JOURNAL OF MATERIALS SCIENCE LETTERS 19, 2 0 0 0, 1139 – 1141 Effects of duplex pre-treatment on diamond deposition on pure titanium BIBO YAN, NEE LAM LOH, YONGQING FU Materials Lab, School of Mechanical and Production Engineering, Nanyang Technological University, Singapore, 639798 E-mail: P145529986@ntu.edu.sg CHANG Q. SUN, PETER HING Gintic Institute of Manufacturing Technology, Singapore, 639798 Titanium alloys are characterized by poor tribological properties and low load bearing capacity; therefore, the traditional use of titanium alloys has been restricted to non-tribological application. The deposition of a well adherent diamond coating is a promising way to solve this problem [1]. However, the significant mismatch of thermal expansion between titanium substrate and diamond coating results in high compressive coating stresses leading to weak adherence of diamond coat- ing [2]. Good adhesion of diamond coatings on tita- nium alloy can be achieved using optimized substrate pretreatment and optimization in deposition parameters to achieve optimized interfacial microstructure [3, 4]. So far, much effort is still needed to decrease the high residual stress and to increase the adhesion strength of diamond coating on titanium. A common mechanical pretreatment process is ei- ther ultrasonic treatment with diamond powders, or scratching using diamond paste. Plasma treatment, such as plasma carbonitriding, can provide both micro- roughness and carbon species to the surface of the Ti substrate, thus improving the nucleation and growth rate of diamond coating [5]. Duplex surface engineer- ing, involving the sequential application of two (or more) established surface technologies, is a promis- ing way to further improve the adhesion strength of diamond coating [6]. In this study, the composite or graded interlayer combining titanium carbonitride and titanium nitride was prepared to improve the adhesion of diamond coating to Ti substrate. The compositional graded interlayer will enhance the chemical bonding between the substrate and diamond coating and pro- vide a lower thermal stress [7]. Plasma nitriding, plasma carbonitriding and diamond deposition were prepared by MPS4 microwave plasma assisted CVD reactor (Coaxial Power System Ltd., UK) using N 2 /CH 4 /H 2 gas mixture. The detail experi- mental parameters used in each treatment are listed in Table I. Surface morphology of the deposited coating was investigated by a Jeol scanning electron microscope. The in-depth nature and distribution of different inter- mediate crystalline structure of diamond coating on titanium substrate were determined by grazing inci- dence X-ray diffraction (GIXD) with different inci- dence angles [8]. The deposited diamond coating shows a dense, well faceted and polycrystalline morphology and is predom- inant in (111) orientation (see Fig. 1a) after 10-hour de- position directly on titanium substrate. Fig. 1b shows the cross-section of diamond coating which clearly re- veals the existence of three different layers: the top diamond coating, the TiC layer and heat-affected and carbon/hydrogen diffused Ti layer [9]. TiC layer, with a thickness of a few microns, is porous and rough which damages the adhesion strength of diamond coat- ing on Ti. The diffusion of hydrogen during the diamond Figure 1 (a) The surface morphology of diamond coating after 10-hour deposition; (b) Cross-section of diamond coating on pure Ti indicating the formation of porous TiC. 0261-8028 C  2000 Kluwer Academic Publishers 1139