SPECIAL SECTION/ADVANCED CERAMICS: Optimizing a Sliding Surface

April 1, 2009
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Sintered silicon carbide helps solve a wide range of sliding bearing material demands.



Extreme demands are placed on sliding bearing materials that are used in high-performance applications. Process fluid-lubricated bearings, such as those found in high-speed chemical pumps, must employ ceramic materials with a low-wear property profile for pumping corrosive or abrasive liquids. Sintered silicon carbide (SSiC) is a ceramic that helps solve a wide range of sliding bearing material demands.

SSiC Benefits

SSiC materials offer many advantages over other high-performance ceramics. The SSiC microstructure can be selectively modified to achieve further improvements in the material. For example, one SSiC material* is characterized by an outstanding load capacity that enables it to withstand extreme pressures and thrust forces. It also includes graphite particles with sizes of 50 to 120 µm homogeneously dispersed in its structure, which reduces the coefficient of friction considerably and improves wear behavior.

The self-lubricating effect of the graphite particles even permits temporary dry running. The material is therefore ideal for tribological applications under poor lubrication conditions, such as in sliding bearings and mechanical seals.

Another SSiC** is also highly corrosion resistant and very resistant to attack by hot water. Since water mainly corrodes the boundary layers between adjacent SiC grains (grain boundaries), a coarse-grained structure was developed with grain sizes up to 1.5 mm. This significantly reduces the grain boundary surface area and provides improved corrosion resistance.

The long SiC grains have another advantage. Since they are anchored deep in the ceramic, hot water or aggressive chemicals, which corrode the material to a depth of 20 µm, have no chance to dissolve the grains and damage the sliding surfaces. Because of its coarse-grained surface, the material has a higher load capacity, which means that the contact pressure of tribologically loaded systems can be further increased. The combination of higher capacity and excellent wear behavior significantly expands this SSiC’s application ranges for bearing and seal systems.

* EKasic® G, developed by ESK Ceramics, a Ceradyne Company
** EKasic® C, developed by ESK Ceramics, a Ceradyne Company

Reduced Break-Away Torque

The coarse-grained microstructure of these SSiC materials also results in a tribological optimization of the sliding surface. Extended SiC grains are randomly oriented at the surface of the part, so the surface is finely undulating on a microscopic scale. This microscopic structuring is a consequence of the anisotropic wear behavior of SiC monocrystals (grains), which, according to their orientation, are abraded to different extents as the bearing stops and starts under poor lubrication conditions.

The bearing surfaces are not polished to a mirror finish; a specific surface roughness remains. Like sharkskin that is not ideally smooth but exhibits reduced flow resistance, the residual roughness of the bearing surface can be seen to improve its flow properties. In addition, the risk of adhesive sticking between the sliding parts is lower, as is the initial break-away torque on start-up of the bearing, which provides particular advantages when these materials are used in highly loaded mechanical seals.

Thrust bearing of SSiC with lubrication groove and hydrodynamic structuring.

Improved Load Capacity

The selective modification of the surface of SSiC sliding bearings significantly improves the hydrodynamics of the lubricating fluid. The introduction of micrometer-sized cavities into the surface by laser ablation provides hydrodynamic support of the liquid film with increased pressure in the lubricant, and, therefore, improved system load capacity.

The cavities are designed such that they form a wedge-shaped gap between the two sliding surfaces of the bearing. This gap is responsible for the hydrodynamic build-up of the pressure maximum. The two sliding parts then experience a lift, similar to that of a water skier, even at low sliding velocities. However, special machining techniques are required to attain the cavities’ necessary wedge shape.

SSiC sliding bearings are micro-machined by a neodymium laser with a wavelength of 1064 nm. When the laser pulses are in the nanosecond range, the laser energy causes the material’s surface to heat up for the duration of the pulse. Since thermal conduction only permits slow energy transfer into the volume, the incident energy is concentrated on a very thin layer. The surface therefore reaches very high temperatures, and the SiC material is suddenly vaporized. The machining accuracy achieved in both the lateral and vertical direction is of the order of less than 1 µm.

In addition, electron microscopy analysis of the structured surfaces does not show any damage to the ceramic material beneath the structures. The mechanical strength, and therefore the reliability of the SSiC bearing part, are not limited by microcracks or other damage patterns.

Wear Performance

SSiC materials, combined with innovative structuring and coating technologies, can significantly improve existing bearing and seal systems. This improvement is necessary because of the continually increasing sliding bearing demands that are the result of strict environmental regulations and increasingly harsh service conditions. A typical example is the exploitation of new gas and oil reserves where modern extraction technology is penetrating into ever-deeper regions where temperatures and pressures are higher.

Pumping out highly abrasive and corrosive oil sludge mixtures from a borehole requires a resistant, high-performance pump with carefully designed bearings and seals. After it was found that harsh stresses were severely reducing the lifetimes of conventional borehole pumps while significantly increasing operating costs, pump manufacturers assisted in the development of the new SSiC bearing system. Not only is it possible to exploit the beneficial corrosion resistance of this material and its resistance to wear by the solid particles in the oil sludge, it considerably improves performance and increases lifetimes by orders of magnitude.

A similar wear challenge was faced by a manufacturer of magnetically coupled cargo pumps. In addition to needing to pump a large number of different fluids with variable viscosities and solids content quickly, the possibility of a breakdown resulting from incorrect operation was a concern. For example, the pump might be started up without opening the shut-off valves, or the system could be shut down after the tanks are empty, leading to an interruption of fluid flow that would cause dry running of the bearings.

Such operating conditions inevitably meant that the critical loading limits of the SSiC thrust bearings were regularly exceeded. The new material has since proved valuable by withstanding extreme cargo pump loads and ensuring reliable operation. A further increase in the load capacity of axial bearings was produced by the laser-induced application of hydrodynamically active fine structures on the sliding surface of the thrust bearing.

Such liquid behavior optimization in the sliding gap also improves lubrication when there is insufficient process fluid. The homogeneous lubrication film that is produced acts as a spring buffer, helping to intercept axial thrust peaks in the bearing system. Overall, the design and material changes that were introduced with the new SSiC materials have increased the critical loading limit more than tenfold.

For more information regarding optimized SSiC materials, contact Hollis Morris, Sales Manager, General Industries, Ceradyne, Inc., 3169 Red Hill Ave., Costa Mesa, CA 92626; (714) 549-0421; e-mail hmorris@ceradyne.com; or visit www.ceradyne.com.

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