Hand-considered hypotheses, 2001–2100
Lubrication, wear and tribology: films and their failure, additive layers, coatings and texture, debris, and contacts under vacuum, cold, heat and pressure
Written one at a time, not composed. Each entry carries its logical form, because the forms are what differ: some claim a quantity is misattributed, some assert a bound or a threshold, some an inversion, some a hidden term, some an artefact that mimics a real effect. The distribution is uneven and was not fixed in advance. None has been tested. Any number followed by “(estimate)” is an order-of-magnitude expectation, not a measurement; nothing here is a datum and no citation is implied.
Fluid film: Reynolds, clearance and the thermal limit
Misattribution
H2001. A journal bearing that loses load capacity as it speeds up is usually said to be suffering viscous thinning of its oil, yet part of the loss is the shaft growing centrifugally and closing the clearance, which happens even if the oil's viscosity is held constant. Centrifugal growth rises with the square of surface speed and with the square of radius divided by the shaft's specific stiffness. A solid shaft and a thin-walled hollow shaft of the same outside diameter, run at one surface speed in one oil, separate the two, because the hollow one grows several times as much (estimate) while the oil cannot tell them apart.
Hidden term
H2002. A capacitance film-thickness reading carries the sensor's own insulating overcoat in series with the oil, and that overcoat's permittivity rises with temperature, so a hot bearing appears to have a thinner film than it has. The overcoat's thickness is fixed but its capacitance is not, and it drifts in the same direction as the thermal closure of the clearance. Two sensors with different overcoat thicknesses in one pad, read at one oil temperature, give offsets proportional to their coatings and expose the term.
Bound
H2003. Load capacity at fixed clearance is bounded by the temperature the oil reaches at the pad's trailing edge rather than by the viscosity at which it entered, because the minimum film sits downstream of the hottest metal. Colder supply buys capacity only until the oil's transit time exceeds the pad's thermal time constant, after which the trailing edge stays at the same temperature. One bearing run at two supply temperatures and three flow rates, with capacity plotted against measured trailing-edge temperature, collapses onto one curve if the bound is thermal.
Threshold
H2004. Starvation begins at a supply-to-entrainment flow ratio near one third (estimate), not when the feed hole runs dry, because the meniscus between pads must retreat past the inlet before replenishment stops. Below that ratio the film collapses quickly; above it, additional supply changes nothing observable. A transparent bearing fed at metered rates, with film thickness read optically, puts the knee at the same ratio for two oils.
Reversal
H2005. A circumferential groove normally costs load capacity by interrupting pressure build-up, but above a surface speed at which the film's own heating would otherwise thin it downstream, the groove pays for itself by admitting cool oil at the hot end. The crossover is where the groove's pressure loss equals the viscosity recovered by that cooling. Grooved and plain pads run through one speed sweep at fixed load cross at that speed, and the crossing moves with inlet temperature as the mechanism requires.
Identity
H2006. Gaseous cavitation and film rupture are one event at one pressure, so changing the oil's dissolved-gas content moves where the film breaks without changing the pressure profile upstream of the break. Two literatures have grown on one phenomenon because the two names were taken to imply two mechanisms. Degassed and aerated batches of one oil, mapped for rupture onset against speed at fixed load, show the boundary moving with gas content while the upstream profile stays fixed.
Artefact mimicking effect
H2007. Near-zero friction coefficients reported for textured thrust washers are sometimes the test frame's axial resonance rather than the film, because a compliant load path stores and returns energy once per revolution and the cycle-averaged force falls. The signature is that the apparent coefficient depends on where the load cell sits, not on the washer. Repeating the measurement with a stiffer load path, same washer and same oil, should raise the coefficient toward the hydrodynamic prediction.
Count
H2008. A tilting-pad bearing with N pads has at most N−1 independent pad loads once the total load is specified, since the pad loads share one equilibrium. A model that fits N independent pad loads is absorbing a violated constraint, and preload, offset and pivot position enter only through that same balance. Fitting a measured set of pad loads with N and with N−1 parameters, and comparing residuals, shows whether the extra parameter bought anything.
Scale argument
H2009. Thermal bow of a shaft grows with the square of its length while the clearance that must absorb it grows only linearly with size, so a geometrically scaled-up bearing has proportionally less room for the same temperature difference. Model bearings therefore run straighter than their full-size counterparts at equal surface speed. Two sizes of one design, run at equal surface speed and equal inlet temperature, show the large one contacting first.
Inversion of standard practice
H2010. Oil is normally fed into the unloaded half of a bearing so that the supply does not disturb the pressure film, yet at high speed the better place is the loaded half, where the film is thinnest and the heat is concentrated, so the supply's cooling outweighs the pressure it disrupts. The crossover is set by the Peclet number of the film. One bearing with two feed positions, run over a speed sweep at fixed load, shows which position wins where.
