Saf titanyum malzeme yüzeyinde titanyum aluminyum intermetalik tabakasının oluşturulması
To constitute a titanium aluminium intermetallic layer on the surface of pure titanium
Yazar: MUSTAFA SAFA YILMAZ
Danışman: PROF. DR. EYÜP SABRİ KAYALI ; YRD. DOÇ. DR. ERDEM ATAR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Kaplama = Coating ; Titanyum = Titanium ; Titanyum alaşımları = Titanium alloys

Özet:
Titanyum ve titanyum alaşımları üstün korozyon direnci, düşük yoğunluk, düşük elastik modülü, biouyumluluk ve yüksek mukavemet gibi özelliklerinin iyi bir kombinasyonuna sahiptir. Bu özellikler son zamanlarda titanyuma olan ilginin artmasını ve bu malzeme üzerinde çalışmaların hızlanmasını sağlamıştır. Birçok alanda diğer metallerden daha yüksek karakteristik özelliklere sahip olan titanyum, zayıf ve yetersiz yüzey özellikleri sergilemektedir.Titanyum malzemelerin yüzeyleri sürtünmenin olduğu kullanımlarda problem oluşturmaktadır. Yüksek sürtünme katsayısına sahip yüzeylere karşı adhesif aşınma gösteren titanyumun yüzey özelliklerinin geliştirilmesi amacıyla bu çalışmaya başlanmıştır. Yüzey karakteristiğini geliştirme yöntemlerinden biri olan Elektro Spark Biriktirme prosesi kullanılmıştır. Bu proseste altlık malzeme ile elektrod arasında mikro alaşımlama prensibine dayalı bir kaplama yöntemi uygulanmıştır. Bu doğrultuda malzeme yüzeyinde titanyum alüminyum intermetalik fazı elde edilmeye çalışılmıştır.Bu yüksek lisans tezinde ticari saflıktaki titanyum malzemenin yüzey özelliklerinin geliştirilmesi için ve geleneksel bir yöntem olan Elektro Spark Biriktirme (ESD, Electro Spark Deposition) prosesi kaplama yöntemi olarak seçilmiştir. Deneysel çalışmalarda altlık malzeme olarak ticari saflıktaki titanyum ( Kalite 2, Cp-Ti Grade2) numuneler, elektrod olarak da ticari saflıktaki alüminyum çubuk kullanılmıştır. Numuneler üzerinde daha kaliteli kaplama tabakası elde etmek amacıyla Elektro Spark Biriktirme (ESD) prosesinin parametreleri incelenmiş ve prosesin daha efektif kullanılması için gerekli şartlar araştırılmıştır.Yapılan ön deneyler sonucunda saf titanyum malzemenin yüzey özelliklerinin geliştirilebileceği düşünülmüş ve detaylı çalışmalar başlatılmıştır.EDS prosesinde kullanılacak parametrelerin belirlenmesi aşamasında hem cihazın teknik yeterlilikleri hemde kullanılan malzemelerin prosese uyumu konuları gözönünde bulundurulmuştur. Bu doğrultuda proseste kullanılan teknolojik parametreleri; akım şiddeti: 100-300-500 Amper, pulse süreleri: 50-100 ?s, olarak seçilmiştir. Belirlenen parametreler ile 1, 2 ve 3 paso kaplamalar yapılması kararlaştırılmıştır. ESD cihazında birçok parametrenin değiştirilmesi mümkündür fakat çalışmanın sonuçlarına önemli bir etkisi olmadığı düşünülen parametrelerin sabit bir değerde tutulmasına karar verilmiştir. Bu amaçla cihazda sabit tutulan parametreler ; kullanılan yük: 2000 mC, her bir kaplama tabakası için harcanan süre: 60 saniye, kullanılan pulse ların şekli: kare ve elektrod tutucu tabancanın titreşim hızı sabit tutulmuştur.Farklı teknolojik parametrelerle saf titanyum malzeme yüzeyine yapılan kaplamanın karakteristik özelliklerindeki değişimlerin incelenmesi için deneysel çalışmalar yapılmıştır.Titanyum malzemenin yüzeyine yapılan kaplamaların karakterizasyon araştırmaları kapsamında; yüzey pürüzlülük deneyleri, elde edilen kaplamaların kalınlığı ölçümleri, yüzeyden ve kesitten elemental mikroyapı incelemeleri, X ışınları difraksiyon analizleri (XRD), taramalı elektron mikroskobu (SEM) ile yüzey morfolojisi ve enine kesit incelemeleri, yüzeyden ve enine kesitten mikro sertlik testleri, yapışma mukavemeti için Rockwell C testi, ESD prosesinde gerçekleşen kütle transferleri analizleri çalışmaları yapılmıştır.Yapılan deneysel çalışmalar neticesinde tezin amacı olan saf titanyum malzemenin yüzeyinde titanyum-alüminyum intermetaliği oluşturulmuştur. Numunelerin yüzeyinde elde edilmesi amaçlanan intermetalikler (TiAl ve Ti3Al) haricinde TiAl3, AlTi-O2, Al11Ti5, TiN fazları da gözlemlenmiştir. Numunelerin yüzeyleri kaplama sayısı arttıkça daha az pürüzlülük sergilemiştir. Yüzeyden alınan sertlik değerlerinde parametrelere bağlı olarak büyük bir farklılık görülmemiştir. Enine sertlik çalışmalarında kaplamadan altlık malzemeye doğru sertliği azalan grafikler görülmüştür. Uygullanan amperin artışı (100, 300, 500 amper) ve uygulama süresindeki (50, 100 ?s) artış kaplama kalınlığında artış sağlamıştır. Kaplamanın kesitinden ve yüzeyinden elemental yapı analizi (EDS) çalışmaları gerçekleştirilmiştir. Son olarak da kaplamanın yapışma karakterini incelemek için yapılan Rockwell C testi kaplamanın iyi yapışma sergilediğini göstermiştir.Tezin son kısmında deneysel sonuçların detaylı irdelenmesi ve yapılan çalışmanın titanyum yüzeyinde ne gibi karakteristik iyileştirmeleri gerçekleştirdiğinden bahsedilmiştir. Kaplamadaki eksikliklerden ve kullanılan ESD cihazındaki yetersizliklerin sebepleri ile birlikte tartışılmış ve bunların giderilmesi için önerilerde bulunulmuştur.

