A tungsten carbide/cobalt hard coating was deposited on steel substrates using plasma-arc and high velocity oxyfuel flame (HVOF) spraying. The characteristics of the coatings made by the two spraying methods were evaluated under identical conditions. The microstructure and the chemical composition ofthe coatings were different depending on the powder heating temperature and the velocity of particles. The sliding wear properties of the coatings against hardened steel at high sliding speeds showed different tendencies from those at low speeds. The plasma sprayed coatings resulted in better abrasive wear properties than HVOF coatings did. Material removal by solid particle erosion depended on the striking angle of particles on the substrate surface but there are no clear differences in wear properties between the two coatings.

In this paper, the corrosion characteristics of two HVOF-sprayed coatings made with self-fluxing NiCrSiB powders are assessed in the as-sprayed condition. One of the coatings is also investigated after post-spray treatment with polymer sealing and after vacuum furnace fusion. It was found that cracking at lamellar boundaries dominates the corrosion behavior of the layers in the as-sprayed and sealed states, while pitting corrosion resulting from the failure of hard phases plays the main role in the melted layer. Paper includes a German-language abstract.

This paper presents the results of a study on WC-Co layers obtained by high-velocity oxyfuel spraying using different commercial systems. It explains how test samples were produced and how coating microstructure, hardness, and wear resistance was examined. The results show that the layers have excellent wear resistance and are suitable for many applications. Paper includes a German-language abstract.

In this work, an artificial neural network (ANN) model was developed to investigate the application of Cr 3 C 2 -25NiCr coatings by HVOF spraying and predict the resulting properties based on flow rates, stand-off distance, and other parameters. HVOF coatings were sprayed and tests were conducted to generate data for training, validating, and testing the model. The model was trained with an R-value of 0.99965 to predict the relationship between spray parameters and coating properties including hardness, porosity, and wear rate. The reliability and accuracy of the model was subsequently verified using independent test sets.

This study assesses the microstructure and properties of SiC-based coatings deposited using liquid and gas-fueled HVOF spraying techniques and a recently developed SiC-YAG ceramic powder. The coatings are shown to be superior to plasma and high-frequency pulse deposition sprayed SiC in terms of density and microstructure and comparable in terms of adhesion values. SEM and EDX analysis of the coatings shows that hard SiC particles are retained in a YAG binder, forming a composite that exhibits good sliding wear and erosion behaviors. Due to its low density (< 4 g/cm 3 ), the SiC composite may be an alternative to coating materials such as WC-CoCr and Cr 3 C 2 -NiCr in weight-sensitive applications.

In the paper, Ni-Al 2 O 3 coatings were prepared by HVO/AF . The microstructure and composition of the Ni-Al 2 O 3 coatings were investigated by metallographic microscope, scanning electron microscope and X-rayed diffraction, Bonding strength and hardness and mechanics properties were tested. The experimental results show the Ni- Al 2 O 3 coatings possessed high microhardness and higher bonding strength with substrate and excellent comprehensive mechanical properties. Abstract only; no full-text paper available.

Due to its oxidation resistance, HVOF thermal sprayed Cr 3 C 2 -NiCr coatings have been widely used for wear resistance at elevated temperatures up to 850ºC. During HVOF spraying deposition, compositional degradation occurs through dissolution of the carbide phase into the metal matrix. The occurrence of carbide dissolution and the high working temperatures affects on the final properties of the coating. The aim of this work is to study the effect on the structure and wear properties at elevated temperatures of Cr 3 C 2 - NiCr coatings using hydrogen as a fuel gas against propylene. The structural characterization was done by SEM-EDS, XRD, and Scanning White Light Interferometry (SWLI). Wear properties at room and high temperatures have been evaluated by Ball-on-Disk (ASTM G99-90). Oxidation resistance of all coatings was carried out using a calorimetric technique Abstract only; no full-text paper available.

The knowledge of thermal fluxes transferred from impinging high temperature jets is always important. For example, during thermal spray applications, the impinging jet acts directly on the transient surface temperature of the substrate. For preheating applications, it acts on the time required to reach the required surface temperature. It is also important to estimate it for some other applications like thermal cycling tests of parts for example. In the present paper, both CFD computations and experiments were carried out in order to quantify thermal fluxes from an impinging HVOF jet. The case of a CDS gun fueled by natural gas was considered. Stand-off distances ranging from 50 mm up to 300 mm were investigated. The model incorporates a 2-layer extension to k-å models for a better estimation of thermal exchanges through the viscous layer on the front body surface. The experiments were performed using a system incorporating two square calorimeters positioned one beside the other in order to attempt defining a radial profile. Abstract only; no full-text paper available.

