Surface structures are of vital importance for the wetting behaviors of hydrophobic coatings. In this work, rare earth oxide coatings with different surface structures were deposited via the solution precursor atmospheric plasma spray (SPAPS) process and solution precursor vacuum plasma spray (SPVPS) process, respectively. The SPAPS coatings showed hierarchical cauliflower-like surface structures composed of micron-sized clusters and nanometer-sized particles, while the SPVPS coatings showed relatively flat topographies with small and short bumps. The formation of different surface structures in the SPAPS and SPVPS processes was investigated by modelling the movement of in-flight particles in the vicinity of the substrate. The properties of plasma jets and the characteristics of in-flight particles in the two processes were correlated. The effects of diverted plasma gas flow on the trajectories of particles impinging on the substrate and the resultant surface structures were elaborated, revealing different shadowing effects in the SPAPS and SPVPS processes. The SPAPS coatings were superhydrophobic due to the presence of hierarchical surface structures, which showed larger water contact angles and smaller roll-off angles than the SPVPS coatings. The correlations between the surface structures and wetting behaviors of different coatings were investigated.

The ingestion of siliceous particulate debris into the gas turbine engines during operation caused the deposition of so-called CMAS (calcium-magnesium-alumino-silicate) on the hotter thermal barrier coating (TBC) surfaces. The penetration of these particles into the TBC at temperatures above 1200°C caused the loss of strain tolerance and premature failure of the TBCs. To mimic real-world conditions, a commercially available CMAS precursor dust powder was sprayed onto 8YSZ coatings using an atmospheric plasma spraying process. The substrate temperature was maintained at an average of 1100°C and 525°C during spraying. The effect of the spraying parameters on the deposition, microstructure, and composition of the CMAS coatings was investigated. In addition, to understand the CMAS build-up on the high-temperature surfaces, the CMAS splat formation behavior was also analyzed on the polished samples at temperatures ~1100°C. SEM/EDS analyzes were performed to identify and quantify the elements of the CMAS deposits. It was found that the surface temperature, deposition time, and different nozzles could play a significant role in having different phases of CMAS deposits.

Thermally sprayed WC-based hardmetal coatings offer high hardness, good sliding wear and abrasion performance and find large applications in mechanical engineering, valve construction, or offshore applications. WC-Co coatings are mainly produced by high-velocity oxy-fuel spraying (HVOF) from conventional spray feedstock powders. In our previous work, the potential of the suspension-HVOF spraying (S-HVOF) to produce dense-structured WC-12Co coatings has been shown. Significant work was devoted to the development of appropriate aqueous hardmetal suspensions starting from commercially available fine WC and Co raw powders feedstock. This contribution proposes a step forward in the development of the S-HVOF WC-12Co coatings and evaluation of their microstructural and tribological properties. Suspension spraying trials were carried out using gas-fuelled HVOF TopGun system. For comparison purposes, liquid-fuelled HVOF K2 was employed to spray WC-12Co coatings starting from commercial available spray powder. Microstructural characterization, X-Ray diffraction and microhardness of the coatings were evaluated. Oscillating sliding wear tests were conducted against sintered Al 2 O 3 and WC-6Co balls. The sliding wear performances of the WC-Co sprayed coatings were discussed in term of the microstructure, phase composition and coating-ball test couples.

