The application of the soft impression technique to evaluate flow stress, creep and frictional deformation of polycrystalline diamond and cubic boron nitride

Metal shaping processes are clear examples of engineering applications where a hard material is worn by a softer one - i.e. the tool and workpiece respectively. The soft impressor technique, introduced by Brookes and Green (1973), has proved valuable in measuring the relevant mechanical properties of tool materials - e.g. the measurement of the flow stress of diamond single crystals at temperatures up to 1500°C (Brookes, 1992). In this work, the technique has been extended further in order to form a basis for the comparison and evaluation of ultra-hard materials. Three main aspects of the performance of these tool materials have been covered: the effect of temperature on flow stress; cumulative deformation under point loading conditions; wear due to repeated traversals (fatigue).In the first part, the technique has been extended to determine the flow stress of polycrystalline diamond and cubic boron nitride as a function of temperature and a mathematical model has been proposed to estimate the flow stress in isotropic polycrystalline materials. This model was first analysed by Love (1928) and was used as the basis on which to identify the threshold pressure above which dislocation movement is initiated in diamond single crystals (Brookes et al (1990)). The applicability of this model for polycrystals was verified by correlating the yield strength of polycrystalline copper, measured in tension, with the determination of minimum contact mean pressure to plastically deform the same material. According to the model, the first evidence of plastic deformation should be observed at the contact periphery and this has been verified in this work. Consequently, using this approach, the effect of temperature on the flow stress of polycrystalline diamond (Syndax) and polycrystalline cubic boron nitride (Amborite) has been established and it is shown that there are three distinct regimes. In regime I, the deformation is brittle and fracture occurs above a given mean pressure; in regime II dislocations are mobile and the flow stress decreases sharply as the temperature rises; and in regime III the flow stress is independent of the temperature.# In the earlier work, the brittle - ductile transition temperature (BOT) has been identified as that temperature where regime I ends and II begins. Above the BDT, time dependent plastic flow has been observed, in all of these materials, leading to a measurable increase in the size of the impression. However, this particular type of cumulative deformation, described as impression creep, is shown to be different to conventional creep as measured under uniaxial stress conditions.Finally, the room temperature friction and deformation of various polycrystalline diamond based specimens, Le. aggregates with a binder phase of cobalt (Syndite) or silicon carbide (Syndax), a polycrystalline coating produced by a chemical vapour deposition processes (CVDite) and cubic boron nitride (Amborite) were studied when softer metallic and ceramic sliders were used. As a result of increasing the number of traversals, significant wear of the CVDite diamond coating by softer metallic sliders (aluminium and mild steel) was observed. This could be attributed to the high level of residual stresses in the diamond layer which is thought to be due to the difference in the thermal expansion coefficients of the coatings and their substrates. Burton et al (1995) reported a strain of 0.3% on the surface of the diamond coating and hence the tensile stress on the upper side of the coating will be equivalent to about 3.0 GPa. This value is comparable to the theoretical cleavage strength of diamond. It is suggested an additional tensile stress, due to the sliding friction, could have caused cleavage of individual diamond crystals. The resultant wear debris then becoming embedded in the metallic slider. These embedded diamond particles in the tip of the slider could be responsible for the increased friction and wear

The Influence of Temperature on the Mechanical Behaviour of Ceramics in Ti1-xAlxN System

