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  • Analysis of the calculated results shows that

    2018-11-15

    Analysis of the calculated results shows that the change of local elastic properties for the “pore-matrix” interface has the most impact on the effective elastic modulus. Fig. 18 shows the effective elastic modulus Eeff of the ceramics samples with modified elastic property of “pore-matrix” interface, which is normalized to the modulus Eeff_0 of the corresponding pure ceramics samples versus fraction of pores with modified properties. One can see that changing Elocal twice leads to change Eeff by +10%/−7% for C2 = 0.05, and by +35%/−25% for C2 = 0.17 for the samples with pore size D = 6 μm. For the samples with smaller pore size (D = 2 μm), this effect is stronger (due to larger value of specific surface of pores): +25%/−23% for C2 = 0.05 and +85%/−48% for C2 = 0.17. The local strength properties of the interface automata are changed to cause a variation of the effective strength of the composite and its fracture pattern; it does not influence on the effective elastic modulus. Fig. 19 shows the effective strength limit σeff of the ceramics samples with modified strength property σlocal of “pore-matrix” interface, which is normalized to the strength limit σeff_0 of the corresponding pure ceramics samples versus fraction of pores with modified properties. Thus, changing σlocal by ±40% leads to change σeff by +14%/−22% for C2 = 0.05, and by +19%/−37% for C2 = 0.17 for the samples with pore size D = 6 μm (Fig. 19). For the samples with smaller pore size (D = 2 μm), this effect is stronger: +27%/−32% for C2 = 0.05 and +40%/−41% for C2 = 0.17. Analysis of specific fracture buy GDC-0879 of the model samples (Efr) shows that the increase in local strength property σlocal leads to the corresponding increase in Efr, whereas the increasing/decreasing of local elastic property Elocal leads to decreasing/increasing of Efr (Figs. 20 and 21). As described in the foregoing, this change is stronger for smaller pore size. Thus, the possible influence of gel soaking of ceramics on its dissipative properties is ambiguous, because the fracture energy is defined by both elastic and strength properties of the material. In particular, if the soaking causes the increase in local elastic properties of pore surface, it may result in decreasing the fracture energy, but the increase in the strength properties always results in increasing the fracture energy.
    Conclusions
    Introduction Ni60 alloy, in combination with hard tungsten carbide (WC), is widely used to improve the wear resistance of steel as surface coating. Owing to an excellent wear resistance, Ni60-WC has been applied in many fields of industry, such as mining, transport, engineering, manufacturing industries, etc. However, Ni60-WC coatings have been found often to have cracks and pores in them. In recent years, the researchers have attempted to apply various deposition techniques on this type of coating to achieve the crack/flaw-free coatings. The traditional techniques, such as thermal spraying, plasma spraying, arc welding, and laser cladding (LC), have often been used to produce WC-reinforced Ni60 composite coatings. Wang et al. [1] studied the abrasive resistance of Ni-based coatings with WC hard phase prepared by plasma spraying with laser post-treatment, and tested the abrasive wear resistances of three Ni-WC composite coatings with different contents of WC particles. The results show that the best coating contains the composition of Ni60 + 60%WC (wt%) but it has the pores in it. Although the cold spraying technique offers high deposition efficiency and low oxidation and retains the initial composition and/or phases of coating materials, it cannot be used to deposit the hard particles. In contrast, the arc welding and laser cladding techniques can be used to deposit the hard particles and offer a good metallurgical bonding because a molten pool is formed on the surface of the substrate, but the coating and substrate can be severely deformed due to excessive energy input. In these processes, WC particles are largely dissolved in the melt pool during deposition, which reduces the hardness of the final coating. Additionally, the thermal stresses induced during the course of welding or cladding can lead to crack formation in the coating, and the low cladding efficiency increases the processing cost of cladding over large areas [2,3]. Zhou et al. [4–6] observed the cracks in the Ni60 coatings deposited by laser cladding but achieved fully dense and crack-free Ni-based WC composite coatings and Ni60 coatings prepared using laser induction hybrid rapid cladding with an elliptical spot. However, this technique restricts the geometry of substrate to simple shape, such as flat, shaft, etc. It is not suited for preparing the complex products. The key reason why Ni60-WC coatings cannot be applied in many areas is that the carbide particles result in the formation of cracks and pores. Therefore, the content and size of WC grain have an important effect on the characteristics of the coatings [2,7]. Matthew et al. [8,9] successfully combined laser cladding with cold spraying together to prepare high-density coatings consisting of Ti and Ti alloy, and used cold N2 as a carrier gas to reduce the cost of cold spraying. The supersonic laser deposition (SLD) method offers many advantages by combining laser with cold spraying, in particular, the use of a laser to control the deposition temperature, which allows hard materials, such as Stellite 6 and carbide [10,11], to be deposited while maintains the key advantages offered by solid-state cold spraying and replaces high cost helium with nitrogen. Moreover, the high-speed impact of particles on the substrate produces severe plastic deformation, resulting in a good bonding of the coating and the substrate. Meanwhile, SLD avoids the melting of the coating material and therefore retains the fine microstructure of the coating and prevents WC from degrading.