Evaluation of Residual Stresses and Their Influence on Cavitation Erosion Resistance of High Kinetic HVOF and HVAF-Sprayed WC-CoCr Coatings

Thermal spray processes have been developing toward lower particle temperature and higher velocity. Latest generation high-velocity oxygen-fuel (HVOF) and high-velocity air-fuel (HVAF) can produce very dense coating structures due to the higher kinetic energy typical for these thermal spray processes. Thermally sprayed coatings usually contain residual stresses, which are formed by a superposition of thermal mismatch, quenching and, in case of high kinetic energy technologies, peening stresses. These stresses may have a significant role on the mechanical response and fatigue behavior of the coating. Understanding these effects is mandatory for damage tolerant coating design and wear performance. For instance, wear-resistant WC-CoCr coatings having high compressive stresses show improved cavitation erosion performance. In this study, comparison of residual stresses in coatings sprayed by various thermal spray systems HVOF (Thermico CJS and Oerlikon Metco DJ Hybrid) and HVAF (Kermetico AcuKote) was made. Residual stresses were determined through thickness by utilizing Tsui and Clyne analytical model. The real temperature and deposition stress data were collected in the coating process by in situ technique. That data were used for the model to represent realistic residual stress state of the coating. The cavitation erosion and abrasion wear resistance of the coatings were tested, and relationships between residual stresses and wear resistance were discussed.publishedVersionPeer reviewe

Microstructure-based thermo-mechanical modelling of thermal spray coatings

This paper demonstrates how microstructure-based finite element (FE) modelling can be used to interpret and predict the thermo-mechanical behaviour of thermal spray coatings. Validation is obtained by comparison to experimental and/or literature data.Finite element meshes are therefore constructed on SEM micrographs of high velocity oxygen-fuel (HVOF)-sprayed hardmetals (WC-CoCr, WC-FeCrAl) and plasma-sprayed Cr2O3, employed as case studies. Uniaxial tensile tests simulated on high-magnification micrographs return micro-scale elastic modulus values in good agreement with depth-sensing Berkovich micro-indentation measurements. At the macro-scale, simulated and experimental three-point bending tests are also in good agreement, capturing the typical size-dependency of the mechanical properties of these materials. The models also predict the progressive stiffening of porous plasma-sprayed Cr2O3 due to crack closure under compressive loading, in agreement with literature reports.Refined models of hardmetal coatings, accounting for plastic behaviours and failure stresses, predict crack initiation locations as observed by indentation tests, highlighting the relevance of stress concentrations around microstructural defects (e.g. oxide inclusions).Sliding contact simulations between a hardmetal surface and a small spherical asperity reproduce the fundamental processes in tribological pairings. The experimentally observed "wavy" morphologies of actual wear surfaces are therefore explained by a mechanism of micro-scale plastic flow and matrix extrusion

High Alpha - Termisesti ruiskutetut kromioksidi- ja piikarbidiseosteiset alumiinioksidipinnoitteet. Loppuraportti

Tässä raportissa on esitetty yhteenveto termisesti ruiskutettujen alumiinioksidipohjaisten pinnoitejauheiden ja niistä eri termisillä ruiskutusmenetelmillä valmistettujen pinnoitteiden ominaisuuksista. Työ liittyy osana TTY:n Materiaaliopin laitoksella toteutettuun Tekes ‐rahoitteiseen High‐ALPHA ‐projektiin, jossa alumiinioksidipinnoitteiden korroosio‐ ja kulumisominaisuuksia pyrittiin parantamaan faasistabiloinnin avulla

Effects of multiple wetting incidents, shear and sliding friction on lubricant stability in SLIPS

