A noncryogenic milling process was recently developed to produce equiaxed nanostructured NiCrAlY powders. In this study, the powders are used to deposit metal bond coats, with and without thermal barrier topcoats, via HVOF and low-pressure plasma spraying. TBCs with bond coats derived from non-cryogenically milled nanopowder show a reduction in porosity and TGO growth rate, delayed formation of mixed oxides, and a 50% increase in cycles to failure during thermal cycle testing. The presence of very fine alumina in the powder and bond coats plays a significant role in these improvements.

This study examines the influence of nano- and near-nano grains in bulk powder metal processing thus providing a baseline for understanding the potential of nanopowders for thermal spray application. Two light alloys (Al and Ti) and two tungsten carbide blends (WC-NiCrBSi and WC-CoCr) are cryomilled into nanocrystalline powders. The nanopowders are consolidated via hot isostatic pressing or spark plasma sintering and tested along with consolidated forms of virgin (micron scale) grains, shedding light on property improvements achieved through nanograined materials. HVOF coatings produced from nano- and micro-crystalline powders are tested as well, and the results are correlated with the improvements observed in the consolidated material forms.

In this study, suspension plasma spraying is used to deposit pseudo eutectic alumina-yttria stabilized zirconia as a potential thermal barrier coating. Process variables including feed rate, powder size, and plasma gas composition were altered to determine the influence of spray parameters on the formation of phases in the composite coating. The most significant variable was found to be the auxiliary gas. The gas influences the formation of phases primarily through its effect on in-flight particle velocity.

This study assesses the effect of carbide grain size on the wear performance of HVOF-sprayed WC-CoCr coatings. The coatings were produced from two powders, one of conventional grain size and one with submicron carbide. Both coatings were subjected to abrasive rubber wheel wear tests in wet and dry conditions with 220 nm titania particles and 368 μm sand particles, respectively. Detailed examination before and after wear tests shows that the wear mechanism depends on the relative size of the carbide and abrasive particles.

This study evaluates the potential of functionally graded composite coatings fabricated with a novel spraying process. The hybrid process embeds varying amounts of ceramic particles in a metal matrix by combining HVOF and wire arc spray streams. In this case, ceramic nanoparticles (SiO 2 , Cr 2 O 3 , Al 2 O 3 ) from liquid precursors are embedded in a matrix synthesized from NiCr wire feedstock. The resulting coatings are evaluated based on high-temperature performance and the composition and dispersion of particles in the NiCr matrix. Uniform size distribution and dispersion of nanoparticles is shown to correlate not only with denser coatings, but also improved corrosion, wear, and oxidation resistance.

Deep-drawing is a widely used sheet metal forming process in the aircraft and automotive industry. The manufacturing of modern parts with complicated shapes and curvatures requires forming tools with highest shape accuracy even at complex surface geometries. However, the application of novel, high-strength sheet metals combined with a continuous increase in productivity impose high tribological demands on forming tools and finally lead to increasing wear. In order to minimize the high costs for the repair and maintenance of such tools it is crucial to enhance their service life by an appropriate surface modification, which is able to preserve the high shape accuracy. Conventional coatings obtained by thermal spraying of coarse grained feedstock materials are not suitable to achieve this aim. In this collaborative study, the feeding and HVOF spraying of WC-Co submicron powders (- 8 + 1 µm) have been investigated to manufacture superfine structured, wear resistant near-net-shape coatings with improved macroscopic properties and smooth surfaces. Special equipment for the powder feeding and a novel HVOF flame spraying system designed by Thermico (optimized for the processing of fine-scaled powder fractions) have been employed. Correlations related to the process dynamics at varying HVOF gas compositions, the thermokinetic particle behavior in-flight and corresponding coating properties have been analyzed.

Plasma spraying was successfully applied for Thermal Barrier Coatings (TBCs), which typically possess a lamellar structure with a porosity of 5-20%. Control of the plasma process presents a big challenge when the goal is to achieve fully dense coatings, as required for some emerging applications such as solid oxide fuel cells (SOFCs) and dense TBCs. Fine powders produce finer lamellae, and result in denser coatings. However, powders finer than 10 microns are very difficult to feed consistently into a plasma torch. Liquid slurries offer a means to deliver fine particles to thermal spray torches. In this paper, an automatic slurry feed system was developed to consistently deliver micro and nano powder slurries. The slurries were injected axially into a high energy/high velocity plasma torch to generate dense coatings. The effects of plasma parameters and different feedstocks on coating microstructures are investigated; dense coatings for various applications are demonstrated.

