Pal Terek

INFLUENCE OF INTERFACIAL LAYER THICKNESS AND SUBSTRATE ROUGHNESS ON ADHESION OF TiN COATINGS DEPOSITED AT LOW TEMPERATURES BY IBAD

Ion beam assisted deposition (IBAD) was applied to produce TiN coatings on carburized steel substrates. Low deposition temperatures (~50°C) were applied to prevent distortion and softening of previously heat-treated substrates. Mechanical properties of all studied coatings are comparable to those obtained at usually used high temperatures. In order to improve adhesion between TiN coating and substrate, an interfacial layer was prepared by ion beam mixing of Ti atoms and steel substrate. The adhesion strength evaluation revealed significant improvement compared to the coatings produced without the ion beam mixed interfacial layer. Adhesion increased with increase in thickness of the interfacial layer. Substrate roughness was varied systematically in order to determine its influence on adhesion strength. The research was conducted for a rarely studied domain of low roughness (Average roughness Ra below 50 nm). The results of scratch tests revealed improvement of adhesion with increase in substrate roughness. This adhesion trend is different from the one reported by other authors who used rougher substrates. Two groups of opposing mechanisms acting during adhesion testing were identified. It appears that there exists an optimum roughness below which adhesion strength increases, and above which it decreases with the increase in substrate roughness. Accordingly, applying an expensive surface finish does not have to be a guarantee for achieving the appropriate adhesion of TiN coatings deposited at low temperatures.

Ejection Performance of Coated Core Pins Intended for Application on High Pressure Die Casting Tools for Aluminium Alloys Processing

In high pressure die casting (HPDC) process of aluminium alloys cast alloy soldering severely damages tool surfaces. It hampers casting ejection, reduces the casting quality and decreases the overall production efficiency. Thin ceramic PVD (physical vapor deposition) coatings applied on tool surfaces successfully reduce these effects. However, their performance is still not recognised for surfaces with various topographies. In this investigation, soldering tendency of Al-Si-Cu alloy toward EN X27CrMoV51 steel, plasma nitrided steel, CrN and TiAlN duplex PVD coatings is evaluated using ejection test. The coatings were prepared to a range of surface roughness and topographies. After the tests sample surfaces were analysed by different microscopy techniques and profilometry. It was found that the ejection performance is independent of the chemical composition of investigated materials. After the ejection, the cast alloy soldering layer was found on surfaces of all tested materials. This built-up layer formed by effects of mechanical soldering, without corrosion reactions. Coated samples displayed a pronounced dependence of ejection force on surface roughness and topography. By decreasing roughness, ejection force increased, which is a consequence of intensified adhesion effects. Presented findings are a novel information important for efficient application of PVD coatings intendent for protection of HPDC tools.