H. Herman

Effects of pores and interfaces on effective properties of plasma sprayed zirconia coatings

It is difficult to establish structure–property relationships in plasma sprayed coatings because of their unique and intermingled splat microstructures incorporating networks of various intrinsic process-dependent microdefects. In this paper, these coatings are characterized using two distinctly different approaches, both based on novel experimental techniques and computational modeling tools. In each approach, detailed finite element models are illustrated to represent the porous coatings fabricated with four different types of zirconia feed powder. One approach is based on smallangle neutron scattering (SANS) studies carried out to quantify microstructure. In this approach, the pore morphology is idealized by artificially rebuilding, based on the collective microstructural information obtained in terms of component porosities of three pore systems, their opening dimensions and orientation. The other method relies on image analysis of real microstructural images obtained using scanning electron microscopy (SEM). The finite element mesh for the actual cross-sectional model, generated by thresholding the SEM images, is constructed with the object oriented finite (OOF) element method. Through these two approaches, the effective thermal conductivity and elastic modulus along the spray, as well as the transverse directions are estimated for thermally sprayed yttria-stabilized zirconia (PSZ) coatings. Our results show the effectiveness of these computational approaches for estimating material properties with each approach having its strength and weakness. However, in comparison with experimentally measured properties, there exist some limitations in both approaches. To further probe the source of discrepancy within the measurements, the coatings are thermal cycled to reduce the effect of splat boundaries on properties. Additional models are constructed for these coatings and their analysis is carried out. For the first time, the influence of the splat interfaces on the effective properties of sprayed coatings is quantified.

Amorphous phase formation in plasma-sprayed hydroxyapatite coatings

The amorphous phase content of air plasma-sprayed hydroxyapatite coatings is dependent upon spraying and deposition conditions. X-ray diffraction and optical microscopy were used to investigate the influence of spray parameters on the formation of the amorphous phase. Results show three factors which most influence the formation of the amorphous phase: dehydroxylation of the molten particle during flight, the cooling rate as it impinges onto the metal substrate, and the substrate temperature. Crystalline regions were identified as unmelted particles and elongated recrystallized areas. Amorphous phase regions vary throughout the coating but are more commonly found at the coating-substrate interface, i.e., the regions decrease toward the surface of the coating. Such an inhomogeneous distribution of phase content is expected to affect the clinical process of bone deposition, and therefore successful implant fixation.

Plasma spray synthesis of nanomaterial powders and deposits

Abstract Conventional plasma spraying was used to process atomized liquid droplets of precursor solutions to produce alumina, zirconia and yttria stabilized zirconia nanoparticles and deposits. An electrostatic precipitator collected the plasma synthesized ceramic particles at a rate of ~0.2 mg s −1 , with ~5–20% collection efficiency. Spray processing produced 1–50 nm size ceramic particles. The size, shape and phase composition of the nanomaterials depend on the spray feedstock. Organo-metallic precursors gave rise to a narrow range of fine-grained material, while aqueous solutions produced wider distributions of larger size grains. Spray processing of liquid feedstock produced nanodeposits with a powdery morphology. Plasma spraying of liquid precursors is a viable technique to produce nanoparticles and deposits.

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.

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.

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.

Microstructural characterization of yttria-stabilized zirconia plasma-sprayed deposits using multiple small-angle neutron scattering

Density, electron microscopy, elastic modulus, and small-angle neutron scattering studies are used to characterize the microstructures of yttria-stabilized zirconia plasma-sprayed deposits as a function of both feedstock morphology and annealing. In particular, anisotropic multiple small-angle neutron scattering data are combined with anisotropic Porod scattering results to quantify each of the three main porous components in these thermal barrier coating materials: intrasplat cracks, intersplat lamellar pores and globular pores. An inverse correlation between the volume of porosity and its surface area is confirmed for the as-sprayed deposits, as is a preferential annealing of intrasplat cracks at elevated temperatures. The average elastic modulus is correlated with the total void surface area while the elastic anisotropy is related more closely to the intersplat porosity. However, depending on the feedstock morphology, globular pores are also shown to play a surprisingly significant role in post-anneal deposit microstructures and properties.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Surface mechanical properties - effects of ion implantation

Abstract Ion implantation has been used to modify the mechanical properties of a wide range of metals and alloys. The affected properties which have been studied include friction and wear, erosion and fatigue. Both BCC and FCC systems have been examined, with the major effort being directed at the former, due to the strong influence of interstitial implantants on mechanical properties of BCC and because of the industrial utility of these alloys. In seeking the microstructural origins of these sometimes dramatic effects, researchers have employed numerous surface analysis techniques, including backscattering and electron spectroscopy, TEM, SEM, X-ray and Mossbauer analysis and internal friction measurements. The interactions of surface dislocation structures with implantation-induced imperfections, surface alloying, and precipitation phenomena are discussed. A review is given of the current status of activities as represented by a number of research groups.

Nanomaterial powders and deposits prepared by flame spray processing of liquid precursors

Ultrafine grained Al2O3, Mn2O3, ZrO2 and Y2O3-ZrO2 powders and deposits were produced by flame spray processing of atomized precursor solutions. Ceramic particles of 1–150 nm size were collected on an electrostatic precipitator at a rate of ~ 5–20 mg/min, with ~ 5–25% collection efficiency. Nanograined oxide deposits with a powder morphology were produced at a deposition efficiency of < 10%. The spray feedstock and processing conditions affected the size, shape and phase composition of the synthesized nanomaterials. Particles with wider distributions of larger size grains were produced from aqueous solutions, whereas narrow range of fine-grained material was produced from organo-metallic precursors. Liquid flame spraying is a viable technique to produce nanoparticles and deposits.