ICMCTF2000 Session B1-3: Hybrid PVD Techniques and Coatings

Since the early fundamental research on superlattice structured hard coatings in the late 1980^,s, rapid progress has been achieved to produce nanoscale compositionally modulated multilayer structures. It has been shown that the periodicity of the multilayers is strongly controlled by the substrate rotation and the actual deposition rate. Appropriate multi-target geometry and controlled target poisoning by optimised pumping conditions lead to deposition conditions similar in its economy to the deposition of typical monolithically grown binary hard coatings. The combined steered cathodic arc/unbalanced magnetron technology guaranties sufficient adhesion (Lc>50N) of the usually highly stressed coatings as well as smooth surfaces due to UBM deposition (Ra < 0.04 micron).

The paper describes the properties of coatings dedicated to high temperature performance: TiAlN/CrN (period 3.8nm), to tribological applications: TiAlYN/VN (period 3.0 nm) and combined wear and corrosion resistance CrN/NbN (period 3.4nm). All the coatings investigated were found to crystallise into B1 NaCl f. c. c. structure and exhibit {111} or {200} preferred orientation for TiAlN/CrN, TiAlYN/VN and CrN/NbN superlattice coatings respectively. The residual stress has been found to be compressive in the range of 4.0 GPa for TiAlYN/VN and between 1.5 GPa to 7.5 GPa for CrN/NbN depending on the stoichiometry and the bias voltage during coating deposition. Corresponding to the high stress values the plastic hardness of the coatings was measured to be 40 GPa for TiAlN/CrN, 60 GPa for TiAlYN/VN system and between 40 GPa and 70 GPa for CrN/NbN depending on the bias voltage. Oxidation temperatures exceeding 900 ⁰C have been typical for TiAlN/CrN. The TiAlYN/VN showed superior tribological properties with coefficient of friction f=0.4 and low sliding wear of 1.26 x 10^-17 m²N^-1 after 1.1 million cycles against Al₂O₃ ball in pin-on-disc test. CrN/NbN exhibit two orders of magnitude lower passive current densities than electroplated hard Cr and pitting potential of 450 mV during polarisation in acetate buffer solution. When Nb⁺ ion etch was used CrN/NbN superlattice coating deposited on 304L stainless steel showed high pitting potentials in the range of 750 mV to 1000 mV in the same corrosive media.

Plasma based processes such as sputtering , ion-implantation and plasma assisted chemical vapour deposition (CVD) have become popular in recent years for the deposition of thin film coatings in number of industrial applications including components for microelectronics, automotive machinery and aerospace. The generation of a plasma above a substrate surface provides extra energy to the gas mixture to enable a coating to be deposited at much lower substrate temperature thus allowing a wider range of potential applications to be developed. Many of the base materials used in the automotive and aerospace industry cannot tolerate high temperature used in techniques such as CVD. At low temperatures, the adhesion and structural integrity of the coatings produced by plasma based methods are often unsatisfactory. Therefore, an improved or new technology is required which can overcome these problems. An attractive alternative to high temperature CVD is to use a new technology called high energy intensified plasma assisted processing (HEPAP).

This work describes a qualitative/ quantitative model for the mechanism involved in a triode glow discharge employed to grow aluminium nitride films. A series of systematic studies have been carried out at varying degrees of cathode voltage, cathode current density, pressure range and ion collector voltage values over a fixed period of time. All films developed have been characterised using Scanning Electron Microscopy, Electron Dispersion Spectroscopy, X- ray Diffraction and micro hardness tests. The conclusions of this project have formed a basic theoretical model that can be applied to produce films of different materials. The use of an RF source to negatively biased substrate has enhanced the growth process. The optimum conditions established have been explained with the use of a theoretical model.

Chemical vapour deposition (CVD) has become a key technology for the synthesis of inorganic films for use in integrated circuitry, solar energy conversion and tribology. Silicon compounds has emerged as one of the most important materials and are widely used in semiconductors. Conventional methods for the preparation of thin silicon nitride films exhibit less acceptable defects in terms of dangling bonds and inefficient doping.

In the past various surface coating techniques such as vapour deposition, sputtering, ion implantation, ion nitriding and dual ion-implantation/plasma nitriding treatments have been applied to produce silicon nitride. Since ion implantation is a line-of-sight process and therefore irregularly shaped pieces will not be uniformly nitrided. Another factor is the depth limitation. Ion nitriding is a suitable alternative although there is a difficulty of removing oxide barrier that forms during the process. This process also operates in high pressures (1-10 torr) and at relatively low voltages thus producing a low energy plasma. As a result of low energy plasma a lengthy sputter cleaning process is required.

As an initial attempt, silicon discs were successfully nitrided using High Energy Plasma Assisted Processing (HEPAP). This process produces ions and neutrals of higher energies than the conventional methods. This also operates at low pressures and eliminates most of the deficiencies of the conventional techniques. The RF powered negative biased silicon wafers were coated with silicon nitride in a triode system where the thermoionic emitter and the ion collector were operated independently. The coatings obtained were characterised using Scanning Electron Microscope and Energy Dispersion Spectrometry. The hardness was tested using Knoop hardness tester and also the surface profilometry was determined. The low temperature coatings have established this High Energy Plasma Assisted Processing as an alternative method in plasma techniques.