Very low pressure plasma spraying (VLPPS) has been used to manufacture thin, dense, finely-structured ceramic coatings for various applications. This paper presents the results of work in which VLPPS is used to deposit metal. Aluminum was chosen as a demonstrative material, due to its moderate vaporization enthalpy (38.23 KJ·cm-3), with the objectives of better understanding the behavior of a solid precursor injected into the plasma jet, leading to the formation of vapors, and controlling the factors affecting coating structure. Nearly dense aluminum coatings were successfully deposited by VLPPS at 100 Pa with an intermediate power (45 kW) plasma torch. Optical emission spectroscopy (OES) was used to observe the behavior of the metal powder injected into the plasma jet, and simplified CFD modeling provided a better understanding of thermophysical mechanisms. The effect of powder size distribution, substrate temperature, and spray distance were studied. Coatings were characterized by SEM observations and Vickers microhardness measurements.

This study investigates the HVOF spraying characteristics of a new WC-FeCrAl powder as compared to a standard WC-Co feedstock. Significantly higher particle temperatures were recorded for the WC-FeCrAl powder during spraying, presumably an effect of phase reactions during particle dwell time in the jet. XRD graphs revealed W2C and δ-Fe2O3 formations. Gibbs free energy calculations propose that energy is being released during the formation of these phases. Comparable correlations between in-flight particle measurements and splat morphologies were found for both powders. Coating hardness was also found to be comparable, although porosity was significantly lower in the WC-FeCrAl samples. This is attributed to the smaller carbide grain size of the new powder, which might help explain the lower viscosity of the molten particles at impact. A response surface analysis of XRD measurements indicates that W2C formation occurs in the spray jet, and it is assumed that δ-Fe2O3 formation occurs on the surface of the substrate after particle deposition.

This study compares the deposition and oxidation behavior of two oxide-dispersed CoNiCrAlY powders, one commercially obtained, the other prepared in a high-energy attrition ball mill using CoNiCrAlY and nanosize α-alumina powders. The custom powder was deposited by HVOF spraying using two sets of parameters, one optimized for CoNiCrAlY powder, the other for fine alumina. Coatings produced under the latter conditions were found to be porous, which can be attributed to a low degree of melting in the dispersed alumina. Isothermal oxidation testing at 1373 K for up to 1000 h in air caused oxidation not only at the surface, but also inside the coatings due to the movement of oxygen through the pores. The coatings deposited under the other set of parameters, i.e., at higher power levels, were free of pores. Isothermal oxidation tests were also carried out on coatings produced from the commercial powder, in this case, by HVOF and as well as vacuum plasma spraying. The coatings obtained by HVOF spraying were found to have a thinner thermally grown oxide layer than not only the VPS coatings, but also conventional metallic bond coats. Internal oxidation in the HVOF coatings is due to insufficient cohesion of the spray particles. Furnace cycling tests were conducted on specimens with an additional ceramic thermal barrier coating. Specimens with VPS bond coats produced from commercial oxide-dispersed powder achieved almost same number of cycles to delamination as specimens with conventional metal bond coats.

This study assesses the effects of dry ice blasting on the lifetime and durability of thermal barrier coatings (TBCs). Three sets of TBCs consisting of a CoNiCrAlY bond coat and YSZ topcoat were deposited by air plasma spraying, each set with a different dry ice blasting treatment. Different microstructures were obtained in both the bond coat and topcoat depending on blasting conditions. Bond coat oxidation and thermal shock lifetime of the TBC are also shown to vary with the blasting treatment. TBCs where both the bond coat and topcoat are dry-ice blasted proved to be the most durable with the biggest improvement in lifetime. They also exhibited the most regular surface roughness.

This investigation employs tensile adhesion tests (TAT) and tubular coating tensile (TCT) tests to measure the adhesion and cohesion bond strength of plasma sprayed YSZ coatings. Tensile adhesion testing measures the bond strength of the YSZ-bond coat system perpendicular to the spray direction, while tubular coating tensile testing measures the intersplat strength of the YSZ coating parallel to the spray direction. In both cases, the failure strength of the coatings can be approximated to a Weibull distribution, indicative of anisotropic behavior as verified by Knoop microhardness indentation tests. The average coating strength parallel to the spray direction is shown to be about 1.5 times greater than the bond strength perpendicular to the spraying direction.

