Results of researches are presented on Figs. 5, 6, 7, 8, and 9.
Analysis and interpretation of oxide-coated films formation on machine parts made with aluminum alloys
The received structure of aluminum Russian-made АД1Footnote 1 (ГОСТ 4784-97) corresponding to American analogue 1230 (ISO 209-1 or ASTM B221 1060) before and after processing shows grains change, size reduction, and stretching them parallel to a plane of processing of aluminum grains.
The visual analysis in Fig. 5 shows that on the investigated surface, there are enormous number of randomly located traces of processing generated ledges and pores (cavities). They have different forms, depths, and sizes. Small-sized deepening and longitudinal multidirectional scratch marks with various forms and sizes are visible as a consequence of direct and inclined collisions. Contours of plots are irregular, that shows the variable nature of sphere’s movement on the processable surface.
On various sides of surfaces, relief pattern crateriform plots showing direct influence of sphere about a surface are visible with deformation directed toward the center of the sample as in Fig. 2. A set of plots received from collision of spheres, directed at an angle to surface are also visible, which shows the sliding impact causing shift and fragile destruction of material. In fact, the direct impact of the medium is helping more in reducing the protrusions and feeding the hollows space until a certain level and slanting impact is obtained assuming the smooth and regular continuity of the surface shape. This testifies that the dynamic random character of the processing medium is necessary to establish relative uniformity to a considered surface being coated as shown in Fig. 2. Deformation processes during vibration exposure are accompanied by plastic deformation of a thin surface layer and the implementation of shock-wave processes. This implies the uniform hardening of a thin surface layer of all elements of the part, finishing and rounding of sharp edges, and finally smooth transitions. It is appropriate to note the high performance among mechanical processes due to the possibility of simultaneous processing of a large number of parts.
The intensity of the process lies on the governing parameters of resulting vibration such as amplitude and frequency. With an increase in the amplitude of oscillations, some mechanical characteristic and the coating formation increases as in usual vibratory machining (Babichev & Babichev, 2008; Essola, Babichev, & Mishnyakov, 2012). Such increase in the amplitude is explained by an increase in the forces of micro-shocks of the particles of the working medium and the path of their active influence on the surface being treated.
The growth in formation of mechanical coating with an increase in the frequency of oscillations is explained by an increase in the forces and the number of micro-blows of abrasive particles on the treated surface per unit of time and consequently an increase in the speed of their relative slip. These forces are due to improvement of the accelerations of abrasive particles with their constant mass. Observation of the behavior of the working medium in the working chamber showed that with an increase in the oscillation frequency, the circulation (rotation) of the entire mass of the working medium accelerates (Babichev & Babichev, 2008). Consequently, the increase in the formation of coating in this case occurs due to more intensive mixing (circulation) of the working medium.
By considering surface samples pictures processed within 30 min (Fig. 5c), the heterogeneity of the processed surface is observed more than the one processed after 15 min (Fig. 5b). It represents the gradual intersection of the initial roughness with processing traces. In the surface of processed sample, the polished sides of the surface as shown in Fig. 5a and sides with traces of destruction differ from each other. The surface is covered with craters (pores) and various types of cavities but less in Fig. 5c, than in in Fig. 5b. This could be explained by the presence of certain discontinuing in the characteristic (mechanical and technological) of processed sample or the effect of shocks in certain places of the surface of parts more than other places. But the technological characteristic of the sample is responsible to the effect of relative leveling shown in Fig. 2; mechanical effect contributes to reduce the length of profile parameter by additive micro-cutting of higher dents, filling the crater with microchips, and fixing it in direct impact. The sliding impact compensate the slope created by dynamic random chocks during processing. Only the duration of the process is favorable through additional repeating impact constraining shifting of the surface with simultaneous better leveling.
In other words, vibration treatment provides additional energy needed to overcome the increasing distance between the metal and the growing oxide film. The vibrating medium in contact with the surface of the growing oxide film loosens it, thereby facilitating the access of the oxidizing solution to the metal surface. This creates the densification of material of surface due to the shocks, compensation of defect, and the destruction of lower stable structure affecting the surface in the beginning of the process.
