Understanding Conductive Layer Deposits: Test Method Development for Lubricant Performance Testing for Hybrid and Electric Vehicle Applications

Advances in hybrid vehicles and electric vehicles (EV) are creating a need for a new generation of lubricants and new lubricant performance tests. Copper corrosion is one prominent concern for hybrid vehicles and EVs and is routinely assessed using a coupon test. This is characterized as metal dissolution, a surface tarnish, or a corrosion layer where a corrosion product remains on the surface and is characterized by a qualitative visual rating. This deficiency does not provide insight into the nature of the corrosion deposit. In an electric drive unit, there are multiple sources of the electric potential present, which can significantly alter the formation of a corrosion deposit which is not assessed in the coupon tests. The formation of a conductive corrosion deposit can result in catastrophic failure of the electric drive unit, either through direct shorting of the motor winding or failure of the power electronics. Here we describe a conductive layer deposit test for use with lubricants for hybrid vehicle and EV applications. Real-time conductance measurements allow insight into the formation of conductive layer deposits. These deposits depend heavily on the chemistry of the system. We intend our essay to be the starting point for a more sophisticated understanding of conductive layer deposits.