Knoop Microhardness Test Research Articles

Pulse current electrodeposition of nanocrystalline zinc

Pulse electrodeposition exhibits marked advantages over direct current electrodeposition in the control of deposit grain size, surface morphology, and preferred orientation. The effect of pulse peak current density ( J p) on the grain size and surface morphology of zinc deposits with additives (polyacrylamide and thiourea) was studied by scanning electron microscopy and field emission scanning electron microscopy. The preferred orientation of zinc deposits was studied by X-ray diffraction, and microhardness of the deposits was measured by a Knoop microhardness tester. Increasing J P dramatically changed the surface morphology and decreased the grain size. Nanocrystalline zinc (56 nm) was produced at J P=2 A cm −2. At J P equal to 0.4 A cm −2, the preferred orientation of zinc deposits was (1 1 2 ̄ 2) and changed to the prismatic (1 1 2 ̄ 0) orientation at J p equal to 0.8, 1.2, and 1.6 A cm −2. However, increasing the peak current density to 2 A cm −2 altered the prismatic (1 1 2 ̄ 0) to the random (1 0 1 ̄ 1). The microhardness increased to approximately 8 times higher than that of pure polycrystalline zinc (0.29 GPa). Microhardness reached a maximum (2.3 GPa) at 1.6 A cm −2, then decreased to 1.5 GPa at 2 A cm −2. The hardness drop was correlated with the presence of additives and the change in texture from (1 1 2 ̄ 0) to the random (1 0 1 ̄ 1) with increasing J p.

Microhardness anisotropy and the indentation size effect on the basal plane of single crystal hematite

Abstract The Knoop microhardness anisotropy of single crystal hematite, Fe 2 O 3 , was investigated on the basal plane (0001) at indentation test loads from 50 to 1000 g. Microhardness maxima were observed for low indentation test loads when the long axis of the Knoop indenter was parallel to the 11 0>. Microhardness minima occurred for the 1 0> orientations. The microhardness anisotropy at low indentation test loads is associated with slip dominance on the primary slip system of the corundum structure, the {0001} 2 0>. An energy balance was applied to the indentation size effect (ISE) and also to address the microhardness anisotropy. The load independent, orientation independent, Knoop microhardness for the basal plane is 5.35 GPa. The ISE was further investigated by lubrication of the specimen test surface just prior to the Knoop microindentation process. Results from the lubricated microhardness tests are compared with the standard dry Knoop microhardness test results and reveal a significant reduction in the ISE. This indicates that friction between the test specimen and the indenter facets is a major contribution to the ISE.

CVD of hard DLC films in a radio frequency inductively coupled plasma source

Chemical vapor deposition (CVD) of hard diamond-like carbon (DLC) films on silicon (100) substrates from methane was successfully carried out using a radio frequency (r.f.) inductively coupled plasma source (ICPS). Different deposition parameters such as bias voltage, r.f. power, gas flow and pressure were involved. The structures of the films were characterized by Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. The hardness of the DLC films was measured by a Knoop microhardness tester. The surface morphology of the films was characterized by atomic force microscope (AFM) and the surface roughness ( R a) was derived from the AFM data. The films are smooth with roughness less than 1.007 nm. Raman spectra shows that the films have typical diamond-like characteristics with a D line peak at ∼1331 cm −1 and a G line peak at ∼1544 cm −1, and the low intensity ratio of I D/ I G indicate that the DLC films have a high ratio of sp 3 to sp 2 bonding, which is also in accordance with the results of FTIR spectra. The films hardness can reach approximately 42 GPa at a comparatively low substrate bias voltage, which is much greater than that of DLC films deposited in a conventional r.f. capacitively coupled parallel-plate system. It is suggested that the high plasma density and the suitable deposition environment (such as the amount and ratio of hydrocarbon radicals to atomic or ionic hydrogen) obtained in the ICPS are important for depositing hard and high quality DLC films.

Microstructural changes due to laser surface melting of an AISI 304 stainless steel

Several techniques can be used to improve surface properties. These can involve changes on the surface chemical composition (such as alloying and surface welding processes) or on the surface microstructure, such as hardening and melting. In the present work surface melting with a 3kW CO2 cw laser was done to alter surface features of an AISI 304 stainless steel. Microstructure characterisation was done by optical and scanning electron microscopy. Vickers and Knoop microhardness tests evaluated mechanical features after surface melting. Phase transformation during rapid solidification is analysed and discussed.

