Huangshi City Friction Material Raw Material Association | Voice of Friction Materials, Issue 3, 2016

The Role of Mineral Reinforcing Raw Materials in Friction Materials

and Its Mechanism (Part 1)

Wang Yiting, Wang Dong, Cao Min, Bai Zhimin

(1. Hubei Xinhai New Material Technology Co., Ltd.; 2. Huangshi Xinyi Mineral Co., Ltd.; 3. China University of Geosciences (Beijing))

 

Friction materials play roles in transmission, braking, deceleration, and parking in moving machinery and equipment, widely used in fields such as automobiles, trains, airplanes, mining, metallurgy, chemical industry, and electricity, with the automotive industry accounting for over 80% of consumption. Friction materials are typical composite materials, usually composed of bonding materials (rubber or resin), reinforcing materials (organic or inorganic fibers), and fillers (mainly mineral powders), where mineral raw materials can serve both reinforcing and filling functions, and they have the largest proportion, significantly affecting the performance of friction materials, making them a key focus in the field of friction engineering. There are various types of minerals used as friction materials, differing in composition and structure, with varying physical and chemical properties and functional effects. The mechanisms and forms of friction action are diverse. A deep understanding of the mineral composition and structural characteristics, and establishing the relationship between composition, structure, performance, and usage efficiency, is crucial for the design and processing of high-quality friction materials. Based on this, the author attempts to clarify the impact of typical mineral materials' composition-structure analysis on the processing technology, product performance, and usage efficiency of friction materials, providing theoretical basis and technical support for the composition design, processing, and development of new products of friction materials.

 

The role of mineral raw materials in friction materials can be divided into three categories: mineral reinforcing raw materials, mineral friction-increasing (friction-enhancing) raw materials, and mineral friction-reducing (friction-reducing) raw materials. This article mainly discusses the composition, structure, performance, role, and mechanism of commonly used mineral reinforcing raw materials, while mineral friction-increasing (friction-enhancing) raw materials and mineral friction-reducing (friction-reducing) raw materials will be discussed in another article.

 

The reinforcing materials in friction materials mainly provide high mechanical strength to friction products, enabling them to withstand the load applied during the production process and the impact, shear, and compressive stresses generated during use, thus preventing breakage and damage. The basic requirements for reinforcing materials in friction materials include: significant reinforcing effect; good heat resistance; appropriate and stable friction coefficient; moderate hardness; and good process operability. The minerals used as reinforcing materials are usually fibrous minerals, mainly including fibrous sepiolite, acicular wollastonite, fibrous talc, and asbestos.

 

1. Fibrous Sepiolite

Sepiolite is a type of magnesium-rich hydrous chain silicate mineral, belonging to the clay group along with kaolinite, montmorillonite, and palygorskite. Sepiolite has two genetic types: leached-hydrothermal and sedimentary, with the former mostly appearing in fibrous form and the latter mostly in fine-grained or flaky form. The fibrous sepiolite used as a friction reinforcing material belongs to the former.

 

The chemical composition and structure of sepiolite are complex, with the chemical formula: R2+(x+y+2z)/2(H2O)8{(Mg8-y-zRy3+z)[(Si12-xRx3+)O30](OH)4(OH2)4}. In this formula, R3+ is mainly Al3+, followed by Fe3+, and the number of R3+ atoms can reach 2; □ represents octahedral vacancies. In the octahedra, Al, Fe, Ni, Ca, Na, etc., mainly replace Mg; in the tetrahedra, Al and Fe replace Si. Its structure can be viewed as a 2:1 type structural layer, characterized by: silicon-oxygen tetrahedra forming two-dimensional continuous sheets, with each silicon-oxygen tetrahedron sharing 3 corner vertices with adjacent tetrahedra; the octahedral gaps between any two silicon-oxygen tetrahedral sheets are filled with cations, forming one-dimensional infinitely extending octahedral sheets (bands); the two silicon-oxygen tetrahedral sheets connected by each octahedral sheet (band) form a banded structure layer similar to the amphibole 'I' beam, extending parallel to the a-axis, which determines its development into rod-like and fibrous forms along the a-axis (Figure 1). The fiber diameter ranges from 0.2 to 7.0 μm.

