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Text | Xiaoxin
Editor|Xiaoxin
The use of WC-Co coatings in applications with intense wear/corrosion has developed into a well-established industrial practice over the past few decades. This metal-ceramic coating is typically applied by thermal spray methods such as atmospheric plasma spray (APS), high-speed oxygen fuel spray (HVOF), and high-speed air-fuel spray (HVAF).
The latter two methods are generally preferred because of their lower deposition temperature and ability to generate high-velocity gas streams, ultimately producing dense, hard coatings in an economically viable manner with minimal thermal decomposition of WC.
●○Raw material powder, spraying process and experimental device ○●
A steel substrate (SAE1070)3 with a size of 160×80×3 mm is coated with WC-17Co mass percentage commercial sintered powder with a particle size distribution of (33+9), and the morphology of the powder particles can be seen in Figure 1. This is an image of a backscattered electron (BSC) detector in a scanning electron microscope (SEM).
Prior to deposition, the substrate is blasted with alumina particles with a median size of 46 microns at a distance of 100 mm, followed by high-pressure air injection and mechanical methods to remove any residual grit on the substrate surface. The coating deposition is done using a patented "compact HVOF" process that includes isentropic plug nozzles to accelerate the exhaust gases to supersonic speeds of Mach 2.7.
The process parameters of the spray gun are previously optimized internally using Oseir's SprayWatch system to achieve the best microstructure (represented by minimum porosity and binder mean free path and maximum WC volume %) and highest microhardness, which occurs at a spray distance of 120 mm and a spray angle of 90°.
Dry sliding tests were carried out on 20 coatings at four intervals (120, 138, 170 and 240 mm) and five spray angles (90, 75, 60, 45 and 30°). All tested samples were sprayed at a consistent gun traverse speed of 502 mm/s.
Average particle velocity and temperature at boundary spray distances of 120 and 240mm, measured by Oseir's SprayWatch system. These samples are named (A1 to E4) to identify their motion states in the correlation plot. Sample names and respective conditions of movement, where wear results are shown.
●○ Dry sliding test and abrasion scar morphology ○●
Wear rate (SWR) and average coefficient of friction (COF) as well as microhardness, WC volume % and WC retention index with their respective names and conditions of motion. Microstructure results have been discussed in detail in previous work and correlated with spray kinematics parameters. The separation distance of 120 mm and the injection angle of 90° are the optimal conditions for the smallest heat generation and the maximum velocity normal component on impact during particle flight.
As a result, they produce optimal microhardness, porosity and WC density, and are therefore expected to show minimal wear damage under the light wear conditions employed in this work. In view of this, the wear properties of coating A1 (90,120mm) can be used as a reference point for the influence of spray distance and spray angle.
The specific wear rate of coating a1 was well consistent, and the sliding wear of HVOF, graphite tungsten carbide-12Co was investigated27 and 28, and WC-FeCrAl and WC-10Co4Cr coatings were detected under similar conditions.
In addition, coating a1 also appears to have the lowest average COF. As can be seen in Figure 2, coatings sprayed at distances of 120 and 138mm behave similarly in terms of SWR over the entire spray angle inspected.
The inclination angle from 90° to 75° does not seem to affect the SWR at these spray distances. After exceeding 75°, a significant increase in the specific wear rate is observed when the injection angle reaches 30°. At a spray distance of 170mm, the wear coefficient of all coatings is significantly higher than the shorter spray distance at all spray angles. The rise associated with the tilted injection angle is mild between 90° and 75° (5% increase) and rises at a steady rate of 30% per injection angle iteration until 30°.
At 240mm, the deterioration of the coating is evident as the spray distance increases, and likewise, regardless of the spray angle, all relevant coatings exhibit significantly higher SWR than coatings sprayed at shorter spray distances. In addition, the effect of the injection angle at 240mm is very different from the shorter spray distance.
Unexpectedly, from 90° (a4) to 75° (B4) injection angles, wear resistance has improved significantly, SWR has been reduced by 23%, and from 75°, SWR steadily increases with the increase of injection angle, increasing by 15-30% per angle iteration. Coating E4 exhibits poor wear resistance, and its SWR is more than six times higher than Coating A1. The reasons for the increase in SWR at 75° can be traced back to the associated coating for microstructure and mechanical properties, which will be discussed in detail later in this article.
Although coatings sprayed at 120 and 138mm behaved similarly in terms of wear volume loss, it was clear that for the rest of the test spray distance, the effect of spray distance was more severe than the effect of reducing the spray angle on the wear behavior of the coating. The effect of spray distance is most severe in cases where tilt angles are small, such as 90° and 75°, in which case the SWR more than doubles with each turf iteration after 138mm.
As the spray angle becomes more inclined, the relative effect of spray distance on SWR is milder, but still greater than the influence of spray angle. Interestingly, the standard error of the measurement increased significantly with the increase of the spray distance, indicating a large dispersion in the measurement of volume loss on the coating of longer turf. Ultimately, the increased measurement dispersion can be explained by the appearance of more and more large pits in the wear trajectories of coatings applied over long distances.
