Wear, 160 (1993)
Effects of additives on friction and wear behaviour coatings
Jianjun Wei and Qunji Xue Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou (China) (Received
16, 1992; revised and accepted
May 21, 1992)
Abstract The effects of several lubricating oil additives on the friction and wear properties of Crz03 coatings were studied using a block-on-ring tester under ambient conditions. The results show that, compared with the base oil, oleic acid, glycol oleate and dibutyl phosphite have a friction-reducing function but sulphurized olefin does not; a remarkable wear resistance is observed for dibutyl phosphite and oleic acid but not for glycol oleate and sulphurized olefin. The concentration of these additives has a great influence on the wear behaviour of the Crz03 coating but little influence on the friction behaviour. Analytical results of X-ray photoelectron spectroscopy show that the antiwear action of dibutyl phosphite is due to the formation of tribochemical reaction films on the rubbing surface and that the wear-increasing action of sulphurized olefin may be related to the physical adsorption of this additive and its decomposed products on the rubbing surfaces.
1. Introduction Effective lubrication to control friction, wear and fracture of ceramic tribocouples is crucial to the success of many advanced technologies. A basic knowledge of the lubrication of ceramics is therefore essential. Recently, the effects of lubricating oils and their additives on the tribological properties of ceramics have been investigated [l-4]. Some results show that, compared with the base oil, a few additives can significantly reduce the friction and wear. However, little fundamental understanding is gained, e.g. the reactivity of ceramics with additives was not clear. As a theoretical basis for developing new lubricants and additives for ceramics in the future, fundamental studies are very important. The purpose of this paper is to investigate the effects of several antiwear additives on the friction and wear properties of ceramic coatings and to clarify the acting mechanisms of these additives.
mechanical properties of the Cr,O, coating are listed in Table 1. Its thickness after grinding was 0.3 mm. The frictional surfaces of specimens were polished with 800-grade emery paper to a roughness less than 0.10 pm centre-line average (c.1.a.). Paraffin oil with a viscosity of 30.19 cSt (25 “C) (CP) was used as a base oil. The additives used were commercial glycol oleate (an ester oil), dibutyl phosphite (DBP) with a phosphorus content of 14.5%-16.0%, sulphurized olefin (trade code T308) with a sulphur content of 42.6% and chemically pure oleic acid. 2.2. Experimental procedure The experiments were carried out using a block-onring test machine in which a lower rotating ring was slid against an upper stationary block. The block is 19.0 X 12.4 X 12.4 mm3 in size and the ring is 49 mm TABLE
2. Experimental details
2.1. Materials The Cr,O, coating used in this study was manufactured by the Shanghai Institute of Ceramics. The specimens (block and ring) were made of a carbon steel. The rubbing surfaces of the block and ring were sprayed with Cr,O, powder. The preparation technique of this coating was described in ref. 5. The physical and
Purity (%) Porosity (%) Grain size (pm)
of CrzO, coating
Value 92.0 4.7 38-76 4.52 4.72 10.0 81.3 SiO,
Bulk density (g cme3) Apparent density (g cmd3) Microhardness (GPa) Bend strength (MPa) Impurity
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in diameter. A normal load of 539 N and a rotation speed of 200 rev min-’ (linear speed 0.51 m s ~ ‘) were used in all tests. Oil lubrication was achieved by a continuous oil-dripping process (five drops per minute). Before testing, the specimens were cleaned by petroleum ether in an ultrasonic bath for 15 min. The friction coefficient was calculated as the ratio of the recorded friction force to the normal load. The wear volume of the block was measured by using a surface profilometer. At least two tests were run (more if the results from the first two tests differed by more than 10%). The worn surface was observed by means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) with an Mg Ka Xray anode operated at a voltage of 8 kV and a current of 40 mA.
