This study investigates the influence of particle size, nozzle diameter, gas flow rate, and stand-off distance on the microstructure and density of suspension plasma sprayed yttrium-oxide coatings and the intermediate effect of particle characteristics. Three ethanol suspensions were prepared, one with coarse Y 2 O 3 , one with fine Y 2 O 3 , and one with submicron YSZ. The suspensions were injected vertically into the plasma jet downstream of the nozzle and a thermal spray sensor was used to measure in-flight velocity and temperature. The coatings were found to have columnar and dense vertically cracked (DVC) microstructure, varying in hardness and density. Text results and examination findings are presented and correlated with spray parameters, particle properties, and possible coating formation mechanisms.

This study investigates the influence of particle size and solid concentration in suspension on the microstructure of suspension plasma sprayed YSZ coatings. It also assesses the influence of plasma gases, plasma power, injector size, spray distance, and suspension feed rate. It is shown that suitable spray distance is necessary to achieve dense coatings with relatively coarse (0.6 μm) particles and that a columnar-like structure tends to be produced when fine (0.2 μm) particles are sprayed. The results also show that using Ar-He plasma and increasing total gas flow is an effective way to create dense vertically cracked (DVC) microstructure.

This paper presents the results of a study on the tribological properties of TiC-based coatings deposited by HVOF spraying. Four powder feedstocks consisting of (Ti,Mo)(C,N) hardmetal with Ni and Co binders were prepared by agglomeration and sintering. The feedstocks differ in composition and particle size distribution, the latter being optimized for fuel type and equipment requirements. Coating specimens are evaluated based on microstructure, hardness, bonding strength, and friction and wear behavior. The results are presented and correlated with spray parameters, equipment differences, and feedstock characteristics.

In this study, MoB-CoCr composite coatings are deposited on low-carbon steel substrates by HVOF spraying and salt spray tests are conducted to qualitatively evaluate coating density. Test samples with optimized dense coatings showed no rust after 300 hours in a salt spray. Samples with porous coatings, on the other hand, showed signs of rust after just 24-48 hours. Test samples protected by the dense composite coatings, as confirmed by salt spray testing, were undamaged after 90 days of immersion in a Zn-0.2%Al galvanizing bath at 460 °C.

In this work, tungsten carbide coatings are deposited by low-pressure cold spraying in order to assess the influence of powder compressive strength and binder materials on coating properties. Powder compressive strength was measured with a micro-compression tester, and cobalt and FeCr in different proportions were used as the metal binder. It was found that compressive strength affects coating hardness as well as deposition efficiency and that the optimum value for deposition efficiency is about 200 MPa. The results also indicate that dense coatings can be produced with either binder material, although coatings with an FeCr binder are the hardest.

In this work, alumina coatings are produced by air plasma spraying (APS) using dense powders ranging in size from 3 to 36 µm and one porous powder with an average particle size of 33 µm. Two spray systems were used, one rated at 40 kW, the other at 100 kW. The powders were applied to grit-blasted Al 6061 and low-carbon steel substrates. Coatings applied to Al 6061 using the high-power sprayer and 3 µm powder peeled off, likely due to thermal shock and mismatch. For all other coatings, the microstructure was examined by cross-sectional SEM, porosity was estimated via optical microscopy, and dielectric strength and volume resistivity were measured. Coatings formed from 3 µm powder were found to be dense with a mostly γ-phase crystal structure. Surprisingly, however, their volume resistivity was lower than that of more porous coatings with high amounts of α-phase. The findings show that, in the case of resistivity, spray equipment has a bigger influence than particle size, but with coating density, the opposite is true.

This paper presents the results of a preliminary study comparing high-velocity oxyfuel and airfuel spraying for the deposition of tungsten carbide coatings as an alternative to electrolytic hard chrome plating. Two tungsten carbide powders with a Co matrix and two with a Co-Cr matrix were sprayed on steel substrates using commercial HVOF and HVAF equipment. The coatings obtained are evaluated by means of SEM and XRD analysis, microhardness and adhesion measurements, and corrosion and wear resistance testing. Detailed results are presented and discussed with emphasis on the role of carbide grain size, carbide contiguity, and binder mean free path. In general, HVOF coatings show significantly higher dry wear resistance, owing to the presence of coarser primary carbides from the initial coarser powder. HVAF coatings, on the other hand, exhibit lower porosity and finer well-distributed primary carbides, giving them an advantage in terms of sliding wear resistance.

