The hardness of a number of coated systems has been measured using a variety of experimental techniques ranging from traditional macro-Vickers indentation to ultra-low-load depth-sensing nanoindentation. This has allowed the hardness response to be measured over scales ranging from those less than the coating thickness, where a coating-dominated response is expected, to much more macroscopic scales where system behaviour is dominated by the substrate. The objective has been to construct a mathematical description of the hardness performance of coated systems which well describes the behaviour over this wide range of scales. Previous attempts at such quantitative descriptions have usually involved models focusing on some particular deformation mechanism (e.g. plasticity, elastic response or fracture). In contrast, this paper presents a new approach to analysing hardness data essentially using dimensionless parameters containing descriptors equally applicable to either plasticity- or fracture-dominated behaviour with all scales measured relative to the coating thickness. The model shows an excellent fit to a wide range of experimental data. Furthermore, once the fit has been made, not only can some deductions be made regarding dominant deformation mechanisms, but the model allows predictions of the contact response of other coated systems to be made.

On the hardness of coated systems

The ultra-low load indentation response of ceramic single crystal surfaces (Al2O3, SiC, Si) has been studied with a software-controlled hardness tester (Nanoindenter) operating in the load range 2–60 mN. In all cases, scanning and transmission electron microscopy have been used to characterize the deformation structures associated with these very small-scale hardness impressions. Emphasis has been placed on correlating the deformation behavior observed for particular indentations with irregularities in recorded load-displacement curves. For carefully annealed sapphire, a threshold load (for a given indenter) was observed below which the only surface response was elastic flexure and beyond which dislocation loop nucleation occurred at, or near, the theoretical shear strength to create the indentation. This onset of plasticity was seen as a sudden displacement discontinuity in the load-displacement response. At higher loads, indentations appeared to be accommodated predominantly by dislocation activity, though microcracks were observed to form at contact loads of only tens of milliNewtons. Possibly such cracks are the incipient slip-induced nuclei for the much larger, indentation-induced cracks usually apparent only on the surface at much higher loads and often used for estimating indentation toughness. By contrast, silicon did not show this behavior but exhibited unusually large amounts of depth recovery within indentations, resulting in a characteristic reverse thrust on the indenter during unloading. TEM studies of indentations in silicon revealed less evidence of obvious dislocation activity than in sapphire (particularly at the lowest loads used) but did show residual highly imperfect–and often amorphous–structures within the indentations, consistent with a densification transformation occurring at the very high hydrostatic stresses produced under the indenter. The reverse thrust is caused by the relaxation of densified material during unloading. Thus, it appears that the low-load hardness response of silicon is controlled by a pressure-sensitive phase transformation. Though SiC has been predicted to undergo a densification transformation similar to silicon, its load-displacement behavior was found to be similar to Al2O3 suggesting that, for these contact experiments at least, the critical resolved shear stress for dislocation nucleation is exceeded before the critical hydrostatic pressure for densification is reached. In all cases, residual, plastically formed indentations were measured to be smaller than the fully loaded indentation depths would suggest, confirming that a significant portion of the deformation is elastic surface flexure. However, there is some doubt as to whether silicon displays elastic-only deformation even at very small penetration depths. The use of microstructural studies to complement nanoindentation experiments is shown to be a key route not only to interpreting the recorded load-displacement responses, but also to examining the deformation mechanisms controlling the mechanical behavior of ceramics to surface contacts at these small spatial scales.

The deformation behavior of ceramic crystals subjected to very low load (nano)indentations

The mechanical properties of wear-resistant coatings: I: Modelling of hardness behaviour

Both hardness testing and scratch adhesion testing are now routinely used for coating evaluation, although the fundamental understanding of both processes is somewhat limited. To establish the hardness of thin coatings independent of substrate, the volume-law-of-mixtures hardness model has been applied to a large number of coating/substrate systems, and extensions to an earlier model have been developed including better deforming volume calculations and the ability to obtain best-fit coating parameters. In order to extend the understanding of the scratch test, the statistical nature of coating failure has been investigated by counting the number of individual failures at a given load for a fixed scratch length. The results are analysed in terms of the Weibull distribution and these are related to the surface roughness of the substrate prior to coating. With increased surface roughness both the critical load and Weibull parameters are reduced, reflecting a change in the distribution of interfacial flaws.

New developments in the modelling of the hardness and scratch adhesion of thin films

Indentation techniques have been extensively used for mechanical characterisation of materials. Development of instrumented indentation techniques at low and ultra low load levels has further improved their utility for understanding the mechanical responses of solids at micro and nano scales. The variation of hardness with the load/depth of indentation, known as indentation size effect, has led to difficulties in using hardness as a fundamental or characteristic mechanical property of materials. Detailed discussions are focused on the issue of indentation size effect in brittle and ductile solids. Various theoretical models accounting for the indentation mechanics are highlighted. The results obtained from these techniques with quasicrystals, bulk metallic glasses and nanomaterials are reviewed. The issues related to phase transformation during indentation tests are briefly discussed. The industrial use of the indentation technique has been pointed out. Some of the current issues and directions fo...

