Nötral Molekül Kaynağı Kullanılarak Yapılan Sıçratma İşleminin Yüksek Hız Takım Çeliklerinin Yüzey Özelliklerine Etkisi

Abstract

Sert seramik ince film kaplamalar, günümüzde giderek daha geniş uygulama alanları bulmaya başlamaktadırlar. Bu tip kaplamalar yüksek sertlik, aşınma direnci ve korozyon direncinin gerektiği uygulamalarda yaygın olarak kullanılmaktadırlar. Aynı zamanda, parlak yüzey özelliklerinden dolayı dekoratif amaçlı olarak da kullanılmaktadırlar. Bu tip ince film kaplamalarda, kaplamadan beklenen en önemli özelliklerden birisi, kaplamanın taban malzemeye olan yapışma mukavemetidir. Kaplama işlemi bir yüzey işlemi olduğu için, yapılan kaplamanın kalitesi direk olarak taban malzemesinin yüzey özelliklerine bağlıdır. Bu yüzey özelliklerinin başında taban malzemesinin yüzey temizliliği ve pürüzlülüğü gelmektedir. Söz konusu yüzey özellikleri, kaplama öncesi yapılan yüzey hazırlama ve temizleme işlemleriyla yakından ilgilidir. Yüzey temizleme amacıyla kullanılan klasik yöntemlerin yanısıra, özellikle son yıllarda plazma altında neutral gaz iyonları veya molekülleriyle yapılan sıçratma yöntemi (iyon bombardımanı) de, özellikle ince film kaplama işlemlerinde kullanılmaya başlamıştır. Bu yöntem ayrıca elektronik sanayinde hassas devrelerin hazırlanmasında da yaygın olarak kullanılmaktadır. Yöntem, diğer temizleme yöntemleriyle ulaşılamayacak yüzey temizliğinde yüzey hazırlanmasına olanak tanımaktadır. Özellikle, ince film kaplama işlemleri öncesi kullanılan freon gazının kullanımının sınırlanması yöntemi daha yaygın kullanılır hale getirmiştir. Yöntemin en büyük sınırlaması, yüksek vakum altında çalışma zorunluluğudur. Bu tez çalışmasında, nötral argon molekülleri kullanılarak yapılan yüzey temizleme işleminin, yüksek hız çeliği takımlarının yüzey özelliklerine etkisi incelenmiştir. Söz konusu yüzey özellikleri, ince film kaplamaların yapışma özelliklerini çok yakından ilgilendiren taban malzemesinin yüzey pürüzlülüğü ve yüzey morfolojisi özellikleridir. Çalışmada, özellikle yapılan sıçratma işlemi sonrası değişen yüzey pürüzlülüğü ve üç boyutlu yüzey profillerinin yapışma üzerindeki etkisi ortaya konmaya çalışılmıştır. Söz konusu değişimler büyük oranda yüzeyi işleme tabi tutulan malzemelerin yüzey morfolojileri ile ilgili olduğu için, Taramalı Elektron Mikroskobu ile yapılan yüzey incelemesi çalışmalarına ağırlık verilmiştir. Yüksek hız takım çeliklerinde ağırlıklı olarak rastlanan yüzey karbür dağılımının gerek yüzey pürüzlülüğüne gerekse de yapışma özelliğine olan etkisi ayrıntılı olarak incelenmeye çalışılmıştır.

