Sanjay Sampath

Impact of high velocity cold spray particles

This article presents experimental data and a computational model of the cold spray solid particle impact process. Copper particles impacting onto a polished stainless steel substrate were examined in this study. The high velocity impact causes significant plastic deformation of both the particle and the substrate, but no melting was observed. The plastic deformation exposes clean surfaces that, under the high impact pressures, result in significant bond strengths between the particle and substrate. Experimental measurements of the splat and crater sizes compare well with the numerical calculations. It was shown that the crater depth is significant and increases with impact velocity. However, the splat diameter is much less sensitive to the impact velocity. It was also shown that the geometric lengths of the splat and crater scale linearly with the diameter of the impacting particle. The results presented will allow a better understanding of the bonding process during cold spray.

Comprehensive microstructural characterization and predictive property modeling of plasma-sprayed zirconia coatings

Quantitative microstructure characterization to better understand processing-microstructure-property correlations is of considerable interest in plasma sprayed coating research. This paper quantifies, by means of small-angle neutron scattering (SANS) data, microstructure (porosity, opening dimensions, orientation and morphologies) in plasma sprayed partially-stabilized zirconia (PSZ) coatings, primarily used as thermal barrier coatings. We report on the investigation of the influence of feedstock characteristics on microstructure and establish its influence on the resultant thermal and mechanical properties. The microstructural parameters determined by SANS studies are then assembled into a preliminary model to develop a predictive capability for estimating the properties of these coatings. Thermal conductivity and elastic modulus were predicted using finite element analysis and ultimately compared to experimental values.  2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Determination of properties of graded materials by inverse analysis and instrumented indentation

Abstract In this paper, a new measurement procedure based on inverse analysis and instrumented micro-indentation is introduced. The inverse analysis is utilized to extract information from indented load–displacement data beyond usual parameters such as elastic modulus. For the initial implementation of this procedure, determination of non-linear functionally graded materials (FGMs) parameters is considered. In FGMs, the compositional profile and the effective mechanical property through the thickness are essential in verifying the fabrication process and estimating residual stresses and failure strengths. However, due to spatial variation of their properties, it is often difficult or costly to make direct measurements of these parameters. In order to alleviate the difficulties associated with testing of FGMs, we propose an effective but simple procedure based on the inverse analysis which relies solely on instrumented micro-indentation records. More specifically, the Kalman filter technique, which was originally introduced for signal/digital filter processing, is used to estimate FGM through-thickness compositional variation and a rule-of-mixtures parameter that defines effective properties of FGMs. Essentially, the inverse analysis processes the indented displacement record at several load magnitudes and attempts to make best estimates of the unknown parameters. Our feasibility study shows promising results when combined data from two differently sized indenters are employed. The procedure proposed is also applicable in estimating other physical and mechanical properties of any coating/layered materials.

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

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.

Rapid Solidification and Microstructure Development during Plasma Spray Deposition

Plasma spray processing is a well-established method for forming protective coatings and free-standing shapes from a wide range of alloys and ceramics. The process is complex, involving rapid melting and high-velocity impact deposition of powder particles. Due to the rapid solidification nature of the process, deposit evolution also is complex, commonly leading to ultrafine-grained and metastable microstruc-tures. The properties of a plasma-sprayed deposit are directly related to this complex microstructure. This paper examines the solidification dynamics and the resultant microstructures in an effort to estab-lish a processing/microstructure relationship. Existing models in the literature developed for splat coo-ling have been extended and applied for examining the rapid solidification process during plasma spraying. Microstructural features of the splats that are produced by individual impinging droplets are examined through scanning and transmission electron microscopy. The relation of dimensions and mor-phologies of these individual splats to the consolidated deposit microstructure is considered. In addition, the distinguishing features in the solidification and microstructural development between air plasma spraying and vacuum plasma spraying are explored, and a unified model is proposed for splat solidifica-tion and evolution of the microstructure.

Measurement of residual stress in plasma-sprayed metallic, ceramic and composite coatings

Abstract Residual stresses in plasma-sprayed coatings were studied by three experimental techniques: curvature measurements, neutron diffraction and X-ray diffraction. Two distinct material classes were investigated: (1) single-material coatings (molybdenum) and (2) bi-material composites (nickel+alumina and NiCrAlY+yttria-stabilized zirconia), with and without graded layers. This paper deals with the effects of coating thickness and material properties on the evolution of residual stresses as a function of composition and thickness in both homogeneous and graded coatings. Mathematical analysis of the results allowed in some cases the separation of the quenching stress and thermal stress contributions to the final residual stress, as well as the determination of the through-thickness stress profile from measurements of different thickness specimens. In the ceramic–metal composites, it was found that the quenching stress plays a dominant role in the metallic phase, whereas the stress in the ceramic phase is mostly dominated by thermal mismatch. The respective thermal expansion coefficients and mechanical properties are the most important factors determining the stress sign and magnitude. The three residual stress measurement methods employed here were found to be complementary, in that each can provide unique information about the stress state. The most noteworthy outcomes are the determination of the through-thickness stress profile in graded coatings with high spatial resolution (curvature method) and determination of stress in each phase of a composite separately (neutron diffraction).

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

Abstract Mechanical properties, such as residual stress and Young’s modulus, play a critical role in the synthesis and performance of thermally sprayed coatings. Thus, it is important to understand their evolution, the influence of processing parameters, and to be able to determine them accurately. The first part of this two-part paper presents a novel in situ curvature method for determination of stresses and Young’s modulus of plasma sprayed coatings. The principle of the method is explained, details of the instrument are provided and the analytical procedure is described. The capabilities of the method are discussed in detail, namely the ability to observe the stress evolution during the entire spraying process, to separate the quenching and thermal stress contributions to final residual stress and to determine the Young’s modulus of the coating. Brief examples of application are also included, and the potential for use of this method for process control of coating quality is addressed. In the second part, a case study for plasma sprayed molybdenum will be presented, focusing on the influence of the key processing parameters.

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

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.

Role of condensates and adsorbates on substrate surface on fragmentation of impinging molten droplets during thermal spray

Abstract We propose that the presence of condensates/adsorbates on low temperature substrate surfaces may be a significant factor responsible for splat fragmentation of impacting molten droplets. Vaporization and rapid expansion of condensates/adsorbates upon molten droplet impact cause instability of the spreading droplet. Plasma spraying experiments, using radio frequency induction processing of ZrO2, were designed to test this hypothesis. In order to obtain different levels of condensates/adsorbates, steel substrates were heated in a vacuum chamber (at 250 torr) and allowed to cool under vacuum for different periods of time, ranging from 1 to 62 h before splat deposition. For comparison, splats were also produced on ambient (25°C) as well as on heated substrates (500°C). It was found that splat morphology changed from highly fragmented to a contiguous, disk-like shape with a decreased level of surface condensates/adsorbates, although the substrate temperature was maintained at ambient temperature.

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

Abstract 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.