Caracterisation des mecanismes d'usure en cavitation de revetements HVOF a base de CaviTec
Why this work is in the frame
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Bibliographic record
Abstract
La cavitation se produit principalement dans les ecoulements turbulents ou des zones de depression-surpressions provoquent l’effondrement de microbulles gazeuses. L’implosion de ces germes de cavitation produit des ondes de choc et des microjets d’eau pouvant causer des impacts de l’ordre du giga pascal sur les surfaces en immersion. Les turbines hydrauliques sont particulierement sujettes a ce type d’erosion pouvant causer plusieurs kilogrammes de degradation sur les pales d’un seul appareil par annee. Il devient ainsi important de mettre au point des revetements protecteurs pour reparer les turbines et autres composantes hydrauliques. Le test d’erosion par cavitation ultrasonique a ete utilise pour simuler un environnement agressif de cavitation. En accord avec la norme ASTM, une distance de 500 μm a ete utilisee entre la tige, oscillant a une frequence de 20kHz et une amplitude de 50 μm, et les echantillons. De l’eau demineralisee maintenue a une temperature de 23°C fut utilise comme milieu d’immersion afin de negliger les effets de corrosion. Chaque echantillon fut erode qu’une seule fois pendant une periode determinee afin d’assurer une plus grande reproductibilite des tests. L’alliage CaviTec, connu pour sa tres grande resistance a la cavitation sous forme massique, fut donc utilise pour produire un revetement de projection thermique HVOF. Cette technique comportant plusieurs avantages pour la production de couches protectrice, les parametres de depots furent optimises pour augmenter la resistance a la cavitation des revetements CaviTec HVOF. Le taux d’erosion fut reduit d’un facteur 3 pour atteindre 1,6 mg/h ce qui se compare tres bien avec celui des revetements Stellite-6 (1,0 mg/h) et WC-CoCr (2,5 mg/h) aussi testes dans cette etude. Cependant, la periode d’incubation du revetement CaviTec (~20 min) est loin d’atteindre celle du meme alliage sous forme massique (>8h). Une analyse du mecanisme de degradation a revele que les defauts dus a la microstructure lamellaire du revetement HVOF accelerent sa degradation. En effet, les particules resistantes de CaviTec sont arrachees de la microstructure due a la propagation de fissures a la frontiere des splats orientes parallelement au substrat. Ainsi, une optimisation du procede de depot, incluant un possible recuit, est toujours envisageable afin d’ameliorer les proprietes mecaniques de ce revetement.----------Abstract Hydraulic turbines used in a constant flow of water can suffer from severe damage. Turbine blade degradation originates mostly from erosion wear, corrosion and cavitation. It has been shown that in fresh water, cavitation erosion is the main wear mechanism. During cavitation erosion, implosion of unstable air bubbles create shock waves and micro-jets of water causing impact forces on immersed surfaces varying from MPa to GPa. The implosion of thousands of bubbles at high frequency induce a low cycle fatigue wear mechanism. The ultrasonic cavitation erosion test is used to investigate the cavitation resistance of material. In this study, the ASTM standard has been respected using a vibrating horn of 20 kHz frequency and 50 μm amplitude. Place in demineralised water (cooled a 23°C), the tip to sample distance was 500 μm. Cavitation erosion curves were built measuring the mass loss of samples after different erosion times to compare the resistance of tested materials. Each sample was eroded once without replacing it in the erosion system. The CaviTec alloy was used to produce a new HVOF coating because this material is known to resist very well, in bulk form, to cavitation erosion. On the other end, the thermal spray deposition offer the advantages of a fast and automated additive techniques. Optimizing deposition parameters, the erosion rate of CaviTec HVOF coatings has been reduced by a factor of 3 reaching 1,6 mg/h which is comparable to Stellite-6 and WC-CoCr coatings (respectively 1,0 and 2,5 mg/h) also tested in this study. However, the incubation period of this new HVOF coating (~20min) is far from reaching the level of the bulk CaviTec (>8h). A systematic study of HVOF coatings degradation mechanism revealed than the lamellar microstructure accelerates the material loss. In fact, the preferential erosion of coating defects (oxide, pores and splat boundary regions) leads to the extraction of cavitation resistant CaviTec splats. A further optimization of the deposition process including a heat treatment of samples surface could be a good solution to recover the incubation period but more study is needed to confirm this hypothesis.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.002 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.002 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it