In this study, the spray-dried Al 2 O 3 /Y 2 O 3 composite powder was prepared using commercially available nanosized Al 2 O 3 and Y 2 O 3 feedstock. The mass ratio of Al 2 O 3 to Y 2 O 3 was 2:1. Atmospheric plasma spraying possesses high thermal enthalpy, large temperature gradient and rapid cooling rate. The microstructure and wear resistant performance of plasma-sprayed Al 2 O 3 .Y 3 Al 5 O 12 (YAG) coating were investigated. As-sprayed Al 2 O 3 .YAG coating was chiefly composed of amorphous phase, which may reveal superplastic feature in supercooled liquid region (from 503.0°C to 906.5°C). The as-sprayed Al 2 O 3 .YAG coating exhibits fine plasticity and toughness. Friction and wear tests of the coatings were executed on a MMU-5GL tribological tester using a ring-on-disk arrangement. The coatings were deposited on end flat surfaces of the wear rings. The graphite disks were prepared. The wear tests were conducted at following conditions: relatively high load of 2000N; a rotational speed of 500rpm (equivalent to a sliding velocity of 0.68m/s). Friction coefficients could be obtained real-timely. The thermocouple was applied to measure the worn surface temperatures. The Al 2 O 3 /YAG amorphous coating/graphite pair possessed lower friction coefficient and worn surface temperature, compared with Al 2 O 3 coating/graphite and Al 2 O 3 -Cr 2 O 3 coating/graphite pairs. After wear tests, many network cracks visible to naked eye appeared the surfaces of Al 2 O 3 and Al 2 O 3 -Cr 2 O 3 coatings. However, no cracks were observed on the worn surface of the Al 2 O 3 /YAG amorphous coating. Therefore, plasma-sprayed Al 2 O 3 .YAG coating possesses excellent wear resistance under severe conditions with high pv values (p: contact pressure; v: friction velocity).

This paper summarizes the results of a decade-long study on nanoparticle reconstitution and its role in thermal spraying. The effect of the reconstitution process on coating nanostructure was investigated for different materials and applications, a number of which are covered in this report, including Al 2 O 3 -TiO 2 , SiC-Al 2 O 3 -ZrO 2 , and zirconia-based TBCs.

Failure is an inevitable consequence with thermal barrier coatings and failure modes are complicated due to irregular microstructure in the coating layers and wide range of external conditions. In this study, three-point bend tests are used to monitor damage evolution in YSZ-CoNiCrAlY TBCs on superalloy and stainless steel substrates. Coating samples, consisting of the bond coat and topcoat, were deposited by atmospheric plasma spraying on test specimens measuring 80 x 6 x 4 mm. The long, narrow specimens were subjected to three-point bend testing, using acoustic emission sensors to detect the formation and propagation of cracks in the coatings and plastic deformation in the substrates. The investigation results indicate that variations in acoustic emission signals correspond well with changes observed in the stress-strain curves of the coatings and substrates and that failure mechanisms can be systematically analyzed based on the amplitude, frequency, and energy of the acoustic emission signals. A detailed description of the actual failure process is provided.

In this work, finite element modeling is used to investigate the influence of segmentation cracks on stress distribution and failure in thermal barrier coatings deposited by atmospheric plasma spraying. The results indicate that the presence of segmentation cracks does not improve thermal insulation, but it may be beneficial in regard to thermal shock resistance, depending on crack density, and residual stress around crack tips, depending on crack length. It may also improve strain tolerance, which is affected by crack density as well as length. A model is proposed to explain the mechanism of failure in thick TBCs exposed to thermal shock. Damage caused by thermal shock can be attributed to the propagation of segmentation cracks and the formation of horizontal cracks at the bond coat-topcoat interface.

In this work, gas-atomized FeCr powders were deposited on aluminum substrates by HVOF spraying, forming dispersion strengthened coatings with a dense layered structure and low porosity. SEM, TEM, and XRD analyses show that the coatings primarily consist of amorphous matrix (40%) with precipitated nanocrystals and hard boride phases. A number of coating properties, including microhardness, bonding strength, and thermal conductivity, were measured and are correlated with spraying conditions.

Lanthanum gallate doped with strontium and magnesium (LSGM) is a good electrolyte candidate for Intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this study, low-temperature sintering is used to increase the density of LSGM coatings prepared by vacuum cold spraying (VCS). LSGM layers with different thickness were deposited by VCS on NiO-YSZ substrates. In order to suppress chemical reactions between Ni and LSGM, the substrates were coated with gadolinium-doped ceria by tape casting. After sintering at 1200 °C, the coatings were found to be denser in most regions due to grain growth, which appears to be accompanied by cracking, particularly in thicker layers. A second layer was deposited on the annealed coatings to seal the cracks and the two-layer structure was further sintered. Gas permeability test results show that the multilayer films are dense enough to consider their use as electrolyte membranes in IT-SOFCs.

Yttria stabilized zirconia coatings were deposited by plasma spraying and heat treated in air at 1100 °C for 50-200 h. Residual stresses in the ceramic topcoat and the thermally grown oxide (TGO) layer were measured before and after thermal exposure. After 50 h of exposure, tensile stress in the as-sprayed topcoat changed to compressive, which then increased with additional exposure time up to 150 h. The average compressive stresses in the cross-section of the TGO layer are shown to be higher than those on the surface of the oxide. In addition to shedding light on the nature and evolution of stresses in plasma-sprayed thermal barrier coating (TBC) systems, the results of the study also provide insights on crack initiation and propagation in the ceramic topcoat and at the topcoat-TGO-bond coat interface and its role in TBC failures.