 

Sepiolite has good thermal stability, losing structural water at 750-820℃, leading to complete structural destruction and transforming into amphibole or pyroxene. The tensile strength of sepiolite is relatively low, only 1/10 to 1/4 of that of chrysotile; therefore, sepiolite cannot independently bear the reinforcing function of friction materials like chrysotile or blue asbestos and usually needs to be used in conjunction with other fibers. However, sepiolite has a large specific surface area (external specific surface area of 214 m2/g, internal specific surface area of 256 m2/g) and strong adsorption capacity, allowing it to create a good interface effect with resins and fillers in friction materials, effectively absorbing the binder during the mixing process, demonstrating good wettability, and achieving uniform mixing with fillers. Additionally, the large specific surface area of sepiolite gives it strong water absorption and moisture retention capabilities, allowing it to absorb 200% to 250% of its own weight in water. This can effectively prevent the foaming phenomenon caused by moisture during the pressing process of bulk friction materials. Furthermore, the pore structure of sepiolite can trap small gas molecules generated from the thermal decomposition of polymer binders within the pores, rather than accumulating on the friction surface, which helps reduce thermal degradation of the friction materials.

 

 

Research has also found that within the temperature range of 1000-1100℃, the fibrous morphology of sepiolite remains intact, with a linear shrinkage rate of only 7.28%, and about 50% of the micropores (average pore diameter of 0.02 μm) are retained. This may be the fundamental reason and unique advantage for sepiolite to exhibit good thermal stability, structural stability, and sound-absorbing effects under high-temperature conditions as a friction material.

 

2. Wollastonite

Talc is a calcium silicate mineral with a chain-like structure. Chemical formula: Ca3[Si3O9]; theoretical composition (wB%): CaO 48.3, SiO2 51.7. In the crystal structure of talc, there are two types of groups: calcium oxide octahedra ([CaO6]) and silicon oxide tetrahedra ([SiO4]). The former connects in a linear chain along the b-axis, forming the pivot of the talc structure; the latter is arranged in a single chain formed by alternating double tetrahedra and single tetrahedra, connecting the calcium oxide octahedra linear chains. This unique connection between silicon oxide tetrahedra and calcium oxide octahedra determines the needle-like and columnar extension characteristics of talc crystals. Its crystal form is needle-like to long columnar, with a length-to-diameter ratio mostly between 7:1 and 8:1, and can reach up to 30:1; the larger the length-to-diameter ratio, the more significant the enhancement effect. The Mohs hardness is between 4.5 and 5.5, relative density is 2.75 to 3.10, and melting point is 1540°C. It has a low expansion coefficient (25~650°C, [010] is 6.23×10-6/°C) and linear expansion characteristics; good insulation properties, with resistance of 1.6~1.7×1014Ω•cm. It has good chemical stability, with a solubility of 0.095mg/L in neutral water at 25°C. Generally, it is resistant to acid, alkali, and chemical corrosion, but decomposes in concentrated hydrochloric acid, forming flocculent substances.

 

As a reinforcing mineral in friction materials, talc has relatively poor toughness, so its enhancement effect is not as pronounced as that of other fibers. However, when mixed with other fibers, the enhancement effect is better. This mineral is hard (high hardness), and its addition in friction materials usually does not exceed 15%, otherwise, the products will produce significant noise during use. This mineral also has a friction-increasing effect, improving the friction coefficient of friction materials at both room temperature and high temperature, and the friction coefficient of the products increases with the amount added.

 

In 2015, global production of talc was about 550,000 tons, of which 10% was used as friction materials.