Hard ceramic coatings whose thicknesses are in the range of 0.5-5 um are widely used in applications requiring high wear resistance owing to their high surface hardness and wear resistance. These thin films have also been using for decorative applications. Increasing number of application fields such as machining technology, making of microcircuits in electronic industry etc., have caused the developments of specific surface preparation and pre-coating applications for these thin coatings. Conventional surface preparation and cleaning techniques such as chemical and ultrasonic are used as-coating treatments in surface plating industry.. The drying process followed by surface cleaning is the second step of the surface preparation. In preparation of surface for hard ceramic film coatings, the quality of surfaces after drying, without stains, is one of the key parameters for ensuring good adhesion of the coating to the substrate. Freon was widely used for drying purposes but its usage is limited due to environmental reasons. Ion Etching (or ion bombardment) is a specific surface preparation technique especially used in thin film operations for the electronic industry. The technique is also called sputtering because of its surface cleaning effect is based on the sputtering of surface atoms of substrate to be cleaned. The ion etching process is mainly based on the generating noble gas ions in a vacuum chamber and accelerating them through to a plasma environment to the substrate to be etched. Argon is the most widely using gas for the sputtering. Because the discharge of electrons which ionise the noble gas atoms only occurs at very high vacuum, the vacuum level of the discharge chamber is an important factor for ion etching. The required vacuum for discharge is about lO^torr. Such a high vacuum level can only be achieved by using special vacuum systems. In order to etch the surfaces of substrates, either gas ions or neutral gas molecules can be used. Although the principles of the discharge is almost the same, using of neutral gas molecules instead of charged gas ions, the effect of the etching on the substrate is somewhat different. Most significant advantage of using neutral gas molecules in a sputtering process is the reduced substrate heating effect, which may affect the surface structure with respect to bombardment with the ions. The generation of the highly energetic electrons in vacuum chamber is the first step of the ionisation process. These energetic electrons are generated at cathode region and then reach the anode region of the chamber. The ions generated by the energetic electrons have almost the anode potential applied. Second step at sputtering process is the accelerating of the noble gas ions in the vacuum chamber toward to substrate to be etched. The accelerating effect is achieved by means of accelerator grids placed in parallel combination with respect to each other. Higher the voltage applied to the these grids, namely accelerating voltage, faster the ions are excited to the substrate with a potential equal to the accelerating voltage. One of the most important advantages of the sputtering process is the easy adjustment of process parameters enabling the setting of the desired cleaning conditions easily during the sputtering operation. The ability of using the sputtering process which is an in situ operation in the same vacuum chamber in which the thin film deposition is also be conducted makes the process especially suitable for thin film deposition techniques. The most important system parameters in sputtering are the accelerating voltage and the sputtering angle. The sputtering angle can be simply described as the angle which the accelerated ions or molecules contact the surface to be etched. In general, sputtering / etching rate increases as the accelerating voltage and the contact angle are increased. Basically, the affectivity of a sputtering operation is described with the sputtering/etching rate, so-called sputtering yield. The sputtering yield is directly related to the ion energy and the sputtering angle up to a certain energy level described as the threshold level. Beyond this level, the effect of the ion energy is not so much pronounced. For this reason, working over this threshold point causing unexpected substrate heating rather than increasing the sputtering yield. The sputtering process is mainly a surface treatment as described above, so the interaction between the gas ions using for sputtering and surface atoms of the substrate to be etched become important. When an energetic ion or molecule contact the surface of a material, there are three possible interactions between two massive particles as following. -The ion or molecule may reflect from the surface with an angle equal to the contact angle. -The ion or molecule sputters away an atom from the surface layer.. -The ion or molecule may penetrate to a certain depth depending on its energy and contact angle. All these interactions may occur at the same time in a sputtering process. As pointed out above, the adhesion between the coating and the substrate is one of the most important thin film properties. As the coating operation is an interface related phenomena, the properties of the substrate on which the thin film coating applied has a tremendous effect on the adhesion of coating. The most important surface properties of substrate affecting the adhesion are the surface roughness, surface cleanness and the surface morphology. Related works on the relation between the surface structure and the adhesion of coatings showed that XI adhesion between surface of substrate and the thin film is directly related the surface roughness and cleanness of the substrate. Referring the works that examined the effect of the sputtering process on the adhesion between TiN onto HSS