TiN sert kaplamaların altlıklarıyla galvanik etkileşimlerinin incelenmesi
Investigation of the galvanic interactions between TiN hard coatings and substrate materials
Yazar: BURÇAK AVCI
Danışman: PROF. DR. MUSTAFA KAMİL ÜRGEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Malzeme Bilimi ve Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Fiziksel buhar biriktirme (FBB) yöntemiyle üretilen sert nitrür kaplamalar 1980'lerin başından beri hem akademik hem de endüstriyel çalışmalara konu olmuştur. Farklı altlıklar üzerine geçiş metallerinin nitrürleri, karbürleri ve karbonitrürleri kaplanabilmektedir. Bunlar yüksek aşınma ve korozyon dirençleri sayesinde altlık malzemelerin kullanım ömürlerini belirgin şekilde uzatmaktadır. Bu yüzden metalik altlık seramik kaplama sistemleri kesici ve delici takım uçlarında tercih edilmektedirler. Bu iki özelliğinin yanı sıra, düşük sürtünme katsayısı nedeniyle tribo-korozyon uygulamaları için de kullanılmaları gündemdedir. Özel olarak bir işleme tabi tutulmayan FBB nitrür kaplamalar, altlık üzerinde kolonsal taneler şeklinde büyürler. Bu tanelerin arasındaki boşluklar tribo-korozyon uygulamaları için istenmemektedir. Bahsedilen boşlukların giderilmesi amacıyla önerilenler arasında çok katmanlı kaplamalar, korozyona dirençli arayüzeyler, amorf ya da nanokompozit kaplamalar vardır.
Sert nitrür kaplamaların korozyon davranışını inceleyen birçok araştırmacı olmasına rağmen sadece kaplama ve altlık malzeme arasındaki etkileşimleri inceleyen çalışmalar oldukça sınırlıdır. Bu etkileşimler kaplamanın hata bölgelerinden kaynaklanmaktadır. Hata bölgesi olarak bahsedilen, tüm kaplamalarda bulunan delikler, boşluklar ve çatlaklardır. Bunun yanı sıra servis ömrü boyunca oluşabilecek çizikler de göz önünde bulundurulmalıdır. Bu bölgelerde elektrolit, altlığa kadar sızarak buralarda etkileşime girmektedir. Her iki malzemenin elektrokimyasal potansiyelleri arasındaki fark, oluşan galvanik korozyon hücresi için itici gücü oluşturmaktadır. Oluşan hücrede, çok büyük bir katotla eşleşen çok küçük bir anot vardır. Böylece korozyon hızının arttığı düşünülmektedir.
Bu çalışmada, TiN kaplamalarla altlık olarak kullanılan düşük karbonlu çelik malzemeler arasındaki etkileşim incelenmiştir. Değişen ortam pH'ına bağlı olarak çeliklerin elektrokimyasal korozyon davranışı belirlenmiştir. İnert altlık üzerine katodik ark fiziksel buhar biriktirme yöntemiyle TiN kaplanmıştır. Ardından kaplamaların elektrokimyasal korozyon davranışları tanımlanmıştır. Bunun için kurulan polarizasyon hücrelerinde açık devre potansiyelleri, korozyon akımları ve potansiyelleri ile anodik ya da katodik polarizasyon eğrileri kaydedilmiştir. Daha sonra, altlık-kaplama ikililerinden oluşan galvanik hücreler kurulmuş ve değişen ortam pH'larına göre bu iki elektrot arasındaki potansiyel fark ile aradan geçen akım kaydedilmiştir. Kurulan bu düzenekle, kaplama-altlık arayüzeyinde oluşabilecek korozyon hasarları hakkında fikir yürütme imkanı doğmuştur.

Abstract:
Hard nitride coatings produced by physical vapor deposition (PVD) methods have been studied extensively by the academia and the industry since their development in 1980s. Nitrides, carbides and carbo-nitrides of the transition group metals are successfully deposited on various substrates such as metals, ceramics or plastics.
With improved wear and corrosion characteristics, these coatings increase tool life drastically. Thus, they are preferred as a metal substrate-ceramic coating system for drilling, working and cutting tools and other machine parts. Several decorative applications are also common due to the wide range of colors possible. In the recent years, biocompatibility of some nitride coatings has been highlighted as well, especially regarding the use in the contact points of joint implants. Apart from those properties, these coating systems are considered for tribo-corrosion applications where low friction coefficients are required.
PVD methods are broadly classified into two groups depending on the generation of vapor phase. In evaporation methods, metal to be deposited is evaporated from a crucible by the help of resistances, inductive currents, electron beam, arc or laser targets. In sputtering methods, however, the surface of the cathode, or target, material is bombarded with energetic ions and the molecules that are knocked off from the cathode surface will form the vapor. In each case, the vapor then condenses on the subsrate material to form the film. By adjusting process parameters, such as gas ratio, pressure, flow rate and deposition temperature, composition and the morphology of the film can be tuned as required by the application.
