X ray diffraction and transmission electron microscopy studies and measurements of hardness and void content were carried out for WC-20% Co coatings produced by detonation flame spraying at various oxygen/acetylene ratios in the detonating gas mixture. It was demonstrated that successive transition from (WC+Co) to (W 2 C+Co 3 W 3 C) to (W+CO 7 W 6 ) occurs as the oxygen content in the mixture is increased, and that amorphous-nanocrystalline structures form in the coating. Two types of these hybrid structures were revealed, one including an amorphous metallic matrix containing precipitates of intermetallic nanocrystals, the other having an amorphous oxide matrix and nanocrystalline precipitates of CO 3 O 4 and WO 3 . The hybrid structures were shown to improve coating density and hardness.

In this paper, the morphology, the microstructure, the thermal stability ranges, the devitrification kinetics, and the hardness of amorphous detonation sprayed FeCrPC coatings are examined and compared with those of amorphous tapes of similar composition. Nanocoatings were made by heat treating the amorphous coatings. It is observed that thermal spraying (using the detonation gun method) produced coatings with an improved thermal stability of the amorphous component. Paper includes a German-language abstract.

The Cold Spray coatings have been sprayed from binary eutectic alloy – Al-12%Si and from the complex composition iron-based alloy. The atomized Al-Si powder had close to microcrystalline (grain size around 1micron) structure, the Fe alloy powder had amorphous-nanocrystalline structure. Aluminum-based alloy was also sprayed with addition of up to 10% of aluminum oxide powder. The coating structure and properties have been investigated using optical microscopy, scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, microcalorimetry, microhadness, tensile, bend, erosion and abrasion tests, and corrosion polarization tests. It has been shown that the Al-Si coatings have microcrystalline structure, and the Fe-based coating have amorphous-nanocrystalline structure similar but not identical to the feedstock powders. Cold Spray process has a specific mechanism to preserve the powders metastable structures. The coatings have enhanced hardness, wear and corrosion resistance. Abstract only; no full-text paper available.

An alternative method to produce bulk nanocrystalline materials and avoid the powder compaction step is to produce amorphous material by rapid solidification followed by controlled heat treatment to introduce nanocrystalline structure. The extremely high cooling rates in plasma sprayed particles give rise to formation of nonequilibrium phases, which may become amorphous for certain materials. Five different materials studied in this work are based on near-eutectic mixtures of alumina, zirconia and silica. The powder feedstock materials have been plasma sprayed using water stabilized plasma torch (WSP) and subsequently heat-treated to prepare nanocomposite materials with varying nanocrystallite size. The as-sprayed materials have very low open porosity and are mostly amorphous. The as-sprayed amorphous materials crystallize at temperatures around 950°C with an associated volume shrinkage of 1-2%. The resulting structure is best described as nanocomposite with very small crystallites (12 nm on average) embedded in inter-crystallite network. Role of the silica compound on phase composition, microstructure, and mechanical properties of the as-sprayed and annealed materials is discussed. Elastic properties were measured for the nanocrystalline materials. The as-sprayed amorphous materials exhibit high hardness and high abrasion resistance. Both properties are significantly improved in the heat treated nanocrystalline samples.

Rietveld analysis was used to measure the phase content of twenty-four coatings sprayed with a multimodal WC-12Co feedstock using three HVOF spray guns. Different correlations were investigated involving, on the one hand, the thermal spray history (particle temperature and velocity) and the mechanical properties of the deposits (microhardness and abrasion resistance), and, on the other, the phase structure of the tested coatings. The results show an inherent effect of the spraying temperature, regardless of the HVOF process used, on the generation of sub-carbide species, mainly W 2 C, W and an amorphous-nanocrystalline phase. In particular, it was found that decarburization of the WC to produce the amorphous/nanocrystalline and tungsten phases is beneficial in enhancing the mechanical properties as long as there is no large amount of tungsten present in the coating. A linear relationship relates this phase with the presence of the amorphous-nanocrystalline phase, independently of the spraying process used in this study. It was also shown that metallic tungsten precipitated in a nanocrystalline structure only with the Diamond Jet HVOF process.

Calcium phosphate (Ca-P) coatings have been used as surface coatings on porous metallic implants in dentistry and orthopaedics for the last twenty years. These Ca-P coatings, nominally hydroxyapatite (HA), have been shown to promote bone fixation and osteconductivity on Ti and Ti alloy substrates used for those purposes. Such coatings can be formed by different methods including plasma spray. In addition to the well known advantages of the plasma spray technique to deposit coatings, a new version of this technique, i.e. solution precursor plasma spray (SPPS), has been reported to produce submicron/nanocrystalline structured coatings. Nanocrystalline HA coatings may improve the resorption of the coating in the body, avoiding the irritant effect of large particles which may be seen in current thermal sprayed HA coatings. The main purpose of this work was to study the suitability of a sol-gel Ca-P solution precursor (calcium nitrate tetrahydrate and ammonium dihydrogen phosphate) as feedstock for the air plasma spray (APS) coating technique. We report on the formation and the characteristics of the coatings so formed on Ti6Al4V substrates. The presence of different Ca-P crystalline and amorphous phases was assessed by X-ray diffraction analysis. The X-ray photoelectron spectroscopy technique was used to characterize the surface chemical composition of the Ca-P coatings. The microstructural features of the coatings were characterized by scanning/transmission electron microscopy combined with image analysis in order to evidence the presence of submicron/nanocrystalline Ca-P features. Final results are discussed in terms of the spraying parameters. Abstract only; no full-text paper available.