3. Results and discussion 3.1. Effect of sliding time Figure 1 shows frictional curves of Cr,O, coatings lubricated by oils with 2% additives. The friction coefficient is always stable in the base oil but decreases slightly with sliding time in the oils with additives. The various additives reduce the friction coefficient to different extents. In particular, oleic acid and DBP have a better friction-reducing function. The variation in the wear volume of the Cr,O,-coated block with sliding time is shown in Fig. 2. As the sliding time increases, the wear volume increases under the various lubricated environments. The order of the antiwear action of the additives can be obtained as follows: DBP> oleic acid > glycol oleate = base oil > T308. Obviously, the type of additive has a great effect on the wear of the Cr,O, coating. Compared with the base oil, DBP shows a pronounced antiwear action, e.g. the wear volume after
Fig. 2. Variation in wear volume of block with sliding time for Cr,O, coatings under oil-lubricated conditions, (load 539 N, sliding speed 0.51 m s-l).
Concentration of' additive (wt%)
Fig. 3. Variation in friction coefficient with concentration of additive for Cr*O, coatings under oil-lubricated conditions (load 539 N, sliding speed 0.51 m s-‘, sliding time 30 min).
40 min is at least half that in the base oil. Oleic acid also shows antiwear action, glycol oleate has little effect on the wear, whereas sulphurized olefin even shows a slight wear-increasing action.
Siding time (min) Fig. 1. Variation in friction coefficient with sliding time for Cr,Oj coatings under oil-lubricated conditions (load 539 N, sliding speed 0.51 In s-y
3.2. Effect of concentration of additive The effects of the concentration of additives on the friction behaviour of the Cr,O, coating are presented in Fig. 3. The data were obtained under a load of 539 N at a sliding speed of 0.51 m s-’ for a sliding time of 30 min. It is observed that the concentration of the various additives has little influence on the friction coefficient. For sulphurized olefin the friction coefficient value is very near to that in the base oil (0.113) and is independent of the concentration. For DBP a slight decrease in friction coefficient with concentration to a minimum value (0.083) at 3% is observed. For glycol
.I. Wei, Q. Xue / Friction and wear behaviour effected by additives
oleate the friction coefficient gradually decreases to a stable value (0.067) with increasing concentration. The effects of the concentration of additives on the wear volume of the block under the same conditions as for Fig, 3 are shown in Fig. 4. The three additives behave differently and their concentration has a great effect on the wear behaviour. For glycol oleate the wear volume remains unchanged at concentrations less than 2% but increases at concentrations above 2%, which indicates that glycol oleate shows wear-increasing action at higher concentrations compared to the base oil. Sulphurized olefin shows a slight wear-increasing action, the wear volume varying slightly with concentration. DBP, however, has a significant wear-reducing function, the wear volume decreasing rapidly from the initial concentration and tending to a stable value. 3.3. Analysis of surface SEM images of the worn surfaces of the Cr,O,coated blocks after testing in oils with 2% additives are shown in Fig. 5. The test conditions are the same as for Fig. 3. Clearly, compared with the original surface, the worn surfaces are relatively smooth. It is noted that microcracks occur on the worn surface with the base oil. Our previous work revealed  that a transition from mild to severe wear occurs by brittle fracture at a critical load (490 N) for Cr,O, coatings lubricated by paraffin oil. The test conditions used here could give rise to high wear of the Cr,O, coatings because a higher load of 539 N was applied. This observation indicates that fracture wear is the main wear mechanism for Cr,O, coatings lubricated by paraffin oil. Thus controlling the fracture process is a key factor for low wear. No obvious microcracks are observed on the worn surfaces with oleic acid, DBP and glycol oleate, which 15.0
. base oil
1 .o Concentration
Fig. 4. Variation in wear volume of block with concentration of additive for Cr203 coatings under oil-lubricated conditions (load 539 N, sliding speed 0.51 m SK’ sliding time 30 min).
