HVOF processes represent the state of art for the spray deposition of wear and corrosion resistant coatings since their supersonic gas velocities in combination with moderate flame temperatures allow the deposition of optimal coatings with very high bond strengths, fine surface finishes and low oxide levels. However, new generation coating materials (fine powders), stringent quality requirements and the high productivity demanded by the industry, push the HVOF technologies to their limits. Recently, a novel air-oxygen controlled high velocity combustion process has been development by Tecnalia. The system operates within the supersonic regime using a broad range of fuel/oxidant ratios thanks to the use of air-oxygen mixtures and a carefully optimized gun design. Extremely low flame temperatures can be achieved while keeping a supersonic flow of combustion products, thus allowing the solid state deposition of almost all industrially relevant metal alloys with superior deposit qualities. In this work, a systematic investigation of the influence of the powder particle size and gun configuration on resulting coating microstructural features has been performed. For comparison, two fine structured commercially available WC10Co4Cr powders with different particle size distributions have been investigated. The coating structure has been characterized with by high resolution SEM cross-section imaging and X-ray diffraction analysis. Resulting coatings are characterized by highly dense structures, a high retention of the primary carbides, average microhardness of up 1885 HV0.3 and fracture toughness varying between 3 and 7 MPa.m -1/2 depending on the powder particle size distribution and the process conditions used.

An air-oxygen controlled high velocity combustion spraying process has been developed that uses a special HVOF gun and a broad range of fuel-oxidant ratios. Extremely low flame temperatures can be achieved while maintaining a supersonic flow of combustion products, thus allowing the solid state deposition of almost all industrially relevant alloys. This work deals with the development of superhard cermet coatings using conventional and fine WC-Co(Cr) powders, optimized spray parameters, and different nozzle geometries. Results are compared based on coating microhardness, toughness, and sliding wear resistance.

The Oxy-Fuel Ionization system (OFI) is a new thermal spray process which consists basically on a high velocity combustion process enhanced by a low energy plasma source. The system is characterized by its stability over a relatively large range of fuel/oxidant conditions, the possibility to use poor fuels like natural one (with low gas consumption) and the high deposition rates that can be achieved in comparison to conventional HVOF guns. The OFI gun has been designed following a modular concept, which in combination with the high flexibility of the system is expected to allow the deposition of coating materials with the most different physical and chemical natures. This work deals with the experimental analysis of the process using methane as fuel gas and its correlation with the deposition of WC-base materials. Two in-flight particle diagnostic systems were used: the Spray Watch diagnostic system (from OSEIR) and the Spray and Deposit Control (SDC) system (developed by the SPCTS laboratory of the University of Limoges). Results are presented for the most representative properties of the optimized coatings (micro hardness distributions on the coating cross section and crystallographic analysis).

The aim of this study is to model a spray process that combines aspects of plasma and HVOF spraying. The process is characterized by its stability over a broad range of fuel-oxidant conditions and ability to produce coatings using relatively little gas with rather low gross heating values The mathematical model developed accounts for the formation of the plasma jet, the combustion process, and supersonic flow issuing from the spray torch. Simulating the new process made it possible to investigate the effect of the plasma on the velocity and temperature of the gas flow inside and outside the gun. The equations were solved using CFD code and predictions were compared with experimental observations. The benefits of the plasma jet are discussed on the basis of predictions and fuel combustion mechanisms.

The high frequency pulse detonation (HFPD) process has shown to be a cost effective spray technique for the deposition of highly dense and erosion resistant YSZ ceramic coatings. In comparison to the coatings produced by conventional APS, a significant improvement of the wear performance can be achieved by the HFPD process as result of the high coating compactness. This work deals with the deposition of different ZrO 2 and Al 2 O 3 based ceramic powders by the HFPD technique, for the development of highly dense and wear resistant ceramic layers. During this development, the gun configuration and the process parameters (gas flows, explosion frequency, spray distance and cinematic conditions) have been optimised to get the best deposition performance. The resulting coatings have been characterised in terms of the microstructure, the microhardness and the sliding wear performance under dry conditions. In comparison to the plasma sprayed coatings produced with equivalent process conditions, the coatings deposited with the HFPD process are significantly harder and their sliding wear resistance is two–three-fold higher for YSZ coatings and five-six-fold higher for Al 2 O 3 coatings. Furthermore, the HFPD process is able to produce highly dense and hard functional coatings in one spray pass, suitable for wear protective applications.

