Azot difüzyonunu önlemeye yönelik 50CrMo4 çeliğinin akımsız nikel fosfor (NiP) kaplanması
Electroless nickel phosphorous coating on 50CrMo4 steel to prevent nitrogen diffusion
Yazar: SEDA YEMENİCİ
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Malzeme Bilimi ve Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Isıl işlem proseslerinden biri olan nitrasyon endüstriyel alanda en yaygın olarak kullanılan bir yüzey sertleştirme yöntemidir. Nitrasyon sırasında sıcaklığı yaklaşık olarak 540 ºC kadar yükseltip azot gazı kullanılarak metal yüzeyine azotun difüze olmasıyla gerçekleşir. Ne kadar tercih edilen bir yöntem olsa da kontrollü olarak yapılması önerilmektedir. Eğer kontrollü nitrasyon yapılmazsa, kullanımda metal yüzeyinde pullanma yada çatlaklar meydana gelmektedir. Kontrollü nitrasyon istenmeyen fazların oluşmasını engellemekte, oluşan diğer fazların kontrolünü sağlamakta ve metalin istenilen mekanik özelliklerini geliştirmektedir. Nitrasyon sırasında iki farklı tabaka oluşmaktadır ve metalin çekirdek yapısı değişmemektedir. Bu tabakalardan yüzeye en yakın olan beyaz tabaka olarak adlandırılıp malzemenin sertliğini, korozyon ve aşınma direncini arttırmaktadır. İkinci tabaka, beyaz tabakanın altındadır ve difüzyon tabaka olarak adlandırılır. Yorulma direncinde iyileşme sağlamaktadır. Kullanım alanına göre hangi özelliğin arttırılması isteniyorsa kontrollü nitrasyonla o tabakanın kalınlığı, fazları ayarlanılmaktadır. Ancak bazen kontrollü nitrasyonun dışında metalin geometrik şeklinden dolayı istenmeyen fazlar oluşur ve kullanımda çatlaklar meydana gelmektedir. Bölgesel çatlakları önlemek için kaplama yapılmaktadır. Nitrasyon özelliklerine en yakın, ekonomik ve hızlı olmasından dolayı akımsız nikel fosfor kaplama tercih edilmiştir. Bu çalışmanın amacı, nitrasyonu engelleyebilecek kaplama kalınlığını belirlemek ve bu kaplamanın malzeme yüzeyinde etkilerini incelemektir. Öncelikle nitrasyonu önleyebilecek kaplama kalınlığı belirlenilmiştir. 50CrMo4 temperlenmiş çeliğe farklı sürelerde toz nikel ve fosfor bazlı kaplama yapılarak nitrasyon uygulanmıştır. Nitrasyondan sonra optik mikroskopta tabaka görüntülerine bakılıp, sertlikleri ölçülerek nitrasyon öncesi ve sonrası değerler karşılaştırılmıştır. Kaplama kalınlığının 3-5 μm arasında belirlenilmiştir. İkinci önemli adım ise kaplama banyosunun parametreleridir. Kaplama banyosunun, kaplama kalınlığını elde edebilecek en verimli olduğu süre, Elnico 571'in kimyasal miktarı, pH ve sıcaklık değerleri üzerinde deneyler yapılmıştır. En iyi çalışma koşulları 10 dakika süre ile 80 ml/L Elnico 571 nikel ve fosfor bazlı kimyasal, 4-6 pH değerinde ve 82-102ºC'de sıcaklık olarak görülmüştür. Kaplama yapıldıktan sonra farklı ortam konsantrasyonlarında kaplamasız ve kaplamalı numunelere nitrasyon uygulanılmıştır. Tüm bu numunelerin optik mikroskopta tabaka kalınlıklarına bakılmış, taramalı electron mikroskobunda EDX analizleri ve elemental haritanlanması yapılmıştır. Kaplamalı numunelere azot difüze olmamıştır. Sonuç olarak denilebilir ki azot difüzyonu engellenildiği için nitrasyon ısıl işlem görevi görmüştür. Bunu daha iyi anlamak için nitrasyon ve nitrasyon ile aynı sürede yapılan ısıl işlemin sertlik değerleri karşılaştırılmıştır. Sertlik ölçümleri hem farklı yüklerde yüzeyden hem de yanal yüzeylerden yapılmıştır. Sertlik sonuçlarına göre ısıl işlemle ile nitrasyonun bir farkı olmadığı görülmüştür. Karakterizasyon işlemlerine XRD ile devam ederek kaplamalı numuneler arasında ısıl işlemin etkisi kontrol edilmiştir. Bu analiz sonucunda kristal Ni ve çökelen Ni3P fazları ortaya çıkmıştır. Bu çökelen fazlar yapılan çizik ve korozyon testinde iyileşme göstermiştir.

Abstract:
Diffusion is material trasnport by atoms motion. In other words, diffusion is phase trasnformation with helping heat treatment. Diffusion encounter with lot of reasons like corrosion, oxidation, creep, solidification, radioactive proces. However, the reasons of the studying diffusion in this project is surface hardening and mechanical improvement with nitration , besides surface improvement with coating.
