In this section, the experimental results from the optical EHL tribometer and twin-disk test rig are presented and discussed.
4.1. Optical EHL Tribometer
Figure 5 shows measured interferograms for an oil temperature of ϑ
Oil = 40 °C, a Hertzian pressure of p
H = 530 N/mm
2, and a slip ratio of s = 0% for all considered lubricants with an example sum velocity of v
Σ = 1.6 m/s.
Figure 6 explicitly shows the averaged measured central and minimum film thickness values h
c and h
m over the sum velocity for ϑ
Oil = 40 °C, p
H = 530 N/mm
2, and s = 0%. All film thickness curves were repeated once. The maximum deviation was 9.3 nm for MIN-10, 7.3 nm for PAO-09, 3.6 nm for PAO-05, 8.1 nm for PAGW-09, 8.1 nm for PAGW-05A, and 5.0 nm for PAGW-05B. For all lubricants, the film thickness curves show a typical behavior with an almost linear increase of h
c and h
m, with increasing sum velocity in double-logarithmic scale. The mineral oil MIN-10 shows the highest lubricant film thickness. The water-containing fluid PAGW-09 shows on average a 33% smaller central film thickness h
c and 23% smaller minimum film thickness h
m than the polyalphaolefin oil PAO-09. The water-containing fluids PAGW-05A and PAGW-05B show almost comparable film thickness values to the polyalphaolefin oil PAO-05 with, on average, a 16% and 20% smaller central film thickness h
c and 13% and 6% higher minimum film thickness h
m.
Based on the measured central film thickness h
c in
Figure 6, the pressure–viscosity coefficients α
p of the lubricants considered can be derived using the Hamrock et al. [
28] formula:
where U is the velocity parameter, G the material parameter, and W the load parameter:
The derived pressure–viscosity coefficients α
p are summarized in
Table 4. The coefficients can be classified according to the base oil type. The mineral oil MIN-10 shows the highest α
p-value, which is approximately 1.7-fold the α
p-values of the polyalphaolefin oils PAO-09 and PAO-05 and approximately 4.3-fold the α
p-values of the water-containing gear fluids PAGW-09, PAGW-05A, and PAGW-05B. Note that the lubricant film thickness formation of the water-containing gear fluids is supported by the approximately 30% higher density, which goes in Equation (5) by the power of 0.67.
The results from the EHL optical tribometer show that the investigated water-containing lubricants have good EHL lubricant film formation capability.
4.2. Twin-Disk Test Rig
Figure 7 shows the averaged measured coefficients of friction µ and bulk temperatures ϑ
M over the slip ratio s for an oil inlet temperature of ϑ
Oil = 60 °C, a Hertzian pressure of p
H = 1200 N/mm
2, and sum velocities v
Σ = {1; 8; 16} m/s. All friction curves measured were repeated once with the same disk pairs. The maximum deviation was 0.0004 for MIN-10, 0.0011 for PAO-09, 0.0022 for PAO-05, 0.0028 for PAGW-09, 0.0013 for PAGW-05A, and 0.0006 for PAGW-05B.
In the case of the conventional gear oils MIN-10, PAO-09, and PAO-05, all friction curves show a typical behavior, as described in
Section 1. A strong increase of the coefficient of friction is followed by a maximum value at low slip ratios. With higher slip ratios, the friction curves become dominated by thermal effects due to the increase of friction power in the disk contact. The bulk temperatures ϑ
M as measure for the friction power increase continuously with the slip ratio and more strongly for higher sum velocities. The two polyalphaolefin oils PAO-09 and PAO-05 show considerably lower coefficients of friction than the mineral oil MIN-10, whereas the influence of the viscosity difference between PAO-09 and PAO-05 is small. These findings correlate well with the general findings of Mayer [
1]. The lowest coefficient of friction of the conventional gear oils was observed for PAO-05.
In the case of the water-containing gear fluids PAGW-09, PAGW-05A, and PAGW-05B, significantly lower coefficients of friction and bulk temperatures were measured compared to the conventional gear oils. In fact, all coefficients of friction were lower than μ < 0.01, which is the value that classifies superlubricity (Hirano et al. [
29]). Contrary to the conventional gear oils, the coefficients of friction increase steadily with increasing slip ratio. No strong increase or pronounced maximum at low slip ratios was found. Some influence of thermal effects can be seen due to the flattening of friction curves at high slip ratios, which was particularly pronounced for higher sum velocities due to higher friction power. Corresponding to the ultra-low coefficients of friction µ, the bulk temperatures ϑ
M were much smaller compared to the conventional gear oils.
For estimation of the lubrication regime in the experiments, the relative film thickness λ
rel can be used:
The minimum film thickness h
m for line contacts was calculated according to Dowson et al. [
30] with values for α
p adopted from
Table 4. Fluid film lubrication was assumed for λ
rel > 3.
Table 5 shows the calculated relative film thickness λ
rel for p
H = 1200 N/mm
2, v
Σ = {1; 8; 16} m/s, and s = {0; 50}%. It indicates fluid film lubrication for all considered operating conditions. The lowest λ
rel values occur for PAO-05, PAGW-05A, and PAGW-05B at v
∑ = 1 m/s, where some asperity contact cannot be excluded. This may correspond to the slightly lower friction of PAGW-09 at v
∑ = 1 m/s in
Figure 7.
Figure 8 and
Figure 9 summarize the averaged measured coefficients of friction µ for an oil inlet temperature of ϑ
Oil = 60 °C, Hertzian pressures of p
H = {600; 1200} N/mm
2, and sum velocities of v
Σ = {1; 2; 4; 8; 16} m/s for a slip ratio of s = 20%. For all lubricants considered, the coefficients of friction measured were lower for the lower Hertzian pressure of p
H = 600 N/mm
2. Again, ultra-low coefficients of friction were measured with the water-containing gear fluids for all operating conditions considered. The coefficient of friction was always lower than μ < 0.01, corresponding to superlubricity.
The friction behavior measured at the twin-disk test rig was very different in comparison to the conventional gear oils and water-containing gear fluids. The conventional gear oils MIN-10, PAG-09, and PAO-05 show coefficients of friction between 0.020 ≤ µ ≤ 0.060 and friction curve behavior typically known from highly-loaded EHL contacts. The water-containing gear fluids PAGW-09, PAGW-05A, and PAGW-05B show ultra-low coefficients of friction below µ < 0.010 in the superlubricity regime, and atypical friction curve behavior with a steady increase of friction over slip ratio.
As the pressure–viscosity coefficients and lubricant film formation of the water-containing gear fluids was evaluated as fairly good, its ultra-low friction cannot be referred to low contact viscosities as stated for water-based fluids with low pressure–viscosity coefficient. Existing model representations for the ultra-low friction of water-containing fluids were summarized in
Section 1. According to Chen et al. [
13], a microscopic layer of FeOOH is built on the substrate in ambient conditions, accumulating a hydrogen-bonded film of glycol and water molecules. Between these films, there is a zone of free water molecules allowing easy sliding, which results in ultra-low friction. A similar mechanism may apply for the water-containing gear fluids considered.
Figure 10 shows light microscope pictures and roughness parameters Ra and Rq of the upper disk after test runs. For all surfaces, typical light circumferential marks on the disk surfaces were observed. The arithmetic mean surface roughness value shows no significant change compared to the initial condition, whereas the root-mean-squared roughness shows a slight increase.