Search Results for hardness

Results concerning the relationship between deposition efficiency and mechanical properties with coating roughness are presented. A commercial partially stabilized zirconia powder was plasma sprayed at different power levels, H2/Ar ratio and spray distance. Coatings sprayed at high deposition efficiencies demonstrated improved mechanical properties. The deposition efficiency is also proportional to the coating roughness. When prepared with a high plasma power and a high H2/Ar ratio, and at short spray distances, these coatings exhibited the highest deposition efficiency and coating roughness. The high coating roughness associated with a high deposition efficiency arises because coarse powder particles are likely to be melted at higher plasma power. However, opposite trends were also observed within different regimes of powder size distribution and torch power. Another objective of this work was to demonstrate that a surface profilometer can be used to understand the deposition efficiency and mechanical properties of thermal spray coatings. Key words: Roughness, Mechanical Properties, DE.

This article discusses the relation between hardness of substrate and WC-Co coating destruction by HVOF. This research uses the indentation test of a hard ball to study WC-Co coating destruction by HVOF thermal spray system. By adjusting the hardness of substrate, the article investigates and evaluates the effects on coating destruction. As for base material hardness, it is necessary to have beyond 500HV (49.1HRC) in order to keep surface hardness of sprayed coating. It was also observed that when sprayed coating received pressure, plastic deformation of the base material made the destruction more likely to occur. By measuring HRC surface hardness of sprayed coating and observing HRC indentation, the research concluded that strength evaluation of sprayed coating is possible.

In this study, finite element analysis and experimental observation are used to evaluate the effect of substrate hardness and spray angle on the deposition of cold-sprayed Ti particles. It is found, in the case of Cu substrates, that both the particle and substrate deform during impact, resulting in a large contact area. Metallurgical bonding is highly likely under such conditions, facilitating formation of thick coatings. In the case of Al substrates, although the contact area is smaller, Ti particles are trapped by the softer substrate material, resulting in mechanical interlocking and a relatively thick coating. In the case of stainless steel substrates, mechanical interlocking does not occur due to the relative hardness of the material, which limits coating thickness. The results of the study also show that decreasing the spray angle reduces interfacial contact area and coating thickness, while increasing porosity.

In automotive industry, thermal spray process is used to reduce engine weight by replacing cast iron liners inserted in cylinder bores. Especially, twin wire arc spray is one of widely used thermal spray processes with inexpensive cost and high deposition rate. In this study, two kinds of wire materials, low carbon steel (0.07 wt.%C) and high carbon steel (0.80 wt.%C) were deposited by twin wire arc spray process using two kinds of process gas (i.e., compressed air and nitrogen) in order to elucidate effects of carbon contents of ferrous coating and process gas type on the hardness and wear resistance of coating. In case of hardness, low carbon steel coatings had higher hardness when air was used as process gas whereas high carbon steel coatings had higher hardness when nitrogen was used, which was caused by the counter effects of carbon loss and oxide formation. The results of sliding wear test in lubricated condition indicated that coatings with higher hardness have better wear resistance and oxides improve wear resistance by playing a role as solid lubricant. The main wear mechanism was splat delamination induced by inter-splat crack, and traces of other wear behaviours such as splat tip fracture and abrasive wear were also observed.

This study assesses the effect of different alloying elements on the microstructure, oxygen content, hardness, and corrosion resistance of stainless steel coatings produced by atmospheric plasma spraying. SUS836L stainless steel powder with Si, Mn, and B additions served as the base feedstock alloy to which different amounts of B, C, Mo, Ti, Nb, V, and Cu were added. The powder mixtures were sprayed on carbon steel substrates and the deposits were examined and tested. The results show that B and C additions of 2-3% have a beneficial effect, but at 5% cause a drop in corrosion resistance that proved to be remediable through the addition of Cu, which improves the corrosion potential of the matrix phase by its combined action with Mo, Si, and B. The effect of Ti, Nb, and V, which are added to suppress Cr oxidation in molten alloy particles during flight, is that it promotes that formation of fine carbide and boride compounds, increasing hardness without sacrificing corrosion resistance. In addition to these findings, the study also shows that the coatings developed are in many ways comparable to Ni-based self-fluxing alloy coatings.