Selection effect
H2011. Published bearing failures over-represent machines that were opened, and machines are opened when they fail, so the apparent distribution of failure modes is conditioned on failure and cannot rank modes for a population that mostly runs. The absent category is slow uniform wear with no event to report. A planned teardown of a random sample of running machines, rather than of failed ones, is the only way to count it.
Unit trap
H2012. The Sommerfeld number is defined with the journal radius in some texts and the journal diameter in others, a factor of two that becomes a factor of four in load because the number is used squared in most charts. The same measured bearing can therefore be called safe or marginal depending on which definition its chart's author used, and nothing in the symbol records which. Recomputing one dataset through both definitions against measured film thickness shows which one the chart was drawn with.
Hidden term
H2013. Squeeze-film damping in a damper fed with aerated oil is overestimated by any model that uses the neat oil's bulk modulus, because entrained gas lowers the mixture's modulus roughly in proportion to the gas fraction at low pressure (estimate). The damper then behaves as a softer spring than the design value and its force lags the displacement. Force and displacement measured at two aeration levels with the same viscosity separate the modulus term from the geometry.
Misattribution
H2014. Whirl a little under half running speed is diagnosed as a film instability, but in a machine with a flexible coupling the same frequency can be the coupling's own kinematic whirl, which does not care about bearing clearance. Film whirl responds to preload; coupling whirl does not. Changing only the bearing preload at constant speed and unbalance separates them, because the amplitude at the whirl frequency follows preload only in the first case.
Bound
H2015. The force a squeeze-film damper can transmit before it cavitates is bounded by the cavitation pressure times the damper's projected area, so beyond that point more oil flow or less clearance buys no additional damping. The bound is independent of viscosity, which is why very viscous dampers disappoint their designers. Driving one damper at increasing amplitude, with force and displacement recorded together, shows the force ceiling flattening while displacement keeps growing.
Threshold
H2016. Surface texture stops acting hydrodynamically at a depth below roughly a tenth of the film thickness (estimate), however much area it covers, because below that depth the step's pressure contribution is short-circuited by the smooth land around it. Area fraction changes the magnitude above the threshold but cannot move it. A texture series of fixed area fraction and stepped depth, run at one speed and load, puts the knee at the same relative depth.
Reversal
H2017. Raising the oil's viscosity in a high-speed bearing normally raises temperature, but over a range it lowers the maximum pad temperature, because the thicker oil is pumped through the bearing as Couette flow and the added cooling outruns the added dissipation. The window closes when the pumped flow becomes limited by the supply pressure. A viscosity series at fixed supply pressure, with pad temperature and throughput flow measured together, shows the window and where it shuts.
Identity
H2018. In steady state the thermal wedge and the elastic wedge of a pad are one wedge, both tilting the film's convergence, and a pad's measured tilt cannot say which contributed. They differ only in time constant, thermal seconds against elastic microseconds, so the identity is exact for capacity and false for dynamics. A step load on an instrumented pad, with film thickness and pad strain recorded together, shows the steady tilt equal to the sum and the two relaxations separating.
Artefact mimicking effect
H2019. A friction drop measured after a viscosity-index improver is added can be the improver's degradation in the test loop rather than a film effect, because the polymer shears down and the oil measured afterwards is not the oil that ran. The apparent benefit then grows with test duration, which no lubrication mechanism would do. Sampling the oil at the start and the end of one run and re-measuring both in a fresh viscometer separates degradation from lubrication.
Count
H2020. Fully flooded isothermal line-contact film thickness collapses onto two dimensionless groups rather than the four usually tabulated, because the load and speed exponents are not independent once supply is unlimited; the extra groups describe the starved branch. Flooded and starved data should therefore need different counts on the same axes. One contact taken from flooded through starved, with the groups counted on each branch, tests whether the second pair earns its place.
Misattribution
H2021. The rumble from a large slow bearing is usually attributed to turbulence in the oil film, but at the Reynolds numbers involved the film is laminar and the noise comes from micro-slip of the pad in its pivot seat. Turbulence would scale with speed; seat slip scales with load reversals. Running the same bearing with two pivot-seat materials at identical speed and load changes the noise only if the seat is responsible.
Null claim
H2022. Filtering an oil finer than the minimum film thickness does nothing for film breakdown, because breakdown is a pressure event and not a particle event; the benefit of filtration saturates once particles are smaller than the film. The saturation is why some machines show no gain from a finer filter and their operators conclude that the filter was defective. One machine run with two filter ratings over a long interval, with film thickness measured directly, shows the flat part of the curve.
