Sliding wear of a self-mated thermally sprayed chromium oxide coating in a simulated PWR water environment

Bearing surfaces in the primary circuit of pressurized water reactors (PWR) are prone to damage due to aggressive chemical and tribological conditions under which they operate, and a wide range of materials have been examined in this regard. One of the most promising candidates is chromium oxide in the form of a thermally spayed coating, and in this work, the behaviour of a commercially available Cr2O3 coating in self-mated sliding was considered. Tests consisted of a number of start-stop cycles of sliding between a crowned pin and a rotating disc in a water environment in an autoclave in an attempt to simulate the most aggressive phase of bearing run-up and run-down. Wear and damage mechanisms were examined at temperatures from ambient up to 250 C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM) and X-ray photoelectron spectroscopy (XPS). Across the temperature range, wear was mild, with no evidence of coating delamination. A five-fold increase in wear was observed between 80 C and 250 C (with wear depths of generally less than 8 µm being observed on the disc samples even at the higher temperature), despite there being only very small changes in hardness of the coating over the same temperature range. Debris was observed on the wear tracks following testing, with the evidence together suggesting that this debris was a very fine-grained mixture of Cr2O3 and amorphous -CrOOH, a corrosion product of Cr2O3

A comparison of the galling wear behaviour of PVD Cr and electroplated hard Cr thin films

Electroplated hard chromium (EPHC) is used in many industries as a wear and corrosion resistant coating. However, the long term viability of the electroplating process is at risk due to legislation regarding the toxic chemicals used. The physical vapour deposition (PVD) process has been shown to produce chromium and chromium-based coatings that could be a possible alternative for EPHC in some applications. This study investigates the microstructure and properties of two PVD chromium coatings as a possible alternative to EPHC to provide resistance to galling. Two PVD deposition processes are investigated, namely electron beam PVD (EBPVD) and unbalanced magnetron sputtering (UMS). Galling wear tests were performed according to ASTM G98-17. The results show that the two PVD coatings are of similar hardness, surface roughness and exhibit similar scratch behaviour. However, the galling wear resistance of the coating deposited by UMS is approximately ten times that of the EBPVD coating, and similar to that of the EPHC. X-ray diffraction reveals that the EBPVD chromium coating has a strong preferred orientation of the {200} planes parallel to the coating surface whilst in the UMS PVD coating, preferred orientations of the {110} and {211} planes parallel to the surface are observed. The EPHC does not exhibit relative peak intensities which conform to the International Centre for Diffraction Data (ICDD) powder diffraction pattern consistent with chromium. The crystal orientation of the PVD chromium coatings appears to play a significant role in influencing galling resistance

The effect of temperature on sliding wear of self-mated HIPed Stellite 6 in a simulated PWR water environment

Cobalt-based Stellite alloys are widely used in the primary circuit of pressurized water reactors (PWR) to protect valve surfaces against wear and galling in a corrosive environment. In this study, self-mated sliding wear of HIP-consolidated (Hot Isostatically Pressed) Stellite 6 (Co − 27.1 Cr − 1.5 Si − 5.0 W − 0.96 C, in wt %) was investigated. A pin-on-disc apparatus was enclosed in an autoclave and wear was measured in water from room temperature up to 250 °C (a representative PWR environment). Samples were characterized before and after wear testing using mass measurements, profilometry, X-ray diffraction and scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD). The bulk HIPed alloy is predominantly two phase and comprises a cobalt-rich fcc matrix and an M7C3 carbide phase. However, surface grinding prior to wear testing causes a surface layer of the matrix to partially transform to hcp Co-rich phase. The wear (mass loss) is very low below 150 °C but increases by approximately an order of magnitude when the temperature is increased from 150 to 250 °C. SEM/EBSD reveals sub-surface damage and partial fcc to hcp transformation of the Co-rich matrix phase to a depth of ~ 15 μm in the disc. However, there is little change in transformation behavior and depth with temperature and this is not regarded as significant cause of the increased wear. The order of magnitude increase in wear is instead ascribed to a tribocorrosion mechanism associated with significantly higher corrosion rates at 250 °C than at 150 °C. As the material removal and factors affecting it are found to be so dependent on temperature, this work demonstrates the necessity of conducting assessments of materials for use in PWR environments under representative conditions

The evolution of subsurface deformation and tribological degradation of a multiphase Fe-based hardfacing induced by sliding contact

Multiphase Fe-based hardfacing alloys, for example Tristelle 5183 Fe-21%Cr-10%Ni7.5%Nb-5%Si-2%C in wt.%, are extensively used for tribological applications, including valves, bearings and drive mechanisms, where two surfaces are unavoidably subjected to loaded sliding contact within engineering systems. In this study, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize, for the first time, the tribologically affected material induced by the self-mated sliding contact of HIPed Tristelle 5183. This provided novel insight into the deformation modes which permit the accumulation of the high levels of subsurface strain required for plasticity dominated (adhesive) wear in a commercial hardfacing. In the subsurface regions furthest from the sliding contact, plastic deformation is accommodated by deformation induced martensitic transformation to e-martensite and α0-martensite, twinning, the generation of planar dislocation arrangements (generated by planar slip) and the generation of dislocation tangles. Closer to the sliding contact, the subsurface becomes unstable, and nanocrystallisation driven by grain boundary mediated deformation mechanisms and crystallographic slip completely engulf the near surface microstructure. It is postulated that nanocrystalisation within the subsurface is a needed in order to accommodate the extremely high strains required in order to permit tribological degradation via plasticity dominated wear. The extrusion of metallic slivers via plastic ratcheting generates ductile shear cracksgoverned by plastic strain, and the failure of these slivers generates plate/flake-like wear debris

Galling of stainless steels as a function of test conditions in an ASTM G196-type test setup -the role of temperature, rotational velocity, interrupted rotation and rotational distance

Austenitic stainless steels have attractive properties for use in corrosive environments, but their use in components where motion under load is experienced (such as valves) is limited by their poor galling behaviour. Whilst stainless steels with improved performance have been developed over many years, the basic understanding of the key parameters in galling of standard stainless steels is not well understood. In this work, the galling behaviour of a dissimilar austenitic stainless steel pair is explored via testing in an instrumented ASTM G196-type test. Key variables examined are environmental temperature (room temperature and 100 °C), rotational speed (2.1 rpm and 5.5 rpm), and the sliding distance (from a quarter of a turn up to five turns). Galling was observed to become more severe with increased temperature, but was not significantly affected by either sliding speed (in the range examined) or interruptions during the rotation. The measurement of friction coefficient along with surface observations revealed that galling does not take place within the initial period of sliding; however, damage is being accrued by the sample surfaces which then results in subsequent observable galling as the sliding distance is increased. The importance of measuring torque during galling tests is illustrated, and the findings provide useful information with regard to those test variables that require critical control (and which do not) during conduct of a galling test programme. WEAR2023_0385 Daure et al. on effect of test parameters on gallin