Buzdolabı Kompresör Yataklarında Mon-cu Nanokompozit Kaplamaların Mangan Fosfatlama Yerine Kullanılabilme Potansiyeli

Abstract

Birbirleri ile sürtünme ve aşınma halinde parça ve yatakların kullanım ömrünü artırmak için bu malzemelere çeşitli modifikasyonlar uygulanır. Bu modifikasyonlar direk taban malzeme ile veya taban malzeme üzerine uygulanacak bir kaplama ile sağlanabilir. Geleneksel yöntemler arasında, taban malzemelere uygulanan ısıl işlem ile sertleştirme veya sadece yüzey sertleştirme yöntemleri hala kullanılmakta olan metotlar arasındadır. Ancak günümüz teknolojisi ile taban malzemeye dokunmaksızın yüzeye uygulanan kaplamalar ile daha üstün tribolojik özellikler elde etmek mümkündür. Geçiş metallerin nitrürleri, üstün sertlik ve korozyon önleyici özelliklerinden dolayı tribolojik amaçlı çok sık kullanılan kaplama türleridir ve özellikle sert kesici takımlarda sıklıkla kullanılmaktadır. Bu kaplamaların tribolojik uygulamalarda verimli hale gelebilmesi için iyi katı yağlayıcı özellikleri göstermeleri önemlidir. Bu tip sert kaplamalara (TiN, CrN, MoN vb.) yapılacak yumuşak faz (Cu, Sn, Ag vb.) katkılandırmaları ile daha üstün katı yağlayıcılık özellikler gösteren nanokompozit kaplamaların eldesi mümkün olup, kaplamalı parçaların aşınma miktarları azaltılarak çalışma ömürleri arttırılabilmektedir. Çeşitli vakum altına kaplama teknikleri ile nanokompozit kaplamalar elde etmek mümkündür. Bu yöntemler arasında en çok kullanılanlar; katodik ark FBB, manyetik alanda sıçratma ve bu yöntemlerin bir arada kullanıldığı hibrit sistemler sayılabilir. Literatürdeki çalışmalar incelendiğinde MoN kaplamaların, TiN ve CrN kaplamalara göre tribolojik özelliklerinin daha iyi olduğu gözlenmiştir ve bu sert kaplamalara yumuşak bir faz olan Cu ilavesi ile nanokompozit kaplamalar elde edilmiş ve aşınma testleri yapılmıştır. En düşük sürtünme katsayısı Cu katkılı MoN kaplamalarda görülmüştür. Mevcut durumda buzdolabı kompresör yataklarına mangan fosfat kaplama uygulanmaktadır. Yağ tutucu özelliği olan bu kaplamalar yatak yüzeylerini belirli bir süre aşınmadan korumaktadır, fakat sert kaplamalar gibi uzun ömürlü olamamaktadırlar. Bu çalışmanın ön aşamasında, mangan fosfat kaplamalı dökme demir taban malzemelere yağlı ortamda ve 80oC’de aşınma testleri (pim-disk) uygulanmış, Stribeck eğrileri çıkarılarak yağlama rejimleri (hidrodinamik, karma ve sınır yağlama) tespit edilmiş ve sabit yük altına uzun süreli (24 saat) aşınma deneyleri yapılarak sürtünme katsayıları tespit edilmiştir. Çalışmanın asıl aşamasında, dökme demir taban malzeler üzerine katodik FBB yöntemi ile %90 Mo- %10 Cu içeriğine sahip tek katot kullanarak azot atmosferinde MoN-Cu nanokompozit bir kaplama uygulanmıştır. Yapılan dropletleri (damlacıkları) hafif zımparalama yolu ile giderilerek kaplama kalınlıkları ve sertlikleri ölçülmüştür. XRD faz analizleri ve SEM analizleri yapılmıştır. Daha sonra mangan fosfat kaplamalı dökme demirlere uygulanan aynı koşullarda Stribeck eğrileri çıkarılarak yağlama rejimleri tespit edilmiş ve sabit yüklü uzun deneyler ile (24 saat) sürtünme katsayıları bulunmuştur. Bütün triboloji deneyleri sonrasına aşınma yüzey profilleri optik profilometre cihazı kullanılarak incelenmiştir. Diskteki aşınma derinlikleri ve pimdeki iz çapları ölçülüp, pim üzerinde meydana gelen aşınma değerlerinden yola çıkılarak özgül aşınma hızı katsayıları hesaplanıp birbirleri içerisinde karşılaştırılmıştır. En düşük özgül aşınma hızı katsayısı MoN-Cu kaplamalı pimde görülmüştür.There are various modifications in order to increase lifetime of bearing parts that are in contact with each other and faced with friction/wear. These modifications can be directly applied to the bulk substrate or the surface of the substrate by coatings. Among traditional methods, heat treatment or surface hardening methods are stil in use. But it is possible to apply different coatings without modifying the original substrate to get superior tribological properties by the help of today’s technology. Nitrides of transition metals are the most commonly used coating type because of their excellent hardness and anti-corrosive features, especially for cutting tools. The most important thing about these type of coatings is to show solid lubricant properties. It is possible to obtain tribologically high performance hard nanocomposite coating by combining a hard phase (MoN, CrN, TiN, etc.) with a soft phase (Cu, Ag, Sn, etc.), so mechanical loss from the surfaces of bearing parts will decrease and lifetime of these pieces will increase. Nanocomposite coatings can be produced by various vacuum technologies. The most commonly used methods among vacuum techniques are cathodic arc physical vapour deposition (PVD), magnetron sputtering and hybrid coating systems that are combining both two methods. There are several recent studies about this topic. In one of these studies, researcher investigated tribological behaviour of MoN, TiN and CrN. After wear tests, he made a comparison between them and found that coefficient of friction (COF) value of MoN coating is lower than the others. But the lowest coefficient of friction value is obtained by adding a soft phase (Cu) into MoN hard coatings that are called nanocomposite coatings. In existing situation, manganese phosphate coating is applied to only