Microindentation Hardness Measurements Research Articles

Surface Hardening of Ceramic Oxide and Metal Alloy Single Crystals by Ultrasonic Cavitation

ABSTRACTCavitation effects have long been considered to be undesirable phenomena that resulted in the damage and failure of metallic components. In the present work, we establish that, in fact, controlled applications of ultrasonic cavitation phenomena can be used to enhance the surface properties of both ceramics and metals. Polished (100) surfaces of single-crystal MgO and single crystals of 70%Fe-15%Ni-15%Cr (stainless-steel) were subjected to ultrasonically induced cavitation by exposure to 20 kHz excitations (at ∼ 100 W/cm2) in isopropanol. Knoop micro-indentation hardness measurements on untreated and ultrasonically treated areas of the surfaces revealed a hardening that increased with the duration of the ultrasonic treatment up to a saturation level. Relative increases in the surface hardness of up to 30% in the case of MgO and ∼250% in the case of Fe-Ni-Cr were obtained. It was found that the rate of hardening was not uniform over the surface but was more rapid on those portions of the surface that were directly under the edge of the ultrasonically vibrating horn tip.

Distinguishing thin film and substrate contributions in microindentation hardness measurements

Microhardness measurements were carried out on silicon, boron, boron nitride, and nickel films deposited on polished stainless steel substrates. A titanium diboride film on a sapphire subsubstrate was also examined. It is shown that a log–log plot of indentation size versus applied load (the Meyer plot) reveals the point at which the substrate begins to influence the hardness measurements. The ratio of indentation depth to film thickness at the critical point varied from 0.06 to 0.4 depending on both film and substrate hardness. The microhardness numbers differ from those calculated from data in the composite (film plus substrate) region by proposed formulas.

Mechanical property changes in ion- irradiated metals: Part I. Ni-Cu alloys

The effect of radiation-induced dislocation loops on hardness in ion-irradiated Ni-Cu alloys has been studied using a recently developed mechanical properties microprobe (MPM). Well-annealed Ni-10 at. pct Cu and Ni-50 at. pct Cu were irradiated with 14-MeV Ni ions to doses of 20 to 100 displacements per atom (dpa) peak damage (5 to 25 dpa at 1 μm) at 0.45Tm (485 °C and 425 °C, respectively). Ultra-low load microindentation hardness measurements and transmission electron microscopy (TEM) were done using cross-sectional techniques. These methods allow for direct hardness measurements, which have been compared to the unirradiated material, of only the small irradiation zone (<3-μm deep). Irradiation induced a high density of dislocation loops, with the size and density of the loops dependent on composition and independent of irradiation conditions. This high dislocation loop density caused a large increase in hardness. A reasonable correlation was found between measured hardness changes and calculated changes based on dislocation loop sizes and densities.

The influence of implanted transition metal ions on the adhesive wear of iron

In a previous study, a model based on the ratio of adhesive to cohesive bonding energy was derived to predict relative wear rates of a variety of metal couples. Surface energy, or reactivity, is a function of the total atomic bonding energy of the atoms comprising the surface and may be varied independently through solid solution alloying. Therefore changes in surface composition should be reflected in the adhesive character and macroscopic wear behavior. Using a reciprocating Be-Cu right cylinder on high purity iron flat as a wear couple, the adhesive character was modified by ion implantation of cobalt, manganese, nickel, niobium, titanium, tungsten and yttrium into the iron flat at a dosage of 1 × 10 17 ions cm −2. The implantation energies were tailored so that the ion concentration vs. depth of penetration below the surface profiles were approximately equivalent. Implantation-induced residual stresses were determined prior to wear testing. The wear-induced surface and subsurface deformation was characterized with microindentation hardness measurements and scanning electron microscopy. A model is presented to explain the enhanced wear resistance of samples implanted with heavier, higher cohesive energy ions compared with those implanted with lighter, lower cohesive energy ions.

