Warm Spray Research Articles

Preparation and Characterizations of NiTi Intermetallic Coatings

Much attention has been paid to NiTi intermetallic coating for enhancing cavitation erosion resistance property due to its shape memory effect. In this paper two processes were used to deposit NiTi intermetallic coatings. Process 1 was the deposition of NiTi intermetallic coating using warm spray process with NiTi intermetallic powder as feedstock, and solution treatment was subsequently performed. Process 2 was a mixture process of warm spray and laser treatment. Ni-Ti coating was first deposited via warm spray process with Ni-cladding Ti as feedstock, and laser-treatment was performed to realize the alloying of Ni-Ti. The microstructure and phase compositions for NiTi intermetallic coatings were characterized by means of SEM and XRD. The fundamental properties for NiTi intermetallic coatings were also analyzed.

Multilayered WC–Co/Cu coatings by warm spray deposition

WC–Co/copper multilayer coatings consisting of 8 layers were fabricated by warm spray deposition in order to investigate the effect of ductile layer inclusion onto their fracture behavior. Bending strength, work of fracture, and surface hardness of freestanding coatings were examined by three point bending tests after removal of the substrates. The multilayered samples showed non-linear stress–strain curves due to crackings in the WC–Co layers and plastic elongation of the copper layers. The multilayered samples with lower volume fraction of copper showed even lower bending strength than the monolithic samples of WC–Co and copper and no beneficial feature in mechanical performance was found. On the other hand, the samples containing higher volume fraction of copper exhibited more than twice higher work of fracture and moderately better bending strength than the monolithic WC–Co coatings, while the surface hardness was almost identical among all samples instead of the monolithic copper. The ductility of copper layers and the plastic constraint by the intact WC–Co layers attributed to enhance their mechanical properties. It has been concluded that cermet/metal laminate coatings can be one alternative approach to further improvement of the mechanical properties of thermal sprayed cermet coatings.

Current Status and Future Prospects of Warm Spray Technology

A modification of high-velocity oxy-fuel (HVOF) thermal spray process named as warm spray (WS) has been developed. By injecting room temperature inert gas into the combustion gas jet of HVOF, the temperature of the propellant gas can be controlled in a range approximately from 2300 to 1000 K so that many powder materials can be deposited in thermally softened state at high impact velocity. In this review, the characteristics of WS process were analyzed by using gas dynamic simulation of the flow field and heating/acceleration of powder particles in comparison with HVOF, cold spray (CS), and high-velocity air-fuel (HVAF) spray. Transmission electron microscopy of WS and CS titanium splats revealed marked differences in the microstructures stemming from the different impact temperatures. Mechanical properties of several metallic coatings formed under different WS and CS conditions were compared. Characteristics of WC-Co coatings made by WS were demonstrated for wear resistant applications.

Effects of Particle Strength of Feedstock Powders on Properties of Warm-Sprayed WC-Co Coatings

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 strengths 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.

Experimental and numerical evaluation of the performance of supersonic two-stage high-velocity oxy-fuel thermal spray (Warm Spray) gun

The water-cooled supersonic two-stage high-velocity oxy-fuel (HVOF) thermal spray gun was developed to make a coating of temperature-sensitive material, such as titanium, on a substrate. The gun has a combustion chamber (CC) followed by a mixing chamber (MC), in which the combustion gas is mixed with the nitrogen gas at room temperature. The mixed gas is accelerated to supersonic speed through a converging-diverging (C-D) nozzle followed by a straight passage called the barrel. This paper proposes an experimental procedure to estimate the cooling rate of CC, MC and barrel separately. Then, the mathematical model is presented to predict the pressure and temperature in the MC for the specific mass flow rates of fuel, oxygen and nitrogen by assuming chemical equilibrium with water-cooling in the CC and MC, and frozen flow with constant specific heat from stagnant condition to the throat in the CC and MC. Finally, the present mathematical model was validated by comparing the calculated and measured stagnant pressures of the CC of the two-stage HVOF gun.

