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

Abstract

Fretting is a serious concern in many industrial components, specifically, in nuclear industry for the safe and reliable operation of the component/system. Till date, a lot of efforts has already been made to understand the basic mechanics and mechanism involved in fretting, but still limited understanding on the following domains exists, (i) No standard experimental set up and procedures exists which could quantify the entire fretting domain. (ii) Limited data available for the designer under controlled environment conditions. (iii) Limited work in correlating fretting damage with the mechanical responses, specifically, for the materials with good adhesion properties. (iv) Limited work to understand the nucleation/initiation of cracks under fretting condition and, the effect of loading on crack propagation. (v) Displacement and shear force distribution at the contact interface accounting the failure mechanism.

The whole efforts in this thesis are focused on the above points and, are investigated in detail. Further, studies are focused to simulate fretting conditions in a Fast Breeder Reactor (FBR). Reactor core components are exposed to sodium environment, which is a low oxygen environment. Experiments under liquid sodium are difficult and as a first step, the tests were done under vacuum condition to simulate condition in sodium environment. Stainless steel (SS316L) is a reactor core component material used in FBRs. Chromium carbide coatings are already qualified based on the performance criteria for friction coefficients, wear rates and galling resistance, but are not evaluated under fretting conditions. Thus, stainless steel and chromium carbide coatings are investigated in detail. In this thesis, mechanics and mechanisms involved in the degradation processes for self-mated stainless steel under fretting conditions are examined in detail. Further, chromium carbide with 25% nickel chrome binder coatings using plasma spray and high-velocity oxy-fuel (HVOF) processes on stainless steel are also investigated. The choices of the coating processes have been made such that the substrate must be maintained in a particular metallurgical condition. The effect of normal load, displacement amplitude, environment conditions, surface roughness, and stress field are critically examined. Stainless steel (SS) is often used in the nuclear industry because of its excellent mechanical properties under high temperature and irradiation environment, but on the other hand, SS is characterized as having relatively poor wear and galling resistance. In nuclear power plants (NPPs), different components move relative to each other, due to differential thermal expansion or flow-induced vibration or during loading and unloading events, and such conditions can be categorized under fretting. The objective of the present work is to understand the mechanics and mechanisms of nuclear grade material (NGM), specifically for sodium-cooled NPPs, under fretting conditions. The first-of-a-kind fretting machine has been designed and developed to simulate fretting condition in both, air and vacuum. The test in vacuum simulates conditions under sodium environment. The major challenge in the design of a fretting machine is to achieve low displacement amplitude, as low as 1µm, between the contact surfaces under constant normal load. The hydraulic actuated machine works under displacement controlled mode, for any frequency between 4Hz and 120Hz, under high vacuum of 10–5mbar and for temperatures up to 873K. A unique feature of the machine is the design of flexural member which provides not only high axial stiffness but also flexibility in the lateral direction. A robust control system with an efficient data acquisition system adds to the reliability of the system. Contact conditions prevailing at the interface were identified on the basis of variation of coefficient of friction (COF) with number of cycles, running condition fretting loops, and total energy dissipation at the contact interface. Gross sliding conditions have been observed under normal load of 70N and 250N and displacement amplitude in the range of 50µm to 200µm, except for normal load of 250N and displacement amplitude of 50µm. The tests were conducted under both ambient and vacuum environment. Higher value of COF observed for self-mated SS, compared to SS versus coated surface, has been attributed to the existence of strong adhesion prevailing at the contact interface. Running condition fretting loops were correlated with damage observed from the scar profile and the micrographs. In addition to elliptical and quadratic loops, triple loops were also identified. The existence of strong adhesion results in an increase of shear force, whereas subsequent drop in shear force is due to third body formation at the contact interface. Higher magnification micrograph reveals fatigue striations at the contact edge, while the fracture features were observed in the central region. The surface morphological features of the material under seizure conditions, which have been observed under a normal load of 250N and displacement amplitude of 50µm, shows large scale cracking on one side of the pin and the flat. Micrographs at higher magnification of the cracked surface just adjacent to the contact interface shows formation of slip bands within the grains, whereas the central region reveals shear fracture. Coated surfaces shows major surface degradation mainly in the form of fracture and spalling of the coatings. Energy dispersive spectroscopy (EDS) shows the occurrences of material transfer between the contacting surfaces. Frictionally constrained conditions have also been investigated at high