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To overcome the critical issue of low density of the MoSi 2 coating prepared by slurry sintering method, a novel strategy of adding diffusion barrier (DB) at the interface was developed to achieve a high density of the coating. The effect of sintering process on the density and microstructure of the MoSi 2 coating was investigated systematically. The addition of DB promotes the formation of dense MoSi 2 coatings with a low area porosity of 0.96–3.98 %, while the value is basically over 41 % for the coatings without DB. The MoSi 2 coating with DB shows an enhanced oxidation resistance due to its dense structure as well as the reduction of the inward diffusion loss of Si by the DB during oxidation. • A novel strategy is proposed to achieve densification of MoSi 2 coating by slurry sintering. • DB can diminish chemical potential gradient to reduce Si diffusion driving force. • The addition of DB promotes the formation of dense coatings with a low area porosity. • The dense MoSi 2 coating exhibits a significantly improved oxidation resistance. • DB can effectively reduce the inward diffusion loss of Si in the coating during oxidation. [ABSTRACT FROM AUTHOR]

A MoSi 2 /NbSi 2 coating with an Al-Si-O interlayer as diffusion barrier was deposited on Nb-based alloy by firstly halide activated pack cementation (HAPC) and then hot press sintering (HPS). The multilayer structure of MoSi 2 /Al-Si-O/NbSi 2 /Nb 5 Si 3 or MoSi 2 /(Nb, Mo) 5 Si 3 /NbSi 2 /Nb 5 Si 3 was formed in the coating with or without the diffusion barrier, respectively. With the addition of diffusion barrier, elemental diffusion was suppressed markedly during formation of the MoSi 2 overlayer and during thermal exposure of the coating sample. Exposed at 1350 °C for 100 h in air, the MoSi 2 /Al-Si-O/NbSi 2 coating exhibited a weight gain about 0.10 mg/cm2 with a slow degeneration in the NbSi 2 transition layer and the thickening rate of surface oxide scale was 0.048 μm2/h, which was increased to 0.15 mg/cm2 and 0.050 μm2/h respectively for the MoSi 2 /NbSi 2 coating without the diffusion barrier. Combining with analyses of the coating formation and the microstructural evolution, mechanism of oxidation resistance and of diffusion for the two coatings were compared and built. • A dense and continuous Al-Si-O interlayer was sandwiched into the MoSi 2 /NbSi 2 coating. • The coating with the interlayer exhibited stronger resistance to corrosion and degeneration. • Interdiffusion was suppressed by the interlayer with strong stability of thermal and structure. [ABSTRACT FROM AUTHOR]

(Ni,Pt)Al coating with varying thicknesses of α-W-based diffusion barrier (DB) were prepared by adding various thicknesses of Ni W plating layer. The oxidation performance of the DB-contained coatings was compared to that of the normal (Ni,Pt)Al coating at 1100 °C. With the thicker of DB, the inter-diffusion of Ni, Al as well as the refractory elements was more effectively inhibited. Moreover, better oxidation resistance was observed on (Ni,Pt)Al coating with a thicker DB, exhibiting a lower oxidation rate, flatter surface rumpling extent and slower phase degradation rate. The effects of DB on the inter-diffusion behaviour and oxidation performance of the (Ni,Pt)Al coating are discussed, and the degradation mechanism of the DB is proposed. • (Ni,Pt)Al coatings with varying thicknesses of in situ formed α-W-based diffusion barrier (DB) are prepared. • (Ni,Pt)Al coatings with thicker DB exhibit better oxidation resistance. • Less inter-diffusion occurs for thicker DB-contained (Ni,Pt)Al coatings. • The DB gets degenerating when the intermittent channels between α-W become wide enough. [ABSTRACT FROM AUTHOR]

The present work focuses on the preparation of amorphous Si–B–C–N coatings to protect γ -TiAl intermetallic alloys against oxidation above 750 °C that otherwise leads to surface degradation and embrittlement. The film fabrication process included the deposition of an amorphous Si interlayer at a temperature of 700 °C to promote adhesion. Subsequently, Si–B–C–N coatings were prepared by pulsed dc and rf magnetron co-sputtering from Si and B 4 C targets in Ar + N 2 gas mixtures. Specifically, we studied the oxidation behavior of the coatings and the evolution of the microstructure, composition, and mechanical properties upon an isothermal oxidation test performed in laboratory air at 800 °C for 100 h. We found that an optimized Si 40 B 5 C 2 N 47 (at.%) coating protects the γ -TiAl alloy substrate against oxidation by a combination of several effects: a) Interdiffusion of Si and Ti results in the formation of Ti x Si y phases on the film-substrate interface promoting adhesion of the coating, b) Diffusion of Ti to the surface is effectively inhibited and no fast-growing TiO 2 is formed at the outer surface, and c) Stable amorphous structure of Si–B–C–N coatings hinders inward oxidation of O. Investigation of the oxidation kinetics at 800 °C up to 1000 h proved that Si–B–C–N coating serves as an efficient oxidation barrier demonstrated by the significantly reduced mass gain and the parabolic oxidation rate constant (1.05 × 10−13 g2·cm−4·s−1) compared to the uncoated γ -TiAl alloy (5.65 × 10−13 g2·cm−4·s−1). • Si–B–C–N coatings were deposited on the γ -TiAl alloy by pulsed dc and rf magnetron sputtering. • A continuous Si interlayer promotes coating adhesion and inhibits outward Ti diffusion. • Si–B–C–N coatings possess stable composition and amorphous structure during oxidation at 800 °C. • The stable amorphous structure of the Si–B–C–N coatings acts as an efficient oxidation barrier. • Si–B–C–N coatings exhibit significantly reduced mass gain and oxidation rate compared to the γ -TiAl alloy. [ABSTRACT FROM AUTHOR]

