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.

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.

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.

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.

Cavitation erosion is one of the serious problems that affect lifetime and performance on hydraulic machineries and its components. Surface modification techniques is required to apply to hydraulic components, such as water turbine and radial flow pump, in general. Here, the techniques, which can easily be applied to large area, is must be required for actual applications in the fields. Thus, thermal sprayed WC cermet coatings with high abrasion resistance under wet conditions are one of the promising solutions as the protective coatings against cavitation erosion. In this study, WC/Cr 3 C 2 /Ni and WC/Co/Cr coatings prepared using a high-velocity oxygen fuel (HVOF) spraying have been evaluated by a magneto-striction vibratory apparatus to investigate the cavitation erosion resistance. The microstructures of the sprayed coatings after the cavitation erosion test have been analyzed by SEM to clarify the erosion mechanism. It has been found that the cavitation erosion resistance of the WC cermet coatings is significantly affected by primary particle size of WC, powder compositions and spray conditions. In the WC/10wt%Co/4wt%Cr, the coating with WC average particle size of 6.3 µm sprayed by optimized spray conditions achieved the highest cavitation erosion resistance than the other WC coatings as well as base materials of stainless steel used in the actual components.

High purity oxide ceramic powders of alumina (Al 2 O 3 ) and yttria (Y 2 O 3 ) for plasma spraying have been developed to apply to semiconductor and flat-panel-display (FPD) production equipments. The ceramic coatings onto inside chamber wall of the equipments are required to have highly erosion resistance against CF containing plasma, widely used in dry etching process for micro-fabrications of the devices. Yttria is increasingly used in this application due to the high resistance compared to alumina. It is found that the yttria coating formed from agglomerated-and- sintered powder consisting of large primary particle has smoother eroded surface. Considering that particle deposition onto the devices, this coating will be effective to decrease generation of large sized particle, which is easily deposit onto the devices. Electric insulating properties of the coatings are also investigated to apply to electrostatic chuck. Electric breakdown voltage of yttria coatings is almost comparable to that of alumina coatings. However, yttria is difficult to apply due to its lower mechanical strength. Using smaller powder as feedstock is effective to improve the electric properties and influence of coating purity is lower than the powder size.

High velocity oxy-fuel (HVOF) sprayed cermet coatings are required in various industrial fields due to their excellent properties, such as combination of wear resistance, corrosion resistance, high hardness, high bonding strength and stability under high temperature. In order to utilize them in the fields, optimization of composition and structure of the coatings are essentially important meaning that both spray powders and spray conditions are key process parameters. In this paper, developed spray powders of cermet materials are introduced for the specific applications, where 1. cavitation erosion, 2. mechanical impact, 3. corrosion by molten alloy and 4. general abrasive wear are major factors that damage the coatings. In order to solve these factors, HVOF coatings of 1. WC/Co/Cr with large WC particle, 2. WC/Cr 3 C 2 /Ni with addition of metal or alloy, 3. MoB/CoCr composed of double boride and 4. WC particle size in WC based cermet, are proposed and these merits are described.

MoB/CoCr with excellent durability against molten metal has been developed to be applied to aluminum die casting parts in automobile and to hot continuous dipping rolls in Zn and Al- Zn plating lines for steel industries. Our previous study has revealed that the MoB/CoCr coating prepared by high velocity oxy-fuel (HVOF) spraying has much higher resistance against attack of the molten Al-45wt%Zn alloys compared to conventional WC/12wt%Co coating from evaluation by laboratory scaled molten metal immersion test. However, performance with no damage is only limited when applying onto the substrate with smaller thermal expansion similar to the MoB/CoCr coatings. This paper describes the solution to prepare the MoB/CoCr coating without damage onto austenitic stainless steel of AISI316L with larger thermal expansion, whose material is commonly utilized for the hot continuous dipping rolls. It has been found that use of undercoat is effective to reduce the negative influence of large difference in thermal expansion between the MoB/CoCr coating and substrate of AISI316L. Optimized thickness of topcoat and undercoat and its combination showed intrinsic performance of low reactive MoB/CoCr without any mechanical damage such as crack and/or peeling.