Boundary lubrication and the additive film
Misattribution
H2023. The friction reduction credited to a friction modifier is often the modifier changing the surface's running-in rather than the boundary film's shear strength, because the same additive added after running-in gives a much smaller effect. If the film were the mechanism, the order of addition would not matter. Adding the additive before and after running-in in two otherwise identical tests separates the two.
Hidden term
H2024. Every boundary friction measurement in a reciprocating rig contains a term from the reversals, where the film is squeezed out and re-formed twice per cycle, and that term grows as the stroke shortens. Coefficients measured at short strokes therefore include a squeeze contribution that is not boundary friction at all. Two stroke lengths at one sliding speed and one nominal pressure separate them, and the difference should scale with the reversal frequency.
Threshold
H2025. An anti-wear additive stops protecting a steel contact above a temperature near 150 °C (estimate) not because it decomposes, but because its film's growth rate falls behind the wear rate there, and the two rates cross. The film is still present above the threshold; it is simply thinner than the wear it must replace. Measuring film thickness in situ against temperature at fixed load puts the crossing where wear rate overtakes growth.
Identity
H2026. Anti-wear and extreme-pressure additives are taught as two classes with two mechanisms, yet in a contact whose flash temperature exceeds the additive's decomposition point they are one: both work by forming a shearable compound where the asperity touched. The classification then records the test that was run, not the chemistry that acted. One additive tested at two loads, with the worn-surface chemistry mapped at both, should show the same compound in the same places.
Reversal
H2027. More anti-wear additive normally means less wear, but past a coverage at which the film is thicker than the asperity's own deformation depth, the film becomes the wearing surface and wear rises again, because it is softer than the metal and is removed faster than it forms. The minimum sits near the film thickness that matches the deformation depth (estimate). A concentration series with film thickness and wear measured together shows a minimum rather than a plateau.
Bound
H2028. A boundary film cannot lower friction below its weakest plane's shear strength divided by the contact pressure, so there is a floor that no additive chemistry passes at a given load, and a reported coefficient below it implies that part of the load was carried by a fluid film. The floor rises with pressure, which is why low-load tests look better than service. One additive at several loads, with film fraction measured independently, should approach the floor asymptotically.
Artefact mimicking effect
H2029. A sudden friction drop when a lubricant is changed is often the rig running in on the new fluid's first few thousand cycles, not the fluid's steady property, and the transient is then averaged into a false steady value. Reversing the order of the two fluids on the same specimens separates the transient from the fluid. If the effect follows the order and not the fluid, it is running-in.
Selection effect
H2030. Additive studies report the concentrations that worked, while the concentrations that caused corrosion, staining or seal swell are reported in a different literature, so a reader of the tribology alone has an optimistically narrow view of the window. The missing results are not random; they cluster at the top of the concentration range. Testing a series to the point of failure, with corrosion and seal swell measured in the same run, restores the shape.
Count
H2031. Coverage, shear strength and thickness are the only three independent chemistry variables that a boundary film's friction can depend on at fixed temperature and load. A study reporting four or more independent chemical drivers is reporting collinear proxies for those three, which is why such studies rarely reproduce. Measuring two of the three independently and predicting the third tests the count.
Threshold
H2032. A bonded solid lubricant stops transferring to the counterface and starts being ploughed out above a contact pressure set by the ratio of the film's shear strength to the substrate's yield pressure, because beyond that ratio the substrate deforms and the film cannot follow it. The threshold is a ratio, not a pressure, so it moves with substrate hardness. Two substrates of different hardness coated identically cross at the predicted ratio.
Misattribution
H2033. Friction anisotropy on a rubbed boundary film is usually assigned to molecular alignment, but much of it belongs to the topography that the rubbing itself created, which is aligned in the same direction and cannot be separated by chemistry alone. Alignment relaxes on mild heating; topography does not. Annealing one specimen after rubbing and re-measuring along both directions separates them.
Hidden term
H2034. Friction measured in humid air includes the capillary term of water bridges at the contact's periphery, and in a boundary contact that term can exceed the film's shear term at low load. Humidity control alone does not remove it, because the bridges re-form on unloading. Two loads at one humidity, with friction plotted against load, give an intercept that is the capillary term.
Reversal
H2035. Water in a boundary contact is usually deleterious, but against a fatty-acid film at low load it can lower friction by ionising the acid and building a soap in place of the physisorbed monolayer. The reversal depends on load, because above the soap film's capacity the water is harmful again. A humidity sweep at two loads, with the surface chemistry mapped after each, shows both directions.
Identity
H2036. The stick and the slip of stick-slip are not two states of the lubricant but one state of the contact sampled at two rig stiffnesses: the boundary film sets only the ratio of static to kinetic friction, while the rig decides whether that ratio produces a visible event. Any model that fits the period is therefore fitting the rig. Two rig stiffnesses with one lubricant at one speed should give one friction ratio and two periods.
