Fretting behaviour of thermal sprayed hardmetal coatings against quenched and tempered steel
Haaja, Vilma (2022)
Haaja, Vilma
2022
Materiaalitekniikan DI-ohjelma - Master's Programme in Materials Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2022-09-02
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202208316848
https://urn.fi/URN:NBN:fi:tuni-202208316848
Tiivistelmä
Fretting movement causes a complex damage mechanism in various mechanical joints involved in machine parts and transmission components. Fretting takes place between contacting surfaces exposed to small amplitude reciprocating motion. It causes wear by combining various wear mechanisms and fatigue damage leading to failure of the component. Due to its severity, various mitigation methods have been studied and proposed.
Different coatings are one notable method to reduce fretting damage. In particular, various hardmetal coatings have shown great potential in different tribological applications due to good mechanical properties and wear resistance. Therefore, fretting behaviour of WC-10Co-4Cr coating deposited by high velocity oxy-fuel (HVOF) and high velocity air-fuel (HVAF) methods was studied in this thesis. Fretting tests were performed with a flat-on-flat test arrangement with a bolted joint contact combining a pair of coated and uncoated quenched and tempered (Q&T) steel specimens. The studied coated specimens were divided into three groups in terms of the coating method, thickness and the surface finishing by grinding. One of the groups contained specimens with thin (90 µm) and an unground coating deposited by HVAF. The specimens of the other two groups had a thick (250 µm) coating, with a ground surface deposited by HVAF and HVOF methods. It has been shown in the literature that cracking in fretting occurs at an early stage, already during the first thousands of cycles. Different loading cycles were used to understand fretting behaviour and crack propagation between coated and plain steel specimens. Damage analysis and characterization were performed by imaging, mainly by the scanning electron microscope.
In terms of fretting fatigue life, the coated and steel specimen pair did not show major change compared to reference data for the steel pair. It is noteworthy that the steel specimens broke in the tests loaded until failure, but the coated specimens remained whole. The cracks observed in tested specimens were limited to the coating and did not progress to substrate material, except in the case of the thin rough coating. Differences in fretting fatigue life between HVOF and HVAF methods were not observed, but there were differences in the amount and shape of cracks. The HVOF sprayed coating was found to be significantly more porous and cracked than the corresponding HVAF coating.
Based on the results, it can be concluded that the coating did not appear to affect the fret-ting of the contact steel, but the progression of damage and cracking in the coating seemed slower. Future studies could elucidate the occurrence of fretting damage between two coated specimens. Although the methods did not directly affect fatigue, based on the characterization, HVAF appears to have better performance and properties in terms of crack delay and wear resistance.
Different coatings are one notable method to reduce fretting damage. In particular, various hardmetal coatings have shown great potential in different tribological applications due to good mechanical properties and wear resistance. Therefore, fretting behaviour of WC-10Co-4Cr coating deposited by high velocity oxy-fuel (HVOF) and high velocity air-fuel (HVAF) methods was studied in this thesis. Fretting tests were performed with a flat-on-flat test arrangement with a bolted joint contact combining a pair of coated and uncoated quenched and tempered (Q&T) steel specimens. The studied coated specimens were divided into three groups in terms of the coating method, thickness and the surface finishing by grinding. One of the groups contained specimens with thin (90 µm) and an unground coating deposited by HVAF. The specimens of the other two groups had a thick (250 µm) coating, with a ground surface deposited by HVAF and HVOF methods. It has been shown in the literature that cracking in fretting occurs at an early stage, already during the first thousands of cycles. Different loading cycles were used to understand fretting behaviour and crack propagation between coated and plain steel specimens. Damage analysis and characterization were performed by imaging, mainly by the scanning electron microscope.
In terms of fretting fatigue life, the coated and steel specimen pair did not show major change compared to reference data for the steel pair. It is noteworthy that the steel specimens broke in the tests loaded until failure, but the coated specimens remained whole. The cracks observed in tested specimens were limited to the coating and did not progress to substrate material, except in the case of the thin rough coating. Differences in fretting fatigue life between HVOF and HVAF methods were not observed, but there were differences in the amount and shape of cracks. The HVOF sprayed coating was found to be significantly more porous and cracked than the corresponding HVAF coating.
Based on the results, it can be concluded that the coating did not appear to affect the fret-ting of the contact steel, but the progression of damage and cracking in the coating seemed slower. Future studies could elucidate the occurrence of fretting damage between two coated specimens. Although the methods did not directly affect fatigue, based on the characterization, HVAF appears to have better performance and properties in terms of crack delay and wear resistance.