Diamond-ceramic composite tool coatings
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We have developed multilayer composite diamond coatings with improved adhesion and wear resistance on WC(Co) tool substrates. The coatings consist of a first layer of discontinuous diamond crystallites that are anchored to the WC(Co) substrate by an interposing layer of ceramic films. These films consist of TiC, TiN, SiC, Si 3 N 4 , or WC deposited to provide a conformal coverage on the first layer of diamond. A second or final layer of continuous diamond film is deposited to provide the cutting edge of the tool. The diamond film in the composite layers is deposited by hot filament chemical vapor deposition (HFCVD) and the interposing layer is deposited by laser physical vapor deposition (LPVD). The different parameters associated with the deposition of diamond and interposing layers are optimized to improve the adhesion and wear resistance. We have studied the adhesion characteristics by indentation tests in which the critical load for peeling of the diamond films is determined. Adhesion and wear resistance of the films are also tested using an overlap polishing on diamond paste with 5 - 6 yu,m particle size. The diamond and interposing layers in the composite are characterized by scanning electron microscopy and Raman spectroscopy. Results of improvement in adhesion and wear resistance are correlated with the quality of the diamond film and the interposing layer. Better accommodation of thermal stresses and strains in the composite layers has been shown to be responsible for improvement in the adhesion and wear resistance of the composite diamond films.
I. INTRODUCTION Four important properties associated with cutting tools for machining are high hot hardness, good fracture toughness, chemical stability in relation to work piece, and good wear resistance.1'2 In addition, ease of fabrication of cutting tools contributes to the lower cost of machining. It is well known that diamond has the highest modulus, low coefficient of friction, high softening temperature, and excellent corrosion and wear resistance in machining metals and alloys in which carbon does not form a solid solution.3 Thus, diamond can be used in machining nonferrous metals, alloys, and composites which require a high hardness and wear resistance.4-5 Machining composites is a difficult task because of large abrasive wear imposed on the machine tool by the relatively hard component present in the composites. Conventional machine tools made of carbides and nitrides are susceptible to failure and shortened tool life. Diamond tools are considered more suitable for machining composites because of improved tool life. A comparison of diamond tools with carbide and nitride tools can be made in order to show the relative advantages. For example, a recent study on machining graphite composites showed that the carbide wear ratio was much higher than that of diamond, and the surface 2850 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 11, Nov 1994 Downloaded: 11 Oct 2015
finish deteriorated rapidly as the flank wear increased.5 8 Machinability tria
