Tungsten carbide-cobalt coatings are extensively used to protect surfaces from wear in many types of applications, such as compressor piston rods, pump plungers, shaft sleeves on centrifugal pumps and fans, and midspans of compressor blades in gas turbines. The wear behavior in any application is strongly influenced by the basic physical and mechanical properties of such coatings. Fracture toughness as a mechanical property indicates the resistance to fracture in the presence of a sharp crack, and thus provides a measure of the intrinsic strength of the cemented carbides coatings. In this study, Vickers indentation tests have been used to quantify the in-plane fracture behavior of various WC-based coatings deposited by the High Velocity Oxy-Fuel (HVOF) spray process. The indentation cracks are analyzed in terms of standardized relations that utilize radial-median crack geometries. It is shown that the fracture properties of HVOF WC-Co coatings are anisotropic, and depend strongly on the microstructure and composition of the coatings. The crack propagation is determined by the porosity, binder mean free path, and the shape, size, and distribution of the reinforcing carbide particles. The erosion resistances of the coatings have also been discussed as a function of the fracture properties and mechanisms. It is shown, in this study, that the Vickers indentation method is a useful and convenient technique for determining the in-plane fracture toughness of HVOF sprayed WC-based coatings.

There is typically a large scatter in the observed failure stress, particularly for ceramic coatings. Failure can thus occur not only at the coating-substrate interface where it is desired but also through the coating, the bonding epoxy, or the epoxy/coating interface. A large number of alternative test configurations have been proposed in an attempt avoid such drawbacks, each with their advantages and disadvantages. A test method developed recently by Troczynski and co-workers, known as the Peel Adhesion Test, is of particular interest for this article. The first specimens examined were Ni-WC coatings applied to 150 micrometer thick 1010 stainless steel foils which were grit blasted in the same manner as a bulk substrate would have been. It was observed that the technique appears to offer the capability of providing a sensitive and reproducible measure of the effects of surface treatments and other deposition parameters on coating adhesion.

Oxygen-free titanium coatings are at present produced using vacuum plasma spray systems. The cold spray process can produce titanium coatings in atmospheric air at reduced cost. Variations of the deposition efficiency as well as the coating characteristics with various process parameters were studied. Results show that the deposition efficiency drops drastically below a critical temperature. The parameters which affect the jet velocity; viz., the type of gas and the nozzle geometry have maximum effect on the process deposition efficiency. Sprayed coatings were porous and exhibited a low modulus and high hardness. Post processing of the coating by machining produced dense, strong and hard coatings. Key words: Cold-Spray, Titanium, Oxidation.

This paper presents a study of the residual stress and microstructural properties of thick, spray-formed components, produced using the High Velocity Oxy-Fuel (HVOF) thermal spraying process. The forming material used is Tungsten carbide cobalt (WC-Co), a material which is more usually processed using expensive press and sinter technology. The aim of this study is to examine the effect of production parameters on the formation of thick components. In order to fabricate thick specimens, certain problems have to be overcome. More specifically these problems include the minimizing residual stresses, which cause shape distortion in the components and maining the integrity of the coating on a microstructural scale. The dependence of residual stress, and sprayed material characteristics on spraying distance, and powder feed rate conditions is presented. Results show that cylindrical WC-Co components up to a thickness of 9mm can successfully be produced, by careful control of these parameters. This represents a significant improvement on maximum thickness values previously reported for WC-Co [1,2].

The high velocity oxy-fuel (HVOF) combustion spray technique has been shown previously to be an excellent solution for depositing crystalline matrix nano-reinforced polymer coatings [1]. Dense polymer coatings can be produced by HVOF combustion spraying by controlling particle dwell time in the jet and through substrate thermal management. Use of an amorphous matrix material, polycarbonate, will enable the role of matrix crystallinity on the structure and properties of thermally sprayed polymer composite coatings to be separated from effects resulting from the reinforcing phase. An amorphous, commercial polycarbonate resin with a broad particle size distribution of irregular particle morphology has been successfully deposited. Results from optical microscopy, X-ray diffraction, scratch and density measurements are presented. The influence of variations in process parameters such as spray distance, nozzle length, chiller temperature, fuel: oxygen ratio, and total gas flow rate on coating microstructure are presented.

Three different types of polyethylene powders were flame sprayed onto pre-heated steel substrate previously coated by electrostatic spray system with a thin epoxy primer layer. Properties of the polyethylene (PE) powders, including powder density, particle size and melt flow rate (MFR) were measured in order to study their influence on the mechanical properties of the coating. The spray experiments started with optimization of spraying parameters. The main variables were pre-heating temperature of the substrate, temperature increase during spraying (influenced by the spraying distance), and thickness of the PE coatings. The laboratory tests performed for the coatings were coating characterization by microscopy and mechanical testing. Porosity and thickness of the coatings were determined by optical and stereo microscopy studies from polished cross-sectional samples. Hardness, impact strength, peel strength, and adhesive strength of the coatings were also investigated. Also some hot water sinking and heat cycling tests were performed. As a result from the present studies it can be concluded that powder properties have great influence on the mechanical properties of the final coating.

This paper compares two methods for determining the composition of Ti/TiN coatings deposited by reactive plasma spraying. The coatings were obtained by spraying titanium powder in a low-pressure N2/Ar atmosphere. The resulting film has a variable nitrogen content in the form of titanium nitrides, depending on gas partial pressure, total pressure, sample-source distance, and other parameters. The composition of the film was determined using X-ray diffraction and X-ray photoelectron spectroscopy. The two techniques provide similar results and either can be used for the compositional characterization of these coatings.