Bound
H2037. A lamellar solid lubricant's friction coefficient in vacuum is bounded below by the ratio of its basal-plane shear strength to the pressure at which its layers are expelled, and both come from the same interlayer potential, so improving one worsens the other. The bound explains why lamellar coatings converge on similar coefficients despite very different chemistries. Shear strength and expulsion pressure measured on one coating, plotted as a ratio against measured friction, tests it.
Unit trap
H2038. Boundary-film wear is quoted as volume per unit load per unit distance in some studies and as a fraction of film thickness per cycle in others, and converting between them needs the contact area and the sliding distance per cycle, which are usually not both reported. Values compared across the two conventions differ by that area, often by orders of magnitude (estimate). Recomputing one dataset both ways, with the area measured, shows the gap.
Scale argument
H2039. Boundary films behave differently in a nanotribometer and in a bench rig not because the chemistry changes but because the ratio of contact size to film thickness does: at nanometre contacts the film is a substantial fraction of the contact, and at millimetre contacts it is a surface layer on a half-space. The transition is geometric, so it can be crossed without changing either material. One chemistry at three contact sizes at equal nominal pressure should show friction converge only when the ratio is matched.
Artefact mimicking effect
H2040. Reports of superlubricity in boundary contacts sometimes rest on a friction force smaller than the rig's own drift, which a null run with the load removed would have measured. A drift of a few micronewtons per minute (estimate) is enough to manufacture a coefficient. The null test, same motion with no contact, is the discriminator, and it is rarely reported alongside the claim.
Inversion of standard practice
H2041. Boundary lubricants are screened at constant load, yet most real contacts see load that varies within each cycle, and a film that survives a steady load can fail under the same peak applied briefly because healing needs time at low pressure that a varying load never provides. Screening at the peak load alone therefore overstates performance. A steady test and a cycled test at equal peak load and equal cycle count isolate the healing time.
Hidden variable
H2042. The friction of a boundary contact depends on its history of direction changes, not only on instantaneous speed and load, because the film is worn into a groove whose axis remembers the last direction. A reversal cuts a new groove, and friction stays transient until that groove forms. L-shaped sliding paths, with friction recorded through the corner, show the memory and its relaxation.
Null claim
H2043. In a contact already fully boundary, adding a viscosity modifier to the base oil changes nothing about friction, because the film's rheology is set by the additive layer and not by the bulk polymer; any measured change is the bulk oil's contribution to the few hydrodynamic pockets that remain. Testing at a load high enough to remove those pockets should make the modifier irrelevant. One oil with and without the modifier, at three loads, shows the difference converging to zero as load rises.
Derived quantity
H2044. The lambda ratio is derived from two measured quantities whose statistics differ, so it carries a variance that is never quoted and is large where the roughness distribution is skewed; a contact at a mean lambda of three can be in boundary contact over a few per cent of its area. The ratio is a mean-field number used as a threshold. Mapping local lambda from a measured topography, rather than averaging it, shows the tail that the average hides.
Coatings, texture and the surface as a structure
Misattribution
H2045. The low friction of a diamond-like carbon coating in dry sliding is usually credited to its smoothness, yet a comparably smooth steel gives several times the friction, and the difference is the coating's ability to passivate its own dangling bonds with a transferred layer. Smoothness sets the contact area; the transfer layer sets the shear. Two counterfaces of equal roughness but different carbon affinity separate them.
Hidden term
H2046. Hardness measured on a coating by indentation includes the substrate's contribution whenever the indent depth exceeds about a tenth of the coating thickness, so a quoted coating hardness is a composite number and the coating is usually softer than quoted. The rule is a ratio of depths, not an absolute depth. Indenting one coating at several depths and plotting hardness against depth over thickness gives the substrate-free value at the intercept.
Threshold
H2047. A hard coating on a soft substrate stops shielding the substrate above a load at which the substrate yields first, and that load is set by the ratio of coating thickness to the contact's elastic half-width. Above the ratio the coating is carried on a plastic foundation and conforms to it instead of spreading the load. A load sweep on two coating thicknesses, with substrate yield detected by acoustic emission, puts the transition at the predicted ratio.
Reversal
H2048. Texturing a coated surface usually extends the coating's life by holding lubricant, but there is a depth at which the texture interrupts the coating's own thickness and its edges become the weakest feature, after which life falls as area fraction rises. The optimum depth is a fraction of the coating thickness (estimate). A depth series on one coating run to a fixed wear criterion shows a maximum rather than a monotone trend.
Identity
H2049. Adhesion measured by scratch and by pull-off are the same interface property sampled at two stress states, so a coating cannot be good in one and poor in the other unless the interface is anisotropic. The usual disagreement between the tests is the substrate's plasticity, which the scratch includes and the pull-off does not. Testing one interface on two substrates of different yield strength separates the interface from the substrate.
