Four-ball test

There are numerous tests for characterizing the mechanicai properties of iubricants cone penetration of greases, extreme pressure tests (as in the four-ball test), etc. 

From this test, antiwear and extreme-pressure data were determined, such as welding load, load wear index and wear scar diameter under a load of 40 decanewtons (daN), 60 daN and 80 daN, respectively. First, evaluations were performed on the three original nonmodified hard-core RMs, then evaluations were performed on the sulfur-functionalized ones. Four-ball test data (results only for one concentration 10 wt% additive) are summarized in Table 3.9 (Delfort et al., 1995 and 1999) for ... 

Table 3.9. The antiwear characteristics and extreme-pressure properties of non-modified and modified hard-core carbonate-sulfonate RMs in mineral oil measured by a four-ball test under 60 decanewtons (daN) load.

A combination of ZDDP and hard-core RMs leads to a synergistic effect of metallic detergents on the degradation of ZDDP. These phenomena are observed in many tests and can be explained in terms of (a) the acid neutralization property of hard-core RMs that leads to the prevention of decomposition of ZDDP (in the valve train wear test and the thin film oxygen uptake test), (b) the competitive adsorption of detergents that reduce the effective concentration of ZDDP on the metal surface (in the four-ball test), (c) the formation of mixed films on the metal surface, formed through the decomposition of ZDDP in the presence of hard-core RM s (the coefficient of friction in the Falex wear test). 

XRF The four-ball test wear surface after thermal decomposition was composed mainly of Zn, S and P (Rounds, 1975). 

Apart from the frictional effects, two other performance characteristics need to be considered, namely load-carrying capacity and wear. Most of the available information on these subjects has been obtained from the practical use of commercial materials and will be discussed later, but Groszek and Witheridge found that molybdenum disulphide ground conventionally in air gave a small increase in Mean Hertz Load and Weld Load in a Four-Ball Test as 5% dispersions in a mineral oil. However, oleophilic molybdenum disulphide, ground in n-heptane, gave a 170% increase in Mean Hertz Load and a 260% increase in Weld Load. 

T. Singh and V.K. Verma, EP activity evaluation of tris(N-arylthiosemicarba-zido)-molybdenum (III) on steel balls in a four-ball test, Wear, 146, 313-323, 1991. 

A. Bhattacharya, T. Singh, V.K. Verma, and N. Prasod, 1,3,4-Thiadazoles as potential EP additivies — a tribological evaluation using a four-ball test, Tribology Int., 28, 3, 189-194, 1995. 

Figure 10-17. "Friction polymer" at rubbing surfaces. (a) Wear scar on stationary ball of four-ball test, leading edge at bottom run in cyclohexane vapor. Fein and Kreuz [50]. (b) Wear scar on conically-ended pin, leading edge at bottom run in white oil. Dorinson, unpublished work. (c) Wear debris in polymer matrix as found in the lubricant. Dorinson, unpublished work.

When tested in the four-ball machine, solutions of sulfur in petroleum oils of moderate viscosity or in white oil raise the critical load for the onset of severe, destructive wear, which is designated as "antiseizure" action in the technological idiom of the four-ball test. Davey [54] found a significant increase in the critical initial seizure load from 834 N (85 kg) for a petroleum base oil to 1275 N (130 kg) for elemental sulfur dissolved in the oil. Sakurai and Sato [55] observed a 3.2-fold increase in the load-wear index (mean Hertz load) for a 0.5 weight-percent solution of elemental sulfur relative to that of the uncompounded white oil. The load-wear index is a specialized result of the four-ball test that can be taken as indicative of the average antiseizure behavior of the lubricant. Mould, Silver and Syrett [56] reported a load-wear index ratio of 3.08 for 0.48% sulfur in white oil relative to that of the solvent oil, and also an increase in the initial seizure load from 441 N to 637 N (45 kg to 65 kg) and in the 2.5-second seizure-delay load from 490 N to 833 N (50 kg to 90 kg). 

In contrast to the foregoing treatment of the additive action of disulfides by an explicit physical and chemical model, the usual approach to interpreting results from lubricant bench testing is to look for empirical correlations. Table 11-7 shows some data obtained by Mould, Silver and Syrett [25] from experiments with organosulfur additives in the four-ball test. The additives are listed in order of in-... 

TABLE 11-7. FOUR-BALL TEST DATA FOR ORGANOSULFUR COMPOUNDS... 

Figure 11-4. Load-carrying action of organosulfides in the four-ball test. "Extreme-pressure" procedure 60 seconds at 1500 rpm. Additives furnished 1.19% sulfur to the lubricant, a Base oil. 1 Di-n-butyl sulfide. 2 Diphenyl disulfide. 3 Di-n-butyl disulfide. 4 Dibenzyl sulfide. 5 Di-t-butyl disulfide. 6 Dibenzyl disulfide. 7 Diallyl disulfide. From data by E. S. Forbes [32].

TABLE 11-8. COMPARISON OF FOUR-BALL TESTS OF ORGANIC SULFIDES... 

Reid [26]. But the low-load one-hour wear data, also listed in Table 11-8, are not in the same relative rank order as the EP data. Because of the empirical and arbitrary character of the four-ball test, it is probable that the contact and rubbing parameters have not been identified precisely enough for a quantitative formulation of the mechanism of additive action. 

Systematic data on the relation between chemical structure or reactivity of chlorine compounds and lubricant additive performance are sparse. Table 11-11 gives some four-ball test data obtained by Mould, Silver and Syrett [35], with the additives listed in order of increasing effectiveness in terms of the wear/load index. The results show numerous departures from expectations based on chemical structure. For example, there is practically as much difference between the wear/load indices for the two primary chlorides, n-hexadecyl (16.2 kg) and n-hexyl (30.4 kg), as for n-hexyl chloride and t-butyl chloride (46.1 kg). A large difference would be expected on the basis of chemical reactivity between the additive effectiveness of primary and tertiary alkyl chlorides, but only a small difference for the two primary aliphatic chlorides. The overall trends are what would be expected in general, primary and aromatic chlorides are less efficacious than secondary chlorides, which in turn... 