Mechanical coating properties like hardness, bond strength and tribological features as well as the layer composite lifetime under thermal and mechanical operation stress conditions are mainly influenced by the residual stress situation in the composite. The final residual stress state in thermally spray coated composites is superimposed by different stress mechanisms occurring during the manufacturing process. By means of the micro milling method, residual stresses are measured in a quasi-nondestructive way over the drilling depth. This paper describes the measurement features as well as experimental and numerical results. Measurements are performed on coated plane composite specimens as well as on inside coated bushing guides of aluminum crankcases for different coating materials. The correlation between residual stresses and coating properties is investigated.

Aluminum coatings reinforced with either Al 2 O 3 or SiC particles were deposited onto aluminum substrates and subjected to various tests. The coatings were made with mechanically alloyed powders via atmospheric plasma spraying (APS). Both types of coatings had uniformly distributed hard particles, porosities in the range of 4 to 5%, and bond strengths of around 20 MPa. The wear resistance of the SiC-reinforced coatings, however, was almost 35% higher than the coatings containing Al 2 O 3 . X-ray examination (XRD) showed that the Al 2 O 3 particles undergo partial phase transformation during spraying, making them more prone to wear.

This paper evaluates a wide range of thermal spray coatings for potential use lining cylinder bores in aluminum engines. Coatings were applied by atmospheric plasma spraying (APS) and high-velocity oxyfuel (HVOF) techniques. More than a dozen coating materials were screened, including ceramics, cermets, and metals. The paper describes the equipment and procedures used in the investigation and assesses the resulting coatings based on their microstructure, hardness, friction coefficient, wear resistance, bonding strength, and residual stress.

Thermal sprayed chromium carbide (Cr3C2)-25% NiCr (Ni-20%Cr) coatings are extensively used in wear resistant applications especially under conditions wherein operating temperatures are likely to be higher than 500°C . The performance of the Cr3C2-NiCr coatings under such conditions depend on a variety of coating properties like the porosity, microstructure, extent of decarburization of Cr3C2 phase and hardness. One of the parameters which affects the above mentioned coating properties is the characteristics of the powder utilized for thermal spraying. In the present study, Cr3C2-25% NiCr powders obtained from four different sources has been utilized to form Cr3C2-NiCr coatings on steel substrates utilizing the detonation spray coating (DSC) system. The Cr3C2-NiCr powders utilized vary from each other in terms of manufacturing route employed (sintered and crushed, pre-alloyed, blended, etc.), particle size distribution, particle shape and even phases present. The influence of each of these powder characteristics on the coating microstructure, porosity, hardness, extent of carbide dissolution and ultimately on coating performance (i.e. sliding and abrasive wear resistance) has been evaluated.

Two types of rutile and anatase powders are used for the deposition of TiO2 coatings. The effects of plasma spraying conditions on the structure of TiO2 coatings are investigated in order to clarify controlling factors of the phase formation and to aim at the development of effective photo-catalyst TiO2. It is found that the amount of anatase TiO2 in the coating is influenced by spray parameters. The decrease of the heat input to spray droplet and an increase in the cooling speed during droplet deposition will increase the amount of the anatase TiO2 in the coating. As a photo-catalyst, the coating deposited under limited plasma power and by HVOF spraying using anatase powder is effective for the decomposition of acetaldehyde gas.

In recent year, there has been an increasing trend to design the turbine coating together with the substrate. In this harsh environment, a failure in one quickly leads to a failure in the other. Materials that are used in structural applications are prone to mechanical vibration which when not attenuated, will lead to fatigue of components and shortening of life cycle. Therefore, it is not uncommon to find that it is important to understand the thermal stability and dynamic mechanical properties of coatings as well, which will be used in the dynamic conditions. In addition to these essentially noise reduction and vibration amplitude control motivated objectives, however, mechanical energy dissipation processes also find intrinsic applications in cases where a thorough understanding of the mechanisms responsible for the damping response of the material are required. This article describes the damping behavior and mechanisms that exist in plasma sprayed NiCoCrAlY coatings.

Effects of spray parameters, such as spray distance, SD, and substrate temperature, Ts, and post heat treatment on the structure and properties of plasma-sprayed zircon coatings were investigated. Zircon was totally decomposed by plasma spray; the coatings were composed of tetragonal zirconia (t-ZrO2) and amorphous silica (a-SiO2), because of the rapid cooling of molten particle right after the impingement to the substrate. Porosity of the as-sprayed coatings was highly affected by both of substrate temperature and spray distance. In all range of the spray distance which had been tried in this study, higher substrate temperature resulted in lower porosity of the coatings; the coating with porosity of 2% was obtained at Ts = 1573K with SD = 95mm. Porosity also decreased with decrease of spray distance. By the heat treatment at 1473K, t-ZrO2 transformed to monoclinic zirconia (m-ZrO2) and a-SiO2 crystallized to cristobalite, respectively. Cracks in the coating disappeared, and open porosity decreased. This can be attributed to sintering of SiO2 and phase transformation of ZrO2. After the heat treatment at 1673K, the coating was composed of ZrSiO4 with dispersed fine m-ZrO2 particle. Open porosity of all the coatings increased up to 10% at this temperature. This is because of volume shrinkage during the formation of zircon.