Build up of strain within thermal barrier coatings has been identified as one of the main reasons for coating failure. The large volume fraction of grain boundaries in nanostructured materials has been predicted to partially relieve the strain in a coating structure. In this study, the difference in morphological characteristics of regular PSZ (r-PSZ) particles and nano-agglomerate PSZ (n-PSZ) particles have been investigated to improve our understanding of the stresses and strains within a coating. The Triple Torch Plasma Reactor, a reduced pressure plasma spray system along with a pair of tungsten and molybdenum apertures was used to isolate spray particles from the plume to deposit splat sample. Cross sections of the splat samples were prepared. Image analysis techniques were used on scanning electron microscopy (SEM) images to characterize both the surface and the cross sectional features of the splat samples. Semi-molten structures were found to be the defining characteristic of the n-PSZ samples. Peak height distribution (PHD) was defined to quantify the distribution of the height of semi-molten structures. PHD was estimated for different samples and was found to track the changes in the morphological characteristics as a function of the deposition conditions.

Plasma sprayed chromium oxides coatings have been widely applied in anilox rolls and pump seal for many years. This paper is researching the effect of nanostructured Cr 2 O 3 5SiO 2 3TiO 2 composite powder preparation when adding nano-size SiO 2 and TiO 2 powder in Cr 2 O 3 powder, to prepare for through spray drying, high temperature sintering and flame density. The microstructure of powder and plasma sprayed coating are analyzed by SEM and XRD, compared with pure Cr 2 O 3 powder and METCO 136F powder. The Microhardness of the coatings are measured by 402MVA TM Vickers hardness tester. Fracture test is used to analysis the ductility of coatings, and the fracture appearance is analyzed by SEM. The result indicates that the mechanical behaviors of nanostructured Cr 2 O 3 5SiO 2 3TiO 2 coating has better performance in the nature of hardness and ductility.

Functional characteristics of materials including the mechanical, electrical, environmental and tribological performance can be significantly improved using nanostructured feedstock. Over the last few years much research has been dedicated to the development of thermal spraying techniques with liquid precursors to prepare fine nanostructured coatings. In this work nanostructured ceramic coatings were prepared using the technique of suspension spraying. Titania and alumina powders with sizes in the nanometre to submicrometre range were used to prepare aqueous and alcoholic suspensions. Atmospheric plasma and high-velocity flame were employed as enthalpy sources. The morphologies and crystalline structures of the sprayed ceramics were mainly characterised by SEM and XRD. The aim of this work was to carry out a comparative study and to discuss the nanostructured ceramic coating microstructures as a function of the physicochemical properties of the feedstock suspensions and spraying parameters.

This paper is a first report of thermally sprayed Ti 60 Cr 32 Si 8 (at-%) alloy coating. It has been reported previously that such an alloy assumes quasicrystallinity in presence of oxygen, when made to undergo rapid quenching. In these cases, either TiO or SiO 2 is used as an ingredient in the alloy so that it can act as a source of oxygen. This paper describes an attempt to produce a quasicrystal by thermally spraying the alloy in air, i.e. in presence of oxygen, either by flame or atmospheric plasma spraying. The spray parameters for both the processes have been varied in the experiment in a wide range. A vacuum plasma sprayed coating served as the reference. This investigation includes a detailed study of the microstructure and phases by optical microscopy, Scanning Electron Microscopy (SEM), X–Ray diffractometry (XRD), microhardness and porosity. An estimate of the composition of the coating has been done using Glow Discharge Optical Emission Spectroscopy (GDOES). The parametric variations have been correlated with the coating microstructure. It has been found in the study that there is a likelihood of obtaining quasicrystalline phase by atmospheric plasma spraying of this alloy under favourable parametric conditions. The tribological properties of these coatings were studied using a ball on disc reciprocating tribometer.

Nanostructured chromium oxide coatings were deposited on stainless steel with an axial powder feeding plasma spray system. Friction and wear properties of the coatings were investigated in view of friction coefficient and volumetric wear loss of the coatings with a SRV oscillating friction and wear tester in a ball-on-disc configuration. The morphology and microstructure of the coatings were evaluated by light microscopy, field emission scanning electron microscopy and X-ray diffraction. The results showed that the nanostructured chromium oxide coatings were harder and had a lower friction coefficient and much better wear resistance than the conventional chromium oxide coatings.

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.

Superior wear performance combined with excellent friction properties against metals makes chromium oxide (Cr 2 O 3 ) an interesting coating material for many industrial applications. However, Cr 2 O 3 is a challenging material for HVOF spraying due to its high melting temperature. Fracture toughness and lamella cohesion of a coating is limited and may be improved by using ceramic-ceramic –nanocomposite powders, which forms phases with improved properties. In this study Cr 2 O 3 -TiO 2 systems were selected aiming to improve the toughness and lamella cohesion of coating without reducing the excellent wear properties.

