جستجوی مقالات مرتبط با کلیدواژه « پاشش حرارتی » در نشریات گروه « مواد و متالورژی »

The thermal spraying method with the aim of increasing the working life of steel parts is one of the most important solutions in surface engineering to solve the problem of wear.

In this research, NiCrBSi and NiCrBSi-MoS2 coatings were applied on 304 stainless steel by thermal spraying, and then the wear behavior was evaluated at ambient temperature and 500 degrees. Phase investigations were done by X-ray diffraction test. The chemical composition of the coatings was checked with the help of energy dispersive spectrometer analysis. Porosity was investigated with the help of optical microscope and scanning electron microscope images. The hardness of the samples was measured using a microhardness test. In order to check the adhesion and tribological behavior of the coatings, VDI3198 and ASTM-G99 pin-on-disk tests were used, respectively, and finally, the wear mechanism was evaluated using SEM images and EDS analysis of the wear surfaces of the samples.

Phase analysis (XRD) and chemical composition (EDS) showed that the coating has an amorphous and crystalline phase and the most important phases of the coating are nickel-gamma, carbide and boride. Porosity measurement results with the help of image analysis software showed higher porosity of NiCrBSi-MoS2 coating. The results of hardness measurement from the cross-section of the samples indicated an increase in the hardness of the substrate in the presence of coatings, and the addition of MoS2 particles decreased the hardness of the NiCrBSi coating. The coating with MoS2 has better adhesive behavior due to its crystalline structure and better plastic deformation ability. The tribological results indicated the superiority of the NiCrBSi-MoS2 coating due to the appropriate ability of plastic deformation as well as the intrinsic lubrication property (based on the crystal structure) in this test. It was found that at ambient temperature, the samples mainly had lamellar wear mechanism, and at high temperature, oxidation wear as a secondary mechanism helped to destroy the surface.

The addition of MoS2 to NiCrBSi coating caused more porosity, more roughness, less hardness and better wear behavior of the coating. The addition of MoS2 to the coating improved the wear resistance of the coating at high temperature.

In this research, silicon carbide (SiC) powder with a particle size of 30 to 45 micrometers and an irregular morphology was used for coating in an electroless nickel bath. The effect of particle surface roughening parameters in an acidic solution and electroless process time on uniformity and thickness of Ni-P coating on the SiC particles was investigated. SiC(Ni)-70vol.%Co blended powder was coated on simple carbon steel substrates by high-velocity oxy-fuel (HVOF) and atmospheric plasma spraying (APS) processes. The composite powder was obtained from the mechanical blending of cobalt powder with coated silicon carbide SiC(Ni) powder. The microstructure and morphology of powder particles and corresponding coatings were investigated by scanning electron microscopy (SEM). The results showed that the surface preparation of SiC powder by roughening the particles with HF/NaF solution and electroless time of 5 minutes led to the formation of a continuous and uniform Ni-P coating with a thickness of 1-3 micrometers on the SiC particles. Electroless nickel coating during thermal spraying by HVOF protected SiC particles against decomposition and oxidation and as a carrier led to the placement and distribution of SiC in SiC(Ni)-Co composite coating. In the APS process, due to the very high temperature of the plasma jet and thermal decomposition, SiC particles was insignificantly contributed to the coating microstructure. The micro-hardness of two mentioned coatings was measured as 570±40 and 460±30 HV0.3, respectively, and this difference was due to the effect of the significant distribution of SiC in the HVOF coating.

In this research, CoNiCrAlY powder was deposited by high-velocity oxy-fuel (HVOF) and low-pressure plasma spraying (LPPS) processes on IN738 nickel-based superalloy substrates. The high-temperature oxidation test was performed on the coatings at a temperature of 1050 ̊C and a time of 200 hours in a muffle furnace. The microstructure and phase composition of the coatings were investigated by SEM and XRD before and after the oxidation test. The porosity (volume percentage) and surface roughness (micrometer) were measured for HVOF coating as 0.6 and 4.4, and for LPPS coating as 2 and 6.62, respectively. The HVOF coating consisted of γ-CoNiCr and β-(Co,Ni)Al, while the LPPS coating included a single phase γ-CoNiCr. The disappearance of the β phase in the LPPS coating after spraying was due to dissolution in the plasma jet and its non-recovery in the conditions of rapid quenching and non-equilibrium solidification. This phase was recovered after heat treatment. The microstructure of the LPPS coating had much less oxide than the HVOF coating due to depositing at low oxygen pressure in the vacuum chamber. After 200 hours of oxidation test, the amount of β phase (as an oxidation resistance criterion) was completely consumed in the LPPS coating, while the HVOF coating contained the retained β deposits. The average thickness of TGO layer for HVOF and LPPS coatings was 5.2 and 7.1 μm, respectively. The dispersed oxides in the microstructure, lower roughness and denser structure of HVOF coating were reasons for the higher oxidation resistance of HVOF coating than LPPS.

