Effects of surface coatings on the fatigue life of AISI P20 steel and Al 7075-T6 subjected to fretting and stress concentrations

Abstract

This thesis investigates the effect of surface coatings on the fatigue behaviour of two engineering alloys; the AISI P20 steel and the Al 7075-T6, which are most commonly used to manufacture plastic injection molds, and in aerospace structures respectively. These alloys are studied for their fatigue life behaviour with and without surface coatings in the presence of machined notches, fastener holes, or clamped region which induces fretting wear. The selected surface coatings are; (1) physical vapour deposition (PVD) coatings which are most commonly used in tool applications for the AISI P20 steel, and (2) electroless Ni-P (EN) coatings as the coating provides a uniform deposition at low process temperatures, which is beneficial for the studied aluminium alloy. First, an investigation was conducted on the fatigue performance of both studied alloys in the presence of stress raisers (notch, fretting, and a combination of notch and fretting). Fatigue tests for both alloys revealed a more dominant effect from notches compared to the fretting wear conditions. However, fractographic analyses confirmed the presence of cracks initiating from the fretting damaged surface of the combined notch and fretting fatigue Al 7075-T6 test specimens. Significant fretting wear marks were also observed for the combined notch and fretting fatigue AISI P20 tool steel specimens. In the second part of this study, the fatigue behaviour of TiAlN and TiAlN/CrN PVD coatings deposited on a v-notched AISI P20 steel was investigated. The major findings are as follows: • Unlike the un-notched (smooth) coated specimens, which are known to exhibit great improvements in fatigue life at high cycle fatigue, considerable reductions in fatigue life of the coated notched specimens were observed. This was understood to be due to the coating's brittleness at the notch tip, which induces fatigue crack initiations. • The effectiveness of the PVD coatings in resisting fretting fatigue failure of the AISI P20 tool steel was also investigated. A significant increase in fretting fatigue life of both PVD coatings was observed. Both PVD coatings showed a beneficial compressive residual stress and substantial increases in its resistance to plastic deformation (H3/E2) as well as surface hardness compared to the uncoated AISI P20 tool steel. Finally, the fatigue behaviour of EN coated Al 7075-T6 fastener holes with symmetrical through slits was studied. Uncoated open-hole, EN coated open-hole, uncoated bolted hole and EN coated bolted hole specimens were fatigue tested. The major findings are as follows: • The S-N curves that were produced, showed improvements in the fatigue life due to the combined beneficial properties introduced by depositing the EN coating and bolt clamping. • The fretting fatigue life of the EN coated Al 7075-T6 specimens tested at high cycle fatigue was significantly improved. The harder EN coated surface was found to reduce the contact stress due to the clamping pressure and thus assisted in improving the fretting fatigue life. The EN coated surface was also found to decrease the friction coefficient which assisted in mitigating surface wear damage, thus reducing the probability for fretting cracks to initiate.