Damage Tolerance of Thermally Sprayed Oxide Coatings

Termisesti ruiskutettuja keraamipinnoitteita käytetään useissa sovelluksissa eri teollisuudenaloilla, kuten paperi- ja prosessiteollisuudessa, avaruus- ja ilmailuteollisuudessa, sekä energiantuotannossa. Pinnoitteille asetetut vaatimukset vaihtelevat kulumisen ja korroosion kestosta funktionaalisiin ominaisuuksiin, kuten alhaiseen pintaenergiaan tai lämmönjohtavuuteen. Oksidipinnoitteet, kuten yttriastabiloitu zirkoniumoksidi, alumiinioksidi, titaanioksidi ja kromioksidi muodostetaan yleisesti termisen ruiskutuksen prosesseilla käyttäen atmosfääristä plasmaa, suurnopeusliekkiruiskutusta tai perinteistä liekkiruiskutusta. Oksidipinnoitteiden suurin varjopuoli on niiden alttius katastrofaaliseen murtumiseen yhtäkkisestä, odottamattomasta iskusta johtuen. Tämänkaltaisten iskujen mahdollisuus on jatkuvasti läsnä useimmissa sovelluksissa, missä keraamisia pinnoitteita käytetään, minkä vuoksi aihe herättää kiinnostusta laajassa skaalassa teollisuuden aloja. Lisäksi tämä pinnoitteiden ominaisuus — tässä työssä nimetty vauriosietoisuudeksi — rajaa mahdollisten sovelluskohteiden määrää. Tämän vuoksi pienikin parannus vauriosietoisuudessa voi avata ovia uusille teknologioille. Useita keinoja keraamipinnoitteiden vauriosietoisuuden parantamiseksi on kokeiltu, kuten metallin lisäystä pinnoitteeseen, oksidisekoituksia ja nanorakenteisia pinnoitteita, mutta toistaiseksi parannukset suorituskyvyssä ovat olleet varsin nimellisiä, tai ovat heikentäneet pinnoitteen muita hyödyllisiä ominaisuuksia. Lisäksi vauriosietoisuuden mittaaminen tarkasti ja toistettavasti on haastavaa. Nykymenetelmiin kuuluu laboratoriomittakaavan kokeet, jotka antavat tietoa materiaalin ja pinnoitteen luonteesta, sekä sovelluspainotteiset kokeet, joista saatu tieto ei ole laajasti hyödynnettävissä muissa ympäristöissä. Tämän tutkimuksen pääpaino oli arvioida eri menetelmiä termisesti ruiskutettujen keraamipinnoitteiden vauriosietoisuuden mittaamiseksi. Vauriosietoisuus jaettiin kahteen selkeästi toisistaan eroavaan ominaisuuteen: särön etenemisen vastustuskykyyn ja matalaenergisten iskujen vastustuskykyyn. Näistä edellinen on lähellä murtositkeyden määritelmää, mutta tähtää tuottamaan laajemmin sovellettavia tuloksia. Särön etenemisen vastustuskyvyn mittausmenetelmiin luetaan nelipistetaivutus akustisella emissiolla instrumentoituna ja suuren energia iskut särön polun tutkimisella. Nämä menetelmät antavat tietoa mikrorakenteen vaikutuksesta pinnoitteen sitkeyteen. Lamellienvälinen koheesio paljastui sitkeyden kannalta heikoimmaksi lenkiksi, sillä heikko rajapinta luo helpoimman etenemisreitin särölle. Edelleen kokeet paljastivat, että tiiviimmät HVOF-pinnoitteet käyttäytyivät hauraammin kuin plasmaruiskutetut vastinparinsa, sillä niissä ei ollut esisäröytyneitä alueita, jotka voisivat vapauttaa pinnoitteeseen muodostuneita jännitystiloja. Matalaenergisten iskujen vastustuskyky on hieman enemmän sovelluslähtöinen tähdäten mukailemaan iskumaista vauriota tosielämän tilanteissa. Sen mittausmenetelmiin taas luetaan väsyttäminen mikrokoon iskuilla, joissa pieni painin iskeytyy pintaan toistuvasti korkealla taajuudella, sekä kavitaatioeroosio, jossa valtava määrä luhistuvista kuplista johtuvia iskeymiä tuottavat tilastollisen lähestymistavan mikrometriluokan pinnoitteen koheesion mittaamiseen. Näiden testien tulokset korreloivat hyvin vauriosietoisuuden käsitteen kanssa, sillä ne mittasivat pinnoitteen ominaisuuksia yleisemmällä tasolla. Koska nämä mittaustavat hyödyntävät pieniä iskuja, pinnoitteen kovuus oli vauriosietoisuuden kannalta määräävä tekijä, kunnes iskujen energia ylitti tietyn raja-arvon. Tätä rajaa suurempienergiset iskut johtivat joko pinnoitteen katastrofaaliseen vaurioitumiseen tai vaiheittaiseen vaurion etenemiseen. Näistä jälkimmäinen on vahvasti suositumpi, sillä silloin tosielämän tilanteissa jää aikaa reagoida ennen komponentin tuhoutumista. Toissijainen painopiste oli parannetun vauriosietoisuuden keraamipinnoitteiden valmistaminen uusia ruiskutusprosesseja käyttäen. Tämän toteamiseksi käytetään ensimmäisessä vaiheessa arvioituja mittausmenetelmiä. Käytetyt ruiskutusmenetelmät olivat suspensiosuurnopeusliekkiruiskutus ja nestemäisen prekursorin ja jauheen syöttäminen samanaikaisesti nk. hybridisuurnopeusliekkiruiskutuksessa. Suspensioruiskutetut kromioksidipinnoitteet osoittivat parannuksia vauriosietoisuudessa säilyttäen tai parantaen kulumisenkestoaan ja kovuuttaan. Alumiinioksidijauheen ja zirkoniumasetaattiliuoksen hybridiruiskutus paljasti tarpeen ruiskutusprosessin lisäoptimoinnille, sillä sulamattomat, prekursorista peräisin olevat nanopartikkeliagglomeraatit heikensivät pinnoiterakennetta koheesion