Abstract:
The aim of this master thesis is to improve the surface properties of commercially pure titanium (Cp - Ti) (Grade 2) by applying of coating with one of the traditional coating methods, Electro Spark Deposition (ESD).Titanium and its alloys have such properties like, high corrosion resistance, low density, low elastic modulus, biocompability and high strength. Due to its suitability in aerospace, medical and dental applications, the interests and studies on titanium and its alloys have increased. Although, titanium and its alloys show better mechanical and chemical properties comparing to some other metals, they have weakness when used in frictional applications.Commercially pure titanium substrates and commercially pure aluminum rods were used in this study. Parameters of Electro Spark Deposition process were investigated experimentally and literarily, in order to form a better coating layer on the substrates.Pulses with 100-300-500 A current intensity, 50-100 ?s duration and 100 ?s pause duration between two pulses were chosen as electro spark deposition parameters. Although many parameters are changeable in this process, some of them were kept constant since it was thought that these parameters had no effect on results of this work. Accordingly, electrical charge of 2000 mC, coating duration of 60 s per pass, rectangle pulse patterns and vibration frequency of the electrode holder were kept constant.1,2 and 3 passed experimental studies were carried out to see the influence of process parameters on the mass transfer, coating phases and surface properties of coatings. After definition of process parameters, experimental studies were conducted and completed. Then, mechanical, chemical and micro structural investigations of coated samples were made.In the preliminary tests, titanium-aluminum intermetallic phase was formed on the surface of the substrates. After obtaining the intermetallic phase on the surfaces, which was also the aim of this study, characteristics of the coatings were investigated.As characterizations of the coated samples; surface roughness analysis with profilometer, cross-sectional microstructure analysis and investigation of surface morphology through Scanning Electron Microscope (SEM), coating thickness measurements via SEM images, phase composition of the coatings via X-Ray diffraction (XRD) analysis, EDS (Energy Dispersive Spectroscopy) analysis, micro hardness investigations both from surface and cross-section with micro hardness tester, Rockwell C test for adhesion properties.200 Metler Toledo Precision Balance, which has 0,001 mg sensitivity, was used for calculating the mass transformation. After calculation, it was detected that there was increasing in the mass transfer due to increase of ampere and pulse duration.In the present research, the surface roughness measurement was performed by using, Perthometer S8P (Perthen-Mahr, Göttingen, Germany) optical profilometer. Roughness values was found as between 3,2 - 7,8 ?m. Roughness values were decreased slightly when the number of passes were increased. By the increaseing with the pulse duration and pulse ampere, roughness of the surface were inceared as well.Rockwell C test (150 kg loading) was used to analyze the adhesion properties of the coatings. In this method we analyzed the cracks and the breaks of coatings. As a result of the tests, it was observed that the adhesion of the coatings had in HF-2 level of the Rockwell C indentation standart for adhesion.Identification of the phases was performed by using a Philips PW 3710 type X-ray diffractometer. X-ray diffraction (XRD) analyses were conducted by using Cu K? radiation with an incident beam angle of 1o. Diffraction angle was between 20-90o, with a step increment of 0.02o