Papers by Keyword: NiCrBSi

Abstract: Thermally sprayed NiCrBSi alloys have attracted interest in various fields of protective coating applications due to their good properties, especially for wear and corrosion resistance. Titanium diboride (TiB₂) is an attractive additive particle because of its properties such as high hardness, high melting point, high chemical resistance, high elastic modulus and low density. In this work, the effect of TiB₂ on dry reciprocating wear behavior of NiCrBSi-TiB₂ coating was systematically studied. NiCrBSi alloy powder was mixed with 15, 20, and 25 wt.% TiB₂ powder. Subsequently, mixed powder was coated on the substrate of 316L stainless steel (∅20 mm and 5 mm thickness) by High Velocity Oxygen Fuel (HVOF) technique. The microstructure of the coating was investigated under an optical microscope and scanning electron microscope (SEM). The hardness of the coating was measured by microhardness. In addition, dry sliding friction and wear behavior of coating were performed on a reciprocating ball-on-flat wear tester machine. The high-chromium steel (AISI 52100) ball of 6 mm in diameter was used as the counterpart. The conditions of the tests were 8 mm in stroke, 1 and 2 Hz in testing frequency with 10 N load, and sliding distance of 100 m. The variation of friction coefficient was recorded during the tests. The results indicated that an increase in TiB₂ content within the coating led to the formation of dark gray phases, oxidation layers and porosities in the microstructure. Consequently, reduction in the coating's hardness and wear resistance were discovered. In addition, the addition of TiB₂ in the NiCrBSi coating resulted in the higher material loss on the counterface ball due to abrasive wear mechanism.

Abstract: Laser Powder Bed Fusion (L-PBF) might be a promising solution for producing complex wear-resistant parts from NiCrBSi self-fluxing nickel alloys. This study investigates a large set of parameters with a baseplate heated up to 500 °C. Furthermore, laser remelting is also explored to further increase surface density and reduce cracking. Material health is deeply investigated by image analysis to quantify different defects (lack of fusion, porosity and cracks). Spatters likely induce lacks of fusion, while the low toughness and high hardness values cause cracks. The lack of fusion surface fraction and crack length decrease with the preheating temperature while the crack width increases. A surface density of 99 % is obtained with careful process optimisation leading to a laser power of 100 W, a laser speed of 750 mm.s^-1 and a preheating temperature of 500 °C.

Abstract: Ongoing increase in demands on efficiency of power plants and other facilities leads to increased interest of new progressive materials and technologies. One of the modern methods which lead to enhancement of surface properties is application of coatings on surface of components composed of less quality materials. Commonly used methods protecting components are based on some form of heat treatment. The current pressure on increasing operating temperatures for enhanced performance of steam turbines caused elevation of operating temperature at limit of the applicability of commonly used protections. From this reasons the demand for using an alternative technologies which would provide such a desired protection rises rapidly. One of the key areas of protection in such environment is protection against hot temperature corrosion. Possible solution can be found in application of coatings based on alloys and cermets prepared by HVOF technology. This paper examines local mechanical and microstructural properties of NiCrBSi coating after exposition to extremely severe hot corrosion environment. Furthermore, the nanoindentation measurements of NiCrBSi coating were performed before and after the corrosion test. In this case the mixture of salts composed from 59% Na₂(SO)₄ with 34.5% KCl and 6.5% NaCl was used. Temperature of test was set on 525°C and 575°C. Duration of exposition to hot corrosion environment was 168 hours in autoclave.

Abstract: effects of slurry fused Ni60 interlayer on bond strength of cermet coating to GH202 substrate were studied. A compact NiCrBSi interlayer was prepared by slurry fused method and the NiCr-HfB2 cermet coating was fabricated on the interlayer. Surface morphology of the NiCrBSi interlayer and NiCr-HfB2 cermet coating and cross-sectional morphology of samples with the NiCrBSi interlayer were examined by means of SEM. The bond strength of the NiCr-HfB2 cermet coating to GH202 substrate with and without the NiCrBSi interlayer was measured by pulling apart tests. And thermal shock resistance of the cermet coating with and without the intermediate layer was measured. The results show that the NiCrBSi interlayer effectively enhance the bond strength of the NiCr-HfB2 cermet coating to GH202 substrate and the thermal shock resistance of the cermet coating .

Abstract: The influence of thermal post-treatment by means of flame and laser energy on the mechanical properties of NiCrBSi coating was studied. The evaluation was done in terms of microstructure, surface hardness, microhardness and instrumented indentation. The microstructure evaluation showed an increase of coatings inner cohesion, together with the creation of metallurgical bond between the coating and the substrate material. In the the case of laser remelted coating, the creation of thick transition layer responsible for lower measured microhardness and hardness value was proved.

