3.1. Morphology and Microstructure Analysis
shows the macro morphology and microstructure of the as-deposited coating. From the cross-section micrographs of the as-deposited coating shown in Figure 3
a,b, it can be seen that the deposited coating with a dense structure is well-bonded to the substrate. No obvious pores, micro-cracks and other defects can be observed on the fusion zone and its vicinity region. A large amount of the solidification-formed coarse columnar grains appeared near the fusion zone as shown in Figure 3
shows the XRD patterns for the coatings before and after ultrasonic impact. The as-deposited coating mainly consists of the austenitic phase (γ-austenite). Furthermore, Fe–Cr intermetallic compounds as well as M2
B (M = Fe, Cr, Mo) are dispersed in the austenitic matrix, which is beneficial to attain a higher surface hardness of the coating. In order to clearly clarify the effect of ultrasonic impact on the microstructure of the near-surface layer, only the main phase γ-austenite is taken into account without considering other phases and metallic compounds. For the ultrasonic impacted coating, the diffraction peaks of martensite phase can be observed in the XRD spectrum illustrated in Figure 4
. The martensite phase is the deformation-induced α’-martensite phase, which is formed mainly from the plastic deformation of austenite matrix during ultrasonic impact. The similar phenomenon was reported in the literature [13
]. Meanwhile, comparing with the as-deposited coating, the peak broadening for the austenitic phase was observed after ultrasonic impact, which can be mainly owing to the grain refinement and microstrain generated near the surface layer during the impact process [23
]. In addition, the average crystallite size of the ultrasonic impacted coating surface is about 11.04 nm measured from (110) diffraction peak, which is about 38.78 nm for the as-deposited coating. It demonstrates that the microstructure of coating surface layer has been refined by ultrasonic impact.
The typical cross-sectional micrographs of the coating before and after ultrasonic impact are shown in Figure 5
. It can be seen that the microstructure of the as-deposited coating is quite different from that of ultrasonic-impacted one. The as-deposited coating presents a homogeneous microstructure as shown in Figure 5
a,b. However, the ultrasonic impacted coating exhibits a gradient microstructure along the thickness direction as shown in Figure 5
Moreover, in Figure 5
c, it is clearly that a conspicuous plastic deformation layer is generated at a depth of approximately 80 μm from the treated top surface of the coating, and a certain amount of the deformation-induced α’-martensitic phase was formed. However, the plastic deformation layer is inhomogeneous. From the SEM micrograph image shown in Figure 5
d, a thin fine grain zone with a thickness of about 34 μm beneath the treated surface can be observed in the near-surface layer. This means that the coating near-surface layer underwent a severe plastic deformation. Additionally, no obvious grain boundaries and crystallographic features can be identified clearly compared with the coarse grain zones, which are far from the top surface of coating.
shows the higher magnification SEM morphologies of the selected regions A and B (in Figure 5
d) on the cross-section of the coating, and the corresponding EDS analysis results after ultrasonic impact. In the selected region A with a fine crystallite near the top surface as shown in Figure 6
a, it is hard to distinguish the consisting phases and the grain boundaries. In the selected region B far from the treated top surface shown in Figure 6
b, the grain morphology is clearly identified. This further indicates that the obvious grain refinement and severe plastic deformation occurred during the ultrasonic impact. Based on the EDS analysis results, it can be found that the concentration of elements Ni, Cr in region B is different from that in region A. This may relate to the formation of the deformation-induced α’-martensite in the ultrasonic affected zone, which may lead to the redistribution of the elements Ni, Cr, and result in different elemental content between the selected regions A and B.
shows EBSD observations of the microstructure micrograph and the average grain-size distribution on the cross-section of the coating near-surface layer with and without UIT processing. Here, Figure 7
a,b is the EBSD inverse pole figures (IPF) superimposed by the orientation map triangle on the bottom right corner, respectively, which present the discernible differences between the two samples. Figure 7
c,d is the statistical analysis between average grain size and number percentage on the cross-section of the coating near-surface layer. It is obviously that the near-surface morphology of the as-deposited and UIT-treated specimens is different. For the as-deposited coating shown in Figure 7
a, the initial cross-sectional microstructure comprises the coarse austenitic grains, and the grain boundaries are basically parallel to each other with <112> preferred orientations. Meanwhile, the size of the austenitic grains is not uniform and the average grain size did not exceed 300 nm according to the histograms of grain size versus number percentage shown in Figure 7
c. In the case of the UIT-treated specimen shown in Figure 7
b, the cross-section microstructure of the coating near-surface layer is mainly composed of fine near-equiaxed crystals with random orientations. The average grain size of the fine equiaxed grains is smaller than 100 nm as shown in Figure 7
d. This also further validates the grain size from the XRD pattern discussed above. Results indicate that the UIT can markedly improve the initial coarse austenitic microstructure of the as-deposited coating, make the bulky columnar crystals change to the fine equiaxed crystals and simultaneously change the grains orientation. Therefore, the initial grains in the coating near-surface layer have been significantly refined and the grain size reaches the nanometer scale after UIT processing.