Computational modelling of titanium particles in warm spray

A warm spray system has been computationally investigated by introducing a centrally located mixing chamber into a HVOF thermal spray gun. The effects of injecting a cooling gas on the gas and particle dynamics are examined. The gas phase model incorporates liquid fuel droplets which heat, evaporate and then exothermically combust with oxygen within the combustion chamber producing a realistic compressible, supersonic and turbulent jet. The titanium powder is tracked using the Lagrangian approach including particle heating, melting and solidification. The results present an insight into the complex interrelations between the gas and particle phases, and highlight the advantage of warm spray, especially for the deposition of oxygen sensitive materials such as titanium. This work also demonstrates the effectiveness of a computational approach in aiding the development of thermal spray devices.

Microstructural Development and Deposition Behavior of Titanium Powder Particles in Warm Spraying Process: From Single Splat to Coating

Warm spraying has been developed by NIMS, in which powder particles are accelerated and simultaneously heated, and deposited onto a suitable substrate in thermally softened solid state. In this study, commercially available titanium powder was sprayed onto steel substrate by the spraying process. Microstructural developments and deposition behaviors from a deposited single particle to a thick coating layer were observed by high resolution electron microscopes. A single titanium particle sprayed onto the substrate was severely deformed and grain-refined mainly along the interfacial boundary of particle/substrate by the impact of the sprayed particle. A successive impact by another particle further deformed the previously deposited particle and induced additional grain refinement of the remaining part. In a thick coating layer, the severe deformation and grain refinement were also observed. The results have demonstrated the complex deposition behavior of sprayed particles in the warm spraying using thermally softened metallic powder particles.

443 ウォームスプレーガン混合室の冷却割合の誤差が粒子の速度と温度に与える影響(固相微粒子による成膜技術とその応用,ものづくりにおける基礎研究と先端技術の融合)

443 ウォームスプレーガン混合室の冷却割合の誤差が粒子の速度と温度に与える影響(固相微粒子による成膜技術とその応用,ものづくりにおける基礎研究と先端技術の融合)

J0401-2-1 WC系サーメット溶射皮膜材料の摩擦磨耗特性([J0401-2]固相粒子成膜技術とその応用(2):溶射・コールドスプレー皮膜の特性評価)

Tribological properties of WC cermet coating layers which are thermal sprayed are researched. 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 is employed as coating material by High Velocity Oxy-Fuel (HVOF) spray and Warm Spray processes. From the experimental results, it is found that the WC cermet sprayed coatings 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.

Effect of Powder Characteristics on Properties of Warm-Sprayed WC-Co Coatings

In high-velocity oxy-fuel (HVOF) spraying of WC-Co coatings, the decomposition and decarburization of WC during deposition are responsible for their much lower toughness compared with a sintered bulk WC-Co. In a previous study, Warm Spray (WS) process, which is capable to control the flame temperature used to propel powder particles, was successfully applied in an attempt to suppress such detrimental reactions by keeping particles’ temperature lower than their melting point. The coatings deposited by WS process showed no or little formation of W2C and η phases and demonstrated moderately improved fracture properties. However, there is still a gap in fracture toughness between WS coatings and the corresponding sintered bulk. In order to optimize the properties of the WS coatings, the effect of original powder sizes were investigated. Microstructural characterization and phase analysis were carried out on deposited coatings by SEM and XRD. The results show that the feedstock powder size has substantial effects on the properties of the coatings, i.e., the smaller powder showed improved properties.

Jetting-Out Phenomenon Associated with Bonding of Warm-Sprayed Titanium Particles onto Steel Substrate

Titanium powder particles accelerated and simultaneously heated by the supersonic gas flow were deposited onto steel substrate by the warm spraying process. The sprayed particles were heavily deformed and bonded to the substrate in solid state. Especially, all the deposited particles showed jetting-out of materials out of the particle-substrate interface triggered by the adiabatic shear instability known to occur under such shock impact conditions. High-magnified images showed that grain refinement occurred in the jetting-out region by dynamic recrystallization. Furthermore, the elemental analysis using the electron energy loss spectrum showed jetting-outs of the substrate as well as the particle. Numerical simulation based on the Johnson-Cook plastic deformation model showed that the jetting-out phenomenon commences about 10 ns after the initial contact of the particle with the substrate and at a position away from the center bottom of particle, where the highest compressive stress is experienced.

