, durable protective barrier, thermal spray coatings improve the performance and longevity of critical parts under extreme operating conditions. The flexibility of thermal spray methods allows for the selection of a wide range of coating materials, offering significant versatility in tailoring coatings for specific needs. Among the various thermal spray coatings, thermal barrier coatings (TBCs) are particularly significant due to their role in protecting metallic substrates from high temperatures. TBCs are widely used in gas turbines, where they prevent thermal damage to the turbine components, enabling the production of more efficient engines by increasing the combustion temperature. By insulating the turbine blades from the extreme heat generated during operation, TBCs allow for a higher combustion efficiency, which in turn leads to improved overall energy production and reduced fuel consumption. This is essential in industries that demand maximum energy efficiency, such as power generation and aviation. As operational temperatures in turbines continue to rise to meet the demand for more efficient engines, traditional metallic materials have begun to show their limitations. Materials such as superalloys, although capable of withstanding high temperatures, eventually fail under the extreme conditions present in modern gas turbines. This has led to the development of ceramic matrix composites (CMCs), which offer superior mechanical and thermal properties at elevated temperatures compared to traditional superalloys. CMCs, particularly those composed of silicon carbide (SiC/C), offer high thermal stability, low density, and exceptional resistance to creep and thermal fatigue, making them ideal for use in combustion chambers and other high-stress environments. Despite their advantages, CMCs are not immune to corrosion. One of the major challenges they face is corrosion caused by water vapor present in combustion gases. This issue is especially significant in environments with high humidity or in applications where the combustion gases contain high levels of water vapor. If left unprotected, CMCs can degrade quickly due to the effects of water vapor, which attacks the material, leading to the formation of detrimental phases that reduce its mechanical properties and thermal stability. As a result, the need for specialized coatings referred to as environmental barrier coatings (EBCs) has arisen. EBCs are designed to protect CMCs from water vapor corrosion, ensuring their long-term durability and performance in high-temperature environments. To address this issue, the selection of suitable EBC materials is essential. Materials for environmental barrier coatings are typically chosen from silicate-based compounds, as they offer thermal expansion coefficients that are more compatible with the CMCs' low expansion properties. This compatibility is crucial to minimize thermal stresses that can occur due to the expansion mismatch between the coating and the substrate. However, while silicate-based materials offer the necessary thermal properties, they also present challenges in terms of water vapor corrosion resistance. Silicates can react with water vapor, leading to the formation of hydrated phases that reduce the coating's effectiveness and durability over time. Therefore, a critical challenge in developing EBCs is balancing the need for both thermal expansion compatibility and resistance to water vapor corrosion. To overcome these challenges, advanced deposition techniques such as Atmospheric Plasma Spray (APS) have become crucial in the development of effective environmental barrier coatings. APS is a widely used thermal spray technology that allows for the deposition of a variety of materials onto substrates with high precision. In the APS process, coating powders are introduced into a high-temperature plasma jet, where they are rapidly heated and accelerated to high velocities. The molten or semi-molten particles are then sprayed onto the substrate surface, where they solidify to form a dense, adherent coating. This process provides several advantages, including precise control over the coating thickness and microstructure, which are essential for achieving the desired performance characteristics in environmental barrier coatings. APS is particularly suitable for the deposition of multi-layered coatings, which is essential when designing coatings with gradient structures. In the case of CMC protection, a multi-layer coating system can be designed to have a silicate-based material near the substrate, where the primary concern is thermal expansion compatibility, and an oxide-based material on the outermost layer, where water vapor resistance is more critical. By using APS, it is possible to achieve coatings that provide both the necessary mechanical properties and corrosion resistance while minimizing thermal expansion mismatch. The versatility of APS also extends to material selection. Coating powders such as mullite (3Al2O3·2SiO2), zircon (ZrSiO4), aluminum oxide (Al2O3), and yttria (Y2O3) are commonly used in EBC applications. These materials were selected for their excellent thermal properties, adhesion strength, and corrosion resistance. For this study, coatings were produced in combinations such as Mullite-Yttria, Zircon-Alumina, and Mullite-Alumina to evaluate their performance in protecting CMCs from water vapor corrosion while maintaining the thermal compatibility needed for high-temperature applications. The results of the study indicate that the multi-layer coatings produced using the APS method demonstrate improved performance over single-layer coatings. The gradient structure, with a transition from silicate-based to oxide-based materials, enhances both thermal expansion compatibility and water vapor resistance. This development not only addresses the issues associated with traditional EBCs but also provides a pathway for designing coatings that can significantly extend the lifespan of CMCs in aggressive high-temperature environments. In conclusion, the integration of APS with advanced material selection offers a promising solution for the protection of ceramic matrix composites in high-temperature applications. The combination of high thermal performance, corrosion resistance, and structural compatibility makes the developed coatings an ideal choice for use in the next generation of gas turbines and other critical components in the energy and aerospace industries. This research provides valuable insights into the future of environmental barrier coatings and their role in enhancing the performance of high-temperature materials in extreme operating conditions. Thermal barrier coatings, which are widely used in gas turbines, in conjunction with protect the metallic components of gas turbines from high temperatures and enable the production of more efficient gas turbines by increasing the combustion temperature. With the requirement for higher combustion temperatures, need for materials resistant to high temperatures (>1300°C) show up. The use of ceramic matrix composites (CMC) in gas turbines has emerged instead of the use of superalloy-based metallic materials. Even though CMC provide superior mechanical properties at high temperatures, they are corroded by water vapour in combustion waste gases. For this reason, a need has arisen for coatings called environmental barrier coating and that it will be protect CMCs from water vapor. CMC used in combustion chambers are generally produced from SiC/C composites, they have relatively low thermal expansion properties compared to oxide materials. For this reason, the materials that can provide the thermal expansion coefficient for Environmental Barrier Coatings (EBC) that can be used as coating materials are generally selected from silicate-based materials and water vapour corrosion problem arises in silicate-based materials. The aim of the study is to obtain at the outermost oxide-based coating layer by changing the composition from the substrate to the outermost with silicate-based oxides whose thermal expansion coefficient is compatible with CMC, which are more resistant to water vapor corrosion even if the thermal expansion coefficient of oxide coatings is high, and to eliminate the CTE incompatibility by applying silicate-based material at the substrate-coating interface. For this purpose, four different coating powders, mullite (3Al2O3.2SiO2), zircon (ZrSiO4), aluminum oxide (Al2O3), yttria (Y2O3), were preferred. Three different coatings were produced as Mullite-Yttria, Zircon-Alumina and Mullite-Alumina. Atmospheric plasma spray (APS) method was used as coating method.