 

3. Fibrous Magnesium Hydroxide

Magnesium hydroxide is a hydroxide mineral with a layered structure; fibrous magnesium hydroxide is a fibrous variant of magnesium hydroxide. Chemical formula: Mg(OH)2. Theoretical composition (wB%): MgO 69.12, H2O 30.88. The magnesium hydroxide structure is one of the important layered structures, with (OH)- approximately arranged in hexagonal close packing, and Mg2+ filling the octahedral voids between the layers, with each Mg surrounded by 6 OH, and each OH having 3 Mg on one side. The [Mg(OH)6] octahedra are connected in layers along {0001} in a shared edge manner, with very weak hydrogen bonds maintaining the layers. The structural characteristics of magnesium hydroxide give it a plate-like crystal form, but when the structure is distorted, it becomes fibrous magnesium hydroxide. Relative density is 2.38 to 2.40, Mohs hardness is 2.5. Vickers hardness is 50.4 to 260.5, and it exhibits significant anisotropy. The fibers are flexible and pliable. The tensile strength is about 900 MPa, making it a medium-strength fiber material. The elastic modulus is 13800 MPa. It is easy to grind into fine powder. The mass resistivity is 8.82×106Ω•cm2, volume resistivity is 5.9×106Ω•cm, and surface resistivity is 3.6×106 to 4.5×106Ω, showing significant anisotropy in resistivity. Its decomposition temperature is around 400°C. The thermal conductivity is 0.46 W/m•K, and for loose fibers, it is 0.131 to 0.213 W/m•K. The thermal expansion coefficients are 16.7×10-7/°C (longitudinal) and 8.8×10-7/°C (transverse), and it exhibits linear expansion. Due to its equivalent crystallization water, it has significant fire resistance, flame retardancy, and resistance to open flames and high-temperature flames. Fibrous magnesium hydroxide is the best in terms of alkali resistance among natural inorganic fibers, but it can be partially dissolved in oxalic acid, citric acid, acetic acid, and Al(OH)3 solutions, and completely dissolved in strong acids; in humid or rainy climates, it is easily eroded by CO2 and H2O in the atmosphere, so the surface of its products needs a waterproof protective layer for protection. Many physical properties of fibrous magnesium hydroxide are similar to those of talc, and it performs well as a reinforcing and flame-retardant component in friction materials, with an addition amount of up to 40%.

 

As a friction material, magnesium hydroxide not only enhances due to its fibrous form but also transforms into periclase and releases crystallization water at high temperatures (above 400°C), which may improve the product's performance. First, the dehydration phase change process of magnesium hydroxide is an endothermic process, consuming some of the heat generated during the friction process; at the same time, the phase change product periclase has a high melting point and high chemical stability.

Mg(OH)2 ⇌ MgO + H2O (Equation 1)

 

4. Asbestos

Asbestos is a general term for fibrous minerals that can be split into extremely fine and flexible fibers containing hydroxyl. As a friction reinforcement material, asbestos includes two types: blue asbestos (amphibole asbestos) and chrysotile asbestos (serpentine asbestos). Both are silicate minerals, but the former belongs to the chain structure of the amphibole group, while the latter belongs to the layered structure of the serpentine group.

 

4.1 Blue Asbestos

Blue asbestos is the commercial name for fibrous amphibole minerals, which have a complex mineral composition, and the chemical composition varies with mineral species, but their structures are similar. The silicon-oxygen tetrahedra of these minerals share apexes, forming double chains along the c-axis, which determines their preferential growth along the c-axis, resulting in fibrous crystal morphology. Between the two double chains, they are usually connected by cations with 6-fold coordination such as Fe2+, Fe3+, Mg2+, Al3+, forming 'I' bundles. The 'I' bundles are mainly connected by cations with 6-8 fold coordination such as Na+, K+, Ca+, Mg2+. When the double chains are connected by low-valence, large-radius cations like Na, K, and Ca, the fiber fineness significantly decreases. Additionally, compared to the internal 'I' bundles, the bond strength between 'I' is relatively weak, which is also the main reason for its significant fibrous nature and good cleavage. The tensile strength of blue asbestos is 98 to 1598 MPa, the tensile elastic modulus is 9709 to 32264 MPa, and the elongation at break is 1.5% to 5.2%. The acid erosion amount is 2.85% to 13.32%, and the alkali erosion amount is 1.32% to 10.06%. It has good heat resistance and a low thermal conductivity coefficient, with the temperature for removing hydroxyl typically between 600 and 700°C, and a melting point above 1200°C. The thermal conductivity coefficient is generally 0.07 to 0.09 W/m•K.

