Šárka Houdková

The Instrumented Indentation Study of HVOF-Sprayed Hardmetal Coatings

Elastic-plastic properties, namely, hardness and Young’s modulus, of four HVOF-sprayed hardmetal coatings were measured by instrumented indentation using Oliver-Pharr method Nanoindenter XP MTS with a continuous stiffness measurement (CSM) module. The results show that with sufficient number of CSM measurements, one can distinguish between indents made in the hard particles and indents made in the binder material. This can be accomplished by analyzing the plots of hardness and Young’s modulus versus load (or versus indentation depth). Further development of the dependence curves enables the load (or indentation depth) to be set to correspond to the point of transition from a single structure component to the composite material and to determine the properties of both. Comparison of results of CSM measurement with the results of single indentation measurement at a defined load reveals a new perspective on the origin of the indentation size effect in hardmetal coatings. The measurements show that the increase in both the hardness and Young’s modulus with decreasing load is caused mainly by the predominant influence of hard particles in the coatings.

Comparison of Isolated Indentation and Grid Indentation Methods for HVOF Sprayed Cermets

This paper compares the results of two approaches of instrumented indentation for characterization of mechanical properties of HVOF coatings. Three types of HVOF sprayed coatings (Cr3C2-NiCr, WC-Co, (Ti, Mo)(C,N)-NiCo) were investigated by the means of isolated nanoindentation and grid indentation methods. The results of the isolated indentation revealed hardness and elastic modulus of the individual phases in very good agreement with the corresponding bulk material. The grid indentation method, based on statistical evaluation of a large number of indentations, was influenced by the carbide-matrix interface, which gave rise to a third peak apart from the two peaks corresponding to the carbides and metallic matrix. As a consequence, the bimodal Gaussian fit was insufficient and a trimodal fit had to be used. The results extracted from low load grid nanoindentations were quite close to the results of isolated indentations whereas higher load grid nanoindentation revealed overall properties of the coating.

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.

High Temperature Resistance of Selected HVOF Coatings

The HVOF (high velocity oxygen fuel) thermal spraying technology is widely used for creation of coatings notable for their resistance against various kinds of loading. Depending on the sprayed material, the coatings suitable for high temperature applications can be sprayed as well. The coatings, based on CrC or Co/Ni alloys, offer the advantageous combination of high temperature oxidation resistance and the wear resistance. In the paper, the attention is paid to the evaluation of the influence of the high temperature on the coatings microstructure and mechanical properties, namely hardness. The stability of the hardness values in respect to the time of the high temperature exposure is presented and related to the phase composition changes.

The Influence of Spraying Parameters on Stresses and Mechanical Properties of HVOF-Sprayed Co-Cr-W-C Coatings

Stellite 6 Co-Cr-W-C coatings were sprayed by HVOF while systematically varying the spraying parameters, namely the equivalent ratio and combustion pressure. During spraying, the in-flight particle temperature and velocity were measured. Deposition, thermal and residual stresses were determined by in-situ curvature monitoring of the sprayed samples. Youngs moduli and hardness of the coatings were determined by instrumented indentation. The relationship between spraying parameters, in-flight particle characteristics and mechanical properties is discussed.

Design of Experiments in the Branch of Thermal Spraying

High quality methods of designing the experiments (DOE) seem to be a very useful exploratory tool at many points of material engineering. Designing the experiments is a natural way to find a systematic influences in the process. It is necessary to have a clear view on how the things are to have the processes under controll. Therefore the importance of DOE increases in management and industry all over the world. In this work, a general use in branch of thermal spraying is shown.

The Influence of Thermally Sprayed Coatings Microstructure on their Mechanical and Tribological Characteristics

The tribological properties of parts surface, namely their wear resistance and friction properties, are in many cases determining for their proper function. To improve surface properties, it is possible to create hard, wear resistant coatings by thermal spray technologies. Using these versatile coatings it is possible to increase parts lifetime, reliability and safety. The thermally sprayed cermet composite coatings show, thanks to their specific properties, excellent resistance to abrasive and erosive wear, as well as corrosion resistance. To predict the behavior, lifetime and application area of thermally sprayed cermet coatings it is necessary to completely understand the relationships between technology, process parameters, microstructure and properties of the coatings. The finding of these relationships and use this understanding to develop deposits with improved wear resistance for coating of various applications is the main aim of the presented work. It was done by studying the coatings microstructure and mechanical properties. Four different tests of wear resistance were done to study the mechanism of surface degradation, to confirm the results of mechanical testing and to predict the lifetime of coated parts - the abrasive wear performance of the coatings was assessed using a dry/sand rubber wheel test according to ASTM G-65, wet slurry abrasion test according to ASTM G-75, pin-on-disc test according to ASTM G-99 and erosion wear resistance for three impact angles. On the basis of obtained data the new possibilities of coatings application was determined, tested and implemented.

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.