Structure, mechanical and tribological properties of nitrogen-containing chromium coatings prepared by reactive magnetron sputtering

Abstract Cr1−xNx coatings were deposited by magnetron sputtering at a substrate temperature of 200°C onto AISI 316 stainless-steel substrates immersed in an Ar/N2 plasma. The goal of this investigation was to study the influence of nitrogen content on the structural, mechanical and tribological properties of the Cr1−xNx coatings (with x being in the range of 0–0.4). The coating composition and microstructure were evaluated utilizing glow discharge optical emission spectroscopy and glancing angle X-ray diffraction, whereas the morphology was evaluated by scanning electron microscopy. Knoop microhardness, scratch adhesion, pin-on-disc sliding, ball-on-plate impact and abrasive wheel wear tests were performed to evaluate the mechanical and tribological properties. The presence of Cr, Cr2N, CrN (and mixtures of these phases) has been identified and related to the film composition. For Cr1−xNx coatings with x≤0.16, only the α-Cr phase could be detected. A progression towards a denser microstructure was found with increasing nitrogen content up to x=0.29, associated with an increase in hardness from 700 up to 2400 HK0.025. Cr1−xNx coatings with x=0.10–0.16 showed good adhesion and the best abrasive and pin-on-disk sliding wear resistance, together with less crack development around the indentation area (and thus improved toughness) in impact tests after 50 000 impacts, against both steel and cemented tungsten carbide balls. The hardest coating (Cr0.71N0.29) performed best in terms of reducing the resulting impact indentation volume.

A comparative study of the influence of plasma treatments, PVD coatings and ion implantation on the tribological performance of Ti–6Al–4V

Abstract The mechanical and tribological properties of the titanium alloy Ti–6Al–4V have been characterized by low-frequency reciprocating wear, rotating–bending fatigue and Knoop microhardness tests and surface roughness (Ra) measurements before and after various treatments. These treatments included ion-beam nitrogen implantation, plasma-assisted physical vapour deposition (PAPVD) of TiN and plasma diffusion of nitrogen and carbon. Using a commercial thermionic triode plasma-assisted PVD system, hot-filament-supported glow discharges were produced with different compositions of nitrogen, carbon and hydrogen. The plasma-diffusion treatments all increased the load support provided by the substrate but reduced the bulk fatigue stength considerably. The PAPVD TiN coating increased surface microhardness but provided limited load support. Nitrogen-ion implantation increased low-load surface microhardnesses and improved the fatigue resistance without improving the overall load support provided by the substrate. It was found that the elevated temperatures and increased process times in plasma-diffusion treatments increased the surface roughness significantly, although this roughening effect could be reduced by depositing a thin titanium layer by PAPVD, prior to the plasma treatment.

Electronic structures and nitride formation on ion-implanted AISI 304L austenitic stainless steel

Abstract A N + 2 implantation technique was employed to improve the surface hardness of stainless steel, and the electronic structures and nitride formation of the ion-implanted layer were investigated and compared with those produced using other techniques, including plasma nitriding. AISI 304L austenite stainless steel was irradiated by 80 keV N + 2 with a dosage ranging from 1.0×10 16 to 1.0×10 18 ions cm −2 at room temperature. The formation of various nitrides was confirmed by X-ray diffraction. The quantitative hardness of the samples was measured by using a Knoop microhardness tester. X-ray photoelectron spectroscopy was also carried out to elucidate the chemical states and electronic structures of the ion-implanted layers. The measurements were repeated after post-annealing at 400 °C for 1 h in a high vacuum. Changes in phase, chemical state and electronic structures were observed according to the ion dose and heat treatment.

