This study assesses the influence of particle size and spray parameters on the structural, mechanical, and electrical insulation properties of alumina coatings deposited by atmospheric plasma spraying. It has been found that the combination of a relatively fine feedstock powder and high velocity plasma spraying promotes the formation of denser coatings with high dielectric strength. Correlations between dielectric strength and deposition efficiency, coating hardness, crystal structure, and surface roughness are also assessed.

This study compares the performance of an ordinary plasma spray gun with that of recently developed gun designed for spraying feedstock suspensions on inner diameter surfaces. To evaluate the guns, yttrium oxide was deposited on 304 stainless steel while varying supply pressure, spraying distance, and gun traverse speed. Different methods of delivering suspension spray material to the gun were also investigated. Although Y 2 O 3 inner-diameter coatings were successfully formed, hardness and cross-sectional porosity need improvement. Based on the findings, it may be necessary to increase substrate temperature, readjust spraying parameters, and optimize feedstock materials.

In this investigation, NiTi coatings are applied by atmospheric plasma spraying. Surface and interface morphology of the as-deposited material is studied using scanning electron microscopy, the presence of the different phases are revealed by X-ray diffractometry, and microhardness is determined by Vickers hardness testing. The as-deposited coatings are exposed to air-borne particle erosion to investigate their wear properties while varying erodent impact pressure and angle. It was found that the plasma sprayed NiTi splats are well formed with significant amounts of intermetallic and oxide phases at the surface and interface, contributing to dense splat formation and higher hardness.

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 work demonstrates the fabrication of a hydroxyapatite (HA) composite material for potential use in biomedical implant applications. A composite powder is prepared by introducing graphene oxide (GO) and F- ions, which are incorporated in the HA crystal structure via in-situ chemical synthesis. The powder is consolidated through spark plasma sintering, resulting in a biocomposite (GO-FHA) material that is mechanically stronger and more chemically stable after implantation than HA. The addition of GO and partial substitution of F- also promote osteoblast proliferation as in-vitro bioactivity tests show.

A new nanoparticle plasma spray process has been developed that uses conventional powder feeders and injectors to produce fine ceramic coatings at high deposition rates. This paper explains how powder feedstocks are prepared and how the resulting coatings compare with coatings by other methods. The feedstock used in the demonstration was produced by adding YSZ nanoparticles to an acrylic liquid resin, which was then solidified, crushed, and screened. SEM images show that the nanoparticles are well dispersed throughout the resin fragments. In the plasma flame, the resin fragments burn down as the nanoparticles are heated and accelerated toward the substrate, producing fine zirconia layers free of microcracks and pores as observed by SEM. The presence of carbon deriving from the resin material is dealt with by post-process heating at different temperatures, the effect of which is assessed by means of thermogravimetry. Vickers hardness of the YSZ phase was measured to estimate the sintered density.

In this study, a design of experiment (DOE) approach is used to investigate the influence of powder size and mixing ratio on the microstructure and properties of thermal spray coatings. Nano and micro sized WC-12Co powders combined in different proportions with Inconel 625 were deposited on carbon steel substrates by HVOF spraying. The resulting coatings are examined and the effect of different powder combinations on hardness and yield strength is assessed. Spraying procedures and test methods are described and the findings are discussed. Of the various coatings produced, one shows great promise for wear protection, particularly in oil and gas applications.

In this study, MCrAlY-Al 2 O 3 composite powders were produced by ball milling and deposited by plasma, HVOF, and cold spraying. The results show that Al 2 O 3 fractions can be well controlled using composite powder due to non-preferential impact debonding of the matrix and Al 2 O 3 . The microstructure of spray powders is well retained in HVOF and cold-sprayed coatings due to the unmelted or partially molten condition of the spray particles. In the case of plasma-sprayed coatings, however, most Al 2 O 3 particles segregate at lamellar interfaces, forming a continuous oxide scale on the splat. The cold-spray coatings exhibit the highest hardness due to the work hardening effect of kinetic deposition.

The present study deals with the production and characterization of Ti-6Al-4V coatings produced by cold spraying. All experiments were performed using nitrogen as the process gas. By systematic variation of spray parameters up to a nitrogen gas temperature of 1000 °C and pressure of 5 MPa, the coatings could be tuned for optimum mechanical properties. Attainable coating properties are described in terms of a newly introduced coating quality parameter based on the ratio of particle velocity to critical velocity. The new parameter, η, is used both to interpret and predict coating properties.

This work investigates the effect of feedstock size on the hardness and wear resistance of metal-matrix composite coatings produced by cold spraying. Feedstocks consisting of Al and Al 2 O 3 powders were prepared for the study. The feedstocks, which differ in regard to Al 2 O 3 particle size (100, 50, 10 µm) and composition (25-50 vol%), were accelerated by compressed air through a Delaval-type nozzle positioned 10 mm from the target substrate. The morphology of the coating resembled that of an Al matrix with embedded Al 2 O 3 particles. Optical microscopy showed that large Al 2 O 3 particles (> 50 µm) fractured into small pieces and embedded in the matrix. It is likely that some of the fragmented particles bounced off, rather than adhering. These collisions (tamping effect) increase coating hardness and density. In the case of the feedstock with 10 µm Al 2 O 3 , particle sizes were unchanged during spraying and the benefits of work hardening were not achieved.