This study investigates the phase stability and thermophysical properties of Y 2 O 3 and Yb 2 O 3 co-doped SrHfO 3 (SHYY) powder and bulk material along with the phase stability and microstructure evolution of as-sprayed SHYY coatings during annealing. The powder was synthesized by a solid-state reaction at 1450 °C, showing good phase stability up to 1400 °C. Dilatometry measurements revealed no abnormal changes in the coefficient of thermal expansion over a temperature range of 200-1300 °C. The thermal conductivity of the bulk material was found to be 16% lower than that of SrHfO 3 . Free-standing SHYY coatings deposited by air plasma spraying were also tested. The coatings consisted of SHYY and a minor amount of secondary phase Yb 2 O 3 and exhibited good phase stability during heat treatment at 1400 °C for 288 h. Coating samples examined after 216 h still exhibited a columnar microstructure.

Various approaches are presented in this paper to adapt conventional twin wire arc spraying (TWAS) for the production of smooth and finely structured coatings. Higher particle velocities were achieved by modifying spraying nozzle geometry. New geometries that incorporate a Laval shape produced the highest particle velocities while also eliminating overspray and extending the high-velocity region. This led to a more focused spraying plume and a change in optimal spraying distance, which was found to be more than 200 mm based on coating roughness. Some of the new nozzles exhibited evidence of particle deposition on the inner walls, which can restrict plume flow if not addressed. The problem is related to the position of the Laval throat in the spray plume as well as changes in gas pressure.

The aim of this work is to optimize molybdenum disilicide coatings for high-temperature oxidation protection of metallic surfaces. Agglomerated and sintered MoSi2 powder was deposited on test substrates by atmospheric plasma spraying. The powders and coatings were characterized by means of optical and scanning electron microscopy. Various tests were carried out to determine the influence of powder size and spray parameters on coating porosity, hardness, and adhesive pull strength.

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.

This study investigates the density, hardness, and wear behavior of WC-Co-Cr coatings produced by high-velocity air fuel (HVAF) spraying. The results indicate that coating hardness, density, and wear performance are improved by increasing gas velocity and using powders composed of fine CoCr particles.

This study investigates the static friction properties of HVOF-sprayed Cr3C2-NiCr coatings. Measurements of the static coefficient of friction (CoF) of as-sprayed coatings show their potential for use in frictionally engaged joints. The form, orientation, and geometric characteristics of Cr3C2-NiCr friction surfaces are assessed as well and slipping curves are determined. The results show a standard deviation in the static CoF depending on nominal contact pressure, but it is not yet possible to establish a correlation with coating properties such as carbide grain size and geometrical parameters such as coating roughness.

Compounds of the material group known as MAX phases combine metallic and ceramic properties. In this work, MAX-phase coatings are deposited from modified Ti3SiC2 and Ti2AlC commercial feedstock powders using HVOF and atmospheric plasma spraying (APS). Feedstock powders and coatings were studied by microscopy and XRD. Despite the use of unoptimized powders, well adhering and relatively dense coatings were produced. HVOF-sprayed layers had denser microstructures with higher amounts of MAX phases. Optimizing the shape and particle-size distribution of feedstock materials is expected to improve coating properties.

This work assesses the challenges of preparing dense technical ceramic substrates for thermal spraying and evaluates the capabilities of laser ablation in comparison with sandblasting. Sintered Si3N4 and AlN substrates were prepared by both methods and surface roughness was measured before and after treatment. Alumina coatings were deposited by suspension-HVOF and atmospheric plasma spraying, and coating cross-sections were analyzed by optical microscopy and SEM. Sandblasting had little or no effect on surface roughness and cracks were observed in coating cross-sections at the near-surface region of the substrate. Laser ablation, on the other hand, significantly increased surface roughness for both ceramics, producing hole patterns that are shown to vary with laser power and pulse timing. In the case of plasma spraying, the best coatings were achieved when the holes in the substrate were less than 100 µm in depth. With suspension sprayed coatings, the best results were obtained on substrates with deeper (> 100 µm) holes.

The temperature distribution of glass fiber-reinforced epoxy flat plates coated with a thin oxy-acetylene flame-sprayed aluminum-12silicon coating was determined experimentally. The composite plates were fabricated by filament winding. Following winding, but prior to and during curing, garnet sand was uniformly distributed on the glass fiber-reinforced epoxy plate surface. The sand roughened the surface such that there was adhesion of the aluminum-12silicon particles to the surface. A resistive heating wire was attached to the coated surface. Thermocouples were attached to the composite and coating surfaces to measure transient and spatial surface temperature distributions. The spatial temperature of the coating and polymer surfaces decayed uniformly throughout the coating-composite ensemble from the heating wire. It was also observed that the coating served to increase the surface temperature of the coating-polymer system compared to uncoated samples. This was attributed to the large thermal conductivity of the metal coating and the low thickness of the samples.

