It is widely recognized that substrate surface roughness, or topography, plays an important role in droplet-substrate interaction and the adhesion of sprayed coatings. A key difficulty in understanding the role that topography plays during droplet impact, wetting and solidification has been the availability of methods for appropriate characterization of the topography. The complex nature of the substrate topographies cannot be adequately characterized by conventional methods such as Ra. In this work, scale-sensitive fractal analyses are considered for advancing the understanding of roughness of grit blasted surfaces in thermal spray applications. Area-scale analysis is performed on 3D data sets acquired from different grit-blasted substrates. From fractal analysis it is known that the apparent area of a rough surface increases as the scale of observation decreases. The area-scale relations are used to guide experimental design for topographical data acquisition and analysis and to better understand the influence of grit blasting on substrates for thermal spray. The potential of these scale-sensitive analysis techniques to fulfill the above bases for supporting statistical correlations and clear physical interpretations is discussed.

The temperature within plasma-sprayed oxide coatings depends strongly on the cooling devices used during spraying as well as on coating pass thickness and powder flow rate. Two oxides, alumina and zirconia, were deposited using dc Ar-H 2 plasma jets and the shape, size, and composition of the columnar structures produced were examined and found to correspond with substrate and coating surface temperatures as measured during and after spraying. The relationship between temperature and thermomechanical properties was also investigated and the results are reported in the paper.

Coatings have been produced by HVOF spraying of four different WC-Co powders, using two fuel gases and two oxygen contents in the flame, and characterised in terms of microstructure and resistance to abrasive wear. It is concluded that there is a close correlation between high levels of chemical reaction, occurring during spraying (and possibly during powder production), and poor wear resistance. Good wear resistance is favoured by using low porosity powders, which interact with the atmosphere less readily during spraying, and also by using a flame with a relatively low oxygen content. This probably minimises the degree of reaction by ensuring that conditions are reducing. Use of propylene rather than hydrogen gives coatings with slightly better wear resistance, despite the fact that the flame temperatures are higher. It is concluded that, for this relatively small rise in temperature, the positive effect on inter-splat cohesion seems to outweigh the negative effect of increased decarburisation.

The phase composition of plasma sprayed (8wt%) YSZ was studied using neutron and X-ray scattering. Comparison shows that neutron scattering is superior for analysis of the phase composition as well as for the analysis of the yttria content of the tetragonal phase. The presence of large amounts of the cubic phase are probably often neglected in standard XRD analysis due to scattering-related limitations and the inherent difficulty of the analysis. The importance of this fact needs to be addressed in future studies. The amount of monoclinic, tetragonal, and cubic phases was determined for as-sprayed deposits and for samples annealed at various temperatures. The as-sprayed deposit was composed of metastable phases, and the phase composition on annealing did not change significantly until 1400 °C for one hour. This indicates the relative stability of the phase composition. The influence of the spray distance is not clear, but the resultant phase composition may be related to the deposition temperature.

The microstructures of as-sprayed and thermally-cycled freestanding and on-substrate deposits of yttria-stabilized zirconia were studied using small-angle neutron scattering (SANS). The SANS analysis allows the interlamellar pores and the intralamellar cracks, which are the two dominant void systems in the microstructure, to be characterized separately. Whereas up to 20% of the void surface area in the as-sprayed deposits was found to be in the cracks, the thermally-cycled deposits contained only a negligible quantity of cracks. At the same time, changes in the pore surface areas between the lamellae (i.e., the interlamellar pores) were much smaller. As a result, the microstructure of the thermally-cycled deposits was much more anisotropic than the microstructure of the as-sprayed deposits. Varying the cooling and the heating rates did not significantly change the microstructure but varying the total time that the deposits were at high temperature did affect the evolution of the surface area. The presence or absence of a bond coat and substrate also did not measurably influence the results.

As part of a characterization and mechanical research about hydroxyapatite (HA) plasma deposits for hip prosthesis, we addressed the problem of determining their crystallinity. A traditional normalization method employed by several laboratories is based on X-ray diffraction by a powder mixture of the investigated HA sample with a standard of crystalline powder, namely Al 2 O 3 . This method is quite unsatisfactory, as very often delivers unreasonable results. In order to overcome these difficulties we investigated some new methods for determining the crystallinity of HA sample, which are based on X-ray diffraction. All these methods provide reasonable results.

Addition of CaSi 2 in steel melt leads to a decrease of oxygen content, an increase of the melt flow capability, an improvement of a metal ductility and weldability. For an estimation of CaSi2 and Si additions effect in conditions of thermal spraying composite powders were produced. A base material of these powders particles is nickel, nichrom and iron. CaSi 2 and Si are additional components of the powders (3-12 wt. %). Plasma coatings produced with using of these powders exhibit high density and adhesion. The coatings structure has a high grade of microcrystallinity with a decreasing size of lamels. Oxides content in the coatings decreases with an increasing of CaSi 2 and Si content in the composite powders, moreover this effect is more essential in case of CaSi2 addition. Addition of CaSi 2 and Si to Ni-base coatings increases a microhardness from 1900-2000 MPa to 2200-2850 MPa. In case of NiCr-base coatings this increasing is from 1800-2100 MPa to 2400-3500 MPa.