Wear behaviour of carbon-containing tungsten coatings prepared by reactive magnetron sputtering

Abstract W 1− x C x coatings (with x being in the range of 0.05–0.19) were deposited by reactive magnetron sputtering on to AISI 316 stainless-steel substrates in order to study the influence of the carbon content on the tribological properties of the coating–substrate composite. Knoop microhardness ( H K ), scratch adhesion, pin-on-disc sliding, ball-on-plate impact and abrasive wheel wear tests were performed to evaluate the mechanical and tribological properties of the coatings. X-ray diffraction (XRD) was used for phase- and peak shape parameter analysis, whilst the coating morphology was evaluated by scanning electron microscopy. It was found that all coatings consist mainly of the b.c.c. α W phase. With increasing carbon content from 5 to 19 at.%, an expansion of the α W lattice occurs progressively, associated with an increase in the peak full width at half maximum (FWHM) and a decrease in intensity. The film density and hardness increased with increasing carbon content up to 15 at.%, where hardness values of 4000 H K0.025 were observed. Coatings comprising W 1− x C x with x ≤0.08 showed the best abrasive wear resistance and adhesion with no through-coating failure in the wear track for dry pin-on-disc sliding and no crack development around the indentation areas in impact tests after 50 000 impacts against both steel and cemented tungsten carbide balls.

Tissue response to nano-hydroxyapatite/collagen composite implants in marrow cavity.

The tissue response to a nano-hydroxyapatite/collagen composite implanted in a marrow cavity was investigated by histology and scanning electron microscopy. A Knoop microhardness test was performed to compare the mechanical behavior of the composite and bone. The ultrastructural features of the composite, especially the carbonate-substituted hydroxyapatite with low crystallinity and nanometer size, made it a bone-resembling material. It was bioactive, as well as biodegradable. At the interface of the implant and marrow tissue, solution-mediated dissolution and giant cell mediated resorption led to the degradation of the composite. Interfacial bone formation by osteoblasts was also evident. The process of implant degradation and bone substitution was reminiscent of bone remodeling. The composite can be incorporated into bone metabolism instead of being a permanent implant. For lack of the hierarchical organization similar to that of bone, the composite exhibited an isotropic mechanical behavior. However, the resistance of the composite to localized pressure could reach the lower limit of that of the femur compacta.

Microhardness anisotropy of lamellar bone

The Knoop microhardness test has been utilised to observe in-plane microhardness anisotropy of rat tibiae. The elongated rhombohedral geometry of the Knoop indenter enables the Knoop microhardness ( H K) to be calculated for a given indenter orientation. Two indenter orientations were used the major axis of the indenter was aligned along the length of, and across the mid-sagittal section. The statistical analysis demonstrated that the variation in H K was primarily due to the orientation of the Knoop indenter ( p < 0.001). H K was consistently greater when the indenter was aligned with the major diagonal radial on the mid-sagittal section.

Vertebral growth disturbance in rapidly growing rats during 14 days of spaceflight.

The effect of 14 days of spaceflight on the vertebrae of rapidly growing rats was studied. The hardness of the vertebrae was measured with a Knoop microhardness tester, and bone mineral density was measured from X-ray photographs. Histomorphometric examination was performed with a microcomputer-aided system. No significant difference (P > 0.05) was observed between flight rats and ground controls with regard to mechanical hardness or bone mineral density. However, histological examination revealed irregular thickening of the endosteal surface of cortical bone in the flight rats, whereas it was uniform in the ground controls. The relative area of lamellar bone showed a significant reduction (P < 0.001) in the flight rats. These findings suggest that the structural disturbances were due to retardation of endosteal modeling and remodeling. We conclude that delay of vertebral maturation can occur in rapidly growing rats after even short-term exposure to microgravity.

Effect of annealing on the wear properties of amorphous chrome-plated steel

Chrome plating is widely used for the manufacture of machines and instruments in the automobile and other industries to improve the wear and corrosion resistance of metallic surfaces [1, 2]. Sargent et al. developed the conventional chrome plating method using a plating bath based on chromium trioxide (CrO3) (which forms dichromic acid (H2Cr207) in aqueous solutions) and containing a sulphate catalyst [3]. The conventional Sargent bath produces a crystalline chromium layer on the metal substrate. The microstructure and properties of the chrome layer are significantly influenced by small amounts of chemical agents in the bath [4]. Furuya et al. observed that the electrodeposited chromium was amorphous when formic acid or other certain organics were added to the sulphate-catalysed chromic acid electrolyte [5,6]. The amorphous chrome layer has fewer defects and smoother surfaces, is more resistant to corrosion by hydrochloric acid solutions and has increased hardness at elevated temperature compared to the conventional chrome layer [7]. Although considerable literature exists concerning amorphous chrome deposits, little information is available concerning the wear properties of the chrome layer. In this study, the vacuum annealing effect on the wear resistance of amorphous chrome deposits was examined in terms of hardness and wear resistance. Chromium deposits were obtained on AISI 1025 steel substrate by the method described by Hoshino et al. [8]. The conventional chrome layers were prepared by electroplating chrome in a Sargent bath containing 250 gl 1 of chrome acid and 2.5 g1-1 of sulphuric acid at ~ temperamre of 55°C and 0.6 A cm -2 for 3 h. Amorphous chrome layers were prepared by electroplating chrome in the modified Sargent bath containing up to 20 roll -1 of formic acid at a temperature of 30 °C and 0.6 A cm -2 for lh . The plating cell was made of acrylic with dimensions of 120 × 200 × 200 mm. The anode was lead/5% antimony alloy plate with dimensions 100 × 100 × 5 mm. The anode and the steel cathode were vertically situated at the ends of the cell. The electroplating of each sample was carried out with a potentiostat (Hokuto Denko HAB-151). Fig. 1 shows the schematic diagram of the amorphous chrome plating apparatus. After deposition, the samples were ultrasonically cleaned in acetone and ethanol and then annealed at up to 600°C in 1.3 × 10 -3 Pa vacuum for 1 h. Knoop microhardness testing was

