Deposition of duplex Al 2O 3/DLC coatings on Al alloys for tribological applications using a combined micro-arc oxidation and plasma-immersion ion implantation technique

Abstract

Micro-arc discharge oxidation (MDO) is a cost-effective plasma electrolytic process which can be used to improve the wear resistance of aluminium alloy parts by creating a hard thick alumina coating on the component surface. However, for sliding wear applications, such alumina coatings often exhibit relatively high friction coefficients against many counterface materials. Therefore, a duplex treatment, combining a load-supporting MDO alumina layer with a low friction diamond-like carbon (DLC) coating, produced by a modified plasma-immersion ion implantation (PI 3) process, has been investigated. PI 3 provides a flexible method of implanting ions into complex-shaped parts using a low temperature, low voltage plasma onto which high voltage pulses are superimposed. It can also be used to enhance the adhesion and growth characteristics of films formed under plasma conditions. In this work, a weakly-ionized, hot-filament supported low-voltage argon–acetylene plasma (with C 2H 2/Ar ratios from 1.0 to 0.15) was used, in combination with a low-frequency dc pulse voltage PI 3 system (in this case 100 μs, 5 kV pulses at 850 Hz) to deposit a low friction DLC top layer onto MDO-treated Al alloy coupons. Microhardness measurements and pin-on-disc sliding wear tests were performed to evaluate the mechanical and tribological properties. Ball-on-plate impact tests were also carried out to assess coating layer adhesion/cohesion. Scanning electron microscopy (SEM) was used to observe coating morphology, and to examine wear scars from pin-on-disc tests and crater scars from impact tests. The work demonstrates that a hard and uniform DLC coating, with good adhesion and a low coefficient of friction, can be successfully deposited on top of an alumina intermediate layer, which provides excellent load support; such that the coating can withstand much higher contact stresses than would normally be the case with aluminium-alloy substrate materials. The C 2H 2/Ar ratio significantly influences the interfacial adhesion between the DLC and alumina layers, but has no significant effect on coating hardness. It is suggested that the C 2H 2/Ar ratio should be selected in the range of 0.25–0.35 to obtain a hard a-C:H carbon film with low-hydrogen-content and excellent adhesion. The investigations indicate that a duplex combination of micro-arc oxidation and PI 3 represents a promising technique for surface modification of Al-alloys for tribological applications in which high contact loads are anticipated.