materials, increasing sputtering conditions such as accelerating voltage causes the increasing adhesion properties showing the effectiveness of the sputtering process. In this study, the effect of the sputtering process on the surface properties of HSS materials was examined. The examined surface properties were the surface roughness and the surface morphology having the major effect on the adhesion properties of thin films. The experimental works conducted in this study is summarized below. -Sputtering of HSS materials with a neutral molecule source at different process parameters, namely varying accelerating voltages/constant sputtering angle and varying sputtering angle/constant accelerating voltage. -Measurement of the surface roughness of the etched/sputtered surfaces by using a surface profilometer. -3-D surface profiling of the surfaces by using parallel recording capabilities of surface profilemeter. -SEM (Scanning Electron Microscopy) investigations of the etched/sputtered surfaces. As pointed out above, the surface roughness and the morphology are the most affective surface properties affecting the adhesion properties of thin films. Varying sputtering process parameters affects the surface properties of materials in different ways. The sputtering process has conducted with different process parameters, namely accelerating voltage and the sputtering angle, in order to show the effect of the varying parameters on the surface properties of the materials to be etched. Two group of sputtering experiments used in the study. In first group of experiments sputtering was conducted by applying constant sputtering angle of 60° and the varying accelerating voltage in order to show the effect of the ion voltage on the surface roughness and morphology of the HSS materials. In thesecond group of experiments, the sputtering angle was varied at constant accelerating voltage. The sputtering process parameters are listed below -First group of experiments: At accelerating voltages of 1.5 keV, 3 keV and 5keV under constant sputtering angle of 60 degrees. -Second group of experiments: At sputtering angles of 0 degrees, 30 degrees and 60 degrees under constant accelerating voltage of 3 keV. Xll During the sputtering process conducted with neutral molecule source METEL, the specimens were partially covered with an alumina plate in order to be able to investigate both as-received and sputtered parts of the same sample. Roughness measurements of surfaces were conducted on the etched and as-received surfaces. A slight difference was observed on etched and as-received surfaces showing that sputtering had an effect on the surface roughness of materials. The roughness parameters of sputtered and as-received surfaces for varying accelerating voltages are summarized below. It was also observed that as the accelerating voltage increases, the surface roughness of etched materials also increase as shown above.. The similar effect was observed in experiments that were conducted with varying different angles. It was found that the effect of sputtering angle on the roughness of surfaces was less pronounced than accelerating voltage. The combined effects of varying accelerating voltages and sputtering angles is shown in figure illustrated below. N 0,46 ? Varying Angles E3 Varying Voltages! 1 2 3 Number of Experiments Xlll As shown in this figure, both variables, namely accelerating voltage and sputtering angle influnce the surface roughness of materials etched. But the effect of the accelerating voltage is higher than the that of sputtering angle. This effect makes the accelerating voltage more effective in a sputtering process at establishing the desired cleaning conditions. The results of SEM investigations of sputtered surfaces revealed a difference between etched and as- received surfaces. The SEM micrographs of as-received surfaces showed a normal carbide distribution on the surfaces. These carbides were analyzed by using EDS ( X- Ray Energy Dispersive Spectroscopy) system and detected as WC particles. SEM investigations conducted on the etched surfaces showed a fine dispersed carbide distribution together with coarse WC particles. These fine carbide particles were detected as VC showing that these particles became visible on the etched surfaces as a result of neutral molecule bombardment. This effect is caused by differential sputtering of matrix material with respect to carbide particles. The changing of surface roughness parameters of etched surfaces detected by surface roughness measurments can be deducated to the changing surface morphologies of etched materials. The changing of the surface profile as a result of neutral molecule bombardment was detected by using 3-D surface profiling. The results verified the surface roughness experiments. As a result of increasing accelerating voltage and sputtering angle, a slight difference was observed at sputtered and as- received surfaces. There was no difference between etched and as-received surfaces of materials sputtered at angle of 0°. This result verified the surface roghness measurements. This result showed that, there was a thereshold angle at which the surface roughness and the profile did not change so much. As pointed out above before, surface morphology and structure have tremendeous effect on the adhesion of thin films. Sputtering process using for surface treatment of the substrates to be coated changes the surface structures of materials. As the accelerating voltage increases in sputtering process with neutral molecule source, the adhesion of TiN films also increase referring the related works. This improvement can be deducted to the increasing of the surface cleanness and the roughness which is at micron scale. The small carbide particles that become visible on the etched surface also contributes to this improvement. XIV