Molybdenum disilicide (MoSi 2 ) has been applied as protective coating material on various substrates fabricated by different methods due to its good oxidation resistance at elevated temperature, relatively low density and coefficient of thermal expansion and high thermal conductivity. In this work, MoSi 2 coatings were fabricated by low pressure plasma spraying technology (LPPS). Their morphology, composition and microstructure characteristics were intensively investigated by SEM, XRD, EDS and TEM. The oxidation behaviors of MoSi 2 coatings were also explored. The results showed that the MoSi 2 coating was compact with porosity less than 5%. Its microstructure exhibited typical lamellar character. The MoSi 2 coating was made up of grains with irregular shapes and different sizes of 0.1-0.2 µm. It was mainly composed of tetragonal and hexagonal MoSi 2 phases. A small amount of tetragonal Mo 5 Si 3 phase formed and randomly distributed in the matrix of MoSi 2 . The MoSi 2 coating exhibited excellent oxidation-resistant behavior at 1773K, which resulted from the continuous dense glassy SiO 2 film formed on its surface.

During the last 20 years, numerous scientists have studied the formation of thermally sprayed WC-Co coatings. Most of them focused on the direct connection between parameter variations and coating properties, such as the microstructure, wear or hardness. As the formation of single splats is the foundation for any thermal spray coating, this work focuses on the investigation of single splat morphologies. The aim of single splat interpretation is to determine the influence of different spray parameters on the morphological distribution of particles inside the flame. This distribution is indispensable to understand the formation of each coating layer during the process. Unfortunately, most of the methods presently used for generating single splats do not allow an assignment of each splat to its radial position in the flame. A method to create a footprint of a spray jet with an extremely short exposure time was used in this paper. The resulting field of splats enables the assignment of the splats on the specimen to their radial position in the cross-section of the spray jet. The footprints were analyzed by correlating the quantities and morphology of the splats to measurements of the spray jet properties and the splat’s radial position inside the jet.

Chromium carbide-based thermally sprayed coatings are widely used for high temperature wear applications. In these extreme environments at those temperatures, several phenomena will degrade, oxidize and change the microstructure of the coatings, thereby affecting their wear behaviour. Although it can be easily conceived that the Cr 3 C 2 -NiCr coating microstructure evolution after high temperature exposure will depend on the as-sprayed microstructure and spraying parameters, very little has been done in this regard. This study intends to develop a better understanding of the effect of spraying parameters on the resulting chromium carbide coating microstructure after high temperature operation and high temperature sliding wear properties. The microstructures of different coatings produced from two morphologies of Cr 3 C 2 -NiCr powders and under a window of in-flight particle temperature and velocity values were characterized through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Sliding wear at 800°C was performed and the wear behaviour correlated to the spraying parameters and coating microstructure. Vickers microhardness (300 gf) of the coatings before and after sliding wear was also measured.

In this study, a ZrO 2 – 7 % Y 2 O 3 (YSZ) powder (-90 +16 µm) was nanostructured by high energy ball milling and sprayed using a modern three-cathode plasma generator TriplexPro- 210 as well as a conventional plasma generator F4MB-XL. The parameters were varied in order to investigate their influence on build-up, microstructure and properties of the thermal barrier coatings (TBC). Powders and coatings were characterized in terms of their morphology, microstructure and phase composition by means of light microscopy (LM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray analysis (XRD). Thermo-shock behavior of TBC was evaluated using thermal cyclic tests at 1300 °C and 1150 °C. The results show that the milled powder contained nano-sized particles. TBC from the nanostructured powder by TriplexPro-210 had high porosities and numerous fine pores, leading to lower microhardness and higher thermos-shock resistance than the reference TBC.

In this work, Al/SiC composite coatings were deposited on the surface of aluminum alloys through atmospheric plasma spray. The effects of SiC volume in Al/SiC composite powders on the deposition behavior and the properties of the Al/SiC coatings were investigated. It was found that there were decarburization and oxidation during the deposition of pure SiC powders in the plasma flame. With the increase of SiC content, the deposition of the Al/SiC composite powder became more difficult through plasma spray. There were cracks between pure SiC and the bond coat on the Al alloy substrate resulting in poor adhesion between them. The hardness of the composite coating became higher with the increase of SiC contents. The Al/SiC(50:50) deposit with a thickness of 70 µm and a hardness of 369 Hv resulted in a strengthening and protection on the surface of Al alloy.