Development of PVD coating processes informed by plasma diagnostics.

Physical vapour deposition technologies have been on the fast track of development for the last two decades due to their ability to meet demands for special materials and performance tools. The ever increasing complexity of the required coating microstructure and chemical composition can be achieved only by the development of PVD technology and in particular plasma sources for vapour generation that can provide the necessary tools.This thesis describes plasma diagnostic studies of plasma discharges, developments of plasma sources and deposition of CrN coatings. Initially the project investigated vacuum arc plasma discharges used in the Hauzer HTC1000/ABS industrially sized coater. The attention was concentrated to the plasma pretreatment by low energy (1200 - 3600 eV) Cr ion implantation into substrates, which contributed to an enhanced adhesion of subsequently deposited TiAIN coatings. Optical emission spectroscopy (OES), electrostatic probes, and time-of-flight (TOF) spectroscopy were used to study the interactions of the arc plasma with the gas atmosphere in the chamber. It was shown that increasing the pressure of Ar gas had a strong effect on the composition of the generated metal ion flux as the density of highly charged metal species reduced significantly to the benefit of gas ionisation. The mechanisms behind these observations are discussed and supported by further experiments. Based on the plasma diagnostic results, a novel two-stage pretreatment method was developed which allowed an enhanced adhesion due to faster sputter cleaning of the substrate surface and more efficient metal ion incorporation in the substrate material.In the final stages of the project a novel high power pulsed magnetron sputtering (HIPIMS) process utilising peak power densities of 3000 Wcm[-2] was investigated. OES studies showed the first evidence of doubly charged Cr and Ti ions generated by the HIPIMS discharge. Peak plasma densities of 10[13] cm[-3] were measured and, in the case of Cr, metal ions were found to constitute 30% of the total deposition flux to substrates. The influence of power on the plasma density, plasma composition and time evolution of the plasma was studied in detail using OES and electrostatic probes. The conditions for glow-to-arc transition were investigated. CrN coatings (thickness 2 mum) were deposited for the first time using HIPIMS of Cr in a nitrogen atmosphere. The microstructure observed in transmission electron microscopy cross sections was highly dense and droplet free and contributed to an excellent corrosion and wear resistance superior to 20 mum thick electroplated hard Cr, and CrN coatings deposited by arc and unbalanced magnetron sputtering. The HIPIMS discharge was used also for pretreatment of substrates with metal ions analogous to the one performed previously with arc discharge. High adhesion was achieved as indicated by the scratch test critical load value Lc = 85 N.Finally, at an intermediate stage of the PhD project, an alternative source providing metal ionisation was studied. It was based on a radio frequency (RF) powered coil that was inductively coupled to a magnetron sputtering discharge. Energy resolved mass spectroscopy and OES in a laboratory-sized version of the plasma source revealed elevated metal ion densities and high ion energies of the order of 60 eV. This source was upscaled, installed, and tested successfully in the industrially sized Hauzer coater. The ion-to-neutral ratio at the substrate position could be increased 5-fold for a similar increase in RF power

Mitigation of oxygen presence in AlN (100) & (002) growth using RF magnetron sputtering

Aluminium nitride (AlN) nucleation layer (NL) is a useful nitride semiconductor for the growth of Gallium Nitride (GaN) on silicon. Major issues related to the fabrication of AlN films are on its crystallographic orientations and high processing temperatures. In order to fabricate AlN NL at low temperatures, radio frequency (RF) magnetron sputtering is used in this study. The aims are to deposit homogeneous and highly crystalline AlN NL with (100) and (002) preferential orientations at low processing temperatures. In this study, the homogeneous deposited AlN with highly crystalline along the (100) and (002) preferential orientations were successfully controlled using sputtering base pressure without any external heating. This will mitigate the presence of oxygen that presents during the deposition process. Moreover, the sputtering parameters such as target-to-substrate distance, working pressure, deposition times, and RF power were optimized. Thus, these highly crystalline AlN along the (100) and (002) preferential orientations were used in the Metal-Insulator-Semiconductor (MIS) structure to investigate their leakage currents. For the (002) preferential orientations of AlN, X-ray diffraction (XRD) showed that the full width at half maximum (FWHM) was smaller with low dislocation density and microstrain. Cross-sectional images from the field-emission scanning electron microscope (FESEM) showed that its exhibited grass-like columnar structures, showing it had a well-aligned structure. Meanwhile, its electrical properties showed that the (002)-oriented AlN NL had high electrical resistivity due to low dielectric permittivity, high capacitance, and low dielectric relaxation. The (100) and (002) preferential orientations leakage currents values in the MIS structure were 4.1 x 10-7 A and 2.0 x 10-8 A, respectively, indicating that (002) preferential orientations AlN NL displayed the lowest leakage current significantly. The effects of oxygen impurity in the layers played a crucial role in the growth of the (002) preferential orientations and acted as defects in the MIS structure, which increased the leakage current