The main parameters of the mechanical action of the combined process are the speed and strength of the impact of the granules of the working medium with the surface of the parts and the contact pressure in the area of impact. An increase in such parameters shows the intensity of the process but till certain level due to the limitation of the vibration machine.
Increase in any of the considered parameter have positive impact to the operation, but as the machine is designed for range of amplitude, the bigger value of frequency can lead to easily cause negative consequences. In this condition, the wear of the working medium becomes so intense that the cost of processing increases dramatically, the load on the shaft bearings on which the imbalance is located increases as the disturbing force increases in proportion to the square of the angular frequency of the vibration, and the noise arising during the operation of the vibration unit increases sharply and acquires a high tone. For these reasons, the optimization of the processing can be reach with an uprising of amplitude.
As a result of the dynamic effect, activation of chemical and physicochemical processes occurring in the surface layer is provided that lead to the change of its geometrical and physicomechanical characteristics.
The process duration has a determinant aspect after choosing the needed equipment for operation. Metal removal rate in time proceeds non-linearly. In the beginning of processing, the incubatory period during which an appreciable loss of weight is not observes proceeds. As shown in complex researches (Babichev & Babichev, 2008; Babichev, Essola, et al., 2011, 2012), during that period dents as imprints are formed; moreover, there is accumulation of the latent energy of destruction and a brittle behavior of superficial layers of processable material.
At longer processing, contact and overlay of a set of individual traces are observed on the surface of the sample in places of accumulation and a large number of processing traces represents a repeatedly deformed layer.
By observing the plots, it is visible that separate spheres during collision with a processable surface leaves on it a discontinuous trace consisting of finer traces caused by the nature of movement of spheres.
At such cross-arrangement nature of the considered processing traces, an original micro relief is formed.
Analyzing the obtained results, it is proper to note that vibratory machining in the polyethylene balls environment on sample’s surface are being formed traces having crater types as a consequence of direct impact. As a result of sliding of spheres on the processed surface, elongated form traces are also formed. Ultimately, on the surface of sample, mixed type traces are present as a consequence of both sliding and direct collision. That is why most parts of processed traces have mixed characteristics.
A large number of processed traces cover mostly all the surfaces of the sample, implying flow of plastic deformation processes at microlevel and lessening of superficial layer. These phenomena have great importance in the formation of oxide-coated film and microrelief of the surface because the direct contact of the sphere occurs on oxide-coated film.
Numerous studies have established the presence of plastic flow of material in a thin superficial layer of the sample in the course of movement of separate granules (Smolentsev, Kondratyev, Ivanov, & Smolentsev, 2017). At direct impact of a sphere, the deformation is directed deeply into the sample. The majority of traces are inherent to impacts of spheres directed with an angle to the surface which causes fragile oxide destruction and shift of separate particles. Such surface destruction allows oxide layer’s lessening with formation of finely broken particles. Part of particles is seized by the juvenile oxide surfaces, falling sometimes into the pores and reducing their volume. The other part is partially carried away by the oxidizing solution, partially compacted by subsequent impacts of the balls and gets a polished appearance.
Initial contact of sphere occurs on top of microroughnesses. During processing, there is an increase in the area of covering’s contact with surface as a result of rounding the radius of the protrusions. Profile record on polished surface after processing in polyethylene balls and overlapping with oxidation shows that as a result of combs’ deformation of microroughnesses under influence of spheres’ impacts, there is reduction of roughness and increase of crest blunt point radius (tops) of ledges.
Hence, as a result of vibration processing, oxide-coated film is lessened and smooth out during its growth (Smolentsev, Ivanov, & Kondratyev, 2017).