parantamisen sijaan. Tästä huolimatta, pinnoitteen saumattoman räätälöinnin lupaava potentiaali kannustaa tutkimaan myös hybridiruiskutusta tulevaisuudessa.Thermally sprayed ceramic coatings are utilized in various applications in industries, such as paper- and process, aerospace and energy production. The requirements for the coatings vary from wear resistance and chemical stability to functional properties, such as low surface energy or thermal conductivity. Oxide coatings, such as yttriastabilized zirconium oxide, aluminum oxide, titanium oxide and chromium oxide are commonly deposited by thermal spray processes using an atmospheric plasma, a high-velocity oxy-fuel or a flame spray torch. The biggest drawback of the oxide coatings is their susceptibility to catastrophic failure from sudden, unexpected impacts, consequently leading to the functional failure of the component. The possibility of such impacts is omnipresent in most applications where ceramic coatings are used, which makes the topic attractive to a wide range of industries. This property of the coatings — named damage tolerance for the purposes of this thesis — additionally limits the number of possible applications. Therefore, any improvement in damage tolerance could open doors to various new technologies. Multiple workarounds have been attempted in improving the damage tolerance of ceramic coatings, such as metallic additions, oxide mixtures and nanostructured coatings, but so far increases in performance have been modest or have deteriorated other beneficial functions of the coating. Furthermore, there lies a challenge in accurate and repeatable measurement of the damage tolerance. Current methodology includes testing in laboratory scale, giving information on the nature of the material and coating, and application-based testing, where the obtained information is not widely applicable in other conditions. In this study, the primary focus was to evaluate different methods of measuring the damage tolerance of thermally sprayed ceramic coatings. Damage tolerance was divided in two distinguishable properties: crack propagation resistance and resistance to low-energy impacts. The former is akin to fracture toughness, but aims to give a more transferable result. Measurement methods of crack propagation resistance evaluated include four-point bending with acoustic emission instrumentation and high-energy impacts from spherical projectiles with crack path tracing. These methods provided insights into the effect of microstructure on the toughness of the coating. Interlamellar cohesion was shown to be the weakest link of toughness in that the weak interfaces provide the path of least resistance for crack propagation. Additionally, denser HVOF coatings proved more brittle than their plasma-sprayed counterparts as they did not have stress-relieving zones from pre-cracked areas. The low-energy impact approach is slightly more application-oriented, aiming to emulate impact damage conditions in real-life environments. The methods used to measure it are micro-impact fatigue, where a small indenter is repeatedly impacted on the surface with high frequency, and cavitation erosion, where a vast number of impacts from collapsing bubbles create a statistical approach to measuring coating cohesion in the micrometer scale. The results of these tests correlated well with the concept of damage-tolerance as they measured the properties of the coating in a more general level. Since these methods rely on small impacts, hardness of the coating was a determining factor of damage-tolerance until the energy of the impact rose past a coating-specific threshold. Above this value, the coatings either failed catastrophically, or showed a more gradual failure propagation. The latter of these behaviors is highly preferred, as it gives time to react before the component fails in real conditions. The secondary focus was to create ceramic coatings with increased damage tolerance through novel spray processes, as measured by the screened testing methods. The spray methods were suspension HVOF-spraying and solution-precursor -powder hybrid HVOF spraying. The suspension sprayed Cr2O3-coatings provided improvements in damage tolerance with similar or improved levels of wear resistance and hardness. The hybrid-spraying of Al2O3 powder and a zirconium acetate based precursor proved to still require further optimization of the spray process, as unmolten agglomerates of precursor-derived nanoparticles rather weakened the coating, instead of improving the cohesion. Nonetheless, promising potential is foreseen for the hybrid-spraying in the future due to its ability to tailor the coating composition rather seamlessly

