3.1. Properties of Powders
The morphologies of the NiCrMoY alloy powders and the Ni60A alloy powders are shown in Figure 1
a,b, respectively. It can be seen that the NiCrMoY powders and Ni60A powders are near-spherical, but the sphericity of the NiCrMoY powders is better than that of the Ni60A powders, and a small number of the NiCrMoY powders have small satellites, while the Ni60A powders have more joint structures.
It is well know that morphology of powders depends on the surface tension of alloy melt, cooling speed, and shrinkage time. Better sphericity and the smoother surface of the powders are formed owing to the increasing surface tension, slow cooling speed, and long shrinkage time [22
]. Due to incorporation of high melting point alloy elements, such as Mo and Nb, into the NiCrMoY powders, the melting point of NiCrMoY powders increases and particle surface tension becomes larger than the powders without addition, i.e., Ni60A alloy powders. Meanwhile the addition of Cu and rare earth Y makes the grains’ surface smooth, improves the malleability [23
], and there is enough time and energy to form a better spherical shape and smoother surface for the NiCrMoY alloy grains (as shown in Figure 1
a) during the formation process of the powders by double-stage coupling fast freezing and low-pressure gas atomizing.
When the particles come out of the nozzle and are atomized by low-pressure gas atomization, the large particles get cooled slowly and have a higher temperature than the small ones, thus, small particles adhere to the surface of the large ones to form joint structure and satellite. Moreover the Ni60A powders easily adhere to the small particles because of its low melting point, so that Ni60A alloy powders have a poor rate of sphere formation and have satellites, as shown in Figure 1
The porosity of the powders can be expressed by the hollow particle ratio of the particles to the ones without porosity in terms of unit area of the cross-section of the powder sample. Figure 2
a,b show the cross-section of NiCrMoY alloy powders and Ni60A alloy powders, respectively, but the samples in Figure 2
were gradually ground and polished without corrosion. It can be seen that the hollow particle ratio of NiCrMoY alloy powders is 6.5%, which is lower than 12.5% forNi60A alloy powders. This is because the rare earth elements interact with oxygen in the alloy melt to form tiny and dispersive rare earth compounds, and Y enhances the non-oxidizability and malleability of the alloy.
Although deoxidization and degasification are carried out in the melting process, there is still small amounts of air existing in the alloy melt, and the temperature and pressure of the atomizing gas (nitrogen) in the atomization barrel increases rapidly during the atomization process, so the cooling velocity of alloy droplets slows so that more gas comes out from the alloy liquids. Meanwhile, due to the stirring induced by the high-pressure, some alloy powders contain nitrogen in the particle-forming process to form porosity in the particles [24
]. However, the hollow particles are easy to burst and form pinholes in the coating layer when spraying. This is an important factor which influences the quality of the coatings, so the hollow particle ratio should be reduced as much as possible.
The particle-size distribution of the NiCrMoY and Ni60A alloy powders are carried out for an appropriate dispersing agent and the dispersion time is shown in Figure 3
a,b, respectively. It can be seen from Figure 3
a that the particle-size distribution of the NiCrMoY alloy powders shows a single peak and fits a normal distribution, and most of particle sizes are in the 38.59–118.15 µm range, and the median particle diameter d
is 68.3 µm. Figure 3
b shows the particle-size distribution of the Ni60A powders, which is bimodal, dispersive, and has a larger median particle diameter.
The NiCrMoY alloy powder has a higher melting point and increasing surface tension due to the addition of the high melting-point alloy, such as Mo or Nb; thus, it requires more energy during gas atomization and the powder size increases compared to those without addition under the same atomizing parameters. However, because the Ni60A alloy powders have many joint structures and satellites, the particle size distribution of Ni60A powders is bimodal and dispersive, as shown in Figure 3
shows the apparent density and flowability of the NiCrMoY alloy powders and the Ni60A alloy powders. The apparent density and flowability of the NiCrMoY powders are 4.300 g/cm3
and 14.07 s/50 g, respectively, while those of the Ni60A powders are 4.031 g/cm3
and 15.05 s/50 g. Thus, it can be seen that the apparent density and flowability of the NiCrMoY alloy powders are better than those of Ni60A alloy powders.
The apparent density and flowability of powders depend on particle-size distribution, morphology, hollow particle ratio, and so on. The flowability of powders increases with better sphericity, and the apparent density of powders increases with the decreasing hollow particle ratio; high apparent density induces particle-size distribution dispersion in spite of large or small powders. This is in good agreement with the particle-size distribution, morphology, and the hollow particle ratio of the two kinds of powders mentioned above.
reveals the oxygen content of the NiCrMoY alloy powders and the Ni60A alloy powders, respectively. The oxygen content of the NiCrMoY alloy powders is 0.042%, lower than that of the 0.072% of the Ni60A powders. This is because rare earth Y improves the non-oxidizability of the alloy powders. The oxygen content of powders has a noticeable effect on the property of the sprayed coatings, and induces defects in coatings, so it should be reduced as much as possible.
3.2. Properties of Coatings
The microstructure of coatings of the NiCrMoY alloy powders and the Ni60A alloy powders are shown in Figure 6
a,b, respectively. As shown in Figure 6
a, boride and carbide structures are distributed in the austenitic matrix of the Ni60A alloy coating. Not only boride and carbide structures be seen from Figure 6
b, but also many needle-like hard phases that are uniformly distributed in the austenitic matrix of the NiCrMoY alloy coating. This is because Mo is a kind of refractory metal with large atomic radius, which can induce noticeable distortion in the crystal lattice of the nickel solid solution. It is reported that carbide is formed uniformly due to the incorporation of the rare earth elements so as to improve the mechanical property of the alloy, especially its shock property [25
The test results of hardness of the Ni60A coating and the NiCrMoY alloy coating are shown in Table 3
. It can be seen that the hardness of the NiCrMoY alloy coating is higher. This is because the addition of the Mo element of the NiCrMoY coating causes grain refinement, increased toughness, decreased crack sensitivity, and enhanced high-temperature hardness and wear resistance. The addition of Nb strongly forms carbide and effectively refines grains. Thus, the appropriate incorporation of rare earth elements refines alloy structures, eliminates impurities, and forms the hard phases, such as carbide and boride, to prevent other new hard phases from forming. This causes the block- and needle-like hard phases to be uniformly distributed in alloy coatings, which increases the hardness of the alloy coatings