Growth defects in CrN/NbN coatings deposited by HIPIMS/UBM techniques

In recent years, high power impulse magnetron sputtering (HIPIMS) has caught the attention of users due to its ability to produce dense coatings. However, microscopic studies have shown that HIPIMS deposited coatings can suffer from some surface imperfections even though the overall number of defects can be significantly lower compared to, for example, arc deposited coatings of similar thickness. Defects can degrade the coating performance thus any kind of defect is undesirable. To better understand the nature of these imperfections and the science of their formation, three sets of chromium nitride/niobium nitride (CrN/NbN) coatings were deposited using HIPIMS technique combined with unbalanced magnetron sputtering (UBM) by varying the deposition parameters, i.e. deposition time (t = 15 to 120 min), bias voltage (Ub = - 40 to - 150 V) and chamber pressure (P = 0.2 to 1 Pa). For each set, one parameter was varied and other two were kept constant. All these experiments were carried out with chamber conditions close to those found in industrial environment. The study revealed that the generated defects were similar for all the coatings and with the increase in deposition time/bias voltage/chamber pressure the surface area covered by optically visible defects (surface defect density) was increased. These defects were categorised as flakes related defects (nodular, open void and cone-like defects) and defects associated with substrate pits (pinhole defects). Depending on their types, the defects influenced the corrosion and tribological properties of the coatings. As the origins of most defects were flakes (generated from the chamber components), an additional study was conducted to understand the influence of chamber cleanliness on defect generation. As expected, surface defect density of the coating produced in a comparatively clean chamber was reduced noticeably (from 3.18 % to 1.37 % after cleaning). Coatings with lower surface defects performed significantly well during corrosion and tribological tests. However, the comparison between pure UBM and combined HIPIMS/UBM deposited coatings suggested that along with the defects, coating structure also had a major role in corrosion, wear and friction mechanisms. Even for deposition conditions where HIPIMS coatings showed higher surface defects, owing to their microstructures, their corrosion resistance and tribological behaviour were superior to the UBM deposited coatings

Invasive and non-invasive diagnostics of High Power Impulse Magnetron Sputtering (HiPIMS) discharges

HiPIMS discharges operated with a titanium and an aluminium-doped zinc target sputtered with the working gas argon were investigated by means of optical 2d-imaging in combination with Abel-inversion. By using optical bandpass filters, the spatial and temporal evolution of the plasma-induced emission of argon atoms and ions and metal atoms and ions were studied. The discharge ignition was found to be accompanied by an intensity maximum observed remote from the target, followed by an asymmetric intensity profile. During the stage of high discharge current, the intensity distribution indicates strong rarefaction of the working gas, efficient ionisation of the sputtered particles above the target and a wider sputter distribution. The off-time is characterised by an initial drop of the intensity by 4 - 6 orders of magnitude and a transition from electron-impact excitation to excitation by electron-electron-ion recombination. Decay constants in the order of 1 ms and the spatial distribution of the emission suggest the loss of electrons and ions due to ambipolar diffusion across the magnetic field. Plasma potential measurements by means of emissive probe revealed strongly negative space potentials of up to -300 V and electric fields in the order of 10000 V/m during discharge ignition, caused by strong charge separation due to the extended sheath. The plasma potential increases to a stable level of more than -25 V during the second half of the discharge pulse, while the electric field is largely reduced to maximal 1500 V/m. It was found that the space potential is consistently 5 V lower when the substrate is kept floating compared to a grounded substrate, which can be explained by the reduced electron loss rate to the substrate due to the potential barrier. This is confirmed by spatially resolved Langmuir probe measurements, showing a density maximum of 1019 m-3 in the confined plasma zone, and an increased density in the vicinity of the floating substrate of 1.3 × �1018 m-3 compared to 0.8 × �1018 m-3 for a grounded substrate. The electron temperature was found to be spatially uniform ranging from 1 eV to 3 eV. A quasi-continuous transport model for sputtered particles confirmed the high degree of ionisation of the sputtered particles of about 90 % and revealed a return probability for titanium ions to the target of 80 % for Ti+ and 96 % for Ti2+. Varying the force caused by a modified two-stream instability [1], showed an increasing sideway defletion of ions also increasing the kinetic energy observed for these particles. The spatial distribution of the relative density confirmed efficient ionisation of sputtered particles in the dense plasma zone adjacent to the target. Average azimuthal velocities of the drifting ion fluid of 3�000 m/s for Ti+ and 7�000 m/s for Ti2+ were obtained

