Effects of anti-wear additives on friction and wear properties of Crz03 coating

Jianjun Wei, Qunji Xue and Hanqing Wang

The effects of some anti-wear additives on the friction and wear behaviour of plasma-sprayed Cr203 coating were investigated using a block-on-ring tester at ambient conditions. The results show that zinc dialkyldithiophosphate (ZDDP), tricesyl phosphate (TCP) and tributyl phosphate (TBP) significantly reduce the wear of Cr2Os coating lubricated by paraffin oil. Additive concentrations as well as sliding time have great influence on the wear. The friction coefficient varies slightly with test conditions. The analysis by XPS of worn surfaces indicates that the wear resistance of these additives is due to the formation of tribochemical reaction films by reacting with Cr203 coatings.

Keywords: anti-wear additives, Cr203 coating, friction and wear properties, worn surfaces, XPS

Introduction Recently, the effects of lubricating oils on the tribolog- ical performance of ceramics have been investigated because, under dry sliding or rolling contact, friction and wear of ceramics are usually too high for practical use ~. Studt, Jahanmir and Fischer found that, at low load and sliding speed, the friction coefficients of Si3N 4 and SiC lubricated by hexadecane exhibit very low values 2,~. A review of the literature shows that very few publications have been concerned with the effects of additives on the tribological properties of ceramics. In particular, fundamental studies such as interaction mechanisms between ceramics and additives are sadly lacking.

The purpose of this paper is to examine the effects of several anti-wear additives on the friction and wear properties of Cr203 coatings and to clarify the interac- tion mechanisms of Cr203 coating with these additives.

Materials

The plasma-sprayed Cr203 coating used in this study was developed by the Shanghai Institute of Ceramics. The preparation technique of this coating is described in detail in reference 4. The specimens (block and ring) were made of a carbon steel (0.45 carbon content). The surface of the block and outer surface of the ring were sprayed with Cr203 powder and their

Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China. Received 31 January 1993; revised 5 April 1993; accepted 17 May 1993.

mechanical properties are listed in Table 1. The sprayed coating had a thickness of 0.3 mm after grinding and a surface roughness Ra of about 0.10 p~m after polishing. It consists of 92.0% Cr203 and 8.0% Si02.

Chemically pure liquid paraffin with a viscosity of 30.19 cSt (25°C) and a boiling point higher than 300°C was used as base oil (produced in Tianjin, China). The additives examined were commercial zinc dialkyldi- thiophosphate (ZDDP, 15.5% S, 7.5% P, 9.0% Zn), chemically pure tricesyl phosphate (TCP, 8.4% P) and tributyl phosphate (TBP, 11.7% P). Their struc- tures are reported elsewhere 5. The oil solutions used were formulated by addition of various content of additives to paraffin oil.

Experimental procedure

The experiments were carried out using a block-on- ring test machine in which a lower rotating ring was

Table 1 Mechanical properties of Cr203 coating

Property Unit Value

Purity % 92.0 Porosity % 4.7 Grain size ixm 38-76 Bulk density g/cm z 4.52 Apparent density g/cm 3 4.72 Microhardness GPa 10.0 Bend strength MPa 81.3 Impurity SiO=

TRIBOLOGY INTERNATIONAL 0301-679X/93/040241-04 © 1993 Butterworth-Heinemann Ltd 241

J Wei et aI--Effects of anti-wear additives on Cr203 coating

sliding against an upper stationary block. The length by width by thickness of the block specimen was 19.0 x 124 x 124 mm and the outer diameter of the ring was 49 mm. '7

The wear tests were performed at ambient conditions. ,--° Lubrication was produced by an oil-dripping process. --- A rotating speed of 200 rev/min (line speed 0.51

o m/s) and a normal load of 539 N were used in all o tests. Prior to testing, the specimens were cleaned by ~ petroleum ether in an ultrasonic bath for 15 min.

