Mikro ark oksidasyon işlemi uygulanmış silisyum karbür takviyeli az91d magnezyum alaşımının korozyon ve aşınma özelliklerinin incelenmesi
Investigation on wear and corrosion properties of micro arc oxidized sic reinforced az91d magnesium alloy
Yazar: MEHMET RAGIP MUHAFFEL
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Makine Mühendisliği Ana Bilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Aşınma dayanımı = Wear resistance ; Kompozitler = Composites ; Korozyon = Corrosion ; Magnezyum alaşımları = Magnesium alloys

Özet:
Magnezyum düşük ağırlık karakteristiği, düşük yoğunluğu, yüksek spesifik dayanımı, iyi dökülebilirliği, kaynaklanabilirlik yeteneği, iyi elektriksel iletkenlik, yüksek ısıl iletkenlik, yüksek sönümlenebilirlik özelliği ile bilinmektedir. Magnezyum ve alaşımları havacılık sanayinde, uzay uygulamalarında, elektronik sanayinde ve özellikle otomotiv sanayinde daha çok tercih edilmektedir. Magnezyumun uygulamalarının artmasının en önemli sebebi dayanım ağırlık oranının yüksek olmasıdır. Buna rağmen kullanım alanının sınırlı olmasının sebebi düşük korozyon direnci, düşük aşınma direnci ve sürünme direncidir.Günümüzde magnezyum matriksli kompozit üretimi için pek çok çalışma yapılmaktadır. Saf magnezyum ve magnezyum alaşımlarının kompozitleri yüksek elastisite modülü, yüksek dayanım, yüksek sıcaklıklarda üstün sürünme ve aşınma direnci gibi özellikleri sebebi ile çalışmalar her geçen gün artmaktadır. Özellikle SiC takviyeli kompozitler iyi mikro sertlikleri, yüksek aşınma direnci ve düşük sürtünme katsayısı sebebi ile dikkat çekmektedir.Bu özelliklerin geliştirilmesi için bazı yüzey modifikasyon teknikleri kullanılmaktadır. Bu teknikler elektrokimyasal kaplamalar, fiziksel buhar biriktirme ve kimyasal buhar biriktirme gibi yöntemler kullanılmaktadır. Mikro ark oksidasyon bu yöntemlerden biridir ve yüzeyde oluşturduğu seramik tabaka sayesinde malzemeye kazandırdığı yüksek sertlik, yüksek korozyon direnci ve yüksek aşınma direnci dışında kolay uygulanabilirliği ve çevre dostu bir yöntem olması sebebi ile tercih edilmektedir.Magnezyum alaşımları mikro ark oksidasyon için uygun bir elementtir. Mikro ark oksidasyon, anodizasyon prosesine benzer bir işlemdir. Farklı olarak yüksek gerilim kullanılmaktadır. Yüksek voltaj uygulandığında oksit tabakası oluşmaya başlar. Belirli bir voltaj değerine ulaşıldıktan sonra malzemenin yüzeyinde mikro arklar oluşmaya başlar. Bu sebeple prosese mikro ark oksidasyon olarak isimlendirilmektedir. Bu prosesin avantajları yüksek adhezyon dayanımı, poroz yüzey, düşük maliyet ve çevre dostu bir prosestir.Bu çalışmada AZ91D alaşımına %2-4-8 oranlarında SiC partikülleri eklenerek kompozit oluşturulmuş ve mikro ark oksidasyon işlemi ile yüzeyde oksit tabakası oluşturulmuştur. İşlem parametreleri olarak gerilim değerleri pozitif 400 V, negatif 80 V ve işlem süresi de 15 dakikadır. SEM görüntüleri ile kaplama yüzeyi ve kesitleri incelenmiştir, XRD ile fazlar tespit edilmiş, Eddy-Current metodu ile kaplama kalınlığı, profilometre ile pürüzlülük değerleri ölçülmüş, aşınma ve korozyon testleri uygulanmıştır.Mikro ark oksidasyon sonrası yapılan karakterizasyon işlemleri sonucunda SiC varlığı yüzeyde düşük miktarda düzensizlik yarattığı tespit edilmiştir. SiC varlığı kaplama kalınlığını etkilememiştir. EDS analizleri sonrası yüzey kompozisyonu Mg ve Si elementlerinden oluşmaktadır. Yüzeydeki kaplama yoğun olarak MgO ve Mg2SiO4 fazları gözlenmiştir. Korozyon dirençleri incelendiğinde %2 ve %4 SiC içeren kompozitlerde daha yüksek korozyon direnci görülmüştür. Yüzeydeki kaplama sebebi ile aşınma dirençleri artmış ve %8 SiC içeren kompozit en iyi aşınma direnci göstermiştir.

