Materials Matter: Scuffing

Figure 1

Much like fatigue-based failure modes, scuffing causes a large degradation of the surface’s geometry, leads to inefficiency in the system, and can potentially cause catastrophic failure. Unlike fatigue-based failure modes where repeated loading and unloading leads to failure after many cycles, scuffing can occur at any point in a component’s life cycle. As a result, designing a component to have adequate resistance to scuffing is critical to avoid premature failure.

There are a number of proposed theories as to why scuffing occurs, but the most commonly accepted line of thought is that it is related to lubricant critical temperature. This concept was first proposed by Blok in 1937 [1-3]. It is based on observations that scuffing is linked to the bulk temperature of the component and the instantaneous flash temperature rise, which occurs as the surfaces pass through the contact zone.

In 1976, Dyson [4] proposed a theory as to why the lubricant would suddenly flash off. The theory states that as the gears come into mesh, the Hertzian contact pressure is increased. As the lubricant is exposed to the peak Hertzian pressures, its viscosity increases. This viscosity increase leads to the lubricant exhibiting properties similar to that of a solid. As the lubricant exhibits more solid-phase properties, this causes a further increase in temperature due to the increase in friction between the two moving surfaces. The increasing temperature starts to offset the viscosity increase, and the competing systems form a dynamic equilibrium in the contact zone. The theory proposes that at a critical temperature, the dynamic equilibrium will cease, causing the viscosity to fall rapidly and leading to a breakdown in the elastohydrodynamic layer (EHL). This viscosity breakdown leads to the direct contact and gross welding of the mating surfaces.

Isotropic Superfinishing and Scuffing

The surface roughness of the mating components and the chosen lubricant (and its operating properties) are primary factors in determining the potential for scuffing. In gear mesh, the lubricant thickness is greatest across the pitch line where the system exhibits pure rolling action. At the extremities of the contact zone, the gear exhibits pure slide and therefore has the lowest lubricant film thickness.

Surface roughness must therefore be taken into account relative to operating lubricant film thickness in these regions. Surface roughness and texture play a critical role in determining the pressure through the concentration of the load in the micro-asperity regions (see the April 2016 Materials Matter column on power density). Furthermore, the inherent friction created by the surface roughness causes the lubricant to build up bulk temperature. Therefore, as a reasoned overview, it is logical that enhanced film thickness associated with a reduced surface roughness will have less susceptibility to a sudden lubricant film collapse.

The various aspects of isotropic superfinishing and isotropic superfinishes that have been discussed in previous Materials Matter columns include how an isotropic superfinish lowers surface roughness, creating a unique surface texture; increases gear performance; and enables maintenance of full EHL over a wide variety of operating conditions. Given the theoretical connection between improved surface roughness and reduction in scuffing risk, isotropic superfinishing should be studied in this aspect. Such a study was conducted by Professor R.W. Snidle and colleagues in conjunction with REM Surface Engineering into the effect of superfinishing on scuffing resistance using Cardiff University’s scuffing twin disc test bench [5]. This test bench operates with 12 sequential, increasing load stages, each lasting three minutes. The highest load stage that the test bench can achieve is 4,150 N of load, representing a Hertzian pressure of 1.7 Gpa.

As part of the study, four surface finishes were reviewed. The investigation concluded that REM’s ISF® Surface provided the most consistent scuffing resistance, as no scuffing occurred even during an endurance cycle that was conducted at the maximum load. A summary of the results is presented in Figure 2.

As shown, isotropic superfinishing has a positive influence on scuffing resistance. However, it is important to note that other studies have shown superfinished surfaces that are completely devoid of texture to exhibit inferior scuffing resistance as compared to isotropic superfinished surfaces, which, by nature, have a subtle, non-directional texture.


Scuffing is a consequence of a rapid shift in the dynamic equilibrium in the contact zone between the compressed solid-like lubricant film and the temperature. Decreasing surface roughness, particularly through isotropic superfinishing, will significantly increase scuffing resistance as a result of the increased lubricant film thickness and the lower inherent friction of the surfaces. Given the difficulty in predicting scuffing and the lack of early warning signs, it seems reasonable that any scuffing-prone applications should consider utilizing isotropic superfinishing to reduce the risk of scuffing failures.


  • Blok, in: Proc. 2nd World Petroleum Congress, Sect. IV, Vol. III (1937) pp. 471486.
  • Blok, in: Proc. General Discussion on Lubrication & Lubricants, u 2 (1937) pp. 225-235.
  • Blok, in: Proc. General Discussion on Lubrication &Lubricants, Vol. 2 (1937) pp. 14-20.
  • Dyson, in: Proc. Institute of Mechanical Engineers, Vol. 190, No. 1 (1976) pp. 52-76.
  • Winkelmann, M. Michaud, R. Snidle, M. Alanou, “Effect of Superfinishing on Scuffing Resistance,” ASME DETC, Chicago, Illinois, 2003, DETC2003.


Martin McCormick has been with REM Surface Engineering since 2008 through REM’s European office in the United Kingdom. He has worked in a number of departments and currently holds a technical sales role focusing on supporting new and existing business. McCormick has an honorary master’s degree in chemistry from Hull University. He can be reached at

This article first appeared in September 2016 edition of Gear Solutions Magazine.  To view more on this Gear Solutions article, please click click here.

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