Wetting Behavior and Functionality Restoration of Cold-Sprayed Aluminum-Quasicrystalline Composite Coatings

Coating design is an efficient strategy to engineer wettability of surfaces and adjustment of the functionality to the necessities in industrial sectors. The current study reveals the feasibility of functional aluminum/quasicrystalline (Al-QC) composite coatings fabrication by cold spray technology. A commercially available Al-based quasicrystalline powder (Al-Cr-Fe-Cu) was combined with aluminum alloy (Al6061) feedstock materials to make coatings with various compositions. A set of cold spray process parameters was employed to deposit composite coatings with different QC-Al ratios and Al6061 coatings as counterparts. The wettability of the coating surfaces was measured by static water droplet contact angles using a droplet shape analyzer and investigation of the dynamic of water droplet impact by high-speed imaging. Through microstructural studies, the Al-QC composites revealed dense structure, well-integrated and adherent deposits, providing structural reliability and enhanced hydrophobic behavior. In the last step of this work, composite coatings were deposited over eroded cold-sprayed Al6061 and a selected composite to demonstrate the feasibility of repairing the damaged part and function restoring. The results and approach used in this work provide understanding of cold-sprayed Al-QC composite coatings manufacturing and their wetting behavior state for cross-field applications.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

Mining tailings as a raw material for glass-bonded thermally sprayed ceramic coatings : Microstructure and properties

Magnesium aluminate, MgAl2O4, spinel powders for thermal spraying, were synthesized from secondary raw materials by spray drying and subsequent reaction sintering. Talc ore mining tailings and aluminium hydroxide precipitate from aluminium anodizing process were studied. A stoichiometric MgAl2O4 spinel coating was prepared as a reference using pure raw materials. Atmospheric plasma spraying resulted in the formation of ceramic coatings. Microstructural investigations revealed that the reference coatings exhibited crystalline lamellar microstructure of MgAl2O4 but secondary coatings contained amorphous areas between the crystalline MgAl2O4 clusters. Abrasive wear test results revealed considerably lower wear rate for secondary coatings. It is suggested that the different structure of coatings, particularly the high degree of amorphous phase between the isolated crystalline MgAl2O4 clusters caused the higher abrasive wear resistance by changing the wear mechanism. The dielectric breakdown strength of the secondary coatings were at the same level, 24 V/μm, as compared to reference coating, 23 V/μm.acceptedVersionPeer reviewe

