Metal Reinforced Diamond Composite Films
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METAL REINFORCED DIAMOND COMPOSITE FILMS C. TSAI*, J. NELSON*, W. GERBERICH*, J. HEBERLEIN** AND E. PFENDER** *Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 "**Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455 ABSTRACT A three-step process for producing a composite diamond film is presented. Plasma deposited diamonds are reinforced by an electroplated metal binder and the diamonds regrown to produce a continuous, strongly adhered diamond film on molybdenum and tungsten substrates. Microscratching and pin-on-disk testing indicate that the composite films are more adherent than plasma deposited diamonds alone. INTRODUCTION With a combination of high hardness, and high thermal and chemical stability, diamond films have high potential as good protective coatings. The current problems with diamond coatings are the poor adhesion and low toughness'. 2 . This paper presents a threestep process for making metal-reinforced diamond coatings to address these problems. THREE-STEP PROCESS FOR MAKING DIAMOND COMPOSITE FILMS The method is comprised of the steps of: (i) deposition of diamond crystals by radio-frequency (RF) thermal plasma chemical vapor deposition; (ii) filling the voids between diamond crystals with metal or alloy using electroplating; and (iii) further growth of diamond on previously deposited diamond crystals. The benefits of this process are threefold: (a) The electrodeposition process favors binder/substrate bonding because of the greater substrate conductivity compared to diamond. This produces small void densities in the binder. (b) An appropriate metal with high thermal expansion coefficient puts the diamond crystals into compression upon heating, increasing their fracture and chipping resistance for abrasion and wear applications. (c) A very dense and high wear-resistant diamond film is obtainable after re-growing diamonds because the upper parts of the diamond crystals interlock with each other while their lower parts are embedded in the metal binder phases as well as being attached to the substrate. (i) Diamond deposition on substrates: Diamond crystals are deposited on metal substrates by RF thermal plasma chemical vapor deposition 3 ,4 . Figure 1 shows a schematic of the diamond deposition apparatus. In a typical 1-hour run, the average particle size is 15 ýLtm.The deposition conditions are summarized in Table 1.
Mat. Res. Soc. Symp. Proc. Vol. 239. 01992 Materials Research Society
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Table I. Conditions for Diamond deposition kV, 4 MHz RF power: 2 A, 6.4 Sheath gas flow rate: 35 L/min (Ar) CH 4 flow rate : 0.05 L/min H2 flow rate: 5.0 L/min Process pressure : 300 torr Substrate temperature : 900-1100°C Deposition time : 20 - 50 min
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Cooling water Figure 1: Schematic of the RF thermal plasma CVD reactor. (ii) Electroplating of metal binder: Electroplating with metal is performed after diamond deposition by standard plating techniques. Since the electrical conductivity of diamond is extremely
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