Alain Grimaud

Influence of substrate roughness and temperature on the adhesion/cohesion of alumina coatings

Abstract This article is devoted to the study of the adhesion/cohesion (A/C) of alumina coatings sprayed on four different substrates: aluminium alloy, titanium alloy, cast iron and mild steel. Fused and crushed alumina particles (−45 + 22 μm) were sprayed with a custom-made torch with a nozzle of internal diameter 7 mm working with an arc current of 600 A and a plasma gas mixture of 45 slm Ar, 15 slm H 2 , which resulted in a voltage of 64 V and a thermal efficiency of 52%. The particles were injected internally and were almost fully molten (less than 3% α-phase in the collected powders after their passage in the plasma jet). The influence of substrate roughness and temperature before (preheating), during and after spraying was systematically studied. R a values were 6–8, 10–13 and 14–21 μm and the substrates were preheated to temperatures T p of 170, 200, 300, 350, 380 and 500 °C. The most important results were the following: 1. • for cold substrates A/C increased with R a up to a maximum of 20 MPa 2. • for substrates whose expansion mismatch with alumina was less than 4.10 −6 K, the highest A/C values (50–60 MPa) were obtained with T p between 350 and 500 °C provided the substrates were not oxidized 3. • at these temperatures the highest values of A/C were obtained with the lowest R a 4. • when particle velocity and surface temperature upon impact decreased, for example by load effect, A/C values decreased too

Comparison of plasma-sprayed alumina coatings by RF and DC plasma spraying

Splat size and shape- factor distributions for plasma- sprayed alumina particles on various substrates were studied using a setup derived from the line- scan test. Direct- current (dc) and radiofrequency (rf) plasma torches were used to study the influence of particle velocity at impact. The influence of substrate temperature prior to spraying also was studied. Splats collected on smooth substrates kept below 100 °C were extensively fingered and had poor substrate contact. When the substrates were heated to 300 °C before spraying, the splats became disk- shaped and their substrate contact was very good. Similar results were obtained for rough substrates. Coating adhesion decreased with particle velocity and was lower for the dc plasma torch when using larger particles, which did not melt as well as smaller ones. Melting and adhesion were much improved with the rf torch.

Deposition of nanoparticle suspensions by aerosol flame spraying: Model of the spray and impact processes

Aerosol flame spraying (AFS) combines the atomization of a colloidal suspension with the lateral injection of the aerosol in a flame. The aerosol droplets are partially dried when crossing the flame and then deposited as a coating onto a substrate. Afterwards, the coating is consolidated by heat treatment without extensive grain growth. In this paper a model of the trajectories, acceleration and vaporization of the droplets is used to predict the impact conditions of the in-flight dried droplets, as well as their size and water content when they impinge onto the substrate. From these calculations and the hydrodynamic properties (viscosity, surface tension, contact angle) of the suspensions, the morphology and size of the lamellae deposited on the substrate are determined by using classic impact models. In spite of the complexity of the mixing of the suspension spray with the flame and the diversity of the thermal histories of the droplets, the observation of the latter after impact shows that the results of the model are quite consistent with measurements. The relationship between droplet impact parameters and coating formation is discussed.

Solar furnace surface treatment of plasma-sprayed thermal barrier coatings

An original process of superficial thermal treatment is described. This process has been successfully applied to partially stabilized yttria-zirconia coatings (ZrO2-Y2O3) to increase the lifetime of the thermal barriers. A thin superficial layer was melted, and morphological transformations were observed. A microstructural comparison between as-sprayed layers and thermally treated layers is presented.

Cast iron substrates reclamation by tape casting of NiCu treated by plasma transferred arc: optimization of the tape and its plasma treatment

Abstract Plasma transferred arc (PTA) is now currently used for reclamation of worn materials or to provide wear or corrosion resistant coatings welded to the base material. Instead of injecting the powder in the plasma and the molten pool created at the coated surface, a tape casting process was used to obtain an adherent powder layer of NiCu (70:30) on the material surface. Tapes deposited on cast iron substrates exhibited a high electrical resistance (a few GΩ). Thus, their PTA treatment has been carried out by starting the electrical arc on the metallic cast iron substrates. The process control by charge coupled device (CCD) camera showed that the NiCu particles fell in the melting pool created at the substrate surface. The study of the obtained alloy compositions has shown the drastic influence of the initial binder concentration in the tape. Some NiCu tapes were sintered on their substrate at 1100°C for 4 h under an inert atmosphere. The resulting coatings, characterized by a low electrical resistance (a few Ω), and a good adhesion to the substrate, were then treated by PTA. The surfacing alloy properties were compared to those obtained without heat treatment.

Low friction stainless steel coatings graphite doped elaborated by air plasma sprayed

A new process has been developed to incorporate graphite particles into a stainless steel coating during its formation. Four means have been tested to inject the graphite particles outside the plasma jet and its plume: graphite suspension, a graphite rod rubbed on the rotating sample, powder injection close to the substrate with an injector, or a specially designed guide. The last process has been shown to be the most versatile and the most easily controllable. It allows the incorporation of between 2 and 12 vol.% of graphite particles (2–15 µm) within the plasma sprayed stainless steel coatings. A volume fraction of 2% seems to give the best results with a slight decrease (6%) of the coating hardness. This volume fraction also gave the best results in dry friction on the pin-on-disk apparatus. Depending on the sliding velocity (0.1–0.5 m/s) and loads (3.7–28 N), the dry friction coefficient against a 100C6 pin is reduced by between 1.5 and 4 compared with that obtained with plasma sprayed stainless steel.

Study of A Nitrogen D.C. Plasma Torch at Different Pressures Using Optical and Thermal Diagnostics

This paper is devoted to the characterization of electrical arid thermal parameters of a d.c. plasma torch and the temperature measurements of the plasma jet at a pressure of 32.8 kPa, the working gas being nitrogen.