Abstract
In this paper, studied was the melting of W powder particles in plasma, their behavior at oxidation as well as the mechanism of hardening on the surface of the substrate. Tungsten coating layers were deposited with vacuum plasma spray technology (VPS) on the test specimens of steel Č.4171 (X15Cr13 EN10027). VPS technology has advantages over the APS technology due to decreased oxidation of melted powder particles, by producing a coating with a controlled proportion of micro pores and greater uniformity of the deposited layers. Evaluation of mechanical characteristics of the layers was done by examining the microhardness using the HV0.3 method and tensile bond strength by tensile testing. The microstructures of the deposited layers were analyzed by means of optical microscopy (OM) and scanning electron microscopy (SEM). The results obtained showed that the tungsten coating consisted of well melted particles that interconnected and were overlapping, which shows a typical lamellar microstructure. Present in the microstructure are micro pores in very small proportion at the inter-lamellar boundaries. Tests have shown that layers of W coating deposited by plasma spray in inert gas shielding at low pressure have good mechanical properties and microstructure, which in the fully enables its use in exploitation.
Highlights
Tungsten has a light gray color, good strength and hardness, high density of 19.3 g/cm3 and the highest melting point of all metals 3422 oC
The values of the vacuum plasma spray technology (VPS)-W coating microhardness indicate that the coating layers show no presence of WO3 oxides - the oxide is yellow and would be seen under a microscope
Low pressure of inert and high-purity Ar in a vacuum chamber and the reducing plasma H2 gas prevented the formation of WO3 oxides which increased the microhardness, and this was confirmed by analysing the image with an optical microscopy (OM)
Summary
Tungsten has a light gray color, good strength and hardness, high density of 19.3 g/cm and the highest melting point of all metals 3422 oC. One of the problems that arise is the mismatch of coefficients of thermal expansion of tungsten coatings and substrate materials, due to which transitional inter layers may develop [11,12] Another problem that occurs is reduced thermal conductivity of the tungsten plasma spray coating. The stage of melting of powder particles of tungsten at low pressure using the plasma spray process affects their deposition, such as the flattening and stacking on the substrate. The tungsten coating was deposited using vacuum plasma spray technology, for the purpose of obtaining coatings of improved resistance to wards different types of defects such as erosion, high temperature and corrosion. For the coating used was tungsten powder with spherical morphology of particles, which were deposited with the F-4 plasma gun onto the test specimens in a vacuum plasma spray chamber of the company Plasma - Technic AG. Tests have shown that layers of VPS - W coatings had mechanical characteristics and microstructure which fully enable the use of the coating for a wide range of purposes
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