Conventional and nanostructured NiCoCrAlY coatings were deposited developed using the high-velocity oxy- fuel (HVOF) thermal spraying technique. The nanostructured NiCoCrAlY powder feedstock for the coatings was produced by the ball milling method. In this regard, the ball-milling method was used to produce the nanostructured NiCoCrAlY powder feedstock. The microstructures examinations of the as-received and nanostructured powders as well as their developed coatings were analyzed investigated by using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) equipped with energy-dispersive electron dispersed X-ray spectroscopy (EDS), and a transmission electron microscope (TEM). Due to the evaluation of the oxidation kinetics, the free-standing coating specimens were subjected to the isothermal and cyclic oxidation testing at 1000 and 1100 ºC respectively, under a laboratory air atmosphere. The results indicated that the as- received NiCoCrAlY coating had a parabolic oxidation behavior in short- and long-term oxidation exposure tests. For the nanostructured NiCoCrAlY coating, in contrast, the long-term oxidation kinetics deviated from parabolic behavior and showed instead sub-parabolic rate behavior. The obtained results also exemplified revealed that the nanostructured NiCoCrAlY coating had a greater oxidation resistance for to both cases of isothermal and cyclic conditions on account of the formation of a dense and slow-growing Al2O3 layer on the coating surface.

In the present research, wear behavior of Cr2O3-20YSZ (CZ) and Cr2O3 (C) coatings created using atmospheric plasma spray (APS) process was evaluated. For this purpose, Cr2O3 and YSZ nanopowders were produced after milling for 5h in a high-energy ball mill and subsequently plasma spraying of the agglomerated powders was carried out on 304L substrates. Microstructural characterization of these ceramic coatings was performed using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and optical microscope. Mechanical tests including adhesive strength, fracture toughness, and micro-hardness were used so as to explain the coatings wear behavior. Sliding wear test was conducted using a ball-on-disk configuration against an alumina counterpart at room temperature. Addition of YSZ particles to the Cr2O3 matrix due to the phase transformation toughening mechanism associated with tetragonal zirconia resulted in an increase in the fracture toughness while decreased the micro-hardness. It was found that the composite coatings had the friction coefficients of the proper order of 0.11-0.15. The CZ composite coating compared to the C coating showed the higher wear resistance so that the weight loss was obtained as 11 and 31 mg, respectively. Observation of the wear tracks of the coatings indicated that the lower wear rate of the CZ coating was attributed to its higher toughness and therefore filling the pores due to the higher plastic deformation of its wear debris.

This study examines the influence of NiCrAlY powder with and without Al2O3 Reinforcement doses of 5, 10 and 20% by weight, produced by plasma spraying method have been investigated. In order to prepare a composite powder, shaft ball mill blender for an hour for each powder were used. The NiCrAlY powder and powder composite NiCrAlY / Al2O3 with values of 5%, 10%, 20% Al2O3 were coated on Inconel 718 substrates by plasma thermal spraying. In order to evaluate the microstructure, morphological and thermal stability evolutions of the powders and coatings were investigated using X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy and optical microscopy. Vickers micro and macro hardness test was used to evaluate the hardness..The pin-on -disk test with 5 N was used to cold resistance assessment of coatings. Findings showed that the composite coatings NiCrAlY/Al2O3 because of the size and morphology of different ingredients, has a higher porosity, but in terms of mechanical properties due to reinforced phase, improvements in composite coatings were observed. Evaluate the wear behavior of Al2O3 10% coatings showed that 37.7× 10-6(mm3/Nm)abrasion resistance better than other coatings, especially coatings from powders without reinforcement 114.6× 10-6 (mm3/Nm) abrasion resistance had.

In this study, microstructure investigation, micro-hardness, fracture toughness and oxidation behavior of stellite6 and NiCrBSi coating produced with HVOF and GTAW methods has been investigate. The microstructure investigation of raw materials and coatings do with Secondary Electron Microscopy (SEM) and Optical Microscopy (OM). A Vickers micro-hardness meter used to study of hardness of coatings with 300 gr load. The fracture toughness of coatings investigated with Vickers hardness meter on the 1.5 to 200 Kg load. Oxidation behavior investigation of coatings carried out at 750 °C for 2, 5, 15 and 50 hrs. The oxide layer formed on the coatings during oxidation test studied with X-Ray Diffraction method. The oxidation results show the layer of Cr2O3 formed on the thermal sprayed Stellite6 coating. It caused to this coating show lower oxidation rate among all coatings. But, on the oxide layer of Stellite6 cladding coating, the spinel oxide phases are major phase, that are formed due to complex phase formed during surface welding.

