D.A. manyetik alanda sıçratma yöntemi ile tungsten karbür ince filmlerin üretimi ve karakterizasyonu

Abstract

Tungsten Karbür ince filmlerin sahip olduğu yüksek sertik, aşınmaya, oksidasyona ve korozyona karşı yüksek direnç, iyi elektrik iletkenliği, kimyasal inertlik ve düşük sürtünme katsayısı gibi üstün özellikler; tungsten karbür ince filmlerin aşınma uygulamalarında, kesici aletlerde ve kalıplarda, metal işleme aletlerinde, madencilik araçlarında, pervaneler ve yüksek basınçlı kompresörler gibi jet motor parçalarında etkin şekilde kullanılmalarını sağlamaktadır. Bu filmlerin gösterdiği mekanik ve tribolojik davranışlar, üretim yöntemleri ve parametrelerine bağlı olarak önemli ölçüde değişmektedir. Bu projede doğru akım manyetik alanda sıçratma yöntemi ile WC-Co hedef malzeme kullanarak, homojen ve taban malzemelere iyi yapışan tungsten karbür ince filmler elde etmek ve biriktirme parametrelerinin üretilen tungsten karbür ince filmlerin kimyasal, mikroyapısal, mekanik ve tribolojik özelliklerine etkilerini incelemek amaçlanmıştır.Bu çalışmada ağırlıkça %7 oranında bağlayıcı metal olarak kobalt içeren sinterlenmiş tungsten karbür hedef malzeme kullanılarak, argon gazı ortamında, silisyum plaka ve yüksek hız çeliği üzerine 450-1060nm arası kalınlıklarda tungsten karbür ince filmler biriktirilmiştir. Kaplama işlemleri 0,3Pa çalışma basıncı altında, 200W hedef gücü, 40cm3/dk argon gaz akışı, 0-200V bias voltajı kullanılarak, oda sıcaklığında ve 250?C?de, 120?şer dakika süreyle gerçekleştirilmiştir. Filmler aynı sürelerde farklı bias voltajı değerlerinde ve farklı sıcaklıklarda biriktirilmiştir. Böylelikle bias voltajı ve sıcaklık gibi parametrelerin film özelliklerine etkisi gözlenmiştir.Biriktirilen filmlerin kalınlık ölçümü ve mikro yapı incelemeleri numunelerin kesitlerinden tramalı elektron mikroskobu ile yapılmıştır. Kaplama kalınlıklarının uygulanan bias voltajı değerinin artmasıyla azaldığı görülmüştür. Yüksek bias voltajı kullanılarak biriktirilen filmlerin kalınlıklarının düşük bias voltajında biriktirilen filmlerin kalınlıklarından daha düşük olduğu görülmüştür. Taramalı elektron mikroskobu ile gerçekleştirilen mikroyapı incelemeleri sonunda harici ısıtıcı kullanılmadan biriktirilen filmlerin artan bias voltajı ile kolonsaldan kolonsuz yoğun bir yapıya geçtiği gözlenirken harici ısıtıcı kullanılarak biriktirilen filmlerin ise kolonsal bir yapıda olduğu ve artan bias voltajı değeriyle yüzey pürüzlülüğünün arttığı gözlenmiştir.Kaplama yapısındaki tungsten, karbon ve kobalt elementlerinin oranları elektron prob mikroanaliz cihazı ile incelenmiştir. Harici ısıtıcı kullanılmadan ve kullanılarak biriktirilen filmlerde uygulanan bias voltajı değeri arttıkça yapıdaki tungsten oranının arttığı, kobaltın azaldığı, karbonun ise yaklaşık olarak sabit kaldığı gözlenmiştir. Aynı cihaz ile yapılan elementel haritalama sonucunda W, C ve Co elementlerinin kaplama yapısında homojen olarak dağıldığı gözlenmiştir.X-ışını difraktometresi ile yapılan faz analizleri sonucunda uygulanan bias voltajının artmasıyla kaplama yapısında kubik tungsten fazından kubik tungsten karbür (WC1-x) fazına geçiş görülmüştür. Düşük bias voltajı değerlerinde biriktirilen filmlerin yarı kristalin (amorf ve nanokristal karışık yapıda) özellik gösterdiği, uygulanan bias voltajı değeri arttıkça kristalin bir yapıya geçtiği, 200V gibi daha yüksek bias voltajı değerlerinde ise yapının amorflaştığı gözlenmiştir.Nanosertlik cihazı kullanılarak biriktirilen filmlerin sertlik ve elastik modülü değerleri ölçülmüştür. Kaplamalarda ~1500-2500Hv arasında değişen sertlik değerleri ve 240-350GPa arasında değişen elastik modülü değerleri elde edilmiştir. En yüksek sertlik ve elastik modülü değerlerinin sırasıyla ~2410Hv ve ~350GPa olarak ölçülen, harici ısıtıcı kullanılarak 250?C?de ve 100V bias voltajı uygulanarak biriktirilen filme ait olduğu görülmüştür.Aşınma testi cihazı kullanılarak filmlerin sürtünme katsayıları belirlenmiş ve aşınan yüzeylerden yapılan profilometre ölçümleri sonucunda elde edilen verilerden filmlerin aşınma hızları hesaplanmıştır. Artan sıcaklık ve bias etkisiyle filmlerin sürtünme katsayılarında önemli oranda azalma gözlenmiştir. Yüksek bias voltajı uygulanarak biriktirilen filmlerin aşınma hızlarının diğer filmlere göre daha düşük olduğu gözlenmiştir.