Neutron absorbers are expected to play an important role in the long-term storage of spent nuclear fuels and nuclear wastes. High neutron absorbing capability, long-term stability, and the capacity to stay with the fuel are important criteria in preventing critical conditions during possible waste package degradation in geological time frames. Existing available neutron absorbing materials are based on boron or boron-10 isotope modifications of austenitic stainless steels or to aluminum based metal matrix composites. Specific rare earths such as gadolinium, samarium, or europium are found to have much higher thermal neutron cross section than boron or boron-10 but have high reactivity which limit their stability and ultimate applicability. In this paper, it is described how it is possible through a nanotechnology approach, to overcome the solubility and stability limitations of conventional materials to allow incorporation of high amounts of boron and rare earths into advanced HVOF coatings. During the development of the NeutraShieldTM Coatings, it was found that high fractions of rare earth elements such as gadolinium along with high concentrations of boron could be dissolved in the liquid melt and then remain soluble in the metallic glass structure. During the transformation of the glass to the nanocomposite structure, the rare earths are found to come out of supersaturated solid solution to form stable nanoscale ternary intermetallic R2Fe14B phases which form in a commensurate fashion and is protected by the highly noble matrix. Abstract only; no full-text paper available.

LTHVOF(Low Temperature High Velocity Oxygen-Fuel) spray is developed by injecting nitrogen into the gun chamber. The flame temperature can be adjusted between 400K to 800K after cooling, which is identical to cold spray. Copper coatings were sprayed by LTHVOF. SEM photos show that the coatings are dense with a lamellar structure. X-ray diffraction pattern shows that there is no oxide in the coatings. LTHVOF may be another choice for coating production of low melt point metal and nanometer size materials.

Al-Sn plain bearings for automotive applications traditionally comprise a multilayer structure. Conventionally, bearing manufacturing involves casting the Al-Sn alloy and roll¬bonding to a steel backing strip. Recently, high velocity oxy- fuel thermal spraying has been employed as a novel alternative manufacturing route. The present project extends previous work on ternary Al-Sn- Cu alloys to quaternary systems, which contain specific additions for potentially enhanced properties. Two alloys were studied in detail, namely Al-20wt.%Sn-lwt.%Cu-2wt.%Ni and Al-20wt.%Sn-lwt.%Cu-7wt.%Si. This paper will describe the microstructural evolution of these alloys following HVOF spraying onto steel substrates and subsequent heat treatment. Microstructures of powders and coatings were investigated by scanning electron microscopy and phases identified by X-ray diffraction. Coating microhardnesses were determined in both as-sprayed and heat treated conditions and differences related to the microstructures which developed. Finally, the wear behaviour of the sprayed and heat treated coatings in hot engine oil was measured using an industry standard test and compared with that of conventionally manufactured Al-Sn bearings.

The paper analyzed microstructure and property of WC-17Co coatings sprayed by High Velocity Oxygen/Air Fuel Spray under three kinds of spray conditions, which are HVOF, HVO-AF and HVAF. Coatings bond well with the substrate. The average bonding strength exceeds 70Mpa. Coatings are dense and hard, and the average porosity is about 1%. Microhardness of coatings is between HV1000 0.2 and HV1200 0.2 . Coatings are mainly composed of WC with little W 2 C and Co 3 W 3 C. With the increasing of Nitrogen, decarburization of WC was reduced.

Since the introduction of the HVOF process by J. Browning in 1982, gun design and spraying parameters have changed to achieve increasing gas resp. particle velocities. One exeption among the up-to-date HVOF systems is the GTV K2 Burner System. From the very first design study, special attention was payed to optimum heat management within the gun. This article discusses the principles of HVOF burner system, that is designed with special regard to heat management, illustrates gun behavior, and describes the results of various GTV K2 applications. The discussion covers the processing of fine powder grades, low melting materials, and oxidization sensitive materials. It is concluded that, solely in case of the K2, the injected kerosene is completely burned inside the combustion chamber.

