Effect of mo content on the microstructure and properties of powder metallurgy iron based friction materials

This article uses powder pressing and sintering methods to prepare iron-based friction materials. The influence of molybdenum on the microstructure and comprehensive properties of powder metallurgy friction materials was studied from the aspects of density, hardness, and wear resistance. The results show that with the increase of Mo mass fraction, the sintering density of iron-based friction materials gradually increases, and the hardness of sintered materials first shows an increasing trend. As the Mo content increases by more than 12%, the hardness of sintered friction materials decreases. With the addition of Mo, the wear amount of the friction material first decreases and then increases. When the mass fraction of Mo added to the sintered friction material is 12%, the wear rate of the material is lower than that of other samples, and it has good wear resistance. Adding Mo element to iron copper based friction materials improves the wear resistance of sintered friction materials due to the strengthening effect of Mo.


Introduction
With the development of the modern transportation industry, the comprehensive performance requirements for friction materials such as strength, hardness, and wear resistance are becoming increasingly high [1,2].We need to develop a new type of friction material with high thermal conductivity and strength.On the basis of the two main friction materials, iron-based and copper-based, a new type of friction material based on iron and copper has been developed by fully utilizing their performance advantages.At present, it has good economic and market prospects [3,4].Due to the small and uniform pore structure of powder metallurgy friction materials, they have high friction coefficients and wear properties.Iron copper based friction materials can be applied in many industrial fields in the future, such as brakes, clutches, motorcycle brake pads, rail transit train braking systems, industrial machinery braking devices, etc.In addition, iron copper based friction materials can also be applied in fields such as friction seals, friction and wear tests, anti-slip brakes, and wind turbines [5,6].
Metal elements such as Cr, Al, and Mo are often added to friction materials based on iron to regulate their properties.The advantage of this friction material lies not only in its good plasticity and heat resistance as metallic materials but also in the lower cost of iron itself.Iron materials have high melting points, high hardness, and good comprehensive mechanical properties [7].Suitable metal elements can be added according to the required performance to form alloys, and the comprehensive mechanical properties of materials can be adjusted according to the application environment.Pure metal substrates have good strength, plasticity, and toughness, but poor wear resistance [8,9].Therefore, it is necessary to add wear-resistant phases to the matrix material to improve its wear resistance.Improving the performance of friction materials through component design has become the main research direction of

