Ceramic-metal matrix composites produced by powder metallurgy provide a solution in engineering applications where materials with high wear resistance are required. In the mining industry, the wear of materials is a crucial and widely recognized industrial problem as over 50 % of components fail as a result of wear damage. Increasing the wear resistance of these components will contribute to a reduction in maintenance and thereby increase efficiency. In this present research, SS316L-50wt.% Al2O3 composites were fabricated using the powder metallurgy route. The effects of the powder metallurgy processing parameters were studied. The produced cermet composites were characterized with respect to microstructure, density, hardness and toughness. Furthermore, the wear behavior of the composites was studied using pin-on-disc testing under dry sliding conditions. The produced test results were used to improve existing wear models, particularly the Wayne’s model. The highest hardness of 1085.2 HV, the highest density of 94.7 % and the lowest wear rate of 0.00397 mm3/m were obtained at a milling speed of 720 rpm, a compaction pressure of 794.4 MPa and sintering at 1400 °C in an argon atmosphere. Compared to commercial SS316 and fabricated SS316L, the composites had 7.4 times and 11 times lower wear rate, respectively. However, it is shown that using better densification methods such as hot isostatic pressing (HIP) or hot pressing can further substantial enhanced densification and improve of the composites wear resistance. Similar to its effects of the strength and the toughness, the remaining porosity was found to substantially affect the wear resistance of the sintered composites. Therefore, the porosity was used to correct the abrasion parameter in the first step of wear model improvement. The porosity represented a further consideration of the microstructure in addition to the reinforcement particle size introduced earlier by Wayne. In a second model improvement step, the test conditions were introduced in the wear resistance calculation. This model allowed the prediction of corrected wear resistance values that are characteristic of the individual test materials and are widely independent of wear test conditions. The coefficient of correlation of the model was 0.91 with respect to Wayne's data and wear test results from this study, and was 0.66 after generalization to a large range of wear data measured on multiple materials tested under varying test conditions. This opens a potential avenue for a model-based assessment of the wear resistance of novel materials as well as changes that can be expected under different wear conditions.

The capabilities of various metal Additive Manufacturing (AM) processes, such as Powder Bed Fusion – Laser (PBF-L) and Direct Energy Deposition (DED) are increasing such that it is becoming ever more common to use them in industrial applications. The ability to print atop a substrate broadens that scope of applications. There is ongoing research regarding the mechanical properties of additively processed materials, but little regarding the interaction between additive material and its substrate. An understanding of the mechanical and performance properties of the AM/substrate interface is imperative. This paper describes a study of the strength properties of AM/substrate interfaces, with respect to torsion and tension, and compares them to their fully wrought and fully additive counterparts. PBF-L and DED are used to produce tensile and torsion test specimens of two different materials, SS316L and M300 steels. This provides sufficient variety in testing for a confident analysis to be made.

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.