Surface icing almost invariably derives from the precursory step of liquid water encountering the surface. Thus, slippery liquid infused porous surfaces, SLIPS, must possess steady wetting durability, and lubricant stability to function as a reliable hydro−/icephobic surface design especially in outdoor applications. Additionally, they should maintain their phobic performance under shear forces, and possess low sliding friction to act as a slippery, multirepellent surfaces. These characteristics are needed in variable applications ranging from moving and rotating blades to steady surfaces, operating in altering climate conditions. More profound durability testing is needed to examine the loss of surface functionality when the lubricant is depleted from the structure via various routes. In addition, the durability tests should be designed to serve the application-related purposes and thus, to reveal performance differences between slippery surfaces for further analysis and targeted end-use development. Here, we tested the wetting durability and stability of SLIPS with multicycle Wilhelmy plate by dipping the surfaces multiple times in water bath. Additionally, we examined the effects of centrifugal and friction-based shear stress to investigate the lubricant depletion from the structure. Tests that measure the durability and the stability of SLIPS designs are in great need in further developing functional slippery surfaces for real outdoor application coatings which encounter environmental stresses, e.g., wetting and icing. Acknowledging the material differences under specific stresses will guide designing the slippery surfaces towards more specific and functionable end-use applications.publishedVersionPeer reviewe

Sliding wear behavior of Cold Metal Transfer cladded Stellite 12 hardfacings on martensitic stainless steel

publishedVersionPeer reviewe

Durability of Lubricated Icephobic Coatings under Various Environmental Stresses

Icephobic coatings interest various industries facing icing problems. However, their durability represents a current limitation in real applications. Therefore, understanding the degradation of coatings under various environmental stresses is necessary for further coating development. Here, lubricated icephobic coatings were fabricated using a flame spray method with hybrid feedstock injection. Low-density polyethylene represented the main coating component. Two additives, namely fully hydrogenated cottonseed oil and paraffinic wax, were added to the coating structure to enhance coating icephobicity. Coating properties were characterised, including topography, surface roughness, thermal properties, wettability, and icephobicity. Moreover, their performance was investigated under various environmental stresses, such as repeated icing/deicing cycles, immersion in corrosive media, and exposure to ultraviolet (UV) irradiation. According to the results, all coatings exhibited medium-low ice adhesion, with slightly more stable icephobic behaviour for cottonseed oil-based coatings over the icing/deicing cycles. Surface roughness slightly increased, and wetting performances decreased after the cyclic tests, but chemical changes were not revealed. Moreover, coatings demonstrated good chemical resistance in selected corrosive media, with better performance for paraffin-based coatings. However, a slight decrease in hydrophobicity was detected due to surface structural changes. Finally, paraffin-based coatings showed better resistance under UV irradiation based on carbonyl index and colour change measurements.publishedVersionPeer reviewe

Composite Finishing for Reuse

Coating processes are emerging for new applications related to remanufactured products from End-of-Life materials. In this perspective, their employment can generate interesting scenarios for the design of products and solutions in circular economy frameworks, especially for composite materials. This chapter would give an overview of coating design and application for recycled glass fiber reinforced polymers on the base of the experimentation made within the FiberEUse project. New cosmetic and functional coatings were developed and tested on different polymer composite substrates filled with mechanically recycled End-of-Life glass fibers. Afterwards, recycled glass fiber reinforced polymer samples from water-solvable 3D printed molds were successfully coated. Finally, new industrial applications for the developed coatings and general guidelines for the coating of recycled glass fiber reinforced polymers were proposed by using the FiberEUse Demo Cases as a theoretical proof-of-concept

Integrity of APS, HVOF and HVAF sprayed NiCr and NiCrBSi coatings based on the tensile stress-strain response

The interlamellar cohesion of thermal spray coatings influences greatly their mechanical properties and ability to use coatings in different loading conditions and wear/erosion resistance. In the present study, micro-tensile testing of free-standing coatings was utilized to evaluate the mechanical response of thermally sprayed coatings. In addition, the longitudinal uniaxial fracture strength of free-standing coatings could be determined by a tensile test. The coating materials studied were NiCr and NiCrBSi coatings sprayed by atmospheric plasma spraying (APS), high velocity oxy-fuel (HVOF), and high velocity air-fuel (HVAF) processes. The different materials used for the coatings sprayed by different methods yield different microstructures, different stress-strain relation in tensile testing. Different tensile test response was found to be related to cohesion strength between lamellas, and thus was affecting the cavitation erosion wear. The effect of other factors such as hardness and residual stresses on cavitation resistance were also discussed. Such results are crucial to understand the suitability of microstructures obtained by TS processes for different wear conditions.publishedVersionPeer reviewe

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.acceptedVersionPeer reviewe