Scratch Hardness Measurements Research Articles

Analysis of the Statistical Comparability of the Hardness and Wear of Polymeric Materials for Orthodontic Applications.

Background: Clinical success depends on the contact strength and wear resistance of medical devices made of polymer materials. The scientific goal resulted from the problem of using different methods of surface evaluation of materials used in the production of orthodontic appliances. The purpose of the work was an experimental comparative assessment of indentation hardness and scratch hardness and the sliding wear of four selected polymeric materials used in the manufacture of orthodontic appliances. Methods: Four commercial materials were compared. Shore hardness tests and a scratch test with a Rockwell indenter were performed. A sliding wear test was performed using the ball-on-disc method. Statistical PCA and correlation analyses were performed. Results: The results of scratch hardness measurements using a contact profilometer correlated with the Shore hardness to a greater extent than measurements made using an optical microscope. PCA showed that Shore hardness explains 45% of the total variance in all the results across the materials. Conclusions: The scratch hardness method allows for a more explicit ranking of orthodontic polymeric materials when measurements are made with a profilometer. The ranking of sliding wear resistance should be made separately.

Enhanced tribological properties of aluminium using laser-assisted tungsten coating

Aluminium (Al) suffers from poor wear and scratch resistance and also low surface hardness. To overcome this, the authors fabricated a ~200 µm coating of fine tungsten (W) powder on Al using laser surface alloying (LSA) with laser power and scanning speed varying from 400-800 W and 800-1,200 mm/min respectively. Material characterisation in the form of SEM and XRD, undertaken on the LSA samples, revealed novel microstructures including formation of intermetallic phases Al4W, Al12W that exhibit substantially higher hardness. Scratch hardness measurements further consolidated that surface hardness of coated samples was more than three times as compared to bulk aluminium. Scratch testing at normal loads of 50-150 N, scratch speed of 30 mm/min, demonstrated a stable friction coefficient of 0.6 and higher coating adhesive strength. When applied to automobile components and aerospace products, the micro-coated LSA samples can provide far better alternatives to the current industrial grade of aluminium from the tribology point of view.

The interaction between Lateral size effect and grain size when scratching polycrystalline copper using a Berkovich indenter

It has been reported previously that, for single and polycrystalline copper (fcc), the indentation size effect and the grain size effect (GSE) can be combined in a single length-scale-dependent deformation mechanism linked to a characteristic length-scale calculable by a dislocation-slip-distance approach (X. D. Hou and N. M. Jennett, ‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). Recently, we identified a ‘lateral size effect (LSE)’ in scratch hardness measurements in single crystal copper, where the scratch hardness increases when the scratch size is reduced (A. Kareer, X. D. Hou, N. M. Jennett and S. V. Hainsworth ‘The existence of a lateral size effect and the relationship between indentation and scratch hardness’ Philos. Mag. published online 24 March 2016). This paper investigates the effect of grain size on the scratch hardness of polycrystalline copper with average grain sizes between 1.2 and 44.4 μm, when using a Berkovich indenter. Exactly the same samples are used as in the indentation investigation by Hou et al. (‘Application of a modified slip-distance theory to the indentation of single-crystal and polycrystalline copper to model the interactions between indentation size and structure size effects,’ Acta Mater., Vol. 60, pp. 4128–4135, 2012). It is shown that, not only does the scratch hardness increase with decreasing grain size, but that the GSE and LSE combine in reciprocal length (as found previously for indentation) rather than as a superposition of individual stresses. Applying the same (as indentation) dislocation-slip-distance-based size effect model to scratch hardness yielded a good fit to the experimental data, strongly indicating that it is the slip-distance-like combined length-scale that determines scratch hardness. A comparison of the fit parameters obtained by indentation and scratch on the same samples is made and some distinct differences are identified. The most striking difference is that scratch hardness is over four times more sensitive to grain size than is indentation hardness.

The nanoscratch hardness of a gold–nickel nanocrystalline nanolaminate at high strain rate

The use of triboindentation testing enables a dynamic range of testing that can span more than eight orders of magnitude in strain rate. The mechanical response of a gold–nickel nanocrystalline nanolaminate is investigated as it is subjected to high strain-rate conditions using the nanoscratch test method. The scratch hardness measurements are compared for scratch widths above which the contributions from grain size and layer pair spacing reaches a plateau and representative interface spacing. It is found that the strain-rate sensitivity increases rapidly beyond a 10+3s−1 strain rate indicative of superplastic behavior.