Influence of long-range order on deformation induced by sliding wear

Samples of ordered and disordered Cu 3Au were worn against a reciprocating sapphire cylinder in an argon environment. While the coefficients of friction were virtually identical for the two structures, the disordered alloy exhibited a wear rate nearly double that of the ordered alloy. The wear-induced strain hardening was characterized with microindentation hardness measurements made in, and adjacent to, the wear track. X-ray line broadening analysis and scanning electron microscopy were combined with residual stress measurements to characterize the surface and subsurface deformation resulting from sliding contact and related to the degree of longrange order. Transmission electron microscopy was used to examine the deformation resulting from the passage of a simulated asperity across the surface.

Hardness and Depth‐Dependent Microstructure of Ion‐Irradiated Magnesium Aluminate Spinel

Stoichiometric polycrystalline magnesium aluminate spinel has been irradiated at 25° and 650°C with 2.4‐MeV Mg+ ions to a fluence of 1.4 × 1021 ions/m2 (∼35 dpa (displacement per atom) peak damage level). Microindentation hardness measurements and transmission electron microscopy combined with energy dispersive X‐ray spectroscopy measurements were used to characterize the irradiation effects. The room‐temperature hardness of spinel increased by about 5% after irradiation at both temperatures. There was no evidence for amorphization at either irradiation temperatures. Interstitial‐type dislocation loops lying on {110} and {111} planes with Burgers vectors along 〈110〉 were observed at intermediate depths (∼1 μm) along the ion range. The 〈110〉{111} loops are presumably formed from 〈111〉{111} loops as a result of a shear on the anion sublattice. Only about 0.05% of the calculated displacements were visible in the form of loops, which indicates that spinel has a high resistance to aggregate damage accumulation. The peak damage region contained a high density of dislocation tangles. There was no evidence for the formation of voids or vacancy loops. The specimen irradiated at 650°C was denuded of dislocation loops within ∼1 μm of the surface.

Adhesion and micromechanical properties of metal surfaces

This paper is part of a long-standing attack on problems of friction and adhesion. It falls into two parts. The first discusses the general mechanism of adhesion and then describes an experimental study of the adhesion and deformation of a model microasperity. This consists of the contact formed between a fine pointed stylus of tungsten and a single crystal of a softer metal (nickel). The experiments are carried out in ultrahigh vacuum and the surfaces are characterized in situ using Auger spectroscopy. The loads range from 0.5 to 1000 μN and contact during loading and unloading is monitored using electrical resistance measurements. With clean surfaces the results suggest that surface forces alone are able to initiate plastic deformation. Oxide monolayers reduce the adhesion but the general behaviour is little changed. In contrast, oxide layers 5 nm thick greatly modify the adhesion and deformation behaviour. The second part describes microhardness measurements carried out over approximately the same load range. A Berkovitch triangular pyramid is used as the indenter. Some of the indents are studied in the electron microscope but the major hardness determinations are based on depth measurements of the indents formed. These measurements are extremely sensitive and may be conveniently monitored so that the plastic indentation process during loading and the elastic relaxation which occurs on unloading may be recorded. Both the adhesion measurements and the microindentation hardness measurements show that the plastic yield stress of very small volumes may be three or four times larger than the bulk values. The results also show that detailed contact models may be of limited value when dealing with contacts involving plastic deformation.

Tribomechanical Properties of Ion Implanted Metals

ABSTRACTA review of tribomechanical studies supported by surface analysis finds ion implantation capable of increasing the sliding wear resistance of ion implanted metals in two ways. First, it can reduce friction by modifying the surface composition (e.g. Ti+ into steel) or by promoting the growth of low friction oxide layers (e.g. N into Ti). Second, it can modify the subsurface composition and structure to resist fracture and debris formation. These modifications harden the surface, change its work-hardening behavior and/or increase residual stresses. Microindentation hardness measurements indicate that many but not all of the wear resistant surfaces are hardened by implantation; thus, surface hardness is a contributing but not necessarily a controlling factor in wear resistance. These mechanisms of wear reduction and the chemical and microstructural modifications responsible for them are discussed. Evidence for wear reduction through the migration of N during wear is critically reviewed. It is concluded that the principal benefit of ion implantation is to prevent or delay the formation of wear particles, thereby changing the wear mode during run-in and permitting metals to reach load-carrying capacities up to their elastic limits.

