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

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.Al ₂O₃ (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 Mo ₅Si₃ second layer, the MoB third layer and the Mo ₂B 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.