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.

Ceramic oxides can be deposited by the High Frequency Pulse Detonation process leading to coatings with unique properties as result of simultaneous melting and high velocities of the sprayed particles. In this paper, several Al 2 O 3 based powders have been HFPD sprayed and the resulting coatings characterized. For this purpose, microstructural evaluation, XRD phase analysis and functional behavior (dielectric strength and wear resistance) have been tested.

A combination of pulsed combustion and advanced thermal spray technology has led to the development of the novel High Frequency Pulse Detonation (HFPD) spray technology by Aerostar Coatings. In the HFPD process, the flow of gaseous products from cycled explosions in the spray gun, is used to accelerate and heat the spray particles. This paper provides a fundamental description of this cyclical process, with the main differences to spray processes with stationary flow and also to traditional detonation spraying techniques in the center. Paper includes a German-language abstract.

The High Frequency Pulse Detonation (HFPD) thermal spray process represents a novel cost-effective alternative for the production of premium quality coatings. In this paper, the HFPD spray technique is characterized by very flexible capabilities which allow to deposit a wide range of materials according to different specifications. The spray parameters are carefully selected and adapted to the respective material or specification. The qualitative effect of each parameter on the coating properties and the reliability of the system are checked. Examples of different coatings are presented. Paper includes a German-language abstract.