Cold-gas dynamic spraying (“cold-spraying”) at low pressure (1034kPa/150 psig) was used to fabricate WC-Ni-Cu metal matrix composite (MMC) coatings. Tungsten carbide (WC)- based powder was mechanically blended with nickel (Ni) and copper (Cu) powder at various compositions. X-ray diffraction (XRD), scanning electron microscopy (SEM), and Vickers micro-hardness testing were conducted on the cold-sprayed coatings. Image analysis was used to determine the WC content in the coatings. XRD profiles showed that no decarburization or oxidation of the WC reinforcing particles occurred in any of the coatings. The WC content in the coatings increased as the WC content in the powder increased, but did not increase further beyond 96 wt. % WC content in the powder blend. The results from Vickers micro-hardness testing confirmed that the coatings with the highest amount of WC had the highest hardness value. The coatings fabricated with a powder composition of 96 wt. % WC + 2 wt. % Ni + 2 wt. % Cu yielded a hardness of 385 ± 73 HV 0.3 /10 (n = 50). These results suggest that it is possible to use cold-spraying at low pressure to fabricate WC-based MMC coatings with improved hardness.

This study evaluates the possibility of depositing hard B 4 C and TiC reinforcing particles in a Ni matrix using low-pressure cold spraying. It also investigates the effect of particle velocity and kinetic energy on deposition efficiency, microstructure, hardness, and wear resistance. B 4 C and TiC powders were blended at 50, 75, and 92 wt% carbide content with Ni powder comprising the remainder of the mixture. The impact velocity of sprayed carbide particles was calculated using a mathematical model based on the thermodynamics of compressible fluid flow through a converging-diverging nozzle. The model showed that the kinetic energy of TiC particles prior to impact was three times smaller than that of B 4 C, resulting in a higher carbide content (18 wt% compared to 8 wt%) due to reduced fracture and rebound of the TiC particles. Although the hardness values of both coatings are within the range of cold-sprayed WC-Co-Ni, wear rates were found to be high.

Hardness and microstructure of Cr 3 C 2 -NiCr coating formed by vacuum plasma spraying process (VPS coating) were investigated in compare with that formed by High Velocity Oxygen Fuel Spraying process (HVOF coating). The results are as follows. (1) The hardness of VPS coatings in as sprayed condition was HV1243 ± 80, which was much higher than that of HVOF coatings, HV958±44, and never went down under HV1100 even after heat treatment at 1273K for 3.6ks. (2) VPS coating presents dense lameller structures composed of Cr 2 C 3 , Cr 7 C 3 and γ-NiCr phase, while HVOF coating presents lameller structures with many fine gaps, composed of Cr 2 C 3 , Cr 7 C 3 , γ-NiCr phase and relatively large amounts of Cr 2 O 3 . (3) The reason why such high hardness was obtained in VPS coating, has been considered due to their dense lamella structures.

High entropy alloys (HEAs) are classified as a new class of advanced metallic materials that have received significant attention in recent years due to their stable microstructures and promising properties. In this study, three mechanically alloyed equiatomic HEA coatings – AlCoCrFeMo, AlCoCrFeMoW, and AlCoCrFeMoV – were fabricated on stainless steel substrates using flame spray manufacturing technique. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Vicker’s microhardness were utilized to characterize the fabricated HEA coatings. Furthermore, Joule heating experiments using a modified version of a two-probe test was used to measure the electrical resistivity of the HEA coatings. To prevent short-circuiting of the metallic coatings, a thin layer of alumina was deposited as a dielectric material prior to the deposition of HEA coatings. The microstructure of the HEA coatings showed the presence of multiple oxide regions along with solid-solution phases. The porosity levels were approximately 2 to 3% for all the HEA coatings. The HEA coatings had a thickness of approximately 130 to 140 μm, whereas the alumina layer was 120 to 160 μm thick. The electrical resistivity values were higher for all the HEA coatings compared to flame-sprayed Ni-20Cr and NiCrAlY coatings and AlCoCrFeNi HEA thin film, which may be attributed to the characteristics of HEAs, such as severe lattice distortion and solute segregations. The combined interaction of high hardness and increased electrical resistivity suggests that the flame-sprayed HEA coatings can be used as multifunctional wear-resistant materials for energy generation applications.

