Experimental investigation and parameter optimization of Cr2O3 atmospheric plasma spray nanocoatings

In this research, Cr2O3 ceramic nano-sized powder particles were prepared using ball milling and then were granulated to reach the proper size for spraying. Afterward, Cr2O3 nano-coatings were deposited by atmospheric plasma spraying (APS) process onto stainless steel substrates. To optimize APS parameters, spraying was carried out under six conditions with different parameters. Microstructures of the elemental/milled powder and coatings were characterized via a field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectroscopy (EDS). In this research, Cr2O3 coatings were deposited under different spraying conditions to understand the effect of APS parameters on the splat formation, deposition efficiency, and porosities of the coatings. After parameter optimization, spraying was performed under a high deposition efficiency of 46.0±1.3%. The optimized Cr2O3 coatings showed porosity content, Knoop microhardness, and adhesive strengths of 8.7±2.2%, 823±27 HK0.2, and 49±4 MPa, respectively; making them a good candidate for industrial use.

Embedding Behavior of Ceramic Particles in Babbitt Coatings and Its Effect on the Tribological Properties of Low-Pressure Cold Sprayed Coatings

The low melting point of Sn-based Babbitt alloys often causes nozzle clogging in the low-pressure cold gas-dynamic spraying (LPCGDS) process, which impacts the process steadiness and the coating quality. Adding hard particles to the feedstock material eliminates this kind of interruption. A certain amount of these particles finds their way in the obtained coatings. These particles also trigger a kind of “hammering effect” due to their impulse forces. These forces are directly dependent on the mass and velocity of the impacting hard particles. However, these forces may lead to a decrease in the porosity and improve the adhesion of the obtained coating. In this study, the effect of the density and size of the hard particle was examined by three different hard materials, Cr3C2, Al2O3, and B4C, which have a material density of 6.68, 3.95, and 2.52 g/cm3, respectively. The used feedstock in this study is a powder mixture that contains 75 vol.% Babbitt and 25 vol.% of either B4C, Cr3C2, or Al2O3. The effect of the size distributions “particles with lower mass” was tested using two different Al2O3. The various hard particles show different embedding behaviors, as well as different effects on the coating build-up. It was found that the blended hard particles were enclosed with the Babbitt matrix, and their interface with Babbitt shows no clear evidence of pronounced diffusion. The size distribution of the blended hard particles has a direct effect on the splat formation and the obtained coating microstructure. It was found that the type of hard particles played a decisive role in the friction behavior. Nevertheless, the hard particle reinforced Sn-Sb-Cu-based composite coatings demonstrated a nearly constant coefficient of friction throughout the load-interval.

Protective Plasma Sprayed Coating forThermo-Sensitive Substrates

Plasma spray is one of the surface treatment techniques that consist on the deposition of a thin coating onto a targeted substrate. Coating is built up by successive accumulation of layered splats resulting from impact and solidification of molten particles into thin ‘‘splats’’ onto the substrate. The process of droplet impact, spreading and solidification is then a crucial process in coating formation. This technique may be also used for thermo-sensitive materials such as wood by applying a metallic coating for protective or decorative purposes. However, when applying a ceramic coating which provides a high protection against hot temperatures like fire, wood may be damaged because of the high temperature at which the ceramic molten particles arrive at the substrate. In this paper, a numerical simulation based on the Finite Elements Method is carried out in order to simulate the process of the first splat formation onto a wood substrate under traditional plasma spraying conditions. The computations are carried out on a fixed eulerian structured mesh using the level set method to track the interface between the molten particle and surrounding gas. The effects of operating conditions as well as the droplet characteristics that allow applying ceramic coating onto a wood substrate without any damage to this thermo-sensitive material are investigated.

An Investigation on Splat and Flattening Behavior of Thermally Sprayed Copper on A Rough Surface: A New Approach

All thermal spray coatings are finally deposited on a rough and active grit-blasted surface of the job. But, available literatures are reporting splat and flattening behavior on a polished surface. There is a gap in thermal spraying to understand actual solidification on a rough surface. Therefore, in the present work an attempt has been made to study of splat formation of thermally sprayed copper onto grit-blasted rough surface. An optimization study is done to collect rounded/semi-rounded disk like splats to set spray parameters. Optimized parameters were also tested at four different gun traverse speeds to fabricate thin Cu coatings (30-50µm).

Heat and Hydrothermal Treatment on the Microstructure Evolution of Plasma Sprayed Hydroxyapatite Coatings Reinforced with Graphene Nanoplatelets

Recent advances and demands in clinical applications drive a large amount of research to hydroxyapatite (HA) composite coatings fabricated by plasma spray. However, lower degree of HA crystallinity related to high temperature exposure in plasma spray usually leads to rapid weakening and disintegration of HA coatings and often promotes inflammatory responses in the surrounding tissue. In this research, graphene nanosheet (GNS) reinforced HA coatings were fabricated using plasma spray and followed by heat and hydrothermal treatment (hereafter referred to as thermal treatment). The addition of GNSs resulted in competing phenomenon to influence HA crystallinity viz. increased portion of the partially melted/unmelted zones and higher cooling rate during splat formation, leading to slight increase in HA crystallinity (~46.0-51.3%) in the as-sprayed coating. XRD and FTIR results showed that thermal treatment was capable of inducing significant transformation of amorphous HA to the crystalline form and removing other foreign non-HA compounds through regaining OH- ion, and therefore HA coatings displayed ~45.5-47.1% improvements in HA crystallinity regardless of addition or not of the GNS nanofillers. Microstructure observations revealed that thermal treatment enabled microcrack propagation due to stresses caused by crystallisation and phase transformations, and the residual partially melted/unmelted zone of the thermally treated GNS/HA coating was significantly decreased in size. More importantly, the added GNSs contributed greatly to the significant increase in surface nanoroughness of the thermally treated HA coatings owing to the fact that much more structural defects along with the GNSs mainly induced by thermal treatment might act as nucleation sites to accelerate HA nanoparticle precipitation, which would be beneficial for the improved adhesion strength of the osteoblast cells on the coating surface.

Thermal Spray Methods and Splat Formation

This chapter describes how surface coatings play a significant role in manufacturing industries due to improvements in the properties of the engineering components that lead to longer service life and reliability. Thermal spray is well suited for large structures and can be easily repaired. Previous publications concerning the principle of thermal spray processing; powder and their fabrication methods; the effect of substrate surface conditions on splat formation and morphologies; the effect of process parameters on splat formation and morphology; and thermal spray coating morphology are reviewed. Also described in detail is physical behaviour, such as thermal conductivity and wetting ability at impact between the thermal spray particle and the substrate, influences the individual splat geometry and coating build-up and, consequently, the coating properties.