Ark Pvd Yöntemi İle Aısı 316 L İmplant Grade Paslanmaz Çelik Üzerine Kaplanmış Zrn Kaplamaların Ö Özellikleri

Abstract

Buhar biriktirme yöntemleriyle üretilen metal-nitrür ince sert kaplamalar, gösterdikleri üstün özelliklerden dolayı, dekoratif amaçlı kullanımlardan, kesme uygulamalarına ve korozyon direncinin gerekli olduğu uygulamalara kadar bir çok alanda geniş kullanım olanağı bulmaktadırlar. Bugüne kadar TİN kaplamaların kalite ve performansını geliştirici çalışmaların üzerinde yogunlaşılmışken ZrN gibi alternatif kaplamalara daha az ilgi gösterilmiştir. Gerek mekanik, gerekse de kimyasal özellikleri ele alındığında kaplamalar dikkat çekici kaplamalar olarak görülmeye başlanmıştır. ZrN, yüksek sertliğe (2300-2600 HV), yüksek ergime noktasına (2980 °C), yüksek ısıl ve kimyasal kararlılığa sahip bir malzemedir. ZrN ve TİN kaplamaların her ikisi de altın şansı renge sahiplerse de ZrN'nin rengi daha parlak ve çekicidir. ZrN, TİNden daha sert olduğu için iyi bir çizme direnci göstermektedir. Taban malzemesi ile arasındaki yapışmanın iyileştirilmesi ise ara katmanların biriktirilmesi ile sağlanabilmektedir. ZrN, korozif ortama maruz kaldığında yüzeyinde kararlı bir oksit film oluşturararak yüksek korozyon direnci gösterebilmektedir. Yapılan bu çalışmada, TİN kaplamalara alternatif kaplamalar izlenimi veren ZrN kaplamaların belirli özelliklerini deneysel olarak belirleyerek, TİN kaplamalar ile karşılaştırma şansı yakalayabilmek amacı ile bir dizi deney gerçekleştirilmiştir. Bu amaç ile 3 16L implant grade paslanmaz çelik numune üzerine ZrN ve TİN kaplamalar ark PVD yöntemiyle kaplanmış ve çeşitli karakterizasyon deneylerine tabi tutulmuşlardır. Mikrosertlik deneyinden elde edilen sonuçlardan, ZrN'nin sertliğinin TiN'den daha yüksek olduğu görülmüştür. Çizme testi sonucunda her iki kaplama da birbirlerine yakın kritik yük değerleri vermişler ancak ZrN'nin TiN'e göre daha gevrek bir malzeme olmasından dolayı çizik kanallarının kenarlarında fazla miktarda çatlama gözlenmiştir. Bu çatlaklar çizme testinin ardından gerçekleştirilen Rockwell C testi sonrasında incelenen iz çevresinde de gözlenmiştir. X-Işınlan analizlerinden elde edilen verilerden ZrN'nin kristalografik olarak (111) düzleminde yönlendiği, TİN' in ise (220) düzleminde yönlendiği gözlenmiştir. Polarizasyon deneyleri ve taramalı elektron mikroskobu incelemeleri sonucunda da ZrN kaplamanın TİN kaplamaya göre daha yüksek korozyon direnci gösterdiği saptanmıştır.