Figure 11-6. Behavior of octyl chlorides in the four-ball test. 8.52% t-Octyl chloride 2 seconds —A. 10 seconds. 8.48% n-Octyl...

Forbes and Silver [40] published data directly comparing the alkyl ester tri-n-butyl phosphate and the aryl ester tricresyl phosphate. Table 11-13 shows the details of this comparison as well as wear data for the acid ester di-n-butyl phosphate. The wear/load index and the initial seizure load show substantially no discrimination between tributyl phosphate and tricresyl phosphate and very little advantage of the compounded oil over the base oil. The low-load wear test distinctly shows better performance with tricresyl phosphate. The data for di-n-butyl phosphate are at variance with the hypothesis that hydrolytic degradation to the acid ester is the first step in the antiwear action of neutral phosphate esters. On the other hand, Bieber, Klaus and Tewksbury [41] separated acidic constituents from commercial tricresyl phosphate by preparative chromatography, and on blending these constituents back into the original tricresyl phosphate at various concentrations they observed enhancement of antiwear action in the four-ball test, as shown in Fig. 11-7. It should be noted that Bieber et at. worked with only 0.051% phosphorus in the lubricant, which may explain the sensitivity they observed to acid impurities. 

TABLE 11-14. COMPARISON OF UNTREATED AND DE-ACIDIFIED TRICRESYL PHOSPHATE IN THE FOUR-BALL TEST... 

A direct comparison of the additive behavior of neutral phosphates and phosphites in the four-ball test under relatively mild conditions is reported in the work of Goldblatt and Appeldoorn [43]. The data shown in Table 11-15 are their findings for 3% of the additive dissolved in a heavy white oil and run in the four-ball machine for 15 minutes at 1200 rpm. In wet air all the phosphates show about the same wear im.provement under a 10 kg load over wear with the base oil in dry argon there is no significant improvement. Tributyl phosphite functions consistently poorer in the wear test than either triallyl or triphenyl phosphite, whose behavior is not significantly different from that of the corresponding phosphates. All the additives improve the performance of the... 

Figure 11-8. Comparison of phosphate and phosphite triesters as additives. Additives in solvent-refined mineral oil. Four-ball test 60 seconds at 1500 rpm. 1 Tributyl phosphate, 120 kg. 2 Tricresyl phosphate, 120 kg. 3 Tricresyl phosphate, 100 kg. 4 Tributyl phosphate, 100 kg. 5 Trixylenyl phosphate, 120 kg. 6 Trixylenyl phosphate, 100 kg. 7 Tributyl phosphite, 140 kg. 8 Tributyl phosphite, 120 kg. 9 Tributyl phosphite, 100 kg. Data by W. Davey [45].

Four-ball test 60 minutes at 15 kg load, 1500 rpm. Data by Forbes and... 

Figure 11-10. Comparison of organic phosphates, phosphonates and phos-phinates as lubricant additives. Four-ball test 60 minutes, 15 kg load, 1500 rpm. 4 mmoles of additive per 100 gm of white oil solution. Neutral esters o. Acids or acid esters A. 1 n-Butyl di-n-butylphosphinate. 1 Di-n-hexylphosphinic acid, la Di-n-octylphosphinic acid. 2 Di-n-butyl n-hexylphosphonate. 3 Di-n-butyl phenylphosphonate. 4 Tri-n-butyl phosphate. 4 Di-n-butyl phosphate. 5 Diethyl benzyIphosphonate. 6 Diethyl o-nitrophenylphosphonate. 7 Di(2-ethylhexyl) phosphate. 8 Dilauryl phosphate. 9 Tricresyl phosphate. From data by Forbes and Silver [40] and by Forbes and Battersby [46].

TABLE 11-19. FOUR-BALL TESTING OF ZINC PHOSPHORODITHIOATE ESTERS... 

Additives 0.006 mole in 100 grams base oil. Four-ball test at 600 rpm. From data by Sanin, Shepeleva, Ulyanova and Kleimenov [42]. 

Figure 11-14 shows data obtained by Dorinson [38] in an investigation of the cooperative action of di-t-octyl disulfide and t-octyl chloride, two independently effective lubricant additives. The criteria for evaluation are the initial seizure load in the 10-second ASTM four-ball test and the magnitude and course of the post-seizure wear. With either 2% sulfur or 2% chlorine as the single active additive element in the lubricant, the post-seizure transition occurs in the load interval 80-100 kg, and the degree of seizure, as judged by the extent of wear, is not severe. With a combination of 1% sulfur and 1% chlorine in the... 

Figure 11-14. Cooperative additive action of t-octyl chloride and di-t-octyl disulfide. Four-ball test 10 seconds at 1750 rpm. Additives in white oil and wear/load index A. 9.1% Di-t-octyl disulfide, 2.08% S 48.0 kg. B. 8.52% t-Octyl chloride, 2.05% Cl 51.9 kg. C. 4.55% Di-t-octyl disulfide + 4.53% t-octyl chloride, 1.06% S, 1.00% Cl 81.0 kg. D. 9.1% Di-t-octyl disulfide + 8.52% t-octyl chloride, 2.1% S, 2.0% Cl 112.1 kg. Data by A. Dorinson [38].

TABLE 11-22. FOUR-BALL TESTING OF MIXED ORGANOSULFUR AND ORGANOCHLORINE ADDITIVES... 

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