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CVD coatings
Published in Kwang Leong Choy, Chemical Vapour Deposition (CVD), 2019
During the pack-cementation process, an engineering component (e.g., turbine blade) is packed in powder containing the coating element (e.g., a pure metal, alloy or compound), an activator (or mixture of activators) and an inert material that acts as a diluent to distribute the pack constituents, in order to prevent sintering and support the component(s) as shown in Figure 6.11a. This is a hot-wall process and it is typically performed in a closed reactor and the gaseous reactants are generated within the packed bed, unlike the usual vapour generation routes in CVD (e.g., Figure 6.11c). The theory and use of pack-cementation coatings for superalloys has been reviewed [33,34]. This process has been first exploited by the Chromalloy gas turbine corporation to improve the performance and lifetime of gas turbine parts in the 1950s [35]. The process has been developed further by introducing Pt and Pd into the coatings system via electroplating prior to aluminising to increase the life of the component further [36–38]. The drawbacks of the pack-cementation method include: disposal of a large amount of wasted starting powder; ease of incorporating contaminants; long coating cycle times; and laborious cleaning of the coated parts after pack-cementation, prior to subsequent heat treatment for the formation of the diffusion coating [29]. This has prompted the development of pack-cementation process into an “out-of-pack” process to address some of the drawbacks. In the “out-of-pack” process, the coating element source and halides are contained in a tray beneath and out of contact with the component. The coating vapour is piped through internal passages using a carrier gas into the reactor. This is also a hot-wall process and is carried out in a semi-open reactor as shown in Figure 6.11b. Such equipment configurations create the possibility of coating internal cooling passages. In the vapour diffusion process, an external generator is used to generate the vapour precursor and this is piped continuously into the substrate holder containing the component. This hot-wall process is performed in an open reactor as shown in Figure 6.11c.
CVD coatings
Published in Kwang Leong Choy, Chemical Vapour Deposition (CVD), 2019
During the pack-cementation process, an engineering component (e.g., turbine blade) is packed in powder containing the coating element (e.g., a pure metal, alloy or compound), an activator (or mixture of activators) and an inert material that acts as a diluent to distribute the pack constituents, in order to prevent sintering and support the component(s) as shown in Figure 6.11a. This is a hot-wall process and it is typically performed in a closed reactor and the gaseous reactants are generated within the packed bed, unlike the usual vapour generation routes in CVD (e.g., Figure 6.11c). The theory and use of pack-cementation coatings for superalloys has been reviewed [33,34]. This process has been first exploited by the Chromalloy gas turbine corporation to improve the performance and lifetime of gas turbine parts in the 1950s [35]. The process has been developed further by introducing Pt and Pd into the coatings system via electroplating prior to aluminising to increase the life of the component further [36–38]. The drawbacks of the pack-cementation method include: disposal of a large amount of wasted starting powder; ease of incorporating contaminants; long coating cycle times; and laborious cleaning of the coated parts after pack-cementation, prior to subsequent heat treatment for the formation of the diffusion coating [29]. This has prompted the development of pack-cementation process into an “out-of-pack” process to address some of the drawbacks. In the “out-of-pack” process, the coating element source and halides are contained in a tray beneath and out of contact with the component. The coating vapour is piped through internal passages using a carrier gas into the reactor. This is also a hot-wall process and is carried out in a semi-open reactor as shown in Figure 6.11b. Such equipment configurations create the possibility of coating internal cooling passages. In the vapour diffusion process, an external generator is used to generate the vapour precursor and this is piped continuously into the substrate holder containing the component. This hot-wall process is performed in an open reactor as shown in Figure 6.11c.
Zn–Al pack co-cementation on AISI 1020 steel: structural, mechanical and tribological appraisal
Published in Philosophical Magazine, 2023
Wei Wang, Mengxin Guo, Caiyuan Sun, Jin Zhang, Songxia Li
Using the pack cementation process, Zn–Al composite coatings were constructed on the AISI 1020 steel surface by tailoring Al addition. A laminated structure across the pack-cementation Al–Zn coating over the steel surface was presented, where the coating thickness reached up to 50 μm. However, as the Al concentration increased, the Fe–Al and Zn–Al layers grew too thick, and an undesirable porous structure emerged when the Al content was 15 wt.%. The resultant coatings, particularly Z50A30, exhibited favourable microhardness and wear resistance.