Elsevier

Thin Solid Films

Volume 514, Issues 1–2, 30 August 2006, Pages 212-222
Thin Solid Films

Impact of plasma-sprayed metal particles on hot and cold glass surfaces

https://doi.org/10.1016/j.tsf.2006.03.010Get rights and content

Abstract

Plasma-sprayed molten molybdenum and amorphous steel particles (38–55 μm diameter) were photographed during impact (velocity 120–200 m/s) and spreading on a smooth glass surface that was maintained at either room temperature or 400 °C. Droplets approaching the surface were identified by a photodetector and after a known delay, a 5-ns laser pulse was triggered to illuminate the spreading splat and photograph it with a charge-coupled device (CCD) camera. A rapid two-color pyrometer was used to collect thermal radiation from particles during flight and impact to follow the evolution of their temperature and size. Particles that impacted the surface at room temperature ruptured and splashed, leaving a small central solidified core on the substrate. On a surface held at 400 °C, there was no splashing and a circular, disk-like splat remained on the surface. Splats on a glass surface held at room temperature had a maximum spread diameter almost three times that on a hot surface. A simple analysis was done to estimate the area of the splat in contact with the non-heated glass surface during spreading. The analysis supports the hypothesis that only a portion of the splat is in good contact with the surface at room temperature, while the rest of the fluid is separated from the substrate by a gas barrier.

Introduction

Fundamental studies of plasma-spray coating processes have found that the temperature of the substrate on which molten droplets impact influences their morphology, size, and extent of splashing [1], [2], [3], [4], [5], [6]. Splat morphology affects coating properties such as porosity, adhesion strength, and microstructure [2], [7]. Aziz and Chandra [8] showed that, for low velocity impacts of tin (1–4 m/s) on mirror-polished stainless steel held at room temperature, as the droplet impact velocity increased, the maximum diameter of the splat increased, and was accompanied by significant splashing. Several investigators [2], [6] have found that, for plasma-sprayed particles, increasing substrate temperature reduced the occurrence of splashing and produced disk-like splats. Jiang, et al. [9] have shown also that removal of condensates and/or adsorbates from a cold stainless steel substrate will eliminate splashing and splat fragmentation of impacting molten zirconia and produce contiguous, disk-like splats.

Many images of the impact and spreading of droplets on flat surfaces have been captured for low velocity impacts [8]. However, it is difficult to capture clear images of the spreading particles in the actual plasma-spray process. Mehdizadeh et al. [5] have photographed plasma-sprayed molybdenum droplets impacting cold glass by using a charge-coupled device (CCD) camera and long-range microscope. A high-speed two-color pyrometer was also used to obtain the temperature evolution during spreading. The two-color pyrometric method, as described by Fantassi et al. [10] and Gougeon et al. [11], calculates the splat temperature from the ratio of the intensities of radiation collected at two different wavelengths. A similar method was employed by Cedelle et al. [12] to photograph the different splashing phenomena of millimeter-sized, plasma sprayed yttria-stabilized zirconia on stainless steel substrates. A CCD camera was oriented either parallel to the cold substrate to photograph the impact splashing or orthogonal, to photograph the splashing during flattening. Tiny droplets were ejected immediately after particle impact during impact splashing, while during flattening splashing, the splat disintegrated during spreading on the substrate. It was found that splashing seemed to occur immediately after impact and continued during flattening.

Fukumoto et al. [2] found that the microstructure of nickel and copper splats on heated AISI 304 steel substrates was fine, columnar, flat, and non-porous, while on the cold steel, it was composed of isotropic coarse grains, indicating that the cooling rate of the splats on the hot substrate was larger than that on the cold substrate. However, actual temperatures during the cooling of the splat were not measured. Moreau et al. [4] measured the temperature evolution of molybdenum droplets that impacted and spread on cold and hot glass. It was found that the cooling rate of the splats on hot glass was on the order of 108 K/s, an order of magnitude larger than the splats on cold glass (107 K/s). Bianchi et al. [13] used a 1-D splat cooling model to show that the thermal contact resistance at the interface of yttria-stabilized zirconia splats and polished stainless steel substrates affects the splat cooling rate significantly. It was found that as the thermal contact resistance increased, the cooling rate decreased.

Photographing droplets in a plasma spray at different stages during impact gives insight into the dynamics of splat formation on both hot and cold substrates. Fukumoto et al. [2], [3] have speculated that during impact on a non-heated surface, only the central portion of the splat is in good contact with the surface, while the rest of the fluid jets out over a gas layer and splashes. However, no direct experimental evidence is available to test this hypothesis.

The objectives of this study were to: (1) use a rapid CCD camera to photograph molybdenum and amorphous steel particles that impacted glass held at room temperature and at 400 °C; (2) use high speed two-color pyrometry to measure the temperature and size evolutions of molybdenum and amorphous steel particles after collision and during spreading; and (3) estimate the area of the splat in contact with the non-heated glass surface during spreading.

Section snippets

Experimental details

A schematic diagram of the experimental setup is shown in Fig. 1. A SG100 torch (Praxair Surface Technologies, Indianapolis, IN) was used to melt and accelerate dense, spherical molybdenum (SD152, Osram Sylvania Chemical and Metallurgical Products, Towanda, PA) and amorphous steel (44% Cr, 48% Fe, 6% B, 2% Si, traces of C and S) (LMC-M, Liquidmetal Technologies, USA) powder particles, sieved to − 60 + 38 μm, with an average diameter of 40 μm. The powder feed rate was less than 1 g/min. It has been

Thermal emission signals and images of spreading splats

Fig. 3 shows images of molybdenum splats at different times after impact on glass held at room temperature or at 400 °C. The figure also shows typical D1 thermal emission signals. D2 thermal emission signals have the same shape and are not shown. For molybdenum, the average droplet impact velocity was 135 ± 2 m/s and the average temperature of the in-flight particles were 2975 ± 10 °C, well above the melting point (2617 °C). 37 samples were available for analysis. The statistical errors, calculated by

Conclusion

The influence of substrate temperature on the area of contact of plasma-sprayed particles was studied. The particles that impacted on a glass surface at room temperature fragmented and splashed, leaving only a small centralized core adhering to the substrate. On a surface held at 400 °C, there was no splashing and a circular splat remained on the surface. The increased splat fragmentation and small contact area on the non-heated surface was attributed to the presence of adsorbates/condensates

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