Michaela Kašparová

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.

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 Variation of Mechanical Properties of WC-Based HVOF Sprayed Coatings with Respect to the Co Content

The indentation methods (HR15N, HV0.3, HIT, EIT) were used to evaluate differences between the WC-based coatings with 12, 17 and 25% of Co binder. The two sets of WC-12%Co coatings with two different sets of parameters were sprayed to evaluate the influence of spraying parameters. The measured mechanical characteristic were correlated to the to the wear resistance of the coatings evaluated by dry sand/rubber wheel test according to ASTM G-65 and pin-on disc according to ASTM G-99. The measured results showed that there is almost no difference between the abrasive wear resistance of WC-Co coating with 12 and 17% of Co binder, while the wear of coating with 25% of Co binder is significantly higher. The importance of the spraying parameters on the WC-12%Co coatings properties has been found to be comparable to the importance of binder content influence.

Fracture Toughness of HVOF Sprayed Cermet Coatings Measured by Vickers Indentation

In the paper, the indentation fracture toughness of the HVOF sprayed WC-12%Co coatings is determined by the Vickers indentation test in dependence on used load. The relations, proposed by Chicot, were used to calculate the value of KIc at condition of the Palmqvist, Intermediate and Radial-median cracking mode. The applicability of using different equations is discussed with respect to the size indentation effect of IFT.

The Damage of a Plunger Operating in Carbamide Environment

The surface of plungers, operating in demanding conditions, is exposed to the effects of pumped media, which may be chemically aggressive, in combination with mechanical wear caused by the interaction between the plunger, the counterpart and possibly the solid particles included in the pumped media. To increase of plunger’s lifetime, their surfaces can be protected by different technologies of a surface treatment. One of them is the progressive technology of thermal spraying. A wide range of sprayed materials is one of the reasons for using these versatile coatings as the protection against corrosion, wear or high temperature exposition in order to increase the parts lifetime, reliability and safety. In this case, the TAFA Praxair JP-5000 spraying equipment was used for the creation of the WC-based hardmetal coating on the plungers operating in carbamide environment. Even though the spraying process and the check sample were without any sign of defects the plungers showed a critical damage after several hours of running. The damage mechanism, its possible causes and the proposed solution are discussed in this paper.Copyright