Mo-N esaslı PVD kaplamaların oksidasyon davranışları

Abstract

Geçiş metal elementlerinin nitrürlerinden oluşan ince film kaplamaların dayanım özelliklerinin çok iyi olduğu bilinir. Bunların kullanım alanları gittikçe artmakta ve teknolojinin birçok sektöründe kullanılmaktadırlar. MoN esaslı ince seramik filmlerin aşınmaya karşı dayanımının yüksek olması makina imalat sanayinde kullanılabilme potansiyelini arttırmaktadır. MoN ince seramik filmler ayrıca dekoratif amaçlı olarak, mikroelektronik (difüzyon engelli) sanayinde, elektriksel aygıtlarda ve süper iletken malzeme üretimlerinde kullanılabilirler. Bu çalışmada ark FBB yöntemi ile alumina plakalara kaplanmış MoN'ün davranışları incelenmiştir. Değişik sıcaklık ve zamanda oksitlenen MoN kaplı numunelerin X Işınları analizleri ile bileşenleri, fazları ve yönlenmeleri belirlenmiştir. Kaplamaların kalınlıkları da taramalı elektron mikroskobu yardımıyla ölçülmüştür. Alumina plakalar, düşük sıcaklıkta kaplamalara imkan tanıyan fiziksel buhar biriktirme yöntemi olan ark FBB tekniği ile 450°C-500°C kaplama sıcaklığında, 3x10" 3 Torr kısmi azot basıncında 5 dakika gibi kısa sürede Mo-N esaslı ince seramik film kaplanmıştır. Literatür araştırmaları sonucunda Mo-N esaslı ince seramik filmlerin 650°C civan ve üzerideki sıcaklık değerlerinde tamamen tahribata uğradığı bilinmektedir. Seçilen oksidasyon deney sıcaklıkları (450°C 30 dak., 400°C 60 dak.; 450°C 5 dak., 450°C 10 dak., 450°C 15 dak.; 500°C 3 dak., 500°C 5 dak.) bu bilgi ışığı altında belirlenmiştir. Oksidasyon deneyi iki ucu açık normal hava akışının mevcut olduğu yatay tüp fırında gerçekleştirilmiştir. Oksidasyon deneyleri sonucunda numune yüzeylerinde renk değişimleri gözlenmiş ve MoN esaslı kaplamanın metalik gri olan rengi mavi renge doğru bir değişim göstermiştir. X Işınları analizlerinden de aşağıdaki sonuçlar elde edilmiştir. 1) Mo-N esaslı ince seramik kaplamalar MoN ve Mo2N fazlarından oluşmaktadırlar. 2) 400-500°C sıcaklık aralığında belli sürelerde atmosferik koşullarda beklentiler Mo-N esaslı ince film kaplamalarda MoN kısa sürede ortadan kalkmakta ve Mo2N fazının yanında Mo02 ve Mo03 fazları oluşmaktadır. 3) Mo-N esaslı ince seramik film kaplamalarda Mo2N fazı, çalışılan 400-500°C sıcaklık aralığında değişik etkinliklerde yer almaktadır.

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 well known PVD method. The basic evaporation process involes the transfer of material to form a coating by vaporization. Unlike the other vapor deposition processes, evaporation is a low energy process with particle energy averaging 0.2 to 0.3 eV. The tecnique is very successfully used in applications where adhesion and good structure are not critical such as decorative and optical uses. Evaporation is not used to coat sustrates subjected to mechanical forces and to corrosive environment. IX 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. Thin films and coatings of transition metal nitrides are well known for their outstanding properties and find continuously incresing interest and application in many field of technology. Most of the investigations about MoN coatings are related to their use as wear and friction-reduction coatings for tools; other applications are in the field of decorative aplications, microelectronics, electrical devices and superconductivity. The microstructure of MoN films depend on the coating parameter such as deposition temperature, nitrogen pressure and bias voltage. Coatings with transition zone microstructure are most dense. MoN coatings have metalic gray color. Adhesion of films to the substrate have major importance in relation to coating performance in wear aplications. MoN coatings have a good adhesion properties. MoN coatings are prefered in cutting industry due to high hardness value (1600 HV). The wear resistance of MoN, TiN, CrN coatings are attributed to their high hardness as well as good chemical resistance to various chemicals. The oxidation threshold temperatures of MoN, TiN, CrN are known to be 400°C,550°C and 700°C respectively. Therefore, these coatings maye have potantial for wear resistance application at elevated temperatures. Iln this study, the PVD method of cathodic arc evaporation is used for deposition. The coating substrate was alumina plates. Alumina was used as the substrate material since the coating is not affected by alumina and the qualities of coating could be determined without the influence of substrate materials. The alumina samples have been coated with mainly MoN which included some M02N ceramic films. Coating was obtained with arc PVD technique at 450-500°C and under particial pressure of nitrogen 3x1 0"3 Torr, in five minutes. The study was coducted in order to determine the oxidation behaviour of MoN coated alumina substrate. The presence of various molybdenum nitrides phases and molybdenum oxides phases was determined using X-Ray difraction methods. The thickness of coatings were determined by scanning electron microscopy. MoN coatings are completely oxidized at temperatures above 700°C. Hence oxidation behaviour of molybdenum nitride coatings were investigated at temperatures between 400°C-500°C in open tube furnace. The oxidation conditions were: 400°C in 30 min., 400°C in 60 min. 450°C in 5 min., 450°C in 10 min., 450°C in 15 min. 500°C in 3 min., 500°C in 5 min. A change in colours was observed during these oxidation experiments: The metalic gray color MoN changed to blue after oxidation. Seven MoN samples were oxidized in tube furnace. Original MoN and oxidized samples were analysed with a X-Ray difractometre and results of analysis are given Fiqure 1,2,3 The original coating was composed of MoN together with a little of Mo2N.With oxidation MoN lost in nitrogen and was converted to Mo2N. Oxidation of Mo-N gave Mo02 and M0O3. At 500°C and longer oxidation times M0O2 and M0O3 were the dominant oxides. XI MftRZHI ıoo. (oounti) J 81.©. 64. O. 49.». 36. ©. as... 16 -O. ».©. 4 O. I... (a) o.o M48030! (b; (counts!. 81.Ö. 80 l°3© (1400&B (C) Flqure 1. The XRD results of original Mo N and oxidized Mo-N samples. a) Original Mo-N sample b) 400üCin30mm. c) 400°C in 60 min. xn f14 J05- (counts 1 J 81.0. 64. O J 49.O. 36.O. 26.0. 16.0. 9.0. 4. O ı.o-l O.o M45810 100.0 t counts ] J 81.0 J 64.O J 49.01 36.O. 25 -O. 16. O. 9.O. 4.©. 1.O. O.O N4S815 tAw (a) (b] (c) Figür» ?" The results of oxldexed Mo-N samples. a) 460öCln5mln. b) 450°Cln10mln. c) 4S0QCln15mln. xiii ıoo.o [ counts 1 J. 1.O. «4.0. «9.O. 36.O. 2B.O 9.0. 4.O. 1.O. O.O 115003 (a) too.o t count» ] J O 1. O Flqurt 3. The results of oxldezed Mo-N samples. a) SÖOPc İn 3 min. b) 500°Cln6mln.