Results and discussion
Figure 1 shows the density variation curves of sintered sample materials with different Mo contents added.From the experimental formula composition table in Table 1, it can be seen that as the Mo mass fraction increases, the Fe mass fraction decreases.As shown in Figure 1, compared with the density of Fe (7.87 g/cm 3 ), the density of Mo (10.2 g/cm 3 ) is relatively higher.When a certain amount of Mo element is added to the sample, the density of the sintered friction sample increases due to the replacement of relatively low-density Fe by high-density Mo.However, the mass fraction of Mo is not very high, and the resulting density will not increase too much.On the other hand, the presence of molybdenum particles can affect the degree of material densification, thereby affecting its mechanical and physical properties.The presence of metallic molybdenum particles promotes the diffusion of copper and iron, accelerating the diffusion rate of copper and iron elements, which to some extent affects the degree of material densification.The presence of metallic molybdenum particles also reduces the number of pores formed after sintering, improves the density of the sintered material, and further increases the sintering density of the material [10,11].As the molybdenum content increases, metallic molybdenum will dissolve in the matrix material to form a solid solution.Within an appropriate range, the dissolution of molybdenum metal can improve the strength and hardness of the material.However, when the molybdenum content exceeds the solid solution limit of the matrix material, excess molybdenum will form precipitates or new phases, leading to a decrease in the hardness of the material.When the molybdenum content increases, lattice distortion may increase, thereby affecting the hardness of the material.After reaching the critical point, lattice distortion may lead to an increase in material plasticity and a decrease in hardness.When the molybdenum content in the metal is too high, it may lead to grain growth or the formation of uneven grain structures, thereby reducing the hardness of the material.Figure 3 shows the variation curve of the wear amount of sintered materials with different Mo contents added.As the Mo content in the friction material increases, the wear of sintered friction materials first decreases and then increases.When the Mo content is 12%, the wear of the friction material reaches its minimum value compared to other formulas.Molybdenum, as a commonly used alloying element in alloy materials, can effectively improve the comprehensive mechanical properties, especially wear resistance, of metal-based friction materials.When the content of molybdenum metal is within the range of 4%-12%, the wear of sintered friction materials gradually decreases.Due to the solid solution strengthening effect, molybdenum metal improves the wear performance of sintered materials to a certain extent.When the Mo content is 12%, the wear of sintered friction materials is the smallest, indicating that adding a certain amount of metal molybdenum to the friction material can significantly improve the wear resistance of friction materials based on iron and copper.Molybdenum metal has good wear resistance and high melting point, which can effectively improve the hardness of sintered friction materials and also improve the strength of friction materials, thus improving the wear resistance of sintered friction materials.Molybdenum metal is an excellent lubricant that can form a lubricating film between the surfaces of two friction materials, reducing direct contact between metals and thus reducing wear.Molybdenum metal has good chemical stability and can effectively inhibit oxidation and corrosion of friction surfaces, thereby reducing wear.Excessive molybdenum metal can form very hard intermetallic compounds, such as Mo2C or MoC.These intermetallic compounds may generate high friction coefficients and wear rates, leading to a decrease in the wear resistance of sintered wear materials.Figure 4 shows the surface morphology of sintered materials with different Mo contents after wear testing.From the surface morphology of the worn material in Figure 4, it can be seen that there are grooves and detachment zones on the surface of sintered friction materials.The frictional heat generated by the repeated friction process of Fe Cu matrix material causes the surface graphite and molybdenum to form a dense film-like substance.When graphite and molybdenum come into contact and wear, molybdenum particles will deposit on the worn surface, forming a dense thin film-like substance.This film-like substance can provide additional lubrication and protection, reduce wear and friction, and improve the bonding strength and hardness of the matrix, thereby reducing wear.When the molybdenum content is 4%-8%, more and more deeper grooves and larger bonding pits can be observed in the wear morphology of the friction surface (Figures 4(a) and 4(b)).When the molybdenum content is 4%-8%, there are a certain number of pores in the sintered friction material, and the surface of the matrix is not dense, resulting in incomplete matrix structure and microstructure.As a result, the surface hardness of sintered materials is low.When the material undergoes repeated friction tests, obvious plow grooves and bonding pits will appear on the surface of the sintered material.Materials with a molybdenum content of 12% have the highest hardness and the matrix is less prone to plastic deformation.The plowing effect between the material and its grinding material is reduced, thereby reducing wear marks and plowing grooves during the friction process [12].Metal molybdenum particles can provide additional lubrication and reduce friction and wear between graphite and worn surfaces.When the molybdenum content is higher than 12%, there are many plows and large bonding pits on the surface of the material after friction (Figures 4(d) and 4(e)), and exceeding a certain molybdenum content may form a large number of molybdenum particles or molybdenum compound particles.These particles are prone to fracture or detachment during friction, thereby exacerbating the deterioration of the material's wear performance.

Conclusions
As the Mo content increases, the density of sintered friction materials gradually increases, and the hardness tends to increase.However, when the Mo content exceeds a certain range, the hardness of the sample decreases.When the Mo content is 12%, the hardness of the sintered friction material reaches its maximum value.As the Mo content increases, the wear of sintered materials first decreases.When the Mo content is 12%, the wear of sintered samples is the smallest and the wear performance is the best.When the Mo content exceeds 12%, the wear of sintered friction materials increases.When the Mo content is between 4% and 12%, the fracture morphology and microstructure gradually become uniform and dense, and the grains become finer; When it exceeds 12%, the tissue begins to become rough and uneven, with an increase in pores.

