REDUCING FRICTION AND WEAR IS CRUCIAL FOR EFFICIENCY AND LONGEVITY IN TRIBOLOGICAL SYSTEMS. SOLID LUBRICANT NANOPARTICLES SHOW GREAT POTENTIAL IN THIS CONTEXT BUT LACK DEPOSITION CONTROL METHODS AND TRIBOLOGICAL PERFORMANCE INVESTIGATIONS UNDER CONDITIONS CLOSER TO REAL SYSTEMS. THE PRESENT STUDY USED DROP-CASTING TO DEPOSIT MULTI-LAYER GRAPHENE (MLG) AND CARBIDE-DERIVED CARBON (CDC) NANOPARTICLES ON AISI1020 STEEL SUBSTRATES. TRIBOLOGICAL PERFORMANCE WAS EVALUATED USING A BALL ON FLAT TESTS WITH AN INCREMENTAL LOAD INCREASE IN A RECIPROCATING MOTION. RESULTS INDICATE A MINIMUM COVERAGE THRESHOLD FOR EFFECTIVE LUBRICATION AND HIGHLIGHT SUBSTRATE TOPOGRAPHY

THIS WORK FOCUSES ON DEVELOPING NOVEL IRON-BASED SELF-LUBRICATING COMPOSITES REINFORCED WITH TI2SNC MAX PHASE PRODUCED BY POWDER METALLURGY. TWO AMOUNTS OF TI2SNC (5 AND 10 VOL%) AND THE ADDITION OF 10 VOL% GRAPHITE WERE EVALUATED. THE MICROSTRUCTURE REVEALED A PARTIAL REACTION BETWEEN THE MATRIX AND THE TI2SNC, EXHIBITING A DEGREE OF DISSOCIATION IN THE PRESENCE OF GRAPHITE, LEADING TO THE PRECIPITATION OF CARBIDES. THE ADDITION OF THE MAX PHASE SIGNIFICANTLY IMPROVED THE HARDNESS AND COMPRESSION STRENGTH. THE DRY COEFFICIENT OF FRICTION WAS AROUND 0.12 FOR FE + 5TI2SNC + 10GR, SHOWING A REMARKABLE REDUCTION IN WEAR RATE UP TO 85 % COMPARED TO PURE IRON. THE RESULTS DEMONSTRATE A SYNERGISTIC EFFECT BETWEEN THE MAX PHASE AND GRAPHITE, ENHANCING TRIBOLOGICAL PERFORMANCE AND WEAR RESISTANCE.

FE-BASED SELF-LUBRICATING COMPOSITES, IN WHICH SIC PARTICLES ACT AS PRECURSORS TO IN SITU GENERATED 2D TURBOSTRATIC GRAPHITE AS A LUBRICATING PHASE, WERE PRODUCED VIA POWDER INJECTION MOULDING (PIM). IN ORDER TO STUDY THE PRECURSOR INFLUENCE ON THE SOLID LUBRICANT STRUCTURE AND PROPERTIES, HEXAGONAL (?) AND CUBIC (?) SIC POLYTYPES WERE USED. THE COMPOSITES WERE TRIBOLOGICALLY TESTED IN DRY AND LOW-VISCOSITY OIL-LUBRICATED CONDITIONS. DISTINCT PRECURSORS DID NOT LEAD TO SIGNIFICANT DIFFERENCES BETWEEN COMPOSITES. HOWEVER, DUE TO THE HIGHLY DISORDERED GRAPHITE-RICH TRIBOLAYER FORMATION, THEY DO PROVIDE LUBRICITY, LOW FRICTION COEFFICIENT, AND HIGH WEAR RESISTANCE, EVEN IN OIL-LUBRICATED CONDITIONS.