Wear resistant composite coatings deposited by electron enhanced closed field unbalanced magnetron sputtering

There is presently considerable interest in wear resistant coatings produced using closed field unbalanced magnetron sputtering technology. For example, layered films of diamond-like carbon (DLC) with tungsten or titanium additions have been widely reported. The benefit is that the mechanical properties are enhanced (e.g. giving greater toughness); also it is possible to control the stress state and enhance adhesion. Here we report the further development of this concept by the addition of TiN, TiCN and TiC layers in DLC-based composites, utilizing an additional source of electrons in the vicinity of substrate to enhance ionisation of the plasma and increase coating density. Composite coatings of ceramics TiN, TiC x N y , TiC, CrN, TiCrN, TiCrCN, TiCrC, metal doped Ti x% -DLC and their combinations were deposited on 316 stainless steel substrates. The mass flow of reactive gases into the chamber was controlled using plasma optical emission monitoring to achieve the desired coating composition. The morphology of the coatings was investigated and correlated with Knoop microhardness, scratch adhesion, pin-on-disc and wet abrasive wheel tests. Dense T-type structures were found for most of the coatings and a high toughness of Ti 30%-DLC coating with a TiC interlayer was observed. Low friction coefficients of 0.15–0.18 for coatings with Ti x% -DLC layers confirmed their benefit in sliding wear applications, while TiCN coatings were found to be the best in abrasive wear conditions.

Effect of processing parameters on the properties of hydroxylapatite films grown by pulsed laser deposition

Thin films of hydroxylapatite (HA) (Ca 10(PO 4) 6(OH) 2) were created on Ti substrates by KrF laser ablation. The layers were deposited in vacuum, in pure H 2O vapours (pressure, 2 × 10 −3 to 2 × 10 −1 mbar) and in an Ar-H 2O vapour mixture. The influence of the laser energy density E T (3 J cm −2, 13 J cm −2) and substrate temperature T s (500–760 °C) on the film parameters was studied. Two different processes were used for HA target preparation. Films and targets were characterized by Rutherford backscattering (RBS) analysis, particle induced X-ray emission (PIXE), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Knoop microhardness and scratch tests. For lower T s and higher E T, the Ca/P ratio in the films was similar to that in the HA target. Crystalline HA peaks were found preferentially in the films deposited in the presence of Ar-H 2O vapours. Time of flight (TOF) spectra of the plasma plume from the HA target were also analysed.

Al2O3-5 wt% Al composites by ICP sintering of synthesized precursor

Microstructure developments during the milling of Al2O3-5wt% Al composite powder in an attritor and subsequent sintering of the precursor by inductively coupled argon plasma are presented. After 4 h of milling the precursor contained tubular ceramic-metal and uniform ceramic regions. With an increase in the milling period the ceramic-metal regions broke into smaller and almost globular regions, and the smaller regions became dispersed in the ceramic regions. After 8 h of milling the composite powder had a stable microstructure and contained 0.25–0.35 μm clusters. The sintered composite was > 99.7% dense and its microstructure consisted of ceramic-metal regions which were dispersed in the matrix of a ceramic region. The sizes of ceramic grains in ceramic-metal regions and the ceramic regions were 0.3–2.2 and 0.8–1.8 μm, respectively. Many ceramic grains in ceramic-metal regions were separated by 30–100 nm wide metal layers. The microstructure of the ceramic-metal region showed many features of interpenetrating phase composites. The Knoop and Vickers microhardnesses of the composites at 5–10 N loads were 410–450. Under 10 N loads in Knoop and Vickers microhardness tests the crack length was 11±3 and 3 ± 0.5 μm, respectively. The crack propogation mechanisms in the indented areas are discussed.