Figure 1 .
Figure 1.Density variation curve of sintered friction materials with different Mo contents.Figure2shows the hardness changes of sintered samples with different Mo contents added.As shown in Figure2, as the Mo content added to the friction material increases, the hardness of the sintered friction material sample first increases and then decreases.When the Mo addition amount is 12%, the hardness of the iron copper based sintered sample reaches its maximum value.The addition of Mo can significantly improve the hardness of iron-based and copper-based powder metallurgy friction materials.As the molybdenum content increases, metallic molybdenum will dissolve in the matrix material to form a solid solution.Within an appropriate range, the dissolution of molybdenum metal can improve the strength and hardness of the material.However, when the molybdenum content exceeds the solid solution limit of the matrix material, excess molybdenum will form precipitates or new phases, leading to a decrease in the hardness of the material.When the molybdenum content increases, lattice distortion may increase, thereby affecting the hardness of the material.After reaching the critical point, lattice distortion may lead to an increase in material plasticity and a decrease in hardness.When the molybdenum content in the metal is too high, it may lead to grain growth or the formation of uneven grain structures, thereby reducing the hardness of the material.

Figure 2 .
Figure 2. Density of sintered friction materials with various Mo contents.Figure3shows the variation curve of the wear amount of sintered materials with different Mo contents added.As the Mo content in the friction material increases, the wear of sintered friction materials first decreases and then increases.When the Mo content is 12%, the wear of the friction material reaches its minimum value compared to other formulas.Molybdenum, as a commonly used alloying element in alloy materials, can effectively improve the comprehensive mechanical properties, especially wear resistance, of metal-based friction materials.When the content of molybdenum metal is within the range of 4%-12%, the wear of sintered friction materials gradually decreases.Due to the solid solution strengthening effect, molybdenum metal improves the wear performance of sintered materials to a certain extent.When the Mo content is 12%, the wear of sintered friction materials is the smallest,

Figure 3 .
Figure 3.The wear amount of sintered friction materials with various Mo contents.Figure4shows the surface morphology of sintered materials with different Mo contents after wear testing.From the surface morphology of the worn material in Figure4, it can be seen that there are grooves and detachment zones on the surface of sintered friction materials.The frictional heat generated by the repeated friction process of Fe Cu matrix material causes the surface graphite and molybdenum to form a dense film-like substance.When graphite and molybdenum come into contact and wear, molybdenum particles will deposit on the worn surface, forming a dense thin film-like substance.This film-like substance can provide additional lubrication and protection, reduce wear and friction, and improve the bonding strength and hardness of the matrix, thereby reducing wear.When the molybdenum content is 4%-8%, more and more deeper grooves and larger bonding pits can be observed in the wear morphology of the friction surface (Figures4(a) and 4(b)).When the molybdenum content is 4%-8%, there are a certain number of pores in the sintered friction material, and the surface of the matrix is not dense, resulting in incomplete matrix structure and microstructure.As a result, the surface hardness of sintered materials is low.When the material undergoes repeated friction tests, obvious plow grooves and bonding pits will appear on the surface of the sintered material.Materials with a molybdenum content of 12% have the highest hardness and the matrix is less prone to plastic deformation.The plowing effect between the material and its grinding material is reduced, thereby reducing wear marks and plowing grooves during the friction process[12].Metal molybdenum particles can provide additional lubrication and reduce friction and wear between graphite and worn surfaces.When the molybdenum content is higher than 12%, there are many plows and large bonding pits on the surface of the material after friction (Figures4(d) and 4(e)), and exceeding a certain molybdenum content may form a large number of molybdenum particles or molybdenum compound particles.These particles are prone to fracture or detachment during friction, thereby exacerbating the deterioration of the material's wear performance.

Table 1 .
Chemical Composition Formulas of Iron-based Friction Materials with Different Mo Content (wt.%).