Mathematical Modeling and Experimental Validation of the Warm Spray (Two-Stage HVOF) Process

The warm spray (WS) gun was developed to make an oxidation-free coating of temperature-sensitive material, such as titanium and copper, on a substrate. The gun has a combustion chamber followed by a mixing chamber, in which the combustion gas is mixed with the nitrogen gas at room temperature. The temperature of the mixed gas can be controlled in the range of about 1000-2500 K by adjusting the mass flow rate of nitrogen gas. The gas in the mixing chamber is accelerated to supersonic speed through a converging-diverging nozzle followed by a straight barrel. This paper shows how to construct the mathematical model of the gas flow and particle velocity/temperature of the WS process. The model consists of four parts: (a) thermodynamic and gas-dynamic calculations of combustion and mixing chambers, (b) quasi-one-dimensional calculation of the internal gas flow of the gun, (c) semiempirical calculation of the jet flow from the gun exit, and (d) calculation of particle velocity and temperature traveling in the gas flow. The validity of the mathematical model is confirmed by the experimental results of the aluminum particle sprayed by the WS gun.

Effects of Temperature of In-flight Particles on Bonding and Microstructure in Warm-Sprayed Titanium Deposits

Micron-sized titanium particles were deposited on steel substrates by the warm spraying, which is a modified high velocity oxy-fuel (HVOF) spraying technique. In the process, nitrogen gas is mixed with the HVOF flame jet to lower the temperature of injected powder particles. Detailed observations of splats formed on polished substrates by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted to investigate the effects of particle temperature on the bonding of splats with the substrate and the microstructure within the splats. At lower nitrogen flow rates, the particles observed were heavily deformed and exhibited diverse splat morphologies and microstructures. At higher nitrogen flow rates, most of the particles were impacted in the solid state and the oxidation of particles was remarkably less. The TEM observation revealed distinctively different microstructures within the splats as well as the splat/substrate interfaces depending on whether the particle was molten or solid before the impact.

Thermal softening effect on the deposition efficiency and microstructure of warm sprayed metallic powder

Micron-size titanium powder was deposited on a steel substrate by a warm spraying process, in which particles were accelerated and simultaneously heated by a supersonic gas flow. The gas temperature was controlled to soften the metal below its melting point before impacting onto the substrate. The particles were deposited with very high efficiency and bonded well to the substrate due to the thermal softening. Remarkably, fully nanocrystalline particles were formed by dynamic recrystallization without any pretreatment of the powder.

Development of WC-Co Coatings Deposited by Warm Spray Process

The high-velocity oxy-fuel (HVOF) process is commonly used to deposit WC-Co coatings. There are some problems with this process; especially the decomposition and decarburization of WC during spraying make a coating brittle. To suppress such degradation, the warm spray (WS) process was applied to deposit WC-Co coatings, which is capable of controlling the flame temperature in the range of 500-2000 °C. The microstructure and phases of the deposited coatings were characterized by using SEM and XRD, and the mechanical properties such as hardness, fracture toughness, and wear properties were also investigated. WS process successfully suppressed the formation of the detrimental phases such as W2C and W, which are usually observed in HVOF coatings. The WS coatings showed the similar trend of the hardness variation for Co content with a sintered bulk material. Improvement of toughness and wear behavior was also observed in WS coatings.

Grain refinement in a single titanium powder particle impacted at high velocity

Titanium powder particles were deposited at high velocity on a steel substrate by a process called warm spraying. Extremely fine grains, several tens of nanometers in size, were observed along the interfacial boundary of the deposited particles known as splats and the substrate, where the most severe deformation had taken place. A schematic model to explain the mechanism of grain refinement of titanium powder is presented, in which dynamic recrystallization caused by adiabatic shear instability plays the major role.