Tribological Assessment of Cold Sprayed Aluminum-Quasicrystal Composite Coatings

Cold spray (CS) technology has proven an enormous potential in the production of composite coatings, enabling a production of materials with superior qualities such as enhanced tribological behavior. This study aims to investigate the tribological properties of CS Al-based composite coatings reinforced by quasicrystalline (QC) particles. Two different Al alloys were used as the matrix, AA 6061 and AA 2024, and mixed with Al-based QC particles (Al-Cr-Fe-Cu) at different Al/QC ratios. A room-temperature ball-on-disc test was then used to evaluate the wear resistance of the CS composite coatings in air and compared to those of the CS non-reinforced Al alloy coatings as well as cast counterparts (AA 6061-T6). We have demonstrated that CS could be employed to produce dense and thick Al-QC composites. Further, the addition of the QC particles into the structure increased the wear resistance of the matrix resistance up to 8 times.Peer reviewe

Damage Tolerance of Thermally Sprayed Oxide Coatings

Termisesti ruiskutettuja keraamipinnoitteita käytetään useissa sovelluksissa eri teollisuudenaloilla, kuten paperi- ja prosessiteollisuudessa, avaruus- ja ilmailuteollisuudessa, sekä energiantuotannossa. Pinnoitteille asetetut vaatimukset vaihtelevat kulumisen ja korroosion kestosta funktionaalisiin ominaisuuksiin, kuten alhaiseen pintaenergiaan tai lämmönjohtavuuteen. Oksidipinnoitteet, kuten yttriastabiloitu zirkoniumoksidi, alumiinioksidi, titaanioksidi ja kromioksidi muodostetaan yleisesti termisen ruiskutuksen prosesseilla käyttäen atmosfääristä plasmaa, suurnopeusliekkiruiskutusta tai perinteistä liekkiruiskutusta. Oksidipinnoitteiden suurin varjopuoli on niiden alttius katastrofaaliseen murtumiseen yhtäkkisestä, odottamattomasta iskusta johtuen. Tämänkaltaisten iskujen mahdollisuus on jatkuvasti läsnä useimmissa sovelluksissa, missä keraamisia pinnoitteita käytetään, minkä vuoksi aihe herättää kiinnostusta laajassa skaalassa teollisuuden aloja. Lisäksi tämä pinnoitteiden ominaisuus — tässä työssä nimetty vauriosietoisuudeksi — rajaa mahdollisten sovelluskohteiden määrää. Tämän vuoksi pienikin parannus vauriosietoisuudessa voi avata ovia uusille teknologioille. Useita keinoja keraamipinnoitteiden vauriosietoisuuden parantamiseksi on kokeiltu, kuten metallin lisäystä pinnoitteeseen, oksidisekoituksia ja nanorakenteisia pinnoitteita, mutta toistaiseksi parannukset suorituskyvyssä ovat olleet varsin nimellisiä, tai ovat heikentäneet pinnoitteen muita hyödyllisiä ominaisuuksia. Lisäksi vauriosietoisuuden mittaaminen tarkasti ja toistettavasti on haastavaa. Nykymenetelmiin kuuluu laboratoriomittakaavan kokeet, jotka antavat tietoa materiaalin ja pinnoitteen luonteesta, sekä sovelluspainotteiset kokeet, joista saatu tieto ei ole laajasti hyödynnettävissä muissa ympäristöissä. Tämän tutkimuksen pääpaino oli arvioida eri menetelmiä termisesti ruiskutettujen keraamipinnoitteiden vauriosietoisuuden mittaamiseksi. Vauriosietoisuus jaettiin kahteen selkeästi toisistaan eroavaan ominaisuuteen: särön etenemisen vastustuskykyyn ja matalaenergisten iskujen vastustuskykyyn. Näistä edellinen on lähellä murtositkeyden määritelmää, mutta tähtää tuottamaan laajemmin sovellettavia tuloksia. Särön etenemisen vastustuskyvyn mittausmenetelmiin luetaan nelipistetaivutus akustisella emissiolla instrumentoituna ja suuren energia iskut särön polun tutkimisella. Nämä menetelmät antavat tietoa mikrorakenteen vaikutuksesta pinnoitteen sitkeyteen. Lamellienvälinen koheesio paljastui sitkeyden kannalta heikoimmaksi lenkiksi, sillä heikko rajapinta luo helpoimman etenemisreitin särölle. Edelleen kokeet paljastivat, että tiiviimmät HVOF-pinnoitteet käyttäytyivät hauraammin kuin plasmaruiskutetut vastinparinsa, sillä niissä ei ollut esisäröytyneitä alueita, jotka voisivat vapauttaa pinnoitteeseen muodostuneita jännitystiloja. Matalaenergisten iskujen vastustuskyky on hieman enemmän sovelluslähtöinen tähdäten mukailemaan iskumaista vauriota tosielämän tilanteissa. Sen mittausmenetelmiin taas luetaan väsyttäminen mikrokoon iskuilla, joissa pieni painin iskeytyy pintaan toistuvasti korkealla taajuudella, sekä kavitaatioeroosio, jossa valtava määrä luhistuvista kuplista johtuvia iskeymiä tuottavat tilastollisen lähestymistavan mikrometriluokan pinnoitteen koheesion mittaamiseen. Näiden testien tulokset korreloivat hyvin vauriosietoisuuden käsitteen kanssa, sillä ne mittasivat pinnoitteen ominaisuuksia yleisemmällä tasolla. Koska nämä mittaustavat hyödyntävät pieniä iskuja, pinnoitteen kovuus oli vauriosietoisuuden kannalta määräävä tekijä, kunnes iskujen energia ylitti tietyn raja-arvon. Tätä rajaa suurempienergiset iskut johtivat joko pinnoitteen katastrofaaliseen vaurioitumiseen tai vaiheittaiseen vaurion etenemiseen. Näistä jälkimmäinen on vahvasti suositumpi, sillä silloin tosielämän tilanteissa jää aikaa reagoida ennen komponentin tuhoutumista. Toissijainen painopiste oli parannetun vauriosietoisuuden keraamipinnoitteiden valmistaminen uusia ruiskutusprosesseja käyttäen. Tämän toteamiseksi käytetään ensimmäisessä vaiheessa arvioituja mittausmenetelmiä. Käytetyt ruiskutusmenetelmät olivat suspensiosuurnopeusliekkiruiskutus