High amorphous phase formation tendency, a desirable microstructure and phase composition and silicon evaporation are the challenges of spraying Yb 2 Si 2 O 7 environmental barrier coatings (EBCs). This research addresses these issues by depositing as-sprayed high crystalline Yb 2 Si 2 O 7 using atmospheric plasma spray (APS) without any auxiliary heat-treating during spraying, vacuum chamber, or subsequent furnace heat treatment, leading to considerable cost, time, and energy savings. Yb 2 Si 2 O 7 powder was sprayed on SiC substrates with three different plasma powers of (90, 72 and 53 kW) and exceptional high crystallinity levels of up to ~91% and deposition efficiency of up to 85% were achieved. The silicon mass evaporation during spraying was controlled with a short stand-ff distance of 50 mm, and an optimum fraction of Yb 2 SiO 5 secondary phases (<20 wt.%) was evenly distributed in the final deposits. The desirable microstructure, including a dense structure with uniform distribution of small porosities, was observed. The undesirable vertical crack formation and any interconnected discontinuities were prevented. Reducing the plasma power from 90 kW to 53 kW, while conducive for mitigating the silicon mass loss, was detrimental for microstructure by increasing the fraction of porosities and partially melted or unmelted fragments. The gradual decrease of the coating temperature after deposition alleviated microcracking but has an insignificant effect on the crystallinity level. Coatings annealed close to their operating temperature at 1300 °C for 24 hours demonstrated sintering and a crack healing effect, closing the tiny microcracks through the thickness. An improved coating composition was detected after annealing by the transformation of Yb 2 SiO 5 to Yb 2 Si 2 O 7 (up to ~10 wt.%).

In the semiconductor industry, plasma etching processes are widely used. Process chamber parts that are located in the plasma etching system are also exposed to the harsh environmental conditions. Thus, parts located close to the process area are typically coated with yttria to increase service life, and thus process performance. However, such yttria coatings are usually porous, and thus can be attacked by fluorine containing plasma. In order to increase the lifetime of the components in the plasma etching system, this research project aimed to improve the protective yttria layer by reducing the porosity of the protective layer. Specifically, a design of experiment was employed in which the porosity was the target value. The main effects of the coating parameters and their interactions including the surface treatment before the coating process were determined. Furthermore, the bonding of the protective coating to the component to be protected, as well as the element distribution and the coating morphology were investigated. The results and their ramifications with respect to the envisaged application will be discussed.

Epitaxial grain growth during the rapid solidification of molten TiO 2 in plasma spraying was studied. The crystallographic structure of the TiO 2 splats deposited on rutile and α-Al 2 O 3 substrates at 150, 300 and 500 °C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homoepitaxial and hetero-epitaxial TiO 2 splats can be formed at the deposition temperature of 500 °C. Epitaxial growth is significantly influenced by the crystal orientation. It is easier to form an epitaxial TiO 2 splat with a <001> orientation in the direction perpendicular to the substrate surface. In order to explain the formation of epitaxial splat during plasma spraying, a competition mechanism between heterogeneous nucleation and epitaxial growth was proposed. It was indicated that the face (001) of rutile crystal exhibits the largest growth velocity, which is conducive to form an epitaxial splat for the melt with a largest undercooling degree. In addition, the effect of deposition temperature and crystalline orientation on the epitaxy was simulated. The simulation results are in agreement with the experimental observations.

This study evaluates a method for producing Gd 2 Zr 2 O 7 /SrZrO 3 , a ceramic-matrix composite considered for use as a thermal barrier coating. GdZrO/SrZrO powders are synthesized by co-precipitation, then cold pressed and sintered to form the bulk composite material. Phase stability of the powder and bulk material is assessed by X-ray diffraction and several bulk material properties are determined, including microhardness, Young’s modulus, fracture toughness, thermal expansion coefficient, heat capacity, thermal diffusivity, and thermal conductivity. The results are presented and discussed.

This study investigates the heat-shock properties of metal-oxide films synthesized from ethylenediamine tetraacetic acid (EDTA) complexes using conventional flame-spray equipment. An EDTA·Y·H powder was placed in the feed unit of the sprayer and transported by a flow of oxygen to the gun. The powder was sprayed using a mixture of H 2 and O 2 as the flame gas, producing a layer of yttrium oxide on a stainless steel substrate. XRD analysis was used to examine the crystal structure of the deposits and SEM imaging revealed the surface and cross-sectional microstructure. A cyclic thermal shock test was conducted and the deposited film was analyzed for the existence of cracks, deformation, and delamination. Although the number of cracks, crack lengths, and cracks per unit area increased due to heat shock, delaminations were not observed. The results show that the Y 2 O 3 films have high thermal-shock resistance and are suitable for use as thermal barrier coatings.

