One approach to improving mechanical seal performance is to upgrade to a sintered silicon carbide composite material. Silicon carbide based materials have been increasing the limits of mechanical seals for more than 25 years, starting with Chemical Vapor Reacted (CVR) silicon carbide whose cross section resembles that of a piece of M&M'S® candy. The porous outer surface is a combination of silicon carbide and graphite. This surface coating encapsulates a carbon-graphite core. This material provided a hardened surface with a unique topography for increased interfacial lubrication.
However, it needed an impregnation to make it impervious for sealing applications. The graphite core also limited desirable physical characteristics such as high elastic modulus. This material did demonstrate that a hard, non-self-lubricating material could be run against itself and improve performance to a primary seal pair.
Reaction Bonded Silicon Carbide
Homogeneous materials such as reaction bonded silicon carbide provided the advantages of lower porosity, higher elastic modulus (stiffness) and improved thermal conductivity and tribological behavior. Initially, the costs associated with the processing of this family of materials somewhat limited their use to high duty applications with less aggressive chemicals. These materials required extensive grinding and the free silicon in the structure was susceptible to highly caustic or acidic environments.
This family of materials quickly gained acceptance in a variety of applications as contact pressures could be increased with less face deflection than that of metallic materials. Normal protocol was to run a high duty, hard, mechanical carbon-graphite against the reaction bonded silicon carbide.
Sintered Silicon Carbide
The need for greater abrasion and chemical resistance drove the development of sintered silicon carbide. Because of the chemical resistance in virtually all corrosive environments sintered (aka, self-sintered, direct sintered or pressure less sintered) silicon carbide began to enter the market. This material could be formed into specific shapes and then sintered to, or nearly to, the finished size, limiting the amount of expensive grinding for the finished item.
This reduction in production cost, coupled with the homogeneous nature of the material, enabled its use in highly corrosive environments and higher volume applications. It began to displace other materials such as alumina where higher pressure-velocity (P-V) capabilities were required. (The P-V parameter is an index of the severity of the application. It is a relative measure of the material pair and is affected by other variables such as the fluid being sealed.)
Unfortunately, this virtually impervious material does not have all the tribological characteristics of a reaction bonded silicon carbide, nor can it run at the higher duties of that material.
Figure 1. Graphite-loaded sintered SiC structure
Graphite-Loaded Silicon Carbide
The marriage of silicon carbide and graphite was a normal progression. Graphite initially was added to reaction-bonded silicon carbide and quickly exhibited an increase in the performance capabilities of hard face combinations. While strength is reduced somewhat and the ability to handle abrasives is not as good as that of pure silicon carbide, tribological behavior is improved significantly. Using this type of material in at least one of the two hard faces extended the seal life cycle in some cases from minutes (i.e., unacceptable) to hundreds of hours.
Graphite-loaded sintered silicon carbide was more recently applied in hard face combinations. This unique family of composite materials has the benefits of sintered silicon carbide while incorporating a second, abradable phase that provides pockets or reservoirs for the "storage" of fluid.
These composite silicon carbide hard face combinations enable higher contact pressures and speeds by improving interfacial lubrication in the same way a matte lap finish does. As shown in Figure 2, both graphite-loaded silicon carbide composites running against themselves have the highest relative P-V values of any of the hard-face pairs. When they are combined with a carbon-graphite mating material, P-V is further enhanced.
The reservoir's configuration is critical so as to not create other issues during operation. Specifically, edge breakdown around the reservoir can create debris that then damages the interface. A graphite-loaded sintered silicon carbide composite with properly configured reservoirs can provide for a smooth transition between the reservoir and the silicon carbide matrix.
Because of the uniformity of these composites, their characteristic performance will not change during operation, which can happen with matte lapping or other shallow surface treatments. Once the benefits of the matte lap have been removed, tribological behavior can change, especially during start-stop situations. Other surface treatments, should they separate from the substrate, can cause rapid and extensive damage to the interface.
Figure 2. Relative pressure-velocity capability
Lubricant: The Essential Ingredient
Silicon carbide primary seal pairs do not perform well in the absence of a lubricant. Fortunately, only a small amount of lubricant is needed to significantly improve tribological characteristics. The abradable graphite phase creates reservoirs that provide dry run survivability, increasing the amount of time a mating pair can operate in the absence of the fluid. In addition, the composite creates a material that is even more resistant to thermal shock than the silicon carbide itself, providing enhanced resistance to those upset conditions that are difficult to predict and control.
Conclusion
These graphite-loaded silicon carbide materials have been around for several years, and new applications are regularly identified. Initially, they were developed to address the highest P-V applications, with a focus on the highest pressures. Now they are being applied to lower pressure and speed applications because they can offer total cost savings to the user through reduction in warranty claims and improved tribological consistency.
It is always best to consult the mechanical seal manufacturer for proper selection and application, since a wide range of primary ring materials are available. The manufacturer can provide further information on their strengths and weaknesses.
Further information on silicon carbide and other hard face seal materials can be found in the December 2005 and January 2006 Sealing Sense columns.
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