Material parts that are required to perform under severe environmental conditions often have to display high hardness and wear, erosion and corrosion resistance. An effective and economical way to meet these requirements is by creating surface coatings that possess the needed properties. Carbides, particularly of the transition metals, have a number of valuable properties, which make them the most promising materials for use in various new fields of technology. Tungsten carbide layers deposited by various techniques appear suitable candidates to be used as resistant and protective coatings.

In this work, we have formed thin hard coatings of tungsten carbides. The samples are thin layers of Tungsten deposited by cathodic sputtering on two types of steel (containing 1 and 0,7% wt. carbon) substrates held at temperature of 500C during 30 min. The samples (W layer + substrate) were submitted to thermal annealing in vacuum, at various temperatures (500-1000C) and during different times.

The formation of tungsten carbides, the evolution of the microstructure and the morphology of the surface of samples were followed by X-rays diffraction (XRD), scanning electron microscopy (SEM). The measurements of micro-hardness were carried out by Vickers tests.

Thermal annealing produces reactions and some other structural changes in the tungsten layers which depend on the carbon content in substrate. It is established that the formation of tungsten carbides occurs above annealing at 800C. The reaction is more rapid on substrate containing 1%C. The mono-carbide WC forms after annealing of samples at 1000C. The hardness of coatings was found to be correlated to their composition. The hardness of coatings increases continuously from 400 to 1200 Kg/mm² as the temperature of annealing increases from 500 to 900C. The samples rich on carbon show higher hardness.

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1. INTERFACIAL REACTIONS OF TANTALUM THIN LAYER ON HIGH STEEL SUBSTRATE