Hull hücresi yardımıyla ni-w alaşım kaplama özelliklerinin belirlenmesi
Optimization of properties of electrodeposited ni-w alloys with hull cell
Yazar: MİRAY EKMEKÇİLER
Danışman: PROF. DR. MUSTAFA ÜRGEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İleri Teknolojiler Ana Bilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Mühendislik Bilimleri = Engineering Sciences
Dizin: Elektrodepozisyon = Electrodeposition ; Metal alaşımlar = Metal alloys ; Metalik alaşımlar = Metallic alloys ; Nikel = Nickel ; Tungsten = Tungsten

Özet:
Ni- W alaşımları son zamanlarda gösterdikleri yüksek ve gelişmiş özelliklerden dolayı ilgi çeken alaşımlar grubuna girmişlerdir. Bu alaşımlar yüksek aşınma, korozyon direnci, yüksek sıcaklıklarda termal kararlılık, gelişmiş tribolojik özellikler gösterirler. Aynı zamanda kaplama endüstirisinde önemli bir yeri olan sert krom kaplamaların zararlı etkilerinden dolayı krom kaplamaların yerini almaktadırlar. Ni-W alaşımlarının üretimi, komposizyonu, yapısı ve özellikleri arasında sıkı bir bağ vardır. Elektro biriktirme yöntemiyle üretilen Ni-W alaşımlarında özellikler kaplama parametrelerine bağlı olarak değişir ve istenilen özellikler geliştirilebilir.Ni-W kaplamalarda W içeriğine ve kaplamanın birikme özelliklerine bağlı olarak yapısal ve işlevsel özellikleri değişmektedir. Bu özellikleri etkileyen en önemli parametrelerden biri akım yoğunluğudur. Hull hücresi deneyleri ile akım yoğunluğu etkisini tek bir deneyle elde ederek diğer çalışma koşullarının alaşımın kimyasal komposizyon, yüzey morfolojisi, kalınlık ve sertlik üzerine olan etkilerini belirleyebiliriz.Bu çalışmayla birlikte Hull hücresi kullanılarak elektrobiriktirme yöntemiyle üretilen Ni-W alaşımlarının akım yoğunluğuna bağlı olarak yapısal ve kimyasal özelliklerindeki değişiminin incelenmesi amaçlanmaktadır.Deneyler iki aşamada gerçekleşmiştir. İlk olarak, Ni-W alaşımlarının özelliklerine akım yoğunluğu, pH ve sıcaklık gibi diğer parametlerin etkisini belirlemek için ve belli özelliklere sahip Ni-W alaşımını optimize edebilmek için Hull hücresi deneyleri yapılmıştır. Bu aşamada pirinç plakalar katot malzemesi olarak kullanılmış ve Hull hücresi deneylerine uygun hale getirilmiştir. Kaplama esnasında nikel banyoları için uygun bulunan 2 amper akım uygulanarak 10- 0.2 A/dm2 arasında değişen farklı akım yoğunluklarına sahip bölgeler elde edilmiş ve işlemler 10 dakika sürmüştür. Öncelikle 0.1 M NiSO4.6H2O, 0.3 Na2WO4.2H2O ve 0,28 M C6H8O7.2H2O içeren çözelti de kaplama işlemi pH=3,5 olacak şekilde 40-50-60oC sıcaklıklarında elektrobiriktirme işlemi yapılmıştır. EDS analizleri sonucu 40oC sıcaklıkta yapılan çalışmada yüksek akım aralığında (10- 5,4 A/dm2) alaşımda oksijene rastlanmıştır. Daha sonra kaplama banyosuna amonyak NH3 ilave edilerek pH= 5 ve 7 için sıcaklıklar 50 ve 60oC'de tutularak deneyler tekrarlanmıştır. Bu deneyler sonucu elde edilen numunelerinde farklı akım yoğunlukları bölgerinin (10.0- 8.6, 8.6- 5.4, 5.4- 3.2, 3.2- 2.2, 2.2- 0.9, 0.9- 0.2 A/dm2) EDS, SEM analizleriyle komposizyon ve mikroyapılarına bakılmış kalınlıkları ölçülmüştür. Sonuçları bakıldığında alaşımdaki W konsatrasyonu,alaşım kalınlığı ve yüzey özellikleri (çatlak içermeyen ve nodülleşmenin olmadığı) esas alınarak uygun pH, sıcaklık ve akım yoğunluğu seçilmiştir. Buna göre yüksek W konsantrasyonu (yaklaşık at % 15-13 W) pH= 5, sıcaklık 60oC ve akım yoğunluğu yaklaşık 3A/dm2 olduğu koşullarda elde ediliyor. Orta W konsantrasyonuna (at %10-11) sahip alaşım yaklaşık olarak sıcaklığın 500C pH'ın 5 ve akım yoğunluğunun 2 A/dm2 olduğu durumda elde edilmiştir. Düşük W konsantrasyonuna (yaklaşık at % 8-6 W) sahip alaşım ise pH=3,5 sıcaklık 50oC ve akım yoğunluğu yaklaşık 1 A/dm2 olduğu koşullarda elde edilmiştir. Yüksek W ve düşük W konsantrasyonlarının sahip olduğu bu deney koşulları esas alınarak çelik plaka üzerine kaplamalar tekrar yapılmıştır. Bu plakaların SEM, EDS analizleri, kalınlık, sertlik ölçümleri yapılmış ve ısıl işlem sonrası tekrar analizler yapılmıştır. Deneyler sonucunda Ni-W alaşımlarının özellikleri ve yapısının banyo koşullarına çok bağlı olduğu görülmüştür. Hull hücresi akım yoğunluğu etkisini tek bir işlemle belirleyebilme avantajı sağlamıştır. Buna göre yüksek W konsantrasyonu yüksek pH'larda yüksek sıcaklıklarda elde edilmektedir. Ancak düşük pH'larda alaşımdaki W konsantrasyonu sıcaklığa bağlı olarak pek değişmektedir. Genel olarak akım yoğunluğuna arttıkça alaşımdaki W miktarı da artmaktadır. Bütün bu işlemlerin sonunda istenilen özelliklere sahip Ni-W alaşımının hangi banyo koşularından biriktirilebiceği belirlenmiştir.