High temperature nanoindentation has been employed to study the influence of temperature on mechanical properties of ceramics in Ti1-xAlxN system. The temperature had a strong effect on the hardness of the bulk single crystal TiN (TiNbulk), leading to a drop from 21.4 ± 0.4 GPa at 22 °C to 13.7 ± 0.4 GPa at 350 °C. Plastic deformation of TiNbulk mainly occurred along the (110) crystallographic planes, over the temperature range 22 °C – 350 °C, suggesting that the drop in hardness with the temperature was attributed to a change of ease of plastic slip. Hardness of magnetron sputtered Ti0.66Al0.34N coatings dropped with temperature in a similar manner to TiNbulk, although from a higher starting value. Approaching the compositional atomic ratio Al : Ti = 1:1, maximum hardness was reached and the thermal stability of hardness improved. It was proposed that the high hardness stability with the temperature of magnetron sputtered coatings is linked to the presence of the two crystallographic domains, fcc TiN and stabilized fcc AlN. The small difference between the lattice parameters of these phases seems to be accommodated by distortion of lattices, in order to form coherent boundaries between domains. It has been shown that stabilised fcc AlN formed during deposition and remained in the structure after annealing at 600 °C. Aluminium addition increased the activation energy for slip from 0.75 eV for TiNbulk to 1.26 eV for Ti0.48Al0.52N coatings. These values indicate the deformation took place by lattice controlled dislocation glide mechanism. The hardness of industrial cathodic arc Ti0.4Al0.6N films decreased with temperature in a similar way to TiNbulk, although at higher values. High deposition energies promoted fcc AlN alongside fcc TiN, and a change of growing direction from (200) to (111). The differences in structure and mechanical properties attributed to different physical vapour depositions are presented.Open Acces

Evaluation of Thermal Management Solutions for Power Semiconductors

This thesis addresses the thermal management and reliability concerns of power semiconductor devices from die to system level packaging design. Power electronics is a continuously evolving and challenging field. Systems continue to evolve, demanding increasing functionality within decreasing packaging volume, whilst maintaining stringent reliability requirements. This typically means higher volumetric and gravimetric power densities, which require effective thermal management solutions, to maintain junction temperatures of devices below their maximum and to limit thermally induced stress for the packaging medium. A comparison of thermal performance of Silicon and Silicon Carbide power semiconductor devices mounted on Polycrystalline Diamond (PCD) and Aluminum Nitride (AlN) substrates has been carried out. Detailed simulation and experimental analysis techniques show a 74% reduction in junction to case thermal resistance (Rth (j-c)) can be achieved by replacing the AlN insulating layer with PCD substrate. In order to improve the thermal performance and power density of polycrystalline diamond substrates further at the system level, direct liquid cooling technique of Direct Bonded Copper (DBC) substrates were performed. An empirical model was used to analyse the geometric and thermo-hydraulic dependency upon thermal performance of circular micro pins fins. Results show that micro pin fin direct cooling of DBC can reduce the number of thermal layers in the system, and reduce the thermal resistance by 59% when compared to conventional DBC cooling without a base plate. Thermal management and packaging solutions for the wide band gap semiconductors, such as GaN, is also described in detail. Comparisons of face up and flip chip thermal performance of GaN on Sapphire, Silicon and 6H-SiC substrates in a T0-220 package system is presented. Detailed thermal simulation results analysed using ANSYS® show that a flip chip mounted GaN on sapphire substrate can reduce junction to case thermal resistance by 28% when compared against the face up mounted technique

A tribological and mechanical study of ion assisted diamond-like carbon thin films