This paper presents the results of a study on the wear resistance of plasma spray coatings made from Cr 2 O 3 -TiO 2 -CaF 2 powders. The composite powders used were produced by self-propagating high temperature synthesis. They were then applied under various conditions in order to optimize the material system, spray process, and application procedures. Based on the results of microstructural examination and wear testing, the thermally sprayed composite coatings have excellent wear resistance, good adhesion, and are self-lubricating at high temperatures.

Chromium carbide-nickel chromium coatings produced by HVOF spraying are widely used for high temperature wear and erosion resistant applications. Examination of the literature shows that whilst the mechanical properties of these coatings have been widely investigated, there has been little research into the physical processes occurring during HVOF spraying of this system, such as carbide dissolution, liquid-metallic phase oxidation, decarburisation and rapid solidification. The purpose of the present work has been to perform a systematic characterisation of the chromium carbide-nickel chromium system in both the initial powder and as-sprayed states with a variety of spraying conditions using optical, scanning and transmission electron microscopy, electron microprobe and X-ray diffraction. The presence of amorphous and nanocrystalline phases has been demonstrated. The nanocrystalline structures tend to be Ni rich, with the amorphous phases rich in Cr. Carbides of the form Cr 3 C 2 were found to be dissolved slightly during spraying, increasing the Cr and C contents of the liquid metallic phase. There was no evidence of chromium carbide oxidation.

The porous microstructure of plasma-sprayed deposits prepared from gray-alumina feedstock and from two different yttria-stabilized zirconia (YSZ) feedstocks were studied as a function of spray distance. For each material, the behavior of the two major void systems—intralamellar cracks and interlamellar pores—was investigated. The results offer the first proof that the quantity and the character of the porosity in these materials can be controlled independently by selecting the appropriate processing protocols.

Hardmetal-like coatings on the base of titanium carbide as a hard phase and nickel as a metal binder were prepared from agglomerated and sintered powders by plasma spray, detonation gun spray and high-velocity oxygen-fuel spray processes. The powders used in the spray experiments were plain TiC-Ni type and alloyed (Ti,Mo)C-NiCo type powders with different binder content. The coatings were characterized by optical and scanning electron microscopy, microhardness measurements, XRD analysis and in an abrasion wear test. The results showed that the sprayability of these novel hardmetal-like powders is good in all spray processes studied and the coatings deposited were found to have dense microstructures and good properties. The XRD analysis showed that the coatings have a phase structure similar to that found in the spray powder. The amount of retained carbides in the coatings was high. Some regions in which the carbides had dissolved with the metallic binder phase during spraying were also found, especially in plasma sprayed coatings. In such microstructural regions submicron size reprecipitated carbides were detected. These were clearly detectable in detonation gun sprayed coatings. HVOF sprayed coatings were found to contain a very high content of retained carbide phase. In this process the heat effect to the material seemed to be the lowest. The wear tests clearly showed the importance of alloying the hard phase and the binder phase in order to improve the wear resistance of the coatings. All studied spray processes produced coatings with nearly similar coating wear properties.

Plasma spraying is under investigation as a method for in-situ repair of damaged beryllium and tungsten plasma facing surfaces for the International Thermonuclear Experimental Reactor (ITER), the next generation magnetic fusion energy device, and is also being considered as a potential fabrication method for beryllium and tungsten plasma-facing components for the first wall of ITER. Investigators at the Los Alamos National Laboratory's Beryllium Atomization and Thermal Spray Facility have concentrated on investigating the structure property relationship between the as-deposited microstructures of plasma sprayed beryllium coatings and the resulting thermal properties of the coatings. In this study, the effect of the initial substrate temperature on the resulting thermal diffusivity of the beryllium coatings and the thermal diffusivity at the coating/beryllium substrate interface (i.e. interface thermal resistance) was investigated. Results have shown that initial beryllium substrate temperatures greater than 600°C can improve the thermal diffusivity of the beryllium coatings and minimize any thermal resistance at the interface between the beryllium coating and beryllium substrate.

Properties and characteristics of a material depend mainly upon its morphological structure. In the case of plasma sprayed deposits, the properties (i.e., mechanical, thermal, electrical, etc.) are especially related to the deposit void content. Voids are coming from pores, cracks and delaminations. Implementing image analysis and stereological protocols, the apparent void content (AVC) of a vacuum plasma sprayed deposit was studied, in terms of the pore size distribution and spatial distribution. Results were compared with data obtained implementing water immersion porosimetry. This study showed on the one hand that the pore size distribution is described by a bimodale function, and, on the other hand, that pores are spatially distributed evenly within the deposit (isotropic distribution). In such a condition, the stereological protocol can be implemented on a single random plane section of the deposit.

