Radial fretting behaviours of dental feldspathic ceramics against different counterbodies

Radial fretting is a fretting phenomenon mainly induced by oscillating normal loads or thermal cycling, where the two surfaces keep in contact and no impact action occurs. In fact, such fretting mode is frequently encountered in oral environment after prosthesis insertion. In this paper, radial fretting tests on two types of ball specimens (52100 steel and Si3N4 ceramics) with diameter of 12 mm against the dental feldspathic ceramics flats (Cerec Vita Mark II) have been carried out. Maximum normal load (Fmax) was varied from 100 N to 800 N with a constant speed of 12 mm/min, and the number of cycles from 1 to 105. In order to maintain the surfaces of flat and ball specimens in contact and avoid an impact effect, a minimum normal load was set at 50 N. The kinetics behaviours of the feldspathic ceramics under different normal loads were analyzed in details. The fretting scars, which were examined by optical microscope and laser confocal scanning microscope (LCSM), were displayed as the worn zones in shape of annular. The critical normal load of cracks initiation was verified. Below the critical normal load, the damage mechanisms were the combination of adhesion and delamination. Above the critical normal load, four damage zones were analyzed. To clarify the experimental results, the analysis of classic Hertzian theory has been discussed. It was also found that the radial fretting damages greatly decreased when a drop of oil was added into the contact interfaces.