Count
H2050. A multilayer coating's wear life depends on at most two independent interface parameters, the weakest interface's work of separation and the spacing between interfaces, so a graded design with many layers cannot beat the best two-parameter design at equal total thickness; extra interfaces only add candidate weak planes. Depositing one total thickness with two, four and eight interfaces shows where life stops improving.
Artefact mimicking effect
H2051. Apparent friction reduction from a soft metallic coating is often the coating conforming to the counterface's roughness and reducing real contact area, which ends as soon as the coating wears through. A short test records the conforming phase and reports it as the coating's property. Running one specimen past coating removal, with wear depth measured, shows the step back to the substrate's friction.
Bound
H2052. A coating cannot reduce a counterface's wear below the rate set by the coating's own debris acting as a third body, because once that debris enters the contact it abrades regardless of the coating's hardness. The bound explains why a soft coating sometimes wears a hard counterface faster than a hard coating does. Two coatings of different hardness but similar debris size, against identical counterfaces, approach the same counterface wear rate.
Scale argument
H2053. A coating's stored elastic energy scales with thickness while its adhesion scales with area, so the ratio of the two grows linearly with thickness; below a critical thickness the coating is stable at any stress it can hold, above it the same stress removes it. The critical thickness is why thin coatings survive what thick ones do not. One material deposited at several thicknesses at one stress level shows which survive and where the boundary sits.
Threshold
H2054. A thermal-spray coating changes wear character at a contact temperature where its oxide grows faster than it is removed, and the transition is sharp because oxide growth is exponential in temperature while removal is linear in load. Below it the coating wears as a metal; above it, as an oxide. A temperature sweep at fixed load and speed, with worn-surface oxygen measured at each point, locates the transition.
Misattribution
H2055. The scuffing resistance of a phosphated surface is credited to the phosphate's own low shear strength, but the phosphate's main function is to hold oil in the pores it creates, and its benefit disappears in a dry contact where no oil is available to hold. Phosphate without oil should then be no better than bare steel. Two phosphated specimens, one run dry and one oiled, test the attribution.
Hidden variable
H2056. Coating performance varies between nominally identical depositions because the coating's residual stress is set by a thermal history the specification does not record, and two runs at one thickness and composition can differ by a factor in wear life. The stress is measurable by substrate curvature but is rarely measured on the parts that are actually tested. Curvature on witness coupons, correlated with wear life across one production batch, tests it.
Reversal
H2057. Harder coatings usually wear less in abrasion, but against a third body of comparable hardness a harder coating can wear faster, because it cannot embed the particle and the particle is pressed against it repeatedly instead of being pressed into a softer surface. Embedding is the softer surface's advantage. Two coatings either side of the abrasive's hardness, run in one slurry, cross where the mechanism changes.
Identity
H2058. The running-in wear of a coating and its steady-state wear are one process at two degrees of surface conformity, and the wear rate is the rate at which conformity is lost and regained. A coating that never conforms wears at the steady rate from the first cycle. Two roughness pairs, one matched and one mismatched, should show or not show the transient, and the transient's length should scale with the mismatch.
Bound
H2059. A surface texture cannot reduce friction in a contact whose film is thinner than the texture's depth, because the texture then acts as a set of edges rather than a reservoir, and the edges raise friction above the smooth value. Useful texture depth is therefore bounded above by the film and below by the roughness. One texture run at two film thicknesses helps in one and harms in the other.
Artefact mimicking effect
H2060. Hardness increases reported after a coating is applied can be the indenter tip blunting on the harder surface rather than the coating's property, because a blunter tip reads a larger contact area at fixed load in several calibration schemes. The artefact grows with the number of indents made with one tip. Imaging the tip before and after a series, and re-indenting a standard, exposes it.
Inversion of standard practice
H2061. Coatings are usually deposited thicker for longer life, but for a coating whose failure is driven by its own stored stress the thinner coating outlasts the thicker at equal finish, and the usual thickness specification sits on the wrong side of the optimum. The optimum is where stored elastic energy equals the interface's work of separation. A thickness series at one finish, run to a common failure criterion, puts the optimum below the standard thickness.
Selection effect
H2062. Coating comparisons are made on coupons that survived deposition, and the runs that flaked in the chamber are absent from the comparison, so the reported coating is a survivor of a process whose yield is not reported. A coating with excellent coupon properties and forty per cent yield is worse than a mediocre one at ninety-eight. Reporting yield alongside wear life, for one production run, changes the ranking.
