Splat Formation Research Articles

Parameters controlling the generation and properties of plasma sprayed zirconia coatings

D.C. plasma jets temperature and velocity distributions as well as the arc root fluctuations at the anode were studied for Ar-H2 (25 vol%) plasma forming gases. The parameters were the arc current up to 700 A, the total gas flow rate up to 100 slm, and the nozzle diameter which was varied from 6 to 10 mm. The trajectories of partially stabilized zirconia particles into the jet were studied by a 2D laser imaging technique and two fast (100 ns) two color pyrometers. The results have revealed the difficulty to inject small particles into the plasma flow since most were found to by-pass the jet rather than penetrate it. The results also show the broad trajectory distribution within the jet and the influence of the arc root fluctuations on the mean particle trajectory distribution within the jet. Beside the measurements of the particle surface temperature and velocity distributions in flight, the particle flattening and the cooling of the resulting splats were studied statistically for single particles all over the spray cone. Such studies have emphasized the drastic influence of the substrates or previously deposited layers temperature on the contact between them and the splats. At 200–300°C this contact is excellent (cooling rates of the order of 100 K/μs for 1 μm thick splats) and it results in a columnar growth within the splats and the layered splats of a bead (up to 500 layered splats). This growth can be observed through passes provided the bead surface temperature has not cooled too much (a few tens of K) before the next bead covers it. A/C values up to 60 MPa were achieved with PSZ coatings. The effect of impact velocity of the particles, of substrate preheating temperature, of relative movments torch to substrate, of substrate oxidation on A/C values and splat formation were also studied.

Influence of particle parameters at impact on splat formation and solidification in plasma spraying processes

A measurement system consisting of two high- speed two- color pyrometers was used to monitor the flattening degree and cooling rate of zirconia particles on a smooth steel substrate at 75 or 150 °C during plasma spray deposition. This instrument provided data on the deformation behavior and freezing of a particle when it impinged on the surface, in connection with its velocity, size, and molten state at impact. The results emphasized the influence of temperature and surface conditions on particle spreading and cooling. When the substrate temperature was 150 °C, the splats had a perfect lenticular shape, and the thermal interface resistance between the lamella and the substrate ranged from 10− 7 to 10− 8 W/m2 · K. The dependence of the flattening degree on the Reynolds number was investigated.

Effects of vacuum plasma spray processing parameters on splat morphology

Several statistical tools (i.e., Gaussian and Weibull distribution analyses, the t-test, and analysis of the variance) were used to examine relationships between vacuum plasma spray processing parameters and the morphology of flattened particles (splats) on a smooth, polished substrate. Astroloy, a nickel-base powder, was vacuum plasma sprayed onto polished copper substrates under various processing conditions. Different flattened particle shape factors, including equivalent diameter, elongation factor, and degree of splashing, were determined using image analysis. The spray parameters (i.e., current intensity, chamber pressure, argon mass flow rate, etc.) strongly influenced splat formation and morphology and thus deposit microstructure and properties.

WC-Co系溶射皮膜形成における粒子扁平化とその積層構造

Lamellar structure, splat formation of molten droplets, coating microstructure, transfiguration of carbide particles and deposition yield of the powders in high velocity oxygen-fuel flame spraying of WC-Co coatings using powders with different morphologies were examined. The ratio of the particle diameter to the thickness of splats determined in this experiment is 7 to 17. These values are in good agreement with the calculated values according to the splat formation model proposed by Jones et al.The formation of coating deposits is followed by the expanding flow of the molten binding phase accompanying solid WC particles. The smaller size of WC particles in the spray powder allows the fluidization and flattening together with the molten binding phase, on the contrary, the flattening behavior of particles are restrained by the size of WC particles in the case that the size of spray powder is large. Susceptivity to flattening of the spray powder tends to result in the compactness or the high density of the coating. Consequently, spray powders composed of the smaller size of WC particles allow the formation of the denser coatings. The selection of the spray powder composed of smaller-sized primary WC particles and the moderately larger size of cermet particles is effective for the higher deposition yield of WC-Co spraying.

Review of impact and solidification of molten thermal spray droplets

The unique properties of coatings created by thermal spray deposition depend on the rapid solidification of individual splats created by impinging molten droplets. However, the impact process has been little studied because of the difficulty of measuring or numerically simulating the process, which occurs very quickly over a small area. Other scientific fields have investigated the impact of liquid droplets on solid surfaces. This paper reviews these studies, along with those conducted specifically on the thermal spray process. Modelers have almost universally ignored droplet solidification during impact; however, some experimental evidence suggests that the solidification process plays a significant role in splat formation. Splashing of impacting liquid droplets, another topic that has been largely ignored, affects deposition efficiency, porosity, and bond strength, and may also affect the amount of oxides incorporated in the coating. The scaling of data from impacting millimeter-size droplets traveling at low velocities to thermal spray conditions is questioned.

Influence of the velocity of plasma-sprayed particles on splat formation

The behavior of individual plasma- sprayed particles as they impinge on a surface was monitored using two high- speed two- color pyrometers, one focused 2 mm before the substrate and the other focused on the substrate surface. The influence of the velocity of the impinging particles (determined from the time of flight between the two focused points of the pyrometers) on the deformation and cooling processes was investigated. Results on zirconia particles impacting on a smooth bare steel substrate are presented in terms of apparent flattening time, splat diameter, and cooling rate determined from the pyrometer signals.

An improved velocity field for the Madejski splat-quench solidification model

Permanent magnetic coatings show potential in micro scale applications such as micro motors and micro generators. Nd–Fe–B magnetic coatings of average thickness 50 μm were produced via the flame spray method where the behavior of Nd–Fe–B particles in the thermal spray process was studied through single splat formation. There are generally five types of Nd–Fe–B splat morphology; which correspond to different solidification routes and varying degree of splashing. Microstructures of coating cross sections exhibit features such as cracks and porosity. The microstructural features were related to the physical properties and the brittle nature of the rare-earth alloy feedstock. Cross sections of the coatings also exhibited the presence of two distinct phases: Nd-rich and Fe-rich regions, which have been validated by EDS analysis. Metastable phase formation and decomposition followed by non-equilibrium solidification of the molten droplet prior to impact have been suggested to cause phase separation and were also identified in the formation of single splats. Hardness tests further confirmed the two distinct phases as Nd-rich and Fe-rich areas.