 

4.2 Chrysotile Asbestos

Tremolite is a commercial name for fibrous serpentine minerals. Its chemical formula is: Mg6[Si4O10](OH)8. The theoretical composition (wB%): MgO 43.0, SiO2 44.1, H2O 12.9. These minerals are typically composed of octahedral sheets of 'hydromagnesite' combined with hexagonal net sheets of [SiO4] tetrahedra in a 1:1 ratio to form structural unit layers. Due to the incompatibility between the octahedral and tetrahedral sheets, the structural layers curl into fibrous shapes, with the fiber axis parallel to the a or b axis. The inner diameter of the fibers generally ranges from 2 to 20 nm, while the outer diameter is about 100 to 500 nm. Tremolite has a tensile strength of 2600 to 3100 MPa and can maintain high strength at elevated temperatures. It significantly dehydrates above 500°C, with structural damage beginning at 650 to 700°C. Its thermal conductivity is approximately 0.233 W/m•K. It has strong alkali resistance but poor acid resistance, which is related to the degradation of 'hydromagnesite' in acidic media.

 

Both chrysotile and tremolite are excellent heat-resistant insulating materials with a long thermal insulation lifespan. Their mass resistivity ranges from 10^4 to 10^8 Ω•g/cm2, classifying them as semi-insulators. Additionally, they exhibit good dispersibility and adsorption capacity, making it easy to form homogeneous mixtures when combined with resins and other fillers, which is beneficial for the processing and preparation of friction products.

 

The good fiber flexibility, chemical stability, and excellent mechanical and thermal properties of asbestos are key to its role as a friction reinforcement material.
 

Asbestos friction materials are characterized by high-temperature resistance, high friction coefficients, low hardness, high strength, and low cost. Although chrysotile is currently considered carcinogenic, and developed countries in Europe and America are gradually reducing or even banning the use of asbestos, China has also completely banned the use of asbestos brake pads since 2003. However, its unique advantages as a friction reinforcement material remain unmatched by other materials.

 

5. Discussion and Outlook

Mineral materials used as reinforcing components in friction materials have unique characteristics due to their composition, structure, and technological properties, leading to different mechanisms and ways of functioning.

 

(1) The advantages of fibrous sepiolite as a reinforcing component in friction materials, aside from its fibrous form, include its good fiber flexibility, which allows it to maintain its original morphology and pore structure at high temperatures, and the stable properties of its phase change products, such as silicate minerals. However, the current development level of fibrous sepiolite resources is still low, with a limited variety of products; the mineral processing and purification technologies to effectively remove impurities like calcite and dolomite, as well as the technology for effectively separating fiber bundles, are not yet mature; surface modification technologies to improve its uniform dispersion and bonding strength in polymers like resins and rubbers still need development; and issues regarding the optimization of formulations, reliability, durability, and environmental properties of products as reinforcing components in friction materials require extensive engineering application practice for validation.

 

(2) As a reinforcing mineral in friction materials, the advantages of wollastonite, aside from its needle-like morphology, include the absence of hydroxyl groups, chemical stability, no phase change at high temperatures, and a high melting point. However, its relatively poor flexibility and high hardness are limiting factors for its large-scale addition. Additionally, during the processing of natural wollastonite crystals into powder, its one-dimensional elongated morphological characteristics are often destroyed. Therefore, developing processing technologies for needle-like wollastonite powders to improve the aspect ratio of crystals in the powder is a key technical challenge affecting its use as a reinforcing component in friction materials. Accelerating the research and industrialization of processes to prepare fibrous wollastonite micro-nano powders from calcium-silicate solid waste is also an effective way to enhance wollastonite's status and economic and social benefits in the field of friction materials.

 

(3) Compared to silicate minerals, the hydroxide mineral talc has a high water content of up to 30%. Its effect on the friction performance and structural stability of friction materials when used at high temperatures, due to the release of H2O during decomposition, still needs extensive engineering application practice for validation. Furthermore, the value (price) of talc as a raw material for producing metallic magnesium and magnesium oxide, as well as its function as a flame-retardant and smoke-suppressing filler in polymer materials, may be more prominent. Therefore, for the relatively scarce mineral resource of talc, correctly handling the relationship between resource development and efficient utilization is key to its application as a reinforcing component in friction materials.