In addition, the fine grains in the near-surface layer gradually turn into the irregular submicron grains and even reach to coarse grains with increasing the depth from the treated surface along the cross-section. This phenomenon can be explained by the fact that the plastic deformation induced by the UIT presents a gradient distribution with the highest plastic strain formed in the treated top surface and gradually decreased deeper from the coating surface.
3.2. Mechanical Properties of Surface Layer
exhibits the load–displacement results of the specimens before and after UIT processing. Six test points 1 to 6 are schematically shown in Figure 5
d at the depths of approximately 0, 30, 60, 90, 120, and 150 μm beneath the treated surface. As can be seen, both the maximum displacement and residual displacement of the deposited and UIT treated specimens increase as the indentation location gets far from the coating surface under the maximum load of 1000 μN. Moreover, the values of maximum and residual displacements of the testing points shown in Figure 8
b are less than that in the same tested position shown in Figure 8
a, which indicates that the microscopic machine property especially the nanohardness of the coating near-surface is enhanced after UIT. Furthermore, the maximum and residual displacements of the near-surface layer are less than that of the deeper locations far from the treated top surface. This can be attributed to the fact that the strengthening effect of UIT tends to be weak with increasing the distance from the treated surface.
The corresponding nanohardness and elastic modulus distribution of the coating cross-section before and after UIT are shown in Figure 9
. Apparently, both the nanohardness and elastic modulus of the UIT treated specimen are obviously higher than that of the as-deposited specimen under the same test conditions. The increments of the elastic modulus and hardness are well-known to favor the enhancement of the stiffness and the resistance of foreign object damage of the components [12
]. Therefore, this means that the UIT can help increase the resistance of foreign object damage of the coating. The increased hardness of the UIT treated coating can be interpreted by the work-hardening effects and grain refinement on the coating surface according to the Hall-Petch relationship [25
]. In addition, the wear behavior of materials can be evaluated in terms of the ratio between the hardness (H
) and elastic modulus (E
). High H/E
ratio is more beneficial to attain excellent wear resistance [27
]. Here, the H/E
ratios of the near-surface layer before and after UIT are 0.027 and 0.033, respectively. Obviously, a higher H/E
value is obtained after UIT. Therefore, it can be concluded that UIT improved the wear resistance of the deposited coating.
3.3. Wear Properties Analysis
a,b show the friction coefficient and wear loss with the applied loads in the range of 200–1000 N at a sliding speed of 380 r/min for 1 h, respectively. It can be found that both the friction coefficient and wear loss of the specimens before and after UIT accordantly increase with increasing the applied load under the same conditions. Meanwhile, both the friction coefficient and wear loss of the UIT-treated specimen are significantly smaller than that of the as-deposited coating. It indicates that the UIT treated coating surface exhibits better wear resistance compared with the as-deposited coating. The enhancement in wear resistance maybe owing to the increase in hardness of the UIT treated surface, which is composed of the fine grains and the deformation-induced α’-martensite phase.
The wear characteristic before and after UIT process are further investigated through SEM. Figure 11
shows the worn surface micrographs subjected to the applied load of 800 N with a sliding speed of 380 r/min after 1 h. Obviously, the UIT-treated specimen (Figure 11
b) shows different worn morphologies from that of the as-deposited specimen as shown in Figure 11
a. For the as-deposited coating, several wide and deep wear grooves parallel to the sliding direction and some randomly distributed abrasive particles and fragments occur on the worn surface. Thus, the dominant wear mechanism for the as-deposited specimen is considered to adhesive and abrasive wear. In the case of the UIT-treated specimen, the parallel wear scars become narrower and shallower and only minor amounts of spots formed. This exhibits the typical characteristics of abrasive wear. The change of the wear mechanisms from adhesive type to abrasive type indicates that the wear resistance of the specimen is enhanced during UIT processing.
shows the worn micrographs subjected to the applied load of 800 N with a high sliding speed of 380 r/min after 5 h. From Figure 12
a, it can be seen that the large area of peelings and spots morphologies occur on the as-deposited coating surface, indicating a strong-adhesion wear feature. In contrast, deeper and wider grooves and cracks morphologies appear on the UIT-treated surface shown in Figure 12
b. Apparently, the wear failure of both specimens is more serious with time going; however, the UIT-treated specimen presents the less worn than the as-deposited specimen. This further demonstrates that the coating possesses better wear resistance after UIT, and the wear mechanism also have changed.
Both the wear characteristics and failure mode of coating have changed after UIT process according to the above-discussed experimental results. It maybe the synthetic effect of the grain refinement and the formation of the deformation-induced α’-martensite phase that benefits the increase of the surface hardness of the UIT-treated specimen during UIT. In consequence, the increased surface hardness induced by UIT will ultimately affect the wear resistance of the specimen based on the Holms and Archards wear theory [26
], which is described as follows:
presents the wear volume loss per unit sliding distance, FN
are the applied load, the hardness of the materials (here, refers to the hardness of coating surface subjected to wear) and the dimensionless wear coefficient, respectively. Therefore, the nanograined structure and the deformation-induced α’-martensite phase formed on the treated coating surface contribute to the improvement in surface strength and wear resistant properties after UIT.