Owing to gas velocities in the super-sonic regimen in combination with moderate flame temperatures, the HVOF processes are preferred for the deposition of wear and/or corrosion resistant carbides as well as Hastelloy, Triaballoy and Inconel alloys. The resulting coatings have usually very high bond strengths, fine as-sprayed surface finishes and low oxide levels. However, the generation of a supersonic flow of combustion products supposes the implementation of relatively high gas flow rates and high energetic gas mixtures, which are intrinsically associated with high production costs, limiting the application of this technology in some industrial fields. This work summarises the first results in the development of a prototype aimed to show the potential of a new thermal spray technology named Oxy-Fuel Ionisation spraying for the development of high quality carbide base coatings. The OFI process is a supersonic combustion process as well, enhanced by the addition of ionised gas specimens. The arising combustion process is characterised by its stability within a broader range of the “fuel/oxidant” correlation in comparison to conventional HVOF systems, because of the presence of ionised gas specimens which are acting as a catalyst. It has been proved that this developed prototype allows the thermal spray deposition of carbide based materials with relatively low oxygen flow rates. For comparison two different coating materials were investigated, WC-17Co and Cr 3 C 2 -NiCr. The process parameters were optimised in terms of the micro hardness, the porosity and the decarburization of the resulting coatings.

Different YSZ powders have been deposited by the high frequency pulse detonation (HFPD) thermal spray process leading to highly dense and hard coatings in comparison to standard APS zirconia layers. During this development, the gun configuration and the process parameters (gas flows, explosion frequency, spray distance and cinematic conditions) have been optimised to get the best deposition performance. In order to explore the potential of the HFPD system to process YSZ base materials, different type of commercial spray powders (agglomerated, fused & crushed, fully or partially stabilized compositions) and size distributions, have been investigated and the resulting coatings characterized. A qualitative comparison of the coating performance at elevated temperatures is presented and discussed. For this purpose, an experimental set-up was designed. Potential applications of these types of zirconia coatings in advanced thermal barrier coating systems, high temperature wear protection or ionic conductors (sensors and fuel cells) are discussed.

Structuring coating materials in the nanoscale can significantly enhance the performance of Fe-based coatings. Nanostructured steel coatings with a very high hardness, the strength of carbon-based fibers and a corrosion resistance superior to nickel-based super alloys are currently commercially available. This work deals with the development of wear resistance nanostructured coatings using a commercial steel powder by high frequency pulse detonation. Results concerning the deposition of this powder using two different high velocity oxy-fuel spray systems are presented for comparison. The process parameters were optimized on the basis of the achieved coating features hardness, thickness, porosity and surface roughness. The microstructure of the coatings achieved with the different systems as well as the resulting adhesion and cavitation erosion resistance are compared. These results are also compared with the performance of standard WC-CoCr coatings.

The technical and economical potential of innovative materials is gaining growing interest for technologically orientated companies. The use of lightweight materials is an extremely important consideration, especially when designing moving components (e. g. in aerospace, automotive or machine construction). Magnesium is a promising alternative to other lightweight materials, such as aluminum and titanium, due to its relatively high specific strength and stiffness. Further advantages are high thermal conductivity and good joining and machining capabilities. However, the use of magnesium alloys is restricted by relatively poor wear behavior and corrosion resistance. In order to overcome the limitations associated with magnesium alloys, a project was founded by the Materials Science Institute (MSI), at the RWTH Aachen, and the Institute of Materials Science (IW), at the University of Hannover, to deal with the application of wear and corrosion protective coatings on Mg alloys by means of thermal spraying. A variety of coating materials were applied on Mg substrates using several thermal spray processes (like Arc Spray and HVOF). The coatings were then characterized particularly with regard to their wear and corrosion properties. To further enhance the overall corrosion resistance two additional approaches were investigated. On the one hand various duplex coating systems were designed and applied. With the objective of decreasing the open porosity coatings were either densified by shot peening or sealed by applying organic sealers on the other hand.

The high velocity oxyfuel flame spraying (HVOF) process appears to be an interesting alternative to low pressure plasma spraying (LPPS) processes for the application of MCrAlY coatings for the use of hot corrosion protection on turbine parts like blades and vanes. Lower investment costs for HVOF facilities compared to LPPS systems combined with adequate coating properties and a stable, easy controllable process can be seen as potential advantages regarding the application of this process. Several recent HVOF systems are screened concerning the application of MCrAlY coatings for hot corrosion protective coatings on turbine blades. In this research project, the Design of Experiments (DoE) is used to built up factorial experimental designs. The aim is, besides a benchmarking, to find out the potential of the HVOF systems to produce high quality hot corrosion protective coatings. The main emphasis of these preliminary investigations is on the evaluation of bonding defects in the interface, the porosity, and the oxide content of the coatings.

This work concerns the production and use of aluminum oxide-NiCr dispersion powders for thermal spraying. It investigates the morphology and hardness of the powders and the microstructure and wear resistance of the resulting coatings. The powders are prepared by high energy ball milling and are used to produce oxide dispersion strengthened NiCr coatings via HVOF spraying. Among the key findings is that the powders are less homogeneous when milled with nanoscale aluminum oxide as are coating properties. Paper includes a German-language abstract.