Micro- and nanoindentation techniques for mechanical characterisation of materials

Load-variant microhardness tests have been used to investigate the hardness behaviour of ion-implanted (1 1 1) silicon wafers. A variety of ion doses, energies and species, including n-type, p-type and isovalent ions, have been implanted. At the high doses used (1 to 8 x 1017 ions cm2−), all implantations resulted in a surface amorphous layer being formed. The microhardness behaviour has been interpreted in terms of the presence of a surface layer of lower hardness than the substrate. The thickness of this layer has been investigated experimentally using Rutherford backscattering and the results correlated with simple theoretical predictions. Finally, the microhardness behaviour of a soft layer on a harder substrate has been modelled in order to try to predict the hardness variations arising from differing layer thicknesses and different indentation sizes. It is concluded that the amorphous layer produced by implantation appears to show no variation of microhardness with load and has a hardness typically between 400 and 700 Vickers (VHN). Further, the previously reported “critical dose” of ∼ 4 x 1017 ions cm−2 necessary to observe significant surface softening seems to correspond to the regime in which the amorphous layer shows a rapid increase with dose.

Surface softening in silicon by ion implantation

Microhardness indentation testing has been used as a means of introducing controlled localized deformation and fracture in both ion-implanted and unimplanted {1 1 1} silicon and {1 0 ¯1 2} sapphire single crystal surfaces The microstructural alterations due to implantation with N
 2
 +
 and Al+ into silicon and Y+ into sapphire have been characterized using channelled Rutherford backscattering, transmission electron microscopy and electron channelling in the scanning electron microscope It was found that sapphire only became amorphous at doses ⪞3×1016 Y+cm−2 which corresponds to a total energy deposition of ∼3×1023 keV cm−3 (∼44 kJ mm−3) The low-load microhardness (<50 gf) was found to be sensitive to the thickness of the amorphous layer produced by implantation into both silicon and sapphire Compared with the parent crystal, this layer was found both to be softer and to behave in a relatively plastic manner with considerable plastic pile-up occurring around indentations in the higher dose specimens The indentation fracture behaviour was found to be dominated by the presence of implantation-induced compressive stresses The resulting effects were: (a) a decrease in the size of the radial crack traces (henceK
IC is apparently increased when evaluated using indentation fracture mechanics), (b) a decrease in the frequency of occurrence of lateral break-out in silicon and subsurface lateral cracking in sapphire, (c) initiation of lateral cracks further below the surface in both silicon and sapphire Thus in general, it is concluded that hardness and surface plasticity are associated with softer amorphous layers whilst indentation fracture modifications are principally stress related

Changing the surface mechanical properties of silicon and α-Al2O3 by ion implantation

Using both cantilever bending and indentation fracture techniques, the generation of near-surface compressive stresses by ion-implantation into sapphire and glass has been monitored and characterized. In all cases, the surface stresses initially increase with ion dose until a critical dose (dependent on material and ion species/ energy) is reached. Beyond this dose, stress relief has been observed and, for sapphire implanted with both Y+ and Ti+, this has been attributed to the formation and growth of an amorphous layer as monitored by hardness testing. The stress relief has been simply modelled and values estimated for the mechanical strength of the amorphous layer produced. For sapphire, the integrated stress produced over the near-surface volume was found to increase linearly with dose; values of the integrated stress produced by the two different species were similar when considered in terms of energy deposition. Estimates of the contribution to the integrated stress of both the implantation-induced damage and the implanted species profile suggest that the implanted profile makes a minor but significant (20%) contribution. Broadly similar behaviour was observed for soda-lime-silica glass specimens implanted with both C+ and N+. While the origins of the compressive stress produced are probably similar to those in crystalline materials (i.e. defect production and ion-stuffing), no microstructural explanations for both the observed hardening with increasing dose and stress relief have been forthcoming. However, high-dose implantation of N+ into glass leads to blistering and concomitant softening.

An investigation of ion implantation-induced near-surface stresses and their effects in sapphire and glass

X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) have been used to study the charge states and atomic environments of Ti+ ions implanted into a single crystal sapphire substrate Fluorescence techniques have been shown capable of detecting signals from the implanted ions even though the implant density in the ∼100μm thick specimen used is ∼ 002 at% (the implant profile all being within 02μm of the surface) By comparison with TiO2, Ti2O3 and TiO standards, XANES data suggest that the implant species environment is disordered Further, EXAFS Fourier transforms show that the first shell radius of the implant species is between that of the TiO and Ti2O3 standards Again, this indicates considerable structural disorder After annealing the implanted specimen in air at 1150° C for 2 h, a shift towards the shell radius of the TiO2 structure is observed together with a similar change in the XANES appearances Our conclusion is that the initial titanium ion implant into sapphire exists in both the Ti2+ and Ti3+ charge states and is located in a range of sites in the radiation-damaged material Annealing produces a shift in charge state towards Ti4+ Implications for the solid solution hardening effect of the implant in sapphire are discussed

P. J. Burnett

Papers

Surface softening in silicon by ion implantation

Changing the surface mechanical properties of silicon and α-Al2O3 by ion implantation

An investigation of ion implantation-induced near-surface stresses and their effects in sapphire and glass

The charge state of titanium ions implanted into sapphire: an EXAFS investigation