J. von Stebut

Progress towards standardisation of ball cratering

The ball cratering (micro-abrasion) test is becoming popular as a method for the abrasion testing of surface engineered materials. It possesses many advantages over more conventional abrasion tests including the ability to test small volumes of material and thin coatings, its perceived ease of use and the low cost of the test equipment, and its versatility. Standards are now being drafted both in the USA and Europe on ball cratering, but further work is needed before this work can be completed on the effect of test variables and the choice of measurement method on the results that are achieved. This paper discusses these aspects of the test and its relevance to industrial wear problems, and describes the results of a preliminary interlaboratory exercise that has been conducted in the UK to determine the effectiveness of the test method. The paper will also give an outline of an EU funded project that has the aim of validating the test and which brings together a consortium of 10 research partners from four European countries.

Tribological properties of quasicrystalline coatings

Coatings of three different compositions (Al 65 Cu 20 Fe 15 , Al 64 Cu 18 Fe 8 Cr 8 , and Al 67 Cu 9 Fe 10.5 Cr 10.5 Si 3 ) were realized by various thermal deposition techniques. They were studied in the as-deposited state and after annealing. In view of potential applications, these quasicrystalline coatings were examined from the point of view of tribology: friction and wear. Some basic components of friction such as roughness, plowing, and adhesion have been studied in scratch testing. The friction resistance of the coating is strongly dependent on its inherent porosity, hardness, and thickness. The damage of the coatings is essentially brittle though some ductile behavior is observed. Static indentation hardness is in the range 500–600 HV 0.03 (5–6 GPa), whereas the scratch hardness varies from 1.4 to 2.4 GPa depending on the percentage of porosity. Friction coefficients (measured at constant load of 20 N) were found to be typically 0.07 and 0.20 for diamond (tip radius R = 0.79 mm) and AISI 52100 (radius R = 0.79 mm) indenters, respectively.

The nano-scratch tester (NST) as a new tool for assessing the strength of ultrathin hard coatings and the mar resistance of polymer films

Abstract At present, functional coatings are used in increasingly demanding applications that require good adhesion and specific resistance to damage. This has prompted a rising interest in improving the mechanical properties of polymer coatings, especially their scratch and mar resistance, as well as ultra-thin hard coatings as used for protective overcoats in the magnetic storage industry. The nano-scratch tester (NST) is a new instrument overcoming the limitations of both the classical stylus scratch test (normal force range) and the atomic force microscope technique (short sliding distances), allowing scratch lengths of up to 10 mm. Tangential force and penetration depth are simultaneously measured during the scratching process, both in a multipass contact fatigue mode. For high resolution inspection of the deformed or damaged area, a scanning force microscope (SFM) is integrated into the system. Experimental results are presented for two very different types of material; polymeric clear-coat samples which give a range of mar resistance and DLC ultra-thin films used as protective overcoats for hard disks. The results indicate very good reproducibility and confirm the application of this new instrument for the accurate characterization of elasticity, hardness, adhesion and mechanical integrity in coated systems where the film thickness is less than 1 μm.

Size-induced enhanced mechanical properties of nanocomposite copper/niobium wires: nanoindentation study

The effect of microstructure dimension δ on plasticity of high strength multifilamentary nanocomposite copper/niobium wires was studied by nanoindentation. Two structures were tested: a Cu matrix containing Nb filaments and a Cu matrix containing Nb tubes filled with Cu. For δ>10μm, no size effect on the composite hardness is observed. In the 1–10 μm range, a strong increase in hardness indicates a change in plasticity mechanism, attributed to the classical Hall–Petch grain size strengthening. In the nanometre range, the hardness of the nanocomposite Cu/Nb regions exceeds that of nanocrystalline Cu or Nb, reaching 5.8 GPa for the finest conductors. The observed size effect on the plasticity of Cu/Nb nanostructures added to the dislocation barrier role of Cu/Nb interfaces confirms previous analyses based on the occurrence of a single dislocation regime at nanometre scale associated with impenetrable Cu/Nb interfaces.

Structure/mechanical properties relationship of titanium–oxygen coatings reactively sputter-deposited

Abstract Ti–O coatings are DC sputter-deposited on HSS substrates from titanium targets in various Ar–O 2 reactive mixtures leading to different structural states, from pure titanium to TiO 2 films. These coatings are structurally (X-ray diffraction measurements) and mechanically (micro hardness and intrinsic stress measurements) characterised as a function of the inlet oxygen flow rate. Special attention is paid to the case of FCC TiO x (0.9 x x layers composition and internal stress are found to depend on deposition parameters like oxygen flow rate, substrate bias and its position with respect to magnetron axis. The intrinsic stress of TiO films is shown to control the cracking damage under high-load Vickers indentation as well as in single pass and multi-pass scratch testing operation. Finally, coating delamination is likely to occur by interface cracking triggered by ‘coalescence along the interface’ from two neighbouring through-thickness crack tips.