THIS RESEARCH AIMS TO EXAMINE HOW A RADIAL GRADED POROSITY DISTRIBUTION AFFECTS THE ELASTIC MODULUS BY CONDUCTING SIMULATIONS ON TI-BASED ALLOY FOAMS WITH FACE-CENTERED CUBIC AND BODY-CENTERED CUBIC CRYSTAL STRUCTURES. FOUR TYPES OF FOAMS WERE ANALYZED; COMMERCIALLY PURE-TI, TI-13TA-6MN (TTM), TI-13TA-(TT) AND TI-13TA-6SN (TTS), (ALL IN AT.%). FOUR RADIAL GRADED POROSITY DISTRIBUTION CONFIGURATIONS WERE MODELED AND SIMULATED USING THE FINITE ELEMENT ANALYSIS (FEA). THE RADIAL GRADED POROSITY DISTRIBUTION CONFIGURATIONS WERE GENERATED USING A MATERIAL DESIGNER (ANSYS) WITH A PORE RANGE OF 200 TO 600 ?M. THESE RADIAL GRADED POROSITY DISTRIBUTIONS HAD AVERAGE POROSITY VALUES OF 0, 20, 30 AND 40%. THE CONSOLIDATED SAMPLES THAT WERE OBTAINED THROUGH A POWDER METALLURGY TECHNIQUE IN TWO STEP SAMPLES WERE SYNTHESIZED USING A POWDER METALLURGY TECHNIQUE, WITH THE ELASTIC MODULI VALUES OF THE AFOREMENTIONED TI BASED ALLOYS BEING MEASURED BY ULTRASOUND USING ~110, ~69, ~61 AND ~65 GPA, RESPECTIVELY. THE RESULTS SHOWED THAT THE MODULUS DECREASED AS A FUNCTION OF POROSITY LEVEL IN ALL SIMULATED MATERIALS. THE TTM, TT AND TTS FOAMS, WITH AVERAGE POROSITIES OF 20, 30 AND 40%, EXHIBITED AN MODULUS SMALLER THAN 30 GPA, WHICH IS A REQUIREMENT TO BE USED AS A BIOMATERIAL IN HUMAN BONES. THE TT FOAMS SHOWED THE LOWEST MODULUS WHEN COMPARED TO THE OTHER FOAMS. FINALLY, CERTAIN THEORETICAL MODELS WERE USED TO OBTAIN THE MODULUS, THE BEST BEING; THE GIBSON?ASHBY MODEL (? = 1 AND N = 2.5) FOR THE CP-TI FOAMS AND KNUDSEN?SPRIGGS MODEL (B = 3.06) FOR THE TTM, TT AND TTS FOAMS.

THE DISPERSION STABILITY OF CARBON-BASED SOLID LUBRICANTS/LUBRICATING OILS REMAINS A CHALLENGE TO OVERCOME. RECENTLY, NOVEL PROCESSING ROUTES WERE DEVELOPED TO OBTAIN 2D TURBOSTRATIC GRAPHITE PARTICLES VIA SOLID-STATE REACTIONS BETWEEN B4C AND CR3C2 (GBC) AND BETWEEN SIC AND FE (GSF) THAT PRESENT OUTSTANDING TRIBOLOGICAL PROPERTIES IN A DRY SCENARIO, AS WELL AS FUNCTIONALIZED GRAPHENE (GNH). THIS WORK INVESTIGATED THE SUSPENSION STABILITY OF GBC AND GSF PARTICLES (0.05 WT.%) DISPERSED IN A LOW-VISCOSITY POLYOL ESTER LUBRICATING OIL AND THEIR TRIBOLOGICAL PERFORMANCE. AMMONIA-FUNCTIONALIZED GRAPHENE (GNH) PARTICLES WERE ALSO USED AS A REFERENCE. IN ORDER TO EVALUATE THE DISPERSION STABILITY, IN ADDITION TO THE CLASSICAL DIGITAL IMAGE TECHNIQUE, A MUCH MORE ASSERTIVE, RELIABLE, QUANTITATIVE AND RARELY REPORTED IN THE LITERATURE TECHNIQUE WAS USED, I.E., THE STEPTM (SPACE AND TIME-RESOLVED EXTINCTION PROFILES) TECHNOLOGY. RECIPROCATING SPHERE-ON-FLAT TRIBOLOGICAL TESTS WERE CARRIED OUT, IN WHICH BEFORE CONTACT, 0.2 ?L OF PURE OIL AND SUSPENSION (POE + 0.05 WT.% OF SOLID LUBRICANT) WAS APPLIED ON A FLAT SURFACE. THE RESULTS SHOWED THAT THE GBC PARTICLES REMAINED REMARKABLY STABLE AND REDUCED THE SPHERE WEAR RATE BY 61.8%. FROM THE TRIBOSYSTEM POINT OF VIEW, THE PRESENCE OF GBC AND GSF REDUCED THE WEAR RATE BY 18.4% AND 2.2%, RESPECTIVELY, WITH RESPECT TO THE PURE OIL, WHILE THE GNH PARTICLES INCREASED THE WEAR RATE BY 4.2%. FURTHERMORE, THE WEAR RATE WAS IMPROVED DUE TO THE HIGHLY DISORDERED CARBON TRIBOLAYER FORMATION IDENTIFIED ON BOTH SURFACES.