In the zone of contact, due to sliding impacts of spheres, oxide-coated film particles are orientated parallel to processing surface of the sample, that testify brilliant sides on the surface. These brilliant sides are well visible in micro photos (Fig. 5b, c) comparatively to initial polished sample picture (Fig. 5a). Additionally, by comparing coated films formation obtained at vibration processing and the one obtained by using traditional method, it is necessary to note that more than 50% of the surface of the covering obtained as a result of vibratory machining has orientation of particles parallel to planes of sliding (motion). That led to conclude that when coating is applied in stationary baths that is without a load being applied, the hydroxide grains have chaotic and random orientation while those obtained during the vibratory machining are oriented parallel to the slip plane.
Interpretation, analysis, and discussion of experimental result of vibration processing coating on the basis of MoS2
Results of conducted researches have shown that vibratory processing solid lubrication coating with MoS2 leads to a decrease in friction factor and an increase in wear resistance of friction pairs during operation. This happens not only in the air environment (dry friction) but also in an oily environment (wet friction). Deterioration within 3 h of processing is not practically observed. It is experimentally confirmed that the presence of layers of various atoms in molybdenum disulfide structure creates conditions of easy sliding.
Large influences on antifrictional properties of coating renders nano-dimensionality, orientation of its particles, and also nanorelief of the surface as seen in “Methods/experimental” section. The formation of molybdenum disulfide coating by means of vibration ensures orientation of particles with base planes parallel to the plane of sliding and the wearing of coatings in this case is not perceived (Babichev & Babichev, 2008; Ivanov, 2017b).
Depending on the conditions of operation and materials of friction pairs, reduction in factor of friction ranges from nine down to three times and the increase in their wear resistance from four up to 20 times.
Figures 10 and 11 present a series of photographs, illustrating the condition of the samples’ surfaces made from steel 45 and MoS2 powder in initial condition and after deposition of molybdenum disulfide coating. Pictures are made with various zooms, from various positions and by tilting. The adsorbed particles of firm greasing have no certain orientation (Fig. 10a), but in the zone of contact, due to the closeness and sliding impacts of indentors, particles are guided with base planes parallel to the processing surface that presents brilliant sides of coating. This can be seen on the photograph shown in Fig. 10b.
The obtained photos give evident representation of the relief’s character of the covered surfaces and surface without solid lubrication coating. These photos open the mechanism of formation with vibration’s impact influence in the steel indentors environment having diameter from 3 to 5 mm. Moreover, they also approve the influence of molybdenum disulfide powder on metal as oxide-coated films formation. In Fig. 11a, parallel lines are precisely visible on metal after machining. In Fig. 11b, the sizes and chaotic arrangement of powder are visible after normal deposition of coatings. By deposition of coating (Fig. 11c) using vibratory machining, the part of powder that was formed precisely on metal edges and which is contributed to chaotic morphological structure has been mechanically removed. The photograph in Fig. 11d gives a lateral metallographic microsections view, which confirms the orientation of molybdenum disulfide particles with respect to the rotation of the machine parts. Apart from the result of the lead researches, it is established that formed layers of firm greasing molybdenum disulfide during vibration processing becomes covered by a thin film having orientation of particles with the base planes parallel to the surface of friction. This result can also be observed with much detail in Figs. 12 and 13.
In fact, as a complex result of (i) the sliding of the balls relative to the surface of the parts, (ii) the mutual oscillation of the atomic groups of the constituent molecules, and (iii) the increased energy of movement of the working medium, the polishing solution is activated by obtaining additional energy, which leads to the formation of a specular thin film on the metal surface.
As seen from the presented figures, the surface with molybdenum disulfide coating in comparison with unprocessed surface has become flatter and the microledges are rounded and more continue. Such structure of a film, as shown in the analysis, is caused by inclusion of nanodimensional structures (Fig. 13). The introduction of nanodimensional structures in vibrating mecano-chemical coatings increases the efficiency of firm greasing (solid lubricants). It is established that with the reduction of grain size from 1 μm down to 2 nm, the volume fraction of grain matter increases up to 88%. This gives possibility to obtain coatings with high and unique properties. For example, they have ameliorated durability as their hardness which range from two to seven times higher than the hardness of coarse-grained coatings; their toughness are 1.5 to 2 times higher and with the reduction of grain size from 10 μm down to 10 nm, the speed of deterioration of coating decreases up to ten times.