- ▸The first pairing principle is soft-against-hard: one ring takes the wear (soft), the other holds geometry (hard), making failure predictable and replacement controllable
- ▸The most universal pair: impregnated graphite against reaction-bonded silicon carbide, covering most chemical pump services
- ▸High-solids and slurry duty: SiC-on-SiC is the exceptional hard-on-hard case, relying on generous flush lubrication
- ▸Avoid hard-on-hard in clean water and low-viscosity light hydrocarbons: a thin film and no graphite fallback can mean instant seizure
- ▸Tungsten carbide (cobalt/nickel bound) suits high pressure and mild solids, but cobalt binder can corrode in certain acids and ammonia media
One of the most common mistakes in seal failure analysis is choosing face materials that are each excellent but paired wrongly. Graphite, silicon carbide and tungsten carbide are all outstanding alone; whether they belong together depends on which side of the wear the duty pushes. This article explains the pairing logic, common combinations and four prohibitions.
I. Why Soft-Against-Hard Comes First
Face wear must land on the sacrificial ring. The soft ring — usually carbon graphite, dry friction coefficient 0.04–0.15, compressive strength 150–300 MPa — runs in, tolerates micro-deviations and self-lubricates when the film fails. The hard ring holds flatness at the 0.0009 mm class and resists media attack. Soft-on-soft doubles wear and destroys geometry; hard-on-hard removes the cushion, so any transient dry contact converts directly into cracking or seizure.
II. Common Combinations and Their Duties
2.1 Graphite Against Reaction-Bonded SiC
The default pair. Silicon carbide conducts heat well, resists wear and most acids and alkalis; the graphite ring takes the wear. The standard choice for chemical centrifugal and process pumps, covering most duties below about 3 MPa.
2.2 Graphite Against Tungsten Carbide
Cobalt- or nickel-bound WC is slightly softer but tougher, fitting higher pressures or mild solids. Note that cobalt binder corrodes in some acidic or ammonia-bearing media — switch to nickel-bound or silicon carbide there.
2.3 SiC on SiC
The hard-on-hard exception: in high-solids, slurry and pulp services the soft ring would be plowed away by particles, so double hard faces are used with generous flush for lubrication. This pair demands a healthy liquid film and fails quickly when misconfigured.
2.4 Graphite Against Alumina
A low-cost alternative for mildly corrosive, clean-water media; ceramic is brittle and cracks under thermal shock — not recommended for high pressure.
III. Four Prohibitions
1.No hard-on-hard in clean water or low-viscosity light hydrocarbons: poor lubricity, no graphite fallback — one dry event can seize the faces
2.No graphite on graphite: both faces wear, flatness collapses quickly, leakage runs uncontrolled
3.Don't mismatch impregnation and media: furan resin handles acids and alkalis, PTFE handles nearly everything — the wrong impregnant chemically kills the soft ring first
4.Don't judge by hardness alone: hard-face materials below roughly 20 W/(m·K) conductivity cannot spread hot spots at high speed, raising thermal-crack risk
IV. Decision Order
First fix the medium and its lubricity — it decides whether hard-on-hard is permissible; then solids content — it decides whether a soft ring survives; finally temperature and pressure — they decide impregnation type and balance ratio. Huahao Sealing manufactures carbon and impregnated graphite rings with made-to-drawing support and can advise on soft-ring material and impregnation; the CTE online selection system launched in August 2024 helps verify thermal-matching parameters.