Microstructure and Mechanical Properties of MoSi2 Coatings via Magnetron Sputtering and Pack Siliconizing

MoSi2 coatings are promising protective materials for hot-section components in aerospace applications, where long-term durability is an important concern. Increasing the coating thickness has been proposed as an important strategy for improving their long-term durability. To prepare MoSi2 coatings with different thicknesses, a Mo precursor layer with a thickness of 30 μm was deposited by direct-current (DC) magnetron sputtering, followed by pack siliconizing at 1300 °C for 3, 5 and 7 h, respectively. The phase composition, thickness, microstructure and mechanical properties were characterized. The results showed that the Mo coating reached a thickness of approximately 30 μm after 18 h of magnetron sputtering, exhibiting a columnar structure without obvious defects. The thicknesses of MoSi2 coatings were 86.5 μm, 94.7 μm, and 104.3 μm, respectively, showing an approximately linear increase with pack siliconizing time within the investigated range of 3–7 h. The coatings obtained after 3 and 5 h consisted of three sublayers, including an outer MoSi2 layer, an intermediate (Mo,Nb)5Si3-type layer, and an inner Mo/Nb-rich layer. After 7 h, the intermediate layer was no longer clearly distinguishable, and no distinct residual Mo/Nb-rich inner layer was observed. The coating was therefore predominantly composed of MoSi2, indicating that 7 h represents a practical threshold for essentially complete silicidation of the 30 μm-thick Mo precursor layer under the present conditions. Within the investigated range of 3–7 h, the mean microhardness, elastic modulus and fracture toughness of the MoSi2 coating increased approximately linearly with pack siliconizing time, with increases of approximately 31.5 HV0.3, 24.6 GPa and 0.56 MPa·m1/2, respectively. The results demonstrate that combining a 30 μm-thick Mo precursor layer with subsequent pack siliconizing provides an effective route for fabricating thick MoSi2-based coatings exceeding 100 μm, together with controllable microstructural evolution and mechanical properties. The potential benefit of such coating thickening for oxidation resistance remains to be verified in future oxidation tests.