Fletcherite, Cu(Ni,Co) 2 S 4 , a new thiospinel from the Viburnum Trend (New Lead Belt), Missouri

Fletcherite, Cu(Ni, Co) 2 S 4 , a new thiospinel mineral, occurs in copper-rich pods in the Fletcher mine of the Viburnum Trend (New Lead Belt) of southeast Missouri. Fletcherite occurs as euhedral to subhedral crystals 5 to 200 mu m in diameter, disseminated in pods of bornite, chalcopyrite, and digenite. Other associated minerals include covellite, pyrite, vaesite, galena, and tetrahedrite. Reflected light examination reveals that fletcherite takes a good polish, is creamy white, has a moderate reflectivity, is isotropic, and has a polishing hardness between chalcopyrite and pyrite. Micro-indentation hardness measurements yield VHN values of 368 to 464 kg/mm 2 . Measured reflectivities, R% (at specific wavelengths) are: 36.4 (420 nm), 40.9 (460 nm), 42.5 (500 nm), 43.4 (546 nm), 45.3 (589 nm), 43.8 (600 nm).The compositional range observed among 39 individual analysis is Cu (sub 1.13) Ni (sub 1.04) Co (sub 0.84) Fe (sub 0.06) S (sub 4.00) to Cu (sub 0.67) Ni (sub 2.11) Co (sub 0.61) Fe (sub 0.01) S (sub 4.00) . The Space Group is Fd3m, Z = 8, a o = 9.520Aa. The name is for the Fletcher mine of the St. Joe Minerals Corporation in which the type material was found.

Relation of Microindentation Hardness to Glass Composition

Microindentation hardness measurements on Na2O‐CaO‐SiO2 and Na2O‐Al2O3‐SiO2 glasses showed that Knoop hardness numbers are linearly related to oxide compositions expressed in mole fraction. Evaluations of the data indicate that these linear relations are unique for each of the primary crystallization phase fields of the glasses studied.

Influence of Defect Structures on Precipitation in Aluminium-Copper Alloys

AbstractThe effect of simultaneous deformation on the ageing kinetics of θ′ in aluminium-copper alloys has been followed by means of micro-indentation hardness measurements and transmission electron microscopy. The influence of deformation on precipitation was found to depend critically upon the initially formed defect structure. In dilute alloys (< 2 wt.-%Cu), deformation at fast and slow strain rates generated a subgrain structure, which resulted in slightly accelerated ageing kinetics. In the more concentrated alloys (2 and 4 wt.-% Cu) slow strain rates gave rise to extensive-arrays of helical dislocations within grains. These helices retarded ageing by absorbing vacancies and solute, thereby minimizing subsequent diffusion through the lattice. As the strain rate was increased, interactions of moving dislocations with helices led to elimination of the helices and the development of a subgrain structure. The substructure then approximated to that of the dilute aluminium-copper alloys, causing similar ac...

Industrial Diamond Substitutes: I, Physical and X‐Ray Study of Hafnium Carbide

A study of hard materials in an effort to find possible substitutes for industrial diamonds led to research on hafnium carbide. This compound was prepared from a mixture of hafnium dioxide and lampblack in a carbon resistance furnace by solid‐state reaction or from a melt. Some factors affecting the combined carbon content of the re‐ action products were qualitatively evaluated. A hafnium carbide prepared from a melt at a temperature slightly above 2800°C. with no holding time had a combined carbon content within 98% of the theoretical value. A curve was obtained by plotting combined carbon against cubic unit‐ cell dimension (a0) for the hafnium carbide‐ “hafnium monoxide” solid solution series. Extrapolation gave 4.641 ± 0.001 a.u. for the cell edge of hafnium carbide of theoretical composition; a0 was observed as high as 4.640 a.u. Density values within 99% of theoretical were obtained. Knoop microindentation hardness measurements with both dry and oil‐immersion objectives indicated a hardness in the silicon carbide range. Owing to its high cost and relatively low hard‐ ness, hafnium carbide is presently not considered to be a promising substitute for industrial diamonds.