[en] This study reports the influence of three High Velocity Oxy-Fuel (HVOF) applied coatings on the high cycle fatigue resistance of a martensitic stainless steel substrate at room and elevated temperatures. It was found that chromium carbide and tungsten carbide coated specimens exhibited significantly lower fatigue capability compared to the substrate material at elevated temperatures while IN625 coated specimens exhibited a small beneficial effect. An attempt is made to explain the observed behavior in terms of elastic modulus mismatch, thermal expansion mismatch, residual stress and coating/substrate properties. It is concluded that coated metallic components must be analyzed as composite structures and that data generated for design properties must be performed on specimens which represent the geometry and characteristics of intended component

[en] Particle injection in a DC spraying plasma jet is a complex problem: in order to try to solve the problem of the diverging particle jet, a double flow injector with a secondary gas injection (gas tunnel) surrounding the injector central tube was tested and investigated with a classical single flow injector. The experiment was coupled with a 2-D laser planar sheet imaging technique. The deposition efficiency of the coatings sprayed with these two injectors were measured to characterize their relative performance

[en] In the presented paper, investigations on HVOF coatings produced from a new family of powders will be discussed. The influence of microstructure, composition and production methods will be discussed in view of powder properties and resulting coating properties. New boride powders and coatings will be compared with regard to their properties deposition, efficiency, hardness, surface roughness, bond strength, and wear against commercial WC-Co and cr3C2-NiCr coatings. Additionally, improved WC- and CrC-based powders and coatings will be compared with regard to oxidation and erosion resistance

[en] Fused and crushed partially stabilized zirconia particles (8 wt % Y₂O₃) were plasma sprayed with an Ar-H₂ plasma jet (45 slm Ar, 15 slm H₂, 600 A, nozzle i.d. 7 mm, internal injection). The study was devoted to the splat, beads and passes (maximum 8 passes) formation. The particles were sprayed on cast iron or stainless steel substrates at which temperature was kept as constant as possible either at 75 or at 350 C with the help of air jets either blown orthogonally to the plasma jet or to the substrate surface. The splat shape and morphology was examined by OM and image analysis, the beads and passes cross sections as well as fractured sections were examined by SEM and their phases determined by XRD. These examinations have shown the drastic influence of the substrate temperature on the splat shapes and contacts with underlying layers, columnar structure growth within one bead or one pass and macrocracks network within the beads and passes

[en] Injection of refractory additions (carbides, borides, oxides etc.) into self-fluxing alloys is a well-known technique for their hardening. Nevertheless the matter of influence of refractory components on the structure and characteristics of composite coatings is not studied well enough. This paper presents the results of investigations of gas thermal coatings (plasma and detonation ones) on the base of stellite with refractory components in the form of borides such as CrB₂, TiB₂, (TiCr)B₂. This study is concerned with the influence of refractory additions (carbides, borides, oxides) on the wear resistance sprayed coatings based on self-fluxing alloys NiCrBSi and CoCrBSi

[en] The rolling contact fatigue (RCF) behavior of tungsten carbide (WC) based cermet coatings (WC-12% Co) deposited by plasma and HVOF spraying was studied. The RCF testing was carried out with a two-roll configuration testing machine under unlubricated rolling conditions without sliding. Loads applied in the tests resulted to Hertzian contact pressures of 420--600 MPa. The influence of spraying method on the RCF behavior of the coatings was studied as a function of Hertzian contact stress. Plasma sprayed coating showed severe surface roughening and subsurface cracking of the coating under all studied load levels. HVOF sprayed coating behavior was clearly different from the plasma coating with smaller structural changes. HVOF coating retained its original surface roughness but vertical cracks penetrating the coating appeared in this coating

[en] Radial injection of powders in traditional DC plasma torches induces powder classification as the larger particles tend to pass through the plasma while the smaller particles tend to bounce off. This effect is minimized with axial injection torches in which powders are injected in the center of the plasma stream. In this configuration, powders can follow a nearly-optimum trajectory with little or no segregation. This promotes high deposition rates and deposition efficiencies. In the present work, the temperature, velocity and diameter of particles sprayed using a new commercial axial injection torch were measured. The particle diagnosis was carried out by detecting the thermal radiation emitted by the hot particles. Particle velocities of 380 and 550 m/sec are reached with WC-Co and titanium powders, respectively. When titanium particles are sprayed in air, their temperature increases as they move away from the torch due to exothermic reaction with air

[en] The effect of pores on the thermal conductivity of plasma sprayed tungsten coatings was investigated. Finite-element models of the actual pore structure observed in the coatings gave estimates of the coating thermal conductivity. The calculated values were compared to measured values for the same coatings. Vacuum plasma sprayed (VPS) tungsten pore structures were found to be well represented by the cross-sectional micrograph image thus leading to an accurate simulation of the conduction phenomena with a 2-D model. Atmospheric plasma sprayed (APS) cross-sectional pore structures required modification for proper phenomenological representation. In addition to thermal conductivity values, values of the maximum effective pore length in the 3-D pore structure were estimated for the APS coatings. Comparison of the calculated thermal properties to measured thermal properties of the same coatings show that the pore structure is the major factor decreasing the thermal conductivity of plasma sprayed tungsten coatings

[en] HVOF thermally sprayed WC/Co coatings are applied onto components which are exposed to wear caused by abrasion, erosion, fretting and sliding. Beside wear attacks and static stresses in lots of cases alternating mechanical stresses caused by dynamic loads occur additionally. Therefore, the fatigue resistance of WC/Co 88/12 and WC/Co 83/17 coated specimens was investigated by high-cycle fatigue tests (HCF). The results of the fatigue tests were documented in statistically ascertained Woehler-diagrams (S-N-curves). Furthermore, the mechanisms of failure are discussed

[en] Coatings have been obtained by electric arc spraying of powder wire under the following regimes: U = 34V, I = 140A. The pressure of gas-spray (air) was equal to 0.6 MPa. Powder wire mixture consists of refractory components Cr₂C₃, CrB₂, (Fe-Cr-B)+Al, B₄C, SiC the latter ones are not completely melted in electric arc and almost do not come into reaction with shell material. Laser treatment has been carried out in such a regime in which the coating and a thin layer of a base are being alloyed. In this case, the structure of a overmelted layer consists of ferrite matrix and dendrites enriched with elements -- carbon and boron. The strength of a coating increases after laser treatment. A porosity after laser treatment decreases from 10 to 0%, making it possible to apply such coatings as corrosion resistant ones. Corrosion resistance evaluation of samples with coatings carried out by analysis of polarization curves obtained in 3% NaCl solution shows that due to surface laser treatment corrosion current decrease from 2.4·10^-2 to 1.3·10^-3 mA/cm² is observed