Alloys of titanium are highly preferred materials for their excellent strength to weight ratio but the tribological issues while using them has been posing challenging issues for the tribological analyst, which are still areas of active research. Ti-6Al-4V (Ti64) is the most popular alloy of titanium and our understanding of the fundamental mechanisms of wear and friction of this alloy is still not complete. Previous investigations related to the tribology of these alloys have suggested a synergistic effect of plastic deformation and tribo-oxidation. The present investigation described in this thesis explores the existence of one more mode, namely the formation of a Mechanically Mixed Layer (MML). The thesis examines the effect of these modes one by one and analyses the synergistic effect of these mechanisms, and also the effect of heat generation during sliding. The tribological condition existing have been varied by doing wear experiments using Ti64 pins sliding against alumina and SS316L (controls MML), diameter of pin (expected to control debris entrapment and thus MML formation), tribo-system (horizontal disc Vs vertical disc, which is also expected to control debris entrapment and thus MML formation), environment (ambient and vacuum, expected to control tribo-oxidation) and sliding speed (expected to control interface temperature and thus plastic deformation mechanism and tribo-oxidations). The division of the main chapters has been so made to present the findings spread over Chapters 5-8, with each chapter dealing with specific tribological test conditions. In each chapter, results from the tribological experimentations in the form of wear and friction are presented, together with the characterization methods which throw light into the tribological mechanisms. These characterization methods include Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDAX), X-Ray Diffraction (XRD) and Electron probe micro-analyzer (EPMA). Wherever possible, the debris collected from the experiments have been subjected to morphological and detailed chemical analysis, and a feature which has not been explored much in detail by tribological investigators, but having a promising potential. Experimental results from tribological testing when Ti64 pins slides against two different materials (Alumina and SS316L) in pin-on-disc tribometers under two different environmental conditions (ambient /vacuum) are analyzed. Each set of experiments looks at two different effects - (1) the effect of sliding speed on the tribological behavior while using a pin of a fixed diameter (all other parameters remaining the same) and (2) the effect of using pins of different diameters for a given set of parameters. Three different pin-sizes were employed (2.1 mm. 4.6 mm and 6.6 mm), the normal loads on these pins were changed according to the pin-size used so that all experiments were done at the same contact pressure (2.8 MPa). By performing the experiments against the ceramic disc (alumina) under vacuum conditions, the effect of this plastic deformation is studied in isolation because the possibility of the Tribo Chemical Reaction (TCR) due to oxidation is inhibited and no MML was found to be formed due to poor compatibility of mixing between the metallic pin and the ceramic disc. For the low speeds/strain rates experiments, the effect of plastic deformation as influenced by the adiabatic shear banding is seen to influence wear which progressively changes to temperature induced plastic deformation and wear. The situation is found to be different when we change the environmental conditions from vacuum to ambient for the same tribo-combination. The tests shows a reduction in wear rate with speed, and this is due to the oxide formations due to TCR as confirmed from the SEM/EDAX characterization. In contrast to previous experiments under vacuum, these permit the effect of TCR also to influence the tribological behavior. The scenario changes when the alumina disc is replaced by a metallic one (SS316L) and tests carried out in vacuum, as the MML was found to be formed with this tribo-pair. Because of the mutual affinity of the materials in the tribo-pair, the wear damage is severe in this case and the flash temperatures crossing the phase transition temperature (~880oC) for Ti64 at high speeds. The growth of the β phase with increase in the sliding (temperature) conditions is captured from the XRD spectra of the wear debris. Synergistic effect of all these mechanisms (plastic deformation, MML, and TCR) is permitted by conducting experiments with Ti64 pin against stainless steel and in ambient conditions. A comparison of the tribological response by presenting results when experiments are run over a range of speeds while using different sized pins under ambient conditions (and compared with similar results in vacuum) while using SS316L disc serve to demarcate the differences in the wear modes which are active/inactive depending on the tribological conditions. In addition a study incorporating the effect of frictional heating and its influence on the tribological phenomena is analyzed. Main conclusions from the thesis are: The wear resistance of Ti64 alloy when sliding against SS316L is found to be influenced by Strain Rate Response (SRR), Tribo Oxidation (TO), Mechanically Mixed Layer (MML) and the prevailing heat flux conditions at the contact. The wear rates were found to decrease marginally with sliding speeds (strain rates) up to a certain speed, which is ascribed to reduction in adiabatic shear band intensity with increase in strain rate. Adiabatic Shear Band (ASB), which allows easy crack propagation, intensity reduces as temperature of deformation of Ti64 is increased. From the results it can be confirmed that the propensity for formation of MML depends on compatibility of the disc and the pin material. The contribution due to of entrapment and retention of debris in the contact zone also would influence formation of the MML. The effect of frictional heating plays an influential role as it can affect the factors (TO, ASB, MML) governing the tribological response. The sensitivity to temperature, which is a marked feature of this alloy in undergoing softening, as confirmed by previous researchers, is reflected in the experimental results. Since the main factor that triggers the micro-structural instability is the energy dissipation that accompanies deformation more fundamental research which can improve the thermal transport properties of this alloy, would be the future scope of work of this thesis. Also, the unique composition of the MML which offers high wear resistance under specific operating conditions opens up the possibility of new such alloy formulations, production routes and techniques which should improve the tribological response of this alloy.