Major damage mechanisms during scratch and wear testing of hard coatings on hard substrates

Abstract Scratch testing and wear testing of hard, brittle TiN coatings on high-speed steel substrates has been done by means of a modified commercial tester. Three-dimensional surface mapping is shown to be a powerful tool for coating failure analysis. Scratch testing damage probably starts by tensile-type crack nucleation behind the trailing edge of the diamond stylus. In a multi-pass scratch-wear operation surface damage is dominated by delamination along the coating/ substrate interface. Fracture mechanics is suggested to model such brittle failure. Sufficient coating ductility to accommodate subsurface plastic strain is shown to be a necessary mechanical property with respect to wear resistance.

Mechanical properties of plasma-deposited silicon-based inhomogeneous optical coatings

Abstract Amorphous hydrogenated silicon nitride, oxide and oxynitride films are deposited by plasma-enhanced chemical vapour deposition (PECVD) using a dual-mode microwave/radio-frequency (MW/RF) plasma system. Optical filters are prepared by varying the film composition either abruptly (discrete, homogeneous multilayer structure) or continuously (graded-index, inhomogeneous structure). The coatings are characterised both optically, by spectrophotometry and spectroscopic ellipsometry, and mechanically, with depth-sensing indentation and low-load microscratch testing. A comparison is made between the properties of the homogeneous multilayer and the inhomogeneous multilayer structure with the corresponding optical performance. This multiple technique approach for characterisation was proved to be efficient for analysis of the optical and mechanical behaviour of coatings, and it provides a possibility for optimising the deposition process. It is demonstrated that the graded system exhibits a higher mechanical strength and a better toughness than the discrete structure.

Novel nanoindentation method for characterising multiphase materials

Abstract We present the nano-hardness tester (NHT), a new depth-sensing instrument with a differential capacitive sensor providing nanometer depth resolution and allowing partial elimination of the frame compliance. With micron lateral positioning of the sample and topographical measurement of the residual indent, using scanning force microscopy, it is possible to gain a better understanding of the indentation method and its effects on the material being tested. Furthermore, parallel imaging allows the material response to be directly investigated. Experimental results are presented for a selection of aged austeno-ferritic stainless steels; these confirm the ability of the NHT to become a common test instrument in quality assurance and process development of multiphase materials.

Acoustic emission monitoring of single cracking events and associated damage mechanism analysis in indentation and scratch testing

Abstract A detailed study on optimisation of single cracking events in indentation and scratch testing of brittle coatings is presented. These results are illustrated for nitrogen-doped stainless-steel coatings deposited by reactive magnetron sputtering. The best results are obtained with a resonant piezo transducer on the indenter shaft and high-speed electronics allowing for the detection of single cracks. Waveforms are seen to contain only information on the crack energy and the instant of nucleation. The latter is exploited for precise synchronisation of all test-specific parameters with the real image of the cracking failure. With this approach, it is possible to carry out an advanced failure mechanism analysis of the test-induced surface damage. This is practised for indentation, where cracking is seen to occur in the elastically strained region outside the real contact area (size of the plastic indent). The shape of the plastic indent is seen to be unmodified elastic recovery on unloading. In scratch testing, a clear correlation between high-energy AE pulses and cracking failure is achieved, and the first attempts to analyse the prevailing failure mechanisms and experimental ‘dead times’ for separation of consecutive events are presented.

Microprobe-type measurement of Young's modulus and Poisson coefficient by means of depth sensing indentation and acoustic microscopy

Abstract Nano-indentation and hyper frequency scanning acoustic microscopy (SAM) are mechanical microprobe techniques allowing measurement of the near-surface elastic response. In the first case, the composite modulus E /(1−ν 2 ) can be assessed using the slope of the elastic unloading plot. The second technique allows measurement of Rayleigh wave velocity, which depends on Youngs modulus, Poisson ratio and density, to be measured through material acoustic signature. In both cases only one of the elastic constants may be determined using a hypothesis on the other one (usually on the Poisson coefficient). In the present paper we show that by joining both techniques for the same specimens, E and ν can be deconvoluted. Effectiveness of this deconvolution method is first tested on two well-known materials. Results on fused silica ( E =72.9 GPa; ν=0.17) as well as bulk aluminium ( E =69.6 GPa; ν=0.33) are in very good agreement with values commonly found in the literature. The method is finally applied on AlCuFe alloys presenting similar compositions but with different phases (icosahedral, tetragonal and cubic). This study shows the importance of crystallographic structure on elastic behaviour, especially on the Poisson ratio which ranges from ν=0.39 for the icosahedral phase down to ν=0.12 for the tetragonal phase.