Plasma spraying at very low pressure (50-200 Pa) is significantly different from atmospheric plasma conditions (APS). Applying powder feedstock it is possible to defragment the particles into very small clusters or even to evaporate the material. As a consequence, the deposition mechanisms and the resulting coating microstructures could be quite different compared to conventional APS liquid splat deposition. Thin and dense ceramic coatings as well as columnar-structured strain-tolerant coatings with low thermal conductivity can be achieved offering new possibilities for application in energy systems. To exploit the potential of such a gas phase deposition from plasma spray-based processes, the deposition mechanisms and their dependency on process conditions must be better understood. Thus, plasma conditions were investigated by optical emission spectroscopy. Coating experiments were performed, partially at extreme conditions. Based on the observed microstructures, a phenomenological model is developed to identify basic growth mechanisms.

Surface state plays an important role in particle bonding and formation of the first layer of coatings in thermal spraying. From a chemical aspect as well as a mechanical point of view, in all cases, the substrate surface needs to be optimized to promote the adhesion of the sprayed particles and then the coating. In order to control such parameters, several works have been conducted to avoid drawbacks on sensitive materials. This study aims at developing a laser surface texturation before the spraying process to improve the coating adherence. According to the laser parameters, different surface morphologies (hole diameters, surface roughness, hole depth, etc.) can be developed. The surface material morphologies were characterised by SEM and bond strength was evaluated through ASTM C633 pull tests. This approach has been applied on the system Al 2017 / NiAl and demonstrates a high influence of the laser treatment. However, the thermal effect induced during the laser-matter interaction has to be controlled to avoid negative effects of the substrate properties particularly tribological properties. In this case, a study of the effect of the conventional processes and texturing process on the fatigue properties of substrate were studied.

The application of fine powders in the thermal spray technology represents an innovative approach to apply dense and smooth near-net shape coatings on tools with complex geometry. However, this aim can only be achieved, as long as the influence of the handling parameters of the spray process such as the spray angle are sufficiently understood. In this study the influence of the spray angle on the deposition rate as well as on the coating properties (microhardness, roughness and porosity) of HVOF-sprayed, fine-structured coatings is investigated. A fine agglomerated and sintered WC-l2Co powder (agglomerate size: 2-10 pm, WC-particle size: FSS = 400 nm) was used as feedstock material. It has been shown that HVOF spraying of fine powders is less susceptible to an alteration of the spray angle than most other thermal spray processes such as plasma- or arc-spraying. The reduction of the spray angle results in a decrease of the deposition rate, while no significant degradation of the coating properties was found up to 30°. However, at spray angles below 30° the coating strength is negatively affected by the formation of pores and cracks.

With the purpose of elaborating high-quality FeAl coatings, a so-called very low pressure reactive plasma spray technique that combines VLPPS and SHS processes was used in the present study. A dense and homogeneous FeAl coating was thus successfully in situ synthesized by reactive plasma spraying of an Al/Fe 2 O 3 composite powder under 1 mbar. The phase composition and microstructural features of the coating were characterized by XRD and SEM. Results indicated that the B2 ordered FeAl phase was synthesized, and the coating featured a dense and defect-free microstructure. The fracture mechanism of the coating remains mainly a brittle failure but the appearance of some dimples in local zones suggests some unexpected toughness.

In this study, porous molybdenum (Mo) materials were prepared by flame spraying semi-molten particles to low velocity levels. The influence of spray particle state, including particle velocity and melting degree, on microstructure and porosity was investigated to understand the formation mechanism of the pore structure and connection between particles. The results showed that Mo sprayed particles at low velocities (<20 m/s) and limited semi-molten state can be generated by flame spraying. The annealed Mo deposits with the porosity ranged from 39% to 61% were deposited. High porosity in the deposit was achieved through the shielding effect of deposited particles, bonded by the settled melt in the particle/particle interface. Moreover, the porosity generally decreased with the increase of melting degree of spray particles prior to impact.

WC-Co coatings are primarily deposited by using the high velocity oxy-fuel (HVOF) spray process. However, the decomposition and decarburization of carbides during spraying result in the degradation of coating wear performance. In this paper, a novel high pressure HVOF with the characteristics of lower particle temperature and higher particle velocity was developed. It exhibits combustion chamber pressures up to 3.0 MPa. The influence of combustion chamber pressure and oxygen/fuel equivalence ratio on WC-Co particle velocity and temperature levels were analyzed by numerical simulation. The experiment results show that the combustion chamber pressure and the oxygen/fuel equivalence ratio have a significant influence on the particle velocity and melt degrees, as well as, on the coating microstructure and microhardness. High velocity WCCo particles in different states, i.e., molten, semi-molten and non-molten can be readily obtained by changing the spray conditions. A comparison to the conventional JP-5000 was also executed.

Yttrium oxide (Y 2 O 3 ) can be used in different applications such as corrosion resistance, high temperature applications and semi-conductor production equipment due to its very high thermal and chemical stability. In the current research, yttria coatings were processed using a new type of DC plasma gun consisted of molecular gases CO 2 +CH 4 . Physical and structural properties were compared with the coating made by SG-100 plasma torch. Gas mixture of CO 2 +CH 4 improves the torch efficiency due to its high thermal enthalpy and conductivity which leads to increased particle temperature and complete fusion of the sprayed particles during the process of coating. SEM study of the structure revealed that the coating has higher density and lower porosity compared to the coating produced by SG-100 torch. No unmelted particles can be observed in the coating. XRD analysis of the coating showed that the coating contains no amount of harmful metastable monoclinic phases. This all proves the better quality of the coatings deposited by CO 2 +CH 4 gas mixture in comparison to the conventional coatings.

