The quality of a plasma sprayed coating is influenced by the plasma jet stability; entrainment of cold air through large scale turbulence can lead to variations in particle heating and trajectories resulting in increased unmelt densities, reduced deposition efficiencies, and oxidation of metal particles. The jet instabilities are in part caused by the swirl flow of the plasma gas. With two modifications to an atmospheric pressure plasma spray torch, we have investigated the influence of reduced swirl flow on jet stability, particle trajectories, and coating quality. The modifications are (1) addition of a shroud consisting of a porous ring surrounding the anode nozzle while simultaneously injecting part of the shroud gas inside the nozzle with a swirl component in the direction opposing the plasma gas vortex, and (2) an injector ring with which part of the plasma gas is injected radially and part tangentially producing reduced vortex flow for the same plasma gas flow rate. Jet stability and particle trajectories are determined using a LaserStrobe system combined with image analysis, and coatings have been evaluated by determining porosity and unmelt density. Results indicate that deposition efficiency is most affected by reduced vortex flow, while the shroud addition reduces unmelt density and porosity.

In Cold Spraying, bonding occurs when the impact velocities of particles exceed a critical value. This critical velocity depends not only on the type of spray material, but also on the powder quality, particle size and the particle impact temperature. For metallic materials, the critical velocity is in the range of 200 – 1200 m/s. In analogy with explosive welding, bonding in Cold Spraying is associated with adiabatic shear instabilities caused by high strain rate deformation during impact. Numerical and experimental methods are developed to investigate the influence of impact conditions and related phenomena on the coating quality. For a deeper understanding of impact phenomena and coating formation, the particle impact was modelled by using the finite element software ABAQUS/Explicit. The numerical analyses indicate shear instabilities localized to the particle surfaces, and thus provide a basis for the calculation of critical velocity in terms of materials properties and process parameters. In addition, modelling is used to obtain information about the effect of process parameters on the bonding quality. For most materials, high-strain-rate data are not available. For a quantitative analysis, therefore, the respective materials behaviour was investigated through individual spraying experiments, which were complemented by additional relevant experiments such as impact tests or explosive powder compaction. In this way, impact dynamics, bonding mechanism and critical velocities could be linked. This type of analysis was proved as a powerful tool to reduce the number of experiments for the optimisation of coating quality in Cold Spraying and also to provide a broader overview of the process.

The size and shape of individual particles are essential characteristics to control their behavior during spraying. Thanks to recently available technology in powder image analysis, this paper investigates the correlation between the geometric properties of several thousands of particles and their flowability (Hall flowmeter) and apparent density (Scott volumeter). Four different chromium carbide Cr 3 C 2 -NiCr 75/25 powders used for HVOF wear / oxidation resistant coatings are studied. The coating quality obtained for each powder by the HVOF spraying process is further investigated using metallographic image analysis.

To obtain the desired functionality and provide sustainability for steel structures, costly maintain and repair is needed. For use in particularly critical environments thermally sprayed coatings based on zinc/ aluminum in combination with a sealing supply many advantages. The subject of the ongoing research project is to deepen the knowledge about the influence of the thermal spray process, the spraying parameters, the manufacturing and especially the sealing process on the durability of sealed ZnAl-corrosion protective coatings. Verification is provided by state of the art testing methods to evaluate the coatings quality. The overall objective of the study is to provide recommendations on approaches and boundary conditions for manufacturing and sealing of thermally sprayed coatings for corrosion protection.

Thermal spray technology became established in various parts of industry (e.g. aircraft industry, medical industry etc.). Some of the applications are highly sensitive, therefore the coating quality plays an important role. Most of the common quality control methods are based on destructive testing methods which are undesirable for economical reasons. In recent years the interest in non-destructive online measurement methods increased and still is growing. More detailed knowledge of the relationship between process parameters can help to improve the coating quality standards and to understand different phenomena in thermal spraying. This will make online process control possible and improves the acceptance of thermal spray technology. In this paper results of fundamental studies on the atmospheric plasma spraying process (APS) are presented. Al 2 O 3 and NiCr powder were sprayed as model materials. In the experiments modem on-line monitoring systems were used to investigate the entire process from gun parameters to the coating quality. The faces in this paper are set on investigations of the in-flight properties of spray particles with the particle sensor DPV 2000 and their relationship with resulting coating properties. A method will be presented to extract characteristic values (velocity and temperature) out of the spray process with DPV 2000. With this method the APS-process was monitored and process parameters were correlated with main coating properties.

In this paper, the quality control in plasma spraying of chromium oxide ceramic coating on large area substrate is studied. This paper consists of three parts. Firstly, basic technical parameters are studied, for example, electric current, voltage, gas, powder, and spray distance. Secondly, the effect of dust from the spraying process on the coating quality is studied. Finally, the character of chromium oxide coating is researched. The results show that the dust from the spraying process greatly affects the coating quality. The effect of the following variables is examined: spray parameters, the powder size and distribution, and the substrate size. We have designed a set equipment to control the coating quality and the result was excellent. Now the equipment has been used in the production of anilox rollers and the patent has been applied. Meanwhile, we have devised a standard to determine the quality when spraying ceramic coating on large area substrate. The method has proved useful and efficient. Abstract only; no full-text paper available.

