M. P. Srivastava

Thermal barrier coatings from sol gel-derived spray-grade Y2O3-ZrO2 microspheres

For the development of ceramic thermal barrier coatings, spray-grade yttria-stabilized zirconia microspheres were prepared by the sol-gel technique. Oxide microspheres were obtained by calcination of the corresponding gel spheres at 1000 °C. Scanning electron microscopic and optical microscopic observations revealed the material thus obtained to have a predominantly spherical morphology and the requisite size distribution (5–50 μm). The dense, calcined microspheres showed good flowability. X-ray diffraction studies indicated the presence of the tetragonal polymorph of ZrO2 as the major phase, in addition to about 14% monoclinic ZrO2. The plasma-sprayed YSZ coatings made from the sol-gel-derived microspheres showed a further decrease in the monoclinic ZrO2 content (6%). The coatings survived 40–50 thermal cycles (30 min at 1200 °C followed by a water quench), indicating good thermal shock resistance.

On the thermal cycling life of plasma-sprayed yttria-stabilized zirconia coatings

Abstract The influence of various factors on the thermal shock resistance of plasma-sprayed yttria-stabilized zirconia (YSZ) coatings was investigated. The coatings, formed using different spraying conditions, were repeatedly exposed to a thermal cycle involving a 30 minute heating period at 1200°C followed by a water quench during this study. The results suggest that, for any given YSZ powder, there exists an optimum plasma arc current for obtaining coatings that best endure such severe temperature cycling conditions. Other coating variables, such as spray distance and powder feed rate, are also observed to effect coating durability. Furthermore, the YSZ overlayer thickness and the bond coat material employed are found to be important coating life determining factors.

Study of Plasma- and Detonation Gun-Sprayed Alumina Coatings Using Taguchi Experimental Design

Atmospheric plasma spraying (APS) is a most versatile thermal spray method for depositing alumina (Al2O3) coatings, and detonation gun (D-gun) spraying is an alternative thermal spray technology for depositing such coatings with extremely good wear characteristics. The present study is aimed at comparing the characteristics of Al2O3 coatings deposited using the above techniques by using Taguchi experimental design.Alumina coating experiments were conducted using a Taguchi fractional-factorial (L8) design parametric study to optimize the spray process parameters for both APS and D-gun. The Taguchi design evaluated the effect of four APS and D-gun spray variables on the measured coating attributes. The coating qualities evaluated were surface roughness, porosity, microhardness, abrasion, and sliding wear. The results show that the coating quality is directly related to the corresponding coating microstructure, which is significantly influenced by the spray parameters employed. Though it is evident that the D-gun-sprayed coatings consistently exhibit dense and uniform microstructure, higher hardness, and superior tribological performance, the attainment of suitable plasma-sprayed coatings can be improved by employing the Taguchi analysis.

Plasma spraying of biologically derived hydroxyapatite on implantable materials

Plasma spraying of hydroxyapatite (HA) coatings on human implants is considered to provide a promising means of enhancing their biocompatibility and improving tissue growth. This paper briefly describes a method of extracting HA powder from a biological source, namely human teeth. The physical and chemical characteristics of the derived powder are studied and the suitability of this powder for plasma spraying applications is ascertained. The deposited coatings are found to retain the chemistry characteristic of the apatite. Typical results of metallographic and scanning electron microscopy (SEM) studies and hardness measurements on the sprayed HA coatings are presented.

Scratch adhesion testing of plasma-sprayed yttria-stabilized zirconia coatings

Abstract The suitability of scratch adhesion testing, usually used for determining the critical load for thin hard coatings like TiC and TiN, in characterizing plasma-sprayed yttria-stabilized zirconia coatings is demonstrated. The effects of loading rate and scratching speed on the critical load of these sprayed coatings were studied. Although some peculiarities in acoustic signal-load plot were observed at high values of loading rate and scratching speed, it was found that these intrinsic parameters, at low and medium values, do not have any prominent effect on the critical load.

