- ▸"Higher hardness means better wear resistance" does not hold for carbon graphite sealing materials: the relationship is non-linear with a critical point
- ▸Critical data: increasing hardness from 60 to 80 HS reduces wear rate by about 40%; from 90 to 110 HS it instead increases wear by 15%-30% due to brittleness and micro chipping
- ▸Two carbon graphite materials at the same 80 HS Shore hardness can differ by several times in wear rate depending on formulation and impregnation, exposing the limits of hardness testing
- ▸Impregnation mechanisms differ: antimony raises hardness (60 to 90-110 HS) and improves wear resistance; PTFE leaves hardness unchanged but reduces wear via low friction
- ▸Silicon carbide (SiC) paired with carbon graphite is the gold-standard friction pair with PV value of 15-25 MPa·m/s; media containing over 0.1% solids raises wear rate 3-5 times
In seal selection, "higher hardness means better wear resistance" is a common intuitive judgment, but for carbon graphite materials this conclusion is not entirely valid. The wear resistance of carbon graphite depends not only on hardness but also on formulation, impregnation type, mating material, and operating conditions. Based on years of test data, Huahao Sealing Co., Ltd. systematically analyzes the true relationship between hardness and wear resistance to correct common misconceptions.
1. Hardness Test Methods and Their Meaning
1.1 Shore Hardness (HS)
The most commonly used hardness index for carbon graphite materials is Shore hardness HS, available in HS (G) and HS (F) scales. Mechanical seal carbon graphite typically ranges from 40 to 100 HS. Shore hardness is a rebound-type hardness reflecting elastic deformation capacity and is well suited to porous heterogeneous materials like carbon graphite.
1.2 Rockwell and Brinell Hardness
Rockwell (HRR, HRM) and Brinell (HB) hardness are also used for metal-impregnated graphite, but due to the heterogeneity of carbon graphite, the data scatter is significant and multiple readings must be averaged. In our outgoing inspection, we take at least five test points per batch.
1.3 Limitations of Hardness Testing
It is worth noting that hardness reflects a material's resistance to local plastic deformation, not its overall wear resistance. Two carbon graphite materials with the same 80 HS Shore hardness may differ by several times in wear rate depending on formulation and impregnation.
2. Non-Linear Relationship Between Hardness and Wear Resistance
2.1 General Trend and Critical Point
Within a reasonable range, increasing hardness of carbon graphite generally corresponds to improved wear resistance, but this relationship has a critical point. When hardness exceeds about 90-100 HS, brittleness increases significantly, and the seal face is prone to micro chipping and spalling during operation, actually accelerating wear. Huahao's experimental data shows that when hardness rises from 60 HS to 80 HS, wear rate drops by about 40%; when rising from 90 HS to 110 HS, wear rate instead increases by 15%-30%.
2.2 Effect of Impregnation Type
Different impregnation processes affect hardness and wear resistance through different mechanisms. Antimony metal impregnation raises hardness from 60 HS to 90-110 HS; with the supporting role of the metallic phase, wear resistance also improves significantly. Phenolic resin impregnation gives a limited hardness increase of about 5-15 HS, but by reducing porosity and improving interface contact, wear resistance still improves. PTFE impregnation causes little hardness change, but its low-friction properties reduce wear rate.
2.3 Formulation and Grain Size
The grain size of the aggregate in the formulation significantly affects hardness and wear resistance. Fine-grain formulations (<75μm) have higher hardness and better surface finish, suited to precision seals. Coarse-grain formulations (>150μm) have slightly lower hardness but better thermal conductivity, suited to high-temperature heavy-load conditions. Huahao offers multiple formulation systems for different applications.
3. Mating Material Matching Is Critical
The wear resistance of carbon graphite sealing depends not only on the graphite itself but on the entire friction pair. Common mating materials include silicon carbide (SiC), tungsten carbide (WC), alumina ceramic (Al₂O₃) and cast iron.
3.1 Silicon Carbide Pairing
SiC paired with carbon graphite is the gold-standard combination in mechanical sealing today, with large hardness differential, good chemical compatibility, PV value up to 15-25 MPa·m/s, and extremely low wear rate.
3.2 Tungsten Carbide Pairing
Tungsten carbides such as YG6 and YG8 paired with carbon graphite are suited to high-pressure and high-speed conditions, but the thermal conductivity of cemented carbide is lower than SiC, requiring attention to face temperature rise.
3.3 Cast Iron Pairing
Cast iron-graphite pairing is low cost and used in ordinary services such as low-pressure water pumps, but the mating part wears faster and requires periodic replacement.
4. Influence of Operating Conditions
Wear resistance is a function of operating conditions. Medium viscosity, solids content, temperature, pressure and speed all affect actual wear rate. For example, a medium containing more than 0.1% solid particles can raise the carbon graphite wear rate by 3-5 times. When medium temperature rises above 200°C, resin-impregnated graphite hardness drops significantly and wear resistance decreases.
5. Conclusion
There is a non-linear relationship between hardness and wear resistance of carbon graphite. Simply pursuing high hardness is not the right selection approach. Engineers should comprehensively consider impregnation type, mating material and operating parameters for systematic matching. Huahao Sealing Co., Ltd. can provide material-level friction and wear test data to help customers make the optimal selection.