| 1. |
- Wani, M. F., et al.
(författare)
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Friction and wear of HPSN bearing materials
- 1997
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Ingår i: American Ceramic Society Bulletin. - 0002-7812 .- 1945-2705. ; 76:8, s. 65-69
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Tidskriftsartikel (refereegranskat)abstract
- Paper presents data on wear and friction of hot-pressed silicon nitride (HPSN) and its composites in contact with ball-bearing steel. The coefficient of friction under unlubricated sliding conditions was 0.45, and it did not change with load under lubricated conditions. The wear coefficient was the least for HPSN sintered with liquid in system containing yttria. HPSN containing 15 % of TiC showed the best wear resistance compared to HPSN with other concentrations of TiC or BN. No adhesive wear was detected.
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| 2. |
- Das, D. K., et al.
(författare)
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Microstructural degradation of plain and platinum aluminide coatings on superalloy CM247 during isothermal oxidation
- 1999
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Ingår i: Materials Science and Technology. ; 15:10, s. 1199-1208
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Tidskriftsartikel (refereegranskat)abstract
- Isothermal oxidation at 1100°C of a high activity plain aluminide coating and a platinum aluminide coating, developed by the pack cementation technique, on cast nickel base superalloy CM247 has been carried out with the primary objective of systematically understanding the coating degradation process during oxidation. While the weight gains during oxidation for both plain aluminide and platinum aluminide coatings follow parabolic kinetics from the very beginning of oxidation exposure, the bare alloy was seen to exhibit a considerably long initial transient oxidation period (∼20 h), beyond which the parabolic law was followed. The parabolic rate constant for the platinum aluminide coating was found to be nearly two orders of magnitude lower than that for the plain aluminide coating. Alumina was identified as the only oxide phase that formed on both plain aluminide and platinum aluminide coatings during most of the oxidation exposure, although NiAl2O4 was also found in the case of the plain aluminide coating beyond ∼200 h. The oxide layer on the bare alloy, however, was found to consist of Al2O3, Cr2O3, and NiAl2O4. The microstructural degradation of both the plain aluminide and platinum aluminide coatings during oxidation was seen to occur in three distinct stages which, however, differed for each coating. This stagewise degradation, which involves final obliteration of the interdiffusion layer in each case, is discussed in detail. © 1999 IoM Communications Ltd.
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| 3. |
- Das, O. K., et al.
(författare)
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Evolution of aluminide coating microstructure on nickel-base cast superalloy CM-247 in a single-step high-activity aluminizing process
- 1998
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Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. ; 29:8, s. 2173-2188
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Tidskriftsartikel (refereegranskat)abstract
- This study deals with the aluminizing of a directionally cast Ni-base superalloy, namely CM-247, by a single-step process using a high-activity pack. It is observed that significant incorporation of Al into the substrate surface during aluminizing continues over a period of about 1 hour and is not restricted merely to the first few minutes, as reported in the literature. Based on the microstructural details of the coatings formed at various stages of aluminizing, it is concluded that the coating growth in the above process takes place primarily by inward Al diffusion initially, followed by an intermediate stage when the growth involves both inward Al and outward Ni diffusion. In the final stages, the outward diffusion of Ni dominates the coating formation process. The above mechanism of coating formation is different from the one that prevails in the conventional two-step high-activity coating process in that the reaction front for the formation of NiAl remains spatially stationary despite the outward diffusion of nickel during the intermediate stage. It is also shown in the present study that the content of the Al source in the pack affects the coating structure significantly. It is further demonstrated that the microstructure of the aluminide coatings depends not only on the amount of Al incorporated in the sample during aluminizing but also on the time over which the uptake of this Al takes place.
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| 4. |
- Krishna, G. R., et al.
(författare)
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Role of Pt content in the microstructural development and oxidation performance of Pt-aluminide coatings produced using a high-activity aluminizing process
- 1998
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Ingår i: Materials Science and Engineering A. ; 251:1-2, s. 40-47
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Tidskriftsartikel (refereegranskat)abstract
- The present study highlights the effect of Pt content on the microstructure of Pt-aluminide coatings produced using a single-step high-activity aluminizing process. The amount of Pt in the coating was varied by changing the thickness of the initial electroplated Pt layer between 1 and 15 μm. The aluminium uptake from the pack was found to be almost the same for all the coatings produced using a Pt layer of thickness 2.5 μm and above, with a somewhat lower uptake for the coating corresponding to a 1 μm thick Pt layer. The coating microstructure, which consisted of an outer two-phase (PtAl2 in a matrix of NiAl) layer, an intermediate NiAl layer and an interdiffusion layer, was also found to be independent of the Pt layer thickness when it was in the range 2.5-10 μm. In the case of the 1 μm Pt layer, however, the whole of the Pt remained in solid solution in the NiAl phase. For a Pt layer thickness exceeding 10 μm, on the other hand, a continuous surface layer of PtAl2 phase was observed. The above mentioned influence of the thickness of the Pt plated layer on the microstructure of the Pt-aluminide coatings observed in the present investigation could be explained in terms of the Pt concentration in the diffusion layer resulting from the interdiffusion between the Pt layer and the superalloy substrate during the pre-aluminizing diffusion treatment. Cyclic oxidation tests on these Pt-aluminide coatings reveal that the presence of Pt in aluminide coatings, in general, enhances oxidation resistance. However, in order to fully realize the beneficial effects of Pt on oxidation behaviour, a certain minimum Pt content in the coating was found to be necessary.
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