Why Is Graphite Soft And Slippery?

Graphite is an important carbon material, which is widely used in life and industry. Its soft and slippery characteristics not only lay the foundation for basic applications, but also help materials science to explore the relationship between micro and macro properties. This provides theoretical support for cutting-edge innovative applications.

Analysis of the crystal structure of graphite

The arrangement of carbon atoms of graphite

Graphite has a layered crystal الهيكل, with each layer of carbon atoms arranged in a hexagonal compact two-dimensional network. Each carbon atom in the plane is connected to the surrounding three carbon atoms by a covalent bond with a bond length of about 0.142nm and a bond Angle of 120°. This covalent bond is highly directional and stable, forming a solid plane skeleton. This gives excellent mechanical and chemical stability in the graphite layer, making it difficult to relative shift the carbon atoms in the layer. And then maintaining the integrity of the plane structure.

Interlayer force

The carbon atoms between the graphite layer and the layer are maintained by a weak van der Waals force, and no covalent bond is formed. Van der Waals forces are intermolecular forces resulting from various dipole interactions of molecules. The van der Waals force between the graphite intermediates is 4-7 kJ/mol, which is much lower than the covalent bond of about 346kJ/mol. This energy difference makes the graphite layers bind loosely, and each layer has the possibility of relative movement. This is the structural basis of the soft slip property of it.

Interpretation of soft properties of graphite from a microscopic perspective

Interlayer sliding under external force

When the graphite is subjected to external forces, the van der Waals forces between the layers are difficult to counter the influence of these external forces. Due to the weak van der Waals force, under the action of small external forces, the relative sliding between the layers can easily occur. The carbon atom can slip smoothly to each other under the push of friction. This kind of interlayer sliding at the micro level is directly reflected in the macro level as the soft texture. And it is easy to change shape under the action of external forces.

Crystal structure factors related to hardness

The hardness of a material essentially depends on the resistance of its crystal structure to external forces. For graphite, although the covalent bonds in the layers give strong mechanical stability in the plane. The weak van der Waals forces between the layers become the key weak link that determines its overall hardness. In the face of external forces, it cannot effectively disperse and resist external forces through a full range of strong interactions between atoms. Like a crystal with a uniform distribution of covalent bonds in three dimensions (such as diamond). In contrast, it is more likely to slip between layers, resulting in lower overall hardness. According to the Mohs hardness standard, its hardness is only 1-2, which is much lower than most common mineral materials. This fully reflects the decisive influence of its crystal structure on hardness.

Discussion on the formation mechanism of smoothness

Relationship between layer slip and friction

The smoothness of graphite is directly due to its easy sliding between layers. When two surfaces are in contact with each other and in relative motion, if graphite is present, the interlayer sliding of graphite can significantly reduce the direct friction between the surfaces. If there is no graphite, the micro-convex and concave on the surface of the object will mesh with each other, resulting in greater friction.When using it as a lubricant, the relative motion of the surface of the object is transformed into sliding between graphite layers. Due to the weak van der Waals forces between layers, the resistance needed to overcome this sliding is very small. Thus greatly reducing the coefficient of friction. After applying graphite to certain metal surfaces, the coefficient of friction can be reduced to 1/3-1/2 of the original. This fully proves its excellent effect in reducing friction.

The embodiment of slippery in life

In daily life, we can intuitively feel the smooth characteristics of graphite through a variety of phenomena. When the hand touches the graphite powder, it will be obvious that the fingers are difficult to grasp. And there is a strong sense of slippiness. This is because the layer structure in the graphite powder slides rapidly between the layers under the touch. And giving the finger a unique tactile feedback.

In addition, the use of pencil writing is another typical embodiment of the smooth properties. During the writing process, the pencil lead is in contact with the surface of the paper. And under the action of pressure, the graphite layers slide between them. So that some of its layers can be transferred and attached to the paper, leaving clear writing. In this process, the smooth characteristics not only ensure the fluency of writing. But also make the writing experience more smooth and comfortable.

Application of graphite soft characteristics

Application in the field of lubricants

Graphite has excellent lubricating properties and you can widely use it in the field of lubricants. Under extreme conditions such as high temperature, high pressure and high vacuum, ordinary liquid lubricants are easy to fail due to evaporation, decomposition or viscosity change. Graphite can form a lubricating film in aviation engines, high temperature furnaces and vacuum equipment. Because it has stable chemical properties and interlayer sliding characteristics reduce friction and wear. And it can improve equipment operating efficiency and life.

The manufacturing principle of pencil lead

Pencil lead is a typical application of its properties. It is made from a mixture of graphite and clay in proportion. When writing, the graphite layer slides on the surface of the paper under the action of pressure. Due to the weak bonding force between the layers, part of the graphite layer is attached to the paper to form writing. Adjusting the ratio of graphite and clay can control the hardness and blackness of the pencil lead. Then making the pencil a common writing tool.

الخاتمة

Graphite is soft and slippery because of its unique crystal structure. This connection between microstructure and macro performance lays the foundation for its application. And this will also promote its innovation in emerging technologies in the future.

FAQ

Why is diamond hard?

Diamond and graphite are allotropes of carbon, but have different crystal structures. Each carbon atom in a diamond is covalently bonded to the four surrounding carbon atoms. Forming a three-dimensional tetrahedral network structure. When stressed, the covalent bond can disperse external forces and resist deformation. So the diamond hardness is extremely high, and the Mohs hardness is 10.

Why does graphite conduct electricity?

In addition to forming σ bonds, the carbon atoms in the graphite layer also have conjugated large π bonds. Each carbon atom contributes a p electron, forming a delocalized electron cloud that can move freely within the layer. When an electric field is applied, the free electrons move in a directional direction to form a current. And the good conductivity makes it widely used in electronics.

Why is graphite used as a lubricant?

First, the inter-layer van der Waals force is weak, and the inter-layer is easy to slide. Second, the chemical properties are stable. When applied, it forms a lubricating film on the surface of the component, replaces direct friction with interlayer sliding. It reduces the coefficient of friction, can maintain performance in different chemical environments, and has a wide range of applications.