Silicon carbide is the harder seal face, the more chemically resistant one in its sintered grade, and the default hard face for abrasive and corrosive duty; tungsten carbide is the tougher face, and it survives impacts and sudden temperature changes that can crack silicon carbide. The metal binder that gives tungsten carbide its toughness also limits the fluids it tolerates and makes it the weaker face when lubrication fails.

What Is a Silicon Carbide Seal Face

A silicon carbide seal face is a ring of hard, non-oxide ceramic, lapped flat and run as the hard face of a pair, against carbon or against another hard face.

It comes in two main grades. Reaction-bonded SiC keeps some free silicon in its structure; sintered SiC, also sold as self-sintered, direct-sintered or alpha SiC, has none.

What Is a Tungsten Carbide Seal Face

A tungsten carbide seal face is a cemented carbide: tungsten carbide grains held together by a metal binder.

Early seal faces used cobalt as the binder. Nickel-bound WC has replaced it in most seal service.

The Difference Between Silicon Carbide and Tungsten Carbide

Wear in Abrasive Service

Neither face reliably outlasts the other on abrasion: an industry rating of seal face materials gives silicon carbide and tungsten carbide the same top mark for abrasive service, and we know of no independent wear test that puts one ahead in the same slurry.

In abrasive slurry service, sintered SiC is the most common face because it combines abrasion resistance with broad chemical resistance, and for the harshest slurries it goes on both faces. Tungsten carbide is used in some mining service, but only where the chemicals used to process the ore will not leach its nickel or cobalt binder out of the face.

Corrosion Resistance

Sintered silicon carbide resists virtually every corrosive fluid a pump handles, while tungsten carbide’s corrosion resistance is only as good as its metal binder.

Cobalt is the binder most vulnerable in acid; nickel-bound WC covers a wider range of fluids, but its free nickel still sets the limit.

Silicon carbide’s resistance depends on the grade. The free silicon in reaction-bonded SiC is attacked by strong caustic and strong acids, so for caustic service such as sodium hydroxide, API 682 recommends sintered SiC on both faces.

Toughness

Tungsten carbide is much the tougher face: the fracture toughness of tungsten carbide is 3 to 5 times that of silicon carbide, and light impacts will generally not fracture a WC face.

Thermal Shock

Which face withstands a sudden temperature change depends on where the change happens: tungsten carbide better resists a change that runs through the whole ring, and silicon carbide better resists heat generated at the sealing face.

A cold flush hitting a hot face, or hot product reaching a cold seal at start-up, stresses the whole ring at once, and silicon carbide can fracture under that thermal stress. Where SiC is needed for other reasons but such transients are expected, a graphite-loaded SiC grade resists thermal shock better than plain SiC.

With frictional heat at the faces, silicon carbide is the better face. Repeated surface heating opens a network of fine cracks across a WC face, known as heat checking, and tungsten carbide is more prone to heat checking than silicon carbide.

Dry Running

Silicon carbide copes better with a loss of lubrication, but neither face should be expected to run dry against another hard face.

Where lubrication is uncertain or dry running is possible, avoid pairing WC against SiC, against alumina or against itself. A carbon face against SiC, or a graphite-loaded SiC grade, lets the seal survive a short loss of fluid.

Cost

Which face costs less varies with the grade and form of each: published price comparisons point both ways, and no independent source ranks them.

The form matters most when comparing tungsten carbide quotes. A WC coating on a stainless steel ring is made to cost less than a solid WC face, and a coating gives the hardness for abrasion but little of the toughness that is the main reason to choose WC.

How to Choose the Right One

Choose silicon carbide, usually the sintered grade, for abrasive slurries, corrosive or caustic fluids, and any duty where lubrication may fail. Choose tungsten carbide for heavy oil service, or where the face must survive impact or a sudden temperature change through the whole ring, and the fluid is compatible with its nickel binder.

First decide whether the duty needs a hard face on both sides. Abrasives or high solids call for two hard faces; in a clean, lubricating fluid, a carbon face against SiC or WC is the usual pair and runs cooler.

High pressure on its own does not decide between them.

Before switching material after a failure, read the failed face:

  • Chipped edges or a cracked SiC ring: impact or a sudden temperature change through the ring; tungsten carbide is the tougher choice.
  • Fine surface cracks on a WC face: heat checking from frictional heat; move to SiC or improve the lubrication.
  • A pitted or etched WC face: the fluid is attacking the binder; move to sintered SiC.
  • Deep wear grooves on either face: abrasive solids reaching the faces; both materials rate the same on abrasion, so the fix is keeping solids away from the faces.