Mechanics and Mechanisams in Fretting Damage for Stainless Steel and Chromium Carbide Coatings

Abstract

Fretting is of serious concern in many industrial components, especially in the nuclear industry, for the safe and reliable operation of various components and systems. Although considerable efforts have been made to understand the basic mechanics and mechanisms involved in fretting, gaps in understanding still exist in the following domains: (i) No standard experimental setup and procedures exist that can quantify the entire fretting domain. (ii) Limited data are available for designers under controlled environmental conditions. (iii) Limited work exists in correlating fretting damage with mechanical responses, especially for materials with good adhesion properties. (iv) Limited studies are available to understand the nucleation/initiation of cracks under fretting conditions and the effect of loading on crack propagation. (v) Limited understanding exists on displacement and shear?force distribution at the contact interface while accounting for the failure mechanism. The entire effort in this thesis is focused on the above issues, which are investigated in detail. Further, studies are aimed at simulating fretting conditions in a Fast Breeder Reactor (FBR). Reactor core components are exposed to a sodium environment, which is a low?oxygen environment. Experiments under liquid sodium are difficult; therefore, as a first step, the tests were conducted under vacuum conditions to simulate sodium?environment behaviour. Stainless steel (SS316L), a reactor?core component material used in FBRs, has been examined. Chromium?carbide coatings have already been qualified based on performance criteria such as friction coefficient, wear rate, and galling resistance, but they have not been evaluated under fretting conditions. Thus, stainless steel and chromium?carbide coatings are investigated in detail. In this thesis, the mechanics and mechanisms involved in the degradation processes for self?mated stainless steel under fretting conditions are thoroughly examined. Further, chromium?carbide coatings with 25% nickel朿hrome binder produced using plasma?spray and high?velocity oxy?fuel (HVOF) processes have been investigated. The selection of coating processes ensures that the substrate remains in a specific metallurgical condition. The effects of normal load, displacement amplitude, environmental conditions, surface roughness, and stress field are critically examined. Stainless steel is often used in the nuclear industry due to its excellent mechanical properties at high temperatures and under irradiation; however, SS is also known for relatively poor wear and galling resistance. In nuclear power plants (NPPs), different components move relative to one another due to differential thermal expansion, flow?induced vibration, or loading杣nloading events. Such conditions fall under fretting. The objective of the present work is to understand the mechanics and mechanisms of nuclear?grade material (NGM), especially for sodium?cooled NPPs, under fretting conditions. A first?of?its?kind fretting machine has been designed and developed to simulate fretting conditions in both air and vacuum. Testing in vacuum simulates sodium?environment conditions. The major challenge in designing a fretting machine is achieving very low displacement amplitudes-as low as 1 祄-between the contact surfaces under constant normal load. The hydraulically actuated machine works under displacement?controlled mode for frequencies between 4 Hz and 120 Hz, under high vacuum of 10?? mbar and temperatures up to 873 K. A unique feature of the machine is the flexural member design, which provides high axial stiffness along with lateral flexibility. A robust control system with an efficient data?acquisition unit ensures measurement reliability. Contact conditions at the interface were identified using variations in coefficient of friction (COF) with the number of cycles, running?condition fretting loops, and total energy dissipation at the interface. Gross sliding conditions were observed for normal loads of 70 N and 250 N and displacement amplitudes ranging from 50 祄 to 200 祄, except for 250 N and 50 祄. Tests were conducted in both ambient and vacuum environments. Higher COF values observed for self?mated SS, compared to SS versus coated surfaces, were attributed to strong adhesion at the contact interface. Fretting loops were correlated with damage observed from wear?scar profiles and micrographs. In addition to elliptical and quadratic loops, triple loops were also identified. Strong adhesion resulted in an increased shear force, whereas a subsequent drop in shear force was due to third?body formation at the interface. High?magnification micrographs revealed fatigue striations at the contacted edge, while fracture features were observed in the central region. Coated surfaces showed severe surface degradation mainly in the form of coating fracture and spalling. Energy?dispersive spectroscopy (EDS) confirmed material transfer between contacting surfaces. Frictionally constrained conditions were also investigated at a high normal load of 600 N and displacement amplitudes from 25 祄 to 200 祄. Constant shear force with cycles and dependence of friction force on displacement amplitude were observed as typical characteristics. Two distinct regions-central stick and annular microslip-indicated partial slip regimes. Junction growth due to plastic flow resulted in an increase of real contact area. Cyclic variations in contact area under tangential loading likely contributed to microslip formation in the annular region, eventually producing two distinguishable zones. Microslip caused material transfer from flat to pin surfaces, as evident from EDS. Circumferential cracks occurred in the annular region, while shear fracture appeared in the central region. Detailed microstructural studies were carried out for two extreme conditions-gross sliding and seizure. Damage was compared based on plastic deformation severity and subsurface crack orientation. Hardness?gradient measurements quantified plastic deformation severity. Gross sliding caused subsurface cracks parallel to the surface, while seizure produced cracks inclined at 45皷54�. Severe plastic deformation under seizure formed shear bands aligned with the direction of macroscopic plastic flow. Initial surface roughness effects on wear were quantified using energy?wear coefficients. Higher energy?wear coefficients were observed for machined pins under sliding, while polished pins showed higher values under seizure. A computer code has been developed to evaluate surface and subsurface stress fields under both partial?slip and gross?sliding conditions. The Cattaneo朚indlin approach was used to model partial slip. Energy?based methods were used to quantify damage under both conditions. Shear?strain energy density and normalized strain?energy release rate were evaluated at the surface and subsurface. Results showed that gross sliding results in higher tensile stress at the trailing edge compared to partial slip. Higher shear?strain energy density governs damage nucleation under both conditions. A criterion for non?growth of subsurface cracks was proposed based on stress?intensity factor distribution and normalized strain?energy release rate. Subsurface cracks can grow to significant depths depending on crack?propagation energy, which depends on coefficient of friction and contact conditions. Analytical results agreed well with experimental observations. Nonlinear finite?element analyses were performed to evaluate stress and strain fields assuming fully stick conditions at the contact interface, simulating strong adhesion. The analyses examined effects of tangential loading on pressure distribution, contact radius, energy dissipation, and damage mechanisms under elastic杙lastic deformation. Accumulation of equivalent plastic strain per cycle is believed to cause ductile fracture. Both cyclic plasticity and ratchetting were involved in the damage mechanism. Ratchetting dominated under cyclic tangential loading. For coated surfaces, stress?based criteria governed the damage due to their limited ductility or brittleness. A continuous?microslip model has been developed to evaluate displacement fields and shear?force distributions for partial?slip and gross?sliding conditions. Studies were also carried out to examine the influence of relative tangential modulus of contacting bodies. Plane?strain and axisymmetric elastic elements were considered, while the interfacial layer was modelled as an elastic杙lastic medium. The model provides shear?force distribution at the interface, which allows estimation of subsurface stress fields once the tangential stiffness of the interface is known. The tangential modulus can be determined through numerical analysis or experiments. The study shows that as the tangential modulus of the interfacial layer increases, the shear force becomes more intense in the stick杝lip transition region, making this location prone to damage nucleation.