Influence of spraying variables on structure and properties of plasma sprayed alumina coatings

Abstract An experimental statistical design study on the plasma spraying of alumina powder has been carried out. Coating experiments were conducted, using a Taguchi full factorial L 16 design parametric approach, to study the effect of four key plasma processing variables on the coating quality, namely, primary gas flow rate, arc current, powder feed rate, and spray distance. Optical microscopy, scanning electron microscopy, XRD, image analysis, and hardness testing were used for characterisation. The resulting as sprayed coating characteristics were quantified with respect to roughness, microhardness, porosity, and microstructure. Through statistical calculation (analysis of variance), the parameters that have significant influence on the structure and properties of the coatings were identified and their relative importance and contribution ratios to overall variance were studied. The Taguchi evaluation employed in the present investigation showed that an improvement in the coating properties could be achieved using an optimum combination of variables.

Experimental study of particle deposition characteristics of alumina using plasma spraying

Plasma spraying is one of the most versatile techniques used to form coatings for protection against oxidation, corrosion, and wear. The plasma spraying is ideally suited for refractory materials, but there are a number of variables that need to be controlled to obtain dense coatings. In spite of considerable progress made in the theoretical understanding of this complex process, there is a need for a simple method to evaluate the interaction between the plasma flame and powder particles that form the coatings. As reported in the literature, this involves metallographic observation of the powders collected from the plasma. In the present study, the structure and morphology of plasma-sprayed splats are experimentally investigated using different power levels and spray distances for alumina powder. The results show that the splashing occurs during splatting of a completely molten droplet. It is found that at higher power levels and shorter spray distances, spreading of molten droplets improves considerably.

Plasma spraying of an indigenous yttria stabilized zirconia powder prepared by the sol-gel technique

An indigenous sol-gel derived yttria-partially stabilized zirconia (Y-PSZ) powder has been characterized and its suitability for plasma spraying applications evaluated. The powder, determined to have about 5·1% yttria content, predominantly consisted of spherical particles with an average equivalent particle diameter close to 25µm. Furthermore, it was found that the powder did not contain any particles >50µm, which is considered the ideal upper size limit for spray-grade ceramic powders in order to ensure complete melting during spraying. The sol-gel produced powder exhibited good flow characteristics and the plasma sprayed coatings developed using this powder were also found to have excellent thermal shock resistance. The corresponding results obtained using an imported Y-PSZ powder are also presented for the purpose of comparison.

Plasma spraying of Wc-Co part II: Experimental study of particle deposition and coating microstructure

WC-Co base wear-resistant coatings deposited by plasma spraying are widely used to enhance component longevity in a variety of wear environments. During spraying of WC-Co, ideally only the cobalt phase should melt and act as a binder for the WC particles. Although it is undesirable to fully melt WC because it can cause decarburization, complete melting of the cobalt phase and its satisfactory flattening on impacting the substrate is necessary to minimize porosity and achieve good substrate/coating adhesion. In this article, the influence of the primary plasma spray variables on the melting characteristics of WC-Co powders is investigated with respect to the microstructure of these coatings. This experimental work complements an analytical study on plasma spraying of WC-Co, and thus, observations are presented to support the predictions of the modeling effort.

Evidence of accelerated thermal cycling test schedules influencing the ranking of zirconia-base thermal barrier coatings

Thermal barrier coatings (TBCs) often encounter temperature cycling in the course of normal operation. In the absence of actual or simulated engine test facilities, accelerated furnace thermal cycling experiments are frequently devised to evaluate the response of various TBCs. This study, which deals with yttria-stabilized and magnesia-stabilized zirconia systems, shows that the performance of a TBC is significantly governed by the severity of the time-temperature schedule employed. More importantly, the ranking of the two zirconia-base TBCs also is influenced by the adopted thermal cycling test schedule. These findings have ramifications in the design of suitable accelerated tests for TBC evaluation.