 

(4) As one of the earliest natural materials used by humans, asbestos currently faces almost no technical barriers as a reinforcing component in friction materials. However, its dust can cause environmental pollution and potentially lead to respiratory diseases in humans, which is the main reason for the gradual reduction in its production and usage. In this article, the author still summarizes and introduces the composition, structure, and role of asbestos as a reinforcing component in friction materials, not to guide, advocate, or encourage the extensive use of asbestos in friction materials, nor to engage in the debate over whether asbestos is harmful, but based on the following three considerations: first, the important role and historical status of asbestos in friction materials; second, the reality that asbestos is still being mined in many countries and that most countries continue to use asbestos products; and third, the attempt to provide ideas and references for the development of superior substitutes for asbestos through this introduction. Research indicates that long-term exposure to asbestos dust significantly increases the likelihood of respiratory diseases. Nevertheless, due to the unique properties and efficacy of asbestos, countries such as Russia, Canada, China, and Kazakhstan continue to mine and process asbestos products, and most countries still use products containing asbestos. Even in the United States, where there are very strict requirements and restrictions on the use of asbestos products, asbestos mining was only completely halted at the end of 2002, yet the apparent consumption of asbestos that year was still 6,850 tons. By 2014, global asbestos production was still 2.02 million tons, and the apparent consumption of asbestos in the United States was still 406 tons.

 

(5) As China gradually enters an automotive society and industrialization increases, the demand for high-performance mineral friction reinforcement materials will gradually rise, with increasing requirements. This presents both opportunities and challenges for the mineral friction reinforcement material processing industry. Therefore, it is recommended that the mineral friction reinforcement materials industry pay close attention to the following three issues: first, to accelerate the research and development of new technologies such as surface modification and crystal shape protection, as well as the technical support and upgrading of equipment, focusing solely on improving the performance and effectiveness of traditional mineral friction reinforcement materials; second, to deepen the research on the intrinsic relationships between the composition, structure, performance, and effectiveness of mineral friction reinforcement materials and their enhancement mechanisms, continuously exploring new mineral friction reinforcement materials; and third, to actively develop industrial preparation technologies and the industrialization process for calcium sulfate whiskers, calcium carbonate whiskers, wollastonite whiskers, etc., to meet the diversified demands for new materials in the field of friction materials to the greatest extent.

 

 

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Unified product pricing and defined sales scope for member companies.

My opinion

(Yuhai Company, Shida Hai)

The Huangshi area is rich in mineral resources, among which many mineral products are important raw materials for the production of friction materials. Relying on resource advantages, there are more than a dozen enterprises engaged in the production and processing of friction material raw materials, holding a certain market share and brand benefits in the national friction material industry. However, due to the large number of production enterprises and the influence of the market economy, it is inevitable that price competition and vicious competition occur among enterprises. Therefore, all enterprises in the association should unify their understanding and thoughts, achieve information sharing and resource sharing, and create a good atmosphere for peaceful coexistence when conducting business. Here are a few personal views I would like to propose:

 

1. The association should conduct statistics and classification of the main products in the region, assess the raw material costs of each product, calculate a cost price, set a minimum selling price, and establish sales guidance prices based on regional differences to maximize the benefits of each enterprise.

 

2. Based on the different situations of each enterprise, relevant products should be divided into high, medium, and low categories, with pricing determined for each category. On the basis of the established selling prices, there can be fluctuations, and a minimum downward adjustment base should be set. Enterprises must not exceed the specified range when conducting business.

 

3. Given the large number and wide distribution of friction material enterprises nationwide, the association's enterprises can be divided into regions. In several major friction material production areas, a few enterprises can take the lead while the remaining enterprises engage in reasonable competition in an orderly manner. This can avoid the perception that the establishment of the association is for price and market monopoly, while allowing enterprises in the Huangshi area to fully leverage their resource advantages for healthy development and mutual benefit.

 

4. Establish a constraint and penalty mechanism to address the frequent occurrences of vicious competition and price undercutting in the market. First, the association should form a supervisory group to conduct irregular inspections of the market or investigate violations based on reports from association enterprises. Relevant situations and results should be reported at the council meeting, and the council should impose penalties based on the relevant actions of the incident according to the penalty measures.