Industrial robots are widely used in thermal spraying operations in order to assure process accuracy and reproducibility, as well as to increase worker´s safety. Robot path planning for the coating of free form geometries keeping constant the kinematic parameters can lead to complex robot paths and in some cases to unreachable robot configurations. An external rotational axis in coordinated movement with the robot can be used to manipulate complex shaped components during the spraying operation and to simplify the robot movement. Moreover, the robot kinematics, which define the spraying path, spray angle, spray distance and spray velocity, are key parameters to control the heat and mass transfer to the substrate during the coating deposition, and they influence therefore the development of residual stresses. The present work describes the off-line planning of robot trajectories for thermal spraying operations using an external rotational axis taking into consideration the heat transfer to the component.

There are different concepts for thermal barrier coating (TBC) application that vary in the method of applying the coating layers. An analysis of the existing part portfolio sprayed with different coating concepts shows that there is a 75% difference in spray performance, depending on which concept is used. Optimizing performance can significantly reduce costs and time. This paper shows the effective use of offline programming tools in combination with a detailed coating process analysis to develop a time-optimized coating concept for vanes and blades. The validation of this optimized coating concept shows an improvement of more than 75% in spray performance leading to a coating time improvement of up to 40%.

The shielding controlled plasma spraying process is investigated to improve corrosion resistance of the metal surface. In this process, a shielding nozzle that covers just the spraying area is attached in front of the tip of a commercial plasma spray gun nozzle, and the environment surrounding the plasma jet is controlled by nitrogen flow. As the oxygen concentration in the shielding nozzle is maintained as low as 0.5%, the metal oxide contents in volume of CoNiCrAlY coating and the porosity of the coating reduced to 0.2% and 0.3% respectively under optimal spray particle size. The corrosion potential of CoNiCrAlY coating sprayed by this process in an acid solution including chloride ions is staying about -150 mV for 1000 hours, and no rust is observed during this test. On the other hand, that of the coating sprayed by atmospheric plasma spraying process changes from about -300 mV to about -500mV for 1000 hours, and the rust comes to the surface of the coating after 10 hours. Therefore the developed shielding controlled plasma spraying process is concluded to improve the corrosion of the metal.

Laser-induced surface acoustic wave spectroscopy (LISAWS) allows quick and non-destructive access to elastic properties such as the Young's Modulus of coatings, surfaces and surface-near bulk materials. Furthermore, the mechanical weakening due to cracks and pores can be evaluated, as they influence the propagation of surface waves as well. Therefore, the method is a quick and powerful tool for surface characterization and can be found today in research and development, quality control and as a precise reference method. The short measuring time of the LISAWS measurement allow the distribution of the effective Young's modulus over the coated surface to be determined with a high accuracy. For this purpose, a LISAWS measurement system was automated to allow for processing of a larger amount of samples and fast mappings. The investigated coating materials were thermally sprayed Al 2 O 3 insulation coatings and WC-reinforced 316L steel coatings on brake discs produced by laser cladding. For the Al2O3 coatings, the correlation of the Young's modulus and its areal distribution is shown for different process parameters, such as spray gun movement direction or spraying distance, and compared with results from pull-off tests. For the WC/316L coated brake discs, the distribution of the wave velocity over the coated surfaces or the two coated sides of different discs with varying coating qualities is used to assess the coating quality and homogeneity.

The aim of this study is to determine how feedstock shape affects deposition efficiency and coating formation in cold spraying. Spherical and irregular-shaped Cu powders were sprayed on Al substrates at gas temperatures ranging from 100 to 650 °C. Rebound particles were collected and examined and various coating qualities were assessed, including deposition efficiency, hardness, stack morphology, and adhesion strength. In order to explain certain discrepancies between predicted and measured data, as-sprayed and heat-treated coatings were compared and powder hardness was analyzed in detail. Contrary to simple theoretical hydrodynamic simulations, the critical threshold velocity for irregular particles was higher than that of the spherical ones. The results and practical implications of the study are discussed in the paper.

