Thermal spraying enables a fast and effective way to additively deposit various ceramics as electric insulators, which are used in conditions where polymers are not suitable. Alumina (Al 2 O 3 ) is among the most widely employed materials in the coating industry because it exhibits good dielectric properties, high hardness, and high melting point, while still being cost-effective. Various parameters (e.g., feedstock type, plasma gas mixture, plasma power) significantly influence the resulting coating in terms of microstructure, porosity, crystallinity, and degree of unmolten and molten particles. As a consequence, these parameters need to be investigated to estimate their impact on the electrical insulating properties of thermally sprayed alumina. This study focuses on the development of a novel electric insulation coating from Al 2 O 3 feedstock powders deposited via atmospheric plasma spray (APS). The microstructure, porosity, and corresponding crystallographic phases have been analyzed with optical microscopy, XRD, and SEM images. To achieve an understanding of the parameters influencing the electrical insulation performance of the manufactured coatings, an in-depth analysis of the fundamental dielectric parameters (e.g., DC resistance, breakdown strength, dielectric loss tangent, and permittivity) is presented.

Many industrial applications require efficient stripping methods for thermal spray coatings, especially when cost-intensive substrate materials are utilized. A typical example is turbine components such as rotor blades with thermal barrier coatings (plasma-sprayed YSZ) in combination with nickel-based bond coatings. Unfortunately, the conventional stripping methods are dirty (contamination with grid blasting material), environmentally harmful (aggressive fluids, strong acid), or relatively ineffective (high tool load in mechanical milling). In order to avoid these disadvantages, alternative techniques such as removal by laser or by water jet stripping and hybrid methods, such as ultrasonic-based mechanical methods and a combination of water, laser, and cryo-techniques have been investigated. In this paper, newly developed stripping methods are compared to classic techniques. The evaluation considers user-friendliness, technological performance such as substrate-interaction, material emission, and economic considerations.

Hot section components of stationary gas turbines such as turbine blades are coated with thermal barrier coatings (TBCs) to increase the high thermal strain tolerance thereby the improvement of the performance for the gas turbines. TBCs represent high-performance ceramics and are mostly composed of yttria-stabilized zirconia (YSZ) in order to fulfil the function of thermal insulation. The microstructure of conventional TBCs should be porous to decrease heat conduction. Besides porous TBCs, the recently developed vertically segmented thermal barrier coatings (s-TBCs) feature outstanding thermal durability. In this work, process parameter development for atmospheric plasma spraying (APS) of s-TBCs is presented. Within the experiments, relevant process parameters such as powder feed rate, surface speed and pathway strategy have been optimized. The aim of this work is to achieve a combination of low internal residual stress and high adhesive tensile strength for s-TBCs. For the formation of vertical cracks, the heat input into the powder feedstock material and the substrate must be controlled precisely.

The process properties for DC plasma spraying are affected by the arc as the source of energy for plasma generation. For instance, the position of the anode attachment point, the arc movement and the arc formation are significantly influencing process stability, reproducibility and coating quality. Following a qualitative and objective assessment of the complete arc movement and plasma generation is leading to an improved process characterization. Therefore, a preventive arc-based analytical method for DC plasma spraying is developed. Based on a representative data volume and realized by an automatic analysis of high-velocity recordings of the arc dynamics and correspondent arc voltage measurements, the evaluations are carried out. A developed software algorithm automatically detects for all images of the video sequence (at least 3200 images) the anode center axis, the arc orientation and the position of anode attachment point on the anode surface referring to the anode center, with simultaneous compensation of aberrations. This allows an objective assessment of the complete arc movement. In further investigations, the detection limit of the developed measuring system is determined and the effects on the arc behavior and the coating process could be quantified. Thus, the developed automated analysis of the arc dynamics in the DC plasma generator corresponds to an arc-based process characterization in DC plasma spraying process with relevance to developers (e.g. new anode nozzle designs) and end users (process control).

This work provides a new in-situ measurement method for the analysis of the spray-spot geometry and the thermal properties of the coating. The new approach is based on infrared detection of the thermal radiation from the coating surface combined with a subsequent automated spray-spot characterization. With this method it is possible to describe the geometry, the axis-position of the torch, the powder injection properties, and the temperature distribution in of the spray-spot. Especially for the automated production in high quantity the spray-spot analysis is a useful assistance for the operator because the detector reacts very sensitive on small changes of the process conditions. With regard on important fields of application (e.g., gas turbine production) the sensor is suitable to detect drifting spray system parameters. Also, the progression of wear at the nozzle, injector and electrode can easily be estimated. In recent research the in-situ spray spot analysis is being developed further for the characterization of multipair electrode plasma generators.

