Seramik kaplamalarda kırılma tokluğunun sonlu elemanlar yöntemiyle analizi

Abstract

ÖZET Anahtar Kelimeler : Sonlu elemanlar yöntemi, termal bariyer kaplamalar, kırılma tokluğu, plazma püskürtme tekniği Bu çalışmada, yaygın olarak kullanılan kaplama-altlık malzeme kombinasyonu için farklı boyuttaki çatlakların, gerilim şiddet faktörleri sonlu elemanlar yöntemiyle hesaplanmıştır. Gerilim şiddet faktörleri üzerine çatlak boyu, kaplama kalınlığı ile malzeme özelliklerinden termal genleşme katsayısı, termal iletkenlik ve elastisite modülleri gibi kaplama parametrelerinin etkileri incelenmiştir. Çalışma şartlarında, mekanik ve termal uyuşmazlıktan dolayı hasara en yatkın olan arayüzeyde bir kenar çatlağı model için kullanılmıştır. Deneysel olarak ise, farklı kaplama kalınlığı kombinasyonları için ZrCb-St 37 çeliği kaplama sisteminin termal çevrim ömrü belirlenmiştir. Sonlu Elemanlar Yöntemi çalışmasında, malzeme özelliklerinin birbirlerine yakınlığı nedeniyle altlık malzeme olarak Küresel Grafitli Dökme Demir (KGDD) ve kaplama malzemesi olarak Z1O2 kullanılmıştır. Z1O2 kaplama kalınlıkları 200 um, 400 um ve 800 jım olarak seçilmiştir. Altlık malzeme kalınlığı 4 mm olarak sabit tutulmuştur. Sonlu elemanlar çalışmalarında, 1.2 mm, 2.4 mm ve 4.8 mm' lik çatlak boylan kullanılmış ve modelin boyu 24 mm olarak sabit tutulmuştur. Malzeme özelliklerinin de sonuç gerilim şiddet faktörü değerleri üzerine etkisinin incelenmesi amacıyla standart malzeme özelliklerinden, her analizde yalnızca biri olmak üzere K2j02> aZr02> EZr02> ve ErGDD değerlerinde değişimler yapılmıştır. Yapılan analizler neticesinde, malzeme özelliklerinin gerilim şiddet faktörleri üzerinde önemli bir etkisinin olduğu bulunmuştur. Bunlardan kaplamanın termal genleşme katsayısının ve her iki bileşen malzemenin elastisite modüllerinin önemli bir etkisinin olduğu belirlenmiştir. Kaplama kalınlığındaki artış, gerilim şiddet faktörleri değerlerinde artışa neden olmuştur. Genel olarak çatlak boyundaki bir artış, gerilim şiddet faktörlerinin artmasına etki etmektedir. 200 um, 400 um ve 800 um' lik MgZrC>3 kaplamalar ve St 37 altlık malzemesi kullanılarak yapılan deneyler neticesinde, en uzun termal çevrim ömrünü ince kaplamalar göstermişlerdir. Termal çevrim deneyleri sonucunda hasar görünüşünden tesbit edilen hasarın, termal yük ve arayüzey oksidasyonu esaslı olduğu bulunmuştur.

