Sıcak daldırma yöntemi ile alüminyum kaplanan düşük karbonlu çeliğe uygulanan mikro ark oksidasyon işleminin karakterizasyonu
Characterization of micro-arc oxidized unalloyed low-carbon steel after hot-dip aluminizing process
Yazar: MUSTAFA AÇIKSÖZ
Danışman: PROF. DR. MURAT BAYDOĞAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Mühendislik Bilimleri = Engineering Sciences

Özet:
Alüminyum kaplanmış çelikler düşük karbonlu çeliklerle kıyaslandığında çok daha iyi fiziksel, kimyasal ve mekanik özelliklere sahiptir. Alüminyum kaplanmış çelikler sıkça yüksek sıcaklık uygulamalarında, oksidasyon ve korozyon direncinin istendiği ortamlarda kullanılırlar. Bu çeliklerin tercih edilmesinin bir diğer sebebi ise paslanmaz çeliğe göre maliyetinin düşük oluşudur. Bu çalışmada doğrudan çelik altlığa uygulanamayan mikro ark oksidasyon işlemine uygun altlık oluşturmak için sıcak daldırma alüminyum kaplama işlemi uygulandı. Bu işlem uygulanırken kaplama sıcaklığı, süresi ve soğutma ortamının intermetalik ve kaplama tabakasının kalınlığına ve yapısına etkisi incelenmiştir.
Son yıllarda sürekli gelişim gösteren yüzey teknolojileri alanında önemli bir yere sahip olan mikro ark oksidasyon yöntemi ile her türlü alüminyum, magnezyum, titanyum ve alaşımlarına kolayca uygulanabilmekte ve yüzey özellikleri geliştirilebilmektedir. Mikro ark oksidasyon yöntemi geleneksel anodizasyon yöntemine benzemektedir ancak işlem parametreleri ve kullanılan yüksek akım/voltaj sayesinde daha gelişmiş kaplamalar elde edilebilmektedir. Bir elektrolit çözeltisi içerisine yerleştirilen anot numuneye yüksek değerlerde voltaj verildiğinde malzeme yüzeyinde oksit tabakası oluşumu başlar. Bu geleneksel bir anodik oksidasyon işlemidir. Yükselen voltaj ile birlikte kritik bir voltaj degerine ulasılır. Bozunum voltajı (dielectric breakdown) olarak adlandırılan bu kritik eşik değeri aşıldığında malzeme yüzeyinde mikro boyutlarda arklar oluşmaya başlar. Bu işlem mikro ark oksidasyon işlemi olarak tanımlanır. Diğer kaplama yöntemlerine göre oluşturduğu oksit tabakasının kalınlığı, yüksek yapışma kuvveti, porozite miktarı, düşük maliyeti ve çevreye zararsızlığı ile avantajlı bir görünüm sergilemektedir.
Bu çalışmada sıcak daldırma alüminyum kaplama yöntemiyle farklı parametrelerde kaplanan AISI1010 kalite çeliklere mikro ark oksidasyon uygulanmasıyla elde edilen oksit tabakasının morfolojik ve yapısal özellikleri incelenmiştir. Daha sonra bu numunelere sertlik deneyleri, aşınma direnci deneyleri ve yüksek sıcaklık oksidasyon deneyleri uygulanmıştır. Sıcak daldırma yöntemiyle alüminyum kaplanan çelikler ve mikro ark oksidasyonu yöntemi ile seramik kaplanan çelikler taramalı elektron mikroskobu kullanarak kalınlıkları ve yapıları incelenmiştir. XRD difraksiyon yöntemiyle faz analizi yapılmıştır. Ayrıca bu kaplamaların EDS analizi, aşınma ve oksidasyon dirençleri incelenmiştir. Elde edilen sonuçlar kıyaslamalı olarak değerlendirildikten sonra en uygun parametreler belirlenmiştir. Bu parametreleri
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etkileyen faktörler göz önünde bulundurularak en iyi sonucun elde edildiği durumlar belirlenmiştir.

Abstract:
Aluminizing is a process through which the surface of a metallic component is coated with a layer of aluminum. This process provides a strong intermetallic layer between the steel and its aluminum coating. Aluminized steel has good formability and good heat reflectivity below 450 °C and also has better resistance to corrosion than low carbon steel. Because of that aluminized steel can't be used for high temperature application. Aluminized steel is generally used for mufflers, high pressure vessels, water heaters and heat exchanger in residential furnaces. In order to use aluminized steel components at higher temperatures, hybrid metal/oxide coating structure has been proposed recently.
