Ark Pvd Yöntemi İle Üretilmiş Crn Kaplamaların Hss Taban Malzeme Yüzeyinden Sökülmesi

Abstract

Bu çalışmanın amacı, CrN kaplı çelik malzemelerin yüzeylerindeki filmin taban malzemeye hasar vermeden sökülebilmesi olanaklarının araştırılması ve bu amaca yönelik bir yöntem geliştirmektir. Krom kaplamalar demir ve çelik taban malzemelerden 100 gr/lt NaOH çözeltisinde anodik çözündürme tekniği kullanılarak sökülmektedirler. Kroma benzer şekilde CrN filmlerin çözündürülmesi için, alkali çözeltilerde anodik çözündürme tekniğinin kullanılabileceği düşünülmüş ve deneyler bu tekniğin CrN kaplamaları çözme potansiyelini araştıracak şekilde tasarlanmıştır. Bu amaca yönelik olarak, çelik malzemelerin CrN kaplanmasından sonra CrN kaplı çelik malzemenin karakterizasyonuna ve CrN kaplamanın galvanostatik olarak çözündürülmesine yönelik çalışmalar gerçekleştirilmiştir. Daha sonra, galvanostatik yöntemle çözündürülen CrN kaplamanın çözünme mekanizmasını belirlemeye yönelik taramalı elektron mikroskobu ve yüzey profilometresi incelemeleri yapılmıştır. Yapılan çalışmalar sonucunda CrN kaplamanın uygulandığı yüksek hız çeliği taban malzemenin yüzeyinden üç mekanizma yardımı ile söküldüğü gözlenmiştir : 1 - Kromun çözünmesi; ark PVD kaplamalarda tipik bir kaplama hatası olan ve saf krom damlacıklarından oluşan dropletler ilk olarak çözünmektedir. 2 - CrN kaplamanın delaminasyonu; gözenek ve oluşan droplet boşluklarının yardımı ile çözelti taban malzeme-kaplama ara yüzeyine ulaşmaktadır. 3 - CrN' ün çözünmesi; kromun çözünmesine ve CrN kaplamanın delaminasyonuna ek olarak CrN de çözünmektedir vıu