Warm Spraying: An improved spray process to deposit novel coatings

Warm Spray is an atmospheric coating process through continuous impact and deposition of solid particles heated and accelerated by a supersonic jet controlled between 800~1900 K and 900~1600 m s − 1 . This paper introduces successful fabrication of dense and less-oxidized metallic titanium (Ti) coatings by Warm Spray and clarification of phenomena occurring upon the spray process. Temperature and velocity of an in-flight Ti particle were compared between measurement by the diagnostic instrument and calculation based on the fluid dynamics simulation. Deformation behaviour of particle from impact to deposition was analyzed through the finite element method (FEM). Densification of stacking particles was attained by applying bi-modal size distribution to the feedstock Ti powder. Qualitative restriction of changes in chemical composition of Ti coating obtained was demonstrated by elemental analysis and by calculation based on the oxidation model. Warm Spray enables various materials to fabricate coatings without thermal deterioration of the original characteristics such as purity and crystallographic phase.

Fabrication of nano-sized oxide composite coatings and photo-electric conversion/electron storage characteristics

Titanium dioxide TiO 2 can be used as a photo-anode to give generated electrons to the metal substrate under illumination. The transition metal oxide such as iron oxide Fe 2O 3 can be used to store electrons generated by the photo-electric conversion function of TiO 2 under the illuminated situation while the electrons are discharged from the transition metal oxide to the metal substrate in the dark. In this paper, coatings of nano-sized composite of TiO 2 and Fe 2O 3 were fabricated by the Warm Spray process, in which the feedstock powder is accelerated by a supersonic gas jet with speed above 1.0 km s - 1 and temperature between 800 and 2500 K, and then impacted onto the target substrate continuously to form coatings. The coatings of TiO 2 and Fe 2O 3 nano-composite fabricated by Warm Spray showed no thermal deterioration such as phase transformation and particle growth of the feedstock during the spray process. The coatings fabricated by the Warm Spray had larger photo-current and the electron charge/discharge capacity than that by a conventional HVOF process. In addition, these characteristics were improved by decreasing the primary particle size of TiO 2 and Fe 2O 3.

Coating Fabrication of Nano-Sized Oxides/Metal Composite by Warm Spray and its Photo-Cathodic Protection Behaviour

By putting photo-electric conversion materials and electron storage materials together on one device, full-time cathodic protection system was expected to function and its coating technique is required to realize the system. So far, Warm Spray, impact deposition of supersonic particle with temperature controllable in a wide range, enabled nano-sized TiO2 and Fe2O3 to be deposited controlling thermal degradation of the particles and it was revealed that each of photo-electric conversion and electron storage were functioned. The rest potential of the coatings as an indicator of overall system performance was not negative sufficiently, which might be related to the less conductive nature. Therefore, metallic Zn was introduced in this paper for shifting the rest potential to more negative direction and increase of the conductivity. The coatings were successfully fabrication with control of thermal degradation of Zn, which worked as a binder to connect the oxide particles. Photo-cathodic protection behaviour indicated that the steady rest potential of the Warm Sprayed coatings of the TiO2-Fe2O3 system was not influenced on the introduction of metallic Zn, implying that it might not be limited by the conductive property of the coatings.

Fabrication and Mechanical Properties of Composite Structure by Warm Spraying of Zr-Base Metallic Glass

Metallic glasses are highly attractive because of their superior strength, relatively low Young’s modulus and superb corrosion resistance[S1], etc. By combining a metallic glass with different materials, synergy effect can be expected in the mechanical properties and corrosion resistance, for example. Fabricating a thick metallic glass layer on a substrate material by thermal spray process is one way to bond these materials. In such practice, however, the temperature of the sprayed metallic glass and that of the substrate must be kept relatively low in order to avoid crystallization and/or oxidation of the amorphous deposits. Warm Spray is a modified version of HVOF spraying developed by National Institute for Materials Science (NIMS) suited for spraying of heat sensitive feedstock materials. In the process, high-velocity solid particles heated to the temperatures below its melting point are projected on to the substrate. A Zr-base metallic glass powder was warm sprayed onto cylindrical substrates of 316L stainless steel to various thicknesses. Negligible crystallization and oxidation were observed in the deposited layer of the glass alloy. The linear relationship between the thickness of the glass alloy layer and the Young’s modulus of the composite bar demonstrated that the mechanical property of such composite structures can be controlled. [doi:10.2320/matertrans.T-MRA2007882]