Bu çalışmada, katodik ark fiziksel buhar biriktirme yöntemi ile krom kaplanan, krom kaplama sonrası farklı sürelerde yüksek bias voltajı etkisi ile krom iyonu bombardımanı uygulanan ve darbeli bias voltajı uygulaması ile krom kaplanan düşük karbonlu çelik malzemelerde meydana gelen spinodal fazlar karakterize edilerek, gelişimleri incelenmiştir. Elektron mikroskopisi, elementel analizler ve kesitlerden alınan derinlik profili çalışmaları ile gerek kaplama sonrası uygulanan krom bombardımanı, gerekse darbeli bias uygulamasında yüksek bias periyodlarındaki krom bombardımanının, krom kaplama ve düşük karbonlu çelik taban malzeme arasında radyasyon destekli yayınma mekanizması ile gerçekleşen bir iyon demeti karışımı prosesine neden olduğu tespit edilmiştir. Bunun sonucunda, taban malzemeden kaplama yüzeyine büyük oranda demir atomu yayınmış ve yüzeylerde çok kısa süreler içerisinde spinodal ayrışma fazları meydana gelmiştir. Bu fazların morfolojileri, literatürde demir-krom (Fe-Cr) sistemindeki spinodal ayrışma çalışmalarında elde edilen mikroyapılar ve bilgisayar simülasyonu ile oluşturulan spinodal yapılar ile oldukça büyük benzerlikler göstermiştir. Ayrıca bombardıman uygulanan örneklerde, spinodal ayrışma yapılarının bir karakteristiği olan tepe/vadi oluşumları ile fazlar arasındaki bileşimsel dalgalanmalar gibi sonuçların gözlenmesi de spinodal fazların oluşumunu doğrulamaktadır. Elde edilen mikroyapılara bakıldığında, taneli taban malzeme üzerinde epitaksik olarak büyüyen kaplamalardaki bazı tanelerde, spinodal ayrışma fazları oluşmadığı görülmüştür. Bununla birlikte, bombardıman süresinin artması ile bu tanelerde de spinodal ayrışma yapılarına benzer oluşumlar gözlenmiştir.

In this study, spinodal decomposition phases formed in low carbon steel samples which were Cr coated, Cr coated and bombarded with Cr ions through applying high bias voltage (1 keV) and Cr coated through applying pulsed bias voltage in a cathodic arc physical vapour deposition (PVD) system were characterized and compared.1.5 mm thick IF (interstitial free) steel sheets were used as substrate. The bombardment periods after the coating were 1 minute, 2 minutes and 3 minutes. In pulsed bias mode, bias voltage was alternately increased to 1 keV for 5 seconds (bombardment section) and decreased to 150 eV for 10 seconds (coating section) along 10 minutes. Total bombardment time was 3.3 minutes in 10 minutes of pulsed bias. In order to determine the spinodal decomposition phases, Field Emission Microscopy (FEM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectral Analysis (EDS) and Glow Discharge Optic Emission Spectroscopy (GDOES) studies were performed. Not only the bombardment applied after the coating process, but also the one applied during the high bias voltage period in the pulsed bias mode caused radiation enhanced diffusion between Cr coating and low carbon steel substrate. This was proven by EDS analysis, GDOES analysis and elemental maps taken from cross sections of the samples. Fe content of coatings increased gradually by increasing bombardment time. As a result of ion beam mixing, severe iron diffusion from the substrate to the Cr coating’s surface took place and this resulted in formation of spinodal decomposition phases in a very short time. These phases’ morphologies were quite similar to microstructures obtained from spinodal decomposition studies in Fe-Cr system and computer simulated spinodal structures in literature. Besides, it was observed through the EDS analysis and FEM studies of the bombarded samples that hill/valley formations and compositional fluctuations which were the characteristics of spinodal decomposition process were also present. Hills were Cr rich and valleys were Fe rich parts of the spinodal structures. It was observed in FEM micrographs that hills and valleys were getting wider with increasing bombardment period and it was another characteristic of spinodal decomposition structures. When the average cohesive energy of the Fe-Cr system (4.19 eV) and the relationship between the cohesive energy and the critical temperature $(T_capprox 100E_{koh})$ at which radiation enhanced diffusion (RED) would be efficient were taken into consideration, ion beam mixing in our system woud take place through RED regime. Altough both metals have similar cohesive energies, dominant Fe movement to the Cr coating was observed in pulsed bias mode instead of an isotropic mixing.While Fe-Cr alloys must be heat treated for very long times (up to 500 hours) in conventional spinodal decomposition processes, it is very interesting that the spinodal decomposition phases formed in 1-3 minutes in this study. It is believed that the diffusion enhanced by the radiation effected the spinodal decomposition kinetics. As a result of thermally activated migration of defects caused by Cr ion bombardment and short range diffusions in very hot thermal spikes formed by the collisions between bombarding ions and target atoms accelerated the diffusion processes which make the spinodal decomposition occur. In some grains of the coatings which grew on grained microstructure of the substrate material epitaxially, spinodal decomposition was not observed. These smooth grains with no hill/valley structure were supposed to be $sigma$ phase which was not decomposed during the bombardment and cooling periods. The coherency strain between the grains may have displaced the spinodal line (coherent spinodal) in some grains resulting in not occuring spinodal decomposition in chemical spinodal line. However, spinodal structures started to form in those smooth grains by increasing the bombardment time and temperature. It was observed via EDS analysis taken from the samples which were not only bombarded after coating but also coated through applying pulsed bias that Fe content increased by increasing the bombardment time. Formation of spinodal structures in smooth grains and increasing of the Fe content concurrently by increasing the bombardment time points that spinodal decomposition occurs in Fe rich part of the spinodal line in those smooth grains. As a potential development of this study, more efficient temperature control should be applied to determine chemical and coherent spinodal lines in Fe-Cr alloys produced by ion bombardment of Cr coated low carbon steels.