CARBON-BASED SOLID LUBRICANTS ARE EXCELLENT OPTIONS TO REDUCE FRICTION AND WEAR, ESPECIALLY WITH THE CARBON CAPABILITY TO ADOPT DIFFERENT ALLOTROPES FORMS. ON THE MACROSCALE, THESE MATERIALS ARE SHEARED ON THE CONTACT ALONG WITH DEBRIS AND CONTAMINANTS TO FORM TRIBOLAYERS THAT GOVERN THE TRIBOSYSTEM PERFORMANCE. USING A RECENTLY DEVELOPED ADVANCED RAMAN ANALYSIS ON THE TRIBOLAYERS, IT WAS POSSIBLE TO QUANTIFY THE CONTACT-INDUCED DEFECTS IN THE CRYSTALLINE STRUCTURE OF A WIDE RANGE OF ALLOTROPES OF CARBON-BASED SOLID LUBRICANTS, FROM GRAPHITE AND CARBIDE-DERIVED CARBON PARTICLES TO MULTI-LAYER GRAPHENE AND CARBON NANOTUBES. IN ADDITION, THESE MATERIALS WERE TESTED UNDER VARIOUS DRY SLIDING CONDITIONS, WITH DIFFERENT GEOMETRIES, TOPOGRAPHIES, AND SOLID-LUBRICANT APPLICATION STRATEGIES. REGARDLESS OF THE INITIAL TRIBOSYSTEM CONDITIONS AND ALLOTROPE LEVEL OF ATOMIC ORDERING, THERE IS A REMARKABLE TREND OF INCREASING THE POINT AND LINE DEFECTS DENSITY UNTIL A SPECIFIC SATURATION LIMIT IN THE SAME ORDER OF MAGNITUDE FOR ALL THE MATERIALS TESTED.

SELF-REPLENISHABLE WAYS OF APPLYING SOLID LUBRICATION IN TRIBOLOGICAL SYSTEMS ARE LIMITED TO THE USE OF SINTERED SELF-LUBRICATING COMPOSITES. THESE ARE OBTAINED BY MIXING SOLID LUBRICANTS WITH METALLIC POWDERS OR BY THE IN SITU PRECIPITATION OF THE LUBRICATING PHASE. BOTH TECHNIQUES CAN PRODUCE SELF-LUBRICATING COMPOSITES ABLE TO PROMOTE LOW FRICTION COEFFICIENTS IN DRY SLIDING CONDITIONS. HOWEVER, THE COMBINATION OF SOLID LUBRICANT AND MATRIX IS LIMITED BY THE THERMODYNAMIC INTERACTIONS DURING SINTERING. FILLING THE PORES OF SINTERED PARTS WITH SOLID LUBRICANTS BY VACUUM IMPREGNATION IS A VERSATILE WAY TO PROMOTE SOLID LUBRICATION, ALLOWING ANY COMBINATION OF SOLID LUBRICANT AND MATRIX TO BE USED, AVOIDING DELETERIOUS PHASES AND OTHER SINTERING PROBLEMS TYPICAL IN SINTERED SELF-LUBRICATING COMPOSITES. IN THIS WORK, LOW ALLOY SINTERED STEEL WAS PRODUCED BY MIXING ASTALOY CRL POWDERS WITH 0.6?WT% C, THE SINTERED SPECIMENS WERE IMPREGNATED WITH GRAPHITE (1.10??M), AND THE DURABILITY AND WEAR RATES OF THE RESULTING COMPOSITES WERE STUDIED USING BALL-ON-FLAT RECIPROCATING TESTS. THE RESULTS SHOW THAT THE IMPREGNATION TECHNIQUE IS ABLE TO FILL THE SURFACE PORES WITH GRAPHITE, PRODUCING FRICTION COEFFICIENTS DOWN TO 0.06 IN DRY-SLIDING CONDITIONS AND WEAR RATES RANGING FROM 10?4 TO 10?7 (MM3/N-M). THE MAIN WEAR MECHANISM OF THE IMPREGNATED SPECIMENS WAS PLASTIC DEFORMATION ON THE SURFACE, WHICH CAUSES SEALING OF PORES, WHILE THE WEAR OF THE COUNTER-BODIES WAS MAINLY ABRASIVE. THE DURABILITY OF THE LUBRICITY REGIME IS DIRECTLY PROPORTIONAL TO THE AMOUNT OF LUBRICANT AVAILABLE AS PORES GET SEALED BY THE SLIDING ACTION OF THE COUNTER-BODY, SO IS PROPORTIONAL TO THE POROSITY.