Alaşım (MoCu), katot kullanarak (Mo-N-Cu) nanokompozit kaplamaların ark fiziksel buhar biriktirme yöntemi ile üretimi ve karakterizasyonu
Production and characterization of Mo-N-Cu, nanocomposite coatings deposited by arc pvd using (Mo-Cu) alloy cathode
Yazar: MORTAZA MOHAMMADIMOGHANJOUGHI
Danışman: PROF. DR. MUSTAFA ÜRGEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İleri Teknolojiler Ana Bilim Dalı
Derece: Yüksek Lisans, 2013
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Nanokompozit kaplamalar geliŞtirilmiŞ mekanik özelliklere sahip olmaları ve kimyasal olarak inert bir yapı sergilemeleri dolayısıyla sürtünme ve aŞınma direncini azaltmaktadırlar. Mo-N-Cu kaplamalar özellikle yağlı ortamda üstün aŞınma ve sürtünme özellikleri sergilemektedir. Bu kaplamaları farklı fiziksel buhar biriktirme yöntemleri kullanarak üretmek mümkündür. Farklı kaplama yöntemlerinin üretim amacı ile kullanılması halinde kaplamaların yapısal ve fiziksel özelliklerinde değiŞiklikler olması beklenen bir durumdur.Bu çalıŞmada, alaŞım katot kullanarak katodik ark fiziksel buhar biriktirme yöntemi ile Mo-N-Cu kaplamalar üretilmiŞ ve tanımlanmıŞtır. Bu kaplamalar manyetik alanda sıçratma yöntemi ile üretilen Mo-N-Cu kaplı numuneler ile yapısal ve mekanik özellikleri gözönüne alınarak karŞılaŞtırılmıŞtır.Ark FBB yöntemi ile yapılan kaplamalarda toz metalürjisi yöntemi ile üretilen ve at %10 bakır içeren katotlar kullanılmıŞtır. Kaplamalar iki farklı katot akımı kullanarak (100 ve 120A) 100 V bias voltajı altında, 450-500 C sıcaklıkta, 5 mtorr azot basıncında yapılmıŞtır. Kaplamalar yüksek hız çeliği taban malzemeler üzerinde biriktirilmiŞtir. Toplam kaplama süresi 60 dakikadır. Manyetik alanda sıçratma yöntemi ile üretilen kaplamalar ise ticari olarak üretilmiŞ kaplama örnekleridir.?ki farklı yöntemle üretilen kaplamalar, XRD yöntemi kullanılarak yapısal olarak tanımlanmıŞtır. Numunelerin üzerinde kaplama kalınlık, sertlik ölçümleri yapılmıŞ, yapıŞma özellikleri belirlenmiŞ ve kırık yüzey ve kesit incelemeleri yapılmıŞtır. Kaplamaların aŞınma özellikleri disk üzerine pim yöntemi kullanılarak yağlı ortamda incelenmiŞtir.XRD sonuçlarına göre manyetik alanda sıçratma ve 100 A katot akımı kullanılarak yapılan kaplamaların kübik Mo2N yapısında olduğu, 120 A katot akımı kullanılarak üretilen kaplamaların ise kübik Mo2N ve hekzagonal MoN karıŞımı halinde olduğu belirlenmiŞtir. Tüm kaplamalara ait piklerde dğiŞik oranlarda düŞük açılara doğru kaymaların varlığı kaplamaların basma yönünde iç gerilme içerdiğini göstermiŞtir.Ark FBB yöntemi ile üretilen kaplamaların sertlikleri beklenildiği üzere daha yüksektir ve 26-28 GPa aralığında değiŞmektedir. Manyetik alanda sıçratma yöntemi ile üretilen kaplamalrın sertlikleri ise 19-20 GPa aralığındadır.Kaplamaların kırık yüzeyleri incelendiğinde ark yöntemi ile üretilen kaplmaların tamamen eŞ eksenli yapıya dönüŞtüğü ve kolonsal yapının ortadan kalktığı, manyetik alanda sıçratma ile üretilen kaplamalarda ise kolonsal yapının hala varlığını sürdürdüğü gözlenmiŞtir.Kaplamaları yapıŞma özellikleri hem Rockwell C hem de çizik yöntemi kullanılarak incelenmiŞtir. Sonuçlar her iki tür kaplamanın da yüzeye yapıŞma özelliklerinin iyi olduğu belirlenmiŞtir.Kaplamaların, yüksek hız çeliği pimler kullanılarak, 20 N yük altında ve 2cm/s hızla formüle edilmiŞ yağda yapılan aŞınma deneyleri sonucunda tüm kaplamaların oldukça düŞük sürtünme katsayıları (0.03-0.06) verdiği belirlenmiŞtir. Deney öncesi kaplamaların yüzeyleri 2500-4000 grid SiC zımpara kullanılarak parlatılmıŞtır. AŞınma deneyleri sonucu sürtünme katsayıları elde edilmiŞtir.Tüm kaplamalarda ve karŞıt yüzeylerde aŞınma izlerine rastlanmamıŞtır.Sonuç olarak her iki kaplama yöntemi kullanılarak yapılan kaplamaların benzer aŞınma davranıŞı gösterdikleri belirlenmiŞtir. Buna göre daha hızlı kaplama olanağı sağlayan katodik ark yöntemi kullanılarak bu kaplamaların istenilen özelliklerde üretilebileciği ortaya konulmuŞtur.

