Papers by Keyword: Pack Cementation

Abstract: Zr-20Ti-5Al (at. %) alloy used as substrates for Si-Y₂O₃ co-deposition experiments was prepared by firstly vacuum non-consumable arc melting and then high frequency induction skull melting. The results showed that Y modified silicide coating prepared at 1250 °C for 4 h possessed a double-layer structure, mainly consisting of a thick (Zr, Ti)Si₂ outer layer and a 15 mm thick (Zr, Ti)Si inner layer. Meanwhile, the growth rate of ZrSi₂ phase changed with temperature, while the growth rate of ZrSi did not vary significantly with temperature. The growth of the coating as well as the two layers followed parabolic laws, and the co-deposition process was controlled by diffusion. ZrSi₂ was not appropriate as oxidation-resistant coatings to protect Nb based alloy from oxidation due to the lack of the formation of good quality glassy SiO₂ layer in the scale.

Abstract: A ZrSi₂-SiC/SiC double layer was fabricated on the surface of graphite spheres by two-step pack cementation with zirconium, silicon, and graphite powders as mainly raw materials. The microstructure, phase, oxidation behavior, and thermal shock resistance of the ZrSi₂-SiC/SiC composite coatings were studied. The results show that the outer coating is composed of ZrSi₂ and SiC and the inner coating is composed of SiC, of which the thickness is 450 μm. Isothermal oxidation test was conducted in air at 1773 K for 20 h and a weight increase of 1.69% were recorded. After 10 times of thermal shock tests between 1473 K and room temperature, the weight of the coated specimens decreased by only 1.14%. The excellent oxidation resistance is due to the production of SiO₂ and ZrSiO₄ in the process of the oxidation of SiC and ZrSi₂, forming a glass phase that can effectively prevent further oxidation.

Abstract: Pack cementation is an effective method in the manufacture of SiC coating on carbon material substrate surface, which is a controllable and simple process. Meanwhile, due to the reactive infiltration process of powder mixture into the substrate, a gradient transition structure layer is formed between the substrate and outer layer. In this paper, SiC was coated on the spherical substrates taken from the matrix graphite pebbles of high temperature gas-cooled reactor (HTR) fuel element. Relations between the Si/C content ratio of the pack mixture and the thickness of SiC layer were studied. Analysis found that Si/C content ratio, powder size and sintering time are factors influenced the thickness of the coating layers. When the Si/C content ratio was higher than 3:1, a uniform thickness coating bonded well with the substrate was obtained. The composition phases and thickness of coating layer etc. also had obvious changes along with the change of the Si/C content ratio in this research.Results also show that sintering atmosphere and particle size of powders are important factors affecting coating microstructure, while a vacuum atmosphere can smaller powder size can help to get a dense structure.

Abstract: In this paper, ductile iron was produced using a rotary furnace. The microstructures of the ductile iron (with and without cyanided coatings) were then characterized using optical microscopy, scanning electron microscopy (SEM) and energy diffraction X-ray spectroscopy (EDS). The surfaces of the ductile iron were then subjected to high temperature carbonitriding using a pack cementation process in which carbon and nitrogen were diffused into the ductile iron from powder mixtures consisting of ground cassava leaves and barium carbonate (BaCO₃) energizers. The wear behavior of the coated and uncoated ductile iron was studied using the pin-on-disk method. The wear mechanisms were also elucidated using a combination of SEM and EDS. The mechanisms of wear were also studied using nanoscratch experiments. The resulting wear rates are then compared with those from micron-scale wear tracks obtained from pin-on-disk experiments. The implications of the results are then discussed for the design of wear resistant ductile irons.