A collection of abrasion marks images taken with an optical microscope is included to compare the macromorphological characteristics of abrasion marks between test coatings. On coatings sprayed at short spray distances and close to vertical spray angles (A1, B1, A2, B2), wear marks are less noticeable. Under these conditions, minimal wear products are produced and some isolated dark areas are visible on the wear trajectory.
As the spray distance increases and the spray angle becomes more inclined, the frequency of leaving pits and dark materials on the wear trajectory increases dramatically. In addition, at spray distances of 170 and 240mm, large spalling and pits can be found on the wear trajectory. EDS and XPS analysis showed that the darker areas on the wear marks were mainly composed of oxides of Co and W and some trace amounts of Al2O3 reflexivity.
This formation is usually found on the wear trajectories of metal-ceramic materials worn in air, and they are often referred to as friction films or transfer films. It can be seen that the coatings (E2, E3, E4) sprayed at an angle of 30° and a distance greater than 120mm are almost completely covered by a friction film. The same applies to all coatings sprayed at 240mm, regardless of the spray angle (A4, B4, C4, D4, E4).
It has been determined that the resulting friction film results from oxidation, compaction/deformation and sintering of wear debris that has been trapped between sliding surfaces and has been smeared onto the wear trajectory, invading any surface cavities caused by previous material loss.
However, the effect of friction film on the wear behavior of cermet coatings is unclear. Some authors believe that it has lubricating properties due to the presence of certain oxides produced by frictional heat generation and contribute to the reduction of COF, while others demonstrate that COF is actually improved in the presence of friction films.
●○Microstructure of wear marks○●
The boundary of the wear trajectory is characterized by the fact that by comparing the protruding WC grain inside the wear trajectory with the smoother surface finish on the outside, the adhesive extrusion effect that occurs during dry sliding is obvious, and the surface finish of the outer coating of the wear mark can be seen in more detail in the figure.
Slight unidirectional marks of polished abrasives can be detected. In addition, due to the smooth surface finish, the WC grain boundary is not significantly different from the surrounding binder. Some inconsistencies were found on the polished coating surface that originated from the pull-out of the WC during the grinding and polishing steps, or from the weakly bonded splash boundary that was preferentially excavated, but they rarely occurred.
Due to the preferential removal of the surrounding binder, the WC grains in the wear trajectory appear to be better qualified, exposing them more to sliding pairs. At a large depth below the sliding interface, there is a plastic flow in the binder, because all the plastic strain generated by the sliding motion is completely absorbed by the binder.
In turn, the plastic flow of the adhesive releases any residual stress, which, together with the adhesive that consumes the surface locally by extrusion, makes the WC grains of the surface easier to pull out and break.
From the figure, the engraved morphology formed by the adhesive extrusion can be seen in more detail, and coating a1 mainly shows isolated WC grain extraction locations. The adjacent WC grains that are pulled out create pits that then join together under repetitive wear and evolve into larger pits. In addition, some sites have large pits with spans close to 10 microns, filled with compacted wear debris.
As mentioned above, the origin of larger pits may be through the gradual enlargement and incorporation of smaller pits; Alternatively, they can be formed by the connection of subsurface cracks and the peeling of larger fragments containing the WC grain and the coating of the adhesive around it. In addition, the fact that such a crater is close to the expected splash size indicates.
To some extent, they may be due to the individual splash delamination that is often observed under such wear conditions. The figure shows where small cracks propagate along intergranular and intergrain propagation. This surface crack has also been observed in previous work inspecting the dry sliding of cermet coatings.
The presence of through-crystal cracks indicates that (1) the high bond strength of the binder-WC couple and (2) the sufficiently high toughness of the binder phase make the fracture of WC priority over the intergranular propagation of the crack, which is relatively less in coating a1.
Indicates the plasticity of the friction film formed at this location and the size of the WC abrasive that wears it. Even if the coating surface outside the wear trajectory is not completely smooth and shows distant pits (due to pull-out during the grinding/polishing phase), there are no cracks on the coating surface, as shown in Figure 4(f).
In addition to WC pull-out, two other main mechanisms of WC removal on the upper surface of the wear trajectory (a1, b1, a2, b2) were observed in coatings sprayed at short spray distances and near-vertical spray angles.
The first issue concerns the survival of areas with a very high ratio of WC to co-binder (WC-cluster) in the coating, which leads to insufficient support due to scarcity of surrounding binder. These WC clusters are derived from the structure of the starting granules, where they are initially formed during the agglomeration and sintering stages of powder production.
It has been shown that such WC clusters are usually retained in the coating microstructure, and it can now be seen that they are a favorable point for the initial removal of unsupported carbide particles during the sliding process. Finally, some surface WC grains break due to excessive local contact stress. With insufficient support, this fracture leads to the gradual separation and removal of fragments and is facilitated when loose WC particles act like third-body abrasives during sliding.
With regard to the friction membrane, its formation is determined by the relative rate at which wear debris is generated and removed from the system. Once the critical rate of wear debris generation is reached, more debris is trapped in the sliding interface rather than removed from the system, and a friction film layer gradually forms.
In addition, friction film coverage on the wear trajectory appears to be positively correlated with COF and negatively correlated with wear resistance. The friction film does not seem to provide any lubrication during the sliding process. Conversely, prolonged sliding under low loads leads to its embrittlement, which keeps COF at a high level due to its continuous cracking and regeneration.