Takumi Nishimoto

Advanced Coatings on High Temperature Applications

To suppress interdiffusion between the coating and alloy substrate in addition to ensuring slow oxide growth at very high temperatures advanced coatings were developed, and they were classified into four groups, (1) the diffusion barrier coating with a duplex layer structure, an inner σ−(Re-Cr-Ni) phase as a diffusion barrier and outer Ni aluminides as an aluminum reservoir formed on a Ni based superalloy, Hastelloy X, and Nb-based alloy. (2) the up-hill diffusion coating with a duplex layer structure, an inner TiAl2 + L12 and an outer β-NiAl formed on TiAl intermetallic and Ti-based heat resistant alloys by the Ni-plating followed by high Al-activity pack cementation. (3) the chemical barrier coating with a duplex layer structure, an inner* γ + β + Laves three phases mixture as a chemical diffusion barrier and an outer Al-rich γ-TiAl as an Al reservoir formed by the two step Cr / Al pack process. (4) the self-formed coating with the duplex structure, an inner α-Cr layer as a diffusion barrier and an outer β-NiAl as an Al-reservoir on Ni-(2050)at% Cr alloy changed from the δ-Ni2Al3 coating during oxidation at high temperature. The oxidation properties of the coated alloys were investigated at temperatures between 1173 and 1573K in air for up to 1,000 hrs (10,000 hrs for the up-hill diffusion coating). In the diffusion barrier coating the Re-Cr-Ni alloy layer was stable, existing between the Ni-based superalloy (or Hastelloy X) and Ni aluminides containing 1250at%Al when oxidized at 1423K for up to 1800ks. It was found that the Re-Cr-Ni alloy layer acts as a diffusion barrier for both the inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate. In the chemical barrier coating both the TiAl2 outermost and Al-rich γ-TiAl outer layers maintained high Al contents, forming a protective Al2O3 scale, and it seems that the inner, γ, β, Laves three phase mixture layer suppresses mutual diffusion between the alloy substrate and the outer/outermost layers.

Two-step Cr and Al diffusion coating on TiAl at high temperatures

Abstract The formation of an oxidation resistive coating layer on a TiAl alloy was investigated with a two- step Cr (at 1573 K for up to 72 ks ) and then Al (at temperatures between 1273 and 1573 K for 36 ks) pack diffusion process at high temperatures. The coated TiAl was oxidized in air at 1173 K for up to 1252.8 ks under a thermal cycling condition. The Cr diffusion coating layer consisted of γ, β, and Laves phases, which were transformed during cooling from the β-phase formed at 1573 K. The coating layer formed by the Al diffusion at 1473 and 1573 K consisted of an outermost TiAl 2 , outer Al-rich γ-TiAl, intermediate γ, β, and Laves phases, and a diffusion zone; there is little Al-diffusion at 1273 and 1373 K. The TiAl coated by the two-step Cr and Al diffusion process at 1573 K showed very good oxidation resistance in air at 1173 K due to the formation of a protective α-Al 2 O 3 scale. After oxidation for up to 1,252.8 ks the coating layer maintained a structure with three phases γ, Laves, and β, which acts as a diffusion barrier to Al and Ti.

Formation of nickel aluminide coating on γ-TiAl alloy

Abstract A nickel aluminide coating process was developed on γ-TiAl alloy by electroplating a Ni film followed by a high Al activity pack cementation carried out in a vacuum with a mixture of fine Al, NH4Cl, and Al2O3 powders at 1273 K for 18 ks. The coating has a duplex layer structure, an outer Ni2Al3 layer and an inner TiAl3/TiAl2/TiNiAl2 layer. The coated TiAl was oxidized in air for up to 3600 ks under thermal cycling between room temperature and 1173 K. A protective Al2O3 scale formed with little oxide exfoliation and the oxidation amount was 8 g/m2 after the 3600 ks oxidation. During oxidation at 1173 K the as-coated layer structure changed from an outer Ni2Al3 and inner TiAl3/TiAl2/TiNiAl2 layers to an outer β-NiAl and inner TiAl2 and TiNiAl2 layers with voids in the outer layer. The voids appear to be formed by the phase change from the Ni2Al3 to β-NiAl during oxidation. A part of the Ti–Al–Ni phase diagram at 1173 K was developed experimentally and it was shown that the solubility limits of Ti into both the Ni2Al3 and β-NiAl phases were less than 0.5at.%. The coating with more Al in the inner layer that in the outer layer would be formed by inward diffusion of Al, caused by a so-called up-hill diffusion phenomenon. The higher Al content in the inner layer was retained after 3600 ks oxidation.

