A new measurement technology in plasma spraying has been established at SULZER METCO AG, Switzerland. Based on the principle of C. Moreau, CNRC, it is now possible to simultaneously measure surface temperature and velocity of melted particles in a plasma flame. Since temperature and velocity are two of the most important parameters affecting coating characteristics in the thermal spray processes, the knowledge and control of these parameters offers new methods and assistance in the development of coating solutions. The object of this paper is to outline some important measurement results, to show correlations to splat formation and coating characteristics and to demonstrate applications in an industrial environment.

Plasma spraying is a complicated process involving many partly interdependent parameters, which are in industrial spray environments difficult to optimise without laborious and time consuming experiments. In this work a non-intensified CCD camera without any external illumination is used for in-flight particle visualisation. Particle visualisation is based purely on the spontaneous light emitted by the hot particles. The motivation for this work is to outline the possibilities to develope a CCD based, low cost and rugged in-situ measurement system suitable also for industrial use. The measurement method has been tested with Plasmatechnik A3000S plasma spraying equipment using fused and crushed Al2O3 powder. Using digital image processing techniques relative particle concentrations and particle velocities have been calculated from the acquired images. These results have been correlated with wear resistance and deposition efficiency of the coatings produced with different powder feed rate and powder port adjustments. Coatings were also produced using both new and worn electrodes. The benefits and limitations of the method are discussed and the measurement results are compared against measurements made using laser sheet illumination, which can give information concerning also the colder and/or smaller particles not visible for the passive CCD system.

The influence of input spray parameters on the state of plasma-sprayed zirconia powder is studied. The particle temperature, velocity and diameter are measured using an integrated optical monitoring system. The monitoring system allows the investigation of the particles behavior in the spray jet. The collected information is correlated to coating characteristics such as deposition efficiency, microstructure and thermal diffusivity. Results show that, by monitoring the state of sprayed particles, a better understanding of the coating microstructure and properties can be achieved.

This paper provides a summary of conventional thermal spray powder feeders, examines the influence of carrier gas flow, powder tube diameter, powder tube type, and gas flow path on the feed precision and continuity of pressure-type powder feeders, and discusses the cause of pulsation associated with the injection of super fine ceramic powders.

Plasma sprayed MCrAlY bondcoats play a major role in thermal barrier coatings. During service, oxide forms on both sides of the bond coat and must be minimized to prevent coating failures. Along with powder chemistry, coating microstructure significantly influences oxide growth. It is known that both coating microstructure and coating strength are strongly related to plasma spraying parameters. This present work examines the effect of inflight particle properties on the adhesion strength and microstructure of NiCrAlY and NiCoCrAlY bondcoats. The relation between particle velocity and temperature and coating properties is particularly important. Relatively small changes in spray parameters such as arc current and gas flows can have a major impact on sprayed particles and consequently coating microstructure. Through online control of particle states it is expected that the quality of plasma-sprayed MCrAlY coatings can be significantly improved.

In an effort to obtain a series of plasma-sprayed coatings of controlled microstructure, three monosized sapphire powders were deposited using an axial injection torch in which the plasma gas composition and nozzle diameter were the only parameters varied. The effect of these parameters on splat morphology, porosity, angular crack distribution, and hardness is reported. Uniform, dense microstructure with a hardness rivaling chromia coatings was achieved through a combination of tightly controlled processing conditions and the use of mono-disperse precursor powders.

Electrodes play an important role in the plasma-spraying process and must be frequently replaced to ensure good coating properties. The purpose of this work was to assess the effect of different plasma gases and oxygen and humidity levels on electrode lifetime. The change in arc voltage over time was recorded during spraying, and the elapsed time for a 5V drop was taken as the electrode lifetime. It was found that variations in oxygen and humidity in Ar-H2 gas mixtures have a major effect on lifetime and that the use of SPRAL22 gas could extend electrode life by a factor of three to four. These and other results are discussed in the paper.

In dc plasma spray guns, the properties of the plasma forming gas largely control the characteristics of the plasma jet and the momentum and heat of the particles injected into the flow. This paper examines the effect of Ar-He-H2 mixtures on the dynamic and static behavior of plasma jets expressed in terms of arc voltage and gas velocity. Correlations between these parameters and operating variables (arc current, gas flow rate, volume composition) were established from a dimensional analysis and supported by the calculation of the thermodynamic and transport properties of the gas mixtures used in the study.

Plasma spraying of metals and metallic alloys performed in controlled atmosphere or soft vacuum results in coatings with a low oxidation level and excellent thermomechanical properties. Unfortunately, the spraying cost is drastically increased by one or two orders of magnitude compared to air plasma spraying (APS). Thus the minimisation of oxidation during APS is a key issue for the development of such coatings. Oxygen concentrations sucked into plasma jets have been measured by an enthalpy probe linked to a mass spectrometer. This technique allows to determine simultaneously plasma composition, temperature and velocity distributions within the plasma plume. Results have been compared to those obtained with a two-dimensional turbulent flow model. The obtained results have shown that surrounding air entrainment is reduced when using adequate Ar/H2/He mixtures which viscosity is higher than that of Ar/H, mixtures, limiting the turbulence in the jet fringes and pumping of the surrounding atmosphere.