Deposition of a dense coating with solid particles by cold spraying requires sufficient deformation of impacting particles and previously deposited underlying particles. The cermet particles and subsequent coating with a high hardness are difficult to deform upon impact. To increase the ability of deformation, the cermet spray particles with a porous structure design is proposed to fulfill the requirements of deformation on impact. To understand the deposition mechanism, the deposition behavior of single WC-Co spray particles impacting on the substrates with different hardnesses during cold spraying were examined using WC-12Co powders with different porosity. The substrates include stainless steel, nickel-based self-fluxing alloy coatings were employed to examine the effect of substrate deformation on the cermet particle deposition. It was found that using two porous powder of the porosity of 30% and 44% the WC-Co cermet particles were deposited on the substrate of different hardness from 200Hv to 800Hv. The deposition of the particles is mainly attributed to the deformation of powders themselves. The properly designed porous cermet powder with certain hardness is necessary condition to deposit hard WC-Co cermet coating.

The effect of the hardness in the substrate surface blasted by a grit blasting process on the adhesive strength of Zn-Al sprayed coatings is investigated to find the adhesive strength is improved by work hardening of the substrate surface. The adhesive strength between a substrate of a carbon steel and sprayed coatings of Zn-Al alloy sprayed by a wire flame spraying process is measured. The substrate is roughened by the grit blasting process with white alumina girt in various blasting angles and blasting time. The hardness is measured in around 20 micro-meter depth from the substrate surface. The adhesive strength increases with increasing the hardness even if the surface roughness is almost same. There is the definite correlation between the adhesive strength and the hardness rather than the surface roughness.

cBN/NiCrAl nanocomposite coating was deposited by cold spraying using mechanically alloyed composite powders. To examine the thermal stability of coating microstructure, the nanocomposite coating was annealed at different temperatures from 750 to 1000°C. The microstructure of composite coatings was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results showed that the nanostructure was retained in the coating when the annealing temperature was lower than 825°C which is 0.7 times of the melting point of NiCrAl matrix. The dislocation density significantly reduced when annealing temperature was higher than 750°C. The cBN particle growth became significant when the annealing temperature was higher than 825°C. The effects of crystal grain refinement and work-hardening strengthening mechanisms were quantitatively estimated as the function of annealing temperature. The effect of annealing temperature on the contribution of different strengthening mechanisms to coating hardness was discussed.

This study shows how the impact resistance of WC-Cr 3 C 2 -Ni coatings can be improved by as much as an order of magnitude through the addition of an undercoat with suitable hardness. It also investigates the effect of powder modifications and substrate hardness. Undercoats with Vickers hardness ranging from400-600 HV provided the biggest increases in impact resistance, but at around 700 HV and above, they are shown to have the opposite effect. The influence of the process used to apply the undercoat and the magnitude of the impact load used for testing are also considered in the study.

In this study, WC-12Co powders with multimodal-sized WC particles were prepared by ball milling and deposited onto stainless steel substrates by cold spraying. Microhardness and fracture toughness were measured on cross-sections by the indentation approach. Coatings produced from powders with small carbide particles, resulting from long milling times, were found to exhibit high microhardness and high fracture toughness. Deposits sprayed with high porosity powders produced by cold compaction had low microhardness, but high fracture toughness.

In this paper, as part of a systematic study on the rolling contact fatigue of HVOF-sprayed hardmetal coatings, the behavior of WC-17%Co coatings on hardened and nonhardened substrates is compared. To obtain a meaningful assessment, the coatings were applied to roller specimens made from soft as well as case-hardened MnCr steel. Two coating thicknesses were used with the expectation that maximum stress will occur in the substrate, in one case, and in the other case, in the coating. Test results show that the durability of HVOF-sprayed WC-Co is significantly better on nonhardened (soft) substrates and that the limiting factor for endurable Hertz pressure is the fatigue strength of the coating.