Effect of coating layer structures and surface treatments on the oxidation behavior of a Ti–50at.%Al alloy

Abstract A Ti–50at.%Al alloy, coated by a two-step Cr (at 1573 K for 7.2 and 18 ks) and Al-diffusion (at 1573 K for 36 ks) process, was oxidized in air under thermal cycling between 1173 K and room temperature for up to 3600 ks. The coated layer consisted of a three layer structure: an outermost TiAl 2 (τ phase), outer TiAl (Al-rich γ-phase), and intermediate γ, β, and Laves phase layers. The coated TiAl with an intermediate layer of a fine, three phase structure oxidized slowly due to a protective α-Al 2 O 3 layer, while a coarse structured coating was oxidized catastrophically due to the formation of cracks. Vickers hardnesses of a β and Laves mixture as well as a γ and Laves one are around 700–800 Hv, whereas the τ phase and a γ and τ mixture showed 300–400 Hv. The Al content and its profile in the outermost and outer layers were almost unchanged before and after the 3600 ks oxidation, and it seems to arise from a slow diffusion of Al, Ti, and Cr through the three-phase intermediate layer. It was suggested that the fine, three-phase structured coating layer was very effective to suppress degradation due to both mutual diffusion and cracking.

Effects of Si Content on the Oxidation Behavior of Fe–Si Alloys in Air

The oxidation behavior of Fe–Si alloys at 1073K in air was investigated. The oxidation kinetics described by the parabolic rate law of diffusion controlled oxidation and the oxidation rate decrease with the increasing Si content. Fe-Si alloys were oxidized for different times at 1073K to obtain the same scale thickness of approximately 30μm. Observations of scale cross-sections indicated the structure of oxide scale and elemental distribution in oxide scales strongly depends on Si content. The oxide scale on Fe-Si alloys with low Si content consisted of three layers with an outer Fe2O3, an intermediate Fe3O4 and an inner FeO and some voids were formed in Fe3O4 and FeO layers. The Si-rich oxide layer was formed at the scale/alloy interface of Fe-Si alloys with high Si content. Furthermore, the amount of internal oxidation zone increased with the increasing Si content. Observations of scale cross-sections indicated that the structure of oxide scale and elemental distribution in oxide scale strongly depend on Si content.

Effect of coatings on the creep and oxidation behavior of TiAl alloys at 1173K in air

The effects of coatings on the creep and oxidation behavior of Ti-50Al alloy were investigated at 1173K in air at a constant loading of 30MPa. The coating was formed by a two-step Cr/Al diffusion treatment and consisted of an outermost TiAl2 layer, an outer Al-rich γ layer, an intermediate γ, Laves and β mixture layer, and a Cr diffusion zone. Creep tests were also carried out with sole Cr or Al coated TiAl and also of uncoated TiAl. The oxide scales formed on the uncoated TiAl and the sole Cr coated specimens were a mixture of TiO2 and Al2O3, which displayed several exfoliations. Both the two-step Cr/Al coated TiAl and the sole Al coated specimens formed a protective Al2O3 layer and little oxide exfoliation was observed here. Significant cracks were observed in the sole Al coated TiAl, while no cracks were observed in the sole Cr coated TiAl; the two-step Cr/Al coated TiAl showed a number of cracks in the coatings. Low creep rates in the two-step Cr/Al coated TiAl could be due to the Laves phase with a hexagonal C14 structure in the intermediate, γ, β and Laves phase mixture, and the high creep rates of the sole Cr coated TiAl may originate in the major β phase component with a B2 structure in the γ, β, and Laves phase mixture.

Formation of Pt-Modified γ’-Ni3Al and Re-Based σ-Alloy Coating System and Cyclic Oxidation Behavior of Coated Superalloy

A duplex layer, outer Pt-modified γ’-Ni3Al + γ-Ni and inner multi-barrier σ- Re(Cr,Ni,W), coating system was formed on a Ni-based single crystal 4th generation superalloy. Oxidation behavior of the coated alloy was investigated under thermo-cycling conditions, and analyzed by EPMA and XRD. During cyclic oxidation 1hr at 1100°C and 20 min at room temperature, a slow growing α-Al2O3 formed for up to 400 cycles and its spallation was rare. The parabolic rate constant of mass change was 6.3x10-16 kg2m-4s-1. The Pt-modified γ’-Ni3Al + γ-Ni contained 19Al, 12Pt, 4Cr, and 3Co in at%, and their concentration profiles were almost flat across the outer layer. The multi-barrier, σ-Re(Cr,Ni,W) contained 40Re, 23Cr, 17Ni, 7Al, 4W, 3.5Mo, and 3Co in at%. Furthermore, the γ’-Ni3Al containing Pt was newly formed between the multibarrier and bulk alloy substrate. It was concluded that the σ-Re(Cr,Ni,W) is compatible with the Ptmodified γ’-Ni3Al in the multi-diffusion barrier coating on Ni-based single crystal, 4th generation superalloy at high temperatures.