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What is PCD?
Polycrystalline Diamond (PCD) is diamond grit that has been fused together under high-pressure, high-temperature conditions in the presence of a catalytic metal. The extreme hardness, wear resistance, and thermal conductivity of diamond make it an ideal material for bearings. Individual diamond crystals cleave quite easily when struck parallel to certain planes (the process used to facet diamond gemstones takes advantage of these relatively weak planes).What is PCD?
Diamond Bearing Tribology Fundamentals
Tribology comes from the Greek word ‘tribos’ which means ‘rubbing’. It is a relatively new science that considers the friction, wear and lubrication at moving contacts. Diamond bearing tribology looks at the physics between two diamond surfaces moving relative to one another, most of the time in the presence of a lubricant.
A magnified view of even the most smoothly machined or ground surfaces will reveal a rough looking surface with high asperities and low valleys. Please refer to figure 1. Two surfaces in sliding motion will contact first at these high asperities. It is the action of welding and shearing that takes place on this uneven surface that results in what we understand as friction. This is largely independent of surface area as first described by
Guillaume Amontons (1699).
As speed increases with the right bearing geometry and lubricant viscosity, a pressurized fluid film can develop between the two surfaces. This film supports the load and keeps the two surfaces completely separated as shown in figure 2. Under these conditions, friction and heat generation are greatly reduced and bearing wear is virtually eliminated. Compare common characteristics of bearing material.
While decreasing the speed, a point is reached where there is not enough relative velocity to maintain pressure in the fluid film and the gap between the two moving surfaces breaks down. Asperities contact, heat and shearing occur and friction increases.
These lubrication regimes are evident in the Stribeck dimensionless curve where friction coefficient, is plotted against absolute viscosity, rotational speed and unit load,. Three identifiable lubrication regions are illustrated in the Stribeck Curve graph: boundary lubrication, mixed and fluid film.
Diamond bearings have the advantage that they can operate effectively in both the fluid film and mixed lubrication regions.