Abstract: WC-NiCrBSi composite coatings on 0.45 % carbon steel were fabricated by laser cladding. The volume fraction (33, 43 and 60 vol. %) of WC particles with the powder size of 2 μm, 5 μm and 50 μm were added respectively into NiCrBSi powders to deposit composite coatings. The microstructure of the powders and the coatings were examined by scanning electron microscope (SEM) and X-ray diffraction (XRD). Results show that the compactness and uniformity of the coatings are good when WC particles size was 2 μm and 5 μm. However nonuniform distribution and deposit of WC particles on the bottom of the coating will occur when the powder size of WC particles was 50 μm and the volume fraction of WC was 33 %. When the volume of WC addition was higher than 60 %, the volume fraction of the retained WC particles in the coatings is obviously decreased. The dissolving and decomposing of WC increase with decreasing the powder size and increasing the volume of WC addition and the laser power.

Abstract: Microstructures and properties of plasma surfaced NiCrBSi+20%WC/12Co layer prepared on the die steel (H13) substrate were investigated. The study showed that the interface boundary existed between the layer and substrate. Compared with the typical microstructures of welded joint, the similar columnar grains in the layer perpendicular to the interface line were observed, which were never seen in the plasma sprayed coating. The conclusion could be drawn that the bonding between the NiCrBSi+20%WC/12Co layer and H13 substrate belonged to the metallurgical cohesion. The XRD analysis showed that the main phases in the layer included γ-(Fe, Ni), Cr₇BC₄, Ni₄B₃, Cr₇C₃ and Co₇W₆, which brought higher strength and higher hardness to the surfaced layer than the H13 substrate.

Abstract: A process of laser cladding of NiCrBSi+2%B4C powder on Ti–6Al–4V substrate to form a coating was carried out, a good coating without cracks and pores was obtained in the process. The microstructure and component of the coating were examined using SEM and EPMA. The interface between the coating and the substrate was metallurgical, The microstructure of NiCrBSi+2%B4C cladding layer consisted of the Ni-based matrix, and the uniformly dispersed reinforcement phases of TiB2, TiC, etc. The microhardness of the coating was measured using a Vickers hardness tester. The average microhardness of the composite coating was HV1200 HV1400, and it was three times higher than that of Ti-6Al-4V substrate.

Abstract: In order to improve wear resistance of titanium alloy, with pre-placed B4C and NiCrBSi powders on Ti-6Al-4V substrate, a process of laser melting-solidifying metal matrix composite coating was studied. The coating was examined using XRD, SEM and EDS. A good metal matrix composite coating was obtained in a proper laser process. There is a metallurgical interface bonding between the coating and the substrate. During laser melting-solidifying process, high energy of laser melted the pre-placed powders and a part of Ti-6Al-4V substrate, which made Ti extend into a melting pool. A reaction between Ti and B4C took place in the melting pool, which in-situ synthesized TiB2 and TiC reinforcements in the coating. The composite coating mainly consists of γ-Ni matrix, TiB2, TiC and CrB reinforcements. Microstructure of the reinforcements obtained using the laser melting-solidifying is not as same as that of reinforcements obtained using general producing methods. Due to high cooling rate of the melting pool, TiC nucleated primarily and grew up in dendrite morphology from undercooled liquid. Encircling TiC, TiB2 precipitated later and grew up in hexagonal prism morphology. TiC and TiB2 formed an inlaid microstructure.

Abstract: The self fluxing NiCrBSi alloys can produce coating layers by means of laser processing techniques. Main procedures are the laser post-treatment of previously thermal or plasma sprayed coats and the laser cladding, for which preplaced or continuously delivered powder (in this case aided by powder feeders) can be used. NiCrBSi alloys have an interesting property due to the presence of boron and silicon in its composition: they exhibit a relatively low melting point, making the laser cladding process easier. The layers obtained on metallic based materials are resistant to high temperature erosion wear and corrosion. However, if additional abrasive wear resistance is needed, the feeding with ceramic powders such as tungsten carbides (WC) is required. The high melting point of ceramics makes the laser cladding process complicated as the melt pool is made up of liquid metal plus not totally melted ceramic particles and the whole suffers the effect of the shielding and carrying gas flows, producing undesired instabilities. In this paper several combination of WC and NiCrBSi powders were tested. It is shown that the WC fraction in the mixture has a major influence on the obtention of pore and crack free clad layers. Bellow a certain ratio the meltpool appears to be more stable and less affected by the different gas flows used in the process, yielding dense NiCrBSi coatings with rather evenly distributed WC particles. In these conditions, the analysis and characterization of the produced coatings shows that the microstructure gains homogeneity without decreasing too much microhardness if compared with the pure ceramic layers.