Antiwear additive

Titanium compounds could be used as an environmentally friendly additive for automotive oil, according to materials scientists from Afton Chemical Corporation and the National Institute of Standards and Technology (NIST).

In a recent study, the researchers established that a titanium compound added to engine oil creates a wear-resistant nanoscale layer bound to the surface of vulnerable engine parts, making it a credible substitute for older compounds.

Modern engine lubricating oil is a complex, highly engineered mixture, up to 20 per cent of which may be special additives to enhance properties such as viscosity and stability and to reduce sludge formation and engine wear.

For years, antiwear additives for high-performance oils have been phosphorous compounds, particularly zinc dialkyldithiophosphate, or ZDDP, that work by forming a polyphosphate film on engine parts that reduces wear.

Unfortunately phosphorus is a chemical poison for automobile catalytic converters, reducing their effectiveness and life span, so industry chemists have been searching for ways to replace or reduce the use of ZDDP. It is not a simple problem because the additive has several useful functions in addition to wear resistance.

Titanium compounds are one candidate replacement. Mechanical tests of one titanium compound at Afton demonstrated that it provided superior wear resistance when added to a fully formulated engine oil.

Just how the titanium compound worked was an open question, however. Surface analysis tests could detect titanium in the wear tracks of test surfaces but not with enough sensitivity to determine its chemical nature or whether, for example, it was just lying there or bound to the metal surface.

To resolve the issue, the researchers turned to NIST’s soft X-ray beamline at the National Synchrotron Light Source (NSLS) in Brookhaven, NY.

The NIST beamline instruments use low-energy ('soft') X-rays that can be precisely tuned to specific elements to measure chemical bonds both at the surface of a sample and deeper into the bulk of the material.

The measurements revealed that the antiwear enhancement comes from titanium that becomes chemically bound into the metal structure of the engine surface, forming a hard oxide, iron titanate.

While considerably more work remains to be done, the results suggest that titanium compounds could play an important role in future low-phosphorus lubricating oils.