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İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Anabilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı

7075 alüminyum alaşımı üzerinde oluşturulan mikro ark oksidasyon kaplamaların aşınma ve korozyon dayanımına zirkonya partikül takviyesinin etkisi

Effect of zirconia particle addition on wear and corrosion resistance of micro arc oxidation coatings fabricated on 7075 aluminum alloy

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Özet:

Alüminyum ve alaşımları şekillendirilebilirlikleri, özgül dayançları, iyi seviyede kimyasal ve fiziksel özellikleri ile ön plana çıkmaktadırlar Endüstride çelikten sonra kullanılan en önemli metal konumundadır Özellikle ağırlık kazancının kritik olduğu havacılık ve otomotiv sektörlerinde geniş kullanım alanına sahiptir Alüminyum alaşımları genel olarak bakıldığında, yüzeylerindeki nano seviyedeki oksit tabakası ile iyi korozyon direnci sergilemektedir Ancak agresif ortamlarda bu oksit tabakası yetersiz kalmaktadır Özellikle 7075 alüminyum alaşımı bileşiminde bulunan demir ve bakır elementleri tane sınırlarında bu elementlerce zengin intermetalikler oluşturarak malzemenin çukurcuk, taneler arası ve eksfoliasyon gibi lokalize korozyon türlerine eğilimini arttırmaktadır Aynı zamanda 7075 alaşımı kullanım alanında sac, plaka veya ekstrüze ürün olarak kullanılması kaynaklı yönlenmiş bir tane yapısına sahip olmaktadır Sıcak işlem sırasında mikroyapıda bulunan çökeltiler tanelerin yeniden kristalleşmesini sınırlayarak taneler işlem yönünde yönlenmiş halde kalmaktadır Korozyon hassasiyeti yanında yanında alüminyum alaşımlarının düşük aşınma direnci servis ömrünü kısaltıp, kullanım alanını daraltmaktadır Alüminyum alaşımlarının kullanım alanını arttırmak adına anodik oksidasyon başta olmak üzere çeşitli konvansiyonel yüzey modifikasyon işlemleri mevcuttur Ancak bu uygulamalar ekonomik, çevresel ve verimlilik açısından bakıldığında yetersiz kalmaktadır Son yıllarda çalışmalar daha verimli yöntemlerin bulunması üzerine yoğunlaşmıştır Mikro ark oksidasyon (MAO), son yıllarda çalışılan, çok yüksek sertlikte ve yüksek korozyon direncine sahip poroz seramik yüzeylerin elde edilebildiği elektro kimyasal bir kaplama tekniğidir Elde edilen kaplamalar alt katmana sıkıca tutunmaktadır Alüminyum alaşımları dışında, magnezyum, titanyum ve alaşımları gibi hafif metallerinde kaplanabildiği çevreci ve verimliliği yüksek bir kaplama yöntemidir MAO yöntemi ile elde edilen kaplamaların en büyük dezavantajları yüksek poroziteye sahip olmaları, sınırlı kompozisyon ve yüksek enerji tüketimidir Özellikle korozyon direnci açısından porozitelerin azaltılması önemlidir Bu amaçla kaplama elektrolitlerine çeşitli partikül takviyeleri yapılarak kolloidal çözeltiler içerisinde kaplamalar yapılmaktadır Partikül takviyeli elektrolitlerde en büyük problem çözelti stabilitesini sağlamak olup çeşitli çözelti stabilizatörleri ile bu durumun önüne geçilmeye çalışılmaktadır Katılan çözelti stabilizatörleri aynı zamanda partiküllerin yüzeylerini daha yüklü hale getirerek kaplama yapısına katılma oranlarını yükselttiği düşünülmektedir ...

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Summary:

