Thermal conductivity plays a critical role in the thermal transport of thermal sprayed coatings. In this paper, a combined image analysis and finite element method approach is developed to assess thermal conductivity from high-resolution scanning electron microscopy (SEM) images of the coating microstructure. Images are analyzed with a collection of image processing algorithms to reveal the microscopic coating morphology. The processed digital image is used to generate a two-dimensional finite element meshing in which pores, cracks and the bulk coating material are identified. The effective thermal conductivity is then simulated using a commercial finite element code. Results are presented for three coating material systems: yttria stabilized zirconia (YSZ), molybdenum and NiAl, and results are found to be in good agreement with experimental values, obtained using the laser flash method. YSZ coatings are also annealed and the analysis procedure repeated to determine if the technique could accurately assess changes in coating morphology.

To implement image-based numerical modeling of the thermal conductivity of coatings, a YSZ coating was considered as a network of two phases, namely the coating material and pores. The variation of the thermal conductivity of the gas trapped within the pores caused by their small size was considered by complying with the Knudsen law. In the present work, the quantification of the Knudsen effect on the effective thermal conductivity of coatings was achieved with the help of image analysis. The thicknesses of the pores were determined by combining the use of the Scion image software with a new in-house algorithm coded in C language. The Knudsen effect was quantified by applying a finite-difference model for both 2D images and a 3D image. In both cases, a decrease of the computed thermal conductivity was found while considering the Knudsen effect. Furthermore, the Knudsen effect was also taken into account in a finite-element model applied on the same images. Despite differences were noticed on the computed thermal conductivities obtained with the two methods, a quite similar decrease was calculated by comparing with data computed with the FDM method.

Release or nonstick properties are not typically achieved through thermal spraying because of the high surface energy of the resulting deposit. Polymer coatings can provide release properties but are low in durability. However, by adding a nonstick low-friction polymer or release topcoat to a hard metal matrix deposit, a variety of properties can be incorporated in the coating solution. This paper explains how such coatings are produced and examines the tradeoffs involved in the selection of materials. It presents numerous examples of successful industrial applications and introduces an accelerated test that has been developed to predict release performance over the life of the coating which correlates well to field conditions.

Tungsten caibide (WC) thermal spray coatings are being used for wear protection on selected components of aircraft. Tungsten carbide coatings are being used on aircraft flap tracks and fan and compressor blade mid-span dampers. However, a larger use of tungsten carbide coatings is being considered for other commercial aircraft applications where it would be used as a replacement for chrome plating. For instance, WC coatings are currently being tested on aircraft landing gear parts. One factor that affects the suitability of WC coatings for these applications is the fatigue life of the coated part. Coatings, whether chrome plating or thermal spray coating, can reduce the fatigue life of the part compared to an uncoated part. This study compares the fatigue life of uncoated 6061 aluminum specimens to the fatigue life of WC thermal sprayed coated 6061 aluminum specimens. The relation between the residual stress level in the coating and the fatigue life of the specimens is also investigated. Fatigue tests were run on cantilever flat beam specimens that were coated on one side. Specimens were cycled in bending so that the coatings experienced tensile fatigue stresses. Residual stress levels for each type of coating were determined using the Modified Layer Removal Method on specimens processed along with the cantilever flat beam specimens. Test results show that the fatigue life of the WC coated specimens is directly related to the level of compressive residual stress in the coating.

During Thermal Spraying material is partially or totally melted in between milliseconds, accelerated to high velocities and propelled onto the surface to be coated. Different temperatures, velocities and cooling times of the particles arise from different spraying techniques and conditions. The microstructure can be widely varied by tuning the spraying parameters. To optimize the coating properties with respect to a specific function one has to know i) the influence of the spraying conditions on the microstructure and ii) the correlation between microstructure and coating properties. Therefore analyzing methods are needed to determine the microstructure and to characterize mechanical, physical and chemical properties of the coatings. The proposed paper summarizes methods to characterize the microstructure including metallographic techniques, electron microscopy and X-ray analysis. Methods to determine the properties of the coatings including various adhesion tests, residual stress measurement, tribological and corrosion tests will be described in more detail. The increasing importance of Quality Management in all industrial sectors call heavily for reliable, destructive and especially non-destructive characterization techniques of coatings. An overview of common characterization techniques as well as new trends will be given. Recent development of International standardized tests will also be reported.