Count
H2063. A coated contact's friction needs at most two independent parameters, the interface's shear strength and the pressure at which it yields, so a study reporting friction varying with five surface treatments is reporting proxies for two quantities. What is measurable is not what matters. Coating one series with graded chemistry, with shear strength and yield pressure measured independently, should collapse the friction data onto one curve.
Threshold
H2064. Below a coating thickness of a few tens of nanometres (estimate) a hard coating cannot remain coherent over a rough substrate, and its wear rate becomes the substrate's because the coating survives only on asperity peaks. The threshold is set by the ratio of coating thickness to the substrate's roughness amplitude, not by the coating's own properties. One coating at several thicknesses on one rough substrate joins the substrate's wear rate at the predicted ratio.
Wear mechanisms and the debris that records them
Count
H2065. Abrasive wear rate at fixed load falls as grit size grows because the number of grits in contact falls as the square of the size while each groove's volume grows only linearly, so the rate follows a count rather than a mechanism. The size effect in abrasion is therefore arithmetic. Counting active grits on the worn surface after tests at three sizes, and computing rate per grit, collapses the data.
Hidden term
H2066. Wear measured by weighing includes the oxygen a worn surface has taken up, so an oxidising metal can gain mass while losing material and the reported wear rate can be negative at low load. The term is the oxide's mass minus the metal it replaced, largest exactly where wear is mildest. Weighing one specimen before and after reducing its worn surface in hydrogen separates metal loss from oxygen gain.
Threshold
H2067. Adhesive wear switches from mild to severe at a load where the real contact area reaches the asperities' collective load-bearing capacity, and the switch is sharp because area grows faster than load once asperities flatten together. The threshold is a ratio of load to hardness times area, not a pressure. A load ramp on one pair, with transferred film measured at each step, shows area jumping at the predicted load.
Reversal
H2068. Adding hard particles to a soft matrix normally reduces wear, but below a particle size comparable to the debris the composite wears faster than the matrix alone, because the particles are too small to resist the debris and instead act as loose abrasive. The crossover is at the ratio of particle size to debris size. Two composites with particles either side of that size, run identically, cross.
Identity
H2069. Two-body and three-body abrasion are one mechanism at two degrees of particle freedom, so a material cannot be ranked differently by the two tests; what differs is the particle's freedom to roll, which cuts less and indents more. One pair tested with loose and with fixed grit of the same material and size should rank the same if the identity holds, and the absolute rates should differ by the rolling fraction.
Misattribution
H2070. Erosion resistance is quoted as a material property, but the ranking depends on whether the particle embeds or rebounds, which is set by the particle's hardness relative to the target, so one sand's ranking does not transfer to another. The particle, not the target, decides which mechanism runs. One target eroded by two sands of equal size and shape but different hardness ranks them differently, and the ordering follows the hardness ratio.
Bound
H2071. Erosion rate cannot exceed the rate at which the target's own debris can leave the crater, so a soft target erodes at a rate bounded by debris clearance rather than by impact energy, and raising velocity beyond that point adds nothing. The bound is why erosion curves flatten at high velocity. Two targets of different hardness, eroded over one velocity range, flatten at velocities that scale with hardness.
Artefact mimicking effect
H2072. Cavitation erosion on a vibratory rig depends on the rig's dissolved-gas content and on the specimen's position in the standing wave, so two laboratories testing one material can disagree by a factor while both are correct. The damage pattern is a map of the acoustic field before it is a property of the material. Mapping damage across the horn's face for two materials shows the same map with a different amplitude.
Hidden variable
H2073. Debris shape is used to diagnose mechanism, but shape is set as much by the debris's transport path as by its formation, because fragments round off while still in the contact. One mechanism can yield angular debris from a fast-escaping contact and round debris from a trapping one. Marking a batch of debris and recovering it after a known path tests the rounding.
Count
H2074. The number of wear mechanisms acting in one contact cannot exceed the number of distinct debris populations in its lubricant, since each mechanism leaves at least one population. A diagnosis naming more mechanisms than populations is naming processes that produced no debris. Filtering one oil sample into fractions and clustering their shapes gives the count to compare against.
Threshold
H2075. Oxidative wear disappears above a sliding speed not because oxidation stops but because the time between contacts falls below the time needed to grow a monolayer, so the transition is set by the ratio of contact interval to oxidation time constant. The same ratio explains why slow machines rust and fast ones do not. A speed sweep at fixed load, with surface oxygen measured at each speed, puts the transition at the predicted ratio.
Reversal
H2076. Lubricating an abrasive contact can increase wear, because the lubricant prevents debris from compacting into a protective third-body layer and keeps it free to cut. Dry abrasion of some pairs is therefore slower than oiled abrasion at equal load. The reversal needs debris harder than both surfaces; one pair run dry and oiled, with debris hardness measured, shows it.
