NiCrBSi Powders Research Articles

Structure and Tribological Characteristics of HVOF Coatings Sprayed from Powder Blends of Cr3C2-25NiCr and NiCrBSi Alloy

HVOF spraying was used to prepare coatings from mechanical blends of Cr3C2-25NiCr and NiCrBSi powders. The aim of this study is to study the tribological behavior of coatings prepared from such powder blends. The coatings were studied under dry sliding conditions particularly at high temperatures. Tribological properties of the coatings were characterized using a specific hot-button tribological tester at the temperature of 300 °C in air, and a pin-on-disk test at room temperature. Addition of NiCrBSi resulted in coatings, which showed low coefficient of friction in high temperatures, and in high levels of contact pressure and sliding speed.

Microstructure and Properties of Ni-Base Coatings Obtained by Micro-Plasma Arc Cladding Processes

Microstructure and properties of Ni-base coatings obtained by micro-plasma arc cladding was investigated. A uniform clad coating was obtained after optimizing the cladding parameters by using NiCrBSi powder. Microstructure of the coatings observed by optical and scanning electron microscopy (SEM) indicated a homogenized microstructure. It has no evident heat-affected zone and no porosity. The phase of the coating was formed by γ-N solid solution, nickel boride Ni3B, carbide Cr21.34Fe1.66C6 and Cr2B3 type chromium boride. Although it likes the laser cladding coatings, micro-plasma coatings has its own characteristic. In addition, the average microhardness and nanohardness of coatings which were much higher than the substrate were observed, and the distribution of the microhardness was drawn.

Characterization of wear resistant plasma sprayed WCCo/NiCrBSi self-fluxing coatings

In this paper, the microstructural, mechanical and tribological properties of WCCoNiCrBSi thermal spray coatings, applied to repair the cylinders of helicopter nasal feet, were analyzed. The process used for coating deposition on steel substrate was the Atmospheric Plasma Spraying (APS).The plasma spray process is a procedure widely used for applying coatings with increased resistance to wear. The combination of WCCo and NiCrBSi powders produced coatings for the protection of metal parts exposed to wear abrasion, erosion and cavitation to 540°C. The WCCoNiCrBSi 50/50% powder is specially developed for the production of coatings of high cohesion and adhesive strength. The coatings are very dense, hard and smooth. Microstructures present in the layers are tungsten carbides and chromium layers that increase resistance to wear and abrasion. Powder is deposited on the substrate rugged with Al2O3. The substrates were heated at temperatures up to 200°C. Powder deposition was performed with the optimal deposition parameters The microstructure and micro hardness of the coating layer was investigated on the cross section of samples using the Pratt & Whitney standard. Micro hardness was tested using the Vickers scale with a load of 300g. The microstructure of the layers was investigated on an optical microscope (OM) with a magnification of 400X. Testing of bond strength between coatings and substrates was performed at room temperature with the speed of twitch of 1mm/60sec. The structure is multi-layered and layers are dense and homogeneous. In the WCCo layers there are dark WC phase particles. The NiCrBSi alloy layers contain fine precipitates of white nickel borida. The WCCo layers have micro hardness values ranging from 1097 - 1390 HV0,3. The tribological evaluation of the coating was performed using a pin - on ring tribometer. Lubricate sliding wear tests were carried out at room temperature according to the ASTM G77 standard. The coated blocks were tested on a hardened and tempered steel ring. A constant load of 400 N and a constant speed of 0,5 m/s were applied for a total distance of 3000 m. The values of micro hardness were also measured in the NiCrBSi layers in a narrow range of 823-930 HV0,3 The values indicate that the layers are homogeneous and thick. The coating bond strength is 59N/mm. The value is high because of the warming substrate. The dynamic coefficient of friction has a value of μ = 0,1 to 0,11. The mass loss was small and on average it amounted to Δmb = 0,11 mg. The results of tribology tests showed good reproducibility of the dynamic coefficient of friction. The coating can be reliably applied to surfaces exposed to wear during operation. The advantage of this coating application is in a significant prolongation of service life of parts, together with reduced operating and maintenance costs.

Comparative examination of the microstructure and high temperature oxidation performance of NiCrBSi flame sprayed and pack cementation coatings

Coatings formed from NiCrBSi powder were deposited by thermal spray and pack cementation processes on low carbon steel. The microstructure and morphology of the coatings were studied by scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). Flame sprayed coatings exhibited high porosity and were mechanically bonded to the substrate while pack cementation coatings were more compact and chemically bonded to the substrate. The microhardness and the high temperature oxidation resistance of the coated samples were evaluated by a Vickers microhardness tester and by thermogravimetric measurements (TG), respectively. Pack cementation coatings showed higher hardness and were more protective to high temperature environments than the flame sprayed coatings.