Chromium carbide-nickel chromium coatings produced by HVOF spraying are widely used for high temperature wear and erosion resistant applications. Examination of the literature shows that whilst the mechanical properties of these coatings have been widely investigated, there has been little research into the physical processes occurring during HVOF spraying of this system, such as carbide dissolution, liquid-metallic phase oxidation, decarburisation and rapid solidification. The purpose of the present work has been to perform a systematic characterisation of the chromium carbide-nickel chromium system in both the initial powder and as-sprayed states with a variety of spraying conditions using optical, scanning and transmission electron microscopy, electron microprobe and X-ray diffraction. The presence of amorphous and nanocrystalline phases has been demonstrated. The nanocrystalline structures tend to be Ni rich, with the amorphous phases rich in Cr. Carbides of the form Cr 3 C 2 were found to be dissolved slightly during spraying, increasing the Cr and C contents of the liquid metallic phase. There was no evidence of chromium carbide oxidation.

This work is aimed at acquiring knowledge on successive deposition of alumina and nickel coatings on a metallic substrate using an RF plasma torch. The composition and morphology of various Ni/Al 2 O 3 steam reforming catalysts, prepared by plasma spraying of precursor nickel suspensions, are studied in this work. As Ni/γ-Al 2 O 3 is a well known reforming catalyst it was decided to study the deposition of a particular catalytic formulation on a metallic substrate. The purpose of this study was to find the means of building 2D monolithic catalysts by plasma deposition. Since 2D monolithic catalysts are non-porous, their specific areas available for hosting the catalytic reactions are low. Thus, It is essential to have very good dispersion of the active catalytic sites. Such dispersion is only possible only if the deposit can take place at the nanometer scale. The suspension plasma spraying is assessed for its ability to produce such structures. This method permits the formation of nanocrystalline structures within a few seconds. The main difficulty is the control of the various spraying parameters in order to obtain satisfactory deposition. The Ni/γ-Al 2 O 3 catalysts so obtained are characterized by scanning electron microscopy, field electron microscopy and X-Ray diffraction.

Hybrid aerosol deposition (HAD) is a new coating method to deposit homogeneous nano-structured ceramic coatings. An accurate evaluation of the fabricated coating properties is required. In this study, α-Al 2 O 3 fine powder was sprayed by HAD. The obtained coatings were dense and uniform with a nanocrystalline structure. An X-ray diffraction measurement revealed that the fabricated HAD Al 2 O 3 coatings mainly consisted of α-Al 2 O 3 phase. The hardness and Young's modulus of the HAD Al 2 O 3 coatings were evaluated by a micro-Vickers method and a nanoindentation method using the Weibull distribution. The hardness of HAD Al 2 O 3 coatings measured by micro-Vickers was ~1400 HV (~15 GPa). The variation of mechanical properties of HAD coatings measured by the nanoindentation method was extremely small compared to those of plasma-sprayed coatings, which also indicates that HAD coatings contain less pores and cracks than plasma-sprayed coatings.

A new WC-WB-Co feedstock material was sprayed with two HVOF torches. During spraying, the in-flight temperature and velocity history were monitored using the DPV2000 tool. Coatings sprayed with both guns had a relatively high microhardness and good abrasion properties, even with a higher level of porosity for some coatings. This was explained by the nature of the matrix which was composed of an amorphous/nanocrystalline structure and W-Co-B phases. It is suggested that the matrix is harder than conventional binders such as cobalt, for instance, and exhibits a better cohesion with the WC hard phase.

Solution precursor plasma spraying has been used to deposit ceramic coatings with submicron/nanocrystalline structures. Previous studies revealed that the deposition mechanism in the solution precursor plasma spraying differs from that in the conventional plasma spraying. To increase the understanding of the deposition mechanism in the solution precursor plasma spraying, a numerical model is used to predict the particle conditions on the substrate. Five types of particle conditions, melted particles; small sintered particles; dry agglomerates; wet agglomerates; and wet droplet are assumed based on the computed temperature distribution of the particles. An analysis of the deposition mechanism in the solution precursor plasma spraying is performed. Experiment results s are also collected to verify the numerical prediction and the analysis of the deposition mechanisms.

A mathematical model was developed and used to design and optimize a supersonic nozzle for a cold spray system. The objective was to spray 20 micron-size aluminum particles. Conventional and agglomerated nanostructured powders were used and successfully sprayed using a radial injection port. The microstructure of the coatings revealed that the nanocrystalline structure is preserved. An increase of 100% of the coating hardness was found for nanostructured coatings compared to conventional coatings. Further work needs to be done to improve the porosity of the coatings by changing some of the process parameter.