The influence of alumina substrate temperature and phase structure : columnar gamma phase, columnar alpha phase and granular alpha phase on splat formation and crystal growth has been studied by SEM and Atomic Force Microscopy. X Ray Diffraction at low angle has allowed to obtain informations on phase structure of layered splats according to substrate phase structure and coating thickness. Column sizes of splats are correlated to a ID model of splat cooling showing the influence of substrate thermal properties and splat thickness on crystal growth kinetic. Finally, coatings adhesion-cohesion values function of spraying parameters are in good agreement with splat morphology and microstructural evolution.

Stainless steel coatings were produced by supersonic air-gas plasma spraying method. Mixture of air and natural gas was used as a plasma forming gas. Powders of 304L and 316L stainless steels were used for plasma spraying. Thickness of coatings was up to 3 mm. Coating structure was studied. Dependence of oxygen content in coatings upon particle sizes and spraying conditions was established. Investigation of electrochemical properties of coatings was carried out by potentiostatic method. Corrosion potential and corrosion rate in sea water, hydrochloric and sulphuric acid solutions were determined.

Flame-sprayed coatings of alumina were produced by an oxyacetylene flame spray system in order to study the phase transformations that occurs on alumina during the spraying. The X-ray diffraction pattern of the alumina powder to be sprayed, showed the main presence of the stable phase alpha alumina and an impurity probably resulting from the process of purification of alumina. For as-sprayed coatings, phase changes occur. The X-ray diffraction pattern shows the presence of the stable form alpha alumina, but also the metastable form gamma alumina and amorphous alumina. True density measurements were done using a helium pycnometer, as an aid to observe the phase transformations. The density of the powder to be sprayed was 3.98 g/cm 3 and the density of the as-sprayed coatings was 3.62 g/cm 3 . This change of density is linked to the phase transformation during spraying. Scanning electron microscopy (SEM) of the incident particles was made after 1 second of deposition onto glass substrates in order to observe the degree of melting of the incident particles, by analyzing their profiles (degree of flattening).

The Central Coatings Laboratory (CCL) is a program that deals with training in coating evaluation techniques. The approach uses a combination of both theoretical and hands-on instruction to teach various concepts. The system is educational based, incorporating a philosophy of making the students problem solvers in lieu of following "cookbook" evaluation procedures. Examples of teaching techniques will be discussed and suggested course material reviewed to highlight this training philosophy.

The heterogeneous or composite nature of the thermal spray deposit can make choosing a metallographic preparation process very difficult. Hard vs. soft phases, brittle ceramics, and mixtures of many phases can create a variety of issues. When a procedure is developed, it becomes a necessity to document all the parameters or intangibles that define the process. Examples of procedure definition will be discussed and a suggested list of critical intangibles such as polishing abrasives (concentration vs. frequency of application), lubricant (pH, type), and many other factors will be highlighted.

A comprehensive approach is presented for facilitating the implementation of advanced plasma spray processing technology in the manufacture, repair, and refurbishment of industrial components. This approach employs an integrated methodology for combining several advanced computer-based methods, including: 1) an interactive multimedia-based education and training tool to effectively store and retrieve plasma spray processing information in a variety of formats; 2) an expert system to select plasma spray feedstock material for a specific coating function; 3) a one-dimensional plasma spray process model that allows simulation of plasma spray processing conditions for identifying operational envelopes for a selected feedstock material; 4) an interface fracture model for identifying appropriate acceptance criteria for reduced cracking along the coating/substrate interface; and 5) a set of computer-based nondestructive test methods for performing quality assurance and control. This comprehensive approach and the integrated methodology provide an advanced engineering tool for the selection, optimization and implementation of specific advances in plasma spray processing technologies. A major outcome is the reduced need for expensive and time-consuming trial-and-error methods in evaluating the application of plasma spray coatings for the manufacture, repair, and refurbishment of specific industrial components. This comprehensive approach and integrated methodology can be extended to include other thermal spray processing technologies as well.

Electrical properties of plasma-sprayed aluminum oxide coatings were measured at temperatures up to 600 °C. High purity (>99.5 wt% pure Al 2 O 3 ) alumina powders were plasma-sprayed on stainless steel substrates over a range of power levels, using two gun configurations designed to attain different spray velocities. Key electrical properties were measured to evaluate the resultant coatings as potential insulating materials for electrostatic chucks (ESCs) being developed for semiconductor manufacturing. Electrical resistivity of all coatings was measured under vacuum upon heating and cooling over a temperature range of 20 to 600 °C. Dielectric constants were also measured under the same test conditions. X-ray diffraction was performed to examine phase formation in the coatings. Results show the importance of powder composition and careful selection and control of spray conditions for optimizing electrical behavior in plasma-sprayed aluminum oxide, and point to the need for further studies to characterize the relationship between high temperature electrical properties, measured plasma-spray variables, and specific microstructural and compositional coating features.