Eva Smazalová

Tribological Properties of Hard Metal Coatings Sprayed by High-Velocity Air Fuel Process

AbstractLowering the thermal energy and increasing the kinetic energy of hard metal particles sprayed by the newly developed HVAF systems can significantly reduce their decarburization, and increases the sliding wear and corrosion resistance of the resulting coatings, making the HVAF technique attractive, both economically and environmentally, over its HVOF predecessors. Two agglomerated and sintered feedstock powder chemistries, WC-Co (88/12) and WC-CoCr (86/10/4), respectively, with increasing primary carbides grain size from 0.2 to 4.0 microns, have been deposited by the latest HVAF-M3 process onto carbon steel substrates. Their dry sliding wear behaviors and friction coefficients were evaluated at room temperature via Ball-on-disk (ASTM G99-90) wear tests against Al2O3 counterparts, and via Pin-on-disk (ASTM G77-05) wear tests against modified martensitic steel counterparts in both dry and lubricated conditions. Sliding wear mechanisms, with the formation of wavy surface morphology and brittle cracking, are discussed regarding the distribution and size of primary carbides. Corrosion behaviors were evaluated via standard Neutral Salt Spray, Acetic Acid Salt Spray, accelerated corrosion test, and electrochemical polarization test at room temperature. The optimization of the tribological properties of the coatings is discussed, focusing on the suitable selection of primary carbide size for different working load applications.

Effect of Heat Treatment on the Microstructure and Properties of HVOF-Sprayed Co-Cr-W Coating

Co-Cr-W HVOF-sprayed protective coatings are used for their high oxidation and wear resistance. Apart from the oxidation resistance, the stability of their mechanical properties in relation to thermal loading is crucial with respect to the most common high-temperature application areas. This work is focused mainly on evaluation of the heat-induced changes in the phase composition and related mechanical properties. It was shown that the original powder, composed fully from face-centered cubic Co-based alloy, partly changes its phase composition during spraying to a hexagonal close-packed (hcp) structure. The annealing further increases the ratio of the hcp phase in the structure. The heat-induced phase changes are accompanied by an increase in the coatings’ hardness and cohesion strength. The abrasive and adhesive wear behavior was evaluated. While the coatings’ heat treatment had a positive effect on the coefficient of friction, the abrasive and adhesive wear resistance of annealed coating was lower compared to as-sprayed coating.

Thermal effects of laser marking on microstructure and corrosion properties of stainless steel

Laser marking is an advanced technique used for modification of surface optical properties. This paper presents research on the influence of laser marking on the corrosion properties of stainless steel. Processes during the laser beam-surface interaction cause structure and color changes and can also be responsible for reduction of corrosion resistance of the surface. Corrosion tests, roughness, microscopic, energy dispersive x-ray, grazing incidence x-ray diffraction, and ferrite content analyses were carried out. It was found that increasing heat input is the most crucial parameter regarding the degradation of corrosion resistance of stainless steel. Other relevant parameters include the pulse length and pulse frequency. The authors found a correlation between laser processing parameters, grazing incidence x-ray measurement, ferrite content, and corrosion resistance of the affected surface. Possibilities and limitations of laser marking of stainless steel in the context of the reduction of its corrosion resistance are discussed.

The effect of the material surface oxidation on laser light absorption

This study examines the influence of surface condition on its interaction with a laser beam. The relation was tested on milled C45 steel samples with a different width of created oxide layer. The milled surface with roughness 1.6 was modified by the creation of oxide layer. The oxide layers were created by solid state pulsed laser with scanning optics. The modified samples were processed by high power diode laser with a wide spread beam. The surface temperature was measured by the thermal camera during this process. The surface hardness and hardness depth profile were measured for all modified samples and the emissivity values were measured. The effect of pretreatment on hardness and microstructure will be discussed. The goal of this paper is to describe the relation between the parameters of created oxide layer and its effect on laser beam absorption.

The Influence of High Temperature Exposure on the Wear of Selected HVOF Sprayed Coatings

The influence of high temperature exposure on the mechanical and wear properties of selected HVOF sprayed CrC and No/Co – based alloy coatings were tested. Comparison of as-sprayed and 600°C/116 h annealed coatings’ microhardness, cohesive strength, abrasive and adhesive wear resistance showed that the Co-based Stellite 6 coating’s wear properties were deteriorated by heat exlosure despite of the increase of its hardness and cohesive strength. The heat exposure was found to be beneficial for both mechanical and wear properties of NiCrBSi self-fluxing coating. On the contrary, mechanical properties of Cr3C2-NiCr coating slightly decreased, while its wear resistance slightly increased as a result of high temperature exposure.

The Potential of AE Sensing in the Indentation Fracture Toughness Measurement

In this study, an attempt is made to characterize the brittleness of the material by crack formation with the Vickers indenter. To assess the critical load, the sensor of AE signal was used. After preliminary tests on a silica glass, the methodology was applied to HVOF sprayed CrC-NiCr coating in as-sprayed and annealed state. The critical value of indentation force obtained from AE sensing was correlated with the standard method of crack length measurement with good agreement

Mechanical Properties of HVOF Sprayed, Flame and Laser Remelted NiCrBSi Coatings

The influence of thermal post-treatment by means of flame and laser energy on the mechanical properties of NiCrBSi coating was studied. The evaluation was done in terms of microstructure, surface hardness, microhardness and instrumented indentation. The microstructure evaluation showed an increase of coatings inner cohesion, together with the creation of metallurgical bond between the coating and the substrate material. In the the case of laser remelted coating, the creation of thick transition layer responsible for lower measured microhardness and hardness value was proved.