Termal bariyer kaplama sistemlerinde yüksek sıcaklıkta düşük çevrimli yorulma şartları altında oluşan hasarların incelenmesi

Termal bariyer kaplamalar (TBC); gelişmiş türbinli motorların yanma odası ve türbin gibi sıcak bölümlerinde, parçaları sıcak gazların olumsuz etkilerinden korumak amacıyla kullanılan ve iki ya da daha fazla katmandan oluşan koruyucu kaplamalardır. Günümüzde güvenilir bir ömür tahmini olmadığından bu kaplamaların potansiyeli tam olarak değerlendirilememektedir. Termal bariyer kaplamalardaki hasar mekanizmaların ın iyi bir şekilde anlaşılması, kaplamanın sağlamlığının ve güvenirliğinin arttırılması için kilit faktördür. Bu çalışmada ilk önce TBC sistemleri kısaca tanıtılmış, ardından yüksek sıcaklıkta düşük çevrimli yorulma deneyleri sonucunda oluşan hasarlar ve olası nedenleri açıklanmıştır. Altlık malzemesi olarak yönlü katılaştırılmış nikel bazlı bir süperalaşım olan Inconel 100 (IN 100 DS), bağ kaplama olarak yaklaşık 120 μm kalınlığında bir NiCoCrAlY tabakası, üst kaplama olarak da yaklaşık 200 μm kalınlığında, ağırlık olarak %7 itriyum oksit ile kısmen kararlı hale getirilmiş zirkonyum oksit içeren seramik tabakası kullanılmıştır. Her iki tabaka da EB-PVD tekniği kullanılarak hazırlanmıştır. Yapılan deneyler sonucunda çatlak sayısı ve çatlak boyutunun uzama aralığıyla doğru orantılı bir şekilde değiştiği saptanmıştır. Çatlaklar TGO/BC arayüzeyinde başlamakta ve altlığa doğru yükleme eksenine dik bir şekilde ilerlemektedirler. Seramik termal bariyer kaplama tabakası çatlak oluşumunu geciktirmektedir. Yalnızca bağ kaplamaya sahip numunelerin yüzeylerinde, türbin kanatçıklarında görülen tipik servis hasarlarından birisi olan buruşma saptanmıştır.

Failure of thermal barrier coatings under high temperature low cycle fatigue conditions

Thermal Barrier Coatings (TBCs) are multi-layer protective coatings used in the hot section components such as combustor and turbine of advanced gas turbine engines to protect them from degrading affects of hot gases. Today, due to lack of a reliable life time assessment, the potential of these coatings cannot be fully used. Understanding of damage mechanisms of thermal barrier coatings is the key factor to increase durability and reliability. In this paper TBCs are shortly introduced and then damages resulting from high temperature low cycle fatigue tests and their probable reasons have been explained. Inconel 100, a directionally solidified nickel based superalloy as a substrate, approximately 120 μm thick NiCoCrAlY bond coat and approximately 200 μm thick 7 wt % Yttria Stabilized Zirconia ceramic top coat have been used. Both layers are deposited by EB-PVD technique. The results of the tests show that number and the size of the cracks changes with the strain range. Cracks initiate in the TGO/BC interface and propagate into the substrate perpendicularly to the loading axes. Ceramic thermal barrier coating retards the formation of cracks. On the surfaces of the specimens having only bond coat, rumpling which is a typical service damage of turbine blades has been detected.

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