implies that these agents can reduce the wear to a limited extent. However, microcracks are clearly observed on the worn surface with sulphurized olefin, showing that this agent cannot control the fracture wear and may even increase it. These observations are generally in line with the results obtained in Fig. 2. Note also that it is difficult from the micrographs in Fig. 5 to define whether deposited films or reaction films cover the worn surfaces. 3.4. Analysis of chemical composition of suflace film The worn surfaces of blocks tested in oils with 2% extreme pressure additives (DBP, T308) under the same conditions as for Fig. 3 were analysed by XPS. The analytical results are listed in Tables 2 and 3. It is found from Table 2 that apart from a large amount of carbon, small amounts of additive active elements are formed on the worn surfaces (6.9% P, 1.2% S). Under DBP lubrication the binding energy of chromium shifts from 577.1 to 578.1 eV (see Fig. 6); the binding energies of chromium and phosphorus may be attributed to chromium phosphate, since that of phosphorus corresponds to PO,. This suggests that, because of the high temperature in the contact region generated during the rubbing process, DBP decomposes to H,PO, and subsequently reacts with the Cr,O, surface to form CrPO,. It has already been demonstrated  that DBP oxidizes at elevated temperatures to produce H,PO, and other volatile products (PH,, C,H,). Under sulphurized olefin lubrication the binding energy of sulphur is attributed to free sulphur (S 2p, 164.0 eV), which indicates that sulphurized olefin decomposes to produce free sulphur. However, the binding energy of chromium shows no variation after testing, indicating that no direct chemical reaction of the Cr,O, coating with sulphurized olefin or its decomposed products takes place. 3.5. Discussion It is found from the above data that the additives, developed on the basis of metal tribosystems, are not completely suitable for Cr,O, coating tribocouples, because their acting mechanisms between two systems behave differently in many cases. Similar to metal systems, oil additives may reduce friction and wear by adsorption of their polar groups on ceramic surfaces. For example, oleic acid effectively reduces the friction and wear of Cr,O, coatings. However, glycol oleate has no wear resistance, which may be due to two factors : one is that larger steric hindrance impedes its effective adsorption on the rubbing surface, and the other is that decomposition of glycol oleate during rubbing produces some small molecules such as glycol which can induce stress corrosion cracking by hydrogen bonding attraction on the Cr,O, coating surface.
Fig. 5. SEM images of worn surfaces after testing in oils with 2% additives: (a) outside of wear track, (b) base oil, (c) base oil +-DHP, (d) base oil fT308, (e) base oil+oleic acid, (f) base oil+glycol oleate.
TABLE 2. Relative percentages in oils with 2% additives Additive
Original Crz03 surface DBP T308
on worn surfaces
TABLE 3. XPS results of worn surfaces tives Additive
Binding energy (eV)
s Original CrrO, surface DBP T308
in oifs with 2% addi-
J. Wei, Q. Xue / Friction and wear behaviour effected by additives
a slight wear-increasing action, but DBP and oleic acid have a wear-reducing function. (3) The concentration of the additives has a great effect on wear. The wear volume increases with increasing concentration of glycol oleate but decreases with increasing concentration of DBP. However, the wear behaviour is almost independent of the concentration of sulphurized olefin. (4) The analysis by XPS of worn surfaces shows that there is a tribochemical reaction between the Cr,03 coating and DBP, resulting in low wear, but no reaction between the Cr,O, coating and sulphurized olefin or its decomposed products.
Fig. 6. X-ray photoelectron spectra of Cr 2p on (a) original Cr203 coating surface, (b) worn surface with T308 and (c) worn surface with DBP.
As in metal systems, dibutyl phosphite also shows extreme pressure antiwear action for Cr,O, coating tribocouples on which tribochemical protective films are formed, resulting in low wear. In contrast to metal systems, sulphurized olefin does not show wear resistance, though it decomposes and forms free sulphur during the rubbing process. The reason is that this additive or its decomposed products such as free sulphur is adsorbed on the rubbing surface in the form of physical adsorption, which even may promote the fracture process.
4. Conclusions The effects of several antiwear additives on the friction and wear properties of Cr,03 coatings have been studied. Several conclusions can be drawn. (1) Compared with paraffin oil, oleic acid, dibutyl phosphite and glycol oleate show friction-reducing action but sulphurized olefin does not. The concentration of these additives has little effect on friction. (2) Under lubrication by oils with 2% additives, glycol oleate has little effect on wear, sulphurized olefin shows
Acknowledgments The authors would like to thank Mrs. Shankui Qi and Zhenshi Chen, Fennian Dai and Chuanxian Ding for their assistance in surface analysis and material preparation.
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