Takashi Nogi

Influence of a Hard Surface Layer on the Limit of Elastic Contact—Part I: Analysis Using a Real Surface Model

A numerical simulation technique for calculating the pressure distribution and the deformed geometry of an elastic half space which has a hard surface layer in contact with a rigid indenter with a rough surface is presented. In order to reduce the computing time, the Conjugate Gradient Method ( CGM ) is applied to solve a set of linear equations for unknown pressures. In each iteration of the CGM, the Fast Fourier Transform (FFT) is used for the task of multiplying a direction vector by an influence coefficient matrix. An FFT-based scheme for evaluating subsurface stresses in the layer and the substrate is also presented. As an example, the pressure distribution and the deformed geometry of a steel surface coated with a TiN layer in contact with a rigid rough indenter are calculated. The subsurface stresses are also compared with von Mises yield criterion to investigate the deformation mode at the asperity contacts. The results show that the limit of elastic contact is highly dependent on the layer thickness and the surface roughness.

Influence of a Hard Surface Layer on the Limit of Elastic Contact—Part II: Analysis Using a Modified GW Model

The Greenwood and Williamson microcontact model of rough surfaces is modified to include the presence of a surface layer which is stiffer and harder than the substrate. The axisymmetric contact between a rigid spherical asperity and an elastic layered halfspace is analyzed numerically and correction factors for the contact area, load and the maximum von Mises stress are approximated to a closed form by using curve fits of the numerical results. The correction factors for the contact area and load are applied to the GW model to reflect the effect of the finite layer thickness and the substrate material. The correction factor for the maximum von Mises stress is used to calculate the plasticity index for layered surfaces. Parametric calculation of the ratio of plastic contact area to real contact area is carried out for a TiN-coated steel surface. The modified GW model is compared with a more rigorous real surface model and the validity of the present model is discussed. When the layer thickness is sufficiently large, the influence of the soft substrate can be neglected. A simple criterion for realizing the contact free of the effect of the substrate is proposed.

Some Tribological Properties of an Ionic Liquid

Some tribological properties of an ionic liquid were investigated by using a pin-on-disc friction and wear tester. Due to running-in, the coefficient of friction of the ionic liquid decreased with time to a very low value of 0.02 which suggests that the lubrication regime was hydrodynamic at the end of the tests. Anti-wear performance of the ionic liquid was substantially comparable to a paraffin-based oil.Copyright

Dynamic Analysis of Minimally Lubricated Ball Bearings for Space Applications

A computer program for dynamic analysis of ball bearings, in which a simple mixed lubrication model for the retainer/race and retainer/ball contact is incorporated, is developed and retainer instability in reaction wheel bearings is investigated. Results show that an increase in the amount of oil promotes the instability and that a retainer with rectangular pockets is more stable than one with circular pockets.Copyright