The main goal of this work is to improve the coating properties of three-cathode atmospheric plasma sprayed coatings with respect to porosity and residual stresses. This was done by use of numerical simulation coupled with advanced diagnostic methods. A numerical model for the triple injection of alumina feedstock, as well as acceleration and heating of the powder particles in the characteristic threefold symmetrical plasma jet cross section produced by a three-cathode-plasma torch was developed. The modeling results for the standard injector’s position “0” were calculated and experimentally verified by Laser Doppler Anemometry (LDA). Based on the criteria defined for concentrated feedstock transport and homogeneous thermal treatment of powder particles in the plasma jet, the optimal injection position was found. In the next step a previously developed, coupled CFD-FEM-simulation model was used for simulations of the coating build-up, describing flattening, solidification and deformation due to shrinkage for alumina particles on a rough substrate surface.

Reactive plasma spraying (RPS) has been considered as a promising technique for in situ formation of aluminum nitride (AlN) based thick coatings. This study investigated the reactive plasma spraying of AlN coating with using Al 2 O 3 powder and N 2 /H 2 plasma. It was possible to fabricate a cubic- AlN (c-AlN) based coating. The phase composition of the coating consists of c-AlN, α-Al 2 O 3 , Al 5 O 6 N and γ-Al 2 O 3 . Understanding the nitriding process during coating deposition is essential to control the process and improve the coating quality. The nitriding process was performed by spraying, collecting the particles into a water bath (to maintain its particle features) and observing their microstructures and cross sections. During the coating process, the sprayed particles were melted, spheroidized and nitrided in the N 2 /H 2 plasma to form the cubic aluminum oxynitride (Al 5 O 6 N). The particles collided, flattened, and rapidly solidified on the substrate surface. The Al 5 O 6 N is easily transformed to c-AlN phase (same cubic symmetry) by continuous reaction through plasma environment. Improving the specific surface area by using smaller particle sizes enhances the surface nitriding reaction and improves the nitriding conversion. Furthermore, using AlN additives enhances the nitride content in the coatings. It was possible to fabricate thick and uniform coatings with high AlN content by spraying fine Al 2 O 3 /AlN mixture. Furthermore, the N 2 gas flow rate improved the nitriding conversion and the coating thickness.

New developments in the field of thermal spraying systems (increased particle velocities, enhanced process stability) are leading to improved coating properties. At the same time innovations in the field of feedstock materials are supporting this trend. The combination of modern thermal spraying systems and new material concepts has led to a renaissance of Fe-based feedstocks. Using modern APS or HVOF systems, it is now possible to compete with classical materials for wear and corrosion applications like Ni basis (e.g. NiCrBSi) or metal matrix composites (MMC, e.g. WC/Co or Cr 3 C 2 /NiCr). The work described in this paper focuses on that combination and intends to give an analysis of the in-flight particle and spray jet properties achievable with two different modern thermal spraying systems (kerosene driven HVOF system K2, 3- cathodes APS system TriplexPro-200/-210) using Fe-based powders. The velocity fields are measured with the Laser Doppler Anemometry (LDA). Additionally, resulting coatings are analyzed metallographically with regard to their properties and a correlation with the particle in-flight properties is given. The experimental work is accompanied by computational fluid dynamics (CFD) simulations of spray jet and particle velocities, leading to a comprehensive analysis and characterization of the achievable particle properties with state-of-the-art HVOF and APS systems.

A high efficiency single electrode (HESE) version of the popular TriplexPro Plasma Gun (Sulzer Metco, Westbury, NY) has been developed and evaluated. A single electrode gun has the advantage that in most cases it can be directly or simply retrofitted to any existing conventional plasma spray system. Three electrode cascade guns like the TriplexPro platform require a unique power supply and control configuration. The design of newly developed single electrode plasma gun was based upon the TriplexPro platform and retains many of the features that contribute to the guns high efficiency characteristics, in particular the cascaded arc configuration. A study was conducted to compare the performance of the newly developed HESE gun against a conventional single electrode design. Factors examined include: voltage stability; in flight particle characterization; coating properties for selected materials and spray spot size.

The basic idea when using common plasma gases for arc spraying with their varying levels of energy was first of all to achieve higher system efficiency and to produce coatings with fewer oxides. The differentiation between the open arc process and plasma spraying is the advantage using active gases with a much higher energy level, for example using fuel gases to influence the process characteristics and to get high end coatings. As these results from this work in arc spraying show, the increased need for research into this technology at the moment is definitely of importance. The main market of arc spraying can be found in the area of corrosion protection, the technology’s growth stems from those areas which are interested in manufacturing high-quality coatings with substantially more cost-effective wire. In order to broaden the range of thermal spray applications, we have endeavored to investigate the issue of the right gas mix for arc spraying. In doing so, it was concluded that, by using wire as the spray material, this cost-effective process can often be used as an equally viable alternative to other methods. The optimization of costs and tailoring of coating properties to suit specific applications are just some of these influencing variables.