Galling - Prevention

Prevention

Adhesive wear and material transfer from one surface to another during sliding, so called galling, occur for a number of different materials and frictional systems. Generally there are two major frictional systems which effect adhesive wear or galling. In terms of prevention, they work in dissimilar ways and set different demands on the surface structure, alloys and crystal matrix used in the materials. The two frictional systems are:

• Solid surface contact
• Lubricated contact

In solid surface contact or unlubricated conditions, the initial contact is characterised by interaction between asperities and the exhibition of two different sorts of attraction. Cohesive surface energy or chemical attraction between atoms or molecules connect and adhere the two surfaces together, notably even if they are separated by a measurable distance. Direct contact and plastic deformation generates another type of attraction through the constitution of a plastic zone with flowing material where induced energy, pressure and temperature allow bonding between the surfaces on a much larger scale than cohesive surface energy.

In metallic compounds and sheet metal forming, the asperities are usually oxides and the plastic deformation mostly consists of brittle fracture, which presupposes a very small plastic zone. The accumulation of energy and temperature is low due to the discontinuity in the fracture mechanism. However, during the initial asperity/asperity contact, wear debris or bits and pieces from the asperities adhere to the opposing surface, creating microscopic, usually localized, roughening and creation of protrusions (in effect lumps) above the original surface. The transferred wear debris and created lumps penetrate the opposing oxide surface layer and cause damage to the underlying bulk material, allowing continuous plastic deformation, plastic flow, and accumulation of energy and temperature. With regard to the previously defined difference between the initial two types of attraction in "solid surface contact" or unlubricated conditions, the prevention of adhesive material transfer is accomplished by the following or similar approaches:

• Less cohesive or chemical attraction between surface atoms or molecules.
• Avoiding continuous plastic deformation and plastic flow, for example through a thicker oxide layer on the subject material in sheet metal forming (SMF).
• Coatings deposited on the SMF work tool, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD) and titanium nitride (TiN) or diamond-like carbon coatings exhibit low chemical reactivity even in high energy frictional contact, where the subject material's protective oxide layer is breached, and the frictional contact is distinguished by continuous plastic deformation and plastic flow.

Lubricated contact sets other demands on the materials surface structure, and the main issue is to retain the protective lubrication thickness and avoid plastic deformation. This is important because plastic deformation raises the temperature of the oil or lubrication fluid and changes the viscosity. Any eventual material transfer or creation of protrusions above the original surface will also reduce the ability to retain a protective lubrication thickness. A proper protective lubrication thickness can be assisted or retained by:

• Surface cavities (or small holes) can create a favourable geometric situation for the oil to retain a protective lubrication thickness in the contact zone.
• Cohesive forces on the surface can increase the chemical attraction between the surface and used lubrication and enhance the lubrication thickness.
• Oil additives may reduce the tendency for galling or adhesive wear.