Low-friction, oxidation-resistant, graphite and ceramic
CEVP's engineers have developed a range of low-friction, oxidation-resistant graphite and ceramic materials for use as bearings and seals in high temperature valve applications. This work has been predominantly for aerospace bleed and butterfly valves, operating at temperatures up to 700 C (in air).
The two main grades we use for the above applications are boronised graphite (grade BG10) and fine-grained hot isostatically pressed boron nitride (grade BN20). These offer the following advantages:
- Very low oxidation rates up to 600 C/900 C in air (BG10/BN20 respectively)
- Self lubricating
- Low friction (hot or cold)
- Good wear resistance
- Excellent thermal shock resistance
- High compressive strength
- Light weight
- Stable in damp or humid conditions
- Good electrical conductor/insulator (BG10/BN20 respectively)
- Good chemical stability
BG10 boronised graphite applications
The properties above make this material ideally suited to triple seals
in hot air bleed valves, in aerospace applications. Triple seals made
from BG10 very quickly polish the bore of the valve they are sealing,
giving very low leakage rates. Once this initial polishing has taken
place, very low wear rates to the seal and the valve bore are experienced
during service. Application expoerience shows that leakage rates remain
constant throughout the valve's service life, as the seal is hardly
affected by oxidation or wear, and does not deteriorate.
This material also has a major advantage over conventional high temperature phosphate impregnated graphites, in that it is very stable in damp and humid conditions. Phosphate impregnated graphite's have been known to leach out the oxygen inhibiting phosphate (which can cause the valve to seize), if the valve becomes wet due to condensation or humidity.
BN20 boron nitride applications
This material has an extremely low friction coefficient and superb
oxidation resistance, up to 900 C in air. It is ideally suited to
bearing applications in hot air valves. It has a major advantage over
metal bearings in that it exhibits low friction hot or cold -enabling
smaller actuators to be used. High-temperature, self-lubricating metal
bearings exhibit low friction when hot, but their friction increases
dramatically when cold. This can give poor valve performance during
start up, or necessitate a larger actuator to be used.
Specification of BG10 grade boronised graphite
Density |
g/cm3 |
1.85 |
Hardness |
Shore D |
60 |
Flexural Strength |
MPa |
70 |
Compressive Strength |
MPa |
120 |
Modulus of Elasticity |
GPa |
20 |
CTE |
10-6/°C |
4.6 |
Max. operating temp. |
In air |
600°C |
Specification of BN20 grade hot isostatically pressed boron
nitride
Density |
g/cm3 |
2.2 |
Flexural strength |
MPa |
32 |
Compressive strength |
MPa |
113 |
Young's Modulus |
GPa |
58 |
CTE |
10-6/°C |
8.0 |
Thermal conductivity @ 20°C |
W/mK |
49 |
Thermal conductivity @ 800°C |
W/mK |
28 |
Max. operating temp. |
In Air |
900°C |
If you would like more information on the above products please contact us by , telephone or fax, or you can use the reply form to ask for further details
