Formation of B-modified MoSi2 coating on pure Mo prepared through HAPC process

• B-modified MoSi2 coating was prepared through pack Si–B co-deposition process.
• Formation mechanism of the B-modified MoSi2 diffusion coating was clarified.
• Oxide B2O3 promoted the formation of a dense scale on the coating during oxidation.
• The degradation rate of MoSi2 to Mo5Si3 during oxidation declined in the coating.

B-modified MoSi2 coatings were prepared on pure Mo through pack Si–B co-deposition process. The effects of pack mixture, co-deposition temperature and time on the formation of the Si–B co-deposition coating were studied by preparing the coatings in the pack mixtures 16Si–xB–4NaF–Bal.Al2O3 (wt.%, x = 0.5–4) at 1100–1400 °C for 2.5–10 h. XRD, EDS, and WDS techniques were used to examine the microstructures of the coatings. Thermodynamics calculation about the equilibrium partial pressures of the volatile fluoride vapors in the pack aided experimental observation was introduced to reveal the coating formation mechanism. The results showed that the Si–B co-deposition coating had four layers: the MoSi2 outer layer with MoB precipitates distributed within its lower part, the Mo5Si3 second layer, the MoB third layer and the Mo2B inner layer. Pack mixture, co-deposition temperature and time made negligible effect on coating structure. The growth of the coating was dominated by the inward diffusion of Si and B. The deposited Si atoms mainly formed MoSi2 phase; the deposited B atoms either diffused into the substrate to form the MoB and Mo2B layers or generated MoB precipitates within the MoSi2 outer layer. The effects of B atoms within the Si–B co-deposition coating on coating oxidation manifested in two aspects: firstly, B atoms diffused to coating surface and oxidized to B2O3, which promoted the formation of a dense scale and reduced the coating weight loss; and secondly, the MoB layer acted as a barrier to the inward diffusion of Si, which slowed the degradation of MoSi2 to poor oxidation-resistant Mo5Si3 and would increase the coating lifetime.