Abstract:
Hardness of materials can be defined as resistance to deformation of materials. Upper hardness limit of materials are related to their crystal structure rigidity. Developments in technology require materials with high strength, hardness and toughness, and that can preserve their stability at high temperature.PVD coating techniques are widely used for the production of hard coatings. The flexibity of the techniques for tuning composition and their ability of producing compounds difficult to prepare by bulk production techniques made them very suitable for the development of compounds with different properties. One of the recent developments in this area is nanocomposite structures. Two main apporaches are used for the production of nitride based hard coatings. One of them iscodeposition of two hard immiscible phases like nc-TiN/a-4?4/a and nc-Ti2, which leads to hardness increase and refinement of the coating structure. Coatings produced in this manner are now used by the industry for machining and high temperature applications.The other approach used for the production of nanocomposite coatings is codeposition of one hard nitride based compound together with a soft metal which does not produce nitrides. Examples of these coatings are Zr-Cu-N, Ti-Cu-N, Cr-NCu, Mo-Cu-N and Al-Cu-N. The coatings produced by this approach also results in an increased hardness and refinement of grain size. The industrial applicaiton of these coatings are not as widespread as hard-hard nanocomposite coatings due to their low oxidation resistance and their reactivity with the work piece during machining processes. However, with a combined reserach activity conducted collaboratively between our research group and Argonne National Laboratories the outstanding property of Mo-N-Cu based coatings has been discovered. These coatings showed a very promising property under lubricated contacts. They not only gave low CoF but also did not wear away the counter bodies under boundary lubrication conditions which made them a very good candidate for using on parts that work under boundary or mixed lubricaiton conditions (piston rings, tappets, piston pins etc) in the automotive industry. These coatings are produced in industrial scale by magnetron sputtering technique using Mo-Cu alloy cathodes. This study aims to produce the same coatings with CAPVD using Mo-Cu alloy cathodes for benefiting from the higher deposition speed and better adhesion properties of CAPVD coated films.In this investigation high speed steel discs (HSS) are used as substrates. Substrates are subjected to a polishing procedure for producing surfaces with a Ra of 0.14-0.15 µm. Before placing the samples into the vacuum chamber, they are cleaned in an hot alkaline bath ultrasonically and then dried in propanol.After evacuation of chamber down to 5*10?5torr, the samples are heated and sputter cleaned by arc plasma of the cathode with succesive application of -600, -800 and -1000 V. each for 1 min. This process not only cleaned the sample surfaces further but also heated them to a temperature of 450-500 C. After heating the samples the coating process is initated by introducing nitrogen gas (5 mtorr nitrogen pressure) and lowering the bias voltage to -100 V. Total deposition duration was 1 hour for all coatings. Only cathode current was used as a parameter during the deposition. Two sets of samples are prepared by using 100 A and 120 A cathode current. As cathodes Mo-10at% Cu alloys cathodes produced by powder metallurgy technique was used. The magnetron sputtered samples was obtained from a batch that was commercially produced using -100 and -120 V bias voltage using Mo-Cu alloy targets.Coated samples produced both by CAPVD and MS are then subjected to characterization studies. Their structures are determined by XRD. The XRD investigation are made in thin film mode of the diffractometer with angle of incidence 2º by using CuK? radiation. Hardness measurements were conducted by using an ultramichardness tester with depth sensing properties. Thickness of the samples are measured with calotest technique. Their chemical composition are analyzed with SEM equipped with EDS. Fracture cross sections of the samples are prepared by breaking them after cooling in liquid nitrogen and investigated with SEM. Adhesion properties of the coatings are measured by using both Rc and scratch tests.Tribological properties of the coating are determined in lubricated environments by using pin on disc technique. Coated circular samples and high-speed steel pins are used as disc and pin in the experiments. Before conducting the tests the surface of samples are slightly polished by using 2500 and 4000 grid SiC paper. During this process droplets that existed on the surface of the CAPVD coated samples are removed. High loads (20N) and low speed (2cm/s) was used during the experiments for ensuring the boundary lubrication conditions. As lubricant commercial fully formulated 10W-40 synthetic oil was used. After the tests surfaces of both pin and disc are investigated by optical 3D surface profilometer.The XRD results showed that the coatings produced by arc PVD by using 100 Acathode current and magnetron sputtering had cubic Mo?N structure. However, the coating produced with a 120 A cathode current possesed a structure composed of a mixture of hexagonal MoN and cubic Mo?N phases. The hardness of the coatings showed a dependence on the coating method. Coatings produced with CAPVD are comparably harder (26-28 GPa) than the magnetron sputtered ones (19-20 GPa). Although the copper contents of all the coatings were close to each other (8-12 at%) the cross sections of the coatings showed appreciable differences; namely CAPVD coatings possesed a very dense featureless structure, on the other hand the columnar structure was still prevailing in magnetron sputtered coatings.The results of Rockweel and scratch tests revealed that there was no adherence problem for of all the coatings invsetigated in this study. The lubricated wear test results showed that there was no appreciable difference on the tribological behavior xxiof CAPVD and MS coatings rubbing against HSS pins. For all coating the CoF was very low (0.03-0.06) and no wear was detected both on the disc and the pin. These results are in accordance with the studies previously conducted in our group.The results of this study has shown that it is possible to use alloyed Mo-Cu cathodes for the production of Mo-N-Cu nanocomposites. These coatings are harder and denser than the magnetron sputtered ones. The deposition duration is for achiving the required coating thicknesses are low. However, there are substantial amount of droplets on CAPVD coatings which require polishing for their removal. The heat input arousing from the cathodes that are running at high currents leads to an increase of the substrate temperatures above 400 C. This makes CAPVD process unsuitable for substrates that may loose their mechanical properties by heating to these temperatures. The tribological properties of the coating under boundary lubrication conditions used in this study did not show any appreciable difference when compared to the ones prepared by magnetron sputtering. As a result this study clearly revealed that MoCu alloy cathodes can be used for the prodcution of Mo-NCu coatings as an alternative to MS.