| 其他摘要 | Thermally sprayed WC-Co coatings are widely used as protective barriers in different industries such as aerospace, automotive or mining owing to their superior wear resistance to sliding, abrasion, erosion and fretting.
Depositing nanostructured coatings by thermal spraying has received considerable attention, but using detonation gun (DG) spraying for nanostructured WC-Co coatings is scarce. Up to date, the investigation of friction and wear performance of WC-Co coatings has been mainly carried out at room temperature. However, temperature, as an important influence factor on tribological performance in the productive practices, has not been fully considered. Moreover, the tribological behaviors of WC-Co coatings in engineering applications have been generally evaluated by a few traditional methods with a steady load. In fact, this simple wear form is not common and the practical cases are more complicated. For these reasons, there are three major parts in the present paper: The tribological characteristics of nanostructured and conventional WC-Co coatings deposited by detonation gun spraying have been brought into comparison; The friction and wear performance of DG sprayed WC-Co coatings at elevated temperature has been studied; The tribological behaviors of WC-Co coatings prepared by different thermal spray processes, respectively, high velocity air fuel spraying (HVAF), low-pressure plasma spraying (LPPS) and air plasma spraying (APS), have been examined with several friction and wear experimental methods.
The investigation results for nanostructured WC-12Co coatings showed that nanosturctured coatings with homogeneous and dense microstructure, less decarburization of tungsten carbide, narrow distribution of microhardness could be obtained by detonation gun spraying through controlling process parameters, such as oxygen to acetylene ration, spraying distance and particle size of feedstock powders, therefore improved wear resistance. In this study, the particle size of feedstock powders played the most important role on microstructure and tribological properties of nanosturctured coatings. Under identical spraying conditions, nanostructured WC-12Co coatings deposited by detonation gun spraying had denser and more homogeneous microstructure than the conventional counterparts. In spite of their similar microhardness, the sliding wear resistance of nanostructured coatings was inferior to that of conventional ones, especially under high load conditions. The worse wear resistance of the nanostructured coatings was attributed to its relatively more decarburization. Under low load conditions, the sliding wear mechanism of the nanosturctured WC-Co coatings was plastic deformation; while under high load conditions, the removal of nanocrystalline carbide particles along with binder phase dominated the wear michanism of nanostructured coatings and spallation occurred at the weak boundaries between splats due to decarburization. The sliding wear mechanism of the conventional coatings was microcutting. In the single pendulum scratching test, the impact wear resistance of the conventional coatings was better than that of the nanostructured coatings as the result of higher levels of retained cobalt of the former.
The tribological performance of DG sprayed WC-25Co coatings at elevated temperature was investigated. The results showed that the oxide formed during the wear test had important effect on the law of friction and wear of WC-Co coatings. There was a close relation between wear loss of the coatings and mating materials. The wear resistance of the coatings increased with increasing temperature from room temperature to 500℃ when WC-25Co coatings were mated by themselves. The change of friction coefficient with temperature was correlated with the tribological characteristics of the oxidation product of WC-Co coating and its mating material, which appeared at 300℃ and enabled the friction coefficient varying slightly in temperature range of 300 ~ 750℃. Furthermore, it was also found that the choice of DG spraying process conditions and the tribological performance of WC-25Co coatings at elevated temperature were dependent on the size distribution and the shape of the particles of feedstock powders. Feedstock powders with particle size distribution of -63mm + 30mm, and mixing spherical and non-spherical particles well could improve the friction and wear properties of coatings at elevated temperature.
In the single pendulum scratching test, the impact wear resistance of five thermally sprayed coatings decreased in the order high velocity air fuel sprayed WC-12Co coatings (H12), high velocity air fuel sprayed WC-17Co coatings (H17), low pressure plasma sprayed WC-17Co coatings (L17), low pressure plasma sprayed WC-12Co coatings (L12) and air plasma sprayed WC-12Co coatings (A12). The wear mechanism of A12 was plastic deformation and spallation, whilst that of H17、L17 and L12 coatings was initial ploughing followed by plastic deformation with increasing impact load. As far as H12 coatings were concerned, there was a transition by generating microcracks to release the stress between ploughing and plastic deformation in the wear mechanism. In the sliding wear test with a steady load, the wear resistance of five coatings decreased in the order H17, H12, L17, L12 and A12, which was correlated well with their microhardness. In the dry sand rubber wheel abrasion test, the wear resistance of H17 coatings was superior to electroplated hard chromium owing to WC hard particles in coatings effectively blocking the cutting of abrasion. The properties of binder phase could influence the abrasive wear resistance of the coatings. The sliding friction coefficient of WC-Co coatings was investigated preliminarily by the orthogonal tests and the results showed that among the principal influencing factors, the importance decreased in the order lubrication condition, thermal spray process, load, mating material and sliding velocity. The result may be used as an important reference in designing tribological experiments and predicting the sliding friction coefficient of WC-Co coatings. |
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