Thermal barrier coatings (TBCs) with nanostructured bond coats have shown significant thermal cycling enhancements over their conventional microstructured counterparts; however, the high cost inherent to the cryomilling processing of the MCrAlY powder limits commercial application. Hence, this study characterizes and evaluates nanostructured bond coats derived from non-cryogenically milled MCrAlY powder with emphasis placed on reduced processing cost and scale-up capability. After extensive development of both a high-energy planetary mill and its operating parameters, fine-grained equiaxed NiCrAlY powder has been produced. XRD and SEM characterization of the milled powder will be presented. Microstructural analyses of the coatings sprayed via the HVOF and cold spray processes will also be carried out, in addition to some preliminary static oxidation test results of the conventional and milled NiCrAlY.

The technology and thermal shock properties and thermal conductivity of plasma sprayed nanostructured yttria-stabilized zirconia thermal barrier coatings (TBCs) are studied in this paper. The TBCs on the substrate of Ni 3 Al based alloy IC-10 were fabricated by using the nanostructured yttria-stabilized zirconia powder under certain plasma spraying conditions. By manipulating the plasma spray process, nanostructured TBCs were obtained. The specimens were thermally shocked from 1000°C, 1100 °C and 1200°C into 20°C water and the morphology and microstructure of the TBCs were evaluated by light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The thermal diffusivity was tested by a laser pulse method. The results showed that the nanostructured TBCs had more than 30% reduction in thermal conductivity and the thermal shock lifetimes were much longer than that of the conventional TBCs.

In this paper, nanostructured 5-8wt.% yttria stabilized zirconia (5-8YSZ) thermal barrier coating (TBC) deposited by air plasma spraying (APS) of powder and new TBC with novel structure deposited by solution precursor plasma spray (SPPS) are compared. Plasma spray conditions, coating forming mechanisms, microstructures, phase compositions, thermal conductivities, and thermal cycling lives of the APS nanostructured TBC and the SPPS nanostructured TBC are discussed. The SPPS process is different from either physical/chemical vapor deposition process or conventional plasma spray deposition in the aspects of substrate temperature, coating forming mechanism, growth rate, and coating structure.

Suspension Plasma Spraying (SPS) allows depositing finely structured coatings. This paper presents an analysis of the influence of plasma instabilities which control the interaction plasma jet-zirconia suspension. A particular attention is paid to the treatment of suspension jet or drops according to the importance of voltage fluctuations (linked to those of arc root) and depending on the different spray parameters such as the plasma forming gas mixture and the suspension momentum. By observing the suspension drops injection with a fast shutter camera and a laser flash triggered by a defined transient voltage level of the plasma torch, the influence of plasma fluctuation on drops fragmentation is studied through the deviation and dispersion trajectories of droplets within the plasma jet.

Al 2 O 3 -ZrO 2 composite coatings were deposited by suspension thermal spraying of submicron feedstock powders. The suspensions were injected internally into a Mettech Axial III plasma torch and a Sulzer-Metco DJ-2700 HVOF gun. The different spray processes induced a variety of structures ranging from finely segregated ceramic laminates to alloyed amorphous composites. Mechanisms leading to these structures are related to the feedstock size and in-flight particle states. Compositionally segregated crystalline coatings, obtained by plasma spraying, showed the highest hardness of up to 1150 Hv 0.3 , as well as the highest abrasion wear resistance (ASTM G65). The HVOF coating exhibited the highest erosion wear resistance (ASTM G75), which was related to the toughening effect of small dispersed zirconia particles in the alumina-zirconia alloyed matrix. The HVOF microstructures also led to low thermal diffusivity, due to high amorphous phase content and limited particle bonding.

Cavitation erosion is one of the major problems of hydraulic machinery and may cause the equipment to reduce power and then to stop working. Now with the study of nanomaterials, some special properties of nanostructured coatings are recognized and HVOF nanostructured WC-12Co coating may possess far more excellent potential to protect equipment from cavitation erosion than the conventional HVOF coatings. In the present paper nanostructured and conventional coatings were deposited by HVOF. Resistance of the coatings to cavitation erosion was studied by ultrasonic vibration cavitation equipment. Cavitation pits and craters were observed by SEM and cavitation mechanisms were explored. The results showed that nanostructured coating demonstrated more excellent performance in cavitation erosion and the erosion rate is approximately one third that of conventional one. The nanostructured existence and the increase in microhardness and toughness of the coating are important factors, which influence the resistance of coatings to cavitation erosion.