Fiziksel buhar biriktirme (Pvd) yöntemiyle yapılan Alüminyum krom nitrür (AlCrN) ince film kaplamaların üretimi ve karakterizasyonu

Abstract

Günümüzde artan rekabet ortamının beraberinde getirdiği daha düşük maliyetli ve üstün performanslı üretim yapabilme hedefi ve azalan hammadde kaynaklarının paralelinde yüzey teknolojileri diye adlandırılan malzemenin bütünü yerine dış kısmında yapılan geliştirmeleri kapsayan alternatif prosesler git gide önemini arttırmıştır. Geleneksel yöntemlerin ötesinde nanometre boyutunda kaplamaların söz konusu olduğu üstün performanslı kaplamalar genel olarak ince film kaplamalar adı altında toplanmaktadır. 1990'lı yıllardan itibaren önemi katlanarak artan bu tipte kaplamalar içerisinde de en geniş kullanıma sahip olanlar ince nitrür film kaplamalar adı verilen alt gruptur.Fiziksel buhar biriktirme tekniği ile üretilen sert nitrür kaplamalar, gelişmiş yüzey özellikleri göstermeleri, takım ömrünü ve verimini arttırmalarından dolayı kesme, delme ve işleme uygulamalarında çok yaygın olarak kullanılmaktadır. Günümüz endüstriyel uygulamalarında en yaygın kullanım alanına sahip sert nitrür kaplama olarak TiN kaplama gözükmesine rağmen temas yükü, temas geometrisi, kayma hızı ve nem gibi etkenlerden dolayı aşınma davranışlarının çok değişkenlik göstermesi sonucu, belirli tribolojik uygulamalarda AlCrN, CrN, ZrN, TiCN, TiAlN, TiCrN ve HfN gibi alternatif nitrür kaplamalara ihtiyaç duyulmuştur.AlCrN kaplamalar yüksek sertliğe, mükemmel oksidasyon direncine ve iyi tribolojik özelliklere sahip ince sert seramik kaplamalardır. AlCrN'ün polisajlı yüzeyinin oda sıcaklığında ve yağlamasız ortamda alümina (Al2O3) bilyaya karşı sahip olduğu görece düşük (0,40) sürtünme katsayısı, talaşlı ve talaşsız imalat proseslerinde kullanılan kesici takım veya kalıp parçalarının AlCrN kaplanması halinde normal olarak takım veya kalıbın adhezif (sıvamalı) aşınma dayanımını arttırmakta, yığıntı köşe oluşumunu ve takım veya kalıp yüzeyinde sarma ve sıvanma sonucu yüzey kaybı oluşumunu önemli oranda azaltmaktadır. Böylelikle, özellikle adhezif aşınmanın baskın olduğu işlemlerde AlCrN kaplamalar aşınma dayanımını yüksek oranda arttırma özelliğine sahiptir.Deneysel çalışmalarda AlCrN kaplamaların, kullanılan reaktif gaz (azot) basıncına bağlı optimizasyonu yapılmıştır. 5, 10, 15, 20 ve 25 mtorr azot basınçlarında üretilen AlCrN filmlerin çeşitli deney yöntemleriyle yüzey pürüzlülüğü, kaplama kalınlığı, kaplama sertliği, kaplama yapışma karakteristiği ve kaplama kompozisyonu gibi özellikleri incelenmiştir.Elde edilen sonuçlara göre, azot basıncının artmasıyla birlikte;1. Yüzey pürüzlülük değerlerinde azalma görülmüştür.2. Kaplamaların azot içeriğinde bir miktar artış gözlemlenmiştir.3. Kaplama kalınlıkları azalmıştır.4. Tane boyutunun genel olarak artış gösterdiği saptanmıştır.5. Sertlik üzerinde doğrudan bir etkisinin olmadığı görülmüştür.6. Kaplamaların yapışma kalitelerine bakıldığında HF2 (kabul edilebilir) yapışma karakteristiğine uygun olduğu görülmüştür.Karakterizasyon sonuçları biraraya getirildiğinde en uygun azot basıncının 25 mtorr olduğu tespit edilmiştir.