From the appearance of high velocity oxygen fuel (HVOF) thermal spray system in the 1980s, WC cermet coatings have been used as anti-wear coatings in many industrial manufacturing applications. Recently, WC cermet spray materials were applied using new thermal spray methods such as warm spray and cold spray, which are still in the research phase. In HVOF spraying, WC-Co and WC-Ni powders are regularly used as coating materials. On the other hand, using cold spray, WC-Fe alloy series can be deposited as dense and thick coatings, better than WC-Co. In this study, WC-Fe alloy powders were sprayed by cold spray to investigate the influence of binder metal on the coating properties and compared with those of HVOF WC-CoCr coatings. It was observed that the lower metal ratio and FeCrNi chemical composition exhibited improved results.

Cold spraying is well known as an attractive coating process that prevents degradation of materials, such as oxidation, decomposition and undesirable reaction due to overheating. High velocity oxy-fuel (HVOF) spraying has been recognized as the most popular technique in particular to produce tungsten carbide (WC) cermet coatings in industrial fields. However, the degradation of WC cermet occurs using HVOF that is quite different from sintered hard metal, which can be defined as an ideal material. Thus, low temperature processes, such as cold and warm spraying, are actively researched to achieve WC cermet coatings, similar to sintered one. In this study, influence of gas conditions in cold spraying on WC cermet coatings has been investigated, where WC-Co, WC-CrC-Ni and WC-Fe alloy are selected. The study reveals that nitrogen gas is effective to form thick, dense and hard coatings of WC-CrC-Ni and WC-Fe alloy compared to helium gas. This suggests optimal temperature and velocity of powder jet has been formed when using nitrogen gas.

From the appearance of high velocity oxygen fuel (HVOF) thermal spray system on 1980s, WC cermet coatings have been used as an anti-wear coating in many industrial manufacturing. Recently, WC cermet spray materials were applied to new thermal spray methods such as warm spray and cold spray under research phase. In HVOF spraying, WC-Co or WC-Ni series powder are used as standard contents. Ni and Co are binder metals for WC because of good wettability and suitable melting point. On the other hand, warm spray and cold spray are lower temperature process than HVOF. It is considered that any other factors of metal material such as hardness, toughness and crystal phase should be investigated in warm spray and cold spray. In this study, WC with Co or Fe alloy powders were sprayed by cold spray and HVOF to investigate the influence of binder metal for spray efficiency and coating property. It is cleared that Ni and Fe were superior to Co in spray efficiency and coating property in cold spray. The detail of above reason was under investigation, however, plastic deformability of binder metal is expected to be an important factor for WC cermet cold spraying.

Mechanical properties of WC-Co coatings prepared by cold spraying (CS) and warm spraying (WS) have been studied with changing material parameters of Co content (12~25%), powder size (-45+15 and -20+5 µm) and WC particle size (0.2 and 1.8 µm) in this paper. The study reveals that a formation of undesirable phases such as W 2 C, W, and amorphous or nanocrystalline Co-W-C (eta) phase has been suppressed in the CS and WS coatings. Both coatings have high hardness, which is comparable to or superior to HVOF coatings as well as higher density (low porosity) than the HVOF. Abrasion wear test has shown that WS coatings has higher resistance than CS coatings within this study. As for powder properties, smaller powder and smaller WC particle sizes are effective to produce hard and dense coatings leading to higher wear resistance.

Warm Spray (WS) process, which can control the temperature of a combustion gas jet used to propel powder, has been successfully applied to deposit WC-Co coatings. Detrimental reactions resulting from dissolution of WC into Co binder and decarburization were suppressed effectively by keeping the WC-Co particles’ temperature below the m.p. of the binder phase. In this study, three nano-structured WC-12Co powders with different particle strength were prepared by changing the sintering conditions of spray-dried powder and were deposited by WS. The deposition efficiency and porosity of the coatings decreased with increasing the particle strength. The coating deposited from the powder with very low particle strength showed significant phase changes, while those deposited from the higher particle strengths showed almost no change. Particle Image Velocimetry revealed significant disintegration of the weakest powder, which explains the changes observed. The hardness and wear properties of the former coating, therefore, were inferior to the other two.