The aim of this work is to optimize molybdenum disilicide coatings for high-temperature oxidation protection of metallic surfaces. Agglomerated and sintered MoSi2 powder was deposited on test substrates by atmospheric plasma spraying. The powders and coatings were characterized by means of optical and scanning electron microscopy. Various tests were carried out to determine the influence of powder size and spray parameters on coating porosity, hardness, and adhesive pull strength.

Due to its low weight and excellent dimensional stability, carbon fibre-reinforced carbon (C/C) gains more and more importance as construction material for light-weight charging racks in industrial furnaces. However, for high-temperature applications above 1,000 °C, C/C has to be protected with a diffusion-inhibiting coating in order to avoid an undesired carburization of components which are in contact with the charging rack. In the present work, coatings were produced by means of atmospheric plasma spraying (APS) and powder flame spraying (PFS). The ceramic powders Al 2 O 3 , Al 2 O 3 /Cr 2 O 3 , Al 2 O 3 /TiO 2 and yttria stabilized zirconia (YSZ) were used as coating materials, while molybdenum and silicon carbide served as adhesion-promoting intermediate layers. In order to reduce the residual stresses in the ceramic coatings, specimens with a defined segmented surface structure were compared with conventional closed coatings. Long-term high-temperature tests in several atmospheres were conducted on laboratory scale as well as in industrial practice.

Cermets like WC/Co or Cr 3 C 2 /Ni20Cr are well-established materials for thermally sprayed wear protection coatings. However, their high price and the adverse health effects of nickel and cobalt cause the motivation for the development of novel materials with excellent wear resistance. Within the AiF/DFG research cluster “Thermal Spraying”, a multi-institutional cooperation of various German research centres, the focus is put on particle-reinforced iron-based composite alloys. High-alloyed steels serve as matrix materials into which hard CrB 2 particles are incorporated by means of high-energy ball milling (HEM). By adjusting appropriate milling parameters, the microstructure of the powder and its level of amorphisation can be influenced effectively. The high-velocity oxygen fuel process (HVOF) allows a transfer of the desired nanocrystalline structure from the particles to the coatings. The deposited coatings exhibit low porosity and high microhardness values of more than 1000 HV0.3. The wear resistance of the coatings was determined by means of Miller test (ASTM G75-01) and compared with conventional wear protection materials and coatings produced with agglomerated and sintered powders. The obtained outstanding results qualify particle-reinforced iron-based materials as a promising alternative for a wide range of applications.

Thermal spray coatings of austenitic materials are mainly used under corrosive conditions. The relatively poor wear resistance strongly limits their use. A selective enrichment of the surface layer region with carbon by means of thermochemical heat treatment improves the residual stresses and increases the wear resistance. The interstitial deposition of carbon causes strong compressive residual stresses and a high surface hardness. The low process temperature of the thermochemical heat treatment avoids the precipitation of chromium carbide, whereas the corrosion resistance is not affected. Increases in the service life of existing applications or new material combinations with face-centred cubic friction partners are possible. In the absence of dimensional change, uniform as well as partial carbon enrichment of the thermal spray coating is possible. In comparative studies between carburized and untreated thermal spray coatings, the influence of the carbon enrichment on the coating properties and the microstructure was investigated. Carburized coatings demonstrate a significant improvement in adhesive wear resistance and an extremely high surface hardness. The cross section micrograph of the carburized coating shows the S-phase formation in the surface layer region. The depth profile of the carbon concentration was determined by GDOS analysis.

This paper presents a new strategy for improving the quality of HVOF sprayed coatings as well as the deposition efficiency of the process. The highly turbulent expanding gas jet is stabilized and focused with the aid of a helical gas shroud. The effect of the design modification is demonstrated by numerical calculations and through the use of a prototype torch.

Well-adjusted processing conditions are the basics for a high quality of thermal sprayed coatings. For HVOF spraying that means super sonic particle velocities and moderate particle temperatures. To fulfil these requirements, a certain speed of the gas jet is necessary. If low fuel and gas consumption is aspired, this can only be realised with adapted nozzle designs. Some efficient ways of nozzle design optimisation are shown in this paper. Novel methods and strategies in computed fluid dynamics are explained and correlated to experimental results. For this purpose, gas jet and particle velocities are investigated by means of laser optical measurements.