Amorphous hydrogenated carbon (a-C:H), diamond & diamond-like (DLC) thin films are some of the many terms used when referring to the generic group of coatings based on hard carbon. They are an emerging technological area within the surface coating discipline and are being increasingly used to improve the efficiency of a wide range of engineering components. In addition, the unique and extreme characteristics of these films result in unequalled material properties, such that in many cases a wide range of new and superior performance devices have only recently begun to be realised.This study focuses on hydrogenated & non-hydrogenated diamond-like thin films deposited by various plasma based, hybrid and beam deposition techniques. The wear resistant and low friction properties of these films are of great importance in many of the potential application areas and has attracted particular interest in recent years. Therefore the major thrust of this research has been on the tribological aspect, particularly in relation to other advanced ceramic coatings, and to highlight the applicability of endurance wear tests used to evaluate diamond-like films. The main findings have been:-a} That carbon can be deposited by several techniques in a hard amorphous phase, the properties of which depend heavily upon the conditions, substrate choice and method of deposition. For a particular technique, material properties can be made to be repeatable by a good understanding of deposition process control.b} The use of plasma based hybrid PVD and beam methods have resulted in a considerably improved structural performance of the films over those produced by the direct evaporation of graphite. The introduction of a hydrocarbon gas into the plasma at the synthesis stage has also been shown to provide further improvements in the physical properties which has correspondingly led to an enhancement in the tribological behaviour. The levels of hydrogen, whether in an unbonded or bonded form, included in the film after deposition has been demonstrated to affect the mechanical and optical properties of the considerably.c) The wear resistant and frictional performance of these coatings has been shown to be variable, depending upon the method and conditions of deposition as well as test parameters such as humidity, surface roughness, film structure, adhesive strength and oxide/impurity formation. In some cases the tribological performance was found to be excellent. The presence of the diamond-like carbon coating has been shown to be beneficial in reducing wear between contacting bodies experiencing relative movement by encouraging the formation of a carbon transfer layer on the surface of the counterface material which acts as a zone of low shear and provides a physical barrier to tribo-chemical interactions. Under certain conditions, such tribo-chemical interactions can occur readily at the interface, facilitating the formation of strong interfacial bonding and increased wear.d) The inclusion of metallic elements into the carbon matrix has been shown to enhance the wear resistant properties of the film to only a small extent, although at the expense of a deterioration in the friction coefficient. The most beneficial effect of doping carbon films with metal species has been the improved resistance to thermal degradation.e) Thin intermediate layers of titanium nitride have also been shown to produce a remarkable improvement in both wear resistance and frictional performance of the diamond-like carbon films to an extent which appears to be related to the level of stoichiometry of the titanium nitride. The main mechanism behind this increased performance appears to be due largely to an enhancement in adhesive strength at the diamond-like carbon/titanium nitride junction, with an increase in load support being provided as a secondary benefit.f) A critical assessment of the available techniques and methodology available for testing hard carbon films has been made and in some cases methods have been found to be either entirely inappropriate or appropriate only when suitable precautionary measures have been taken. These difficulties largely stem from the exacting demands of thin, hard layers of diamond-like carbon due to its unique and extreme mechanical, electrical and optical properties

Thick, Adherent Diamond Films on AlN with Low Thermal Barrier Resistance.

The growth of >100-μm-thick diamond layers adherent on aluminum nitride with low thermal boundary resistance between diamond and AlN is presented in this work. The thermal barrier resistance was found to be in the range of 16 m2·K/GW, which is a large improvement on the current state-of-the-art. While thick films failed to adhere on untreated AlN films, AlN films treated with hydrogen/nitrogen plasma retained the thick diamond layers. Clear differences in ζ-potential measurement confirm surface modification due to hydrogen/nitrogen plasma treatment. An increase in non-diamond carbon in the initial layers of diamond grown on pretreated AlN is seen by Raman spectroscopy. The presence of non-diamond carbon has minimal effect on the thermal barrier resistance. The surfaces studied with X-ray photoelectron spectroscopy revealed a clear distinction between pretreated and untreated samples. The surface aluminum goes from a nitrogen-rich environment to an oxygen-rich environment after pretreatment. A clean interface between diamond and AlN is seen by cross-sectional transmission electron microscopy

AlNxOy thin films deposited by DC reactive magnetron sputtering

AlNxOy thin films were produced by DC reactive magnetron sputtering, using an atmosphere of argon and a reactive gas mixture of nitrogen and oxygen, for a wide range of partial pressures of reactive gas. During the deposition, the discharge current was kept constant and the discharge parameters were monitored. The deposition rate, chemical composition, morphology, structure and electrical resistivity of the coatings are strongly correlated with discharge parameters. Varying the reactive gas mixture partial pressure, the film properties change gradually from metallic-like films, for low reactive gas partial pressures, to stoichiometric amorphous Al2O3 insulator films, at high pressures. For intermediate reactive gas pressures, sub-stoichiometric AlN x O y films were obtained, with the electrical resistivity of the films increasing with the non metallic/metallic ratio.FEDER - Program COMPETE - Programa Operacional Factores de CompetitividadeFundação para a Ciência e a Tecnologia (FCT) - Project PTDC/CTM/69362/2006; PhD grant Nº SFRH/BD/47118/200