Tribological properties of MoN-Ag nanocomposite coatings

Abstract

Nanostructured coatings have been recently of great interest because of the possibility of synthesizing unique physical and chemical properties. Different approaches exist for the production of nanostructured coatings such as superlattice coatings, nanoscale multilayer coating, super lattice coating and nanocomposite coatings. The nanocomposite type coatings have attracted the greatest interest by the researchers. Completely new properties are exhibited by nanocrystalline coatings with decreasing the grain size to about 10 nm. The mechanical properties of these materials are determined by processes in boundary regions because the number of atoms in grain is smaller than in the boundary regions. High fracture toughness can be obtained in the nanocomposite structure due to the nano size grain structure. Currently, two main practices exist for the production of nanocomposite coatings. In the first one, the nanocomposite structure is obtained by the combination of two hard and immiscible phases in one coating (e.g., nc–TiN/a–Si3N4; a–TiSi2–nc–TiSi2) while in the other practice, the mixture of one hard (MeN) and one soft (X) immiscible phase (e.g., Zr–Cu–N, Cr–N– Cu, Ti–Cu–N and Mo–Cu–N) are used. In these coatings, one metal may be converted into nitride as a nanocrystalline phase and the other participated in the growing film without any reaction. Nanocomposite coatings based on hard/hard phases have attracted in recent years and there are now numerous studies that deal with the production, characterization, and application of these coatings. The coatings with hard–soft structure have a very promising structure with respect to tribological applications. One can easily formulate these coatings by selecting the hard and soft phase combinations for the required tribological application. Examples of such formulations have been put forward in our research group especially for coating applications suitable for systems working under boundary lubrication conditions. The main approach used in these formulations relies on functioning of the hard phase (such as MoN) as load bearing material and soft phase (copper, silver, tin, antimony etc.) or their compounds. In this study, further verification of this approach is aimed using a hard-soft nanocomposite system based on Mo-N-Ag. MoN is selected as the hard phase because of its better tribological properties compared to other hard phases. On the other hand silver is an easily shearable soft material that is mainly used in high temperature applications as solid lubricant. Silver also has a potential of forming solid lubricant compounds with the oxides of the hard phase (such as silver molybdates) and it can also react with the formulated oil ingredients and form lubricous structures. For achieving this aim, not only the effect the composition of the coating material but also the effects of different sliding velocity on the tribological behavior is investigated and the results are correlated with the character of the debris that are determined with micro-Raman spectroscopy. Hence, this is a unique study, in which the tribological behavior of the Mo-N-Ag nanocomposite coatings is investigated in detail by taking into consideration both formulation of the coatings and the tribo -mechanical and -chemical properties of the system. Moreover, in this study a hybrid coating system based on cathodic arc and magnetron sputtering that is previously used extensively in our group for the production of other MeN-X type structures, is used for the first time for the production of MoN-Ag nanocomposite coatings. Molybdenum nitride and MoN-Ag nanocomposite coatings were deposited using cathodic arc and cathodic arc + magnetron sputtering hybrid physical vapor deposition techniques. Hardened M2 discs were used as substrates. Hardness of the coatings were measured with a dynamic ultra micro-hardness tester (Fischerscope H 100) equipped with an elongated Vickers type indenter; a total load of 20 mN was applied in 120 steps with a time interval of 0.5s at each step. The phase structure of the coatings was analyzed by a glancing angle X-ray diffractometer. EDS equipped field emission scanning electron microscope (Jeol JSM 700F) was used for investigation of coating growth and wear tracks morphology. Wear tracks and chemical composition of wear debris were examined by 3D profilometer and micro- Raman, respectively. Observations of the wear traces with the micro-Raman investigation showed that formation of lubricious oxide and silver molybdate into the wear track caused decreasing the wear and friction. Silver addition showed its beneficial effect by lowering the wear of counter face material and decreasing the coefficient of friction.

Özet

Birbirleri ile temas halinde olan yüzeylerden, mekanik etkilerle oluşan malzeme kaybı, yüzyıllardır insanoğlunun mekanik problemlerinin başında gelmiştir. Dünyadaki mekanik enerjinin yüzde otuzunun mekanik kayıplara harcandığı düşünülürse, sürtünme ve aşınmanın önemi daha da iyi anlaşılabilir. Teknolojik ilerlemelerle birlikte sürtünme ve bunun neden olduğu aşınma problemlerinin aşılması için sert yüzey kaplamaları kullanılmıştır. TiN, CrN gibi ince film kaplamalar endüstriyel uygulamalar için en yaygın olarak kullanılan kaplamalardır. Molibden esaslı nitrür kaplamalarda, sürtünme ve aşınma açısından oldukça üstün özelliklere sahiptirler. Molibden esaslı kaplamalar da düşük sürtünme katsayısı elde edilmesine rağmen temel problem, karşıt malzemede meydana gelen aşınmadır. Çok sert olan molibden kaplamlara yumuşak Ag, Cu gibi elementlerin ilavesi ile elde edilen yapılar, kuru şartlarda ticari olarak kullanılan kaplamalardan daha üstün sürtünme-aşınma özelliklerine sahiptirler. Bu çalışmada katodik ark ve manyetik alanda sıçratma sistemlerinin hibrit kullanıldığı fiziksel buhar biriktirme yöntemi ile yüksek hız takım çeliği malzemelere üzerine kaplanan farklı Ag içeriğine sahip MoN-Ag kaplamaların Al2O3 karşıt malzeme kullanılarak atmosferik koşullardaki sürtünme ve aşınma özellikleri incelenmiştir. Aşınma deneylerinde katkısız MoN kaplamaların yanı sıra düşük, orta ve yüksek (at.%1.4, %8, %24Ag) Ag içeriğine sahip kaplamalara sürtünme-aşınma deneyi yapılmıştır. MoN yapılara Ag ilavesi ile sürtünme deneyleri sonucunda yüksek Ag içeren kaplamalar dışındaki kaplamalarda önemli bir aşınma görülmemesinin yanında Ag’ün yararlı etkisi özellikle at.%8 oranında Ag içeren kaplamalarda tespit edilmiştir.