INTRODUCTION: Scuffing is a kind of catastrophic surface failure which suddenly occurs in severe sliding conditions. When scuffing occurs, friction and surface temperature rapidly increase with strong adhesion between sliding surfaces. In the worst case, scuffing causes a system shutdown of machinery. It is necessary to investigate mechanisms of the scuffing progression for prevention of the worst case. It is known that crystalline structure and mechanical properties of scuffed surfaces change compared to bulk material. Markov et al.1 suggested that change of mechanical properties such as hardness and microstructure due to frictional heat at macro-scale can influence the scuffing progression. Yagi et al.2 indicated that instantaneous heat generation due to plastic flow and phase transformation occurred during scuffing. In this study, in-situ observation of the contact area during scuffing was conducted. After the test, a cross section of a scuffed pin was observed and crystallographic structure was analyzed.
EXPERIMENTAL PROCEDURE: In-situ observation system was mainly divided into two parts comprising a tribometer and a capturing system located above the tribometer. The tribometer was a pin-on disc type. A contact area was created between a rotating sapphire disc and a fixed steel pin. Load was applied by an air cylinder below the steel pin. The material of the pin was SUJ2, which is equivalent to AISI52100. The diameter of the steel pin was 4 mm and the curvature of the top surface was 12.7 mm. Sapphire can transmit visible and near infrared light and thus was used as material of the disc to in-situ observe the contact area. Lubricant was supplied to the leading edge of the contact area with controlling the temperature. The capturing system consists of a microscope, a xenon flashing lamp, a visible camera and a near infrared camera. Visible and near infrared images can be captured simultaneously by using beam splitter to disperse a light toward two cameras.
After the test, the test pin was cut along the sliding direction. The cross section of the pin was processed by 5 % nital etching after polishing and observed by a laser microscope. The cross section was polished again and measured by EBSD (Electron Back Scatter Diffraction Patterns) analysis.
RESULTS: A test was conducted with a step load of 1000 N/m, a sliding speed of 3 m/s and an oil temperature of 80ºC. The frame rate of the cameras were set to 60 fps. Figure 1 shows visible and near infrared images of the contact area during scuffing at the test time of t = 122.00 s. Large deformation and heat generation on the entire contact area could be captured. Figure 2 shows the cross section of the scuffed pin processed by nital etching. Figure 2a shows general view which could capture large deformation of the friction surface. Figure 2b shows enlarged view of the cross section at the top surface which can indicate the formation of a white layer with a thickness of 100 µm. Figure 3 shows crystal orientation maps of the white layer and bulk material. At the white layer, crystal grains are much finer than those of the bulk material. To approximately 3 µm depth from the top surface, crystallographic information could not be identified. To approximately 15 µm depth from the unidentified region, equiaxed grains were formed. Under the equiaxed grains region, crystal grains were deformed along the sliding direction.
The equiaxed grains in the white layer as shown in Fig. 3a are commonly seen as a result from dynamic recrystallization which affects mechanical properties of materials.3 Dynamic recrystallization may occurs due to high pressure, high shear stress, and heat generation during scuffing. Relationship between change of