one surface of bearing parts (counterfaces are un-coated) of refrigerator compressors due to its oil- absorbing properties. By the help of this feature, operating life of bearing parts become longer for specific period, but not as long as hard coatings. In this study, primary investigations was about exploring tribological behaviour of manganese phosphate coated cast iron materials. Tribological experiments of pin on disc samples by using reciprocating motion are conducted at 80oC in oil bath. The oil is a kind of mineral oil-based refrigeration oil for use with refrigerant isobutane (R600a) containing various additives. First of all, Stribeck curves are found out in order to discover lubrication regimes (hydrodynamic, mixed and boundry lubrication regime) of manganese coated and uncoated samples. In hydrodynamic lubrication regime, there is a stable oil film layer between tribo-couples, coefficient of friction value is low and it is possible to say that there is no real wear between the surfaces. Mixed lubrication regime is a transition region from hydrodynamic to boundry regime. The most aggresive wear conditions are observed in boundry lubrication regime, because stability of oil film between surfaces spoils and tribo-couples begin to be directly in contact. That’s why coefficient of friction value gives a rise in this area. Boundry regime is the most significant region that shows high wear rate. After discovering lubrication regimes from the graphs of Stribeck curves, long wear experiments (24 hours) under constant load and same temperature (80oC) in oil bath are conducted in order to determine coefficient of friction values. Coefficient of friction value of uncoated pin/ uncoated disc is 0.24, manganese phosphate coated pin/ un-coated disc is 0.20, both sides are coated is 0.14. The main purpose of this study is to investigate tribological behaviour of MoN-Cu nanocomposite coated cast iron samples and compare them with manganese phosphate coated cast irons in existing situation. For this aim, cast iron disc and pin samples coated with MoN-Cu by cathodic arc physical vapour deposition technique (PVD) by using one cathode (target) material that contain %90 Mo- %10 Cu. This cathode material is produced by the method of spark plasma sintering (SPS). Before coating process, samples are cleaned with bombarding the surface of sample by cathode atoms under Argon atmosphere and during coating process nitrogen is used as reactive gas to obtain MoN phases. The disadvantage of cathodic arc PVD system is droplet formation. Droplets formed on sample surface spoil the surface roughness and it is not useful tribological applications. So the surfaces must be polished after the coating process with fine sandpaper. After polishing the surface, thickness and hardness of the coating are measured. The thickness is approximately 3-4 micron and the hardness is 22283 N/mm2. Additionally, XRD (X-Ray Diffraction) and SEM-EDS (Scanning Electron Microscope- Energy Dispersive Spectroscopy) analysis are applied to the samples. Mo and Mo2N phases are determined in XRD pattern, Mo peaks are assumed to come from droplets and Mo2N phase is coming from coating region. After characterization, MoN-Cu coated cast iron discs and pins are tribologically tested under same conditions (80oC, 24 hours, constant load: 50 N) that applied to manganese coated samples. Coefficient of friction value of MoN-Cu coated pin/ uncoated disc is 0.11, uncoated pin/ MoN-Cu coated disc is 0.06. If coefficient of friction versus time curves of wear tests are compared, the most stable curves are obtained in MoN-Cu coated samples. Wear track of all pin and disc samples are analyzed using optical profilometer in order to measure wear depth of disc, track diameter of pin and view 2D/3D surface morfology. The deepest and biggest value of wear depth and track diameter are observed in uncoated pin/ uncoated disc and smallest values are obtained in wear tests of MoN-Cu coated pin/ uncoated disc. Addition to these wear depth and diameter measurements, wear volumes of pin materials are calculated depending on pin geometry using original radius and diameter of wear track after wear experiments. Wear volumes are estimated directly proportinal to track diameter of pins. After finding out wear volume of pins, specific wear rate coefficients are calculated by using Archard Equation (1953); V=k x F x s V: Wear volume (m3) k: Specific wear rate coefficients (m3/Nm) F: Normal load (N) s: Sliding distance (m) The specific wear rate coefficient of uncoated pin/ uncoated disc tribo-couple is 3.78 m3/Nm, while manganese phosphate coated pin/manganese phosphate coated disc is 1.92 m3/Nm and MoN-Cu coated pin/ un-coated cast iron disc is 0.15 m3/Nm. Amount of wear depending on time also investigated in this study. Long wear experiment (24 hours) of MoN-Cu coated pin and uncoated disc is divided into four sub-durations ( 3 hours- 6 hours- 12 hours- 24 hours). In every duration step, experiment was stopped and wear depth of the disc, track diameter of pin were measured using optical profilometer. It is observed that the rate of change of wear depth that occurs in the uncoated disc decreases as time progresses; however on the MoN-Cu coated pin, it can be said that the wearing stops after 12 hours.