This paper describes a new composite coating technology developed for hot dip galvanizing rolls used in automotive manufacturing. The multilayer coating consists of HVOF sprayed WC and electroless Ni-P plating. Coating surfaces and cross-sections are examined and adhesion strength, friction, and wear resistance are measured. In addition, changes in surface roughness are recorded on galvanizing rolls after three months of service.

In this work, finite element analysis is used to investigate the effect of pore size on the stress intensity factor (SIF) of plasma sprayed coatings. Test samples with different pore sizes were obtained by spraying wollastonite powders with particle sizes of 60-75 μm, 75-95 μm, and 95-150 μm onto Ti 6 Al 4 V coupons. The results show that coating stress varies in proportion to the length of 2D pores and to a lesser extent the diameter of 3D pores. This implies that reducing the length of 2D pores may be considered as a way to increase the fracture resistance of plasma sprayed coatings.

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.

A Fe48Cr15Mo14C15B6Y2 alloy with high glass forming ability (GFA) was selected to prepare amorphous metallic coatings by atmospheric plasma spraying (APS) process. The as-deposited coatings present a dense layered structure and low porosity. Microstructural studies show that some nanocrystals and a fraction of yttrium oxides formed during spraying process, which induced the amorphous fraction of the coatings decreasing to 69% compared with fully amorphous alloy ribbons of the same component. High thermal stability employs the amorphous coatings to work below 910K temperature without crystallization. Corrosion behavior of the amorphous coating was investigated by electrochemical measurement. The results show that the coatings exhibit extremely wide passive region and low passive current density in 3.5% NaCl and 1mol/L HCl solutions, which illustrate their superior ability to resist localized corrosion. Moreover, the corrosion behavior of the amorphous coatings in 1mol/L H 2 SO 4 solution is similar to their performance in chlorine ions contained conditions, which manifests their flexible and extensive ability to withstand aggressive environments.

The influence of spray parameters on particle with the improved plasma gun was studied by using SpryWatch-2i System. The results show that the particle velocity increases and exceeds the velocity of sound when using a new designed nozzle (C2 nozzle) during the plasma spraying. Under a certain condition, the particle velocity varies as the spray distance rise firstly, decline then, as the primary gas pressure increases, the particle velocity increases, the particle temperature drops firstly then increase, the velocity increase a little. But as the steadily increase of the flow rate, the influence on the particle temperature drops, the influence of arc voltage on particle velocity and temperature is slightly. Arc current influent little on particle temperature, but the particle velocity increase in a range.

The plasma torch is one of the most important equipment during air plasma spraying, is used to produce the plasma arc in the nozzle for the acceleration and heating of particles. The voltage-current characteristic of the plasma arc is very important to the fusion of particles. The volt-ampere characteristic is affected by many factors, such as gas species, gas flow rate, nozzle geometry and the type of gas injection. So the factors influencing the voltage-current characteristics in high velocity plasma spraying were studied. As the results showed, the arc voltage is increased with the increase of the primary gas flow rate in some range. The secondary gas flow rate has an important effect on the electric arc voltage. The influence of nitrogen on the arc voltage is more than argon when they are chose as the primary gas in plasma spraying. The arc voltage is decreased with the increase of the nozzle’s compression angle and the decrease of the nozzle’s aperture, when the length of the nozzle is unvarying.

Nickel clad hexagonal boron nitride (h-BN) powders were prepared by reducing nickel ions from a solution under hydrogen pressure in the presence of ammonia as a complexing agent, and plasma spraying was carried out to deposit the corresponding coating. The microstructure, morphology and phase composition of the powders and the coating were characterized by optical microscope (OM), Scanning Electronic Microscope (SEM) and X-ray Diffraction (XRD), respectively. The results show that alkali solution pretreatment and activation procession are necessary for acquiring a dense and uniform nickel coating on the surface of the h-BN particles, and the h-BN particles are distributed well throughout the coating with the porosity of about 26%, which indicate that the coating was potential for abradable sealing application.

The jet characteristics of supersonic plasma jet spraying (SPJS) system were studied. Comparison of jet property was made between conventional plasma spray system and SPJS system. The results showed that the characteristics of the SPJS system are superior to the latter system. The particles velocities in the range of 400~800 m/s are obtained for supersonic plasma jet guns with 4.5 mm diameter nozzles and 60 l /min total gas flow rates, SPJS operate about 36 KW. The particle velocity is more than two times as much as conventional plasma jet spraying. The supersonic plasma jet is much longer than that of the conventional torch. It can bring about improvement for flying powder heated and accelerated and also prolong spray distance.

The adhesive strength between substrate and sprayed coatings seems to be mainly due to the mechanical interlocking effect. The physical bonding of spray particles to the substrate is the second factor. In this paper, the bonding mechanisms of sprayed coatings were studied on basis of experiments with the substrate at different surface roughness. The substrate surface roughness was quantitatively evaluated using a surface roughness tester. The tensile adhesion strength test method was used to evaluate the adhesive properties. The relationship between surface roughness and adhesive strength was investigated. The adhesive strength of plasma sprayed coating decreased with an increase in substrate surface roughness, as the size of the grit particles for roughening increased. On the contrary, the adhesive strength of arc sprayed coating increased.

Several effective numerical techniques, based on finite element analysis, have been developed and computed independently in this paper. Results are presented describing the impacting process, and the subsequent temperature and residual stress fields of a molten nickel particle impacting onto a flat substrate. Problems of this type, especially the prediction of the thermal residual stresses, are of major practical interest in thermal spray operations as a pioneering approach.