Tungsten-based cermets are well-known engineering materials finding applications in aerospace, nuclear equipment, and many other fields. Plasma spraying is an interesting industrial process to manufacture those refractory materials. Original plasma sprayed hard coatings for wear protection composed of a stainless steel matrix and inclusions of tungsten carbide (WC) nanoparticles were developed. To built-up the coatings, two precursors were injected separately in the plasma jet : a stainless steel micrometric powder was classically injected into the plasma jet using a carrier gas whereas WC nanoparticles were injected with a liquid carrier, like in the so-called process suspension plasma spraying. One of the challenges is to maintain the WC phase stoichiometry in the deposit, without decomposing the carbide into brittle W 2 C, W 3 C, and metallic tungsten, phenomenon usually occurring with thermal spraying techniques. Another issue is to succeed in including homogeneously the carbide nanoparticles in a sufficiently dense stainless steel matrix. Coatings with different WC contents were deposited on stainless steel substrates and investigated with respect to their microstructure by optical and scanning electron microscopy, porosity level using the Archimedean method, phase composition by X-ray diffraction and Vickers micro-hardness. Results have shown that coatings consisting of a stainless steel matrix containing inclusions of carbide nanoparticles can be produced by plasma spraying. The phase composition analysis indicated that nanoparticles are largely composed of the WC phase and contain a small amount of WC1-x phases. A slight increase of the porosity level was measured for coatings containing nanoparticles, compared to the pure matrix, probably due to the cooling effect of the WC carrier liquid on the in-flight characteristics of the stainless steel particles. Micro-hardness measurements gave similar values for with or without nano-sized particles, showing that the amount of WC included in the samples was insufficient to improve the hardness property.

Wall Colmonoy Ltd has developed a new generation of Iron-based powder alloys providing superior resistance to dry wear and excellent corrosion resistance. These alloys are economically interesting because they contain none or very low levels of expensive and market sensitive metals such as nickel and cobalt. These powders are marketed under the trade name Colferoloy. This paper details the properties and advantages of Colferoloy when compared to current “traditional” surface treatment methods and consumables. Metallurgical examination of the Colferoloy deposits showed that the alloy formed sub-micron structures which afford excellent dry-wear resistance, whilst the high chromium content provides good resistance to corrosion. Colferoloy alloys were applied using HVOF techniques to engineering components producing dense coatings with a hardness in the range of 950-1000 Vickers. The cost benefits of using an Iron-based alloy with low density hard phases were studied and comparisons with traditional materials were performed. This study showed that Colferoloy is a viable alternative to current hardfacing alloys and treatments.

The PAS method was used to produce W-ZrC and W-HfC composite powder, and the LVPS process technologies were used to create W composite coating layers. In addition, the mechanical properties, high-temperature resistance, and ablation characteristics of the W composite coatings were compared and analyzed for different types of carbides. For comparison and analysis, Vickers hardness, porosity, and adhesive strength were measured, and plasma torch tests were conducted. The use of the LVPS technologies led to successful production of W composite coatings (W-HfC; W-ZrC), approximately 1,000 μm or above in thickness. ZrC particles were observed in the layers of W-ZrC coating. The porosity was 3.59 % in W-HfC and 7.74 % in W-ZrC, indicating the W-HfC coating had a better pore quality than W-ZrC. Vickers hardness was approximately 120Hv higher in W-ZrC than in W-HfC due to the presence of ZrC particles in the W-ZrC coating. Adhesive strength was found to be nearly identical in both coatings. Results of the evaluation of high thermal resistance characteristics of the W composite coating materials showed that W-ZrC coating performed better in resisting high thermal conditions than W-HfC coating, due to the strengthening effects of ZrC particles in the layers and the generation of ZrO 2 phase with high levels of stability in high temperatures.

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.

To improve the deposition efficiency of copper particles, especially fine particle, in cold spray process onto metallic substrate, optimization of nozzle shape and dimension was performed by numerical simulation. Maximum velocity of the particle reached up to 685 m/s under the optimum conditions by using self-designed nozzle based on the simulation results. In the spraying of copper particle onto steel substrate, lamellar-like unique micro-structure was observed near the interface region of the steel substrate. Correspondingly, hardness increase in this region of steel substrate was recognized. Work hardening was induced in the steel substrate due to the higher velocity of copper particles. Furthermore, to reduce the bow shock effect on the substrate surface region in cold spray process, special shaped nozzle was newly developed. While nominal particle velocity decreased in the special nozzle, deposition efficiency, Vickers hardness and adhesion strength increased significantly especially in the case of fine particle and higher pressure levels of the working gas. Numerical simulation showed that pressure level on the substrate surface decreased effectively in the special nozzle. In the observation of a cold sprayed individual particle onto mirror polished substrate, extended metal jet was recognized at particle’s periphery when the particle was sprayed by the special nozzle. The results indicate that the decrease of particles velocity by bow shock was suppressed effectively in the special nozzle as compared with conventional nozzle.