The fabrication and characterisation of metal oxide thin films for microelectronic and optical applications

Metal oxide thin films are the subject of considerable research due to their novel optical and electrical properties which make them suitable for use in many applications. These applications include gas sensors, solar cell arrays, anti dazzling rear view mirrors, smart windows, gate dielectrics, semiconductors and many more. The possibilities for new metal oxide based materials is forever growing with the introduction of novel deposition methods which allow precise control of the deposition parameters and the ability to dope in order to tailor properties. The conditions used for the deposition of these coatings has an influence on the microstructure which in turn plays an important role in determining physical properties, such as the optical transmission and electrical conductivity. In addition, for many metal oxide materials the structure-property relationship is not well understood. The aim of this thesis was two-fold. Firstly, to deposit some metal oxide thin film coatings using several physical vapour deposition techniques and characterise their microstructure and physical properties. Secondly, to make a comparison between films deposited using different techniques to determine how the properties of a film depend on the conditions under which they are formed. To achieve these aims, tungsten oxide, zinc oxide doped with aluminium (also known as aluminium zinc oxide - AZO) and hafnium oxide coatings were deposited and the microstructure and physical properties were investigated. For tungsten oxide deposition, films were deposited using pulsed magnetron sputtering and pulsed cathodic arc. It was found that films deposited using magnetron sputtering were highly disordered. In contrast, those deposited with cathodic arc were a highly ordered and exhibited a tetragonal phase, usually only observed at high temperatures. In the case of AZO, films were deposited using pulsed cathodic arc, pulsed magnetron sputtering and high power impulse magnetron sputtering (HIPIMS). The pulsed cathodic arc films were found to have both good transmittance in the visible region and the best resistivity of all of the samples. It was found that magnetron and HIPIMS produced films that exhibited non-uniform properties across their surface due to in situ oxygen bombardment during deposition. This undesirable effect was eliminated by incorporating a novel magnetic filter into the deposition setup which acted to improve both the crystallinity and the resistivity. This thesis also performed the first comprehensive investigation of hafnium oxide films prepared using a filtered cathodic vacuum arc. Samples deposited at high substrate biases were found to damage readily which made them electrically leaky. Samples deposited at room temperature were found to have a disordered microstructure and had a good electrical breakdown. At elevated temperatures the crystallinity of the samples increased, resulting in a microstructure containing large monoclinic crystals. However, it was also found that the electrical breakdown worsened at elevated temperatures, in agreement with other researchers who also find that hafnium oxide films with disordered microstructure have the best electrical characteristics. Ab initio calculations of the near edge structure found in x-ray and electron loss edges were found to be a powerful way of distinguishing between the phases of tungsten oxide

Nanoscale multilayer Me-graphite coatings grown by combined steered cathodic arc/unbalanced magnetron sputtering.