In order to produce plasma sprayed coatings with best economical, technical and environmental qualities several factors need to be taken into consideration. These include properties of the powder feedstock, equipment reliability, optimized process parameters and stability of the spray process. In case of plasma spray guns with conventional and frequently used radial powder feeding configurations, proper and constant powder feeding during spraying plays an important role. Once optimized spray parameters need to be preserved and optimal spray conditions for best coating quality and highest deposition efficiency should also be kept constant during possible changes of the spray parameters, e.g. due to electrode wear, powder quality variations etc. Various spray process monitoring tools are available and useful for this purpose; especially if they are cost-effective to be adopted by industrial thermal sprayers. In the present work, a conventional 40 kW plasma spray process equipped with a typical radial single point powder feeding was studied with the robust and industrially viable “SprayWatch G” system with an on-line monitoring CCD camera. The plasma spray torch was run under slightly varying spray conditions typical to normal industrial spraying; the spray process was monitored, and various actions, e.g. powder carrier gas flow rate adjustment, were realized in order to restore the most optimum conditions. Special attention was paid on finding correlations between the temperature, brightness, shape, and direction of the spray plume and the properties, e.g. deposition efficiency and physical properties of the sprayed alumina coatings.

Direct current (DC) plasma spraying, in atmosphere (APS) or under vacuum condition (VPS), is a standard spraying technology to produce high performance coatings for important applications. Although inductive coupled (IC) plasma torches have been known since the early sixties, its use for industrial coating production remained limited due to the static gun requirement. An adequate substrate manipulation opens up the IC-spraying for its practical use. A VPS system was realized which combines IC- with DC-spraying. The potential of this new facility is highlighted by coatings of W and Yb 2 O 3 . The key parameters to achieve an optimized coating structure are explained.

The shear test in accordance with DIN EN 15.340 is a method to test the bond between substrate and coating in thermal spraying. It has been developed in order to avoid the disadvantages of the test mainly in use, the adhesion test (DIN EN 582). In quality management in a thermal spraying job shop often results of coating analyses are necessary within a very limited time frame; very important is a low number of necessary specimens and a fast and reliable test method. It is described how the shear test can be implemented into the practice of a job shop and the experiences of the OBZ innovation GMBH, which has used the shear test for more than four years, are explained.

The goal of this work is to have standardized quality control of thermal spray coatings incorporated into the Standard Practices Manuals of all aircraft engine original equipment manufacturer’s around the world. With the many manufacturers’ criteria for evaluation of thermal spray materials, laboratories are forced to have multiple criteria/systems to test and analyze coatings. As airlines/repair shops are moving towards overhaul of more variable engine types/models in their shops, the need to have a common evaluation system has been identified and is currently being addressed. A sub-committee of the European Airline Committee for Materials Technology is currently working to formulate this common system for thermal spray coatings evaluation. Participants are from GEAE, Pratt & Whitney, Rolls Royce, SNECMA and KLM Royal Dutch Airlines. The progress and goals of this group will be documented and reported.

This paper investigates the influence of nozzle design on the quality of Al 2 O 3 coatings produced by atmospheric plasma spraying. It compares the thermal and deposition efficiency of different plasma guns along with the porosity and adhesion strength of the resulting coatings. The intent of the study is the application of convergent-divergent nozzles as an economic alternative to the nozzles typically used for atmospheric plasma spraying. Paper text in German.

The development of thermal spray processes usually requires an analysis of the complex coating microstructure. In order to inspect critical areas of a coating, destructive testing methods such as the preparation of cross-sections are commonly used. In this work, the suitability of largely non-destructively measured polarisation curves for the quality assessment of thermal sprayed AISI 316L coatings is investigated. Therefore, a 3.5 % NaCl gel electrolyte was developed to prevent the corrosive medium from infiltrating the porous and micro-cracked microstructure characteristic for thermal sprayed coatings. In addition, a measuring cell based on the 3-electrode arrangement was designed to simplify the setup, reduce the measurement time and enable mobile measurements directly on the component surface at a later stage of development. The effects of process-related differences in the microstructure of HVAF and APS AISI 316L coatings on the polarisation curve was investigated by determining the corrosion current density. The microstructure of the AISI 316L coatings was analysed by optical microscope, SEM and EDS, focussing on the porosity and oxide content. The results clearly show that the potentiodynamic polarisation curves of the AISI 316L coatings differ significantly depending on the spray process used and microstructure created. Even small changes in the oxide content within a coating can be detected. Therefore, electrochemical measurement methods using gel electrolyte offer an interesting opportunity to evaluate the quality of thermal sprayed AISI 316L coatings in a largely non-destructive manner.

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.

Due to its easy handling and low operating costs, wire arc spraying has become one of the most established processes for applying protective coatings to components used in waste incineration plants. This paper discusses the development of relatively low-cost Fe-Cr-Si coating materials for incinerator applications and the corrosion and wear properties that have been achieved using conventional arc spraying methods.

Aim of this project is to produce anti-corrosion coatings for high temperature applications by wire arc spraying. It is necessary to adjust the feedstock material as well as the spray equipment in order to control the coating composition and the coating morphology. The substitution of expensive Ni- and Co-based feedstock by Fe-based alloys leads to an economic benefit. In combination with wire arc spraying, the total coating costs can be reduced significantly. There is also an enomous potential for reduction of service costs in waste incineration plants due to an enhanced coating service life and shorter downtime for the anti-corrosion coating repair.