Without any treatments, PVD nitride coatings have columnar structure with intrinsic porosity. The porosity issue is solved with various scenarios. The production of multilayered coatings, corrosion resistant interfaces, amorphous or nanocomposite coatings are all probable solutions by which the nature and the morhology of the coating is optimized to yield the required resistance to electrolyte through service conditions.
There is a vast literature regarding the corrosion behavior of hard nitride coatings on different substrates with a focus on steels, stainless steels and titanium alloys. These mostly rely on conventional electrochemical methods. The commercial coating systems and the most commonly studied coatings range from single nitride coatings (such as TiN and CrN) to more complex ones (like TiAlN). Even though the electrolytes used in these studies are usually acidic (in the range of pH 0-4.5, usually), in the recent years there is an interest in more biocompatible solutions such as Hank's or other stimulated body fluid solutions with higher pH values and higher temperatures. Some work related to the corrosion behavior of nitride coatings in supercritical, ionic fluids or basic solutions is also present.
However extensive the corrosion research may be, the literature regarding the corrosion behavior of the sole coating and the substrate is highly limited. The interactions between the coating and the substrate are thought to occur mainly at the defect sites of the coating. These are pores, pinholes, cracks and scratches from both production and service conditions. At these sites, the electrolyte diffuses through the openings in the coating and interacts with the substrate underneath. The difference between the potentials of the substrate and the coating will be the driving force required. A galvanic cell will form in which the coating acts as a big cathode and the substrate acts as a small anode. As a result, the corrosion is accelerated.
This study aims to investigate the interactions between TiN coatings and low alloy carbon steel substrates. The electrolyte used is % 3.5 NaCl with pH varying between 1 to 3. It is deaerated with N2 gas before and throughout the experiments. The substrate is 99.6% Fe, 0.0361% C, 0.0082% Si, 0.249% Mn, 0.0014% P, 0.0053% S, 0.0188% Cr, 0.0018% Mo and 0.0412% Ni. It is polished upto 2500 grid paper. TiN is deposited on inert polished alumina substrates by using cathodic arc PVD using a titanium target and 1 Pa N2, 150 V bias voltage and 70 A arc current. The electrochemical cell is set up by using saturated calomel electrode as the reference electrode and graphite rod as the counter electrode. With this conventional system, both the substrate and the sole coating are characterized. Open circuit potentials, corrosion currents and potentials, cathodic or anodic polarization behaviours are all recorded. Finally, galvanic cells composed of the substrate and the coating as electrodes are set up to observe the galvanic potentials and currents. The effect of the anode to cathode ratio is also investigated. The cathode area is kept constant at 3 cm2. However, the anode area varies between 3, 1.5 and 0.5 cm2.
The electrochemical parameters regarding the steel substrates are similar to those found in literature. With decreasing pH, the open circuit potentials or corrosion potentials shifts towards more noble potential values, from -550 mV at pH 1 to -587 mV at pH 3. Meanwhile, the exchange current density increases as the pH decreases. The anodic Tafel slopes are around 60 mV/decade for all instances. TiN coatings, have relatively different open circuit potentials than titanium metal itself, around +250 mV instead of -250 mV. Those values are consistent with what is being reported in the literature. During cathodic polarization of TiN coatings, however, it is interesting to note that the cathodic reaction (hydrogen evolution reaction for the scope of this study) are very slow on the surface and thus only a very limited current can pass. The intristic porosity of the TiN coating in fact plays a role in the formation of a galvanic cell, as demonstrated by the current flow during the coupling of the two independent electrodes. Even though the open circuit potential of TiN is very much higher than of the steel in every case, TiN doesn't provide an easy pathway for current flow because of the limited current flow in cathodic reactions. Normally, when a galvanic cell is formed, the anode to cathode ratio is a critical parameter that defines the extent of the corrosion failure. If the anode is very small compared to the cathode, then it is said to be problematic under service conditions since the anode will try to replenish the electrons required by the cathodic reactions as quickly as possible. Meanwhile, in the TiN-steel system, the currents passing through are hardly above 15 μA in each case. These small currents aren't able to polarize the anode materials towards very noble potentials as well. This particular situation is visible by overlaying cathodic polarization data of the TiN and the anodic polarization data of steel.
Eventually, it must be stated that far greater cathode areas coupled with much smaller anodes might be able to show drastic changes before and after. On the other hand, these coupling values can only be compared with polarization data when the scan rate for polarization is very slow or if verification by potentiostatic scans is available.