WC-Co thermal sprayed coatings are mainly used for wear protecting functions in various industries, for which high velocity oxy fuel (HVOF) spray is considered to be the best suited process. However, WC-Co HVOF coatings still have some defects as compared with sintered bulk, such as decarburization of WC and porous structure. Recently, experiments of WC-Co coatings using warm spray (WS) and cold spray processes have demonstrated some improvements in reduction of these defects. In particular, WS process seems to be a more promising process for WC-Co coatings from the previous work. In this study, wear resistant functions of WC-12%Co coatings prepared by HVOF and WS were investigated by abrasion and erosion tests. In addition, in-flight particles were captured and their characteristics such as the amount of decarburization, crystal phase, particle strength and particle size distribution were investigated to clarify the difference between HVOF and WS processes. The result shows that the wear resistances of the WC-Co WS coatings are comparable or superior to those of the HVOF coatings, which can be attributed to the difference in the amount of W 2 C and coatings porosity revealed by the in-flight particles and the coating microstructure. The result of the in-flight particle analysis also indicates that wear resistance of WS coatings can be further improved by optimizing the powder shape and chemical composition.

Yttrium oxide (Y 2 O 3 ) coatings have been prepared with high power axial injection plasma spraying using fine powder slurries. It is clarified that the coatings have high hardness, low porosity and high erosion resistance against CF4 contained plasma in the previous study. This suggests that the plasma spraying of Y 2 O 3 with slurry injection techniques is applicable to fabricating equipments for semiconductor devices, such as dry etching. Surface morphologies of the slurry coatings with splats are almost similar to conventional plasma-sprayed Y 2 O 3 coatings, identified from microstructural analysis by field emission SEM in this study. However, no lamellar structure has been seen from cross sectional analysis, which is apparently different from the conventional coatings. It has also been found that crystal structure of the slurry Y 2 O 3 coatings mainly composed of metastable phase of monoclinic structure, whereas the powders and the conventional plasma spray coatings have stable phase of cubic structure. Mechanism of coating formation by plasma spraying with fine powder slurries will be discussed based on the findings.

WC-Co cermet coatings were fabricated by using Warm Spraying, which is a modification of HVOF spraying to lower the temperature of the propellant gas below the melting point of Co. By changing the processing parameters, specimens were prepared for hardness, abrasion wear and particle erosion tests. Their microstructures were examined by SEM and XRD. The microstructure clearly showed the effects of suppression of the dissolution of WC into the Co phase, which is the major cause of embrittlement of the conventional HVOF sprayed WC-Co coatings. By combinations of adequate feedstock powder and processing parameters, it was possible to take advantage of fine WC grain size to prepare coatings with higher hardness (HV > 1400), smoother surface (Ra < 2 μm), and moderately improved wear performances compared with conventional HVOF coatings.

This paper describes tribological properties of WC cermet coating layers which are thermally sprayed with HVOF. On the assumption of the severe sliding condition in which large compress and tensile stress are applied to a surface layer simultaneously, relatively hard material is chosen as counter parts and additional tensile stress is applied. WC/12%Co, WC/10%Co/4%Cr and WC/20%CrC/7%Ni are employed as coating material. From the experimental results, it is found that the WC cermet spray coatings by the HVOF are inferior to sintered WC/Co with regard to fracture toughness near the surface and pseudo plastic deformation which are main wear behavior in severe sliding condition. It is also found that a heat treatment has certain effect to eliminate the wear caused by the surface fracture, while, for the pseudo plastic deformation, it does not exhibit much effect.

Molybdenum disulfide (MoS 2 ) films are widely used to improve friction performance, but they are difficult to fabricate using conventional thermal spray processes due to thermal decomposition of the feedstock powder. In this study, Cu-MoS 2 composite coatings are fabricated by cold spraying using mechanically milled powders containing different concentrations of MoS 2 . Investigators found that increasing the concentration of MoS 2 in the powder improved some coating properties while degrading others. Through testing it was determined that the ideal concentration of MoS 2 is 5wt%. Increasing the milling time of the powder mixture also provided benefits in terms of hardness and wear resistance.

This study investigates the influence of plasma spray conditions on the structural, mechanical, and plasma-erosion properties of yttrium oxide coatings. Powder feed rate, plasma power, and primary particle size appear to have little effect on plasma erosion properties, hardness, surface morphology, and coating structure. However, a Y 2 O 3 coating produced with fine powder retained a smooth erosion surface despite its porosity and relatively low hardness.

In this study, high-power axial-injection suspension plasma spraying is used to synthesize yttrium oxide coatings from fine powder slurries. The coatings are assessed based on microstructure, hardness, and porosity and compared with coatings produced by other spraying methods. Resistance to erosion from CF 4 containing plasma is also investigated. Test results show that the suspension plasma sprayed Y 2 O 3 coatings are superior in terms of density, hardness, uniformity, and plasma erosion resistance. They also retain a smoother surface when exposed to plasma that contains CF 4 .

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.