FRACTURE TOUGHNESS ANALYSIS OF CERAMIC COATINGS BY FINITE ELEMENT METHOD Keywords ; Finite element technique, thermal barrier coatings, fracture toughness, plasma spray technique SUMMARY New materials are needed, because of meeting more stringent requirements for technological developments. Usually materials which will be used in severe operating conditions are more expensive than conventional materials. Therefore, surface preparation techniques were used to make convenient material combinations in usage of high technological requirements. Coatings are mostly used for many engineering applications in order to improve the surface properties of components. High temperature coatings can be used to reduce the base metal temperature regarding thermal barrier coatings; however, resistance to hot corrosion and oxidation is again mandatory. Moreover, thermal barrier coats reduce substrate air cooling requirements in the case of gas turbine engines. Plasma is the highly ionized state of mass, consisting of molecules, atoms, ions, electrons and light quantums. H2, N2, Ar and He are extremely used as plasma gases. A plasma is a very high energy state and thus transfers heat very fast to the powder, reducing the necessary dwell time at high temperature which minimizes by the inert nature of the heat source. Plasma spraying equipment consists of a complex of individual apparatuses and devices, for example the plasma torch, power unit, cooling system, powder and gas feeder units. In such a torch, the electric arc is ignited between the thoriated tungsten cathode and copper anode in the form of nozzle and heated by the electric arc energy to a high temperature, causing the dissociation of one atom gases and dissociation and partial ionization of two atom gases. The plasma gas transfers via dissociation and ionization processes, into a plasma state, and a large volume of thermal and kinetic energy is released which leaves the nozzle in the form of a plasma beam. The temperature in the plasma arc center even attains around 30000 K. The powder is melted by temperature taken up by the beam and thrown on the substrate at high xivelocity. Metals, ceramics, cermets and plastics can be coated by spraying. Plasma spraying rarely heats the substrate over 300°C and it is required to keep the substrate temperature using air cooling in the range of 200-250°C. Optimization of plasma spraying processes has been attempted to decrease coating porosity and achieve better adherence. The deposition efficiency is strongly influenced by both particle size and distribution. Most of the particles must be molten before impingement to produce dense deposits and must have sufficient velocity to splat into the irregularities of the previous splats. Interaction of the molten material with the plasma beam and surrounding atmosphere affects a physical and chemical the transformation of the particles in the plasma beam melt. Coating properties are affected by many technological parameters. Most important are the electrical current, flow rates of the plasma forming gases, the spraying distance and environment. Thermal barrier ceramic coatings have exhibited very good oxidation resistance. At high temperature, oxygen can easily diffuse to metallic bond coat, e.g. NiAl owing to the high porosity, segmentation and thermal conductivity of ZrCb and MgZrC»3 in the top coat. The bond coat suffers from oxidation attack. An oxide layer (CT2O3 and AI2O3) has attained critical thickness, cracks first be observed. With increasing oxidation attack the ceramic coatings start to spall. Finite element technique is a computerized technique for the most widely used, large scale, and a general purpose for engineering analysis. This technique is used worldwide for solutions to design challenges in the aerospace, automotive, power, consumer machinery, biomechanics and electrical/electronics industries. Finite element technique uses nodal displacements at any time increment for transient analysis. These nodal displacements can be then converted into the stresses. In this finite element study, fracture toughness were investigated using ZrCb coating and spheroidal cast iron substrate under the thermal stresses. For computer analysis a Z1O2 coating thickness was chosen as 200 urn, 400 urn and 800 urn. Substrate thickness was chosen as 4 mm. An interfacial crack was used in the analysis, having lengths of 1.2 mm, 2.4 mm and 4.8 mm. Some material properties of coating and substrate were hypothetically changed to identify material properties against fracture toughness. In this experimental study, thermal cycling life of MgZrC»3 coating and St 37 steel substrate was investigated. Coating thickness was chosen as 200 urn, 400 (am and xii800um. The thickness of substrate was kept as 4 mm. Thermal cycling life tests were` conducted at a constant temperature of 1025°C. Each coated sample was put into a furnace heated to a desired temperature before, retrieved at regular intervals and cooled down in air atmosphere to room temperature in order to measure the thermal cycling failure. The finite element analysis and experimental results can be summarized as follow : 1. Material properties of coatings have very significant importance in the result of fracture toughness. 2. While the coating thickness is high, failure probability will also be high. 3. The stress intensity factors of coatings are increased with increasing the interfacial crack length. 4. Metallographic evaluations of coatings showed pores in coating layer and lamellar structure of coating. 5. Thermal cycling tests exhibited two different failure mechanisms as thermal stresses and interfacial oxidation. xm