Thermal stability and mechanical properties of boron enhanced Mo–Si coatings

Abstract

A steady progress in industrial applications requires enhanced material performance, especially concerning the thermal stability and oxidation resistance. Motivated by outstanding mechanical properties and high temperature oxidation resistance reported for refractory-metal-based Mo1-x-ySixBy bulk materials, we studied the ternary system Mo–Si–B by detailed investigations of various sputter deposited Mo1-x-ySixBy coatings. We focused our coating development on the Mo-rich area of the ternary system Mo–Si–B with compositions x and y≤50at.% as materials composed of Mo3Si, T1-Mo5Si3, and T2-Mo5SiB2 (Akinc triangle) are known to provide the highest resistance against oxidation. Structural investigations of as deposited coatings showed, that no ternary Mo1-x-ySixBy based phase crystallizes and that an increasing B content tends to form amorphous structures. After vacuum annealing treatments, the formation of crystalline T1 as well as T2 structured phases can be detected, accompanied by a pronounced hardness increase with a maximum of 27GPa for Ta=900°C. The oxidation resistance is closely linked to the B/Si ratio and the condition that the Si content is about 25at.% or higher. These ingredients allow for the formation of dense borosilica oxide scales, protecting the coatings underneath. Best results are obtained for Mo0.58Si0.28B0.14, which could withstand all oxidation treatments tested. In agreement with earlier studies on bulk materials, also for our coatings, the consumed layer thickness of ≈0.4μm (about 12% of the coating thickness) during oxidation at 1300°C is even below the ≈1μm during oxidation at 900°C.