and account time of 1s. In consequence of analysis conducted, there were found TiAl, Ti3Al, TiAl3, AlTi-O2, Al11Ti5, Al2O3 and TiN phases in the coatings. It was come out that TiAl and Ti3Al intermetallic phases were shown in all one passed coating specimens. The increasing number of coating passes resulted in transformation of phases into aluminum reach ones phases such as TiAl3 and Al11Ti5. TiN phase was also seen in all coatings.Microstructures of the composites were investigated by computer equipped Jeol JSM 7000F type field emission Scanning Electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). Pictures were taken from surface and cross sectional areas through SEM. By the help with these pictures, the analysis of EDS area and EDS lines were made. The findings of these analysis were supported the result of XRD.The pictures from SEM were also used to measure the coating thicknesses. The measurement was made by taking 6 values from each specimens. After averaging the measurement values, the coating thickness was found out as between 15 ? 25 ?m. It was also detected that there were an increase on the coating thickness as a result of increasing in pulse duration and pulse amplitude.Cross sectional pictures of coatings showed that there were rather homogeneous mixture of coating layers on specimens which were produced under 100 ampere currency. On the other hand, there were heterogeneous mixture of coating layers on specimens which were produced under 300-500 ampere currency.Additionally, it was shown that there were not only some cracks and some splashing in the surface image of coating, but also some cracks in the cross-sectional image of coatings. These findings were evaluated as the nature of the process.Cross-sectional hardness tests were carried out with a Vickers V-F 34 pyramid indenter, using a CSM micro-hardness tester. During tests, load?indentation depth?time data were recorded. Hardness measurements were performed under 5 gr load and was applied to the samples with a 2s pause between loading and unloading parts. The hardness values were taken as the average of a minimum of 6 measurements. As a result of analysis, on the surface of the coatings, it was measured that hardness is averaged 1800 VHN. The value of hardness in the solid solution was as around 800 VHN.During hardness works on the surface of specimens 50, 100, 200, 500 and 1000 gram loads were used. Before hardness tests, all specimens were sandpapered for a short period due to prevent of rough surface effect. The hardness measurement were varied between 350-550 Vickers according to increasing the test load.SEM analysis showed micro cracks lying throughout the coating from surface to interface. Variations on process parameters were found to cause no significant changes on amount and size of these cracks. These cracks are thought to be the result of cooling rate and thermal expansion coefficient differences.Compensation of technical shortages of the EDS equipment can provide coatings with a better surface quality and interface (coating-substrate) characteristics, and opportunities such as faster and more efficient production. Study on electrical equipment (voltage, capacitance, ampere, inductance, pulse intensity and duration), working medium (gas medium, temperature), electrode material (composition, density, shape, travelling speed, contact intensity) and substrate material (material choice, surface preparation, cleaning, temperature, shape) of equipment can be carried out [23].Cracks forming in the structure of coatings are thought to be the paramount deficiency of the process. These cracks are seen in all studies carried on using these material couples. Certain treatments can be suggested before and after coating process to prevent crack formation. These treatments can include heating of the substrate material before coating process and, controlled cooling and repairing cracks by laser melting treatment after coating process.In the final part of this thesis, experimental findings were discussed in a detailed way. The way of improvement of surface characteristics of titanium were also argued. It was analyzed the efficiency of the parameters and was discussed about the optimum parameters The lack of structural integrity (cracks, heterogeuos mixture layers, etc) in coatings and low performance of ESD equipment were detailed.