Selecting a material for an industrial component exposed to friction and wear may appear to be a decision based primarily on mechanical strength or hardness.
However, components that operate in contact—such as bearings, seals, wear rings, guides, pistons, and valves—do not fail only because the material is weak.
They may also fail because the material is incompatible with the counterface, generates excessive heat, loses stability at operating temperature, or depends on a lubricant that cannot be maintained under actual conditions.
Industrial material selection requires evaluating the complete system in which the component will operate.
Load, speed, temperature, motion, lubrication, chemical environment, geometry, and maintenance interact with one another. Changing any of these variables can affect friction, wear, and equipment service life.
NASA explains that tribological behavior depends on the interaction among surfaces, lubricants, and operating conditions. Even identical materials can produce different results when load, temperature, or environmental conditions change.
For this reason, the correct material is not necessarily the hardest, strongest, or most expensive. It is the material that provides the appropriate combination of properties for the application’s actual operating conditions.
MATERIAL SELECTION BEGINS WITH THE APPLICATION
Before comparing materials, an engineer must understand what the component is expected to do.
It is not enough to define a part as a bearing or seal. Engineers must also identify:
- The function it must perform
- The type of motion involved
- The load it must support
- The speed at which it will operate
- The temperature it will reach
- The surface against which it will run
- The fluids or substances that will be present
- The type of lubrication available
- The maintenance it can receive
- The consequences of failure
These factors determine which material properties are truly important.
For example, a material with high wear resistance may appear suitable for a bearing. However, it could damage the shaft if the counterface is softer, generate excessive heat if its coefficient of friction is high, or lose stability as temperature increases.
Materials engineering for contact components is not about selecting one isolated property. It requires balancing mechanical, thermal, chemical, and tribological characteristics.
LOAD DEFINES THE STRESSES THE MATERIAL MUST WITHSTAND
Load is one of the first criteria that must be evaluated.
A component may be exposed to:
- Compression
- Tension
- Bending
- Impact
- Radial loads
- Axial loads
- Cyclic loads
- Combined stresses
For components exposed to friction, the total magnitude of the load is not the only important factor. Engineers must also determine how that load is distributed across the surface.
A concentrated load can create high contact pressures, localized deformation, and lubricating film breakdown. Cyclic loading can cause fatigue even when each individual cycle remains below the material’s static strength limit.
When the load exceeds the material’s capabilities or is distributed unevenly, the following problems may occur:
- Permanent deformation
- Fractures
- Localized wear
- Loss of tolerances
- Overheating
- Surface fatigue
- Material transfer
Hardness can help resist certain abrasive wear mechanisms, but it does not guarantee satisfactory performance by itself. NASA wear research shows that resistance also depends on microstructure, adhesion between surfaces, and the active wear mechanism.
The material must therefore be evaluated according to actual contact pressure, not only the equipment’s nominal load.
SPEED CHANGES FRICTION AND HEAT GENERATION
Sliding or rotational speed directly influences material performance.
As speed increases, the following may also increase:
- Heat generation
- Surface temperature
- Lubricant demand
- Risk of deformation
- Wear rate
- System instability
However, the relationship between speed and wear is not always linear.
In lubricated systems, increasing speed may promote the formation of a hydrodynamic film that separates the surfaces. In other cases, speed may raise the temperature enough to reduce lubricant viscosity and weaken the film.
SKF notes that speed, load, lubrication, and temperature are interdependent variables. The permissible speed of a rotating component is often limited by operating temperature and the mechanical capabilities of the system.
To evaluate speed correctly, engineers should determine:
- Whether motion is continuous or intermittent
- Whether the component rotates, oscillates, or slides
- Maximum and average speed
- Frequency of starts and stops
- Cycle duration
- The system’s ability to dissipate heat
Starts and stops can be particularly demanding because the system may pass through periods of boundary lubrication, during which greater contact occurs between the surfaces.
TEMPERATURE CHANGES MATERIAL PROPERTIES
Operating temperature should not be confused with ambient temperature.
A component may reach much higher temperatures because of:
- Friction
- Heat generated by the process
- Proximity to ovens or turbines
- Fluid compression
- Inadequate cooling
- High speeds
- Loss of lubrication
Temperature can alter fundamental material properties, including:
- Strength
- Hardness
- Stiffness
- Dimensional expansion
- Thermal conductivity
- Chemical stability
- Oxidation resistance
- Lubricant behavior
A polymer that provides low friction at room temperature may deform or lose strength as temperature increases. A metal may expand and reduce operating clearances. A lubricant may lose viscosity, oxidize, or evaporate.
