Studies of specimens from the VT5 alloy after nitriding showed that the surface roughness Ra was in the range of 0.58-0.64, and the hardness of the HV5 surface was 525±20 kgf/mm² with a base hardness of 325±15 kgf/mm². Fig. 2 shows the structure of the digestive in a mixture of hydrofluoric and nitric acids of a cross section of a nitrided layer. In the structure of the nitrided layer, there is a thin, 3-4 µm thick, dark outer layer, then a light uniform continuous layer 25-30 µm thick, behind which there is an intermediate layer between the base and the solid light layer. The intermediate layer has a mixed structure of light-colored fragments, similar to a continuous layer, interspersed in a field with a base structure. Such a three-layer structure of the surface nitrided layer is formed during the nitriding of titanium and α-alloys in a certain temperature range above the temperature of the α → β phase transition, which for the VT5 alloy, according to various literature data [12,16], is in the temperature range (940 ÷ 990)°C. The outer layer is titanium nitride TiNx, then the layer in which the transition from the β phase to the α phase occurred due to its saturation with nitrogen to a concentration sufficient for such a transition at the nitriding temperature. In the intermediate layer, only in some fragments of its structure, a nitrogen concentration was reached that was sufficient for the formation of the -phase at the nitriding temperature in the -phase layer. On cooling, these fragments, having the same color as the overlying layer, did not experience a polymorphic transformation, while the base and the rest of the intermediate layer experienced a β→α transformation on cooling. The core structure of the nitrided sample did not change compared to the alloy structure it had after annealing at 850°С.
The results of tests of nitrided samples in contact with different materials under lubricated friction conditions are presented in Table 2. They can be conditionally divided into two groups. One group of materials is friction pairs that have shown satisfactory results in terms of wear and friction coefficient for these test conditions, which include all types of tested bronzes, cast iron, cemented steel 20Cr3MoWV and steel 1.3505. Another group of tested materials in friction pairs, which included nitrided steel 20Cr3MoWV and steel 1.4878, had high values of both the coefficient of friction and the amount of wear of the contacting materials.
In the first group of materials, the lowest friction coefficient was recorded in the friction pair of nitrided titanium with bronze VB-23NTS, Br.O10C2N3, VB-24, steel 1.3505, cast iron GGG80, as well as 20Cr3MoWV cemented steel, had a minimum wear capacity with respect to nitrided titanium (2.4x10-4 mm3/s), in a pair after testing even some increase. Friction tracks with this group of materials are hardly noticeable on the surface of nitrided samples during visual inspection and practically did not change in the surface structure compared to the original one (Fig. 3). Another group of materials tested in friction pairs, which included nitrided steel 20Cr3MoWV and steel 1.4878, had high values of both the friction coefficient and the amount of wear of the contacting materials. A higher wear capacity in relation to nitrided titanium, as well as the value of intrinsic wear, had nitrided steel 20Cr3MoWV. The friction coefficient of this group of materials turned out to be the same and had a value of 0.54. Fig. 4 shows a view of the friction track after tribological tests of a nitrided titanium sample paired with nitrided steel 20Cr3MoWV
Table 2
Test results of friction and wear pairs lubricated with TS-1 fuel.
№
|
Counterbody material
|
"Pad" wear, g
|
Counterbody wear,
g
|
Friction coefficient
|
Pad wear rate mm3/Nм
|
Counterbody wear rate mm3/Nм
|
1
|
bronze VB 23NTS
|
0,0000
|
0,0030
|
0,08
|
˂1,13х10-6
|
1,81х10-5
|
2
|
bronze Br.O10C2N3
|
0,0001
|
0,0010
|
0,10
|
1,13х10-6
|
6,25х10-6
|
3
|
bronze VB 24
|
0,0000
|
0,0002
|
0,10
|
˂1,13х10-6
|
1,21х10-6
|
4
|
cast iron GGG80
|
0,0000
|
0,0016
|
0,11
|
˂1,13х10-6
|
1,14х10-5
|
7
|
steel 1.3505
|
0,0002
|
0,0011
|
0, 10
|
2,26х10-6
|
7,22х10-6
|
8
|
steel 20Cr3MoWV cemented
|
+0,0002
|
0,0003
|
0,12
|
Increase
2,26х10-6
|
1,97х10-6
|
9
|
steel 20Cr3MoWV nitrided
|
0,0020
|
0,0090
|
0,54
|
2,26х10-5
|
5,92х10-5
|
10
|
steel 1.4878
|
0,0014
|
0,0031
|
0,54
|
1,58х10-5
|
2,42х10-5
|
The track has contours with torn edges, and its surface is a rough striped relief with differences in the depth of furrows of tens of microns (Fig. 5) at a maximum average track depth of ≈40 μm.
The friction track after tribological tests paired with steel 1.4878 has a similar appearance at a somewhat shallower depth. With such a depth of the friction track, wear in the area of contact of the nitrided sample with the counterbody must be completely subjected to the upper layer of titanium nitride, the continuous nitrided layer and partially the intermediate layer between the nitrided layer and the base material of the titanium alloy, shown in Fig. 1.Fig. 6 shows the structure of transverse sections of titanium samples with a nitrided layer in the area of friction tracks with nitrided steel 20Cr3MoWV and steel 1.4878.
