Today, powder particles diameter used for thermal spraying is generally comprised between 5 and 100µm with a preferred range around 40µm for APS applications. Actually, the future trends in plasma spraying are directed to the use of fine or ultrafine powders and the reduction of the steps between raw materials and coatings. So, the present paper investigates the way to use directly spray dried ceramic powders in suppressing the sintering stage. AI2O3 based powders were obtained by the spray drying process. By optimizing the parameters (slurry composition and injection as well as drying characteristics), a narrow grain size distribution was achieved. Chemical composition and shape of synthesized powders were analyzed by scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The crystallographic structure was identified by X-ray diffraction (XRD). Demonstration was made that it is possible to obtain coatings using directly spray dried ceramic powders. The plasma spray process parameters (such as current intensity, gas flow rate, powder feed rate and injection mode, cooling stage,...) have to be managed to achieve cohesive coatings. The structure and chemical composition of these coatings were studied. In this way, the direct use of spray dried powders appears as a promising way to realize ceramic coatings.

In general, thermal spraying involves high temperatures that can be deleterious for the microstructure and deformation of the substrate. As a consequence, the use of a cooling system during spraying is often necessary. Meanwhile, in some cases, a too low surface temperature can induce a loss of properties, in particular concerning adherence and coating density. Therefore, it would be sometimes interesting to combine pre-heating and cooling stages with the plasma spray. A specific process, named HeatCool, was developed and patented to ensure a precise control of the temperature at the spraying location. The present work was focused on the study of the influence of pre-heating and cryogenic cooling conditions on the microstructure and mechanical characteristics of NiCrFeBSi self-fluxing alloy deposited by d.c. plasma spray technique. Firstly, a comparison between air and CO2 cooling was conducted to assess the efficiency corresponding to the specific use of cryogenic CO2. The main characteristics studied were the microhardness, roughness, porosity, mechanical deformations, morphology and crystallographic structures. Optimising the cooling methods and conditions combined with the process parameters improved microhardness of the plasma sprayed metal alloy and induced lower strain deformation of the substrate. Secondly, the pre-heating system was added to the device and the HeatCool process was evaluated. The process was demonstrated to be an efficient mean to enhance the structural and mechanical characteristics of coatings made of self-fluxing alloy.

The quality of plasma sprayed coatings depends strongly on substrate surface preparation, especially roughness, grit residue, and oxidation stage; particle spray jet position and size relative to the plasma jet; impacting particle distribution; particle velocity, temperature, and size prior to impact; substrate temperature; and pass thickness. A simple and low-cost spray and deposit control system developed in our laboratory allows to monitor on-line the position, shape, and centroid of the hot particle spray jet. Such a tool has proved to be very sensitive to any drift in powder injection conditions and torch input parameters. Although it gives no direct information on particle velocity and temperature, this system can be easily implemented in an industrial environment and help to maintain constant the particle parameters during spraying. A CCD camera is used in conjunction with a pyrometer making it possible to measure simultaneously substrate temperature. The system can monitor coating parameters such as deposition efficiency and residual stresses. This paper describes how the system can be used to set the tolerance range of process input parameters to obtain coating parameters within given specifications.

Water turbine parts damaged by cavitation erosion (CE) and/or slurry erosion (SE) may cause excessive operational costs for plants worldwide. The damages can be reduced by choosing more resistant materials and right technology in the first-production or at repair and overhaul. Thermal spray technologies have a great potential in the field of repairing works. Thick multilayered coatings deposited by wire electric arc spraying (WAS) has been developed and applied as CE and SE protection at the repair of stationary Francis turbine blades. Repair technology by WAS was performed on large eroded areas (1-3 m 3 ) of preguide blades of Francis turbine: 1) local damaged depths 30-35 mm maximum were repaired by sprayed materials, 2) subsequently wire arc spraying of functional coating was applied. Three types of functional coatings with total thickness 10 mm a) duplex high - Cr stainless steel with NiAl bond coat, b) graded NiAl - Cr stainless steel coatings, and c) multilayered graded NiAl - Cr stainless steel coatings were compared by means of stress measurements and structural analysis. The coating structure influences very strongly the residual stress level and adhesive-cohesive strength. Multilayered graded NiAl - Cr stainless steel coatings showed the best results and were sprayed on water turbine blades in 4 Czech water power station during regular cut-off repair periods. After 30 - 36 months' continuous operation, Francis turbine blades repaired by WAS technology show better behaviour in comparison with original material from the point of wear resistance, reliability, cost-effect and life-time.