SKF indicates that operating temperature depends on load, speed, component size, lubrication conditions, and the application’s cooling capacity. Therefore, the part and its adjacent components must be thermally evaluated as a system.
Material selection must account for both continuous operating temperature and any thermal peaks or excursions that may occur.
THE ENVIRONMENT CAN COMPLETELY CHANGE PERFORMANCE
A material that performs well under clean, dry conditions may fail rapidly when exposed to moisture, dust, chemicals, or process fluids.
Environmental conditions that should be evaluated include:
- Humidity
- Dust
- Abrasive particles
- Corrosive substances
- Steam
- Vacuum
- Oxygen
- Low-viscosity fluids
- Food or pharmaceutical products
- Refrigerants
- Fuels
- Sanitary environments
The operating environment can cause corrosion, contamination, swelling, chemical degradation, or loss of lubricity.
NASA has documented that very small amounts of gases, surface films, or contaminants can significantly change adhesion, friction, and wear between materials.
This means that tribological properties do not belong solely to the material. They also depend on the environment in which it operates.
In environments containing particles, for example, a lubricant may trap contaminants and become an abrasive medium.
In chemical environments, a material may retain its mechanical strength while still experiencing surface degradation.
In sanitary applications, the material must resist fluid absorption and allow effective cleaning.
THE COUNTERFACE IS ALSO PART OF MATERIAL SELECTION
No material exposed to friction operates in isolation.
There is always a counterface: a shaft, race, seal face, guide, or another surface.
The interaction between both materials determines:
- Friction
- Material transfer
- Wear of each surface
- Temperature distribution
- Contact stability
- The risk of galling or seizure
A material may show a low wear rate while still causing significant damage to its counterface.
The objective should therefore not be limited to protecting the replaceable component. It should optimize the behavior of the entire material pair.
When evaluating the counterface, engineers should consider:
- Hardness
- Roughness
- Surface finish
- Thermal conductivity
- Chemical resistance
- Dimensional stability
- Tribological compatibility
- Replacement cost and difficulty
In many designs, it is preferable for the wear component to be easier and less expensive to replace than the shaft or primary structure.
THE SURFACE MATTERS AS MUCH AS THE MATERIAL
Surfaces that appear smooth to the naked eye contain microscopic irregularities known as asperities.
Actual contact occurs at the highest points of these asperities, rather than across the entire apparent area.
When two surfaces slide, their asperities may:
- Deform
- Adhere
- Fracture
- Cut into the counterface
- Generate particles
- Disrupt the lubricating film
For this reason, surface finish can significantly affect friction and wear.
A surface that is too rough may accelerate initial wear and create localized heat. A surface that is excessively smooth may make it difficult to retain lubricant or prevent certain materials from running in properly.
The direction of machining marks can also affect lubricant distribution and particle movement.
Industrial material selection should include specifications for:
- Roughness
- Flatness
- Roundness
- Surface hardness
- Treatments
- Coatings
- Finish direction
- Tolerances
Selecting the correct material with an unsuitable surface can produce results similar to selecting the wrong material.
THE TYPE OF LUBRICATION INFLUENCES MATERIAL SELECTION
Lubrication reduces friction, limits wear, protects against corrosion, and can help remove heat from the contact area.
Bearings and other rotating components require suitable lubrication to operate reliably. SKF indicates that lubricant selection depends primarily on temperature, speed, and load.
However, not every application can depend on oils or greases.
Conventional lubrication may be limited by:
- High temperatures
- Cryogenic temperatures
- Contamination
- Lubricant washout
- Sanitary environments
- Restricted access
- Long maintenance intervals
- Dry running
- Vacuum
- Leakage risks
In these situations, engineers must determine whether the system requires:
- A liquid lubricant
- Grease
- Solid lubrication
- A coating
- A transfer film
- A self-lubricating material
- A combination of solutions
NASA notes that lubricants may include oils, greases, solid films, and easily sheared surface layers. Selection depends directly on the application and its thermal and environmental conditions.
MAINTENANCE MUST BE CONSIDERED DURING DESIGN
A material may perform correctly in controlled testing and still be impractical in an industrial plant if it requires frequent maintenance.
Before selecting a material, engineers should ask:
- How frequently will it require lubrication?
- Is the component accessible?