Based on the HVO-AF (High Velocity Oxygen-Air Fuel) thermal spray system, the Low Temperature High Velocity Air Fuel Spray was realized by additional liquid feed stocks. In this paper, the microstructure and characteristics of composite coatings sprayed by this spray technology were analyzed. Composite powders were composed at three mass fractions, Fe, 5mass%Fe -polymer, 15mass%Fe-polymer. In the experiments, the coatings properties were tested. The results indicated that all the coatings microstructure is dense and low porosity; metal particles were dispersed with polymer in the coatings. There were little oxide phase in the coatings. The coatings were closely combined with substrate, the reflectance coefficient of 5mass%Fe-polymer composite coatings is better than others, the reflectance coefficient curve of the coatings is 2~6dB at 2~18GHz.

Thermally sprayed Inconel 718 coatings have been deposited by high velocity oxy-fuel (HVOF) spraying on Inconel 718 substrates. The aim of the on-going study is to understand and control the adhesion mechanisms and the residual stress state of the deposit/substrate system, in order to build up thick coatings for maintenance purposes. The coating adhesion strength was evaluated by the standard ASTM C633 tensile test. Coating shear strength was evaluated by the recently developed prEN15340 Shear Test. A modified Layer Removal Method (MLRM) test was carried out to measure residual stresses. The work is a part of an ongoing study for evaluation of relationships between process parameters, residual stress distribution and adhesion strength.

Flame and particle velocity mathematical model were constructed according to jet dynamics. Velocity of flame, and WC-17Co and NiCrBSi particles were simulated. Results show that velocity of the flame decreases continuously from supersonic to subsonic with the increasing of spray distance. Velocity of particles distributes in 300~800m/s, which relates to particle diameter, density and flame velocity.

Molybdenum disilicide (MoSi 2 ) is a suitable material for high temperature applications especially because of its excellent high temperature oxidation resistance. For several high temperature applications MoSi 2 shows high potential to be used as a protective coating. The oxidation behaviour of HVOF sprayed MoSi 2 coatings is studied at 1500 °C. The oxidation tests are carried out in a simultaneous thermogravimetric device and the mass change is measured in dependence on the oxidation time. The microstructure of the coatings before and after oxidation is examined by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXS). The mass of the coating increases according to a parabolic function. During the oxidation test the microstructure changes significantly from a typical thermal spray coating microstructure with lamellae, pores and a phase mixture of MoSi 2 and Mo 5 Si 3 to a two phase system with sharply separated grain boundaries. On the surface of the coating a silicon dioxide layer with a thickness of less than 10 µm is formed.

The Airbus A380 program marked the Goodrich Landing Gear introduction of High Velocity Oxygen Fuel (HVOF) applied tungsten carbide cobalt chrome (WC-Co-Cr) coating as a replacement coating for electrolytic hard chrome. HVOF is a new coating technology when applied to aircraft landing gear so specifications for coating application, finishing, powder and supplier qualification were developed to reflect the unique function of landing gear components. Since the materials, sizes and shapes of landing gear components are dissimilar to other aerospace parts currently HVOF sprayed, the capabilities of each spraying and grinding supplier needed careful assessment. Both suppliers and internal customers required training on the requirements specific to landing gear. This paper will discuss the development of HVOF specifications specific to aircraft landing gear, the methods developed for qualifying HVOF suppliers, and some challenges encountered when introducing HVOF-applied WC-Co-Cr as a hard chrome replacement.

The physical bonding of spray particles to the substrate is a main factor which contributes to the high adhesive strength of HVOF WC-Co coatings in addition to the mechanical interlocking effect. The adhesive strength between substrate and coatings deposited with partially melted particles under HVOF spray conditions has a significant contribution from physical bonding compared with conventional thermal spray processes having molten particles with relatively low velocity. The physical bonding results from the high impact velocity of partially-molten droplets during the HVOF spraying process.

Environmental problems of hard chrome plating are raising its cost and shrinking availability. HVOF and Detonation spray technologies for application of tungsten carbide and chrome carbide based coatings have proved to be cleaner and more effective than chrome plating. In this paper, the results of fatigue tests for WC-17Co coating deposited onto AISI 4340 steel by HVOF are compared to those for hard chrome plating. The fatigue life distributions as a function of the probability of failure for the coated AISI 4340 steel specimens showed that the HVOF coated specimens exhibited extraordinarily higher fatigue lives compared to the uncoated specimens whereas the hard-chrome-plated imparted fatigue strength degradation to the AISI 4340 steel.