Abstract: Characterization of microstructure of pack cemented silicide coatings based on MoSi₂ phase was described in this article as well as a short term oxidation resistance of these types of coatings at temperature 1200oC. Silicide coatings were deposited on top surface of Mo and Mo alloys of TZM type by diffusional process of siliconizing in activated powders. Morphology of coatings was characterized by evaluation of chemical and phase’s composition of deposited coatings by EDS and XRD methods respectively. In the case of Mo sheet after diffusion process of siliconizing single layer of MoSi₂ type was obtained, but in the case of TZM alloy, in the same conditions of process, double layered coating was generated. Thick outer layer was build form MoSi₂ phase, but very thin internal layer consist Mo₅Si₃ phase. The thicknesses of coatings were ca. 45μm, and were uniform. Morphology of coatings was free from inclusion and contaminations in their cores. The isolated cracks were observed, but their volume was very small. The oxidation test was performed at temperature 1200oC. The specimens were tested at 25, 500 and 100 hours of test. After each period of test characterization of coatings top surface was made. Total destruction of specimens was observed in range between 50 and 100 h of test.

Abstract: Alumincoating was prepared on a Ni-Cr-W-Al nickel-base superalloy by pack cementation, the effects of Al content and aluminizing temperature on coating formation were investigated. Coating microstructures were investigated using optical and scanning electron microscopes, EDS and X-ray diffraction (XRD) techniques. The results showed that the coatings prepared at 800°C in the powders with 15%, 30% and 99% Al were different. NiAl₃ coating was obtained in the conditions of aluminizing temperature of 700°C, 750°C, 850°C and of Al content of 99%.

Abstract: Si-Al-Y co-deposition coatings on γ-TiAl alloy were prepared by pack cementation process at 1050°C for 3 h, Scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS), X-ray diffractometer(XRD) and microhardness tester were employed to investigate the surface morphologies, surface element distributions, microstructures, phase constitutions and microhardness of the coatings. Friction-wear tests of the γ-TiAl alloy substrate and the coatings were conducted in air at ambient temperature. The results show that the coating prepared by co-depositing Si-Al-Y had a multiple layer structure: the outer layer was composed of TiSi₂, TiSi, Ti₅Si₄, Ti₅Si₃, the middle layer was composed of TiAl₃, TiAl₂, and the inner layer was Al rich layer. The microhardness of the surface is up to 1538HK_0.49.The wear mass loss is only one half as much as that of γ-TiAl matrix, and the wear resistance increases significantly.

Abstract: Mo-Si-B alloys have been received an attention due to the high temperature strength and phase stability. However, the nature of poor oxidation resistance often limits the application of the alloy system. In order to resolve the poor oxidation resistance of the alloy system, in this study, the oxidation behaviors of Si diffusion coated Mo-Si-B alloys have been investigated in order to identify the underlying mechanism for the effect of the constituent of the phase combination of Mo-Si-B alloys. The oxidation tests performed at 1100 °C show that the produced MoSi₂ phase, as a result of the coatings, give an excellent oxidation resistance at prolonged high temperature exposure in air. The oxidation behaviors of uncoated and Si coated Mo-Si-B alloys have been discussed in terms of microstructural observations during oxidation tests.

Abstract: The hybrid Ni₂Al₃/Ni coating was formed on creep resistant ferritic steels by firstly nickel electroplating and then partially aluminising the Ni layer at 650 °C by pack cementation process using powder mixtures of Al, AlCl₃ and Al₂O₃. The effect of pack Al content (W) on growth kinetics of the outer Ni₂Al₃ layer of the coating was investigated by varying it from 2 to 10 wt% whilst keeping the pack AlCl₃ content constant at 2 wt% and aluminising conditions at 650°C/4h. It was revealed that, once W was above a minimum level, the growth of the outer Ni₂Al₃ layer thickness depended linearly on W^1/2. The possible reasons for such growth kinetics were discussed.

Abstract: Pure Ni was aluminized with an Ni₃Al, NaCl and Al₂O₃ pack powder mixture at 1100°C for 12 h. The Ni(Al) solid solution formed in aluminized Ni was internally oxidized at 1000°C for 2 to 12 h with a Co/CoO buffer. The internal oxidation zone consisted of 2 regions: granular precipitate zone near the surface and rod-like zone near the oxidation front. The growth of internal oxidation zone deviated from the parabolic law due to the non-constant Al profile developed by the aluminizing process. The granular zone grows linearly from the surface of the alloy.