From Thermal Stress to Varnish: Lab Tools for Lubricant Deposit Assessment

Deposit formation remains one of the most critical limiting factors for lubricant performance in both automotive and industrial applications. Deposits such as lacquer, varnish, and carbon residues may impair heat transfer, restrict fluid circulation, promote component wear, and ultimately reduce equipment reliability and lubricant lifetime. Their formation is mainly driven by thermal stress and oxidation mechanisms but is strongly influenced by lubricant formulation, additive chemistry, and operating conditions.

This presentation provides a practical overview of some laboratory‑scale tests used to evaluate deposit formation tendencies in lubricants, with a particular focus on high‑temperature deposit formation methods and thermo‑oxidative tests. The objective is to illustrate how complementary laboratory tests can be combined to characterize deposit risks over a wide range of lubricant types, including engine oils, hydraulic fluids, turbine oils, and other industrial lubricants.

Special emphasis will be placed on panel‑based thermal deposition tests, including the Panel Coker Test (PCT) GFC and FTM, which simulate lubricant exposure to hot metal surfaces and enable visual and gravimetric assessment of deposit formation.

In parallel, the Micro Cooking Test (MCT) will be presented as a rapid screening tool to assess thermal residue formation under confined high‑temperature conditions, allowing early discrimination between formulations.

To extend the assessment beyond purely thermal stress, several thermo‑oxidative aging tests such as Cincinnati Milacron, TOST, and DKA will also be discussed. These tests provide valuable information on oxidation stability, sludge formation, and insoluble generation, which are closely linked to varnish and deposit formation in service.

Finally, such an approach allows better ranking of lubricant formulations, supports formulation optimization, and improves the prediction of field performance under severe operating conditions.