Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments

Thermally sprayed tungsten-carbide (WC) coatings have proven to be one of the most wear resistant coatings available and a respectable replacement for hard-chromium coatings. They are used in paper machine parts such as calender rolls. However, improved lifetime and performance are continuing considerations, as well as finding more economical alternatives. This study researched the wear phenomena of tungsten-carbide coatings in a paper machine environment. To achieve this, five different feedstock materials and coatings manufactured from these were compared by electron microscopy as well as dry abrasion-, high-speed slurry abrasion- and cavitation erosion tests. Improvements in ductility by changing the matrix material were found, while changing the particle strength had no effect on the behavior of the coatings. The findings suggest further research on altering the matrix of the feedstock could lead to overall improvements in coating quality and component lifetime

Characterizing the micro-impact fatigue behavior of APS and HVOF-sprayed ceramic coatings

The fatigue life of thermally sprayed Al2O3- and Cr2O3-based coatings has been studied under low-energy (0.7–5 mJ) impact conditions. A threshold impact energy and amount of repetitions the coatings can endure with said energy before catastrophic failure was obtained. The catastrophic failure was determined to occur when the fracture mode of the coating switched from brittle cone cracking to quasi-plastic radial cracking. The results are examined relative to the microstructural features along with other properties of the coatings - hardness and cavitation resistance. The experiment provided a new approach for a straightforward comparison of the micro-scale impact fatigue life of thermally sprayed coatings unachievable with previous methods.proofPeer reviewe

Evaluating the toughness of APS and HVOF-sprayed Al2O3-ZrO2-coatings by in-situ- and macroscopic bending

Thermally-sprayed ceramic coatings are commonly used in applications where high wear and corrosion resistance are essential. However, their inherently low toughness and resistance to impacts often limit their use. In bulk ceramics, the toughening effect of ZrO2 has been successfully implemented in different compositions of Al2O3-ZrO2. Successful toughening leads to increased wear resistance and higher reliability. In this study, APS- and HVOF-sprayed Al2O3-40ZrO2 coatings were characterized with SEM and XRD techniques. The toughness of the coatings was evaluated by measuring their strain tolerance with in-situ (SEM) three-point-bending and macroscopic four-point bending with acoustic emission instrumentation. The APS-coatings had a higher strain-to-fracture but failed abruptly. In HVOF-coatings, the cracking commenced earlier but proceeded slower with more crack deflections. The observed behaviour is likely to derive from the coarser microstructure of the APS-coatings, which allows strain distribution in a larger area unlike the finer structure with a lesser melting degree of the HVOF-coatings.acceptedVersionPeer reviewe

Characterization of powder-precursor HVOF-sprayed Al₂O₃-YSZ/ZrO₂ coatings

Abstract Thermal spraying using liquid feedstock can produce coatings with very fine microstructures either by utilizing submicron particles in the form of a suspension or through in situ synthesis leading, for example, to improved tribological properties. The focus of this work was to obtain a bimodal microstructure by using simultaneous hybrid powder-precursor HVOF spraying, where nanoscale features from liquid feedstock could be combined with the robustness and efficiency of spraying with powder feedstock. The nanostructure was achieved from YSZ and ZrO₂ solution-precursors, and a conventional Al₂O₃ spray powder was responsible for the structural features in the micron scale. The microstructures of the coatings revealed some clusters of unmelted nanosized YSZ/ZrO₂ embedded in a lamellar matrix of Al₂O₃. The phase compositions consisted of γ- and α-Al₂O₃ and cubic, tetragonal and monoclinic ZrO₂. Additionally, some alloying of the constituents was found. The mechanical strength of the coatings was not optimal due to the excessive amount of the nanostructured YSZ/ZrO₂ addition. An amount of 10 vol.% or 7 wt.% 8YSZ was estimated to result in a more desired mixing of constituents that would lead to an optimized coating architecture

Tribological properties of plasma sprayed Cr2O3, Cr2O3–TiO2, Cr2O3–Al2O3 and Cr2O3–ZrO2 coatings

Plasma sprayed Cr2O3 is widely used to protect industrial components against wear. The present study seeks to clarify how its properties can be modified by alloying with other oxides. Therefore, pure Cr2O3 and Cr2O3–25%TiO2, Cr2O3–16%Al2O3, Cr2O3–35%Al2O3, Cr2O3–10%ZrO2 and Cr2O3–20%ZrO2 coatings were studied. All samples were obtained from pre-alloyed feedstock, resulting in rather homogeneous solid solutions. Compared with pure Cr2O3 and Cr2O3–Al2O3 coatings, the Cr2O3–25%TiO2 and Cr2O3–ZrO2 ones exhibit lower indentation hardness (HIT) but higher toughness, qualitatively assessed by scratch testing. Cr2O3 and Cr2O3–16%Al2O3 also exhibit higher hardness/elastic modulus ratios (HIT/E*, HIT3/E*2) than all other samples. The sliding wear resistance of the coatings against Al2O3 and ZrO2 balls is most closely correlated to indentation hardness and, secondarily, to the hardness/modulus ratios. Pure Cr2O3 is therefore the most sliding wear resistant of all samples, whilst Cr2O3–25%TiO2 suffers very severe wear. However, ZrO2 counterparts cause systematically more severe wear than do Al2O3 ones. Dry particles' abrasion, which proceeds through flake formation, is controlled by toughness. The resistance to abrasive wear is, therefore, predicted by scratch testing. The various coatings rank almost the opposite as they did in sliding wear tests, with comparatively lower wear losses for Cr2O3–25%TiO2 and (most of all) Cr2O3–ZrO2 samples

High temperature corrosion properties of thermally sprayed ceramic oxide coatings

Oxides are chemically stable and wear resistant materials. Because of these properties, they are often applied as protective coatings in harsh environments. However, their chemical and mechanical stability at high temperature in chlorine containing environments is uncharted. These conditions are present in waste-to-energy and biomass boilers in which the currently available metallic and metal matrix composite coatings provide unsatisfactory protection. To be effective in these conditions the coatings should be chemically inert, erosion resistant and act as environmental barriers. For this purpose, this research studies the corrosion behavior and microstructural features of HVOF- and APS-sprayed Al2O3-, Cr2O3-, TiO2-based coatings. Their chemical stability was evaluated by high temperature corrosion testing of self-standing coatings under KC1 salt deposit at 550, 650 and 720 °C for the duration of 72 h