Low carbon steel is one of the important material for structural application because of the lower cost. The hot-dip Al coating is one of the technique for improving the oxidation resistance of low carbon steel. To protect the steel substrate from oxidizing environment, the Al layer on top of steel substrate can be oxidized to aluminum oxide layer which has high melting point and great hardness by performing high temperature oxidation. Hot-dipped aluminized steel can be used as component of exhaust system. As we know, steel is used widely in industry but its performances in bearing wear and corrosion are not optimal. Recently, some researchers report that the PEO techniques can be applied to steel, with good adhesion achieved between the steel and ceramic coating.
There are two distinct ways of plasma electrolysis techniques for making ceramic coatings on steel surface from the viewpoint of electrodes. One is cathodic micro-arc electrodeposition where the steel is used as the cathode and cathodic discharges occur during treatment. Studies of this method are limited, but it appears that the coating content comes mainly from the electrolyte. The other technique exploits an anodic barrier layer on the substrate. Since the steel material has low passivation in PEO electrolytes, the anodic voltage does not increase with treatment time, and the anodic discharge cannot occur. In order to enhance the passivation of steel, an anodic barrier layer is first deposited on substrate by a separate process.
Micro-arc oxidation (MAO), also known as plasma electrolytic oxidation (PEO), is a new processing technique which can generating oxide coatings on light metals such as Al, Mg, Ti, etc. and their alloys. It can be used to grow thick, hard ceramic coatings. This technique can offer protection against wear, heat and corrosion. This technique can be used in automotive, aerospace, medicine application. But, MAO is not applicable to steel surface directly.
There are many indirect way to apply advanced surface modification techniques to steel surface. One of them is hot dipping and micro-arc oxidation hybrid method. Steel
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surface prepared to MAO process by using HDA. So the hybrid method of MAO after HDA is very encouraging method to get ceramic coating on steel surface. Three different layer was obtained this hybrid coating method. Al-Fe intermetallic layer near the steel substrate, Al rich layer between Al-Fe intermetallic layer and as a top layer ceramic coating. The aluminum-rich intermetallic compounds are emerged in hot-dip aluminizing process; thereby it enhances the resistance of wear and oxidation on steel.
In this study, aluminum layer was prepared by HDA on AISI1010 steel surface and then ceramic surface was obtained on the aluminum layer by using MAO. Dry sliding wear tests were conducted to all aluminized and MAO'ed samples and uncoated sample at room temperature. The resistance to high temperature oxidation of aluminized steel was studied. The progress of the HDA and MAO behaviors was researched. The phase composition, the thickness of coated layers, the cross sectional micro hardness and surface morphology was investigated.
Circular samples of 1010 steel were used as the substrate for HDA in this study. The samples were cut to diameters of 5mm and 10mm length then prepared for HDA by grinding (120-400#) with abrasive paper. All the samples cleaned ultrasonically in phosphoric acid and acetone for 15 min separately and then dried. These samples were tied with stainless steel wire for dipping procedure.
As a hot dipping material, pure aluminum was utilized. Aluminum was heated to 750°C and 850°C in graphite melting pot using electric melting furnace. After the reaching desired temperature, flux added to the melted aluminum and waited for 15 min. At the end of the process, prepared samples are immersed into the molten aluminum for 2 min, 5 min and 10 min and cooled by air, furnace and water quenching
Micro arc oxidation performed 450 AC power supply for 5 min to the aluminum surface in order to build the ceramic coating which prepared with HDA for the MAO process. The solution prepared for the usage of coating confected as 4.5 g/l NaAlO2 and 2 g/l KOH in the distilled water. Stainless steel container used as an electrolyte. Also stainless steel container used as a cathode and sample used as an anode. The samples which prepared for MAO were dipped for 5min at 750°C and 850°C then cooled by air, water and furnace.
The samples were mounted for metallographic examination. To observe the cross sectional micrograph, scanning electron microscope (SEM) were used. The sample used for the examinations are polished with silica paste after grinding with the SiC sandpaper (240-2500#). Thin gold film sprayed on the prepared samples for enhance the conductivity. The phase composition of coating layers examined by EDS. Crystallographic structure obtained by using X-ray diffraction.
Micro hardness measurements were conducted by a micro Vickers hardness tester (Wilson) at the maximum load of 25 g. At least ten measurements were done from every layer and then averaged.
Wear test samples of dimension diameter 5mm and length 10mm were prepared. The surface of the wear test samples cleaned by acetone before the tests. The wear tests are performed at a normal load of 3N corresponding to vertical movement of 2mm. The worn surfaces of the samples were examined using 3D optical profilometer and SEM.
The high temperature oxidation tests were done at 1000°C for 1h, 6h, 24h and 48h. The samples aluminized at 750°C for 5 min and quenched in water due to the highest thickness of Al layer and uncoated steel samples were placed into resistance furnace. The samples were heated in air to 1000°C over 1 hour heat-up period. After reaching
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the required temperature, waited for 1h, 6h, 24h and 48h. Then, the samples were cooled to the room temperature in air.