Thermally sprayed coatings are applied to various substrates for the purpose of providing a wear-resistant, corrosion-resistant, or heat barrier surface. The materials used to coat a substrate will vary in hardness with respect to each other and with respect to the substrate. Denser coatings produce more reliable hardness data than less dense coatings due to the absence of pores and other inconsistencies in the coating. Variations in coatings will cause preparation problems such as pullout, edge rounding, or coating-to-substrate relief which effects the reliability of hardness determinations. The repeatability of hardness data is directly related to the density of the coating. This paper deals with the effect of coating characteristics (density) on the reliability of manual and automated hardness measurements.

In this investigation reactive plasma spraying was used to produce wear resistant coatings of Ti-carbides/titanium or Ti-nitrides/titanium composites. Ti-powders with different powder size distributions were used as raw materials. Methane and nitrogen were used as reactive gases to form carbides and nitrides. A reactor was adapted to the plasma gun F4 of a Sulzer Metco vacuum plasma spraying equipment to increase the degree of the expected reactions. Phase analysis and micrography of the coatings reveal that the Ti-hardphases were synthesized during spraying and embedded in the titanium matrix. The in situ synthesized hardphases show different forms and sizes. Most of them are non-stoichiometry. Compared to the titanium coating the coatings produced by reactive plasma spraying are much harder and more resistant against both sliding and abrasive wear.

An overview on potential non-destructive testing (NDT) methods for the inspection of thermally sprayed coatings in production lines is given. The suitability of several NDT techniques has been investigated with view on their sensitivity for the detection of defect sizes possibly being critical for Thermal Spray coatings. Eventually, a method is presented that shows qualification for detection of subsurface flaws in coatings such as delaminations and cracks and offers flaw distinction by image analysis procedures. A storage and archiving of test results is possible thus completing full quality control and meeting possible warranty claims. The major advantage of the NDT process is its very high detection speed which guarantees high productivity even for high volume production systems. Thus a full-area detection of large coated components becomes possible. The process is not restricted to plain components, but is usable for the detection of internally coated cylinders as well.

Thermal barrier coatings with a zirconia top coating and a NiCoCrAlY bond coating were plasma sprayed onto a nickelbase alloy. The pre-heating of the bond coated substrates and the cooling during the top coating spraying were varied to produce five different spray sets. A finite element model was developed to predict the heat transfer and the resulting thermal stresses during the spraying. A layer removal technique was used to measure the residual stresses in the as-sprayed samples. The measurements revealed low residual stresses in the top coatings and tensile stresses in the order of 150 MPa in the bond coating. A correlation between the measured top coating residual stresses and the substrate temperature in the end of the top coating spraying was found. In general, good agreement between modelled and measured residual stresses was found. The top coatings were found to contain vertical microcracks and the densities of the cracks were point-counted in the spray sets. A slight increase in microcrack densities was found as the spraying was performed onto a colder substrate. The densities of vertical microcracks were correlated to modelled in-elastic strain in the top coatings.

Nondestructive methods previously developed for the detection of flaws, such as delaminations, or for the evaluation of the elastic properties of materials, could be judiciously adapted in order to get a better assessment of plasma-sprayed coatings. Laser ultrasonics is a nondestructive evaluation method which relies on both optics and ultrasonics. A short-pulse laser generates an ultrasonic wave into the material to be inspected, and a long-pulse laser, coupled to an interferometer, detects the resulting ultrasonic displacement. Laser ultrasonics is a remote sensing method and therefore could be used for the monitoring of hot plasma-sprayed coatings during the deposition process. In this work, experiments were performed on samples composed of ZrO 2 sprayed under different conditions onto thick copper substrates. The samples were first probed by conventional ultrasonic transducers and then by a non contact laser-ultrasonic scanning system. The two series of measurements agreed well. These experiments showed that, after calibration, the coating thickness could be measured during the deposition process, with a relatively good accuracy, by laser ultrasonics. The laser-ultrasonic scanning system also revealed non uniformity of more than 10% in the coating thickness of the tested samples. This thickness variation is possibly caused by a temperature gradient induced in the coatings during spraying.