Physical properties of coatings based on Fe-B, Fe-Ni-B, Fe-Cr-P-C, Fe-Ni-Si-B, Ni-P, Ni-Nb and Co-Fe-B-Si, deposited by the methods of flame, plasma-arc, and detonation spraying were investigated. The coatings have mostly the amorphous structure with the volume content of the amorphous phase equal to 75-95 %. Values of the distribution and temperature coefficients of electric resistance of the coatings, depending upon a method and conditions of spraying, as well as upon their treatment parameters, were determined. Comparative studies of these coatings and thin amorphous strips produced by the melt spinning method were conducted. The amorphous coatings of ferromagnetic iron and cobalt alloys are shown to be magnetically soft materials and are characterized by a high magnetic induction combined with a high magnetic permeability. As compared with the amorphous strips, Curie temperature of the amorphous ferromagnetic coatings is by 50-140 K higher and their anisotropy of magnetic properties is lower.

An integrated approach was developed for investigation of thermal spray coatings with the amorphous-crystalline structure. The new approach combines methods of metallography, differential thermal and X-ray phase analysis, scanning electron microscopy and X-ray microanalysis. This makes it possible to reveal structural, phase and chemical heterogeneity, determine the degree of amorphization of coatings, temperature and heat of crystallization of the amorphous phase during heating. The new integrated approach was used to study amorphous-crystalline coatings of the Ni-P, Fe-Ni-B and Fe-B systems produced by thermal spraying.

Microscopic fracture mechanisms of thermal spray coatings under bending stress are investigated. Samples of thermally sprayed coatings were made using three distances. The sprayed powder was pure molybdenum. Vertical microcracks occur in lamellae and subsequently, these cracks join together and form vertical macrocracks in the samples sprayed with a short spraying distance. On the other hand, horizontal microcracks occur at the lamellae interfaces, and these cracks link together in the samples sprayed with a long spraying distance. These tendencies can be explained in terms of the hardness of the lamella and the bonding strength between each lamella. It is clarified that the bonding strength between each lamella corresponds to the applied strain at the point of rapid increase of the acoustic emission (AE) event. The amplitude and rate of AE beyond the point of rapid increase are high in the coatings which formed macrocracks. It is concluded that the coating which has high resistance to crack formation has a high point of AE increase, low AE amplitude and low AE increasing rate.

Many components in helicopter dynamics systems depend on hard, wear resistant coatings for reliable performance. Component replacement times are currently limited by performance of these coatings in many cases. Thermal spray coatings have been evaluated for these applications to replace nickel and chromium electroplate. The effect of coatings on fatigue strength is quantified by a strain limit concept to reduce test requirements during development phase. Full scale bench testing and coupon wear testing were conducted to reveal large improvement in sliding and fretting wear resistance. A fine porosity network in thermal spray coatings, high hardness and high fatigue strength are shown to be the main contributors to improved wear performance. Thermal spray coatings also exhibited enhanced corrosion resistance in salt fog and crevice corrosion conditions. These data permitted implementation on several critical helicopter components.

Fiber-reinforced polymer composites are an important class of structural materials, offering high strength-to-weight ratios and high rigidities. For many applications, however, their wear resistance is less than desirable. Wear-resistant thermal spray coatings have the potential to improve the surface properties of fiber-reinforced polymer composites, although some require the application of a bond coat to achieve sufficient adhesion. The present study was conducted to find acceptable bond coat materials and compare their performance. Materials such as polyamides, polyimides, polyether-ether-ketone, or simply aluminum or nickel were found to be suitable bond coats for many composite substrates.

Three-body wear is important in many industrial processes. A laboratory study was performed to follow the response of highly polished thermal spray coating surfaces to degradation by hard titanium dioxide particles. A scanning electron microscope, a white light interferometer and a gloss meter were used to evaluate surface structure and texture of the aspolished and abraded surfaces. Tungsten carbide coatings with differing binder material and applied by different processes all degrade by the same mechanism: softer material is worn away by the hard TiO 2 particles until the remaining structure consists of the hardest carbides protruding above the general surface plane. This structure is obtained in spite of significant differences in other surface properties such as roughness (Ra) and gloss number. If a more uniform wear surface is desired the carbide spacing must be reduced so that the coating more effectively presents a carbide surface to the TiO 2 particles.

The goal of this work is to have standardized quality control of thermal spray coatings incorporated into the Standard Practices Manuals of all aircraft engine original equipment manufacturer’s around the world. With the many manufacturers’ crit