Nitration that is one of the heat treatment process is a surface hardening method and the most widespread used in the industrial area. Nitration is occured by higher temperature (about 540 ºC) and diffusion of nitrogen. Nitrogen is obtained with decomposition of amonium. When temperature is increasing, amonium is decomposed to nitrogen and hidrogen molecule that is get off the furnace. Residual nitrogen start diffusing on surface of the metal. It is suggestted to nitride controlledly altough still it prefers. If it is not nitride controlledly, the surface of substrate material occurs peeling or crack. Controlled nitration prevent to form undesired phase, provide to control other forming phase and improve necessary mechanical properties of the metal. During nitration, two layers are formed and the nuclear structure of metal does not change. One of the layers that is the closest to the surface is called white layer. It is improved hardness, corrossion and wear resistance. The second layer is called diffusion layer, and improved fatigue life. Thickness of the layer and phases are set up with controlled nitration if which properties need to improve according to used area. However, undesired phase is sometimes formed, and cracks are occured out of controlled nitration e.g. because of geometric shape of the metal. These area is coated to prevent regional cracks. The preferred coating is electroless nickel phosphorous coating. The reasons of the choice are economic, fast and have similar properties to nitration. Aim of the project is determined coating thickness to prevent nitration and analyzed effects of the coating on the metal surface. This project was get through step by step like this: Determining coating thickness to prevent nitration, determining coating bath parameters, applying structural characterisation (Energy-Dispersive X-Ray Spectroscopy, Optic Microscorp, Scanning Electron Microscopy, Elemental Mapping and X-Ray Diffraction), appliying mechanical tests (hardenning and sctract test) and finally corossion test.
Firstly, thickness of the coating to prevent nitration is determined. 50CrMo4 tempered-steel as the substrate material was coated different time like 5,10,15,20 minute. After that, all samples nitride that area concentration was set up 2. The reason of this controlled nitration is that it has the high area concentration in this project, in other words it diffuse more nitrogen. If nitrogen at this concentration can be prevented with coating thickness, all processes can be prevented. To analyze whether nitrogen diffuse or not, layer images was controlled with optic microscope. According to optical images, coated sample with 5,10,15 and 20 minutes have diffused nitrogen. They have had white and diffusion layers, only coated sample with 20 minute have not have these layers. After that, the samples were measured hardness value. Hardness values before and after nitration was compared. The result of it has supported the optic images. So the determined coating thickness is 3-5 μm. The second step in this project was to determine parameter of the coating bath. The most effective parameters of the coating bath to determine coating thickness is time, chemical quantity, pH and temperature. The first experiment was about time and quantity. The samples were coated with 60ml/L, 80 ml/L and 100 ml/L and 5,10,15,20 and 25 minute. The best coating was seen sample that was used 80ml/L and 100 ml/L chemicals with 10 minute. The second experiment was about pH and temperature according to these parameter. pH value was used between 2 and 4, 4 and 6, 6 and 7. The temperature was also between 72 and 82 ºC, 82 and 92 ºC, 92 and 102 ºC. The most efficient coating was determined 80 ml/L chemical with 10 minute at 4-6 pH value and 82-92 ºC. The last experimet about the coating bath was related to surface roughness. Before coating, two group sample was formed; polished and unpolished. The roughness value of the polished and unpolished sample were in order 0.08 and 0,52. The polished sample has more coating thickness at same time, pH and temperature. It is said that roughness tend to improve coating thickness. After determining coating parameters, uncoated and coated samples nitrided different area concentration 0.4 and 2. Some samples were preoxidation before nitriding. Thus, four groups occured in this experiments. All nitriding witout/ with preoxidation and uncoated/ coated samples were analyzed layers thickness with optic microscope. They were made EDX analyse, elemental mapping, because it was investigated differences between uncoated and coated sample. Coated samples which has 3-5 μm thickness was not seen nitrogen. It can be said as the result of that nitrogen act as a heat treatment to prevent nitrogen diffusion. The effective of heat treatment was controlled between coated sample threby it continued chacterisation examination with XRD. According to the XRD result, the untreated coated sample has amorph structure. It was seen only Ni element peak. However, after heat treatment, the structure transform crystall structure. The Ni3P phase was formed and appear Ni3P peak and crystall Ni peak. These peak is formed at 400 ºC, and after this temperature, the phases are precipitated. This precipiated phase changes mechanical properties. The coated sample was also examined mechanically hardness, corrosion, stratch and wear resistance. Firstly, the hardness was measured by two diffrent way. One of them was applied different load from 50 grams to 2000 grams. It was compared coated and uncoated sample. Other way was measured to cross sectional that was gone away each 50 μm from surface. Other mechanical test was corrosion. The corrosion test was made salt spray test and evaluate according to ASTM-D610-08 standarts. The best result belongs to treated, nitrated and coated sample due to longer treatment time between coated sample, because it had longer treatment time and more precipitated phase. In a similar way, sctratch test had same result. Only coated sample was seen flaking, so acoustic emission and friction coefficient was higher. When the nitraded and coated sample gave better result than only coated sample. Its acoustic emission and friction coefficient were decreasing.
Consequently, 3-5 μm coated thickness is enough for preventing nitrogen diffusion, treated. When they were analyzed with optic image, EDX analyze, elemental mapping, the nitrogen didn't find on surface of metal. Heat treatment was investigated on the coated materials, new phases were determined to form like cyrstal Ni and precipitate Ni3P. This phases help to improve some properties. Treated, nitraded and coated sample gave the best result corrosion test, stratch test due to these forming and precipitating phase.