Low friction, nanoscale multilayer carbon/chromium (C/Cr) coatings were successfully deposited by the combined steered cathodic arc/unbalanced magnetron sputtering technique (also known as Arc Bond Sputtering or ABS) using a Hauzer HTC 1000-4 PVD coater. The work described in this thesis has been directed towards understanding the effect of ion irradiation on the composition, microstructure, and functional properties of C/Cr coatings. This has been achieved by varying the bias voltage, U[B], over a wide range between -65 V and -550 V. C/Cr coatings were deposited in three major steps: (i) Cr+ ion etching using a steered cathodic arc discharge at a substrate bias voltage of -1200 V, (ii) deposition of a 0.25 mum thick CrN base layer by reactive unbalanced magnetron sputtering to enhance the adhesion, and (iii) deposition of C/Cr coatings by unbalanced magnetron sputtering from three graphite targets and one chromium target at 260°C. The coatings were deposited at different bias voltages (U[B]) from -65 V to -550 V in a non-reactive Ar atmosphere.C/Cr coatings exhibit excellent adhesion (critical load, L[C] > 70 N), with hardness ranging from 6.8 to 25.1 GPa depending on the bias voltage. The friction coefficient of C/Cr coatings was found to reduce from 0.22 to 0.16 when the bias voltage was increased from U[B] = -65 to -95 V. The relevance of C/Cr coatings for actual practical applications was demonstrated using dry high-speed milling trials on automotive aluminium alloy (Al-Si8Cu3Fe). The results showed that C/Cr coated cemented carbide ball-nose end mills prepared at U[B] = -95 V (70 at.% C, 30 at.% Cr) enhance the tool performance and the tool life compared to the uncoated tools by a factor of two, suggesting the potential for use in dry high-speed machining of "sticky" alloys such as aluminum. Different film morphologies were observed in the investigated bias voltage range between U[B] = -65 and -550 V using XTEM. With increasing bias voltage from U[B] = -65 to -95 V, the structure changed from columnar, with carbon accumulated at the column boundaries, to a dense structure which comprised randomly distributed onionlike carbon clusters. A novel nanostructure was observed within this bias voltage range, in which the basic nano-lamellae obtained as a result of substrate rotation in front of the C and Cr targets were modified by an ion-irradiation induced nanocolumnar structure. Further increases in the bias voltage to U[B] = -350 V and U[B] = -450 V led to segregation and self-organisation of the carbon atoms induced by the high energy ion bombardment and, finally, to the formation of a new type of self-organised multilayer structure. A coating growth model accounting for the influence of ion bombardment on the growing C/Cr film was introduced to explain the phase separation and formation of the selforganised layered nanostructure.A novel experimental set-up for the investigation of tribocorrosion was built based on a modification of the conventional Scanning Reference Electrode Technique (SRET). The device comprises a ball on rotating cylinder contact configuration combined with a SRET electrochemical device. This combination may contribute significantly to the understanding of wear-corrosion synergism

Nanostructured Multilayer Ceramic Coatings for Wood-Cutting Tools

Lo sviluppo di questo dottorato di ricerca è stato supportato e finanziato dal progetto europeo Interreg ITA-SLO NANOREGION. La missione del progetto NANOREGION consiste nel costituire un consorzio tra università ed enti di ricerca locali atti a fornire una piattaforma scientifica di supporto alle imprese del territorio comprensivo delle regioni dell'Italia nord-orientale e la Slovenia. In tali regioni, l'impatto economico-sociale dell'industria del legno è rilevante. È noto che nella manifattura di materiali legnosi la necessità di una frequente sostituzione degli utensili da lavorazione, necessaria a garantire gli standard di qualità del prodotto finito, ha un forte impatto sull'economia del processo di lavorazione. Gli utensili da taglio sono generalmente protetti con rivestimenti a film sottile costituiti da materiali duri e refrattari per aumentarne la durabilità, riducendo la frequenza di sostituzione e quindi aumentando la produttività delle aziende. Tuttavia, a causa della natura del legno come materiale da lavorazione e dei rigorosi requisiti tecnici specifici della lavorazione del legno, il rapporto costi/benefici del processo di rivestimento degli utensili si è finora dimostrato non sufficiente a giustificare i costi addizionali legati al processo di rivestimento. Questo risulta in una minore penetrazione nel mercato degli utensili rivestiti rispetto a quanto accade invece nell'industria metalmeccanica. Un'eccezione, seppure in misura limitata, è rappresentata dai rivestimenti a base di cromo, come il nitruro di cromo (CrN), che forniscono la massima protezione contro l'usura dovuta a processi di corrosione. Questi, infatti, sono riconosciuti come una delle principali cause di usura nella lavorazione del legno, favorendo la successiva usura abrasiva dei fragili prodotti d’ossidazione operata da inclusioni di minerali duri presenti all'interno di materiali a base legnosa, soprattutto prodotti secondari già sottoposti a lavorazioni precedenti. Tuttavia, a causa della relativamente bassa durezza, il CrN fornisce di per sé una scarsa resistenza all'usura abrasiva. Una possibile soluzione consiste nel sostituire rivestimenti monostrato di CrN con rivestimenti multistrato. Ricerche e applicazioni pratiche approfondite hanno dimostrato che la progettazione di rivestimenti multistrato offre numerosi vantaggi rispetto ai rivestimenti a strato singolo, soprattutto se lo spessore degli strati costitutivi è ridotto al regime nanometrico. Tuttavia, la ricerca per lo sviluppo di rivestimenti multistrato mirati a rispondere ai severi requisiti dell'industria del legno è molto limitata. Pertanto, lo scopo di questo progetto di ricerca era di effettuare preliminari studi sulle proprietà protettive dei sistemi di rivestimento PVD multistrato a base di CrN accoppiato con nitruri di indurimento. L’abbinamento di proprietà anticorrosive e migliori caratteristiche meccaniche potrebbero rendere tali sistemi di rivestimento dei potenziali candidati per il rivestimento di utensili da taglio del legno. I nitruri di tungsteno e molibdeno (WN e MoN, rispettivamente) sono stati scelti in quanto già applicati per la realizzazione di sistemi multistrato con altri nitruri, e anche per fornire uno studio comparato su due sistemi che raramente vengono studiati in parallelo nelle stesse condizioni sperimentali. L'obiettivo fondamentale dell'aggiunta di questi materiali aggiuntivi è fornire al CrN una migliore durezza meccanica mantenendo o migliorando l'elevata resistenza alla corrosione, e la letteratura pregressa suggerisce ottime potenzialità di questi materiali come nitruri indurenti.The research activity of this Ph.D. thesis is grounded is founded by the European Interreg ITA-SLO NANOREGION project. The mission of NANOREGION was to provide a scientific support to local enterprises in the region comprehending North-eastern Italy and Slovenia. In such regions the impact of the wood-industry on local economics is relevant. It is well known that the impact of tools replacement has a strong impact on the economics of the wood-working process to maintain the manufacturing standards required to obtain high quality products. Cutting tools are usually coated with hard, refractory thin film coatings to increase their service life and hence increase the manufacture productivity. Nonetheless, owing to the nature of the wood as a workpiece and to the strict technical required specific of wood-machining, the cost-to-benefits ratio of tools coating has so far proved not enough to allow the same market penetration of coated tools that is instead recorded in the metal-working industry. An exception to some extent is represented by Chromium-based coatings such as CrN, which provide the highest protection against corrosive wear, which in turn is recognized as of the main causes of wear in wood-machining, as it favors the subsequent abrasive wear of the brittle oxidation products mediated by hard mineral inclusions within the wood-based materials. Nonetheless, CrN alone is known to provide poor resistance to abrasive wear, due to its relatively low hardness. A large amount of research has demonstrated that the design of multilayer coatings offers several advantages over single-layer coatings, especially if the thickness of the constituent layers is reduced to the nanometric regime. Nonetheless, the research investigating the development of multilayer coatings specifically targeting the strict requirements of the wood industry is very limited. Hence, the purpose of this research process was to investigate preliminary protective properties of multilayer PVD coating systems based on oxidation-protective CrN coupled with hardening nitrides as potential candidate coatings for wood-cutting tools. WN and MoN were chosen due to reported literature on successful coupling of such coatings with other nitrides such as TiN or ZrN and CrN, and because of the limited number of scientific and systematic comparative investigations on these systems. The fundamental aim of the addition of these additional materials is providing CrN with improved mechanical hardness while retaining or improving high corrosion resistance