Misattribution
H2077. The wear-in of a new machine's first hours is attributed to asperity removal, but much of the metal released in that period is manufacturing residue, loose grit and burrs and lapping compound, and the machine would settle faster if it were cleaned first. The residue's share is measurable by flushing before start-up. Two identical machines, one flushed and one not, with debris counted hourly, separate the two contributions.
Scale argument
H2078. Wear per unit sliding distance scales with contact size only if pressure is held constant; at constant load a larger contact has lower pressure and wears proportionally less, so laboratory pins always wear faster than the parts they represent. Scaling by pressure rather than load is what makes the comparison valid. Two contact sizes at one pressure, and one pair at one load, show which scaling the data obey.
Hidden term
H2079. Every wear test's nominal contact pressure includes the specimen's thermal expansion against its holder, which at high load and speed can add a pressure that is neither measured nor reported, and it scales with holder stiffness and temperature rise. Instrumenting the holder at the specimen rather than at the actuator exposes it. One test run with a compliant and a stiff holder at equal applied load gives two wear rates whose difference is the term.
Threshold
H2080. Abrasive wear of a metal stops depending on its hardness above a hardness ratio near 0.8 between abrasive and metal (estimate), because above that ratio the abrasive's edges blunt against the metal instead of cutting it. The ratio, not the hardness, is the variable. One abrasive against a hardness series spanning the ratio shows the wear rate flattening at the predicted value.
Selection effect
H2081. Published wear maps are drawn from tests that reached steady state, and tests that seized or transferred material are excluded as not being wear, so a map's boundaries mark where the experiments stopped rather than where mechanisms changed. The excluded region is the interesting one. Testing deliberately into that region at the map's boundary conditions shows whether the boundary is a mechanism change or an apparatus limit.
Derived quantity
H2082. Specific wear rate is derived from mass loss, density and a nominal contact area, and when the contact is conformal and the wear is non-uniform the derived number depends on how the worn volume was estimated, by a factor of two or more (estimate). Two estimation methods on one worn specimen, profilometry and mass loss, give that factor and show which one is being quoted.
Extreme conditions: vacuum, cold, heat and pressure
Misattribution
H2083. High friction in vacuum is usually attributed to the absence of oxide, but a clean metal in vacuum has high friction because its surfaces cold-weld over the whole real contact, and the oxide's only role is to reduce that area. A metal with a thin but continuous oxide in vacuum has friction closer to air than to clean vacuum, so thickness and not presence is the variable. One metal at three oxide thicknesses in one chamber shows friction following thickness.
Hidden term
H2084. Cryogenic lubrication by a solid film includes the film's thermal contraction against its substrate, and at liquid-nitrogen temperature that mismatch can matter more than the film's shear strength advantage, pre-stressing an adherent film toward removal. The term is fixed by the two expansion coefficients and is absent from room-temperature screening. Cooling one coated specimen while measuring its curvature gives the stress, and running it at temperature shows whether that stress predicts failure.
Threshold
H2085. A grease stops lubricating above a shear rate at which its thickener network breaks faster than it re-forms, and the transition is sharp because re-formation is cooperative. Past it the grease behaves as its base oil with the thickener as a contaminant. A shear-rate sweep in a couette cell, with film thickness read optically, shows the drop at one rate regardless of temperature.
Reversal
H2086. At high temperature a more volatile lubricant can protect a contact better than a less volatile one, because its vapour condenses in the cooler inlet and forms a film there, so volatility is the delivery mechanism. The reversal needs a temperature gradient along the contact. Two lubricants of different volatility, run with and without a gradient, show the effect only with the gradient.
Bound
H2087. The maximum temperature at which a perfluorinated lubricant works is bounded by its decomposition rate at the contact's hot spot rather than by its bulk decomposition temperature, and the hot-spot rate is set by how long the flash lasts. A lubricant can therefore survive a bulk temperature above its quoted limit if the contact is fast and lightly loaded. One lubricant run above its limit at two speeds survives at the speed where the flash is short.
Identity
H2088. Scuffing and seizure are one event at two stages, so a surface that scuffs will seize if the load is held, and the scuffing load is the seizure load minus the time the scuff needs to spread. The two are separated by time, not by mechanism. Holding a scuffed contact at constant load and recording time to seizure against load should reproduce the load-ramp curve.
Hidden variable
H2089. Friction in a vacuum chamber depends on the chamber's history of previous experiments, because the residual layer that passivates the surfaces is deposited by whatever ran before, and a clean vacuum is not a repeatable environment. The history term decays with pumping time but not to zero. Running one experiment in a fresh chamber, and again after a hydrocarbon-bearing experiment at the same pressure, shows the difference.