- Can it be inspected without stopping the process?
- Will failure be detected before it causes additional damage?
- Is trained personnel available to maintain it?
- What happens if maintenance is delayed?
- How long does replacement require?
- What is the cost of downtime?
In difficult-to-access or continuous-service applications, a material with a higher initial cost may be more economical if it reduces intervention requirements.
Total cost should include:
- Component purchase
- Installation
- Lubricants
- Labor
- Spare-parts inventory
- Planned shutdowns
- Unplanned failures
- Damage to adjacent components
- Expected service life
Selecting the least expensive material without evaluating these factors can significantly increase the system’s operating cost.
WHY THE HARDEST MATERIAL IS NOT ALWAYS THE BEST
Hardness is often associated with wear resistance, particularly in abrasive applications. However, wear does not have a single cause. A component may fail because of:
- Adhesion
- Fatigue
- Abrasion
- Corrosion
- Erosion
- Deformation
- Thermal shock
- Loss of lubrication
Each mechanism requires different material properties.
A hard material may resist abrasive particles but remain brittle under impact.
A material with high compressive strength may fail under tension.
A metal may support a high load but experience galling when it runs against another metallic surface.
A polymer may provide low friction but deform under elevated temperature or load.
For this reason, wear-resistant materials should be selected according to the expected damage mechanism, not only according to a hardness value.
COMPARE MATERIALS ACCORDING TO THE COMPLETE SYSTEM
Before making a decision, it may be useful to create a material-selection matrix that evaluates:
- Load capacity
- Permissible speed
- Continuous temperature
- Temperature peaks
- Coefficient of friction
- Wear rate
- Chemical resistance
- Thermal conductivity
- Thermal expansion
- Dimensional stability
- Counterface compatibility
- Lubrication requirements
- Machinability
- Availability
- Total cost
- Required maintenance
The objective is not to identify a material that performs perfectly in every category.
The correct choice is the material that satisfies the application’s critical variables while maintaining an adequate margin under the most severe operating conditions.
WHEN TO CONSIDER CARBON GRAPHITE
Carbon graphite is one of the materials for friction applications that can be considered when oils, greases, metals, or certain polymers present limitations.
Its properties may include:
- Self-lubrication
- A low coefficient of friction
- Wear resistance
- Dimensional stability
- Thermal conductivity
- Chemical resistance
- Dry-running capability
- Performance at extreme temperatures
- Reduced tendency to gall against metallic surfaces
Metcar develops carbon graphite grades for dry-running, submerged, high-temperature, and chemically aggressive applications. Formulation and manufacturing processes can be modified to adjust properties such as strength, lubricity, impermeability, conductivity, and oxidation resistance.
This does not mean that a single grade is suitable for every application.
Metcar notes that differences among grades can significantly affect performance and that selection must consider the component’s specific operating conditions.
For example:
- Metal impregnation can increase strength or load capacity.
- Resin impregnation can reduce porosity and improve chemical resistance.
- An oxidation inhibitor can improve high-temperature performance.
- A graphitized grade can provide greater lubricity and thermal conductivity.
Porosity can also be used to introduce impregnation materials that modify component performance.
KEY QUESTIONS BEFORE APPROVING A MATERIAL
Before approving a material selection, the engineering team should answer:
- What is the most likely wear mechanism?
- What is the actual load, and how is it distributed?
- What speed and type of motion are involved?
- What are the continuous and maximum temperatures?
- Which substances will contact the material?
- How will the system be lubricated?
- What occurs during starts and stops?
- What is the counterface?
- What surface finish is required?
- What maintenance can be performed?
- What would be the consequence of failure?
- Has the material been evaluated under representative conditions?
If any of these questions cannot be answered clearly, the selection may still be based on assumptions rather than actual operating conditions.
FROM COMPARING PROPERTIES TO DESIGNING FOR RELIABILITY
Industrial material selection should not be reduced to choosing among metal, polymer, ceramic, or carbon graphite.
The primary question is not: Which material is stronger?
The correct question is: Which material can maintain the required performance under the actual combination of load, speed, temperature, environment, and maintenance conditions?
Answering this question requires understanding the component as part of a complete system.
A suitable material can reduce friction, control wear, protect the counterface, and extend equipment service life.
A material selected only according to cost, hardness, or availability can transfer the problem to other components and cause premature failures.
When operating conditions are demanding, material selection should be supported by data, testing, and application-specific knowledge.
