Effect of Carbonaceous Components on Tribological Properties of Copper-Free NAO Friction Material

Our plain-English summary of Effect of Carbonaceous Components on Tribological Properties of Copper-Free NAO Friction Material, by H.Y. Lin, H.Z. Cheng, K.J. Lee, et al (Materials, 2020).

The study

• Authors: H.Y. Lin, H.Z. Cheng, K.J. Lee, C.F. Wang, Y.C. Liu, Y.W. Wang
• Journal: Materials
• Published: 2020;13(5)
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Copper helps to accelerate heat transfer during the braking process, allowing the brake materials to produce a stable coefficient of friction (COF), which in turn reduces wear loss and braking noise. However, its properties are also quite harmful to aquatic organisms. Finding a suitable replacement that fits all functions of copper for brake materials is not an easy feat. In this paper, six different carbonaceous components (coke, carbon black, carbon fiber, artificial graphite, natural graphite and expanded graphite) were substituted for copper in non-asbestos organic (NAO) friction materials. The hardness, thermal conductivity and tribological behaviors of these copper-free NAO friction materials were examined. Experimental results indicate that carbonaceous components improve lubrication and assist the friction composites with generating friction layers on the worn surface. Specimens containing coke, carbon black or carbon fiber exhibit broken friction layers, whereas specimens containing artificial graphite, natural graphite or expanded graphite exhibit quite adherent and smooth friction layers. Among all the copper-free carbon containing specimens, the specimen containing expanded graphite appears to be the best choice. It has the highest thermal conductivity, a relatively low wear loss and a relatively high and stable COF.

In plain English

This laboratory study, from researchers in Taiwan, tested brake friction materials. It did not study asbestos or human health; "non-asbestos organic" (NAO) is the name of the brake material type. The authors made a copper-containing control specimen and six copper-free versions. In these, copper was replaced by 5.1 percent by volume of coke, carbon black, carbon fiber, artificial graphite, natural graphite or expanded graphite. They then measured hardness, heat conduction, friction and wear against cast iron. The expanded graphite specimen had the highest thermal conductivity (2.6 W/mK) and the lowest hardness (about 40 HRS). The coke specimen had the lowest thermal conductivity (1.4 W/mK). The copper control had the highest wear loss (0.023 cm3), and wear in the copper-free specimens was 30 to 57% lower. The coke, carbon black and carbon fiber specimens formed broken friction layers and rougher worn surfaces. The three graphite specimens formed smoother, adherent layers. The authors concluded that expanded graphite has potential to replace copper in copper-free NAO friction materials.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: laboratory materials experiment comparing brake friction material formulations (hardness, thermal conductivity, friction and wear testing)
• Who was studied: test specimens of NAO brake friction material made in the laboratory: one copper-containing control (NAO) and six copper-free formulations; at least three specimens per condition
• Exposure: no asbestos was studied; the variable was a 5.1 vol % 'theme ingredient' that was either copper (powder and fiber) or one of six carbonaceous components (coke, carbon black, carbon fiber, artificial graphite, natural graphite, expanded graphite)
• Main results:
  • The expanded graphite (EG) specimen had the highest thermal conductivity, 2.6 W/mK
  • The carbon black and carbon fiber specimens had 1.6 W/mK; the coke specimen had the lowest, 1.4 W/mK
  • The EG specimen had the lowest hardness, about 40 HRS
  • Among copper-free specimens, carbon fiber had the highest average coefficient of friction (0.50); artificial and natural graphite had the lowest (0.39)
  • The average coefficient of friction of the EG specimen was 13% lower than that of the copper control
  • The copper-containing control had the highest wear loss (0.023 cm3)
  • Wear volume loss of the copper-free specimens was 30 to 57% lower than the copper control
  • Coke, carbon black and carbon fiber specimens had broken friction layers on the worn surface
  • Artificial and natural graphite specimens had smooth, adherent friction layers
  • Worn surface roughness was about 1.70 to 1.80 micrometers for the coke, carbon black and carbon fiber specimens, and 1.42 micrometers for the EG specimen
• The authors' conclusion: The authors concluded that the expanded graphite specimen had the highest thermal conductivity, relatively low wear loss and a relatively high and stable coefficient of friction, and that expanded graphite has potential to replace copper in copper-free NAO friction materials
• Limitations the authors state:
  • The authors did not disclose the detailed proportion of each ingredient, citing business confidentiality

Topics

Medical terms in this study, explained in our Lexicon: Hardness (Mohs Scale).

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