Deposition of duplex Al2O3/DLC coatings on Al alloys for tribological applications using a combined micro-arc oxidation and plasma-immersion ion implantation technique

Micro-are discharge oxidation (MDO) is a cost-effective plasma electrolytic process which can be used to improve the wear resistance of aluminium ahoy parts by creating a hard thick alumina coating on the component surface. H owever, for sliding wear applications, such alumina coatings often exhibit relatively high friction coefficients against many counterface materials. T herefore, a duplex treatment, combining a load-supporting MDO alumina layer with a low friction diamond-like carbon (DLC) coating, produced by a modif ied plasma-immersion ion implantation (PI3) process, has been investigated. PI3 provides a flexible method of implanting ions into complex-shaped part s using a low temperature, low voltage plasma onto which high voltage pulse s are superimposed. It can also be used to enhance the adhesion and growth characteristics of films formed under plasma conditions. In this work, a we akly-ionized, hot-filament supported low-voltage argon-acetylene plasma (wi th C2H2/Ar ratios from 1.0 to 0.15) was used, in combination with a low-fre quency de pulse voltage PI3 system tin this case 100 mus, 5 kV pulses at 85 0 Hz) to deposit a low friction DLC top layer onto MDO-treated AI alloy cou pons. Microhardness measurements and pin-on-disc sliding wear tests were pe rformed to evaluate the mechanical and tribological properties. Ball-on-pla te impact tests were also carried out to assess coating layer adhesion/cohe sion. Scanning electron microscopy (SEM) was used to observe coating morpho logy, and to examine wear scars from pin-on-disc tests and crater scars fro m impact tests. The work demonstrates that a hard and uniform DLC coating, with good adhesion and a low coefficient of friction, can be successfully d eposited on top of an alumina intermediate layer, which provides excellent load support; such that the coating can withstand much higher contact stres ses than would normally be the case with aluminium-alloy substrate material s. The C2H2/Ar ratio significantly influences the interfacial adhesion betw een the DLC and alumina layers, but has no significant effect on coating ha rdness. It is suggested that the C2H2/Ar ratio should be selected in the ra nge of 0.25-0.35 to obtain a hard a-C:H carbon film with low-hydrogen-conte nt and excellent adhesion. The investigations indicate that a duplex combin ation of micro-are oxidation and PI3 represents a promising technique for s urface modification of AI-alloys for tribological applications in which hig h contact loads are anticipated. (C) 2000 Elsevier Science B.V. All rights reserved.