Count
H2090. At high contact pressure the number of mechanisms that can act is smaller than at low pressure: adhesion, abrasion and oxidation collapse into one because real area saturates and debris cannot escape. A high-pressure test therefore cannot distinguish mechanisms that a low-pressure test separates. One pair across a pressure range, with debris populations counted at each pressure, shows them merging.
Artefact mimicking effect
H2091. Reports of friction vanishing at very high sliding speed are often the measurement's bandwidth, because the force transducer averages over the cycle and a friction force that alternates sign within the cycle averages near zero without the contact ever being frictionless. The apparent superlubricity is a sampling artefact. Recording force above the excitation frequency rather than below resolves the alternation.
Threshold
H2092. A solid lubricant's friction rises irreversibly above a contact temperature lower than its oxidation onset, because the rise is caused by its own decomposition products accumulating at the interface rather than by bulk oxidation, and those products are stickier than the parent. A temperature ramp with the interface composition measured at each step, and a second ramp in an inert carrier, separates the two temperatures.
Misattribution
H2093. Low friction at cryogenic temperature is credited to the absence of water, but much of it is the rise in hardness and modulus, which reduces real contact area at fixed load; the same materials dried at room temperature do not reach the same friction. The area term is calculable from the modulus change. One pair tested dry at two temperatures, with contact area measured by contact resistance, separates the two.
Inversion of standard practice
H2094. Greases are chosen for hot service by dropping point, but the dropping point measures bulk oil loss and not the thickener's survival in a thin film, and a grease with a lower dropping point can outlast one with a higher because its thickener oxidises more slowly in the film the bearing actually has. The ranking inverts between bulk and film tests. Both greases in a thin-film test at one temperature and in the standard dropping-point test give opposite orders.
Bound
H2095. A lubricant's extreme-pressure capacity is bounded by the rate at which its additive reaches freshly exposed surface, so a faster contact is harder to protect not because the film is weaker but because the additive's diffusion time exceeds the interval between exposures. The bound is a ratio of diffusion time to contact interval. One additive at several speeds, with surface coverage measured after each, shows capacity falling where the ratio passes one.
Hidden term
H2096. High-pressure viscosity measured in a diamond cell includes the cell's own anvil deflection, so the reported viscosity at the highest pressures carries a compliance correction that is assumed rather than measured, and the correction is largest where the data are most quoted. Measuring the cell's deflection with a separate interferometer at each pressure removes it. One fluid measured in two cells of different stiffness shows the discrepancy growing with pressure.
Reversal
H2097. Adding a solid lubricant to a grease normally lowers friction at high temperature, but above the temperature at which the solid's own oxide forms, the additive becomes abrasive and raises both friction and wear. The crossover is at the solid's oxidation onset, which lies below the base oil's. A temperature sweep with and without the additive, in one rig, shows the crossing.
Scale argument
H2098. Under extreme pressure the film thins exponentially with pressure while the asperities flatten only in proportion to pressure over hardness, so the ratio of film to roughness falls faster than either, and extrapolating protection from low-pressure data overestimates it. The two quantities have different pressure dependences and cannot be scaled together. Measuring both at three pressures gives the two slopes and the crossing.
Null claim
H2099. For a contact already limited by its boundary film, raising the base oil's pressure-viscosity coefficient changes nothing, because the film's rheology is not the bulk oil's; the coefficient matters only where a hydrodynamic film survives. Any measured effect in a boundary contact is a measurement of how much hydrodynamic film remained. Two oils a decade apart in coefficient, tested at a load high enough to remove the film, should give equal friction.
Misattribution
H2100. The high friction of a ceramic in dry sliding is assigned to its brittleness, but single crystals of the same ceramics show the same friction, so the term belongs to the surface's inability to relieve shear by plastic flow and not to bulk toughness. The distinction matters because a tough ceramic with a low-shear surface layer can be made to slide like a metal. Two ceramics of equal toughness but different surface shear strength, tested dry, show friction following the surface.
Status
One hundred entries, 2001 to 2100, contiguous. The forms were allowed to fall where the subject put them; a list with equal counts per category would have been a grid wearing labels.
Three that could be settled with apparatus a tribology laboratory already owns: H2002, that a capacitance film reading carries its sensor's overcoat permittivity and so reports a thinner film as the pad heats; H2036, that the period of stick-slip belongs to the rig while only the ratio of static to kinetic friction belongs to the lubricant; and H2044, that the lambda ratio is a mean-field number and a contact at a mean lambda of three can be in boundary contact over a few per cent of its area.
If one of these is worth an afternoon, it is H2036 — two rig stiffnesses, one lubricant, one speed, and the periods should differ while the friction ratio does not. It needs a leaf-spring carriage and a load cell, nothing more.
Incoprea Research · hand-considered hypotheses 2001–2100 · lubrication, wear and tribology · 3 November 2026 · ~/Desktop/Nov3/