©

The friction coefficient was calculated as the ratio of ¢ ~A the recorded friction force to the normal load. The ,~

wear volume of the block was measured using a surface o profilometer and the standard deviation was less than > 10%. The worn surfaces of the blocks were examined using X-ray photoelectron spectroscopy (XPS). In order to ascertain chemical change of additives before and after testing, the binding energies of active elements of the additives were analysed by XPS under nitrogen-cooling conditions. The binding energy of C,~ (284.6 eV) was used as a reference.

Results and discussion

Effect of sliding time

Friction traces under various lubricated conditions are given in Fig 1. In paraffin oil, the friction coefficient was low and always remained stable during the test. In oils with 2% additives, the friction coefficient was close to that in the base oil.

Figure 2 represents the variation of the wear volume of the Cr203 coating block with sliding time. In paraffin oil, the wear volume increased rapidly with sliding time and became a linear increase after 10 min. In oils with 2% additives, however, the wear volume initially increased slowly with sliding time, and then

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big 2 Wear volume of the block as a function of sliding time for Cr20~ coating lubricated by oils" with 2% additives (load 539 N, sliding speed 0.51 m/s)

gradually tended to an asymptotic value, independent of sliding time. The three additives provided remark- able wear resistance but presented no obvious differ- ences at this concentration (2% wt).

Effect of additive concentration

The concentrations of the additives had little effect on the friction of the Cr203 coating, as shown in Fig 3, whereas the wear behaviour was greatly influenced by the amount of additives in the base oil (see Fig

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O

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Fig 1 Friction coefficient as a function of sliding time for Cr20~ coating lubricated by oils with 2% additives (load 539 N, sliding speed 0.51 m/s)

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Fig 3 Friction coefficient as a function of concentration of additive (load 539 N, sliding speed 0.51 m/s, sliding time 30 min )

242 1993 V O L U M E 26 N U M B E R 4

J. Wei et aI--Effects of anti-wear additives on Cr20s coating

4). When small concentrations of additives were added to the base oil a remarkable decrease in the wear volume of the block was found. However, above 1% the variation in wear was different for the three additives. For ZDDP, the wear volume fell to a minimum as its concentration increased up to 2%, then rose to 2-3% and reached a stable value above 3%. For TCP, the wear volume gradually decreased with its concentration and finally reached a steady stage above 3%. For TBP, the wear volume rose slightly as its concentration increased.

Analyses of chemical compositions of surface films

The results of the wear tests indicate that there may be interactions between the Cr203 coating and the additives during the rubbing process, leading to low friction and wear. In order to clarify their interaction mechanisms, XPS was used to analyse the worn surfaces tested in oil with 2% additives. The conditions of wear tests were the same as in Fig 4 and XPS results are shown in Table 2. Under ZDDP lubricated conditions, it was found that the binding energies of three elements (S, P, Zn) were different from those in the original ZDDP. The binding energies of P2p and Zn2p are attributed to Zn3(PO4)2, and that of S2p corresponds to disulphides or mercaptan, in comparison with the standard spectra 6. It is interesting to note that these compounds have already been identified in the case of thermal decomposition of ZDDP under static heating conditions 7. This finding suggests that, like the thermal decomposition mechanism, ZDDP decomposes thermally during the rubbing process and its decomposition products are deposited on the Cr203 coating surface. It was also found that the binding energy of Cr2p has a slight variation, and it is likely that the decomposed products of ZDDP react with the Cr203 coating, forming new products.

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Fig 4 Wear volume of the block as a function of concentration of additive (load 539 N, sliding speed 0.51 m/s, sliding time 30 min)

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Table 2 XPS results of the worn surfaces of CrzOa coating blocks under lubricating conditions

Lubricant Binding energy (eV)

Cr2p3/2 S2p P2p Zn2p3/2 Zn (A)

Cr203 577.1 original surface ZDDP a 162.7 133.7 1024.1 986.9 2% ZDDP 577.4 163.1 134.2 1022.5 986.6 TB Pa 133.4 2% TBP 577.8 134.6 TCP ~ 133.9 2% TCP 577.8 134.0

aData obtained from original additives under liquid nitrogen cooling conditions

Under TBP lubricated conditions, it was found that the binding energy of P2p shifts from 133.4 eV to 134.6 eV after testing. A shift of binding energy of Cr2p from 577.1 eV to 577.8 eV was also observed. This indicates that TBP decomposes and reacts with the Cr203 coating during the rubbing process. Since phosphorus exists in the form of PO4, the binding energies of P and Cr are probably attributed to CrPO4.