Nickel aluminide diffusion coatings act like an aluminium reservoir to form protective alumina scales at high temperatures. However, interdiffusion phenomena play a critical role in the metallurgical integrity of these coatings. This work explores a new type of self-healing coating for high temperature oxidation applications. Micro-reservoirs consisting of an aluminium-rich intermetallic core (Al 3 Ni 2) and an aluminium oxide shell are embedded in a nickel aluminide matrix (NiAl). This new type of coating has been synthesized by an electrochemical chemical route to trap the micro-reservoirs in an electrodeposited nickel matrix followed by aluminization via a slurry route. Under high temperature oxidation conditions, the micro-reservoirs allow to form a diffusion barrier and to release Al into the coating matrix. These two simultaneous phenomena make it possible to maintain a sufficient amount of aluminium and ensure the unique formation of α-alumina at the surface of the coatings, hence to possibly increase the lifespan of the aluminium diffusion coatings. [Display omitted] • Successful synthesis of a self-regenerating aluminide coating • Coating made of preoxidized Al-rich NixAly reservoirs in a nickel aluminide matrix • Reservoirs trapped in a Ni electrodeposit that is subsequently aluminized by slurry • Distribution of reservoirs in high energy areas of the coatings • Loss of Al by interdiffusion reduced to 6 % vs. 30 % in the absence of the reservoirs [ABSTRACT FROM AUTHOR]

This paper studied the effects of internal oxide contents on the high temperature oxidation and β-phase depletion behaviour of high velocity oxygen fuel (HVOF) thermally sprayed CoNiCrAlY coatings. Two oxygen-to-kerosene ratios were employed in the HVOF thermal spraying process to generate different internal oxide contents in the as-deposited CoNiCrAlY coatings. The isothermal oxidation behaviour of the as-deposited CoNiCrAlY coatings were examined in air at 1100 °C. The as-deposited HVOF CoNiCrAlY coatings before and after oxidation were characterised by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was shown that the high oxygen-to-kerosene ratio caused significant oxidation of in-flight particles, resulting in a large volume fraction of the internal oxides in the as-deposited CoNiCrAlY coatings. Furthermore, it was found that the CoNiCrAlY coating with a higher content of internal oxides exhibited larger mass gain and significant β-phase depletion during oxidation. This is likely due to that the Al in the coating was initially consumed during the spraying process and the discrete internal oxide stringers were not effective diffusion barriers. • Two CoNiCrAlY coatings were prepared at different oxygen-to-kerosene ratios. • The internal oxides were found to be Al-rich oxides by EDS analysis. • High mass gain was found in the coating with high internal oxide content. • Rapid β-phase depletion occurred in the coating with high internal oxide content. [ABSTRACT FROM AUTHOR]

Cr 2 AlC coating has potential applications as high temperature protective coating. However, inevitable interdiffusion at coating/substrate interface promotes Al depletion and deteriorates its oxidation resistance. In the present work, the oxidation behavior of a triple-layered Cr 2 AlC coating, with (Cr,Al) 2 O 3 inner layer, amorphous intermediate layer and crystalline Cr 2 AlC outer layer, was investigated at 900 and 1000 °C in air. The effect of the (Cr,Al) 2 O 3 inner layer as diffusion barrier was evaluated during the oxidation process. It is showed that the triple-layered Cr 2 AlC coating could effectively improve the oxidation resistance of the M38G alloy substrate, due to the Cr 2 AlC outer layer. The (Cr,Al) 2 O 3 inner layer could prohibit internal nitridation and retard interdiffusion at the coating/substrate interface, improving oxidation resistance of the triple-layered Cr 2 AlC coating. Furthermore, cracking of the triple-layered Cr 2 AlC coating was found after oxidation at 900 and 1000 °C, due to the thermal stress and internal stress introduced by intermediate amorphous layer crystallization during oxidation. • The triple-layered coating improves the oxidation resistance of the M38G alloy effectively. • The (Cr,Al) 2 O 3 inner layer prohibits inter diffusion and internal nitridation during oxidation. • Thermal stress and stress due to amorphous layer crystallization lead to cracking of the coating. [ABSTRACT FROM AUTHOR]