Wear properties of WC/Co cermet coatings have been investigated prepared by cold spraying. Influence of cobalt contents (12~25wt.%), WC particle size (0.2 and 1.8 µm) and agglomerated-and-sintered powder size (-20+0 and - 45+15 µm) on abrasive wear resistance, micro hardness and coating structure is studied, in detail. It has been found that both smaller WC particle and decrease of cobalt content are effective to produce dense, hard and highly wear resistant coating. Smaller powder size is also favorable to make a coating with high mechanical properties. As a result, the cold sprayed coating from WC(0.2 µm)/12wt.%Co with powder size of -20+0 µm has best mechanical properties within this study. This coating has high uniformity and high density with little pores compared to conventional HVOF sprayed coatings. Abrasive wear resistance of the cold sprayed coating, investigated using Suga-abrasion tester, has been almost comparable to HVOF sprayed coating prepared from same feedstock. Strong correlation is also seen between Vickers hardness and abrasive wear resistance. Microstructural analysis suggests that further improvement of coating uniformity by decrease of small pores with the size of sub micron and homogeneous dispersion of WC grains in the cobalt matrix is required to improve the mechanical properties.

The demand of industry for metallic thermal sprayed coatings with controlled porosity until now is fulfilled by the spraying of metallic powders mixed with additives (organic element in many cases) which play the role of pores. The new technology of cold spray can lead to the formation of innovate coatings of controlled porosity by using pure metallic (or alloy) powder without any further addition. A fine Al-12Si powder (<45 µm) was sprayed with a cold spray system (CGT Kinetic 3000-M) on stainless steel substrate under different spraying conditions. In the present study, the new polymeric nozzle PBI-33 of CGT was used for the formation of al-based coatings. The microstructure, the porosity, the Vickers microhardness and the superficial Rockwell hardness (R15Y) of the produced coatings are examined.

In low-pressure plasma spraying, a plasma jet generator with a supersonic expansion nozzle is useful for spray coating hard and large-area films adhering strongly to substrates. In the expansion nozzle, the pressure and the electron density drastically decrease downstream, and therefore the plasma is in thermodynamic nonequilibrium state. Additionally, the supersonic expanding plasma jet is expected to be in chemical nonequilibrium state in which excited plasma particles are carried downstream in chemically-active state. In this study, titanium nitride (TiN) reactive spraying was carried out under a low-pressure environment using a DC arc plasma jet generator with a supersonic expansion nozzle. Titanium powders were injected using a hollow cathode with argon gas, and the plasma gas was nitrogen or nitrogen and hydrogen mixture. Microstructure and properties of the coatings were examined using scanning electron microscope (SEM) and X-ray diffraction (XRD). A dense and high-quality TiN coating with a Vickers hardness of 2000 was formed at a low substrate temperature of 700 °C with a low input power of 5.3 kW. All results showed that the supersonic plasma jet in thermodynamic and chemical nonequilibrium state had high potentials for reactive spraying.

In magneto-plasma-dynamic (MPD) arc jet generators, plasma is accelerated by electromagnetic body forces. Silicon nitride reactive spraying was carried out using an MPD arc jet generator with crystal silicon rods and nitrogen gas. Because higher-velocity, higher-temperature and higher-density and larger-area plasmas are produced with the MPD arc jet generator than those with conventional thermal plasma torches, nitriding of silicon can be enhanced. A dense and uniform β-Si 3 N 4 coating 30 µm thick was formed after 200 shots at a repetitive frequency of 0.03 Hz with a discharge current of 9 kA and a substrate temperature of 700 °C. The Vickers hardness reached about 1300. Furthermore, silicon carbide and aluminum nitride sprayings were conducted with the same spraying system. Surface modification is under study with lots of chemically reactive gases. All results showed that the MPD arc jet generator had high potential for spraying and surface modification.

The steadily increasing requirements to the properties of thermal spray coatings have to lead to the development of new characterization tools, in particular for non-destructive testing. Laser acoustic surface waves relate to the most promising methods for cost-effective non-destructive testing. In this work seven HVOF-sprayed WC-based coatings were systematically studied by laser acoustic surface waves using the LAWave device. Due to short measurement and calculation times the coating can be easily multiply tested. Young`s modulus and densities of the coating were obtained by this method. The values of the Young's modulus were compared with those derived from a micro-indentation method using Vickers indents and were found to be in a good agreement. Moreover, Vickers hardness values of the coatings obtained by using different loads were compared and the phase composition was studied by X-ray diffraction. The coating porosity was determined by image analysis of optical micrographs of metallographic cross-sections. It is proposed that in the case of WC-based coatings changes in the theoretical density of the material composition due to phase transformations induced by the spray process (formation of W2C and solid solutions on their base) prevent a direct access to the porosity values.