The thermal spray industry requires universal and economical HVOF systems for the production of high-quality coatings with high deposition efficiencies. In the last years classic HVOF guns have been adapted insufficiently to these requirements. This paper shows how modern numeric simulation and new inventions in gas safety engineering enable the development of a spray system for powder and wire feedstock that is optimized especially for the needs of the market. The new IBEDA TopGunAirJet is equipped with an air-cooled ‘de Laval’-like nozzle. The optimized expansion of the gas leads to high gas and particle velocities as well as to moderate flame and particle temperatures. Advantages of the TopGunAirJet are the achievable high coating quality, the low energy consumption (propane, ethene), the utilization of powder or wire and the efficient air cooling of the thermally loaded nozzle parts. Additionally, as a result of the axial powder injection, the free jet divergence and the nozzle wear are minimized. By varying the powder gas flow, the dwell time of the particles inside the flame can be influenced in order to avoid fusing of hard phases. As an example, WC-Co 88-12 coatings are presented and compared to coatings sprayed with a standard HVOF system of the 3rd generation. Different 316L coatings are produced and tested in comparison to standard HVCW coatings to show the workability of solid and cored wires. Finally, extensive LDA measurements are used for in-flight particle analysis, and investigations concerning the achievable deposition efficiencies are carried out.

Though wire flame spraying is a relatively old thermal spray process, modern equipment permits production of high quality coatings featuring outstanding homogeneity, high density and low roughness due to increased particle velocities as a result of increased combustion gas velocity. Typically spray particles are accelerated to velocities exceeding 250 m/s, if the wires are atomized adequately. In order to make a wide spectrum of coating materials available for wire flame spraying use of cored wires needs to be considered. A high speed camera is used to determine the particle velocity depending on process conditions for massive, grooved cored and tube cored AISI 316L wires. Thereby the influence of the wire design without simultaneous influence by the chemical composition is studied. Additionally nickel based carbide reinforced coatings are sprayed and characterized concerning their microstructure and properties in use.

For constant conditions concerning substrate state, feedstock and environment properties of thermal spray coatings depend only on temperature and velocity of particles at impact on the substrate. Two different HVOF spraying guns, the kerosene fuel system Tafa JP5000 with radial powder injection and the ethylene fuel system Sulzer Metco Diamond Jet Hybrid 2700 with axial powder feeding, are characterized concerning the evolution of space resolved velocity and density of particles by LDA. Also influence of process parameter variations is examined. The region of shock diamonds is studied specifically. The influence of different characteristics concerning impact velocity and trajectories on the coatings microstructure is determined by means of optical microscopy and microhardness testing.

There are constantly increasing requirements concerning light-weight concepts in automotive design due to energy saving demands. Additionally reduction of component weight is desired for demountable parts to permit easier handling. One innovative development example in this field is a draw bar consisting of precipitation hardened aluminum featuring significant weight reduction compared to presently applied steel draw bars. However, low wear resistance of aluminum alloys makes sophisticated addition of wear resistant parts in the area of the positive fit necessary. This leads to increased machining time, further costs for purchasing and machining of adapted additional components and further assembling steps. A study on the capability of thermal spray coating deposition in the positive fit area for substitution of the sophisticated wear protection system is carried out. Different HVOF coating systems with variable thicknesses are tested concerning their capability to withstand bending stress. The optimum combination of Ni20Cr bond coats and Cr 3 C 2 - 25 Ni20Cr top coats is applied for prototype production. The prototypes are tested concerning their performance under dynamical load and in corrosion tests. All produced prototypes pass both mechanical and corrosion tests. Prototypes, on which arc-sprayed Ni20Cr coatings are deposited as a low-cost solution, are still in tests.

HVOF sprayed cermet coatings are used for wear protection in numerous application fields. Most feedstock powders can be processed successfully with different spraying guns but optimized parameters for high quality coating production cannot be transferred directly. Besides substrate temperature only particle velocity and temperature inflight and at impact on the substrate determine the resultant coating properties. Two typical HVOF powders, WC/Co 88/12 and Cr 3 C 2 /Ni20Cr 75/25, are sprayed using the kerosene fuel system GTV K2 and particle parameters are recorded just prior to impact on the substrate with the passive diagnostic tool Spray Watch. Detailed correlation of particle parameters and the coating properties porosity and micro hardness as well as deposition efficiency is worked out in order to deduce particle parameter ranges providing coatings with defined optimum properties. The transferability to another kerosene fuel HVOF spraying gun, Tafa JP5000, is tested. It is also shown that effects like wear of the nozzle can be detected by use of the Spray Watch. Secondary correlation of particle parameters and machine settings is carried out to build a base for straight forward closed-loop control of the HVOF spraying process applying process diagnostics