Under TCP-lubricated conditions broader P2p peaks around 134.0 eV were observed. A shift of Cr2p binding energy from 577.1 eV to 577.8 eV took place. This also indicates that TCP decomposes and reacts with the Cr203 coating during the rubbing process. As with TBP, the reaction product could be speculated as being CrPO4 by referring to the standard spectra.

Discussion

Based on the XPS results, the wear behaviours under different lubricated conditions could be reasonably elucidated. The wear tests shown in Fig 2 indicate that, with 2% additive lubrication, the wear volume of the Cr203 coating block was almost independent of sliding time after 20-30 min. This was due to the formation of tribochemical reaction films and the deposition of additives or their decomposed products on the rubbing surfaces. Figure 4 shows that the anti- wear properties between ZDDP and phosphates (TBP and TCP) are different at higher concentrations, perhaps as a result of the presence of sulphur in ZDDP. The good anti-wear properties of ZDDP at lower concentrations is mainly due to the deposition of ZDDP and its decomposed products on the rubbing surface, while the worse properties at higher concen- trations may be caused by a tribochemical reaction in which Cr203 reacts with its decomposed products such as sulphides and leads to excessive corrosive wear. On the other hand, the good anti-wear properties of phosphates are due to the formation of tribochemical reaction products such as CrPO4 and the deposition of their decomposed products on the rubbing surfaces. Moreover, for TBP, a slight increase in the wear at higher concentration is a result of corrosive wear caused by tribochemical reactions,

243

J. Wei et aI--Effects of anti-wear additives on Cr20a coating

Conclusions 1. Under three additive lubricated conditions (2% j.

ZDDP, 2°/,, TCP, 2% TBP) the wear volume of a Cr203 coated block tends to be independent of sliding time after 20-30 min testing. Compared with base oil, these additives can significantly reduce the 2. wear but have little effect on the friction.

2. The concentrations of ZDDP, TCP and TBP have an influence on the wear behaviour of a C r 2 0 3 3.

coating, and the effect of Z D D P differs from that of organic phosphates. The friction coefficient is not affected by the concentration.

3. XPS analyses of worn surfaces show that the anti- 4. wear properties of these additives are due to the tribochemical reaction with the CreO~ coating 5. and the deposition of the additives or their decomposition products. ~.

Acknowledgements The authors wish to acknowledge Mrs Chuanxian Ding, 7. Fengnain Dai and Shangkui Qi for their assistance in this experiment.

References

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Studt P. Influence of lubricating oil addit ives on friction of ceramics under condit ions of boundary lubrication. Wear F)87, 115, I85 191

J a h a n m i r S. and Fischer T.E. Friction and wear of silicon nitride lubricated by humid air, water , hexadccanc and hexadecane ~-0.5 percent stearic acid. STI.E Trans. 10~8, 31. 1. 32-43

Wei J . , Xue Q. and Wang H. Friction and wear of (?re() coat ing in inorganic salt solut ions. Wear 1092, 152. 161-170

Wei J. and Xue Q. Effect of additive interaction on the friction and wear proper t ies of a WC coating. Wear 19~2, 157, 163 172

Wagner , C.D. et al. tlandbook ~)/" X-ray I'hotoehwtron Spec- tro.scopy, Pcrk in-Elmer Corpora t ion , Physical Electronic Div- ision, Minnesota . 1979

Wei J. and Xue Q. Study tm thermal decomposi t ion and anti-oxidative per formance of zinc d ia lkyldi th iophosphatc . Pemdeum Proces,sing 1991, 11. 27 32 (in Chinese)

244 1993 VOLUME 26 NUMBER 4