Despite the fact that plasma spraying has been a widely used technology over the past three decades, industries using this technology still need higher quality products. Presently, only a small degree of process control is used in most plasma spraying systems. Improved process control should lead to more consistent results and higher quality products. We discuss a relatively simple control scheme consisting of a microphone as a primary sensor and a fuzzy logic look-up model indicating the condition of the anode. Selected frequency peaks in the power spectrum of the microphone signal are analyzed online, and the results are correlated with an average jet length obtained from a series of high speed images. The jet length, in turn, is correlated with coating characteristics. A simple feedback control system is proposed which will counteract the negative effects of an eroded anode on coating quality.

The integration of thermocouples into thermal spray deposits and especially into vacuum thermal spray coatings could provide temperature monitoring between the substrate and the coating or between two different coatings during the spray process and later during post treatments and service life. Thermocouples of 251µm in diameter were made using Chromel and Alumel wires. Electrical insulation was obtained using a ceramic cement. Astroloy and Copper coatings were successfully sprayed over these sensors and the temperature given by an embedded thermocouple was compared to the response of an infrared pyrometer during the spraying process.

This paper presents some results of an inspection program to detect coating-substract defects using a Michelson type of shear interferometer. Debonding defects are detected by properly loading the surface of the specimen in such a way that the interference fringe pattern is modified, rendering the debond readily visible. Image processing techniques are applied to enhance the detection and better define the debonding geometry. Load conditions, leading to a better detection of debondings, are also discussed. The technique allows to obtain information over a large area of the tested part, with a resolution one hundred of the wavelength of the light being used.

For the adhesion of thermally sprayed coatings, the substrate roughness, or topography, plays an important role in droplet-substrate interaction. The lack of availability of methods for appropriate characterization of the real topography is a major difficulty in understanding the role that topography plays during droplet impact, wetting and solidification. The complex nature of the usually chaotic substrate topographies cannot be fully characterized by conventional roughness numbers such as Sa or Ra. In our study a scale-sensitive fractal analysis method is used for describing the morphology of grit blasted surfaces. Area-scale analyses are performed on 3D data sets acquired from different substrate materials, treated by various grit blast parameters. 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 on one side to guide experimental design for topographical data acquisition and analysis, and on the other side to understand the influence of the grit blast process on the different kind of substrates. The potential of these scale-sensitive analysis techniques for supporting statistical correlations and clear physical interpretations will be discussed.

Microscopic fracture mechanisms of thermal spray coatings under bending stress are investigated. Samples of thermally sprayed coatings were made using three distances. The sprayed powder was pure molybdenum. Vertical microcracks occur in lamellae and subsequently, these cracks join together and form vertical macrocracks in the samples sprayed with a short spraying distance. On the other hand, horizontal microcracks occur at the lamellae interfaces, and these cracks link together in the samples sprayed with a long spraying distance. These tendencies can be explained in terms of the hardness of the lamella and the bonding strength between each lamella. It is clarified that the bonding strength between each lamella corresponds to the applied strain at the point of rapid increase of the acoustic emission (AE) event. The amplitude and rate of AE beyond the point of rapid increase are high in the coatings which formed macrocracks. It is concluded that the coating which has high resistance to crack formation has a high point of AE increase, low AE amplitude and low AE increasing rate.

This study evaluates a nondestructive method for determining the adhesion strength and porosity of thermally sprayed coatings. The method is based on ultrasonic surface echo measurements. Porosity is surmised by the impedance of the coating and adhesion by the transmission coefficient. Experimental results confirmed that the method is effective for its intended use.

A high-speed video recording system was developed to measure the velocity and temperature of thermal spray particles in flight. The system consists of three image sensors and a cubic beam-split prism that directs light, based on color or wavelength, to three imaging planes. In this study, one of sensors is used to measure particle velocity while the other two measure particle temperature. The prospect of simultaneously measuring the temperature and velocity of individual particles in flight is discussed along with the insight it provides on the formation of spray coatings.

In this paper, we will discuss how to incorporate several of the QS-9000 quality system requirements into an existing ISO 9000 quality system. This expansion of the existing quality system will be cost effective and ensure that the continuous improvement program will meet its goals. The QS-9000 quality system was developed in 1994 by the "Big Three" automotive and truck producers in Detroit, Michigan. This document, originated in the United States, is based upon the International Standard: ISO 9001-94. It is mandatory that any supplier of goods and/or services to either Chrysler, Ford or General Motors must have their quality system registered to the QS-9000 standard. We believe that there can be benefits to non-automotive organizations if they will incorporate some of the QS-9000 clauses into their current ISO 9000 quality systems. We will discuss actual cases in the following areas: purchasing, safety, process control, training, business management and continuous improvement. We will show how these examples of quality system enhancement are both effective and efficient ways of driving cost reduction and process improvement programs. Cost of quality initiatives can be better managed.

Although high-velocity oxyfuel (HVOF) spray coating is a relatively new thermal spraying process, interest is growing rapidly along with the pace of development in areas such as torch design, powder quality, and modelling. The gases used in HVOF spraying are also important because they directly influence the state of the particle striking the substrate. This presentation reviews the HVOF combustion process with an emphasis on the gases used and their influence on coating quality.

Deformation stages and specific features were studied by mesomechanical methods under tension of plasma sprayed coating-steel matrix composites. The effect of coating quality on the deformation of the matrix at the mesolevel was revealed. The results obtained make it possible to predict reliability and durability of this kind of coating and of the entire composite.

This paper discusses the basic design and operating characteristics of a precision heavy-duty thermal spray turntable. The system incorporates eccentricity and speed control and accommodates working pieces up to 100 mm in diameter and 2000 mm in height weighing as much as 1500 kg. Site tests, as reported in the paper, indicate that the turntable also meets dust and heat tolerance requirements.