In this research, effects of different coating process parameters on the final properties of thermal spray coatings obtained from HVOF method are investigated. For this aim, five different levels are considered for four effective parameters (Fuel injection rate, oxygen injection rate, powder injection rate and spray distance) on HVOF process by response surface method (RSM) and minimum porosity and minimum oxidation layer thickness are considered as the responses. To investigate the microstructure of coatings, Thermal Grown Oxide (TGO) thickness and porosity, a Scanning Electron Microscopy (SEM) and image analysis method are applied. Oxidation test has done at1100ºC for 50 hours on coated samples. Results show that (oxygen injection rate and powder injection rate), (fuel injection rate and powder injection rate), (powder injection rate and spray distance) have a reaction on each other in the TGO thickness value. Also, (oxygen injection rate and powderinjectionrate), (fuel injection rate and powder injection rate) have a little reaction on the porosity against the other parameters which haven’t any effect on it. Investigation of the coated sample with optimized parameters reveals that porosity of the coating is about 0.5% and TGO thickness is about 5.3 micrometer.

Tungsten carbide powders have been known as one of the hardness and wear resistance materials. The combinations of carbide with soft metals such as cobalt as a binder are widely used to improve the wear resistance, toughness and prevent brittle failure. In this research, WC-12Co powders were deposited on steel substrate by high velocity oxygen fuel (HVOF) spraying. Due to the existence of pores and microcracks in the coatings, the corrosion and wear resistance properties of the coatings could be improved by sealing the porosities and cracks via a post deposition treatment. A thin layer of Al2O3 were deposited on the HVOF thermally sprayed WC-Co coatings by Sol-Gel method. The phase composition of coatings was investigated by X-Ray Diffraction (XRD) analysis and the morphology and microstructure of the coatings as well as their thicknesses were investigated by Scanning Electron Microscopy (SEM) followed by image analysis. To study the wear resistance of the coatings a pin on disk wear tester was used. The results showed that the coefficient of friction was reduced from 0.6 to about 0.3 after sealing. The wear rates of the sealed coatings were decreased from 5.5x 10-3 in the as-sprayed coatings to 3.4 x 10-3.in the sealed coatings. The wear resistance of coatings after the sol-gel sealing process was improved due to the filling of the pores and cracks as well as formation of the aluminum and cobalt oxides on the surface during wearing test.

In this study, high-velocity oxygen-fuel (HVOF) process was applied to produce Stellite WC composite coatings on the surface of carbon steel samples. For this purpose, various amounts of WC powder 0%, 10%, 20% and 30% were mixed with Stellite6. The structural and mechanical behavior of the coatings were evaluated using Xray diffractometry, scanning electron microscopy, microhardness and roughness test. The results showed that the micro structure of the coatings were composed of a cobalt and chromium solid solution dendritic structure with the interdenderitic phases of cobalt enriched phase, carbides and WC particle. The XRD evaluations indicated that the coatings included Co-rich phase, CoCx , Co3W3C, Co6W6C as well as Cr23C6 and Cr7C3 carbides. Also, It was found that the hardness was increased, the adhesion was improved and the porosity was decreased by increasing the tungsten carbide as reinforcement.

In this study, satellite 6 and NiCrBSi coatings deposited on hot work tool steel DIN-2344/1 by thermal spraying and cladding were investigated. Stellite 6 and NiCrBSi powders were deposited on the substrate by HVOF and Stellate 6 wire were cladded by GTAW. Scanning electron microscopy with energy dispersive spectroscopy and optical microscopy, pins on disk wear machine, macro and micro hardness meter were used to analyze the coatings. Results showed that the hardness and wear resistance of thermal spray coating were higher than cladding coating. At high temperature, the results of wear test show, that the NiCrBSi coating has the better wear resistance rather than satellite 6 coating. This behavior of NiCrBSi coating attributed to silicate and boride particles in this coating. On the other hand, the thermal spray satellite 6 coating show the very low friction coefficient due to chromium oxide formed on the surface of coatings.

In this research, the effect of adding chromium carbide to stellite-6 thermal spray coatings was investigated. For this reason,the various amount of Cr3C2 powder (10%, 20% and 30 wt . %) were mixed with stellite-6 powder. The mixed powders deposited on the Inconel-617 substrate with high velocity oxygen fuel (HVOF) process. The coatings properties were evaluated using optical microscopy, scanning electron microscopy equipped to Energy Dispersive Spectroscopy (EDS) analysis, microhardness and X-Ray Diffraction (XRD). XRD analysis results showed that no oxide phases formed in coatings. In order to evaluate tribological properties of the coatings ball on disk wear test was performed in 1000m with 40N load. Results show that hardness and wear resistance of the coatings increased with increasing of chromium carbide reinforcement. Stellite6-30 wt. % chromium carbide coating had the highest wear resistance than others coatings.

