Subsurface crack/interfacial crack subjected to IIertzian moving load is analyzed by the finite element method to understand the phenomena of the spalling of coatings in wear. The effects on the behavior of subsurface crack or interfacial crack, of various factors such as the crack length, Hertzian contact length, coating thickness, friction coefficients of the crack surface and the loading surface and combinations of the material of the coating layer and substrate are discussed in terms of the mode II stress intensity factor.
If the Hertzian contact length and the crack length are sufficiently large, compared to the depth of the subsurface crack(or thickness of the coating layer), the crack surface is completely closed while the load passes above the crack, which is quite distinguished from the case of the concentrated loading(i.e. zero hertzian contact length). Moreover as the Hertzian contact length is increased, $ΔK_II$ is decreased.
When the friction coefficient of the loading surface is smaller than that of the crack surface $ΔK_II$ shows a maximum value and decrease monotonically as the coating thickness gets smaller. On the other hand, $ΔK_II$ shows a maximum value, subsequently a minimum value and increses monotonically as the coating thickness gets smaller when the friction coefficient of the loading surface is larger than that of the crack surface.
The friction coefficient of the crack surface generally decreases $ΔK_II$ and its effect is more evident when the ratio, the coating thickness/the crack length is large. However the ratio is small, the value of $ΔK_II$ is dominantly affected by the difference between the friction coefficients of the crack surface and the loading surface.
The dependency of $ΔK_II$ on the thickness of the coating layer with the Dundurs' parameter α and β varying is examined and it is found that the trend is somwhat similar to that of the surbsurface crack in the homogeneous material. As α is algebrically increased, $ΔK_II$ gets smaller and the thickness $t_min$ which is defined to be the thickness of the coating layer when $ΔK_II$ is a minimum, gets smaller. As β is algebraically increased, $ΔK_II$ gets smaller, however, the thickness $t_min$ gets larger in the case.