Plasma deposition and characterization of silicon oxide-containing diamond-like carbon films obtained from CH 4:SiH 4:O 2 gas mixtures

In this work silicon oxide-containing diamond-like carbon films were deposited from gaseous mixtures of CH 4 + SiH 4 + O 2 and characterized. The effects of the self-bias voltage and O 2 content on deposition and properties of the films were investigated. At low self-bias voltages (| V B| < 200 V) films present reduced residual stress due to the smaller internal stress of a silicon oxide-rich (SiO x ) network. At high values of self-bias (| V B| > 200 V), on the other hand, the film residual stress is comparable to oxygen-free films although a considerable concentration of oxygen is present. Hardness of the films are somewhat reduced by oxygen incorporation in the whole range of bias voltages investigated. An increasing value of hardness was observed as | V B| is increased. The refractive index of films increased with increasing | V B| due to both the relative reduction of the SiO x content and to the increased sp 2 character of the diamond-like carbon network. Oxygen incorporation results in important structural changes: i. a strong reduction of the density of C–H bonds with a reduction of C–H n sp 3/sp 2 ratio and ii. a dramatic increase of Si–O bonds that substitute Si–H n ones, suggesting the formation of a non-hydrogenated a–SiO x -rich network. Refractive index measurements indicated a strong decrease of the material density as a result of the formation of Si–O–Si bonds. Scratch hardness measurements revealed that the obtained films can be used as highly transparent scratch resistant coatings for polycarbonate substrates. These results show that films with properties equivalent to those obtained with liquid siloxane precursors can be produced when using CH 4 + SiH 4 + O 2 mixtures as gas feeds.

Combined surface characterization and tribological (friction and wear) studies of CVD diamond films

The tribological properties of polycrystalline chemically vapor deposited (CVD) diamond films grown on silicon substrates and containing varying amounts of amorphous carbon impurities were investigated. Films were characterized by secondary electron microscopy (SEM) and atomic force microscopy (AFM) for surface morphology and roughness and by spatially resolved Raman spectroscopy for amorphous carbon (a-C) content. Friction measurements were performed with a Rockwell C hemispherical diamond tip in ultrahigh vacuum (UHV) and in ambient air. In vacuum, the friction coefficient rises monotonically from 0.6 in a region with substantial a-C to 0.85 in a region with pure diamond. Under ambient conditions, the friction coefficient is substantially lower than that in vacuum and deceases slightly (from ∼0.19 to ∼0.16) with the decreasing a-C content. Under both vacuum and ambient conditions, the friction coefficient was observed to be independent of load over the range of 0.1–0.5 N. The friction values are discussed in terms of adhesion between the diamond tip and the film. Qualitative scratch hardness measurements were performed in UHV by measuring the minimum load at which plastic deformation occurs for a single traversal of the tip. Scratch hardness is found to increase with increasing diamond content of the films. The wear mechanism of the pure diamond regions was evaluated by examining wear tracks with SEM and AFM. The wear tracks showed evidence of spalling, buckling, and grain pull-out indicative of a cohesive mode of failure (failure at grain boundaries). A decrease in surface roughness in the wear tracks indicates asperity wear. Adhesive failure at the Si substrate interface or of a phase transformation of the diamond film was not observed in this load regime.

Anisotropy in the scratch hardness of cubic crystals

A smooth parallel-sided groove can be formed in all single crystals, by a harder slider, when the experimental conditions are adjusted to facilitate extensive plastic flow. Under these conditions scratch hardness measurements can be based on the width of the groove and expressed in terms of a mean pressure. The hardness values depend on crystal face and sliding direction and results obtained for a range of metallic and non-metallic crystals indicate that the observed anisotropy is determined by the relevant slip systems. For example, the nature of anisotropy in the scratch hardness of diamond cubic crystals is directly analogous to that of facecentred cubic crystals. While there is generally a significant difference in the hardness of materials in these two categories, plastic flow occurs on {111}&lt;110&gt; slip systems in both types of crystal structure. The deformation and displacement of material beneath, and in front of,a moving slider has been considered on the basis of resolved shear stress criteria. An analysis of the scratch hardness process is presented in which the relative magnitude of the ‘effective resolved shear stress ( r ' e )’, as a function of sliding direction, can be determined. Sliding in crystallographic directions which correspond to the minimum values of r ' e , on specific crystallographic surfaces, are those of maximum hardness and conversely. The nature of anisotropy predicted by this analysis, and based on the relevant slip systems, is shown to be consistent with the results for cubic crystals.

Some observations on scratch and indentation hardness measurements

Measurements have been made in order to compare the scratch hardness with the corresponding indentation hardness of lead, copper and mild steel at room temperature. Both types of measurement were made using the same conical indenter, and the effects of lubrication, normal load and apical angle were investigated. It is shown that the scratch hardness values were most reproducible when measured with conical indenters of apical angles in the range 120°-170°. However, these values were rarely the same as the indentation hardness for a given cone. The hardness characteristics of lead, at room temperature, differed from those of the other metals in that the scratch hardness was generally 1·6 times greater than the indentation hardness, and both kinds of measurement were independent of cone angle. It was concluded that this was due to ideal plasticity in a metal deformed at, or above, half the melting point, and that this type of behaviour would be anticipated in other metals indented at sufficiently high homologous temperatures. Finally, the scratch hardness measurement was used to demonstrate the greater degree of anisotropy observed in single crystals, under these experimental conditions, compared with the identation technique.