Technologies of Coatings and Surface Hardening for Tool Industry

The innovative coating and surface hardening technologies developed in recent years allow us to obtain practically any physical–mechanical or crystal–chemical complex properties of the metalworking tool surface layer. Today, the scientific approach to improving the operational characteristics of the tool surface layers produced from traditional tools industrial materials is a highly costly and long-lasting process. Different technological techniques, such as coatings (physical and chemical methods), surface hardening and alloying (chemical-thermal treatment, implantation), a combination of the listed methods, and other solutions are used for this. This edition aims to provide a review of the current state of the research and developments in the field of coatings and surface hardening technologies for cutting and die tools that can ensure a substantial increase of the work resource and reliability of the tool, an increase in productivity of machining, accuracy, and quality of the machined products, reduction in the material capacity of the production, and other important manufacturing factors. In doing so, the main emphasis should be on the results of the engineering works that have had a prosperous approbation in a laboratory or real manufacturing conditions

Physical Vapor Deposited Biomedical Coatings

The book outlines a series of developments made in the manufacturing of bio-functional layers via Physical Vapour-Deposited (PVD) technologies for application in various areas of healthcare. The scrutinized PVD methods include Radio-Frequency Magnetron Sputtering (RF-MS), Cathodic Arc Evaporation, Pulsed Electron Deposition and its variants, Pulsed Laser Deposition, and Matrix-Assisted Pulsed Laser Evaporation (MAPLE) due to their great promise, especially in dentistry and orthopaedics. These methods have yet to gain traction for industrialization and large-scale application in biomedicine. A new generation of implant coatings can be made available by the (1) incorporation of organic moieties (e.g., proteins, peptides, enzymes) into thin films using innovative methods such as combinatorial MAPLE, (2) direct coupling of therapeutic agents with bioactive glasses or ceramics within substituted or composite layers via RF-MS, or (3) innovation in high-energy deposition methods, such as arc evaporation or pulsed electron beam methods