Tribological behaviour of AFNOR grade Z6 NCT DV 25-15 precipitation hardening austenitic stainless steel with NiCr-B coating in high temperature liquid sodium

摘要

The sliding wear and friction behaviour between AFNOR Grade Z6 NCT DV 25-15 precipitation hardening austenitic stainless steel and NiCr-B hardface coating was investigated in high purity liquid sodium at 823 K employing an indigenously developed reciprocating-type tribometer. The specimens for testing were in the form of pins and disc, with two pins in contact with both the surfaces of a disc specimen. The pin and disc specimens were made from precipitation hardened Z6 NCT DV 25-15 alloy and NiCr-B alloy deposited on 316L stainless steel substrate using Plasma Transferred Arc (PTA) welding process respectively. Testing was carried out at sliding speeds of 2 mm/s for a total sliding distance of 200 m under contact stress of 10 and 20 MPa respectively. The static friction coefficient (μ_s) and dynamic friction coefficient (μ_d) were estimated from the maximum value of friction force at the start of the movement and from the friction force measured during movement respectively. In both the tests μ_d was found to be significantly lower than μ_s. Weight loss measured for both the pins and disc at the end of the tests were negligible and this indicates that wear is very low in both the tests carried out at contact stress of 10 and 20 MPa. Worn scar profilometry on disc specimens was also carried out using Talysurf CLI 1000 Optical Profilometer with laser triangulation gauges, which revealed scars with larger scar depth on disc tested at 20 MPa stress level than on disc tested at stress level of 10 MPa. Scanning Electron Microscopy (SEM) on worn disc specimens revealed the presence of deformed and delaminated layer zone in both the cases. However this was significantly more in disc tested at 20 MPa, which is due to higher localized stress between rubbing surfaces at this stress level. It is concluded from this study that the static and dynamic friction coefficients (μ_s and μ_d) values are about 0.2 and 0.10 obtained at 20 MPa which are more than the friction coefficients values of about 0.11 and 0.05, respectively, at 10 MPa stress level. This is attributed to higher surface degradation caused by deformation and delamination at 20 MPa stress than at 10 MPa stress level. Results also indicate negligible wear loss in liquid sodium, which could be due to the lubricating action of liquid sodium.