Vapour deposited hard coatings like TiN, ZrN and a great number of ternary or multicomponent compounds of the transition elements find application areas such as decorative and tribological applications. There is substantial amount of studies concerning improvement of the quality and performance of TiN coatings and less attention was paid to an alternative coating such as ZrN. ZrN has high hardness (2300-2600 HV), high melting point (2980 °C), high thermal and chemical stability. Although both ZrN and TiN show golden colour, the optical properties of ZrN is better than that of TiN. As it is generally harder than TiN, it offers good scratch resistance. ZrN has a light, bright and beautiful golden colour and has found decorative uses where the colours of brasses and gold are of interest. It is used in a variety of jewellery applications. As mentioned in the early studies, nitrides are noble and corrosion resistant in many environments, existing defects in the coatings such as pinholes may lead to a rapid corrosion of less noble substrates. ZrN has the ability to react with the corrosive environment and form a protective layer, hence less defect sensitive. The protective passive layer is an oxide or an oxi-nitride film but it should be noted that the high amount of oxygen content of ZrN leads to the change of the colour from gold-silver to bluer tinge. In some applications, changing of the colour may not allow ZrN to be used. In this study, several experiments were conducted in order to investigate the properties of ZrN coated 316L stainless steel and compaire with TiN coated 316L stainless steel. In the experimental studies, first step was thw deposition of the coatings onto AISI 3 16L substrates and the second was the characterisation of the coatings. The coatings were deposited on AISI 316 L implant grade stainless steel substrate 40 mm in diameter and 3 mm thick. The substrate materials were ultrasonically cleaned in hot alkaline bath and then dipped into a propanol and trichloroethylene bath. After the chemical pretreatments, the substrate materials were coated with ZrN and TİN in arc PVD unit (Model NVT-12, NOVATECH-SIE, Moscow). The coating parameters are given in table 1. In order to characterise and compare the properties of the coatings, thickness measurement (calotest), dynamic ultra micro hardness test, scratch test, Rockwell C test, X-Ray diffraction analysis, colorimetric measurements, polarisation experiments and scanning electron microscopy investigations were done. The results of the experiments are given in tables 2 to 7 and in figure 1 : Table 2. Results of thickness measurements Table 3. Results of micro hardness measurements. Table 4. Resuts of scratch test measurements. Table 5. Resuts of X-Ray diffraction analysis. Table 7. Corrosion potential, current density, polarisation resistance and cathodic taffel slope values of TiN coated 31 6L stainless steel, ZrN coated 316L stainless steel and uncoated 316L stainless steel substrate in İN H2SO4 solution. Microhardness test showed that Zrn coating is harder than TiN (Table 3). Both samples give close critical load values after scratch test (Table 4) but more delaminated regions could be seen at the scratch pattern of ZrN in optical microcope observation since it is more brittle than TiN. The same observation could be made after Rockwell C test. More cracks and delaminated regions were seen at the indentation pattern of ZrN than that of TiN. Low crirical loads are because of the low hardness of the substrate and the low thickness of the coatings. According to the XRD investigations, ZrN was the only phase present (with (111) preferred oriantation) in ZrN coatings, and TiN was the only phase (with (220) preferred oriantation) in TiN coating (table 5). 1.E+00 1.E-01 1.E-06 1.E-07 1.E-08 -1.80 -1.50 -1.20 -0.90 -0.S0 -0.30 0.00 0.30 0.60 0.90 1.20 1.50 1.80 Potential, V (vs. SCE) Figure 1. Cathodic and anodic polarisation curves of ZrN coated 316L, TiN coated 3 16L and uncoated 3 16L in 1 N H2SO4 solution. The polarisation experiments in 1 N H2SO4 conducted on ZrN coated 316L, TiN coated 316L and uncoated 316L samples, revealed better corrosion resistance of both TiN and ZrN coated 3 16L substrates than uncoated 3 16L(figure 1). The active-passive transition region which is clearly seen on the polarisation curve of uncoated 316L substrate, cannot be seen on the curves of both ZrN and TiN coatings. From this point of view, it can be understood that the coatings exhibit a passive state. According to table 7, it was found that ZrN has the minimum corrosion current density, maximum resistance of polarisation and maximum cathodic taffel slope values. This indicates that the corrosion resistance of ZrN is better than the others. The main reasons for this resistance can be the protectivity of the stable and self-healing passive layer formed on the surface on ZrN film (since the passive- transpassive transition region can not be seen), the uniformity and stability of the interface between the ZrN coating and the substrate and the less porosity content in ZrN coating compared to TiN. The better protective properties of the ZrN coatings were also observed in passive - transpassive transition region. The depressive effect of ZrN coating was clearly seen in the anodic polarisation curves. The value of cathodic taffel slope (be) of ZrN coated samples was the highest which means that the occurance of cathodic reaction (evolution of hydrogen) was more difficult on the ZrN surface. This is probably caused by the surface properties of ZrN coating.