Abstract:
Ni-W alloys have become one of the preferred alloys group because of their excellent and improvable properties. Ni- W alloys exhibit higher wear, corrosion resistance, thermal stability in high temperatures, and andvanced tribological behaviours. Moreover, when Ni- W alloys are compared with Ni coatings, they show better properties. At the same time, due to the toxic and cancerogenic effects of hexavalent Cr6+ ions, hard chrome coatings which have an important role in coating industry have been replaced by the Ni- W alloy coating. They also exhibit good electrocatalytic, magnetic, electrical properties. The interest of Ni- W alloy has increased because of their applications in electromechanical systems (MEMS), barrier layers or capping layers in copper metallization for ultra large scale integration (ULSI) devices, resistors, mold inserts, otomotive and aerospace industry, magnetic heads, magnetic relays, resistors and instead of chromium plating which has hazardous effects to environment and people.Electrodeposition is one of the adjustable, accessible and inexpensive method that is used to produce advanced materials in bulk form or as coatings. Electroplating of tungsten alloys attrack interests because this metal enhance the hardness and corrosion resistance of its alloys. However tungsten is not electrodeposited from its aqueous or nonaqueous solvent. Iron group transition metals like Ni are required to deposite tungsten alloys. Electrodeposition is one of the best method to get Ni- W alloy in inexpensive way. A typical bath for Ni- W alloy consists Na2WO4, NiSO4 and complexing agents for both nickel and tungstate ions. It was found that concentration of nickel ions limits the tungsten content of the Ni- W alloy. However it is interesting that tungsten can not be electrodeposited unless there is at least one of the iron group metals.There is a strong relation between the production methods, composition, structure and properties of Ni- W alloy coating. Therefore the properties of these alloy depend on the parameters that are used during the electrodeposition and can be developed in accordance with the desired properties.The functional and structural features of Ni- W coatings can vary depending on the tungsten content in the alloy and the deposition parameters used during production. One of the most important parameters is current density that effects the properties of alloy. Hull cell was developed to indicate the effects of current density by reducing the number of experiments. Hull cell provide us to get different current density regions on one plate in one experiment because the distance between anode and cathode change along the cell. The closest distance between anode and cathode results the highest current density, therefore causes burned region. The farest distance between anode and cathode results lower current density.The other parameters that have affects on the alloy properties like chemical composition, surface morphology, thickness and hardness of coating can be determined by utilizing the hull cell.The purpose of this study is to determine the chemical and structural variations of electrodeposited Ni- W alloys depending on the exchange in pH, temperature and current density that is provided by using hull cellThe experiments was done in two steps which include hull cell and then water jacketed cell experiments. In the first step hull cell was used to observe the parameters impacts like current density, pH, temperature on electrodeposited Ni- W alloy?s properties. In this step the parameters effects on chemical and structural properties was optimized through Hull cell. In this step brass plates were used as cathode material and pure nickel (%99.8) was used as anode material. The plates were cut so as to have 100 mm length and 75 mm height and were accomodated to hull cell experiments. According to literature, for the hull cell experiments which contain nickel bath, 2 amper current was chosen appropriate and therefore we used 2 amper and get different current density regions that vary between 10- 0.2 A/dm2 along the brass plate. The information taken from the literature defines hull cell experiments contain nickel solution last 10 minutes. For this reason ,our experiments were also endured 10 minutes.The bath composition consisted 0.1 M NiSO4.6H2O, 0.3 Na2WO4.2H2O and 0,28 M C6H8O7.2H2O. Citrate was used for the complexing agent. Firstly, the pH of the bath was 3.5 and three selected temperature (40-50-600C) was used. The results of the energy dispersive X-ray spectroscopy (EDS) analysis