Nitrogen plasma source ion implantation of aluminum

Nitrogen plasma source ion implantation has been applied to pure aluminum and aluminum alloy 7075 (Al–Cu–Mg–Zn). Nitrogen implantation modified the surface properties through transformation of the surface to aluminum–nitride (AlN). A uniformly thick (∼0.15 μm), polycrystalline (∼30 nm grain size) and continuous AlN layer was formed on both pure aluminum and 7075. Knoop microhardness tests showed an increase in surface hardness at low loads. Pin-on-disk wear tests showed nitrogen implantation decreased the wear rates by a factor of ∼10 for alloy 7075. This study demonstrates significant modification of the tribological properties of an aluminum surface can be accomplished using nitrogen plasma source ion implantation.

The effect of honey on human tooth enamel in vitro observed by electron microscopy and microhardness measurements

Various fruit juices with relatively low pH are known to have erosive effects on human tooth enamel in a reasonably short time. Honey, also with a relatively low pH, could do the same, but scanning electron microscopy showed no erosion of enamel by honey over a period of 30 min, neither did Knoop microhardness tests show any deterioration of enamel structure. The absence of any effect could be only partially attributed to the calcium, phosphorus and fluoride levels in honey.

The properties of a-C:H films deposited by plasma decomposition of C2H2

Diamondlike a-C:H films have been deposited by plasma decomposition of C2 H2. 50 samples were prepared with a systematic variation of the deposition parameters: the substrate bias voltage was between −100 and −1400 V and the C2 H2 gas pressure was between 4×10−4 and 2.6×10−1 mbar. The following properties of the films were measured: the density by Rutherford backscattering, the total concentration of hydrogen by elastic recoil detection, the bonding ratio sp3 /sp2 by infrared spectroscopy, the internal stress by the bending beam method, and the hardness with a Knoop microhardness tester. It has been shown that the hardness and other mechanical properties cannot be correlated to the average carbon coordination number mc. This is because mc is calculated under the assumption of a homogenous single-phase model, which does not seem to be justified. It is demonstrated that the mechanical properties can be explained by the application of a void model.

The friction and hardness of ion-implanted sapphire

A lubricated pin-on-disc technique has been used to determine ion-implantationinduced changes in the kinetic coefficient of friction between {011̄2} single-crystal sapphire surfaces and both iron and titanium pins. Implantations of Ti + (to doses of 3.2 × 10 16 and 7.7 × 10 16 ions cm −2) and Zr + ions (to a dose of 1.7 × 10 16 ions cm −2) were chosen and, for all specimens, 0.5 mm diameter pins sliding at about 60 cm s −1 in light mineral spirit were used. The near-surface hardness changes arising from implantation were also evaluated using low load (25 gf and 50 gf) Knoop microhardness testing. For all specimens it was found that ion implantation resulted in increases in both the coefficient of friction and the near-surface microhardness. These results are discussed in terms of both the microstructure and the chemistry of the surface.

Deposition of boron nitride thin films by ion beam assisted deposition

The deposition of boron nitride as well as other refractory metal nitride thin films (TiN, HfN etc.) normally requires elevated temperatures to avoid problems with film adhesion, residual stress and chemical stability. A system will be described which has produced adherent, stable thin films. The method uses the low temperature technique of ion beam assisted deposition in which the metal is evaporated onto the substrate and bombarded with energetic nitrogen ions. Coatings as thin as 120 nm have been produced using a 5 mA ion beam of 120 keV N 2 +. Boron nitride coatings with a Knoop microhardness in excess of 2500 kg/mm 2 have been produced. Non-mass-analyzed ion beams have been simulated during the depositions and correlated to film properties. The films are characterized using Knoop microhardness tests, ball-on-disc wear tests and coefficient of friction tests. Film properties are studied by optical and scanning electron micrography, spectral reflectivity and index of refraction measurements.