The gas tunnel type plasma spraying enables to produce high quality ceramic coatings. Moreover a high hardness coating was obtained at a short spraying distance in the case of alumina coating. A high hardness zirconia (ZrO 2 ) coating could also be obtained at an atmospheric pressure by using the gas tunnel type plasma spraying. The Vickers hardness of the ZrO 2 coating at a short spraying distance was very high: a high hardness of more than Hv = 1200 was achieved at the surface side of the coating. In this study, the characteristics of these high hardness zirconia coatings produced the gas tunnel type plasma spraying were investigated by the measurement of the Vickers hardness of the coating. The microstructure of the obtained high hardness zirconia coatings were also investigated by the microscopic method and by the X-ray diffraction method.

During grinding of thermally sprayed WC-Co, the grinding ratio G ( ratio of volume of work removed to the volume of wheel consumed) is usually low and the finish produced sometimes is inadequate. Improvement in surface finish accompanies increase in grinding ratio. The objective of this investigation is to study the effect of type of abrasive, table speed, and depth of cut on the surface finish and hardness of WC-Co. Thermally sprayed WC-12 wt % Co and WC-17 wt % Co produced using the high velocity oxygen fuel (HVOF) process, have been ground using silicon carbide and diamond wheels under different operating conditions. The surface profile reveals the significant role played by the above parameters on the surface finish. The grinding ratio, G in case of diamond grinding was found to be larger than silicon carbide grinding however, the quality of the surface finish produced by silicon carbide was better than the diamond. The surface structure of the ground WC-Co was examined by SEM. Surfaces ground using a silicon carbide wheel exhibited extensive plastic flow, while surfaces ground with diamond wheels are highly fractured with localized flow which suggests two different mechanisms of material removal. The surface hardness after grinding, was found to depend on the type of abrasive and table speed. Silicon carbide grinding has shown higher hardness and better surface finish than diamond grinding.

Commercially available coating techniques such as "open arc" and "spray & fuse" methods were used to compare the microstructural development with the plasma transferred arc (PTA) coating process for the hardfacing of NiCrBSi and Stellite 6 alloys. Denser eutectic structure was observed in the case of PTA coated layers of the Stellite 6 alloys than those of open arc weld-surfacing process. The shape of both carbides and borides in the 16C alloy coated by PTA processing were also obtained to have coarse morphology of carbides and borides while the "spray & fused" layers show a needle shape with finer distributions. Possible thermal history during each coating process is discussed. Based on microstructural observation, the hardness, wear resistance, and corrosion behaviors are reported. As expected, the alloy properties are directly related to their constituents of microstructure.

Nanostructured WC-Co powders were cold sprayed on different substrate materials at different accelerating gas temperatures. Splat morphology and microstructure were examined, showing that the splats are partially embedded in plain carbon and stainless steel substrates with a contour similar to that of the feedstock powder. Gaps and revers were observed around the splats and corrugations or ripples were found on the surface. In contrast, splats on the surface of WC-Co substrates are relatively flat with ejectas on the periphery. A comparison of splats also shows that particle deformation increases with increasing gas temperature and substrate hardness.

Using thermal barrier coatings on metallic gas turbine or diesel engine components is widely preferred to enable increased operation temperature with reduced component temperature. A standard thermal barrier coating is applied on a metallic substrate and has two layers. Namely, a metallic bond coat for resisting oxidation and bonding top coat to substrate and a ceramic top coat for thermal insulation. Mechanical properties are important for thermal barrier coatings due to their effects on coatings’ life and quality. Density and cohesive strength are the indispensable characteristics of coatings, which can be observed by improved hardness. In this paper three parameters, substrate temperature, substrate roughness and spray distance are selected and experiments are designed using Taguchi methods. Each parameter has three levels and a total of nine experiments are performed according to Taguchi’s L9 design. A bond coat is sprayed on a stainless steel substrate using HVOF thermal spray system. Particle temperature and velocity are monitored during the experiments. A series of micro hardness characterizations are made for each coating.