In this study, superhydrophobic samaria-doped ceria coatings are produced by plasma spray physical vapor deposition (PS-PVD) followed by fluorination treatment. Samples are sprayed at distances of 300, 400, and 500 mm in order to obtain surfaces with different morphology. SEM examination shows that the surfaces have a hierarchical structure with island-like features consisting of nanoparticles, the size of which is shown to influence sliding behavior. The superhydrophobic coating surfaces also exhibited good stability in repeated adhesive-tape tear tests.

This work deals with ZrB 2 -based coatings prepared by inert plasma spraying and their behavior under high heat flux in moist atmospheres. ZrB 2 coatings with different compositions and microstructures were produced and subjected to high-temperature oxidation testing in order to identify the most oxidation-resistant sample. It is shown that coating microstructure can significantly influence oxidation kinetics and that uniformly dispersed nanoscale additives are particularly effective for slowing oxidation.

In this work, Yb 2 Si 2 O 7 powder was synthesized from Yb 2 O 3 and SiO 2 powders and applied to aluminum substrates by atmospheric plasma spraying. Phase composition and microstructure were examined and density, porosity, and hardness were assessed. Thermal stability was evaluated by thermogravimetry and differential thermal analysis and thermal conductivity was measured. The as-sprayed coating was mainly composed of crystalline Yb 2 Si 2 O 7 with a small amount of Yb 2 SiO 5 and amorphous Yb 2 Si 2 O 7 . It had a dense structure containing pores, microcracks, and other types of defects. TG-DTA tests showed that there was almost no mass change from room temperature to 1200 °C, although a sharp exothermic peak appeared at 1038 °C, indicating that the amorphous phase had crystallized. The thermal conductivity of the coating decreased with increasing temperature, reaching a minimum of 0.68 W/(m·K) at 600 °C, followed by an increase at higher temperatures.

In this work, micro-plasma spraying is used to produce hydroxyapatite coatings on Ti-6Al-4V substrates. To understand coating formation mechanisms, in-flight particle velocity and surface temperature were monitored under different spraying conditions. XRD measurements show that the resulting coatings have a high degree of crystallinity with little amorphous or metastable phases. Some of the coatings were also found to have a uniformly distributed columnar structure, corresponding to a strong (002) texture and excellent stability in Hanks’ salt solution even after 14 days of immersion.

A transition in the flattening behavior of thermally sprayed metals has been observed in previous studies. It has been proposed that ultra-rapid cooled chill structure preferentially formed at the bottom part of the splat may play a role in the generation of disk-shaped metallic splats. The applicability of this hypothesis to other materials was verified experimentally for several ceramic oxides. To accomplish this, Al 2 O 3 , Y 2 O 3 , and YSZ particles were plasma sprayed onto stainless steel substrates and the fraction of disk-shaped splats was measured as a function of substrate temperature. Splat microstructure was also examined. Unique amorphous and chill structures were observed in the bottom portion of Al 2 O 3 and Y 2 O 3 splats, indicating that similar formation mechanisms may be at work. However, only a columnar microstructure was observed in the YSZ splats, which calls for additional study.

This study compares the dielectric properties of annealed forsterite (Mg 2 SiO 4 ) and alumina coatings deposited on mild steel substrates by atmospheric plasma spraying. As-sprayed coating samples were electrically characterized then submitted to a series of one-hour annealing treatments at temperatures from 300 to 800 °F. After each treatment, impedance measurements were recorded over a frequency range of 30 to 100 kHz. An electrical model was fitted to Nyquist data (Im Z vs. Re Z) using a least-mean-square algorithm with a weighting function. Although impedance spectroscopy measurements were obtained at different temperatures, this paper focuses on the acquisition, modeling, and comparison of room temperature properties, particularly electrical resistivity and dielectric constant. It also compares the microstructure of as-sprayed and annealed forsterite and alumina coatings and discusses coating degradation mechanisms stemming from differences in CTE.