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.

Computational metallurgy is a technique being used and developed in the field of bulk alloys to design and develop novel amorphous and nanocrystalline materials. This technology can be transitioned to develop chemistries for both wear and corrosion resistant thermal spray coatings. Using computational metallurgy and small scale laboratory experiments, nanostructured and amorphous chemistries can be designed to specifically accommodate one of the many environmental conditions challenging the oil and gas industry. This study reviews the design procedures behind developing three unique chemistries intended to function in different environments: 1) an Fe-based chemistry designed for metal to metal sliding wear resistance, 2) an Fe-based chemistry containing elevated refractory content intended specifically for spray and fuse applications to resist sulfurous corrosion, and 3) a Ni-based chemistry similar to Alloy C276 for high temperature corrosion resistance. All three alloys were designed using computational techniques and eventually manufactured into cored wires for use within the twin wire arc spray (TWAS) process. The fine grained structure provides unique benefits to each application including 1) high hardness, 2) ability to rapidly form protective scale, 3) low melting temperature and creep resistance.

Iron-based wire feedstocks represent a technical as well as economical alternative to carbide reinforced feedstocks for wear protection applications. To assess the potential of such feedstocks, iron-based cored wires were developed with up to 6 wt% boron. The feedstocks were deposited by electric arc spraying, forming hard, partially amorphous coatings with embedded nanocrystalline boride precipitations. To further improve wear resistance, chromium carbide was blended into the powder filling in some wires. Coatings produced from all feedstocks were evaluated by means of optical microscopy, X-ray diffraction, and microhardness measurements.

Ultra fine grain Fe-Si based coatings were synthesised by HVOF thermal spraying of nanostructured powders obtained from mechanical milling. Magnetic measurements revealed a soft magnetic character for all the coatings. Additions of boron, niobium and copper were investigated. The thermal stability and the evolution of the coercivity with temperature were observed to be remarkable.

The present paper describes the synthesis of nanocrystalline 316-stainless steel coatings by high velocity oxy-fuel (HVOF) thermal spraying. The feedstock powders were synthesized by mechanical milling to produce flake-shaped agglomerates with an average grain size of less than 100 nm. The powders were introduced into the HVOF spray to successfully produce nanocrystalline coatings. X-ray diffraction analysis and transmission electron microscopy were used to determine the average grain size of the milled powders. Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the microstructure of the as-sprayed coatings. The properties of various coating materials were characterized by microhardness measurements performed on the polished surface of the cross section.

Cryomilled Cu-Al powder and the Cu/Al mixture were sprayed onto aluminum substrate using the cold spraying process. This study focused on the wear properties of the nanocrystalline (NC) Cu-Al coating in comparison to its coarse-grained (CG) Cu/Al counterpart. The results showed that the as-sprayed deposit presented a dense microstructure. Investigations on the worn surface of the NC coating revealed that the plastic deformation with grooves and some debris were prominent with no visible cracking. Nanocrystalline Cu-Al coating showed a good wear resistance with a low friction coefficient. The enhancement of the wear properties of the NC Cu-Al was attributed to the grain refinement and the superior hardness.

In the present study, a nanostructured FeAl coating was prepared by cold spraying of ball milled powder. Annealing treatment was applied to the coating to investigate its effect on the phase structure, grain size and microhardness of the cold-sprayed nanostructured FeAl coating. The results showed that the FeAl phase was kept unchangeable when the coating annealed at the temperature above 500°C. Annealing temperature significantly influenced the microstructure and microhardness of cold-sprayed FeAl coating. With raising annealing temperature, the lamellar structure in the as-sprayed coating disappeared and a dense coating microstructure with fully bonding of deposited particles at their interfaces was achieved after annealing at 950°C. Nanograin growth of the FeAl phase occurred at an annealing temperature higher than 800°C. The microhardness of cold-sprayed FeAl coating remained about 400 Hv 0.1 at the annealing temperature below 800°C and decreased to 300 Hv 0.1 at 1100°C.

WC-Co coatings were deposited using conventional High Velocity Oxy-Fuel Jet-Kote (HVOF-JK) and Suspension HVOF (S-HVOF) methods. Microstructural and mechanical properties along with the wear resistance of coatings were investigated. Reciprocating sliding wear tests were conducted against sintered Si 3 N 4 counter-body with a normal load of 25N and total sliding distance of 500m following ASTM G133-2 standard. Coatings were characterised by Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD) and nano-Indentation techniques. HVOF-JK coating showed good retention of WC whereas S-HVOF coating showed the presence of W, W2C and amorphous/nanocrystalline phases. Nano-indentation of HVOF-JK and S-HVOF showed that the relative hardness of the HVOF-JK coating was higher but their elastic modulus was lower. The lower total wear rate was exhibited by the HVOF-JK coating. This difference in wear performance is attributed to the difference in hardness of the coatings and decarburisation of WC particles.