Erosion properties of high purity alumina (Al 2 O 3 ) and yttria (Y 2 O 3 ) coatings prepared by plasma spraying have been investigated against process plasma in semiconductor and liquid-crystal-display (LCD) production equipments for dry etching. Studies on plasma power dependency have revealed that yttria coatings have showed higher anti-plasma erosion resistance than alumina coating as well as sintered alumina bulk at every condition. Although yttria coatings are inferior to sintered yttria bulk, the difference of the resistance becomes smaller with increasing plasma power. This result positively promotes the application of the yttria spray coating because of increasing plasma power in actual process. Erosion properties of the coatings against Ar/CF 4 /O 2 plasma, whose composition is increasingly being used in commercial process, are also investigated where physical etching effect becomes stronger than the typical CF 4 /O 2 case. It has been found that order of durability (yttria bulk > yttria coatings > alumina bulk > alumina coating) is independent of the gas species used in this study. The mechanism of the plasma erosion is discussed through micro-structural analysis of the eroded surfaces.

The structural and mechanical properties, in terms of surface roughness, hardness and wear resistance, of WC/12wt%Co coatings are investigated as a function of powder properties, such as powder size and WC grain size, as well as high velocity oxygen-fuel (HVOF) spray conditions. It has been found that the WC/12wt%Co coatings with relatively smoother surface, about 2 µm in average surface roughness (Ra), retaining high wear resistance can be formed by optimizing the powder and spray parameters. Further, we have found that powder size can control the surface roughness over a wide range; from about 2 to 7 µm (about Δ5 µm) in Ra. On the other hand, Ra changed only about Δ1 µm or more when changing spray conditions, such as barrel length and spray distance.

Application of plasma sprayed ceramic coatings with a high purity of more than 99.9% has been sharply increasing in semiconductor and liquid-crystal-display (LCD) production equipments for dry etching, sputtering and ashing in the last few years. The size of the equipments becomes larger with increasing Si wafer size and the LCD size that promotes the replacement from conventional techniques, such as alumite film and bulk ceramics, to plasma spray coatings, where the high durability against the plasma erosion (anti-plasma erosion resistance) is required. However, as far as we know, no systematic studies on the plasma-erosion properties are reported. In this work, durability of plasma sprayed ceramic coatings against CF 4 /O 2 plasma are investigated by reactive ion etching (RIE) system and are compared to that of the conventional techniques. The erosion mechanism is also discussed through the micro-structural analysis.

MoB/CoCr, a novel thermal spray material, with high durability in molten Al and/or Al-Zn alloys has been developed to utilize for die casting parts of Al alloy, and for hot continuous dipping roll in Zn and Al-Zn plating lines. The durability of the MoB/CoCr coatings prepared by high velocity oxy-fuel (HVOF) spraying has been investigated using a molten metal immersion tester. The immersion tests revealed that the MoB/CoCr coating has much higher durability without dissolution in the Al alloys than conventional spray coatings, such as WC/12%Co and WC/10%Co/4%Cr, as well as various other surface modification methods. The MoB/CoCr coating also has showed higher durability in the molten Al-Zn alloy when the coefficient of thermal expansion (CTE) of the substrate is similar to that of the MoB/CoCr. The lifetime of the coating on stainless 316L substrate, widely used as the hot continuous dipping roll, was insufficient because of the generation of cracks due to a large difference in the CTE between the MoB/CoCr coating and substrate. Basic anneal examinations have revealed that optimization of coating structure by changing undercoat material and its thickness is effective to improve the lifetime.

A novel thermal spray material of MoB/CoCr with high durability in molten aluminum and/or zinc alloys has been developed to utilize for die casting parts and for galvanizing bath parts. The durability of the MoB/CoCr coatings prepared by high velocity oxy-fuel (HVOF) spraying has been investigated using a molten-metal immersing tester. It has been found that durability of the MoB/CoCr coating in the molten aluminum and Al-45%Zn alloys is much higher than that of the conventional surface treatments, such as physical vapor deposition, nitridation and spray coatings of conventional materials, such as WC/12%Co, WC/10%Co/4%Cr, Cr 3 C 2 /NiCr, Al 2 O 3 and ZrO 2 -8%Y 2 O 3 . Preservation of the crystal structure and no decrease in coating thickness due to dissolution or delamination of the MoB/CoCr coating were seen after long term immersing test of about 600 hours. Further, the molten alloys were easily dropped from the specimens of the MoB/CoCr coatings during the immersing test, suggesting much lower reactivity. On the other hand, an adhesion of the alloys on the coating surface of other specimens was clearly observed. This means that the MoB/CoCr coating is excellent in increasing the lifetime of mechanical parts, which come into contact with molten metal and/or alloys.