In present time, in the name of the greater challenge in marketing, the goal of manufacturing low cost materials and efficient productivity, and in the parallel of reducing raw material sources, alternative processes, which are called surface technologies, that include the improvements on the outer part of the material rather than the whole material, get great importance day by day. Beyond the traditional methods, generally nano-meter scaled, supreme performance coatings studied under the topic of thin film coatings. Thin nitride film coatings which are the sub-family of this type coatings have increased their importance after nineteen nineties.Metals are protected with metallic, ceramic or organics coatings. These coatings are used for preventing or for reducing corrosion of the substrate, for improving the physical or mechanical properties of the substrate material and for giving desired decorative apperance.The choice of substrate material is usually governed by cost, weight and general physical, mechanical and manufacturing properties. A chosen material often does not have ideal wear and corrosion resistance in service condition. Coatings improve chemical and physical performans of the substrate.Ceramic coatings due to its refractory properties have been succesfull and widely used on cutting tools where wear is critical.Vapor deposited coatings are not only used in cutting industry but also in the other areas including optical, electrical, electronic, chemical and decorative applications. These coatings are extensively used on glass frame for optical and decorative functions, on watchs and automotive parts for decorative function, on capacitors for electronical function, on cutting tools for mechanical function and manufacturing corrosion resistant parts.Thin ceramic films based on carbides, nitrides and borides of transition metals are generally produced by Chemical Vapour Deposition (CVD) and Physical Vapour Deposition (PVD) methods. PVD methods have capability to deposit refractory materials as thin films and to coat uniformly complex shaped parts at high deposition rates.PVD processes can be divided into two groups based on different mechanism in bringing material to be coated, into vapour phase. These PVD processes are evaporation and sputtering.Evaporation is the oldest and the simplest PVD method. Material to be deposited is placed in a boat or crucible and then heated resistively or by the high current electron beam or laser beam or arc. In all cases the material evaporates and form a vapour flux in the vacuum chamber. Condensation of this vapour onto the substrate produces the desired film.PVD by cathodic arc evaporation is often prefered for its low substrate coating temperature, good target economy, high deposition rate and high degree of ionization. Synthesis of nitrides involve an arc evaporation of the metal such as titanium, chromium, hafnium etc. in the reactive nitrogen gas.One of the main characteristics of arc evaporation is macroparticles which are generated by the action of the cathode spots. Coatings that include significant macroparticles have a surface roughness and matt apperance. Applications where macroparticle would clearly be detrimental include optical and microelectronic coatings. In order to reduce the generation of macroparticle it is necessary to reduce the cathode temperature or to use steered arc deposition techique and to filter macroparticles from plasma by using low angle collectors and increasing substrate bias voltage.The cathodic arc deposition technique is one of the important physical vapour deposition (PVD) methods, successfuly used for the preparation of hard, wear resistant coating for tools and different mechanical parts. Cathodic arc method provides deposition rates up to several micrometers per minute when depositing pure metals or alloys. As a consequence, cathodic arc makes many applications economically feasible. With appropriate source and process developments to eleminate microdroplets, applications of the arc process will be extended to the areas of magnetic and optical disc, hybrid circuit and flexible circuit.When the surface of a material is bombarded with high energetic particles, generally ions, the physical erosion of the material from the surface is occured. This effect is known as sputtering. Sputtering is widely used as a source of vapour for thin film deposition. In all sputtering PVD processes the ions for sputtering is produced by glow discharge plasma.Introducing of reactive gas/gases into the chamber while depositing thin films causes compound film formation onto the substratewbich is called as reactive PVD.Ionizing the vapour to be deposited and applying negative potential (BIAS) to the substrate relative to the vacuum chamber walls is named as ion plating. Ion plating improves the film properties - such as adhesion, the increase of film density - and deposition rate.All thin film processes contain four sequential steps. A source of film material is provided, the material is tranported to the substrate, deposition takes place and finally it is analyzed to evaluate the process. The results of the analysis are then used to adjust the conditions of the other steps for the film property modification.Because of the interesting colors of many thin films (such as carbides, nitrides and borides of the transition metal elements) produced by PVD technique, decorative applications of this type of coatings are available. These films give not only interesting colors but also wear resistance, ease of changing color and reproducibility of the film.Hard nitride coatings deposited by means of PVD technique are extensively used in many types of cutting operations, where they enhance tool life, improve surface finish and increase productivity. Accompanied with that development, a large number of studies have been done so far on tribological properties of thin nitride coatings. In current industrial applications, TiN coatings are widely used. Since the wear of TiN coatings change dramatically with adjustment of parameters such as contact load, sliding speed, contact geometry and humidity, for certain specific applications alternative coatings such as AlCrN, CrN, ZrN, TiCN, TiAlN, TiCrN and HfN are developed.AlCrN provides high hardness, superior oxidation resistance and good tribological behavior. The relatively low friction coefficient of the polished surface of AlCrN coating at room temperature and non-lubricated conditions against alumina (Al2O3) enhances adhesive wear resistance of AlCrN coated tools and considerably reduces the built-up edge formation and the material loss during machining and forming processes respectively. Therefore AlCrN coating has a significant effect to improve the wear resistance of the tools especially used in adhesive wear conditions.In the experimental studies, AlCrN coating is optimized due to reactive gas (nitrogen) pressure. The basic properties such as surface roughness, thickness, hardness, adhesion and chemical composition of AlCrN films deposited in various nitrogen pressures (5, 10, 15, 20 and 25 mtorr) are determined and compared.The investigations revealed that thickness and surface roughness decrease, grain size and nitrogen content increase respectively and hardness values are not affected linearly with increasing nitrogen pressure and all coatings have acceptable adhesion strength.As a result, the coating produced under 25 mtorr nitrogen pressure exhibited the best properties.