It can be seen from Fig. 6 that in the region of the maximum depth of the friction track after tribological tests with nitrided steel 20Cr3MoWV and steel 1.4878, the wear limit of the titanium sample material is in the region of the intermediate nitrided layer, not reaching the base material. Deformation distortions in the structure of the intermediate layer are also clearly visible, caused by the influence of the friction force between the rotating roller and the surface of the test sample.
Sufficiently good tribological properties of cast irons in friction pairs with various materials are primarily associated with a high content of carbon in them, which, upon contact of rubbing surfaces, participates in the formation of surface films that provide sliding with a low coefficient of friction. From these positions, it is possible to explain why the results of tribological tests in friction pairs with nitrided and cemented, steel 20Cr3MoWV differ so much, in which, according to [14], the surface concentration of carbon, depending on the carburizing modes, can range from 2.14 % to 5 .7 %, but at a distance of 0.2 mm from the surface, the carbon concentration can be from 0.95 % to 1.45 %.
At operation of different friction pairs with grease in conditions of limiting friction the coefficient of friction has value at level 0,08 - 0,15, occupying intermediate values between values, characteristic for liquid friction conditions and conditions of dry friction. Exactly in this range there is value of coefficient of friction for the first group of friction pairs. In the second group of friction pairs, the obtained value for the friction coefficient indicates a significant contribution of adhesive interaction between materials, despite the presence of a lubricating liquid, which leads to seizure and formation of scuffs in some parts of the contacting surfaces in the friction pair. This is evidenced by the type of friction paths of these groups of materials.
In order to predict the behavior of units and mechanisms under conditions of insufficient lubrication or interruptions in its supply, it is necessary to know the characteristics of the selected tribopair also in dry friction conditions. Besides, for many friction units the use of lubricating oils according to the conditions of their operation is simply unacceptable. Cemented steel 20X3MVF is widely used in the production of various units. Therefore, it was interesting to determine the tribological characteristics of nitrided alloy VT5 when working under dry friction conditions, which showed an increase in pad weight rather than wear when tested with lubrication in pair with this material. Given the non-lubricated friction pair operating conditions used in the units produced, the tests were conducted at a load of 100N. The tests showed that at the initial point during the next 75 seconds of testing, the coefficient of friction had an almost constant value of 0.34. After this period of time the coefficient of friction became inconsistent and lower with values fluctuating first from 0.32 to 0.2 and increasing until the end of the test to values from 0.26 to 0.52. Table. 3 shows the results of measuring the wear of the studied samples in this experiment.
Table 3
The results of tests without oil of the friction pair "nitrided alloy VT5-cemented steel 20Cr3MoWV".
path,
m
|
"Pad" wear, g
|
Counterbody wear,
g
|
Friction coefficient
|
Pad wear rate mm3/Nм
|
Counterbody wear rate mm3/Nм
|
1413
|
Increase
+0,0005
|
0,0046
|
0, 2 ÷0,52
|
Increase
6,51х10-7
|
4,46х10-6
|
After testing without oil the friction pair "nitrided alloy VT5 - cemented steel 20X3MVF" the shoe added weight, as well as after testing such a pair with grease. But if, after testing such a steam with lubrication, the surface of the nitrided titanium alloy in the area of contact with the counterbody did not undergo significant changes, then in this case a friction track was formed, which, as measurements showed, had a depth of about 13 μm. With such a track depth from the counterbody Ø 50 mm, the calculated weight drop should have been at least 0.0006 g. The discrepancy between the calculated change in the block weight and the actually measured one indicates that during the sliding of this pair, material is transferred from the roller. Fig. 7 shows a photo of the friction track on the pad with material enveloping on the edge of the roller exit with the surface of the pad and numerous areas with the products of interaction of rubbing materials on the surface of the friction track.
The change in the value of the friction coefficient during testing can be explained by a change in the composition of the contacting material of the pad surface during its wear. The initial period, apparently, corresponds to the time during which the sliding was carried out over the titanium nitride layer, during which the adhesive interaction with the roller material was constant and, therefore, the friction coefficient did not change. With a further increase in the depth of the development of the area of contact with the roller, the adhesive interaction of the material of the roller is enhanced due to the appearance of contact with the titanium alloy, the sliding of which is accompanied by the process of transferring the material of the roller and acts of local setting of the contacting materials. This leads to instability of the value of the friction coefficient and an increase in the amplitude of its fluctuations as a result of an increase in the contact area of the counterbody with the titanium alloy with subsequent wear of the nitrided layer. A similar character of the change in the friction coefficient was also observed in [5] when testing according to the “disk-ball” scheme of the nitrided VT6 alloy paired with a ceramic ball without lubrication.
The studies carried out in this work have shown that, depending on the material contacting in the friction pair with the nitrided alloy VT5, it is possible to provide sliding with sufficiently low values of the friction coefficient and wear of the tribopair materials under the test conditions chosen in this work. The presence of titanium nitride layer on the surface of titanium nitrided alloys ensures their high wear resistance. The reduction of adhesion interaction and tendency to adhesion with contacting materials during sliding is also associated with the presence of a nitride layer on the surface of nitrided alloys. The processes of transferring the counterbody material to the nitrided surface of titanium alloys may play an important role. Based on these positions, an explanation of the results in this work, obtained by testing the friction pair "nitrided alloy VT5 - cemented steel 20Cr3MoWV " without lubrication, is proposed.