The residual stresses present in coatings and layer composites are influenced not only by the thermal and mechanical loads generated during manufacturing, but also by the mechanical and thermophysical properties of the coating and substrate materials. In-process measurement of transient, process-induced stresses may thus enable the manufacturing of coated parts with a residual stress state that lies within a predefined application-oriented stress regime. This paper presents a quasi-nondestructive method by which such measurements may be obtained. A small amount of material is removed from the surface of a part by laser ablation, while optical interference sensors monitor surface deformation caused by stress relaxation and heating due to absorbed laser energy. The new method is evaluated by four-point bend testing using Al5754 plates coated with Al/TiO 2 by atmospheric plasma spraying.

This study evaluates candidate coatings for potential use in the manufacture of metal-seated ball valves for hydrometallurgy service. All coatings were deposited on grit-blasted titanium coupons by air plasma spraying to a nominal layer thickness of 500 µm. The feedstock powders used were selected based on literature review and field experience and include Cr 2 O 3 , TiO 2 -Cr 2 O 3 , nano TiO 2 , and a novel mixture of nano TiO 2 and conventional Cr 2 O 3 . The resulting coatings are compared based on microhardness, shear strength, friction properties, and wear resistance. Specimen preparation procedures and test methods are described in the paper along with the findings and potential implications of the study.

This study employs a combination of numerical analysis and experimental testing to obtain a better understanding of the changes that occur in hollow spherical metal-oxide powders during detonation spraying and how they affect coating quality. The heating and melting characteristics of hollow spheres are initially calculated for the general case then refined based on a simple detonation spraying model. The estimates are compared with experimental results obtained from detonation-sprayed Al 2 O 3 coatings produced using fused and crushed, dense spherical, and hollow spherical powders. The coatings as well as the powders are characterized based on morphology, particle size distribution, splat formation, cross-sectional microstructure, porosity, and hardness. Important findings, observations, and correlations are identified and discussed in the paper.

This study investigates the effects of operating environment and temperature on the friction behavior of self-mated WC-CoCr coatings in sliding contact. Nickel superalloy substrates were coated with 86WC-10Co-4Cr powder using a warm spray gun. Coating cross-sections and surfaces were examined by SEM, XRD, EDX, and x-ray photoelectron spectroscopy (XPS). Tribological tests were conducted on a high-load tribometer at various temperatures in air, nitrogen gas, and distilled water. Test samples were examined by SEM and XPS, revealing wear patterns and elemental compositions while providing insights on oxide formation.

Dysprosia stabilized zirconia coatings with large globular pores have good potential as TBC topcoats. In previous work, such coatings have been produced by air plasma spraying with the aid of a polymer pore former. The aim of this work is to optimize the spraying parameters. A design of experiments approach was used to create a two-level full factorial test matrix based on spray distance, powder feed rate, and hydrogen flow. An agglomerated and sintered dysprosia stabilized zirconia (DySZ) powder mixed with polymer particles was sprayed on Hastelloy X substrates that had been prepared with NiCoCrAlY bond coats. The coatings obtained were evaluated based on thermal conductivity, thermocyclic fatigue life, and morphology, which are shown to correlate with spray parameters and in-flight particle properties.

In this study, acoustic emission sensing is used to monitor interfacial cracking in thermal barrier coatings during uniaxial tensile adhesion testing. The TBCs consist of a ZrO 2 topcoat and a NiCrAl bond coat, both of which are applied by atmospheric plasma spraying. Tensile testing was performed to failure and the resulting fracture surfaces were examined by SEM and XRD analysis. Experimental results show that cracks usually initiate in the ceramic layer then propagate toward the metallic-ceramic interface where failure occurs. Finite element simulations were also conducted, confirming the experimental findings.