Abstract:
Magnesium is the best known for its light weight characteristics and also for low density, high specific strength, good cast and welding ability, better electrical conductivity, high thermal conductivity, high dimensional stability and high damping capacity. Magnesium and its alloys are attractive for applications in the aircraft, aerospace, automotive, and electric electronic industries. Newer applications, such as drive shafts, radiators, cylinder heads, and suspension systems have proven to be most advantageous when dealing with weight - strength considerations. However, magnesium alloy have a number of undesirable properties including poor corrosion and wear resistance, poor creep resistance and high chemical reactivity, which have limited their more extensive use in many applications.Nowadays numerous studies have been made on magnesium matrix composites as they exhibit many advantages over monolithic magnesium or magnesium alloys, such as high elastic modulus, high strength, superior creep and wear resistances at elevated temperatures. More recently, studies observed that the distribution of silicon carbide particles (SiCp) reinforcements in a magnesium matrix influenced the tribological properties of the magnesium matrix composites during abrasive sliding. It was reported that well-dispersed SiCp led to better microhardness, lower coefficient of friction and higher wear resistance.In order to improve these properties, surface modification is an effective approach and techniques such as electrochemical plating, anodization, physical vapor deposition (PVD), and chemical vapor deposition (CVD) have been proposed. Micro-arc oxidation (MAO) is derived from theconventional anodic oxidation technology and has attracted increasing interests in the surface treatment of lightweight metals to enhance their wear and corrosion resistance.Magnesium alloys can be coated easily by the new and improvable technology in surface process area called micro arc oxidation. This method is similar to conventional anodization process but we can obtain advanced coatings owing to using high voltage, current density and other process parameters of micro arc oxidation. When high voltage is applied to the anode in an electrolytic solution, an oxide layer starts to form on the specimen surface. This is the conventional anodic oxidation process. As the increasing voltage approaches to a critical voltage, ?dielectric breakdown?, micro arcs start to occur on the surface of the material. This process is defined as micro arc oxidation process.According to other coating technologies, this process shows some advantages as high layer thickness, high adhesion strength, porosity structure, low cost and friendship to the environment. Micro arc oxidation (MAO), has attracted much interest as an effective method to improve the wear and corrosion resistance of aluminum alloys, by creating a thick ceramic film called MgO and Mg2SiO4.Both intrinsic factors (electrolyte compositions and pH) and extrinsic factors (electrical parameters and electrolyte temperature) affect the formation and microstructure of microarc oxidation coatings. The composition and the concentration of electrolyte and electrical parameters during the process play a crucial role in obtaining the desired coatings of special phase component and microstructure. Among them, it is assumed that the intrinsical effects of electrolytes may be summarized as follows:1.Transmitting the essential energy needed for anode oxidizing, occurring in the interface of metal and electrolyte, as the medium of current conduct.2.Providing the oxygen source in the form of oxysalt needed for oxidation.3.Components presenting in the electrolyte incorporated into the coatings can further modify or improve the properties of micro arc oxidation coatings.Coating properties mainly depend on treatment time, type of substrate, electrolyte composition and electricalregime (DC or AC), with AC receiving recent interest due toimproved coating performance compared with coatings formedunder DC.Metal matrix composites exhibit an outstanding combination of low density, high specific strength and high specific stiffness. However, the corrosion susceptibility is usually higher than the unreinforced materials due to the