normal load of 600N and for displacement amplitude in the range of 25µm to 200µm. Constant shear force with number of cycles and dependence of friction force on displacement amplitude were observed as the typical characteristics of frictionally constrained bodies. Two distinct regions, viz. center stick region and annular micro slip region, indicate the existence of partial slip regimes. Junction growth due to plastic flow of the material resulted in an increase of real area of contact at the contact interface. It is believed that the cyclic variation in the contact area, under cyclic tangential loading, may have given rise to micro slip in the annular region, and finally resulted in two distinguishable regions. It has been observed that the occurrence of micro slip in the annular region resulted in the material transfer from flat to pin surface, as evident from EDS responses. Damage in the form of circumferential cracks has also been observed in the annular region, whereas the center region shows features of shear fracture. Detail micro structural studies have been carried out for two extreme conditions, viz., gross sliding and seizure conditions. The conditions were identified mainly based on shear force variation with number of cycles and running condition fretting loops. Subsurface damage under both conditions has been compared based on the severity of plastic deformation and the orientation of subsurface cracks. Severity of plastic deformation has been quantified based on hardness variation along depth. A steep gradient of hardness indicates that the damage is very much confined in the region just beneath the contact interface. Gross sliding condition at the contact interface resulted in the propagation of subsurface cracks parallel to the surface, whereas under seizure condition the cracks were found inclined at an angle between 450 and 540 to the surface. Further, severe plastic deformations under seizure condition have resulted in the formation of shear bands and were found oriented in the direction of macroscopically imposed plastic flow. Influence of initial surface roughness on wear damage has also been quantified based on energy wear coefficient. Higher energy wear coefficient has been found for machined pin under sliding condition, whereas, under seizure condition polished pin shows higher energy wear coefficient. A computer code has been developed for the evaluation of surface and subsurface stress field, under both partial slip and gross sliding condition. Cattaneo and Mindlin approach has been adopted to model the partial slip condition. Energy based approach has been adopted for the quantification of damage observed under both contact conditions. Shear strain energy density and normalized strain energy release rate have been evaluated at the surface and in the subsurface region. Effect of contact conditions and the influence of coefficient of friction on stress field have been studied in detail. Analysis results shows that gross sliding results in higher tensile stress at the trailing edge, as compared to the stress induced under partial slip condition. Further, it has been observed that higher shear strain energy density at the surface and in subsurface region controls the nucleation of damage under both partial slip and gross sliding conditions. A criterion for the no growth of subsurface cracks has been discussed based on the distribution of stress intensity factor and normalized strain energy release rate as a function of crack size. It has been observed that subsurface cracks can grow up to significant depth depending on the crack propagator energy. The availability of crack propagator energy depends on coefficient of friction and contact conditions prevailing at the contact interface. The analytical results were found in good agreement with experimental observations. Non-linear analyses have been carried out using finite element analysis to evaluate stress and strain fields, assuming the existence of fully stick condition at the contact interface. Fully stick condition simulates the contact condition under strong adhesion. The analysis investigates the effect of shear or tangential loading on pressure distribution, contact radius, energy dissipation, and damage mechanism involved under elastic-plastic deformation. The accumulation of equivalent plastic strain in each cycle is believed to be responsible for ductile fracture. It has been observed that both cyclic plasticity and ratcheting are involved in the damage mechanism. Ratcheting has been observed as the governing damage mode under cyclic tangential loading condition. In contrast to this, due to limited ductility or brittle nature of coated surfaces, stress based criteria governs the damage. Continuous micro slip model has been developed to evaluate the displacement field and shear force distribution for partial slip and gross sliding condition. Further, the studies have been carried out to study the influence of relative tangential modulus of the contacting bodies on displacement field and shear force distribution. Plane strain and axisymmetric elastic elements are considered in the modeling while the interfacial layer has been modeled as an elastic-plastic layer. The model gives the shear force distribution at the contact interface and subsequently subsurface stress field can be estimated, once the tangential stiffness of the contact interface layer is known. The value of tangential modulus can be estimated either from numerical analysis or from experiments. Further, the study shows that as the relative tangential modulus of an interfacial layer increases, the shear force becomes more intense in the stick-slip transition region making this location more prone for damage nucleation.