Thermal spray offers a variety of sub-sets of processing approaches to produce coatings. The various processes are classified based on the thermal spray source (from low velocity combustion spray to high temperature plasma jets) and method of material injection (in the form of powder, wire or rod). However, it is this intrinsic versatility which sets-up variations in characteristics of the applied coatings. Properties of thermally sprayed coatings, including process induced residual stress, are controlled by various parameters of the spraying process. This study examines three thermal spraying techniques with significantly different particle temperatures and velocities. They are air plasma spraying (APS), twin wire-arc spraying (TWA) and high velocity oxy-fuel (HVOF) spraying. For comparison purposes the recently developed cold spray processed materials were included in the study. For each method, in-flight particle diagnostics was performed; Ni–5 wt.%Al splats and deposits were fabricated and analyzed. Porosity, elastic modulus and thermal conductivity of the deposits were evaluated and correlated to the process variables. Using indentation at different loads and analysis of the indented region, stress–strain relationships for these coatings were obtained. Surprising differences in the properties were observed and were explained based on the fundamental variations in microstructure development. Through-thickness residual stress profiles in Ni–5 wt.%Al coatings on steel substrates were determined non-destructively by neutron diffraction. The stresses range from highly tensile in the APS coating to compressive in the HVOF coating. Various stress generation mechanisms—splat quenching, peening and thermal mismatch—are discussed with respect to process parameters and material properties.

Role of thermal spray processing method on the microstructure, residual stress and properties of coatings: an integrated study for Ni–5 wt.%Al bond coats

For plasma sprayed thermal barrier coatings (TBCs), control of thermal conductivity is critical since low thermal conductivity depends not only on the intrinsic property of the yttria-stabilized zirconia (YSZ) TBC, but also on the morphology of pores and cracks introduced during spray process. They are closely linked to process methodology as well as to chemistry, structure and morphology of the ceramic feed materials. This paper addresses the influence of feedstock characteristics on particle state in the plasma and the resultant coating properties. In addition, substrate temperature, angle-of-impact and thermal cycling effects on porosity (quantity and morphology) and its resultant influence on thermal conductivity and elastic modulus of plasma sprayed YSZ TBCs. The results show increased porosity with particle size, due to an increase in the degree of particle fragmentation and unmelted particles, leading to lower thermal conductivity and modulus. Furthermore, higher substrate temperatures and low particle velocity lead to lower porosity and improved inter-splat contact and, thus, enhanced coating properties. Sintering during thermal cycling reduces porosity and increases thermal conductivity and modulus.

Processing effects on porosity-property correlations in plasma sprayed yttria-stabilized zirconia coatings

Splashing occurs when a liquid drop hits a dry solid surface at high velocity. We report experimental studies of how the splash depends on the roughness and the texture of the surfaces as well as the viscosity of the liquid. For smooth surfaces, there is a "corona" splash caused by the presence of air surrounding the drop. There are several regimes that occur as the velocity and liquid viscosity are varied. There is also a "prompt" splash that depends on the roughness and texture of the surfaces. A measurement of the size distribution of the ejected droplets is sensitive to the surface roughness. For a textured surface in which pillars are arranged in a square lattice, experiment shows that the splashing has a fourfold symmetry. The splash occurs predominantly along the diagonal directions. In this geometry, two factors affect splashing the most: the pillar height and spacing between pillars.

https://arxiv.org/pdf/physics/0702080.pdf

Liquid drop splashing on smooth, rough, and textured surfaces.

The spraying distance, substrate temperature, coating thickness and surface roughness of substrate during deposition play an important role on the plasma spray coating process and effect the final properties of the coatings. Al 2 O 3 coatings on AISI 304 L stainless steel substrate were prepared to investigate the effects on the coating of these parameters. The results indicated that the parameters such as the spraying distance, substrate temperature, coating thickness and substrate roughness were fairly effected the hardness, porosity and surface roughness of Al 2 O 3 coatings. The lowest surface roughness and the lowest porosity and the highest hardness values of Al 2 O 3 coating were obtained for the spraying distance of 12 cm and the surface roughness of 3.28 μm and the substrate temperature of 500 °C. It also found that the increases of coating thickness were lowered the hardness and enhanced the porosity and the coating roughness.

Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process

In situ measurement of residual stresses and elastic moduli in thermal sprayed coatings: Part 1: apparatus and analysis

In recent years it has been observed that the substrate surface temperature during thermal spray deposition has a profound effect on the morphology of the impacted droplet (splat) and consequently on the microstructure and properties of the deposits. In this set of two papers (one for metal and one for ceramic), the substrate temperature effects have been studied in an integrated manner relating the initial splat formation to microstructure development and eventually to the properties of the deposit. Isolated impacted splats have been obtained on polished steel substrates at two different temperatures (high and low) and these have been analyzed quantitatively for their shape factors and thicknesses. The deposits have been formed nominally at these two different temperatures and their microstructures and properties have been analyzed. The results confirm that there exists a threshold transition temperature for the substrate surface beyond which the splat morphology changes from a fragmented (splashed) to a more contiguous (disk-shaped) morphology. In the case of zirconia this temperature appears to be in the range of about 250‐300°C, which is roughly 10% of the melting temperature of zirconia. It has been further observed that the splat‐substrate and inter-splat contact is significantly improved at higher temperatures, leading to reduced porosity, increased thermal conductivity and strength. These results are assimilated to develop an integrated structure‐property relationship and preliminary arguments are presented as to the reason for such transitions. © 1999 Elsevier Science S.A. All rights reserved.

Substrate temperature effects on splat formation, microstructure development and properties of plasma sprayed coatings Part I: Case study for partially stabilized zirconia☆

It has been observed that substrate temperature has a dramatic effect in modifying the morphology of splats formed by impacting molten droplets on substrates during plasma spraying. This effect also has important implications on the deposit microstructure and properties. With an increase of substrate temperature from room temperature to a few hundred degrees (200–400°C), the splashing tendency is reduced and the splat morphology changes from fragmented to a contiguous disc-shape. Consequently, deposit microstructural integrity improves, and a number of properties are significantly enhanced with concomitant implications for coating performance. The mechanism of this splat morphology change has been subject of considerable investigation within the thermal spray community. It has been suggested by various researchers that the cause of this effect can be attributed to a number of factors such as: a higher solidification rate at the low substrate temperature; poor wetting and/or a contaminated substrate surface at lower temperatures etc. This paper summarizes, the experimental findings and reviews the various proposed hypotheses. The results are synthesized based on some recent experimental findings.

Splat formation and microstructure development during plasma spraying: deposition temperature effects

A schematic representation referred to as “process maps” examines the role of process variables on the properties of plasma-sprayed coatings. Process maps have been developed for air plasma spraying of molybdenum. Experimental work was done to investigate the importance of such spray parameters as gun current, primary gas flow, auxiliary gas flow, and powder carrier gas flow. In-flight particle temperatures and velocities were measured and diameters estimated in various areas of the spray plume. Empirical models were developed relating the input parameters to the in-flight particle characteristics. Molybdenum splats and coatings were produced at three distinct process conditions identified from the first-order process map experiments. In addition, substrate surface temperature during deposition was treated as a variable. Within the tested range, modulus, hardness and thermal conductivity increases with particle velocity, while oxygen content and porosity decreases. Increasing substrate deposition temperature resulted in dramatic improvement in coating thermal conductivity and modulus, while simultaneously increasing coating oxide content. Indentation reveals improved fracture resistance for the coatings prepared at higher substrate temperature. Residual stress was significantly affected by substrate temperature, although not to a great extent by particle conditions within the investigated parameter range. Coatings prepared at high substrate temperature with high-energy particles suffered considerably less damage in a wear test. The mechanisms behind these changes are discussed within the context relational maps, which have been proposed.

X.Y Jiang

Papers

Role of thermal spray processing method on the microstructure, residual stress and properties of coatings: an integrated study for Ni–5 wt.%Al bond coats

Substrate temperature effects on splat formation, microstructure development and properties of plasma sprayed coatings Part I: Case study for partially stabilized zirconia☆

Splat formation and microstructure development during plasma spraying: deposition temperature effects

Development of process maps for plasma spray: case study for molybdenum