Aluminum and its alloys can be characterized by their good formability, low density, high specific strength, good chemical and physical properties, high thermal and electrical conductivities It is the most important metal used in the industry after steel Aluminum alloys, which have a wide range of application field from automotive to households, are also attractive for aircraft industries, where weight saving is critical In general, aluminum alloys exhibit good corrosion resistance owing to presence of compact nano-oxide layer on their surface However, this oxide layer may not be sufficient in certain aggressive environments Especially, 7075 aluminum alloy is prone to localised corrosion types such as pitting, exfoliation and intergranular corrosion due to intermetalics rich in copper and iron present in microstructure Furthermore, recrystallization of the grains is limited during hot forming process because of the precipitates present in the microstructure and grain structure remains elongated in the process direction For all these reasons, corrosion resistance decreases Except corrosion resistance, the low wear resistance of aluminum alloys shortens the service life and narrows the usage area In order to overcome these problems, it is necessary to improve the properties of aluminum alloys by applying various surface treatments There are various conventional surface modification processes like anodic oxidation, chromate based conversion coating, electroplating and paint applications for aluminum alloys However, these practices are insufficient in terms of economical, efficiency and environmental Therefore, the use of an alternative technique to these methods has become widespread Micro arc oxidation (MAO) process, which is also known as micro-arc discharge oxidation, spark anodisation or plasma electrolytic oxidation (PEO), is an electrochemical coating technique which porous, very hard and high corrosion resistant coatings can be obtain on light metal alloys and the resulting coatings are firmly attached to the substrate metal MAO technique has been widely researched and applied in the last 10 years as a new surface coating process In addition to aluminum alloys, magnesium, titanium and zirconium alloys, which are called as valve metals can be coated with this easy, environment friendly and efficient method System does not use fossil fuel so there is no toxic gas emission Another advantage of the process is the complex parts can be coated This technique is based on that the workpiece, which is immersed in an electrolyte, is coated with hard, wear and corrosion resistant oxide layer due to the plasma discharges caused by plasma chemistry and electrochemical reactions in the process electrolyte However, high porosity and high energy consumption are main restrictions for MAO coating In some fields, like biomedical applications, porosity is an advantage But this is a disadvantage in aircraft, automotive and shipping industries where the threat of corrosion is high Generally, features of MAO coatings depends on their morphology and composition and these are determined by electrolyte and process parameters Applied electrical parameters change MAO coatings characteristics but it is insufficient to avoid hight porosity for MAO coatings Different additives to electrolytes can be used to change the coating morphology and composition In recent years, it has been remarkable that low porosity, corrosion and wear resistant composite coatings have been obtained by the additives made to MAO electrolytes The additives can be compounds, which dissolve in the electrolyte, containing different metal ions or may be various sizes of non-water soluble particles such as SiO2, Al2O3, B4C, ZrO2 and non-oxide particles Especially zirconia (ZrO2) is promising due to its high strength, chemical and mechanical stability and excellent corrosion resistance However, homogeneous dispersion of additive particles in electrolyte is the most difficult task Various stabilizers are used for better suspension of the addivies in the electrolyte The surface charge and stability in the solution of the particles are evaluated according to the Zeta potential A higher absolute zeta potential particle exhibits more stable behaviour in solution A high zeta potential is preferable in electrophoretic process as this utilizes the rate of particle movement in a given electrical field In MAO process, substrate material is anode and it has positive charge So the negative zeta potential is preferred for the process In this study, we aimed to improve the corrosion and wear resistance of the 7075 aluminm alloy by MAO process For this purpose MAO process was applied in the three different type of electrolyte First one was the base electrolyte consisting of sodium aluminate (NaAlO2), sodium phosphate (Na3PO4) and potassium hydoxide (KOH) To examine the effect of zirconia addition into the electrctrolyte on the charactersitics of the coating, m-ZrO2 particles were introduced into the base electrolyte As the third different electrolyte, to provide more negative Zeta potential for the zirconia particles to ensure their better suspension, sodium polyacrylate (Na-PA) was also added into the electrolyte For the determination of phases in the coating, analysis was performed by X-ray diffractometer (XRD-Philips PW3710) Surface rougness of samples was measured by Veeco Dektak surface profilometer Thickness of coating was measured by Fischer Dualscope MP20E-S which works with Eddy Current principle Surface and cross sections of samples were examined light optical microcope and, by energy dispersive spectrometer (EDS) equipped JEOL NeoScope JCM6000 Plus and Hitachi TM-1000 scanning electron microscopes (SEM) After characterization of the samples wear test was carried out at room temperature on a reciprocating wear tester (Tribotech Oscillating Tribotester) Test load, sliding distance, counterface were, 4N, 100 m and Al2O3 ball with 6mm radius, respectively Tests were carried out at three different speeds 5, 10 and 15 mm/sec Immersion corrosion test was applied in the solution prepared accordig to BS EN ISO 11846:2008 standard and the results were also evaluated according to the standard Phase analysis showed that coatings which formed in base electrolyte contained meta-stable phase of alumina (γ-Al2O3) By addition of zirconia particles coatings contained phases of γ-Al2O3, monoclinic and tetragonal zirconia (m-ZrO2 and t-ZrO2) Presence of zirconia particles in the base electrolyte provided better growth rate for the oxide layer, while Na-PA addition effected negatively the thickness of the coating ...

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