indicated the alloy which was handled in 400C temperature included oxygen in the 10- 5.4 A/dm2 current density region. After that, NH3 was added to the solution and the pH of the solution was increased to 5 and 7. At this time two selected temperature (50-700C) were chosen for these pH levels because we get oxygen in the 400C therefore we eliminated this temperature and rehearsed the experiments. We get different current density regions that varies between 10-8.6, 8.6- 5.4, 5.4- 3.2, 3.2- 2.2, 2.2- 0.9, 0.9- 0.2 A/dm2. Energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) analysis were made to obtain composition and micro structure of all of these regions. The thickness of the samples were measured by Fisherscope Xray XDL. We used Ni/CuZnPb product for the thickness measurement. In additition we could calculate the Ni thickness (µm) of the plate.When all the results such as W concentration in the nickel- tungsten alloy and the thickness and microstructure and the surface characteristic (no cracked and nodular structure) of the alloy were examined, appropriate pH, temperature, current density were selected for the water jacketed experiments. According to that high W concentration (about at W % 15-13) in the alloy was obtained at pH= 5, temperature 600C and current density was about 3 A/dm2. Medium W concentration (about at W % 10- 11) in the Ni- W alloy was obtained at pH was about 5, the temperature was about 500C and the current density was about 2 A/dm2. Low W concentration in the alloy was obtained (about at W % 8- 6) at pH was about 3.5, the temperature was 500C and current density is about 1 A/dm2.In the second step, the experiments were repeated for the bath condition which provided us to get high, medium and low tungsten concentration in the alloy. In this step water jacketed cell was used and steel plates were chosen as a cathode material and pure nickel (%99.8) was selected as an anode material. The electrodeposition process had proceeded one hour to get the thickness 3 µm for low, 7 µm for medium and 13 µm for high tungsten concentrated Ni- W alloy. The experiments were conducted at three different conditions that provide high, medium, and low tungsten concenrated in the alloy.One of these condition was that the temperature was about 600C, pH was about 5, current density 3 A/dm2 and the solution included 0.1 M NiSO4.6H2O, 0.3 Na2WO4.2H2O and 0,28 M C6H8O7.2H2O and NH3 for the pH adjustment.The other condition was that the temperature was about 500C, pH was about 5 and current density was about 2 A/dm2 and the solution included 0.1 M NiSO4.6H2O, 0.3 Na2WO4.2H2O and 0,28 M C6H8O7.2H2O and NH3 for the pH adjustment.The third condition was that the temperature was about 500C, pH was about 3.5 and current density was about 1 A/dm2 and the solution included 0.1 M NiSO4.6H2O, 0.3 Na2WO4.2H2O and 0,28 M C6H8O7.2H2O. This type of bath did not include NH3. The energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis thickness and hardness of these Ni- W coatings on steel substrate were conducted. Moreover heat treatment was applied to all samples. The samples were kept in the oven at 4000C and 5000C in one hour. All hardness measurements and XRD analysis were performed again after the heat treatment for all samples. It was observed that the hardness values of the Ni- W alloys were increased after the heat treatment.Consequently, the relation between the Ni- W alloy properties and structures and the bath conditions is very close and strong. The characteristics of the alloy depends on the production method, temperature, pH, current density, concentration of Ni2+ ions, concentration of WO42- ions and also concentration of citrate and the other complexing agents in the solution. Controlling of bath conditions helps us to get Ni- W alloys which have desired properties. We took the advantage of hull cell, the effects of current density were determined in a few number of experiments. The other effects like temperature and pH were investigated.In conclusion, high tungsten concentration of the Ni- W alloy is achieved at high pH levels and high temperature. However when pH is reached at seven, then precipitation is observed in the solution. Commonly, the concentration of tungsten of the nickel- tungsten alloy is rised while the current density is promoted on the substrate. At the end of all of these processes, using proper bath solutions and conditions provides us Ni- W alloy that have desired properties. Moreover, the properties and characteristic of the Ni- W alloy can be improved by using appropriate conditions.