ja nestemäisen prekursorin ja jauheen syöttäminen samanaikaisesti nk. hybridisuurnopeusliekkiruiskutuksessa. Suspensioruiskutetut kromioksidipinnoitteet osoittivat parannuksia vauriosietoisuudessa säilyttäen tai parantaen kulumisenkestoaan ja kovuuttaan. Alumiinioksidijauheen ja zirkoniumasetaattiliuoksen hybridiruiskutus paljasti tarpeen ruiskutusprosessin lisäoptimoinnille, sillä sulamattomat, prekursorista peräisin olevat nanopartikkeliagglomeraatit heikensivät pinnoiterakennetta koheesion parantamisen sijaan. Tästä huolimatta, pinnoitteen saumattoman räätälöinnin lupaava potentiaali kannustaa tutkimaan myös hybridiruiskutusta tulevaisuudessa.Thermally sprayed ceramic coatings are utilized in various applications in industries, such as paper- and process, aerospace and energy production. The requirements for the coatings vary from wear resistance and chemical stability to functional properties, such as low surface energy or thermal conductivity. Oxide coatings, such as yttriastabilized zirconium oxide, aluminum oxide, titanium oxide and chromium oxide are commonly deposited by thermal spray processes using an atmospheric plasma, a high-velocity oxy-fuel or a flame spray torch. The biggest drawback of the oxide coatings is their susceptibility to catastrophic failure from sudden, unexpected impacts, consequently leading to the functional failure of the component. The possibility of such impacts is omnipresent in most applications where ceramic coatings are used, which makes the topic attractive to a wide range of industries. This property of the coatings — named damage tolerance for the purposes of this thesis — additionally limits the number of possible applications. Therefore, any improvement in damage tolerance could open doors to various new technologies. Multiple workarounds have been attempted in improving the damage tolerance of ceramic coatings, such as metallic additions, oxide mixtures and nanostructured coatings, but so far increases in performance have been modest or have deteriorated other beneficial functions of the coating. Furthermore, there lies a challenge in accurate and repeatable measurement of the damage tolerance. Current methodology includes testing in laboratory scale, giving information on the nature of the material and coating, and application-based testing, where the obtained information is not widely applicable in other conditions. In this study, the primary focus was to evaluate different methods of measuring the damage tolerance of thermally sprayed ceramic coatings. Damage tolerance was divided in two distinguishable properties: crack propagation resistance and resistance to low-energy impacts. The former is akin to fracture toughness, but aims to give a more transferable result. Measurement methods of crack propagation resistance evaluated include four-point bending with acoustic emission instrumentation and high-energy impacts from spherical projectiles with crack path tracing. These methods provided insights into the effect of microstructure on the toughness of the coating. Interlamellar cohesion was shown to be the weakest link of toughness in that the weak interfaces provide the path of least resistance for crack propagation. Additionally, denser HVOF coatings proved more brittle than their plasma-sprayed counterparts as they did not have stress-relieving zones from pre-cracked areas. The low-energy impact approach is slightly more application-oriented, aiming to emulate impact damage conditions in real-life environments. The methods used to measure it are micro-impact fatigue, where a small indenter is repeatedly impacted on the surface with high frequency, and cavitation erosion, where a vast number of impacts from collapsing bubbles create a statistical approach to measuring coating cohesion in the micrometer scale. The results of these tests correlated well with the concept of damage-tolerance as they measured the properties of the coating in a more general level. Since these methods rely on small impacts, hardness of the coating was a determining factor of damage-tolerance until the energy of the impact rose past a coating-specific threshold. Above this value, the coatings either failed catastrophically, or showed a more gradual failure propagation. The latter of these behaviors is highly preferred, as it gives time to react before the component fails in real conditions. The secondary focus was to create ceramic coatings with increased damage tolerance through novel spray processes, as measured by the screened testing methods. The spray methods were suspension HVOF-spraying and solution-precursor -powder hybrid HVOF spraying. The suspension sprayed Cr2O3-coatings provided improvements in damage tolerance with similar or improved levels of wear resistance and hardness. The hybrid-spraying of Al2O3 powder and a zirconium acetate based precursor proved to still require further optimization of the spray process, as unmolten agglomerates of precursor-derived nanoparticles rather weakened the coating, instead of improving the cohesion. Nonetheless, promising potential is foreseen for the hybrid-spraying in the future due to its ability to tailor the coating composition rather seamlessly