Metals are protected with metallic or ceramic coatings for one or more of the following reasons : 1 - to prevent or to reduce corrosion of the substrate material; 2 - to improve the physical or mechanical properties of the substrate material; 3 - to give desired decorative appearance. The choise of substrate material is usually governed by cost, weight and general physical, mechanical, and manufacturing properties. Chosen single material often does not have the necessary bulk and surface properties. If it has these properties, it is usually uneconomic. Thus, the substrate material should be a cheap material, in good supply, which can provide the necessary structural properties, and it should be coated to provide the required surface characteristics. Coatings develop chemical and physical performance of the substrate. Hard ceramic coatings have been successfully and widely used in many engineering applications, especially on cutting tools due their excellent tribological properties and stable structures since begnning of the 1980s. CrN coatings were introduced recently in industrial areas, such as paper, textile and plastics industry where severe abrasive loads are encountered with good results. Because of their lower stressed structure thicker coatings up to 50 um can be obtained. The corrosion protection properties of CrN coatings are also good due to their higher thickness and lower porosity. As a result, CrN is an ideal coating material candidate for applications where high abrasive loading present such as forming, punching, forging, hot forming, drawing, plastic moulding, etc. CrN coated tools are also used in machining of copper, and some aluminum alloys where TiN coating can not be used due to its chemical reactivity towards these materials. Becauser of CrN coatig' s low chemical reactivity toward titanium, also this material can be machined with CrN coated tools. CrN is also a better alternative for die casting molds because of its higher oxidation resistance. When compared with TiN, CrN coatings exhibit high hardness values, very good toughness, relatively smooth surfaces and, even in the range of high coating thickness, lower friction coefficient in addition to their higher thickness and better IX corrosion protection properties. At high temperatures, up to 700 °C, they show improved oxidation behaviour. As in other coatings such as electrolytic metal coatings stripping of the coating material from the substrate is also needed for hard ceramic coatings. Stripping of hard ceramic coatings is needed in the following cases: 1 - Especially expensive coated tools which are used for some period of time becomes blunt and they are further used after resharpening and recoating. Before the application of the new coating stripping of the old coating is needed. 2 - Tools with defective coatings needs to be recoated after stripping of the defective coating. Stripping especially provides important economic savings if the tools to be coated are expensive, such as gear cutting hobbs and gear shaper cutters. The gear shaper cutter tools used in automobile industry can be recoated with TiN after regrinding after each cycle of use up to 25 times. The most widely used hard ceramic coating TiN can easily be stripped from steel substrates in alkaline solutions containing hydrogen peroxide. This technique is industrially widely used. However CrN coatings are disadvantageous from this point of view. There is not any technologically available simple chemical stripping technique for CrN coatings. It is not possible to strip CrN coatings in alkaline peroxide containing solutions by chemical means as in the case of TiN coatings. In this study, 4 group of experiments carried out in order to evaluate a technique for stripping of CrN films on steel substrates without damaging the substrate material. 1 - Deposition of CrN films on steel substrates 2 - Characterization of CrN films : Coating Thickness and Roughness Coating Structure and Copper Decoration Technique 3 - Galvanostatic experiments 4 - Scanning Electron Microscope ( SEM ) and Surface Profilometer investigations for observation of dissolution mechanism of CrN films There are two techniques for stripping of chromium films : 1 - Immersion in dilute hydrochloric acid : Chromium may be removed from copper, brass and bronze without attack on the underlying metal by this method. 2 - Electrolytic stripping in a solution of caustic soda : It is the preferable technique for the removal of chromium deposits from iron or steel substrates as these metals readily attacked by hydrochloric acid. The potential (E) - pH diagram of CrN - H20 system is constructed in order to consider the electrochemical behaviour of CrN. It has been concluded that, chromium and CrN exhibit similar electrochemical behaviours according to this diagram except the greater stability region of CrN. As a consequence, it is presumed that anodic dissolution technique can be used in order to strip CrN films on steel substrates in an alkaline solution. Thus, the experiments were planned to investigate if this technique has the potential for dissolving CrN films. The substrates were high speed steel sheets in 3 mm x 3 mm size. The sheets were used without hardening. Before coating, the samples were grinded up to 1000 emery paper and then polished with 1 um diamond paste. The surface roughness of the substrates were between Ra = 0.07-0.08 um at this stage. All samples were then ultrasonically cleaned in an alkaline solution. After that, they were rinsed in clear water and left to dry in air. The substrates were sputter cleaned before deposition with a Metel neutral molecule source using a negative voltage (Bias) of 4000 V and under a 5 x 10"3 Torr Argon pressure for a period of 15 minutes. Coating process was conducted in NVT1 2-Novatech Arc PVD coating unit The first step of the deposition process was evaporating the chromium target with arc effect and accelerating the chromium ions formed by ionization towards the substrate using a high bias voltage of 1000 V. By this way, heating of the substrate to coating temperature is provided. At the same time, surface of the substrate is cleaned by sputtering. After reaching a substrate temperature of 440 °C, deposition is started by decreasing bias voltage to 120 V. A thin chromium film was formed at this stage of the process. Then reactive gas, nitrogen is introduced into the vacuum chamber. The total deposition time was 30 minutes. The deposition conditions are summarized in the following table : H The thickness of the CrN films were determined by Calotest and their thickness were in the range of 1.58-2.10 urn. The surface roughness of the CrN coated HSS substrates were in the range of 0. 14-0. 16 um (Ra ). Copper decoration technique was used to identify the coating defects. The principle of the decoration technique is based on simple cementation. The coatings are introduced into the the solution containing copper ions which are noble relative to the steel substrate but base relative to nitride coatings. Copper metal preferentially deposits on defective, coating free sites reaching the substrate and decorate them while the substrate is oxidized to give away electrons that are consumed in the reduction process. CrN coated samples and decorated sample were examined by scanning electron microscope, SEM, ( Jeol JSM 5410 ). It was observed that, CrN coating has a porous structure with droplets. It is well known that films prepared by PVD techniques most often exhibit columnar growth with inherent porosity. In arc PVD coatings, besides inherent porosity, other coating defects originating from the operation parameters can also be a part of the process. A type of defect most frequently encountered in arc PVD coating is the formation of metallic droplets. In stripping of chromium in alkaline solutions the potential difference applied between anode and cathode is 6 Volts. In the experimental trials for stripping CrN similar conditions were used namely; 1 A/cm2 constant current density was applied which gave a 6 V potential difference in the experiments. Under these conditions it was clearly evident that it is possible to strip the CrN coatings by the application of this procedure. However the dissolution rate was too fast with vigorous gas evolution. In order to be able to follow the dissolution of the coatings lower current densities were chosen. The details of controlled galvanostatic experiments are given below: Galvanostatic experiments were conducted in 100 gr/1 NaOH solution at room temperature under 40 mA/cm2 constant current density in order to observe the dissolution mechanism of CrN coating. The dissolution times were 1, 2, 3, 4 and 5 minutes for each sample respectively. One sample was completely dissolved in order to identify the dissolution time. The results of galvanostatic experiments were not consistent, probably due to the different pore and droplet densities on different regions of the coatings on the samples. The surface of the dissolved samples were examined by scanning electron microscope and by surface profilometer, ( Perthen Perthometer S8P ) in order to clarify the dissolution mechanism of CrN coating. Three different type of mechanisms were observed which are effective in stripping of CrN coatings from high speed steel substrate : Xll 1 - Dissolution of Chromium : It was observed that droplets, which are composed of pure chromium, initially dissolved rapidly. Chromium transpassively dissolves in alkaline solutions by the application of positive voltage as expected. In the galvanostatic experiments under 40 mA/cm2 constant current density the potential of the coated material was between 700-900 mV vs SCE. The products of this reaction are chromate ions ( Cr04"2 ), which are yellow in color. Transpassive dissolution reaction for chromium is as follows : Cr + 4 H20 = Cr04""+ 8 H+ + 6 e' 2 - Delamination of CrN Layer : Alkaline solution gets into contact with thin chromium layer between coating and substrate by the help of pores and holes caused by the rapid dissolution of droplets. This thin layer of chromium dissolves readily and causes delamination of the CrN layer. Solid particles which were found at the bottom of the cell after termination of the dissolution process supports this conclusion. The delaminated coating does not dissolve in the alkaline solution because it is not anodically polarized. 3 - Dissolution of CrN : Dissolution of CrN is the slowest mechanism among the mechanisms which are effective in stripping of CrN coatings. The oxidation of CrN occurs first according to the reaction given below : 2 CrN + 3 H20 = Cr203 + 6 H* + N2 + 6 e" Cr2C«3 is further reacts to give chromate ions according to the following reaction : Cr203 + 5 H20 = 2 Cr04~" + 10 H+ + 6 e" The tree dimensional surface profiles of the dissolved samples showed that, the CrN layer is getting thinner by the time which indicates the dissolution of CrN. Decrease of the thickness of the coating was not homogeneous due to the local character of the dissolution. The surface examination of the substrates after stripping of chromium showed pitting of the substrate material. The behaviour of high speed steel in alkaline solutions is not the subject of this study. However, a study investigating behaviour of high speed steels in alkaline solutions would be worthwhile in highlighing the problem encountered in stripping of CrN on high speed steel substrates and in finding possible solutions to this problem.