Transition metal carbides such as tungsten carbide present high interest because of their specific physical and mechanical properties. Tungsten carbide has high melting point that is 2870?C for WC phase. It exhibits extreme hardness, low friction coefficient, chemical inertness, oxidation resistance and good electrical conductivity. These specific properties make this material an ideal candidate for industrial applications like wear-resistant coatings, cutting and drilling tools. Tungsten carbide coatings are deposited via various deposition processes such as plasma spraying, physical vapor deposition (PVD) and chemical vapor deposition (CVD). Sputtering deposition which is used to deposit thin films in this study is a low temperature process in contrast with the CVD processes which is increasingly used for industrial hard coating.In this study, tungsten carbide thin films were deposited on high speed steel (AISI-M2) and Si wafer substrates by DC magnetron sputtering of tungsten carbide target. The microstructural, chemical, mechanical and tribological properties of the coatings have been modified by the change in the bias voltage from grounded to 200V and by the deposition temperature from room temperature to 250?C. External heater was used to deposit films at 250?C. The microstructure and thickness of the films were analyzed by scanning electron microscobe (SEM). The chemical compositions of the film were analyzed by electron probe microanalyse (EPMA) device. XRD was used for phase analyses. Nanoindentation tests and wear tests were conducted to determine the hardness, elastic modulus and wear resistance of the films. The results of the analyses provided key information about the relationship between the deposition parameters and the microstructural and mechanical properties of WC films.The sputtering target material was a 150mm diameter 7mm thick sintered tungsten carbide target which contains 7% cobalt (Co) as binder material. Substrates were previously cleaned in ethanol in an ultrasonic bath for 15 minutes. Before each deposition the sputtering chamber was evacuated to approximately 10-5Pa by a combination of rotary and turbomolecular pump system. Sputtering process was carried out by using pure argon as sputtering gas. All coating processes were carried out under an operating pressure of 0.3Pa. The target was pre-sputtered and substrates were bias etched in Ar plasma during 10 minutes by increasing the bias voltage from 50V to 250V. Various thin films were deposited on substrates by changing the bias voltage from 0V to 200V and by changing the substrate temperature from room temperature to 250?C. For both substrate temperatures four samples were prepared with 0V, 50V, 100V, and 200V bias voltages. Coating process lasted for 120 minutes in all experiments to reach the desired coating thickness. In all the runs, the sputtering DC power was fixed at 200W. External power supply was operated during the all experiments.The thicknesses of the coatings were determined from the cross-sectional SEM images of the Si samples. The film thickness were differed from 450nm to 1060nm. It is observed that coating thicknesses were decreasing with increasing bias voltages and films, which were deposited with higher bias voltages, were thinner than the other films. The decrease in the film thicknesses were explained by the resputtering effect with the increase in bias voltages. The morphology of the films was also determined from the cross-sectional SEM images of the Si samples. Well-adherent, homogeneous and dense film were