accumulation of defects, discontinuity of the passive layer and/or galvanic coupling at the matrix/reinforcement interfaces. Therefore, surface treatments of metal matrix composites are often needed. One of the main advantages of the MAO process, compared with conventional anodizing, is that uniform coatings can be obtained on metal matrix composites without the ceramic reinforcement disrupting the continuity of the anodized layer.The corrosion susceptibility of magnesium MMCs is usually higher than that of equivalent unreinforced materials. Addition of reinforcing phase increases the number of structural flaws, such as crevices and pores, and may also lead to the formation of interfacial reaction products, galvanic coupling and increased localized corrosion of the matrix. The electrochemical activity of magnesium is also a significant factor in the corrosion of these materials, even under ambient conditions. Recent studies of Al2024/SiC MMC revealed that the dimensions of SiC particles within the coating gradually reduces with treatment time, until they almost completely disappear, contrary to conventional anodizing, where SiC particles disrupt the continuity of the anodic film.In this study, the cast SiCP/AZ91D magnesium matrix composites were examined. The nominal composition of matrix AZ91D magnesium alloy was 8.5?9.5 wt.% Al, 0.6?1.4wt.% Zn, and Mg balance. The reinforcement was 2-4-8 vol.% SiC particles with average diameter of 32 µm. Rectangular materials (10mm×10mm×4mm) of composite were cut and polished. Polished samples were then immersed in electrolyte for micro arc oxidation treatment. The coatings were fabricated using an alternating current MAO system (70 Hz). The sample and stainless steel container of 10 litres were used as two electrodes. The electrolyte was an aqueous solution of KOH, KF, Na2SiO3. The electrolyte temperature was retained in the range of 12-16o C during MAO process in order to decrease the solution evaporation and the effect of electrolyte temperature on chemical reaction rate. A bipolar asymmetric voltage was selected with +400 V in the positive half cycle and -80 V in the negative half cycle. The duration of coating process was 15 min. The coating thickness was measured using an Eddy current thickness gauge and cross sectional SEM images.Characterization of the samples were carried out by, macroscopic and microscopic examinations via optical microscopy and scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction analysis, reciprocal wear tests and electrochemical corrosion tests.The morphology, microstructure, cross-section microstructure and composition of the coatings were analyzed using scanning electron microscope (SEM, Hitachi, TM-1000) and energy dispersive spectroscope (EDS, Oxford Instruments, Swift-ED). Surface phase analysis was made with X-ray diffractometer (XRD, GBC, MMA 027) using Cu K? radiation (? = 0.154 nm) with a scan range between 20-100o at a step of 0.020o. The potentiodynamic polarization curves in 3.5wt.% NaCl solution were carried out by electrochemical workstation to evaluate the corrosion behaviours of bare and coated SiCP/AZ31 composites. A three-electrode cell with sample as working electrode, saturated calomel electrode (SCE) as reference electrode and platinum coil as counter electrode was employed. Wear tests were conducted for wear resistance properties of coating surfaces. Wear resistance of both unreinforced Mg and its composite were evaluated using reciprocal wear test machine at a sliding speed of 5 ms?1 and at load of 5 N. All the tests were carried out at a constant sliding distance of 25 m. The wear rate of the worn out specimens was calculated from the volume loss measurements.Presence of SiC particles in the coating caused discontinuities along the oxidized layer. However SiC content did not affect the thickness of the oxide layer. The EDS results showed that the main elements in the oxide layer were Mg and Si. The phases that occurred after the MAO treatments investigated by XRD. The main phases in the oxide layer were MgO and Mg2SiO4. The corrosion resistance of the composites which contained %2 and %4 SiC was higher than other samples. Although the oxidized composite which contains %8 SiC was the highest wear resistant sample at all.