Thermal shock behaviour of thermal sprayed ZrO2-8%Y2O3 coating on aluminized superalloy In-738LC has been investigated. Thermal shock experiments consisted of rapid thermal cycling between 1100 ºC and 300 ºC. Visual observations were conducted after each cycle in order to identify macroscopic features such as macrocraking and spallation. Coating characterization before and after the thermal cycling was conducted by means of optical and scanning electron microscopes as well as EDS analysis. The results demonstrated that the coating response to the thermal cycling was consisted of TGO thickening، extending of the interdiffusion zone and transformation of β-NiAlγ''-Ni3Al in the aluminide coating. In addition، failure of the coating system comprised of pitting oxidation at aluminide topcoat، nucleation of cracks in TGO، and finally cracks propagation into the ceramic layer.

Thermally sprayed coatings are inherently associated with residual stresses in the coatings. These stresses vary in nature and magnitude، and have a pronounced effect on the mechanical behavior of the system. This study develops the EDM drilling process to measure the residual stress in WC-Co thermally sprayed coatings، the stress state of which is difficult to obtain by a widely used method called high-speed hole-drilling method (ASTM Standard E837). The experimental results reveal that the stress curves are not uniform throughout the coating thickness. It has been found that the mean residual von Mises stress is of approximately 126MPa. Obtained results were confirmed by the value of the residual stress measured by postmortem curvature method in this study (-164MPa). This technique confirms the feasibility of residual stress measurement on HVOF thermally sprayed WC-Co coatings using ED-hole drilling method. Morphological and crystallographical studies were conducted using optical microscopy، scanning electron microscopy (SEM)، and X-ray diffraction، respectively، to evaluate the powder and coating characteristics.

In this study, in order to produce intermetallic compound of MoSi2, pure molybdenum and silicon powders were milled using and attritor mill for 20 hrs with the molar ratio of Mo:Si being equal to 1:2. Mechanically alloyed powders were annealed in an atmosphere controlled furnace at 1100˚C for different times. The result of X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicated formation of pure MoSi2 which was achieved by mechanical alloying for 20 hr followed by annealing for 7hrs at 1100˚C. Since the produced powders were extremely fine and not suitable for thermal spraying, they were agglomerated by carboxymethyl cellulose (CMC) binder and were meshed. This allowed a better flow of the powders and indicated a suitable flow for thermal spraing.

In this study, an attempt was made to synthesize Mo-Si-B multiphase alloy coatings using a combination of mechanical alloying (MA) and air plasma spraying (APS). Mo-14Si-10B (at%) elemental powders were milled using an attritor mill. Mechanically alloyed (MAed) powders as compacted buttons were annealed in an atmosphere controlled furnace at 1100ºC for different times. The annealed Mo–Si–B buttons were crushed, sieved and prepared for coating. Then, powders of Mo-Si-B as alloyed and agglomerated, were plasma sprayed under different conditions onto plain carbon steel. Metallurgical characteristics of powders and coatings were evaluated by SEM, OM, XRD and AAS. The results did not show any related intermetallics after MA. However, Molybdenum silicides were identified when the MAed powders were subjected to high temperature annealing over 10 h. Also, the critical parameters in APS to maintain the starting stoichiometry were identified; this included thermal spraying in an inert environment to minimize the oxidation of coatings. No intermetallic compounds could be identified when alloyed powders were sprayed.

In this paper, the inclined impact of a droplet on a solid surface in a spray coating process is studied using both numerical and analytical models. The numerical simulation is based on a previously developed model that includes the solution of Navier-Stokes equations along with an equation for the liquid free surface. The contact angle is modeled using two advancing and receding angles at the liquid front on the solid surface. The close agreement between the results of the numerical model with those of the experiments shows that the model can accurately predict the droplet impact behavior. Using the balance of droplet energy before and after the impact, a simple analytical model is presented for the maximum spread of a droplet during an inclined impact. This model is an extension of a previously developed model for the normal impact of a droplet on a solid surface. A comparison between the results of the analytical model with those of the simulations and experiments verifies the accuracy of the analytical model predictions for the maximum spread of a droplet in a spray coating process. Based on the analytical model, for high Reynolds numbers and small angles of impact, the effect of Weber number on the maximum spread is increased. For low Reynolds numbers and impact angles close to 90o, the maximum spread remains nearly constant