Method and surface roughness aspects for the design of DLC coatings

Abstract

The design of coatings for highly loaded component contacts, such as bearings, gears and valve train components involves several important factors, including load, friction, lubrication, surface characteristics and material parameters. This paper presents an investigation of the influence of the material, coating thickness and surface roughness on tensional stress levels for coatings that are more compliant than the substrate material. Specifically the effect of multiple asperity contact is studied in three dimensions. The simulation is based on a finite element model where the load is applied as several interacting Hertzian pressure distributions.

The results show that the surface structure, in combination with the elastic properties of the coating, has a large influence on the tensional stress level in the coating. The highest tensional stress level in the coating occurs when contact spots almost overlap neighbouring cells and at the same time the size of the contact spots is in the same order of magnitude as the coating thickness.

Introduction

One way to increase the service life and tribological performance of machine elements is to apply a thin coating of hard diamond-like carbon (DLC). Such coatings have been shown to have the potential to greatly increase the frictional and wear performance of machine elements [1]. DLC coatings can be made in a large number of variants [1], [2]. One example of a machine element that is suitable for coating is a roller follower in a valve train system, shown in Fig. 1. In the same figure a measured form of topography is also presented. Under load, the pressure distribution profile between the two components will depend on the roughness of their surfaces. Where there is an asperity, there will be higher normal pressure than in a valley. The coating thickness, elastic modulus and surface topography parameters influence the stress in the coating when loaded. Several researchers [3], [4], [5] have calculated the stress and strain in the coating and substrate for both two and three-dimensional cases. Most of the work in the literature is related to stiff coatings on a relatively compliant substrate. However, for DLC coatings, which are of interest in the motor vehicle industry, the substrate is stiffer than the coating [6]. The brittle ceramic-like structure of DLC coatings makes them better at withstanding compressive stresses than tensional stresses [7]. Due to surface roughness effects in contacts between machine elements, there will be pressure spikes at asperities in contact and much lower pressures between asperities in contact. Where one asperity is in contact, the situation can be compared to a ball on a flat contact. In such cases, the largest principal stress for a homogenous material outside the contact circle is given by Eq. (1):

$${\sigma }_1=p_{\text{o}}(1-2\nu )\frac{a_0^2}{3r^2}$$

where p_o is the maximum Hertzian pressure, ν the Poisson's ratio, a₀ the contact radius and r the radius in a cylindrical coordinate system. The maximum tensional stress is reached at the edge of the contact and the stress then decays with increasing distance. Fig. 2 is a schematic representation of two surfaces in contact, where the size and shape of the contact areas depend strongly on the surface topography of the mating surfaces. In Fig. 2, the distance between the contact spots is one wavelength defined as 2λ. When two asperities are in close proximity, they will influence each other in such a way that as λ decreases and a₀ increases, the function for σ₁ will have a local maximum between the asperities. This paper investigates the influence of the density and size of asperities in contact on the tensional stress for a coated surface.