The effect of the deposition temperature and activator concentration on the structure of NiCrBSi coatings deposited on low carbon steels by pack cementation process

The feasibility of NiCrBSi powder deposition on low alloy steels with pack cementation is investigated at different temperatures (800–1000 °C) and with various halide salt activator content in the pack powder (1–6 wt.%), while the deposition time was set at 4 h. The as-coated samples were examined with Scanning Electron Microscopy (SEM) associated with Energy Dispersive Spectroscopy (EDS), and X-Ray diffraction analysis (XRD). It was revealed that both parameters affect the coating thickness, but not the phase composition of the as deposited coating, which in every case is composed by a Cr 2O 3 thin layer on the top, which enhances the corrosion resistance and Cr–B, Ni–B and Fe–Ni phases in the rest of the coating which increase the hardness and wear resistance. The activator content has also effect on the coating quality and integrity. In cases where its concentration is low or at higher levels (> 6%wt.) the as formed coatings are inhomogeneous and of low cohesion.

In-Situ Synthesis of Metal Matrix Composite Coating with Laser Melting-Solidifying Processes

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.

Laser Cladding of Ni Based Cermets

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.

Laser cladding of Ni-base composite coatings onto Ti–6Al–4V substrates with pre-placed B 4C + NiCrBSi powders

NiCrBSi + 5 wt.%B 4C powders were pre-placed and then laser clad onto Ti–6Al–4V substrates. Microstructure and morphology of the coatings were examined using an X-ray diffractometer, an electron probe X-ray microanalyzer and a scanning electron microscope with an energy dispersive spectroscopy. The microhardness of the coating was measured using a Vickers hardness tester. After laser cladding, a good metallurgical bond without cracks and pores was obtained. A reaction between B 4C and Ti occurred, leading to an in-situ synthesis of TiB 2 and TiC reinforcements in the coating. The coating mainly consists of γ-Ni as the matrix phase and TiB 2, TiC and CrB reinforcements. The average microhardness of the composite coating is HV 1300 and it is three times higher than that of the Ti–6Al–4V substrate.

Relationships between NiCrBSi particle characteristics and corresponding coating properties using different thermal spraying processes

Because the microstructure and physical properties of a thermally sprayed coating are determined by the dynamics of the particles interacting with the spray jet, different processes were tested in order to explore the effects of process variables to particle characteristics and to coating quality. The NiCrBSi powder was sprayed successively with plasma, flame and HVOF process. Standard parameters specified by previous researches were followed for this operation. The DPV 2000 measurement system was used to monitor particle characteristics during their flight into the spray. Then, the coatings were realized at the position corresponding to the DPV measurements. The coatings were characterized in term of microstructure and composition, hardness, Young modulus, porosity and oxide level. The results revealed significant modifications in coating properties depending on the used spray process. Comparing the particle characteristics obtained successively with the different processes allows to better understand the properties of the formed coatings and to investigate the causes of a coating quality changes. Differences were interpreted based on in-flight diagnostic results.

Laser cladding of Ti-6Al-4V alloy with TiC and TiC+NiCrBSi powders

Laser cladding of Ti-6Al-4V alloy with TiC and TiC+NiCrBSi powders was carried out, and microstructure, as well as microhardness profile, in the clad layers was examined. The results showed that, in the TiC clad layer, TiC was melted and solidified to form dendrites in the clad zone, and dissolved into the melted Ti-alloy substrate and precipitated to form well-developed dendrites in the dilution zone. With increasing specific laser energy, the dilution effect of the Ti-alloy substrate was enhanced, and the microhardness decreased in both the clad and the dilution zones. In the TiC+NiCrBSi laser clad layers, TiC particles dissolved into the melted Ni-based alloy (binder material) in the clad zone. With increasing specific laser energy, the degree of solution of TiC particles was increased. During cooling, fine spherical particles and dendrites of TiC precipitated from the Ni-based alloy. When the TiC volume fraction increased to more than 50%, clustering of TiC particles was observed in the clad zone. The clustering of TiC particles resulted in a decrease in the homogeneity of the microstructure and microhardness distribution in the clad zone. The dilution zone of the TiC+NiCrBSi clad layers is a mutually melted region of the Ni-based alloy and titanium-alloy substrate and presents a microstructure of dendrites.

Microstructure and wear resistance of NiCrBSi laser clad layer on titanium alloy substrate

Laser cladding of NiCrBSi powders on Ti-6Al-4V alloy substrate was performed, and microstructure, microhardness and wear resistance of the clad layers were evaluated. Results show that the laser clad layer is divided into three regions: the clad, the dilution and the heat-affected zones. In the clad zone, fine particles of TiB 2, TiC and M 23(CB) 6 are distributed in the matrix of the primary γ-Ni and the multi-phase eutectics consisting of γ-Ni, Ni 3B and silicides. Microhardness of the clad zone is very high, being approximately HV 1000. The dilution zone is a mixture of melted Ni-base and Ti-base alloys, and possesses a characteristic of directional crystallization. The heat-affected zone has an acicular martensitic structure, and the microhardness is HV 360–380. Compared to titanium alloy, the wear resistance of clad layer is improved. The mechanism of wearing of clad layer is a mixed type of slight peeling-off and abrasion.