Characterization of Powder-Precursor HVOF-Sprayed Al2O3-YSZ/ZrO2 Coatings

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.publishedVersionPeer reviewe

Microstructure and Wetting Performance of High-Pressure Cold Sprayed Quasi-Crystalline Composite Coatings

High-pressure cold spraying has shown significant potential in manufacturing metallic composite coatings for a wide range of industrial applications, including wear and corrosion protection. Quasi-crystalline materials, in turn, are promising candidates due to their unique microstructural features. Combining these concepts, metallic compo- site coatings were generated using high-pressure cold spraying to produce functional and protective coatings. Several spray trials were done to detect the effect of compositions and size of quasi-crystalline feedstock materials mixed with metal powders, Al6061, and stainless steel 316L, on coating microstructure, integrity, and surface properties. A scanning electron microscope was used to examine the microstructure of the feedstock materials and composite coatings. A 3D surface optical profilometer was also used to investigate surface texture. The wettability of the coating surfaces was measured by static water contact angles using a droplet shape analyzer. Cold-sprayed quasi-crystalline composite coatings showed denser and well-integrated deposits with a random distribution of phases across the composite surface, indicating promising structural reliability and hydrophobic behavior.publishedVersionNon peer reviewe

Process Parameter Impact on Suspension-HVOF-Sprayed Cr2O3 Coatings

Chromium oxide (Cr2O3) is commonly used as an atmospheric plasma-sprayed (APS) coating from powder feedstock in applications requiring resistance to sliding wear and corrosion, as well as amenability to texturing, e.g., in anilox rolls. Recently, high-velocity oxy-fuel spray methods involving suspension feedstock have been considered an extremely promising alternative to produce denser and more homogeneous chromium oxide coatings with lower as-sprayed surface roughness, higher hardness and potentially superior wear performance compared to conventional APS-sprayed coatings. In this study, the impact of process parameters namely auxiliary air cleaning nozzles and a transverse air curtain on suspension high-velocity oxy-fuel-sprayed Cr2O3 suspensions is presented. The produced coatings are characterized for their microstructure, mechanical properties and wear resistance by cavitation erosion. The results reveal the importance of optimized air nozzles and air curtain to achieve a vastly improved coating structure and performance.publishedVersionPeer reviewe