obtained according to SEM analyses. It was observed that films deposited at room temperature had columnar structures and a transformation from columnar to non-columnar, featureless morphologies occurs with the increase in bias voltages. Films deposited at 250?C were also exhibited columnar structure and it is seen that the surface roughness were increased with increasing bias voltage. It is also observed that films deposited at room temperature have less surface roughness than the films deposited at 250?C.The chemical composition of the films was analyzed by EPMA for the films deposited on steel substrates. The concentration of tungsten, carbon and cobalt in the films were investigated. It was observed that the carbon content in the films is quite constant between ~7-8% by weight. The weight difference of carbon in the films is negligible when it is compared with other elements; tungsten and cobalt. Tungsten content in the films increased from 86% to 91% by weight while the cobalt content decreased from 7% to 0.5% with the increase in bias voltages.Phase analyses of samples were carried out by X-Ray Diffraction analysis for the films deposited on steel substrates. XRD analysis was carried out with a glancing angle attachment using Cu-K? radiation over the range of 10-90?. The ? scan mode with a fixed incidence angle of 1? was used. It is observed that films deposited at room temperature with 0V (WC-1) and 50V (WC-2) bias consist of a nanocrystalline or amorphus+nanocrystalline mixed metallic tungsten phase. While the bias voltage increases, it transforms to a nonstoichiometric WC1-x phase. It is determined that films deposited at room temperature with 100V (WC-3) and 200V (WC-4) bias exhibit WC1-x phase. It is observed that film deposited at 250?C without applying any bias voltage (WC-5) consists of a cubic tungsten phase and the films deposited at 250?C with 50V (WC-6), 100V (WC-7) and 200V (WC-8) bias voltages exhibit WC1-x phase. Higher bias voltage causes a phase formation in the tungsten carbide films. The XRD peaks that obtained from the samples WC-1, WC-2 and WC-5 exhibit amorphous-like structure. With the increase in bias voltage, a phase transformation was observed for WC-3, WC-6 and WC-7. These peaks exhibit nano crystalline structure. Further increase in bias voltage led an amorphization(WC-4, WC-8).Nanoindentation tests were carried out to determine the hardness and elastic modulus of the films. Hardness and elastic modulus of the films were determined by the CMS Instruments Nano-Hardness Tester for the films deposited on steel substrates with a load of 20mN. Five measurements were carried out for each sample. The results of these analyses provided information about the relationship between the various parameters and the microstructural and mechanical behavior of the films. Film hardnesses and elastic modulus values were differed from 1500-2500Hv and 240-350GPa, respectively. Hardness test results showed that sample WC-3 and WC-7, which were deposited with 100V bias at room temperature and 250?C respectively, exhibit the highest hardness and elastic modulus values which can be explained by the phase transformation from W to WC1-x.Wear tests of the films were carried out by Tribo Technic Oscillating Tribo Tester for the films deposited on steel substrates. Friction coefficients and wear rates of the films were determined. Friction coefficients of the films were between 0.15 to 0.50. The friction coefficients of the films decreased with increasing bias voltage and substrate temperature. Wear depths and widths of the samples were measured with profilometer and by using these data the wear rates of the films were calculated. Wear rates of the films were between 1,8x10-6-1,5x10-7mm3/Nm. It is found that films deposited with higher bias voltages, have lower wear rates. The sample WC-6, which was deposited with 50V bias at 250?C, exhibits the highest wear resistance.