Robust, chromium, semi-reflective coatings have been applied to transparent polymethylmethacrylate and polyurethane discs and this has enabled conventional, normal incidence optical interferometry to be used to measure lubricant film thickness in soft EHL conditions for the first time. High quality interferograms comparable to those obtained from coated glass discs are obtained. Measured film thickness has been compared with existing soft EHL film thickness equations obtained using computer modelling and revised central and minimum film thickness equations have been proposed. These film thickness measurements and measurement technique have applicability to our understanding of the performance and design of lubricated gears and bearings manufactured from polymeric materials.

Viscosity versus shear rate curves have been measured up to 10 7 s −1 for a range of VM solutions and fully formulated oils of known composition at several temperatures. This shows large differences in the shear thinning tendencies of different engine oil VMs. It has been found that viscosity versus shear rate data at different temperatures can be collapsed onto a single master curve using time-temperature superposition based on a shear rate shift factor. This enables shear thinning equations to be derived that are able to predict the viscosity of a given oil at any shear rate and temperature within the range originally tested. One of the tested lubricants does not show this time temperature superposition collapse. This fluid also exhibits extremely high viscosity index and shear thins more easily at high than at low temperature, unlike all the other solutions tested. This unusual response may originate from the presence on the VM molecules of two structurally and chemically different components. In a companion paper, the master shear thinning curves obtained in this paper are used to explore how VMs impact film thickness and friction in a steadily loaded, isothermal journal bearing [1].

In a companion paper, the temporary shear thinning behaviour of a series of viscosity-modifier (VM)-containing blends was studied over a wide shear rate and temperature range [Marx et al. in Tribol Lett, https ://doi.org/10.1007/s1124 9-018-1039-5]. It was found that for almost all VMs the resulting data could be collapsed on a single viscosity versus reduced strain rate curve using time-temperature superposition. This made it possible to derive a single equation to describe the viscosityshear rate behaviour for each VM blend. In the current paper, these shear thinning equations are used in a Reynolds-based hydrodynamic lubrication model to explore and compare the impact of different VMs on the film thickness and friction of a lubricated, isothermal journal bearing. It is found that VMs reduce friction and especially power loss markedly at high shaft speeds, while still contributing to increased hydrodynamic film thickness at low speeds. The model indicates that VMs can contribute to reducing friction in two separate ways. One is via shear thinning. This occurs especially at high bearing speeds when shear rates are large and can result in a 50% friction reduction compared to the equivalent isoviscous oil at low temperatures for the blends studied. The second is via their impact on viscosity index, which means that for a set viscosity at high temperature the low-shear-rate (and thus the high shear rate) viscosity of a high-VI oil, and consequently its hydrodynamic friction, will be lower at low temperatures than that of a low-VI oil. The identification and quantification of these two alternative ways to reduce friction should assist in the design of new, fuel-efficient VMs.

The friction properties of a range of viscosity modifier-containing oils in an engine bearing have been studied in the hydrodynamic regime using a combined experimental and modelling approach. The viscometric properties of these oils were previously measured and single equations derived to describe how their viscosities vary with temperature and shear rate (Marx et al. Tribol Lett 66:92, 2018). A journal bearing machine has been used to measure the friction properties of the test oils at various oil supply temperatures, while simultaneously measuring bearing temperature using an embedded thermocouple. This shows the importance of taking account of thermal response in journal bearings since the operating oil film temperature is often considerably higher than the oil supply temperature. For Newtonian oils, friction coefficient measurements made over a wide range of speeds, loads and oil supply temperatures collapse onto a single Stribeck curve when the viscosity used in determining the Stribeck number is based on an effective oil film temperature. Journal bearing machine measurements on VM-containing oils show that these give lower friction than a Newtonian reference oil. A thermo-hydrodynamic model incorporating shear thinning has been used to explore further the frictional properties of the VM-containing oils. These confirm the findings of the journal bearing experiments and show that two key factors determine the friction of the engine bearing; (i) the low shear rate viscosity of the oil at the effective bearing temperature and (ii) the extent to which the blend shear thins at the high shear rate present in the bearing.

The ultrashear viscometer (USV) has been adapted and employed to investigate the permanent shear thinning of polystyrene solutions in a series of phthalate ester base fluids. The permanent shear stability index based on viscosities measured at 10 6 s -1 , PSSI(10 6 ), has been found to be a convenient way to express the magnitude of permanent shear thinning. When comparing permanent shear thinning at various shear rates in the USV, it is very important to take account of the different times of shear that are present at different shear rates. The PSSI(10 6 ) value divided by the total time of shear is then a useful way of quantifying and comparing permanent shear thinning rates. Tests using polystyrene in different viscosity base fluids have shown that this rate of permanent shear thinning depends on shear stress and not shear rate and varies linearly with polymer concentration. The rate of permanent shear thinning also varies exponentially with shear stress, suggestive of a stress-promoted polymer breakdown process. By using a small volume of test fluid in the USV and solvent extraction after a test, it has proved possible to obtain molecular weight distributions of polymer after shear using gel permeation chromatography (GPC). This indicates that the polymer breakdown process is different at low and high polymer concentrations, with molecule fragmentation at low polymer concentration but mid-chain scission at high concentration. A key feature of the USV is that, unlike other methods currently used to measure permanent shear thinning behaviour of engine oils, it subjects the test fluid to well-defined, controllable high shear conditions. Coupled with the use of GPC, this makes it possible for the first time to relate quantitatively the permanent shear thinning of engine oils to shear conditions and to polymer degradation response.